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  • 1995-1999  (252,572)
  • 1990-1994  (22)
  • 1920-1924
  • 1997  (121,238)
  • 1996  (131,356)
Years
Year
Language
  • 1
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physics Letters B 294 (1992), S. 466-478 
    ISSN: 0370-2693
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physics Letters B 317 (1993), S. 474-484 
    ISSN: 0370-2693
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 3
    Book
    Book
    Wien [u.a.] :Springer, ; 1.1977 - 16.2003; damit Ersch. eingest.
    Title: Computing : archives for informatics and numerical computation; Supplementum
    Publisher: Wien [u.a.] :Springer,
    Year of publication: 1977-2003
    Dates of Publication: 1.1977 - 16.2003; damit Ersch. eingest.
    Type of Medium: Book
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  • 4
    Online Resource
    Online Resource
    Berlin ; Heidelberg :Springer, ; 1.1995(1996) - 8.2002
    Title: Journal of molecular modeling
    Publisher: Berlin ; Heidelberg :Springer,
    Year of publication: 1996-2002
    Dates of Publication: 1.1995(1996) - 8.2002
    ISSN: 0949-183X
    Type of Medium: Online Resource
    Language: Undetermined
    Subsequent Title: Forts. ---〉:Journal of molecular modeling
    Note: Kumuliert jeweils den abgeschlossenen Jg. der Internetausg.
    Parallel Title: Druckausg. ---〉:Molecular modeling annual
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  • 5
    Journal/Serial
    Journal/Serial
    New York, NY :ACM, ; 1.1969 - 7.1975/76; N.S. 1.1976 - 21.2001,1; damit Ersch. eingest.
    Title: SIGBIO newsletter /
    Author: Association for Computing Machinery / Special Interest Group on Biomedical Computing
    Publisher: New York, NY :ACM,
    Year of publication: 1969-2001
    Dates of Publication: 1.1969 - 7.1975/76; N.S. 1.1976 - 21.2001,1; damit Ersch. eingest.
    ISSN: 0163-5697
    Type of Medium: Journal/Serial
    Language: Undetermined
    Parallel Title: Internetausg. ---〉:Biomedical computing
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  • 6
    Journal/Serial
    Journal/Serial
    Los Alamitos, Calif. :Soc., ; 5.1997 - 8.2000; damit Ersch. eingest.
    Title: IEEE concurrency /
    Author: Institute of Electrical and Electronics Engineers / Computer Group
    Publisher: Los Alamitos, Calif. :Soc.,
    Year of publication: 1997-2000
    Dates of Publication: 5.1997 - 8.2000; damit Ersch. eingest.
    ISSN: 1092-3063
    Type of Medium: Journal/Serial
    Language: Undetermined
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  • 7
    Journal/Serial
    Journal/Serial
    New York, NY :North-Holland, ; 1.1984 - 46.2000
    Title: ¬The¬ journal of logic programming
    Publisher: New York, NY :North-Holland,
    Year of publication: 1984-2000
    Dates of Publication: 1.1984 - 46.2000
    ISSN: 0743-1066
    Type of Medium: Journal/Serial
    Language: Undetermined
    Subsequent Title: Forts. ---〉:¬The¬ journal of logic and algebraic programming
    Parallel Title: Internetausg. ---〉:¬The¬ journal of logic and algebraic programming
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  • 8
    Book
    Book
    New York, NY :ACM, ; Nachgewiesen 2.1971 - 20.1999,4; damit Ersch. eingest.
    Title: Computer personnel : a quarterly publ. of the Special Interest Group on Computer Personnel Research, SIGCPR
    Publisher: New York, NY :ACM,
    Year of publication: 1971-1999
    Dates of Publication: Nachgewiesen 2.1971 - 20.1999,4; damit Ersch. eingest.
    ISSN: 0160-2497
    Type of Medium: Book
    Parallel Title: Internetausg. ---〉:Computer personnel
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  • 9
    E-Resource
    E-Resource
    Oxford :Univ. Press, ; 1.1996/97 - 3.1999; damit Ersch. eingest.
    Title: Medical image analysis CD : an international journal of computer vision, visualization and image- guided intervention in medicine
    Publisher: Oxford :Univ. Press,
    Year of publication: 1996-1999
    Dates of Publication: 1.1996/97 - 3.1999; damit Ersch. eingest.
    ISSN: 1361-8431
    Type of Medium: Electronic Resource
    Language: Undetermined
    Parallel Title: Druckausg. ---〉:Medical image analysis
    Parallel Title: Internetausg. ---〉:Medical image analysis
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  • 10
    Journal/Serial
    Journal/Serial
    Amsterdam :CWI, ; 1.1988 - 12.1999; damit Ersch. eingest.
    Title: CWI quarterly
    Author: Centrum voor Wiskunde en Informatica 〈Amsterdam〉
    Publisher: Amsterdam :CWI,
    Year of publication: 1988-1999
    Dates of Publication: 1.1988 - 12.1999; damit Ersch. eingest.
    ISSN: 0168-826X , 0922-5366
    Type of Medium: Journal/Serial
    Language: Undetermined
    Former Title: Vorg. ---〉:Centrum voor Wiskunde en Informatica 〈Amsterdam〉: CWI newsletter
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  • 11
    Journal/Serial
    Journal/Serial
    Woodbury, NY :AIP, ; 1.1987,1(Nov./Dez.); 2.1988 - 12.1998
    Title: Computers in physics /
    Contributer: American Institute of Physics
    Publisher: Woodbury, NY :AIP,
    Year of publication: 1987-1998
    Dates of Publication: 1.1987,1(Nov./Dez.); 2.1988 - 12.1998
    ISSN: 0894-1866
    Type of Medium: Journal/Serial
    Language: Undetermined
    Subsequent Title: Forts. ---〉:Computing in science & engineering
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  • 12
    Title: SIGART bulletin : a quarterly publ. of the Special Interest Group on Artificial Intelligence
    Author: Association for Computing Machinery / Special Interest Group on Artificial Intelligence
    Publisher: New York, NY :ACM,
    Year of publication: 1990-1998
    Dates of Publication: 1.1990 - 9.1998
    ISSN: 1053-4830
    Type of Medium: Journal/Serial
    Language: Undetermined
    Former Title: Vorg. ---〉:Association for Computing Machinery / Special Interest Group on Artificial Intelligence: SIGART newsletter
    Subsequent Title: Forts. ---〉:Intelligence
    Parallel Title: Internetausg. ---〉:Artificial intelligence
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  • 13
    Journal/Serial
    Journal/Serial
    San Francisco, Calif. :Miller Freeman, ; 1.1983 - 16.1998,3
    Title: Unix review : the publication for the Unix community
    Publisher: San Francisco, Calif. :Miller Freeman,
    Year of publication: 1983-1998
    Dates of Publication: 1.1983 - 16.1998,3
    ISSN: 0742-3136
    Type of Medium: Journal/Serial
    Language: Undetermined
    Subsequent Title: Forts. ---〉:Unix review's performance computing
    Parallel Title: Internetausg. ---〉:Unix review.com
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  • 14
    Journal/Serial
    Journal/Serial
    Amsterdam [u.a.] :Elsevier [u.a.], ; 9.1985 - 30.1998
    Title: Computer networks and ISDN systems : the international journal of computer and telecommunications networking
    Publisher: Amsterdam [u.a.] :Elsevier [u.a.],
    Year of publication: 1985-1998
    Dates of Publication: 9.1985 - 30.1998
    ISSN: 0169-7552 , 0376-5075
    Type of Medium: Journal/Serial
    Language: Undetermined
    Subsequent Title: Vorg. u. Forts. ---〉:Computer networks
    Note: Computer networks for research in Europe
    Additional Information: In 14,1=15 von:Networkshop: Conference report
    Additional Information: 16,1/2=4; 17,4/5=5 von:European Networkshop: European Networkshop
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  • 15
    Journal/Serial
    Journal/Serial
    New York, NY :ACM, ; 4.1969 - 33.1998,2; damit Ersch. eingest
    Title: SIGNUM newsletter
    Author: Association for Computing Machinery / Special Interest Group on Numerical Mathematics
    Publisher: New York, NY :ACM,
    Year of publication: 1969-1998
    Dates of Publication: 4.1969 - 33.1998,2; damit Ersch. eingest
    ISSN: 0163-5778
    Type of Medium: Journal/Serial
    Former Title: Vorg. ---〉:Association for Computing Machinery / Special Interest Committee on Numerical Mathematics: SICNUM newsletter
    Additional Information: 16,3=3,2 von:Association for Computing Machinery / Technical Committee on Fortran: FORTEC forum
    Parallel Title: Internetausg. ---〉:Numerical mathematics
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  • 16
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3345-3351 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrationally and rotationally resolved spectra of the first-tier E0+(v′) ion-pair state ←X0+(v′′=0) ground-state transition of jet-cooled BrCl near 145 nm were recorded by exciting the halogen with a tunable vacuum ultraviolet "laser" generated by four-wave difference mixing in Kr gas, and detecting the ions produced by (1+1′) resonance enhanced multiphoton ionization in a linear time-of-flight mass spectrometer. J-numberings for thirty individual isotopic bands were established by ground-state combination differences, and per band rotational constants were derived. Extensive perturbations precluded the determination of an unambiguous absolute v′-numbering for the observed features or equilibrium rotational constants. However, excited-state vibrational isotope shift measurements do indicate that transitions from v′′=0 of the ground-state probe vibrational levels between v′=173 and 205. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 17
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3363-3375 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this work we analyze clusters between the methoxy radical (CH3O, an open-shell molecule) and the nonpolar solvents Ar, N2, CH4, and CF4. CH3O is formed through the photolysis of CH3OH vapor in a supersonic expansion of CH3OH and a solvent gas (Ar, N2, CH4, CF4) seeded in a carrier gas of He. The radical and solvent molecules are cooled to ∼15–20 K and form clusters. These clusters are probed using laser induced fluorescence (LIF) of the CH3O radical. An extensive set of calculations, including ab initio and atom–atom potential calculations and rotational contour simulations are performed for each cluster in order to elucidate the cluster structure and the nature and relative importance of the limiting types of interactions that are responsible for cluster binding. A final minimum energy structure is presented for each cluster, together with the analysis of the limiting type of interactions that generate the van der Waals binding of the cluster. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 18
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3402-3416 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational eigenvalues with estimated errors 〈5×10−2 cm−1 and their corresponding eigenfunctions for J=0 5D (planar) acetylene modeled by the Halonen–Child–Carter potential-energy surface are obtained using an energy-shifted, imaginary-time Lanczos propagation of symmetry-adapted wave packets. A lower resolution (∼4 cm−1) vibrational eigenspectrum of the system is also calculated by the Fourier transform of the autocorrelation of an appropriate wave packet. The eigenvalues from both approaches are in excellent agreement. The wave function of the molecule is represented in a direct-product discrete variable representation (DVR) with nearly 300 000 grid points. Our results are compared with the previously reported theoretical and experimental values. We use our 69 computed eigenstates as a basis to perform an optimal control simulation of selective two-photon excitation of the symmetric CH-stretch mode with an infrared, linearly polarized, transform-limited, and subpicosecond–picosecond laser pulse. The resulting optimal laser pulses, which are then tested on the full DVR grid, fall within the capabilities of current powerful, subpicosecond, and tunable light sources. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 19
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3447-3459 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The high resolution laser induced fluorescence spectra of the A˜ 2Σ+−X˜ 2Π3/2 electronic transition of the R⋅SH/D (R= Kr, Ar, Ne) van der Waals (vdW) complexes are reported. Analysis of these bands requires the inclusion of rotation, fine and hyperfine structure, spin-rotation interactions, and parity splittings. A number of molecular parameters are determined, along with internuclear bond distances between the R and the SH moiety. Comparison of the present results for R⋅SH/D is made with the analogous R⋅OH/D species where applicable. In addition, the detailed "rotational" structure and the highly precise determinations of the band origins of the different heavy atom isotopomers are critical for absolute vibrational quantum number assignment in the A˜ state. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 20
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3675-3683 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The temporal dependence of the time correlation function detected by quasielastic light (QEL) scattering in solution consisting of two homopolymers with arbitrary hydrodynamic and thermodynamic properties is investigated. New results appropriate for the ternary polymer solution in various concentrations are obtained. These new results should facilitate the analysis of the QEL scattering data of a ternary solution in any concentrations. Information about tracer diffusion coefficients of the constitutive polymers and their excluded volume interaction parameter is shown to be obtainable from the QEL experiment of the ternary polymer solution. Applications to ternary polymer solutions with one polymer isorefractive with the solvent, solutions with two polymers differing only in molecular weight, and solutions with two polymers having arbitrary refractive index increments are discussed. Clarification in regard to the ternary solution under the zero average contrast (ZAC) condition is also made. It is shown that under the ZAC condition, the autocorrelation function of the composition fluctuation in general decays with two exponentials, even at 1:1 symmetric composition. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 21
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3726-3729 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A differential in the relaxation rates of the ice surface and subsurface during Ostwald ripening near 138 K is the basis for a new approach to determination of the separate surface and subsurface vibrational spectra of ice nanocrystals. The resulting FT-ir spectrum of the ice subsurface matches that previously obtained from adsorbate-induced relaxation of the subsurface, confirming both the form of the surface and subsurface spectra, and the disordered nature of the surface of the nanocrystals. From these data, together with the results of simulation studies, ice emerges as the only molecular crystalline substance for which the surface and subsurface spectra are known and assigned. Semiquantitative rates of the structural relaxations near the surface, for bare ice and ice coated with the adsorbate SO2, have also been determined. Finally, spectral data are presented that track the insertion of SO2 into the vulnerable strained H bonds common to simulated structures of the ice surface. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 22
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3738-3739 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate all-electron Dirac–Fock–Roothaan calculations were performed for the 1Σ+ ground state of ThO using Gaussian basis sets of Th (24s20p16d12f)/[19s15p12d5f] and O (10s6p1d)/[7s4p1d]. The relativistic effects in ThO were found to slightly extend the bond length re, to increase the dissociation energy De, and to decrease the harmonic frequency ωe. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 23
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2734-2737 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Systematic analysis is carried out for molecular phase in (ω1,ω3) coherent control scheme. Two mechanisms for nonzero molecular phase are proposed. It is shown that the imaginary part of the three-photon transition operator, which is due to the contribution of the intermediate continuua, can give different phase factors for the one- and three-photon transition amplitudes, yielding very important effects of molecular phase in (ω1,ω3) coherent control. We also show that the interactions between the final continuum states can result in nonzero molecular phase, when the intermediate states are discrete. These findings are discussed in relation to Gordon and co-workers' recent observations [Zhu et al., Science 270, 77 (1995)] on phase lag. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 24
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2752-2759 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Line intensities in the (0,0) band of NF(b 1Σ+→X 3Σ−) spin forbidden emission have been observed in a microwave apparatus. Deduced line strengths of the OP and SR well resolved branches are shown differently, in contrast with the theoretical prevision, and are interpreted by taking into account some peculiarities of the NF radical. An intermediate (a) and (b) Hund case is seen to apply toOP and SR in the whole rotational range examined. Accordingly, a reliable value for the effective transition momentμe is evaluated from the experiment. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 25
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2798-2806 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reactions of laser-ablated Cr atoms with O2 gave a very strong, sharp 965.4 cm−1 band and weak, sharp 1869.7, 984.3, 914.4, 846.3, 716.2, and 643.1 cm−1 bands. The 1869.7, 965.4, and 914.4 cm−1 bands track together on annealing, show 52Cr, 53Cr, 54Cr isotopic splittings appropriate for a single Cr atom and triplets with statistical 16,18O2 for two equivalent O atoms, and are assigned to the ν1+ν3, ν3 and ν1 modes of the bent (128°±4°) chromium dioxide OCrO molecule. The 984.3 cm−1 band shows chromium isotopic splittings for two Cr atoms and 16,18O2 components for two O atoms, and is attributed to the bent CrOCrO molecule. The weak 846.3 cm−1 band exhibits proper oxygen isotopic behavior for CrO and is redshifted 39 cm−1 from the gas-phase value, the maximum shift observed for a first row transition metal monoxide. The sharp 716.2 and 643.1 cm−1 bands track together; the former reveals Cr isotopic splittings for two Cr atoms and the latter 16,18O2 splittings for two sets of dioxygen subunits; the branched-puckered-ring dimer O(Cr2O2)O is identified. Annealing produces new bands due to CrOO, CrO3, Cr(OO)2 and the ring dimers (Cr2O2) and (Cr2O2)O, which are identified from isotopic shifts and splitting patterns. © 1997 American Institute of Physics.
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  • 26
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2839-2844 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the capture of lithium atoms by a beam of large cold helium clusters and subsequent ion production by electron impact. Li atoms are efficiently picked up by the He droplets, although with a cross section significantly lower than that for other atoms and molecules. The mass spectrum reveals the presence of Li atom and dimer ions, as well as the weakly bound complex HeLi+, confirming that capture by He-cluster beams can be used for efficient soft ionization of fragile species. The electron-energy dependence of the Li+, Li2+, and HeLi+ yield shows that they are formed primarily by Penning ionization in a collision with a metastable He atom in the droplet. This leads to the conclusion that lithium metal atoms are not submerged in the helium clusters but locate on the surface, corroborating theoretical predictions for bulk helium surfaces and spectroscopic measurements. © 1997 American Institute of Physics.
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  • 27
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2845-2852 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The determination of rate constants for chemical reaction systems is of considerable practical importance. This paper introduces a temporal inversion method for parameter identification of dynamical systems, and butene isomerization is used to illustrate the method. The rate constants are artificially treated as temporal functions and an explicit equation describing the time evolution of the rate constants is obtained by minimizing the error along the reaction course. With appropriate experimental data the unknown rate constants are guided from arbitrary initial values to their best values for the available experiments at the final time. The method leads to an iterative algorithm. Numerical results show that accurate estimates of all the rate constants can be obtained. © 1997 American Institute of Physics.
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  • 28
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2872-2880 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Within the universal integral formalism of encounter theory, we described (a) the bimolecular charge recombination that follows the geminate one beginning with photoionization and (b) bimolecular quenching of singlet and triplet excitations parallel to an inter-system crossing (ISC). In both cases the new features appear which represent either the geminate ion pair density (positive) or pair vacancies density (negative), which accounts for singlets quenched before ISC. It was justified that the shortage of near-contact excitations resulting from static singlet quenching may be built into the initial conditions for subsequent evolution of triplets and lead to the "anti-Smoluchowski time behavior" of their annihilation rate constant. However, only an integral theory can account for the same effect when singlet quenching is diffusion accelerated. © 1997 American Institute of Physics.
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  • 29
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2903-2912 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose a common rigorous foundation to the classical collision theory and that of the classical activated complex. Based on the notion of activated complex, this foundation relies on a stochastic approach showing up the different influence factors of a chemical reaction. The thermodynamic formulation is obtained here by assuming the exponential statistical distribution within each stable chemical state. The general model we obtain yields two stochastic formulations called the stochastic transition state theory (denoted STST) and the stochastic activated collision theory (denoted SACT) respectively, depending on whether the rate of the reaction is of the same scale as, either the rate of passing over the potential energy barrier, for the STST, or the rate of reaching the activated complex state (which is a generalization of the collision rate), for the SACT. The modeling is first done in the case of a closed small system undergoing stochastic changes of chemical state and then, by extension, in the frame of classical chemical kinetics. Some properties of these two theories are studied. Furthermore the formulation is applied to the stochastic example of RNA structure changes and to the classical simple example of a reaction between two atoms A and B to form the diatomic molecule AB. © 1997 American Institute of Physics.
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  • 30
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2920-2929 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Raman excitation profiles for twelve vibrational modes of the solvatochromic dye betaine-30 have been measured in CH3OH and CD3OD solutions at wavelengths that span the S0→S1 charge transfer transition. Though the absorption spectra of the dye are the same in the protonated and deuterated forms of the solvent, Raman cross-sections for all modes were found to be generally lower in the CH3OH solution than inCD3OD. The time-dependent theory of Heller was applied to model the absorption and Raman profiles, and both mono- and bi-exponential solvent relaxation were considered in order to account for solvent induced electronic dephasing. The two models lead to different physical pictures for the relevant solvent dynamics, but in either case the amplitude of solvent dephasing is reduced in deuterated compared to protonated methanol. The effect is interpreted in terms of stronger solvent–solute and solvent–solvent hydrogen bonding in deuterated methanol solution. Comparing to previous results for betaine-30 Raman cross-sections in acetonitrile [Y. Zong and J. L. McHale, J. Chem. Phys. 106, 4963 (1997)], it is concluded that slower solvent dynamics and perturbations to the electronic structure of betaine-30 lead to larger Raman intensities in methanol. The data suggest that solvent dynamics depend strongly on the electronic state of betaine-30. © 1997 American Institute of Physics.
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  • 31
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2974-2989 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the investigations of nonadiabatic scattering processes (reactive as well charge-transfer) occurring in B++H2 reaction in the gas phase on the triplet electronic surfaces utilizing a mixed quantum-classical approach to scattering of three particle systems in hyperspherical coordinates. The time-dependent Schrödinger equation is solved in diabatic representation using wave packet propagation method on a grid in two quantum dimensions. The potential-energy surfaces have been obtained using the valence-bond diatomics-in-molecule approach. © 1997 American Institute of Physics.
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  • 32
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3032-3041 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a new integral equation formulation of the polarizable continuum model (PCM) which allows one to treat in a single approach dielectrics of different nature: standard isotropic liquids, intrinsically anisotropic medialike liquid crystals and solid matrices, or ionic solutions. The present work shows that integral equation methods may be used with success also for the latter cases, which are usually studied with three-dimensional methods, by far less competitive in terms of computational effort. We present the theoretical bases which underlie the method and some numerical tests which show both a complete equivalence with standard PCM versions for isotropic solvents, and a good efficiency for calculations with anisotropic dielectrics. © 1997 American Institute of Physics.
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  • 33
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3085-3097 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new parametric crossover model for the phase behavior of a binary mixture is presented that corresponds to the Leung–Griffiths model in the critical region and is transformed into the regular classical expansion far away from the critical point. The model is optimized to, and leads to excellent agreement with, isothermal vapor–liquid equilibrium data for dilute aqueous solutions of sodium chloride by Bischoff and co-workers. It then accurately predicts constant-composition phase equilibrium loci as measured by independent workers. This crossover model is therefore capable of representing the thermodynamic surface of ionic solutions in a large range of temperatures and densities around the critical points of vapor–liquid equilibrium.
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  • 34
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3144-3158 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by performing molecular dynamics simulations. The equations of motion have been augmented by a director constraint torque that fixes the orientation of the directors. This makes it possible to fix them at different angles relative to the stream lines in shear flow simulations. In equilibrium simulations the constraints generate a new ensemble. One finds that the Green–Kubo relations for the viscosities become linear combinations of time correlation function integrals in this ensemble whereas they are complicated rational functions in the conventional canonical ensemble. We have evaluated these Green–Kubo relations for all the shear viscosities and all the twist viscosities. We have also calculated the alignment angles, which are functions of the viscosity coefficients. We find that there are three real alignment angles but a linear stability analysis shows that only one of them corresponds to a stable director orientation. The Green–Kubo results have been cross checked by nonequilibrium shear flow simulations. The results from the different methods agree very well. Finally, we have evaluated the Miesowicz viscosities [D. Baalss, Z. Naturforsch. Teil A 45, 7 (1990)]. They vary by more than 2 orders of magnitude. The viscosity is consequently highly orientation dependent. © 1997 American Institute of Physics.
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  • 35
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3196-3203 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cluster-cluster collisions and cluster dissociation into two smaller clusters have an effect on steady-state size distributions and nucleation rates. Clustering also affects nucleation rates by influencing the saturation vapour pressure, which is often ignored in nucleation studies. Both the kinetic and the vapour pressure effect of clustering on nucleation in one and two component systems are investigated. For water, the effect of cluster-cluster kinetics is in the order of 10%, and the effect via vapour pressure in the order of 100% in nucleation rate. The approximate method [Shugard et al., J. Chem. Phys. 75, 5298 (1974)] to take the kinetic effect of clustering into account works well for water. In acetic acid vapour the dimer concentrations are an order of magnitude higher than monomer concentrations. Even in this extreme case the approximate way to describe the kinetic effect of clustering gives reasonable estimates of the correct rates. In sulphuric acid-water mixture the kinetic effect is from one to four orders of magnitude in nucleation rate, depending on whether the clustering is accounted for in the saturation vapour pressure or not. The effect of clustering via vapour pressure is 5-10 orders of magnitude, depending on the kinetic model used. The approximate way to describe cluster kinetics works only in some of the cases. © 1997 American Institute of Physics.
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  • 36
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3239-3246 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Fokker–Planck equation is solved by the method of distributed approximating functionals via forward time propagation. Numerical schemes involving higher-order terms in Δt are discussed for the time discretization. Three typical examples (a Wiener process, an Ornstein–Uhlenbeck process, and a bistable diffusion model) are used to test the accuracy and reliability of the present approach, which provides solutions that are accurate up to ten significant figures while using a small number of grid points and a reasonably large time increment. Two sets of solutions for the bistable system, one computed using the eigenfunction expansion of a preceding paper and the other using the present time-dependent treatment, agree to no fewer than five significant figures. It is found that the distributed approximating functional method, while simple in its implementation, yields the most accurate numerical solutions yet available for the Fokker–Planck equation. © 1997 American Institute of Physics.
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  • 37
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3262-3266 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The local structure of a Lennard-Jones fluid confined to a mesoscopic slit-pore with structureless (i.e., "smooth") walls is investigated by computing the local density ρ(z) in grand canonical ensemble Monte Carlo (GCEMC) simulations. For noncritical thermodynamic states of a corresponding bulk fluid, a homogeneous core region exists in the pore characterized by the bulk density ρbulk. Sufficiently close to the bulk critical point, however, the core is depleted, that is its local density ρcore(z) is less than ρbulk. Along a bulk near-critical isotherm T*=1.36(similar, equals)Tc,bulk* GCEMC data are analyzed in terms of the equations of state μbulk=μ(ρbulk,Tc,bulk) and μcore=μ(ρ¯mid,Tc,bulk), respectively, where ρ¯mid is the average midpoint density of the core fluid. At densities sufficiently above and below the bulk critical density thermodynamic states of core and bulk are the same, i.e. μbulk=μcore. However, if depletion is observed the density dependence of μbulk and μcore is generally different. By applying a mean-field theory to μbulk and μcore depletion is related to a reduced degree of criticality of the core fluid compared with the bulk phase in a self-contained way. © 1997 American Institute of Physics.
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  • 38
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3309-3323 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociative chemisorption of H2 on Cu(100) has been studied using a six-dimensional wave packet method. All molecular degrees of freedom are treated quantum mechanically, with no dynamical approximations. The potential-energy surface used is an analytical fit to calculations employing density functional theory (DFT), using the generalized gradient approximation (GGA) and a slab representation of the metal surface. The dependence of the reaction probability on normal incidence energy has been obtained for both (v=0, j=0) and (v=1, j=0) H2. Comparison to experiment suggests that, on average, the DFT method overestimates the barriers to dissociation by ∼0.15 eV for H2+Cu(100). In broad agreement with experiments on H2+Cu(111) and H2+Cu(110) the calculations show large vibrational inelasticity. A prediction of the survival probability of (v=1, j=0) H2 as a function of incidence energy is also presented. The measurement of this quantity is now feasible and should provide another sensitive test of the DFT potential-energy surface. © 1997 American Institute of Physics.
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  • 39
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3333-3336 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An integral equation approach for a binary hard-sphere mixture interacting with a planar semipermeable wall (membrane) is formulated by using the second-order nonuniform or pair Ornstein–Zernike equation as well as the usual singlet Ornstein–Zernike equation. The results of the pair theory are compared with those obtained from the singlet theory and with computer simulation data. The pair approach is more accurate than the singlet theory. © 1997 American Institute of Physics.
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  • 40
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    The Journal of Chemical Physics 107 (1997), S. 2175-2179 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A summary and selected results are presented for calculations of the rotational strengths of circular dichroism transitions in the microwave for the three chiral deuterated oxiranes, methyl- and trans-dimethyloxirane, and methylthiirane. The calculations, up to J=10, were carried out using a modified version of the spectral cataloging programs of Pickett. A selection of transitions, with rotational strengths between 10−44 and 100×10−44 esu2 cm2, which are considered to be the most likely candidates for observation of rotational circular dichroism, is given. © 1997 American Institute of Physics.
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  • 41
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    The Journal of Chemical Physics 107 (1997), S. 2205-2214 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mercury atoms excited to the 3P1 state in rare gas and molecular (CH4, CD4, CF4, and SF6) matrices relax by a direct 3P1→1S0 fluorescence with a ca. 60 ns lifetime or by the 3P1(functional relationship)3P0 radiationless transition followed by the 3P0→1S0 long-lived (τ in the 10 to 500 ms range) emission. The main difference between atomic and molecular hosts is the appearance, in the latter case, of a well resolved vibrational structure in the spectrum of the 3P0→1S0 transition involving the frequencies of nontotally symmetric modes of the host molecules. This structure, related to a shortening of the lifetime of the 3P0→1S0 emission is explained by the intensity borrowing from the allowed 3P1→1S0 transition, i.e., by the 3P1↔3P0 mixing induced by nontotally symmetric vibrations of neighbor molecules (analog of the Herzberg–Teller effect). Relative intensities of bands corresponding to different vibrational modes may be deduced from estimation of electric fields induced in the Hg atom by displacements of electric charges of the vibrating molecule © 1997 American Institute of Physics.
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  • 42
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    The Journal of Chemical Physics 107 (1997), S. 2242-2248 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Relative predissociation rates of OH (A 2Σ+, v′=3) have been determined for N′ up to 17 by simultaneously measuring laser-induced fluorescence excitation and cavity ring down absorption in the same experimental setup. The results are in overall agreement with calculated predissociation rates by Yarkony [J. Chem. Phys. 97, 1838 (1992)], but show a stronger increase of the predissociation rate with increasing N. The obtained values for the quantum yield can be used in the application of the laser-induced predissociative fluorescence detection technique in combustion diagnostics. © 1997 American Institute of Physics.
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  • 43
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    The Journal of Chemical Physics 107 (1997), S. 2284-2291 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodetachment-photoelectron (PD-PES) and anion-zero kinetic energy photoelectron (anion-ZEKE) spectra of FeO have been measured. The vibrational progression bands of the X 5Δi(FeO)←X 5Δ7/2(FeO−) transition in the PD-PES spectrum exhibit substructure which could not be resolved in earlier PD-PES spectra. A comparison with the high resolution anion-ZEKE spectrum clearly shows the existence of a second low energetic electronic state which could be the a 7Σ+ of neutral FeO proposed by several authors. In addition, for the A 5Σ+ state of FeO an excess energy of 4050 cm−1 was found. Vibrational frequencies for the X 5Δ, a 7Σ+, and A 5Σ+ states have been determined as 882, 887, and 800 cm−1. All spin orbit splittings of the neutral and anionic ground states could be measured directly or deduced from spin orbit combination transitions. We succeeded in resolving the rotational envelope of the vibrational origin of the neutral–anion ground states transition with indicated single rotational lines of the ΔJ =+3/2 branch. © 1997 American Institute of Physics.
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  • 44
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2340-2350 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this article a global potential energy surface for the 1A′′ state of H2O based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio results is presented. The resulting 1A′′ surface is used in conjunction with a previously determined 1A′ surface to study the O(1D)+H2(HD,D2) reaction dynamics, with emphasis on the influence of the 1A′′ excited state on measurable properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of about 2 kcal/mol on the 1A′′ surface, and even at 5 kcal/mol, the 1A′′ reactive cross section is only a small fraction (∼20%) of the barrierless 1A′. However, the 1A′′ surface populates very specific product vibrational states (v=3–4) and gives strongly backward peaked differential cross sections, so certain types of measurements are quite sensitive to the presence of this excited state. In particular, better agreement is found with experimental vibrational and angular distributions with 1A′′ included, especially at translational energies of 4 kcal/mol and above. A rough estimate of the influence of the 2A′ surface indicates that this state also makes an important contribution to vibrational and angular distributions at high energies. The 1A′′ and 2A′ surfaces also measurably affect the energy dependence of the integral cross sections at energies above 2 kcal/mol, and the temperature dependence of thermal rate coefficients for O(1D)+H2 at temperatures of 1000 K and above. © 1997 American Institute of Physics.
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  • 45
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    The Journal of Chemical Physics 107 (1997), S. 3253-3261 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this work we present a Monte-Carlo simulation of propionic acid in aqueous solution and look into the structural and thermodynamic behavior of an amphiphile at temperatures of 285, 298 and 330 K. All Monte-Carlo simulations were done with one solute and 338 water molecules, a NVT ensemble, a polarizable ab initio potential, Metropolis sampling, and periodic boundary conditions. On the technical side we show that very large statistical samples (8×108) are needed to attain reliable results and that accounting for microscopic detail is needed to find patterns that are hidden by averaging. Related to the molecular basis of hydrophobicity we found that solute–water interactions, even around the hydrophobic tail, lead to a favorable enthalpic effect, in agreement with other simulations. We also found that the tail enhances water–water interactions as compared to pure water, in agreement with the experiment. An important finding is that the anomalous solubility seems to come from an induced structure of waters neighboring the hydrophobic tail. On the contrary, at low temperatures the polar head loses its hydration shell as the water network becomes important. From a detailed analysis we propose an interplay of several factors as the reason for hydrophobicity. © 1997 American Institute of Physics.
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  • 46
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    The Journal of Chemical Physics 107 (1997), S. 3289-3294 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rigid rodlike polymers have considerable technological importance due to their excellent mechanical properties. The polymerization kinetics of such condensation polymers are qualitatively different from flexible polymers, and exhibit significant slowing at the later stages of the reaction. This is due to the slow rotational diffusion of the molecules to an appropriate configuration for reaction. In this work we have carried out Brownian dynamics (BD) simulations to obtain the effective rate constant for reaction between rodlike molecules in the presence of diffusional limitations. The theory of Northrup et al. [J. Chem. Phys. 80, 1517 (1984)] for pairwise BD simulation of reactions is extended to the case of rodlike molecules assuming isotropic translational diffusion. The computed results are compared to exact analytical predictions. Good agreement between computation and theory is obtained over a wide range of parameter values. © 1997 American Institute of Physics.
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  • 47
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    The Journal of Chemical Physics 107 (1997), S. 2589-2596 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The behavior of simple fluids under shear is investigated using molecular dynamics simulations. The simulated system consists of a fluid confined between two atomistic walls which are moved in opposite directions. Two approaches for shear flow simulations are compared: in one case, the sheared fluid is not thermostatted and only the confining walls are maintained at a constant temperature, while in the other, a thermostat is employed to keep the entire mass of the sheared fluid at a constant temperature. In the first case the sheared fluid undergoes significant viscous heating at the shear rates investigated, consistent with experimental observations and with theoretical predictions. Most simulations to date, however, have used the second approach which is akin to studying a fluid with infinite thermal conductivity. It is shown here that results for transport coefficients are significantly affected by the thermostat; in fact, the transport properties of the fluid determined using the two methods exhibit a qualitatively different shear rate dependence. It is also shown that the temperature profiles observed in our simulations can be described by continuum mechanics, provided the temperature dependence of the viscosity and thermal conductivity is taken into account. © 1997 American Institute of Physics.
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  • 48
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    The Journal of Chemical Physics 107 (1997), S. 2267-2283 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have developed a theory of the fifth-order off-resonant spectroscopy to study the effect of anharmonicity of molecular vibrational modes. The anharmonicity, as well as nonlinear dependence of polarizability on nuclear coordinates, can be the origin of the fifth-order Raman signal. A profile of the signal varies depending on the relative importance of the two effects—the anharmonicity and the nonlinearity. The anharmonicity of a potential can be distinguished from the other effects such as the nonlinearity or the inhomogeneity of vibrational modes. In order to carry out calculations analytically, we employ the multimode Brownian oscillator model and treat anharmonicity as perturbation to the harmonic vibrational modes. A simple analytical expression for the fifth-order polarization is obtained through a diagrammatic technique, called Feynman rule on the unified time path. Physical pictures for the analytical expression are given for a single mode system through numerical calculations and through double-sided Feynman diagrams. Applications to CHCl3 and CS2 are made where the third-order experiments are used to extract parameters. In the CS2 case, the theoretical fifth-order signals are compared with recent experiment, which suggests some sign of anharmonicity. © 1997 American Institute of Physics.
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  • 49
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    The Journal of Chemical Physics 107 (1997), S. 2313-2328 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Tunneling in the one-dimensional Eckart system is treated by a semiclassical method that describes the S-matrix in terms of an integral over the initial momenta of real-valued classical trajectories. The results are found to be sensitive to a certain parameter γ which is expected to be essentially arbitrary for classically allowed processes. Analysis of the semiclassical error allows formulation of conditions for the validity of the tunneling treatment. This, in turn, leads to an explanation for the sensitivity of the results to γ and an understanding of how this parameter should be chosen. With an optimized choice, the semiclassical method is found to yield very accurate tunneling results even for probabilities as small as10−10. The relationship between the present method and the conventional uniform semiclassical treatment of barrier tunneling is discussed. © 1997 American Institute of Physics.
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  • 50
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    The Journal of Chemical Physics 107 (1997), S. 2300-2307 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The highly structured visible absorption spectrum of the bromine dioxide radical, OBrO, has been observed in the 15 500–26 000 cm−1 region. The spectrum is dominated by a long progression in the Br–O symmetric stretching motion (ν1′) and a series of short progressions built on the bending mode (ν2′); there are no features associated with the excitation of the antisymmetric stretching mode (ν3′). The spectrum also contains numerous transitions originating from the (0,1,0) and (1,0,0) vibrational levels of the electronic ground state, X(2B1). A simultaneous fit to all of the observed vibronic features yielded the frequencies ν1″=799.4 cm−1, ν2″=317.5 cm−1, ω1′=641.5 cm−1, ω2′=210.7 cm−1, and a band origin T0=15 863 cm−1. Franck–Condon simulations combined with ab initio calculations of the four lowest OBrO doublet electronic states identify the spectrum as arising from the C(2A2)←X(2B1) electronic transition. © 1997 American Institute of Physics.
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  • 51
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    The Journal of Chemical Physics 107 (1997), S. 2407-2414 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We studied the dynamics of intramolecular proton transfer in the glycolate anion, and of the analogous processes in the deuterated and tritiated derivatives, by means of variational transition state theory with various multidimensional semiclassical models of tunneling. Rate constants were obtained directly from MP2/6-31++G** ab initio results scaled so that the height of the barrier coincided with the value given by QCISD/6-31++G** calculations, 3.81 kcal/mol. In this kind of system, the effective barrier height can be significantly lower for the lighter isotope. Analysis of the kinetic isotope effects (kH/kD and kH/kT) showed them reduced by tunneling but favored by certain vibrational modes. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 2395-2406 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The correlation diagram of the LiNC/LiCN isomerizing system as a function of (h-dash-bar), taken as a parameter, is considered in conditions of widespread classical chaos. Two series of isolated avoided crossings of very different nature, involving states related by the same 1:8 quantum resonance condition, are characterized and the corresponding interaction matrix elements between adiabatic states analytically calculated and analyzed. One of these series has been found to separate two distinct regions: one for smaller values of (h-dash-bar) (at any given energy) corresponding to regular states, i.e., with a definite nodal pattern, and the other of much more stochastic behavior. Moreover, the effect of the interaction involved in these avoided crossings is to mix the regular wave functions giving rise to wave functions strongly scarred by the periodic orbits of the associated 1:8 classical resonance. This result constitutes an interpretation of the scarring effect from a new perspective. © 1997 American Institute of Physics.
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  • 53
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    The Journal of Chemical Physics 107 (1997), S. 2419-2423 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the photoelectron–energy-selected photoion coincidence technique, we find that photoions of BF3 with kinetic energy larger than 4 eV are predominantly F+ in the F 1s ionization region whereas B+ is more intense than F+ at the F 1s→2a2″ excitation. Two partial cross sections σ|| and σ⊥ for A1′→E′ parallel and A1′→A2″ perpendicular transitions are obtained from the angular distribution measurement of energetic photoions and the symmetries of some F 1s core excited states are confirmed. The dissociation dynamics of the F 1s−12a2″ core excited state are discussed in detail. © 1997 American Institute of Physics.
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  • 54
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    The Journal of Chemical Physics 107 (1997), S. 2463-2480 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio Hartree–Fock 6-31G longitudinal polarizabilities and second hyperpolarizabilities of polydiacetylene and polybutatriene oligomers are computed and, then, extrapolated to the infinite polymer limit. Both the electronic and vibrational components are obtained with the latter evaluated in the double harmonic approximation. For static fields the electronic hyperpolarizability of the butatrienic form is almost 50 times larger than the acetylenic form; the ratio of the vibrational to the electronic component is 0.86 for the latter and 3.3 for the former. Thus, the two components are sensitive to the difference in bond length alternation (BLA) in different ways. Depending upon the particular nonlinear process it is shown that the vibrational hyperpolarizability may remain large even at optical frequencies. Three intense Raman-active k=0 modes dominate the vibrational component. At the level of theory used here these modes occur in the frequency range 1000–2500 cm−1 and they consist of changes in the BLA as well as hydrogen wagging motions. For polydiacetylene, there is a 1:1 relation with the corresponding vibrations that make the most important hyperpolarizability contributions in polyacetylene and polyyne.© 1997 American Institute of Physics.
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  • 55
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    The Journal of Chemical Physics 107 (1997), S. 2499-2515 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Zero-electron-kinetic-energy photoelectron spectroscopy ("ZEKE-PES") is based on the pulsed field ionization of long lived Rydberg states (ZEKE states); it is generally accepted that ZEKE states have large angular momentum l, which quenches electron-core interactions, but how they acquire it remains a matter of dispute. We show that {nl}→{nl′} ion-Rydberg collisions are a viable and prominent mechanism for the excitation of large-l Rydberg states. We elucidate the dynamics by an exactly solvable classical model which provides a transparent and intuitive picture of the excitation of high-l states. By a geometric interpretation of the dynamics we are able to predict for which values of the impact parameter and reduced velocity of the incoming ion a change of the angular momentum of the state becomes possible. We pay particular attention to the influence of the quantum defect, δl, on the {nl}→{nl′} cross section and demonstrate that, for small initial angular momenta, δl is itself a major contributor to the experimentally observed scaling of the cross section as ∼n5. This classical-quantum defect model is not only able to explain the dependence of the experimentally measured fractional population of high-l states on the velocity of the incoming ion, but leads to the prediction that dipole-forbidden transitions are likely to dominate l transitions induced by ion-Rydberg collisions under ZEKE-PES conditions. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 1936-1940 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational contributions to the first and second hyperpolarizabilities of a push–pull polyene are theoretically studied by using the valence-bond charge-transfer model. The formal relationships between the vibrational and electronic contributions to the hyperpolarizabilities are derived. It is found that there exist strong correlations between the vibrational contributions and bond length alternation of a push–pull polyene. By calculating these contributions numerically, it is proven that the vibrational contributions, estimated from the IR, Raman, and hyper-Raman measurements, to the hyperpolarizabilities can also provide a measure of the nonlinear optical property. © 1997 American Institute of Physics.
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  • 57
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    The Journal of Chemical Physics 107 (1997), S. 2673-2682 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Some important unique phenomena are encountered in gels of interpenetrating polymer networks (IPN) which arise from the coupling between phase transition, elasticity, and mutual entanglements and interaction of the underlying subnetworks. The relevant order parameters of IPN–gels are obtained as the local composition ratio fluctuations, the characteristic length of microphase separation and the volume fraction of the liquid. On the basis of a preaveraged Landau–Ginzburg theory, both microphase separation and volume phase transition are considered. There exists a common critical point at which the IPN–gel becomes critical for both types of phase transitions. © 1997 American Institute of Physics.
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  • 58
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    The Journal of Chemical Physics 107 (1997), S. 2698-2698 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
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  • 59
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    The Journal of Chemical Physics 107 (1997), S. 1703-1716 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photoinduced Rydberg ionization (PIRI) spectroscopy has been applied to the problem of recording vibrationally resolved spectra of the dipole-forbidden B˜ 2E2g←X˜ 2E1g transition of C6H6+ and C6D6+. PIRI spectra of the B˜ state have been recorded via a number of vibrational states of the ionic ground state. A combination of Herzberg–Teller and pseudo-Jahn–Teller vibronic coupling between the B˜ state and a close lying C˜ state were used to explain the complex B˜ state vibrational structure. The two pseudo-Jahn–Teller vibrational modes, ν16 and ν17, were both found to be active in the PIRI spectra. In addition, evidence for a strong pseudo-Jahn–Teller interaction within each of these modes was found. The perturbed vibronic bands resulting from this pseudo-Jahn–Teller activity were successfully modeled using a two-mode pseudo-Jahn–Teller vibronic coupling model. Using a combination of the spectral results and the pseudo-Jahn–Teller calculations, we were able to generate absolute vibronic symmetry assignments for many of the lower B˜ state vibrational levels, from which vibrational assignments were made. The calculations also produced unperturbed vibrational frequencies for both pseudo-Jahn–Teller active modes along with values for their respective linear coupling constants. © 1997 American Institute of Physics.
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  • 60
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    The Journal of Chemical Physics 107 (1997), S. 1732-1739 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational spectrum of the cyanomethyl radical CH2CN in its 2B1 ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer equipped with a free-space cell. TheCH2CN radical was generated in the cell by dc-glow discharge in pureCH3CN, and CD2CN inCD3CN. Fifty a-type R-branch rotational transitions were measured in the frequency region of 80 to 282 GHz, where fine structure and hyperfine structures due to the nitrogen and hydrogen nuclei were resolved for lower-N transitions, but only the fine structure for higher-N transitions. Similarly fine-structure resolved rotational transitions were measured for CD2CN in the frequency region of 190 to 286 GHz. The molecular constants of CH2CN, including the fine-structure constants and hyperfine coupling constants due to both the nuclei of N and H, were precisely determined by least-squares methods from 146 observed spectral lines, including 36 astronomically observed lines for the Ka=0components of N=1–0 and 2–1 from a dark cloud TMC-1 [Kaifu et al., Astrophys. J. Suppl. (submitted 1997)]. Those ofCD2CN were similarly observed from 92 observed spectral lines. The rotational constants of both the species were used to derive molecular structural parameters of CH2CN:r0(CN)=1.1919(13) Å, r0(CC)=1.3680(12) Å, andr0(CH)=1.0894(7) Å on the assumption of 120.22° for (angle)HCH. The spin densities of unpaired electrons estimated from the hyperfine coupling constants for the N and H atoms are consistent with the molecular structure obtained. © 1997 American Institute of Physics.
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  • 61
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    The Journal of Chemical Physics 107 (1997), S. 1765-1778 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Fourier transform infrared spectrum of the in-plane CH3-rocking fundamental of 13CH3OH has been investigated at 0.002 cm−1 resolution. The rocking band is principally of parallel character and has a double-peaked Q branch and relatively wide spread subbands indicative of a substantial change in torsional barrier height. All A subbands from K=0 to 11 and all but one E subband from K=0 to 9 have been assigned in the n=0 torsional state and fitted to J(J+1) power-series expansions to obtain the subband origins and excited-state energy structure. The effects of vibrational interactions between the CH3-rocking and CO-stretching modes are prominent in the spectrum. Coriolis coupling between rocking (K−1) and CO-stretching K levels is observable for K≥6, and makes significant contributions to the subband origins and effective B values. Several J-localized perturbations due to level-crossing resonances with CO-stretch states have been observed and characterized. Two reported strong far-infrared laser lines optically pumped by the 10R(26) CO2 laser line have been found to arise through such a ΔK=3 level-crossing resonance. Modeling of the rocking-state torsion-K-rotation energies yields a height of V3r=469.2(38) cm−1 for the torsional potential barrier, a 26% increase over the ground state. The asymmetry K-doubling pattern in the excited state is qualitatively consistent with this barrier for K=2 to 4, but the K=5 rocking substate displays strongly enhanced splitting. © 1997 American Institute of Physics.
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  • 62
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    The Journal of Chemical Physics 107 (1997), S. 1375-1387 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a theoretical analysis of the effect of nonlinear dipole solvation on steady-state optical spectra and intramolecular electron transfer (ET) reactions. The solvation nonlinearity is attributed to saturation of a dipolar liquid produced by the solute dipole. The treatment explores the perturbation expansion over the solute-solvent dipolar interaction truncated in the form of a Padé approximant. The optical line shape and the free energies along the ET reaction coordinate are related to the chemical potential of solvation of a fictitious solute with a complex-valued dipole moment. Due to solvent dipolar saturation the spectrum of dipolar fluctuations is confined by a band of the width 2Elim. Solvation nonlinearity was found to manifest itself for optical transitions with high dipole moments in the initial state, most often encountered for emission lines. In this case, the spectral line approaches the saturation boundary Elim bringing about "line squeezing" and decrease of the line shift compared to the linear response prediction. In the nonlinear region, the line shift dependence on the solute dipole variation Δm switches from the quadratic linear response form ∝Δm2 to a linear trend ∝|Δm|. The bandwidth may pass through a maximum as a function of |Δm| in the saturation region. Nonlinear solvation results thus in a narrowing of spectral lines. For a transition with solute dipole enhancement, the bandwidth in emission Δe is therefore lower that in absorption Δa: Δe〈Δa. As a result, the plot of βΔa,e2, β=1/kBT against the Stokes shift (h-dash-bar)Δst demonstrates the upward deviation of βΔa2 and downward deviation of βΔe2 from the linear response equality βΔa,e2=(h-dash-bar)Δωst. We also explored the nonlinearity effect on charge separation/charge recombination activation thermodynamics. The solvent reorganization energy was found to be higher for charge separation (λ1) than for charge recombination (λ2). Both are smaller than the linear response result. For the reorganization energies, the discrepancy between λ1 and λ2 is relatively small, whereas their temperature derivatives deviate significantly from each other. The theory predictions are tested on spectroscopic computer simulations and experiment. Generally good quantitative agreement is achieved. © 1997 American Institute of Physics.
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  • 63
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    The Journal of Chemical Physics 107 (1997), S. 1441-1450 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Based on weak field quantum control theory, a semiclassical approximation relates the characteristic parameters of the tailored light field to corresponding classical dynamical quantities and thus reveals the underlying physical basis of wave packet focusing. A coordinate-dependent two-level-system approximation is employed to further analyze the molecular dynamics induced by short laser pulses, thus leading to a simple interpretation of the observed correlation between the pulse chirp and vibrational focusing and defocusing. Though our study is presented in the context of quantum control, the conclusions are general, providing an intuitive picture of the quantum coherence of light–matter interaction and a guideline for the design of tailored laser fields. © 1997 American Institute of Physics.
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  • 64
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    The Journal of Chemical Physics 107 (1997), S. 1904-1914 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that the newly completed accurate semiclassical theory for time-independent curve crossing problems can be usefully utilized to study various time-dependent curve crossing problems. Quadratic time-dependent problems can be solved exactly with use of the theory developed for the time-independent linear potential model. Furthermore, accurate and compact semiclassical theory can be formulated for general curved potentials. Even diabatically avoided crossing cases can be nicely treated. Multi-level problems can also be handled without difficulty with use of a new method to evaluate the necessary basic parameters directly from adiabatic potentials on the real axis in the fully diagonalized adiabatic representation. This method does not require a search for complex crossing points in the multi-level system, which is practically very difficult especially when the number of levels exceeds three. © 1997 American Institute of Physics.
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  • 65
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    The Journal of Chemical Physics 107 (1997), S. 1953-1962 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using cloud point determinations, the phase boundaries (binodals) for metastable liquid–liquid (L–L) separation in supersaturated hen egg white lysozyme solutions with 3%, 5%, and 7% (w/v) NaCl at pH=4.5 and protein concentrations c between 40 and 400 mg/ml were determined. The critical temperature for the binodal increased approximately linearly with salt concentration. The coexisting liquid phases both remained supersaturated but differed widely in protein concentration. No salt repartitioning was observed between the initial and the two separated liquid phases. After the L–L separation, due to the presence of the high protein concentration phase, crystallization occurred much more rapidly than in the initial solution. At high initial protein concentrations, a metastable gel phase formed at temperatures above the liquid binodal. Both crystal nucleation and gel formation were accelerated in samples that had been cycled through the binodal. Solutions in the gel and L–L regions yielded various types of precipitates. Based on theoretical considerations, previous observations with other proteins, and our experimental results with lysozyme, a generic phase diagram for globular proteins is put forth. A limited region in the (T,c) plane favorable for the growth of protein single crystals is delineated. © 1997 American Institute of Physics.
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  • 66
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    The Journal of Chemical Physics 107 (1997), S. 2034-2045 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations for polar Kihara fluids are reported for linear models of different lengths at several dipole and quadrupole values and at three different thermodynamic states. Two of these states are close to the vapor–liquid equilibrium curve and the third one is at the same density as the first and at the same temperature as the second. Self-correlation functions and translational and orientational times are calculated and analyzed. Transport properties, diffusion, thermal conductivity, and shear viscosity are also reported and discussed in terms of multipolar forces. Correlation terms are used to calculate band broadening in different kinds of molecular spectra. Finally, it is shown how it is possible to discriminate between two models of acetonitrile that fit equilibrium properties equally well by using dynamic properties. © 1997 American Institute of Physics.
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  • 67
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    The Journal of Chemical Physics 107 (1997), S. 2089-2095 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For general irreversible deposition processes, a relation between the variance σ2 of the number of deposited particles on subsystems out of a large surface and the available surface function Φ is obtained. This relation is based on a mean field assumption and follows the resolution of a master equation system. It is valid at low to intermediate values of the surface coverage θ as shown by comparison with exact results and with numerical simulations for special deposition models. In the low coverage limit, if the available surface function is written as a series expansion of the coverage θ, whose first nontrivial term varies as θk, the reduced variance has a similar expansion. However, the prefactor of θk derived in this article is in general different in both series expansions. This result has also been obtained by a rigorous argument based on the evolution of the k-particle distribution function with the coverage. © 1997 American Institute of Physics.
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  • 68
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    The Journal of Chemical Physics 107 (1997), S. 2930-2941 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground-state potential energy surface (PES) for linear arrangements of the N2H+ molecular ion is numerically computed by the multireference single- and double-excitation configuration interaction (MRD-CI) technique. An analytical representation of the potential energy function is obtained by fitting a power series in the Simons–Parr–Finlan coordinates to the numerical data. For investigating the intramolecular dynamics we describe the nuclear motion by a Gaussian wave packet located initially in the strong interaction region of the PES. The vibrational eigenvalue spectrum is calculated by Fourier transforming the time autocorrelation function. The spectrum is then analyzed statistically in the light of random matrix theory (RMT) to understand the nature of the intramolecular dynamics. We examine the short-range correlation in the spectrum through the nearest neighbor level spacing distribution P(s) and the long-range correlation through Δ3 and Σ2 statistics. The spectrum in the time domain is analyzed by computing the ensemble averaged survival probability 〈〈P(t)〉〉. The above four quantities obtained from the spectrum are compared with the distribution predicted for regular, irregular, and mixed (intermediate) spectra by the RMT. We find the system is of mixed type and the fractional irregularity is 0.7±0.05. In order to reveal a possible correspondence to the classical dynamics, we have carried out the spectral analysis of the dynamical variables for classical trajectories over a wide range of internal energies. In addition the classical dynamics of proton collisions with N2 molecules has also been preliminarily studied on the same PES, in particular the dependence of the final vibrational action nf on the initial vibrational phase φi of N2 and, furthermore, the Poincaré surface-of-section superimposed with the zero-order separatrix; we find a large number of trapped trajectories. © 1997 American Institute of Physics.
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  • 69
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    The Journal of Chemical Physics 107 (1997), S. 2990-2999 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Glutamic acid in the gas phase and in aqueous solution was studied with ab initio quantum chemistry calculations. The effective fragment potential (EFP) has been used to study the effects of up to ten water solvent molecules on the neutral and zwitterionic isomers of glutamic acid, as well as to model the reaction path connecting these two isomers. The results with one and two water molecules have been compared to full ab initio "super molecule" calculations. This comparison shows that the EFP successfully models the effects of solvent molecules on structure and energetics. In the calculations including zero, one, or two water molecules, the neutral isomer was shown to be more stable than the zwitterion. On the other hand, in the EFP calculations with ten water molecules, where Monte Carlo methods were used to assist in finding the lowest energy conformations, the zwitterion was found to be more stable, which is the expected aqueous phase result. © 1997 American Institute of Physics.
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  • 70
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    The Journal of Chemical Physics 107 (1997), S. 3021-3031 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio self-consistent-field (SCF), two-configuration SCF (TCSCF), and density functional theory (DFT) calculations on the charge–transfer complexes of doubly Li/Cl-doped oligothiophenes and oligo(p-phenyls) and on respective charged systems without counterions have been carried out in order to study polaron to bipolaron transitions. Oligomer chains up to octamers and the ring structures cyclo-dodecathiophene and cyclo-dodeca(p-phenyl) have been investigated. Special attention is paid to the open-shell biradical character of two isolated polaronic defects. It is found that the TCSCF and the spin-unrestricted DFT methods can be successfully applied. A bipolaron structure is obtained when the doping atoms are located on neighboring rings and when there is one undoped ring separating the two doped ones. If there are two or more undoped rings in between a two-polaron configuration (biradical) is found. The bipolaron system is calculated to be more stable than the two-polaron case when counterions are taken into account. The stabilities are reversed if the bare, doubly-charged systems are considered. A theoretical estimate for the barrier height of the polaron to bipolaron transition is given using model reaction coordinates. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 3080-3084 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Structure of liquid and gaseous CCl4 at elevated temperatures from 195 °C to the subcritical point 275 °C (Tc=285.15 °C) along the liquid–vapor coexistence curve have been studied by means of pulsed neutron diffraction. Structure factor in a wide range of momentum transfer Q from 0.03 to 20 Å−1, covering from the small angle scattering region to atomic-scale structure region, has been obtained. Long-range density fluctuation and atomic scale structure have been analyzed: (1) molecular orientational correlation still exists in liquidCCl4 at 275 °C while it disappears in its saturated vapor; (2) correlation lengths of density fluctuation are 10–13 Å for the liquid at 275 °C and 13–17 Å for its saturated vapor; and (3) molecular-density fluctuation (〈Nm2〉−〈Nm〉2)/〈Nm〉 is estimated to be 2.4 for the liquid at 275 °C and 9.3 for its saturated vapor. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 1163-1172 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A formalism for relating dynamic observables of collision processes to the electronic structure of the colliding species is illustrated for the case of He collision-induced rotational excitation of H2. Expressions are presented for the functional derivative of transition cross sections and rate constants with respect to the electronic wave function. Regions of electronic coordinate space in which the electron probability densities have particularly strong influence on one or more inelastic transitions are identified. The possible role of functional derivatives of this sort for guiding electronic structure calculations to produce intermolecular potentials is discussed. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 1212-1217 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method is described to incorporate the Ewald lattice-sum method into quantum mechanical calculations in hybrid quantum and molecular mechanical (QM/MM) fluid simulations. The procedure is illustrated through standard free energy perturbation calculations in the context of Monte Carlo simulations. The free energy of hydration of chloride ion was computed using the hybrid QM/MM-Ewald method, and comparison was made to results obtained with standard spherical cutoff. The results indicate that the hybrid QM/MM-Ewald method can be effectively used to include long-range electrostatic interactions in quantum mechanical calculations of condensed media. © 1997 American Institute of Physics.
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  • 74
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    The Journal of Chemical Physics 107 (1997), S. 1195-1211 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The long-standing problem of the topography, energetics, and vibrational dynamics of the ground-state surface of SiC2 is systematically investigated by means of the gamut of state-of-the-art electronic structure methods, including single-reference correlation techniques as extensive as the coupled-cluster singles and doubles method augmented by a perturbative triples term [CCSD(T)], the Brueckner doubles method (BD) with analogous contributions from both triple and quadruple excitations [BD(TQ)], and second-through fifth-order Møller–Plesset perturbation theory (MP2–MP5), as well as the multiconfigurational complete-active-space self-consistent-field [CASSCF(12,12)] approach. The one-particle basis sets for these studies ranged from Si[6s4p1d], C[4s2p1d] toSi[7s6p4d3f2g1h], C[6s5p4d3f2g1h]. The methodological analysis resolves the polytopism problem regarding the mercurial potential energy surface for the circumnavigation of Si+ aboutC2− in silicon dicarbide, whose topography is shown to exhibit almost all conceivable variations with level of theory. It is concluded that the X˜ 1A1 global minimum of SiC2 is a T-shaped(C2v) structure connected monotonically to a linear transition state 5.8 kcal mol−1 higher in energy, thus ruling out any metastable linear isomer. Previously undocumented bent transition states and L-shaped minima are encountered at relatively high levels of theory, but ultimately these stationary points are shown to be spurious. High-level focal-point thermochemical analyses yieldD0(Si–C2)=151 kcal mol−1, and hence a substantial revision is made in the heat of formation, viz.,ΔHf,0&convolu;(SiC2)=+155 kcal mol−1. A complete quartic force field about the T-shaped minimum is determined at the CCSD(T) level with the aug-cc-pVTZ (Si[6s5p3d2f],C[5s4p3d2f]) basis set and then employed in a preliminary probe of contours for large-amplitude motion, anharmonicity of the vibrations, and zero-point effects on the molecular structure. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 3376-3381 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-lying rotational Rydberg states (p series) of NO near the ionization threshold were prepared by two-color (1+1′) resonant excitation, and then detected by a ZEKE (zero kinetic energy) electron technique as well as by a MATI (mass analyzed threshold ion) technique using one- and two-pulsed field ionization (abbreviated by 1PFI and 2PFI, respectively). The MATI-2PFI spectrum has been interpreted in terms of a single progression due to p-series Rydberg states (n=37–48) converging into the ion core with N+=11, perturbed by the d-series Rydberg states converging into N+=12. From a ZEKE-1PFI spectrum obtained at a high field strength of F=−33 V/cm, it has been found that (1) the band width (ΔE) is narrower than expected from the relation ΔE=C(F)1/2 (C=4∼6) [W. A. Chupka, J. Chem. Phys. 98, 4520 (1993)], and (2) the field ionization threshold was observed at n=82 (principal quantum number) for the p-series Rydberg states at a static dc field of 0.25 V/cm. From a MATI-2PFI spectrum obtained at F1=20 V/cm and F2=−33 V/cm, it has been found that the lifetimes of the p-series Rydberg states (n≥37) below the field ionization threshold are lengthened at the same static electric field. These experimental results may be interpreted in terms of two factors; one is the lifetime lengthening due to the Stark mixing between the lower and the higher l states, and the other is the rotational coupling of between a low-n high-N+ state and a high-n low-N+ state, lengthening the apparent lifetime of the low-n high-N+ state. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 3417-3427 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By using simple mode coupling equations, we investigate the depolarized light scattering spectra of two so-called "fragile" glassforming liquids, salol (phenylsalicylate) and CKN (Ca0.4K0.6(NO3)1.4), measured by Cummins and co-workers. Nonlinear integrodifferential equations for the time evolution of the density-fluctuations autocorrelation functions are the basic input of the mode coupling theory. Restricting ourselves to a small set of such equations, we fit the numerical solution to the experimental spectra. It leads to a good agreement between model and experiment, which allows us to determine how a real system explores the parameter space of the model, but it also leads to unrealistic effective vertices in a temperature range where the theory makes critical asymptotic predictions. We finally discuss the relevance and the range of validity of these universal asymptotic predictions when applied to experimental data on supercooled liquids. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 3471-3477 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A generalized eigenequation is derived for the optimal pairs of phase-unlocked pump-dump fields in the control of pure state molecular systems in the weak response regime. The associated eigenvalues are further correlated to the yields of pump-dump control, thus the globally optimal pair of phase-unlocked pump-dump fields is identified unambiguously. Presented is also a hierarchy of reduction from the general Liouville-space density matrix control formulation in the strong response regime to various linearized control equations in the weak response regime. Application to the control of a molecular ro-vibronic level in the ground electronic surface reveals a novel symmetry relation between the optimal pump field and its counterpart of the optimal dump field. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 3492-3497 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nickel clusters were produced in a high intensity cluster ion source and thermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass- and charge-selected nickel clusters then passed through a linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monoxide. The reaction products were then analyzed by a quadrupole mass-spectrometer. Using this technique, saturation limits for Nin+ clusters with n=4–31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation was observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluster growth may be explained by a pentagonal sequence of structures for n=4–7, capping of the pentagonal bipyramid to buildup an icosahedron at Ni13+, and further capping of this icosahedron to form a double icosahedron at Ni19+. © 1997 American Institute of Physics.
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  • 79
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    The Journal of Chemical Physics 107 (1997), S. 2257-2266 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrationally excited levels of the a 4Π state, the lowest electronically excited state of the NO molecule, can be studied through their collisional energy transfer to the radiating B 2Π state. Following one-photon excitation in the 11–0 band of the forbidden a 4Π–X 2Π transition with the output of a tunable dye laser, we monitor the temporal evolution of the emission from the v=2 level of the B 2Π state. From pressure-dependence analysis, we determine the removal rate coefficients for a 4Π(v=11) with NO, O2, N2O, N2, CO2, He, and Ar. In addition, using direct B 2Π excitation of the v=2, 3, 4 levels, we obtain removal rate coefficients for the same colliders. These vibrational levels span the region around a 4Π(v=11). In general, a 4Π removal is quite fast, ranging from rate coefficients of 1.0×10−12 cm3 s−1 to more than 1.0×10−10 cm3 s−1, and is substantially faster than B 2Π removal for N2, N2O, CO2, and Ar. NO(B 2Π, v=3) removal is generally much faster than that for the v=2 and v=4 levels, because of a localized perturbation between the v=12 level of the a 4Π state and v=3 of the B 2Π state. Preliminary measurements on transfer between the b 4Σ− and B 2Π states are reported. © 1997 American Institute of Physics.
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  • 80
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    The Journal of Chemical Physics 107 (1997), S. 2308-2312 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoinduced electron transfer between conjugated polymers and a series of functionalized fullerenes was studied. A new photoluminescence signal was observed in the near IR (∼1.4 eV). This weak IR photoluminescence does not result from direct excitation of the fullerene, but from radiative electron-hole recombination between the fullerene excited state and the polymer ground state. The intensity of this recombination luminescence depends on the electrochemical nature of the functional group; it is observed only for fullerenes with first reduction potential higher than that of C60. © 1997 American Institute of Physics.
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  • 81
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    The Journal of Chemical Physics 107 (1997), S. 2351-2356 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dependencies of the integral cross sections for the title reactions on collision energies were determined from 0.6 kcal/mol to nearly 6 kcal/mol. The result provides, for the first time, direct and unambiguous experimental evidence for the existence of an abstraction pathway, in addition to the widely accepted inserted one. A reaction barrier of about 1.8 kcal/mol was found for this elusive abstraction channel. The branching of these two microscopic pathways was estimated. An intriguing H/D isotope effect was revealed, which called for further studies. © 1997 American Institute of Physics.
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  • 82
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Free-jet absorption millimeter-wave spectroscopy and gradient-corrected density-functional calculations are combined to study trans-1-(2-pyridyl)-2-(4-pyridyl)-ethylene, 2,4-DPE, a stilbenelike molecule. The interplay between experiments and computational theory backed by the large amount of data and modeling already available for stilbene allows us to establish the parameters of the two-dimensional potential of the pyridyl torsions which well reproduce the experimental defect of inertia (−1.295 uÅ2). The large difference between the defects of inertia of 2,4-DPE and stilbene ((approximate)−15 uÅ2) and further computer simulations suggest that the latter may warrant a reinvestigation. The effect of introducing a weakly binding N(centered ellipsis)H interaction in the stilbene frame is discussed and a very simple model based on the ab initio calculations is found to rationalize the energetics and structural differences of the two conformers of 2,4-DPE. © 1997 American Institute of Physics.
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  • 83
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    The Journal of Chemical Physics 107 (1997), S. 3684-3690 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The normal mode analysis in dihedral angle space is carried out on a famous double-stranded DNA dodecamer d(CGCGAATTCGCG) to reveal the dynamical characteristics and internal motion of this molecule. We have observed the coupled motion of sugar and base. We have also noted the dependence of internal motion in the dodecamer on location of the nucleotide relative to the helical ends, and the dependence of atomic fluctuations on distance from the helix axis. The similarity of atomic fluctuations between two strands suggests the presence of a twofold pseudosymmetry axis with its origin located at the center of the molecule. Our results show that the atomic fluctuations of duplex DNA are mainly determined by a small number of low-frequency normal modes, the normal modes with frequencies below 30 cm−1 make major contributions to the site-dependence of atomic fluctuations. © 1997 American Institute of Physics.
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  • 84
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    The Journal of Chemical Physics 107 (1997), S. 3723-3725 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of applying an electric field on the MJ levels of the R(1) feature (υ=17 255.684 cm−1) in the AΩ=0+−X 1Σ+ band system of 174YbO has been examined using the technique of high resolution laser induced fluorescence spectroscopy. The results were used to obtain values for the permanent electric dipole moments, μ, in the X 1Σ+ and AΩ=0+ states of 5.89±0.02 and 5.93±0.04 D, respectively. © 1997 American Institute of Physics.
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  • 85
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    The Journal of Chemical Physics 107 (1997), S. 3734-3737 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose here a new mechanism of droplet coarsening in phase-separating fluid mixtures. In contrast to the conventional understanding that there are no interactions between droplets in the late stage of spinodal decomposition, we demonstrate the existence of interactions between droplets that is caused by the coupling between diffuse concentration change around droplets. We show the possibility that this mechanism plays an important role in droplet phase separation together with Brownian-coagulation mechanism. We also discuss the coupling between hydrodynamic and diffusion modes, namely, "collision-induced collision" phenomena. © 1997 American Institute of Physics.
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  • 86
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    The Journal of Chemical Physics 107 (1997), S. 2720-2727 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first spectroscopic identification and characterization of ytterbium monoacetylide (YbCCH) is reported. By combining resonance-enhanced two photon ionization (R2PI), laser-induced fluorescence (LIF), and photoionization efficiency spectroscopy (PIE) with density functional calculations the X˜ 2Σ+ and the A˜ 2Π1/2,3/2 states of YbCCH as well as the X˜ 1Σ+ state of YbCCH+ have been characterized. The A˜ 2Π1/2,3/2–X˜ 2Σ+ system whose 0-0 band for the A˜ 2Π1/2 component lies at around 16 848 cm−1 for YbCCH has been studied at 0.3 cm−1 resolution. The excitation spectra, both R2PI and LIF are characterized by progressions involving the YbCC bending mode (ν5) whose wave number has been determined to be 96 and 103 cm−1 for the X˜ 2Σ+ and A˜ 2Π1/2,3/2 state, respectively. The dispersed fluorescence spectra show a progression in the Yb-C stretching vibration with a wave number of ω(ν3)=328 cm−1. Density functional calculations confirmed the vibrational assignment and yielded a linear geometry for both the X˜ and A˜ state of YbCCH as well as for the X˜ 1Σ+ state of the cation. Photoionization efficiency spectroscopy yielded an adiabatic ionization potential of 47 165(10) cm−1 [5.8477(12) eV]. Rydberg series converging to the 51 and 52 level of YbCCH+ were observed and combined with the appearance potentials led to ω(ν5)=97 cm−1 for the YbCC bending mode of YbCCH+. © 1997 American Institute of Physics.
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  • 87
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    The Journal of Chemical Physics 107 (1997), S. 2529-2541 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The atomic electron affinities of the eight first row (H,Li,...,F) atoms as well as the adiabatic electron affinities of 12 first row diatomic and 15 first row triatomic molecules were determined using six different density functional or hybrid Hartree–Fock/density functional methods. The 35 species were selected for having relatively well-established experimental electron affinities. Harmonic zero-point vibrational energy corrected electron affinities are also reported for the diatomic and triatomic molecules. Equilibrium geometries and harmonic vibrational frequencies are given for the 27 molecules and their anions as determined with each density functional method. Discussion focuses on comparison of theoretical and experimental electron affinities. For the atomic, diatomic, and triatomic electron affinities the average absolute error is reported for each exchange–correlation functional. Since many of the molecular anion structures and vibrational frequencies are unknown, the work suggests new experimental directions. © 1997 American Institute of Physics.
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  • 88
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    The Journal of Chemical Physics 107 (1997), S. 2557-2572 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A generic model has been proposed with which to understand the unusual properties of liquid crystal dimers, in particular their behavior at the nematic–isotropic transition. In the model the many conformational states available to real dimers are replaced by just two, a linear and a bent conformer which are allowed to interconvert. The nematic behavior of the model has been explored with the aid of the molecular field approximation and found to be in good qualitative agreement with that of real dimers, thus providing a valuable aid to our understanding of them. To see to what extent the use of the molecular field approximation influences the predictions we have undertaken a Monte Carlo computer simulation study of the generic model. To facilitate the simulations the particles have been confined to the sites of a simple cubic lattice with the anisotropic interactions being restricted to nearest neighbors and having the form −(ε/4)P2(cos β), where β is the angle between the anisometric groups. The other advantage of this model is that for the linear conformer alone it is identical to the Lebwohl–Lasher model for nematics which has been studied in depth and so allows the influence of the presence of the interconverting bent conformer on the liquid crystal behavior to be readily discerned. We have also extended the discrete generic model, with just two interconverting conformers, to a continuous model in which the torsional angle defining the conformational state varies continuously from 180° (linear) to 0° (bent); this makes the model far more general and more analogous to real dimers. A wide range of properties have been determined from the computer simulations; these include the second rank orientational order parameter for the anisometric groups, the singlet orientational distribution function for these, the second rank short range orientational order parameter, the internal order parameters and the distribution function for the torsional angle. Armed with this wealth of information we are able to make a searching test of the molecular field theories for the two models as well as commenting on the behavior of real liquid crystal dimers. © 1997 American Institute of Physics.
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  • 89
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    The Journal of Chemical Physics 107 (1997), S. 2597-2602 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The present calculations were performed on the basis of the Sanchez-Lacombe-Balasz lattice fluid theory. The two system specific parameters cursive-epsilon12* and δcursive-epsilon* required for that purpose have been obtained from the spinodal temperatures measured (SANS) for mixtures of poly(vinyl methyl ether) (PVME) and deuterated polystyrenes (d-PS) by Schwahn and coworkers. The experimental data reported for atmospheric pressure and six representatives of the present system are well described theoretically, where cursive-epsilon12* does not depend on molar mass and δcursive-epsilon* decreases only slightly as the chain length of d-PS is raised. The measured pressure influences on the spinodal conditions correspond to an approximately linear reduction of δcursive-epsilon* with increasing P; this observation should reflect the volume changes associated with the formation of specific interactions. According to the present calculations the critical composition shifts markedly towards pure PVME as P is raised. Since experimental data are commonly expressed in terms of the Flory-Huggins theory, the current results were also translated into Flory-Huggins interaction parameters and evaluated with respect to the contributions of enthalpy and of entropy. The agreement between experimental information and that calculated from the Sanchez-Lacombe-Balasz lattice fluid theory is reasonable. © 1997 American Institute of Physics.
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  • 90
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    The Journal of Chemical Physics 107 (1997), S. 2643-2652 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Equilibrium states of the system of self-assembled monolayers (SAM's) of n-alkanethiol molecules CH3(CH2)n−1 chemiabsorbed on the Au(111) crystal surface are considered in relation to temperature and external pressure applied normally to the surface. Couplings between the atoms (C, H) of the n-alkanethiols are approximated both by the Morse potential and by the Lennard-Jones one. Couplings between the n-alkanethiols and the crystal surface are approximated by the 12-3 potential. Because of the symmetry Z2 of the n-alkanethiols in the tilted state the system is reduced to the Ising model on the triangular lattice with two competing exchange parameters. Calculated p–T diagrams of the tilted system include the following phases: para, ferro, incommensurate, and structure 2×1. The incommensurate phase results from competition between the exchange parameters. It is shown that for some specific choices of the coupling constants the p–T diagram has tricritical points. The temperatures of phase transitions and equilibrium tilt, twist, and azimuthal angles depending on the coupling constants are found. All phase transitions are of the first order. Temperature behavior of heat capacity is calculated. © 1997 American Institute of Physics.
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  • 91
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    The Journal of Chemical Physics 107 (1997), S. 2942-2949 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutron diffraction experiments of heavy water at several supercritical states (380〈T〈500 °C and200〈P〈900 bar) corresponding to densities ranging from 0.2 to 0.7 g/cm3 are performed. They allow the static structure factor and the pair correlation function to be derived once absorption, multiple scattering, and inelasticity corrections have been made. The experimental results are compared with previous data obtained by Postorino et al. [Nature 366, 668 (1993)] and with new molecular dynamics simulations using the extended simple point charge (SPCE) potential. They lead to the conclusion that hydrogen bonding is still present in supercritical water. This conclusion does not agree with the previous findings of Postorino et al. but looks rather in agreement with the results of computer simulations. © 1997 American Institute of Physics.
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  • 92
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    The Journal of Chemical Physics 107 (1997), S. 2968-2973 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It has long been assumed that dipole-bound anionic states possess two properties; (i) the electron binding energy is dominated by the electrostatic electron–dipole interaction, and (ii) the geometrical distortion of the polar molecule upon attachment of a distant electron is negligible. Our results indicate, however, that the dispersion interaction between the loosely bound electron and the electrons of the neutral dimer is as important as the electrostatic electron–dipole stabilization. In addition, the hydrogen bond in (HF)2 is susceptible to a deformation upon attachment of a distant electron. This deformation enhances both the electrostatic and dispersion components of the electron binding energy. The calculated Franck–Condon factors indicate that neutral dimers formed in electron photodetachment experiments may be vibrationally excited in both stiff intramolecular HF stretching modes and soft intermolecular modes. The predicted value of the adiabatic electron detachment energy for (HF)2− is 396 cm−1 and the theoretical photoelectron spectrum is in remarkable agreement with the experimental data of Bowen et al. J. H. Hendricks, H. L. de Clercq, S. A. Lyapustina, and K. H. Bowen, Jr. [J. Chem. Phys. 107, 2962 (1997)]. © 1997 American Institute of Physics.
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  • 93
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    The Journal of Chemical Physics 107 (1997), S. 1725-1731 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A molecular mechanics calculation of the rotational potential experienced by methyl groups in molecular crystals is presented. Good agreement for seven out of the eight cases considered is obtained and the methyl group becomes a quantitative probe of its molecular environment. The calculation is used to gain insight into the coupling between methyl groups. For the smallest molecules, comparison between the generalized semiempirical calculation and a limited ab initio calculation on dimers of molecules suggests the simple rotational model employed is adequate, and that discrepancies between measured and calculated values are due to incorrect modeling of the atom–atom potentials. © 1997 American Institute of Physics.
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  • 94
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    The Journal of Chemical Physics 107 (1997), S. 1744-1752 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The deuterium quadrupole coupling tensors were calculated for the methyl halides CD3X (X=F, Cl, Br, and I) using ab initio effective core potential (ECP) methods up to the QCISD level. The effects of basis set quality and electron correlation were investigated, and a comparison with all-electron calculations was made. We also determined a new experimental value for methyl chloride by the liquid crystal nuclear magnetic resonance (LC NMR) method. Locally dense basis sets were used for the deuterium atoms and ECPs for the halogens. The Br and I ECPs were scalar relativistic. Computations at the equilibrium (re) geometry result in slightly too negative tensor elements along the molecular C3 symmetry axis for all X, as compared to the experimental LC NMR and microwave spectroscopic results. Values corresponding to the rα(300 K) geometry are in a very good agreement with experiment. As the tensors are sensitive to the length of the associated CD bond, we applied approximate vibrational corrections to the theoretical results. The current corrected values for the deuterium quadrupole coupling constants and asymmetry parameters are likely to be the most reliable available for the present set of molecules, despite that residual computational errors still remain. The common approximation of a cylindrically symmetric electric field gradient tensor at the deuterium site is shown to lead to an underestimation of the corresponding quadrupole coupling constant in methyl halides. © 1997 American Institute of Physics.
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  • 95
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    The Journal of Chemical Physics 107 (1997), S. 1753-1758 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: p-chloronitrobenzene (PCNB) and p-chlorobromobenzene (PCBB) crystallize in the centrosymmetric space group P21/c with two molecules per unit cell. The space lattice will have an equal number of points with molecules facing in opposite directions. As a consequence, these compounds exhibit an orientational rigid disorder. In this work, we have measured the temperature dependence of the chlorine nuclear quadrupole spin–lattice relaxation time (T1), linewidth, and resonance frequency for both compounds for temperatures higher than 80 K. Both compounds exhibit an inhomogeneously broadened line shape and a "normal" Bayer-type temperature dependence of the resonance frequency. The analysis focuses on the identification of the dominant relaxation process at high temperatures (T〉240 K in PCNB and T〉260 K in PCBB). It is shown that T1(T) reflects the existence of 180° molecular reorientations through a modulation of the crystalline contribution to the electric field gradient. © 1997 American Institute of Physics.
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  • 96
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    The Journal of Chemical Physics 107 (1997), S. 1779-1793 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum coherence and its dephasing by coupling to a dissipative environment play an important role in time-resolved nonlinear optical response as well as nonadiabatic transitions in the condensed phase. We have discussed nonlinear optical processes on a multi-state one-dimensional system with Morse potential surfaces in a dissipative environment. This was based on a numerical study using the multi-state quantum Fokker–Planck equation for a colored Gaussian–Markovian noise bath, which was expressed as a hierarchy of kinetic equations. This equation can treat strong system-bath interactions at a low temperature heat bath, where quantum effects play a major role. The approach applies to linear absorption measurements as well as four-wave mixing including pump-probe spectroscopy. Laser induced photodissociation and predissociation have been studied for the potential surfaces of Cs2. We have calculated nuclear wave packets in Wigner representation and their monitoring by femtosecond pump-probe spectroscopy for various displacements of potentials and heat-bath parameters. Numerical calculations of probe absorption spectra for strong pump pulse are also presented and discussed. The results show dynamical Stark splitting, but, in contrast to the Bloch equations which contain an infinite-temperature dephasing, we find that at finite temperature their peaks have different heights even when the pump pulse is on resonance. © 1997 American Institute of Physics.
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  • 97
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    The Journal of Chemical Physics 107 (1997), S. 1861-1871 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The gas-phase reactions of nitrogen, hydrogen, and carbon monoxide with Ni38 are studied as a function of reagent pressure at several reaction temperatures. Saturation coverage of the cluster is found at Ni38(N2)24, Ni38H36, and Ni38(CO)36. These saturation levels are consistent with the metal core of the ligated cluster having the structure of a truncated octahedron in each case. An alternate fcc structure derived from a 40-atom truncated tetrahedron is consistent with the nitrogen data, but not with the hydrogen or carbon monoxide results. In addition, the nitrogen uptake data indicate that the bare Ni38 cluster also has the structure of a truncated octahedron or possibly a deformed truncated octahedron. There is no indication that Ni38 has an icosahedral or polyicosahedral structure. The nature of the binding of the three reagents to the cluster is discussed. Evidence is presented that CO initially binds to atop sites, but following saturation of these sites a local rearrangement to bridge sites occurs that allows an increase in coverage to the observed saturation at Ni38(CO)36. At high reagent pressures all three reagents cause adsorbate-induced structural changes to isomers that bind more ligands and whose structures have yet to be determined. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 1890-1895 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Limiting mutual diffusion coefficients of aromatic compounds in acetone have been measured at 298.2 K by using the chromatographic peak-broadening method. The data of the polar and nonpolar pseudoplanar solutes are compared, and the effects of molecular association on diffusion as well as the solvation numbers are determined. It is found that the effects of hydrogen bonding are such that –OH〉–NH2〉–SH, and that the solvation numbers are approximately equal for solutes containing the same polar group. Using ab initio molecular orbital theory, molecular solute–acetone interaction energies have been calculated. There exists a nearly linear relationship between the interaction energy and the effect of solute–solvent association on diffusion. The applicability of the rough-hard-sphere theory to the diffusion of associated molecules is discussed. © 1997 American Institute of Physics.
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  • 99
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    The Journal of Chemical Physics 107 (1997), S. 1948-1952 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general theory is presented for the problem of condensed-phase particle nucleation from gas-phase precursors via a sequence of reversible chemical reactions, where no supersaturated vapor exists. We consider a system in which nucleation is initiated by the reaction between an "initiating species" and a "growth species."Subsequent steps in cluster growth involve reversible reactions between a cluster and the growth species, producing a larger cluster and a volatile byproduct, which may be considered a "suppressing species." Following the mathematical formalism of homogeneous nucleation theory, a steady-state nucleation rate is derived in the form of a summation over discrete cluster sizes. The resulting nucleation rate is linearly proportional to the product of the concentrations of the initiating species and the growth species, while the ratio of the concentrations of the growth species to the suppressing species, relative to a suitably defined equilibrium value, is seen to play a similar role as the vapor saturation ratio in homogeneous nucleation. © 1997 American Institute of Physics.
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    The Journal of Chemical Physics 107 (1997), S. 1970-1980 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Structural and spectroscopic properties of the equilibrium excess electron solvated in methanol are evaluated via adiabatic quantum molecular dynamics simulations employing an electron-methanol pseudopotential based on that of Zhu and Cukier et al. [J. Chem. Phys. 98, 5679 (1993)]. The solvated electron is localized in a roughly spherical ground electronic state of 2.5 Å radius. The electron is surrounded by approximately six or seven methanol molecules in the first solvent shell with mostly O–H bond orientation. Due to the strong electron-methanol coupling, solvent fluctuations result in substantial fluctuations of energy, size, and shape of the electron. The influences of radial fluctuations and asymmetric distortions of solvent cavities are illustrated on both the solvation structure and the optical spectrum. The optical-absorption spectrum of the solvated electron is dominated by transitions to three clearly nondegenerate p-like excited states. The fluctuations influence the s-p energy gap and modulate the p-p energy splitting leading to the calculated broad, featureless optical band. Comparison between systems containing flexible and rigid methanol molecules is also reported. Unlike in water, characteristic fluctuations of approximately 20-ps time scale can occur in methanol. The structural and optical consequences of these long timescale fluctuations are examined. © 1997 American Institute of Physics.
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