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Notes: The Doppler-selected time-of-flight method was applied to map out directly the differential cross sections of the title reaction. These new results supersede those reported earlier from this laboratory using the conventional Doppler-shift technique. More significantly, clear structures are now revealed, which correspond to a series of subgroups of partially resolved states of OH(v,j). They offer considerable promise in unraveling the complicated dynamical attributes associated with the two microscopic pathways, insertion and abstraction, which play in this reaction. © 1997 American Institute of Physics.

Notes: In this letter we present preliminary results of full six dimensional quantum dynamics calculations for dissociative adsorption of a hydrogen molecule on a Cu(111) surface. We utilize the time-dependent wave-packet approach to simulate the dissociation process on a full dimensional LEPS potential energy surface which has incorporated the latest ab initio data [Hammer et al. Phys. Rev. Lett. 73, 1400 (1994)]. We use a novel partitioning of the angular momentum operator in the split-operator method so that a direct product DVR can be rigorously implemented. The most interesting observation in the present rigorous quantum dynamics study is the site-averaged effect, i.e., the averaged dissociation probability of the four dimensional calculations over the three symmetric impact sites strongly resembles the exact dissociation probability of the six dimensional calculations. In accord with the low dimensional calculations, initial vibrational excitation of H2 effectively reduces the translational threshold energy. The rotational orientation effect observed in the four dimensional studies remains in the present full dimensional dynamics with the cartwheel orientation yielding dramatically lower dissociative efficiency than the energetically equivalent helicopter orientation. We focus on normal incident scattering. The diffractive scattering and more detailed results will be presented in a later paper. © 1997 American Institute of Physics.

Notes: Anharmonic force fields and ro-vibrational spectroscopic properties of AlF3 andSiF3+ have been investigated in detail, using the coupled cluster method with single and double substitutions augmented by a perturbative treatment of triple excitations [CCSD(T)] with a triple zeta basis set of 124 contracted Gaussian-type orbitals (cGTOs). A complete set of ro-vibrational spectroscopic constants for each species has been calculated using second order perturbation theory. The geometry only was calculated with a quadruple zeta basis set (224 cGTOs). Our best estimates of the equilibrium bond distances re (Al–F) and re (Si–F) are 1.624 Å and 1.508 Å, respectively, based on the quadruple zeta CCSD(T) bond distances and corrections derived from spectroscopically related known molecules. The CCSD(T) fundamental frequencies are689 cm−1(ν1),301 cm−1(ν2),951 cm−1(ν3), and241 cm−1(ν4) forAlF3 and853 cm−1(ν1),357 cm−1(ν2),1187 cm−1(ν3), and307 cm−1(ν4) forSiF3+. Energies of a number of low lying vibrational states(≤2400 cm−1) have been calculated by three methods: (1) standard second order perturbation theory formulas; (2) Van Vleck perturbation theory based on raising and lowering operators to second order and to fourth order; and (3) a variational method. © 1997 American Institute of Physics.

Notes: Resonance states of HCO are calculated for total angular momentum J=0, 1, and 3 using the artificial boundary inhomogeneity (ABI) method of Jang and Light [J. Chem. Phys. 102, 3262 (1995)]. Resonance energies and widths are determined by analyzing the Smith lifetime matrix. A resonance search algorithm and a method for resolving overlapping resonances are described. The accurate prediction of J=3 resonances from J=0 and 1 data is tested with good results for excited stretch resonances and less accurate results for bending resonances, demonstrating the degree of separability of vibration from overall rotation for these quasi-bound states. © 1997 American Institute of Physics.

Notes: Our previously developed analytical infinite order sudden (IOS) quantum theory of triatomic photodissociation is extended to describe indirect photodissociation processes through a real or virtual intermediate state. The theory uses the IOS approximation for the dynamics in the final dissociative channels and an Airy function approximation for the continuum states. These approximations enable us to evaluate the multi-dimensional non-separable transition amplitudes analytically (as one-dimensional quadratures), despite the different natural coordinates for the initial bound, the intermediate resonant, and the final dissociative states. The fragment internal energy distributions are described as a function of the initial and final quantum states and the photon excitation energy. The theory readily permits the evaluation of rotational distributions for high values of the total angular momentum J in the initial bound molecular state, a feature that would be very difficult with close-coupled methods. In paper II we apply the theory to describe the photofragment yield spectrum of NOCl in the region of the T1(13A″)←S0(11A′) transition. © 1997 American Institute of Physics.

Notes: We present a general methodology for calculating free-energy profile of reaction in solution using quantum mechanical methods coupled with the dielectric continuum solvation approach. Particularly, the generalized conductorlike screening model (GCOSMO) was employed in this study, though any continuum model with existing free-energy derivatives could also be used. Free-energy profile is defined as the steepest descent path from the transition state to the reactant and product channels on the liquid-phase free-energy surface. Application of this methodology to calculate the free-energy profile of the Menshutkin NH3+CH3Cl reaction in water is discussed. The efficiency of the GCOSMO method allows characterization of stationary points and determination of reaction paths to be carried out at less than 20% additional computational cost compared to gas-phase calculations. Excellent agreement between the present results and previous Monte Carlo simulations using a combined quantum mechanical/molecular mechanics (QM/MM) potential confirms the accuracy and usefulness of the GCOSMO model. © 1997 American Institute of Physics.

Notes: A new Brownian dynamics simulation technique is presented for the calculation of the effective rate constant for diffusion controlled reactions with a finite intrinsic reactivity. The technique is based on the calculation of the recollision probability of a molecule with a reactive site using a large number of Brownian trajectories, when the probability of reaction upon collision with the reactive site (cursive-phif) is less than unity. The technique is a modification of the earlier work of Northrup et al. [J. Chem. Phys. 80, 1517 (1984)], and is applied to the case of a uniformly reactive target sphere and a target sphere with axially symmetric reactive patches. A theoretical analysis is presented to relate cursive-phif to the intrinsic surface reaction rate constant (k). Computational results for the uniformly reactive sphere are in excellent agreement with theory, and those for the sphere with patches are in very good agreement with the results obtained using a different computational technique [Allison et al., J. Phys. Chem. 94, 7133 (1990)]. The proposed method requires the computation of the recollision probability to a high accuracy; however, this does not result in computational times greater than those of Allison et al. [J. Phys. Chem. 94, 7133 (1990)]. The new method has the advantage that the results of the Brownian dynamics simulation are independent of k and can subsequently be used to calculate the effective rate constant for any given value of k. © 1997 American Institute of Physics.

Notes: A molecularly detailed simulation method, designed to be efficacious for modeling conduction properties of closed shell atoms or molecules in solids, liquids, and at interfaces, has been developed. This approach successfully predicts the effective mass of a conduction electron in both solid xenon, and liquid xenon over a wide density range, as compared to experimental results. To model the electron-atom interaction, angular momentum and density-dependent semi-local pseudopotentials are employed. The pseudopotentials are first fit to reproduce the gas phase electron-xenon scattering phase shifts, and are then corrected to include many-body polarization effects in a reliable mean field fashion. The effective mass of a conduction electron is calculated by solving the Schrödinger-Bloch equation using Lanczos grid methods to obtain the Bloch wave vector (k) dependent energies in both the solid and the liquid. In the liquid phase, a representative sample of the fluid is replicated to form the "periodic" infinite system. This approximation is shown to be reliable as the effective mass does not depend on the system size or the particular configuration which is chosen. It is shown that the l=0 scattering in the condensed phase determines the k=0 ground state energies; these are coincident with the conduction band energy in this system. In contrast, the l=1 scattering is shown to determine the effective mass of the conduction electrons. © 1997 American Institute of Physics.

Notes: In this study we report on the application of the diffusion process-controlled Monte Carlo method to a 20 amino acid αβ peptide (Ac-E-T-Q-A-A-L-L-A-A-Q-K-A-Y-H-P-M-T-M-T-G-Am). The polypeptide chain is represented by a set of 126 particles, the side chains are modeled by spheres, and the backbone dihedral angles φ and ψ of each of the amino acid residue are essentially restricted to a set of ten high probability regions, although the whole φ-ψ space may be visited in the course of the simulation. The method differs from other off-lattice Monte Carlo methods, in that the escape time from one accepted conformation to the next is estimated and limited at each iteration. The conformations are evaluated on the basis of pairwise nonbonded side chain energies derived from statistical distributions of contacts in real proteins and a simple main chain hydrogen bonding potential. As a result of four simulations starting from random extended conformations and one starting from a structure consistent with NMR data, the lowest-energy conformation (i.e., the αβ fold) is detected in ∼103 Monte Carlo steps, although the estimated probability of getting the αβ motif is ∼10−12. The predicted conformations deviate by 3.0 Å rms from a model structure compatible with the experimental results. In this work further evidence is provided that this method is useful in determining the lowest-energy region of medium-size polypeptide chains. © 1997 American Institute of Physics.

Notes: Heat capacity curves as functions of temperature were calculated using Monte Carlo methods for the series of Ne13−nArn clusters (0≤n≤13). The clusters were modeled classically using pairwise additive Lennard-Jones potentials. The J-walking (or jump-walking) method was used to overcome systematic errors due to quasiergodicity. Substantial discrepancies between the J-walking results and those obtained using standard Metropolis methods were found. Results obtained using the atom-exchange method, another Monte Carlo variant for multi-component systems, also did not compare well with the J-walker results. Quench studies were done to investigate the clusters' potential energy surfaces. Only those Ne-Ar clusters consisting predominately of either one or the other component had lowest energy isomers having the icosahedral-like symmetry typical of homogeneous 13-atom rare gas clusters; non-icosahedral structures dominated the lowest-energy isomers for the other clusters. This resulted in heat capacity curves that were very much different than that of their homogeneous counterpart. Evidence for coexistence behavior different than that seen in homogenous clusters is also presented. © 1997 American Institute of Physics.

Notes: We describe atomistic simulations of the free energy and entropy of hydration of ions in aqueous solution at 25 °C using a simple point charge model (SPC/E) for water and charged spherical Lennard-Jones solutes. We use a novel method with an extended Lagrangian or Hamiltonian in which the charge and the size of the ions are considered as dynamical variables. This enables us to determine thermodynamic properties as continuous functions of solute size and charge and to move smoothly from hydrophilic to hydrophobic solvation conditions. On passing between these extremes, the entropy of solvation goes through maxima. For example it shows a double maximum as a function of charge at constant size and a single maximum as a function of size at constant (non-zero) charge. These maxima correspond to extremes of structure-breaking and are associated with the disappearance of the second solvation shell in the radial distribution function; no anomalies are seen in the first shell. We also present direct evidence of the asymmetry in the free energy, enthalpy and entropy of hydration of ions on charge inversion arising from the asymmetry in the charge distribution in a water molecule. Our calculation only includes local contributions to the thermodynamic functions, but once finite size corrections are applied, the results are in reasonable agreement with experiment. © 1997 American Institute of Physics.

Notes: We have measured the turbidity for a series of solutions of polymethylmethacrylate in 3-octanone (PMMA/3-OCT) with various polymer molecular weights. The obtained correlation length (ξ) and the osmotic compressibility (χ) show power law dependence on both the reduced temperature ε=(T−Tc)/Tc and the degree of polymerization N of the polymer chains, i.e., ξ∼N0.15ε−0.63 and χ∼N−0.06ε−1.23, with the associated scaling exponents in good agreement with theoretical predictions. When the results of the present experiment are combined with those from an earlier coexistence curve measurement [K.-Q. Xia, X.-Q. An, and W.-G. Shen, J. Chem. Phys. 105, 6018 (1996)], it is found that the concept of two-scale-factor universality applies to systems consist of the same polymer/solvent pair but with different molecular weights. © 1997 American Institute of Physics.

Notes: In this paper we report on the angle-resolved ultraviolet photoelectron spectroscopy on ultrathin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) (BTQBT) deposited on a MoS2 surface with synchrotron radiation, and describe the quantitative analysis of the angular distribution of photoelectron from the highest occupied state. The observed angular distributions were better explained by those calculated with the single-scattering approximation combined with molecular orbital calculation considering intramolecularly scattered waves than the previously used independent-atomic-center approximation combined with molecular orbital calculation. Further, the low-energy-electron diffraction measurements were performed on the film. From the low-energy-electron diffraction, the two-dimensional lattice of the ultrathin films on the MoS2 was found to be MoS2(0001)−(13×13, R=±13.9°)-BTQBT, and from the analysis of the photoelectron angular distributions, it was found that at the lattice points the molecules lie flat with azimuthal orientations of 19° and 47° with respect to each surface crystal axis of MoS2. On the basis of these results, the full structure of the thin film, the two-dimensional lattice, and full molecular orientations at the lattice points, is proposed. © 1997 American Institute of Physics.

Notes: [S0021-9606(97)00230-4]

Notes: A formulation is presented for calculating double quantum two dimensional electron spin resonance (DQ-2D ESR) spectra in the rigid limit that correspond to recent experimental DQ-2D ESR spectra obtained from a nitroxide biradical. The theory includes the dipolar interaction between the nitroxide moieties as well as the fully asymmetric g and hyperfine tensors and the angular geometry of the biradical. The effects of arbitrary pulses (strong but not truly nonselective pulses) are included by adapting the recently introduced split Hamiltonian theory for numerical simulations. It is shown how arbitrary pulses in magnetic resonance create "forbidden" coherence pathways, and their role in DQ-2D ESR is delineated. The high sensitivity of these DQ-2D ESR signals to the strength of the dipolar interaction is demonstrated and rationalized in terms of the orientational selectivity of the "forbidden" pathways. It is further shown that this selectivity also provides constraints on the structural geometry (i.e., the orientations of the nitroxide moieties) of the biradicals. The theory is applied to the recent double quantum modulation (DQM) experiment on an end-labeled poly-proline peptide biradical. A distance of 18.5 Å between the ends is found for this biradical. A new two pulse double quantum experiment is proposed (by analogy to recent NMR experiments), and its feasibility for the ESR case is theoretically explored. © 1997 American Institute of Physics.

Notes: Quantum mechanical analysis is presented on the control of the vector properties of the photoproducts by vibrationally mediated photodissociation of OH. The angular distributions and alignment of O(3P) fragments are calculated near isolated and overlapping asymmetric resonances. The vector properties depend very sensitively on the vibrational levels (νi=0–4) of the initial X 2Π state. The variations of the anisotropy parameters as a function of the excitation energy near the asymmetric resonances change markedly depending on νi. The widths of the variations tend to increase with increasing νi, which is very similar to the corresponding behaviors of the product branching ratios studied earlier [J. Chem. Phys. 104, 1912 (1996)], indicating that νi could be a useful tool for choosing the proper linewidths in the experiments for the control of the product branching ratios, angular distributions, and m distributions near asymmetric resonances. It is also found that the vector properties may exhibit splitting of the overlapping resonances for high νi, in contrast to scalar properties. © 1997 American Institute of Physics.

Notes: The dissociation dynamics of H2 and D2 on a rigid Ni13 cluster has been investigated using a quantum mechanical model. The model is based on the spectral grid/fast Fourier transform technique and includes three variables which are treated quantum mechanically; the translational motion of the molecule normal to the cluster, the vibrational coordinate, and the polar orientation angle. The remaining three variables are fixed during the simulations. The dependence of the dissociation probability on the incident beam energy, initial molecular state and impact site has been examined. The probabilities for rovibrational excitation of the scattered flux have also been computed as function of incident beam energy and impact site. In addition, the dissociation probability has been averaged over the remaining three variables that define the impact site configuration. © 1997 American Institute of Physics.

Notes: In this publication is presented a three-dimensional quantum mechanical study, within the coupled states approximation, of the process H+H2+(v1=0,j1)→H++H2. Both reactive (exchange) and inelastic processes were considered. The main findings are: (a) The charge transfer process takes place at large distances (∼3.5 Å) and so the reagents are essentially on the lower potential energy surface when they approach the close interaction region; (b) The main contributions to the reaction (exchange) are from large impact parameters; (c) The initial rotational states have at most a minor effect on the results (whether being charge transfer or chemical exchange); (d) The deep potential well in the interaction region of the lower surface has only a secondary effect on the results. © 1997 American Institute of Physics.

Notes: An analytic method to evaluate nuclear contributions to electrical properties of polyatomic molecules is presented. Such contributions control changes induced by an electric field on equilibrium geometry (nuclear relaxation contribution) and vibrational motion (vibrational contribution) of a molecular system. Expressions to compute the nuclear contributions have been derived from a power series expansion of the potential energy. These contributions to the electrical properties are given in terms of energy derivatives with respect to normal coordinates, electric field intensity or both. Only one calculation of such derivatives at the field-free equilibrium geometry is required. To show the useful efficiency of the analytical evaluation of electrical properties (the so-called AEEP method), results for calculations on water and pyridine at the SCF/TZ2P and the MP2/TZ2P levels of theory are reported. The results obtained are compared with previous theoretical calculations and with experimental values. © 1997 American Institute of Physics.

Notes: Quantum molecular similarity (QMS) techniques are used to assess the response of the electron density of various small molecules to application of a static, uniform electric field. Likewise, QMS is used to analyze the changes in electron density generated by the process of floating a basis set. The results obtained show an interrelation between the floating process, the optimum geometry, and the presence of an external field. Cases involving the Le Chatelier principle are discussed, and an insight on the changes of bond critical point properties, self-similarity values and density differences is performed. © 1997 American Institute of Physics.

Notes: We examine the influence of attractive interactions on the phase behavior of rodlike colloids. We model the rodlike particles by spherocylinders, for which the phase diagram, in the absence of attraction, is known for arbitrary aspect ratio. We consider the case that the attraction is due to depletion forces caused by the addition of nonadsorbing polymer. The range of this attraction is determined by the size of the polymer. If the radius of gyration of the polymer is small compared to the diameter of the rods, we can model the polymer-induced attraction by a suitable generalization of the square-well model for spherical particles. However, for longer ranged attractions, pairwise additive attractions lead to phase behavior that is very different from what is found when the nonadditivity of depletion forces is taken into account. In our simulations, we find evidence for demixing transitions in the isotropic, nematic and solid phases. We compare our simulation results with predictions based on the perturbation theory of Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. A crucial input in this theory is the so-called free-volume fraction of the hard spherocylinder reference system. In the work of Lekkerkerker and Stroobants, this quantity is estimated using scaled-particle theory. We test the validity of this approach by comparing it to numerical results for the free-volume fraction. © 1997 American Institute of Physics.

Notes: We report the first gas phase observation of a bending vibration in an open shell molecule by far-infrared laser magnetic resonance. The spectrum is due to FeD2; its observation shows that rovibrational transitions can be detected with such a spectrometer. The molecule was produced in the reaction between deuterium atoms and iron pentacarbonyl. FeD2 has a linear 5Δ ground state and shows the effects of Renner–Teller coupling in the excited vibrational (010) level. © 1997 American Institute of Physics.

Notes: A model which treats spin diffusion and spin-lattice relaxation in multiphase polymers on the same footing is proposed. This new approach allows for more accurate determination of domain sizes and interfacial thickness in the probed polymers. In a poly(styrene-b-isoprene) copolymer example a significant improvement in the agreement between the NMR measurements and the simulated results can be obtained with the incorporation of a spin-lattice relaxation term into the spin diffusion process. The obtained microphase structural parameters are similar to the small angle X-ray scattering results. In addition, the spin-lattice relaxation times (T1) in different domains can be predicted reasonably well based on T1s of the component homopolymers and on the microphase structure in the present model. © 1997 American Institute of Physics.

Notes: A theory to describe the effect of the Donnan osmotic pressure on the volume phase transition of hydrated gels is presented by extending the previous theory. The theory takes into consideration a coupling of the volume change affected by the Donnan osmotic pressure and the dehydration of the gel chain. The theory reveals that the swelling force suppresses the dehydration and diminishes the volume change at the transition. When applied to a model accounting for the volume phase transition of N-isopropylacrylamide (NIPA) gel, the theory predicts that the transition disappears as the Donnan osmotic pressure increases. © 1997 American Institute of Physics.

Notes: The Ionization Potentials of small LiN clusters are calculated with a Shell Correction Method. They are used to illustrate that, within the jellium approximation, deformed cluster shapes provide an adequate description of the observed systematic size dependence of the properties of simple metal clusters. Such deformation effects were overlooked in the analysis of Gardet et al. © 1997 American Institute of Physics.

Notes: King and Weinhold assert in J. Chem. Phys. 103, 333 (1995) that the description of the hydrogen bond in terms principally of electrostatic interactions is incapable of describing the cooperative properties of hydrogen-bonded systems. However, provided that induction is included in the electrostatic interactions, the cooperative energy and dipole moment of HCN clusters can be quantitatively described. © 1997 American Institute of Physics.

Notes: We report a distributed approximating functional (DAF) fit of the ab initio potential-energy data of Liu [J. Chem. Phys. 58, 1925 (1973)] and Siegbahn and Liu [ibid. 68, 2457 (1978)]. The DAF-fit procedure is based on a variational principle, and is systematic and general. Only two adjustable parameters occur in the DAF leading to a fit which is both accurate (to the level inherent in the input data; RMS error of 0.2765 kcal/mol) and smooth ("well-tempered," in DAF terminology). In addition, the LSTH surface of Truhlar and Horowitz based on this same data [J. Chem. Phys. 68, 2466 (1978)] is itself approximated using only the values of the LSTH surface on the same grid coordinate points as the ab initio data, and the same DAF parameters. The purpose of this exercise is to demonstrate that the DAF delivers a well-tempered approximation to a known function that closely mimics the true potential-energy surface. As is to be expected, since there is only roundoff error present in the LSTH input data, even more significant figures of fitting accuracy are obtained. The RMS error of the DAF fit, of the LSTH surface at the input points, is 0.0274 kcal/mol, and a smooth fit, accurate to better than 1 cm−1, can be obtained using more than 287 input data points. © 1997 American Institute of Physics.

Notes: We consider the kinetics of the two-dimensional, stoichiometric A+B→0 reaction under confined-scale turbulent mixing and concentrate on the interplay between the kinetic patterns and the spatial organization of the system. We study the properties of the arising clusters and of the reaction zones, both in the presence and in the absence of mixing. We show that the two- point correlation function CAB(r)=〈cA(r′+r)cB(r′)〉/〈cA(r)〉2 is closely related to the effective reaction rate, while the functional form of the quartic correlation function Q(r,t)=〈cA(r′,t)cB(r′,t)cA(r′+r,t)cB(r′+r,t)〉/〈cA2(r,t)cB2(r,t)〉 is connected to the geometry of the reaction zones. We pay special attention to the occurrence of time windows of fast (classical) concentration decay even when the reactants show strong segregation. © 1997 American Institute of Physics.

Notes: Molecular dynamics simulations of many degree of freedom systems are often comprised of classical evolutions on quantum adiabatic energy surfaces with intermittent instantaneous hops from one surface to another. However, since quantum transitions are inherently nonadiabatic processes, the adiabatic approximation underlying the classical equations of motion does not hold in the regions where quantum transitions take place, and the restriction to classical trajectories for adiabatic quantum states is an approximation. Alternatives which employ classical paths that account more fully for nonadiabaticity can be computationally expensive and algorithmically complicated. Here, we propose a new method, which combines the surface hopping idea with the mean-field approximation for classical paths. Applied to three test systems, the method is shown to outperform the methods based on an adiabatic force without significant extra effort. This makes it an appealing alternative for modeling complex quantum–classical processes. © 1997 American Institute of Physics.

Notes: A recently proposed energy switching scheme is used to improve the two-valued many-body expansion potential energy surface of Murrell, Carter, Mills, and Guest [Mol. Phys. 42, 605 (1981)] for H2O by merging it with the spectroscopically accurate polynomial-type form of Polyanski, Jensen, and Tennyson [J. Chem. Phys. 105, 6490 (1996)]. An attempt is also made to improve its long range forces, and Coulombic behavior at the collapsed molecular limits. The resulting ES two-valued surface has almost spectroscopic accuracy up to 13 650 cm−1, and like the original many-body expansion form may be used for studies of reaction dynamics. A brief analysis of the Σ–Π locus of conical intersection is also presented. © 1997 American Institute of Physics.

Notes: Korona, Williams, Bukowski, Jeziorski, and Szalewicz [J. Chem. Phys. 106, 1 (1997)] constructed a completely ab initio potential for He2 by fitting their calculations using infinite order symmetry adapted perturbation theory at intermediate range, existing Green's function Monte Carlo calculations at short range and accurate dispersion coefficients at long range to a modified Tang–Toennies potential form. The potential with retardation added to the dipole-dipole dispersion is found to predict accurately a large set of microscopic and macroscopic experimental data. The potential with a significantly larger well depth than other recent potentials is judged to be the most accurate characterization of the helium interaction yet proposed. © 1997 American Institute of Physics.

Notes: The unimolecular decomposition of size selected cluster cations of trivalent metals (Aln+, Gan+, and Inn+), induced by high fluence laser ionization, has been investigated in the n=7 to n=85, 55, and 75 size ranges, respectively. This method is applied for the first time to photoexcited trivalent clusters generated in an evaporative ensemble and the experimental data cover a size range that was not explored in previous pioneering experiments on their dynamics. Small clusters dissociate through the loss of a neutral or a charged atom whereas clusters larger than a well defined critical size merely dissociate through the first channel. In the framework of the RRK statistical theory, the measured evaporation rates provide some information about the size evolution of the cluster dissociation energies and their ionization potentials in the low size range. The competition between the ion and the atom evaporation is found to be consistent with the size evolution of the ionization potentials independently measured by direct photoionization. The agreement between theory and experiment is discussed in relation to cluster structure, especially in the case of gallium. © 1997 American Institute of Physics.

Notes: Using the laser photolysis/laser-induced fluorescence (LIF) "pump-and-probe" technique, the dynamics of H atom formation in the photodissociation of CH3–CF2Cl (HCFC-142b) after excitation at 193 nm and the Lyman-α wavelength were studied under collision-free conditions in the gas-phase at room temperature. The H atoms produced were detected by (2p2P←1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα(approximate)121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV photodissociation experiments the Lyman-α laser radiation was used both to photodissociate the parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The following quantum yields ΦH for H atom formation were determined by a photolytic calibration method: ΦH(193 nm)=(0.06±0.02) and ΦH(Lα)=(0.53±0.12). From the measured H atom Doppler profiles the average H atom kinetic energy was determined to be ET(193 nm)=(51±10) kJ/mol and ET(Lα)=(72±4) kJ/mol, respectively. © 1997 American Institute of Physics.

Notes: Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited S1 state indicated that the gas phase thermal rate at room temperature is much smaller than the thermal rate in the liquid phase. This was based on the observations that: (a) A combination of measured energy-dependent rates and RRKM calculations led to an isolated molecule thermal rate at T=300 K of 2×109 s−1; (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the S0 to S1 zero point to zero point electronic transition energy (000), gave a lifetime in the excited state of ∼780 ps. The liquid state lifetime in ethane is ∼30 ps. In this paper we present theoretical computations of the rate in the gas and liquid phases, based on a new potential model of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. We find that: (a) RRKM rates are in agreement with measured energy-dependent rates; (b) The thermal rate derived from the new RRKM rates is the same as the thermal rate in liquid ethane; (c) The laser excitation experiment of Balk and Fleming leads to laser cooling of the excited state suggesting that their measured lifetime is longer than the lifetime in the liquid. The surrounding liquid heats up the molecule on a time scale which is faster than the isomerization lifetime. Experiments are suggested to verify this interpretation. © 1997 American Institute of Physics.

Notes: Time-of-flight spectra of the Cl photofragments were measured for molecular beams of o-, m-, and p-chlorotoluene (ClC6H4CH3) and o-, m-, and p-dichlorobenzene (ClC6H4Cl) irradiated by a 193 nm excimer laser pulse. The observed translational energy distributions of photofragments revealed that these chlorinated benzene derivatives dissociate via three different channels: (1) very fast predissociation and/or a direct dissociation, (2) predissociation through the triplet state, and (3) predissociation via highly excited vibrational levels of the ground electronic state (hot molecules). The three dissociation channels for dichlorobenzene have similar probabilities (∼0.3) in accord with those for chlorobenzene, indicating no significant change caused by the additional chlorine atom. The methyl substituent on chlorobenzene (chlorotoluene), however, remarkably induces dissociation through triplet states, probably due to the enhanced intersystem crossing by the methyl group. The angular distribution of the photofragment was also measured for p-chlorotoluene and p-dichlorobenzene excited by linearly polarized laser light. Angular distributions of the Cl fragments via the 2nd and 3rd channels were isotropic, while the fastest fragment via the 1st channel has an anisotropic distribution, confirming that the dissociation rate of the 1st channel is shorter than a molecular rotation. © 1997 American Institute of Physics.

Notes: An integral-direct implementation of first-order one-electron properties in the coupled cluster singles and doubles (CCSD) model is presented. The implementation increases the range of applicability of CCSD first-order one-electron property calculations significantly compared to nondirect approaches. As an application a thorough basis set investigation is performed on five diatomic molecules at the Hartree–Fock and CCSD levels for the molecular electric dipole moment, the molecular electric quadrupole moment, and the electric field gradient at the nuclei. In general, basis sets of polarized triple-zeta quality are the smallest to be recommended, and the convergence towards the basis set limit is faster at the Hartree–Fock level than at the CCSD level. Among the properties considered, the electric dipole moment is the easiest to converge. The electric dipole and especially the electric quadrupole moment require diffuse functions for high accuracy. With standard basis sets, it is not possible to calculate electric field gradients consistently within three thousandths of an atomic unit of the basis set limit—for this purpose, elaborate nonstandard basis sets are required. The electric field gradients at the nuclei in HCN and the electric dipole moment of the furan molecule are calculated at the CCSD level employing up to 417 basis functions, further demonstrating the large-scale applicability of the implementation. © 1997 American Institute of Physics.

Notes: We present measurements of binary homogeneous nucleation of sulfuric acid and water vapors at 298 K at two different relative humidities (38.2% and 52.3%), H2SO4 vapor concentrations ranging from about 1⋅1010 to 3⋅1010 molecules/cc. The experimental setup consisted of a turbulent mixing unit combined with a flow chamber where the nucleation took place. Particular attention was paid to the generation of sulfuric acid vapor. The measured nucleation rates vary between about 2 and 3000 cm−3 s−1. The results are compared with other measurements found in the literature. Our experimental rates are in agreement with predictions of the revised classical nucleation theory incorporating the hydration effect. © 1997 American Institute of Physics.

Notes: We present a gauge-origin independent method for calculating the electric-field dependence of the molecular magnetizability—that is, the hypermagnetizability, related to the Cotton–Mouton Effect (CME)—of solvated molecules. In our approach, the solvated molecule is placed in a spherical cavity surrounded by a linear, homogeneous, and polarizable dielectric medium. We apply the model to investigate the dielectric-medium effects on the CME of liquid water. The effects of electron correlation, molecular geometry, and the surrounding dielectric continuum on the hypermagnetizability and the CME are investigated. The change induced in the hypermagnetizability anisotropy by the dielectric medium is the dominating effect, being almost twice as large as the correlation contribution. The combined effect of electron correlation and the dielectric continuum leads to a doubling of the hypermagnetizability anisotropy when going from the SCF gas phase value (Δη=17.89 a.u.) to the value obtained for the MCSCF wave function in the dielectric medium (Δη=39.74 a.u.). The effects of change in geometry are shown to be small. Our result for the static Cotton–Mouton constant averaged in the temperature range 283.15 K to 293.15 K, mC=15.2×10−20 G−2 cm3 mol−1, differs from experiment still by the sign and by a factor of almost 8. The major reason for this discrepancy is the neglect of short-range interactions such as hydrogen bonding and van der Waals interactions not accounted for by the continuum model. © 1997 American Institute of Physics.

Notes: The initial dissociative chemisorption probability, S0, of O2 on Ir(111) has been investigated with molecular beam techniques and electron energy loss spectroscopy (EELS). The adsorption process on the clean surface occurs by distinct dynamical mechanisms. At incident kinetic energies, Ei, of 0.1 eV and below, the dissociative chemisorption probability decreases with increasing kinetic energy, indicating the dominance of a trapping-mediated mechanism. A decrease in the value of S0 with increasing surface temperature, Ts, is also characteristic of this regime. This temperature dependence reflects the participation of a physically adsorbed state and molecularly chemisorbed state in the dissociation scheme. Additionally, the dependence of S0 on incident angle, θi, in the low kinetic energy regime exhibits near normal energy scaling. At high kinetic energy (Ei〉0.1 eV), the initial dissociative chemisorption probability rises with increasing Ei indicating that translational energy is effective in surmounting a potential barrier to adsorption. Direct access of a molecularly chemisorbed state followed by dissociation, rather than direct access of the dissociated state, is hypothesized to be the primary initial adsorption step. Several observations support this mechanism, including a temperature dependence in the high kinetic energy regime and no observed increase in oxygen saturation coverage with increasing kinetic energy. In addition, EEL spectra show that molecularly chemisorbed states of oxygen are formed on the Ir(111) surface at Ts〈70 K after exposure to a 1.36 eV beam and partial saturation of the atomic overlayer. Attempts to identify molecularly chemisorbed oxygen at low coverages were unsuccessful and limited by the experimental setup which provides cooling of the iridium crystal to only ∼68 K. © 1997 American Institute of Physics.

Notes: A kinetic model is developed for the electrocatalytic oxidation of formic acid on Pt under potentiostatic control. The model development proceeds stepwise via a simple model of the electrocatalytic CO oxidation. The full model consists of four coupled, nonlinear ordinary differential equations. The scanned and stationary current/outer potential (I/U) behavior, stationary current oscillations, two-parameter bifurcation diagrams and stirring effects are simulated using realistic model parameters. The numerical findings are found to be consistent with the experimental results given by Strasser et al. The model reproduces period-1 as well as mixed-mode oscillations. Furthermore, a mechanistic analysis of the model was performed: two suboscillators are identified whose characteristics allow a plausible interpretation of the observed dynamics. After a classification of the suboscillators into previously described categories, an attempt is made to identify the minimal mechanistic requirements for electrochemical current oscillations. © 1997 American Institute of Physics.

Notes: A single mode cw dye laser along with velocity modulation was used to record weak bands of TiCl+ in the spectral region 17 100–18 600 cm−1. The ions were produced in the positive column of an ac glow discharge with a gas mixture of He/TiCl4. These new bands have been identified as linking the upper already known [17.8]3Δ(3d4s) state to respectively the X 3Φ(v=1) state and to a newly observed A 3Δ(v=0) state located 350 cm−1 above the X 3Φ(v=0) state. As suggested by Focsa et al. (J. Chem. Phys. 106, 9044 (1997)) the A 3Δ(3d2) state was found to be responsible for the perturbations observed in the X 3Φ(3d2) state. The spin-orbit components of the A 3Δ(v=0) and the X 3Φ(v=1) states revealed interaction leading to homogeneous and heterogeneous perturbations. A set of deperturbed parameters has been determined with the help of a full matrix-based handling of the A 3Δ(v=0) and the X 3Φ(v=0,1) states. Study of the contributions of the basis functions to the eigenvectors of the diagonalized matrix displayed evidence of a reversal of the leading character of the Ω=3 spin-orbit components of the A 3Δ(v=0) and the X 3Φ(v=1) states. A remarkable example of avoided crossing has been observed between the A 3Δ1(v=0) and the X 3Φ2(v=1) spin-orbit components. © 1997 American Institute of Physics.

Notes: We compare two models describing the dynamics of phase separation of incompressible mixtures: A local model proposed by de Gennes, and a nonlocal model proposed by E and Palffy-Muhoray. We find that in the interfacial regime, the local model gives rise to interfacial motion via the Mullins-Sekerka law for moderate quenches, and surface diffusion for deep quenches. The interface dynamics is slowed down sharply as the quench depth is increased. The nonlocal model, on the other hand, has an additional convective mechanism which gives rise to motion by Hele-Shaw dynamics. This additional mechanism is insensitive to the quench depth. While both models explain qualitatively the observed pinning phenomenon for deeply quenched off-critical mixtures, only the nonlocal model predicts the correct dependence of the coarsening rate on the quench depth. © 1997 American Institute of Physics.

Notes: We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the "molecular lens" are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed. © 1997 American Institute of Physics.

Notes: The previous matrix-theoretical analysis of the time lags and mean first passage times for reaction-diffusion transport in the Laplace domain [J. Chem. Phys. 106, 8022 (1997)] has been extended to the problems of membrane permeation, absorption and desorption. A matrix transport equation has been derived to calculate the flux (in the Laplace domain) for specified initial and boundary conditions corresponding to membrane permeation, absorption or desorption. Seven equality relations between fluxes for reaction-free diffusion and one for diffusion accompanying reaction have been found. Among them are two new equality relations which have been discovered by this matrix analysis. © 1997 American Institute of Physics.

Notes: Three different, linear, three-site binding models are presented for the binding of proteins, such as a repressor, to DNA. The first model assumes that the DNA is rigid and all correlations between the ligands are due to direct ligand–ligand interactions. In the second and third models conformational changes in the DNA are induced by the binding process. In these models we find both direct and indirect correlations between the ligands. Examination of the indirect correlation reveals two characteristic features that, in general, are negligible in the direct correlation. (1) Long range correlation, between nonadjacent sites. (2) Nonadditivity of the triplet correlations. It is shown that these two features cannot be neglected in the indirect correlations. The arguments for explaining these features are similar to those which explain long range and nonadditivies of correlations in the liquid state. © 1997 American Institute of Physics.

Notes: The mean value and the distribution of the penetration depth of F atoms is determined from samples composed of three layers with controlled thickness in the monolayer range and with a test of the compactness of the films via the intensity of surface excitons. F atoms with an average kinetic energy of 4.3 eV are generated in the top layer (Ar doped with F2) by photodissociation of F2 with 10.15 eV in a spin forbidden repulsive state. The F atoms are injected into an Ar spacer layer of variable thickness. Those reaching the interface to the Kr bottom layer are monitored via the intensity of the Kr2F fluorescence at a wavelength of 444 nm, which allows one to discriminate between F in Ar (439 nm), in Kr (453 nm), and at the Kr/Ar (444 nm) interface. The F content at the interface is kept below 1/20th of a monolayer to suppress recombination, and the detection sensitivity is increased to 1/1000th of a monolayer by excitation via Kr exciton energy transfer. The probability for F atoms to penetrate the Ar spacer layer decreases exponentially with increasing thickness down to 10% for a thickness of 23 monolayers, and an average penetration depth of 10 monolayers is derived. These very large penetration depths exceed those of F+ and F− ions by more than one order of magnitude. They are consistent with those molecular dynamics calculations, which predict a rather rectilinear motion in channels of the Ar lattice. An average length of travel of up to 27 monolayers with a mean-free path (large angle scattering) up to four monolayers is compatible with the results. © 1997 American Institute of Physics.

Notes: This paper focuses on the structural characteristics of confined squalane and tetracosane under shear flow conditions. Nonequilibrium molecular dynamics simulation is used to explore the rheology of these model lubricants. A preceding paper describes the molecular model and the simulation method, and examines interfacial slip. The lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. In this paper we examine the density profiles and chain conformations of the alkanes under shear flow conditions. Our results indicate a profound influence of the walls on the fluid structure. In particular, when the wall spacing is close to an integral multiple of the molecular diameter, tetracosane shows the formation of distinct layers with the molecules being in a fully extended state. This behavior is not observed for squalane. Under shear flow conditions the molecules tend to orient parallel to the walls, as would be expected, with a greater degree of orientation (a) close to the walls, (b) at the positions of local density maxima, and (c) at higher strain rates. © 1997 American Institute of Physics.

Notes: Double-resonance overtone excitation prepares HOCl molecules in single rovibrational states above the unimolecular dissociation threshold in the ground electronic state. Detecting the OH dissociation fragments allows us to observe which reactant states lie above or below the dissocation threshold and determine that threshold to be 19 290.3±0.6 cm−1. Dissociation rates from single, well-characterized eigenstates of the parent molecule exhibit fluctuations of more than an order of magnitude.© 1997 American Institute of Physics.

Notes: We present an analysis of the contributions of the first- and second-derivative tensors of the many-body polarizability to third- and fifth-order nonresonant spectroscopies in isotropic media. Collision-induced effects are shown to have a notable influence on the second-derivative polarizability tensor (Π(2)) for intermolecular modes. As a result, polarization selectivity in nonresonant intermolecular spectroscopies can be achieved in fifth-order spectroscopies. Additionally, terms in fifth-order spectroscopy that arise from three interactions through Π(2) may not be negligible in many liquids. Our analysis shows that there exists no straightforward relationship between the observables in third- and fifth-order intermolecular spectroscopies. The predictions of this analysis are tested against the available experimental data for CS2. © 1997 American Institute of Physics.

Notes: Photoionization of W(CO)6 in the photon energy range of 8–40 eV produced a variety of intermediate ions of the form (WCm(CO)n)x+. Photoionization efficiency (PIE) curves of these ions have been measured for the first time by using a time-of-flight mass spectrometer (TOFMS) with vaccum ultraviolet (VUV) photons from a synchrotron radiation source. Appearance potentials (AP) of all the observed ions have been determined from their PIE curves. Based on these AP values, we obtained a series of bond dissociation energy (BDE) data for the intermediate ions produced by the dissociative photoionization. © 1997 American Institute of Physics.

Notes: A theoretical study of vibrational dephasing of molecular vibrations in liquids is presented with an aim to understand the experimentally observed sub-quadratic quantum number (n) dependence of the vibrational dephasing rate, in systems like CH3I and CHCl3 and their deuterated analogues. The analysis is based on Oxtoby's theory of vibrational dephasing but with a detailed microscopic description of the frequency dependent frictional forces on the vibrational mode. The friction on the normal coordinate in liquids is found to have a pronounced biphasic behavior with a dominant Gaussian initial component followed by a slow exponential-like relaxation. While the exponential relaxation usually assumed in Kubo's stochastic theory leads to a quadratic n dependence of the dephasing rate, the biphasic friction is shown to give rise to the sub-quadratic n dependence. As the biphasic frictional response is expected to be a generic feature of the friction on any vibrational coordinate in dense liquids, the sub-quadratic quantum number dependence is predicted to be common to most ultrafast overtone dephasing. In addition, the calculated rates (without any adjustable parameter), are found to be in good agreement with the experimental results for the C-I stretching mode in liquid CH3I and for the C-H stretching in liquid CHCl3. © 1997 American Institute of Physics.

Notes: High-sensitivity, microwave–infrared double-resonance measurements can be made in molecular-beam spectrometers employing a single state-focusing device. The key feature of the double-resonance technique is the achievement of large signal modulations of infrared signals using microwave transitions, even in cases where the infrared transition cannot be saturated. A series of measurements is presented that shows that the technique is based on the transition moment and state-focusing properties of dressed molecular states in the presence of a strong microwave field. Using a state-focusing device, the spectroscopic measurements are doubly sensitive to the composition of the dressed states. The technique can be extended to other types of spectroscopy, such as electronic spectroscopy and the spectroscopy of weakly bound complexes. © 1997 American Institute of Physics.

Notes: A stochastic theory of combined radiative and nonradiative transport is presented. The stochastic approach is physically clear and versatile, allowing the consideration of the combined effect of radiative and nonradiative transport, carried out here for the first time. The stochastic approach is formulated for delta-pulse excitation and for the photostationary state. General equations for the intensity, polarization, and anisotropy decays are derived. © 1997 American Institute of Physics.

Notes: We consider the electronic spectroscopy of dilute CH3I in supercritical Ar fluid. Absorption line shapes for the B←X transition of CH3I have been measured previously in low-density argon, which yielded results for the CH3I/Ar pair potentials. Using these potentials, Kalbfleisch et al. [J. Chem. Phys. 105, 7034 (1996)] have performed molecular dynamics simulations to calculate the absorption line shapes at higher densities, and also the solvation correlation function. We compare the results of several analytic theories to the simulated line shapes and solvation correlation functions. © 1997 American Institute of Physics.

Notes: In this work we present a new global fit for the potential energy surface of the LiFH system. This fit is an improvement of a recently published one [Aguado et al., J. Chem. Phys. 106, 1013 (1997)] for which more ab initio points have been calculated (from 644 to 2323). The reaction dynamics is studied using a time dependent treatment in reactant Jacobi coordinates in a body-fixed frame in which the internal coordinates are represented on a grid while Eulerian angles are described in a basis set. The centrifugal sudden approach is tested for total angular momentum J=5 and used to calculate the reaction cross section. The reaction cross section shows oscillations as a function of kinetic energy. This is a consequence of strong interference effects between reactant and product channels and is in agreement with the recent experimental data. © 1997 American Institute of Physics.

Notes: The seminal work of Nijboer and De Wette [Physica 23, 309 (1957)] enables the calculation of lattice sums of spherical harmonics, but has long been overlooked. In this article, their central result is recast in a simplified form suitable for modern multipole algorithms that employ the solid harmonics. This formulation makes possible the imposition of periodic boundary conditions within modern versions of the fast multipole method, and other fast N-body methods. The distinction between the extrinsic values obtained with the lattice sums M of the multipole interaction tensors, and the intrinsic values associated with Taylor's expansion of the Ewald formulas, is made. The central constants, M, are computed to 32 digit accuracy using extended precision arithmetic. Timings and corresponding errors obtained with a periodic version of the fast multipole method are presented for particle numbers spanning [103,106], and a range of expansion orders. A qualitative comparison is made between the present implementation, other periodic versions of the fast multipole method, and fast Ewald methods. © 1997 American Institute of Physics.

Notes: Expressions for the even Cauchy moments for nonvariational methods have been derived using the time-averaged quasienergy Lagrangian technique. The expressions obtained require the solution of linear equations but do not involve a sum over individual excited-state contributions. An implementation is reported for the coupled cluster models CCS, CC2, and CCSD and calculations have been performed for the Cauchy moments and the Verdet and Cotton–Mouton constants of the Ne atom and for the C6 dispersion coefficient of the Ne2 dimer. © 1997 American Institute of Physics.

Notes: We present a simple one-dimensional model with molecular interactions favoring the formation of clusters with a defined optimal size. Increasing the density, at low temperature, the system goes from a nearly ideal gas of independent molecules to a system with most of the molecules in optimal clusters, in a way that resembles the formation of micelles in a dilution of amphiphilic molecules, at the critical micellar concentration. Our model is simple enough to have an exact solution, but it contains some basic features of more realistic descriptions of amphiphilic systems: molecular excluded volume and molecular attractions which are saturated at the optimal cluster. The comparison between the exact results and the mean field density functional approximation suggests new approaches to study the more complex and realistic models of micelle formation; in particular it addresses the long-standing controversy surrounding the separation of internal degrees of freedom in the formulation of cluster association phenomena. © 1997 American Institute of Physics.

Notes: We have studied the relaxation and transport properties of a ferrofluid in an elongational flow. These properties are influenced by the bistable nature of the potential energy. Bistability comes from the irrotational character of the flow together with the symmetry of the dipoles. Additionally, the presence of a constant magnetic field destroys the symmetry of the potential energy magnetizing the system. We have shown that at a moderate temperature, compared to the height of the energy barrier, the viscosity decreases with respect to the value it would have if the potential were stable. This phenomenon is known as the "negative viscosity" effect. Thermal motion induces jumps of the magnetic moment between the two stable states of the system leading to the aforementioned lowered dissipation effect. © 1997 American Institute of Physics.

Notes: In this three part study, nonequilibrium molecular dynamics simulation of the rheology of confined films is used to explore the microscopic properties and response of model lubricants under shear. The rheological behavior of two alkanes that differ in molecular structural complexity is examined: tetracosane (C24H50), which is a linear alkane, and squalane (C30H62), which has six symmetrically placed methyl branches along a 24 carbon backbone. The model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. Shear flow is generated by moving the walls at constant velocity, and various properties are calculated after attainment of steady state. Heat generated by viscous dissipation is removed by thermostatting the first two atoms of the tethered molecules at 300 K, which allows a temperature profile to develop across the width of the lubricant layer. This paper details the molecular model and simulation method, and examines interfacial slip at the interface between the tethered chains and the fluid alkane. The effects of various parameters on the slip behavior are presented. Two subsequent papers respectively address the structural features of these liquid alkanes under shear flow and compare the viscosities from independent calculations of the bulk and confined fluids. © 1997 American Institute of Physics.

Notes: Analysis of the nuclear spin relaxation rates of lipid membranes provides a powerful means of studying the dynamics of these important biological representatives of soft matter. Here, temperature- and frequency-dependent 2H and 13C nuclear magnetic resonance (NMR) relaxation rates for vesicles and multilamellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in the liquid–crystalline state have been fitted simultaneously to various dynamic models for different positions of the acyl chains. The data include 2H R1Z rates (Zeeman order of electric quadrupolar interaction) acquired at 12 external magnetic field strengths from 0.382 to 14.6 T, corresponding to a frequency range from ωD/2π=2.50–95.3 MHz; and 2H R1Q rates (quadrupolar order of electric quadrupolar interaction) at 15.3, 46.1, and 76.8 MHz. Moreover, 13C R1Z data (Zeeman order of magnetic dipolar interaction) for DMPC are included at six magnetic field strengths, ranging from 1.40 to 17.6 T, thereby enabling extension of the frequency range to effectively (ωC+ωH)/2π=938.7 MHz. Use of the generalized approach allows formulation of noncollective segmental and molecular diffusion models, as well as collective director fluctuation models, which were tested by fitting the 2H R1Z data at different frequencies and temperatures (30 °C and 50 °C). The corresponding 13C relaxation rates were predicted theoretically and compared to experiment, thus allowing one to unify the 13C and 2H NMR data for bilayer lipids in the fluid state. A further new aspect is that the spectral densities of motion have been explicitly calculated from the 2H R1Z and R1Q data at 40 °C. We conclude that the relaxation in fluid membrane bilayers is governed predominantly by relatively slow motions, which modulate the residual coupling remaining from faster local motions (order fluctuations). Only the molecular diffusion model, including an additional slow motional process, and the membrane deformation model describing three-dimensional collective fluctuations fit the 2H NMR data and predict the 13C NMR data in the MHz range. Orientational correlation functions have been calculated, which emphasizes the importance of NMR relaxation as a unique tool for investigating the dynamics of lipid bilayers and biological membranes. © 1997 American Institute of Physics.

Notes: The raised points related to heterogeneous two-dimensional (2-D) nucleation growth model are answered. The function f(m,x) characterizing the height of 2-D heterogeneous nucleation barriers in reference to homogeneous nucleation, has been shown to be different from the 3-D case. The relation between 2-D heterogeneous nucleation and dislocation growth, the condition for "dust-induced kinetic roughening" have been discussed in detail. © 1997 American Institute of Physics.

Notes: The laser fluorescence excitation spectrum of the origin band of an electronic transition in the CH2CFO radical was recorded with partial rotational resolution using a supersonic, rotationally cold beam. The radical was prepared in a pulsed free jet by 193 nm photolysis of acetyl fluoride diluted in helium or argon. The rotational structure of the band is consistent with an in-plane electronic transition of this near oblate rotor. In analogy with the vinoxy radical, this transition is designated as B˜2A′′−X˜2A′′. Spectroscopic constants were derived from a fit to the assigned rotational transitions. The lower state rotational constants agree with those calculated from an ab initio CH2CFO equilibrium structure [M. Furubayashi, I. Bridier, S. Inomata, N. Washida, and K. Yamashita, J. Chem. Phys. 106, 6302 (1997)]. The present study thus provides confirmation of the assignment of the molecular carrier as CH2CFO and eliminates the alternative assignment to FCO [B. A. Williams and J. W. Fleming, J. Chem. Phys. 106, 4376 (1997)]. © 1997 American Institute of Physics.

Notes: The electron–electron coalescence I(0) and counterbalance E(0) densities are probability densities of finding any two electrons, respectively, at the same position and at the reflection points in the three-dimensional space. For a single Slater determinant wave function, these electron-pair properties are shown to be exactly expressible in terms of the spin-traced one-electron density function ρ(r) and its orbital components ρi(r): I(0)=(1/4){〈ρ〉−ΔI} and E(0)=2{〈ρ〉−ΔE}, where 〈ρ〉 is the average electron density, and ΔI and ΔE are linear combinations of overlaps between two orbital densities, that depend on the electronic configuration and the LS multiplet state of the atom under consideration. For the atoms He through Ne in their experimental ground state, the explicit forms of ΔI and ΔE are derived, and the electron–electron coalescence and counterbalance densities obtained from the numerical Hartree–Fock calculations are discussed. © 1997 American Institute of Physics.

Notes: A method of structural analysis, based on restricted averages of the angular pair correlation function, is proposed and applied to liquid water. Minimum free energy paths for a molecule moving in an average local structure are obtained. Around a central molecule, accepting and donating neighbors form hydrogen bonds having the same energy minima but differing for the availability of low free energy states. Some bounded states have specific bridging functions and connect hydrogen-bond and transition states along negative free energy pathways, thus favoring local diffusion. Bridging and transition states play the role of structural defects and include bifurcated hydrogen-bond and interstitial water geometries. Our methods and results can be relevant to understand, from a structural point of view, the anomalous behavior of stressed and confined water. © 1997 American Institute of Physics.

Notes: Higher order correlation functions as observed in four-dimensional NMR experiments on glass-forming liquids are interpreted in terms of a simple model for molecular reorientations. Its key assumption is an intrinsic link between the rotational and structural relaxation of the liquid. It is shown that the introduction of an additional time scale as proposed previously is not necessary. Furthermore, the model naturally accounts for a number of features associated with rotational motions in supercooled liquids. Applications of the model to other aspects of the α relaxation in supercooled liquids are discussed. © 1997 American Institute of Physics.

Notes: Calculations for the Gibbs–Thomson effect and the intergranular melting of the ice droplets in (water) emulsions at temperatures below 273.16 K show that water and ice coexist at thermodynamic equilibrium in an apparently frozen emulsion. The fraction of water at this equilibrium increases on heating, which alters further the thermodynamic properties of the emulsion. As some of the ice in the emulsion has already melted, the increase in the enthalpy, H, and heat capacity, Cp, and the decrease in the volume measured on the normal melting at 273.16 K, are less than the values anticipated. The ratio of this increase in H, or Cp, on melting of the emulsion to the corresponding value for pure ice, underestimates the emulsion's water content which, when used for scaling the difference between the Cp of the unfrozen and frozen emulsion at lower temperatures, as in earlier studies, leads to a larger Cp of supercooled water than the actual value. Similar scaling of the corresponding difference between the volume leads to higher volume, or lower density, than the actual value. A formalism for this premelting effect is given for both the adiabatic and differential scanning calorimetry (DSC), and its magnitude is calculated. New experiments show that the rise in the DSC signal, or equivalently in the apparent Cp observed on heating the frozen emulsion, occurs over a temperature range much wider than the Gibbs–Thomson effect and intergranular melting predict, for which reasons are given. It is shown that Cp of the dispersant phase is also affected by the melting of ice droplets. There are four consequences of the premelting effects for all finely dispersed materials, for frozen water emulsions below 273.16 K: (i) water and ice coexist in the emulsion, (ii) its apparent Cp will increase with increase in the heat input used to measure it, (iii) the apparentCp will increase with decrease in the average size of the droplets, and (iv) the apparent Cp will decrease on annealing the frozen emulsion for a period long enough to allow the ice-grain growth in the frozen droplets. Calorimetry of emulsions has verified consequences (i) and (iv). The corresponding effects on the molar volume are briefly discussed. A substantial fraction of the anomalously high Cp and volume of supercooled water is due to the observed premelting effects. © 1997 American Institute of Physics.

Notes: A (–Cu–O–) string on a Ag(110) surface defined by scanning tunneling microscopy (STM) was studied using the x-ray photoelectron spectroscopy (XPS), high resolution electron energy loss spectroscopy (HREELS). These data give the information about bonding properties which cannot be derived from the STM images. The combinative studies could show the structure as well as the growth mechanism of (–Cu–O–) strings on the Ag(110) surface. Deposition of Cu atoms on a p(2×1)-O Ag(110) surface resulted in a new O(1s) peak at 529.9 eV in the XPS and a new electron energy loss peak at 35 meV in the HREELS. These new peaks are assignable to the (Cu–O) bonding state on Ag(110), which is corresponding to the growth of new (–Cu–O–) strings in the [11¯0] direction shown by the STM. These results suggest a stoichiometric reaction producing (–Cu–O–) strings according to an equation of (–Ag–O–)+Cu→(–Cu–O–)+Ag on the Ag(110) surface. The fact that the Ag atoms released by the chemical reaction make layered Ag islands and the (–Cu–O–) strings also grow on the Ag islands in the [11¯0] direction is indicative that the reaction proceeds perfectly in stoichiometric manner over the terrace. © 1997 American Institute of Physics.

Notes: Dehydration of porous glass hydrated with a 5 mM solution of the spin-label molecules 2,2,6,6-tetramethyl-4-acetamido-piperidine-1-oxyl (TMAPO) was studied with electron paramagnetic resonance at 9 GHz. The spin-label molecules' concentration remains constant, i.e., it follows the Langmuir isotherm when glass pores of 292±20 nm diameter are dehydrated. When the pore diameter is only 127±11 nm, or less, the Langmuir isotherm appears not to apply despite the fact that in smaller pores the surface-to-volume ratio is larger. The reason for this apparent failure could be the establishment of a water cluster phase which does not dissolve TMAPO, and/or it could be a kinetic effect of the spin-label molecules. In this report the possibility that the failure of the Langmuir isotherm is the consequence of ordered water clusters is examined. To account for the lower concentration of TMAPO in smallest pores, as much as 70% of total water volume has to consist of such water clusters, in agreement with an earlier proposal. © 1997 American Institute of Physics.

Notes: Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with room-temperature methane gas over Ir(111) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, these calculations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to the results of a previous study of methane dissociation on Ir(110), although the reactivity of thermalized methane is approximately an order of magnitude higher on the (110) surface of iridium. © 1997 American Institute of Physics.

Notes: Using the Han equation derived from the blob theory we obtain that the chains of wormlike polymers consist of one Kuhn statistical segment at the onset of excluded volume behavior. This holds when these polymers are dissolved in any solvent. This is in accordance with the predictions of Yamakawa–Stockmayer based on the statistical mechanics theory. Combining the same equation of the blob theory with a relation derived from the two-parameter theory, and accepting that at the onset of excluded volume behavior we have one Kuhn statistical segment for the wormlike polymers, we obtain a relation from which we calculate the statistical segment length of these polymers from the K and a(underbar) parameters of the Mark–Houwink–Sakurada equation. The obtained values are in good agreement with the values given in the literature. © 1997 American Institute of Physics.

Notes: This study uses nonequilibrium molecular dynamics simulation to explore the rheology of confined liquid alkanes. Two alkanes that differ in molecular structural complexity are examined: tetracosane (C24H50), which is a linear alkane, and squalane (C30H62), which has six symmetrically placed methyl branches along a 24 carbon backbone. These model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. This paper, the third of a three part series, compares the viscosities of the confined fluids to those of the bulk fluids. The alkanes are described by a well-documented potential model that has been shown to reproduce bulk experimental viscosity and phase equilibria measurements. Details of the simulation method, and structural information can be found in the preceding two papers of this series. The measured strain rates in these simulations range between 108 and 1011 s−1, which is typical of a number of practical applications. The confined fluids undergo extensive shear thinning, showing a power-law behavior. Comparison of results for the confined fluid to those for the bulk fluid reveal that, for the conditions examined, there is no difference between the bulk and confined viscosities for these alkanes. This observation is in contrast to experimental results at much lower strain rates (10–105 s−1), which indicate the viscosities of the confined fluid to be much larger than the bulk viscosities. In making the comparison, we have carefully accounted for slip at the wall and have performed simulations of the bulk fluid at the same conditions of strain rate, temperature, and pressure as for the corresponding confined fluid. The viscosity is found to be independent of the wall spacing. The calculated power-law exponents are similar to experimentally observed values. We also note that the exponent increases with increasing density of the fluid. © 1997 American Institute of Physics.

Notes: A theoretical analysis is given of the photofragment anisotropy of long-lived vibrational states as a function of the rotational level and as a function of the predissociation mechanism. The anisotropy of the lower rotational levels is a strong analytical tool for the identification of the electronic symmetry in the final continuum state. Experiments have been performed on the (4pπ)e 1Πu(v=0) Rydberg state and b′ 1Σu+(v=16) valence state in molecular nitrogen using fast beam translational spectroscopy. The rotational levels of the e 1Πu state yield both N(4S)+N(2P) and N(4S)+N(2D) products. We found different angular distributions for 4S+2P and 4S+2D fragments, although originating from the same rotational level. The observed anisotropy parameters, also called β parameters, have been interpreted and point to the presence of complex predissociation pathways involving mixing with known nearby bound and continuum states. The b′ 1Σu+(v=16) state produces dominantly 4S+2D fragments. We give a suggestion for the most likely dissociation mechanism based on the observed angular distribution. © 1997 American Institute of Physics.

Notes: The low-lying singlet states of biphenylene have been studied using ab initio methods. Vertical excitation energies were calculated by multiconfigurational perturbation theory (CASPT2), starting from a complete active space self-consistent field (CASSCF) reference. The geometries of the most important low-lying excited states were individually optimized at the CASSCF level to study the difference between vertical and adiabatic excitations. Extended atomic natural orbital (ANO)-type basis sets were used to calculate state energies. Geometry optimizations were done with smaller ANO-type basis sets. Excitations from the ground state to the 1 1B3g and 1 1B2u excited singlet states lead to pronounced geometry changes which alter the bond alternation pattern. The theoretical results provide a solid basis for the assignment and interpretation of experimental spectra. © 1997 American Institute of Physics.

Notes: One- to two-exciton transitions have been examined in molecular aggregates with linear and circular geometries at various strengths of the exciton–exciton interaction. For the interaction parameter a sufficiently different from its critical value acrit=1, the exciton–exciton interaction has been shown to have little influence on the transition dipole moments, as well as on the corresponding transition energies between the one-exciton states and the dissociated two-exciton states. The interaction between the excitons then may be represented in an effective manner by the replacement of the actual number N of molecules per aggregate by a nearby effective number Neff, the latter being a-dependent. Hence, inclusion of the exciton–exciton coupling does not affect substantially the previous analysis of one- to two-exciton transitions based on the model of noninteracting one-dimensional excitons. That is, effects such as the blue shift of the excited-state absorption and the enhancement of nonlinear susceptibilities are not sensitive to the exciton–exciton interaction. These findings are relevant, inter alia, to J-aggregates in which there is no evidence for the coupling parameter a to be in the critical region or beyond. On the other hand, for the critical value of the exciton–exciton interaction (a=acrit), the blue shift is either totally absent in the excited-state absorption, or extremely small compared with the ordinary case. The above is in full agreement with earlier calculation of the pump–probe spectrum showing a weak dependence on the exciton–exciton interaction for a〈1, as well as a strong bleaching of the exciton band in the critical region. © 1997 American Institute of Physics.

Notes: The pure rotational spectrum of the free radical MgBr has been measured in its 2Σ+ ground electronic state by Fourier transform microwave spectroscopy. Transitions have been observed for both 24Mg79Br and 24Mg81Br in the v=0 and v=1 vibrational states. Rotational and centrifugal distortion constants have been determined for each isotopomer in each vibrational state. Equilibrium rotational constants have been calculated and an accurate equilibrium bond length has been determined. Spin-rotation constants, for both the unpaired electron and the bromine nuclei, have been calculated along with magnetic and nuclear quadrupole hyperfine constants for the bromine nuclei. From these constants, the electronic structure of MgBr has been investigated and comparisons have been made to similar compounds. The unpaired electron spin density on the bromine nucleus has been found to be very small, suggesting that this is a very ionic compound. However, the Mg–Br bond has been found to have more covalent character than the bond in other alkaline earth monobromides. © 1997 American Institute of Physics.

Notes: In this work we introduce a theoretical model for the description of doubly charged xenon clusters. It is based on the assumption that the charges migrate inside the cluster by isotropic hopping through a Hubbard Hamiltonian. The Xe atoms are considered classical polarizable particles. For their interaction we use a 2-body potential to which we add charge–charge, charge–dipole, and dipole–dipole interactions. The calculations are carried out within the ground state approximation. We perform molecular-dynamics calculations on doubly charged clusters of up to 55 atoms. We investigate the role that the quantum degree of freedom plays on the critical size for the appearance of the doubly charged clusters. The incorporation of the quantum hopping results in a fragmentation energy barrier for clusters larger than the experimentally observed critical size, so that the calculated critical appearance size is in agreement with experiment. © 1997 American Institute of Physics.

Notes: The effects of temperature, solvent mass, ground-state solute–solvent interaction potential, and difference potential on the time scale for the decay of an electronic transition energy gap correlation function (ECF) are investigated within the context of a linear instantaneous normal mode (INM) model of fluid dynamics. This correlation function is also known as the solvation autocorrelation. The system described here is the B-state transition of methyl iodide in the nonpolar solvents argon and methane. The required ground- and excited state interaction potentials have been determined in previous experimental spectroscopic studies. The solvation time scale is of the order of 100–200 fs for solvent densities ranging from ρ*=0.08 to ρ*=0.8. The molecular properties responsible for determining the solvation time scale of this nonpolar system are delineated here. Via this INM approach, the nonpolar solvation time scale can be approximated by the ratio of a characteristic solute–solvent separation distance scaled by the shape of the difference potential and the inertial velocity of the solvent particles. The time scale of solvation is found to be independent of the magnitude of the difference potential (solute–solvent coupling strength). Thus by changing the coupling strength and leaving the shape of the difference potential constant, the corresponding electronic absorption spectrum passes from the inhomogeneous to the motional narrowing limit. This is due to the change in the decay time of the static dipole correlation function and not to any change in system dynamics. Only very modest changes in this decay time are found for realistic temperature increases and mass changes of the solvent. Similarly, changes in the ground-state solute–solvent potential are found to have only a minimal effect on the ECF decay time. Finally, if the shape of the difference potential is similar for two different observables in a given solvent, the use of the spectral density of one for the description of the (ultrafast) solvent response of the other observable is rationalized. © 1997 American Institute of Physics.

Notes: Vibrational relaxation cross-sections and rate constants have been calculated for the deactivation of CO(v=1) by 3He and 4He on a new intermolecular potential with vibrational coordinate dependence [T. G. A. Heijmen, R. Moszynski, P. E. S. Wormer and Ad van der Avoird, J. Chem. Phys. 107, 9921 (1997)]. The new surface is found to resolve the qualitative discrepancy between theory and experiment which existed in earlier theoretical calculations. The low impact energy regime has also been investigated focussing in particular on impact energies of less than 15 cm−1 above the vibrational (v=1) threshold. Resonance structure has been found to occur and a comparison is made with an earlier investigation of the low temperature region. © 1997 American Institute of Physics.

Notes: Some comments concerning heterogeneous two-dimensional nucleation on crystal surfaces are made: the similarity to three-dimensional heterogeneous nucleation, the fate of the nuclei, a possible condition for kinetic roughening on dirty crystal surfaces, and recurring nucleation at favored nuclei. © 1997 American Institute of Physics.

Notes: Experimental measurements are reported for the temperature dependence of the vibrational lifetime, T1, of the asymmetric CO stretching mode of tungsten hexacarbonyl in supercritical ethane at constant density from just above the critical temperature to substantially higher temperatures. T1 is found initially to increase with temperature along an isochore (reaching a maximum at about 70° above the critical point of ethane), and then subsequently to decrease. Using a recent classical theory of vibrational relaxation, we attempt to rationalize the T1 data. This behavior can be semiquantitatively reproduced by the theory if quantum corrections to the classical rate expressions are assumed to be temperature independent in the limit when the transition energy is much greater than thermal energy. In this case, the theory indicates that the initial increase in T1 with temperature arises because of a competition between properties of the solvent which are changing rapidly as the temperature is raised above the critical temperature. At sufficiently high temperature, properties of the solvent vary slowly with temperature, and the explicit temperature dependence of the vibrational relaxation dominates, producing a decrease in T1 with increasing temperature. The predictions of the theory are also examined when other postulated forms of the quantum correction factors are used, and the implications of these results for theoretical approaches to vibrational relaxation are discussed. © 1997 American Institute of Physics.

Notes: We investigated the coherent motion of vibrational wave packets in the |B〉 1Πu state of the potassium dimer applying two color pump/probe spectroscopy with a sub 100 fs time resolution. Special interest was paid to the ionization probe step which was analyzed carefully by varying the probe energy over a wide range. Time-dependent quantum calculations explain the experimental outcomes by introducing a nonconstant transition dipole moment between the |B〉 and the ionic state |X+〉 and by taking into account the excitation of long lived autoionizing Rydberg states. © 1997 American Institute of Physics.

Notes: The change of spectroscopic properties of sodium-water and sodium-ammonia complexes upon deuteration is investigated. The ionization potential of NaD2O is shifted by 70 cm−1 towards lower energies compared to NaH2O. A shift of 81 cm−1 between NaND3 and NaNH3 is observed again towards lower energies. From these shift the vibrational frequencies for the ground state of the neutral and ionic complex have been estimated. The first electronically excited state (A˜2E) of NaND3 has been investigated by resonant two color two photon ionization. The comparison with the formerly observed NaNH3 spectrum enables us to do an unequivocal assignment of the vibrational structure. © 1997 American Institute of Physics.

Notes: The photoionization spectrum of the threshold region of CH3 equilibrated at room temperature has been recorded and compared to the zero electron kinetic energy (ZEKE) spectrum of Blush et al. [J. Chem. Phys. 98, 3557 (1993)]. The ionization onset region is at ∼70 cm−1 higher energy than previous high-temperature photoionization work [Chupka and Lifshitz, J. Chem. Phys. 48, 1109 (1967)], but still ∼34 cm−1 lower than that implied by invoking only direct ionization. The residual discrepancy can be accounted for by including fully allowed quadrupole-induced and partially allowed dipole-induced rotational autoionization, thus making the observed onset completely congruous with the ZEKE ionization potential. In addition, the fragment appearance potential of CH3+ from CH4 was redetermined by accurate fitting as AP0(CH3+/CH4)=14.322±0.003 eV. With the very precise ZEKE ionization potential, this yields the best current value for the bond dissociation energy in methane, D0(H–CH3)=4.484±0.003 eV=103.40±0.07 kcal/mol (104.96±0.07 kcal/mol at 298 K). © 1997 American Institute of Physics.

Notes: The reduced distribution function formalism of diffusion-influenced bimolecular reactions is generalized to incorporate a quantum-mechanical gating mode in the description. An analytical expression for the reaction rate coefficient is obtained in the Laplace transform domain for a general initial condition. For a simple reaction model, the time-dependent reaction rate coefficient and the product yield are calculated numerically for two representative initial conditions. Dependence of the rate coefficient and the product branching ratio on various reaction parameters is discussed. © 1997 American Institute of Physics.

Notes: The effect of an external boundary and diffusive interaction between reactants on the rate constant for diffusion-controlled bulk reactions in an external electric field is considered. Appropriate boundary-value problems for "wall–sink" and "sink–sink" systems in an electric field are solved approximately by the method of reflections. Mainly we are interested in calculation of the deviation of the reaction rate constant from the value calculated by Smoluchowski approach. A thorough analysis is made of the time-dependent case of "wall–sink" system in absence of an electric field. It has been shown that the wall effect leads to a nonexponential long time tail for the bulk concentration of diffusing particles. Special attention is given also to the investigation of competitive effect between two sinks which is found to be of interest for many applications. The rigorous theoretical study of this problem provides a way of quantitative estimation of the shadow effect. Drift of diffusing particles in some arrays of ideal sinks is treated as well. © 1997 American Institute of Physics.

Notes: A simple artificial neural network (ANN) is developed and applied to collision processes. A general discussion of how ANNs can be introduced to study general phenomena in scattering problems is presented and neural networks are proposed to predict classical rainbow trajectories in atomic and molecular collisions. As a result of modeling the collision process, based on the neural network approach, analytical equations were obtained to calculate classical atomic and molecular rainbow trajectories. However, these analytical results just translate the behavior of the input/output data and do not contain any general physical meaning. Although a fitting procedure could be easily used in the present case, the cost of function approximation using ANNs increases only linearly with the number of input variables. This contrasts with classical polynomial fitting procedures for which the computational cost increases exponentially with the input space dimension. This makes the ANN approach worth considering when modeling scattering processes, as shown throughout this paper. At last, an articial network strategy is pointed out to study inversion problems in collision processes. © 1997 American Institute of Physics.

Notes: A new method for simulating the effect of outgoing-wave boundary conditions in the calculation of quantum resonances is presented. The Hermitian Hamiltonian operator H is multiplied on each side by a damping operator D, consisting of a real function d(R), which is unity in the resonance region and falls gradually to zero in the asymptotic region. The spectrum of the symmetrically damped Hamiltonian operator, DHD is shown to provide an excellent approximation to the resonance energies of the Hamiltonian with outgoing-wave boundary conditions. Applications to the calculation of resonance energies for collinear H+H2 scattering and for HO2 dissociation are presented. In addition, we explore the feasibility of extracting resonance widths by using the DHD operator within a filter diagonalization (FD) scheme. Application of the FD scheme to HO2 yields encouraging results. © 1997 American Institute of Physics.

Notes: We consider the problem of calculating the electronic absorption spectrum of a chromophore with intramolecular degrees of freedom coupled to a condensed phase environment. We approach this calculation in the framework of the imaginary-time path integral Monte Carlo techniques, and focus on the problem of the analytic continuation of the imaginary-time data to the real-time axis. Two alternative analytic continuation methods are considered: the maximum entropy method and the singular value decomposition method. An exactly solvable model is introduced to test the accuracy of these methods. Exact numerical results for the absorption spectra are compared to the spectra reconstructed by the analytic continuation methods; it is found that the singular value decomposition method gives systematically higher resolution than the maximum entropy method and is capable of reproducing the fine vibronic structure of the absorption spectrum. © 1997 American Institute of Physics.

Notes: The N,N'-diacid of meso-tetrakis(4-sulfonylphenyl)porphin forms both J- and H-aggregates. Both the B- and Q-transitions exhibit a J-band. Under typical experimental conditions, the monomer and H-bands are also observed. We have measured the pressure dependence of the various spectral bands up to 40 kbar. All data show linear dependence of spectral red shift with pressure. The monomer Soret band has the highest pressure sensitivity. The pressure slope of the B-band is 3.6 times greater than that for the Q-band for the J-aggregates. A simple excitonic theory predicted a ratio of nearly 11. It appears that there is a substantial solvent shift contribution to these pressure derivatives. © 1997 American Institute of Physics.

Notes: We present experimental results for the alignment of N2O+(A 2Σ+) photoions over an extended energy range (16.4≤hνexc≤240 eV). The polarization of the N2O+(A 2Σ+→X 2Π) fluorescence is used to interpret the oscillator strength distributions for normally unresolved degenerate ionization channels. The results clearly show the influence of a near-threshold 7σ→kσ shape resonance, and help to clarify the results of previous fluorescence and photoelectron studies. At high photon energies, the photoelectrons are not favored to exit via a particular channel, in contrast to recent results on N2 and CO, where photoelectrons are ejected preferentially via the kσ channel and the photoions retain significant alignment even at the highest measurable energies. These results demonstrate that even well above threshold the spectral dependence of the alignment (i.e., polarization) is very sensitive to the molecular environment for photoejection. © 1997 American Institute of Physics.

Notes: Molecular dynamics (MD) of stationary chemical kinetics is used to simulate oscillating chemical reactions in a system of N classical mechanical particles with Lotka–Volterra kinetics. The MD includes oscillations in a (closed) system with conserved energy and time reversible dynamics as well as oscillating chemical reactions in an open and driven non-equilibrium system, and with and without a competing phase separation of the different components in the reactions. The approach allows a detailed investigation of the kinetics and demonstrates on a molecular level, the phenomenon oscillating reactions for various chemical and reaction kinetics details. When phase separation takes place during the oscillations the kinetics is no longer simple diffusion driven. © 1997 American Institute of Physics.

Notes: The excess free energy per unit area of an uncharged curved interface separating a phase with a low dielectric constant from a solution containing electrolyte is considered. The contribution due to the electrolyte to the excess free energy per unit area is expanded in the curvature, and the coefficient of the linear term or spontaneous curvature is calculated. Its sign implies a tendency for an interface to curve toward the nonaqueous phase. The zeroth order term in this expansion is an earlier result obtained by Onsager and Samaras. The expansion parameter is the curvature times the Debye screening length. The opposite regime of large curvature will also be considered. To obtain these results the Gibbs adsorption equation has been used. © 1997 American Institute of Physics.

Notes: The theory describing nuclear magnetic resonance cross-polarization using adiabatic sweeps of the rf spin-lock fields through the Hartmann–Hahn matching condition is extended to small homonuclear coupled systems of the type INS. In particular, the connection is made between such experiments and the associated theoretical limits on polarization transfer—the "unitary bounds"—demonstrating that these techniques can achieve the maximum transfer of polarization from the I spins to the S spins, subject to the constraint of angular momentum conservation imposed by spin-locking. Factors such as permutation symmetry of the spins, imperfect adiabaticity of individual crossings and fast sample spinning are shown to have no fundamental impact on the validity of these results. © 1997 American Institute of Physics.

Notes: Monte Carlo simulations of linear polyelectrolyte solutions containing mixed valency simple ions in the cylindrical cell model are reported. The equilibrium distributions of the simple ions and the osmotic pressure of the solution are calculated at various concentrations of the monomer units of the polyelectrolyte. Specifically, the following systems are studied—monovalent counterions with added 2:2 salt, divalent counterions with added 1:1 salt, and systems containing mono- and divalent counterions only, and mono- and trivalent counterions only. The simulation results are compared with the corresponding predictions from the Poisson–Boltzmann and modified Poisson–Boltzmann theories applied to the cell model. It is seen that upto moderate concentrations of the polyion, the modified Poisson–Boltzmann theory provides a very good description of the systems with deviations occurring at higher concentrations. The theory also reproduces the charge reversal observed in the simulations when strongly correlated counterions overscreen the charge of the polyion. On the other hand, the classical Poisson–Boltzmann results begin to show discrepencies from the Monte Carlo results at relatively lower concentrations. Comparisons of the simulated osmotic pressures with available experimental results confirm the validity of the cell model in a spectrum of practical situations of interest. © 1997 American Institute of Physics.

Notes: We have investigated particle diffusion through different obstacle geometries by computer simulations. The model structures used in this work — randomly placed point obstacles and cage-like structures — were chosen with the aim of represent a broad range of geometrical structures similar to gels and in order to be compared with our previous simulations of particle diffusion through polyacrylamide gels. The diffusion behavior was studied as a function of tracer size and obstacle concentration. The isomorphism between the diffusion of finite-sized tracers and the diffusion of point tracers in the presence of expanded obstacles was applied. Only hard-sphere interactions of the tracer with the immobile obstacles were considered and the theoretical description was made in terms of theory of the obstruction effect. In the case of randomly placed point obstacles an analytical expression for the dependence of the diffusion coefficient on tracer radius and obstacle concentration, applying the model of spherical cells, could be deduced. The same description was applied numerically to the other model systems. Up to moderatly high fractions of excluded volume this description was found to be successful. For very high fractions of excluded volume — higher concentrations or larger tracers — the validity of Fick's second equation for describing diffusion breaks down and anomalous diffusion was found. The anomalous diffusion exponent diverges as the tracer size becomes comparable to the size of the pores. Analysis of the trajectory of tracers in the cases where an anomalous diffusion takes place shows a Levy-flight-like characteristic. © 1997 American Institute of Physics.

Notes: Free energies of ionic solvation calculated from computer simulations exhibit a strong system size dependence. We perform a finite-size analysis based on a dielectric-continuum model with periodic boundary conditions. That analysis results in an estimate of the Born ion size. Remarkably, the finite-size correction applies to systems with only eight water molecules hydrating a sodium ion and results in an estimate of the Born radius of sodium that agrees with the experimental value. © 1997 American Institute of Physics.

Notes: Helium pressure broadening cross sections for the (J,K)=(1,1), (2,2) and (3,3) inversion transitions of ammonia were measured at temperatures from 10 to 35 K. Measurements were taken in a quasiequilibrium cell using the collisional cooling technique. In contrast to the situation at room temperature where there is little state-to-state variation in helium pressure broadening cross sections, at low temperature the (3,3) cross sections were found to be, on average, three times larger than the (1,1) cross sections while the (2,2) cross sections were twice as large as the (1,1) values. The major factor determining the relative size of the low temperature cross sections appears to be the contribution from inelastic collisions into lower energy levels. The relative size of the cross sections thus scales with the number of rotational levels underlying a given (J,K) state. Cross sections were also calculated for comparison with the experimental data using three existing NH3–He potential surfaces. Overall, none of the three surfaces gave consistent agreement with the experimental results for all three observed transitions. The discrepancy between experiment and theory was greatest for the (3,3) data. This may be a manifestation of a previously observed tendency of NH3–He potential surfaces to underestimate the contribution from parity-conserving collisions in calculated state-to-state rates. © 1997 American Institute of Physics.

Notes: The k3Π state of the CO molecule is investigated in the region between 91 000 and 97 000 cm−1 via 1+1 resonance enhanced multiphoton ionization spectroscopy on CO molecules prepared in a single quantum level of the a3Π(v=1) state. A new vibronic band is found which is at lower energy than the vibrational ground state reported in the literature, leading to a reassignment of the vibrational numbering of the k3Π state. The rotationally resolved spectra of the k3Π (v=0–6)←a3Π(v=1, J=1, Ω=1) of 12CO and 13CO have been observed and analyzed, confirming the new vibrational labeling and providing a full set of molecular constants of the k3 Π valence state. © 1997 American Institute of Physics.

Notes: Recently obtained spectroscopic constants for excited vibrational states of HCN–HF with up to five quanta in the ν7 intermolecular bending mode, which can be characterized as being mostly HCN hindered rotation, and one quanta in the ν4 intermolecular stretching mode were combined with earlier experimental data on this system to obtain a reduced intermolecular potential energy surface where the HF fragment was treated as a pseudoatom and the HCN fragment was treated as a rigid rotor. A functional form was assumed for the interaction potential and the vibrational dynamics was studied using a vibrational configuration interaction method. The parameters in the potential were varied to obtain the best fit to all available experimental data. The resulting interaction potential was found to have a strong coupling between the ν4 and ν7 vibrational modes. The locus of constrained minima on this surface was in excellent agreement with the hard-sphere assumption used in the Buckingham–Fowler model [A. D. Buckingham and P. W. Fowler, J. Chem. Phys. 79, 6426 (1983)] for weakly interacting molecular dimers. © 1997 American Institute of Physics.