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Notes: The polarized absorption spectra in the bc face of an α-sexithienyl (T6) single crystal have been measured at 4.2 K. The origin of the lowest electronic transition is at 18 360 cm−1 and has been assigned to the lowest b-polarized au Davydov component of the 1 1Bu molecular level. The second optically allowed Davydov component (bu) is polarized in the ac crystal plane and is located at 20 945 cm−1. Therefore, the interchain interaction leads to a Davydov splitting of about 2600 cm−1. A quantum chemical model, which considers the total molecular wavefunctions for each transition, shows good agreement with the experimental findings for the energy and polarization of the optically allowed crystal levels. The vibronic manifold in the absorption spectra has been interpreted in terms of the Herzberg–Teller and Franck–Condon vibronic coupling. In particular, two false origins due to the coupling of the molecular electronic levels 1 1Bu and 2 1Bu have been identified at 18 486 and 18 657 cm−1. Consistently, the emission counterpart of the two false orgins has been identified in the polarized fluorescence spectra. The totally symmetric modes involved in the coupling are in excellent agreement with the Raman scattering data of the single crystal. © 1998 American Institute of Physics.

Notes: In order to develop a consistent nucleation theory, the main assumptions of the theory should be revised. One of the questionable problems is the role of the carrier gas in nucleation and the surface tension for the critical embryo as a function of cluster size. Using a flow diffusion chamber, the vapor nucleation rates were measured with high precision and phase transitions in critical embryos containing two and more dozen molecules were detected. Phase transitions in critical embryos were used as markers to detect that the new phase critical embryos contain two components. Phase transitions of the first order related with critical point second-order phase transitions in the pure CO2 carrier-gas were used as markers to demonstrate the presence of CO2 in critical embryos of condensate. Results of this research, in our opinion, very clearly demonstrate that vapor nucleation in a gaseous atmosphere is a binary process and must be interpreted from the point of view of nucleation theory within a binary system. "Supercritical" nucleation is a virtual term born by interpretation of binary vapor–gas nucleation by using the nucleation model of a single component. A critical condition for the binary system could be a higher level for the single component critical pressure and/or temperature, which can produce the illusion of supercritical nucleation. One component interpretation can be used far from the critical condition. On the other hand, the Laplace pressure practically always is able to approach the nucleation condition to the critical pressure. This level of detail is a problem for future studies. The traditional application of classical nucleation theory for vapor–gas nucleation should be modified to consider the nucleation conditions in pressure-temperature-composition space. © 1998 American Institute of Physics.

Notes: We have studied the structure and free energy landscape of a semiflexible lattice polymer in the presence of the surface of a polymer crystal. At low temperatures coexistence of two-dimensional integer-folded crystals is observed. As the temperature is increased there is a transition from these crystalline configurations to a disordered coil adsorbed onto the surface. The polymer then gradually develops a three-dimensional character at higher temperatures. We compute the free energy as a function of increasing crystallinity and compare with the free energy profiles assumed by the Lauritizen–Hoffman surface nucleation theory of polymer crystallization. Our free energy profiles exhibit a "sawtooth" structure associated with the successive formation of chain folds. However, in the early stages of crystallization our profiles significantly deviate from those assumed by surface nucleation theory because the initial nucleus is not a single stem but two incomplete stems connected by a fold. This finding has significant implications for the theoretical description of polymer crystallization. © 1998 American Institute of Physics.

Notes: We study the surface tension for thin, amorphous polymer films by means of computer simulation. Using molecular dynamics, we present surface tension measurements via the fluctuation spectrum of capillary waves in the long-wavelength limit for sufficiently large systems. We find good agreement with a theory based on continuum mechanics. In addition, we observe the spreading of the surface thickness with increasing lateral system size, an effect which allows another estimate of the surface tension. Furthermore, we studied the correlation between two surfaces and measured the transverse length scale by varying the film thickness. We also present data of the temperature dependence of the bulk density of the polymer film and the thickness of the surface region in the regime above the glass transition temperature. © 1998 American Institute of Physics.

Notes: The vibrational frequency of the I2〈sup ARRANGE="STAGGER"〉− chromophore in size-selected I2〈sup ARRANGE="STAGGER"〉−(Ar)n and I2〈sup ARRANGE="STAGGER"〉−(CO2)n clusters has been measured to wave-number accuracy. The frequencies are determined by creating a coherent superposition of vibrational levels with a femtosecond laser pulse via a resonance impulsive stimulated Raman scattering process. The resulting wave-packet oscillations are detected with femtosecond photoelectron spectroscopy. Blueshifting of the frequency occurs upon solvation, with larger shifts observed for solvation with CO2. The nature of the shifting is discussed and related to specific cluster geometries. © 1998 American Institute of Physics.

Notes: A rigorous and transparent treatment for determining electronic–vibrational hyperpolarizabilities in the context of the finite-field method is developed. Our emphasis is on the vibrational contributions. The new formulation is more general than previous ones in that it yields properties over the entire frequency range. © 1998 American Institute of Physics.

Notes: A basis set of generalized nonspherical Gaussian functions (GGTOs) is presented and discussed. As a first example we report on Born–Oppenheimer energies of the hydrogen molecule. Although accurate results have been obtained, we conclude that H2 is too "simple" to allow for a substantial gain by using nonspherical functions. We rather expect that these functions may be particularly useful in calculations on large systems. A single basis set of GGTOs was used to simultaneously calculate the potential energy curves of several states within each subspace of 1,3Σg,u symmetry. We hereby considered the entire region of internuclear distances 0.8≤R≤1000 a.u. In particular the results for the fourth up to sixth electronic states show a high accuracy compared to calculations which invoke explicitely correlated functions, e.g., the relative accuracy is at least of the order of magnitude of 10−5 a.u. Energies for the 4 1Σu+ and 4–6 3Σu+ were improved and accurate data for the 6 3Σg+, 5 1Σu+, and 6 1Σu+ state are, to the best of the authors' knowledge, presented for the first time. Energy data for the seventh up to the ninth electronic state within each subspace were obtained with an estimated error of the order of magnitude of 10−4 a.u. The 7 1Σg+ and the 6 1Σu+ state were found to exhibit a very broad deep outer well at large internuclear distances. © 1998 American Institute of Physics.

Notes: The optical absorption spectrum of a vanadium tetramer ion, V4+, was obtained by measuring a photodissociation efficiency of an ion complex, V4+Ar, as a function of the photon energy of the laser pulse used for the photodissociation. The optical absorption spectrum thus obtained was simulated by a density functional calculation to search for the most probable geometric structure which reproduces the measured spectrum. The analysis showed that V4+ is most likely to have a "distorted" tetrahedral structure with C2v symmetry. The ground electronic state of V4+ was found to be a low spin state, 2A1. The relatively broad spectral profile is explained in terms of the distortion related to a low-energy vibration. Geometry optimization of the Amsterdam density functional (ADF) calculation also predicts C2v symmetry for the structure of V4+. © 1998 American Institute of Physics.

Notes: We observed bound levels of the I′ state in H2 and D2, confined in the outer well of the lowest 1Πg adiabatic potential close to its (1s+2p) dissociation limit, with an equilibrium internuclear distance of (approximate)8 a.u. Rovibronic levels (v=0–2, J=1–5 for H2 and v=0–5, J=1–6 for D2) are populated with pulsed lasers in resonance enhanced XUV+IR (extreme ultraviolet+infrared) excitation, and probed by a third laser pulse. Level energies are measured with an accuracy of (approximate)0.03 cm−1, and are in reasonable agreement with predictions from ab initio calculations in adiabatic approximation; the smallness of Λ-doublet splitting indicating that nonadiabatic interactions with 1Σg+ states are generally weak. Additional resonances are observed close to the n=2 dissociation limit, some of which can be assigned as high vibrational levels of the EF 1Σg+ state. © 1998 American Institute of Physics.

Notes: Perturbed states representing the response to an external electric field are obtained in the multireference averaged coupled pair functional formalism based on orbitals which are optimized in a perturbed multireference self-consistent field procedure. For each perturbing operator perturbed wave functions for several frequencies of the perturbing field are obtained simultaneously. From these sets of perturbed states reduced spectra are derived which represent effective oscillator strength distributions. The broad shape of the Schumann–Runge continuum leads to a poor description of the dynamic polarizability for frequencies approaching the first pole when only vertical transitions are considered. To account for this effect the absorption spectrum in the Schumann–Runge region is calculated and the polarizability is described up to the range of anomalous dispersion. The dynamic polarizabilities and the interaction coefficients are found to be in excellent agreement with experimental data where it is available. Significant differencies of up to 20% are observed between our anisotropic interaction coefficients and earlier estimates obtained from semiempirical anisotropic dipole oscillator strength distributions. © 1998 American Institute of Physics.

Notes: Photoelectron spectra of mass-separated Nbn− clusters reveal an even/odd alternation for n=6–17, indicating a closed electronic shell of the neutral even-numbered clusters. The HOMO–LUMO gap of Nb8, Nb10, and Nb16 is found to be larger than that of the other even-numbered clusters, which correlates with the low H2 reactivities of these species. The spectrum of Nb15− is different from all other clusters in this size range, which might be an indication for a geometric bcc shell closing. The influence of the electronic structure of the clusters on the reactivity is discussed. © 1998 American Institute of Physics.

Notes: Initial vibrational excitation of a state containing three quanta of N–H stretch (3ν1) decreases the fractional photolysis yield of NH (a 1Δ) relative to NH (X 3Σ−) by a factor of approximately two compared to the isoenergetic photodissociation of a 300 K thermal sample of HNCO. At a total energy of 43 480 cm−1, NH (a 1Δ) accounts for 24% of the total NH yield in the direct photolysis but only 10% in the photodissociation of 3ν1. At 44 440 cm−1, the NH (a 1Δ) yields are 65% and 32% in the single photon and two-step photodissociations, respectively. The variation in branching ratio may arise from dynamical behavior that is closely related to the preferential production of NCO in the photolysis of vibrationally excited HNCO. The initial vibrational excitation has no influence on the rotational and vibrational distributions of NH (X 3Σ−), but it significantly increases the amount of energy in rotation of NH (a 1Δ). These results, along with several recent experimental and theoretical studies, suggest the participation of at least three different potential energy surfaces in the photodissociation of isocyanic acid. © 1998 American Institute of Physics.

Notes: The size dependence of the electronic spectrum of InAs nanocrystals ranging in radius from 10–35 Å has been studied by size-selective spectroscopy. An eight-band effective mass theory of the quantum size levels has been developed which describes the observed absorption level structure and transition intensities very well down to smallest crystal size using bulk band parameters. This model generalizes the six-band model which works well in CdSe nanocrystals and should adequately describe most direct semiconductor nanocrystals with band edge at the Γ-point of the Brillouin zone. © 1998 American Institute of Physics.

Notes: Hartree–Fock and coupled-cluster calculations have been performed for cubic AgCl and for AuCl having a cubic or the observed structure with space group I41/amd. Cohesive energies and lattice constants are in excellent agreement with experiment for AgCl; for AuCl we find good agreement, and the experimental structure is correctly predicted to be lower in energy than the cubic one. Electron-correlation effects on lattice constants are very large, of up to 0.8 Å for cubic AuCl. We especially discuss the strength of the closed-shell interactions, and for the first time a quantitative analysis of the so-called "aurophilic" Au(I)–Au(I) interaction is presented in solids. © 1998 American Institute of Physics.

Notes: A simple yet reasonably accurate perturbation theory for the Gay–Berne model, capable of describing the uniform isotropic and nematic phases, as well as the layered smectic-A phase, is presented. The theory, in line with a previously proposed theory, is based on a perturbative scheme, but the reference system, a hard Gaussian overlap model, is treated using the nonlocal approximation of Somoza and Tarazona. This approximate scheme, which reduces to the well-known decoupling approximation for nematics, is a simple generalization of the decoupling approximation designed to include smectic structures. The attractive free energy is calculated using a mean-field approximation. Underestimation of the attractive energy implied by this approximation is alleviated by introducing some scale factors, set to reproduce the critical point and two triple points involving the smectic phase. The choice of scale factors, which is valid for a particular set of molecular parameters, is shown to reproduce accurately the phase diagram corresponding to other parameter values. The theory is used to examine the global liquid-crystalline phase behavior of the Gay–Berne model, paying particular attention to the effect of the anisotropy attraction parameter κ′ on the location of the various phase boundaries. Comparison of the results with the available computer simulations for this system indicates that the theory leads to qualitatively correct predictions. The theory could be useful to predict the phase behavior of realistic systems with respect to molecular elongation and energy anisotropy. © 1998 American Institute of Physics.

Notes: The screening length of the charge-asymmetric electrolyte is calculated in the hypernetted chain approximation. Results are presented for 1:1, 2:1, 3:1, and 4:1 electrolyte with concentrations ranging from 10−5 to 1 M. A number of analytic approximations for the screening length are compared to the hypernetted chain data. We find that the second moment approach of Attard [Phys. Rev. E 48, 3604 (1993)] gives good results for monovalent but not asymmetric electrolyte. A nonlinear version of the second moment approach is found to give good results for asymmetric electrolytes. We also find that the asymptotic formula of Mitchell and Ninham [Chem. Phys. Lett. 53, 397 (1978)], although giving a null result for monovalent electrolyte, is a satisfactory estimate for asymmetric electrolytes. © 1998 American Institute of Physics.

Notes: Results of differential scanning calometry (DSC), x-ray diffraction (XRD), and 19F nuclear magnetic resonance (NMR) of InF3-based glasses, treated at different temperatures, ranging from glass transition temperature (Tg) to crystallization temperature (Tc), are reported. The main features of the experimental results are as follows. DSC analysis emphasizes several steps in the crystallization process. Heat treatment at temperatures above Tg enhances the nucleation of the first growing phases but has little influence on the following ones. XRD results show that several crystalline phases are formed, with solid state transitions when heated above 680 K. The 19F NMR results show that the spin–lattice relaxation, for the glass samples heat treated above 638 K, is described by two time constants. For samples treated below this temperature a single time constant T1 was observed. Measurements of the 19F spin–lattice relaxation time (T1), as a function of temperature, made possible the identification of the mobile fluoride ions. The activation energy, for the ionic motion, in samples treated at crystallization temperature was found to be 0.18±0.01 eV. © 1998 American Institute of Physics.

Notes: Spectroscopic and dynamic features of the vibrationally excited D2O/zeolite system have been investigated by two-color infrared–infrared pump–probe experiment. The frequency- and delay-scanned probe intensities were measured by tuning the pump laser to the OD stretching bands of the D2O molecule hydrogen bonded to the acidic OD group of mordenite zeolite. Two types of pump-induced signals were observed: the ones which have the population lifetime of 43±5 ps and display frequency shift by the pumping frequency, and the others, which have the lifetime of about 15 ps and exhibit no such frequency shift. Possible origins of the signals are discussed. © 1998 American Institute of Physics.

Notes: We have investigated desorption and dissociation of O2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms by high resolution electron energy loss spectroscopy and supersonic molecular beam technique. The cross section for both processes increases rapidly both as a function of angle of incidence and of total impact energy of the inert gas atom. While the increase with energy is expected, the increase with the angle is somewhat surprising and is sensibly larger than observed for previously investigated systems. The cross section for desorption decreases moreover with coverage. In the limit of high impact energy and high coverage its value is always larger than the one for dissociation. The branching ratio between the two processes depends thereby on energy and angle of incidence of the inert gas atom. Atomic oxygen is not removed under any impact condition, because of its larger binding energy. In order to explain the experimental results, molecular dynamics simulations have been performed using a simple model including multiple scattering. We find that the angular dependence of the cross section is determined by surface corrugation and by multiple scattering which suppresses desorption at normal incidence while the energetic threshold is determined by energy loss to the substrate. © 1998 American Institute of Physics.

Notes: The fullerenes C70 and C60 have been studied by excitation and luminescence spectroscopy. Highly resolved phosphorescence-excitation spectra of C70 in n-pentane at 1.5 K are reported. Besides the origin and e1′ vibronic bands of the lowest-energy A2′←S0 excitation, the origin, and an a1′ vibronic band of the E1′←S0 excitation are assigned. For both C70 and C60 the fluorescence spectrum changes with temperature, which is explained in terms of thermally induced population of excited states above S1. For C70, this concerns the E1′ state and the corresponding fluorescent state is found to be 130 cm−1 above the A2′ state in decalin/cyclohexane. For C60 in the same matrix, it concerns the state of predominant Gg character at 85 cm−1 above the T1g state. © 1998 American Institute of Physics.

Notes: The absolute laser phase dependence of the time-dependent populations of the molecular states, including the steady-state (long time) populations of the states, associated with the interaction of a molecule with a pulsed laser is investigated using illustrative two-level examples. One-photon transitions, including the effects of permanent dipoles, are discussed as a function of the pulse duration, intensity, and (absolute) laser phase, for selected laser frequencies. The effects of laser phase can be large, depending on the values of the pulse duration for a given frequency and intensity. The effects of permanent dipoles, relative to no permanent dipoles, are significant for large laser field strengths ε0. When the laser-molecule coupling parameter b=μ12ε0/E21≥0.2, where μ12 and E21 are the transition dipole and energy difference between the ground and excited states, respectively, the dynamics of the pulse-molecule interaction are (strongly) phase dependent, independent of pulse duration, whereas the corresponding steady-state populations of the molecular states may or may not be phase-dependent depending on the pulse duration. Analytical rotating wave approximations for pulsed laser-molecule interactions are useful for interpreting the dynamics and the steady-state results as a function of field strength and pulse duration, including the effects of permanent dipole moments. The results reported in this paper are based on molecular parameters associated with an S0→S1 electronic transition in a dipolar molecule. However, they are presented in reduced form and therefore can be scaled to other regions of the electromagnetic spectrum. Short, intense pulses at or beyond the limits of current laser technology will often be required for the types of absolute laser phase effects of this paper to be appreciable for electronic excitations. The discussion, in the UV-VIS, also suffers from the use of a two-level model and from the requirement of field intensities that can be beyond the Keldysh limit. For other spectral regions, these absolute laser phase effects will be much more readily applicable. © 1998 American Institute of Physics.

Notes: The lowest doublet (2A′) potential energy surface for the reaction N2O+Br→N2+OBr was investigated using ab initio and nonlocal density functional theory calculations. Geometries, energies and vibrational frequencies for stationary points were evaluated at HF/6-31G(d), MP2/6-31G(d) and B3LYP/6-31G(d) levels of theory. All levels of calculation give a similar geometry for the transition state, but the MP2 barrier is narrower. Intrinsic reaction coordinate (IRC) calculations starting from the transition state and proceeding toward the two local minima confirmed that IRC trajectories reached the reactant and product regions, respectively. Calculations of kinetic isotope effects were also performed. They are influenced by the theoretical level and the B3LYP method gives results in best agreement with experimental data. The HF method predicts the relative values of both the primary and secondary nitrogen kinetic isotope effects less accurately. At the MP2 level of calculations only the oxygen kinetic isotope effect is reproduced satisfactorily. Finally, the best value for the activation energy is again provided by the B3LYP method. © 1998 American Institute of Physics.

Notes: We report a relativistic all electron ab initio calculation of the ground and excited state potential energy curves of the CsHg molecule along with a determination of the spectroscopic parameters. Spin-orbit and kinematical relativistic effects were taken into account using the Douglas–Kroll-transformed no-pair Hamiltonian. © 1998 American Institute of Physics.

Notes: The microwave rotational spectra for seven isotopomers of tetracarbonyldihydroruthenium were measured in the 4–12 GHz range using a Flygare–Balle type microwave spectrometer. The measured transition frequencies could be fit to within a few kilohertz using a rigid rotor Hamiltonian with centrifugal distortion. The rotational constants for the most abundant isotopomer are A=1234.2762(4), B=932.7016(6), and C=811.6849(6) MHz. The dipole moment is aligned with the c axis of the complex. The 21 measured rotational constants were used to determine the following structural parameters: r(Ru–H)=1.710(23) Å, r(Ru–C1)=1.952(21) Å, r(Ru–C3)=1.974(28) Å, (angle)(H–Ru–H)=87.4(2.4)°, (angle)(C1–Ru–C2)=160.6(4.3)°, (angle)(C3–Ru–C4)=101.4(1.5)°, and (angle)(Ru–C1–O1)=172.6(7.6)°. The axial carbonyl groups are bent slightly toward the hydrogen atoms. These structural parameters are in excellent agreement with the substitution coordinates determined from the Kraitchman equations, and with the structural parameters calculated using density functional theory. There was no previous structural data on this complex. The results of the microwave data and theoretical calculations both indicate C2v molecular symmetry, and show that the H atoms are separated by about 2.36 Å. These results indicate that this complex is clearly a "classical dihydride" rather than an η2-bonded, "dihydrogen" complex. Fairly large deuterium isotope effects were observed for the Ru–H bond length and H–Ru–H angle. The r0, Ru–D bond lengths were observed to be 0.03(2) Å shorter than the r0, Ru–H bond lengths. The D–Ru–D angle is 1.1° less than the H–Ru–H angle indicating that the anharmonicity effects are comparable for the bond lengths and for the interbond angle. The new results on this complex are compared with previous results on the similar dihydride complexes, H2Fe(CO)4, and H2Os(CO)4. © 1998 American Institute of Physics.

Notes: Proton transfer energies of phenol and 14 chlorophenols with H2O as a base are analyzed in the gas phase and in solution using quantum chemical methods at the semiempirical and ab initio level of computation. The effect of aqueous solution was accounted for by applying the density functional theory (DFT) implementation of the conductor-like screening model (COSMO) as well as semiempirical continuum-solvation models. The results reveal substantial and systematic overestimations of the free energies of proton transfer as derived from experimental solution-phase pKa data. This can be traced back to both deficiencies in the current model parameterization as well as to limitations of the underlying gas-phase quantum chemical models, which is further illustrated by additional complete-basis-set (CBS) calculations for the proton transfer reaction with phenol. In contrast, the relative pKa trend is reflected well by COSMO-DFT calculations with correlation coefficients (adjusted for degrees of freedom) of 0.96. Decomposition of the dissociation energy in aqueous solution into a gas-phase term and a term summarizing the solvation contributions provides new insights into the effect of solvation on proton transfer energies, and yields mechanistic explanations for the observed differences in the gas-phase and solution-phase acidity orders of various subgroups of the compounds. © 1998 American Institute of Physics.

Notes: We develop the macrostate variational method (MVM) for computing reaction rates of diffusive conformational transitions in multidimensional systems by a variational coarse-grained "macrostate" decomposition of the Smoluchowski equation. MVM uses multidimensional Gaussian packets to identify and focus computational effort on the "transition region," a localized, self-consistently determined region in conformational space positioned roughly between the macrostates. It also determines the "transition direction" which optimally specifies the projected potential of mean force for mean first-passage time calculations. MVM is complementary to variational transition state theory in that it can efficiently solve multidimensional problems but does not accommodate memory-friction effects. It has been tested on model 1- and 2-dimensional potentials and on the 12-dimensional conformational transition between the isoforms of a microcluster of six-atoms having only van der Waals interactions. Comparison with Brownian dynamics calculations shows that MVM obtains equivalent results at a fraction of the computational cost. © 1998 American Institute of Physics.

Notes: Cross sections of the electron impact ionization as well as the dissociative ionizations of ethane have been measured for electron energies from threshold to 600 eV. The complete collection of all the ionic fragments has been verified directly in the experiment. The results are thus believed to be reliable. The results for the products of Hn+(n=1–3) are obtained for the first time. The appearance potentials of the ionic products are also measured. The disagreement with respect to the appearance potentials between the present work and previous measurements is explained by the dipole-forbidden transitions in the electron impact excitation process. The starting channels for the ionic products are discussed based on the appearance potentials, the kinetic energy distributions of the ionic products, and the ejection of the electrons out of the different orbitals of the ethane molecule. © 1998 American Institute of Physics.

Notes: In the course of mechanistic studies on HCN+ isomerization in reaction with CO and CO2, we have fortuitously determined the heats of formation of HNC and HNC+ with good precision. The uncertainty is the uncertainty in the heat of formation of HCN, ±1 kcal mol−1. This appears to be the first precise experimental determination of these energies. Several prior theoretical calculations have produced reasonable energy values. We also observed the reaction of HCN+ with CF4 to be slightly endothermic, which allows the determination of an upper limit on the threshold energy for CF4→CF3++F of 14.28 eV, in excellent agreement with several prior determinations but in sharp disagreement with the most recent determination. © 1998 American Institute of Physics.

Notes: In this work, photodissociation of O2 at 157 nm has been studied using the photofragmentation translational spectroscopic technique. Two product channels O2+hν→O(1D)+O(3P), O2+hν→O(3P)+O(3P) have been observed. The relative yields and anisotropy parameters of both channels are determined. Anisotropy mixing of dissociation resulting from a perpendicular excitation and a parallel-type excitation has been observed in the dissociation channel O2+hν→O(3P)+O(3P). The observed results can be used to look at the detailed dynamical processes of the O2 dissociation through the Schumann–Runge band. © 1998 American Institute of Physics.

Notes: We calculate 1542 ab initio points for the HF2+ ground state at the QCISD(T)/6-311++G(2df,2pd) level for a wide range of geometries. We fit the ab initio points to a multiparameter analytic function to obtain a multidimensional potential energy surface (PES) valid for large amplitude hydrogen motion. We then calculate and assign vibrational levels for this PES. There is intramolecular proton transfer when the hydrogen atom tunnels through a triangular transition state separating the two equivalent equilibrium geometries. The barrier to proton transfer is 9547 cm−1 (8340 cm−1 with zero-point correction). Below the barrier to proton transfer, the energy levels are split and measurable splittings are predicted for relatively low-lying vibrational levels that may be experimentally accessible. The first three levels with splitting greater than 0.01 cm−1 are, in order of increasing energy, (0,3,1), (0,4,0), and (0,3,2), while the first three levels with splitting greater than 0.1 cm−1 are, in order of increasing energy, (0,4,1), (0,5,0), and (0,4,2), where ν2 is the H–F–F bend quantum number and ν3 is the F–F stretch quantum number. We conclude that H–F–F bend excitation is essential for proton transfer, and that F–F stretch excitation facilitates proton transfer. In addition, there is a 3:1 Fermi resonance between the HF stretch (harmonic frequency 3334 cm−1) and the H–F–F bend (harmonic frequency 1141 cm−1), and levels with HF stretch excitation can have significant splittings. For example, the splitting is greater than 0.01 cm−1 for the (1,1,1) level, which is Fermi resonant with (0,4,1) and greater than 0.1 cm−1 for the (1,2,1) level, which is Fermi resonant with (0,5,1). This is relevant for the experimental observation of the vibrational splittings since the IR intensity of the HF stretch is four times that of the H–F–F bend. © 1998 American Institute of Physics.

Notes: The role of frequency variation in the transition between two crossing diabatic electronic states originating in a conical intersection is studied. We have considered a minimal model with two vibrational degrees of freedom, a coupling mode, and a tuning mode, and performed computations varying the frequency of both in the initially unoccupied diabatic state. It is shown that neglecting this effect may result in a wrong estimate of the transition probability. Two simple rules for evaluating qualitatively such an effect are proposed. © 1998 American Institute of Physics.

Notes: The adiabatic ionization potentials of TiO, ZrO, NbO, and MoO have been measured using two-color photoionization efficiency (PIE) spectroscopy and mass-analyzed threshold ionization (MATI). From the sharp ionization thresholds in the PIE and MATI spectra the following ionization potentials were derived: IP(TiO)=6.8197(7) eV, IP(ZrO)=6.812(2) eV, IP(NbO)=7.154(1) eV, and IP(MoO)=7.4504(5) eV. These values have been combined with the ionization potentials of the metal atoms and the bond energies of the transition metal oxide cations, D0(MO+) [M. R. Sievers et al., J. Chem. Phys. 105, 6322 (1996)] to derive the bond energies, D0(MO), of the neutral metal monoxides; D0(TiO)=6.87(7) eV, D0(ZrO)=7.94(11) eV, D0(NbO)=7.53(11) eV, D0(MO)=5.44(4) eV. It is argued that these values are more accurate than the currently accepted values and hence are recommended for future work. Experimental evidence suggests that the ground state of MoO+ is the 4Σ− state arising from the δ2σ1 configuration. © 1998 American Institute of Physics.

Notes: We report turbidity, light scattering, and coexistence curve data for a solution of triethyl n-hexyl ammonium triethyl n-hexyl borate in diphenylether. We recently reported that the present sample shows much higher turbidity than that of K. S. Zhang, M. E. Briggs, R. W. Gammon, and J. M. H. Levelt Sengers [J. Chem. Phys. 109, 4533 (1998)] for an earlier sample. An analysis of the data shows that nonclassical critical behavior is favored in the reduced temperature range from 10−5 to 10−2. At fixed reduced temperature, the correlation length is about twice as large as that of the previous sample. The correlation length amplitude calculated from the fit is 1.4 nm±0.1 nm. A detailed data analysis points out the limitations of turbidity measurements far away from the critical point. The intensity of scattered light was measured at 90°. Multiple scattering is relevant in the wider vicinity of the critical point and was corrected for by a Monte Carlo simulation method. An Ising-type exponent for the correlation length was obtained: ν=0.641±0.003, and the amplitude of the correlation length ξ0=1.34 nm±0.01 nm agrees with that of the turbidity experiment. Mean-field behavior can be ruled out. The refractive indices of coexisting phases were measured in the reduced temperature range from t=10−4 to 0.04. These measurements disagree with results reported by R. R. Singh and K. S. Pitzer [J. Chem. Phys. 92, 6775 (1990)]. The present data lead to an exponent β=0.34±0.01, close to the Ising value. The coexistence curve is much narrower than that of Singh and Pitzer. Crossover could not be detected in any of the experiments. Two-scale-factor universality could be confirmed for this and another ionic system within the experimental uncertainty. © 1998 American Institute of Physics.

Notes: We have measured the excitation photon energy dependence of the resonance Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluids of trifluoromethane and carbon dioxide. We have found that the band peak positions of the C(Double Bond)N stretching mode of PB appear to change with the excitation laser frequency in polar solvents. On the other hand, no dependence of the band peak positions on the excitation frequency is found in nonpolar solvents. The peak positions of the C(Double Bond)O stretching mode also show small excitation energy dependence in chloroform and in methanol. The extent of the excitation energy dependence is correlated with the amount of the fluctuation of the local field on PB molecule exerted by the solvent, which had been estimated from the bandwidths of the absorption and the Raman spectra of PB in solution. The IR absorption spectra of PB are measured in chloroform and carbon tetrachloride, and compared with the Raman spectra in the same solvents. The excitation energy dependence is explained by the solvation state selective excitation in the resonance Raman spectroscopy, that is, the inhomogeneity that appears in the absorption spectra of PB also appears in the resonance Raman process. © 1998 American Institute of Physics.

Notes: Octadecanol monolayers at the air/water interface are studied by grazing incidence x-ray diffraction. At low lateral pressures one observes the diffraction patterns inherent to the next-nearest neighbor tilt of the molecules. The unit cells are found to be distorted at both low (6 °C) and high (33 °C) temperatures due to tilt of the molecules and ordering of their backbones. Unexpectedly, at an intermediate temperature of 22 °C, the unit cell remains hexagonal in the water plane, despite the tilt of the molecules. This behavior is treated in terms of hydrogen bonding with the water molecules and does not imply the existence of a separate phase. Our diffraction study also does not show any indication of a phase transition S*–L2* corresponding to the singularities in the isotherms within the tilted phase. The tilting phase transitions from untilted phases S and LSII seem to be continuous, since the transition pressures obtained by extrapolation of the tilt angle to zero tilt coincide with the ones determined by Brewster angle microscopy. © 1998 American Institute of Physics.

Notes: Optical absorption and photoluminescence spectroscopy have been carried out on a new class of (phenylene) ring substituted p-pyridylvinylenephenylenevinelyne polymers used as active materials in light emitting diodes. The effects of the ring substitutions on the optical absorption and photoluminescence energies are qualitatively explained through the use of semiempirical quantum chemical modeling of the ring torsion angles. Reduced aggregation through the use of "strap" substituents on the phenylene rings also is discussed. © 1998 American Institute of Physics.

Notes: It is shown that the hydration correction for the free energy of cluster formation in the classical binary nucleation theory is not applicable for the thermodynamic data of the water–surface acid system commonly used in nucleation calculations. A new form of the hydration correction is presented. For the commonly used thermodynamic data the nucleation rates of the new hydration correction are in the temperature range of 248–323 K, and at a relative humidity greater than 40%, 103–106 times lower, respectively, than the nucleation rates of the former theory. The predictions of acid–water nucleation rates of the thermodynamically consistent version of the classical binary homogeneous theory [G. J. Wilemski, Chem. Phys. 80, 1370 (1984)] with the new hydration correction are in accordance with experimental results [B. E. Wyslouzil, J. H. Seinfeld, R. C. Flagan, and K. J. Okuyama, Chem. Phys. 94, 6842 (1991)] obtained at 298 K and with experimental results [D. Boulaud, G. Madelaine, D. Vigla, and J. Bricard, J. Chem. Phys. 66, 4854 (1977)] obtained at 293 K, but at a given number concentration of acid and relative humidity, they predicted nucleation rates are rather more stronger functions of temperature than the experimental rates. The theoretical nucleation rate of water–sulfuric acid vapor is due to the hydration correction sensitive to the thermodynamic properties of dimers and timers. © 1998 American Institute of Physics.

Notes: In this paper the use of the second-order memory function to represent experimental results of dielectric relaxation data of amorphous and glass forming materials is presented, and particularly, its interpretation in terms of the frequency dependence of the complex shear modulus, shear viscosity, and diffusion coefficient is pointed out. The method used for its evaluation is applied to consider the experimental results for three ester substances, and their particular features are discussed. © 1998 American Institute of Physics.

Notes: The bimolecular reaction of O(3P) with ethylene and the unimolecular photodissociation of acetaldehyde and formaldehyde have been studied using a picosecond pump/probe technique. The bimolecular reaction was initiated in a van der Waals dimer precursor, C2H4⋅NO2, and the evolution of the vinoxy radical product monitored by laser-induced fluorescence. The NO2 constituent of the complex was photodissociated at 266 nm. The triplet oxygen atom then attacks a carbon atom of C2H4 to form a triplet diradical (CH2CH2O) which subsequently dissociates to vinoxy (CH2CHO) and H. The rise time of vinoxy radical production was measured to be 217 (+75−25) ps. RRKM theory was applied and a late high exit barrier was invoked in order to fit the measured rise time. The structure and binding energy of the van der Waals complex have been modeled using Lennard-Jones type potentials and the results were compared with other systems. The unimolecular side of the potential energy surfaces of this reaction has been investigated by photodissociating acetaldehyde at the same pump energy of 266 nm. The resulting photoproducts, acetyl radical (CH3CO) and formyl radical (HCO), have been monitored by resonance enhanced multiphoton ionization (REMPI) combined with a time-of-flight mass spectrometer. The similarity in the measured evolution times of both radicals indicates the same photodissociation pathway of the parent molecule. The photodissociation rate of acetaldehyde is estimated from RRKM theory to be very fast (3×1012 s−1). The T1←S1 intersystem crossing (ISC) rate is found to be the rate determining step to photodissociation and increases with energy. The REMPI mechanism for the production of CH3CO+ is proposed to be the same as that of HCO+(2+1). The HCO product from the photodissociation of formaldehyde at 266 nm reveals a faster T1←S1 ISC rate than in acetaldehyde. © 1998 American Institute of Physics.

Notes: The application of the zeroth-order regular relativistic approximation (ZORA) for molecular density functional calculations is investigated. By introducing a model potential to construct the kinetic energy operator, stationarity of the energy with respect to orbital variations is gained and most problems connected with gauge dependence of the regular approximation are eliminated. The formulation of a geometry gradient is greatly facilitated using this formalism. Calculations for the coinage metal hydrides (CuH, AgH, AuH) as well as for the homonuclear (Cu2, Ag2, Au2) and heteronuclear (CuAg, CuAu, AgAu) diatomics show that the results of ZORA calculations within the electrostatic shift approximation, as introduced by van Lenthe and co-workers, can be duplicated using the simpler scheme proposed in this work. Results for the coinage metal fluorides (CuF, AgF, AuF) and chlorides (CuCl, AgCl, AuCl) are presented as well. First-order relativistic calculations have been performed for all systems to assess the applicability of leading-order relativistic perturbation theory. © 1998 American Institute of Physics.

Notes: The potential energy surface for the gas-phase SiH4+H→SiH3+H2 reaction and its deuterated analogs was constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. Using this surface, the rate constants were calculated with variational transition-state theory over the temperature range 200–1000 K, finding good agreement with experiments. We also provide a detailed analysis of the kinetic isotope effects and a comparison with the scarce experimental results. © 1998 American Institute of Physics.

Notes: Discovery of the photo-induced chaos in the Briggs–Rauscher system is reported. The chaotic oscillations were observed between the large- and the small-amplitude simple oscillatory states existent in low and high light intensity regions, respectively. Period-doubling sequence from the large-amplitude oscillations to the chaos was observed. Deterministic nature of the chaos was confirmed by the next-amplitude return map. The stretching and folding mechanism of the trajectories was revealed through the three-dimensional attractor reconstructed via the singular value decomposition method. The chemical origin of the photoinduced chaos is discussed based on the photoautocatalysis of HIO2. © 1998 American Institute of Physics.

Notes: The spin-lattice relaxation time for nuclei possessing electric quadrupole moments is determined mainly by the electric quadrupolar interactions between the nucleus and its environment. Here we give a microscopic formulation of the nuclear quadrupolar relaxation problem for a nucleus of a monatomic solute dissolved in molecular fluids. Our formulation is based on classical statistical mechanics and the interaction site model representation of the intermolecular potential. We assume that the fluctuating field gradient felt by the nucleus is caused mainly by the charge distribution of the surrounding solvent molecules, modulated by the Sternheimer (anti)shielding factor of the nucleus. In the extreme narrowing condition, the problem reduces to the determination of a time integral of the field gradient time correlation function G(t) on the nucleus position. By separation of G(t) into a static contribution G(t=0) and a normalized time correlation function, we seek microscopic expressions for both G(t=0) and its correlation time τQ. Within certain approximations we express τQ in terms of the wavevector-dependent polarization charge correlation time τμ(k), and G(t=0) in terms of the pure solvent charge structure factor Sμ(k) and an analytical function of the solute cavity radius a. Taking as input τμ(k) from molecular dynamics simulations of the pure solvent and Sμ(k) from the extended reference interaction-site model (XRISM) calculation, we apply the theory to the spin lattice relaxation rate of seven quadrupolar nuclei in acetonitrile solution. The solutes considered cover a wide range of size, charge, and nuclear spin quantum number. With reasonable choices of the solute cavity radii, the theory successfully reproduces the experimentally measured 1/T1 for these solutes. Using molecular dynamics simulation, we also investigate the effects on 1/T1 of neglecting the solute mobility. Our simulated data suggest that the solute mobility can reasonably be neglected for spin relaxation of heavy quadrupolar nuclei such as Kr and Xe. Finally, the dielectric continuum limit of our theory is discussed and compared with the related theory developed by Hynes and Wolynes. © 1998 American Institute of Physics.

Notes: Molecular and atomic radicals from electron-impact dissociation of methane and a variety of fluoroalkanes are detected mass spectometrically as organotellurides produced by the reaction of the radicals at the surface of a tellurium mirror. The radicals detected include CH3 from CH4; CF3 from CF4 and CHF3; CHF2 from CHF3 and CH2F2; CH2F from CH3F; and CF3 and C2F5 from C2F6 and C3F8 produced by electron impact at energies between 10 eV and 500 eV. Relative cross sections are measured. These are placed on an absolute scale by comparison with related measurements. For the collision energies relevant to processing plasmas, 10–30 eV, it is shown that dissociation into neutrals rather than dissociative ionization is mainly responsible for the production of molecular radicals. © 1998 American Institute of Physics.

Notes: According to the singularity-free interpretation of the thermodynamics of supercooled water, the isothermal compressibility, isobaric heat capacity, and the magnitude of the thermal expansion coefficient increase sharply upon supercooling, but remain finite. No phase transition or critical point occurs at low temperatures. Instead, there is a pronounced but continuous increase in volume and a corresponding decrease in entropy at low temperatures, the sharpness of which becomes more pronounced the lower the temperature and the higher the pressure. We investigate the behavior of the response functions, equation of state, and entropy of a schematic waterlike model that exhibits singularity-free behavior, and thereby illustrate the simplest thermodynamically consistent interpretation that is in accord with existing experimental evidence on water's low-temperature anomalies. In spite of its simplicity, the model captures many nontrivial aspects of water's thermodynamics semiquantitatively. © 1998 American Institute of Physics.

Notes: The adsorption transitions of a single self-avoiding polymer chain at chemically heterogeneous surfaces have been investigated using Monte Carlo methods and analyzed using scaling arguments. Evidence is provided that the crossover exponent φ, characterizing the transition between the weakly and the strongly adsorbed states, depends linearly on the dilution 0.6≤p≤1 of adsorbing sites on the surface, φ(p)(approximate)0.35+0.2p. The transition temperatures of chains of length N scale according to Tc(N,p)/Tc(∞,p)∼N−φ(p). In particular, we have analyzed the adsorption energies and the parallel and perpendicular components of the end-to-end distance. © 1998 American Institute of Physics.

Notes: Numerical simulation based on the modified time-dependent Ginzburg–Landau (TDGL) model has been performed on the domain growth and related rheological properties of binary mixtures under oscillatory shear. The simulation results reveal that the domain growth is anisotropic and depends on the quench depth. It is found that, in the deep quench case, the disclike domain with the normal parallel to the velocity gradient direction is observed, while in the shallow quench case, the rodlike domain with rod axis aligned along the flow direction is observed. The scattering functions for different light incident directions are calculated and suggest that the undulated rodlike morphology is formed in the shallow quench case. This undulated rodlike morphology shows the anomalous rheological response. A plausible interpretation for the anomalous rheological property is proposed based on the deformation of the undulated rodlike morphology under oscillatory shear. © 1998 American Institute of Physics.

Notes: A benchmark comparison is presented of two direct dynamics methods for proton tunneling, namely variational transition-state theory with semiclassical tunneling corrections (VTST/ST) and the instanton method. The molecules chosen for the comparison are 9-hydroxyphenalenone-d0 and -d1, which have 64 vibrational degrees of freedom and show large tunneling splittings for the zero-point level and several vibrationally excited levels of the electronic ground state. Some of the excited-level splittings are larger and some smaller than the zero-level splitting, illustrating the multidimensional nature of the tunneling. Ab initio structure and force field calculations at the Hartree–Fock/6-31G** level are carried out for the two stationary points of the tunneling potential, viz. the equilibrium configuration and the transition state. The VTST/ST calculations are based on both the small- and the large-curvature approximation; the additional quantum-chemical calculations required at intermediate points of the potential are performed at the semiempirical modified neglect of differential overlap (MNDO)/H2 level. The VTST/ST computations use the MORATE 6.5 code developed by Truhlar and co-workers. The instanton dynamics calculations are based on the method we previously developed and applied to tropolone, among others. It uses the transition state rather than the equilibrium configuration as reference structure and approximates the least action analytically. The computations use our "dynamics of instanton tunneling" (DOIT) code. It is found that the large-curvature approximation and the instanton method both reproduce the observed zero-level splitting of the d0 isotopomer if the calculated barrier is reduced by a factor 0.87. With this adjusted barrier, the instanton method also reproduces the zero-level and excited-level splittings of the d1 isotopomer. However, both the small- and the large-curvature approximations severely underestimate all these splittings. These methods, which use relatively inflexible trajectories, do not handle the isotope effect well and also are not developed to the point where they can deal satisfactorily with vibronic level splittings. In addition, there is a striking difference in efficiency between the two methods: the MORATE 6.5 code took 40 h on an R8000 workstation to perform the dynamics calculations, whereas the DOIT code took less than 1 min and produced superior results. The main reason for this superior performance is ascribed to the effective use made of the least-action principle by the instanton method and to the avoidance of the adiabatic approximation, which is not valid for modes with a frequency equal to or lower than the tunneling-mode frequency. © 1998 American Institute of Physics.

Notes: When contributions from the interfacial energy become significant and comparable to the bulk energy, liquid and crystalline phases can coexist at a temperature much lower than the usual melting point. A formalism for this coexistence is given, and thermodynamic conditions for the melting of nanometer-size cubic ice crystals are derived when both the ice and water are at an equilibrium vapor pressure. By using the approximate values of surface energy and the enthalpy and entropy of melting, it is shown that nanometer-size water droplets can coexist with cubic ice particles of about the same size at temperatures in the 150–180 K range. The unusually large decrease in the temperature of a liquid-solid phase equilibrium is expected to be a general phenomenon in the nanometer-size films, clusters, and particles of materials.© 1998 American Institute of Physics.

Notes: We report results of extensive dynamical Monte Carlo investigations on self-assembled equilibrium polymers (EP) without loops in good solvent. (This is thought to provide a good model of giant surfactant micelles.) Using a novel algorithm we are able to describe efficiently both static and dynamic properties of systems in which the mean chain length 〈L〉 is effectively comparable to that of laboratory experiments (up to 5000 monomers, even at high polymer densities). We sample up to scission energies of E/kBT=15 over nearly three orders of magnitude in monomer density φ, and present a detailed crossover study ranging from swollen EP chains in the dilute regime up to dense molten systems. Confirming recent theoretical predictions, the mean-chain length is found to scale as 〈L〉∝φαexp(δE) where the exponents approach αd=δd=1/(1+γ)(approximate)0.46 and αs=1/2[1+(γ−1)/(νd−1)](approximate)0.6,δs=1/2 in the dilute and semidilute limits respectively. The chain length distribution is qualitatively well described in the dilute limit by the Schulz-Zimm distribution p(s)(approximate)sγ−1 exp(−s) where the scaling variable is s=γL/〈L〉. The very large size of these simulations allows also an accurate determination of the self-avoiding walk susceptibility exponent γ(approximate)1.165±0.01. As chains overlap they enter the semidilute regime where the distribution becomes a pure exponential p(s)=exp(−s) with the scaling variable now s=L/〈L〉. In addition to the above results we measure the specific heat per monomer cv. We show that the average size of the micelles, as measured by the end-to-end distance and the radius of gyration, follows a crossover scaling that is, within numerical accuracy, identical to that of conventional monodisperse quenched polymers. Finite-size effects are discussed in detail. © 1998 American Institute of Physics.

Notes: Theoretical design of global optimization algorithms can profitably utilize recent statistical mechanical treatments of potential energy surfaces (PES's). Here we analyze the basin-hopping algorithm to explain its success in locating the global minima of Lennard-Jones (LJ) clusters, even those such as LJ38 for which the PES has a multiple-funnel topography, where trapping in local minima with different morphologies is expected. We find that a key factor in overcoming trapping is the transformation applied to the PES which broadens the thermodynamic transitions. The global minimum then has a significant probability of occupation at temperatures where the free energy barriers between funnels are surmountable. © 1998 American Institute of Physics.

Notes: In a recent paper [H. L. Schmider and A. D. Becke, J. Chem. Phys. 108, 9624 (1998)], we applied a systematic method for the determination of exchange-correlation functionals within the generalized gradient approximation (GGA) to the extended G2 test set of standard heats of formation of Curtiss et al. [J. Chem. Phys. 106, 1063 (1997)]. In the present work, we apply a similar methodology that goes beyond the GGA by taking second-order gradients and the (noninteracting) kinetic-energy density into account. The resulting improvement in the reproduction of thermochemical data brings us very close to the quality of G2 theory itself. Our lowest mean absolute error for standard heats of formation, 1.60 kcal/mol, is only marginally greater than the G2 value (1.58 kcal/mol). The corresponding largest deviation is 9.97 kcal/mol, as compared to 8.2 kcal/mol for G2 theory. © 1998 American Institute of Physics.

Notes: We present the results of self-consistent quantum Monte Carlo simulations of the structures of the liquid-vapor interfaces of alkali metals (Na, K, Rb, Cs) using a modified semiempirical empty-core model potential. The purpose of this investigation is to simplify the analysis of inhomogeneous metals sufficiently to permit qualitative inferences to be drawn about the properties of families of metals. Both electronic and ion density profiles along the normal to the surface show oscillations in the liquid–vapor transition zone. These oscillations closely resemble those found in previous simulation studies of the liquid–vapor interfaces of alkalis, based on sophisticated nonlocal model potentials. Because of its semianalytical representation, the model potential used in this paper allows considerable simplification in the computational scheme relative to the effort involved in the previously published simulations. We find liquid Na, K, Rb, and Cs to exhibit similar surface layering. Moreover, our results suggest the existence of a corresponding states representation of the properties of this class of metals. We expect this new analysis will be useful in predicting the qualitative properties of the surface structures of a broad range of pure liquid metals. © 1998 American Institute of Physics.

Notes: Simple polymer blends were studied by simulations with the intent of probing the response of structurally asymmetric chains to progressively stronger attractive interactions strengths. It was found, for miscible blends, that the intermolecular pair correlation functions, the g(r)'s, varied by less than 10% from those of the associated blend with strictly repulsive interactions. When used for the purposes of calculating changes in pressure due to the attractive strength, the g(r)'s from either the repulsive or attractive systems could be used interchangeably in the perturbation theory. On the other hand, calculations of the enthalpic χ parameter were sensitive to small variations in the g(r)'s, although, at least for the systems studied here, the associated predictions of the phase boundaries were relatively insensitive. © 1998 American Institute of Physics.

Notes: A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (±0.8 cm−1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric fields involved, and scanning the exciting photon energy, it was possible to determine the field-free energetic threshold for H35Cl+hν→H++35Cl−. Using this energy, together with the known values of the ionization potential of H and electron affinity of Cl, a new estimate for the dissociation energy of HCl was obtained: D0(H35Cl)=35 748.2±0.8 cm−1. © 1998 American Institute of Physics.

Notes: The reduced frozen-core approximation (RFCA) that has been previously used for nonrelativistic calculations is extended to relativistic calculations. The RFCA adopts a new method for the orthogonalization of valence basis functions to core orbitals. Orthogonalization is performed using corelike basis functions consisting of fewer primitive basis functions than core orbitals. Dirac–Fock–Roothaan calculations on HI and ThO show that the relativistic RFCA can reduce computing time and closely reproduce the total and valence orbital energies and spectroscopic constants obtained by all-electron calculations. © 1998 American Institute of Physics.

Notes: The dependence of spin–spin nuclear magnetic resonance (NMR) coupling constants on the basis set and electron correlation has been investigated in methane using Hartree–Fock and multiconfigurational self-consistent field wave functions (HF-SCF and MCSCF). The effect of the size, contraction, and tight s functions of the basis sets is analyzed. Some suggestions about the contraction scheme are indicated. MCSCF wave functions with different numbers of active orbitals and different numbers of excited electrons were used. An approximation to determine spin–spin coupling constants at a high level of electron correlation from three calculations with a smaller level of correlation and reduced computational cost is investigated. The best calculated 1JCH and 2JHH couplings are 120.63 and −13.23 Hz, respectively, which are 0.24 and 1.24 Hz smaller than those experimentally obtained for the equilibrium geometry. The remaining error in these coupling constants can be attributed mainly to correlation and not to basis set effects. © 1998 American Institute of Physics.

Notes: The spin–spin time correlation function of the spin-boson model is studied using a maximum entropy imaging procedure founded on knowledge of early time derivatives or moments of the corresponding spectral density. The coherent–incoherent boundary is reproduced in agreement with the results of dynamical path-integral Monte Carlo and the procedure is shown to be stable numerically at both long time and low temperature. © 1998 American Institute of Physics.

Notes: A spectral line shape model accounting for both the collisional confinement narrowing of the Doppler distribution and the inhomogeneous effects due to the radiator speed dependence of the collisional broadening and shifting parameters is proposed. The velocity changes are assumed to be induced as well by hard as soft collisions. Doppler-collision correlations and speed-class exchanges are taken into account. A comparison with the previous models used for the Doppler regime and for the collisional one is done. The present model also applies for the intermediate regime where the two above mechanisms are simultaneously efficient. The spectral line shape characteristics are exemplified through the H2–Ar and C2H2–Xe prototype molecular systems. © 1998 American Institute of Physics.

Notes: The effect of temperature on electron attachment to dichlorodifluoromethane (CCl2F2) has been investigated for temperatures up to 500 K and for mean-electron energies from thermal to 1.0 eV using an electron swarm method. The measurements were made in mixtures of CCl2F2 with nitrogen. The electron attachment rate constant increases with temperature over the entire temperature and mean-electron energy range investigated. The variation of the thermal value of the electron attachment rate constant with temperature compares well with earlier measurements of this quantity and shows an increase by a factor of 10 when the temperature is raised from 300 to 500 K. From a comparison of published data on the electron affinity, electron attachment using the swarm method, electron attachment using the electron beam method, electron scattering, electron transmission, indirect electron scattering, and related calculations, the lowest negative ion states of CCl2F2 have been identified with average positions as follows: a1(C–Clσ*) at +0.4 eV and −0.9 eV, b2(C–Clσ*) at −2.5 eV, a1(C–Fσ*) at −3.5 eV, and b1(C–Fσ*) at −6.2 eV; an electron-excited Feshbach resonance is also indicated at −8.9 eV. © 1998 American Institute of Physics.

Notes: We confirm that the recently discovered field-dependent nuclear quadrupolar splitting of 131Xe [T. Meersmann and M. Haake, Phys. Rev. Lett. 81, 1211 (1998)] arises from a distortion of the electron density due to the applied magnetic field. This distortion depends both linearly and quadratically on the applied field. The existence of the former is due to the coupling to the field and the nuclear spin. The latter is a manifestation of the quadratic Zeeman effect. In addition to confirming the order of magnitude for the observed effect, we show that there should be an asymmetry introduced in the spectra due to the linear coupling with the nuclear spin. This effect has not been seen experimentally. © 1998 American Institute of Physics.

Notes: Based on an ab initio potential energy surface, the features of the quantum spectrum of HCP have been recently discussed in terms of the periodic orbits of the exact classical Hamiltonian [J. Chem. Phys. 107, 9818 (1997)]. In particular, it was shown that the abrupt change in the bending character of the states at the lower end of the Fermi polyads, at about 15 000 cm−1 above the origin, can be ascribed to a classical saddle node bifurcation. The purpose of the present article is to show that the use of a very accurate Fermi resonance Hamiltonian, which was derived very recently from high-order perturbation theory [J. Chem. Phys. 109, 2111 (1998)] can provide a still deeper insight into the highly excited vibrational motion. The principal advantages of the resonance Hamiltonian compared to the exact one rely on the remaining good quantum numbers and classical action integrals, which enable one to consider HCP as a formal one-dimensional system parametrized by the polyad number i and the number v3 of quanta in the C–H stretching motion. It is shown in this article that all the quantum observations can be interpreted and explained in terms of the positions and bifurcations of the fixed points of this one-dimensional system: the shape of the quantum wave functions depends on the stable elliptic fixed points, whereas the dip in the gap between neighboring quantum levels is governed by the unstable, hyperbolic fixed points. The dependance on v3 of the bending character of the lowest states in each polyad i is discussed in some detail, whereas the previous work was fundamentally limited to v3=0. Moreover, the dependence on i and v3 of the form of the dip in the distribution of the gap between neighboring levels is given a clear explanation. © 1998 American Institute of Physics.

Notes: Isotopically labeled (benzene)13 clusters, (C6H6)(C6D6)12, were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization (R2PI) spectroscopy through the C6H6B2u←A1g601 transition. Because of the spectrum's simplicity, it serves as a sensitive monitor of the environment and dynamics of the C6H6 chromophore. We report experimental evidence for both evaporation and isomerization dynamics. Initially, the observed (C6H6)(C6D6)12 cluster population undergoes a transition from fluxional to rigid, resulting from the evaporation of a single C6D6 molecule from (C6H6)(C6D6)13. "Solidification" is followed by isomerization, in which the C6H6 moiety migrates from the surface of ordered, rigid clusters to their interior. The "freezing" temperature of (C6H6)(C6D6)12 is inferred to be near 137 K, in good agreement with theoretical simulations [Bartell and Dulles, J. Phys. Chem. 99, 17107 (1995)]. © 1998 American Institute of Physics.

Notes: We report on molecular beam experiments and molecular dynamics simulations of xenon scattering with incident energies E=0.06−5.65 eV from graphite. The corrugation felt by an atom interacting with the surface is found to be influenced by both surface temperature, Ts, and E. Angular distributions are significantly broadened when Ts is increased, clearly indicating corrugation induced by thermal motion of the surface also at the highest E employed. Direct scattering dominates for high E, while trapping becomes important for kinetic energies below 1 eV. The coupling between atom translation and surface modes in the normal direction is very effective, while trapped atoms only slowly accommodate their momentum parallel to the surface plane. The very different coupling normal and parallel to the surface plane makes transient (incomplete) trapping-desorption unusually pronounced for the Xe/graphite system, and atoms may travel up to 50 nm on the surface before desorption takes place. The nonlocal and soft character of the Xe-graphite interaction compared to interactions with close packed metal surfaces explains the observed high trapping probabilities and the lack of structural corrugation effects at high kinetic energies. Experimental results and simulations are in good agreement for a wide range of initial conditions, and we conclude that the model contains the most essential features of the scattering system. © 1998 American Institute of Physics.

Notes: Gas-phase products of liquid surface photochemistry in high vacuum were analyzed by time-of-flight/quadrupole mass spectroscopy. A thin liquid film of a long-chain alkyl iodide, C18H37I, dissolved in squalane (C30H62) was irradiated with nanosecond laser pulses at 275 nm. The photoproducts leaving the liquid after a low-fluence laser pulse (0.5 mJ/cm2) were I, HI, and I2. Since these species may desorb at times delayed by diffusion in the liquid phase, time-of-flight profiles were also recorded using a chopper wheel in front of the surface. This allowed a reconstruction of the time-dependent flux from the surface. The flight time distributions were compared to model calculations which take into account laser photolysis of C18H37I, diffusion and surface evaporation of I, HI, and I2, and the condensed-phase kinetics of radical reactions, and allow for a component of direct photodissociation of surface layer molecules, leading to hyperthermal I atoms. Simulations based on a comprehensive kinetic scheme are in good agreement with our measurements, with no specific surface processes other than evaporation of thermalized species formed at, or diffusing to the surface. However, compared to results previously found for liquid C2H5I, the caging of the geminate pair is much stronger in our system, and the fraction of I atoms promptly reacting to HI is smaller. The absence of prompt hyperthermal I fragments is interpreted by a preferential orientation of the C18H37I molecules in the topmost liquid layer with the I atom pointing into the liquid. The sensitivity of the method is discussed with respect to surface-specific processes, as well as primary and secondary radical reactions occurring in the bulk liquid. © 1998 American Institute of Physics.

Notes: Monte Carlo (MC) simulations of electronic excitation transfer (EET) among a small number of chromophores covalently incorporated into copolymer molecules are presented and used to test the results of previously developed analytical EET theories that are useful for the study of polymer chain structure [K. A. Peterson and M. D. Fayer, J. Chem. Phys. 85, 4702 (1986)] and phase separation in polymer blends [A. H. Marcus and M. D. Fayer, J. Chem. Phys. 94, 5622 (1991)]. The simulations and theory account for EET among chromophores bound to a single chain and among chromophores attached to different chains. The calculated quantity, 〈Gs(t)〉, which is the probability that an initially excited chromophore is still excited at time t, is related to time-resolved fluorescence depolarization experiments. The theories, particularly the treatment of interchain EET, depend on a series of approximations whose efficacy has not been determined. Close agreement between the MC simulations and the analytical theory are found for a variety of situations, including those that mimic real polymer systems. The limits beyond which agreement is weakened provide specific guidelines for the design of polymer structure and phase-separation experiments. © 1998 American Institute of Physics.

Notes: We present a simple, physically motivated equation for the radial distribution function g(r) for molecular liquids, valid for polymers interacting via soft potentials. It is constructed to perform properly at low density for polyelectrolyte solutions. However, it also accounts for intermolecular correlations at both the molecular and monomer level, above those contained in the random phase approximation. We show that this theory reduces in various limits to some well-known polymer theories for g(r). In a preliminary analysis, we apply the equation for soft potentials to two very different systems: a solution of rod polyelectrolytes and a solution of flexible van der Waals chains. © 1998 American Institute of Physics.

Notes: DFT schemes based on conventional and less conventional exchange-correlation (XC) functionals have been employed to determine the polarizability and second hyperpolarizability of π-conjugated polyacetylene chains. These functionals fail in one or more of several ways: (i) the correlation correction to α is either much too small or in the wrong direction, leading to an overestimate; (ii) γ is significantly overestimated; (iii) the chain length dependence is excessively large, particularly for γ and for the more alternant system; and (iv) the bond length alternation effects on γ are either underestimated or qualitatively incorrect. The poor results with the asymptotically correct van Leeuwen–Baerends XC potential show that the overestimations are not related to the asymptotic behavior of the potential. These failures are described in terms of the separate effects of the exchange and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitive to the polarization charge induced by the external electric field at the chain ends. © 1998 American Institute of Physics.

Notes: We address the points raised by Giordano and Leporini (GL) and show that accounting properly for the nonexponential decay of the rotational correlation function leads to improved agreement with the Stokes–Einstein–Debye (SED) relation above the crossover temperature TC for those probes 3,3′-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL), and perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide) (MOTA) that are well-coupled to the viscous modes of o-terphenyl (OTP) when the average relaxation rate 〈fraction SHAPE="CASE"〉16〈τ〉 is plotted versus 1/T. On the other hand, 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) shows simple Arrhenius behavior in this regime, because of weak coupling to the solvent cage, inconsistent with SED, which was clearly shown in our paper. We also suggest that the difference in chemical structure of the PDT probe, studied by us, compared to 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO), studied by GL, accounts for the difference in the low-temperature relaxation behavior of the two probes. © 1998 American Institute of Physics.

Notes: Predictions for double-step strain flows are presented using a newly proposed reptation theory that accounts for segment connectivity, chain-length breathing, segmental stretch and constraint release in a self-consistent, full-chain theory. In this part of the work emphasis is on double-step shear strains where the second step is reversed and the imposition time of the second strain is earlier than the estimated retraction time, for which the Doi–Edwards model and single-integral models have been found to be incapable of describing experimental trends. Transient stress relaxation properties of two types of reversing flows, types B and C, have been examined and compared to the predictions obtained from the Doi–Edwards model and a single-integral model. The simulations show excellent agreement with the experimental trends based on recent mechanical and optical measurements. © 1998 American Institute of Physics.

Notes: We report on the shear modulus G of colloidal crystals formed from thoroughly deionized suspensions of charged latex spheres. G is measured as a function of particle number density n. Body- and face-centered-cubic (bcc and fcc) crystal structures are observed by simultaneously performed static light scattering, and a broad coexistence region is found between (2.7±0.1)×1018 m−3≤n≤(4.8±0.2)×1018 m−3. Below n=1019 m−3, G closely follows theoretical predictions for both bcc and fcc, while it stays constant throughout the transition region. Above n=1019 m−3, G still observes the predicted n-dependence but with values larger than expected. While in that region, an upper bound of particles per crystallite is estimated from scattering data to be on the order of 104; the abrupt change in G cannot be solely attributed to the gradual morphological transition from polycrystalline to nanocrystalline materials. © 1998 American Institute of Physics.

Notes: A one-electron model for the photodissociation and recombination dynamics of a diatomic anion has been proposed. The main purpose of the study is to provide a simple model to better understand the effect of strong coupling between the solvent polarization and the extra charge in a system like I2−. The model diatomic anion consists of two identical nuclei and an extra electron whose dynamics are treated explicitly. The effect of solvent polarization is modeled by introducing an effective solvent field representing a nonequilibrium solvent configuration. Nonadiabatic theoretical calculations, in which the electronic and the nuclear dynamics are treated simultaneously, can reveal the importance of nonadiabatic effects by including intrinsically all the electronic states involved. It is found that a purely dissociative excited state can support recombination due to coupling of the anomalous charge distribution with the solvent polarization. The charge switching and the subsequent charge separation for the dissociating fragments are strongly coupled with the fluctuating solvent polarization, as represented by the time-dependent solvent field in the present model. The results of the calculations with varying time scales for the solvent response have demonstrated the possibility of numerous diverse phenomena resulting from nonadiabatic transitions. In particular, we found charge transfer induced by changing solvent polarization. The general model presented in the study provides a reasonable interpretation, at least on a qualitative level, for the interesting features obtained from recent experiments and nonadiabatic molecular dynamics studies on the photodissociation of I2− in molecular clusters. © 1998 American Institute of Physics.

Notes: The interfacial density profiles, pressure tensor, and Tolman-length δT are determined for the critical droplets of nonane in the framework of the semiempirical van der Waals/Cahn–Hilliard theory. It is shown that the Tolman's length is strongly size dependent, and tends to a negative value for large drops; indicating that Tolman's approximation for the curvature dependence of the interfacial free-energy γ(approximate)γ∞/(1+2δT,eq/R) based on the equilibrium values δT,eq, and γ∞, is inaccurate for typical liquid nuclei. © 1998 American Institute of Physics.

Notes: Expressions for the analytic energy gradients and the nonadiabatic derivative couplings are derived for the effective valence shell Hamiltonian theory (a variant of degenerate/quasidegenerate many-body perturbation theory) using the diagonal and off-diagonal Hellmann–Feynman formulas and a generalized set of coupled perturbed Hartree–Fock equations to evaluate the derivatives of the molecular orbitals. The method is designed for efficiently treating the energy derivatives and nonadiabatic couplings for several states simultaneously. The generalized coupled perturbed Hartree–Fock equations arise because the reference space orbitals are optimized for simultaneously describing the ground and excited states, a feature lost with the traditional partitioning where the virtual orbitals provide a poor choice for representing the low lying states. A simple correspondence emerges between the new generalized coupled perturbed Hartree–Fock and the traditional coupled-perturbed Hartree–Fock methods enabling the use of the former with straightforward modifications. The derivatives of the second and higher order portions of the effective Hamiltonian are readily obtained using a diagrammatic representation that will be described elsewhere. © 1998 American Institute of Physics.

Notes: An ab initio potential energy surface for the quartet electronic state of NH3+ has been constructed at the MP2/6-31G(d,p) level of theory. The accuracy of this surface has been verified by comparison with high levels of theory. Classical simulations of the collision of N(4S) and H3+(1A1′) showed no reaction to form NH2++H at thermal energies. The possibility of reaction via surface hopping to the doublet electronic state has been investigated by calculation of the quartet–doublet energy gap at the MRCI/6-311+G(2df,p) level of theory. No evidence of surface crossing could be found for configurations accessible at thermal energies. © 1998 American Institute of Physics.

Notes: Potential energy curves of the ground and excited states for the dissociation of the Rydberg NH4 radical into (NH2+H2) have been calculated using ab initio Hartree–Fock and singly and doubly excited configuration interaction methods with a large basis set including Rydberg basis functions. The ground potential curve (2A1) of the (NH4+)(e−)3s radical adiabatically correlates to the [NH2*(A˜ 2A1)+H2(X˜ 1Σg+)] asymptote, while the first excited state (2T2) of (NH4+)(e−)3p correlates to [NH2(X˜ 2B1)+H2(X˜ 1Σg+)]. Two diabatic valence curves emerging from the [NH2*(A˜ 2A1)+H2(X˜ 1Σg+)] and [NH2(X˜ 2B1)+H2(X˜ 1Σg+)] asymptotes are repulsively represented, while two diabatic curves from [NH2+(A˜ 1A1)+H2−(X˜ 2Σu+)] and [NH2+(X˜ 3B1)+H2−(X˜2 Σu+)] are attractively represented. At shorter than R(NH)(similar, equals)2.0 Å, the avoided curve crossings between the dissociative diabatic states of the [(NH4+)(e−)Rydberg] radical and the repulsive diabatic states emerging from the antibonding interactions of the [NH2+H2(X˜ 1Σg+)] asymptote are found mainly. While, at larger than R(NH)(similar, equals)2.0 Å, the avoided curve crossings between the repulsive diabatic states emerging from H2 and the Rydberg states of NH2 and the attractive diabatic states from [NH2+(A˜ 1A1)+H2−(X˜ 2Σu+)] and [NH2+(X˜ 3B1)+H2−(X˜ 2Σu+)] are found. © 1998 American Institute of Physics.

Notes: The recently proposed (explicitly correlated) r12-MR-CI and r12-MR-ACPF (averaged coupled-pair functional) methods are applied to the computation of the clamped-nuclei nonrelativistic ground-state energies of the first-row atoms and their positive ions. For the neutral atoms we obtain accuracies of −0.05 (He and Li), −0.013 (Be), +0.12 (B), −0.1 (C and N), +0.3 (O) and +0.6 (F and Ne) mEh. Our energies of B–F are by far the best available. In all cases, the energy eigenvalues of the Schrödinger equation are calculated to better than chemical accuracy (1 kcal/mol). Since our method is completely general, this, for the first time, implies the possibility of performing quantum chemical calculations of general many-electron systems where the error of the computed energy is not any more very large compared to the desired accuracy. © 1998 American Institute of Physics.

Notes: Dielectric measurements of water mixtures of ethylene glycol oligomer (EGO) with 1–6 repeat units were carried out in the frequency range of 100 MHz–30 GHz at 25 °C. One relaxation process due to water and EGO was observed for each mixture. If the number of repeat units of EGO is larger than three, the water mixtures show a broad and symmetric relaxation curve. On the other hand, if the number of repeat units of EGO is two or less, the mixtures show a broad and asymmetric relaxation curve. The two types of relaxation curves observed in the EGO–water mixtures reflect the size of the EGO molecule. The asymmetric relaxation curve is due to the cooperative motion of water and EGO molecules in the EGO–water cluster for smaller EGO–water mixtures. In contrast, the symmetric dielectric relaxation curve is a result of the variation of local structure in larger EGO–water mixtures. The larger EGO molecules cannot move cooperatively and behave as a geometrical constraint to the motion of water clusters. © 1998 American Institute of Physics.

Notes: The thermal spin transition in the diluted mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x=0.00025, (6-mepy)3tren=tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔHHL0 and ΔSHL0 of 1551(50) cm−1 and 7.5(5) cm−1/K and a molecular volume of reaction, ΔVHL0, of 22(2) Å3 result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)3tren](PF6)2 from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm−1 and an interaction constant, Γ, of 109(15) cm−1. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn1−xFex(6-mepy)3tren](PF6)2 and the LS system [Zn1−xFex(py)3tren](PF6)2 in the pressure range between 1 bar and 1 kbar are presented. Above (approximate)100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in [Zn1−xFex(py)3tren](PF6)2 is more than three orders of magnitude larger than the one in [Zn1−xFex(6-mepy)3tren](PF6)2. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation. © 1998 American Institute of Physics.

Notes: We report a numerical study of homogeneous gas–liquid nucleation in a binary mixture. We study the size and the composition of the critical nucleus as a function of the composition and supersaturation of the vapor. As we make the (Lennard-Jones) mixture increasingly nonideal, we find that there is a regime where the critical nucleus is still miscible in all proportions, even though the bulk liquid phase is not. When these critical nuclei grow, their composition "bifurcates" to approach the value of one of the two bulk phases. For more strongly nonideal mixtures, the two species in the critical nucleus are no longer completely miscible: we observe droplets that are either rich in one species, or in the other. However, we do not find evidence for phase separation inside the critical nucleus—a scenario suggested by Talanquer and Oxtoby [J. Chem. Phys. 104, 1993 (1996)]. In fact, our simulations show that such demixed clusters have a higher free energy than critical nuclei that have an asymmetric composition. © 1998 American Institute of Physics.

Notes: We present the results of experimental studies of light scattering in the isotropic phase of two mesogens, p-n-pentyl-p′-cyanobiphenyl (5CB) and p-n-pentyl-(p′-cyanophenyl)-cyclohexane (5PCH) and in their solutions in CCl4, a globular molecules solvent. Integral intensities of various Raman bands and Rayleigh wing polarized spectra were studied. Dilution experiments have shown that in mesogen systems, due to molecular interactions, intensity of light scattering may be substantially (about 30%) reduced. Isotropic component of the scattered light is influenced by orientation-translational molecular correlations, while for anisotropic component pure orientational molecular correlations are also important. As a result, depolarization ratio in the condensed state may be substantially different from that for the isolated molecule and may vary through the mesomorphic phase transitions. © 1998 American Institute of Physics.

Notes: Polar solvation dynamics of water sequestered inside Aerosol OT (AOT) reverse micelles have been investigated as a function of the surfactant countercation, specifically replacing Na+ for K+ and Ca2+. For Ca-AOT reverse micelles, the solvation dynamics for the smallest micelles probed occurs on a subnanosecond time scale. The K-AOT reverse micelles display an additional ultrafast component that is attributable to bulklike water motion. As previously reported for Na-AOT reverse micelles [Riter, Willard, and Levinger, J. Phys. Chem. B 102, 2705 (1998)], solvent mobility increases with increasing micellar size for both Ca-AOT and K-AOT reverse micelles. The solvation dynamics in strongly ionic aqueous solutions of Ca2+ and K+ have also been investigated. The 10 M electrolyte solutions display water motion on significantly shorter time scales with substantial ultrafast components. These results show that the micellar interfacial structure plays a significant role in immobilizing intramicellar water and that solvent immobilization in the reverse micelles is not merely a result of solvent–ion interactions. © 1998 American Institute of Physics.

Notes: A variable size nanoscale particle sieve with openings between 2 and 50 nm has been used for determining, selecting, and manipulating the size of large liquid helium clusters in the range from 7×103 to 3×106 atoms. The variable openings of the sieve are obtained by rotating a nanofabricated free-standing transmission grating with a 100 nm period and 50 nm wide slits around an axis parallel to the slits. The new nano-sieve can be applied to clusters of various species as well as to large molecules with sizes down to 2 nm. © 1998 American Institute of Physics.

Notes: A method to excite multiquantum transitions in a two-level system by application of continuous irradiation with a four-frequency microwave field is presented. The generated signal is composed of many intermodulation sidebands at different frequencies, which arise from the multiquantum transitions. These processes involve an odd number of photons, that are absorbed from, or emitted to two, three, or four fields. The correspondence between the frequency spectrum of the signal and possible pathways of multiquantum processes is found with recourse to the many mode Floquet theory. The reported experiments demonstrate the multiquantum origin of the detected intermodulation sidebands and their properties. © 1998 American Institute of Physics.

Notes: Monoiodogermylene has been detected for the first time using pulsed discharge and laserinduced fluorescence techniques. HGeI and DGeI were produced by an electric discharge through argon seeded with H3GeI or D3GeI. Although the vibronic structure in the spectra was very limited, all three excited state vibrational frequencies have been obtained for both isotopomers. Analysis of the partially resolved rotational structure of the 000 bands gave the following approximate r0 structures, with the bond angles constrained to previous ab initio values: r0″(Ge–I)=2.525(10) Å, r0″(H–Ge)=1.593(15) Å, θ0″(HGeI)=93.5°, r0′(Ge–I)=2.515(10) Å, r0′(H–Ge)=1.618(15) Å, and θ0′(HGeI)=116.2°. The fluorescence lifetime of monoiodogermylene in the lowest rovibronic levels is 1.515±0.004 μs and shows significant variations on deuteration and with rotational and vibrational level. © 1998 American Institute of Physics.

Notes: A general coordinate transformation is used to derive smooth exterior scaling (SES). Different complex paths are discussed and it is also shown how to derive a complex absorbing potential (CAP) from the SES. Accurate resonance values are computed both for short range and long range potentials. It is shown that the SES absorbs outgoing flux very effectively. The approximation of not scaling the potential and its relation to CAPs is discussed. It is emphasized that the SES can be implemented as easy as CAPs for grid methods. © 1998 American Institute of Physics.

Notes: We have modified a Dirac–Fock program package, MOLFDIR [L. Visscher et al., Comput. Phys. Commun. 81, 120 (1994)], to perform two-component molecular spinor calculations based upon the relativistic effective core potentials (REPs) incorporating effective spin–orbit operators. The modified MOLFDIR can be used to perform two-component REP calculations instead of four-component all-electron calculations at various levels of theory including the configuration interaction (CI) level. As a test case for the multireference CI methods, the low-lying states of Tl2 are calculated with the two-component CI method and the more conventional spin–orbit CI method. Results indicate that the two-component method has some advantages in describing the ground state of Tl2 while excited states are similarly described by both methods. © 1998 American Institute of Physics.

Notes: We have performed a systematic ground state geometry search for the singly charged Sin cations in the medium-size range (n≤20) using density functional theory in the local density approximation (LDA) and generalized gradient approximation (GGA). The structures resulting for n≤18 generally follow the prolate "stacked Si9 tricapped trigonal prism" pattern recently established for the lowest energy geometries of neutral silicon clusters in this size range. However, the global minima of Sin and Sin+ for n=6, 8, 11, 12, and 13 differ significantly in their details. For Si19 and Si20 neutrals and cations, GGA renders the prolate stacks practically isoenergetic with the near-spherical structures that are global minima in LDA. The mobilities in He gas evaluated for all lowest energy Sin+ geometries using the trajectory method agree with the experiment, except for n=18 where the second lowest isomer fits the measurements. The effect of gradient corrections for either the neutral or cationic clusters is subtle, but their inclusion proves to be critical for obtaining agreement with the mobility measurements in the n=15–20 range. We have also determined ionization potentials for our Sin neutral geometries and found that all experimental size-dependent trends are reproduced for n≤19. This particularly supports our structural assignments for Si9, Si11, Si12, and Si17 neutrals. The good overall agreement between the measured and calculated properties supports the elucidation of the "prolate" family of silicon clusters as stacks of trigonal prisms. © 1998 American Institute of Physics.

Notes: Using the diffusion quantum Monte Carlo (DMC) method with importance sampling, we calculate the binding energy and annihilation rate of positronium hydride, PsH. We get 1.0661±0.0014 eV for the binding energy, and 2.463±0.020 ns−1 for the annihilation rate. The binding energy is in agreement with the results of the best Ritz variational calculations and the best DMC calculation reported to date, and the annihilation rate agrees with the results of the Ritz variational calculations but not that of the only other DMC result known to us. A new method for correcting expectation values calculated from mixed estimators is proposed and demonstrated. © 1998 American Institute of Physics.

Notes: The electron-spin magnetic moments of O3−, O3Li, and O3Na, as parametrized by the g factors, are studied at the uncorrelated restricted open Hartree–Fock (ROHF) and correlated multireference configuration interaction (MRCI) ab initio levels. The present method, which uses a perturbative approach complete to second order, is based on a Breit–Pauli Hamiltonian. The calculated Δg values, with Δg=g−ge, are very similar for all three species, confirming that the O3− moiety is retained in the ozonides O3M. In the standard C2v notation, Δgyy〉Δgzz(very-much-greater-than)|Δgxx|. The perpendicular component Δgxx is small and negative, while the in-plane components Δgyy and Δgzz are large and positive. The MRCI results for Δgxx, Δgyy, Δgzz (in ppm) are: −475, 16 673, 10 121 for O3−; −679, 13 894, 9308 for O3Li; and −494, 12 298, 8690 for O3Na. The ROHF values of Δgyy and Δgzz are smaller than the MRCI data, due to a general overestimation of the excitation energies. The MRCI Δg values for isolated O3− reproduce the experimental results for O3− trapped in crystals or adsorbed on MgO surfaces (in ppm, −500–1200 for Δgxx, 12 400–16 400 for Δgyy, and 6900–10 000 for Δgzz). For O3Na, the experimental Δgxx, Δgyy, Δgzz data (−100, 14 200, 9800 ppm) are again satisfactorily described by our correlated results. No experimental g shifts are available for O3Li. In all systems studied here, the Δgxx component is dictated by first-order terms (ground state expectation values); Δgyy is governed by the second-order magnetic coupling between X 2B1 and 1 2A1 [electron excitation from the highest a1 molecular orbital (MO) into the b1(π*) singly occupied MO]; and Δgzz, by the coupling with two 2B2 states (excitations from the two highest b2 MOs into π*). © 1998 American Institute of Physics.

Notes: Femtosecond transient absorption studies of I2–arene complexes, with arene=hexamethylbenzene (HMB), mesitylene (MST), or m-xylene (mX), are used to investigate the effect of solvent–solute attractive forces upon the rate of vibrational relaxation in solution. Comparison of measurements on I2–MST complexes in neat mesitylene and I2–MST complexes diluted in carbontetrachloride demonstrate that binary solvent–solute attractive forces control the rate of vibrational relaxation in this prototypical model of diatomic vibrational relaxation. The data obtained for different arenes demonstrate that the rate of I2 relaxation increases with the magnitude of the I2–arene attractive interaction. I2–HMB relaxes much faster than I2 in MST or mX. The results of these experiments are discussed in terms of both isolated binary collision and instantaneous normal mode models for vibrational relaxation. © 1998 American Institute of Physics.

Notes: The concentration and the temperature dependencies of 1H and 13C chemical shifts in NMR of aqueous acetone mixtures were studied, together with the concentration dependence of the frequency of the C–H stretching vibration of acetone in IR spectra. 1H and 13C chemical shifts were measured at 1 °C, 23 °C, and 48 °C by the external double reference method using a capillary with a blown-out sphere at the bottom for tetramethylsilane as the external reference substance. By this method, it is possible to determine the volume magnetic susceptibility of a sample solution at each temperature, for which the observed chemical shifts may be corrected exactly. Thus, we revealed the detailed electronic polarization in acetone as well as water as functions of concentration and temperature. On diluting acetone with water, the chemical shift of water protons, δH2O, is 3 ppm at the mole fraction of water Xw=0,05 and increases to the value for pure water, ca. 5 ppm, at Xw=0.96, with increasing Xw. In the region of Xw〉0.96, δH2O is slightly larger than the value, indicating the presence of more polarized water species than pure water. The chemical shifts of C–H proton, δCH(underbar)3, and C–H carbon, δC(underbar)H3, also increase slightly with increasing Xw up to Xw=0.96. The frequency for the C–H vibration of acetone, νC–H, increases from the value for pure acetone, 3005 cm−1, to 3013 cm−1 at Xw=0.96, while it decreases sharply with further increase in Xw. These results of IR and NMR measurements show that the hydration of acetone accompanies electronic redistribution in the C–H bonds in cooperated with the change in the polarization of the surrounding water molecules, and that two different types of hydration of acetone are predominant in different concentration regions, Xw〈0.96 and Xw〉0.96. In the region of Xw〈0.96, the results can be explained satisfactorily if we consider that a part of the electron about the C–H proton is pushed out into the C–H bond due to a repulsive interaction between the C–H hydrogen and water oxygen. In the region of Xw〉0.96, we can interpret the results well by considering that the pushing by the water oxygen becomes strong enough to induce the polarization of the C–H bonds compared to the pushing at Xw≤0.96. Since the polarization of the C–H bond was found to increase with decreasing temperature, the repulsive interaction seems to have the property of hydrogen bonding and to be denoted as C–H(centered ellipsis)OH2((centered ellipsis)OH2)n, where OH2((centered ellipsis)OH2)n expresses water molecules hydrogen-bonded cooperatively and responsible for the more polarized water than pure water. The ratio of water to acetone seems to be a predominant factor to cause the transition of the hydration state from the repulsive interaction to hydrophobic hydration of acetone. © 1998 American Institute of Physics.

Notes: Two, previously unobserved, low-frequency vibrational modes of c(4×2) CO/Pt(111), measured using high-resolution helium atom scattering, are reported. The modes, at frequencies of (h-dash-bar)ω=8 meV and (h-dash-bar)ω=10 meV, are nearly dispersionless, similar to the previously known low-frequency mode at (h-dash-bar)ω=7 meV, which was also observed in the present measurements. From the intensity dependence on wave vector and relative intensity to the (h-dash-bar)ω(approximate)7 meV mode, we suggest that the new modes are also polarized parallel to the Pt(111) surface and, further, that they are associated with the bridge bonded CO molecules. © 1998 American Institute of Physics.

Notes: We present the results of nonequilibrium molecular dynamics (NEMD) studies of self-associating polymer systems composed of flexible telechelic chains with associating end-groups ("stickers"). Formation of micellar aggregates, their structure and structural characteristics of associative polymer network (micellar gel) are studied under the influence of external shearing forces. When the association energy εc is quite large (εc≥εc*, where εc* is a critical association energy corresponding to the gelation transition at rest), the spatial organization of the system as a whole is characterized by a typical network architecture with bridging chains connecting different micellar aggregates. The shearing forces cause only a slight perturbation in the structural properties of the sol (at εc〈εc*). However, when the association energy εc becomes quite large (εc(approximately-greater-than)εc*), we observe sharp structural variations as the shear force is increased. At sufficiently strong attraction between stickers, the shear flow facilitates development of the aggregation process: even at rather weak shearing forces, the cluster-size distribution functions become considerably wider when the association energy is close to εc*. Thus, aggregation processes become considerably more pronounced under shear, as compared to the same system at rest. As the shear force increases, the content of looplike chains decreases while the fraction of bridgelike chains rises. This process is accompanied by stretching of the chains. However, under a condition of high shear, many chains belonging to the associative network tend to be coiled up. This leads to the redistribution of structural elements in favor of loop chains. This exchange process can decrease the internal stress in the associative network which is mainly determined by the number and spatial topology of elastically active bridgelike chains.© 1998 American Institute of Physics.

Notes: A semiempirical model is developed, based on ab initio calculations, to provide an analytic representation of excited-state potential energy surfaces for (H2O)n, n=2–6. Using quantum calculations of the ground vibrational states, the UV absorption spectra are computed by a semiclassical approximation, showing a strong blue-shift with extended blue tails relative to the monomer, but with an additional red tail in the case of the dimer absorption band. The nature of the excitonic states is discussed. © 1998 American Institute of Physics.

Notes: The first three-dimensional (3D) simulation of meso-phase formation in a specific polymer system—55% aqueous solution of the triblock polymer surfactant (EO)13(PO)30(EO)13—under simple steady shear is performed. The method is based on dynamic mean-field density functional theory. The hexagonal phase is investigated. The simulations reproduce recent experimental observations on the same polymer system. © 1998 American Institute of Physics.

Notes: This paper explores the quantum fluid dynamical (QFD) representation of the time-dependent Schrödinger equation for the motion of a wave packet in a high dimensional space. A novel alternating direction technique is utilized to single out each of the many dimensions in the QFD equations. This technique is used to solve the continuity equation for the density and the equation for the convection of the flux for the quantum particle. The ability of the present scheme to efficiently and accurately describe the dynamics of a quantum particle is demonstrated in four dimensions where analytical results are known. We also apply the technique to the photodissociation of NOCl and NO2 where the systems are reduced to two coordinates by freezing the angular variable at its equilibrium value. © 1998 American Institute of Physics.

Notes: Ion imaging is employed to analyze the fragments from timed Coulomb explosion studies of femtosecond (fs) molecular dynamics. The technique provides high detection efficiency and direct recording of the two-dimensional velocity of all ionized fragments. We illustrate the approach by studying photodissociation of iodine initiated by a weak femtosecond pump pulse and probed by an intense, delayed femtosecond pulse. The ion images, recorded as a function of the pump-probe delay, reveal the evolution of the internuclear separation of the dissociating I2 molecule. The experimental results are in good agreement with quantum mechanical wave packet simulations. We discuss the perspectives for extending the studies to photochemical reactions of small polyatomic molecules. © 1998 American Institute of Physics.

Notes: A classic example of a barrierless reaction, CH3+H→CH4 is used as a model to test the extent of nonadiabatic coupling on the reaction rate constant. This coupling has two contributions. The first arises from the anisotropy of the CH3+H potential and the second is Coriolis coupling. A method is presented which calculates adiabats formally equivalent to those calculated by statistical adiabatic channel model (SACM) while still permitting the determination and inclusion of nonadiabatic coupling. Using the discrete variable representation (DVR) for the interfragment distance R, the ro-vibrational Hamiltonian is solved at particular R values. The eigenvalues and eigenfunctions are calculated at each of these R values to create the surface and the coupling elements used in a wave packet propagation. The dynamics of the reaction are investigated through a study of the cumulative reaction probability N(E,J) using energy resolved flux methods. We find that for J=0, 1, and 2, neglecting the coupling due to the changing anisotropy as a function of R results in a 20% error in N(E,J). Neglecting the Coriolis coupling results in average errors of 2% lending support to the helicity-conserving approximation. Finally, within the adiabatic approximation, the calculated adiabats provide a more realistic view of the barriers than the analytic functions of SACM, require no fitting parameter, and are obtained at reasonable computational cost. © 1998 American Institute of Physics.