bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational predissociation of 9,10-dichloroanthracene—Mixed and homo rare gas atom clusters
Authors: Penner Abraham , Amirav Aviv
Published: July 2002

Clusters of the form DCA(R1R2), where DCA=dichloroanthracene and R1, R2=Ar, Kr, and Xe were synthesized in a supersonic molecular beam. The mixed clusters were efficiently formed by the coexpansion of DCA in a mixture of two different rare gases with the heavier rare gas being in the minority. The clusters’ vibrational predissociation dynamics was probed using a nanosecond excimer pumped dye laser spectroscopy and energy resolved emission diagnostics. DCA was chosen for this study because of its high emission quantum yield and relatively few spectral interferences at high vibrational energy. The emission quantum yield of DCA–rare gas atom complexes was found to be unity at the electronic origin. The emission quantum yield is greatly reduced upon the increase of the vibrational energy being 0.052 at the 1390 cm−1 vibrational level. It was more than three times higher in the DCA–rare gas atom clusters at the 1390 cm−1 vibration, in comparison with that of the bare molecule, due to vibrational predissociation. The vibrational predissociation products have been identified using energy resolved emission. At 1390 cm−1, excess vibrational energy two argon atoms or one xenon atom could dissociate. For DCA(Kr)n, it is not clear whether one or two krypton atoms had dissociated. In DCA(XeAr) or DCA(XeKr) excited to the 1390 cm−1 vibration, either one of the two rare gas atoms could dissociate, but not both of them.

The results indicate that predominantly the weakest bound rare gas atom dissociates, although its vibrational modes seems less effectively coupled to the excited skeleton modes. The dissociation rates were determined by the relative emission intensity before and after the dissociation, which could be spectrally identified. The time scales for vibrational predissociation of all the various DCA clusters were found to be about 1 ns, independent of the rare gas identity. The results are interpreted by assuming the excitation of a vibrationally mixed 1390 cm−1 state which undergoes a secondary intramolecular vibrational energy redistribution (IVR) within the DCA chromophore to a combination mode which contains a low lying promoting vibrational character. This secondary IVR is the ‘‘bottleneck’’ precursor process whose time scale is intramolecular, being independent of the rare gas atom attached to the DCA. After this secondary IVR, the vibrational energy flows on a much shorter time scale to and between the rare gas atom–DCA vibrational modes, and the weaker bound atom statistically dissociates.
Effect of phonon coupling on hydrogen tunneling rates at gas–surface interfaces
Authors: Wonchoba Steven E. , Truhlar Donald G.
Published: July 2002

The centrifugal-dominant small-curvature semiclassical adiabatic tunneling approximation is used with variational transition state theory to calculate diffusion coefficients for hydrogen, deuterium, and tritium atoms on the (100) face of copper for temperatures in the range 80–1000 K. The system is modeled by the embedded cluster method, and the copper lattice is constructed with a lattice constant optimized for the assumed potential energy function. Calculations are presented with up to 171 nonfixed degrees of freedom. The results are well converged with respect to the number of lattice atoms whose motion is allowed to couple to the adatom. The difference between the diffusion coefficients calculated with rigid and nonrigid lattices at 300 K are 3.7, 3.3, and 3.1 for H, D, and T, respectively, increasing to factors of 24.1, 19.4, and 17.2 at 120 K. The effect levels off for lower temperatures, e.g., the ratio for H is 27.3 at 100 K and 24.4 at 80 K. The convergence with respect to the number of moving copper atoms is nonmonotonic; detailed examination of the intermediate results shows that such nonmonotonicities result from the nonsmooth cancellation of a large number of competing effects attributable to many surface phonon modes. We compare the present results to those predicted by path integral transition state theory and to those predicted by transition state theory with quantum effective potentials, and we find them to be in reasonably good agreement. This is very encouraging since tests of multidimensional semiclassical tunneling approximations have been limited to systems with only a few degrees of freedom in the past.
Influence of retardation on the higher-order multipole dispersion contributions to the helium dimer potential
Authors: Luo Fei , Kim Geunsik , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

In this note, we evaluate the influence of the relativistic retardation corrections on the dipole–quadrupole, dipole–octupole and quadrupole–quadrupole contributions to the He–He dispersion potential, which affect the attractive r −8 and r −10 terms. These higher-order corrections influence the binding energy of 4He2 an order of magnitude less than does the retardation correction to the leading r −6 dipole–dipole term.
Ab initio correlation calculation for metallic lithium
Authors: Heilingbrunner Andrea , Stollhoff Gernot
Published: July 2002

Ab initio correlation calculations for the ground state of metallic lithium are reported. They are based on Hartree–Fock calculations with the program Crystal and are carried out with the local Ansatz. The electrons are found to be weakly correlated. Long range correlations can be well treated in a real space representation, but sizeable corrections to the linearized coupled cluster approximation are expected for the energy. Other quantities such as charge distribution, equilibrium distance, and bulk modulus are given as well.
A molecular dynamics study of the structure of a long chain amphiphile monolayer adsorbed on ice Ih
Authors: Bell Kyle-Pierre , Rice Stuart A.
Published: July 2002

We report the results of molecular dynamics simulations of a monolayers of long chain carboxylic acids and alcohols supported on the 0001 face of ice Ih. The simulations are designed to provide information concerning the influence of the atomic structure of the head group and of the nature of the head group–surface interactions on the collective tilt of the molecules in the monolayer. The results of the simulations show that the structure of the head group is a more important influence on the collective tilt of the molecules than is the specific character of the head group–surface interactions. It is also shown that the constraint imposed by the potential field of the rigid ice Ih 0001 surface renders the ice supported monolayer different from the liquid water supported monolayer. Specifically, the collective tilt of the molecules in the ice supported monolayer is not accompanied by a distortion of the projection of the unit cell of the monolayer in the plane of the surface; that projection remains hexagonal for all tilt angles. In contrast, the introduction of a collective tilt of the molecules in a liquid water supported monolayer always leads to a distortion of the projection of the unit cell in the plane of the surface, typically from hexagonal at zero tilt to centered rectangular for all nonzero tilt. The tilting of the monolayer supported on ice Ih is favored by the molecular spacing induced by the field of the 0001 surface, which is slightly greater than closest packing separation of the molecules. The equilibrium tilt angle appears to be determined primarily by the chain–chain interactions since the surface pressure attained at equilibrium is sensibly independent of the precise value of the tilt angle and of the character of the head group–head group and the head group–surface interactions.
Self-avoiding walks on a simple cubic lattice
Authors: Eizenberg N. , Klafter J.
Published: July 2002

Dynamic Monte Carlo simulations, using the pivot algorithm, have been performed on a three-dimensional simple cubic lattice in order to study statistical properties of self-avoiding walks of lengths up to about 7200 steps. The scaling properties of the end-to-end distribution function and its second moment have been investigated and compared with previous works. The distribution function is found to be in agreement with the enhanced Gaussian scaling form of des Cloizeaux. Some properties of the pivot algorithm, such as the acceptance fraction and the integrated autocorrelation times in three dimensions, are also discussed.
Infrared diode laser spectroscopy of the ν3 fundamental and ν3+ν5−ν5 sequence bands of the C4 radical in a hollow cathode discharge
Authors: Moazzen-Ahmadi N. , Thong J. J. , McKellar A. R. W.
Published: July 2002

The infrared absorption spectrum of the linear C4 radical has been studied in an extension of the original observation of gas-phase C4 by Heath and Saykally [J. Chem. Phys. 94, 3271 (1991)]. The experiment was performed using a flowing mixture of acetylene and helium subjected to a hollow-cathode discharge, which was probed in the 1525–1570 cm−1 spectral region using a tunable diode laser spectrometer. Transitions with N-values up to 60 were measured. Their analysis yielded band origins, rotational, and centrifugal distortion parameters for the lower and upper vibrational states, and l-type doubling parameters for the degenerate bending states ν5 and ν3+ν5. In particular, the ν3 origin was determined to be 1548.6128(4) cm−1, the ground state rotational and centrifugal distortion parameters were B=4979.89(21) MHz and D=0.848(44) kHz, and the l-doubling parameters for ν5 was q5=10.98(13) MHz. This value for q5 was used to estimate the ν5 frequency of gas-phase C4 to be 160±4 cm−1. Both the l=0 and 2 components of the ν3+2ν5−2ν5 sequence band were also tentatively observed, but a detailed analysis was not yet possible. The results were completely consistent with a linear structure for the triplet ground state of C4, and showed no effects of quasilinearity such as that exhibited by C3.
Resonant diffusion of molecules in solids
Authors: Tsekov Roumen , Ruckenstein Eli
Published: July 2002

In the present paper the stochastic dynamics of a hydrocarbon molecule moving in a solid body is examined and a generalized Langevin equation which describes the evolution of the coordinate of its center of mass is derived when the molecule behaves as a linear rigid one. An explicit expression for the friction tensor is obtained which accounts both for the molecule–solid interaction and for the thermal vibrations of the lattice of the solid. The diffusion coefficient D of the hydrocarbon molecule is calculated as a function of the number N of the carbon atoms it contains. D exhibits a periodic dependence on N, with maxima when the length of the molecule is an integer multiple of the length of the repeating unit of the lattice and minima when it is a half integer, phenomenon which could be called resonant diffusion. The results are compared with the experimental data for the diffusion of n-paraffins through zeolite T and a qualitative agreement is observed.
Electronic coupling and conformational barrier crossing of 9,9′-bifluorenyl studied in a supersonic jet
Authors: Smith P. G. , Gnanakaran S. , Kaziska A. J. , Motyka A. L. , Hong S. M. , Hochstrasser R. M. , Topp M. R.
Published: July 2002

Fluorescence excitation, dispersed fluorescence, and picosecond time-resolved fluorescence spectroscopies have been used to study the dimeric molecule 9,9′-bifluorenyl, isolated under supersonic jet conditions. The excitation spectrum reveals a splitting in several principal resonances of the S0→S1 excitation spectrum, which can be attributed to electronic coupling between the two fluorenyl moieties. The splitting, which for different vibronic transitions correlates with Franck–Condon factors, is consistent with an exciton model that includes higher electronic states. The splitting is reasonably well reproduced by a multipole interaction potential. Calculations have verified that the electron exchange coupling is small. Furthermore, the relative intensities of the resonances allow an estimate of the equilibrium geometry, suggesting that the molecular long axes have an angular displacement of 60°–70°, consistent with the geometry found by x-ray diffraction. A most interesting feature of this species is that it is weakly fluorescent in fluid solution, which has been attributed to activated conformational barrier crossing of the excited molecule. In contrast, the fluorescence quantum efficiency of the isolated molecule can be close to unity, the lifetime ranging from 18 to 20 ns at vibrational energies <1400 cm−1. Above this region, the fluorescence decay time decreases steadily, to ≊2 ns by 2550 cm−1, indicating the onset of a nonradiative relaxation channel. Since the molecule was also seen to exhibit vibrational relaxation at low energies (i.e., ≤400 cm−1), the relaxation dynamics observed above 1400 cm−1 reflect the existence of a conformational potential energy barrier in the isolated molecule.
Low temperature inversion of optically detected electron spin resonance spectra due to different rates of electron tunneling for singlet and triplet radical–ion pairs
Authors: Doktorov A. B. , Spirina O. B. , Molin Yu. N.
Published: July 2002

A theoretical model based on the assumption about different rates of electron tunneling from different spin states is suggested to explain the experimentally observed low temperature inversion of OD ESR spectra of radical–ion pairs. Mathematical background of the model based on the conception of the effective tunneling spheres of different radii is given. The model suggested is in excellent agreement with the qualitative picture of OD ESR signal formation at the required conditions. On the basis of the model the inversion point of spectra is found and the behavior of the OD ESR signal intensity in the neighborhood of the inversion point depending on various physical parameters is predicted. The dependencies obtained may serve as a basis for experimental identification of the suggested mechanism.
Calculation of electron spin resonance linewidths for hydrogen atom impurities in solid para-hydrogen
Authors: Li Daohui , Voth Gregory A.
Published: July 2002

The linewidth of the electron spin resonance (ESR) spectra for H impurities trapped in solid para-hydrogen is calculated. The equation for the linewidth, which was originally formulated for F-center problems, is modified to take account of the changes of the molecular hydrogen electronic orbitals, as well as the vibrations and rotations of the H2 molecules. By virtue of constant pressure path integral Monte Carlo simulations, it is found that zero point vibrations make solid hydrogen so compressible that the solid cannot sustain the local stress created by interstitial impurities. As a result, the local environment for an impurity at an interstitial site is the same as that for an impurity at a substitutional site, and the calculated ESR linewidth for a H impurity in either site is the same and in excellent agreement with the experimental observation.
Threshold photodetachment zero-electron kinetic energy spectroscopy of Si−3
Authors: Arnold Caroline C. , Neumark Daniel M.
Published: July 2002

The threshold photodetachment zero-electron kinetic energy (ZEKE) spectrum of Si−3 is presented and discussed. The spectrum shows well-resolved vibrational structure. A comparison with several ab initio calculations shows that the spectrum is due to transitions to the 3A′2 (D3h) state of Si3. The symmetric stretch and degenerate e′ frequencies for the Si3 3A′2 state are determined to be 501±10 and 337±10 cm−1, respectively. Additional structure observed in the spectrum suggests that the negative ion is a fluxional, Jahn–Teller distorted species. The ZEKE spectrum shows no evidence for transitions to the 1A1 state of the neutral, which is predicted to lie very close to the 3A′2 state. A comparison of the ZEKE and previously obtained photoelectron spectrum of Si−3 suggests that these two states are close in energy, but that transitions to the singlet state are very weak in the ZEKE spectrum.
Two isomers of SF•5 and SF+5: Structures and energetics
Authors: Becker Hansjürgen , Hruŝák Jan , Schwarz Helmut , Bohme Diethard K.
Published: July 2002

The apparently conflicting results for the ionization energy of SF•5 and the standard enthalpy of formation of SF+5 reported over the last 25 years have been reinterpreted and brought into line in terms of the existence of square pyramidal (C4v) and trigonal bipyramidal (D3h) isomers of SF•5 and SF+5. New experimental results and theoretical calculations are reported which are consistent with this interpretation. Values are recommended for the ionization energy and standard enthalpies of formation for the two isomers of SF•5 and SF+5.
Photodissociation of ketene: CH2(ã 1A1)(0,0,0) rotational state distributions
Authors: Garcia-Moreno I. , Lovejoy Edward R. , Moore C. Bradley
Published: July 2002

Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from the threshold for production of CH2(ã 1A1)(0,0,0)+CO(X̃ 1Σ+) to 2900 cm−1 above. By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence spectrum is recorded and the 1CH2 product state distributions determined. The appearance thresholds and yield curves of individual 1CH2 rotational states are obtained by scanning the photolysis laser frequency with a fixed 1CH2 probe laser frequency. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. PHOFEX yield curves combined with the distribution data demonstrate that the ketene absorption cross section is constant within 10% over the 0–1000 cm−1 range of the PHOFEX spectra. PHOFEX curves up to 200 cm−1 and rotational state distributions for singlet methylene at 50, 100, and 170 cm−1 above its threshold are in good agreement with phase space theory (PST) calculations. The lowest energy methylene rotational states of + and − symmetry are formed in the ratio predicted by phase space theory at all energies. States of mixed singlet/triplet parentage are populated with equal probability for triplet characters ranging from 30% to 70%. For energies greater than 300 cm−1 above the threshold, the 1CH2 rotational distribution is substantially colder than the statistical distribution given by PST. These differences increase with excess energy above the singlet threshold. In this energy range, the dynamics of motion between the transition state and separated product fragments is in neither the adiabatic nor the statistical limit though the data do suggest that motion may well be adiabatic for molecules fragmenting to yield the lowest energy rotational states of ortho methylene.
Construction principle for stable multiply-negative charged molecular systems. Part II. Triply-negative charged systems
Authors: Scheller M. K. , Cederbaum L. S.
Published: July 2002

Following a recently introduced construction principle, triply-negative charged compounds are designed and investigated. The specific series of systems discussed are M2X3−5 alkali and M2X3−7 earth alkali halides. For the highly ionic members in these series considerable evidence for their stability to fragmentation and to electron autodetachment is obtained at various levels of theory.
Time dependent solution of generalized Zusman model of outersphere electron transfer reactions: Applications to various experimental situations
Authors: Roy Srabani , Bagchi Biman
Published: July 2002

The Zusman model of the environmental effects on the outersphere electron transfer reaction has been widely used to study solvent effects on various important electron transfer reactions. We present here a generalized treatment of the Zusman model using a powerful Green’s function technique. This generalization enables us to obtain the time dependent solution of the model for various complicated situations often encountered in experiments. In addition, the present formulation allows for a unified description of the barrierless and the high barrier reactions for both the nonadiabatic and the weakly adiabatic limits of electron transfer reactions. A merit of the present description is that one need not assume an initial equilibrium population of the reactants and therefore, this method is particularly suitable for the treatment of photoelectron transfer reactions. The following four model situations have been studied. (a) Ground state, symmetric, and asymmetric electron transfer reactions. The reactant surface population exhibits a markedly nonexponential decay. (b) A low barrier, photoelectron transfer reaction where an additional photochemical reaction occurs from the charge transfer state. In this case, the decay of the reactant population is largely exponential when the funnel is efficient. (c) A high barrier photoelectron transfer reaction with a radiative decay from either or both of the reactant and product surfaces. In specific situations, the radiative decay can even compete with the electron transfer to give rise to a fractional dependence of the fluorescence quantum yield (of the locally excited state) on the average solvation time. (d) A multichannel nonradiative decay of population from an excited state surface having multiple sinks. Here an interesting nonlinear interaction between the sinks is observed. In all these cases, it is found that the average rate of electron transfer can be considerably different from the long time rate. The generality of the present formulation has been demonstrated by deriving the rate expressions of the theory of outersphere electron transfer reaction due to Zusman, that due to Rips and Jortner and also that due to Sumi, Nadler, and Marcus.
The f Rydberg series in the absorption spectrum of N2
Authors: Huber K. P. , Jungen Ch. , Yoshino K. , Ito K. , Stark G.
Published: July 2002

The nf Rydberg levels of 14N2 converging to the X 2Σ+g ground state of N+2 have been studied from n=4–9 in the high-resolution absorption spectra of supersonically expanding nitrogen at wavelengths ranging from 843.2 to 802.6 Å (118 600–124 600 cm−1). The best experimental results, achieving rotational temperatures on the order of 20 to 40 K and a resolution of 0.5 cm−1, were obtained by photographing the jet absorption against the background continuum from a synchrotron radiation source. Complementary data for 14N2 and 15N2 come from the analyses of spectra recorded under equilibrium conditions at 70 K with a resolution of 1 cm−1, using the He continuum as background source. The observations are interpreted with the help of multichannel quantum defect calculations and lead to the conclusion that, to varying degrees and with the exception of 4f(v=0), all the complexes studied here show the effects of interactions with core excited d and s Rydberg levels built on the A 2Πu first excited state of N+2. Also, the 9f(v=0) complex is shown to be perturbed by 10p(v=0), and the strength of the p–f interaction has been determined.
Monte Carlo simulations and kinetic theory of homogeneous and heterogeneous adsorption with lateral interactions
Authors: Cortés Joaquin , Valencia Eliana , Araya Paulo
Published: July 2002

Using the kinetic form of the Monte Carlo (MC) simulation method, a comparison has been made between the results that predict the phenomenological equations for adsorption kinetics and MC experiments in systems with and without lateral interactions on homogeneous and heterogeneous surfaces with random topography and varying degrees of surface heterogeneity. A very good correspondence exists between the theoretical results and those obtained from the MC experiments.
Energy activation of adiabatic and nonadiabatic electron transfer
Authors: Burshtein A. I. , Georgievski Yu.
Published: July 2002

Low-level and two-level approaches to above-barrier reactions are used to establish the borders between adiabatic and nonadiabatic electron transfer to either stable or unstable products. The multiple curve-crossing probability is equal to unity in a narrow ‘‘energy window,’’ just above the barrier, and slowly decreases with energy over it. When the reactants are the Langevin oscillators and the energy dispersion per period is less than the width of energy window, Mel’nikov’s one-level theory is appropriate. Otherwise (at higher friction) it must be replaced by the two-level nonadiabatic theory of Burshtein and Zharikov. The transition from an energy diffusion regime to transition state theory (TST) or perturbation theory results occurs within the Langevin model at much lower friction than within the ‘‘strong collision model’’ considered previously.
On the factors determining the isomers of metal clusters
Authors: Yang Liqiu , DePristo Andrew E.
Published: July 2002

Two similar semiempirical interatomic potentials were used to study 13-atom Pt clusters. One predicts very compact icosahedral structure with high symmetry while the other predicts open and fluxional structures with low symmetries. This can be understood in terms of the low-coordination atomic binding energies produced by the potentials.
An ab initio study of the structure and infrared spectrum of Si2C3
Authors: Rittby C. M. L.
Published: July 2002

The geometry and vibrational spectrum of the previously not studied silicon–carbon cluster Si2C3 has been investigated. Geometries and frequencies for a number of isomeric structures are presented at the Hartree–Fock level. In agreement with a concurrent experimental observation the ground state is found to be linear. Harmonic frequencies, isotopic shifts, and infrared intensities calculated using many-body perturbation theory to second order are found to be in good agreement with the experimental results supporting the identification of a new penta-atomic silicon–carbon cluster. The general behavior of penta-atomic silicon–carbon clusters is discussed and preliminary ground state geometries and harmonic frequencies are presented for Si3C2 and Si4C.
Vibrational spectra of penta-atomic silicon–carbon clusters. II. Linear Si2C3
Authors: Presilla-Márquez J. D. , Graham W. R. M.
Published: July 2002

Fourier transform infrared matrix measurements carried out in conjunction with ab initio calculations reported in a companion paper by Rittby have resulted in the first identification of two fundamental vibrations, the C=C stretching mode ν3(σu)=1955.2 cm−1, and the Si–C stretching mode ν4(σu)=898.9 cm−1, of the SiC3Si cluster formed by trapping the products of the vaporization of silicon/carbon mixtures in Ar at 10 K. The observed frequencies, relative intensities, and 13C, 29Si, and 30Si isotopic shifts for the ν3 and ν4 vibrations are in good agreement with the results of the ab initio calculations at the second-order many-body perturbation theory [MBPT(2)] level which predict a linear centrosymmetric geometry for the ground state of SiC3Si. The results of force constant adjustment calculations are consistent with the proposed vibrational assignments and structure.
Theoretical study of the effect of substituent and backbone conformation on the electronic properties of symmetrically substituted poly(di-n-alkylsilanes)
Authors: Crespo Raúl , Piqueras Mari Carmen , Tomás Francisco
Published: July 2002

We present the results of ab initio 3-21G* geometry optimizations and valence effective Hamiltonian (VEH) band structure calculations aimed at determining the evolution of the geometric and electronic (ionization potential, electron affinities, and band gaps) properties of all-trans poly(dimethylsilane), poly(diethylsilane), poly(di-n-propylsilane), and poly(di-n-butylsilane) when increasing the size of the alkyl group. In the latter polymer, we have also studied the 7/3 conformation, in order to analyze the effect of the backbone conformation on the geometric and electronic structure. The VEH ionization potentials of all-trans poly(di-n-alkylsilanes) are almost equal, and as experimental photoemission data show, only slight differences are appreciated. The band gap decreases in going from poly(di-n-butylsilane) to poly(di-n-propylsilane) and to poly(diethylsilane), and increases when passing to poly(dimethylsilane), which coincides with experimental evidences on poly(di-n-alkylsilanes). The change from all-trans to 7/3 conformation of poly(di-n-butylsilane) implies an increase of both, ionization potential and band gap, in perfect agreement with experimental photoemission and absorption data. The applicability of VEH to deal with poly(di-n-alkylsilanes) is discussed.
A lattice dynamics model of lipid bilayer systems
Authors: van der Sijs Dick A. , Levine Yehudi K.
Published: July 2002

A lattice dynamics model study of the orientational order and dynamics of a lipid bilayer based on the Monte Carlo dynamics technique is presented. The underlying premise of the method is that the conformational dynamics of the hydrocarbon chains can be described as the superposition of local structural rearrangements involving small chain segments. Although these simulations only take into account intramolecular conformational energy and excluded volume interactions, they reproduce the order parameter and mobility gradients along the length of the lipid molecules and the electron density profile of the bilayer. It is shown that this approach provides a useful and efficient approach for simulating the effects of cholesterol on the dynamic behavior of the lipid molecules. Moreover, the simulations also provide insight into the orientational motions of the cholesterol molecules in the bilayer system.
Dissociative adsorption of H2 on Cu(110): A mixed quantum-classical study
Authors: Kumar Sudershan , Jackson Bret
Published: July 2002

A mixed quantum-classical approach is used to study the dissociative sticking of H2 on Cu(110). The method includes all six molecular degrees of freedom: three quantum mechanically and three classically. Dissociation probabilities are computed as a function of translational energy for several rotational and vibrational states. These results are averaged together to compute total dissociative sticking probabilities for both hot nozzle and seeded beam conditions. The contributions of various types of motion to dissociation are discussed. The nature of rotational orientation during dissociation is also examined.
Molecular beam optical Stark spectroscopy of CaSH
Authors: Scurlock C. T. , Henderson T. , Bosely S. , Jung K. Y. , Steimle T. C.
Published: July 2002

The rotationally resolved spectrum of the (000–000) B̃ 2A″–X̃ 2A′ band system [T000–000=15 859.4638(15) cm−1] of a supersonic molecular beam sample of calcium monohydrosulfide, CaSH, was recorded in the presence of a variable static electric field. The magnitude of the a-principal axis component of the permanent electric dipole moment, ‖μa‖, was determined to be 5.36(4) and 3.78(7) D for the X̃ 2A′ and B̃ 2A″ states, respectively. An analysis of the field free spectra produced values for the (000) B̃ 2A″ vibronic state fine structure parameters of (in cm−1); A′=10.433 56(16), B′=0.143 43(15), C′=0.141 14(15), εaa=−5.779 3(19), εbb=0.107 67(88), and εcc=0.032 70(83). An interpretation of the parameters is presented.
Ab initio potential energy surfaces and trajectory studies of A-band photodissociation dynamics: ICN*→I+CN and I*+CN
Authors: Amatatsu Yoshiaki , Yabushita Satoshi , Morokuma Keiji
Published: July 2002

The photodissociation reaction of ICN in the A continuum has been theoretically studied based on ab initio potential energy surfaces and classical trajectories. Ab initio contracted spin–orbit configuration interaction calculations have been carried out to obtain potential energy surfaces (PES’s) of 3Π1, 3Π0+ and 1Π1 excited states, where results are fit to five diabatic potential functions and their couplings as functions of all three internal degrees of freedom. The transition dipoles at the Franck–Condon region have also been calculated. All the PES’s involved in photodissociation are bent near the Franck–Condon region. Classical trajectory calculations performed on these potential surfaces have produced results that are in agreement with various experimental findings and provide a basis for their interpretation. The calculations indicate that the absorption is a mixture of parallel and perpendicular transition. A reasonable I/I* branching ratio can be obtained by considering the effect of initial bending vibrations in addition to the character of mixed transitions. The I/I* channel selectivity of the CN rotation can be compared to the shape of PES’s with respect to the bending angle. The rotational excitation of the CN fragment is determined by the shape of PES’s on which trajectories travel before and after the transition.

The higher rotational component in the I channel is attributed to the energy gradient of 1Π1 with respect to the bending angle at the transition region where the C–I distance is between 5.0 and 8.0 a.u. The lower component in the I channel emerges from 3Π1. The average rotational distribution obtained with the proper weight of Boltzmann populations and transition intensities is in agreement with the experiment. This interpretation can also be applied to the rotational quantum number dependence of anisotropy parameters. Trajectory calculations on the 3Π1 surface alone, give a single Boltzmann rotational distribution. Reflecting the shape of PES’s with respect to the CN distance, the product CN vibration on 3Π0+ and 1Π becomes suppressed while that on 3Π1 becomes slightly more excited. The anisotropic parameter was also analyzed. Some comments on the femtosecond transition spectroscopy are also made.
Temperature dependence of ion pairing of a potassium salt in nonaqueous liquid and polymer electrolytes: X-ray absorption studies
Authors: Yang X. Q. , Lee H. S. , McBreen J. , Xu Z. S. , Skotheim T. A. , Okamoto Y. , Lu F.
Published: July 2002

Near-edge x-ray absorption fine structure spectroscopy was used to study the effect of temperature on ion pairing of a potassium salt in a modified carbonate (MC3) solution and in a poly(ethylene oxide)-potassium salt complex that used MC3 as a plasticizer. The modified carbonate was made by attaching three ethylene oxide units to the four position of ethylene carbonate. Spectra were obtained, at the K edge of potassium, over the temperature range of 25–110 °C. Studies of reference systems showed a correlation between ion pairing and white line splitting in the near-edge region of the spectra. The degree of white line splitting was used as a qualitative indicator of the degree of ion pairing as a function of temperature. The results indicate that, in both systems, the number of ion pairs increases with increasing temperature.
Dissociative attachment in selected monochloroalkanes
Authors: Pearl D. M. , Burrow P. D.
Published: July 2002

The total cross sections for the dissociative attachment (DA) process are reported for eight normal chloroalkanes CnH2n+1Cl for n=2–9, two secondary compounds 2-chloropropane and 2-chlorobutane, one tertiary compound t-butyl chloride, and three cyclic chloroalkanes c-CnH2n−1Cl for n=4–6, for electron energies below 4 eV. Using vertical attachment energies (VAEs) determined by electron transmission spectroscopy elsewhere and in this work, we show that the resonance widths vary linearly with VAE in this series of compounds and that the peak DA cross sections decline exponentially with increasing VAE. The small upper bound to the DA cross section reported previously for methyl chloride is discussed and is consistent with the measurements made on the remaining chloroalkanes.
Optical absorptions of Li atoms in mixed Ar/Xe matrices
Authors: Corbin Robert A. , Fajardo Mario E.
Published: July 2002

We present the results of matrix isolation spectroscopy experiments on mixed Ar/Xe matrices containing Li atoms produced by laser ablation of solid lithium. Optical absorption spectra of Li/Ar/Xe matrices containing ∼0.01% guest Li atoms and ≊0, 3%, 10%, 30%, 50%, 70%, 90%, 97%, and 100% Xe as the matrix host are included. In all cases well defined ‘‘triplet’’ absorption features (i.e., three main peaks) are observed for the Li atom 2p←2s absorption. We also present new data on the photobleaching of the well known ‘‘red triplet’’ absorption in Li/Xe matrices, which show changes to the fine structure observed on the sharp 655 nm component. In these dilute guest systems, the Li atom absorption line shape is determined completely by guest–host interactions, which depend strongly on the local Li atom trapping site structure. In single Rg host matrices, it is possible that the trapping site structures may correspond to single or multiple Rg atom substitutional sites in otherwise crystalline regions of the rare gas solid. In these cases, the observed triplet line shapes would be due to the removal of the threefold degeneracy of the excited Li atom 2p state by dynamical distortions of the system away from the high symmetry equilibrium trapping site structures. In the mixed Ar/Xe matrices, the Li atom trapping sites necessarily have lower equilibrium or static symmetries due to the possibly amorphous nature of these solids, and to the differences in the Li–Ar and Li–Xe interactions. The observed spectra in mixed host matrices thus contain contributions from the many and varied Li atom trapping site structures, yet they still exhibit the familiar triplet absorption pattern. While we do not settle the long-standing question as to the crystalline vs amorphous nature of the single Rg host matrices, the present observations do provide new data for the comparison of the relative importance of static vs dynamic effects on the spectra of matrix isolated alkali atoms.
A three dimensional quantum mechanical theory to treat tetra-atom reactions: State-to-state cross sections for the H2+OH→H2O+H process
Authors: Szichman H. , Baer M.
Published: July 2002

A quantum mechanical approach to treat diatom–diatom exchange processes of the type AB+CD→ABC+D is presented. The approach is based on three types of wave functions ψ0ν, ψ0λ, and χ, where the first two are (asymptotic) elastic distorted wave functions for the two arrangement channels ν and λ, and χ is a short range (square integrable) wave function describing the system in the close interaction region. The functions ψ0ν and ψ0λ are presented within the jz approximation, and to calculate χ, a perturbative-type Schrödinger equation which contains negative imaginary potentials (to form absorbing boundary conditions) is solved. The variationally stable solution is obtained employing Gaussians and local adiabatic basis sets. The actual calculation of χ is done employing the quasibreathing sphere model, where the relevant angular coordinates are selected randomly (about 50 sets like that were used) employing a Monte Carlo approach. With this approach, most detailed state-to-state cross sections were calculated for the reaction H2(n1=0, j1=0)+OH(n2=0, j2=0)→H2O(vj̄vKΩK)+H, where (vj̄v) are the usual vibrational-bending states of the water molecule, and K and ΩK are, respectively, the overall rotational quantum number and the corresponding magnetic component. The results were compared with those due to other treatments and with experiment.
Locating transition states by quadratic image gradient descent on potential energy surfaces
Authors: Sun Jun-Qiang , Ruedenberg Klaus
Published: July 2002

An analysis is given of the so-called ‘‘image function’’ approach to finding transition states. It is demonstrated that, in fact, such functions do not exist for general potential energy surfaces so that a plain minimum search is inappropriate. Nonconservative image gradient fields do exist, however, and their field lines, defined by Euler’s equation, can lead to transition states as exemplified by quantitative integrations of these equations for the Müller–Brown surface. As do gradient fields, image gradient fields contain streambeds and ridges, but their global structure is considerably more complex than that of gradient fields. In particular, they contain certain singular points where the image gradients change sign without passing through zero. They are the points where the two lowest eigenvalues of the Hessian are degenerate. Some of them can act as singular attractors for the image gradient descent and any algorithm must contain safeguards for avoiding them. (Such regions are equally troublesome for quasi-Newton-type transition-state searches.) Image gradient fields appear to have considerably larger catchment basins around transition states than do quasi-Newton-type or gradient-norm-type transition-state searches. A quantitative quadratic image-gradient-following algorithm is formulated and, through applications to the Müller–Brown surface, shown to be effective in finding transition states.
Application of the total-correlation function formulation of ordered systems to nematic hard ellipsoids
Authors: Kromhout Robert A. , Linder Bruno
Published: July 2002

Our formulation of ordered systems in terms of the single-particle distribution and the two-particle (total) correlation function, g(x, x′), is applied to hard ellipsoids of revolution with an aspect ratio of 5. It is found that such a system exhibits an isotropic-nematic phase transition at ρv0=0.3806 (ordered) and 0.3791 (disordered), where ρ is the particle density and v0 the molecular volume, with an order parameter of 0.20.
Transient coexisting nanophases in ultrathin films confined between corrugated walls
Authors: Curry Joan E. , Zhang Fushan , Cushman John H. , Schoen Martin , Diestler Dennis J.
Published: July 2002

Grand-canonical Monte Carlo and microcanonical molecular dynamics methods have been used to simulate an ultrathin monatomic film confined to a slit-pore [i.e., between solid surfaces (walls)]. Both walls comprise atoms rigidly fixed in the face centered cubic (100) configuration; one wall is smooth on a nanoscale and the other is corrugated (i.e., scored with regularly spaced rectilinear grooves one to several nanometers wide). Properties of the film have been computed as a function of the lateral alignment (registry), with the temperature, chemical potential, and distance between the walls kept constant. Changing the registry carries the film through a succession of equilibrium states, ranging from all solid at one extreme to all fluid at the other. Over a range of intermediate registries the film consists of fluid and solid portions in equilibrium, that is fluid-filled nanocapillaries separated by solid strips. The range of registries over which such fluid–solid equilibria exist depends upon the width of the grooves and the frequency of the corrugation. For grooves of width comparable to the range of the interatomic potential, fluid and solid phases cease to coexist. In the limit of very wide grooves the character of the film is similar to that of the film confined by strictly smooth walls. The rich phase behavior of the confined film due to the coupling between molecular (registry) and nano (corrugation) scales has obvious implications for boundary lubrication.
Simulation study of the ferrous ferric electron transfer at a metal–aqueous electrolyte interface
Authors: Smith B. B. , Halley J. W.
Published: July 2002

We report a new simulation study of the rate of ferrous–ferric electron transfer at a metal electrolyte interface. In contrast with earlier work, new features in our study include a detailed account of the effects of the field associated with the charging of the electrode, inclusion of entropic effects in the calculated free energy barriers, and a study of the dependence of the relevant free energy surfaces on the distance of the ion from the electrode. The qualitative picture of the reaction mechanism which emerges is significantly more detailed than that in earlier work. The dominant factors in determining the rate and mechanisms of electron transfer are the distance dependence of the work function of the metal, the redox species concentration profile, and the electronic matrix element. Calculated free energy barriers are consistent with experimentally measured ones. We also estimate the equilibrium potential for this reaction from the model, and find it to be consistent with the experimental equilibrium potential.
A molecular-dynamics simulation study of the adsorption and diffusion dynamics of short n-alkanes on Pt(111)
Authors: Huang Daoyang , Chen Yin , Fichthorn Kristen A.
Published: July 2002

Using molecular-dynamics studies and static potential-energy minimization, we have resolved the mechanisms by which n-alkanes (ethane through n-decane) diffuse on a model Pt(111) surface in the low-coverage limit of a single adsorbed molecule. Our simulations reproduce all of the experimental trends seen for the adsorption and diffusion of C3–C6 on Pt(111) and Ru(001). The short alkanes (C2–C8) behave as rigid rods and their motion involves coupled translation and rotation in the surface plane. For this series, there is a linear increase of the diffusion barrier with the molecular chain length. We have analyzed the compliance of the motion of the assumptions of a nearest-neighbor hopping model. Although hopping motion can be observed for all of the molecules at sufficiently low temperatures, the hopping involves a significant fraction of long jumps. As the temperature increases, the adsorption becomes virtually delocalized. Despite the extensive deviations of the motion from the assumptions of a nearest-neighbor hopping model, the static diffusion-energy barriers, arising from the minimum-energy path for hops between nearest-neighbor binding sites, agree well with those obtained from the tracer-diffusion coefficients for butane, hexane, and octane. For these molecules, multiple-site hops and long flights appear to influence the values of the preexponential factors, which are too large. Neither the diffusion barrier nor the preexponential factor for ethane agrees well with theoretical estimates. We attribute these discrepancies to the smallness of the static diffusion barrier and/or the existence of unique dynamical behavior for this molecule. Due to the increased difficulty of in-plane rotation and increased mismatch between the geometries of the molecule and the surface, the diffusion barrier for n-decane drops below that for n-hexane. The characteristic mechanism of motion for n-decane involves significant C–C–C bond-angle bending.
On the relation between unimolecular reaction rates and overlapping resonances
Authors: Peskin Uri , Reisler Hanna , Miller William H.
Published: July 2002

Unimolecular decay processes are studied in the regime of overlapping resonances with the goal of elucidating how unimolecular reaction rates depend on resonances widths (the imaginary part of the Siegert eigenvalues). As illustrated analytically for one-dimensional models and numerically for a more general random matrix version of Feshbach’s optical model, transition state theory (TST, Rice–Ramsperger–Kassel–Marcus) provides the correct average unimolecular decay rate whether the resonances are overlapping or not. For all studied cases, the explicit ‘‘universal’’ dependence of the TST average rate on the average resonance width (for a given energy, or an energy interval) is that of a saturation curve: in the regime of nonoverlapping resonances (i.e., weak coupling) the standard relation ‘‘unimolecular decay rate=resonance width /ℏ’’ holds, but as the resonance overlap increases (strong coupling) the rate saturates, becoming practically independent of the average resonance width in the strong overlapping limit. On the basis of these conclusions, a discussion of what has been or can be measured in experiments of unimolecular decay that relates to the average decay rate and to the resonance widths is given.