bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of rotation on the translational and vibrational energy dependence of the dissociative adsorption of D2 on Cu(111)
Authors: Michelsen H. A. , Rettner C. T. , Auerbach D. J. , Zare R. N.
Published: July 2002

We have investigated the dependence on the rotational and vibrational states of the translational energy of D2(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D2 on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D2 molecules were detected in a quantum-state-specific manner using three-photon resonance-enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D2+ ions in a field-free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as translational energy, in promoting adsorption. Rotational motion is found to hinder adsorption for low rotational states (J≤5) and enhance adsorption for high rotational states (J≥5). Even for high J states, however, rotational energy is less effective than either vibrational energy, which is 30%–70% more effective than rotational energy, or translational energy, which is 2.5–3 times more effective than rotational energy in promoting adsorption. The measured internal state distributions for the rovibrational states listed above are consistent with the observed dependence of the kinetic energy of the de- sorbed molecules with the rotational state. In addition, the analysis performed yields the dependence of the adsorption probability on kinetic energy separately for each rovibrational state. These functions have very similar sigmoidal shapes for all states examined. Changing the quantum state is primarily associated with a shift in the position, or threshold energy, for the curves. The level at which these functions saturate or level off at high energy is independent of rotational state but varies nonmonotonically with the vibrational state.
Solvation effects on reactive intermediates: The benzyl radical and its clusters with Ar, N2, CH4, C2H6, and C3H8
Authors: Disselkamp R. , Bernstein E. R.
Published: July 2002

Mass resolved excitation spectra are presented for the benzyl radical and its clusters with Ar, N2, CH4, C2H6, and C3H8. The cluster spectra exhibit small redshifts (≤50 cm−1) relative to the unclustered benzyl radical for the D1(1 2A2)←D0(1 2B2) and D2(2 2B2)←D0(1 2B2) electronic transition regions. A unique set of low energy van der Waals modes is observed for these clusters for each excited electronic state investigated. The cluster spectra also reveal significant vibronic coupling between the two excited electronic states of the benzyl radical, as evidenced by a single vibrational predissociation threshold for each cluster. Ab initio calculations are performed on the benzyl radical to examine excited electronic state structure, predict transition energies, estimate ionization energy, and determine partial atomic charges in the electronic states of interest. The resulting partial charges are used in empirical atom–atom potential energy calculations to aid in the understanding of cluster spectroscopic shifts, binding energies, and van der Waals modes.
Manifestations of spatially dependent friction in classical activated rate processes
Authors: Straus Jay B. , Gomez Llorente Jose M. , Voth Gregory A.
Published: July 2002

Classical activated rate processes are examined for systems which are influenced by a spatially dependent friction on the reaction coordinate motion. In these systems, the spatial dependence of the friction kernel arises from nonlinear couplings between the reaction coordinate and a many-dimensional harmonic oscillator representation of the bath fluctuations. This model is thought to provide a reasonable representation of many charge transfer reactions in polar solvents. One of the systems studied corresponds to the case where the friction along the reaction coordinate is weaker in the region of the barrier than in the reactant and product states, while the second system exhibits the opposite behavior. In the first system, significant effects due to the nonlinear dissipation are found over the entire range of the friction strength. In the second system, the nonlinear effects are largely confined to the weak to intermediate damping regime, but they can be quite large. Interestingly, for some values of the nonlinearity parameters, the second system does not exhibit a simple single exponential rate behavior for a long period of time. Mathematical analysis is provided to help explain the computer simulation results in both the weak and strong damping limits of each system.
Efficient molecular dynamics and hybrid Monte Carlo algorithms for path integrals
Authors: Tuckerman Mark E. , Berne Bruce J. , Martyna Glenn J. , Klein Michael L.
Published: July 2002

New path integral molecular dynamics (PIMD) and path integral hybrid Monte Carlo (PIHMC) algorithms are developed. It is shown that the use of a simple noncanonical change of variables that naturally divides the quadratic part of the action into long and short wavelength modes and multiple time scale integration techniques results in very efficient algorithms. The PIMD method also employs a constant temperature MD technique that has been shown to give canonical averages even for stiff systems. The new methods are applied to the simple quantum mechanical harmonic oscillator and to electron solvation in fluid helium and xenon. Comparisons are made with PIMC and the more basic PIMD and PIHMC methods.
Discrete variable representations of differential operators
Authors: Szalay Viktor
Published: July 2002

By making use of known properties of orthogonal polynomials the discrete variable representation (DVR) method [J. C. Light, I. P. Hamilton, and J. V. Lill, J. Chem. Phys. 82, 1400 (1985)] has been rederived. Simple analytical formulas have been obtained for the matrix elements of DVRs of differential operators which may appear in the rovibrational Hamiltonian of a molecule. DVRs corresponding to Hermite, Laguerre, generalized Laguerre, Legendre, and Jacobi polynomial bases and to the Lanczos basis for Morse oscillator, that is, to basis sets often used in calculating rovibrational energy levels, have been discussed.
The advantages of the general Hartree–Fock method for future computer simulation of materials
Authors: Hammes-Schiffer Sharon , Andersen Hans C.
Published: July 2002

The general Hartree–Fock (GHF) method is a quantum mechanical method for electronic structure calculations that uses a single determinantal wave function with no restrictions on the one-electron orbitals other than orthonormality and the use of a specific basis set. The more familiar restricted Hartree–Fock (RHF) and unrestricted Hartree–Fock (UHF) methods can be regarded as special cases of the GHF method in which additional restrictions are imposed on the occupied orbitals. We propose that the GHF method is very suitable as an electronic structure method to be incorporated into computer simulations that combine the calculation of the Born–Oppenheimer ground state surface with the simulation of the motion of the nuclei on that surface. In particular, for many problems of interest there is only a single GHF minimum of the energy, and the GHF wave function is a continuous function of nuclear positions. The RHF and UHF methods, in comparison, typically have a multiplicity of local minima with curve crossings that generate a discontinuous behavior of the ground electronic state wave function as a function of nuclear positions. In this paper, we use energy minimization techniques to identify and characterize the UHF and GHF electronic minima at fixed nuclear positions for three model systems. The results verify the above assertions and suggest that the GHF method would be more suitable than the RHF or UHF methods for computer simulations.
Temperature and supersaturation dependent nucleation rates of heterogeneous water by molecular cluster model calculation
Authors: Lutrus Chen K. , Hagen Donald E. , Salk Sung Ho
Published: July 2002

A statistical mechanical method to evaluate the energy of formation of water clusters attached to a foreign particle surface is described, with the binding energy being evaluated on a molecular level, using semiempirical modified neglect of diatomic overlap (MNDO) theory. The model is applied to water nucleation on a silicon oxide surface. The binding energy contribution, which represents the energy of formation at T=0 K, is found to slightly (but not negligibly in the thermal sense) increase with the number of hydrogen bonds between the water cluster and the condensation nucleus whose surface is made of silicon oxide. An analytic expression is developed to fit the binding energy contribution as a function of cluster size. At lower temperatures, a linear relationship is found between the log of the nucleation rate and reciprocal temperature for fixed saturation ratio. However, at higher temperatures, this relationship deviates from linearity. The deviation is sufficient to suggest the existence of a critical temperature for which the nucleation rate reaches a maximum. Furthermore, another kind of critical temperature is found, which corresponds to a minimum cluster critical size (at fixed saturation ratio). These are found to almost coincide for the cases of heterogeneous and homogeneous nucleation.
Proton transfer dynamics and cluster ion fragmentation in phenol/ammonia clusters
Authors: Hineman Max F. , Kelley David F. , Bernstein Elliot R.
Published: July 2002

Excited-state proton transfer dynamics are reported for the phenol(NH3)n cluster system. Excited-state proton transfer is shown to occur for this system by isotopic substitution of the hydroxyl proton and ammonia protons. The observed dynamics slow from 80 to 600 ps upon cluster deuteration. The effects of cluster vibrational energy and ammonia concentration in the expansion gas are also studied. The results of these experiments are compared with a previous study [Syage and Steadman, J. Chem. Phys. 95, 2497 (1991)] which used a single excitation energy and varied ionization energy. Our experiments indicate that a significant amount of cluster ion fragmentation occurs for this system and caution must be exercised in assigning the decays observed in a given ion mass channel to a particular parent cluster. Due to the difficulties of associating a given decay curve with a cluster of particular mass, a previously proposed model of proton transfer in isolated clusters [Hineman et al., J. Chem. Phys. 97, 3341 (1992)] which details the effect of cluster vibrational energy on the proton transfer rate cannot be applied to the phenol/ammonia system.
Method for deriving Hopf and saddle-node bifurcation hypersurfaces and application to a model of the Belousov–Zhabotinskii system
Authors: Clarke Bruce L. , Jiang Weimin
Published: July 2002

Chemical mechanisms with oscillations or bistability undergo Hopf or saddle-node bifurcations on parameter space hypersurfaces, which intersect in codimension-2 Takens–Bogdanov bifurcation hypersurfaces. This paper develops a general method for deriving equations for these hypersurfaces in terms of rate constants and other experimentally controllable parameters. These equations may be used to obtain better rate constant values and confirm mechanisms from experimental data. The method is an extension of stoichiometric network analysis, which can obtain bifurcation hypersurface equations in special (h,j) parameters for small networks. This paper simplifies the approach using Orlando’s theorem and takes into consideration Wegscheider’s thermodynamic constraints on the rate constants. Large realistic mechanisms can be handled by a systematic method for approximating networks near bifurcation points using essential extreme currents. The algebraic problem of converting the bifurcation equations to rate constants is much more tractable for the simplified networks, and agreement is obtained with numerical calculations. The method is illustrated using a seven-species model of the Belousov–Zhabotinskii system, for which the emergence of Takens–Bogdanov bifurcation points is explained by the presence of certain positive and negative feedback cycles.
A state-selective multireference coupled-cluster theory employing the single-reference formalism
Authors: Piecuch Piotr , Oliphant Nevin , Adamowicz Ludwik
Published: July 2002

A new state-selective multireference (MR) coupled-cluster (CC) method exploiting the single-reference (SR) particle-hole formalism is described. It is an extension of a simple two-reference formalism, which we presented in our earlier paper [N. Oliphant and L. Adamowicz, J. Chem. Phys. 94, 1229 (1991)], and a rigorous formulation of another method of ours, which we obtained as an approximation of the SRCC approach truncated at triple excitations (SRCCSDT) [N. Oliphant and L. Adamowicz, J. Chem. Phys. 96, 3739 (1992)]. The size extensivity of the resulting correlation energies is achieved by employing a SRCC-like ansatz for the multideterminantal wave function. General considerations are supplemented by suggesting a hierarchy of approximate schemes, with the MRCCSD approach (MRCC approach truncated at double excitations from the reference determinants) representing the most important one. Our state-selective MRCCSD theory emerges through a suitable selection of the most essential cluster components appearing in the full SRCCSDTQ method (SRCC method truncated at quadruple excitations), when the latter is applied to quasidegenerate states. The complete set of equations describing our MRCCSD formalism is presented and the possibility of the recursive intermediate factorization [S. A. Kucharski and R. J. Bartlett, Theor. Chim. Acta 80, 387 (1991)] of our approach, leading to an efficient computer algorithm, is discussed.
Computer simulations of NaCl association in polarizable water
Authors: Smith David E. , Dang Liem X.
Published: July 2002

Classical molecular dynamics computer simulations have been used to investigate the thermodynamics and kinetics of sodium chloride association in polarizable water. The simulations make use of the three-site polarizable water model of Dang [J. Chem. Phys. 97, 2659 (1992)], which accurately reproduces many bulk water properties. The model’s static dielectric constant and relaxation behavior have been calculated and found to be in reasonable agreement with experimental results. The ion–water interaction potentials have been constructed through fitting to both experimental gas-phase binding enthalpies for small ion–water clusters and to the measured structures and solvation enthalpies of ionic solutions. Structural properties and the potential of mean force for sodium chloride in water have been calculated. In addition, Grote–Hynes theory has been used to predict dynamical features of contact ion-pair dissociation. All of the calculated ionic solution properties have been compared with results from simulations using the extended simple point charge (SPC/E), nonpolarizable water model [J. Phys. Chem. 91, 6296 (1987)]. The dependence on polarizability is found to be small, yet measurable, with the largest effects seen in the solvation structure around the highly polarizable chlorine anion. This work validates the use of some nonpolarizable water models in simulations of many condensed-phase systems of chemical and biochemical interest.
Thermodynamic perturbation theory: Lennard-Jones chains
Authors: Banaszak M. , Chiew Y. C. , O’Lenick R. , Radosz M.
Published: July 2002

The compressibility factors for freely-jointed Lennard-Jones chains are determined in the context of Wertheim’s first-order thermodynamic perturbation theory (TPT1). In the TPT1 treatment, nonbonded Lennard-Jones spheres are used as the reference system. The compressibility factors of the chain system, at temperature T*, segment density ρ*, and chain length m, are obtained based on the compressibility factors of the Lennard-Jones spheres, and the density derivative of the radial distribution function, ∂ ln gLJS(σ;ρ*,T*)/∂ρ*, evaluated at the same temperature T* and segment density ρ*. The exact values for these two quantities, obtained from Monte Carlo simulations, are used in our calculations. The TPT1 predictions are found in agreement with Monte Carlo simulation results for the 8-mer, 16-mer, and 32-mer Lennard-Jones chains over a wide range of densities and temperatures. When values of the density derivative of the reference Lennard-Jones sphere system are estimated from the approximate Weeks–Chandler–Andersen perturbation theory, however, the TPT1 predictions do not match the simulation results.
(σ3s) Rydberg states of cyclohexane, bicyclo[2.2.2]octane, and adamantane
Authors: Shang Q. Y. , Bernstein E. R.
Published: July 2002

In this effort the effect of Rydberg electronic excitation on the structure of cyclic and polycyclic alkanes is investigated. Two-photon resonant, one-photon ionization, mass-resolved excitation spectroscopy is employed to observe the (σ3s)←(σ)2 Rydberg transitions of cyclohexane, bicyclo[2.2.2]octane, and adamantane cooled in a supersonic jet expansion. Rydberg spectra of these three molecules display sharp, well-resolved vibronic structure. Analysis of the spectra is assisted by isotopic substitution, circular/linear polarization, vibronic feature widths (rotational selection rules), as well as comparison to the ground-state vibrational energies. A significant reduction of vibrational energies in the excited electronic state and a 381 cm−1 blue shift of the transition origin upon deuterium isotope substitution for cyclohexane are interpreted as due to the promotion of an electron from a σ-bonding orbital to a nonbonding Rydberg orbital upon optical excitation. Extensive vibronic coupling is observed for both cyclohexane and adamantane in their excited (σ3s) Rydberg electronic states. Jahn–Teller splitting is small for adamantane but quite substantial for cyclohexane. This difference is attributed to the basic stability difference for the two different ring systems (mono- and tri-cyclic). A progression in a nontotally symmetric mode is observed in the Rydberg spectrum of bicyclo[2.2.2]octane suggesting a change in the geometry of this molecule upon (σ3s)←(σ)2 excitation.
Scattering state-selected NO+ on GaAs(110): The effect of translational and vibrational energy on NO− and O− product formation
Authors: Martin J. S. , Greeley J. N. , Morris J. R. , Feranchak B. T. , Jacobs D. C.
Published: July 2002

The scattering of state-selected NO+(X 1Σ+, v=0–6) on GaAs(110) was explored across the hyperthermal energy region. Relative yields and velocity distributions for scattered anionic products NO−(X 3Σ−, v=0) and O−(2P) were measured as a function of the number of vibrational quanta and collision energy for NO+(X 1Σ+, v=0–6) incident on the surface. Facile neutralization along the inbound trajectory forms vibrationally excited NO(2Π) immediately prior to surface impact. Electron attachment to form NO−(X 3Σ−, v=0) occurs near the distance of closest approach between the molecule and surface. With regard to O−(2P) emergence, a collision-induced dissociation mechanism is consistent with the observed 25 eV threshold. Incident vibrational energy is as much as ten times more effective than translational energy in forming O−(2P). This paper represents the first experimental investigation into the effect of vibrational energy on electron transfer and dissociation of ions at surfaces and highlights the unique interplay between translational and vibrational motions in an ion/surface encounter.
Stochastic model for the A+B2 surface reaction: Island formation and complete segregation
Authors: Mai J. , Kuzovkov V. N. , Niessen W. von
Published: July 2002

In this paper we introduce a stochastic model for the A+1/2B2→0 reaction on a square lattice. Reaction between an A and a B particle occurs if they are nearest neighbors on the lattice. To this system which includes adsorption and reaction steps we add the effect of A-diffusion and A-desorption. We describe the model in terms of master equations using the Markovian behavior of the system. The equations are truncated at a certain level via a modified Kirkwood approximation. The reaction is in this paper introduced between particles which are nearest neighbors on the lattice. This approach which is different from a previous article [J. Mai et al., J. Chem. Phys. 98, 10017 (1993)] requires a special treatment of the stochastic equations and the correlation functions. In particular the Kirkwood superposition approximation, which is used to truncate the hierarchy of equations, has to be modified. The resulting system of lattice equations is solved in a small region around a reference point. The solution is connected to continuous functions which describe the system behavior for larger distances. This system shows kinetic phase transitions which separate the reactive regime from two nonreactive states where the lattice is completely covered by A or B. We study the location and the character of the phase transitions in detail. With the help of correlation functions we identify the different phases of particles on the lattice. Island formation and segregation of the particles on the lattice are found to be dominant processes. It is established that finite lattices which have to be used in simulations can be seriously inadequate and miss physical processes. This problem does not appear in the ansatz presented here.
A complete basis set model chemistry. IV. An improved atomic pair natural orbital method
Authors: Montgomery J. A. , Ochterski J. W. , Petersson G. A.
Published: July 2002

An improved complete basis set-quadratic configuration interaction/atomic pair natural orbital (CBS-QCI/APNO) model is described in this paper. It provides chemical energy differences (i.e., D0 I.P., and E.A.) with a mean absolute error of 0.53 kcal/mol for the 64 first-row examples from the G2 test set, and is computationally feasible for species with up to three first-row atoms. A set of 20 CBS-QCI/APNO bond dissociation energies of hydrocarbons also agree with known experimental values to within less than 1 kcal/mol. Calculations on the cyclopropenyl radical and cyclopropenylidene provide new dissociation energies which are in accord with an interpretation of the thermochemistry emphasizing ring strain and aromaticity.
New monolayer phases of n-alkane thiols self-assembled on Au(111): Preparation, surface characterization, and imaging
Authors: Camillone N. , Eisenberger P. , Leung T. Y. B. , Schwartz P. , Scoles G. , Poirier G. E. , Tarlov M. J.
Published: July 2002

We report the observation by scanning tunneling microscopy (STM) and low energy atom diffraction, of new, striped, structures at the surface of monolayers of n-alkane thiols [CH3 (CH2)n−1 SH with n=8,10,12] self-assembled on the (111) face of single crystal gold. These structures can be prepared by slow (room temperature) or thermally accelerated treatment of the well known c(4√3×2√3)R 30° phase formed by self-assembly in solution, or can be accessed directly by molecular beam deposition. With respect to the unit mesh of the gold substrate, the new striped structures can be described as p×√3 overlayers where 7.5≤p≤13. The discovery of these phases has implications for the understanding of the growth mechanisms and the pursuit of applications of this widely studied class of materials.
Reaction-path potential and vibrational frequencies in terms of curvilinear internal coordinates
Authors: Jackels Charles F. , Gu Zhen , Truhlar Donald G.
Published: July 2002

We present a general formulation that allows physically intuitive curvilinear internal coordinates to be used for the calculation of potential energy expansions and generalized normal-mode vibrational frequencies in reaction-path calculations. The reaction path is defined, as usual, as the minimum-energy path in the mass-scaled Cartesian coordinate system, and curvilinear coordinates are used for vibrational frequency calculations at nonstationary points. The method is well adapted for use in variational transition state theory with semiclassical multidimensional tunneling (VTST/MT) approximations to calculate thermal rate constants. We present VTST/MT calculations for five reactions, H+H2→H2+H, O+H2→OH+H, CH3+H2→CH4+H, H+O2→HO2, and Cl+HBr→HCl+Br, to illustrate the use of the new curvilinear coordinates, and we compare the results to calculations employing rectilinear coordinates. We make detailed comparisons not only of the calculated rate constants but also of the vibrationally adiabatic ground-state potential energy curves and bound-state vibrational frequencies as functions of the reaction coordinate.
Molecular dynamics simulations of a pressure-induced glass transition
Authors: Shumway Shelly L. , Clarke Andrew S. , Jónsson Hannes
Published: July 2002

We simulate the compression of a two-component Lennard-Jones liquid at a variety of constant temperatures using a molecular dynamics algorithm in an isobaric–isothermal ensemble. The viscosity of the liquid increases with pressure, undergoing a broadened transition into a structurally arrested, amorphous state. This transition, like the more familiar one induced by cooling, is correlated with a significant increase in icosahedral ordering. In fact, the structure of the final state, as measured by an analysis of the bonding, is essentially the same in the glassy, frozen state whether produced by squeezing or by cooling under pressure. We have computed an effective hard-sphere packing fraction at the transition, defining the transition pressure or temperature by a cutoff in the diffusion constant, analogous to the traditional laboratory definition of the glass transition by an arbitrary, low cutoff in viscosity. The packing fraction at this transition point is not constant, but is consistently higher for runs compressed at higher temperature. We show that this is because the transition point defined by a constant cutoff in the diffusion constant is not the same as the point of structural arrest, at which further changes in pressure induce no further structural changes, but that the two alternate descriptions may be reconciled by using a thermally activated cutoff for the diffusion constant. This enables estimation of the characteristic activation energy for diffusion at the point of structural arrest.
Tensor propagator for iterative quantum time evolution of reduced density matrices. II. Numerical methodology
Authors: Makri Nancy , Makarov Dmitrii E.
Published: July 2002

In a recent Letter [Chem. Phys. Lett. 221, 482 (1994)], we demonstrated that the dynamics of reduced density matrices for systems in contact with dissipative harmonic environments can be obtained in an iterative fashion by multiplication of a propagator tensor. The feasibility of iterative procedures in reduced dimension spaces arises from intrinsic features of the dissipative influence functional in Feynman’s path integral formulation of quantum dynamics. Specifically, the continuum of frequencies characteristic of broad condensed phase spectra disrupts phase coherence to a large extent, such that the dynamics of an augmented reduced density tensor becomes Markovian. In a preceding article [J. Chem. Phys. 102, 4600 (1995)] we examined in detail the formal properties of the tensor propagator. In the present paper we show that the tensor propagator can be further decomposed into a product of small rank tensors, resulting in an extremely simple and efficient numerical scheme that scales almost linearly with the dimension of the augmented reduced density tensor. Numerical application to a model electron transfer reaction is presented.
Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes
Authors: Yang Wen-Hui , Schatz George C. , Van Duyne Richard P.
Published: July 2002

We present a discrete dipole approximation (DDA) method to determine extinction and Raman intensities for small metal particles of arbitrary shape. The Raman intensity calculation involves evaluation of surface electromagnetic fields, and thus is relevant to surface enhanced Raman scattering (SERS) intensities. We demonstrate convergence of the method by considering light absorption and scattering from an isolated spheroid, from an isolated tetrahedron, from two coupled spheroids, and from a spheroid on a flat surface. We also examine comparisons with traditional T-matrix methods. Extensions and simplifications of the method in studies of clusters and arrays of particles are presented.
Rotational excitation and molecular alignment in intense laser fields
Authors: Seideman Tamar
Published: July 2002

Rotational excitation and spatial alignment in moderate intensity radiation fields are studied numerically and analytically, using time-dependent quantum mechanics. Substantial rotational excitation is found under conditions typically used in time-resolved spectroscopy experiments. The broad rotational wave packet excited by the laser pulse is well defined in the conjugate angle space, peaking along the field polarization direction. Both the rotational excitation and the consequent spatial alignment can be controlled by the choice of field parameters. Fragment angular distributions following weak field photodissociation of the rotational wave packet are computed as a probe of the degree of alignment. In the limit of rapid photodissociation the angular distribution is peaked in the forward direction, reflecting the anisotropy of the aligned state. Potential applications of the effect demonstrated range from reaction dynamics of aligned molecules and laser-control to material deposition and laser-assisted isotope separation.
Mass-resolved multiphoton ionization spectra of XeKr in the region of Xe* (6p,5d)
Authors: Lipson R. H. , Dimov S. S. , Hu X. K. , Mao D. M. , Cai J. Y.
Published: July 2002

Single isotopomer (2+1) resonantly enhanced multiphoton ionization (REMPI) spectra of jet-cooled XeKr have been recorded using time-of-flight (TOF) mass detection. Vibrational analyses for several transitions involving excited states that dissociate to Kr(1S0)+Xe* (6p,5d) between 77 560 and 80 150 cm−1 are reported for the first time. Equilibrium bond lengths were established by Franck–Condon calculations, while excited state symmetries were inferred from excitation spectra recorded using circularly polarized light. A local vibrational perturbation evident in the dominant band system dissociating to Kr(1S0)+Xe*5p56p[5/2]2 has also been analyzed. The identity of the perturbing state is proposed, based in part, on predissociation spectra obtained by monitoring atomic fragments in TOF detection.
Statics and dynamics of adsorbed polymer chains: A Monte Carlo simulation
Authors: Lai Pik-Yin
Published: July 2002

Monodispersed polymer chains under good solvent condition adsorbed on a short-range attractive impenetrable wall (xy-plane) is investigated by dynamic Monte Carlo simulation using the bond-fluctuation model. Chain conformation, segment orientation, fraction of segment adsorbed, surface coverage, chain dimensions and volume fraction profiles are measured for systems with a different number of chains over a wide range of temperatures. An adsorbed layer begins to form at the same adsorption transition temperature Ta of a single chain. However, the second-order phase transition at Ta in the single chain adsorption is suppressed in the multi-chain system as is indicated in the disappearance of the peak at Ta in the specific heat. Volume fraction profiles near the adsorption regimes are found to be in agreement with previous theoretical results. For the dynamics, the time auto-correlation function, relaxation time, and mean square displacements are measured. The auto-correlation function can be described by a stretched exponential form and the relaxation time extracted from it starts to increase dramatically with a Vogel–Fulcher behavior at a temperature T2 which is much lower than Ta. The diffusion coefficients parallel and perpendicular to the z-axis are measured and analyzed for the dilute and multi-chain semi-dilute systems. Both the static and dynamic results indicate a glass transition occurs at T2 below which the system shows a glassy behavior. The low temperature transport properties and glassy behavior are analyzed and discussed in term of appropriate physical pictures.
Microshock wave propagation in molecular clusters
Authors: Schek Israel , Jortner Joshua
Published: July 2002

Microshock wave propagation in ArN (N=55–555) clusters generated by high-energy cluster-Pt surface impact (cluster velocities v=1–10 km s−1) is explored by molecular dynamics simulations. The gross features of the dynamics of the intracluster microshock wave propagation at this impact velocity range are not sensitive to the details of the repulsive potential (i.e., the Lennard-Jones or the exp-6 form). The propagation of the microshock within the cluster was quantified by the time dependence of the first moment of the total energy. A linear dependence between the microshock (compression) velocity us and the cluster impact velocity v is observed and for sufficiently large clusters (N≥321) us≊v. For large clusters (N≳321), the cluster Hugoniot temperature–pressure relations are qualitatively similar to those for the compression of macroscopic fluid Ar.
A six dimensional quantum study for atom–triatom reactions: The H+H2O→H2+OH reaction
Authors: Zhang Dong H. , Light John C.
Published: July 2002

A time-dependent wave packet method has been developed to study atom–triatom ABC+D→AB+CD reactions in full six dimensions (6D). The approach employs a body-fixed coupled angular momentum basis for three angular coordinates, and three 1D bases for three radial coordinates. It permits the calculation of diatom AB vibrational state resolved total reaction probability for an initial rovibrational state of the triatom ABC. The approach is applied to study the H+H2O→H2+OH reaction on the modified Schatz–Elgersman potential energy surface. Initial state-selected total reaction probabilities are presented for initial ground and several vibrationally excited states of H2O for total angular momentum J=0, along with the final OH vibrational state distributions. We also report the cross sections for reaction from the initial ground vibrational and the first bending excited states of H2O. Comparisons are made between our results and those from other theoretical calculations and experiments.
Full nine-dimensional ab initio potential energy surfaces and trajectory studies of A-band photodissociation dynamics: CH3I*→CH3+I, CH3+I*, and CD3I*→CD3+I, CD3+I*
Authors: Amatatsu Yoshiaki , Yabushita Satoshi , Morokuma Keiji
Published: July 2002

The full nine-dimensional potential energy surfaces (PESs) of the 3Q0 and 1Q1 states of CH3I have been calculated with the ab initio contracted spin–orbit configuration interaction method. The results are fitted to three diabatic potential terms and their couplings as functions of all the internal degrees of freedom. The transition dipole at the Franck–Condon region has also been calculated. Surface hopping quasiclassical trajectory calculations on these potential energy surfaces have been performed to examine the photodissociation dynamics of both CH3I and CD3I in the A-continuum. The results are in general good agreement with the recent experimental findings. The reasonable I*/(I*+I) branching ratio can be obtained with these PESs when the contribution of direct transition to the 1Q1 state is considered. The rotational distribution of the CH3 and CD3 fragments and its I*/(I*+I)-channel selectivity are determined by the shape of the PESs with respect to the bending angle outside the conical intersection region. The vibrational distribution of umbrella mode is closely related to the shape of PESs for the umbrella angle; the sudden switch of reaction coordinate from 3Q0 to 1Q1 at the conical intersection is the origin of vibrational excitation in the I* channel. The larger umbrella excitation of the CD3 fragment in both I and I* channels, in comparison with the CH3 fragment, is related to the larger separation of the reaction coordinate from the Franck–Condon geometry. The symmetric stretching energy increases during the dissociation, which is related to the shape of PESs with respect to this coordinate, and the excitation of symmetric stretching mode seems to be possible.
Linear scaling computation of the Hartree–Fock exchange matrix
Authors: Schwegler Eric , Challacombe Matt
Published: July 2002

Thresholding criteria are introduced that enforce locality of exchange interactions in Cartesian Gaussian-based Hartree–Fock calculations. These criteria are obtained from an asymptotic form of the density matrix valid for insulating systems, and lead to a linear scaling algorithm for computation of the Hartree–Fock exchange matrix. Restricted Hartree–Fock/3-21G calculations on a series of water clusters and polyglycine α-helices are used to demonstrate the 𝒪(N) complexity of the algorithm, its competitiveness with standard direct self-consistent field methods, and a systematic control of error in converged total energies.
A theoretical study of CH4 dissociation on pure and gold-alloyed Ni(111) surfaces
Authors: Kratzer P. , Hammer B. , No/rskov J. K.
Published: July 2002

We present a density functional theory study of the first step of CH4 adsorption on the Ni(111) surface, dissociation into adsorbed CH3 and H. The rupture of the C–H bond occurs preferentially on top of a Ni atom, with a dissociation barrier of about 100 kJ/mol (including zero point corrections). The transition state involves considerable internal excitation of the molecule. The active C–H bond is both stretched to 1.6 Å and tilted relative to the methyl group. A normal mode analysis shows that the reaction coordinate is mainly a C–H stretch, while the orientation of the C–H bond relative to the surface is responsible for the highest real mode. Alloying the surface with gold also affects the reactivity of the Ni atoms on adjacent surface sites. The dissociation barrier is increased by 16 and 38 kJ/mol for a Ni atom with one or two gold neighbors, respectively. We attribute these changes to a shift in the local density of d states at the nickel atoms in the neighborhood of gold.
Autodetachment spectroscopy and dynamics of vibrationally excited dipole-bound states of H2CCC−
Authors: Yokoyama K. , Leach Gary W. , Kim Joseph B. , Lineberger W. C. , Boldyrev A. I. , Gutowski M.
Published: July 2002

Direct observation of the rotational fine structure levels of a vibrationally excited negative ion dipole-bound state (DBS) is reported. Autodetachment resonances of H2CCC− were observed for the 2A1−2B1 transition in one quanta of ν6, ν4, and ν2 and two quanta of ν6 vibrational modes of the DBS. Rotational assignments for both the electronic ground state and the DBS were performed. Strong type (a) Coriolis coupling between ν6 and ν9 in both the electronic ground and excited states was observed, and coupling constants were determined. QCISD ab initio calculations were performed for the ground state, the negative ion, and the neutral state of H2CCC. The calculations on the neutral agree well with measured vibrational frequencies of the dipole-bound state. The autodetachment resonances contain information about the detachment dynamics via the observed linewidths, showing, e.g., that motions along the dipole moment axis significantly enhance autodetachment, indicating that the DBS is unstable with respect to neutral core motion which modulates the magnitude value of the dipole moment.
Amino groups in nucleic acid bases, aniline, aminopyridines, and aminotriazine are nonplanar: Results of correlated ab initio quantum chemical calculations and anharmonic analysis of the aniline inversion motion
Authors: Bludský Ota , Šponer Jiří , Leszczynski Jerzy , Špirko Vladimír , Hobza Pavel
Published: July 2002

The amino group nonplanarity in nucleic acid bases, aniline, aminopyridines, and aminotriazine was investigated by ab initio methods with and without inclusion of correlation energy utilizing medium and extended basis sets. For all the systems studied, the amino group was found to be nonplanar and the coupled cluster method [CCSD(T)] ‘‘nonplanarities’’ and inversion barriers slightly higher than their second-order many-body perturbation-theory (MP2) counterparts. To assess the reliability of the calculations, inversion splittings for aniline and aniline-ND2 were evaluated by solving a two-dimensional vibrational Schrödinger equation for the large-amplitude inversion and torsion motions, while respecting the role of small-amplitude C–N stretching and H–N–H bending motions. Because a large number of points is required for the description of the aniline potential energy surface, the Hartree–Fock (HF) method with 6-31G* basis set was utilized. The vibrational calculations were performed within the framework of the semirigid bender Hamiltonian of Landsberg and Bunker. Excellent agreement between experimental and theoretical inversion-torsion frequencies for fundamental, overtone, and combination modes was found, which gives strong evidence for the adequacy of the theoretical model used in general, and potential energy surface in particular. Similarity between the HF, MP2, and CCSD(T) aniline inversion barriers and amino group nonplanarities gives us confidence that the MP2 and CCSD(T) inversion barriers and amino group nonplanarities of the DNA bases, aminopyridine, and aminotriazine, are close to the actual values which are still experimentally unknown.
Isothermal study of the kinetics of carbon monoxide oxidation on Pt(111): Rate dependence on surface coverages
Authors: Zaera Francisco , Liu Jinyao , Xu Mingde
Published: July 2002

The kinetics of the oxidation of carbon monoxide on Pt(111) surfaces was studied isothermally by using an effusive directional molecular beam in an arrangement based on a variation of the dynamic method originally devised by King and Wells. Three temperature regimes were identified for this reaction on surfaces precovered with atomic oxygen. Below 300 K no reaction is observed, and the presence of preadsorbed atomic oxygen on the surface does not significantly affect the initial sticking coefficient of CO but only reduces its saturation coverage by less than half, which it does by preferentially blocking the bridge sites. Above 400 K, on the other hand, the desorption of CO2 from oxygen-covered surfaces is controlled by the impinging frequency of the incoming CO. The most interesting temperature range is that between 300 and 400 K, where the rate of surface recombination of CO with oxygen competes with that of CO adsorption; under those conditions the overall dynamic behavior is fairly complex, and not all the surface oxygen is reactive. Furthermore, the reaction rates in this regime not only depend on the coverages of the reactants, but also on how the surface is prepared. Two kinetically distinct types of oxygen atoms develop during the course of reaction in spite of the fact that they all sit on identical sites at the start of the kinetic runs, suggesting that the reactivity of chemisorbed CO depends on the local oxygen coverage of neighboring sites. We propose that such local arrangements modify the adsorption energy for atomic oxygen, and that this in turn changes the activation energy for the oxidation reaction. Previous reported molecular beam experiments were also extended to cover a wider range of surface coverages in order to better determine the dependence of the rate constant for the surface oxidation step on the coverages of CO and oxygen. It was found that while the presence of oxygen on the surface helps the production of CO2, increasing CO coverages augment the activation barrier for this reaction, an observation that is in direct contrast with previous reports. Finally, the adsorption sites for CO during the surface CO+O recombinatory reaction were characterized by reflection–absorption infrared spectroscopy. The data reported here is analyzed and discussed in terms of possible kinetic models.
A fast algorithm for evaluating multidimensional potential energy surfaces
Authors: Hollebeek Timothy , Ho Tak-San , Rabitz Herschel
Published: July 2002

A recently introduced general interpolation method based on reproducing kernel Hilbert space (RKHS) theory has been quite successful in constructing a number of potential energy surfaces. A straightforward implementation is slow when large numbers of ab initio  points are involved, since the computation time is proportional to the number of points. The algorithm introduced here precomputes and stores the sums necessary to generate the surface, allowing the surface computation time to be independent of the number of points. The method is tested on its ability to reproduce a prior global analytic fit to the potential surface for the N(2D)+H2 system based on a 7×7×7 grid of points. The RKHS interpolated surface is found to exactly reproduce the 343 points on which it is based, and has a root mean square (rms) error of 14.2 kJ mol−1 elsewhere, while the prior analytic fit has a rms error of 25.1 kJ mol−1 at the points used to fit the surface. With a 16×16×16 grid as input the RKHS surface had a rms error of 1.32 kJ mol−1. The interpolated surface with the new algorithm can also be evaluated twice as fast as the prior analytic fit.
Demixing in hard ellipsoid rod-plate mixtures
Authors: Camp Philip J. , Allen Michael P. , Bolhuis Peter G. , Frenkel Daan
Published: July 2002

The phase behavior of fluid mixtures of hard uniaxial ellipsoids with elongations e and 1/e, and equal molecular volume, has been studied using constant-pressure Gibbs ensemble Monte Carlo simulations for e=15 and e=20. Four distinct phases are observed: isotropic (I), uniaxial nematic (N+ and N−) and biaxial nematic (B). The region of stability of the biaxial phase is found to be limited severely by demixing into two coexisting uniaxial phases. This is in agreement with recent theoretical predictions. The theory, however, does not account for the surprising asymmetry of the phase diagram that we find in our simulations.
Connection between quasielastic Raman scattering and free volume in polymeric glasses and supercooled liquids
Authors: Novikov V. N. , Sokolov A. P. , Strube B. , Surovtsev N. V. , Duval E. , Mermet A.
Published: July 2002

Quasielastic light scattering (QLS) in the frequency interval 100–1000 GHz is measured in some polymers: polycarbonate, polybutadiene, polystyrene, and poly(methyl methacrylate). To describe the spectra, a model of the fast picosecond relaxation processes responsible for the QLS, which is based on the damping of the boson peak vibrations by the dynamic hole volume fluctuations, is used. Within the frame of the model, the intensity of the fast relaxation process is proportional to the fractional dynamic hole volume (which above the glass transition temperature Tg is known as the fractional free volume). The hole volumes can be measured using the positron annihilation lifetime spectroscopy (PALS). The comparison of the literature PALS data in the four polymers with the QLS shows an apparent correlation between the relaxation strength and the fractional dynamic hole volume in good agreement with the predictions of the model.