bluuebunny/crossref_metadata_2025 huggingface dataset:
A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001). II. The dispersion of copper
Authors: Hoffmann F. M. , Paul J.
Published: July 2002

The dispersion of copper adsorbed on a Ru(001) substrate has been investigated by using Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) and carbon monoxide as a molecular probe. Copper films evaporated at 85 K show a drastically different CO adsorption behavior compared to annealed films and exhibit a variety of adsorption sites. Characteristic C–O stretching frequencies allow us to identify small copper clusters of 1–4 atoms (2138–2123 cm−1), two-dimensional (2120–2110 cm−1) and three-dimensional (2098 cm−1) copper aggregates. After annealing to 250 K copper films at sub- and monolayer coverages form well-ordered small two- and three-dimensional copper aggregates. Formation of the epitaxial monolayer or islands of copper (2082 cm−1) requires a surprizingly mild annealing temperature of 350 K. Further annealing to 540 K results in increasing domain size of the copper islands or annealing of defect sites of the epitaxial monolayer. Multilayer coverages of copper evaporated at 85 K exhibit C–O stretching frequencies found for high-index copper single crystal surfaces, e.g., (211) and (755). This indicates a large number of surface steps and protruding copper atoms associated with rough films. Annealing to 540 K results in a smooth copper layer with preferential (111) orientation (2075 cm−1). The vibrational data presented here for Cu–Ru(001) agree well with previous reports of CO adsorption on copper single crystals, supported or evaporated films, and matrix-isolated clusters. They further allow us to determine the dispersion of supported Cu–Pt and Cu–Ni catalysts from data in the literature.
A determination of the stabilities of N+2(N2)n and O+2(N2)n with n=1–11 from measurements of the gas-phase ion equilibria
Authors: Hiraoka Kenzo , Nakajima Genei
Published: July 2002

The gas-phase equilibria of solvation reactions of N+2 and O+2 ions with N2 were measured with a pulsed electron-beam high-pressure mass spectrometer. van’t Hoff plots of the equilibrium constants lead to the determination of the thermochemical stabilities for N+2(N2)n and O+2(N2)n clusters with n=1–11. While a covalent bond was formed in the cluster N+2⋅⋅⋅N2, the interaction is largely electrostatic for ligands in the clusters N+2(N2)n with n=2–11. The decrease in the −ΔH0n−1,n values of N+2(N2)n with n for n=2→11 are rather smooth, i.e., the clusters N+2(N2)n do not have any particularly stable shell structures except for n=1. On the contrary, O+2(N2)n was found to have the shell structure with n=4.
Scaled field particle theory of the structure and the thermodynamics of isotropic hard particle fluids
Authors: Rosenfeld Yaakov
Published: July 2002

The pair structure and the thermodynamics of isotropic fluids composed of hard particles of different shapes and concentrations are considered by a new approach which (i) unifies the diagramatic Percus–Yevick theory and the geometric scaled particle theory, and (ii) leads to analytic accurate approximations for the direct correlation functions and cavity distribution functions. The scaled-particle interpolation idea of Reiss et al. is achieved diagramatically, approximating the full expansion of the direct correlation functions by appropriately renormalized low order graphs, in which the size of the field particle (‘‘black circle’’) is scaled. It is shown that the lowest order scaled field particle approximation, involving only pair excluded volumes as variables, and pair overlap volumes as functions, contains the exact solution of the Percus–Yevick integral equation for the general hard sphere mixture in D dimensions. An exact geometric relation obeyed by the excluded volume (covolume) of two fused convex bodies and another single convex body, is used to show that the scaled field particle theory is equivalent to the

scaled particle expansion of the chemical potential in terms of the fundamental measures of the hard particles. This is in contrast to the conventional scaled particle Taylor series expansion in powers of the linear scale of the solute particle. The first order scaled field particle approximation and the ‘‘fundamental measure’’ scaled particle theory become identical when the fluid mixture contains only convex particles. For a D-dimensional (D=odd) hard spheres fluid mixture our new analysis enables to obtain the complete exact solution of the Percus–Yevick integral equation for the structure directly from its compressibility equation of state. This leads to the derivation of simple, analytic, geometric approximations for the direct correlation functions and cavity distribution functions of the general isotropic hard particle fluid, conformal to those for the hard sphere fluid mixture.
A determination of the stabilities of O+2(O2)n and O−2(O2)n with n=1–8 from measurements of the gas-phase ion equilibria
Authors: Hiraoka Kenzo
Published: July 2002

The gas-phase equilibria of solvation reactions of O+2 and O−2 ions with O2 were measured with a pulsed electron-beam high-pressure mass spectrometer. van’t Hoff plots of the equilibrium constants lead to the determination of the thermochemical stabilities for O+2(O2)n with n=1–8 and for O−2(O2)n with n=1–7. The fashion of the bonding in the clusters O+2(O2)n and O−2(O2)n changes drastically from covalent to electrostatic one between n=2 and 3, and n=1 and 2, respectively. That is, the first shells formed for these positive and negative cluster ions are O+2(O2)2 and (O2---O2)−. The formation of the second shells, O+2(O2)2(O2)2 and (O2---O2)−(O2)2, are also observed, but they are only slightly more stable than larger clusters toward dissociation.
A new contribution to spin-forbidden rare earth optical transition intensities: Gd3+ and Eu3+
Authors: Downer M. C. , Burdick G. W. , Sardar D. K.
Published: July 2002

Quantitative calculations show that numerous spin-forbidden linear optical transitions observed in trivalent rare earth ions acquire a major fraction of their intensity from hitherto neglected contributions involving spin–orbit linkages within excited configurations. Motivated by the importance of analogous linkages previously demonstrated in two-photon absorption, we derive a general expression applicable for all lN→lN transitions which can be incorporated into a revised Judd–Ofelt analysis of observed intensities. Presenting this revised analysis of observed linear absorption intensities for Gd3+ and Eu3+, we show that the new contribution is often comparable to standard contributions. With substantial modification of previously fitted phenomenological parameters, an improved fit to observed intensities is achieved, suggesting that reanalysis of linear intensity data for all trivalent rare earths is warranted.
Direct observation of adsorbate dynamics from low-frequency vibration on a step defect—CO on Pt(112)
Authors: Henderson M. A. , Szabó A. , Yates J. T.
Published: July 2002

The relative shape of the hindered translation potential energy well for CO chemisorbed on the step sites of a Pt(112) single crystal was determined with digital electron stimulated desorption–ion angular distribution (ESDIAD). The angular displacement of the CO molecule in its hindered translation well predominately determines the half-width at half-maximum (HWHM) of the neutral a3π CO species (CO*) produced by ESD, a species which may be imaged in an ESDIAD apparatus. Variations in the CO* ESDIAD HWHM as a function of temperature are due to population of higher levels of the hindered translational mode. By monitoring the CO* ESDIAD HWHM for CO coverages on the step below 50% step saturation (0.17 monolayer) as a function of temperature, we show that the steepness of the hindered translation potential well is different for CO vibrations up, down, and along the step edge, following the trend: down the steps>up the steps>along the steps. There is no coverage dependence in the CO* ESDIAD HWHM values up or down the steps, but the HWHM values along the steps at 0.17 monolayer CO are significantly broader than those from lower coverages in the temperature range of 150–350 K. We interpret this to indicate the production of transient structures of tilted CO on adjacent step sites formed from preferential diffusion of CO molecules laterally on the step sites. Thus, the experiments probe directly the anisotropy of lateral diffusion of the adsorbate on step sites.
Two-dimensional microwave Fourier transform spectroscopy
Authors: Vogelsanger B. , Bauder A.
Published: July 2002

New two-dimensional correlation techniques in the field of pulsed microwave Fourier transform spectroscopy are presented. Five different pulse sequences were used to demonstrate connections between rotational transitions with well separated or with closely spaced frequencies. The introduction of two- and four-step phase cycles to select the desired coherence transfer pathways simplified the two-dimensional spectra considerably. Experiments were performed on methyl formate, norbornadiene, 1-chloro-1-fluoroethene, and vinyl cyanide. A theoretical treatment is given in terms of the density matrix formalism. The relationship between our experiments and similar nuclear magnetic resonance (NMR) techniques is also discussed.
Molecular-thermodynamic approach to predict micellization, phase behavior and phase separation of micellar solutions. I. Application to nonionic surfactants
Authors: Puvvada Sudhakar , Blankschtein Daniel
Published: July 2002

We present a detailed description of a molecular-thermodynamic approach which consists of blending a molecular model of micellization with a thermodynamic theory of phase behavior and phase separation of isotropic (surfactant–water) micellar solutions. The molecular model incorporates the effects of solvent properties and surfactant molecular architecture on physical factors which control micelle formation and growth. These factors include (i) hydrophobic interactions between surfactant hydrocarbon chains and water, (ii) conformational effects associated with hydrocarbon-chain packing in the micellar core, (iii) curvature-dependent interfacial effects at the micellar core–water interface, and (iv) steric and electrostatic interactions between surfactant hydrophilic moieties. The free energy of micellization gmic is computed for various micellar shapes Sh and micellar-core minor radii lc. The ‘‘optimum’’ equilibrium values, l*c, Sh*, and g*mic, are obtained by minimizing gmic with respect to lc and Sh. The deduced ‘‘optimum’’ micellar shape Sh* determines whether the micelles exhibit two-dimensional, one-dimensional, or no growth. These results are then utilized in the thermodynamic theory to predict a broad spectrum of micellar solution equilibrium properties as a function of surfactant concentration and temperature. These properties include (1) the critical micellar concentration, (2) the micellar size distribution, (3) the critical surfactant concentration for the onset of phase separation, and (4) other thermodynamic properties such as the osmotic compressibility. The proposed molecular-thermodynamic approach provides an excellent description of a wide range of experimental findings in aqueous solutions of nonionic surfactants belonging to the polyoxyethylene glycol monoether and glucoside families.
Numerical solution of Schrödinger’s equation in polyatomic molecules
Authors: Becke A. D. , Dickson R. M.
Published: July 2002

We have developed a fully numerical, basis-set-free algorithm for solution of the Schrödinger single-particle equation in polyatomic molecules. As a test of the algorithm, the Hartree–Fock energy of H+3 is computed and compared with previous momentum-space benchmarks. The present calculations are the first successful basis-set-free calculations in coordinate space on a polyatomic molecular system.
Quantum nonadiabatic effects in the photodissociation of vibrationally excited CH3I
Authors: Guo Hua , Lao Kai Qin , Schatz George C. , Hammerich Audrey Dell
Published: July 2002

Photodissociation of vibrationally excited CH3I is studied using a time-dependent quantum mechanical formalism based on the fast Fourier transform (FFT) method. The dissociation dynamics is modeled with two active degrees of freedom, i.e., the dissociation coordinate and the C–H3 umbrella coordinate. The ground state vibrational wave functions are calculated using a time-dependent relaxation method proposed by Kosloff and Tal-Ezer. Two coupled excited states are explicitly considered in this model and the potential energy functions are taken from a previous study that was able to reproduce experiments for photodissociation of the CH3I ground state. We investigate the dissociation dynamics of the system after initial vibrational excitation, with particular attention paid to nonadiabatic transitions during the dissociation process. Our calculations show that vibrational excitation can significantly change the product I*/I branching ratio. In particular, it is found that there are significant dips in the I* yield at energies associated with minima in the absorption spectrum. These dips can be attributed to differences in Franck–Condon factors associated with the two excited state potential surfaces. Other observables of the dissociation process, such as the absorption spectrum and fragment vibrational state distributions, have also been investigated.
Equations of state for star polymers
Authors: Yethiraj Arun , Hall Carol K.
Published: July 2002

Monte Carlo simulations are performed to determine the equation of state of star polymers which are modeled as a collection of freely- jointed hard spheres with a central bead that has arms of equal length protruding from it. Three- and four-arm stars are considered with three and five beads per arm, respectively. The generalized-Flory dimer theory is extended to star and branched molecules. The theory’s predictions for the compressibility factor are in good agreement with the simulation data for the model star polymers.
Theoretical study of Si2Hn (n=0–6) and Si2H+n (n=0–7): Appearance potentials, ionization potentials, and enthalpies of formation
Authors: Curtiss Larry A. , Raghavachari Krishnan , Deutsch Peter W. , Pople John A.
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular-orbital theory, is used to calculate the energies of Si2Hn (n=0–6) and Si2H+n (n=0–7). The resulting G2 energies are used to calculate appearance potentials and ionization potentials. The results are in general agreement with the recent photoionization studies of Ruscic and Berkowitz [J. Chem. Phys. 95, XXXX (1991); 95, XXXX (1991)], supporting their observation of the species Si2H2, Si2H3, Si2H4, and Si2H5 for the first time. Atomization energies of the neutrals are calculated and used to derive enthalpies of formation. In most cases the results are in good agreement with previous theoretical values. In addition, new results for the structures of Si2H+6, Si2H3, and Si2H+, the enthalpies of formation of the cations, and proton affinities are presented.
Optimal control of curve-crossing systems
Authors: Gross Peter , Neuhauser Daniel , Rabitz Herschel
Published: July 2002

Controlling curve-crossing dynamics of a model diatomic system between two dissociative electronic states through radiative coupling with a third bound state is examined. Starting with an initial wave packet on one of the crossing surfaces, optimal control theory is used to design the radiative field to either enhance or eliminate (at our choice) selectivity of one product channel over another. A new optimization procedure is introduced which filters out dc and low frequency components from the optimal field, but still allows for resonant transitions to a third bound state. This procedure forces the fields to employ interesting physical mechanisms involving the bound state in order to control the electronic branching ratios rather than directly negating or enhancing the diabatic coupling term in the Hamiltonian. A new propagation scheme for a multisurface Hamiltonian using Pauli matrices is also presented.
Solvation dynamics in liquid water. A novel interplay between librational and diffusive modes
Authors: Roy Srabani , Bagchi Biman
Published: July 2002

A microscopic calculation of the solvation dynamics of an ion in liquid water is presented. The calculated solvation time correlation function shows an ultrafast Gaussian decay which carries about 70%–90% of the strength followed by a biexponential decay with time constants equal to 250 fs and 1 ps. These results are in excellent agreement with the computer simulations of Maroncelli and Fleming and also with the experimental findings of Barbara and Jarzeba. In addition, we find that both the rotational librations and the intermolecular translational vibrational modes of water contribute significantly to the initial Gaussian decay.
Accurate global fit of the H4 potential energy surface
Authors: Aguado Alfredo , Suárez Cristina , Paniagua Miguel
Published: July 2002

A global potential energy surface (PES) for the adiabatic ground state of the H4 system which fit published ab initio data [Boothroyd, et al. J. Chem. Phys. 95, 4331 (1991)] at a quantitative level has been obtained (root-mean-square error about 2 mhartree or 1–2 kcal/mol), and without any quantity of ad hoc character, preserving the accuracy of the ab initio points. The global fitting procedure used here is an extension of the corresponding procedure for triatomic systems including the functional form previously proposed by the authors. The global H4 PES obtained here is totally symmetric with respect to permutations of the hydrogen atoms and satisfies the criteria needed to be used in scattering calculations.
Study of the ArBr−, ArI−, and KrI− anions and the corresponding neutral van der Waals complexes by anion zero electron kinetic energy spectroscopy
Authors: Zhao Yuexing , Yourshaw Ivan , Reiser Georg , Arnold Caroline C. , Neumark Daniel M.
Published: July 2002

Three rare gas halide (RgX−) anions, ArBr−, ArI−, and KrI−, and the corresponding open-shell van der Waals complexes, ArBr, ArI, and KrI, were studied with anion zero electron kinetic energy (ZEKE) spectroscopy. Photodetachment of each anion accesses the three lowest-lying electronic states (the X1/2, I3/2, and II1/2 states) of the neutral complex. The spectra for each system reveal well-resolved progressions in the low frequency vibrations of the anion and the three neutral electronic states, providing a detailed spectroscopic probe of the Rg⋅X− and Rg⋅X interaction potentials. The line shapes observed in the ZEKE spectra are analyzed in terms of the line strengths of the underlying rotational transitions. From our data, we construct the potential energy curve for each neutral state as well as for the anion, and these interaction potentials are compared to potentials obtained from scattering and ion mobility experiments.
Dissociative electron attachment cross sections for halofluoromethanes
Authors: Underwood-Lemons Theresa , Gergel Thomas J. , Moore John H.
Published: July 2002

Dissociative electron attachment cross sections for the halofluoromethanes, CF3X and CF2X2 (X=Cl, Br, I), have been measured for electron energies less than 6 eV under single-collision conditions at temperatures of 393 and 563 K. Peaks in the cross sections as a function of electron energy are observed near threshold and in the vicinity of previously-identified negative ion resonances. The magnitude of the threshold enhancement depends on the nature of the halogen substituent. Changes with temperature include a slight increase in the cross section at threshold and a broadening of the peaks below 1 eV.
Path integral Monte Carlo study of SF6-doped helium clusters
Authors: Kwon Yongkyung , Ceperley David M. , Whaley K. Birgitta
Published: July 2002

The path-integral Monte Carlo technique is applied to study the SF6He39 cluster at low temperatures. The method employs as input only pair potentials, the number of atoms, and the temperature, and is thus independent of the trial function bias which can affect calculation of structural quantities in variational and diffusion Monte Carlo. We thereby obtain an unambiguous answer to the question of the location of SF6 in small clusters (39 He atoms), as well as the temperature dependence of the cluster structure. The cluster is found to undergo a gradual transition to a superfluid between 0.625 K and 1.25 K, and to evaporate significantly at temperatures above 2 K. We also calculate spectral shifts for the ν3 vibrational mode of SF6, using the instantaneous dipole-induced dipole mechanism. The results are compared with infra-red absorption measurements and with the previous ground-state quantum Monte Carlo calculations.
Simulation of electron transfer in polar solvents: Effects of nonequilibrium initial state preparation
Authors: Evans Deborah G. , Coalson Rob D.
Published: July 2002

An approximation method recently developed for computing the time evolution of electronic state populations in nonadiabatically coupled systems prepared in nonequilibrium nuclear configurations is applied to a model of a complex chemical system. A master equation formalism is used to describe an intramolecular electron transfer reaction in a polar fluid which is initiated by injection of an electron onto the donor site of the electron transfer complex. Time-dependent rate constants are obtained from molecular dynamics simulations based on electrostatic interactions between the solvent dipoles and the charge distribution on the electron transfer complex. (Appropriate Lennard-Jones potentials are also included to represent the finite size of the atoms involved.) From these rate constants electronic state populations may easily be obtained. The molecular dynamics data is also utilized in an attempt to construct an effective harmonic oscillator environment which can satisfactorily mimic the properties of the actual condensed phase medium. Some difficulties in constructing such an effective oscillator bath in cases typified by the electron injection scenario are pointed out.
Generalized Flory equations of state for hard heteronuclear chain molecules
Authors: Gulati Harpreet S. , Wichert John M. , Hall Carol K.
Published: July 2002

The generalized Flory (GF) and generalized Flory-dimer (GFD) equations of state have been extended to fluids containing hard heteronuclear chain-like molecules. Compressibility factor expressions have been derived for block, alternating and random ‘‘copolymer’’ fluids. The effect of composition and the relative size of the segments of a heteronuclear chain on the compressibility factor are studied. We have also performed molecular dynamics computer simulations on these systems using an extension of Rapaport’s algorithm in which the chains are effectively treated as hard spheres of different sizes held together by invisible strings. The compressibility factors predicted by the GFD theory for heteronuclear chain fluids are in very good agreement with our computer simulation results. The predictions of Chiew’s Percus Yevick theory are also in very good agreement with our computer simulation results on block copolymers.
Molecular dynamics studies of minicascades in electronically stimulated sputtering of condensed-gas solids
Authors: Johnson R. E. , Liu M.
Published: July 2002

An analytic model and molecular dynamics (MD) simulations are used to describe the collisional energy transport and sputtering initiated by a 1–4 eV atom or molecule in low temperature, solid Ar, O2, and N2. In these systems energetic exothermic processes, such as repulsive relaxation events following electronic excitation by a fast ion, an electron or an ultra violet photon, can give kinetic energy to an atom or molecule initiating a sequence of low energy collisions, a minicascade. When such an event occurs near the surface in a low-temperature, condensed-gas solid, atomic or molecular ejection can result. Using MD calculations the moving particles are tracked and the energy and angular spectra of the ejected particles determined. For a distribution of excitations which is uniform with depth, the average number of particles ejected is shown to be proportional to the average initial kinetic energy divided by the sublimation energy. The proportionality constant is only weakly dependent on the condensed-gas solid, since, for the molecular solids studied, the amount of the initial energy transferred into vibrational excitation of the ejecta is small. Therefore, measured electronic-sputtering yields provide an estimate of the energy release in energetic, nonradiative relaxation events. The ejecta exhibit a nearly E−2 kinetic energy spectra for recoil energies, E, much greater than the sublimation energy. The MD calculation of the yields and of the ejecta energy and angular distributions are interpreted here using an analytic model of a cascade of collisions in which the collisions in the solid are not assumed to be binary.
Self-consistent integral-equation theory of chain-molecular liquids. II. Improved intermolecular equations
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

Improved self-consistent intermolecular integral equations for a chain-molecular liquid are derived from the polymer Kirkwood hierarchy. The present work is a continuation of our recent work reported in a previous paper [J. Chem. Phys. 103, 2140 (1995)]. It is shown that the reference interaction site model (RISM) equation and extensions thereof can be obtained from the new intermolecular equations. The solutions of the new self-consistent set of intra- and intermolecular equations are compared with computer simulation data for chains with repulsive interaction potentials whose lengths N vary from 4 to 100 sites. The intermolecular correlation functions obtained from simulations are accurately reproduced. Comparisons with simulation data for the pressure equation of state and excess chemical potential show that the predictions of the self-consistent theory are accurate for packing fractions up to 0.4. These thermodynamic functions are found to scale as N for N≳16, implying that results obtained for short chains can be extrapolated to much longer chains.
Development and parametrization of continuum solvent models. II. A unified approach to the solvation problem
Authors: Horvath D. , Lippens G. , van Belle D.
Published: July 2002

In order to avoid the computational expense required to obtain precise numerical solutions of the Poisson–Boltzmann equation, different hypotheses have been introduced which led to less rigorous, but fast and simple solvation models. However, few systematic studies of the predictive features of such models have been reported. Comparisons between different continuum models are made difficult by the large variety of simplifying hypotheses or thermodynamic reference states used by their authors, so that the relationships between the proposed solvation terms is not straightforward. In the present work we consider various continuum models whose common feature is the description of the solvation process in terms of displacement of the high dielectric solvent by the low dielectric molecular bulk. We adapt these different models to work within the frame of the nonlinear fit algorithm developed in our previous study. This leads to a benchmark allowing to run rigorous comparative tests between all the different schemes. The unification of these allows us to derive new solvent models that combine the advantages of the original ones. We address also the problem of the optimal parametrization of the solvation terms and propose a new strategy of assignment of the atomic radii. It is shown that the exact solvation energies, which are in theory linearly related to the solvent displacement terms, cannot be expressed as a linear combination of the displacement effects evaluated with the usual simplifications. Nevertheless, nonlinear empirical models based on these simplified displacement terms are found to yield high quality predictions of vacuum-to-water transfer energies.
Critical behavior of viscosity for alginate solutions near the gelation threshold induced by cupric ions
Authors: Zheng Honghe , Zhang Qingzhi , Jiang Kai , Zhang Hucheng , Wang Jianji
Published: July 2002

The critical behavior of relative viscosity and intrinsic viscosity near the gelation point was experimentally examined for 1 g/100 mℓ alginate solution induced by cupric ions. The critical exponent of relative viscosity was found to be 1.27, which is in good agreement with the prediction of the percolation model based on a Rouse cluster approximation gives the exponent as 1.3. Critical behavior of intrinsic viscosity reveals that the constant of Huggins equation k′ for alginate clusters formed near the gelation point was considerably higher than that of linear alginate polychains in aqueous solution. Typically intramolecular crosslinking between polychains in the low cupric concentration could be detected in the plots of [η] vs molar fraction of cupric ions for the system. The weak divergence of intrinsic viscosity resembles the critical behavior of Zimm clusters predicted by the three-dimensional percolation theory rather than clusters of classical theory.
Spin relaxation of muonium-substituted ethyl radicals (MuCH2ĊH2) in the gas phase
Authors: Fleming Donald G. , Pan James J. , Senba Masayoshi , Arseneau Donald J. , Kiefl Robert F. , Shelley Mee Y. , Cox Stephen F. J. , Percival Paul W. , Brodovitch Jean-Claude
Published: July 2002

The spin relaxation of the muonium-substituted ethyl radical (MuCH2ĊH2) and its deuterated analog (MuCD2ĊD2) has been studied in the gas phase in both transverse and longitudinal magnetic fields spanning the range ∼0.5–35 kG, over a pressure range from ∼1–16 atm at ambient temperature. The Mu13CH213ĊH2 radical has also been investigated, at 2.7 atm. For comparison, some data is also reported for the MuCH2Ċ(CH3)2 (Mu-t-butyl) radical at a pressure of 2.6 atm. This experiment establishes the importance of the μSR technique in studying spin relaxation phenomena of polyatomic radicals in the gas phase, where equivalent ESR data is sparse or nonexistent. Both T1 (longitudinal) and T2 (transverse) μSR relaxation rates are reported and interpreted with a phenomenological model. Relaxation results from fluctuating terms in the spin Hamiltonian, inducing transitions between the eigenstates assumed from an isotropic hyperfine interaction. Low-field relaxation is primarily due to the electron, via both the nuclear hyperfine (S⋅A⋅I) and the spin rotation interactions (S⋅J), communicated to the muon via the isotropic muon–electron hyperfine interaction. At the highest fields, direct spin flips of the muon become important, due to fluctuations in the anisotropic part of the muon–electron hyperfine interaction. In the intermediate field region a muon–electron ‘‘flip–flop’’ relaxation mechanism dominates, due partly to the anisotropic hyperfine interaction and partly to modulation of the isotropic muon–electron hyperfine coupling. In the case of the T2 rates, electron relaxation mechanisms dominate over a much wider field range than for the T1 rates, and inhomogeneous line broadening also contributes. The fluctuations that induce both the T1 and T2 relaxation rates are described by a single correlation time, τc, inversely proportional to the pressure. An effective spin-reorientation cross section is deduced from this pressure dependence, σJ∼100±20 Å2, for all isotopically substituted ethyl radicals. This is similar to the geometrical cross section, but about a factor of 4 larger than values of σJ found for similar-sized diamagnetic molecules by gas phase NMR, primarily reflecting the longer range of the electron-induced intermolecular potential.
Spatial distribution of N2 and NO desorbing from a Rh(533) surface
Authors: Ikai M. , Janssen N. M. H. , Nieuwenhuys B. E. , Tanaka K.
Published: July 2002

The spatial distribution of N2 and NO desorbing from Rh(533), Rh(S)-[4(111)×(100)], was measured along the [65̄ 5̄] direction (along the steps) after adsorption of NO at 320 K. The N2 desorption at 450 K and 600 K showed a specific spatial distribution, that is, there was no peak either normal to the (111) terraces (θ=+14.4°) or normal to the (100) steps (θ=−40.3°), but a desorption maximum around θ=−15°. Integrated TPD spectra of N2 from the NO preadsorbed Rh(533) surface were very similar to those on Rh(111), but the spatial distribution of N2 from Rh(533) indicates desorption of N2 from the boundary between the (111) terraces and the vacant (100) steps. The spatial distribution of NO desorbing from the Rh(533) surface at 420 K obeyed cos θ, indicative of desorption normal to the crystal surface. Contrary to this, the spatial distribution of the desorption of NO molecules produced by the recombination reaction of adsorbed N and O desorbed at 550 K exhibits a peak intensity at θ=+15° (normal to the (111) terraces).
Semiclassical theory of multi-channel curve crossing problems: Landau-Zener case
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

A two-by-two diabatic propagation method is developed to deal with general one-dimensional multi-channel curve crossing problems. Each crossing is treated within the newly completed two-state semiclassical theory in the diabatic representation and is represented by a nonadiabatic transition matrix (I-matrix). A product of all the I-matrices yields the reduced scattering matrix for the entire system. This theory can handle the multi-channel curve crossing problems as simply as a two-state problem. An analysis of the complex crossing points in the momentum space can provide a comprehensive illustrative criterion of this method. A detailed severe numerical test is made by taking a seven-state system with 6 and 12 nonzero diabatic couplings. It is demonstrated that dominant processes among the state-to-state transitions are well reproduced by the present semiclassical theory.
Molecular dynamics study of a lipid bilayer: Convergence, structure, and long-time dynamics
Authors: Shinoda Wataru , Namiki Nobuo , Okazaki Susumu
Published: July 2002

Long time molecular dynamics simulations for the dipalmitoylphosphatidylcholine lipid bilayer in the liquid crystal phase could successfully be performed in the isothermal-isobaric ensemble using the Nosé-Parrinello-Rahman extended system method. Three independent 2 ns calculations show excellent convergence to the same equilibrium state of the system in about 0.5 ns. Various structural properties such as atomic distribution, order parameter, gauche fraction in the alkyl chains, and bent structure of the head group and sn-2chain were satisfactorily reproduced. Dynamic quantities such as trans-gauche transition were qualitatively in good correspondence to the experiment. The calculations presented a microscopic picture of the whole molecular conformations, including the finding that there is not a collective tilt in the bilayer. Some interesting dynamical observations concerning large structural fluctuations and pendulum motion of the alkyl chains were also made.
The state-to-state photodissociation dynamics of HOD(Ã)
Authors: Brouard M. , Langford S. R.
Published: July 2002

HOD, rotationally state selected in the third and fourth OH stretching overtone (|04〉, |05〉) levels, has been photodissociated via the Ã state at λ≃ 288 nm. In accord with previous studies, H + OD is the dominant dissociation channel, with the yield of OH photofragments being below the detection limit in both sets of experiments. Model Franck-Condon calculations suggest that the OD photofragment rotational state distributions, determined by OD(A-X) laser induced fluorescence (LIF), reflect primarily the rotational and zero-point bending motions in the selected rovibrational state of HOD(X̃). However, in contrast to the state selected photodissociation of H2O, the OD rotational distributions from HOD also reveal the influence of a small exit channel torque, which is enhanced in the photodissociation of the deuterated molecule by the shift in the OD centre-of-mass relative to that in OH. A modified Franck-Condon model, which accounts approximately for this modest impulsive torque, qualitatively reproduces the observed behaviour.
Quadratic configuration interaction versus coupled-cluster theory: Importance of orbital relaxation phenomena in CuH and CuF
Authors: Hrušák Jan , Ten-no Seiichiro , Iwata Suehiro
Published: July 2002

The potential energy surfaces, dipole moments, and spectroscopic constants of the ground states of CuH and CuF are calculated by using several single reference (SR) many electron theories. The methods used in this particular study are the coupled-cluster doubles (CCD), singles and doubles (CCSD), Brueckner doubles (BD), and quadratic configuration interaction singles and doubles (QCISD). All these methods are supposed to give the same results when the Hartree–Fock (HF) orbitals coincide with the approximate Brueckner orbitals. Furthermore, we examine the effect of the inclusion of perturbational triples to these methods, i.e., CCD(T), CCSD(T), BD(T), and QCISD(T). Since the chosen molecules, CuH and CuF, have large T1 amplitudes, the comparisons of the different methods offer critical examinations of the SR theories. For CuH, all the strict SR theories, i.e., the CCD, CCSD, BD, CCD(T), CCSD(T), and BD(T), result in parallel potential energy curves. The QCISD energy is, however, too low in comparison with the results of the other methods. Furthermore, the inclusion of triples, QCISD(T), gives an obviously wrong potential energy curve. Analyses of the calculated dipole moment based on the finite field method and the diagnostics of T1 amplitudes clearly demonstrate that in the QCISD method overestimates the single electron excitations around the equilibrium distance. The absence of disconnected T1 products in the QCISD is responsible for this failure. These effects are even more pronounced in the CuF case. The QCISD equilibrium bond distance, re=1.767 Å, matches the results of the other methods. However, the calculated dipole moment does not compare with the experimental nor with the other CC results, and De is overestimated. Severe failure was found for the QCISD(T) energy: the calculated curve possesses an unphysical double well profile. The dipole moment is overestimated by a factor of 3 while the calculated De is too low. The results indicate that the HF orbitals become less suitable at the bonding region and the QCl results consequently become less reliable within the SR many electron theories.
Electronic structure of H adsorbed on Pt13 clusters
Authors: Watari Noriko , Ohnishi Shuhei
Published: July 2002

Electronic structures of Pt13 clusters and those adsorbed with hydrogen atoms based on the first principles calculations are studied for Pt13 clusters of icosahedral (Ih) symmetry, cuboctahedral (Oh) symmetry and the systems of 8,12,14,20 hydrogen atoms adsorbing to them. Calculations have been done by the self-consistent local density functional scheme using the norm-conserving pseudopotential in the linear combination of the atomic orbital method. The equilibrium distances of Pt–Pt and Pt–H are calculated. The electronic structures of H adsorbed on Pt clusters are turned out to be the shell structure of “the giant atom.” H-1s electrons fill n=1 shell of Pt13H cluster with Pt-6s electrons. The energy levels of shell-2d of naked Pt13 clusters and shell-n=3,4 states of H adsorbed Pt13 clusters, which come from H-1s antibonding are compared with experimental results of in situ x-ray absorption near-edge structure (XANES).
Charge transfer and dipole moments of polyatomic systems
Authors: Winkler R. , Pantelides S. T.
Published: July 2002

Heteronuclear molecules have electric dipole moments because of electronic charge transfer among the constituent atoms. Quantum mechanical calculations reproduce the values of observed dipoles quite well, but easy-to-use model theories have so far failed to produce dipole moments in agreement with experiment. By combining density-functional theory and classical concepts, we obtain a simple predictive model for charge transfer which overcomes the shortcomings of earlier models based on the concept of electronegativity equalization. It yields dipole moments for many diatomic molecules and for the water molecule that are in satisfactory agreement with experiment. The model has promise as a supplement of classical molecular dynamics simulations for multicomponent polyatomic systems.
Experimental studies of cold electron attachment to SF6, CF3Br, and CCl2F2
Authors: Le Garrec J. L. , Sidko O. , Queffelec J. L. , Hamon S. , Mitchell J. B. A. , Rowe B. R.
Published: July 2002

A new technique has been used for the measurement of electron attachment rate coefficients for the molecules, SF6, CF3Br, and CCl2F2 at temperatures between 48 and 170 K. The results demonstrate very clearly the strong effect that internal vibrational energy of the molecules has on the attachment process.
Inelastic tunneling in long-distance biological electron transfer reactions
Authors: Medvedev Emile S. , Stuchebrukhov Alexei A.
Published: July 2002

The effect of protein dynamics on the long-distance biological electron transfer reactions is discussed. Computer simulations reported recently by our group [Daizadeh, Medvedev, and Stuchebrukhov, Proc. Natl. Acad. Sci. USA 94, 3703 (1997)] have shown that in some cases a strong dynamic coupling of a tunneling electron to vibrational motions of the protein matrix can exist. This results in a modification of the conventional picture of electron transfer in proteins. The new element in the modified theory is that the tunneling electron is capable of emitting or absorbing vibrational energy (phonons) from the medium. As a result, some biological reactions may occur in an activationless fashion. In the present paper we study analytically the probabilities of such inelastic tunneling events and show how they affect the overall dependence of the reaction rate on the driving force, temperature, and the strength of electron–phonon coupling. Harmonic and anharmonic models are proposed for vibrational dynamics of the intervening medium.
Ab initio characterization of the structure and energetics of the ArHF complex
Authors: van Mourik Tanja , Dunning Thom H.
Published: July 2002

The ArHF complex has been investigated using correlation consistent basis sets at several levels of theory, including Mo/ller–Plesset perturbation theory (MP2, MP3, MP4) and coupled cluster techniques [CCSD, CCSD(T)]. The three stationary points (the primary linear Ar–HF minimum, the secondary linear Ar–FH minimum, and the interposed transition state TS) on the counterpoise-corrected potential energy surface have been characterized. Calculations with the aug-cc-pV5Z basis set predict De for the Ar–HF minimum to be (with estimated complete basis set limits in parentheses) −215 (−218) cm−1 for MP4 and −206 (−211) cm−1 for CCSD(T). For the Ar–FH minimum and the TS, calculations with the d-aug-cc-pVQZ sets predict De’s (and CBS limits) of −97 (−99) and −76 (−78) cm−1 (MP4) and −93 (−94) and −75 (−76) cm−1 [CCSD(T)], respectively. The corresponding values for the H6(4,3,2) potential of Hutson [J. Chem. Phys. 96, 6752 (1992)] are −211.1±4 cm−1,−108.8±10 cm−1, and −82.6±10 cm−1. While the agreement of our CCSD(T) estimate with Hutson’s value is excellent for the global minimum, it is less so for the other two stationary points, suggesting that the H6(4,3,2) potential may be too attractive around the secondary minimum and the transition state.
Smart walking: A new method for Boltzmann sampling of protein conformations
Authors: Zhou Ruhong , Berne B. J.
Published: July 2002

A new Monte Carlo algorithm is presented for the efficient sampling of protein conformation space called the Smart-Walking (S-Walking) method. The method is implemented using a hybrid Monte Carlo protocol. The S-Walking method is closely related to the J-Walking method proposed by Frantz et al. (J. Chem. Phys. 93, 2769, 1990). Like the J-Walking method, the S-Walking method runs two walkers, one at the temperature of interest, the other at a higher temperature which more efficiently generates ergodic distributions. Instead of sampling from the Boltzmann distribution of the higher temperature walker as in J-Walking, S-Walking first approximately minimizes the structures being jumped into, and then uses the relaxed structures as the trial moves at the low temperature. By jumping into a relaxed structure, or a local minimum, the jump acceptance ratio increases dramatically, which makes the protein system easily undergo barrier-crossing events from one basin to another, thus greatly improving the ergodicity of the sampling. The method approximately preserves detailed balance provided the time between jumps is large enough to allow effective sampling of conformations in each local basin.
Energy transfer in rare gas collisions with self-assembled monolayers
Authors: Bosio Sylvie B. M. , Hase William L.
Published: July 2002

A molecular dynamics simulation is presented of a beam of neon atoms scattering off a n-hexyl thiolate self-assembled monolayer adsorbed on a Au {111} surface. Ab initio QCISD(T)/6-311++G** calculations, for a model system consisting of a neon atom and a methane molecule, were used to derive an accurate interaction potential between the neon projectile and the monolayer. Four initial translational energies of 1.2, 5, 20, and 40 kcal/mol and five incident angles (with respect to the surface normal) of 10, 30, 45, 60, and 80 degrees were investigated in the simulations. Both trapping desorption and direct inelastic scattering collisional events were observed. The fraction of trapping desorption decreases as the initial translational energy and/or the angle of incidence increases. For high initial translational energy, trapping desorption may involve penetration of the monolayer. The overall energy transfer probability and the fraction of trapping desorption are in good agreement with previous experiments [J. Chem. Phys. 99, 7056 (1993)] and computational studies [J. Chem. Phys. 100, 8408, 6500 (1994)] of neon atom beams colliding with liquid squalane surfaces. There is a small but measurable change in the energy transfer efficiency for neon atoms colliding parallel versus perpendicular to the “tilt angle” of the SAM. The distributions of the change in the azimuthal angle and the scattering angle between the neon final velocity vector and surface normal provide additional information about the collision dynamics. A general discussion of these simulation results is given, in the context of existing models and theories for gas/surface collisions, to achieve a more fundamental understanding of the dynamics of interfaces.
Rotational energy transfer of CH in the B (v=0) state by collisions with Ar and N2O using a time-resolved Fourier Transform spectrometer
Authors: Wang Chaung-Chi , Chin Thou-Long , Lin King-Chuen
Published: July 2002

By using a step-scan Fourier Transform Spectrometer, we studied rotational energy transfer (RET) by Ar and N2O collisions of the CH B (N⩽16, v=0) state. The findings of multiquantum changing collisions and markedly large rate constants imply that the RET collisions are dominated by attractive force.
Mean field curvature corrections to the surface tension
Authors: van Giessen A. E. , Blokhuis E. M. , Bukman Dirk Jan
Published: July 2002

Expressions for the surface tension σ, Tolman length δ, and for the rigidity constants k and k̄ of a curved liquid-vapor interface in mean field approximation are presented. The local free energy comes from the Carnahan-Starling equation of state in mean field, and the usual square gradient term is replaced with a more general, integral nonlocal term. The surface tension and the Tolman length are calculated numerically, and the behavior as the critical temperature is approached is discussed; this behavior is compared with results from a molecular dynamics simulation. Two appendices are included; the first presents derivations of alternative expressions for σ and δ, and the second gives an analytical calculation of the value of δ at the critical point. The Tolman length is found to be negative and approximately 0.2 molecular diameters in magnitude for all temperatures investigated. It is also found to approach a constant value of −0.20263 molecular diameters at the critical point, in agreement with previous results.