bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical study of atomic fluorine chemisorption on the Ni(100) surface
Authors: Siegbahn Per E. M. , Pettersson Lars G. M. , Wahlgren Ulf
Published: July 2002

Cluster size convergency of the chemisorption energy has been studied for the case of fluorine on Ni(100). Bond preparation of the cluster is found to be equally important for fluorine as for the previously studied hydrogen chemisorption. An estimate of the chemisorption energy for fluorine in the fourfold hollow site of Ni(100) is reached based on the average value for the bond-prepared clusters, a correction for the use of one-electron ECP’s, an estimate of the basis set limit and finally adding the effect of 3d correlation. The chemisorption energy is in this way estimated to be about 120 kcal/mol. Even though the bonding between fluorine and the surface should be regarded as almost totally ionic, there is still no correspondence between the chemisorption energy for a cluster and the highest ionization energy (Fermi level) of that cluster. The critical feature of bond preparation is that it allows the fluorine lone-pair electrons pointing down towards the surface to be fitted into the electronic structure of the cluster.
Spectroscopic constants and dipole moment functions of the 22 electron dications SiNe++, PF++, SO++, NCl++, and CAr++
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

The potential energy functions and spectroscopic constants of a series of eighteen 22 electron diatomic molecules with charge −1, 0, +1, and +2 have been obtained using version 1 of the coupled electron pair approximation (CEPA-1) with a Gaussian basis set of 101 functions, containing both f orbitals and diffuse s and p orbitals. All of these except the negative ions have also been treated by singles and doubles configuration interaction (CI-SD) and by the latter including a size consistency correction [CI-SD(s)] with a slightly smaller basis set (93 functions) that omitted the diffuse s and p functions. Both methods [CEPA-1 and CI-SD(s)] produce values of re and ωe for the neutral and monocation species that give deviations from experiment that are both small and smoothly varying across the series. Corrections based on these deviations have been applied to the re and ωe values of the spectroscopically unknown dications, SiNe++, PF++, SO++, NCl++, and CAr++, which possess either great kinetic stability, or in some cases, even thermodynamic stability, as published ab initio calculations have already demonstrated. Our estimated uncertainties for the corrected re and ωe values are different for the five dications, depending upon the available spectroscopically known comparison species and the consistency achieved between methods, but in the most favorable case (PF++) they are ±0.002 Å and ±10 cm−1. The dipole moment functions of the doubly charged ions have been calculated for the first time (using CI-SD) and used to estimate the intensities of the rotational and vibrational spectra.
Excited-state preparation and relaxation in the vibrational quasicontinuum
Authors: Marshall Kenneth T. , Hutchinson John S.
Published: July 2002

The preparation of highly vibrationally excited states in polyatomic molecules is analyzed by quantum mechanically modeling the vibrational quasicontinuum. Our model includes 10 000 coupled states in an energy range of 200 cm−1, grouped into ten ‘‘tiers’’ according to their first-order couplings to other states. Our results reveal that state preparation in the quasicontinuum is dominated by the presence of ‘‘intramolecular vibrational relaxation (IVR) resonance’’ states embedded in the quasicontinuum. This hypothesis is confirmed by calculations using the method of optical potentials of the resonance positions and widths. These IVR resonance states correspond to high-energy vibrational motions which are only partially localized and which relax slowly into the dense state background, and which are possibly analogous to unstable classical periodic orbits.
Ultrathin films under shear
Authors: Lupkowski Mark , van Swol Frank
Published: July 2002

We study the dynamic response of confined liquid films to shear with molecular dynamics simulations. Both the hydrostatic pressure and chemical potential are controlled during the shearing process. We introduce grand canonical molecular dynamics to allow for material fluctuations.
Dynamics of triatomic photodissociation in the interaction representation. I. Methodology
Authors: Williams Carl J. , Qian Jiwen , Tannor David J.
Published: July 2002

This paper presents a new, quantum mechanical, time dependent approach to the photodissociation of triatomic molecules in Jacobi coordinates. The algorithm is based on a nested interaction representation, designed to make the representation of the time evolving wave packet as compact as possible. The new equations of motion are solved numerically using a synthesis of grid techniques: the fast Fourier transform (FFT) method is used in Cartesian-like coordinates, and the discrete variable representation (DVR) method in the angular or bending coordinate. A variant on the short iterative Lanczos (SIL) procedure is used for the temporal propagation of the wave packet. Rotational state distributions obtained from this new algorithm are presented for the single surface photodissociation of ClCN and for the two surface photodissociation of ICN. The ClCN results are in good agreement with the semiclassical results of Barts and Halpern [J. Phys. Chem. 93, 7346 (1989)] and in excellent agreement with the time independent quantum results of Schinke [J. Chem. Phys. 92, 2397 (1990)]. Rotational state distributions for the two electronic surface photodissociation of ICN are in good agreement with the time independent quantum results of Guo and Schatz [J. Chem. Phys. 92, 1634 (1990)] and illustrate the flexibility of the method for dealing with nonadiabatic processes. The numerical efficiency of the method is comparable with standard time independent techniques, but has the attractive feature of yielding final state distributions at all energies from a single wave packet propagation.
Hole transport in bis(4-N,N-diethylamino-2-methylphenyl)-4-methylphenylmethane
Authors: Borsenberger P. M. , Pautmeier L. , Bässler H.
Published: July 2002

By time-of-flight photocurrent techniques, hole transport has been investigated in vapor deposited films of the title compound. The measurements were made over a wide range of fields and temperatures which encompassed the glass transition temperature. The results are described within the framework of the disorder formalism due to Bässler and co-workers. The formalism is based on the assumption that transport occurs by hopping in a manifold of states subject to both energetic (diagonal) and positional (off-diagonal) disorder. In this paper, we describe (1) the field and temperature dependencies of the photocurrent transients, (2) the role of polymer dynamics on charge transport at temperatures above the glass transition temperature, and (3) the spectroscopy of interfacial hole states derived from an analysis of photocurrent transients in terms of computer simulations conducted under the premise of energy selective carrier injection into a Gaussian distribution of states.
Theoretical study of the low-lying bound states of O2
Authors: Partridge Harry , Bauschlicher Charles W. , Langhoff Stephen R. , Taylor Peter R.
Published: July 2002

A b initio results are presented for the low-lying bound states of O2 that dissociate to ground-state atoms. Spectroscopic constants and dissociation energies (De) are reported for the X 3Σ−g, a 1Δg, b 1Σ+g, c 1Σ−u, A′ 3Δu, A 3Σ+u, 3Πu, 5Σ−u, 5Πu, and 5Πg states. For the first six states, which have been experimentally characterized, we obtain accurate results at the multireference configuration-interaction level after correction for higher excitations. For example, our computed De value for the X 3Σ−g state corrected for basis-set superposition error is only 0.7 kcal/mol less than the experimental value. The 5Πg state is estimated to have a De of 0.16±0.03 eV, suggesting that the importance of this state in the nightglow should perhaps be reconsidered.
Sequential clustering reactions of SiH+ with silane: An a
b i
n
i
t
i
o molecular orbital study
Authors: Raghavachari Krishnan
Published: July 2002

The mechanisms for the sequential clustering reactions of SiH+ with SiH4 have been studied by means of accurate quantum chemical techniques. Detailed reaction pathways including the necessary transition structures have been investigated to understand the structures and stabilities of the reaction intermediates and products. In accordance with the experimental observations of Mandich and Reents, the reactions lead to the formation of Si2H+3, Si3H+5, Si4H+7, and Si5H+11 without overall endothermic barriers. As seen experimentally, both reactive (dibridged) and unreactive (tribridged) isomers of Si2H+3 can be formed in the first reaction step. While Si2H+3 and Si3H+5 involve reactive structures with bridging hydrogens, Si4H+7 corresponds to a cyclic isomer (the second row analog of the cyclobutyl cation) which only forms a complex with silane. Silicon isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism.
Liquid–liquid immiscibility loops predicted with the simplified-perturbed-hard-chain theory
Authors: van Pelt A. , Peters C. J. , de Swaan Arons J.
Published: July 2002

In this study, it will be shown that in binary mixtures, type VI phase behavior, according to the classification of van Konynenburg and Scott [Philos. Trans. R. Soc. London, Ser. A 298, 495 (1980)], can be obtained from a simple, semitheoretical equation of state. The applied equation of state was derived from the simplified-perturbed-hard-chain theory (SPHCT). In literature, there are no known examples of type VI phase behavior being obtained from a simple equation of state. In addition, the systematic changes in phase behavior from type I via type V towards type VI will be discussed in this contribution. Surprisingly, a new type of phase behavior was found that was foreseen by Schneider [Ber. Bunsenges. Phys. Chem. 70, 497 (1966)]. It is proposed to call this new phase behavior type VIII.
Quantum reactive scattering of four-atom reactions with nonlinear geometry: OH+H2→H2O+H
Authors: Clary David C.
Published: July 2002

A quantum mechanical method is described for calculating state-selected cross sections and rate constants for four-atom reactions of the general form AB+CD→ABC+D with nonlinear geometry. The method involves using hyperspherical coordinates to describe the BC and CD bonds, accounting for both the rotation of the AB molecule and the bending mode of the ABC molecule with a spherical harmonic basis set, holding the AB spectator bond length fixed and applying a version of the bending-corrected-rotating line approximation to treat the rotation of the CD molecule. The method is applied to the OH(j)+H2(v)→H2O(n,m)+H reaction, and its reverse reaction, where v and j are vibrational and rotational quantum numbers, and n and m label bending and local OH-stretching vibrational states of the H2O molecule. A modified potential energy surface based on a fit to ab initio data is used. Comparisons of the calculated cross sections are made with quasiclassical trajectory calculations. The effect of the bending and stretching modes of the H2O molecule on the reaction is examined in detail. The calculated rate constants for the OH+H2 reaction agree very well with experiment over the whole temperature range of 250–1050 K.
Light scattering at the liquid to glass transition
Authors: Fuchs M. , Latz A.
Published: July 2002

Light scattering in a viscoelastic liquid is studied. The change from liquid to solid-like Brillouin spectra is explained using frequency dependent elasto-optic constants. Within a standard dielectric model microscopic formulas for these generalized frequency dependent constants and the spectra are obtained. The phenomenological pictures of light scattering in an amorphous solid and in a liquid are obtained as high and low frequency limits. The observation of a Rytov Dip in a system of optically isotropic particles is predicted for slow enough viscoelastic relaxation. Approximations using the Mode Coupling Theory allow the spectra to be expressed in terms of density fluctuations alone. The predictions of this theory for the spectra are discussed.
Thermal electron mobility and scattering in carbon tetrafluoride gas
Authors: Gee Norman , Freeman Gordon R.
Published: July 2002

Electron mobility has been measured in gaseous CF4 at low electric field strengths, E/n<1 Td (Td=10−21 V m2/molecule), at densities 0.7<n/1025 molecule m−3 <2.3, and temperatures 121<T/K<502. At E/n<0.2 Td the field-independent mobility μ0 of thermal electrons was attained. In the vapor at equilibrium with the liquid nμ0 increased on increasing n from 0.7 to 2 (1025 molecule/m3). Away from the gas/liquid coexistence curve nμ0 increased slightly as the gas was heated from 130 to ∼200 K, then decreased with further heating. The experimental values of nμ0 are compared to those calculated using cross sections recently reported in the literature, and the cross section values are modified.
Temperature and cluster size dependence studies of reactions of protonated water clusters with acetonitrile
Authors: Yang X. , Castleman A. W.
Published: July 2002

The temperature dependence of reactions of protonated water clusters H+(H2O)n with acetonitrile CH3CN was studied in a fast-flow reactor over the temperature range of 300–130 K and for cluster sizes from n equals 1 to 22. The measured rate constants were found to agree well with the calculated values based on a parametrized trajectory method [T. Su and W. J. Chesnavich, J. Chem. Phys. 76, 5183 (1982); over the entire range of the temperatures and cluster sizes studied. The rate constants measured in this work also agree very well with other reported values at room temperature. These results demonstrate that collisional rate constants for the type of the cluster-ion–molecules reactions studied here can be calculated accurately by the theoretical method at room as well as low temperatures. This conclusion not only has a direct bearing on other research fields, e.g., the modeling atmospheric ion chemistry at low temperatures, but also offers insight into the reaction dynamics of cluster-ion–molecule reactions as well.
Cluster ions: Gas-phase stabilities of NO+(O2)n and NO+(CO2)n with n=1–5
Authors: Hiraoka Kenzo , Yamabe Shinichi
Published: July 2002

Thermodynamic stabilities of cluster ions NO+(O2)n and NO+(CO2)n with n=1–5 were determined with a pulsed electron beam mass spectrometer. The measured binding energies of NO+(O2)n were found to be considerably smaller than those of NO+(N2)n, i.e., the solvating power of O2 toward NO+ is weaker than that of N2. For both NO+(O2)n and NO+(CO2)n, the cluster ions with n=3 were found to be relatively more stable toward dissociation. The experimental energies of NO+(CO2)n agree well with ab initio ones and are much larger than those of NO+(N2)n and NO+(O2)n. Structural difference between NO+(N2)n and NO+(CO2)n is also discussed. It is postulated that not NO+(O2) but NO+(CO2) is the key intermediate for conversion of NO+ to the hydronium series ions in the D region of the Earth’s atmosphere.
The direct photodissociation of ClNO(S1): An exact three-dimensional wave packet analysis
Authors: Untch Agathe , Weide Klaus , Schinke Reinhard
Published: July 2002

We present the results of a three-dimensional wave packet study on the photodissociation of ClNO through excitation of the first singlet state S1. The calculations employ an ab initio potential energy surface depending on the Cl–N and N–O bond coordinates and the ClNO bending angle. By expanding the wave packet in terms of the eigenfunctions of the NO rotor, the time-dependent Schrödinger equation is transformed into a coupled set of 60 two-dimensional partial differential equations which are solved by discretization on a grid. The wave packet yields the absorption spectrum and all partial dissociation cross sections for producing the NO fragment in a particular vibrational–rotational state (nj). The photodissociation of ClNO via the S1 state is a relatively fast process and the necessary propagation time is on the order of 50 fs. The calculated data agree well with recent experimental results. For the first time, we can directly compare the wavelength dependence of partial photodissociation cross sections for a single vibrational–rotational fragment state state with experiment. Our results suggest that the photodissociation of ClNO(S1) proceeds mainly adiabatically for the vibrational degree of freedom.
A coupled-cluster study of inversion symmetry breaking in the F+2 molecular ion
Authors: Watts John D. , Bartlett Rodney J.
Published: July 2002

Several coupled-cluster methods have been used to calculate equilibrium properties (re, ωe, and De ) of the ground state (2Πg) of the F+2 molecular ion. Two unrestricted Hartree–Fock reference determinants have been used. The first was made up of inversion symmetry constrained D2h orbitals, the second of inversion symmetry broken C2v orbitals. The results of the coupled-cluster calculations are rather insensitive to the choice of reference determinant, in contrast to what is observed for finite-order perturbation theory. It follows that for certain symmetry breaking problems, single reference coupled-cluster methods are sufficiently powerful to overcome a symmetry broken reference function, thus in principle obviating the need for either a multireference starting point or a symmetry constrained single reference starting point. Some extended basis set coupled-cluster calculations of equilibrium properties of F+2 and F2 were performed. Very good agreement with experiment was obtained for F2, suggesting that the results for F+2 are also reliable.
An analytical representation of the lowest potential energy surface for the reaction O(3P)+HCl (X 1Σ+)→OH(X 2Π)+Cl(2P)
Authors: Koizumi H. , Schatz George. C. , Gordon Mark S.
Published: July 2002

We present a new analytical representation of the lowest 3A″ energy surface for the reaction O(3P)+HCl(X 1Σ+)→OH(X 2Π)+Cl(2P). This surface is derived by fitting ab initio calculations, and adjustments are made to the barrier height so that rate constants derived from accurate quantum scattering calculations match the room temperature thermal rate constants. Contributions to thermal and state resolved rate constants arising from reaction on the lowest 3A′ surface are also considered. Comparisons of thermal and state resolved rate constants with experiment are good except at high temperature where the 3A′ contributions are significant.
Time-dependent treatment of vibrational predissociation within the golden rule approximation
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

The time-dependent formulation for vibrational predissociation of weakly bound complexes within the golden rule treatment is presented. The quantum wave packet propagation in this formulation eliminates the difficulty of strenuous long time propagation of the full wave function due to long-lived resonance which occurs in the more exact time-dependent treatment. As a result, the new time-dependent treatment of vibrational predissociation essentially parallels that of direct photodissociation and therefore requires only short time propagation of the wave function. The wave packet propagation is efficiently carried out in the interaction picture and numerical calculations of rotational state distributions and the total decay width are presented for vibrational predissociation of HeCl2. Correct rotational state distribution can be obtained with only a few time propagation steps owing to the use of large step size allowed in the interaction picture. Our approach provides an efficient method for the calculation of vibrational predissociation for more complex systems such as those with more than three atoms.
Numerical simulation of rodlike polymers in a uniaxial extensional flow: The distribution-function version versus the order-parameter version of Doi’s theory
Authors: Hu T. David , Ryskin G.
Published: July 2002

The molecular theory of nematic liquid crystals, proposed by Doi in 1981, exists in two versions: the original kinetic equation written in terms of the orientational distribution function and an averaged approximation of the original kinetic equation written in terms of the order parameter tensor. In this study, the two versions, as well as a third (hybrid) approach by Edwards and Beris (1989), are compared. Due to the complexity of the original kinetic equation, only uniaxial extensional flow in a homogeneous monodomain is considered. The solution of the original kinetic equation without simplifications is obtained for the first time—using a finite difference scheme with an iterative procedure for an integral equation. The solution of the kinetic equation written in terms of the order-parameter tensor (a nonlinear algebraic equation) is obtained by Newton’s procedure. For the ordered phase, the order-parameter version provides acceptable approximation only in the low-concentration region, the extent of which decreases with increasing velocity gradient. The hybrid approach of Edwards and Beris (1989) provides no improvement over the order-parameter version; in the presence of a strong flow, it actually gives worse results at low concentrations.
Scaling behavior of polyelectrolytes and polyampholytes: Simulation by an ensemble growth method
Authors: Higgs P. G. , Orland H.
Published: July 2002

We use a nondynamical Monte Carlo method to simulate chains with charged monomers interacting via Coulomb potentials and short-range excluded volume potentials. An ensemble of chains of length N is created by adding monomers to the end of chains in an existing ensemble of length N−1. This is done in such a way that the ensemble remains at equilibrium at each stage of the process. For a weakly charged polyelectrolyte in a θ solvent the chain remains Gaussian for short lengths (R2∼N) but becomes linear (R2∼N2) for N greater than a ‘‘blob’’ size N0. The crossover point N0 decreases with increasing charge density. The results support the predictions of scaling theories and show that the ‘‘electrostatic blob’’ idea can be taken quite literally. For a polyelectrolyte in a poor solvent the Coulomb repulsions are in competition with attractive short-range interactions. The chain configuration is found to be collapsed (R2∼N2/3) for NNτ, where the blob size Nτ is a function of a charge density and the excluded volume. If the Coulomb interactions are screened then there is a configurational change of the polymer when the screening length κ−1 is reduced to the electrostatic blob radius. This is shown to happen very gradually over a range of κ−1 for the θ solvent case, but abruptly for the poor solvent case, again in agreement with theory. We also consider a neutral polyampholyte chain containing an equal number of positive and negative charges. The effect of the Coulomb interactions is shown to be attractive, but we are unable to see the long-chain scaling behavior for the chains of length 220 generated here. We discuss the statistical errors and correlations in the ensemble growth method. The efficiency of the method compares favorably with dynamical Monte Carlo methods (e.g., reptation algorithm) since it permits the whole range of lengths 1−N to be studied in a single run.
Size and temporal dependence of the metastable decay probabilities of ionized rare gas clusters
Authors: Ji Y. , Foltin M. , Liao C. H. , Märk T. D.
Published: July 2002

The temporal dependence of the metastable fractions have been measured for monomer evaporation from Ar+20, Ar+50, Ar+100, Kr+20, Kr+50, and Kr+100 ions produced by electron impact ionization of the respective neutral cluster beams. Size and temporal dependence are found to be in reasonable agreement with the predictions of the evaporative ensemble model. Small variations from the model predictions observed (in particular for the evaporation channel involving the cluster with the magic number 20) may be rationalized within this model by adjusting the appropriate parameters.
Photodissociation of molecular chlorine in xenon matrices
Authors: McCaffrey J. G. , Kunz H. , Schwentner N.
Published: July 2002

The photodissociation of Cl2 in crystalline Xe is examined using synchrotron radiation for all state of Cl2 from the C 1Π1u state to the region of Xe exciton absorption. Isolation of atomic chlorine in two dominant trapping sites is observed following photodissociation throughout the spectral region studied. The production efficiency of the thermally stable trapping site was found to increase significantly with temperature and was most pronounced in the region of the onset of the Xe2Cl excitation feature. Correlation between these two processes is explained in terms of a charge-transfer-induced dissociation mechanism involving the formation of a Xe+(Cl2)− intermediate. In contrast to Ar and Kr matrices, photodissociation of Cl2 in Xe appears to occur without a pronounced cage effect. This observation is rationalized in terms of the larger lattice parameters of Xe which allows isolation of Cl2 and Cl at single substitutional and octahedral interstitial sites, respectively. From this structural information, simple models are presented for the production of Cl in the two different kinds of trapping sites. Thus production of the thermally unstable site involves the symmetric dissociation of Cl2 with both Cl atoms occupying octahedral interstitial sites separated by a single lattice constant of Xe. The thermally stable site involves the isolation of one Cl atom at the substitutional site originally occupied by the Cl2 parent molecule and the other at an octahedral interstitial site.
Monte Carlo evaluation of real time coherent state path integrals
Authors: Marchioro T. L. , Beck Thomas L.
Published: July 2002

An alternative formulation of the discrete real time coherent state path integral is examined. Following the development of a previous work [T. L. Marchioro II, J. Math Phys. 31, 2935 (1990)] the discrete coherent state action is transformed to second order form. The resulting path integral expression contains an integrand which factors into a Gaussian probability density and an oscillatory term, making the integral directly amenable to Monte Carlo techniques. Several simple examples in one and two spatial dimensions are presented and the possibilities of extending such an approach to general multidimensional systems are discussed.
The 846 nm A′ 3Σ−u←X 3Σ−g band system of jet-cooled V2
Authors: Spain Eileen M. , Behm Jane M. , Morse Michael D.
Published: July 2002

The 846 nm band system of jet-cooled 51V2 has been recorded using resonant two-photon ionization spectroscopy, and is assigned as the A′ 3Σ−u←X 3Σ−g band system. Both the Ω′=1u←Ω″=1g and Ω′=0+u←Ω″=0+g subbands of the 0–0 band have been rotationally resolved and analyzed. Although the transition could, in principle, correspond to a π*g←πu or σ*u←σg excitation, it is assigned as a δ*u←δg excitation. Either of the other two possibilities makes it difficult to rationalize the small reduction in bond length which accompanies the excitation [re(X 3Σ−g)=1.77 Å, r0(A′ 3Σ−u)=1.756 Å]. In addition, the second order spin–orbit splitting of the ground X 3Σ−g state is reanalyzed to predict the existence of a 1Σ+g state, arising from the same sσ2gdσ2gdπ4udδ2g electronic configuration as the X 3Σ−g ground state, at an energy about 1821 cm−1 above the ground state.

This allows a state previously observed in an electronic resonance Raman effect 1860 cm−1 above the ground state to be positively identified as the sσ2gdσ2gdπ4udδ2g, 1Σ+g state, which is responsible for the anomalous spin–orbit splitting in the ground X 3Σ−g state. A similar analysis of the spin–orbit splitting of the A′ 3Σ−u state predicts the location of the sσ2gdσ2gdπ4u dδ1gdδ*1u, 1Σ+u state at 13 426 cm−1 above the V2 X 3Σ−g(0+g) ground state.
Experimental determination of the specific opacity function for the Ba+HI→BaI(v=0)+H reaction
Authors: Vaccaro Patrick H. , Tsekouras Athanassios A. , Zhao Daqing , Leach Christine A. , Zare Richard N.
Published: July 2002

Through the use of laser-induced fluorescence spectroscopy, product rotational population distributions were recorded for the Ba(1S0)+HI(X 1Σ+)→BaI(X2 Σ+,v=0)+H(2S1/2) reaction under well-defined, crossed-beam conditions. In this kinematically constrained reaction, orbital angular momentum of the reagents Lreag is channeled almost exclusively into rotational angular momentum of the products Jprod. Consequently, ‖Jprod‖≂μvrelb, where μ is the reduced mass of the reactants, vrel is their relative velocity, and b is the impact parameter of the reactive collision. For relative velocity distributions with mean values ranging from 860 to 1000 m s−1, the BaI v=0 rotational distributions were found to peak sharply at high J values (≳420). Nonlinear least-squares analysis showed the specific opacity function (impact parameter distribution) for the formation of vibrationless BaI product to be exceptionally narrow (∼0.3 Å FWHM) with a pronounced maximum at the highest energetically allowed impact parameter ∼4.5 Å.
Liquid-state methods for random media. II. Spin glasses
Authors: Given James A.
Published: July 2002

The continuum replica method allows one to use equilibrium liquid-state theory to treat those nonequilibrium systems in which the quenched and annealed degrees of freedom correspond to distinct subsets of the particles in the system. In this paper, we provide a new generalization of the replica method that applies to a much larger class of continuum models. This involves using methods from the theory of chemical association to represent a particle as a bound state of pseudoparticles of different types or ‘‘species,’’ each of which carries some of the degrees of freedom of the particle. We use this method to study a realistic continuum spin glass. In particular, we show how to construct thermodynamic perturbation theory for the correlation functions of the system. We also show in detail how to apply association methods to study models of growth and aggregation treating, in particular, the Eden model and self-avoiding walks (SAWs).
Effects of magnetic field on the perturbation between the B 1Π and c 3Σ+ states of NaK
Authors: Baba Masaaki , Nishizawa Kiyoshi , Yoshie Naoki , Ishikawa Kiyoshi , Katô Hajime
Published: July 2002

Zeeman spectra of the strongly perturbed lines between the B 1Π and c 3Σ+ states of NaK were measured by the Doppler-free laser polarization spectroscopy. Magnetic field dependence of the Zeeman pattern was studied and the line shapes were found to change remarkably because of the mixing of the B 1Π and c 3Σ+ states. The patterns of Zeeman splitting were found to be different depending on whether the level was located on the high energy side or the low energy side of the interacting level. The observed Zeeman spectra were studied by taking account of the spin–orbit, hyperfine, and Zeeman interactions. The Zeeman spectroscopy is shown to be useful to analyze the perturbation between the excited states.
Size-selective Raman spectroscopy of carbazole–(Ar)n clusters at sub-wave-number resolution
Authors: Venturo Vincent A. , Maxton Patrick M. , Henson Bryan F. , Felker Peter M.
Published: July 2002

Size-selective Raman spectra of carbazole–(Ar)n clusters (n=0–14) obtained between about 1305 and 1325 cm−1 by mass-selective ionization-loss stimulated Raman spectroscopy are reported. The spectra are interpreted with respect to vibrational energy flow and argon evaporation, and are compared with vibronic spectroscopic results on the species.
Comment on the rate of vibrational predissociation of some RgCl2 and RgICl molecules
Authors: Zhao Meishan , Rice Stuart A.
Published: July 2002

We report the results of calculations of the rates of predissociation of HeCl2, NeCl2, HeICl, and NeICl using a version of the classical theory of unimolecular reaction rate which includes a balanced treatment of both the intramolecular energy transfer and fragmentation processes. The agreement between theory and experiment is very good.
Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. VI. CH3X→X−+CH+3 (X=F, Cl, Br)
Authors: Suzuki Shinzo , Mitsuke Koichiro , Imamura Takashi , Koyano Inosuke
Published: July 2002

Ion-pair formation from photoexcited halomethanes, CH3X*→X−+CH+3 (X=F, Cl, Br) has been studied by measuring photodissociation efficiency curves of X− using synchrotron radiation in the 9.9–27.5 eV photon energy range. A new spectral feature is observed in each of the curves near the threshold for the removal of an na1 electron from CH3X (n=4, 6, and 8, respectively, for CH3F, CH3Cl, and CH3Br ). This feature, composed of two or three peaks in each case, is interpreted as resulting from photoexcitation to the Rydberg states converging to CH3X+(C̃ 2A1), which then predissociate into ion pairs through avoided potential energy surface crossings. The interpretation is based on the results of the inner-shell electron energy loss study by Brion and co-workers and the photoabsorption study by Hochmann and co-workers. Peak features are also observed in the X− efficiency curves near the ionization threshold for CH3X+(X̃ 2E). The origins of these peaks are also discussed.
The use of magnetic rotation spectroscopy to simplify and presort spectra: An application to NiH and CeF
Authors: McCarthy Michael C. , Field Robert W.
Published: July 2002

The application of magnetic rotation spectroscopy (MRS) to the electronic spectra of gas phase NiH and CeF is examined. Experimental results for both MRS and laser excitation spectroscopy are presented for the NiH B 2Δ5/2–X 2Δ5/2 (1,0) and 2Φ7/2–X 2Δ5/2 (0,0) bands. A comparison between these two techniques clearly illustrates the ability of MRS to simplify and presort spectra. The power of MRS originates from its specificity for low-J, ΔΩ=ΔJ transitions and its ability, based on the sign and magnitude of the MRS effect, to partially presort rotational branches. A simple theoretical explanation of the MRS results, based on the structure of the underlying Zeeman pattern, is presented. Also, the extension of MRS to the more complicated and congested spectrum of CeF is presented. Using MRS, a previously unassigned CeF band, near 568 nm, has been definitively assigned as Ω′=4.5←Ω″=3.5.
Inversion of gas–surface scattering data for potential determination using functional sensitivity analysis. II. Extraction of the full interaction potential from low energy diffraction data
Authors: Ho Tak-San , Rabitz Herschel
Published: July 2002

We demonstrate that the full gas–surface potential can be successfully recovered from low energy elastic scattering intensities within the framework of functional sensitivity analysis and Tikhonov regularization as formulated previously. Specifically, we consider an extensive, but finite, set of simulated discrete data from specular scattering and three in-plane diffraction orders over a wide range of incident polar angles for the rigid, periodic, and corrugated He–Xe/C(0001) system at fixed beam energy and fixed azimuthal angle. It is found that the potential extracted from these input data unambiguously resembles the intended model potential. The recovered potential is only slightly steeper in the upper portion of the repulsive wall, and its minima are slightly deeper. Moreover, the repulsive walls of the recovered and true potentials cross the zero at almost the same distances from the surfaces. Finally, all of the details of the input data, namely, the positions and shapes of the bound-state resonance extrema for all scattering channels under study, are accurately reproduced by the recovered potential.
Finite size effects for the simulation of phase coexistence in the Gibbs ensemble near the critical point
Authors: Mon K. K. , Binder K.
Published: July 2002

In the Gibbs ensemble gas–liquid phase coexistence can be studied by obtaining the density distribution function in a finite system from the study of two subsystems exchanging particles. The temperature dependence of the peak of this distribution function is studied near the critical point, and Monte Carlo simulations for the simple special case of the two-dimensional lattice gas model are presented. This case is a ‘‘restricted Gibbs ensemble’’ where the particle numbers of the two systems fluctuate but their volume fluctuations are suppressed. From formal analysis and physical arguments, we predict that the density difference of the peak positions vanishes according to a classical power law [1−T/Tc(L)]1/2, where Tc(L) is a shifted critical temperature of the finite system of linear dimension L, for temperatures within a regime where fluctuations are significant (L does not exceed the correlation length ξ there). This behavior is verified by Monte Carlo simulations for L×L lattices with periodic boundary conditions and L in the range from 4≤L≤20. It is also shown that Tc(L) approaches the critical point of the infinite system from above, according to a law Tc(L)−Tc(∞)∼L−1/ν, ν being the correlation length exponent, as expected. Our discussion also applies to the standard grand canonical ensemble of the lattice gas or the equivalent canonical ensemble of an Ising magnet, respectively. Some Monte Carlo results for this ensemble are also presented. They are consistent with the theoretical predictions.
Effect of skeletal relaxation on the methyl torsion potential in acetaldehyde
Authors: Ozkabak Ali G. , Goodman Lionel
Published: July 2002

Fully-relaxed model ab initio calculations at Hartree–Fock/6–31G(d,p) and Mo/ller–Plesset (MP2)/6–31G(d,p) levels for acetaldehyde methyl conformers indicate significant skeletal flexing (e.g., the CH3 –C bond length changes by 0.006 Å) and methyl hydrogen folding. Thirteen methyl conformer energies at 15° intervals are used to assess the magnitudes of the torsional potential function expansion terms. Only two terms V3=373.8 and V6=3.4 cm−1 (both significantly different from those obtained from microwave and infrared analyses) are found to be important. These calculations clearly show that relaxation during methyl rotation (i.e., skeletal flexing and methyl hydrogen folding) is an important determinant of the torsional potential. Energy levels obtained from internal rotation potentials which include flexing simulate infrared torsional fundamental frequencies in CH3CHO and CD3CHO to within 1–2 cm−1 of the experimental values. In the absence of relaxation infrared torsional fundamental frequencies are poorly simulated by ab initio calculations.
State-to-state rate constants for quenching of xenon 6p levels by rare gases
Authors: Alford W. J.
Published: July 2002

State-to-state rate constants have been measured for xenon 6p[3/2]1,2, 6p[5/2]2,3, 6p[1/2]1, and 6s′[1/2]1 levels quenched by helium, neon, argon, and xenon. Some total rate constants and 6p radiative transition probabilities have also been measured. The time-dependent fluorescence from pumped and collisionally populated levels following pulsed excitation of a single level is observed. Data for different gas pressures are fit using a rate equation model that includes all relevant levels. Neon is found to have small total rate constants, while xenon has large total rate constants. Both neon and xenon quench to a number of nearby levels. Helium and argon have total rate constants varying greatly with 6p level. Helium tends to quench to the next lower level while argon usually quenches to many levels.
Electronic spectral shifts of the 1L
b(α) excitation of aromatic molecule–rare-gas heteroclusters
Authors: Shalev Eli , Ben-Horin Narda , Jortner Joshua
Published: July 2002

Microscopic spectral shifts of the lowest spin-allowed weak 1Lb(α) transitions of heteroclusters consisting of inert-gas atoms bound to naphthalene, phenanthrene, and pyrene were evaluated utilizing the multicenter monopole representation for the dispersive intermolecular interactions, in conjunction with a semiempirical scaling of the Hückel transition monopoles. These calculations predict that the spectral shift for the electronic origin of the 1La(p) transition is higher by a numerical factor of 2–4 than for the weak 1Lb(α) transition, in accord with experiment. The semiempirical theory was applied for the calculation of cluster size and isomer structure dependence of the 1Lb(α) spectral shifts in these heteroclusters.
Rotationally resolved photoelectron spectra in resonance enhanced multiphoton ionization of H2O via the C 1B1 Rydberg state
Authors: Lee M.-T. , Wang Kwanghsi , McKoy V. , Machado L. E.
Published: July 2002

In this paper, we extend a previous formulation of molecular resonance enhanced multiphoton ionization (REMPI) photoelectron spectra of diatomic molecules to treat rotationally resolved photoionization of nonlinear polyatomic molecules. Useful parity selection rules, which govern changes of angular momenta ΔKa and ΔKc, are also derived. As an example, we use this formulation to study rotational branching ratios and photoelectron angular distributions resulting from (3+1′) REMPI of H2O via the C 1B1 (3pa1) Rydberg state. Cooper minima are predicted to occur in the d wave (l=2) of the ka1 (λ=0) and kb1 (λ=1) photoelectron continua. The effects of these Cooper minima on rotationally resolved photoelectron spectra are also investigated.
Quantum free-energy calculations: Optimized Fourier path-integral Monte Carlo computation of coupled vibrational partition functions
Authors: Topper Robert Q. , Truhlar Donald G.
Published: July 2002

The Fourier coefficient path-integral representation of the quantum density matrix is used to carry out direct, accurate calculations of coupled vibrational partition functions. The present implementation of the Fourier path-integral method incorporates two noteworthy features. First, we use a Gaussian in Fourier space as a probability density function, which is sampled using the Box–Muller algorithm. Second, we introduce an adaptively optimized stratified sampling scheme in Cartesian coordinates to sample the nuclear configurations. We illustrate these strategies by applying them to a coupled stretch–bend model which resembles two of the vibrational modes of H2O. We also apply a simple, yet accurate method for estimating the statistical error of a Metropolis integration, and we compare the Box–Muller and Metropolis sampling algorithms in detail. The numerical tests of the new method are very encouraging, and the approach is promising for accurate calculations of quantum free energies for polyatomic molecules.
A molecular beam study of alkali promotion of NO sticking on Si(100): Local promotion in a single collision regime
Authors: Namiki A. , Suzuki S. , Kato H. , Babasaki Y. , Tanaka M. , Nakamura T. , Suzaki T.
Published: July 2002

Alkali–metal (K and Cs) promotion for sticking of nitrogen oxide molecule (NO) on Si(100) has been studied using a molecular beam method combined with an Auger electron spectroscopy (AES) and a laser ionization spectroscopy [resonance enhanced multiphoton ionization (REMPI)]. The observed sticking probability S shows a good correlation with alkali coverage, indicating that the alkali promotion is local in nature. The decay of S as a function of NO dose as observed with AES shows an anticorrelation with the evolution of the direct-inelastic scattering intensity as obtained with REMPI. This fact is understood as follows: since the direct-inelastic scattering occurs mostly in a single collision process with the surface, local alkali promotion is realized in a single collision of the incident NO molecule with the alkali–metal adsorbates. The decay of S as a function of NO dose is then analyzed with a reaction cross section. The evaluated reaction cross sections are close to the area of the 2×1 unit cell, and thus the estimated reaction radii are almost equal to but somewhat larger than the covalent radius of a K atom, with a increasing trend with alkali coverage. The alkali promotion is explained in terms of local electron charge transfer from the nonionized alkali adatoms to the affinity level of NO molecules based on the adatom density of state around EF.
Ab initio and semiempirical molecular orbital studies of surface enhanced and bulk hyper-Raman scattering from pyridine
Authors: Yang Wen-Hui , Schatz George C.
Published: July 2002

Ab initio and semiempirical molecular orbital methods have been used to calculate the bulk and surface Raman and hyperRaman spectra of pyridine. Similar calculations for benzene are also reported. For both molecules, we compare the results with available bulk and surface experiments. In all calculations, the polarizability and hyperpolarizability derivatives are evaluated using normal coordinates that are obtained from Hartree–Fock (HF) calculations. The Raman results are somewhat sensitive to the basis set used in determining the normal coordinates, and the hyper-Raman spectra are strongly sensitive. The Raman and hyper-Raman spectra have been calculated at zero frequency using HF methods (GAUSSIAN 90), and finite frequency using INDO (intermediate neglect of differential overlap) methods (zindo). The frequency dependence of the results is found to be minor. Comparisons with experiment indicate that the inclusion of diffuse basis functions is crucial to the accuracy of the HF calculations. For such basis sets, the benzene Raman and hyper-Raman spectra provided by HF are in excellent agreement with experiment. Raman spectra from HF for pyridine also agree well, but hyper-Raman spectra agree only in part. Changes in Raman and hyper-Raman spectra calculated in going from bulk to surface generally agree with observations. The corresponding bulk and surface spectra from zindo are usually less accurate, especially the hyper-Raman spectra, and especially the out of plane modes. Estimates of the surface hyper-Raman enhancement factor are in the range 1011–1012 for pyridine on silver electrodes.
Is TeF2−8 the MX2−n dianion with the largest electron detachment energy (5 eV)
Authors: Boldyrev Alexander I. , Simons Jack
Published: July 2002

TeF82− is predicted to be electronically stable with respect to TeF8− by ca. 5 eV and to have a geometrically metastable square antiprism structure that lies 43 kcal/mol above the TeF7−+F− asymptote but is separated from this asymptote by a barrier. Geometries and local harmonic frequencies have been calculated for TeF82− and TeF7− as well as for the related TeCl82− and SeF82− dianions and TeCl7− and SeF7− anions. The TeF82− species is found to have the largest electron detachment energy among these three candidates.
Collisional quenching of highly rotationally excited NH (A 3Πi)
Authors: Chappell Ellen L. , Jeffries Jay B. , Crosley David R.
Published: July 2002

Collisional removal of the v′=0 level of the A 3Πi state of the NH molecule has been studied as a function of rotational level with particular attention to high N′. Multiphoton dissociation of NH3 at 193 nm produces highly rotationally excited ground state NH (to N″=30), which is subsequently electronically excited with a pulsed dye laser, and quenching rate coefficients determined by the pressure dependence of the time-resolved fluorescence. Colliders investigated are NH3, CO, CH4, H2, and D2. The quenching rates at first decrease with increasing N′, but become nearly constant at highest N′. This is consistent with dynamic effects on an attractive, anisotropic surface. Radiative rates for A 3Πi were also determined and found to decrease with increasing N′ at a rate in excellent agreement with recent ab initio calculations.