bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational excitations in CH4 II: Effect of Kr impurities
Authors: Grondey S. , Prager M. , Press W.
Published: July 2002

The perturbation of the partial orientational order in CH4II caused by dilution with Kr(xKr≤0.20) has been studied with inelastic incoherent neutron scattering. Both, the lines assigned as rotational tunneling and almost free molecular rotation, display a characteristic asymmetric broadening which gets more pronounced with increasing Kr concentration. At low Kr concentrations (xKr≤0.05) the tunneling spectra are well explained by a model which is based on a statistical distribution of local rotational potentials. In this model only the number of Kr atoms replacing orientationally ordered CH4 nearest neighbors is relevant for the local potential at the sites of tunneling molecules. The different local symmetries are not considered in the model. It was not possible to formulate a quantitative model for the scattering from molecules at nearly free rotor sites.
Phase transitions in phospholipid monolayers at air–water interfaces
Authors: Roland C. M. , Zuckermann M. J. , Georgallas A.
Published: July 2002

In this study we present a unified theoretical description of neutral, saturated, phospholipid monolayers at air–water interfaces. This model encompasses the liquid-condensed (LC) to liquid-expanded (LE) and liquid-expanded to surface gas (SG) phase transitions observed in such systems. The model is a lattice model of lipid hydrocarbon chains which allows for the introduction of free volume. The lipid chains can be upright in a ground or excited state or collapsed relative to the substrate. Furthermore, the chains interact via short range potentials due to steric, van der Waals, and dipolar forces. We show that the LC/LE phase transition is to be understood as a chain melting transition and through the growth of lipid domains across the transition. We further show that the LE/SG transition involves the creation of large amounts of free volume into which the lipid chains can collapse.
A theoretical application of coherent multicolor laser spectroscopy to selective control of singlet and triplet excitations in carbon monosulfide
Authors: Holme Thomas A. , Hutchinson John S.
Published: July 2002

A theoretical proposal for preparation of chemically interesting coherent superpositions of molecular eigenstates using more than one laser was recently presented [T. A. Holme and J. S. Hutchinson, Chem. Phys. Lett. 124, 181 (1986)]. In this paper, the proposed coherent two-color excitation process is applied to the separation and selective control of excitations of singlet and triplet manifolds. The method is applied to diatomic CS, using experimentally derived vibronic states and spin-orbit couplings, and realistic laser intensities. We show that frequency tuning of two lasers can yield control of branching ratios between different excited triplet manifolds. We also generalize the previous two-laser proposal to multicolor excitations, and illustrate the extension with a three-laser highly selective excitation of a singlet vibronic state.
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o calculations of transition probabilities and potential curves of SiH
Authors: Larsson M.
Published: July 2002

The complete active space SCF (CASSCF) method has been used to calculate the SiH A 2Δ–X 2Π electronic transition moment curve. The radiative lifetime deduced for v′=0 on basis of this transition moment curve and averaged over a number of rotational levels was 508 ns, which is in very good agreement with experimental results. The contracted CI (CCI) method was used to obtain the electronic potential functions of the A 2Δ and X 2Π states and the ground state electric dipole moment function. The dipole moment calculated at the equilibrium geometry (re=1.520 Å), μe=0.123 D, is in excellent agreement with previous theoretical results. The ground state dissociation energies of SiH and CH were calculated with comparable basis sets in order to resolve a discrepancy regarding the SiH dissociation energy which prevails in the literature. The calculated dissociation energy D0(SiH)=3.00 supports the value obtained from observation of a predissociation in the B 2∑+ state of SiH, D0≤3.06 eV. Very recently this upper bound to the SiH dissociation energy also found support from a photoelectron spectroscopy experiment. The previously observed predissociation in the A 2Δ state, which suggested a 10% higher dissociation energy, is discussed in some detail and it is shown that the prior interpretation of this predissociation as an interaction between the A state and the ground state continuum is reasonable, but only yields an upper limit to the dissociation energy.
Infrared absorption and microwave–infrared double resonance studies of Ne⋅OCS molecular beams
Authors: Hayman Garry D. , Hodge Jeremy , Howard Brian J. , Muenter John S. , Dyke Thomas R.
Published: July 2002

Infrared absorption spectra for molecular beams of Ne⋅OCS have been observed with a diode laser for the vibrational transition near 2062 cm−1 correlating with the monomer ν3 mode. The linewidths were ∼150 MHz (FWHM), giving rotationally resolved spectra and allowing the upper and lower vibrational state A, B, and C rotational constants to be determined along with the frequency of the band origin. No broadening in excess of that expected from Doppler effects and laser linewidth was observed, setting a lower limit of 10−9 s on the lifetime of the upper state. Rotational transitions for the vibrational ground state were observed by microwave–infrared double resonance experiments. The ∼150 kHz linewidths in these experiments increased the precision of the rotational constants and permitted the quartic centrifugal distortion constants for the ground state to be determined. The effective structure of the Ne⋅OCS complex was calculated from the rotational constant data. The vibrational frequency and structural results are discussed in relation to similar work on other rare gas–OCS complexes.
On the Franck–Condon behavior of the H+Cl2 reaction
Authors: Laganà Antonio
Published: July 2002

Extended quantum collinear calculations of the H+Cl2 reaction have been compared with model results in order to single out distinctive properties of reactive probabilities for a reaction thought to be governed by a Franck–Condon mechanism. Such an analysis has shown that even a simple model is able to rationalize the Franck–Condon contributions to the product vibrational distribution when the energy dependence of the exact quantum results is taken into account properly. On top of that, it has been shown that the product vibrational distribution of a Franck–Condon driven reaction need not have v+1 peaks (v being the initial vibrational state of reactants).
Model calculation of PDA–PA heterostructures: Influence of topological defects on localized states
Authors: Ruckh R. , Sigmund E. , Kollmar C. , Sixl H.
Published: July 2002

Model calculations for mixed polydiacetylene (PDA)–polyacetylene (PA) chains are generalized to the inclusion of mobile-soliton-like bond alternation defects. The properties of the midegap state of the solitary defect as well as the influence of the radical electron properties on localized states (‘‘quantum well states’’) of the chain are investigated on the basis of a Hückel band structure calculation. It turns out that the quantum well character of the chain strongly depends on the position of the bond alternation defect, e.g., whether it is located on the PA or PDA part of the chain.
The open-shell coupled-cluster method: Ionization potentials and electron affinities of the alkali atoms, Li to Cs
Authors: Kaldor Uzi
Published: July 2002

The open-shell coupled-cluster method with single and double excitations (CCSD) is applied to the alkali atoms, Li to Cs, with moderately large basis sets. The calculated electron affinities are all within 0.01 eV of experiment, with valence-shell correlation contributing almost all the correction to the (poor) Hartree–Fock results. Errors in the 2S ionization potentials range from 0.02 (Li) to 0.22 eV (Cs); with relativistic corrections estimated from numerical Hartree–Fock, IPs are within 0.1 eV of experiment. 2P IPs are even more accurate (0.02–0.04 eV).
Corrected effective medium method. I. One-body formulation with applications to atomic chemisorption and diatomic molecular potentials
Authors: Kress Joel D. , DePristo Andrew E.
Published: July 2002

We have derived a corrected effective medium (CEM) theory which describes the binding between an atom and an inhomogeneous host. As in all EM theories, the zeroth order term of the interaction energy is provided by the embedding energy of the atom into a spin-unpolarized homogeneous electron gas, and is obtained from self-consistent calculations within the local density approximation. Higher order terms provide corrections of two sorts: (1) the Coulomb interaction is accounted for by an explicit evaluation of the electrostatic interaction between the atom charge density and the host charge density; and (2) the difference in kinetic, exchange, and correlation energies between the atom/inhomogeneous system and the atom/homogeneous system is provided by a spin-polarized density functional evaluation. Both the Coulomb and difference energies are calculated non-self-consistently within the superposition of atomic densities approximation. A sampling procedure to obtain the homogeneous electron density from the inhomogeneous host density is derived by minimization of the contributions from the non-self-consistent difference term. Applications of the CEM theory are made to three types of systems that reflect a measure of difference in the spin polarization and inhomogeneity of both the atom and host spin density. We first describe the interaction of an H atom embedded into a spin-polarized homogeneous electron gas. Next, we calculate the binding potentials for a set of diatomic hydrides. Finally, we predict the interaction potentials for the chemisorption of H atoms on three different transition metal surfaces, Ni(100), Cu(100), and Fe(110).
Unimolecular dissociation of energy-selected methyl formate ion
Authors: Nishimura Toshihide , Zha Qingmei , Meisels G. G.
Published: July 2002

Fragmentation mechanisms of the methyl formate and methyl formate-d1 ions have been investigated using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The breakdown diagrams and ion kinetic energies of fragment ions were measured at internal energies of the molecular ion from 0 to 7 eV, and fragmentation pathways were elucidated. The formation of HCO+ showed a large kinetic isotope effect. The formation of CH3O+ from methyl formate-d1 is accompanied by a large kinetic energy release, about twice that calculated using quasiequilibrium theory (QET). Its relative abundance in the breakdown curve also appears to be inconsistent with statistical theory. Direct dissociation from an isolated state of the molecular ion was one possible explanation. Slow dissociations were observed for the formation of methanol ions resulting from a migration of the formyl hydrogen while QET predicts no metastable dissociation for this process. Two-component reaction rates for methanol ion formation suggest participation of more than one state or isomer. The heat of formation of HCOO was found to be −40±6 kcal/mol, indicating that it is less stable than COOH (ΔHf =−58+4 kcal/mol).
Electrophoretic light scattering from macromolecular solutions and conformational dynamics
Authors: Hubbard J. B. , McCammon J. A.
Published: July 2002

We propose a theory for the influence of conformational dynamics of solvated macromolecules on the translational self-correlation functions and associated laser light scattering spectra of the solution in an external electric field. This theory assumes that the electrophoretic drift velocity of a macroion may be coupled to its internal degrees of freedom via an unspecified auxiliary stochastic process to which we refer as ‘‘electrophoretic mobility fluctuations.’’ A surprisingly diverse set of fluctuation-generated spectral signatures, which depend strongly on the underlying statistical model, emerge from the analysis. These effects range from skewed Lorentzian profiles to satellite lines having a field-dependent amplitude.
Random tempering of Gaussian-type geminals. II. Molecular systems
Authors: Alexander S. A. , Monkhorst H. J. , Szalewicz K.
Published: July 2002

We use random tempering formulas to create a basis set of explicitly correlated Gaussian-type geminals for the calculation of the second-order energy of lithium hydride. Like our earlier calculations on the beryllium atom, this technique matches results obtained by the full optimization of all nonlinear parameters but requires considerably less computational effort.
Third-order a
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o calculations of the intensity parameters for the Pr+3 ion in LaAlO3, NdAlO3, and LaCl3 hosts
Authors: Smentek-Mielczarek Lidia , Hess B. Andes
Published: July 2002

The values of intensity parameters for the Pr+3 ion in LaAlO3, NdAlO3, and LaCl3 hosts evaluated with complete ab initio calculations based on the third-order f↔f transition theory are reported. In this theory, the intensity parameters, in addition to the standard second-order contributions, contain the static and dynamic third-order contributions which arise from electron correlation within the central ion. In the numerical procedure performed, all the one-particle effective operators which originate from singly excited intermediate configurations were included and the radial factors were evaluated within the perturbed-function approach. For the first time the discussion of the influence of electron correlation upon the values of intensity parameters is presented, and a comparison with experiment is made.
Molecular dynamics simulation of alkali-silicate glass structures
Authors: Tesar A. A. , Varshneya A. K.
Published: July 2002

The method of molecular dynamics (MD) was used to simulate the structure of R2O–SiO2 (R=Li, Na, K) glasses in an effort to identify compositional fluctuations which could act as precursors to phase separation. Radial distribution functions (RDF) for various ion pairs, vibrational frequencies, and coordination numbers for the various ions, the Si–O–Si and Si–O–R bond angle distributions were calculated. No definite composition fluctuations could be observed with variations in alkali type or content. Significant changes in the basic Si–O network characteristics were, however, observed when the volume was forced to vary. In addition to the usual objections of lack of directionality and unreasonably high fictive temperatures associated with the MD calculations, the problem of defining a molar volume correctly could be equally critical to determine composition fluctuations in the R2O–SiO2 glasses.
Dimerization in mixed-stack organic charge–transfer crystals
Authors: Luty Tadeusz
Published: July 2002

A model of dimerization of uniformly spaced chain of donor and acceptor molecules is suggested. It is based on a local instability of intracharge–transfer mode which can be electronically driven due to coupling of the intermolecular mode to charge–transfer exciton and bond charge. This local dimerization, enhanced by indirect coupling between intermolecular and intramolecular vibrations (via the charge–transfer exciton) originates a noncollective process of growing new phase, the process which can be seen as a solid state chemical reaction.
Energy transfer kinetics of singlet molecular oxygen: The deactivation channel for O2(b 1∑+g)
Authors: Knickelbein Mark B. , Marsh Kenneth L. , Ulrich Otho E. , Busch George E.
Published: July 2002

A tunable alexandrite laser was used to produce O2(b 1∑+g) in the gas phase, with quantitative measurement of O2(b 1∑+g) and O2(a 1Δg) densities achieved by detection of the b→X fluorescence and dimol emission signals, respectively. A simple kinetics analysis indicates that collisional deactivation of O2(b 1∑+g) by ground state oxygen molecules occurs with unit production of O2(a 1Δg). These results are in accord with previous assumptions regarding collisional deactivation of O2(b 1∑+b) based on spin conservation.
High resolution spectroscopy of diphenyldecapentaene and diphenyldodecahexaene
Authors: Horwitz James S. , Itoh Takao , Kohler Bryan E. , Spangler Charles W.
Published: July 2002

Fluorescence and fluorescence excitation spectra of single crystal and polycrystalline solutions of diphenyldecapentaene in n-decane and diphenyldodecahexaene in n-dodecane have been measured at 77 and 4.2 K. At 4.2 K the spectra are well resolved (FWHM bandwidths ≂10 cm−1) and, for the principal site, the energy for the 2 1Ag(S1) to 1 1Ag(S0) origin band of diphenyldecapentaene in n-decane is 20 133.3 cm−1 and that of diphenyldodecahexaene in n-dodecane is 18 261.4 cm−1. Analysis of these data includes the determination of fluorescence decay rates and C–C and C=C vibrational frequencies in the ground and excited states.
Observation of the gas-phase infrared spectrum of BH3
Authors: Kawaguchi Kentarou , Butler J. E. , Yamada Chikashi , Bauer S. H. , Minowa Tatsuya , Kanamori Hideto , Hirota Eizi
Published: July 2002

The gas-phase spectrum of BH3 was detected for the first time by diode laser spectroscopy through the observation of the ν2 band. The borane was produced by photolysis of B2H6 or BH3CO with an ArF excimer laser. The same spectrum was obtained by a discharge in a B2H6 and He mixture. Line assignments were made for the Q-branch series with J=K of the ν2 band; effects due to Coriolis perturbations by the ν4 state were recognized.
The populations of bridge and top site CO on Rh(100) vs coverage, temperature, and during reaction with O
Authors: Gurney Bruce A. , Richter Lee J. , Villarrubia J. S. , Ho W.
Published: July 2002

The intensities of the stretch modes of CO adsorbed in bridge and top sites on Rh(100) are presented as a function of coverage, temperature, and during reaction with O. Following adsorption at 90 K, the top site is predominantly occupied at low coverages, and at 0.5 atomic monolayers (ML) only the top site is occupied and a c(2×2) low energy electron diffraction (LEED) pattern is observed. With higher coverages the bridge site is increasingly occupied, and at saturation a p(4(2)1/2×(2)1/2)R45° LEED pattern is observed. The occupations of bridge and top sites at a fixed (0.5 ML) total coverage are observed to vary reversibly when the temperature is linearly ramped from 87 to 371 K and back down to 90 K; the difference in the binding enthalpy of bridge and top sites is determined to be 1.10±0.06 kcal mol−1. The enthalpy difference between bridge and top sites is also obtained from temperature programmed EELS (TP-EELS) which follows the site occupations as the temperature is increased past desorption. Surface order is found to have a significant effect on the normalized intensities of CO in conventional EEL spectra. The normalized intensities in angle integrated spectra, obtained by modulating the voltage difference applied to the two halves of the split analyzer input lenses, are insensitive to changes in the degree of surface order. The effect of low frequency frustrated rocking motions of diatomic molecules on the temperature dependence of the intensities observed in electron energy loss spectroscopy (EELS) is discussed. TP-EELS of coadsorbed CO and O allows the direct observation of the changes in the occupation of CO binding sites during reaction, and the results are correlated with LEED and temperature programmed reaction spectroscopy (TPRS) measurements of the reaction. Segregation of the reactants during the reaction is important.
Dynamical analysis of molecular decay at spherical surfaces
Authors: Leung P. T. , George Thomas F.
Published: July 2002

A dynamical approach to the classical decay rates for molecules near a dielectric sphere is presented through the application of the diffraction theory for a dipole antenna established by Van del Pol and Bremmer. This theory is somewhat simpler than but formally equivalent to that established by Ruppin and preserves a feature which is closer to the method of the theory established by Chance, Prock, and Silbey for a flat surface. The results, when compared to those obtained from the static image theory, show that this latter theory can be very inaccurate for large molecule-sphere distances or highly conducting spheres, consistent with previous findings for surfaces with perfect flatness or small roughness.
Coupling between channels with Δv≠0 and/or Δl≠0 in the Rydberg states of He2
Authors: Ginter Dorothy S. , Ginter Marshall L.
Published: July 2002

Interactions among bound Rydberg states of the helium dimer are descri A summary of the character and strengths of these interactions is given. (AIP)
Laser measurements of the unimolecular kinetics of energy-selected molecular ions: Isotope effects in benzene
Authors: Kühlewind H. , Kiermeier A. , Neusser H. J. , Schlag E. W.
Published: July 2002

Metastable ion decay rates for isotope decay channels differing only by one mass unit have been measured in a reflectron mass spectrometer with high mass resolution. Several decay channels with H, H2, C2H2, and C3H3 (and their isotopic analogs) elimination from C6H+6, C6D+6, and C6D5H+ are investigated. We present examples of intermolecular and for the first time of intramolecular kinetic isotope effects for internal energy selected reactants. The reaction coordinate for the decay channels can be characterized as C–H or alternatively as C–C stretching vibrations and the total looseness of the activated complex with respect to the remaining 3N−7 vibrations can be attributed to the subsets of the C–H and C–C vibrations. Furthermore, from the good agreement of RRKM calculations and all inter- and intramolecular isotopic results for a given decay channel it is concluded that C–H vibration participate in the random energy flow before dissociation.
Explicit solutions of normal form of driven oscillatory systems
Authors: Tsarouhas George E. , Ross John
Published: July 2002

We consider an oscillatory dissipative system driven by external sinusoidal perturbations of given amplitude Q and frequency ω. The kinetic equations are transformed to normal form and solved for small Q, near a Hopf bifurcation to oscillations in the autonomous system, for ratios ωn to the autonomous frequency of irrational so that the response of the system is quasiperiodic. The system is assumed to have either two variables or is adequately described by two variables near the bifurcation, and we obtain explicit solutions for this general case. The equations show interesting effects of external perturbations on limit cycles, both stable and unstable. Next we treat a specific model (Brusselator) and show by comparison with results of numerical integration that the theory predicts well the shape of the perturbed limit cycle, its variation with changes in constraints and parameters, and the point of transition from quasiperiodic to periodic response.
Assessment of the spherical-averaging approximation for the interpretation of electron spin-echo modulation for cupric ion complexes
Authors: Anderson Michael W. , Kevan Larry
Published: July 2002

A comparison is made between three methods for the simulation of deuterated ligand modulation from cupric ion complexes as powders using electron spin-echo modulation (ESEM) spectroscopy: (a) the spherical-averaging approximation which assumes both isotropic g and a random distribution of deuterium nuclei about the paramagnetic center, (b) an average which is performed over specific orientations of the field H0 with respect to g∥ using analytical expressions for the spin-echo intensity for an axially symmetric g tensor, and (c) a similar approach to (b) but with full geometrical correlation between the deuterium nuclei. Approach (c) is found to be unnecessary because of the degree of freedom of the ligand molecules to rotate about the ligand axis before freezing locks them into a particular configuration. This, in effect, removes any strict correlation between the deuterium nuclei. Approaches (a) and (b) are found to be equivalent when (i) the electron–nuclear distance r is greater than 0.35 nm, (ii) r is less than 0.35 nm but the number of interacting nuclei, N, is 6 or greater, and (iii) r is less than 0.35 nm, N is less than 6, and the angle θI between g∥ and r is greater than 45°. In all cases microwave irradiation must be near g⊥. When the situation is none of the above, then parameters defined by the spherical-averaging approximation should be interpreted with caution.
Electron energy loss spectroscopy of disilane
Authors: Dillon Michael A. , Spence David , Boesten Ludwig , Tanaka Hiroshi
Published: July 2002

Electron energy loss spectra of disilane have been recorded over an excitation energy range of 20 eV employing electrons of 20 and 200 eV incident energy for scattering angles of 0°–90°. Every transition detected except one appears at an energy consistent with the first observed members of Rydberg series converging to one of four possible ion states. The first two observed transitions belong to (2a1g)2→1,3(2a1g,4s) dipole forbidden channels appearing at excitation energies of ∼6.3 and 7.05 eV for the triplet and singlet, respectively. Evidence is presented for the identification of additional forbidden transitions as well as possible low-lying valence transition.
Gas-phase infrared spectroscopy of ClHCl−
Authors: Kawaguchi Kentarou
Published: July 2002

The ν3 band of the hydrogen bichloride anion in a D∞h structure has been detected by infrared diode laser spectroscopy. The anion was produced in a discharge of a CHCl3, H2, and He mixture. Analysis of the observed spectra gave the fundamental frequencies; ν3(35ClH35Cl−)=722.8965(2) and ν3(37ClH35Cl−)=722.9589(2) cm−1 with one standard deviation in parentheses. The anomalous isotope shift was explained by an interaction between the ν3 and 2ν1 states in 37ClH35Cl−. From the rotational constant obtained, the Cl–Cl internuclear distance in the ground state was calculated to be r0=3.146 76(5) for 35ClH35Cl− and r0=3.146 64(6) Å for 37ClH35Cl−.
Estimating the relativistic energy by diffusion quantum Monte Carlo
Authors: Vrbik Jan , DePasquale Michael F. , Rothstein Stuart M.
Published: July 2002

We report formulas to estimate the relativistic corrections for atoms or molecules (treated as the first order perturbation) by diffusion quantum Monte Carlo (DQMC) simulations. Our formulas involve primarily quantities which are routinely computed in nonrelativistic simulations. Hence our approach entails only minor additional modifications of existing (all electron) DQMC codes. We illustrate our algorithm by estimating various relativistic corrections to the ground state energy of LiH.
Electronic excitations in polyaniline: An INDO/S-CI study
Authors: Sjögren B. , Stafström S.
Published: July 2002

We investigate the electronic excitation in aniline tetramers at various levels of oxidation and protonation. We show that for certain geometrical conformations there is excellent agreement between our results and recently published experimental data. The neutral forms of the tetramer are shown to have excitation spectra almost identical to polymer equivalents. For the doubly protonated emeraldine tetramer we propose a bipolaron type of defect state, unlike in its polymer equivalent for which a polaronic metal model has been proposed. The electronic transitions associated with the bipolaron are shown to involve only one bipolaron level in contrast to the common picture of two bipolaron levels.
General transform technique for simultaneous frequency shifts and linear non-Condon coupling in resonance Raman scattering
Authors: Lu H. M. , Page John B.
Published: July 2002

The optical absorption→resonance Raman profile ‘‘transform’’ technique is generalized to the case of first-order profiles of modes having simultaneous linear non-Condon coupling and frequency shifts under electronic excitation. It is shown how the new transform follows, within well-defined approximations, from our recent general time-correlator analytic model expressions for the absorption and RR profiles of nonzero temperature multimode systems having both types of coupling. Mode mixing is assumed negligible for the Raman mode, but may be present for the remaining modes. It is found that the argument of the shift operators occurring in the transform is the Raman mode’s excited state frequency, in both the Condon and non-Condon terms. Moreover, it is shown that the Raman mode’s linear non-Condon and linear electron–phonon coupling constants are renormalized by the mode’s frequency shift. It is these model parameters which a transform analysis allows one to extract directly from measured absorption and profile data, a priori of explicit multimode modeling. Numerical illustrations of the new transform for a four-mode model system are presented, and the importance of including both non-Condon coupling and frequency shifts is demonstrated.
Long-range electron transport in random trapping models
Authors: Kundu Kalyan , Izzo Dora , Phillips Philip
Published: July 2002

We extend the standard random trapping (RT) model of electron transport on d-dimensional hypercubic lattices to include long-range charge transfer. We show that for RT models, it is possible to decouple exactly the site energy disorder from the distance dependence of the hopping rate. We then apply the exit probability approach developed by us to the formulation of the transport properties. At short times for exchange-type rates, we show that it is the site energy disorder rather than the distance dependence of the hopping rate that determines the short-time dependence of the mean-square displacement. The analytic structure of the short-time expansion is shown to be consistent with the corresponding limit of a typologically disordered random hopping problem. At long times we consider exchange and multipolar models for the hopping rates. For a particle initially placed at the origin, it is shown that the long behavior of the diffusion coefficient [D(t)−D(t=∞)]∼t−d/2, is identical to the asymptotic behavior of D(t) for nearest-neighbor transport. The implications of these results on photoconductivity are discussed.
Relaxation of excited normal modes in benzene
Authors: Guan Yuhua , Thompson Donald L.
Published: July 2002

Classical trajectories have been used to investigate energy relaxation of various excited C–C stretching, CCH bending, and CCC bending normal modes in benzene. These normal modes were initially excited up to about 10 000 cm−1. The calculated relaxation of the excited C–C ring breathing mode is in agreement with the recent experimental results of Chernoff, Myers, and Pruett [J. Chem. Phys. 85, 3732 (1986)]. In general, the relaxation of the excited normal modes involving C–C stretching, CCH bending, and CCC bending are at least several times slower than the relaxation of the CH stretch local mode. Mode-to-mode energy transfer was also investigated. Only those modes with frequencies close to that of the excited mode participate in the energy transfer.
A dynamic light scattering study of 1,3,5-tri-α-naphthyl benzene: A supercooled liquid
Authors: Ma R.-J. , He T.-J. , Wang C. H.
Published: July 2002

A dynamic light scattering study of 1,3,5-tri-α-naphthyl benzene in the supercooled state using Fabry–Perot interferometry is carried out. The frequency, linewidth, and Landau–Placzek ratio of the spectrum associated with the longitudinal hypersonic wave, the frequency of the transverse hypersonic wave, and the orientational relaxation time are determined as functions of temperature. The linewidth maximum of the longitudinal hypersonic wave is observed at 420 K, at which the transverse frequency becomes vanishingly small; the longitudinal frequency also changes the temperature dependence behavior at this temperature. Combination of the interferometric data with the data obtained using photon correlation shows that over 13 orders of magnitude, the temperature dependence of the orientational relaxation time follows the Debye–Stokes–Einstein equation.
Period lengthening and associated bifurcations in a two-variable, flow Oregonator
Authors: Gáspár Vilmos , Showalter Kenneth
Published: July 2002

The dynamic behavior of a two-variable Oregonator with CSTR flow terms is examined as a function of the stoichiometric factor f. Three steady states are exhibited over a range of f and seven bifurcation points are identified and characterized. Slowing down of the limit cycle oscillations on approaching the upper limit of oscillatory behavior is explained by a saddle-loop bifurcation.
Numerical study of nonlinear differential equation for decomposition process in a two-dimensional system
Authors: Hatta N. , Kokado J. , Ishii R. , Takuda H.
Published: July 2002

The diffusion equation which was originally derived by Cahn has been found to give various numerical solutions depending upon whether the average composition exists inside or outside spinodal regions. We have investigated the behavior of numerical solutions to the equation in a two-dimensional system in such a way as to pursue the time evolution of composition modulation by computer. Again, the wavenumber method has been adopted. In this paper, it is demonstrated how the growth of a composition modulation proceeds according to the difference in the average composition, and in the initial perturbation corresponding to the composition amplitude. The results so obtained are discussed from a numerical point of view.
A lattice model of a liquid supported monolayer of amphiphile molecules: Mean-field analysis
Authors: Wang Zhen-Gang , Rice Stuart A.
Published: July 2002

We describe a lattice model for amphiphile monolayers in which amphiphile configurational flexibility and amphiphile–amphiphile interaction in and out of the surface layer are explicitly treated. The analysis employs a mean-field approximation based on the work of Scheutjens and Fleer. The density distribution as a function of surface coverage and the equation of state of the monolayer are predicted. Monte Carlo simulations of the same model as treated in the mean-field approximation show that the predicted density profile is quite accurate except near φ=0 and that the film thickness as a function of surface coverage is well represented except near φ=0, but that the predicted liquid–gas phase boundary is rather inaccurate.
Brownian dynamics study of the relaxation behavior of linear and ring polymers
Authors: van Waveren Matthijs , Bishop Marvin , Michels J. P. J.
Published: July 2002

The relaxation behavior of single linear and ring polymers, with and without excluded volume, is studied by Brownian dynamics. The velocity, angular momentum, and end-to-end vector autocorrelation functions are investigated. It is found that the velocity and angular momentum autocorrelation functions decay exponentially and that the time dependence of the vector end-to-end autocorrelation function is mainly determined by the time dependence of its direction.
Consistently averaged hydrodynamic interaction beyond the Oseen approximation for Rouse–Zimm–Bueche dumbbells in steady shear flow
Authors: López de Haro M. , Rubí J. M.
Published: July 2002

The effect of including corrections to the Oseen hydrodynamic interaction in the rheological properties of polymer solutions is investigated. The case of a dilute suspension of Rouse–Zimm–Bueche dumbbells in steady shear flow is examined within the consistent averaging approximation due to Öttinger [J. Chem. Phys. 83, 6535 (1985)]. A reduction of the values of the material functions as compared to the results obtained in the Oseen approximation is observed. The effect is most important at zero shear rate and the highest allowed value of the hydrodynamic interaction parameter. It decreases when either the shear rate is increased or the hydrodynamic interaction parameter is lowered. The ratio of the second to the first normal stress coefficients is always a small positive quantity.
Chain polymer ensembles by computer simulation
Authors: Tuthill G. F. , Sui Zhifeng
Published: July 2002

Monte Carlo methods on the square and simple cubic lattices are used to study thermal and configurational properties of an equilibrium (polydisperse) ensemble of polymers which can undergo reversible chain growth and fracture. Phase transitions (continuous on the square lattice, discontinuous on the cubic lattice) to states with orientational order are observed for stiff polymers. The power-law dependence of the average square radius of gyration upon size is found to depend on both stiffness and temperature; in the low-temperature limit the effective Flory exponent approaches that which relates the average size to the radius of gyration averaged over all sizes. The average asphericity of the longest chains in our samples approaches that of isolated self-avoiding walks.
Effective collision cross sections for SF6 from nuclear magnetic relaxation
Authors: Jameson Cynthia J. , Jameson A. Keith
Published: July 2002

The thermal average cross sections for transfer of rotational angular momentum in SF6 on collision with Ar, Kr, Xe, N2, CO, HCl, CO2, CH4, CF4, and SF6 have been determined from pulse measurements of 19F nuclear spin relaxation times in binary mixtures of gases at 260–400 K.
Inhomogeneous Coulomb fluids with image interactions between planar surfaces. III. Distribution functions
Authors: Kjellander Roland , Marčelja Stjepan
Published: July 2002

The anisotropic hypernetted chain approximation is used to calculate the one- and two-particle distribution functions for an ionic fluid confined between two charged surfaces. Particular attention is given to the anisotropic distribution functions g(R,R′), c(R,R′), and ρ(R)g(R,R′) (where g, c, and ρ are the pair distribution, direct correlation, and density functions, respectively). They are plotted for various cases and are used to illustrate the mechanisms of the double-layer interaction and other aspects of the system. The effects of image charges (when each surface is a dielectric boundary) and discrete surface charges are investigated.
Jahn–Teller stabilization energies for ZnS:Mn and ZnSe:Mn excited states obtained by the angular overlap model
Authors: Stavrev K. K. , Kynev K. D. , Nikolov G. St.
Published: July 2002

Linear Jahn–Teller coupling constants are obtained by the angular overlap model. A comparison with the data published recently [Parrot et al., J. Chem. Phys. 87, 1463 (1987)] is made in order to demonstrate the considerable improvement in the calculated Jahn–Teller coefficients when the parameters of the angular overlap model are extracted from a spectral analysis of the systems under consideration.
Probing of extremely low configurational barriers in an organic glass
Authors: Köhler W. , Friedrich J.
Published: July 2002

By measuring the reduction of a spectral hole burnt at 500 mK due to temperature cycling experiments, we show that the lower bound of the distribution function of barrier heights is below 10 cm−1.
The correlation between electron transmission and inner shell electron excitation spectra
Authors: Benitez Alexander , Moore John H. , Tossell John A.
Published: July 2002

Shape resonances are prominent features observed in both electron transmission spectroscopy (ETS) and inner shell electron energy loss spectroscopy (ISEELS) or x-ray absorption spectroscopy (XAS). For the inner shell excitation process the resonant state is stabilized by the positive core created. In comparing analogous features in ET and ISEEL spectra, a measure of this stabilization is given by the sum of the attachment energy from ETS and term value from ISEELS. Employing ETS and ISEELS data from the literature as well as new spectra, it is shown that within an homologous series (such as halobenzenes, vinyl halides, or methyl halides) this stabilization is nearly constant with a value in the 6 to 8 eV range. Furthermore, the intensities of shape resonances in ETS are shown to be related to intensities in ISEELS.
Hindered internal rotation in some singly methylated t
r
a
n
s-stilbenes
Authors: Spangler Lee H. , Bosma Wayne B. , van Zee Roger D. , Zwier Timothy S.
Published: July 2002

The fluorescence excitation and several dispersed fluorescence spectra for supersonically cooled p-methyl-trans-stilbene and the two m-methyl-trans-stilbenes are presented. Despite significant spectral congestion present in these molecules which contain low frequency modes and internal rotation structure, nearly complete assignments of the excitation spectra are given. One notable transition is assigned as a combination of a nontotally symmetric vibration and a nontotally symmetric level of the methyl rotor. All three species have a ground state barrier to internal rotation of ∼28 cm−1. Excited state barriers and conformational changes observed on excitation are 150 cm−1 and 35° for p-Me, and 80 cm−1 and 60°, and 186 cm−1 and 30° for the two m-Me compounds. The p-Me compound shows a threefold barrier to internal rotation rather than a sixfold one, indicating that the two meta positions are inequivalent. The large differences in barrier height and conformational change on excitation between the two meta isomers, whose origins are separated by 207 cm−1, confirm this inequivalence.
Competition between direct-inelastic and trapping desorption channels in the scattering of NO (v=0, J) from Ir(111)
Authors: Hamers R. J. , Houston P. L. , Merrill R. P.
Published: July 2002

The competition between direct-inelastic and trapping-desorption scattering of NO from IR(111) has been studied using multiphoton ionization and time-of-flight mass spectrometry. Molecules interacting by each mechanism were observed and characterized by their angular, velocity, and internal state distributions. For Ts<300 K both channels are observed, while for Ts>300 K only trapping-desorption is observed. At Ts<300 K, the trapping fraction was ≂0.85. The shift in scattering mechanism appears to be caused by a change in the surface composition due to dissociation of chemisorbed NO near room temperature.
The WCA reference system for four- and five-dimensional Lennard-Jones fluids
Authors: Bishop Marvin
Published: July 2002

The WCA reference system is investigated for four- and five-dimensional Lennard-Jones fluids by molecular dynamics simulations. It is found that the WCA prescription for the scaling of the reference system to a hard hypersphere one is a very good approximation in the fluid region.
Brillouin scattering studies of the SF6 crystal
Authors: Kiefte H. , Penney R. , Clouter M. J.
Published: July 2002

The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants and the elasto-optic coefficient ratios of oriented single crystals of SF6 at temperatures near the triple point. The values of the elastic constants at 221.0 K are C11 =34.1, C12=22.2, and C44=13.2 (in units of kbar) and the elasto-optic coefficient ratios are P12 /P11 =0.92 and P44 /P11 =0.09. The temperature dependences of the elastic constants were determined and were found to be linear down to 187.0 K with ΔC11 /ΔT having a particularly large value of −0.46 kbar K−1 .
Subthermal photoproduct surface desorption distributions arising from excimer laser excitation of adsorbed trimethylaluminum
Authors: Higashi G. S.
Published: July 2002

Excimer laser photolysis of adsorbed trimethylaluminum has been studied using a time of flight mass spectrometric technique. Methyl molecules are found to desorb with reasonable efficiency at 193 nm, but leave the surface with surprisingly small amounts of kinetic energy. Whereas, direct excitation of a single Al–C bond with a 6.4 eV photon would result in ∼3.5 eV of excess energy, only 0.025 eV is observed in translation experimentally. Maxwell–Boltzmann fits to these desorption distributions yield a temperature of ∼150 K even though desorption occurs from surfaces at room temperature. Possible interpretations of this result are discussed.
Dipoles induced by the interactions of HD with He, Ar, H2, or HD
Authors: Borysow Aleksandra , Frommhold Lothar , Meyer Wilfried
Published: July 2002

The collision induced dipole components of molecular HD–X pairs (X=He, Ar, H2, or HD) are obtained with a numerical precision in the range of a few percent from previous ab initio calculations of the induced dipole components of H2–X pairs, using a coordinate transformation that accounts for the fact that in the HD molecule, the center of mass, and the center of electronic charge do not coincide.
Two photon spectra of electronic states of Kr2 and ArKr near the Kr* 4p55p states
Authors: Dehmer P. M. , Pratt S. T.
Published: July 2002

The two photon resonant, three photon ionization spectra of Kr2 and ArKr were determined in the energy region of the Kr* 5p[3/2]1, 5p[3/2]2, and 5p[1/2]0 states. The spectrum of Kr2 displays three new band systems that arise from two photon transitions from the ground state to excited states of gerade symmetry. The analysis of the observed vibronic structure of these band systems provides new information on the dissociation energies and the potential energy curves of the low-lying dipole forbidden excited gerade states of Kr2. A photoelectron spectrum determined following (2+1) excitation via an intense peak of the band system dissociating to Kr 1S0+Kr* 5p[1/2]0 provides the first observation of the weakly bound C 2Π3/2u state and the repulsive B 2Π1/2g and D 2Σ+1/2g states of Kr2. The (2+1) ionization spectra for both ArKr and KrXe were examined in the same energy region. For ArKr, a band system analogous to that observed in Kr2 was observed near the Kr* 5p[1/2]0 state, and an additional band system was observed near the Ar* 4s[3/2]01 state. No structure was observed for KrXe.
An effective Golden Rule decay rate expression for quasidissipative IVR processes
Authors: Voth Gregory A.
Published: July 2002

An effective Golden Rule rate expression is derived for the intramolecular vibrational decay dynamics of an initially prepared nonstationary state in an isolated polyatomic molecule. It is shown that the decay rate of the initial state depends on the properties of those states which are directly coupled to the initial state. In many cases, there may be a suppression of the exponential decay rate. An explanation for the insensitivity of some experimental C–H overtone linewidths with increasing excitation energy is also proposed.
Intensity distribution in rotational line spectra. II. Analysis of the fluctuations in the Doppler-free S1←S spectrum of benzene
Authors: Engel Y. M. , Levine R. D.
Published: July 2002

The fluctuations in the intensities of rotational bands in the S1 manifold of benzene are analyzed in detail. The extent of mixing is found to increase with energy and those bands which could not be assigned are found to have an intensity distribution characteristic of the chaotic limit. At a given energy, those bands which borrow intensity due to resonance coupling are more mixed and the C6D6 bands are more strongly coupled than those of C6H6.
The Jahn–Teller distortion in SiH+4
Authors: Frey Regina F. , Davidson Ernest R.
Published: July 2002

The three states that correspond to the triply degenerate 2T2 state of SiH+4 at the tetrahedral configuration are studied using configuration-interaction (CI) calculations. The potential-energy surfaces for the three states of interest are modeled using a functional form of the potential that is based on normal-mode distortions from the tetrahedral configuration. It is shown that this form of the potential, which includes quadratic terms in the normal-mode displacements, is not able to correctly describe the region containing the van der Waals Cs minimum structure. However, using single-root CI calculations, the ground and excited states of the Cs, C2v, and SiH+2 +H2 structures are then studied in detail.
The thermal conductivity of polyatomic molecules
Authors: van den Oord R. J. , Korving J.
Published: July 2002

We show that by means of a linear transformation of the trial functions in the second approximation to the distribution function, a simple expression for the thermal conductivity of polyatomic molecules can be obtained. This is made possible using our results of state-selective measurements of the nonequilibrium distribution in a heat conducting gas. Also a direct relationship between the pure gas transport coefficients can be deduced, which gives an accurate description of both monatomic and polyatomic gases. This is illustrated by a comparison between calculated and experimental data of the thermal conductivity.