bluuebunny/crossref_metadata_2025 huggingface dataset:
Visible and ultraviolet resonance enhance multiphoton ionization photoelectron spectroscopy of H2S in the one-photon wavelength region 143–158 nm
Authors: Steadman Jhobe , Cole S. Keith , Baer Tomas
Published: July 2002

The Rydberg states of H2S have been investigated by multiphoton ionization, resonant enhanced at the two-photon and three-photon level (REMPI). The REMPI spectra (ion signal vs laser wavelength) are compared to the one-photon absorption spectrum. In addition, photoelectron spectra (PES) were collected at most of the absorption peaks in both the 2+1 and 3+1 REMPI processes. The vibrational state distribution of H2S+ formed by REMPI at a number of laser wavelengths is highly peaked at a single vibrational level. These intermediate Rydberg states could therefore be assigned on the basis of the PES. The REMPI-PES at other laser wavelengths were more complex probably because of perturbed intermediate Rydberg states. A comparison of the 2+1 and 3+1 REMPI wavelength scans shows significant peak intensity differences, which we attribute to the different Δl selection rules in the two- and three-photon absorption steps. New H2S 4p Rydberg states were observed in the two-photon resonance study, and a number of previously observed peaks have been reassigned on the basis of these results.
π bonded intermediates in alcohol oxidation: Orientations of allyloxy and propargyloxy on Ag(110) by near edge x-ray absorption fine structure
Authors: Solomon J. L. , Madix R. J. , Stöhr J.
Published: July 2002

The orientation of allyl alcohol (CH2=CHCH2OH), propargyl alcohol (CH 3/4 CCH2OH), and their surface alkoxy groups has been examined on Ag(110) using near edge x-ray absorption fine structure (NEXAFS) measurements. The planes containing the π* orbitals of allyl alcohol and its alkoxy species, allyloxy (CH2=CHCH2O−), are tilted 26±4° and 32±5° from the surface normal, respectively. Allyl alcohol itself shows no azimuthal ordering, but the π* orbital in allyloxy is azimuthally oriented 33±15° from the close-packed direction ([11̄0] azimuth). Propargyl alcohol is randomly oriented on the Ag(110) surface. Its alkoxy group, propargyloxy (CH 3/4 CCH2O−), is strongly oriented with the triple bond axis parallel to the plane of the surface and directed along the [001] azimuth, perpendicular to the troughs. The position of the σ* resonances indicate that no loss of the allylic hydrogen has occurred and that the double and triple bonds are slightly, but not significantly, perturbed from their normal gas phase bond lengths.
Microscopic theory of dynamics in an orientationally ordered fluid
Authors: Kirkpatrick T. R.
Published: July 2002

A variety of dynamical properties are studied in a model complex liquid composed of hard parallel spherocylinders. For this fluid a revised Enskog kinetic theory (RET) is introduced. In the long wavelength and long time limit the RET leads to liquid crystal nematic-like hydrodynamic equations with explicit expressions for all transport coefficients in terms of an assumed known equilibrium two-point distribution function. The transport process of self-diffusion in this fluid is also studied in the Enskog approximation. This model has a nematic to smectic phase transition (N→A) where a one-dimensional translational order occurs. Near this phase transition the RET is shown to lead to a van Hove-like theory of critical dynamics. The critical mode near the N→A transition is analogous to the so-called soft extended heat mode recently discussed for simple fluids. Near the N→A transition the critical singularities in the nematic viscosities are discussed. It is shown that the singularities arise from the same mode coupling mechanism that is responsible for the anomalously slow relaxation of the stress-tensor autocorrelation function in dense simple liquids.
Application of the zero separation theorem to the restricted primitive model of electrolyte solutions
Authors: Sloth Peter
Published: July 2002

Calculations of activity coefficients by application of the zero separation theorem is discussed for the restricted primitive model of electrolyte solutions. Numerical results within the framework of the hypernetted chain approximation are given. These indicate that the bridge functions—at zero separation—as a first approximation might be quite well represented by the corresponding function for an uncharged hard sphere system.
Zero potential energy criterion applied to Hartree–Fock wave functions
Authors: Koga Toshikatsu
Published: July 2002

The zero potential energy criterion, which is a necessary condition for the exact wave function, is applied to Hartree–Fock wave functions for a closed-shell system with doubly occupied spatial orbitals. With the help of the known long-range asymptotic behavior of Hartree–Fock orbitals, we first derive a single-electron zero potential energy criterion to be satisfied by Hartree–Fock orbitals. The Hartree–Fock wave function is then shown to never satisfy the zero potential energy criterion, which implies that the Hartree–Fock approximation cannot describe the correct long-range asymptotic behavior of many-electron wave functions. Some numerical illustrations are given.
Determination of non-Boltzmann vibrational distributions of N2(X,v″) in He/N2 microwave-discharge afterglows
Authors: Piper Lawrence G. , Marinelli William J.
Published: July 2002

We have extended a technique for studying the vibrational distributions of ground-electronic-state, molecular nitrogen in the afterglow of a microwave discharge through mixtures of helium and nitrogen. The technique is based upon adding metastable helium atoms to the afterglow. The He*(2 3 S) excites the N2(X,v) to N+2(B 2Σ+u) in a Penning-ionization reaction. Since Penning ionization is a Franck–Condon process, the vibrational distribution of the N+2(B) product is determined by that of the N2 (X,v) from which it was produced. The measurements show that the ground-state nitrogen distribution is highly non-Boltzmann, with vibrationally hotter distributions being produced with lower mole fractions of nitrogen in the discharge. We have also observed the production of N+2(C 2Σ+u) from He* Penning ionization of molecular nitrogen. This process is energetically allowed only if the vibrational energy in the ground-electronic-state nitrogen exceeds 3.8 eV or 15 vibrational quanta.
The time propagation of the stationary states of a Morse oscillator by the Gaussian wave packet method
Authors: Hansen F. Y. , Henriksen N. E. , Billing G. D.
Published: July 2002

The Gaussian wave packet method has been developed for the simulation of processes like molecular collisions, photodissociation of molecules, and laser excitations of molecules. So far a necessary condition for an accurate result is that the fragment states are propagated accurately. We have considered one-dimensional bound states described by a Morse potential, and carried out a systematic study of the ability of the Gaussian wave packet method to propagate the stationary states. It is found that the complete set of equations of motion as derived by the minimum error method (MEM) cannot be used in practical calculations because of numerical problems. These are eliminated by the introduction of simplifications such as the independent Gaussian approximation (IGA), where each wave packet is propagated independently. The conditions for an accurate propagation within that assumption are developed, and a simple method is devised to identify those states, which are propagated accurately. This procedure may be used to investigate when the Gaussian wave packet method is appropriate for the simulation of a given process.
An isothermic coexistence curve in the nitrobenzene–decane–benzene solution
Authors: Rzoska S. J. , Chrapeć J.
Published: July 2002

An isothermic coexistence curve in the nitrobenzene–decane–benzene solution has been determined. Its shape can be described by the single, power relation with the exponent β′=0.376±0.008. This result is consistent, within the limits of error, with theoretical predictions based on the Fisher’s renormalization.
Direct simulation of dichotomous noise-induced transitions in a bistable system
Authors: L’Heureux Ivan , Kapral Raymond
Published: July 2002

A numerical study of dichotomous noise-induced transitions in a one-dimensional bistable system is presented. Using projection operator techniques on an extended phase space that includes the stochastic variable, expressions for the rate coefficients are derived. When a phenomenological law is valid, these expressions provide an efficient algorithm for estimating the rate coefficient. Numerical results are presented and compared with analytical theories.
Condensation of a supersaturated vapor. VIII. The homogeneous nucleation of n-nonane
Authors: Hung Cheng-Hung , Krasnopoler Mitchell J. , Katz Joseph L.
Published: July 2002

Homogeneous nucleation rates for n-nonane were measured as a function of supersaturation at nine temperatures (233 to 315 K) using an upward thermal diffusion cloud chamber. On each isotherm, the supersaturations were set to values which produced nucleation rates ranging from about 5×10−5 to 100 drop cm−3 s−1. The observed dependences on both temperature and supersaturation were compared to the predictions of several nucleation theories. Closest agreement was obtained with classical theory. Nonetheless, a multiplicative temperature-dependent correction that ranges from 2×10−5 at 233 K to 4×103 at 315 K was required to make classical theory agree with experiment. A comparison of our nucleation rate measurements to measurements made using two different expansion cloud chambers showed consistent deviations from classical theory.
Inelastic neutron scattering determination of phonon dispersion curves in the molecular crystal s
y
m-C6F3Cl3
Authors: Dove M. T. , Powell B. M. , Pawley G. S. , Chaplot S. L. , Mierzejewski A.
Published: July 2002

Phonon dispersion curves have been obtained along the Δ, Σ, and T directions in a single crystal of sym-C6F3Cl3 at 5 K by inelastic neutron scattering measurements. These have been interpreted within the framework of rigid-molecule lattice dynamics. A model intermolecular potential reproduces the overall behavior, but there remains scope for improvement in the model. Other models based on transferable potentials are found to be less satisfactory.
Solvent dynamical effects in electron transfer: Numerical predictions of molecularity effects using the mean spherical approximation
Authors: McManis George E. , Weaver Michael J.
Published: July 2002

The influence of solvent molecularity upon the adiabatic barrier-crossing frequency νn and barrier height ΔG*, for electron–exchange reactions involving only solvent reorganization is examined numerically on the basis of a mean spherical approximation (MSA) treatment of the reaction coordinate time-correlation function, Δ(t) [Refs. 3(c) and 7(c)] for Debye solvents. The calculated ΔG* values for a spatially isolated redox couple fall increasingly below the corresponding dielectric continuum quantity ΔG*con as the ratio of the solvent to reactant radii (rsol /rre ) increases. For the experimentally common circumstance (rsol /rre )≲2 and for zero and infinite-frequency dielectric constants ε0 and ε∞ over the ‘‘typical’’ ranges 20–100 and 1.75–2.5, respectively, the calculated ΔG* values are up to ∼30% below ΔG*con , the deviations tending to be larger for smaller ε0 and ε∞ values. Two kinetic models are utilized to extract νn values from Δ(t): Hynes’ approach derived from the generalized Langevin equation (GLE), and that based on the Smoluchowski diffusion equation. The νn values derived using both models decrease progressively below the corresponding dielectric continuum frequency factor νconn as (rsol /rre ) increases, the deviations between νn and νconn being greater for smaller ε∞ and/or larger ε0 . The magnitude of these deviations, however, is noticeably smaller for the Hynes GLE than the diffusion model. Thus for (rsol /rre )≲2 with the above dielectric parameters, νn is calculated to be up to ∼two and five fold smaller than νconn on the basis of the Hynes GLE and diffusion model, respectively. These differences can be understood in terms of the relative influences upon νn of the slower relaxation components arising from short-range solvation, responsible for the deviations from νconn , in comparison with the faster dynamics associated with more distant solvent

molecules. Slightly larger deviations from the continuum predictions are obtained using the GLE approach in the presence of barrier-top curvature. At least for (rsol/rre)<2, the MSA-prescribed rate constants also do not deviate greatly from the corresponding continuum prediction, the corresponding decreases in νn and ΔG* being partly compensatory.
Molecular dynamics simulations of energy flow at a solid surface. New methods using a small number of atoms
Authors: DePristo Andrew E. , Metiu Horia
Published: July 2002

We present a number of new methods for the treatment of the dynamics of a strongly perturbed, finite sized, anharmonic system embedded in a structured, infinite system. All of the methods divide the full system in three classes of particles: Those that are treated by molecular dynamics; those that obey some form of stochastic molecular dynamics; and, those that are fixed. The methods are all simpler than the well-known generalized Langevin equation technique, and can be applied easily to much more complex processes than is possible for the GLE. A detailed illustration is provided of the energy flow through a solid lattice following a sudden disturbance of one surface atom. We compare the different methods and establish the size of the region of moving atoms needed to describe the lattice response over various time scales for the different dynamical methods.
Valence bond approach to exact nonlinear optical properties of conjugated systems
Authors: Soos Z. G. , Ramasesha S.
Published: July 2002

Diagrammatic valence-bond (DVB) theory is extended to dynamic nonlinear susceptibilities of interacting π electrons in Pariser–Parr–Pople (PPP) or other quantum cell models whose correlated ground state ‖G〉 is known. Corrections φ(1)(ω) and φ(2)(ω2,ω1) to ‖G〉 due to oscillating electric fields are found directly as linear combinations of VB diagrams. Any nonlinear optical coefficient is reduced to matrix elements that implicitly include all excited states, as verified for shorter polyenes. Static and dynamic χ(3) coefficients for cis and trans polyenes to N=12 carbons illustrate the importance of retaining the full spectrum. The coefficients βijk(ω,ω) for second harmonic generation are found for polar molecules like aniline and nitroaniline. Divergent responses are treated by lifetimes Γ for the resonant states, as shown for third harmonic generation in hexatriene with Γ=0 and in octatetraene with Γ>0. Electron–electron interactions reverse the sign of γijkl(ω,ω,ω) in linear polyenes, except for the largest xxxx coefficient along the chain, and strongly decrease their magnitude and size dependence. The DVB procedure is checked against transition moments, noninteracting π electrons in Hückel models, explicit sums over all excited states, and is compared to recent approximate PPP calculations for the nonlinear optical (NLO) coefficients of linear polyenes.
Electrochemical infrared spectroscopy of carbon monoxide on ordered rhodium (111): Comparisons with vibrational spectra on Pt(111) and in related surface–vacuum environments
Authors: Leung Lam-Wing H. , Chang Si-Chung , Weaver Michael J.
Published: July 2002

Surface infrared spectra are reported for carbon monoxide adsorbed on a well-ordered Rh(111) crystal in 0.1 M HClO4 , and in neutral and alkaline electrolytes as a function of CO coverage θ as well as electrode potential. The results are compared in detail with corresponding vibrational spectra obtained recently for CO at the Pt(111)–aqueous interface, and on Rh(111) and Pt(111) in ultrahigh vacuum (uhv) in the absence and presence of coadsorbed water, in order to explore the possible influences of the aqueous electrochemical environment upon the CO surface coordination geometry. The potential-dependent surface infrared spectra for high CO coverages in 0.1 M HClO4 exhibited a major C–O stretching band (νCO ), due to terminal (i.e., linearly bound) CO at 2025–2042 cm−1 over the potential range −0.25 to 0.25 V vs SCE; a weaker νCO band, attributed to CO bound to twofold bridging sites, was observed at 1792–1815 cm−1 under these conditions. The latter form undergoes electro-oxidation preferentially, as deduced from spectral sequences obtained during slow voltammetric sweeps. At CO coverages close to saturation, θ≊0.75, the linear/bridged site occupancy ratio is estimated to be roughly 2. At lower CO coverages within the hydrogen adsorption region, a band at about 1870–1875 cm−1 appears at the expense of the terminal νCO feature. Corresponding spectra obtained in neutral 0.1 M NaClO4 and alkaline electrolytes also yielded similar terminal and

bridging νCO bands, downshifted in frequency from those in 0.1 M HClO4 to an extent (≲50 cm−1 ) commensurate with the more negative electrode potentials involved. Comparison of the electrochemical infrared spectra with vibrational spectra obtained for the analogous Rh(111)/CO system in uhv indicates that the presence of the aqueous environment yields only small (≲20 cm−1 ) downshifts in the νCO band frequencies. Essentially the same conclusion is obtained from a corresponding comparison between vibrational spectra obtained at the electrochemical and uhv Pt(111)/CO interfaces. Cognizance is taken in these analyses of the known (or anticipated) differences in the potentials of zero charge and work functions in the electrochemical and uhv environments, respectively. These findings are also compared with recent vibrational spectra obtained for CO coadsorbed with water on Rh(111) and Pt(111) in uhv at low temperatures.
Dynamics of conformational transitions to isomeric states favoring intramolecular excimer formation in polymeric chains. Application to dimer models of polystyrene
Authors: Bahar Ivet , Mattice Wayne L.
Published: July 2002

The dynamic rotational isomeric states model, which has been recently proposed to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. The transition rate matrix governing the rate of passage between the subsets {φ}a and {φ}b, consisting of excimer-forming and non-excimer-forming conformations, respectively, is modified such that microstates in {φ}b are nonrecurrent, in contrast to the stationary process of conformational transitions in chains in equilibrium. The theory allows for the calculation of the fraction Pt(a) of sites that undergo transition to excimer-forming states at least once prior to time t, as well as mean first passage time from {φ}b to {φ}a, as a function of real chain conformational characteristics and structural properties. Application of the model to the meso and racemic dyads in polystyrene confirms the fact that conformational mobility of the chain plays a major role in intramolecular excimer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions.
Correlated van der Waals coefficients. II. Dimers consisting of CO, HF, H2O, and NH3
Authors: Rijks W. , Wormer P. E. S.
Published: July 2002

Correlated frequency-dependent polarizabilities are calculated for the molecules HF, H2O, NH3, and CO by the use of a many-body perturbation theory method, to second order in the correlation potential. We computed from these polarizabilities the dispersion coefficients Cn, up to n=10, for all the dimers consisting of these molecules and for all their combinations with He, Ne, H2, and N2 for which the polarizabilities were calculated previously. The results are compared with (semi-) empirical and theoretical values from literature as far as possible.
Mean-force-field and mean-spherical approximations for the electric microfield distribution at a charged point in the charged-hard-particles fluid
Authors: Rosenfeld Yaakov
Published: July 2002

The linearized mean-force-field approximation, leading to a Gaussian distribution, provides an exact formal solution to the mean-spherical integral equation model for the electric microfield distribution at a charged point in the general charged-hard-particles fluid. Lado’s explicit solution for plasmas immediately follows this general observation.
Spin–orbit state-selected reactions of Xe+(2P3/2 and 2P1/2) with H2, D2, and HD
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

Spin–orbit state-selected reactions of Xe+(2PJ ), J=3/2 and 1/2, with isotopic molecular hydrogen (H2, D2, and HD) to form XeH+ and XeD+ are studied using guided ion beam mass spectrometry. Reaction cross sections are determined as a function of reactant kinetic energy from near thermal energy to 15 eV c.m. Although the reaction of Xe+(2P1/2)+H2 to form ground state products XeH++H is exothermic, no reaction is observed at low energies and the reaction cross section is zero or small at higher energies. The Xe+(2P3/2)+H2 reaction has an apparent threshold near its endothermicity, but the cross section rises slowly above this threshold. The reaction energetics and isotope effects indicate two separate mechanisms for Xe+(2P3/2). The dominant mechanism is a direct, impulsive process with an apparent activation barrier. The second mechanism allows reaction near the thermochemical threshold and involves more interaction among all three atoms. These results are interpreted in relation to the spin–orbit coupled potential energy surfaces of XeH+2 system.
Picosecond degenerate four-wave mixing study of nonlinear optical properties of the poly-N-vinyl carbazole: 2,4,7-trinitrofluorenone composite polymer photoconductor
Authors: Ghoshal Suniti K. , Chopra Pratibha , Singh Bhanu P. , Swiatkiewicz Jacek , Prasad Paras N.
Published: July 2002

Resonant third-order nonlinear optical susceptibility χ(3) of poly-N-vinyl carbazole: 2,4,7-trinitrofluorenone composite polymer photoconductor has been measured at 602 nm for various compositions by picosecond degenerate four-wave mixing. The origin of effective third-order nonlinearity of this system is attributed to the charge–transfer excitation which creates thermalized correlated electron–hole pairs. The optical nonlinearity of this polymeric system is characterized by a long relaxation time of hundreds of picoseconds. A progressive enhancement of the signal intensity and hence effective χ(3) accompanied by an increase in the decay rate of the degenerate four-wave mixing signal has been observed with an increase in the mole fraction of trinitrofluorenone.
Branching ratios and lifetimes for the dissociative decay of triplet H2*
Authors: Koot Wouter , van der Zande Wim J. , Post Paul H. P. , Los Joop
Published: July 2002

Molecular hydrogen is selectively excited to low vibrational levels in the triplet 3sσ and 3dλ states. We have observed the decay to both the b 3Σ+u and the c 3Π+u state for all excited levels. The dissociative final states have been investigated simultaneously using translational spectroscopy with a time and position sensitive detector. In this way the branching ratios were determined which depend on the mixing between the n=3 states. For some excited states we also measured the radiative lifetime. The experimental results are compared to a model which, in its most elaborate form, contains some transition dipole moments that depend on the internuclear coordinate. Even though the absolute values of the dipole moments are varied to fit the experiment, overall quantitative agreement cannot be obtained.
The use of distributed Gaussian basis sets for calculating energy levels of weakly bound complexes
Authors: Peet Andrew C.
Published: July 2002

A basis set method for calculating the energy levels of weakly bound complexes is investigated. The basis is constructed from distributed Gaussian functions in both the bending and stretching coordinates. This produces a set of functions which are localized in the full internal space and allows the construction of nondirect product basis sets which model the known characteristics of the wave function. Low lying states which are restricted to a small portion of space may be described by functions placed just in the well region. Highly excited states usually occupy large regions of space but may still be efficiently described by modeling the regions of greatly differing wave function curvature. Application to the van der Waals complex Ar–CO2 shows that such basis sets give a reduction of about a third in basis set size when compared with a more conventional basis of Legendre functions for the monomer rotation and distributed Gaussians for the stretching motion. Such savings should increase for larger, more anisotropic systems.
A time-resolved study of rotational energy transfer into A and E symmetry species of 13CH3F
Authors: Everitt Henry O. , De Lucia Frank C.
Published: July 2002

Rotational energy transfer processes into the A and E symmetry species of the symmetric top molecule 13CH3F have been studied. In this time-resolved double resonance experiment a tunable millimeter/submillimeter wave spectrometer was used to monitor the change in strength of rotational transitions in the ν3 vibrational state after a Q-switched CO2 laser pumped the K=3, J=5 level in ν3. A simple numerical simulation of rotational energy transfer allowed the 13CH3F system to be modeled and collisional energy transfer rates to be obtained from the data. Two important processes were studied. The first, a process that obeys the spin statistic selection rule ΔK=3n has a rate of 29±6 ms−1 mTorr−1. The second, a vibrational quantum number swapping collision that effectively transfers population between the A and E symmetry species and thereby transcends the spin statistic selection rule, has a rate of 6.6±0.7 ms−1 mTorr−1, about 1.4 gas kinetic collisions. The numerical simulations and these rates, along with previous measurements of the ΔJ=±1 rate and vibrational decay rates, provide an accurate characterization for a large body of varied experimental data.
Schrödinger equation in terms of linearly averaged position moments
Authors: Koga Toshikatsu
Published: July 2002

Linearly averaged position moment (LAPM) [Rk11Rk22⋅⋅⋅ Rk3N3N] is defined as the linear (not quadratic)

average of the position moment operator Rk11Rk22⋅⋅⋅ Rk3N3N over the N-electron wave function Ψ({Rj}), where {Rj} are 3N Cartesian coordinates of electrons and {kj} are non-negative integers. When all the LAPM’s are well defined, it is shown that the Schrödinger equation is equivalent to a set of an infinite number of equations between LAPM’s involving the potential-energy operator. The kinetic energy operator does not appear. The spherical polar representation of the LAPM equation is also presented. Illustrations are given for simple one- and two-electron atoms, where the LAPM equation is applied to the determination of approximate wave functions and associated energies.
Kinetics of the thermal desorption of indium from GaAs(100)
Authors: McCaulley J. A. , Donnelly V. M.
Published: July 2002

We report studies of the kinetics of thermal desorption of In from Ga-stabilized GaAs(100) in ultrahigh vacuum. The relative coverage of In was monitored by x-ray photoelectron spectroscopy (XPS), while the substrate temperature was accurately measured using infrared laser interferometric thermometry. The In was deposited on GaAs by dosing, at room temperature, to saturated monolayer coverage with trimethylindium, yielding In ∼2×1014 cm−2. Subsequent heating to 400 °C desorbs all hydrocarbon species without affecting the In coverage. Further heating leads to first-order desorption of In, as confirmed by exponential decay of In(3d 5/2) XPS signal in isothermal desorption experiments at 473 and 503 °C. From temperature programmed desorption studies through the range 450 to 530 °C, differentiation of the In coverage vs time yielded desorption rates from which Arrhenius parameters were extracted. The unit-weighted average values of the preexponential factor and the desorption energy, obtained from three experiments with heating rates from 0.6 to 3.2 °C/min, are: log10A(s−1)=12.2±0.5 and Ed=53.5±1.2 kcal/mol, where the uncertainties are 1σ sample standard deviations. The Arrhenius parameters of desorption of In from Ga-stabilized GaAs(100) are found to be similar to those of vaporization of pure In. This is consistent with the known tendency of In to form islands on GaAs surfaces, but could also reflect the similarity of the local environments of an In atom adsorbed on an In island and an In atom adsorbed on a Ga-terminated surface. The observed first-order kinetics and the constancy of the In(3d 5/2) XPS signal up to the temperature where desorption occurs indicates that if islands are formed they are small (<10 Å diameter) at the low coverage used here.
Vibrational assignments and intensity considerations of the fluorescence excitation spectra of jet-cooled S1 t
r
a
n
s-stilbene and its three isotopic analogues
Authors: Urano Taeko , Hamaguchi Hiro-o , Tasumi Mitsuo , Yamanouchi Kaoru , Tsuchiya Soji , Gustafson Terry L.
Published: July 2002

Vibrational mode assignments of S1 trans-stilbene (tSB) in a jet, particularly those for fundamental vibrational levels above the isomerization barrier, are reported with three partially isotope-substituted analogues (C6 H5 –13CH=13CH–C6 H5 , C6 H5 –CD=CD–C6 H5 , and C6 D5 –CH=CH–C6 D5 ). Vibrational levels are assigned on the basis of the frequency shifts due to isotopic substitution. The established mode assignments then enable us to discuss the individual band intensities in the fluorescence excitation spectra which bear mode specific information on the isomerization dynamics in isolated tSB molecules. Upon deuteration, the intensities of several CH deformation modes increase drastically as their frequencies become lower than the isomerization barrier. Intensities of several fundamental bands are found much smaller than those of nearby combination bands, contrary to what is expected from ordinary absorption spectra in condensed phases. The intensity ratio of the 71 (C=C stretch) to the 00 band is semiquantitatively studied with reference to a molecular-orbital calculation, the resonance Raman intensity, and the absorption intensity in a low-temperature crystal. The unusually low fluorescence excitation intensities are ascribable to large isomerization rates specific to certain kinds of fundamental vibrational levels.
Nonequilibrium molecular dynamics of liquid sulfur in Couette flow
Authors: Rustad James R. , Yuen David A. , Spera Frank J.
Published: July 2002

Previous work in nonequilibrium molecular dynamics (NEMD) has been restricted to systems subject only to pair interactions. We use methods of homogenous NEMD to investigate the nature of liquid sulfur under extreme shear using the potential model developed by Stillinger and Weber which involves explicitly three-body interaction. Simulations with up to 2048 particles have been carried out at a temperature of 1583 K and a density of 1.805 g cm−3 for shear rates between 0.005 and 1.75 in reduced units. We find that the fluid separates in sheets alternating from high to low density in planes perpendicular to the velocity gradient. No evidence is seen for the transition to the ‘‘string’’ phase as exhibited by two-body systems. The molecules show a tendency to align in the direction of shear. Data are presented describing the magnitude of this effect.
The structure of liquid 1,2-dichloroethane by neutron diffraction. II. Pulsed neutron and isotope difference studies
Authors: Alvarez M. , Bermejo F. J. , Howells W. S. , Chieux P. , Enciso E. , Alonso J. , Garcia N.
Published: July 2002

Pulsed neutron diffraction measurements have been made on liquid 1,2-dichloroethane-d4 (DCE). The wide momentum-transfer range (∼0.3–50 Å−1) available has been used to further refine previously measured molecular structure parameters as well as to test the validity of the inelasticity corrections applied. A measurement using chlorine isotopes on a steady (reactor) source served to partially separate the chlorine–chlorine and the chlorine–carbon plus chlorine–deuterium correlations. The isotopic difference curves were then analyzed and the relevant features of the distribution of internal dihedral angles [P(τ)] obtained by adequate inversion of the experimental difference–functions. The intermolecular pair correlation function was then derived and both sets of functions (from pulsed and steady sources) are compared and tentatively assigned.
Novel a
b i
n
i
t
i
o self-consistent-field approach to molecular solids under pressure. III. Second-order Mo/ller–Plesset correlation corrections
Authors: Raynor S.
Published: July 2002

The ab initio subcell approach for determining the electronic structure of molecular solids and clusters is extended to include correlation at the MP2 level. The approach is demonstrated with calculations on solid H2 at four pressures (∼3×106, 1×106, 0.5×106, and 0.1×106 atm) for which contributions of correlation at the MP2 level are found to be small (∼3%–5% of the calculated interaction energy per molecule). It is found that the correlation energy at the MP2 level is far less sensitive to the number of nearest-neighbors included than the Hartree–Fock energy.
Operator algebra for the many-body problem in the spin eigenfunction basis
Authors: Duch Wl/odzisl/aw
Published: July 2002

In the many-body perturbation theory and related methods equations involve sums over orbital indices. Reduction of the many-body equations to such a form is not trivial when spin eigenfunctions are used. A method to achieve such reduction, based on the algebraical properties of the unitary group generators, is described. As an example of application a formula for the second order MBPT with an arbitrary open shell reference state of the RHF type is derived.
The solution of the reference hypernetted chain equation for the dipolar hard diatomic fluid
Authors: Lomba E. , Lombardero M. , Abascal J. L. F.
Published: July 2002

The reference hypernetted chain equation (RHNC) for a fluid of dipolar hard diatomics was solved numerically. Three choices for the reference bridge function B0(12), were examined. The simplest, B0(12)=0 (HNC) and B0(12) corresponding to the uncharged homonuclear hard diatomic fluid, computed from simulation data and by the Percus–Yevick approximation. The computed fluid structure [i.e., the g(12) expansion coefficients] showed a remarkable quantitative agreement with the structure obtained from a mean reaction field Monte Carlo simulation. The same applied to the configurational energy. The values for the dielectric constant, however, furnished only qualitatively indications of the density dependence of this quantity. The probable origin of this discrepancy is analyzed.
NH3 inversion in condensed media. Static, dynamic, and inhomogeneous environmental effects on the internal tunneling
Authors: Girardet C. , Lakhlifi A.
Published: July 2002

The ν2 umbrella mode of vibration–inversion of ammonia trapped in condensed media exhibits typical tunneling slow-down due to the statics, the dynamics, and the inhomogeneity of the ambiant solid. The reduction of the level splitting of the ν2 mode of ammonia from 37.6 cm−1 in gas to 24 cm−1 in argon, to 22 cm−1 in xenon, and to 1.65 cm−1 in nitrogen is interpreted in terms of the statical and dynamical influence of the matrix. In argon, the static environment accounts for 92% of the tunneling slow-down. In xenon, 45% of the effect is due to the dynamical inertia of the surrounding matrix. In nitrogen, the Franck–Condon dynamical influence represents about 70% of the effect. These results are in good agreement with experimental data and are consistent with results observed in other physical cases (phase transitions, superconductors, proton transfer). It is also shown that the environmental inhomogeneity due to the presence of other ammonia molecules cannot be neglected for the determination of the inversion level splitting in nitrogen matrix, even for high dilution (c≤1/20 000) of ammonia molecules in the solid.
Molecular multipole polarizabilities and the intensity of vibrational lattice modes: The s-triazine and pyrazine crystals
Authors: Gamba Z.
Published: July 2002

The intensities of vibrational lattice modes of α and β s-triazine and pyrazine are calculated using four models of increasing complexity for the description of the molecular multipole electrostatic moments and polarizabilities. For s-triazine crystals it is found that high-order electrostatic interactions should be taken into account in order to reproduce the experimental infrared absorption intensities. For pyrazine crystal it is found that high-order molecular polarizabilities give a non-negligible contribution, and that it cannot be reparametrized in terms of a molecular dipolar polarizability.
Dynamics of NH3 trapped in N2 matrix: Tunneling, motional narrowing, and vibrational relaxation
Authors: Girardet C. , Lakhlifi A.
Published: July 2002

A quantitative interpretation of the features observed in the high-resolution infrared profile and in the vibrational relaxation of the umbrella ν2 mode of ammonia trapped in a nitrogen matrix is given through the coupled dynamical study of the molecule and the crystal. The motional narrowing that occurs in the quadruplet structure of the vibration–inversion spectrum, as temperature rises, is due to the closing of the inversion levels of the two isotopic ammonia species (A2 and E) by dynamical coupling with the ambient solid. Indeed, this coupling is responsible for the tunneling slowdown of the ammonia frequency due to the tendency for the bath to localize the molecule in a given inversion state. The temperature behavior of the experimental profile in the range 5.5–25 K is well explained on the basis of inversion–libration relaxations assisted by one and two bulk phonon–libron processes. Moreover, the fast vibrational relaxation is also interpreted, within our approach, as being assisted by the ammonia orientational modes. The calculated relaxation time (0.05 ns) agrees with the experimental one (0.07 ≤τ≤ 2ns).
Structure of the radical resulting from the addition of H on a P=P bond: A theoretical and single crystal electron spin resonance study
Authors: Cattani-Lorente Maria , Geoffroy Michel
Published: July 2002

Room temperature ESR spectra of x-irradiated single crystals of bis (2,4,6-tri-tert-butylphenyl) diphosphene provide evidence for the trapping of a R(H)PṖR radical produced by hydrogen addition on the P=P bond. The various experimental hyperfine tensors are compared with those calculated for H2PṖH and CH3(H)PṖCH3 after optimization of the structure of these model radicals by ab initio methods. The barrier to internal rotation about the P–P bond is calculated to be equal to 2.8 kcal/mol for H2PṖH.
Magnetic and optical properties of triplet excitons and traps in 1,4-dibromobenzene
Authors: Jongenelis A. P. J. M. , Schmidt J.
Published: July 2002

An optical and magnetic resonance study on neat 1,4-dibromobenzene crystals shows that the molecules in their lowest triplet state are distorted into a chair-like shape and confirms previous conclusions about the one-dimensional character of the triplet excitons. From a study of the phosphorescence spectrum at various temperatures it is found that the band width of the triplet excitons is (−1.0±0.15) cm−1 and that the k=0 state is at the bottom of the band.
Prediction and design of first super-strong liquid-crystalline polymers
Authors: Dowell F.
Published: July 2002

This paper presents the details of the theoretical prediction and design (atom by atom, bond by bond) of the molecule chemical structures of the first candidate super-strong liquid-crystalline polymers (SS LCPs). These LCPs are the first LCPs designed to have good compressive strengths, as well as to have tensile strengths and tensile moduli significantly larger than those of existing strong LCPs (such as Kevlar). The key feature of this new class of LCPs is that the exceptional strength is three dimensional on a microscopic, molecular level (thus, on a macroscopic level), in contrast to present LCPs (such as Kevlar) with their one-dimensional exceptional strength. These SS LCPs also have some solubility and processing advantages over existing strong LCPs. These SS LCPs are specially-designed combined LCPs such that the side chains of a molecule interdigitate with the side chains of other molecules. This paper also presents other essential general and specific features required for SS LCPs. Considerations in the design of SS LCPs include the spacing distance between side chains along the backbone, the need for rigid sections in the backbone and side chains, the degree of polymerization, the length of the side chains, the regularity of spacing of the side chains along the backbone, the interdigitation of side chains in submolecular strips, the packing of the side chains on one or two sides of the backbone, the symmetry of the side chains, the points of attachment of the side chains to the backbone, the flexibility and size of the chemical group connecting each side chain to the backbone, the effect of semiflexible sections in the backbone and side chains, and the choice of types of dipolar and/or hydrogen bonding forces in the backbones and side chains for easy alignment. The chemical syntheses of several of the theoretically designed SS LCPs presented in this paper are nearing completion at Los Alamos. This paper also makes some predictions about chemical crosslinking of some of the structures in SS LCPs.
Application of the adhesive sphere model to the structure of colloidal suspensions
Authors: Regnaut C. , Ravey J. C.
Published: July 2002

The fluid of adhesive sphere (AS) is considered a tool for the investigation of the structure and thermodynamics of dispersed colloidal systems. We compare this model to a more realistic fluid such as the ‘‘square well fluid’’ where the interactions are of finite range and strength. Considering the Percus Yevick (PY) approximation, we show that the analytical solution of the structure factor of the AS fluid and the numerical solution for the square well fluid agree fairly well at all volume concentrations if the range of the square well does not exceed 10% of the particles diameter. By comparison to the modified hypernetted chain approximation (MHNC) and simulation data we show that the PY approximation is convenient for hard core potentials with a short range tail. Selected examples of application of the AS model as a reference system are given either for the inverse micelles with short range interactions or for nonionic systems invoking the external structurated water shell contribution as a repulsive barrier which is state dependent.
Infrared study on phase transition in β-quinol-methanol or -acetonitrile clathrate
Authors: Kanesaka Isao , Ishida Mitsuo , Kawai Kiyoyasu
Published: July 2002

Half-bandwidths of O–H and O–D stretchings of methanol in clathrates were measured in the temperature range of 16–300 K. They changed abruptly at 65 and 75 K for the H and D systems, respectively, with change in curvature at 100 K. The splitting of O–D stretchings of a host lattice in a 10% deuterated compound was found below 100 K. A similar feature was found for O–H out-of-plane bendings. These special features at 100 K were explained in terms of a coupled relaxation process of a libration of methanol with an orientation of hydrogen bonds in a six-membered hydrogen-bonded ring in a host lattice. It was reasonably suggested that a phase transition at 67 K is ascribed to the order–disorder change for the inversion on the c axis. The splitting of CH3 asymmetric bendings in a β-quinol-acetonitrile clathrate was observed below 50 K. The temperature dependence was explained in terms of a rotational motion about the C3 axis of acetonitrile.
Studies of a version of scaled particle theory for nonadditive hard disks. II. Fluid–fluid phase equilibria
Authors: Bearman R. J. , Mazo R. M.
Published: July 2002

The scaled particle theory of Tenne and Bergmann [J. Chem. Phys. 70, 1952 (1979)] studied in paper I [J. Chem. Phys. 88, 1235 (1988)] has been examined for fluid–fluid phase equilibria in binary solutions of equal size particles. For a large deviation from additivity, changes in curvature of the Gibbs free energy identify regions of complete miscibility, partial miscibility, and immiscibility. The compositions of coexisting phases are determined using the double tangent construction. The simplicity of the intermolecular potential leads to an inverse relationship between the pressure–mole fraction phase diagram at constant temperature and the temperature–mole fraction phase diagram at constant pressure. The binary solutions are found to possess a lower solution critical pressure at constant temperature and an upper solution critical temperature at constant pressure. The critical exponent β is found numerically to be classical as would be expected from a scaled particle theory. The phase equilibria reported in the present paper differ from those reported by Tenne and Bergmann which appear to result from nonphysical mathematical vagaries of the model already discussed in paper I.
He diffraction study of the structural phase transition on the Ge(111) surface at 550 K
Authors: Ha Jeong Sook , Greene E. F.
Published: July 2002

A structural phase transition on the Ge(111) surface is investigated using thermal energy He scattering by observing the temperature dependence of several diffraction peaks. The presence of strong quarter-order peaks confirms that the Ge(111) surface has three orientational domains of a c(2×8) reconstruction at room temperature. As the temperature rises, all the superlattice peaks decrease in intensity without broadening and disappear above 550 K. In addition there is one striking feature which has not been observed in earlier low energy electron diffraction and reflection high energy electron diffraction experiments. The half-order peak shows continuous broadening and a shift of its maximum toward the specular peak as the temperature increases above 550 K. Consideration of several possible explanations for such a peak shift suggests that the transition, which may be driven by a charge density wave, is to an incommensurate phase.
Photodissociation dynamics of ethylene sulfide at 193.3 nm
Authors: Kim H. L. , Satyapal S. , Brewer P. , Bersohn R.
Published: July 2002

We have investigated the photodissociation dynamics of ethylene sulfide (thiirane) at 193.3 nm by studying Doppler broadened laser induced fluorescence spectra of atomic sulfur fragments. The only dissociative channel found produces S(1D) atoms and ethylene molecules. The measured average translational energy of the fragments is 20.5±4.5 kcal/mol. We have found an anisotropy in the velocity distribution of the atomic sulfur fragments (β=1.03±0.19), which confirms that the 193 nm transition is a parallel transition. We conclude that the dissociation is direct and symmetric, both C–S bonds breaking at once. The dissociation time is estimated to be about 300 fs.