bluuebunny/crossref_metadata_2025 huggingface dataset:
Site–site correlations in short chain fluids
Authors: Yethiraj Arun , Hall Carol K. , Honnell Kevin G.
Published: July 2002

Intramolecular and intermolecular site–site correlations in short chain fluids are obtained via Monte Carlo simulation for volume fractions ranging between 0.05 and 0.35. The chains are modeled as pearl necklaces of freely jointed hard spheres; chains composed of 4 and 8 beads are studied. The intramolecular distribution between a pair of beads separated by a fixed number of segments along the chain is found to be remarkably independent of the position of the pair along the chain. At low densities the intermolecular site–site pair distribution function at contact is found to be much less than one due to the ‘‘correlation hole’’ effect. The contact value increases as the density is increased, and decreases as the chain length is increased. We use the intramolecular correlations measured to obtain polymer reference interaction site model predictions for the intermolecular site–site distribution function. We find that the theory accurately reproduces the local structure of the fluid, but significantly overestimates the contact value of the distribution function, especially at low densities. A comparison of freely jointed chain results with simulations of chains with fixed bond angles and torsional rotations treated in the rotational isomeric state approximation shows that the correlation hole is more pronounced in freely jointed chains. We test a superposition approximation used to evaluate the three body term in the pressure equation for chain molecules. We find that the three-body term is sizeable, and that the superposition approximation significantly underestimates the three-body contribution.
Facile alignment of molecular rotation in supersonic beams
Authors: Pullman David P. , Friedrich Břetislav , Herschbach Dudley R.
Published: July 2002

We have obtained substantial alignment of I2(X 1Σ+g;v″=0;J″=13,15) seeded in supersonic beams of light carrier gases. Laser-induced fluorescence and a variant of the magnetic precession technique were used to measure the ratio n⊥/n∥ of molecules with the rotational angular momentum vector J perpendicular to the beam axis z to those with J parallel (or antiparallel) to z. As the nozzle stagnation pressure P0 is increased, this ratio increases markedly, reaches a maximum, and then decreases steadily. At the maximum, n⊥/n∥=1.6, 1.7, and 2.2, respectively, for He, D2, and H2 as the carrier gases; this occurs at different pressures of the order of 103 Torr for nozzle diameter d=50 μm and temperature T0=315 K and corresponds to nearly the same rotational temperatures of about 6–8 K. We compare the observed dependence of alignment on P0⋅d with a J-dependent model that invokes two mechanisms for alignment, macroscopic gas transport, and anisotropic rotational cooling. The transport processes involve reorientation of J and give rise to alignment with n⊥/n∥>1; this dominates the initial increase with P0⋅d up to the maxima. The anisotropic cooling processes do not in our model involve reorientation of J but are fostered by the anisotropy of the rotational relaxation cross section; the alignment arises from different Boltzmann weights for molecules with J⊥z and J∥z due to their slightly different rotational temperatures (about 5%). At high P0⋅d the net alignment is dominated by the anisotropic cooling term. At the correspondingly low rotational temperatures, this term counteracts the effect of gaseous transport, so the net alignment can reverse. Indeed, at high P0⋅d we do observe n⊥/n∥<1. For a higher rotational state (J″=39,41) we find lower alignment that peaks at a higher rotational temperature (about 27 K), as predicted by the model.
Solvation structure and the time-resolved Stokes shift in non-Debye solvents
Authors: Fried Laurence E. , Mukamel Shaul
Published: July 2002

We develop a microscopic theory of the time-resolved Stokes shift of a chromophore in a polar solvent which incorporates both non-Debye dielectric relaxation and solvation shell structure. The present theory depends on the direct correlation function of the pure solvent, the measured frequency-dependent dielectric constant, and a microscopically derived translational diffusion parameter. We compare the predictions of the theory given here to a variety of experimental results on solvation in protic and aprotic solvents. Good agreement with experiment is found. Our theory compares favorably with the dynamical mean spherical approximation (MSA) theory of time-dependent solvation.
Three-point extension hypernetted chain, conventional hypernetted chain, and superposition approximations: Numerical results for the force between two plates
Authors: Lozada-Cassou Marcelo , Díaz-Herrera Enrique
Published: July 2002

The conventional hypernetted chain/mean spherical (HNC/MS) equation and a superposition approximation to the Born–Green–Yvon (BGY) equation are derived and numerically solved for the force between two charged plates immersed in a primitive model electrolyte or a point-ion electrolyte. The results are compared to the force calculated through the three-point extension to the HNC/MS approximation. The force between two hard plates, immersed in a hard-sphere fluid, is also calculated with these three theories. Excellent agreement is found among the three theories for the force between two hard uncharged plates. For charged plates, the agreement goes from good to total disagreement, depending on the plate potential and/or electrolyte concentration. While the three theories predict attractions between the plates, the conventional HNC/MS and BGY equations predict a nonexistent attraction at low electrolyte concentration. The corresponding conventional method and superposition approximation method calculations of the force between two plates immersed in a point-ion solution are in surprisingly good agreement with the Verwey–Overbeek results, when the potential on the plates is low,and/or the electrolyte concentration is high.
Multiple intermolecular bend vibrational excitation of a hydrogen bond: An extended infrared study of OCOHF
Authors: Nesbitt David J. , Lovejoy Christopher M.
Published: July 2002

We report the use of near infrared tunable difference frequency laser absorption methods to investigate low-frequency bending of the intermolecular hydrogen bond in OCOHF complexes. By deliberate thermal warming of the slit jet expansion to 16 K, we observe bending ‘‘hot band’’ transitions built on the fundamental vHF=1←0 HF stretch from the lowest five internally excited bending states (i.e., vlbend=00←00, 11←11, 20←20, 22←22, and 31←31) which correspond to low-frequency, skeletal bending of the intermolecular hydrogen bond. In addition, much weaker parallel (Δl=0) combination band transitions (vlbend=20←00 and 31←11 ) are observed at ≲5% of the 00←00 intensity. Furthermore, measurements of the extremely weak 11←00 perpendicular (Δl=1) band are obtained at ≲1% of the 00←00 intensity. The fundamental, hot band, and combination band data permit quantitative measurement of the absolute vibrational energies of all vibrational levels for the l=0 and 1 manifolds in both HF excited (vHF=1) and ground-state (vHF=0) complexes.

The bending frequencies are surprisingly low (∼10 cm−1 ) and exhibit positive anharmonicity (i.e., the energy level spacings increase with vlbend ). The results suggest nearly unrestricted bending of the hydrogen bond in a very flat, highly anharmonic angular potential. In contrast with many other weakly bound complexes, the lowest bending frequency decreases dramatically upon HF excitation, which signals a vHF vibrationally induced shift from a linear to a nonlinear equilibrium geometry for the vHF=1 excited OCOHF potential surface. Excess Lorentzian line widths are observed in all OCOHF transitions, attributable to vibrational predissociation lifetimes that vary smoothly from 1.2 ns (vlbend=00) to 650 ps (vlbend=31) as a function of intermolecular bending excitation.
Theoretical study of the hyperfine parameters of OH
Authors: Chong Delano P. , Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

The magnetic hyperfine parameters of 17OH are studied as a function of the one- and n-particle spaces. All of the parameters, except the spin density on oxygen, bF(O), are well described by modest levels of theory. Thus, we have directed much of our effort to understanding the computational requirements for accurately determining bF(O). Full configuration-interaction (FCI) calculations in six Gaussian basis sets give unambiguous results for the effect of uncontracting the oxygen s and p basis sets, of adding diffuse s and p functions, and of adding polarization functions to oxygen. FCI calculations in a [6s 2p 1d/2s] contracted basis give a bF(O) value that is in good agreement with experiment. A critical factor in any n-particle treatment is a balanced treatment of core and valence correlation. The size-extensive modified coupled-pair functional method provides bF values in reasonably good agreement with the FCI. Methods that account quantitatively for the valence correlation, such as complete-active-space self-consistent-field/multireference configuration interaction, appear to introduce an imbalance between core and valence contributions to bF when the one-particle basis sets are flexibly contracted in the core.
An experimental and theoretical study of the bond selected photodissociation of HOD
Authors: Vander Wal R. L. , Scott J. L. , Crim F. F. , Weide K. , Schinke R.
Published: July 2002

Experimental and theoretical studies of the photodissociation of single vibrational states in HOD provide a qualitative and quantitative understanding of the dissociation dynamics and bond selectivity of this process. Vibrationally mediated photodissociation, in which one photon prepares a vibrational state that a second photon dissociates, can selectively cleave the O–H bond in HOD molecules containing four quanta of O–H stretching excitation. Dissociation of HOD(4νOH) with 266 or 239.5-nm photons produces OD fragments in at least a 15 fold excess over OH, but photolysis of the same state with 218.5-nm photons produces comparable amounts of OH and OD. Wave packet propagation calculations on an ab initio potential energy surface reproduce these observations quantitatively. They show that the origin of the selectivity and its energy dependence is the communication of the initial vibrational state with different portions of the outgoing continuum wave function for different photolysis energies.
Fragmentation mechanisms from three-dimensional wave packet studies: Vibrational predissociation of NeCl2, HeCl2, NeICl, and HeICl
Authors: Gray Stephen K. , Wozny Christopher E.
Published: July 2002

We present three-dimensional, time-dependent quantum studies on the van der Waals vibrational predissociation reactions of NeCl2, HeCl2, NeICl, and HeICl. A wave packet/basis set expansion approach is employed. The results for these systems agree reasonably well with experiment and time-independent quantum calculations, where available. The similarities and differences among the four systems are explored by detailed inspection of the propagating wave function. The rotational product distributions can be understood in terms of a unified fragmentation mechanism that depends on the product of the initial probability density with a classical force, and short time dynamics. Quantum interference effects are shown to play an important role in the helium cases. A semiclassical model is also advanced to account for some features of the product distributions.
An effective barrier model for describing quantum mechanical activated rate processes in condensed phases
Authors: Voth Gregory A. , O’Gorman Eileen V.
Published: July 2002

An analytic variational model is developed to treat quantum activated rate processes in condensed phase systems. An effective parabolic barrier curvature in this model describes both the barrier nonlinearity along the reaction coordinate as well as the influence of dissipation from the fluctuating condensed phase environment. The theory is developed by exploiting the relationship between the statistics of Feynman path integral centroid densities and the rate of quantum activated events. The result is numerically tested on a model activated dynamics problem and applied to estimate the quantum tunneling correction to an SN2 reaction in water.
Theoretical study of the NO β system
Authors: Langhoff Stephen R. , Partridge Harry , Bauschlicher Charles W. , Komornicki Andrew
Published: July 2002

A b initio calculations including high levels of correlation treatment have been performed for the B 2Π–X 2Π β system of NO. The calculations are complicated by the fact that the B 2Π state gains considerable Rydberg character in the Franck–Condon region of this transition, as it approaches an avoided crossing with the C 2Π Rydberg state at shorter r values. The calculated lifetimes vary from 2.12 to 1.17 μs for v′=0 to 6, as compared with the recent measurements using laser induced fluorescence that vary from 2.00 to 0.85 μs. Nearly exact agreement with the experimental lifetimes can be achieved by shifting the transition moment function to larger r values to simulate a more rapid onset of the Rydbergization of the B 2Π state. The calculated Einstein coefficients for v′=6 to v″=0–16 are in overall good agreement with experiment, but systematic differences remain that may be partially attributable to limitations in the calibration of the spectrometer.
Study of correlation effects in an exactly solvable model two-electron system
Authors: Ghosh Swapan K. , Samanta Alok
Published: July 2002

The correlation energies for a system of two electrons moving under mutual Coulomb repulsion augmented by a linear interparticle potential and bound to a central harmonic oscillator potential are studied through an exact analytical solution proposed recently by the authors. The same are also obtained for a related model with no linear term through numerical solution. Several plots of the local as well as averaged correlation holes for varying strengths of the binding potential for both the models provide insight into the shape and radius of the holes. The effect of correlation on the single particle density is manifested through reduction of the probability density near the center with a consequent increase in the outer region.
Intensities in local mode overtone spectra of dimethyl ether and acetone
Authors: Kjaergaard Henrik G. , Henry Bryan R. , Tarr Allan W.
Published: July 2002

Gas phase overtone spectra of dimethyl ether (ΔvCH=2–5) and acetone (ΔvCH=3) are measured using conventional near infrared (NIR) spectroscopy. Intracavity dye laser photoacoustic spectroscopy (IDL-PAS) has been used to measure the ΔvCH=5–7 gas phase spectra of both dimethyl ether and acetone. Oscillator strengths are calculated using a harmonically coupled anharmonic oscillator local mode description to obtain the vibrational wave functions and ab initio molecular orbital (MO) calculations to obtain the dipole moment function. The calculations, which use no adjustable parameters, can account for the magnitude of the intensities and for the falloff in intensity with increasing v, for both molecules. It can also account reasonably well for the relative intensity of various peaks within a given vibrational manifold and for the relative intensity in the spectra of these two molecules and of propane.
On the fallibility of variational calculations: A
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o versus empirical potential energy functions for HCN
Authors: Smith Alice M. , Klemperer William , Lehmann Kevin K.
Published: July 2002

HCN is one of the most advantageous candidates among the polyatomics for an empirical determination of the anharmonic potential energy function because of the extensive list of precise spectroscopic data available for a number of isotopomers. We have determined an empirical stretching potential energy surface for HCN with a rms error of less than 3 cm−1 for vibrational energy levels of up to 18 400 cm−1. The best ab initio stretching potential has a rms error of 10.5 cm−1 for the same levels. However, using the ab initio dipole moment surface and vibrational wave functions from the empirical vs ab initio potentials, we find that the predicted intensities are remarkably different, a factor of 4 for the (0,0,6) band at 18 377 cm−1. Furthermore, the predicted intensities from the theoretical potential agree significantly better with the experimental values. The most natural explanation of these results is that the ab initio potential is closer to reality than an empirical surface obtained from exhaustive spectroscopic data.
Collision-induced state-to-state flow of vibrational energy in S1 toluene
Authors: Kincaid Edward H. , Worah V. , Schuh Merlyn D.
Published: July 2002

State-to-state flow of vibrational energy in the S1 state of toluene vapor under ‘‘single collision’’ conditions has been measured. The 6b1 level (ν′6b=530 cm−1 ) was pumped by pulsed laser excitation, and resolved fluorescence spectra revealed which vibrational levels in the S1 state were produced by collisions with three added gases. Energy flow was found to be selective and occurred into four channels. Endergonic flow of energy to identified, accessible vibrational levels is not observed. Collision cross sections increased with increasing molecular complexity of the collision partners, but similar energy flow patterns were observed with all collision partners. Energy flow between 6a1 and 6b1 levels, which have a spacing of about 90 cm−1, was found to be very inefficient.
Highly asymmetric electrolyte: Comparison between one- and two-component models at different levels of approximations
Authors: Linse Per
Published: July 2002

An accurate structure of a two-component asymmetric (charge asymmetry 1:20) electrolyte is used to investigate the mean spherical approximation (MSA), the hypernetted-chain (HNC), and the Rogers–Young (RY) equations. It is demonstrated that the inherent approximations of the MSA and HNC theories cause the micellar charge currently extracted from small-angle neutron scattering experiments to be smaller than the nominal one. Thus, the extracted micellar charge should be viewed as a purely fitting parameter and a value smaller than the nominal one does not necessarily imply an incomplete ionization of the counterions from the micelle, as previously suggested. The HNC theory gives a considerable improvement over the MSA theory, but is still not sufficiently accurate for providing quantitative correct results (15% error). The accurate structure is also used to assess the usefulness of using a simpler one-component model with an effective macroion potential, either with simulation or with liquid-state theories. It was found that the screened Coulomb potential is an excellent approximation and moreover the optimal prefactor and screening length fulfill the DLVO relation provided that the micellar charge is regarded as a fitting parameter. However, when one of the liquid-state theories is employed, the DLVO relation is not fulfilled. The RY approximation gave an improved structure as compared to the HNC and the MSA equations.
Ultrafast transient Raman investigation of geminate recombination and vibrational energy relaxation in iodine: The role of energy relaxation pathways to solvent vibrations
Authors: Xu Xiaobing , Yu Soo-Chang , Lingle Robert , Zhu Huiping , Hopkins J. B.
Published: July 2002

Direct observation of geminate recombination and vibrational energy relaxation in the X state of iodine has been accomplished using picosecond Raman spectroscopy. The dynamics of energy relaxation from vibrational levels ranging from v=52 to v=1 have been observed. These levels correspond to absolute energies of 9300 to 210 cm−1 above the zero point in the X potential. The effect of relaxation to solvent vibrations in resonance with I2 vibrations has been studied. The efficiency of these vibrational–vibrational relaxation channels is found to be very solvent dependent. The results suggest that the vibrational coupling between the excited iodine oscillator and the solvent is drastically affected by the nature of the normal mode character of the solvent vibration.
Transition state theory, Siegert eigenstates, and quantum mechanical reaction rates
Authors: Seideman Tamar , Miller William H.
Published: July 2002

The ‘‘good’’ action variables associated with a transition state (i.e., the saddle point of a potential energy surface), on which a general semiclassical transition state theory is based, are shown to be the semiclassical counterpart of the Siegert eigenvalues of the system. (Siegert eigenvalues are the complex eigenvalues of the Schrödinger equation with outgoing wave boundary conditions.) By using flux correlation functions, it is then shown how the exact quantum mechanical reaction rate can be expressed in terms of the Siegert eigenvalues (and eigenfunctions). Applications to some test problems show these Siegert-based rate expressions to be rapidly convergent with respect to the sum over Siegert states.
Equation of state of shock-compressed liquids: Carbon dioxide and air
Authors: Nellis W. J. , Mitchell A. C. , Ree F. H. , Ross M. , Holmes N. C. , Trainor R. J. , Erskine D. J.
Published: July 2002

Equation-of-state data were measured for liquid carbon dioxide and air shock-compressed to pressures in the range 28–71 GPa (280–710 kbar) using a two-stage light-gas gun. The experimental methods are described. The data indicate that shock-compressed liquid CO2 decomposes at pressures above 34 GPa. Liquid air dissociates above a comparable shock pressure, as does liquid nitrogen. Theoretical intermolecular potentials are derived for CO2 from the data. The calculated shock temperature for the onset of CO2 decomposition is 4500 K at a volume of 17 cm3/mol.
Fractals and resonances in collinear (He,H+2) collisions
Authors: Balasubramanian V. , Mishra B. K. , Bahel A. , Kumar S. , Sathyamurthy N.
Published: July 2002

Oscillations in the chattering region in plots of final action and collision time as a function of the initial vibrational phase of the diatom in collinear He+H+2(ni=0) collisions are shown to have characteristics of fractals with a capacity dimension 1.38–1.68 over a wide range of translational energies. For energies above the reaction threshold, the fractal zones are shown to occur between reactive and nonreactive bands and are related to known quantal reactive scattering resonances.
Quantum scattering calculations on the OH+H2(v=0,1), OH+D2, and OD+H2 reactions
Authors: Clary David C.
Published: July 2002

Quantum reactive scattering calculations are reported for the four-atom reactions OH+H2(v=0,1)→H2O+H, OH+D2→HOD+D, and OD+H2→DOH+H, and their reverse reactions. The method involves using hyperspherical coordinates to describe the H2 vibration and one local OH stretching vibration of H2O, accounting for both the rotation of the OH and the bending mode of H2O with a spherical harmonic basis set, and applying a version of the bending-corrected rotating-line approximation to treat the rotation of H2 and the vibration of initial OH. The method gives cross sections and rate coefficients for these reactions which are state selected in the initial OH(j) rotational and H2(v) vibrational states and in the H2O(n,m) product states where n and m label bending and local OH-stretching vibrational states of H2O. A modified potential-energy surface based on a fit to ab initio data is used. The calculated rate coefficients for both the OH+H2(v=0) and OH+D2(v=0) reactions agree very well with experiment over the whole temperature range of 250–1050 K. The rate coefficients for OH+H2(v=0) are significantly larger than those for OH+D2(v=0) at lower temperatures due to quantum tunneling. The rate coefficient for the OH+H2(v=1) reaction at room temperature also agrees well with experiment, and is larger than that for the OH+H2(v=0) reaction by factors ranging from 103 at 150 K to 10 at 1000 K. The rate coefficients for the OD+H2(v=0) reaction are almost identical to those for the OH+H2(v=0) reaction. The rotational product distributions of OH(j) arising from the reverse reaction H2O+H→OH(j)+H2 are found to be correlated with excitation in the initial vibrational bending mode of H2O.
Dynamics of ion solvation in a Stockmayer fluid
Authors: Perera Lalith , Berkowitz Max L.
Published: July 2002

Molecular dynamics computer simulations were performed to study the dynamics of the ionic solvation in a Stockmayer fluid. The simulations show that the solvent relaxation proceeds in two time regimes. Most of the relaxation occurs in a short time period during which the relaxation process can be described by a Gaussian function. The long time regime can be described by an exponential relaxation. The decay exponent of the relaxation function in this regime is the same as the exponent describing the decay of the single dipole correlation function. In addition, the contribution of the rotational and translational modes of the solvent to the energy relaxation was investigated. It was found that when the rotational mode is the dominant mode of the solvent motion the relaxation occurs from the outside–in, in accordance with the Onsager ‘‘snowball’’ picture. When the influence of the translational mode is increased the Onsager picture breaks down.
Polarizability effects in a four-charge model for water
Authors: Kozack R. E. , Jordan P. C.
Published: July 2002

A planar four-charge model for the water molecule, with a spherically symmetric Lennard-Jones potential and a point polarizability, is examined. The model parameters are determined by fitting gas-phase data for the one and two-molecule systems. A reasonable description of water clusters of up to five molecules is obtained. The model gives qualitatively accurate properties in liquid and solid phases, but not enough condensation for agreement with experiment. It is suggested that the use of a point polarizability only partially accounts for many-body effects. Alternatives to the above model are discussed.
Intramolecular vibrational relaxation in a triatomic van der Waals molecule: ArCl2
Authors: Halberstadt Nadine , Beswick Alberto , Roncero Octavio , Janda Kenneth C.
Published: July 2002

Exact quantum mechanical calculations of the vibrational predissociation of ArCl2 are reported. The results confirm the hypothesis that Δv=−2 dissociation of the B state, v′=10 level occurs by preliminary coupling to the Δv=−1 manifold followed by coupling to the Δv=−2 continuum. The intensity borrowing due to the coupling is consistent with intramolecular vibrational relaxation (IVR) in the sparse limit, ρV≤1.
Spin-Hamiltonian analyses of grafted rod phase transitions
Authors: Costas María Eugenia , Wang Zhen-Gang , Gelbart William M.
Published: July 2002

We highlight here the symmetry breaking which occurs as rodlike particles are adsorbed onto a planar surface. A spin-1 Ising-variable Hamiltonian is defined which includes both bilinear and biquadratic interaction terms. Nearest-neighbor couplings are also introduced which distinguish between parallel pairs of particles having different orientations with respect to the surface normal; finally, a single-particle term describes the anisotropy in adsorption energy. By solving for the thermodynamics of this Hamiltonian in both mean-field and renormalization-group approximation, we establish the necessary conditions for the appearance of two successive fluid–fluid phase transitions.
Canonical variational transition state theory for dissipative systems: Application to generalized Langevin equations
Authors: Frishman Anatoli , Pollak Eli
Published: July 2002

A numerical solution for the canonical variational dividing surface of two degree of freedom conservative systems is presented. The method is applied to reaction rates in dissipative systems whose dynamics is described by a generalized Langevin equation. Applications include a cubic and a quartic well using Ohmic and memory friction. For Ohmic friction, we find that in almost all cases, curvature of the optimal dividing surface may be neglected and the Kramers spatial diffusion limit for the rate is in practice an upper bound. For a Gaussian memory friction and a cubic oscillator, we compare the present theory with numerical simulations and other approximate theories presented by Tucker et al. [J. Chem. Phys. 95, 5809 (1991)]. For the quartic oscillator and exponential friction, we discover a strong suppression of the transmission coefficient and the reaction rate whenever the reduced static friction is of the same order of the reduced memory time. We also show that in this case, there is a strong suppression of the energy diffusion process in the reactants’ well.
Frequency-dependent hyperpolarizabilities for argon, krypton, and neon: Comparison with experiment
Authors: Rice Julia E.
Published: July 2002

Accurate frequency-dependent hyperpolarizabilities which incorporate the effects of electron correlation have been calculated for neon, argon, and krypton. Specifically, these values are derived from a previous ‘‘best estimate’’ static value based on coupled-cluster calculations, and use of a second-order perturbation theory frequency-dependent correction. The values for γK(−ω;ω,0,0) and γ∥(−2ω;ω,ω,0) are compared with experiments for the d.c. Kerr effect and second harmonic generation, respectively. No negative dispersion is observed for either of these processes for neon, argon, and krypton. In particular, the second harmonic generation values for neon show a positive dispersion effect which is in agreement with other recent theoretical calculations and in line with recent measurements. This study demonstrates that it is possible to achieve quantitative accuracy for the frequency-dependent hyperpolarizabilities of dilute gas-phase many-electron systems.
A complete description of the electric dipole moment, polarizability, and hyperpolarizability tensors of hydrogen peroxide
Authors: Maroulis George
Published: July 2002

We report a complete description of the electric dipole moment tensor (μα), dipole polarizability (ααβ), first (βαβγ), and second (γαβγδ) dipole hyperpolarizability tensors of hydrogen peroxide. All the independent components of the four tensors are extracted from self-consistent field (SCF) and Mo/ller–Plesset perturbation theory (MP) calculations of the energy of the molecule perturbed by a static homogeneous electric field. Electron correlation decreases the dipole moment and increases the dipole polarizability of the molecule, has a strong effect on the first dipole hyperpolarizability and an even stronger effect on the second dipole hyperpolarizability. The best values of the permanent dipole moment and the mean polarizabilities ᾱ, β̄, and γ̄ obtained in this work are 0.6165 ea0, 14.95 e2a20E−1h, −8.56 e3a30E−2h, and 1601 e4a40E−3h, respectively.
Photodissociation of H2O2 at 193 and 222 nm: Products and quantum yields
Authors: Vaghjiani Ghanshyam L. , Turnipseed Andrew A. , Warren Rachel F. , Ravishankara A. R.
Published: July 2002

The primary quantum yields of OH(X 2Π),H(2S), and oxygen atoms [O(1D)+O(3P)] produced in the photodissociation of H2O2 at 193 and 222 nm have been measured at 298 K. At 193 nm, the primary quantum yields were observed to be 1.51±0.18, 0.16±0.04, and <0.02, for Φ(OH), Φ(H), and the sum of Φ(O) and Φ(O 1S), respectively. At 222 nm, the OH yield was Φ(OH)=2.02±0.35, the H atom yield was Φ(H)=0.024±0.012, and Φ(O) was <0.002. The errors quoted above are 2σ, precision plus estimated systematic errors. The OH product was directly monitored by pulsed laser-induced fluorescence, and the atomic species were detected via cw resonance fluorescence. The OH quantum yields reported here were measured relative to known product quantum yields in the dissocation of H2O2 at 248 nm. H(2S) yields were measured relative to those in photolysis of HBr and HCl, (at 193 nm) or CH3SH (at 222 nm), whereas O atoms yields were measured relative to O3 photolysis at both wavelengths. The present results indicate unit dissociation of H2O2 at both 222 and 193 nm with only two major products OH (∼80% at 193 nm, 98% at 222 nm) and H(2S) (∼20% at 193 nm, 2% at 222 nm). Up to 15% of the OH produced in the 193 nm photolysis may be vibrationally excited; however, no evidence for vibrationally excited OH was observed at 222 nm.
Thermodynamic formulas of liquid phase nucleation from vapor in multicomponent systems
Authors: Nishioka K. , Kusaka I.
Published: July 2002

Interfacial thermodynamics is extended to noncritical liquid clusters in vapor for multicomponent systems so as to clarify the uncertainty over whether or not size and composition dependence of interfacial tension must be taken into account in taking the extremity condition of the reversible work of forming a noncritical cluster to derive the size and the composition of a critical nucleus. It is found that the differential of interfacial tension does not arise in the extremity condition due to the Gibbs–Duhem relation derived for a system of a noncritical cluster and the vapor which is in equilibrium under an additional constraint to maintain the number of molecules contained in a cluster.
Note on the number dependence of nonequilibrium molecular dynamics simulations of the viscosity of structured molecules
Authors: Rowley Richard L. , Ely James F.
Published: July 2002

Nonequilibrium molecular dynamics simulations have been performed for a system of 512 four-site models representing n-butane molecules. The results are compared to simulations previously reported for 125 molecules to check a recently reported number dependence in such simulations. A small system size dependence was observed at low shear rates, but the rheological behavior is not affected. Considerably greater certainty is obtained using larger systems.
The lowest excited singlet state of isolated 1-phenyl-1,3-butadiene and 1-phenyl-1,3,5-hexatriene
Authors: Buma Wybren Jan , Kohler Bryan E. , Nuss John M. , Shaler Thomas A. , Song Kyuseok
Published: July 2002

We report vibrationally resolved S0→S1 excitation spectra and vibronic level decay times for the phenyl-substituted polyenes 1-phenylbutadiene and 1-phenylhexatriene seeded in supersonic He expansions. This information was obtained using one- and two-color resonance-enhanced multiphoton ionization techniques. The shift in the excitation energy of the lowest excited singlet state upon deuteration of the phenyl ring demonstrates that in 1-phenylbutadiene S1 is mainly an excitation of the benzene ring while in 1-phenylhexatriene S1 is mainly the 2 1Ag state of hexatriene. Analysis of the excitation spectrum of 1-phenylhexatriene shows that the spectrum contains contributions from two species, the trans,trans and the trans,cis isomers, whose 2 1Ag state excitation energies differ by about 155 cm−1. The lifetimes of 2 1Ag zero-point levels and, in particular, the activity of the C–C and C=C stretch vibrations are markedly different for the two isomers. From these results we obtain a picture of the influence that the phenyl group has on such aspects of the electronic structure of the 2 1Ag state of hexatriene as the effective conjugation length, the symmetry properties, and the fluorescence quantum yield.
Dynamics of ethylene photodissociation from rovibrational and translational energy distributions of H2 products
Authors: Cromwell Evan F. , Stolow Albert , Vrakking Marcus J. J. , Lee Yuan T.
Published: July 2002

The dynamics of H2 elimination from ethylene was studied via a pump and probe technique utilizing an ultrahigh resolution vacuum ultraviolet laser system. H2 product internal and translational energy distributions were obtained for the photodissociation at 193 nm. The distribution of energy in H2 product from the dissociation of (1,1)-dideuteroethylene is also presented. Two separate H2 elimination channels are inferred: a 1,1 elimination producing the vinylidene radical and a 1,2 elimination producing the acetylene molecule. Differences between the vibrational, rotational, and translational energy distributions for these two channels are discussed and correlations between product internal and translational energy are presented. A comparison with ab initio calculations of the transition state configurations for these processes is made. We suggest that the H2 elimination process may be nonstatistical in nature. The D atom elimination from C2D4 was examined and kinetic energy distribution for this product measured.
Correspondence of phase separation in several charged particle systems
Authors: McGahay Vincent , Tomozawa Minoru
Published: July 2002

This paper demonstrates the close correspondence in phase-separation behavior of a variety of systems which contain charged particles. These include electron-hole fluids in semiconductors, oxide melts and glasses, and molten salts. Reduced critical parameters for the fifteen systems considered are calculated using the theory of corresponding states for ionic fluids. The reduced critical parameters are found to be in close agreement, indicating that the different systems exist in corresponding states at their critical points to a good approximation. A model of phase separation based on Debye–Hückel theory is presented. The usual limitations of the theory due to the linearization of the Poisson–Boltzmann equation as well as neglect of excluded volume effects due to hard core interactions are avoided by employing the dissociation/association equilibrium of Fuoss. The theoretical critical parameters which are derived are in excellent agreement with experimental data. The degree of dissociation is calculated to be about 3% at the critical point, which is in the expected range of a few percent.
Structure and thermodynamic properties of water–methanol mixtures: Role of the water–water interaction
Authors: Tanaka Hideki , Gubbins Keith E.
Published: July 2002

Thermodynamic properties and structures of water–methanol mixtures at various temperatures have been investigated by means of Monte Carlo simulations and subsequent analyses. The OPLS model by Jorgensen was used for the methanol–methanol interaction and both the Caravetta–Clementi (CC) potential and TIP4P potential by Jorgensen et al. were used for the water–water interaction. We show that the role of water–water interaction is very important in discussing aqueous solutions of alcohols, and examine the origin of the exothermic mixing processes. We have investigated the sensitivity of the temperature dependence of the enthalpy of mixing to the water–water interaction. The CC potential is able to reproduce the temperature dependence observed in experiments, although the absolute values of the mixing enthalpy were larger than the experimental ones. While the TIP4P potential results in better agreement for the excess enthalpy and volume near room temperature, the temperature dependence of the excess enthalpy did not agree with experiment. The difference in the magnitude of the exothermic hydration for different water–water interactions is explained in terms of the energetic stability of the clathrate hydrate compared with ice, on the basis that the structure of water in the vicinity of a methanol molecule is similar to the clathrate hydrate. It is found that the energetic stability of the clathrate hydrate for the CC model is higher than that for TIP4P, and this is responsible for the larger exothermic hydration. The higher stability of the clathrate hydrate structure for the CC potential, in turn, arises from the difference in the pair interaction energy surface between two kinds of potential functions; the minimum energy structure and the flexibility of the hydrogen bonded pair.
Isotope effects in the vibrational deactivation of large molecules
Authors: Toselli Beatriz M. , Barker John R.
Published: July 2002

Collisional deactivation of highly vibrationally excited gas phase toluene-d8 and benzene-d6 pumped at 248 nm, has been investigated by monitoring the time resolved infrared fluorescence from the C–D stretch modes near 4.3 μm. For toluene-d8, energy transfer data were obtained for about 20 collider gases, including unexcited toluene-d8; for benzene-d6, only a few colliders were investigated. For both systems the data were analyzed by an inversion technique that converts the fluorescence decay to the bulk average energy, from which is calculated the average energy transferred per collision, 〈〈ΔE〉〉inv. Data obtained earlier for benzene-d0 were reanalyzed and the revised results are reported. Results for both normal and deuterated excited species show 〈〈ΔE〉〉inv to be nearly directly proportional to the vibrational energy 〈〈E〉〉inv of the excited molecule from 5 000 to 25 000 cm−1. However, for pure toluene-d8, benzene-d6, and a few other collider gases at high energies, the slope of the 〈〈ΔE〉〉inv vs 〈〈E〉〉inv curve is reduced and even becomes negative at sufficiently high energies. The results obtained for normal and deuterium-containing species are discussed in terms of possible quantum effects and mechanisms for energy transfer. In particular, it is considered likely that V–T/R energy transfer dominates over V–V, and the lowest frequency vibrational modes are the conduits for the energy transfer, in agreement with results for small molecules. Attention is called to a fundamental difference between classical and quantum statistics and how this difference may adversely affect classical trajectory simulations of large molecules.
Adsorption and reaction of formic acid on NiO(100) films on Mo(100): Temperature programmed desorption and high resolution electron energy loss spectroscopy studies
Authors: Truong Charles M. , Wu Ming-Cheng , Goodman D. Wayne
Published: July 2002

Adsorption and reaction of formic acid on well-defined NiO(100) films prepared on Mo(100) have been investigated using combined temperature programmed desorption (TPD)/high resolution electron energy loss spectroscopy (HREELS). A novel approach to HREELS of oxide materials is utilized to acquire spectral data. This approach enables the direct observation of weak loss features due to the excitation of adsorbates without serious interference from intense multiple surface optical phonon losses associated with oxide materials. The results indicate that formic acid initially adsorbs associatively on NiO(100) surfaces at 90 K and undergoes heterolytic dissociation upon heating to ≥200 K to form a formate intermediate. The adsorbed formate species is proposed to bond to a cation site via one of the oxygen atoms of the formate in a monodentate configuration. The presence of a dimeric formic acid species is also indicated and a bonding configuration for this species has been proposed.
Fine-structure branching ratios and Doppler profiles of Cl(2P
j) photofragments from photodissociation of the chlorine molecule near and in the ultraviolet region
Authors: Matsumi Yutaka , Tonokura Kenichi , Kawasaki Masahiro
Published: July 2002

Photofragment chlorine atoms in the 2Pj states from Cl2 at 266–500 nm are measured by a resonance-enhanced multiphoton ionization technique. The measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] increases monotonically with dissociation wavelengths from (9.5±1.8)×10−3 at 308 nm to (4.7±0.5)×10−1 at 475 nm. At 375 nm≤λ≤475 nm, Doppler profiles of the chlorine atoms indicate adiabatic formation of two Cl(2P3/2) from the 1Π(1u) state of Cl2 and Cl(2P3/2)+Cl*(2P1/2) from the B 3Π(0+u) state. On the other hand, at a shorter wavelength (308 nm), Cl*(2P1/2) is generated from the 1Π(1u) state through nonadiabatic transitions during the dissociation. The relative contribution of the 1Π(1u)←X 1Σ(0+g) and B 3Π(0+u)←X 1Σ(0+g) transitions in the photoabsorption is estimated from the obtained branching ratios at 375 nm≤λ≤475 nm.
High resolution electronic spectroscopy of Ar⋅OH and Ar⋅OD vibronic bands
Authors: Chang Bor-Chen , Williamson James M. , Cullin David W. , Dunlop James R. , Miller Terry A.
Published: July 2002

Laser-induced fluorescence spectra with resolved rotational, fine, and hyperfine structure have been observed for the U bands of the Ã←X̃ electronic transition of the Ar⋅OH/D complex. The appearance of these spectra are very different from the previously analyzed A band system of the complex. Detailed analysis of the spectra reveals that their spectral differences are caused by changes in values of molecular parameters (due to the different regions of the potential surface sampled in the Ã state) and their resulting effect upon the coupling of the angular momenta in the complex. Numerical results are given for the molecular parameters in the Ã state levels examined in these experiments and the implication of these values for the geometry and the potential surface of the complex are discussed.
The Cs(7p)+H2→CsH+H reaction. III. Quasiclassical trajectory study
Authors: L’Hermite J.-M.
Published: July 2002

A quasiclassical trajectory study of the Cs(7p)+H2→CsH+H harpooning reaction at thermal energies is carried out on a single potential energy surface. Extensive calculations are performed in a 2D space since 3D calculations show that all trajectories are nearly confined into the collision plane. Calculated rotational and angular distributions of CsH products are in good agreement with experimental data. The best agreement is obtained using a potential energy surface with no potential barrier in the exit valley. Some dynamical features are demonstrated through an analysis of correlations between various collision parameters. Migrations of the electronic charge ‘‘−’’ from one H atom to the other during the collision induce important dynamical effects; in particular, they increase the collision time and reduce CsH product vibrational energy. The duration of the collision, the deflection angle, the angle between J′ and L′ (internal and orbital output angular momenta) and the translational energy of products are closely related to the impact parameter; there is a radical change in the behavior of the system according to the impact parameter being smaller or greater than the critical value bc≂2.8 Å. For b≤bc, the collision time is constant, the deflection function is linear, J′ and L′ are either parallel or antiparallel; for b≥bc, the collision time increases with b, an isotropic component appears in the deflection function, and J′ and L′ are always parallel. The geometry of approach has no influence on the dynamics of the collision. Angular and rotational distributions are not modified when taking into account the nonreactive channels correlated to Cs(6s,6p,5d,7s)+H2.
Electronic spectra of jet-cooled 5-bromotropolone and 5-chlorotropolone. Influence of symmetrical substitution on proton tunneling in the S1 state
Authors: Tsuji Takeshi , Sekiya Hiroshi , Nishimura Yukio , Mori Ryoji , Mori Akira , Takeshita Hitoshi
Published: July 2002

The S1–S0 fluorescence excitation and dispersed fluorescence spectra have been measured for jet-cooled 5-bromotropolone (5BTR-h), 5-chlorotropolone (5CTR-h), and their OD derivatives in order to investigate substituent effects on proton tunneling. Both the 0++ and 0−− transitions were identified in the fluorescence spectra of these molecules. The tunneling doublet splittings in the electronic origin band of 5BTR-h (16 cm−1) decreased by 4 cm−1, whereas that of 5CTR-h (23 cm−1) increased by 3 cm−1 as compared with tropolone (TRN-h). Relatively small deviation in the tunneling separations for these molecules from that for tropolone is ascribed to conjugation in the S1 state. An electron-withdrawing character of chlorine and bromine atoms is almost canceled by a conjugative electron-releasing effect.
Core-hole excited states of H2O: Symmetries and relative oscillator strengths
Authors: Kim D. Y. , Lee K. , Ma C. I. , Mahalingam M. , Hanson D. M. , Hulbert S. L.
Published: July 2002

Tunable and polarized soft x-ray radiation was used to excite core electrons of H2O. Angle-resolved proton yields were measured by retarding potential analyzers at 0° and 90° with respect to the polarization vector of the radiation. The ability of angle-resolved detection to reveal hidden resonances in rich molecular spectra is demonstrated, and the anisotropy parameter obtained from the two ion yields identifies the symmetries and the relative oscillator strengths of the core-hole excited states in the near-edge region of the spectrum. The analysis of the data substantiates the validity of the axial recoil approximation and provides evidence for ultrafast dissociation. Comparisons with the results of theoretical calculations are made.