bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of spectral line shape. I. Formulation and line coupling
Authors: Roney P. L.
Published: July 2002

A theoretical expression for spectral line shape, applicable to the conditions for atmospheric infrared transmission, is derived. The theory is based on a previously derived kinetic equation for the density matrix for a system of absorbing molecules, in a thermal bath. An iterative solution for the linearized kinetic equation is developed as a density expansion about an isolated line approximation, based on Fano’s frequency-dependent collision operator. The absorption coefficient is determined by the application of linear response theory, together with the fluctuation-dissipation theory and detailed balance. The isolated line shape has non-Lorentzian wings due to frequency dependent shift and width parameters. New expressions are found for the line coupling terms, which depend on the line strengths of coupled lines as well as their width and shift parameters.
Coherent anti-Stokes Raman spectroscopy study of collisional broadening in the O2–H2O Q branch
Authors: Fanjoux G. , Millot G. , Saint-Loup R. , Chaux R. , Rosenmann L.
Published: July 2002

The fundamental isotropic Raman Q branch of oxygen perturbed by collisions with water vapor has been studied at pressures up to 1.5 atm and for temperatures between 446 and 990 K. The spectra have been recorded by using coherent anti-Stokes Raman spectroscopy (CARS) which has been preferred to stimulated Raman spectroscopy (SRS) in order to obtain more signal and higher sensitivity as the mixture has a small percentage of oxygen. The high resolution CARS spectrometer uses a seeded Nd:YAG laser actively stabilized on an external Fabry–Perot interferometer to prevent any frequency drift during the course of the experiment. The line broadening coefficients have been determined for several rotational quantum numbers (up to N=31 at 990 K). The effect of the splitting into triplets at lower pressure and the effect of interferences between neighboring lines at higher pressure have been taken into account. The influence of Dicke narrowing has also been considered and special care has been taken to avoid Stark broadening. The line broadening coefficients have been calculated according to a semiclassical model. The rotational quantum number and temperature dependencies of the experimental line broadening coefficients have also been studied with another approach based on fitting and scaling laws. Among several laws, the modified exponential energy gap law (MEG), the statistical power-exponential gap law (SPEG), and the energy corrected sudden law with basis rate constants taken as a hybrid exponential-power law (ECS-EP) have given good results. We have used the fitting and scaling laws to extrapolate in temperature the linewidths at 2000 K.
Three-dimensional quantum mechanical study of the Li+HF→LiF+H process: Calculation of integral and differential cross sections
Authors: Baer Michael , Last Isidore , Loesch Hans-Jurgen
Published: July 2002

In this work is presented a detailed quantum mechanical study of the Li+HF(v=0,j) →LiF(v′,j′)+H reaction in the low energy region 0.001≤Etr≤0.15 eV. The theoretical-numerical treatment was carried out within the inelastic jz approximation coupled with negative imaginary potentials to account for the reactivity of the system. Integral and differential state-to-state cross sections were calculated and compared with experiment and other calculations. The theoretical numerical study revealed the following. (a) In the low energy region namely 0.001≤Etr≤0.04 eV the exchange process is governed by a potential well which is responsible for the build up of a long-lived-activated complex. (b) At higher energies the reaction process is direct but is dominated by strong interference effects reminiscence of the Fraunhofer diffraction phenomenon. (c) The quantum mechanical treatment supported the existence of the quasiselection rules related to the orbital and the internal angular momenta as well as the expected right-hand-angle ejection of the hydrogen atom. The calculated cross sections were compared with experiment. The following was found: (a) the theoretical treatment approximately reproduced the laboratory measured differential cross sections and (b) integral cross sections were found to be in the range 0.30≤σ≤0.65 Å2 which overlap rather well with the single measured value (at Etr=0.14 eV) of σ=0.6±0.30 Å2.
Density functional study of symmetric proton transfers
Authors: Stanton Robert V. , Merz Kenneth M.
Published: July 2002

Proton transfers are important in many organic and biological systems. As a first step towards examining such reactions in condensed phases we evaluate here the ability of density functional theory (DFT) to model symmetric proton transfers. The DFT results are calculated using the local density approximation, as well as with the incorporation of nonlocal (NL) corrections to the exchange and correlation. Through a comparison of the results we find that NL corrections are necessary to obtain accurate representations for reactions involving proton transfer. Hartree–Fock (HF) and post-HF calculations are also reported for the same systems using identical basis sets in order to evaluate the correlation between HF results and those of DFT.
Experimental and theoretical study of the O+HCl transition state region by photodetachment of OHCl−
Authors: Davis Michael J. , Koizumi Hiroyasu , Schatz George C. , Bradforth Stephen E. , Neumark Daniel M.
Published: July 2002

We present measured and calculated photodetachment spectra of OHCl−, and we interpret the results in terms of the vibrational structure of the transition state of the O+HCl→OH+Cl reaction. The measured spectra exhibit two distinct features—an intense broad peak at high electron kinetic energies and a less intense shoulder at lower energies. Superimposed on these broad features are several sharper structures, but they are barely discernible from noise in the spectrum. To interpret these spectra, we have used a recently developed global 3A″ potential surface for the O+HCl reaction to calculate Franck–Condon factors, using an L2 method (i.e., expansion in terms of square integrable basis functions) to approximate the scattering wave functions on the reactive surface. Assignment of the spectrum has been assisted using the results of quantum coupled channel calculations for the same surface. The resulting calculated spectrum shows the same broad features as the measured spectrum. There is also fine structure with spacings and energies that are similar to the experiment, but specific features do not match. To interpret both the broad and fine features in the theoretical spectrum, a hierarchical analysis is applied wherein this spectrum is decomposed by a tree construction into components of increasingly higher resolution. The physical meaning of each of these components is then determined by plotting ‘‘smoothed states’’ that are obtained from the tree coefficients. This leads to the conclusion that the two broad features in the spectrum are made up of progressions in hindered rotor states of the Cl–OH complex, with the most intense feature corresponding to OH(v=0) and the weaker shoulder corresponding to OH(v=1). There is evidence for Feshbach resonance features in the v=1 feature, but it appears that most of the fine structure is due to hindered rotor states.
Extended stochastic description of the new phase formation kinetics
Authors: Manankova A. V.
Published: July 2002

The effect of equilibrium fluctuations of stable variables of state of a new phase nucleus on nucleation kinetics in addition to fluctuations of the unstable variable is reconsidered. For a description of kinetics of the process, we use the stochastic differential equations (the Langevin approach). The advantages of this approach are discussed. In the reduced description (the averaging over additional variables) the equation similar to that of the classical nucleation theory is obtained. But the coefficients of this equation are renormalized due to fluctuations of the additional degrees of freedom. Thermal energy fluctuations of a drop (the phase transition vapor-to-liquid, nonisothermal homogeneous nucleation) are discussed in detail. The results are presented in the simple analytical form. A suggestion is made on how the renormalized coefficients of the equation can be recalculated into the renormalized parameters of the classical nucleation theory. The re-evaluated correction factor is found to differ from results of previous studies significantly, by an order of magnitude.
Evidence for quantum interference in collision-induced intramolecular energy transfer within CO singlet–triplet mixed states
Authors: Sha Guohe , He Jinbao , Jiang Bo , Zhang Cunhao
Published: July 2002

The quantum interference effect associated with one type of radiationless transition, the collisional energy transfer between singlet–triplet mixed molecular states, is studied. The experiments are conducted on CO(A 1Π,e 3Σ−)–He(Ar) via the ultrasensitive optical–optical double resonance multiphoton ionization (OODR-MPI) technique which measures state-to-state cross sections to an accuracy of ±10%, irrespective of the lifetime of the excited state. Distinct evidence of a quantum interference effect on the transfer rate has been obtained for mixed state CO intramolecular energy transfer processes. A simple, explicit expression for the cross section for mixed state energy transfer, based on the first order Born approximation of time dependent perturbation theory, is derived. The use of a transition phase angle θST is incorporated in the expression to describe the phase angle difference between singlet and triplet channels. This greatly refines the existing theory of quantum interference for collisional processes in that it successfully calculates the energy transfer cross sections and interprets the observed quantum interference effect under various quantum transitions. Experimental values of θST obtained for the CO–He system are 66° for J=9 and 73° for J=13. These results indicate that the infinite-order-sudden approximation which calls for θST=0 cannot satisfactorily account for the quantum interference effect. In addition, measured θST values for CO–Ar system are consistent with our theoretical expectations.
A detailed three-dimensional quantum study of the Li+FH reaction
Authors: Parker G. A. , Laganà A. , Crocchianti S. , Pack R. T
Published: July 2002

Accurate quantum reactive scattering calculations in the full three-dimensional physical space have been carried out for the Li+FH reaction at zero total angular momentum using the adiabatically adjusting principal axis of inertia hyperspherical coordinate formalism. The procedures for fitting the potential energy surface, calculating the surface functions, and propagating the solutions in a coupled channel treatment are given and discussed. Features of the resulting reactive probability plots are analyzed, and simple explanations of a number of the quantum resonance and oscillatory features are found.
Laser initiated half reaction study of H+O2→OH+O
Authors: Honma Kenji
Published: July 2002

The H+O2 reaction system was studied under geometry limited half reaction conditions. The weakly bonded complex O2–H2S was formed by supersonic expansion, and reaction was initiated by 193 nm photoirradiation of the complex. Rotational, spin-orbit, and lambda doublet state distributions of product OH were determined by a laser-induced fluorescence (LIF) technique. The populations of the two spin-orbit states were observed to be statistical. The population of the Π(A′) level was almost twice that of the Π(A″) level, and the planar geometry was suggested for reaction path. These populations of the fine structures of OH were similar to those of OH formed under bimolecular reaction conditions. On the other hand, the rotational state distribution of OH from the half reaction has two components and the dominant one shows a very cold rotational distribution, in sharp contrast with that of the bimolecular reaction where rotation is highly excited. This cold rotational distribution could be partially explained by the absorption of a part of available energy by the internal motion of SH. However, the distribution with a peak at the lowest rotational level could not be explained by this effect, but ascribed to the exit interaction between SH and OH and/or the entrance channel specificity, i.e., the reaction occurs in limited impact parameters.
Highly efficient collisional stabilization and the symmetry constrained dynamics of high temperature complex formation
Authors: Gole James L.
Published: July 2002

The highly efficient collisional stabilization of high temperature complexes of some considerable spatial extent is demonstrated. A series of near single collision and well defined multiple collision (following paper) chemiluminescent and laser induced fluorescent studies extending over six decades of pressure demonstrate the stabilization of electronically excited group IIA dihalide collision complexes via a radiative three body recombination process (R3BR) operative at microTorr pressures. Over the pressure range 1×10−6–5×10−4 Torr, a comparative study of the emission from M (M=Ca, Sr, Ba)–X2 (Cl2,Br2,I2) and M–XY (ICl,IBr) reactive encounters identifies a symmetry constrained dynamics associated with the formation of the dihalide product complexes. The onset of the monitored R3BR process at 1×10−6 Torr signals an extremely large stabilization cross section (σS≳3000 Å2) which may not be readily explained within the RRKM framework. Comparisons between the highly ionic dihalides and the isoelectronic CO2 molecule are noted as they affect excited state dynamics. The pressure dependence of the light emission from these complexes in the near single collision pressure range displays a striking correlation with the periodicity of dihalide molecular electronic structure and the resultant nature of the low-lying dihalide electronic transitions. The absence of a difluoride emission associated with the M(Ca,Sr,Ba)–F2 reactive encounters signals an important periodic trend in these systems. A simple first order model within the electron jump framework is presented to explain the qualitative trends inherent to these reactions.
Theoretical studies of the effects of matrix composition, lattice temperature, and isotopic substitution on isomerization reactions of matrix-isolated HONO/Ar
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

Theoretical molecular dynamics studies of matrix composition, lattice temperature, and isotopic substitution effects upon cis–trans isomerization rates and the vibrational relaxation rates to lattice phonon modes of matrix-isolated HONO, DONO, and H18ON18O systems are reported. The results show that isomerization is usually slower in an argon matrix than in xenon. The calculated ratios of the rates for different initial vibrational energy distributions correlate well with the ratio of the well-depth parameters for the lattice/HONO interactions. In all cases examined, the matrix-isolated isomerization rate is enhanced relative to the gas-phase rate. This behavior is attributed to a vibration → lattice phonon modes → rotation → torsional vibration) isomerization mechanism. Isomerization in both Xe and Ar matrices is nonstatistical with pronounced mode specificity present in both environments. In the gas phase, deuterium and 18O substitution produce small, positive enhancements of the isomerization rate by 13% and 26%, respectively, due to an increased kinetic coupling to the torsional modes. In the matrix, however, the isotope effects are negative and larger in magnitude. This reversal is attributed to a reduced rate of energy transfer from the lattice to rotation of DONO and H18ON18O due to the increased moment of inertia. In general, all of the present results support a matrix HONO isomerization mechanism via a (vibration→lattice phonon modes →rotation→torsional vibration) energy transfer pathway.
Rotational dynamics of axially symmetric solutes in isotropic liquids. I. A collective cage description from molecular dynamics simulations
Authors: Polimeno Antonino , Moro Giorgio J. , Freed Jack H.
Published: July 2002

An operational definition of collective cage variables previously introduced for liquid argon is extended, via a molecular dynamics study, to the rotational properties of axially symmetric molecules. Quantitative measures of the static and dynamic cage properties are extracted for liquid Cl2 near the triple point. The collective cage variables are well described by the potential acting on an arbitrary molecule (i.e. solute) for a fixed configuration of the other molecules (i.e. solvent). A dynamic separability of the solute orientation relative to the cage potential and of the relative solute displacement is justified in part by the faster relaxation found for the latter. Large and persistent orientational cage potentials (∼15–20 kBT) lead to substantial alignment of the solute in the cage with an average local order parameter of 0.87. The reorientational correlation times for the cage are consistent with axially symmetric Brownian motion. The reorientational correlation times for the solute are nearly equal to the equivalent ones of the cage, consistent with the strong coupling of solute within its cage which leads to a collective reorientation of solute and cage (e.g. τcage(2)=1.4 ps, and τsolute(2)=1.2 ps). Solute librations within the cage are much faster (τ libr(2)=0.12 ps) and are comparable to the relaxation of the relative solute displacements (τr=0.15 ps). The solute angular momentum exhibits the fastest correlation time (τ J=0.06 ps). While the orientational cage potential shows rapidly and slowly relaxing components (τωf=0.14 ps and τωs=2.87 ps, respectively), its dominant portion shows a very long persistence.
Investigation of free volume percolation under the liquid–glass phase transition
Authors: Voloshin Vladimir P. , Naberukhin Yuri I. , Medvedev Nikolai N. , Jhon Mu Shik
Published: July 2002

Mutual arrangement of large volume atomic cells (defined as the Voronoi polyhedra) is investigated in molecular dynamics models of Lennard-Jones liquid, crystal and amorphous solid by the percolation theory methods on the Delaunay network. In addition to ordinary percolation some extended formulations of the percolation problem are realized. All variants of the percolation analysis lead to characteristics of percolating clusters and to the percolation threshold values which do not practically differ for all the three phases. This provides evidence that the phase transition liquid–amorphous solid is not associated with percolation through regions with a large free volume.
Statistical adiabatic channel calculation of accurate low-temperature rate constants for the recombination of OH radicals in their ground rovibronic state
Authors: Maergoiz A. I. , Nikitin E. E. , Troe J.
Published: July 2002

Accurate low-energy capture cross sections and low-temperature capture rate constants for two OH radicals in their ground rovibronic states X 2Π3/2(v=0, j=3/2) were calculated within the statistical adiabatic channel approach. The rate constants calculated in first order provide a good approximation to the true rate constant below 4 K. The rate constants calculated in second order provide a correction of about 25% to the first order rate constant at 20 K and indicate an only weak temperature dependence at T≳20 K. At higher temperatures deviation of the potential from long-range electrostatic interaction have to be accounted for.
A perturbational study of some hydrogen-bonded dimers
Authors: Langlet Jacqueline , Caillet Jacqueline , Caffarel Michel
Published: July 2002

We present a detailed study of several hydrogen-bonded dimers consisting of H2O, NH3, and HF molecules using the Symmetry Adapted Perturbation Theory (SAPT) at different levels of approximations. The relative importance of each individual perturbational components and the quality of the total interaction energies obtained are discussed. The dependence of the results on the relative orientation of the molecules of the dimers and on the intermonomer distance is also investigated.
Phase instabilities in charged hard-sphere mixtures. II. Binary mixtures of salts
Authors: Kenkare Paresh U. , Hall Carol K. , Caccamo C.
Published: July 2002

Charged hard-sphere mixtures consisting of two positively charged species and one negatively charged species (common anion mixtures) are used to represent binary mixtures of salts. Phase separation in the mixture is studied using the Gibbs free energy expression for common anion mixtures derived in paper I of this series. A detailed description of the dependence of the resulting phase diagrams on molecular size and charge of the species, and on the osmotic pressure of the mixture is presented. Binary mixtures of salts containing equal-sized ions exhibit type III phase behavior whereas binary mixtures of salts containing ions of unequal size exhibit either type II or type IV phase behavior. The type of phase behavior observed in binary mixtures of salts is characterized as a function of the critical pressures and critical volumes of the pure salts. Our results suggest that phase separation in mixtures of charged hard spheres is influenced by a competition between mixing effects (entropy), which encourage miscibility and ion-pairing effects (enthalpy), which encourage phase separation. Potential applications of the model to experimental systems are discussed.
Model of the glass transition
Authors: Mansfield Marc L.
Published: July 2002

This paper examines the three well-known empiricisms that accompany the glass transition of fragile glass-forming liquids: the unusual thermodynamic behavior of supercooled liquids that is usually called the Kauzmann paradox, the temperature dependence of the relaxation time as embodied in the Vogel–Fulcher law, and the time-domain relaxation law usually known as the Kohlrausch–Williams–Watts relaxation function. Mutually consistent descriptions of all three phenomena are presented. The Kauzmann paradox can be explained by recognizing that the ground states of both the liquid and the crystal, and also the dominant excitations of these two ground states, are similar in many ways. This implies that the two phases have similar thermodynamics at low T. The Vogel–Fulcher law results from the assumption that localized regions of the liquid must be excited above a certain threshold enthalpy before they can relax; as the temperature falls, this threshold enthalpy becomes less accessible and the apparent activation enthalpy to affect relaxation increases. The resulting model is also consistent with the Montroll–Schlesinger–Bendler description of Kohlrausch–Williams–Watts relaxation, which explains the third empiricism.
Some integral relationships for distribution functions of fluids in disordered media
Authors: Madden William G.
Published: July 2002

The Yvon–Born–Green, Kirkwood and Kirkwood–Salsburg integral equation hierarchies have been obtained for the case of a fluid adsorbed into a host medium made up of immobile particles. Despite earlier work which showed that the Ornstein–Zernicke equations for this situation were fundamentally different from those of a binary equilibrium fluid mixture, the pure-fluid and mixed-fluid-matrix Yvon–Born–Green and Kirkwood–Salsburg equations for the matrix-averaged distribution functions, g(n)f and for g(n)mf, are found to be identical to those for the equilibrium mixture. However, the equilibrium mixture equations for g(n)m do not apply. At present, the Kirkwood equation does not appear in a matrix-averaged form suitable for numerical work. The Kirkwood–Salsburg equations can be used to generate the fundamental graph theory for the problem. In practical calculations, the special role of the matrix enters principally in the closures used to truncate the hierarchy of equations. The standard Kirkwood superposition approximation is appropriate in this application, and circumstances in which practical corrections to the superposition approximation can be employed are considered.
Internal motion and the tunneling rates of CH+4 and CD+4
Authors: Wang Jian , Laaksonen Aatto , Eriksson Leif A. , Boyd Russell J.
Published: July 2002

The thermal motion of CD+4 in the gas phase at 4 and 40 K is studied by means of a molecular dynamics method using the MP2/6-31G(d,p) force field. The thermal motion of CD+4 in the temperature range from 4 to 40 K does not involve a deuterium permutation on the picosecond time scale. The electronic energy fluctuations of CD+4 due to thermal motion are less than 0.7 kcal/mol at 4 K and 1.6 kcal/mol at 40 K, which is less than the energy barrier (3.4 kcal/mol at MP2 level) required for the permutation between two energy minimum C2v structures. On the basis of these calculations it is suggested that the observation of four equivalent hydrogen atoms in the ESR spectrum of CH+4 can be attributed to fast quantum tunneling, with an estimated rate of 4.3×106 s−1 for CH+4 and 1.6×103 s−1 for CD+4.
Static and transport properties of symmetric and asymmetric electrolytes in micropores at fixed surface potential
Authors: Olivares Wilmer , Sulbarán Belky , Lozada-Cassou Marcelo
Published: July 2002

We study some static and kinetic properties of model electrolyte solutions, confined by two charged plates, for example, ionic exclusion, the force balance between the two plates, and the electrokinetic phenomenological Onsager’s coefficients Lmn. The three point extension hypernetted chain/mean spherical integral equation (TPE-HNC/MSA) was solved to obtain the ionic concentration profiles of a confined restricted primitive model electrolyte and the plate–plate force. We compare the results of this theory with the classically used modified Gouy–Chapman (MGC) point ion theory. It is shown that the properties of the system depend strongly on the charge of the counterions and, hence, the asymmetric 2:1 and 1:2 salts give results similar to those of the symmetric 1:1 and 2:2 salts, respectively. The MGC theory fails to predict the force oscillation between repulsive and attractive and other important features that appear for ultranarrow pores. These discrepancies are enhanced by the presence of doubly charged ions. The TPE-HNC/MSA theory predicts a nonlinear behavior of the capillary conductivity as a function of the slit width.
Nanodomain formation in a liquid polymer blend: The initial stages of phase separation
Authors: Marcus Andrew H. , Hussey Deborah M. , Diachun Nathan A. , Fayer M. D.
Published: July 2002

The morphology of nanodomain structures in binary polymer blends of a random copolymer and a homopolymer is determined using electronic excitation transport (EET) studies. The experimental system employed is a copolymer, 6.5% atactic poly(methyl methacrylate-co-2-vinyl naphthalene) [P(MMA-2VN)], in atactic poly(vinyl acetate) (PVAc). The naphthalene groups serve as chromophores in the EET experiments. The mixtures are prepared such that initially the P(MMA-2VN) chains are randomly distributed in the PVAc matrix. The nanodomains are formed while low-concentration mixtures of the P(MMA-2VN) in PVAc are held at constant temperature in the melt state (T≳Tg), above the temperature at which phase separation occurs. In the melt the chains diffuse, and P(MMA-2VN) chains aggregate until the temperature is quenched below Tg. The structures of the resulting domains are examined with time-resolved fluorescence depolarization measurements, and the data are analyzed using an analytical theory to model EET among interacting polymer chains. The agreement between theory and data is very good. The results of the analysis indicate that the nanodomains correspond to aggregates with a characteristic size equal to the radius of gyration of the copolymer, Rg. The number of P(MMA-2VN) chains in aggregates prepared under different conditions is determined.
Structure of binary polymer blends: Multiple time step hybrid Monte Carlo simulations and self-consistent integral-equation theory
Authors: Gromov Dmitry G. , de Pablo Juan J.
Published: July 2002

A newly developed self-consistent formulation of the polymer reference interaction site model (PRISM) theory is used to predict the structure of binary polymer blends. Theoretical radial distribution functions are compared to those obtained from hybrid Monte Carlo simulations of mixtures of Lennard-Jones chains. A multiple time step method is implemented to increase the efficiency of the simulations. We examine both the cases of atomic and molecular closures and consider both conventional and self-consistent PRISM. We find that, overall, theoretical distribution functions are in good agreement with simulation.
Statistical properties of surfaces covered by deposited particles
Authors: Wojtaszczyk P. , Mann E. K. , Senger B. , Voegel J. C. , Schaaf P.
Published: July 2002

The statistical properties of surfaces covered by irreversibly adsorbed colloidal particles are studied as a function of the Péclet number (or equivalently as a function of their rescaled radius R*). More precisely, the radial distribution function g(r) is determined as a function of the coverage θ for five systems corresponding to different values of R*. Also measured is the reduced variance σ2/〈n〉 of the number n of adsorbed particles on surfaces of given area out of the adsorption plane. Finally, the evolution of 〈n〉 with the concentration of particles in solution during the deposition process is determined for the different systems. This allows us to obtain information on the available surface function Φ. All these parameters are compared to their expected behavior according to the random sequential adsorption (RSA) model and to the ballistic model (BM). It is found that the radial distribution function of the system of particles characterized by R*<1 is well predicted by the RSA model whereas for R*≳3 the BM can serve as a good first approximation. On the other hand, one finds surprisingly that the available surface function Φ and the reduced variance σ2/〈n〉 vary with the coverage θ in a similar way for all the systems over the range of value of R* investigated. Their behavior corresponds, in first approximation, to the expectations from the BM. In particular, the reduced variance σ2/〈n〉 exhibits a horizontal tangent at low coverage whereas the RSA model predicts an initial slope of −4. This result is the more intriguing that σ2/〈n〉 is directly related to the radial distribution function g(r), which does vary with R*. Finally, higher order moments of the distribution of the number of particles n adsorbed on our surfaces are also determined as a function of the coverage. They behave, within experimental errors, like those of a Gaussian distribution as predicted by the central limit theorem.
Near UV optical second harmonic generation studies of surface-adsorbed tryptophan residues
Authors: Smiley Beth L. , Vogel Viola
Published: July 2002

A new optical method intended for interfacial protein studies has surfaced from the observation of near UV second harmonic generation (SHG) from the aromatic amino acids. Here, the first hyperpolarizability, β=4.72±0.52×10−30 esu, of t-butyloxycarbonyl-L-tryptophan is measured for SHG at 266 nm, and its polar orientation at the air–water interface is deduced. The indole ring nitrogen atom is found to point upwards at the interface. From a monolayer surface density of 1.74±0.15 adsorbate molecules/nm2, it is estimated that a minimum of 0.49 Trp residues/nm2 of surface area can be detected above the water background for this wavelength. SHG studies of tryptophan-containing peptides and proteins based on these results have the potential to complement existing linear optical techniques by providing new information on interfacial molecular ordering.
Absorption and Raman spectroscopy of mass-selected tantalum tetramers in argon matrices
Authors: Wang Huaiming , Craig Robert , Haouari Hanae , Dong Jian-Guo , Hu Zhendong , Vivoni Alberto , Lombardi John R. , Lindsay D. M.
Published: July 2002

We have examined both the absorption and resonance Raman spectra of mass-selected Ta tetramers. The tetramers are produced in a sputtering source and mass filtered with a Wien filter, then neutralized and deposited in an Ar matrix at low temperatures. The absorption spectrum indicates two broad transitions, one in the red at 768 nm and another to the blue at 532 nm. Raman resonances could be excited in both regions giving three distinct fundamental frequencies at 270.2(1) cm−1, 185.1(1) cm−1, and 130.6(2) cm−1. The lowest vibration shows a long progression (up to seven overtones) with alternating separations of 135 cm−1 and 126 cm−1, indicative of a weak Jahn–Teller effect in the tetramer ground state. This, along with the observation that the fundamental frequency ratios are close to 2:√2:1 indicate that the molecule has a tetrahedral ground state geometry with an electronic E state symmetry. The appearance of three fundamentals in the resonance Raman spectrum indicates that both excited states corresponding to the observed absorption bands arise from severely distorted T electronic states.
Molecular dynamics simulations of liquid, interface, and ionic solvation of polarizable carbon tetrachloride
Authors: Chang Tsun-Mei , Peterson Kirk A. , Dang Liem X.
Published: July 2002

In this study, we construct a nonadditive polarizable model potential to describe the intermolecular interactions between carbon tetrachloride, CCl4, based on classical molecular dynamics techniques. The potential parameters are refined to accurately describe the experimental thermodynamic and structural properties of liquid CCl4 at 298 K. We then carried out additional liquid CCl4 simulations at temperatures in the range of 250–323 K to examine the temperature dependence of the thermodynamic properties. The computed liquid densities and the enthalpies of vaporization are in excellent agreement with experimental values. The structures of liquid CCl4 can be analyzed by examining the radial distribution functions and angular distribution functions. It is found that the liquid CCl4 forms an interlocking structure and that a local orientational correlation is observed between neighboring CCl4 molecules. We also investigate the CCl4 liquid/vapor interface using this potential model. The density profile shows that the interface is not sharp at a microscopic level and has a thickness of roughly 5 Å at 273 K. The results of angular distribution function calculations suggest that CCl4 molecules do not have a preferred orientation at the interface. The calculated surface tension is 31±2 dyn/cm, in good agreement with the experimental value of 28 dyn/cm. This model potential is also used to examine the interactions between Cs+ and small (CCl4)n (n=1–6) clusters. A tetrahedral configuration is found for the minimum structure of the Cs+(CCl4)4 cluster.

It is noticed that the polarization energy is the dominant component of the total interaction of these ionic clusters, indicating the importance of including explicitly the polarization in the ionic interactions. In the study of Cs+ solvation in liquid CCl4, we observe a well-defined solvation shell around the Cs+ with a coordination number of six CCl4 molecules. It is also found that Cs+ induces a strong local orientational order in liquid CCl4. Accurate ab initio electronic structure calculations were also carried out on the CCl4 dimer and the Cs+(CCl4) cluster to compare to the results from the molecular dynamics simulations.
Electron energy distribution functions and thermalization times in methane and in argon–methane mixtures: An effect of vibrational excitation processes
Authors: Krajcar-Bronić Ines , Kimura Mineo
Published: July 2002

Electron thermalization in methane and argon–methane mixtures is studied by using the Boltzmann equation. The presence of low-lying vibrational excited states in methane significantly changes electron energy distribution functions and relaxation times. We found that (i) the mean electron energy just below the first vibrational excited state is reached faster by 1000 times when the vibrational states are taken into account, and (ii) electron energy distribution functions have distinct peaks at energy intervals equal to the vibrational threshold energies. Both these effects are due to large vibrational stopping cross section. The thermalization time in mixtures of argon–methane (without vibrational states) smoothly changes as the mixture composition varies, and no significant difference in the electron energy distribution function is observed. When the vibrational excited states are taken into account, thermalization is almost completely defined by CH4, even at very low fractional concentrations of CH4. The sensitivity of the electron energy distribution functions on the momentum transfer cross sections used in calculation on the thermalization is discussed.
Determining nuclear hyperfine populations in the ground electronic state of atomic hydrogen produced by the 193 nm photolysis of HBr
Authors: Cowen Kenneth A. , Lorenz K. Thomas , Yen Yu-Fong , Herman Michael F. , Koplitz Brent
Published: July 2002

A variation of velocity-aligned Doppler spectroscopy is used to study F=0 and F=1 nuclear hyperfine populations in the ground electronic state of atomic hydrogen as the result of a chemical reaction. In this case, the reaction is the 193 nm photolysis of HBr. Not only can the nuclear hyperfine states be studied, but the method allows unequivocal correlation with the spin–orbit states of the bromine atom, i.e., Br(2P1/2) and Br(2P3/2). For H atoms correlated with Br(2P3/2), the F=0/F=1 is 1.2±0.2. Future directions involving the study of this fundamental chemical product state distribution are also discussed.
Polar solvation dynamics of polyatomic solutes: Simulation studies in acetonitrile and methanol
Authors: Kumar P. V. , Maroncelli M.
Published: July 2002

This paper describes results of simulations of solvation dynamics of a variety of solutes in two reference solvents, acetonitrile and methanol. Part of these studies involve attempts to realistically model the solvation dynamics observed experimentally with the fluorescence probe coumarin 153 (C153). After showing that linear response simulations afford a reliable route to the dynamics of interest, experimental and simulation results for C153 are compared. Agreement between the observed and calculated dynamics is found to be satisfactory in the case of acetonitrile but poor in the case of methanol. The latter failure is traced to a lack of realism in the dielectric properties of the methanol model employed. A number of further simulations are then reported for solvation of a number of atomic, diatomic, and benzenelike solutes which are used to elucidate what features of the solute are important for determining the time dependence of the solvation response. As far as large polyatomic solutes like C153 are concerned, the solute attribute of foremost importance is shown to be the ‘‘effective moment’’ of its charge distribution (actually the difference between the S1 and S0 charge distributions). This effective moment, determined from consideration of continuum electrostatics, provides a simple measure of how rapidly the solute’s electric field varies spatially in the important regions of the solvent. Simulations of fictitious excitations in a benzene solute show that this single quantity is able to correlate the dynamics observed in widely different solutes.

Also explored is the effect of solute motion on its solvation dynamics. While of minor relevance for large solutes like C153, in small solutes of the size of benzene, solute motion can dramatically enhance the rate of solvation. A model based on independent solvent dynamics and solute rotational motion is able to account for the bulk of the observed effects. Finally, the influence of solute polarizability on solvation dynamics is considered. Simulations of diatomic molecules with a classical polarizability show that the rate of solvation decreases roughly in proportion to the polarizability of the solute. This dynamical effect can be understood in terms of the change that polarizability produces on the solvation force constant. These simulations indicate that the magnitude of the effect should be relatively small (10%–25%) in real systems, at least in the linear response limit.
The effect of a laser field on electron transfer in metal complexes: Quantum degrees of freedom
Authors: Dakhnovskii Yuri , Evans Deborah G. , Kim Hyung J. , Coalson Rob D.
Published: July 2002

The effect of a strong cw laser field on Fe(OH2)6+2/Fe(OH2)6+3 electron transfer in aqueous solution is considered. On the basis of a kinetic master equation for the time-dependent population, an analytical solution for the forward and backward reaction rates is obtained. The presence of a strong ion–ligand vibration at ℏωq=432cm−1 qualitatively changes the intensity dependence of the sum of forward and backward rate constants (denoted as the ‘‘total rate constant’’). It is shown that the total rate constant is strongly dependent on the laser frequency. For the symmetric exchange reaction this rate constant exhibits resonances as a function of the cw field amplitude. For the activationless reaction (achievable by application of an additional dc field) it decreases sharply with the cw field intensity. It is shown that the asymptotic populations of the reactant and product states are non-Boltzmann; their ratio can be varied by 18 orders of magnitude. Contrary to the case with no ac field, the equilibrium constant exhibits a nonmonotonic dependence on the reaction heat, ε. In some regions of ε it is insensitive to the reaction heat. The analytical solution is verified via numerical solution of the original master equation. Excellent agreement is obtained.
Ultraviolet elimination of H2 from chloroethylenes
Authors: He Guoxin , Yang Yuangan , Huang Yibo , Hashimoto Satoshi , Gordon Robert J.
Published: July 2002

The elimination of H2 in the photodissociation of mono- and di-chloroethylenes was studied with a pump-and-probe technique. A 193 nm excimer laser was used to photodissociate the parent molecules, and a tunable dye laser was used to probe the H2 fragment by 2+1 resonance-enhanced multiphoton ionization (REMPI). The nascent rotational state distributions of H2(X 1Σ+g,v″=0–4) were extracted from the REMPI spectra, and were found to have Boltzmann-type distributions. The maximum and average translational energies for some of the rovibrational levels of H2 were measured using magic angle Doppler spectroscopy. The translational energy of the fragments plus the internal energy of H2 was found to exceed the available energy for a three-center elimination mechanism. It is concluded that a migration mechanism plays a significant role in H2 elimination.
Dielectric response and a phenomenon of a narrow band absorption for a classical rotor in a double well potential
Authors: Gaiduk Vladimir I. , Gaiduk Vladimir V. , Novskova Tamara A. , Tseitlin Boris M. , McConnell James
Published: July 2002

The theory of dielectric relaxation in a planar ensemble of polar molecules is presented for a model where dipoles may rotate in a conservative double well potential (DWP) U=U0 sin2 ϑ. The evolution of wideband dielectric spectra as a function of the potential well depth U0 is studied; both isotropic and anisotropic media being taken as examples. The spectra comprise the Debye relaxation and the quasiresonant Poley absorption regions. The exact theory is compared with a simple quasielastic bond/extended diffusion approximation. The latter is valid for a quantitative description for any value of the field parameter p=[U0/(kBT)]1/2. For p≫1 the spectrum comprises one narrow absorption band, while the Debye relaxation region is markedly shifted to low frequencies. It is shown that in the long lifetime τ limit there exists a minimum absorption band Δν0(p). The quantity Δν0 becomes small for values of parameter p≫1. The dielectric relaxation in ice 1 is discussed in relation to this phenomenon.
Equation of state for classical hard-particlelike fluids
Authors: Edgal U. F. , Boukahil A. , Huber D. L.
Published: July 2002

We extend earlier studies of the equation of state of classical hard-particle fluids to potentials where there is an attractive tail in addition to a repulsive hard core. Like the earlier work, the approach is based on the arbitrary point, nearest-neighbor probability density function. In the high temperature (hard-particle) limit, a parametrization of the integrated distribution is introduced. By matching the parameters against the coefficients in a seven-term virial expansion, we obtain an equation of state that is in excellent agreement with the results from Monte Carlo, molecular dynamics calculations in both two and three dimensions. The theory is extended to finite temperatures by treating deviations from the hard-particle limit as small corrections that can be evaluated using hard-particle distribution functions. A comparison is made with the results from a five-term finite temperature virial expansion for a three-dimensional hard-particle system with a square well attractive potential.
Dynamics of coarse-grained helical wormlike chains. I. Diffusion equation
Authors: Yoshizaki Takenao , Yamakawa Hiromi
Published: July 2002

An alternative representation of the diffusion equation is derived for the discrete helical wormlike (HW) chain model by treating the constraints in a manner different from that previously adopted. In contrast to the previous diffusion equation for the same HW model, which can describe local motions but not global to quasi-global motions of flexible polymers in dilute solution because of the defect arising from the preaveraging of the constraining matrix, the present equation may be considered to be suitable for the description of the latter since it can give explicitly the Rouse–Zimm eigenvalues in the ground (global) state. It is shown that the present and previous equations are exactly the same before the constraining matrix common to them is preaveraged, as is natural, and that the above difference between their characteristics arises from the difference in the effect of the preaveraging approximation. A comparison is also made of the present model (or diffusion equation) with other models for polymer chain dynamics. In particular, the present theory is shown to be rather similar to the Fixman–Kovac theory for the conventional bond chain in the physical meaning as well as the formulation.
New orientationally ordered phases of a homopolymer
Authors: Kuznetsov Yu. A. , Timoshenko E. G. , Dawson K. A.
Published: July 2002

We present results of Monte Carlo simulations of a homopolymer with three-body interactions on a lattice. At equilibrium we find three collapsed phases with different orientational order. The first one may be described as a conventional phase with effective three-body attraction. Two other phases are observed in the region of the phase diagram corresponding to three-body repulsion. The second phase is characterized by local orientational order within the globule. The third one possesses global orientational order and it appears to be analogous to nematic phase of a liquid crystal. Kinetics of the collapse transition to the phase III is crucially different from that of phase I and II due to formation of clusters with different orientations at the intermediate stage. This is the origin of a metastable long-lived state in kinetics.