bluuebunny/crossref_metadata_2025 huggingface dataset:
Pair dynamics in hard sphere fluids: Recollisional time correlation functions
Authors: Vesely Franz J. , Evans Glenn T.
Published: July 2002

Molecular dynamics calculations are reported on dense hard sphere fluids at three packing fractions (η=0.30,0.40,0.48). Attention is directed to the analysis of correlated recollisions of pairs of particles. The short time dynamics [t≂0(τc) where τc=1/collision frequency] is governed by correlated backscattering collisions as evidenced by persistence of vector time correlations. For longer times a crossover takes place from the correlated backscattering regime (or geminate recombination) to the diffusive-like regime (nongeminate recombination) in which vector correlations decay faster than scalar correlations. Recollisional events are categorized as first recollisions, second recollisions, and all recollisions. The resulting correlation functions exhibit different evolutionary time scales. Contributions to the time correlation functions (tcfs) from collision sequences involving three and four particles are also separated and analyzed.
The repulsive wall of the Ar–Ar interatomic potential reexamined
Authors: Aziz Ronald A. , Slaman M. J.
Published: July 2002

The repulsive wall of the Ar–Ar interaction is reexamined with the purpose of rationalizing the apparent inconsistency between high energy beam results and high temperature transport data. A new piece-wise experimental potential is proposed which predicts not only the beam results but a large set of transport data up to 7000 K. Lower temperature/energy data are predicted almost equally as well as other state-of-the-art empirical potentials.
The electric field response of periodic chemical wave trains in folded slow manifold systems
Authors: Sultan Rabih , Ortoleva Peter J.
Published: July 2002

The properties of periodic chemical wave trains in an applied electric field are analyzed for a two variable model. The model reduces to a free boundary problem in the limit where certain reactions are fast. The dispersion relation (frequency vs wave vector) was obtained and compared with the field-free case studied earlier. Electrochemical periodic wave trains were shown to emerge as a bifurcation at a fixed wave vector as the field strength decreases through a wavelength dependent critical value. As the field exceeds its critical value, periodic waves of a given wavelength are annihilated at finite amplitude. The results presented here differ significantly from the small amplitude bifurcation analysis whereby the wave amplitude passes through zero as the field exceeds its critical value. No finite amplitude extinction is observed.
Pseudospectral Hartree–Fock calculations on glycine
Authors: Ringnalda Murco N. , Won Youngdo , Friesner Richard A.
Published: July 2002

The pseudospectral method for Hartree–Fock calculations is applied to the glycine molecule, a test case with 100 basis functions. Several algorithmic improvements are reported, including a Newton–Raphson convergence scheme, Fock matrix updating, a multigrid technique, and optional recalculation of integrals. The pseudospectral method is shown to accurately reproduce the Roothaan–Hall relative and total energies for three conformations of glycine. Timing results show the pseudospectral code to be substantially faster than conventional Hartree–Fock codes.
The intramolecular kinetic isotope effect for the reaction O(3P)+HD
Authors: Robie Daniel C. , Arepalli Sivaram , Presser Nathan , Kitsopoulos Theofanis , Gordon Robert J.
Published: July 2002

The branching ratio for the reaction O(3P)+HD to produce OH and OD was measured over the temperature range 339–500 K using a discharge-flow reactor. The OH and OD products were detected using laser-induced fluorescence under steady-state conditions. In order to determine the relative concentrations of OH and OD produced in this reaction, calibration runs were performed using the reactions of O(3P)+H2 and D2 to produce known relative amounts of the same species. Kinetic modeling showed that secondary reactions and the production of vibrationally excited OD did not affect the measured branching ratio. We found that the OH/OD ratio increased rapidly with decreasing temperature, in qualitative agreement with theory, showing that the reaction is dominated by tunneling below 400 K. At the lowest temperatures, the observed branching ratio is larger than predicted. The reason for this discrepancy is not known.
Free-energy model for the inhomogeneous hard-sphere fluid mixture: Triplet and higher-order direct correlation functions in dense fluids
Authors: Rosenfeld Yaakov , Levesque Dominique , Weis Jean-Jacques
Published: July 2002

Extensive Monte Carlo simulation results for the pair and triplet direct correlation functions of the one-component, hard-sphere system near freezing are presented. These results are in good agreement with the analytic predictions of a recently derived free energy density functional for the inhomogeneous hard-sphere fluid. This free energy model is based on the fundamental geometric measures of the particles. It provides the first unified derivation of the analytic description of the hard-sphere thermodynamics and pair structure as given by the scaled-particle and Percus–Yevick theories, and yields simple explicit expressions for the high order direct correlation functions of the uniform fluid.
Rotational energy transfer in CH3F: The ΔJ=n, ΔK=0 processes
Authors: Everett Henry O. , DeLucia Frank C.
Published: July 2002

We report the measurement of the rates o ΔJ=n, ΔK=0(‖n‖≤10) processes for CH3F–CH3F collisions at 300 K. The data are derived from a time-resolved millimeter/submillimeter-infrared double resonance investigation of both the 12CH3F and the 13CH3F isotopic species. The rates were obtained via a nonlinear least-squares analysis of the data using a numerical simulation of rotational energy transfer in methyl fluoride. These rates are shown to be quantifiable in terms of the scaling law of infinite order sudden collision theory and the statistical power gap law. As a result, the numerous ΔJ=n, ΔK=0(‖n‖>1) rates can be understood in terms of only two parameters, independent of isotopic species. Using these results and the results of our earlier studies of K-changing processes, we discuss how rotational energy transfer in the CH3F system in general can be described in terms of a small number of collisional processes and parameters.
High-temperature equilibrium studies of the gaseous thorium chlorides
Authors: Lau K. H. , Hildenbrand D. L.
Published: July 2002

High-temperature gaseous equilibria among the species Th, ThCl, ThCl2, and ThCl3 were studied by mass spectrometry over broad temperature ranges; both enthalpy and entropy data were derived by the second law procedure. In addition, the sublimation of ThCl4 was studied by the torsion-effusion method and the thermodynamics of the gaseous tetrachloride were evaluated. Individual bond dissociation energies in ThCl4 fall within the range 489–545 kJ mol−1, and vary with ligand number in a way similar to ThF4. Only ThCl appears to have a significant electronic entropy in the experimental temperature range. For ThCl4 the experimental entropy indicates the likelihood of a regular tetrahedral structure as with ThF4, but unlike the uranium tetrahalides which appear to be distorted tetrahedra. In the mass spectrum of ThCl4, the fragment ions ThCl+3, ThCl+2, and ThCl+ show a larger and more nearly correct temperature dependence than the parent ion ThCl+4. Results are compared with those of other thorium and uranium halides.
Molecular dynamics simulations of hydrocarbon chains
Authors: Karaborni S. , O’Connell J. P.
Published: July 2002

Molecular dynamics methods are used to study the conformation of single model hydrocarbon chains in a monatomic Lennard-Jones fluid of methylene segments. Simulations of 7, 9, 11, 13, 15, 17, and 21 segment chains were made with 100–500 spheres. The forces of the skeletal chains involve intramolecular effects of bond vibration, angle bending, and rotation among quartets of adjacent segments. The average trans fraction of the hydrocarbon chains shows no significant effect of chain length, and the end-to-end distance and radius of gyration vary linearly with chain length.
An assessment for the full coupled cluster method including all single, double, and triple excitations: The diatomic molecules LiH, Li2, BH, LiF, C2, BeO, CN+, BF, NO+, and F2
Authors: Scuseria Gustavo E. , Hamilton Tracy P. , Schaefer Henry F.
Published: July 2002

The full coupled cluster singles, doubles, and triples (CCSDT) model for molecular electronic structure theory has been used in conjunction with two less complete coupled cluster methods (CCSD, CCSDT-1) to predict the equilibrium bond distances and harmonic vibrational frequencies of a family of closed-shell ground state diatomic molecules. The results are compared with the configuration interaction method including all single, double, triple, and quadruple excitations (CISDTQ) and experiment. With previous CCSDT results for HF, OH−, N2, and CO, the present work provides a realistic assessment of the CCSDT method. The BeO molecule in particular places great demands on the theory, and the linearized triple excitation model CCSDT-1 fails badly.
Electric dipole moment and hyperfine properties of bromoacetylene in the ground and first excited C–H stretching vibrational states
Authors: Bass Steven M. , DeLeon Robert L. , Muenter J. S.
Published: July 2002

Molecular beam electric resonance spectroscopy, combined with color center laser excitation, has been used to measure the electric dipole moment and Br hyperfine properties of bromoacetylene in its ground and first excited C–H stretching vibrational states. For the HCC79Br, v=0: μ=0.229 62(1) D, eQq=648.113(3) MHz, CBr=6.4(3) kHz, and B=4000.07 MHz. For the HCC79Br, v=1: μ=0.248 82(1) D, eQq=648.160(2) MHz, CBr=7.0(3) kHz, and B=3992.79 MHz. For the HCC81Br, v=0: μ=0.229 56(1) D, eQq=541.430(1) MHz, CBr=7.4(1) kHz, and B=3978.46 MHz. For the HCC81Br, v=1: μ=0.248 61(1) D, eQq=541.464(3) MHz, CBr=7.6(4) kHz, and B=3971.34 MHz. The vibrational band origin for the C–H stretching vibration is 3367.7(2) cm−1.
Collision-induced angular momentum reorientation and rotational energy transfer in CaF(A 2Π1/2)–Ar thermal collisions
Authors: Norman Jeffrey B. , Field Robert W.
Published: July 2002

We have carried out an experimental study of collision-induced rotational angular momentum laboratory frame reorientation and energy transfer in CaF(A 2Π1/2, v=0) in thermal collisions with ground state Ar atoms. An optical–optical double resonance (OODR) technique has been used in which the J=1/2, M=+1/2, f-symmetry level of the CaF A 2Π1/2 state is initially populated, from the ground X 2Σ+ electronic state, using circularly polarized cw dye laser radiation at 606 nm. Collision-induced population of nearby magnetic sublevels of the A state, belonging to both e and f symmetry components of J′=1/2 and 3/2, is probed with a second circularly polarized cw dye laser via the E 2Σ+←A 2Π1/2 transition at 560 nm while monitoring subsequent E 2Σ+→X 2Σ+ ultraviolet fluorescence at 290 nm. This experiment has yielded M-dependent thermal rate constants and velocity-averaged cross sections, ratios of which are in partial agreement with those predicted by Alexander and Davis [M. H. Alexander and S. L. Davis, J. Chem. Phys. 79, 227 (1983)] in an infinite-order-sudden, irreducible tensor treatment of the collision dynamics of an open-shell diatomic molecule and a structureless collision partner.
Kinetic energy release measurements of ammonia cluster ions during metastable decomposition and determination of cluster ion binding energies
Authors: Wei S. , Tzeng W. B. , Castleman A. W.
Published: July 2002

The dissociation dynamics of protonated ammonia clusters is investigated following their production from neutrals using multiphoton ionization. A very useful method of measuring the kinetic energy release arising from evaporative unimolecular dissociation is established; it employs a time-of-flight/reflectron to separate daughters and parents and enables the energy release to be determined from the peak shapes. The method is found to lead to values of high precision. Using a modified statistical theory analysis suggested by Engelking, it is shown that binding energies for large cluster ions can be readily determined. The results compare very well with those derived by other methods, ones which generally have much more severe size limitations. In the case of (NH3)n H+, the observed drop in binding energy from n=5 to 6 is consistent with the particularly stable protonated pentamer structure. Heretofore, unavailable results are available from n=7 to 17, with hints of slightly more stable cluster ions at n=12 and 14, than for neighboring size clusters. The unimolecular dissociation is found to proceed by a statistical evaporative mechanism, with average kinetic energy releases showing a general decrease with size beyond n=5; small local maxima are found in regions of particularly stable structures.
A time-dependent interpretation of the absorption spectrum of CH3ONO
Authors: Engel Volker , Schinke Reinhard , Hennig Steffen , Metiu Horia
Published: July 2002

We use time-dependent quantum theory to interpret the absorption spectrum of CH3ONO in terms of the nuclear motion on the upper potential surface. The model uses one excited potential energy surface and two nuclear coordinates: the NO stretch and the CH3O–NO bond. The latter bond breaks upon excitation leading to dissociation. The spectrum consists of a broad band and two progressions corresponding to predissociation resonances. The band width is inverse proportional to the time scale on which the NO bond length increases to adjust to the longer equilibrium bond length of the upper potential energy surface. The progression of intense narrow resonances corresponds to the NO stretching motion. The other progression is due to oscillations along the reaction coordinate of the wave function temporarily trapped in the predissociation well. Our calculations show that important dynamic information can be obtained by ‘‘smearing off’’ the high resolution spectrum to generate a series of low resolution versions which reveal the time scales on which various spectral features develop in the spectrum. We also show that time-dependent theory can be used efficiently to calculate the lifetime of relatively long lived resonances.
Light scattering theory from monodisperse spheroidal particles in the Rayleigh–Debye–Gans regime
Authors: Buehler Christopher S. , Caruthers James M. , Franses Elias I.
Published: July 2002

A general expression for the scattered intensity by populations of Rayleigh–Debye–Gans (RDG) spheroids has been derived for single and independent scattering from nonabsorbing monodisperse particles. The form factor—which contains information on the particle size, shape, and orientation—and the orientation distribution function are expanded in terms of a complete set of spherical harmonics for the polar and azimuthal angles, θ and φ. These expansions lead to a systematic solution procedure for the inverse problem of determining the orientation distribution function for populations of RDG spheroids from scattered intensities in two dimensions. With Fourier sine and cosine transforms of the form factor with respect to φ, the two-dimensional problem (θ, φ) is reduced to several linear one-dimensional problems at various scattering angles, θ. A quadratic optimization routine is used to solve these problems for obtaining the most pertinent orientation distribution coefficients. As an example, the form factors have been calculated for populations of RDG spheroids undergoing rotary diffusion. The diffusion time, initial orientation, aspect ratio, and particle volume have been examined in these simulations with two-dimensional contour plots. Changing the incident beam polarization changes the scattered intensity contours but not the form factor contours, hence yielding no additional information. The inverse problem has been solved and tested with simulated data from azimuthally symmetric populations of spheroids. The performance of the inverse algorithm has been examined for various particle aspect ratios, sizes, and angles, θ. With error-free simulated data, the orientation distribution functions can be precisely determined over the complete angular range from data obtainable over only a small angular range. These results show the robustness of the new approach. When up to 1% random error is introduced in the simulated intensity data, the solution to the inverse problem becomes less precise but is still reasonable. The implications of the inverse algorithm for analyzing experimental data are discussed.
Reaction path study of conformational transitions in flexible systems: Applications to peptides
Authors: Czerminski Ryszard , Elber Ron
Published: July 2002

A new protocol to find minima and transition states in flexible systems is proposed. It is tested on alanine dipeptide (AD) with four minima and eight transition states and applied to a flexible molecule, isobutyryl–(ala)3–NH–methyl (IAN), the shortest peptide which can form a full helical turn. There were found 138 minima and 490 transition states for IAN. It is shown that the connectivity network between the minima is very dilute and composed of transitions localized on one or two neighboring amide units. Most of the transitions involve one or two dihedral flips. The reaction coordinate from an extended chain conformation to a helix follows a sequence of nonoverlapping rotations around bonds. The rotations along the reaction coordinates are such that the hard repulsions between different atoms are minimized but at the same time the hydrogen bonding and the electrostatic interactions are maximized. A melting-like transition which is associated with the sudden opening of a large number of hopping channels is identified between minima at approximately 5 kcal/mol above the lowest energy minimum. The calculated minima and transition states are used to construct a master equation for the two molecules. The vibrational spectrum of the N–H stretch as a function of time and temperature is finally calculated. It is demonstrated that time-dependent lineshape analysis can be a useful tool to investigate the properties of the large number of minima and the intervening transitions.
Reduced partial dissociation cross sections for a molecule photodesorbed from a surface
Authors: Messina Michael , Coalson Rob D.
Published: July 2002

An implementable formalism is given for computing reduced partial cross sections for photodissociation into final internal fragment states of selected primary modes of motion, but unresolved (i.e., summed over all possible outcomes) with respect to other secondary modes. For concreteness, we focus on the case of photodesorption of a diatomic molecule from a crystal lattice. Using time-dependent wave-packet propagation techniques, numerical illustration is presented for a simplistic model of carbon monoxide initially bound to monoatomic carbon and aluminum crystals.
Vibrational and rotational energy distributions of CH3 and IF formed in the reactions of F atoms with CH4 and CH3I
Authors: Sugawara Ko-ichi , Ito Fumiyuki , Nakanaga Taisuke , Takeo Harutoshi , Matsumura Chi
Published: July 2002

The two reactions F+CH4→CH3+HF (1) and F+CH3I→CH3+IF (2) have been investigated by using time-resolved diode laser absorption spectroscopy to probe the reaction products CH3 and IF. The fluorine atoms have been generated by the pulsed CO2 laser photolysis of SF6 in the presence of CH4 or CH3I at a total pressure of 5 Pa. Rotational lines of CH3 in the v2 =1–0, 2–1, and 3–2 bands (out-of-plane bending vibration) and those of IF in the v=1–0, 2–1, 3–2, 4–3, 5–4, and 6–5 bands have been observed and analyzed to determine time-dependent product energy distributions. No clear evidence for population inversion has been found in the vibrational levels of both CH3 and IF. Nascent vibrational distributions of v2=0:1:2 of CH3 are 1:0.36:0.15 for reaction (1) and 1:0.32:0.14 for reaction (2), and that of IF v=0:1:2:3:4:5 is 1:0.70:0.40:0.22:0.10:0.04. The nascent CH3 produced in reaction (1) is rotationally cold (around room temperature) and only 3% and 2% of total available energy are partitioned to the ν2 vibrational mode and rotational motion of CH3, respectively. The vibrational and rotational energy distributions of CH3 and IF produced in reaction (2) have been well reproduced by a statistical model calculation.
Weak predissociation of the EF, GK, and H 1Σ+g states of the H2 molecule by nonadiabatic coupling with the electronic ground state
Authors: Quadrelli P. , Dressler K. , Wolniewicz L.
Published: July 2002

The energy shifts and widths generated in the vibrational levels of the EF, GK, and H 1Σ+g states of H2 by their nonadiabatic interactions with the electronic ground state have been calculated using ab initio wave functions. The shifts of the 44 quasibound vibronic levels of these excited states range from +0.02 to +1.00 cm−1. Along the EF progression the shifts are found to depend on vibrational kinetic energy in surprisingly good accord with the Van Vleck approximation. The calculated nonradiative decay probabilities range up to 3.3×106 s−1, and in one case the predissociation rate is as large as 39% of the radiative one. Experimental tests of these predictions are not yet available.
The electronic spectra of SNS. Low-lying doublet states
Authors: Yamaguchi Yukio , Xie Yaoming , Alberts Ian L. , Grev Roger S. , Schaefer Henry F.
Published: July 2002

The present study aims to characterize the electronically low-lying doublet states of the SNS molecule in detail. From an analysis of the eigenvalues and eigenvectors of the molecular orbital (MO) Hessian at the self-consistent-field (SCF) level, the corresponding rotations amongst the high-lying occupied and low-lying unoccupied molecular orbitals highlight nine low-lying doublet states worthy of study. Three bent (2A1, 2B2, and 2A2), three ring (2B1, 2A2, and 2A1), and three linear (2Πu and two 2Πg) structural isomers with doublet electronic states have been investigated at the SCF, configuration interaction with single and double excitations (CISD), and complete active space (CAS) SCF levels of theory with five different basis sets, double zeta (DZ) through triple zeta plus double polarization (TZ+2P). At the SCF level of theory, the stability of the wave functions and the energetics of the above mentioned doublet states are discussed in terms of the molecular orbital (MO) Hessian. The MO Hessian is often shown to properly predict the energetic ordering of the different electronic states.
Brute force in molecular reaction dynamics: A novel technique for measuring steric effects
Authors: Loesch H. J. , Remscheid A.
Published: July 2002

We present the theoretical foundation of a novel technique for the orientation of ∑-molecules and symmetric tops with permanent electric dipole moment. The method bases on the adiabatic transformation of free rotational states into those of librational oscillations taking place during the passage of a molecule into a strong electric field. Several examples of calculated spatial distributions of the molecular axis are given. The computational results demonstrate that in connection with highly relaxed supersonic nozzle beams only moderately strong electric fields are required to generate a marked orientation of the axis. In a first application of this technique we studied steric effects of the reaction K+CH3I→KI+CH3 at two elevated collision energies (0.79 and 1.24 eV). All observed steric effects could be rationalized in terms of a simple impulsive reaction model. We find that backward scattered products are more likely formed if the approaching atoms face the I-end of CH3I—in agreement with low energy results—while forward scattered products are favorably formed if the CH3-end is encountered.
Desorption of vibrationally excited adsorbates in competition with relaxation: A classical picture
Authors: Guan Y. , Muckerman J. T. , Uzer T.
Published: July 2002

The competition between desorption and relaxation of vibrationally excited CO adsorbed on a series of model surfaces is examined theoretically using a classical mechanical description of the dynamics. The analogy between these processes and the vibrational predissociation of van der Waals molecules in the gas phase is emphasized. This study, which is restricted to a collinear arrangement of the adsorbate and surface atoms explores the effect of a systematic variation of the parameters of the simple model systems. The parameters varied include the surface Debye frequency and the strength of the adsorbate–surface interaction. Rate constants for the competing processes of predesorption, relaxation, and thermal desorption of both excited and relaxed adsorbate molecules are extracted as a function of the parameter values. Surface vibrations in models of NaCl and Si were found to accelerate the classical predesorption rate compared to the case of a stationary surface. This effect can be rationalized in terms of an impulsive collision mechanism. Raising the surface Debye frequency was found to enhance relaxation at the expense of predesorption. Mode mixing between adsorbate and surface modes appears to be a more important factor in this regard than Fermi resonance, or mode matching. Thermal desorption was not found to be a significant process in these classical calculations. Comparisons with parallel quantal calculations are made in the following paper.
Monte Carlo dynamics of a dense system of chain molecules constrained to lie near an interface. A simplified membrane model
Authors: Milik Mariusz , Kolinski Andrzej , Skolnick Jeffrey
Published: July 2002

The static and dynamic properties of a dense system of flexible lattice chain molecules, one of whose ends is constrained to lie near an impenetrable interface, have been studied by means of the dynamic Monte Carlo method. It is found that increasing the surface density of the chains in the layer increases the orientational order. The value of the order parameter of the chain segments decreases with increasing distance from the interace. The short time dynamics of the model chains are similar to those observed in polymer melts. Then, there is a time regime of strongly hindered collective motion at intermediate distance scales. Finally, for distances greater than the chain dimensions, free lateral diffusion of the chains is recovered. It is shown that the model exhibits many features of the real systems such as detergents on a surface and lipid bilayers.
The emission spectrum of helium hydride. II. Bands near 5500 and 6400 Å
Authors: Ketterle Wolfgang
Published: July 2002

The emission bands of helium hydride near 5500 and 6400 Å were analyzed for 4HeH, 3HeD, and 4HeD. They are assigned to the emission of the coupled states D 2Σ+, (3d, ℒ=2) and (for the deuterides) C 2Σ+, v=3 to the A 2Σ+ (5500 Å) and B 2Π (6400 Å) states. The 3d, ℒ=2 state is treated as pure Hund’s case (d). The coupling of the electronic states is homogeneous and described by constant matrix elements. Only in the case of 4HeH, strong predissociation of the D state was observed for N′≥3. The emission spectra were observed after neutralization of a fast (15 keV) mass-selected HeH+ beam in potassium vapor.
Theoretical interpretation of the photoelectron detachment spectra of Na−2–5 and of the absorption spectra of Na3, Na4, and Na8 clusters
Authors: Bonačić-Koutecký Vlasta , Fantucci Piercarlo , Koutecký Jaroslav
Published: July 2002

The configuration-interaction (CI) study of excited states of alkali metal clusters accounts for spectroscopical patterns obtained from (i) the photoelectron detachment spectra of their anions and from (ii) the photodepletion spectra of the neutral species, reproduces observed excitation energies, intensities for allowed transitions, and permits an assignment of cluster structures. For Na−2–4 the linear anionic geometries are responsible for the photoelectron detachment spectra. In the case of Na−5, both planar and linear anionic isomers seem to contribute to the recorded spectrum. The calculation of optically allowed states for Na3(C2v) and Na4(D2h) structures and oscillator strengths yield rich spectra which have been fully assigned to the observed ones. In the case of Na8, the Td and the related D2d forms give rise to an intense transition located at ∼495 nm and the weak fine structure shifted to the red in full agreement with the measured spectrum. A molecular versus collective excitation interpretation of absorption spectra is discussed.
Determination of the transition dipole moment μi→b(R) in H2 from the measurement of vibrational wave functions
Authors: Schins Juleon M. , Siebbeles Laurens D. A. , Los Joop , Kristensen Martin , Koch Henrik
Published: July 2002

In this work we present a theoretical and experimental study of the i 3Π−g→b 3∑+u transition dipole moment in molecular hydrogen. By means of translational spectroscopy the functional dependence on internuclear distance of the transition dipole moment was directly probed. Selective excitation of the i 3Π−g, N=1 level, being unaffected by rotational couplings with other 3d singly excited states, allows for straightforward comparison with ab initio calculations. Excellent agreement is found.
Use of the corrected centrifugal sudden approximation for the calculation of effective cross sections. I. The H2–He system
Authors: Thachuk Mark , McCourt Frederick R. W.
Published: July 2002

A detailed examination is made of the corrected centrifugal sudden (CCS) approximation originally introduced by McLenithan and Secrest [J. Chem. Phys. 80, 2480 (1987)]. A brief review of the method is given, and comparisons are made between eleven energy dependent cross sections calculated using the exact quantum mechanical close-coupled (CC), centrifugal sudden (CS) and corrected centrifugal sudden (CCS) procedures for the H2–He system. Further, three L-labeling choices, initial, final, and average L-labeling, are compared within the CS and CCS approximations. The energy dependent cross sections include those of both the relaxation and production type used in the description of the shear viscosity, thermal diffusion and thermal conductivity and their magnetic field-effects. These cross sections are then thermally averaged, and comparisons are again made at the temperature dependent level. These studies show that the CCS approximation provides significantly improved results over those of the CS approximation for the average L-labeling scheme, but provides poorer results for the initial and final L-labeling schemes.
Structure of liquid benzene and naphthalene studied by pulsed neutron total scattering
Authors: Misawa M. , Fukunaga T.
Published: July 2002

The structure factor Sm(Q) of liquid benzene and naphthalene have been measured by means of pulsed neutron total scattering. Both intramolecular and intermolecular structures are discussed. The intermolecular structures are analyzed by taking account of orientational correlations introduced between the nearest neighbors and a packing of uncorrelated molecules. The experimental Sm(Q) and the radial distribution function of liquid benzene are reproduced reasonably well by introducing a perpendicular orientation just between the nearest neighbors. The analysis of liquid naphthalene suggests an orientation somewhat similar to that in liquid benzene.
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o molecular orbital calculations on HBr−2: Geometry, frequencies, and enthalpy changes
Authors: Ikuta Shigeru , Saitoh Toshikazu , Nomura Okio
Published: July 2002

HBr−2 has D∞h symmetry at both the second-order (MP2) and third-order (MP3) Mo/ller–Plesset perturbation levels of theory with the extended basis sets, whereas the Hartree–Fock level of theory predicts that it has C∞v symmetry. A potential energy map is calculated with the MP2 method as a function of two parameters, i.e., the distance of two Br atoms and the displacement of H from the center of them. Two-dimensional vibration analysis is performed by using this potential energy map. The calculated ν1 is 200 cm−1 and the ν3 is 837 cm−1 in HBr−2, and those in DBr−2 are 199 and 569 cm−1, respectively. Thus the ratio of the asymmetric vibration (ν3)H/(ν3)D is 1.47, being somewhat greater than the value of √2 expected for a harmonic oscillator. The enthalpy changes (ΔH°) in the reaction of Br− with an HBr molecule are determined by both the MP3 total energy changes and the thermal energy changes between the reactants and the product. The calculated enthalpy change of −21.2 kcal/mol is in good agreement with the experimental one (−21 kcal/mol) recently observed by Caldwell and Kebarle. These calculated data on HBr−2, which will provide us with a useful guide for future measurements of gas-phase vibrational spectra, are compared with those on HCl−2 and HF−2.
First-order correlation orbitals for the spin-unrestricted Hartree–Fock zero-order wave function. Electron affinities of C4 clusters
Authors: Adamowicz Ludwik
Published: July 2002

The second-order Hylleraas functional and the Newton–Raphson optimization technique have been used to generate first-order correlation orbitals (FOCOs) for the spin-unrestricted Hartree–Fock (UHF) zero-order wave function. The correlation orbitals are linear combinations of the UHF virtual orbitals and are different for electrons with α and β spins. We show that even the number of FOCOs is significantly reduced with respect to the number of all UHF virtual orbitals, the decrease of the second-order correlation energy is rather small. The primary application of the FOCO UHF method is to study larger open-shell molecular systems at higher correlated levels of the theory. The coupled-cluster calculations with FOCOs on the electron affinities of the linear and rhombic carbon C4 clusters, presented here, illustrate the capabilities of the proposed methodology.
Separation and semiclassical quantization of bending motion near linear geometries of a triatom
Authors: Natanson Gregory A.
Published: July 2002

It is shown that separation of internal bending motion from other degrees of freedom in the vicinity of linear configurations of a triatom leads to the one-dimensional Schrödinger equation for the centrifugal oscillator for a very broad family of coordinate transformations. This feature makes the approach especially attractive for semiclassical quantization of bending motion, compared with the alternative, broadly used approach treating bending motion as two mutually perpendicular bending modes. Significant errors due to the neglect of the quartic potential term coupling the mutually perpendicular bending modes are demonstrated using bending resonances in partial cross sections for the H+H2 exchange reaction as an example. Another important issue addressed in the paper is how the choice of rotating axes affects the rotational energy. It is shown that the correct expression for the rotational energy is obtained only if the body-fixed axes are directed along the principal inertia axes of the triatom in its accessible nearly linear configurations.
Validity of time-dependent self-consistent-field (TDSCF) approximations for unimolecular dynamics: A test for photodissociation of the Xe–HI cluster
Authors: Alimi R. , Gerber R. B. , Hammerich A. D. , Kosloff R. , Ratner M. A.
Published: July 2002

The photodissociation dynamics of a collinear model of the van der Waals cluster Xe–HI is used as a testing ground for time-dependent self-consistent field (TDSCF) approximations. In this study, the quantum-mechanical TDSCF and a combined classical/quantal TDSCF (in which the light atom is treated quantum mechanically, the heavy atoms are treated classically) are compared to numerically exact wave packet calculations. Very good agreement is found between the TDSCF approximations and the exact result over the entire subpicosecond time duration of the process. In particular, all the properties related to the quantal degree of freedom in the combined quantal/classical TDSCF method reproduce almost perfectly the exact results. However, the classical mode in the hybrid approximation is somewhat less well described due to insufficient representation of energy transfer between the modes. The conclusions are very promising as to the applicability of TDSCF methods, in particular the hybrid quantal/classical scheme to more complex systems in which only a few degrees of freedom can be treated quantum mechanically.
A quantum electrodynamical study of intermolecular line broadening and line shift
Authors: Blake Nick. P.
Published: July 2002

In this paper quantum electrodynamics is used to investigate the effects of radiation field-induced energy transfer as a mechanism for the manifestation of frequency shift and both homogeneous and inhomogeneous line broadening. Using resolvent operator methods and the Power–Zienau–Woolley multipolar Hamiltonian to derive an effective Hamiltonian, it is illustrated how all coherent and incoherent field-induced line shift and line broadening including Förster energy transfer, dispersion, and cooperative Rayleigh scattering can be described solely in terms of a single multipolar interaction. The detailed analysis reveals that a description of intermolecular interactions in terms of virtual photon coupling leads a detailed range dependence that is not only different to nonretarded semiclassical theory, but also complex. This paper therefore illustrates for the first time that all intermolecular processes contribute to both line broadening and energy shift, and that the imaginary contributions to virtual photon coupling lead to significant retardation effects in condensed media. Finally by applying the results within a density matrix framework important aspects regarding the statistical averaging of these additional terms in the Hamiltonian are discussed, and it is illustrated how intermolecular coupling leads to non-Lorentzian, redshifted, asymmetric line profiles.
Photodissociation of RNCS and RSCN (R=H, CH3, C2H5) : Evidence for an excited state isomerization and energy deposition in the NCS product
Authors: Northrup F. J. , Sears Trevor J.
Published: July 2002

The 248 and 193 nm photodissociation of the series of molecules RNCS and RSCN (R=H, CH3, C2H5) has been investigated. Laser-induced fluorescence of the radical NCS was used to probe the energy deposition in the R+NCS channel of this dissociation, however excitation to many vibrational states of NCS caused spectral congestion which prohibited direct measurement of populations of individual levels. A single-photon dissociation channel leading to RS and CN was observed for both isomers of all of the precursor molecules. While this was expected for the thiocyanate species, no direct channel to these products is available for the isothiocyanates. This observation is discussed in terms of an excited state isomerization of these molecules. Measured ratios of CN to NCS production for all compounds provide additional support for this mechanism and suggest a significant barrier to the process. Comparison of nascent laser-induced fluorescence (LIF) spectra of NCS with spectra obtained following different amounts of collisional relaxation suggests that all three vibrational modes were equally excited with a vibrational temperature of roughly 4500 K. The rotational temperature was estimated to be less than 1000 K. A spin–orbit population inversion was observed for all vibrational levels with approximately 75% of the population in the upper spin–orbit component of the vibrationless and low lying bending excited levels. No simple model is found to explain the photodissociation dynamics or the isomerization.
A crossed-beam study of the state-resolved dynamics of CH(X 2Π)+D2. I. The inelastic scattering channel
Authors: Macdonald R. Glen , Liu Kopin
Published: July 2002

The state-to-state integral cross sections for the inelastic scattering of CH(X 2Π) and D2 to produce rotationally excited CH(X 2Π) product have been measured in a crossed-beam apparatus by the laser-induced fluorescence method. Two types of measurements were performed: (1) the translational energy dependence of an individual quantum state of the product and (2) the state distribution of the products at fixed and well-defined translational energy. For the inelastic scattering channel, the cross sections gradually increased from a dynamical threshold to a broad maximum and then slowly decreased as the translational energy increases. Evidence for multiple-impact rotational rainbows was found and a possible frequency-locking phenomenon between the two receding rotors resulted. Moderate orbital alignment was observed except for the highest rotational levels of the product. By comparing and contrasting the kinematically similar system CH(X 2Π)+He, the influence of a strongly attractive potential energy surface on the inelastic scattering of CH+D2 was inferred. Combining the results of the inelastic scattering and the isotopic exchange channels (the following paper) provide an unprecedented look into the dynamics of collisions between CH(X 2Π) and D2.
High pressure nuclear magnetic resonance study of the dynamical solvent effects on the rotation of coordinated ethylene in an organometallic compound
Authors: Peng Xiangdong , Jonas Jiri
Published: July 2002

The effect of temperature and pressure on the internal rotation rate of coordinated ethylene in π–cyclopentadienylethylenetetrafluoroethylene–rhodium in liquid solution has been investigated by using 1H Fourier transform (FT) nuclear magnetic resonance spectroscopy. The solvents used in this study are n–pentane–d12, carbon disulfide, and methylcyclohexane–d14. The activation energy (13.4±0.2 kcal/mol) for the internal rotation of ethylene is independent of solvent and pressure as determined from conventional Arrhenius type plots and isoviscosity plots. It is found that the rotation of the coordinated ethylene is initially accelerated by pressure, reaches a maximum and then decreases at high pressure. The strong pressure dependence of the observed activation volume for the rotation suggests a strong collisional contribution to the activation volume and the presence of dynamical solvent effects. The experimental data, as interpreted in terms of stochastic models of isomerization reactions, indicate a Kramers’ turnover for the pressure dependence of the rotation of coordinated ethylene in the Rh complex in solution. The observation of the energy-controlled regime in this system may be the consequence of the so-called heavy metal atom bottleneck effect which reduces the intramolecular energy transfer within the molecule. The experimental dependences of the rates upon solvent viscosity and/or Enskog collision frequency show that solvent shear viscosity represents only an approximative measure of the coupling of the reaction coordinate to the medium.
J dependence of predissociation rates and hyperfine constants as measured by high resolution laser spectroscopy
Authors: Katzenellenbogen Nir , Prior Yehiam
Published: July 2002

Highly excited rotational states in electronically excited bromine molecules are measured with full hyperfine resolution, by means of sub-Doppler polarization spectroscopy. The dependence of molecular parameters such as predissociation rates and hyperfine coupling constants on the rotational quantum number is extracted, and found to be proportional to the rotational energy J(J+1), extending previous data to much higher rotational states. By fitting a large set of hyperfine resolved rotational spectra from one vibrational band to a single set of parameters, very small dependencies may be extracted. The predissociation rates measured here in the frequency domain are compared to previous measurements in the time domain, and found to be smaller by more than an order of magnitude. We show that extracting decay times from lines of unresolved hyperfine structure may be misleading, and attribute this discrepancy to dephasing due to interferences between hyperfine components.
New insight into experimental probes of cluster melting
Authors: Adams John E. , Stratt Richard M.
Published: July 2002

Experiments are now appearing which attempt to probe melting in small clusters, a notable example of these being the recent studies of benzene–Arn clusters by Hahn and Whetten [Phys. Rev. Lett. 61, 1190 (1988)]. We report a study of the dynamics of these same benzene–Arn clusters which seeks to clarify further the nature of ‘‘phase transitions’’ in small systems. The techniques used here, involving an instantaneous normal mode analysis based on the results of Monte Carlo calculations, have been shown previously to yield a picture of argon cluster melting which is more complete than the one which emerges from a mere enumeration of low-energy structures. Although the bare argon clusters are found to undergo dynamical transitions as the cluster temperature is increased, these transitions are inhibited by the presence of an embedded benzene molecule, which provides a template for ordering of the argon atoms. The calculations also suggest a possible explanation for the doubly peaked spectra observed in the experiments of Hahn and Whetten: the benzene may be found in two general types of bonding environments, either surrounded by or on surface of the Arn cluster. This occurrence is consistent with the finding of Bösiger, Knochenmuss, and Leutwyler [Phys. Rev. Lett. 62, 3058 (1989)] that the solute–solvent interaction in clusters can be either wetting or nonwetting. We discuss how the evidence necessary to resolve such issues can be derived from an analysis of cluster dynamics.
A subpicosecond, subnanosecond and steady-state study of diffusion-influenced fluorescence quenching
Authors: Eads Daniel D. , Dismer Berit G. , Fleming Graham R.
Published: July 2002

Subpicosecond and subnanosecond time resolved experiments are combined with steady-state fluorescence measurements to examine the diffusion influenced fluorescence quenching reaction of rhodamine B and ferrocyanide. The classic models of Smoluchowski, and Collins and Kimball are unable to consistently explain both the rapid initial decay and the slower decay seen at long times (>1 ns) in the experimental data. Neither the short nor the long time data can be reconciled with the steady-state data using these models. Better agreement is found between the data and a simple model incorporating a position dependent intrinsic reaction rate [A. Szabo, J. Chem. Phys. 93, 6929 (1989)] in addition to a diffusional rate. This model suggests a rate of electron transfer for the rhodamine B–ferrocyanide system of (27.5±4 ps)−1. Use of a bare Coulomb potential between reactants is found to be inappropriate in all of the models investigated.
Semiempirical study of rare gas and rare gas–hydrogen ionic clusters: R+n, (RnH)+, and (RnH2)+ for R≡Ar, Xe
Authors: Last Isidore , George Thomas F.
Published: July 2002

The ionic rare gas clusters Ar+n and Xe+n and rare gas–hydrogen clusters (ArnH)+, (ArnH2)+, (XenH)+ and (XenH2)+ are studied by the semiempirical diatomics-in-ionic-systems (DIIS) method. The Ar+n clusters (n>3) are seen to have a structure of a linear Ar+3 core surrounded by n−3 neutral or almost neutral Ar atoms. For Xe+n (n>3), a symmetrical Xe+4 ionic core with the geometry of regular pyramid is formed. The rare gas–hydrogen clusters with one H atom have a simple Rk(RH)+ structure with k neutral rare gas atoms attracted to the (RH)+ molecule by polarization forces. Two H atoms can bind with Ar atoms to form quasistable clusters ArnH+2 which dissociate to (n−1)Ar+H+(ArH)+ through a high barrier of roughly 0.75 eV. Two H atoms and one Xe+ ion are shown to form a collinear valence-bound (XeHH)+ cluster whose dissociation energy is 0.46 eV.