bluuebunny/crossref_metadata_2025 huggingface dataset:
V–V energy transfer in the HCl–CO system: A comparison between measurements in the gas phase and in liquid xenon solution
Authors: Vlahoyannis Yannis P. , Krueger Herman , Weitz Eric
Published: July 2002

Both UV photolysis and overtone excitation(followed by energy relaxation) are used to produce vibrationally excited HCl in HCl–CO–liquid xenon (LXe) solutions. Rate constants for V–V transfer from HCl (v=1) to CO are reported both for the gas phase (210–295 K) and for liquid xenon solutions (210 and 229 K). In the gas phase, the probability of V–V transfer is found to increase with temperature (by approximately 50%) over the measured range. Liquid phase rate constants are approximately twice as fast as gas phase rate constants at the same temperature. The experimental ratio of liquid to gas phase rate constants is close to that predicted by a simple isolated binary collision (IBC) model employing hard sphere pair distribution functions. Such a model is clearly oversimplified for the present system, and both its inadequacies and possible reasons for its ability to reproduce the experimentally measured ratios are discussed. Despite evidence of H and/or Cl atom chemistry in solution, the technique of UV (193 nm) photolysis of HCl–CO–liquid xenon solutions, which produces vibrationally excited HCl, is also found to yield an accurate value of the HCl (v=1) →CO V–V rate constant. Possible reaction products and uses of this technique to generate mobile radical species in rare gas solution are considered.
Fragment analysis of molecular electronic energies. I. Theory
Authors: Ro/eggen I. , Nilssen E. Wislo/ff
Published: July 2002

A molecular system is partitioned into fragments. Within the context of extended geminal models, it is shown that the total electronic energy can be written as a sum of intra- and interfragment energies. On the basis of this type of analysis, the difference in equilibrium bond angles in H2O and H2S, NH3 and PH3, is discussed.
Renormalization group theory of the Rouse–Zimm model of polymer dynamics to second order in ε. II. Dynamic intrinsic viscosity of Gaussian chains
Authors: Wang Shi-Qing , Freed Karl F.
Published: July 2002

The dynamical intrinsic viscosity [η(ω)] of Gaussian chains is studied within the Rouse–Zimm bead-spring model of polymer dynamics using conformational space renormalization group methods. Calculations are performed through order ε2 (ε=4−d, where d is the spatial dimensionality) both with and without the preaveraging approximation. We find an 8% preaveraging error in the steady state limit. Preaveraging corrections to [η(ω)] involve couplings between all Rouse modes, leading to more complicated expressions for the Rouse mode relaxation rates and to weight factors for mode contributions to [η(ω)].
Semiclassical collision theory: Self-consistent classical path method
Authors: Billing Gert Due
Published: July 2002

A new method for obtaining self-consistent classical trajectories in the semiclassical ‘‘classical path’’ approach is suggested. The method is obtained by introducing a reformulation of the effective potential obtained from the Feynman path integral approach to inelastic scattering. It is then possible to reduce the search for the self-consistent path to a search in a one-parameter space. It is shown that in general several solutions may exist and that quantum interference between the trajectories may be included using techniques available from classical S-matrix theory.
Multiphoton ionization of fluoranthene in tetramethylsilane
Authors: Faidas H. , Christophorou L. G.
Published: July 2002

The multiphoton ionization of fluoranthene in liquid tetramethylsilane in the laser excitation wavelength λl range 370–550 nm is reported. From measurements of the laser wavelength and power dependencies of the ionization signal the ionization threshold was determined to be ∼5.7 eV. Two ionization mechanisms were identified in this wavelength region: (1) direct two-photon ionization for λl ≲435 nm and (2) three-photon ionization—involving two-photon excitation followed by relaxation to either the first and/or the second excited singlet state and subsequent one-photon ionization—which becomes the sole photoionization mechanism for λl ≳435 nm. In the wavelength region 400≤λl ≤435 nm both mechanisms occur.
Dissociation energies of the hydrogen-bonded dimers RCN⋅⋅⋅HF (R=CH3, HCC) determined by rotational spectroscopy
Authors: Legon A. C. , Millen D. J. , North Hazel M.
Published: July 2002

Measurements of absolute intensities of rotational transitions have been used to determine the zero-point and equilibrium dissociation energies D0 and De of the hydrogen-bonded dimers CH3CN⋅⋅⋅HF and HCCCN⋅⋅⋅HF. For CH3CN⋅⋅⋅HF reported values are D0=26.1(0.6) kJ mol−1 and De=29.0(0.9) kJ mol−1 while for HCCCN⋅⋅⋅HF the corresponding values are 20.4(0.7) and 23.4(0.9) kJ mol−1, respectively. The De values are used in conjunction with De=26.1(1.6) kJ mol−1 previously obtained for HCN⋅⋅⋅HF to investigate the suitability of the Morse and Lennard-Jones functions for describing the hydrogen-bond radial potential energy.
The adiabatic and related approximations for quantum particles in harmonic systems
Authors: Vail John M.
Published: July 2002

The adiabatic approximation is set up to estimate variationally the energy of a quantum particle interacting with a quantum harmonic oscillator. This example is seen as a prototype for a wide range of problems, including molecular clusters, free or in condensed matter. A heirarchy of approximations is introduced, labelled simplified adiabatic, average field, and classical field. The quasiadiabatic limit is then introduced, in which the oscillator takes account, as much as possible, of the instantaneous particle position, and an interpolation procedure between the average field adiabatic and quasiadiabatic limits is derived. Questions of implementation are discussed. In particular, an approximation is introduced, in which the quasiadiabatic generalization of the average field case is found to be energetically favored, and which reduces to a problem similar to the static-field case, for which many-electron computational methods are well developed. Now, however, oscillator zero-point effects are incorporated in a renormalized oscillator self-energy and a modified effective particle-oscillator interaction.
Alkali 〈I⋅S〉 wall relaxation in dichlorodimethylsilane coated resonance cells
Authors: Camparo J. C.
Published: July 2002

Alkali 〈I⋅S〉 wall relation rates for Pyrex resonance cells coated with dichlorodimethylsilane have been studied. In particular, by considering the ratio of the Rb85 wall relaxation rate to the Rb87 wall relaxation rate, evidence is presented showing that dimethylsiloxane surfaces and alkane (paraffin) surfaces relax alkali polarization with similar interaction strengths and correlation times. This conclusion indicates that the outer most functional group of an organic surface molecule has the primary influence on the alkali polarization’s surface relaxation.
Disordering mechanisms in polymethylene molecules: A chain in a channel
Authors: Jona P. , Bassetti B. , Benza V. , Zerbi G.
Published: July 2002

The disordering mechanisms which characterize the pretransitional states of a polymethylene chain inserted in a host material are studied with statistical mechanics. While the intramolecular conformational energy of the chain is chosen according to Flory’s theory, the interaction with the host material is modeled in terms of a given external potential. The competition between end-gauche and kink formation is in particular examined: it is shown that chain-end distorsions are initially favored and, with increasing temperature, kink formation and longitudinal translation of the molecular chain sustain the disordering process. The results are compared with the experimental data from vibrational spectroscopy.
Chemical ion association and dipolar dumbbells in the mean spherical approximation
Authors: Lee Song Hi , Rasaiah Jayendran C.
Published: July 2002

The sticky electrolyte model (SEM) is solved analytically in the mean spherical approximation (MSA) for binding between oppositely charged ions of a symmetrical electrolyte at a distance L=σ/4 and σ/5, where σ is the atomic diameter, extending earlier analytic studies for L=σ, σ/2, and σ/3. The excess energy of a fluid of dipolar dumbbells of elongation L<σ/2 is calculated in this approximation by considering the saturation limit when all the ions are associated to form dimers and steric hindrance prevents polymerization. The results are in good agreement with Monte Carlo simulations for dipolar dumbbells and compare favorably with the solutions to the MSA and the HNC approximation using the site–site Ornstein–Zernike equation.
Electron correlation, basis sets, and the methylene singlet–triplet gap
Authors: Carter Emily A. , Goddard William A.
Published: July 2002

The effect of basis set and electron correlation on the singlet–triplet splitting (ΔEST) of CH2 is examined using the generalized valence bond (GVB) approach. For a standard double zeta plus polarization basis, the GVB based calculation (with only 20–25 spin eigenfunctions) approaches the full CI result (∼220 000 spin eigenfunctions) of Bauschlicher and Taylor to within 0.5 kcal/mol for this basis, but both differ substantially from experiment (errors of 2.4 and 2.9 kcal/mol for GVB and full CI, respectively). We have studied the convergence of ΔEST with basis set and find that an extremely extended basis (triple zeta sp, diffuse sp, triple zeta d, double zeta f ) for GVB yields ΔEST=9.03 kcal/mol, in excellent agreement with the experimental value of 9.09±0.20 kcal/mol.
Comment on: On the evaluation of potential parameters in fluorite-type AB2 crystals
Authors: Sharma S. C. , Kumar M. , Shanker J.
Published: July 2002

Some errors are pointed out in the method of Singh et al. for calculating the potential parameters in fluorite-type AB2 crystals. It is found that the correction of these errors gives improved agreement between the calculated and experimental values of crystalline state properties. It has been shown that the agreement is further improved if one uses different values of hardness parameters for different pair interactions.
Analysis of torsional spectra of molecules with two internal C3v rotors. XXIV. High resolution far infrared spectra of acetone-d, -d3, and -d6
Authors: Groner P. , Guirgis G. A. , Durig J. R.
Published: July 2002

The high resolution (0.10 cm−1) far infrared spectra of gaseous acetone-d0, -d3, and -d6 were measured between 370 and 40 cm−1. The region of the methyl torsional transitions below 130 cm−1 was analyzed in terms of the semirigid rotor model for molecules with two internal C3v rotors and the two-dimensional torsional potential functions were determined. Effective barriers of 291 cm−1 (832 cal/mol) and sine–sine interaction terms of −l08 cm−1 (−309 cal/mol) were found for all three isotopic species. The results are compared with those from ab initio calculations and from the analysis of splittings in the microwave spectrum.
Theoretical study of the dipole moment function of OH(X 2Π)
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

The convergence of the electric dipole moment function for the X 2Π state of OH is studied as a function of extensions of both the one-particle and n-particle basis sets. Full configuration-interaction (FCI) energies and dipole moments in both [4s3p1d/2s1p] and [4s3p2d/2s1p] Gaussian basis sets are used for the calibration of a variety of approximate methods of including electron correlation. A CASSCF/MRSDCI treatment, which accurately reproduces the FCI potential and dipole moment function, is then performed in an extended [6s5p4d2f1g/4s3p2d] one-particle basis set. The dipole moment is evaluated both as an expectation value and as an energy derivative. The energy derivative is found to be generally superior, especially for the coupled pair functional approaches. Our best dipole moment function gives a permanent dipole moment within 0.01 D of experiment for v=0, and within 10% of the accurate experimental value for the difference in dipole moments between v=0 and v=1. However, our theoretical value for v=2 does not lie within the experimental error bounds.
Test of the Wigner method for the photodissociation of symmetric triatomic molecules
Authors: Henriksen N. E. , Engel V. , Schinke R.
Published: July 2002

The validity of the semiclassical Wigner method for photodissociation of symmetric triatomic molecules is studied. The photodissociation of H2O in the first absorption band using an ab initio potential energy surface and a collinear model for CO2 are considered. In both cases comparison is made with exact quantum mechanical calculations. The overall agreement with the exact results is reasonable, and the general features of the partial and total cross sections are reproduced. However, finer details such as ‘‘vibrational’’ structures superimposed on the broad absorption cross section are poorly resolved. The agreement is generally better in the case of CO2 for which the potential energy surface is more harmonic.
Kinetic anharmonic coupling in the trihalomethanes: A mechanism for rapid intramolecular redistribution of CH stretch vibrational energy
Authors: Green William H. , Lawrance Warren D. , Moore C. Bradley
Published: July 2002

The coupling of CH stretching and CH bending vibrations in trisubstituted methanes is analyzed. Improved spectroscopic constants, especially the cubic anharmonic stretch–bend coupling constants, are extracted from Fermi resonances in the overtone spectra of HCF3 and HCCl3. Both harmonic oscillator and Morse oscillator basis functions are used in the analysis and the results compared. That part of the coupling which arises from the kinetic energy as expressed in curvilinear coordinates is calculated and compared to the coupling calculated using the more conventional rectilinear treatment. Use of curvilinear coordinates is found to provide significant advantages. The formalism for curvilinear normal coordinates is clarified and generalized. From these calculations and the spectral analysis, one of the cubic anharmonic constants of the potential energy surface is extracted for comparison with ab initio calculations. The curvilinear model of the CH stretch–bend interaction tested for these isolated CH chromophores is expected to be useful in understanding CH bonds and vibrational energy flow in larger hydrocarbons.
Evidence for vibronic coupling contributions to overtone intensities in alkyl phenyl ketones
Authors: Goswami Prabhat C. , Swanton David J. , Henry Bryan R.
Published: July 2002

Liquid phase overtone spectra are measured in the spectral regions of CH-stretching local modes corresponding to ΔvCH=2–7 for acetophenone, ΔvCH=2–8 for isobutyrophenone, and to ΔvCH=2–6 for biacetyl and isobutyrophenone-d5. The spectra are assigned in terms of the local mode model. The intensities of the aryl CH overtones are quantitatively determined as a function of ΔvCH. The intensities of the alkyl CH-stretching overtones of all four molecules and the intensities of the aryl CD-stretching overtones of isobutyrophenone-d5 appear to decrease smoothly and exponentially with increasing ΔvCH (ΔvCD). However, the intensities of the aryl CH–stretching overtones of isobutyrophenone actually increase from ΔvCH=6 to ΔvCH=7 to ΔvCH=8. A similar but less pronounced anomaly is observed for the intensities of the aryl overtones of acetophenone. The results are interpreted in terms of a vibronic coupling contribution to overtone intensity which involves excited electronic states.
Rotational spectra and structures of the Ar2–H35Cl/37Cl trimers
Authors: Klots T. D. , Chuang Carl , Ruoff R. S. , Emilsson T. , Gutowsky H. S.
Published: July 2002

Microwave rotational spectra have been observed for both Cl isotopes of the Ar2–HCl trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. The Cl nuclear quadrupole hyperfine structure was analyzed for each of the transitions and the coupling constants and line centers determined. Sixteen transitions were observed in the 2 to 15 GHz region for the 35Cl species and 11 for 37Cl. The line centers were fitted to obtain ground state rotational and quartic centrifugal distortion constants A″, B″, C″, τ1, τ2, τaaaa, τbbbb, and τcccc. For Ar2–H35Cl, the values are 1733.857, 1667.932, 844.491, −0.1170, −0.0292, −0.1199, −0.0802, and −0.0079 MHz, respectively, and for Ar2–H37Cl: 1733.824, 1606.877, 828.497, −0.1121, −0.0279, −0.1205, −0.0737, and −0.0075 MHz. The equilibrium geometry is determined to be T shaped with C2v symmetry and the H end of the HCl closest to the Ar2. Large amplitude slightly anisotropic torsional motion of the HCl is evident from the hyperfine constants. The chlorine isotopic substitution enables the torsional displacement of the Ar2 dimer to be estimated. It is found to be an average of 7.3° with respect to the b axis. With allowance for the torsional effects, a value of 3.861 Å is obtained for the Ar–Ar distance and 4.005 Å for the Ar to Cl distance. An approximate, harmonic force field analysis is based on the centrifugal distortion constants. A comparison is given of the force constants and Ar–Ar distances for the Ar2–HCl and Ar2–H/DF trimers.
The spans of a polyelectrolyte near a charged surface
Authors: Gaylord Richard J. , Zhang Huanzhi
Published: July 2002

The mean spans of a polyelectrolyte strongly adsorbed on a charged surface are calculated. The spans in the directions parallel to the surface depend only on the number of chain segments while the span in the direction perpendicular to the surface depends only on the Debye screening length and the surface–segment interaction energy.
Mean field theory of a dipolar excitonic insulator transition in matrix-bound systems
Authors: Logan David E.
Published: July 2002

A mean field theory is developed for a dipolar excitonic insulator transition occurring in matrix-bound systems, paradigms of which are low temperature alkali-doped rare gas solids at low impurity concentration. The excitonic insulator transition is driven primarily by changes in the host matrix density, and is a transition from the normal insulating domain in which the impurity electronic ground state is spherically symmetric, to an excitonic phase in which the impurity atoms possess electric dipole moments. A description is given of the electrical and optical characteristics of the system in both the normal insulating phase and the dipolar excitonic state, and it is shown that a variety of properties characteristic of the excitonic state can be deduced from experiment. Comparison is made with experimental results, with which the theory is shown to be compatible, and it is suggested that the dipolar excitonic state may have been observed widely over many years.
Mutual diffusion in binary polymer mixtures as measured by dynamic light scattering
Authors: Brereton M. G. , Fischer E. W. , Fytas G. , Murschall U.
Published: July 2002

We report light scattering measurements of the static structure factor and mutual diffusion coefficient for the compatible polymer mixture poly (phenylmethylsiloxane)/poly(styrene). The measurements were made for a series of molecular weights of the poly(styrene) of 2000, 4000, and 9000 for different volume fractions of the poly(styrene) over a range of temperatures from 350 to 520 K. The principal aim was to try and distinguish between the various theoretical approaches which have appeared in the literature and which give radically different results for the concentration dependence of the mutual diffusion coefficient in a binary polymer mixture. Since the static structure factor was measured in the same light scattering experiment as the diffusion coefficient we have also been able to obtain the concentration, molecular weight, and temperature dependence of the Onsager coefficient directly from experiment. A concentration-dependent glass transition temperature complicates the comparison with theory and we propose a method for the data analysis to deal with this problem. Our results are consistent with the expression for diffusion coefficient based on the dynamic random phase approximation.
Local ergodicity as a probe for chaos in quantum systems: Application to the Henon–Heiles system
Authors: Ramachandran B. , Kay Kenneth G.
Published: July 2002

Classical chaos is usually accompanied by ‘‘local’’ ergodicity—ergodic behavior in regions of phase space that are generally smaller than, but of the same dimensionality as, the energy surface. If there are strong bottlenecks in phase space impeding the relaxation to statistical equilibrium in the full region of ergodicity, it is often possible to define an approximate form of ergodic behavior in a smaller subregion where relaxation occurs temporarily but quickly. Such ‘‘pseudoergodicity’’ is also a symptom of chaos. We use the presence of quantum behavior that mimics local ergodicity and pseudoergodicity as a probe for the influence of classical chaos on quantum dynamics. We show that the quantum analog of a pseudoergodic region in phase space is generally formed by superpositions of energy eigenstates. The superposition states spanning a given pseudoergodic zone have similar expectation values and small off-diagonal elements to other states with similar energy for a certain class of operators. We perform calculations which identify the ergodic regions for two versions of the quantum Henon–Heiles system. For the ‘‘more classical’’ of these systems we find good agreement between the proportion of quantum states in pseudoergodic zones and the proportion of classical phase space occupied by chaotic trajectories. We also find that the quantum pseudoergodic regions can be identified with classical vague tori of the precessing and librating types. Different ergodic regions are separated from one another by what appear to be the quantum analogs of the precessor–librator separatrix and other classical bottlenecks. For the ‘‘less classical’’ of the systems, we find that classical chaos is reflected in the quantum dynamics to a smaller, but still noticeable, degree.
Statics and dynamics of concentration fluctuations in nonionic micellar solutions
Authors: Wilcoxon J. P. , Kaler E. W.
Published: July 2002

Measurements of the angular distribution of light scattered from a nonionic surfactant, C12E6, are used to obtain the divergence of the static correlation length and isothermal compressibility near the critical point. The dependence of the apparent diffusion coefficient on momentum transfer yields a dynamic correlation length which agrees closely with the static values for the temperature regime (Tc−4 °C)<T<Tc. Outside this regime the scattering measurements are consistent with micelle growth. Dynamic scaling holds for this system near Tc but a significant background contribution to the diffusion coefficient is present due to the large micelle size.
Intramolecular vibrational relaxation of CH stretch overtones in benzene
Authors: Bintz Karen L. , Thompson Donald L. , Brady John W.
Published: July 2002

Energy flow from CH overtones in benzene has been examined using quasiclassical trajectory calculations. The potential energy surfaces for this study are of the form reported by Pulay, Fogarasi, and Boggs [J. Chem. Phys. 74, 3999 (1981)]. The effect of out-of-plane motion on energy transfer in benzene was examined by comparing results for planar and nonplanar models. The methods of analysis include the calculation of the energy in each of the CH bonds and the energy in each of the planar normal modes as a function of time. It is shown that there are two distinct sets of normal modes: low frequency modes which do not significantly participate in energy redistribution and higher frequency modes which appear to be strongly coupled to the excited CH bond. Within the participating set of modes, those which appear to possess the most CCH bend character are the most instrumental in absorbing energy from the excited CH bond. This study shows that rapid energy transfer out of an excited CH stretch mode can occur for a purely harmonic potential-energy surface.
Inclusion of the Förster-rate orientation factor into the theory of concentration self-quenching by statistical traps
Authors: Knoester J. , Van Himbergen J. E.
Published: July 2002

The incorporation is studied of the orientation factor occurring in the complete Förster rate of incoherent energy transfer, into the theory of concentration self-quenching by statistical pairs of luminescent molecules. Within Burshtein’s theory of hopping transport, exact results for the steady state donor fluorescence yield and emission anisotropy are obtained for three-dimensional systems. Two opposite limits to treat the orientations of the molecular transition dipoles within statistical pairs are considered, and it appears that in both cases the fluorescence yield is substantially influenced by inclusion of the orientation factor into the transfer kinetics.
Sound absorption and concentration fluctuations in water–propanol mixtures
Authors: Madigosky W. M. , Warfield R. W.
Published: July 2002

The excess ultrasonic sound absorption in 1-propanol water mixtures has been measured from 45 kHz to 474 MHz and is shown to result from a coupling of the sound wave with the long-wavelength spectrum of the composition fluctuations. The results are shown to be in agreement with the fluctuation theories of Fixman, Romanov and Solovyev, and Kawasaki and the more recent theories of Kroll and Ruhland, and Bhattacharjee and Ferrell. It is further suggested that the large absorption observed in alcohol–water mixtures may be due to the presence of large associated groups, or clusters.
Structure of solid t-butyl cyanide: A study by means of constant-temperature, constant-pressure, molecular-dynamics simulations
Authors: Ferrario Mauro , Klein Michael L. , McDonald Ian R.
Published: July 2002

The structural phase transition that occurs in solid t-butyl cyanide just before melting has been studied by constant-temperature, constant-pressure, molecular-dynamics simulations. The simulations were based on a six-site potential model with parameters obtained from static-energy calculations for the observed, low-temperature, monoclinic structure. Changes in the lattice constants and in order parameters for the molecular orientations show that an apparently first-order transition to a tetragonal, orientationally disordered phase occurs at a temperature in the range between 250 and 275 K; experimentally, a similar transition is observed at 233 K. Comparison is made with neutron-scattering results for the tetragonal phase, and deficiencies in the potential model and in the model used to parametrize it are discussed.
Electron mobility in dense gases of deuterated methanes as functions of electric field strength, temperature, and density
Authors: Floriano M. Antonio , Gee Norman , Freeman Gordon R.
Published: July 2002

Values of electron mobility μ were measured in gaseous deuterated methanes CHxD4−x(x=0–4) as a function of density normalized electric field strength at 0.2≤n/1026 molecule m−3≤61=nc (subscript c denotes critical fluid), and 98≤T/K≤433. The mobility at low E/n had a constant value μ0 but at fields above a threshold (E/n)th, μ increased. In the saturated vapor the ratio of the electron drift velocity at (E/n)th to the speed of sound in the vapor was determined mainly by T, while nμ0 was mainly determined by n; there was no clear isotope effect. The effect of density on nμ0 is discussed in terms of the deformation potential and dielectric screening models. The former fits better.
Quantum theory of atomic fluorescence near a metal surface
Authors: Arnoldus Henk F. , George Thomas F.
Published: July 2002

Quantum electrodynamics of an atom near a surface is a timely problem in current theoretical research. It appears, however, that a full dynamical theory, which includes both the time evolution of the atomic density operator and the details of the fluorescence radiation (temporal photon distribution) has never been formulated. In this paper the quantum theory of an atom near a perfect conductor is presented, and it is indicated how the formalism can be modified to account for more realistic optically active substrates. An expression is derived for the atomic spontaneous-decay Liouville operator from the Hamiltonian, which recovers the familiar results for the lifetimes and energy shifts. Furthermore, the emitted power is calculated as a function of time from the explicit expression for the radiation field. Comparison of the atomic-decay rates with the power of the emitted radiation shows the consistency of the theory, as far as the properties of the fluoresence are concerned. An unusual energy interference in the fluorescence, which is emitted by a multilevel atom, is predicted. Similarities and discrepancies with other theories are pointed out, and it is shown that especially the mirror theory has a very restricted applicability.
A simple kinetic theory model of reactive collisions. IV. Laboratory fixed orientational cross sections
Authors: Evans Glenn T.
Published: July 2002

The differential orientational cross section, obtainable from molecular beam experiments on aligned molecules, is calculated using the line-of-normals model for reactive collisions involving hard convex bodies. By means of kinetic theory methods, the dependence of the cross section on the angle of attack γ0 is expressed in a Legendre function expansion. Each of the Legendre expansion coefficients is given by an integral over the molecule-fixed cross section and functions of the orientation dependent threshold energy.
Theoretical and experimental study of the vibrational excitations in ethane monolayers adsorbed on graphite (0001) surfaces
Authors: Hansen F. Y. , Taub H.
Published: July 2002

The collective vibrational excitations of two different crystalline monolayer phases of ethane (C2H6) adsorbed on the graphite (0001) surface have been investigated theoretically and experimentally. The monolayer phases studied are the commensurate 7/8 ×4 structure in which the ethane molecules lie on their side in a herringbone arrangement and the commensurate 7/8 × 7/8 structure in which the molecules stand on-end with the C–C bond perpendicular to the surface. Semiempirical atom–atom potentials have been used to model the intermolecular and molecule–substrate interactions in calculations of the monolayer structure and lattice dynamics. The observed inelastic neutron scattering spectra below 4.1 THz are compared with calculated one-phonon incoherent cross sections. The calculated cross sections qualitatively reproduce the rather different spectra of the two phases and enable an identification of the observed vibrational modes. Moreover, they suggest that an important feature of the herringbone phase dynamics is a coupling of the lowest-frequency librational mode to the vibratory mode perpendicular to the surface. Calculations of the phonon dispersion relations, the phonon density of states, and the phonon contribution to heat capacities of the two monolayer phases are also presented and discussed.
Raman trace scattering intensity parameters for propane
Authors: Gough K. M. , Murphy W. F. , Stroyer-Hansen T. , Svendsen E. No/rby
Published: July 2002

Absolute Raman trace scattering cross sections have been measured for gaseous propane-h8, propane-d8, 1,1,1,2,3,3,3-propane-d7, and the two rotational isomers of 1,1,2,2,3,3,3-propane-d7. A set of ∂ᾱ/∂Sj intensity parameters were determined from the experimental cross sections and compared with the ones obtained from quantum chemistry calculations. The anomalous intensity ratio observed for the CH stretching bands of the two conformers of CHD2CD2CD3 is explained by an unanticipated difference in the ∂ᾱ/∂rCH value for the two types of methyl CH bonds.
The benzene ground state potential surface. II. Harmonic force field for the planar vibrations
Authors: Ozkabak Ali G. , Goodman Lionel
Published: July 2002

A complete harmonic force field in terms of nonredundant coordinates has been generated from experimental frequencies for D6h, D3h, and D2h isotopically labeled benzenes and degenerate mode Coriolis constants predicting broken symmetry labeled benzene frequencies to ±0.1% and Coriolis constants to ±0.01 units, on the average. Exact solutions have been obtained for the six E1u force constants from D6h symmetry frequency data with the inclusion of 13C6H6 information. Some modes (e.g., the e2g mode ν8, in Wilson notation) are significantly altered from previous experimental force field predictions, rationalizing unclearly understood vibronic features of phosphorescence and two-photon spectra. A conundrum regarding the e1u Coriolis constant for ν18 (Wilson notation) is identified: no harmonic force field is capable of predicting the reported experimental magnitudes for this constant for both C6H6 and C6D6. The Pulay et al. scaled ab initio force field is in qualitative agreement with the experimental field for every symmetry class, with significant quantitative discrepancies involving CH stretching constants.
Photon driven charge transfer half-collisions: The photodissociation of CO2⋅O+2 cluster ions with resolution of the O2 product vibrational states
Authors: Kim Hyun-Sook , Kuo Chau-Hong , Bowers Michael T.
Published: July 2002

The photodissociation dynamics of the CO2⋅O+2 cluster are studied in a crossed ion beam–laser beam apparatus from λ=590 nm to λ=357 nm. At all wavelengths only CO+2/O2 products are observed even though O+2/CO2 products are lower in energy by 1.71 eV. The absolute cross section for photodestruction of CO2⋅O+2 is measured and increases smoothly from less than 1×10−18 cm2 at 590 nm to approximately 7×10−18 cm2 at 357 nm. In the range 590–458 nm ground electronic state CO+2(X 2Π)/O2(X 3∑) products are formed. Arguments are made that indicate that the O2 product is selectively vibrationally excited to the highest level energetically allowed (ν=0,1, or 2 depending on the wavelength). Asymmetry parameter analysis indicates that the photoexcited state of CO2⋅O+2 accessed is repulsive and the cluster dissociates in times short compared to a rotational period. At 357 nm the mechanism changes. Arguments are made that unambiguously indicate the O2 product is electronically excited at this wavelength and the product states are CO+2(X 2Π)/O2(a 1Δ). In addition, 10%–25% of the photoexcited clusters at 357 nm may initially be formed in a bound state that subsequently vibrationally predissociates. The large majority of the products at this wavelength are formed by direct dissociation from a repulsive state, however, similar to the longer wavelength data.
The bonding in FeN2, FeCO, and Fe2N2: Model systems for side-on bonding of CO and N2
Authors: Bauschlicher Charles W. , Pettersson Lars G. M. , Siegbahn Per E. M.
Published: July 2002

CO and N2 side-on bonding to one Fe atom are found to be very similar in many respects. In both cases the bonding leads to an increase in the CO or N2 bond length and a decrease in the vibrational frequency. The CO or N2 stretching modes lead to a large dipole derivative along the metal–ligand bond axis. The populations show an almost identical, large donation from the Fe 3d orbitals into the CO or N2 π*. For FeN2, the shift in the observed N2 frequency is smaller than observed for the α state of N2/Fe(111). The shift in the N2 vibrational frequency is found to increase when the N2 interacts with two Fe atoms, either at the Fe–Fe nearest neighbor distance or at the first layer Fe–Fe distance, when the side-on N2 axis is oriented perpendicular to an Fe–Fe bond. The very large shifts in the N2 ωe are similar to those observed in experiment. When the N2 and Fe2 bond axes are parallel, the N2 vibrational frequency is similar to that found for the interaction with one Fe atom. The Fe2N2 calculations yield two different low-lying Fe occupations; in one, the two Fe atoms have occupation 3d74s1 while in the second, 4s promotion to 3d mixes in the occupation 3d8 and a larger charge transfer from Fe to N2 is observed. This might be of importance for breaking the N–N bond on an Fe surface.
Symmetric mixtures of hard spheres with positively nonadditive diameters: An approximate analytic solution of the Percus–Yevick integral equation
Authors: Gazzillo Domenico
Published: July 2002

We study the properties of symmetric binary mixtures of hard spheres with positive nonadditive diameters Rij, namely two-component systems with R11=R22=R and R12>R, at equimolar concentration. The functional form of the direct correlation functions cij(r) is investigated, within the Percus–Yevick approximation, by using Hiroike and Fukui’s version of the Ornstein–Zernike integral equation for multicomponent fluids. It is shown that, by introducing simple polynomial expressions for the cross term C12(r)=rc12(r), the problem of finding an approximate analytic solution of the abovementioned integral equations can be reduced to an algebraic one, i.e., to solving a closed set of a few nonlinear algebraic equations for some unknown parameters. Results corresponding to three different approximations are presented for the radial distribution functions at contact, the virial pressure and the so called bulk modulus. Comparison is made with our exact numerical solutions of the Percus–Yevick integral equation and a very good agreement is found. Finally, calculations based on a simple first-order perturbation method, which gives a slight extension of analytic expressions derived from the Barker–Henderson perturbation theory, are reported and discussed.
Ligand field theory for the orbit–lattice coupling coefficients to E strains of d5 ions in tetrahedral symmetry
Authors: Boulanger D. , Parrot R.
Published: July 2002

A molecular orbital model has been elaborated to determine the orbit–lattice coupling coefficients to strains of symmetry E of the orbital triplet states of d5 ions in tetrahedral symmetry. The wave functions and energies of the monoelectronic molecular orbitals have been determined for a tetrahedral molecule MnX4 in a crystal and for a slightly distorted molecule of symmetry D2d corresponding to a strain Eθ. It is shown that in D2d symmetry, the splitting of the monoelectronic molecular orbitals e and t2 is due to the variations of the group overlap integrals and the variation of the crystal electrostatic field with respect to their values in cubic symmetry. The orbit–lattice coupling coefficients which are independent of the spin–orbit interactions, are obtained by diagonalizing the Tanabe–Sugano matrices for the orbital triplet states and then using the molecular orbitals in D2d symmetry as zero-order wave functions in a first-order perturbation scheme. The proposed model for the orbit–lattice coupling coefficients of the orbital triplet states 4T1 and 4T2 at lower energy for Mn2+ in ZnS and ZnSe gives theoretical values which differ at most by 30% from the experimental values. It is shown that three fifths of the theoretical values for the orbit–lattice coupling coefficients are due to the variations of the group-overlap integrals, the remaining part of the theoretical values being due to the variations of the crystal electrostatic field describing the Coulomb’s interaction in the molecular model.
Experimental and molecular dynamics study of depolarized Rayleigh scattering by O2
Authors: Zerda T. W. , Song X. , Jonas J. , Ladanyi B. M. , Geiger L. C.
Published: July 2002

Collision-induced effects on depolarized Rayleigh scattering (DRS) in gaseous oxygen at 300 K in the pressure range from 10 to 3000 bar have been studied experimentally. DRS spectra for four thermodynamic states at this temperature and in the pressure range from 130 to 3000 bar have also been obtained from molecular dynamics (MD) simulation. Molecular trajectories were calculated using the Lennard-Jones atom–atom potential and the system polarizability was modeled using the first order dipole–induced dipole approximation. The frequency dependence of DRS band shapes is well reproduced by the MD results, which predict that the orientational contribution dominates the overall band shape. The agreement between experimental and calculated second, fourth, and sixth spectral moments is very good. The observed density dependence of the spectral moments is analyzed in terms of contributions from the collision-induced (CI) polarizability component and orientational pair correlations. The second spectral moment, M2, is found to decrease with increasing density at low densities and to increase at higher densities. The MD results indicate that the initial decrease in M2 is due primarily to slow relaxation of the CI polarizability at low densities. The increase in M2 at higher densities is found to be mainly due to the contribution from orientational-CI cross correlations. MD results as well as analytical calculations, carried out for the low density limit, predict that the CI contribution to the total DRS intensity remains relatively small (≤4%) throughout the entire density range. The observed density dependence of the intensity is not consistent with these calculations. Possible sources of this disagreement are discussed.
Paramagnetic resonance evidence for anion fluctuations in the paraelectric phase of potassium dihydrogen phosphate
Authors: Owens F. J.
Published: July 2002

A marked nonlinear decrease of hyperfine splitting of the electron paramagnetic resonance spectrum of the I= 1/2 SeO3−4 radical doped into potassium dihydrogen phosphate and an increase in the linewidth is observed with increasing temperature above 40 °C. The effects are shown to be a result of slow tumbling of the radical in the lattice. The slow tumbling is believed to be driven by order–disorder fluctuations of the intrinsic phosphate anions which has been observed by NMR and neutron diffraction studies.
Determination of dissociation energies and thermal functions of hydrogen-bond formation using high resolution FTIR spectroscopy
Authors: Wofford B. A. , Eliades M. E. , Lieb S. G. , Bevan J. W.
Published: July 2002

A technique which employs high resolution Fourier transform infrared spectroscopy is demonstrated for evaluation of hydrogen bond dissociation energies D0 and De. Results for HCN--HF give a D0=20.77(22) and De =28.77(45) kJ/mol which are compared with previously determined values obtained from microwave absolute intensity measurements and ab initio molecular orbital calculations. Rovibrational band information available for HCN--HF also permits evaluation of thermal functions of dimer formation in kJ/mol: ΔU○298.2 =20.1(2), ΔH○298.2 =22.6(2), ΔG○298.2 =59.4(2), ΔS○298.2 =−0.1235.
Depolarized light scattering and neutron scattering spectra for noble gas fluids
Authors: van Well A. A. , de Schepper I. M. , Verkerk P. , Huijts R. A.
Published: July 2002

Using the technique of interaction-induced depolarized light scattering (DILS) the four-point dynamic correlation function is probed. Here, we discuss three different decoupling approximations which express the four-point dynamic correlation function in terms of two-point dynamic (Van Hove) correlation functions. We show that the experimental results of the integrated DILS intensity of Ar as a function of density can be reproduced satisfactorily by one of the decoupling approximations for densities up to the critical density. The line shape of the DILS spectrum of Xe in the liquid phase is compared with the decoupling approximation using the two-point dynamics correlation function obtained from neutron scattering experiments on Ar at the corresponding state. It is shown that the low-frequency part of the model calculation is consistent with the experimental DILS results. In this frequency range, in the liquid phase, the DILS spectrum appears to be mainly determined by density fluctuations with wavelengths of the order of the diameter of the particles. The influence of different expressions for the pair polarizability on the model calculations is discussed.
Pair potentials for the interaction energy of Li+, Na+, K+, and NH+4 with organic molecules
Authors: Portmann Pius , Maruizumi Takuya , Welti Martin , Badertscher Martin , Neszmelyi Andras , Simon Wilhelm , Pretsch Ernö
Published: July 2002

Interaction energies of K+ and NH+4 with the model ligands methyl-propyl-ether, N-ethyl-N-methyl-acetamide, 1,2-dimethoxy-ethane, 2-methoxy-N,N-dimethyl-ethylamine, and ethyl-propionate were determined by performing ab initio SCF-LCAO-MO calculations for about 1000 complex structures. Atom pair potentials were obtained by fitting the calculated interaction energies with an analytical potential. Both the form of the analytical potential and the assignment of atoms to classes, i.e., in groups having the same atom pair potential values, were amended relative to earlier contributions. Previously determined Li+ and Na+ interaction energies were fitted into a consistent form of analytical potential and classification. The pair potentials obtained were applied for the calculation of interaction energies between the ions and 18-crown-6 and the results were compared with corresponding ab initio calculations.
Numerical Hartree–Fock calculations on diatomic chromium
Authors: Richman Kent W. , McCullough E. A.
Published: July 2002

Numerical restricted Hartree–Fock (RHF) calculations on diatomic chromium have been carried out with the partial-wave procedure. The results are compared with the large Slater basis results of McLean and Liu. Most of the residual error in their largest basis is shown to be asymptotic error in the energy of atomic chromium. The true molecular error is small and has little effect on the spectroscopic properties of the molecule at the RHF level.
The ordered and disordered phases of crystalline F2
Authors: Gamba Z. , Halac E. B.
Published: July 2002

We report a molecular dynamics simulation study of the ordered and disordered phases of crystalline F2. Several macroscopic and microscopic statical equilibrium properties have been calculated at different temperatures; among the latter we include orientational probability functions of molecules in both crystalline phases. Dynamical properties were analyzed by calculating several auto- and cross-correlation functions at different temperatures and their characteristic times are given. A comparison with several theoretical models is also performed.
Observation of a new electronic state of C2 by resonance ionization spectroscopy
Authors: Goodwin Peter M. , Cool Terrill A.
Published: July 2002

A study of the resonance enhanced multiple photon (REMPI) spectrum of C2(A 1Πu) radicals formed in the two-step photodissociation of C2H2 at 193 nm has been completed for laser wavelengths from 370 to 480 nm. Rotational analyses for 12 rovibronic bands of two-photon resonant C2(1 1Δu←A 1Πu) transitions provide preliminary spectroscopic constants for the previously unobserved 1 1Δu state.
The absorption line shape for a molecular system stochastically coupled to a phonon thermal reservoir
Authors: Faid Karim , Fox Ronald F.
Published: July 2002

In this paper we extend earlier results regarding the relaxation properties of a molecular system, stochastically coupled to a phonon thermal reservoir, and interacting with a radiation field. The absorption line shape is derived in the white as well as the colored noise regime. Broadening, shift, and asymmetry are predicted. The expressions for the parameters, which characterize the features of the line, have an explicit dependence on the temperature, which our earlier, microcanonical theory did not include.