bluuebunny/crossref_metadata_2025 huggingface dataset:
The interaction potentials of SF6 ions in SF6 parent gas determined from mobility data
Authors: Brand K. P. , Jungblut H.
Published: February 2003

All available data, including some as yet unpublished, concerning the mobility of SF6 ions in SF6 as the parent gas are critically reviewed. All mobilities can be attributed to one of the following five ions: SF+5, SF−5, SF−6, SF−6(SF6), and SF−6(SF6)2. Starting from the SF+5 ion, which represents the only complete data set over the whole E/n range, we found (9,6,4) potentials, which reproduce the measured mobilities of all ions very well. The potential parameters corresponding to the above mentioned sequence of ions are γ=0.660, 0.625, 0.683, 0.709, 0.753, and r0=4.0, 4.0, 4.05, 4.72, 5.03 Å, respectively.
Mesomorphic properties and molecular structure. I. Structure of the smectic A phase in the 4-acetyl-4′-n-alkanoyloxy-azo-benzene series
Authors: Albertini G. , Fanelli E. , Guillon D. , Melone S. , Poeti G. , Rustichelli F. , Torquati G.
Published: February 2003

This paper reports an investigation by dilatometry and x-ray diffraction of the structure at the molecular level of the smectic A phases occuring in homologous series of liquid crystals, namely the 4-acetyl-4′-n alkanoyloxyazobenzene series. First, several structural parameters referring to the aromatic part and to the paraffinic chain were obtained; then, on the basis of these parameters, a model was proposed concerning the structural organization of the molecules inside the smectic A layers. According to this model, a bilayer organization of the molecules was found to occur with a strong segregation between paraffinic and polar parts of the molecules, along with a pronounced spreading of the aliphatic chains at the interface.
Hydrogen in aromatics. III. Chemisorption of hydrogen in graphite–alkali metal intercalation compounds
Authors: Enoki Toshiaki , Sano Mizuka , Inokuchi Hiroo
Published: February 2003

The hydrogen chemisorption mechanism in the graphite–alkali metal intercalation compounds has been investigated by means of ESR and electrical resistivity under hydrogen atmosphere. In the first stage rubidium compound C8Rb, hydrogen molecules are dissociated into atoms which are stabilized in the interstices of Rb atoms in the gaps of the graphite layers. C8K has a two-stage hydrogen absorption. In the initial stage, the dissociated hydrogen atoms are transiently yielded and then, in the second stage, the formation of hydride ions H− takes place through the charge transfer from C8K to H. C8Cs shows no change in the ESR signal under hydrogen atmosphere. This finding suggests that its catalytic activity for an H–D exchange reaction is due to the surface activity. For the second stage compounds C24K and C24Rb, there exists a one-stage hydrogen absorption process, i.e., absorbed hydrogen directly becomes hydride ion H−. C24Cs shows no change in the ESR signal under hydrogen atmosphere. For the chemisorption activity, the order of the strength is K≳Rb≳Cs in both first and second stage compounds.
Electron thermalization in gases. V. Diatomic molecules H2, N2, and CO
Authors: Tembe B. L. , Mozumder A.
Published: February 2003

The method introduced by one of us (AM) for calculating the thermalization times, evolution of the effective electron temperature, and other quantities of interest in electron thermalization, is extended to the diatomic molecules H2, N2, and CO. Starting with a displaced pseudo-Maxwellian distribution function for energetic electrons, the evolution of the thermalization process is studied using available cross section data for elastic and inelastic processes. Careful attention has been paid to microscopic reversibility and detailed balancing. We find that, while the major contributions to thermalization are due to rotational and vibrational excitations of the gas molecules, the contribution due to elastic collisions is not negligible. We also find that the relaxation time (obtained from the relaxation time due to inelastic and elastic processes) is a significant part of the thermalization time. The relative contributions of elastic and inelastic processes are evaluated as a function of time as well as gas temperature. Calculated results are in reasonable agreement with experimental data.
Limiting thickness of an adsorbed polymer chain. Random flight model
Authors: Rubin Robert J. , Weiss George H.
Published: February 2003

The probability density of the span of an N-step adsorbed polymer chain in the direction normal to the solution surface is derived in de Gennes’ model for weakly adsorbed chains. It is noteworthy that the following two different measures of thickness of an adsorbed polymer chain have a different N dependence. The average span is proportional to ln N, and the scale length of the exponential step density distribution is independent of N. The average span is identically equal to the distance from the adsorbing solution surface at which the exponential step density obtained in the de Gennes model is equal to unity.
Crossover between Gaussian and self-avoiding limits via finite order self-avoiding walk: Conformation space renormalization for polymers. VII
Authors: Bagchi Biman , Oono Y.
Published: February 2003

The crossover between the random walk and the self-avoiding walk via finite order self-avoiding random walk (FSAW) is studied analytically with the aid of conformation space renormalization group theory. Explicit expressions for the end-vector distribution function and 〈R2〉 are given up to order ε=4−d (d the spatial dimension). Since the excluded volume parameter for FSAW is contour length dependent, it is very awkward to study the present crossover behavior by using the polymer–magnet analogy. In contrast, our calculations in conformation space are simple and transparent, showing the power of this RG approach. The crossover behavior along FSAW is compared with the crossover obtained when the magnitude of the excluded volume interaction is decreased. The crossover via FSAW may occur in situations where a very long single chain is immersed in a solution of (shorter) chains and the concentration of the (shorter) chains is increased.
Dielectric effects on the spectra of condensed phases
Authors: Warner J. W. , Wolfsberg Max
Published: February 2003

The single-resonance damped oscillator model for molecular polarizability is combined with the Lorentz model for the local field in order to obtain expressions describing the dielectric constant and absorption coefficient of pure condensed phases. These expressions are functions of three parameters: the transition dipole matrix element, the molecular resonance frequency, and the lifetime. It is found that condensed phases exhibit a macroscopic resonance frequency different from the molecular resonance frequency, and equations are supplied whereby one may calculate this shift. The shape of the condensed phase absorption coefficient is found to be unsymmetrical about the resonance frequency for intense bands; a method is described in which one exploits this lack of symmetry in order to find the transition dipole matrix element. Attenuated total reflection data for four liquid phase infrared bands are analyzed in terms of the absorption model.
Infrared diode laser spectroscopy of the CF3 ν3 band
Authors: Yamada Chikashi , Hirota Eizi
Published: February 2003

The ν3 degenerate C–F stretching vibration–rotation band of the trifluoromethyl radical was observed in the gas phase by infrared diode laser spectroscopy and was analyzed to determine precisely the rotational constants, centrifugal distortion constants, and the band origin. The spectrum was found to be perturbed by the so-called Δl,ΔK=2,2 and 2,-1 interactions, and an analysis of the perturbation yielded the C0 rotational constant which was otherwise difficult to determine. The two r0 structure parameters, the C–F distance and the F–C–F angle, were calculated from the observed B0 and C0 rotational constants to be 1.318±0.002 Å and 110.76±0.4°, respectively, where the errors were entirely due to the uncertainties of B0 and C0. A general quadratic force field was derived using the fundamental frequencies of 12CF3 and 13CF3 and also the ζ3 Coriolis coupling constant obtained by the present work. The observed vibration–rotation constants α3B and α3C were used to estimate two cubic potential constants φ133 and φ233, and two more constants φ333 and φ334 were obtained from the observed q(2,2 interaction) and r(2,-1 interaction) constants.
Electron impact ionization–excitation of CO in a supersonic beam
Authors: Dagdigian P. J. , Doering J. P.
Published: February 2003

Electron impact ionization–excitation of cold, supersonic CO beams has been investigated in order to understand the rotational selection rules when the excited ionic state has 2Π symmetry. Variable electron energy (150–900 eV) was used, and the recorded emission spectra had the rotational structure almost completely resolved. Assuming the electron dipole selection rule to be valid, a simple model of rotational transitions within ionization–excitation to a 2Π state was developed. Synthetic spectra, calculated on the basis of this model, were in excellent agreement with the experimental spectra at 900 eV. The possibility that there might be deviations from the electric dipole selection rule at lower electron impact energies was considered. However, because of the complicated relationship between ground and excited state populations, we did not expect to be able to draw any definitive conclusions about such effects. Experimentally, definite evidence was obtained for such deviations at electron impact energies below 200 eV.
Concentration-dependent collapse of a large polymer in a solution of incompatible polymers
Authors: Tanaka Fumihiko
Published: February 2003

DNA of high molecular weight is observed to collapse into a very compact particle in a salt solution in the presence of second neutral polymers, poly(ethylene oxide) [(EO)n] of lower molecular weight, above a certain critical concentration. We show in a statistical mechanical way that the phenomenon is caused by the repulsive interaction between the DNA and (EO)n’s. A self-consistent (SC) potential is introduced to treat both intramolecular and intermolecular repulsive interactions of short-range nature. Appearance of a bound state in this SC potential is necessary and sufficient condition for a collapsed state to be stable. The critical concentration of (EO)n is found to be proportional to the inverse degree of its polymerization. The average size of a DNA is expected to change from N3/5 to N1/3 at the transition point, where N is the number of base pairs in the DNA. The transition is of second kind.
Formation of CO+ (A 2Πi) in collisions between metastable Ne* (3s3P2,0) and CO
Authors: Bel Bruno Joseph , Krenos John
Published: February 2003

Optical emission from CO+(*) formed in electronic energy transfer collisions between metastable neon atoms and ground-state carbon monoxide molecules has been measured in the spectral range 300–870 nm by a beam plus scattering gas technique. An indirect procedure allows the estimation of the relative reactivity of the two metastable states present in the beam. (AIP)
Reply to ‘‘Comment on ‘Dirac states for unit position and momentum: Phase consistency of their angular momentum representations’ ’’a)
Authors: Snider R. F.
Published: February 2003

It is stressed that the defined states and their relationship deal exclusively with the quantum mechanics of directions and angular momentum. In particular, they have nothing to do with the possibility of factoring three dimensional motion into magnitude and directional components. A magnitude independent derivation of the previously obtained result is given.
On the Chapman–Enskog first approximation for mixtures: A reply to Karkheck, Kincaid, and Stella)
Authors: Eu Byung Chan
Published: February 2003

A reply is given to the criticism of Karkheck, Kincaid and stell 1, on the Chapman. Enskog solution method for gaseous mixtures.(AIP)
The production of large concentrations of molecular ions in the lengthened negative glow region of a discharge
Authors: De Lucia Frank C. , Herbst Eric , Plummer Grant M. , Blake Geoffrey A.
Published: February 2003

A technique for enhancement of positive molecular ion concentrations in a glow discharge is presented. The technique consists of modifying an anomalous glow discharge by the addition of a longitudinal magnetic field of up to 300 G. Enhancements in the ion signal strength, as measured by millimeter and submillimeter wave spectroscopy, are approximately two orders of magnitude. Evidence is presented that the magnetic field increases the length of the ion rich negative glow by restricting inside a small diameter tube the ionizing electrons accelerated by the large cathode drop of an anomalous glow discharge.
Pressure dependence of conductance of the lithium and cesium ions in cooled and supercooled water
Authors: Takisawa N. , Osugi J. , Nakahara M.
Published: February 2003

Limiting molar conductances (λ0) of the Li+ and Cs+ ions in water have been determined at 0, −5, and −10 °C as a function of pressure up to 2 kbar. In the studied ranges of pressure and temperature, λ0(Li+) increases monotonically with increasing pressure and λ0(Cs+) has a maximum against pressure. The low-temperature conductances fit well an empirical equation of the critical law form over the pressure range studied; λ0=A(T/TS−1)γ. The parameter TS decreases with a rise in pressure below 1.5 kbar, but above that TS is almost constant. The Hubbard–Onsager (HO) dielectric friction theory has been critically tested against the experimental results obtained under the extreme conditions after λ0 are transformed into the drag coefficient (Δζ) subtracted by that due to Stokes’ law for perfect slip (4πηR). The observed pressure coefficient dΔζ/dP is negative for the small ion Li+ even in cooled or supercooled water as predicted by the HO theory but positive for the large ion Cs+ in contrast to the theoretical prediction. The following three kinds of anomalies in Δζ(Cs+) are revealed: Δζ<0, dΔζ/dP≳0, and dΔζ/dT≳0. The PTC mechanism proposed for explaining the found anomalies has been supported by model calculations; it turns out that the potential for the PTC process can be flat with a reasonable size of cavities.
Microscopic surface tension and binary nucleation
Authors: Flageollet-Daniel C. , Garnier J. P. , Mirabel P.
Published: February 2003

Experimental results concerning the binary homogeneous nucleation from the vapor phase of water–alcohols mixtures are reexamined. It is shown that the large discrepancies between theory and experiment can be explained by the fact that, for those mixtures, the use of macroscopic surface tension values is not applicable to binary droplets. A lattice model is developed to describe a microscopic cluster. This allows the determination of the surface monolayer composition, and the calculation of the actual surface tension as a function of the cluster size. The introduction of this model in the classical theory of binary nucleation restores good agreement with the experimental results.
Rate constants for intermediate and high temperature 4He–H2 and 4He–D2 vibrational relaxation
Authors: Jolicard Georges , Grosjean Alain
Published: February 2003

Rate constants for 4He–H2 and 4He–D2 vibrational relaxation are computed using the phase modulated semiclassical sudden approximation and compared with experiments for 100 K<T<3000 K. The Gordon–Secrest potential is used with a rotating Morse oscillator description of the H2 and D2 molecules. We carefully analyze the influence of the initial rotational distribution on the relaxation process and show that the upper rotational states (O≤j≤5) play an important role in the intermediate temperature range.
Predissociation rates of the A 1Π state of CH+
Authors: Graff M. M. , Moseley J. T. , Durup J. , Roueff E.
Published: February 2003

Levels of the A 1Π1 state of CH+ which lie above the C+(2P1/2)+H(2S) dissociation limit may be predissociated by radial coupling with the a 3Π1 state and by rotational coupling with the X 1Σ+0 state. In addition, predissociation by tunneling may also occur for shape resonance levels above the C+(2P3/2) +H(2S) dissociation limit. The spin-orbit coupling matrix element has been derived as a function of internuclear distance. Predissociation widths, and hence lifetimes, for all levels have been calculated for both radial and rotational processes, as well as for the previously considered tunneling process. The calculations indicate that radial and rotational coupling are of major importance in determining the lifetimes of these levels, often dominating even for levels where tunneling is possible. Almost all of the levels between the two dissociation limits are, according to this calculation, significantly predissociated.
Direct fitting of spectroscopic data to near-dissociation expansions: I2(D′ → A′), Br2(D′ → A′), and XeCl(B → X and D → X)
Authors: Tellinghuisen Joel
Published: February 2003

The utility of near-dissociation expansions (NDEs) for diatomic vibrational energies is tested through least-squares fitting of vibrational bandhead data for selected electronic transitions in I2, Br2, and XeCl. In these test cases, the NDEs show efficiency comparable to that for the traditional polynomials in (v+1/2). For data sets which span an intermediate range of v levels, the NDEs are clearly superior to the polynomials for extrapolating to higher v, and of comparable reliability in extrapolation to low v. However, in their approach to dissociation, the NDEs can show unphysical behavior, the correction of which requires further constraints on the form of the NDE. Also, for lengthy extrapolations the NDEs may yield optimistically precise yet erroneous values for the dissociation energy. The present best estimates of the dissociation energies (𝒟e) for the lower states involved in these calculations are 2506.0(3) cm−1 for I2(A′), 2828(8) cm−1 for Br2(A′), and 281.1(7) cm−1 for XeCl(X).
Multichannel distorted wave Born approximation for reactive scattering
Authors: Hubbard Lynn M. , Shi Sheng-hua , Miller W. H.
Published: February 2003

Previous applications of the distorted wave Born approximation (DWBA) to reactive scattering have often given reasonably good relative cross sections, i.e., angular distributions, product state distributions, etc.—but absolute reactive cross sections have been poor. It is argued in this paper, however, that the DWBA for reactive scattering should be accurate if (1) the reaction probability is sufficiently small (as it is, e.g., in the threshold region of a reaction) and (2) the nonreactive scattering is described sufficiently accurately. To describe the nonreactive scattering ‘‘sufficiently accurately’’ this paper utilizes a nonreactive coupled-channel scattering wave function for distorted waves in the DWBA. Application to a standard test problem (collinear H+H2) shows this multichannel DWBA indeed to be extremely accurate if the reaction probability is no larger than 0.1 and if ∼3 to 4 vibrational states are included in the nonreactive coupled-channel expansion. This approach thus provides an excellent description of the threshold region of a reaction which has an activation barrier.
Bounds for the atomic electronic density
Authors: King Frederick W.
Published: February 2003

Several upper bound estimates for the atomic electronic density are derived. The bounds are formulated in terms of the kinetic energy and, in some of the cases considered, the expectation values 〈Ψ ‖ r1−1 ‖ Ψ〉 or 〈Ψ ‖ r1−2 ‖ Ψ〉 are employed. All the inequalities follow from the application of Block’s theorem. A numerical test of the bounds has been carried out for some two electron species.
On the slipping boundary condition for low density gas flow
Authors: Peralta-Fabi Ramón , Zwanzig Robert
Published: February 2003

We discuss the slipping boundary condition in fluid dynamics. We argue that when the boundary is a well defined and smooth surface, the slipping boundary condition is the correct choice. The Grad 13-moment equations are used to describe the hydrodynamic fields for the slow stationary flow of a viscous compressible and low density fluid. We study a planar Couette flow and flow around an arbitrary obstacle. The equations are solved for the flow past a sphere. The drag on the sphere is calculated to second order in the mean free path.
Molecular dynamics study of Rayleigh light scattering from molecular fluids
Authors: Ladanyi Branka M.
Published: February 2003

In this article, we present the results of a molecular dynamics simulation of Rayleigh light scattering (LS) from molecular fluids. We include the orientational and collision induced (CI) contributions to the depolarized LS time correlation and the CI contribution to the isotropic LS time correlation. The intermolecular potential is represented by a two site Lennard-Jones model. The bond length and potential parameters are chosen to correspond to those of O2 and CO2. We consider two pair polarizability models, one based on the dipole-induced dipole (DID) interactions between molecular centers and the other on the DID interactions between Lennard-Jones sites. For both models, we study the variations of the LS time correlations, integrated intensities, and spectral moments with density and temperature. In the case of the more anisotropic CO2, we find that the two models predict very different behavior of the CI part of the Rayleigh LS spectra. The site DID model is found to agree slightly better with the available experimental data. We discuss the implications of our results to the interpretation of Rayleigh spectra in terms of structure and dynamics of molecular fluids.
The a 4Σ−–X 2Π transition in CF: A measurement of the term energy and bond length of a fluoromethylidyne metastable
Authors: Grieman F. J. , Droege A. T. , Engelking P. C.
Published: February 2003

We report the first observation of the (0,0) and (0,1) vibrational bands of the weak, spin forbidden a 4Σ−–X 2Π transition of the CF radical. The spectra, obtained in a flowing afterglow, yield values for T0, B0, and r0 of the a 4Σ− state of 28 564±1 cm−1, 1.3035±0.002 cm−1, and 1.326±0.001 Å, respectively. The intensity distribution in the rotational subbands is consistent with the mixing of the a 4Σ− state predominantly with the X 2Π state and the uncharacterized C 2Σ+. This result is discussed with regard to previous calculations.
M dependence in the analysis of NH3–He microwave double resonance experiments
Authors: Davis Stephen L. , Green Sheldon
Published: February 2003

New close-coupled calculations of laboratory-frame, m-dependent cross sections for rotational excitation in NH3–He collisions are used to examine the validity of using degeneracy averaged values in the analysis of four-level double resonance experiments. It is found that the proper use of m-dependent cross sections and absorption probabilities produces only minor changes in the calculated ΔI/I and does not, therefore, resolve the discrepancies between theoretical and experimental values that were noted in previous studies.
Rotational and vibrational analysis of the O2(A′ 3Δu → a 1Δg) system
Authors: Slanger Tom G. , Huestis David L.
Published: February 2003

The oxygen afterglow spectrum published by Degen in 1968 has been reanalyzed. It is found to contain three O2(A′ 3Δu → a 1Δg) bands and four O2(A 3Σu+ → X 3Σg−) bands, and by computer simulation, rotational and vibrational constants have been determined for particular levels of the A′ 3Δu(Ω=3) and a 1Δg states.
Multiple phase-coherent laser pulses in optical spectroscopy. II. Applications to multilevel systems
Authors: Warren W. S. , Zewail Ahmed H.
Published: February 2003

The effects of intense laser pulse trains in coupled multilevel systems (such as pure or mixed molecular crystals) are analyzed by calculating exact density matrix evolutions. It is shown that two-level approximations are inadequate. The contributions of exchange couplings, inhomogeneous broadening, permanent multipole interactions and transition multipole interactions to absorption and photon echo line shapes are calculated. The absorption line shape of 1,4-dibromonaphthalene (DBN) is shown to be predominantly an isotopic substitution effect, as our Monte Carlo results give quantitative agreement with experiment for this model. Average t-matrix approximations to the Green’s function, which have been used to propose a different mechanism for the DBN line shape are shown to be qualitatively inadequate. Dipole–dipole interactions are shown to be an important photon echo decay mechanism in mixed crystals, with the relative importance of permanent and transition multipole interactions dependent on the resonance frequency distribution. Multiple pulse trains, including multiple pulse echoes and optical multiple-quantum sequences, are shown to be capable of distinguishing different types of interactions in the molecular Hamiltonian and reducing optical density effects. Specific pulse sequences are proposed and their effects are calculated.
Smoluchowski–Vlasov theory of charge solvation dynamics
Authors: Calef Daniel F. , Wolynes Peter G.
Published: February 2003

The dynamics of polar solvent reorganization near an instantaneously created charge are investigated. A linear, Vlasov-type equation is derived, where the interactions between the polar molecules are treated using a mean field term containing the direct correlation function. For the diffusive limit, a Smoluchowski–Vlasov equation for reorientation is studied. The results, for model solvents, are compared to the results from continuum dielectric theories, to experimental results, and to results from recent computer simulations.
Motion mechanisms in framework solid electrolytes: Correlated hopping and liquidlike diffusion
Authors: Jacobson Solomon H. , Ratner Mark A. , Nitzan Abraham
Published: February 2003

Motion mechanisms for ions in framework solid electrolytes are investigated. The results are obtained from numerical studies on a one-dimensional model system, utilizing the method of stochastic Langevin dynamics. We find that, for commensurate systems (for which one mobile ion occurs exactly every l lattice sites), the mechanism always involves correlated hops, and the ion–ion repulsion decreases (always) the total conductivity. For incommensurate systems, the conductivity changes from hopping to liquidlike as the interaction forces are increased to dominate the potential due to the framework lattice. Different assumed ion–ion potentials produce different correlations, both local and overall; the nearest-neighbor harmonic forces, such as are assumed in the Frenkel–Kontorova model, will generally produce substantially different correlation effects from the Coulomb repulsion. The frequency-dependent conductivity at low frequency is shown to be proportional to the square of the frequency; the proportionality coefficient is positive for correlated hopping mechanisms. A double-peaked structure in the frequency-dependent conductivity, due to local oscillation and to long-time, long-range diffusive behavior, is observed when particle–particle interactions are absent and damping is weak.
Nonequilibrium solvation dynamics in solution reactions
Authors: van der Zwan G. , Hynes James T.
Published: February 2003

We construct a theoretical framework for the description of nonequilibrium solvation and solvent participation in the reaction coordinate for solution reactions. The framework is illustrated by a model of reactive dipole isomerization. We show that a multidimensional reaction coordinate picture is equivalent to a one dimensional description in which a generalized friction characterizes and quantifies nonequilibrium solvation effects on the reaction rate. The adiabatic regime where equilibrium solvation and mean potential ideas are correct is identified. Several distinct regimes of nonequilibrium solvation are identified and described in molecular terms. In the effective mass regime, equilibrium solvation ideas give the reaction barrier curvature correctly, but solvent inertia modifies the barrier passage rate. In the nonadiabatic regime, the solvent is ‘‘frozen’’ during the barrier passage and cannot provide equilibrium solvation. In the polarization caging regime, the reacting species adjust to the moving solvent, rather than vice versa, and the solvent is heavily involved in the reaction coordinate. The rate constant in each of these regimes is related to reactive and solvent dynamics.
On the stretching of alkyl chains by nematics
Authors: Ben-Shaul Avinoam , Rabin Yitzhak , Gelbart William M.
Published: February 2003

When a semiflexible chain molecule is subjected to an anistropic solvent, e.g., leads to a partial elongation and alignment of the molecule. The ordering of the CC bonds as a funciton of position along the chain was treated in an earlier communication by solving a lattice version of the rotational isomer state model in the presence of an external field. In the present paper, we extend this approach to the explicit calculation of solvent-induced size and anisotropy effects. Specifically, we determine the increase in 〈Z2N〉1/2 as a function of chain conformation (i.e., gauche vs trans) energy ε and of monomer–solvent coupling strength f. Here 〈Z2N〉1/2 is the root-mean-square value of the Z component of the end-to-end vector for an N-bond molecule (the space-fixed Z axis being taken to lie along the nematic direction). The elongation of the molecule is described in terms of the deformation of a sphere with radius R0=(3〈Z2N〉f=0)1/2 into an ellipsoid of revolution with eccentricity determined by 〈Z2N〉f≳0.
Quantum theory of solvent effects on electronic spectra: Predictions of the exact solution of the mean spherical model
Authors: Schweizer Kenneth S. , Chandler David
Published: February 2003

A microscopic quantum statistical mechanical theory for the solvent modification of the electronic spectra of nonpolar solutes is developed. Attention is focused on the solute frequency shift as a function of molecular parameters, solvent dielectric constant, and density. Comparison of the predictions of the quantum theory and dielectric continuum models (Onsager–Böttcher and Wertheim) based on the macroscopic reaction field picture are made. Significant differences are found which indicate the importance of solvent structure and nonzero frequency effects absent in the dielectric models. The theory is also applied to several aromatic hydrocarbon molecules in dilute solution with emphasis on the isothermal density dependence of the electronic frequency shift. Generalization of the quantum theory to treat polar solutes is briefly outlined.
Ã 2Π←X̃ 2Σ+ infrared electronic transition of C2H
Authors: Carrick P. G. , Merer A. J. , Curl R. F.
Published: February 2003

Five bands of the Ã 2Π←X̃ 2Σ+ lowest electronic transition of C2H have been observed in the region 3600–4200 cm−1 in an argon discharge over polyacetylene using color center laser spectroscopy with magnetic rotation sensitivity enhancement. The band origins are approximately 3692, 3773, 3785, 4011, and 4106 cm−1. Rotational assignments and preliminary fittings of all five bands have been made. Four of the bands have a 2Σ+ lower vibronic state which is undoubtedly the ground vibronic state of the molecule, and different 2Π upper vibronic states which correspond to mixtures of the lowest vibronic state of the Ã electronic state and three different vibrationally excited levels of the X̃ electronic state with an odd number of quanta in the bending mode and overall 2Π symmetry. These three ground state vibrational levels have been tentatively assigned as (1,1,0), (0,1,2), and (0,5,1). The four 2Π vibronic levels are mixed by vibronic coupling between the X̃ and Ã electronic states thereby providing electronic oscillator strength for the vibrational transitions. The fifth band is a 2Σ−←2Π hot band with the lower state the first excited bending level (0,1,0) of the ground electronic state and upper state the 2Σ− component of the (0,1,0) excited bending level of the excited electronic state. From these observations a value of 1.30 Å for the CC bond length in the excited electronic state and a value of ∼375 cm−1 for the bending frequency of the X̃ ground electronic state are obtained.
Vibrational energy transfer in atom–polyatomic collisions: The role of intramolecular dynamics in Li+/CO2 and Li+/N2O
Authors: Vilallonga E. , Micha D. A.
Published: February 2003

Using a quantal many-body theory of hyperthermal atom–polyatomic collisions, we present a detailed study of the role of intramolecular dynamics in vibrational energy transfer. In this approach, differential vibrational cross sections are given by time-correlation functions (TCFs) of the motions of the target atoms. Slow rotations of polyatomic molecules are decoupled from vibrations, and we concentrate on vibrational energy transfer. The dependence of the TCFs on collision energy, scattering angle, and changes in vibrational quantum numbers is examined in detail for a general polyatomic, and it is shown how each target atom contributes to vibrational energy transfer. For the systems Li+/CO2 and Li+/N2O, we present vibrational differential cross sections calculated with new multicenter short-range potentials obtained from the uniform electron-gas model. Comparison of theoretical results with experimental measurements of the Li+/CO2 system shows good agreement for several collision energies and scattering angles. Furthermore, we show how the structure of the vibrational cross section is related to the atomic parameters of the target and to the relative sizes of the repulsive cores of the intermolecular potential.
Complex coordinate calculations of Feshbach resonance energies and widths for a collinear triatomic system
Authors: Christoffel Kurt M. , Bowman Joel M.
Published: February 2003

Complex resonance energies are calculated for a model triatomic system studied previously by Eastes and Marcus [W. Eastes and R. A. Marcus, J. Chem. Phys. 59, 4757 (1973)]. The complex coordinate method is employed to obtain resonance positions and widths for six states. The positions are all in excellent agreement with previous scattering and stabilization results and there is also good agreement with the one width obtained previously from scattering calculations. The additional widths obtained are given for the first time. Some speculations about possible laser induced absorption and emission among these resonances and true bound states are made.
On the onset of strong mode coupling in small polyatomic molecules
Authors: Berry R. Stephen
Published: February 2003

The number of vibrational states corresponding to separable, quasiperiodic motion of low dimensionality is estimated by a generalization of the analysis of the quantum-mechanical Hénon–Heiles problem by Hose and Taylor. The number of nonseparable states with strongly coupled modes appears to overwhelm the number of separable ‘‘quasiperiodic’’ states whenever the molecule in question contains five or more vibrational quanta, and may dominate for states with four or even three vibrational quanta. Some comments are made to try to clarify the concepts of separability and quasiperiodicity.
Semiclassical quantization and extraction of eigenfunctions using arbitrary trajectories
Authors: De Leon N. , Heller Eric J.
Published: February 2003

We present in this paper a coordinate independent semiclassical quantization method. We demonstrate that in order to extract accurate eigenvalues and eigenfunctions the trajectory does not necessarily have to reside on the quantizing torus, rather, one can use information obtained on arbitrary tori. Because the method is coordinate independent, no difficulty is encountered in quantizing within classical resonance zones. Furthermore, nearby eigenstates and eigenvalues (nearby in action space) may be extracted from the same trajectory—this is especially convenient when the density of states becomes large.
Molecular clustering about a positive ion. Structures, energetics, and vibrational frequencies of the protonated hydrogen clusters H+3, H+5, H+7, and H+9
Authors: Yamaguchi Yukio , Gaw Jeffrey F. , Schaefer Henry F.
Published: February 2003

The positive hydrogen clusters H+(H2)n for n=1, 2, 3, 4 have been studied via nonempirical molecular electronic structure theory. Using double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets, wave functions are reported at both the self-consistent field (SCF) and configuration interaction including all single and double excitations (CISD) levels of theory. In each case analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies. The effects of electron correlation are shown to be greater for these loose molecular complexes than for ordinary molecules. Although H+5 in its lowest energy conformation is not qualitatively described as H+3⋅H2, the larger molecular ions do fit the qualitative picture H+3(H2)n, with H+3 as a nucleating center. Of special interest here are the ‘‘new’’ normal modes of these clusters, i.e., those modes having no counterpart in the isolated H+3 or H2 species. There are 15 such vibrational degrees of freedom for H+9, and the resulting harmonic vibrational frequencies range from 775 cm−1 all the way down to 63 cm−1. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Kebarle.
Ab initio study of electronic coupling in the aqueous Fe2+–Fe3+ electron exchange process
Authors: Logan Jean , Newton Marshall D.
Published: February 2003

Electronic Hamiltonian matrix elements between initial and final zeroth order states associated with electron exchange in the hexa-aquo Fe2+/Fe3+ redox system have been calculated in terms of self-consistent field (SCF) ab initio wave functions. The face-to-face and apex-to-apex approach geometries of the quasioctahedral reactants have been modeled, respectively, by the [Fe(H2O)3–Fe(H2O)3]5+ cluster (S6 symmetry) and the [Fe(H2O)–Fe(H2O)]5+ cluster (D2h symmetry). For the latter cluster, the Condon approximation has been tested and found to be accurate to within ∼1 cm−1 for the important range of inner-shell FeO distances. The calculations employ ab initio effective core potentials for inner-shell electrons and explicitly include all metal and ligand valence electrons. Due to weak 3d–3d overlap, the energy-preferred SCF solutions are charge localized (i.e., symmetry broken: S6→C3 and D2h→C2v). The present results for the interpenetrating face-to-face approach geometry are quite similar to earlier results based on a crystal-field model, implying an electronic transmission factor 〈κ〉 of ∼1/5 at the most probable reactive encounter separation (5.3 Å). The alternative apex-to-apex approach geometry is found to be less kinetically favorable. Attempts to fit calculated values of the electronic matrix elements to functions of the form Pn(r)rm exp(−αr) for the range r∼5–8 Å, where r is the Fe⋅⋅⋅Fe separation in Å and Pn(r) is a polynominal, yield values of α ranging from 0.8 to 2.4 Å−1, depending on the values of n and m, the orientation of reactants, and the model employed for the ligands. The calculated matrix elements are found to be rather insensitive with respect to variation of certain features of the SCF wave functions.
Some theoretical considerations for magnetic vibrational circular dichroism spectra
Authors: Laux Leo , Pultz Vaughan , Marcott Curtis , Overend John , Moscowitz Albert
Published: February 2003

Expressions are derived for the A terms, B terms, and associated partial anisotropy factors relevant for the magnetic vibrational circular dichroism (MVCD) spectra of molecules with nondegenerated ground states. These expressions are derived subject to the approximations of the harmonic fixed partial charge model and the condition of magnetic coupling among only the ground electronic state vibrational levels. Numerical values for partial anisotropy factors calculated from these expressions, when taken in conjunction with the recent MVCD data of Keiderling, suggest the importance of magnetic coupling of the ground electronic state with higher electronic states in the case of B terms. Symmetry constraints on the nonvanishing of such coupling terms also suggest the possible utility of MVCD data for providing spectroscopic and molecular structural information.
One- and two-photon spectra of NaF:Cu+: Jahn–Teller and vibronic coupling effects
Authors: Payne Stephen A. , Goldberg Andrea B. , McClure Donald S.
Published: February 2003

Notwithstanding the fact that CuF does not exist, the dilute solid solution NaF:CuF is stable. One- and two-photon spectra of the cubic single crystals were obtained for the near ultraviolet 3d10→3d94s transitions of Cu+ over a range of temperatures from 2 to 300 K. The two-photon polarized spectra allowed the separation of the 1Eg from the 1T2g spectra, and revealed the double humped band characteristic of the Jahn–Teller effect. Zero phonon lines were observed in the two-photon spectra, and a progression in a low frequency t1u mode was observed with even quanta in the two-photon, odd quanta in the one-photon spectra. From the fine structure and from the temperature dependence of the broadbands, information about the potential function of the Cu+ in NaF was derived for a1g, eg, and t1u coordinates. The Cu+ is on center in the ground and excited states. The value of EJT is 2250 cm−1 and 10 Dq is 1800 cm−1 for the d9s configuration.
Kinetics of the Cl–H2 system. II. Abstraction vs exchange in D+HCl
Authors: Miller John C. , Gordon Robert J.
Published: February 2003

The gas phase reaction of D with HCl was studied at room temperature by the method of flash photolysis, with resonance fluorescence detection of D, H, and Cl atoms. The D and Cl atoms were observed to decay exponentially, whereas no H atom signal from the exchange reaction was detectable. From the calibrated sensitivity of the H atom detector, an upper limit of 0.002 was set for the ratio of rate constants for the exchange vs abstraction channels.
The state distribution of OH radicals photodissociated from H2O2 at 193 and 248 nm
Authors: Ondrey G. , van Veen N. , Bersohn R.
Published: February 2003

Photodissociation of H2O2 at 248 and 193 nm yields largely vibrationless OH radicals in their ground electronic state. At 248 nm, on the average about 64 and 3 kcal/mol of energy are relased as translational and rotational energy, respectively. At 193 nm the corresponding quantities are 92 and 8 kcal/mol. The distribution of the OH radicals over K″ peaks near K″=5 when dissociated at 248 nm and near K″=6 at 193 nm but the latter distribution is somewhat broader. Doppler width anisotropy data imply that at 248 nm a single upper state is reached but that at 193 nm several upper surfaces may be responsible for the absorption. It is concluded that the upper state potential functions(s) may be respresented by the sum of a large repulsive term depending only upon the distance between centers of mass of the OH radicals and a small angularly dependent term which generates the rotational excitation.
The unimolecular reaction of (CH3)3CNO following n−π* excitation with a tunable dye laser
Authors: Reisler H. , Pessine F. B. T. , Haas Y. , Wittig C.
Published: February 2003

The predissociation of (CH3)3CNO following the 570–710 nm laser excitation of the n-π* transition is reported. The small fluorescence quantum yield and its short lifetime (<20 ns) indicate efficient radiationless processes prior to dissociation. NO is the major dissociation product and its concentration was monitored in real time by using the two-photon laser excited fluorescence technique. The NO yield as a function of wavelength followed very closely the absorption contours of (CH3)3CNO. The NO production rate was ≳108 s−1 at all wavelengths studied and this rate is too fast to be consistent with a mechanism involving a radiationless transition to the ground electronic state followed by energy randomization and dissociation. Alternative predissociation mechanisms are considered.
IR laser induced loss of fluorescence in UO2(hfacac)2⋅TMP. I. Real time detection of multiple photon excitation of the ground electronic state
Authors: Hall R. B.
Published: February 2003

The infrared multiple photon excitation and unimolecular reaction of UO2(hfacac)2⋅TMP is monitored in real time by the loss of dye laser induced fluorescence (LIF) intensity. The loss of fluorescence results from a change in the chemical structure and not from a change in the vibrational state population of the ground or excited electronic state. Absorption into the emitting states also becomes forbidden following IR excitation. The observed rates of fluorescence depletion are greater than 106 s−1 with excitation the rate limiting step over the range of laser intensities studied. At low extents of depletion the observed rate follows the single photon absorption rate. At higher yields, the observed rate is within a factor of 2 of the rate derived from the measured dependence of the yield on laser power using the analysis given by Quack. Such an analysis can provide a reasonable estimate of the reaction rate when no rate measurements can be made. The ensemble dynamics contribute significantly to the observed kinetics. The excitation and reaction behavior is characteristic of molecules in the quasicontinuum excitation regime from the absorption of the first photon. No fluence or power threshold for reaction is observed. At a fixed fluence, the yield remains constant over a change in the peak power of more than an order of magnitude and thus appears to be independent of the rate at which the photons are absorbed. In addition, the absorption profile is homogeneous with respect to reaction; the yield remains proportional to the product of fluence times the small signal absorption cross section for any laser transition within the v=0 to v=1 absorption profile of any of the accessible vibrational modes. Collision cross sections for fluorescence quenching by UO2(hfacac)2⋅TMP and by the products of the multiple photon excitation UO2(hfacac) and TMP are also obtained.
Rotational rainbow scattering in collisions of Ar with Cl2
Authors: Hoffbauer Mark A. , Burdenski S. , Giese Clayton F. , Gentry W. Ronald
Published: February 2003

The pulsed molecular beam technique has been used to measure speed and angle-resolved differential cross sections for scattering of Ar by Cl2, over the range of initial relative kinetic energies from 0.09 to 0.16 eV. Angular rainbows are observed at small scattering angles. At large scattering angles, the product speed distributions at constant center-of-mass scattering angles display rotational rainbow structure which corresponds to a large fraction of the initial relative kinetic energy appearing as rotational energy in the Cl2 product. The large-angle data may be interpreted in terms of the classical rigid-ellipsoid model, which expresses the anisotropy of the potential energy surface with a single parameter.
The rotational spectrum and molecular properties of the hydrogen cyanide hydrogen bromide complex
Authors: Campbell E. J. , Legon A. C. , Flygare W. H.
Published: February 2003

Rotational spectra have been assigned for four isotopic species of a linear hydrogen-bonded complex formed between hydrogen cyanide and hydrogen bromide. The vibrational ground state spectroscopic constants are The order of the atoms in HCNHBr with a N⋅⋅⋅Br separation of 3.61Å.
The infrared absorption spectra of (HCCH)2 and (DCCD)2
Authors: Pendley Rex D. , Ewing George E.
Published: February 2003

The infrared absorption bands of acetylene dimers have been investigated with long path low temperature Fourier transform infrared spectroscopy. Five bands were found, three of (HCCH)2 and two of (DCCD)2. Four of these bands can be assigned as coupled monomer modes. Normal mode analysis based on a theoretical intermolecular potential confirms these assignments. One band shows evidence of fast vibrational predissociation, and a symmetry based argument is made to support the claim that the predissociation fragments are rotationally excited.