bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo variational transition-state theory study of recombination and desorption of hydrogen on Si(111)
Authors: Raff Lionel M. , NoorBatcha I. , Thompson Donald L.
Published: July 2002

A variational transition-state theory study of the recombination/desorption rate of H2 from a Si(111) surface using a previously described Monte Carlo procedure [J. Chem. Phys. 83, 1389 (1985)] is reported. The potential-energy surface is expressed as the sum of a lattice potential, a lattice-adatom interaction term, and an adatom–adatom interaction. Keating’s formulation with the parameters suggested by Weber is used for the lattice potential. The adatom–lattice term is written as a pairwise sum of 60 Morse potentials each multiplied by a hyperbolic switching function that limits each absorption site to one bond. The adatom–adatom interaction is the product of an H2 Morse potential and a switching function that attenuates the H–H interaction as the Si–H bonds form. The parameters of the potential are adjusted to fit the results of hydrogen atom–silicon cluster calculations, the experimental and theoretical results for the H2 insertion barrier into Si and SiH2, and the measured H2(g) bond dissociation energy, fundamental frequency, and equilibrium bond distance. The minimum-energy path is obtained using a Monte Carlo random walk procedure with importance sampling. The potential surface predicts a 2.52 eV barrier for H2 recombination/desorption and a 0.182 eV barrier to the back reaction. Variational rate calculations are carried out by expressing the dividing surface as a linear combination of the recombination/desorption coordinates and minimizing the computed flux across the surface with respect to the expansion coefficients using a partial grid search. An Arrhenius plot of the minimized flux yields an activation energy and a frequency factor of 2.41 eV and 0.202 cm2 s, respectively. This activation energy is in good agreement with one reported experimental value and is 0.60 eV greater than that found in two other experiments. The frequency factor lies in the middle of the range of the reported experimental values.
Distance dependence in photo-induced intramolecular electron transfer
Authors: Larsson Sven , Volosov Andrey
Published: July 2002

The distance dependence of the rate of photo-induced electron transfer reactions is studied. A quantum mechanical method CNDO/S is applied to a series of molecules recently investigated by Hush et al. experimentally. The calculations show a large interaction through the saturated bridge which connects the two chromophores. The electronic matrix element HAB decreases a factor 10 in about 4 Å. There is also a decrease of the rate due to less exothermicity for the longer molecule. The results are in fair agreement with the experimental results.
Temperature dependence of the low- and high-frequency Raman scattering from liquid water
Authors: Walrafen G. E. , Fisher M. R. , Hokmabadi M. S. , Yang W.-H.
Published: July 2002

Low frequency Δν̄=0–350 cm−1, Raman intensity data were obtained from liquid water between 3.5 and 89.3 °C using holographic grating double and triple monochromators. The spectra were Bose–Einstein (BE) corrected, I/(1+n), and the total integrated (absolute) contour intensities were treated by an elaboration of the Young–Westerdahl (YW) thermodynamic method, assuming conservation of hydrogen-bonded (HB) and nonhydrogen-bonded (NHB=bent and/or stretched, O–H O) nearest-neighbor O–O pairs. A ΔH°1 value of 2.6±0.1 kcal/mol O–H ⋅⋅⋅ O or 5.2±0.2 kcal/mol H2O (11 kJ/mol O–H ⋅⋅⋅ O, or 22 kJ/mol H2O) resulted for the HB→NHB process. This intermolecular value agrees quantitatively with Raman and infrared ΔH° values from the one- and two-phonon OH-stretching regions, and from molecular dynamics, depolarized light scattering, neutron scattering, and ultrasonic absorption, thus indicating a common process. A population involving partial covalency of, i.e., charge transfer into, the H ⋅⋅⋅ O units of linear and/or weakly bent hydrogen bonds, O–H ⋅⋅⋅ O; is transformed into a second high energy population involving bent, e.g., 150° or less, and/or stretched, e.g., 3.2 Å, but otherwise strongly cohesive O–H O interactions. All difference spectra from the fundamental OH-stretching contours cross at the X(Z,X+Z)Y isobestic frequency of 3425 cm−1. Also, total integrated Raman intensity decreases occurring below 3425 cm−1 with temperature rise were found to be proportional to the total integrated intensity increases above 3425 cm−1, indicating conservation among the HB and NHB OH-stretching classes. From the enthalpy of vaporization of water at 0 °C, and the ΔH°1 of 2.6 kcal/mol O–H ⋅⋅⋅ O, the additional

enthalpy, ΔH°2, needed for the complete separation of the NHB O–O nearest neighbors is ∼3.2 kcal/mol O–H ⋅⋅⋅ O or ∼6.4 kcal/mol H2O (13 kJ/mol O–H ⋅⋅⋅ O or 27 kJ/mol H2O). The NHB O–O nearest neighbors are held by forces other than those involving H ⋅⋅⋅ O partial covalency, i.e., electrostatic (multipole), induction, and dispersion forces. The NHB O–O pairs do not appear to produce significant intermolecular Raman intensity because they lack H ⋅⋅⋅O bond polarizability, but the corresponding NHB OH oscillators do contribute weakened Raman intensity above 3425 cm−1. An ideal solution thermodynamic treatment employing ΔH°1 =2.6 kcal/mol O–H ⋅⋅⋅ O, the HB mole fraction, and the vapor heat capacity, yielded a very satisfactory specific heat value of 1.1 cal deg−1 g−1 H2O at 0 °C. The NHB mole fraction, fu, from the YW treatment is negligibly small, 0.06 or less, for t<−50 °C. However, fu increases to 0.16 at 0 °C, and fu≊1 at 1437 °C, where recent shock-wave Raman measurements indicate loss of all partially covalent, charge transfer hydrogen bonding.
The thermodynamics of ammonium scheelites. III. An analysis of the heat capacity and related data of deuterated ammonium perrhenate ND4ReO4
Authors: Brown R. J. C. , Callanan J. E. , Weir R. D. , Westrum E. F.
Published: July 2002

An analysis of the heat capacity of deuterated and undeuterated NH4ReO4 has been carried out in which the effects of the anisotropy of the thermal expansion have been considered, an approach hitherto not used for ammonium compounds. In the ammonium scheelites, the axial thermal expansion coefficients are very large, but of opposite sign, and as a result the volume of the scheelite lattice is nearly independent of temperature. It is shown that the correction from constant stress to constant strain results in a major contribution to the heat capacity of this highly anisotropic lattice. The difference between the experimental and calculated values of heat capacity, referred to as ΔCp, is expressed as the sum of the contributions from the anisotropy and the rotational heat capacity. The results of the analysis show that the rotational contribution is much smaller then previously thought. However, the exact contribution of the anisotropy cannot be calculated at this time because the elastic constants are not known. In calculating the heat capacity, maximum use has been made of external optical mode frequencies derived from spectroscopic measurements.
A statistical method for identifying transition states in high dimensional problems
Authors: Pratt Lawrence R.
Published: July 2002

A statistical method based on Monte Carlo sampling of paths associated with Markovian stochastic processes is proposed for the location of transition states in molecularly complex reactions. It is discussed in what respects the proposed method would identify transition state regions, and an algorithm for Metropolis sampling of Metropolis paths (based on the Metropolis random walk process) is devised. Physical aspects of this new algorithm are examined, and the relation of this method to simulated annealing algorithms for physical design of circuits is noted.
A determination of the stabilities and structures of F−(C6H6) and F−(C6F6) clusters
Authors: Hiraoka Kenzo , Mizuse Susumu , Yamabe Shinichi
Published: July 2002

The temperature dependence of the equilibria F−+C6H6=F−(C6H6) and F−+C6F6=F−(C6F6) was measured using a pulsed electron beam mass spectrometer. The −ΔH° values obtained are 15.3 and 27.5 kcal/mol, respectively. The measured enthalpy changes are reproduced by the ab initio MO calculations. While F−(C6H6) involves a moderately strong hydrogen bond, the covalent bond is formed in the cluster F−(C6F6) (i.e., tetrahedral structure).
Studies of the intramolecular dynamics of model polyatomic molecules
Authors: Sumpter Bobby G. , Thompson Donald L.
Published: July 2002

The intramolecular dynamics of several model systems are investigated by using classical trajectories. Power spectra of auto- and cross-correlation functions obtained from the classical trajectories are used to classify the type of motion for excited local vibrational modes and to investigate the mechanisms of energy flow at various excitation energies. Coherency spectra are used to investigate interactions between two vibrational modes. Various models of four systems, HC3, HNNH, HCCH, and H2O2, are examined. The emphasis is on hydrogen peroxide. The motion is quasiperiodic at low energies of excitation. The onset of chaotic motion occurs at approximately 89% of the energy required for dissociation of the OH bond in H2O2. For acetylene the classical motion becomes chaotic only for energies very near the dissociation limit of the CH bond. For HC3 and HNNH, chaotic motion occurs at energies much less than that required for dissociation of the CH or NH bond. In general, the chaotic limit is lowered and the rate of energy flow is enhanced by the presence of bending degrees-of-freedom. The amount of energy transferred from a local mode is found to be invariant to the number of degrees-of-freedom in the chaotic regime while it is strongly dependent upon the number of degrees-of-freedom in the quasiperiodic regime. However, the initial rate of energy transfer in the chaotic regime is substantially affected by the number of degrees-of-freedom.
Kinetics of diffusion-influenced bimolecular reactions in solution. I. General formalism and relaxation kinetics of fast reversible reactions
Authors: Lee Sangyoub , Karplus Martin
Published: July 2002

A general theoretical framework for treating the kinetics of diffusion-influenced bimolecular reactions in solution is presented. It is based on a hierachy of phenomenological kinetic equations for the reduced distribution functions of reactant molecules. With this formalism, a perturbation series expression for the rate coefficient for irreversible reactions involving a long-ranged sink function is derived. For a delta-function sink, it reduces to that obtained previously by Northrup and Hynes [Chem. Phys. Lett. 54, 244 (1978)]. It is demonstrated that the correctness of the Smoluchowski’s expression for the rate coefficient in the low concentration limit results from a cancellation of errors. For diffusion-influenced reversible reactions involving a delta-function sink, explicit expressions for the time-dependent forward and reverse rate coefficients are derived. Experimental data on the relaxation kinetics of the triiodide ion formation reaction are reinterpreted, and consideration of diffusion effects is found to be essential. It is shown that the rate coefficient for the diffusive encounter of reactant molecules is not the upper bound to the bimolecular rate coefficient for reversible reactions.
Laser induced fluorescence study of the B̃ 2B2→X̃ 2A2 transition of the furan cation in a supersonic free jet expansion
Authors: Smith R. Seth , Anselment M. , DiMauro L. F. , Frye J. M. , Sears T. J.
Published: July 2002

The first experimental observation of laser induced fluorescence from the furan cation, C4H4O+, is reported. The ion is formed in a supersonic free jet via a two photon ionization involving the 193 nm output of an ArF excimer laser. The B̃→X̃ electronic transition is probed and vibrational assignments are presented for both states. Comparisons to the infrared, Raman, multiphoton ionization, and photoelectron spectrum of the parent molecule are made. The term value T00 for the B̃→X̃ electronic transition is located at 24 675 cm−1, which differs drastically with previously reported values from both experimental and theoretical investigations. These differences are discussed and probable explanations proposed. The lifetime of the vibrationless level of the B̃ state is measured to be 330 ns.
High pressure rate constants for unimolecular dissociation/free radical recombination: Determination of the quantum correction via quantum Monte Carlo path integration
Authors: Klippenstein Stephen J. , Marcus R. A.
Published: July 2002

The determination of a quantum correction factor for the transitional modes of a unimolecular dissociation/free radical recombination reaction having a transition state of varying looseness is described. The quantum correction factor for the high pressure canonical rate constant is calculated via Monte Carlo path integral evaluation of partition function ratios, and is applied to the recombination reaction 2CH3→C2H6.
The structure of liquid 1,2 dichloroethane by neutron diffraction. I. Molecular structure, full pair correlation function, and temperature effects
Authors: Bermejo F. J. , Enciso E. , Dore J. C. , Chieux P. , Garcia N. , Santoro J.
Published: July 2002

Neutron diffraction measurements have been made in liquid 1,2 dichloroethane-d4 (DCE) at temperatures of −20, 21, and 70 °C. The intramolecular structural parameters have been determined by analysis of the high momentum transfer region of the derived structure factor as well as the relative populations of trans and gauche conformers. The separation between intra- and intermolecular contributions is achieved by means of previously reported procedures based upon maximum entropy estimation techniques. The full (composite) single-particle pair correlation function has been evaluated and show significant structural features up to 15 Å. A tentative assignment is made based upon results from a Monte Carlo simulation. The temperature variation produced some marked changes due to both the density decrease at high temperatures as well as to the displacement of the conformational equilibrium.
Antiresonance in autoionizing Rydberg series of naphthalene
Authors: Syage Jack A. , Wessel John E.
Published: July 2002

Asymmetric Fano line profiles have been observed by autoionization for vibrational Rydberg series of naphthalene. Specific vibrational excitation in the Rydberg series was made possible by two-color excitation that excites the corresponding vibration in the intermediate S1 level. An observed series of autoionizing states involving the ν8̄×ν7 vibration was determined to belong to members n=11 to 22 of a Rydberg series having a quantum defect of δ=0.10 and converging to 1537 cm−1 above the adiabatic ionization potential. The line shapes were analyzed in terms of degenerate resonance interference between optically active discrete and continua states. Observed linewidths were as narrow as 6 cm−1 for naphthalene and 3 cm−1 for naphthalene-d8.
Higher order interaction-induced effects on Rayleigh light scattering by molecular liquids
Authors: Geiger Lynn C. , Ladanyi Branka M.
Published: July 2002

Molecular dynamics (MD) computer simulation was used to study Rayleigh light scattering (LS) in oxygen and carbon disulfide liquids. Results are presented for one thermodynamic state of oxygen (82 K, 1.215 g/cm3) and two thermodynamic states of CS2 (193K, 1.42 g/cm3 and 293 K, 1.30 g/cm3). The molecular trajectories were generated using Lennard-Jones atom–atom potential models. LS spectra were calculated using the dipole–induced dipole (DID) model for the interaction-induced polarizability. The results obtained for the LS intensities, time correlations, and line shapes, using the first-order perturbation theory for the system polarizability, based on this model, are compared to the exact DID model results. We find that the relative contribution of the higher order DID terms to LS spectra of these liquids can be correlated with the magnitudes of the isolated molecule isotropic and anisotropic polarizabilities. As a result of this, these terms have a very small effect on O2 LS spectra, but that they make a large contribution to CS2 spectra. In CS2, the main consequence of including these terms is to substantially increase the orientational component and decrease the collision-induced component of the depolarized Rayleigh spectrum. Higher order DID interaction terms also lead to a faster initial relaxation of the collision-induced components of isotropic and depolarized spectra. The MD effective polarizability and depolarized LS line shape data for CS2 at 293 K are compared with experimental results. We find that the exact DID model results are in much better agreement with experiment than those obtained using the first-order DID model.
Activationless solvent-controlled electron transfer
Authors: Rips Ilya , Jortner Joshua
Published: July 2002

Activationless outer-sphere electron transfer (ET) in polar solvents is studied using the stochastic Liouville equation (SLE) formalism. The role of the initial conditions on the dynamics of activationless ET is explored. A solution of the SLE for arbitrary initial conditions is presented. Numerical results for the total population in the initial state for the equilibrium initial conditions and for the localized initial conditions are compared with approximate analytic solutions for activationless ET. For the equilibrium initial condition the approximate solution constitutes an excellent description of the ET dynamics over a broad time domain. For the initially localized state the ET dynamics exhibits sequential thermalization and activationless ET.
Beyond Poisson–Boltzmann: Images and correlations in the electric double layer. II. Symmetric electrolyte
Authors: Attard Phil , Mitchell D. John , Ninham Barry W.
Published: July 2002

An extension of the Poisson–Boltzmann theory of the electric double layer is applied to a symmetric electrolyte between two charged planar surfaces. This analytic treatment includes the effects of images and of ion correlations via the Debye–Hückel closure for the direct correlation function. It is demonstrated that at large separations the corrections to Poisson–Boltzmann theory appear as an effective surface charge. A means of correcting the apparent ion binding inferred from fitting experimental force data is also given. The extended theory shows good agreement with accurate numerical calculations for systems with low coupling. The generalization of the Onsager–Samaras limiting result to higher concentrations and to the case of the surface free energy of a charged dielectric interface is also presented.
Measurements of XeF ground state dissociation and vibrational equilibration
Authors: Bott J. F. , Heidner R. F. , Holloway J. S. , Koffend J. B. , Kwok M. A.
Published: July 2002

The removal rates of the lower levels of the XeF(B→X) excimer laser transitions strongly affect the overall efficiency of the E-beam-pumped devices. We have deduced the removal rates of XeF(X,v) in argon, helium, and neon by measuring the populations of vibrational levels v=0, 1, 3, and 4 formed by the photolysis of XeF2. The time history of each vibrational population is monitored with a cw tunable dye laser tuned to an absorption feature of the selected vibrational/rotational level. The studies show a rapid vibrational relaxation followed by a common decay rate of the coupled vibrational levels. Helium and neon were found to remove these levels with the same rate coefficient of (2.0±0.2)×104 (s Torr)−1 at room temperature. The corresponding rate coefficients for argon were larger by about 25%. The removal rate coefficients increased with temperature in the range of T=23–95 °C. The rate coefficient for the vibrational excitation of v=0 by helium was measured to be (3.6±0.7)×105 (s Torr)−1 at room temperature. This is considerably larger than the value of (4.8±1.5)×104 (s Torr)−1 previously reported by Fulghum et al. Fast vibrational relaxation is required to empty the lower levels of the laser transitions so that vibrational ‘‘bottlenecking’’ does not terminate the laser prematurely.
Theory of dynamics of entangled rod-like polymers by use of a mean-field Green function formulation. I. Transverse diffusion
Authors: Teraoka Iwao , Hayakawa Reinosuke
Published: July 2002

A mean-field Green function theory is developed for investigating the dynamics of entangled rod-like polymers in solutions. We regard the hindrance of surrounding rods as a multiple (sequential) perturbation to the free transverse or lateral diffusion of a test rod (a rod in consideration) and incorporate the effects of the perturbation elements one by one into the mean-field Green function. Thus, an explicit expression for the transverse diffusion constant is obtained as a function of a rod length and a polymer concentration for solutions from dilute to semidilute.
Higher-order interaction-induced effects on the allowed Raman spectra of liquid CS2
Authors: Geiger L. C. , Ladanyi B. M.
Published: July 2002

Molecular dynamics (MD) computer simulation is used to study nonvibrational contributions to the ν1 (symmetric stretch) mode Raman spectra of liquid carbon disulfide in two thermodynamic states: 193 K, 1.42 g/cm3 and 293 K, 1.30 g/cm3. A computational method for evaluating the Raman polarizability to all orders of dipole-induced dipole (DID) interactions is developed. The exact DID model results for Raman intensities, effective polarizabilities, time correlation functions and line shapes are compared to their counterparts obtained using the same model and first order perturbation theory for the interaction-induced Raman polarizability. Inclusion of higher-order DID interaction terms causes a dramatic change in the relative importance of collision-induced (CI) and orientational components of the depolarized Raman spectrum. The first order model predicts that the CI contribution to the spectrum is considerably larger than the orientational one, while the exact DID model predicts the reverse. The presence of higher-order DID interactions results in appreciably faster relaxation of the CI components of both isotropic and depolarized spectra. It is shown that the depolarized CI spectral component in both models contains a significant slowly relaxing portion due to mutual diffusion of pairs of molecules. The results for Raman and Rayleigh spectra are compared and their high-frequency line shapes are discussed in terms of the Madden–Cox theory. MD Raman depolarization ratios and depolarized spectra are compared to experiment at 293 K. The inclusion of higher-order DID terms improves agreement with experiment, but the DID model does not reproduce accurately the high frequency line shape. The sensitivity of this line shape to the changes in potential parameters is tested by varying the atom–atom Lennard-Jones parameters within reasonable limits and by adding atom–atom Coulomb potential terms to represent quadrupolar interactions. It is found to be only slightly sensitive to these changes.
CO(A–X) electric dipole transition moment
Authors: DeLeon R. L.
Published: July 2002

The internuclear dependence of the CO (A 1 π– X 1 Σ+ ) transition moment has been obtained for the range 1.0<r̄<1.8 Å. This was done by making laser induced fluorescence measurements on highly vibrationally excited CO. These data were jointly fit with fluorescence lifetime data from a previous study done by Field et al. The second-order fit obtained was 〈Re(r̄)〉 =10.57(41)[1−1.177(22)r̄+0.350(13)r̄2] D. This fit provides a good description of the transition moment function in that it quantitatively fits the fluorescence branching ratio data and lifetime data and agrees with the qualitative observation of CO(A–X) transition strengths.
Molecular dynamics study of an isomerizing diatomic in a Lennard-Jones fluid
Authors: Straub John E. , Borkovec Michal , Berne Bruce J.
Published: July 2002

The behavior of the reaction rate of an isomerizing diatomic molecule solvated in a Lennard-Jones fluid is studied by molecular dynamics simulations. A comprehensive study of solvation effects on the rate constant, using the reactive flux absorbing boundary approximation of Straub and Berne, is presented. We provide simulation data over three orders of magnitude in solvent density for four systems differing in the mass of the solvent atoms and frequencies of the internal potential. Rate constants are also calculated for the model system using both Langevin Dynamics with exponential memory and impulsive collision dynamics of the BGK model. A simple method for calculating the average energy transfer and collision frequency is used to determine the collision efficiency for systems in which the mass of the solvent atoms is lighter than, equal to, or heavier than that of the atoms composing the isomerizing diatomic. We find that for solvents of equal and heavy mass compared to the solute the impulsive collision model provides the best description of the dynamics. Finally, we employ a method recently introduced by us to calculate the spatial dependence of the dynamic friction; we compare the reaction coordinate friction at the transition state separation with an approximation based on the single particle friction. This directly calculated reaction coordinate friction, when combined with the Grote–Hynes theory for barrier crossing, gives good agreement with the simulation data at high density.
Monte Carlo integration of the repeated ring equation for the Lorentz gas
Authors: Palmer Bruce J. , Keyes Thomas
Published: July 2002

We propose a stochastic simulation of the repeated ring equation for the Lorentz gas based on a Monte Carlo integration of the scattering series obtained by formally inverting the repeated ring equation. We use this simulation to calculate the properties of the Lorentz gas in two and three dimensions.
Quantization of a classical analog for the E⊗e Jahn–Teller system at intermediate couplings
Authors: Zwanziger Josef W. , Grant Edward R.
Published: July 2002

The Meyer–Miller classical analog for the linear E⊗e Jahn–Teller system is quantized for vibronic coupling strengths ranging from 0 to twice the magnitude of the zeroth-order force constant. The dynamics of the classical analog, which range from near diabatic at small coupling to near adiabatic at large coupling, are strongly chaotic in this intermediate regime. To effect quantization we use a method recently proposed by Jaffé, in which a classical analog Hamiltonian matrix is obtained from the Liouville formulation of the problem. The eigenvalues of this matrix are the semiclassical energies, and the eigenvectors reflect the interaction of the zeroth-order basis distributions. The method is shown to yield exact agreement with quantum mechanics for the classical analog of a model, constant-coupled two-state Hamiltonian, provided the Langer modification is used. In the fully coupled Jahn–Teller system, good agreement with quantum mechanics is obtained over the parameter range, with deviations (at avoided crossings) that reflect need for more complete uniformization.
Some properties of generalized irreversible thermodynamics founded by Bearman–Kirkwood equations
Authors: Bartoszkiewicz M. , Miekisz S.
Published: July 2002

Generalized thermodynamics founded by Bearman–Kirkwood equations is considered. It is shown that the Prigogine theorem on the entropy production invariance also holds when inertia and local stresses contribute to the diffusion-dependent part of the dissipation function. A comparison with other approaches is made and conditions which reduce the generalized theory to the classical phenomenological approach are obtained for the case of mechanical equilibrium. The role of coupling between diffusion and viscous flow in barodiffusion processes is discussed. Bearman–Kirkwood partial salt viscosities are estimated for the low concentration region. The generalized theory predicts that in the presence of viscous effects and in the low concentration region, electrolyte barodiffusion depends mainly on local stresses. As a consequence, under the mentioned conditions, electrolyte barodiffusion is directed ‘‘against’’ the motion of the mass center. Thus, for some porous membranes the ‘‘practical’’coefficient LπP may be interpreted by the diffusion–viscous flow coupling. The possibility of a measurement of the Bearman–Kirkwood ionic partial viscosities is shortly discussed.
The rotationally resolved fluorescence excitation spectrum of 1-fluoronaphthalene
Authors: Majewski Wojciech A. , Plusquellic David F. , Pratt David W.
Published: July 2002

A rotationally resolved fluorescence excitation spectrum of the 000 band in the A 1A′←X 1A′ (S1←S0) transition of 1-fluoronaphthalene, at ∼3138 Å, has been observed using a newly constructed laser spectrometer operating in the ultraviolet. The band in question is a hybrid band exhibiting 75% a-type and 25% b-type character, and consists of ∼4000 lines at a rotational temperature of 9 K, each with a FWHM=3 MHz, all of which have been assigned. The band origin is at 31 866.508±0.002 cm−1 (vac), and the rotational constants are A″=1920.6, B″=1122.2, C″=708.5, A′=1891.5, B′=1102.1, and C′=696.6±0.1 MHz. The S1←S0 optical transition moment is rotated away from the a inertial axis by an angle of ±30°, an effect that could have dynamic consequences. Some operating characteristics of the spectrometer are described.
Dependence of effective desorption kinetic parameters on surface coverage and adsorption temperature: CO on Pd(111)
Authors: Guo Xingcai , Yates John T.
Published: July 2002

The effective desorption kinetic parameters of CO on the Pd(111) surface have been studied by thermal desorption spectroscopy. The zero coverage effective desorption activation energy and the preexponential factor were found to be 35.5 kcal/mol and 1013.5 s−1, respectively. As a function of CO coverage, a four-stage correlation between Ed(θ) and the development of stable low-energy electron desorption (LEED) structures was observed for the first time at Tads= 200 K. Ed and ν1 showed a strong compensation effect with Tc=519 K. The adsorption temperature dependence of Ed from Tads=87 to 200 K was observed and interpreted qualitatively by a model involving the production of different domain structures at various adsorption temperatures and the preservation of domain structures at higher coverages during temperature programmed desorption.
Contribution of covalent bond force to pressure in polymer melts
Authors: Gao J. , Weiner J. H.
Published: July 2002

Molecular dynamics simulations of the freely jointed effective hard sphere model of a polymer melt are performed, with continuous covalent and noncovalent potentials employed for computational convenience. The virial theorem is used to determine the melt pressure from the simulations. Excellent agreement is found over a wide range of density and degree of polymerization with the recently introduced equation of state of Honnell and Hall. The atomic pressure is defined as the contribution made by each atom to the compressibility factor of the system; it is found to be uniform along the interior atoms of the polymer chain, with slightly different values for the end atoms. This uniform value is almost independent of N, the number of bonds in the chain, and explains the small dependence on N of the pressure that is observed. Particular attention is paid to the contribution to the pressure made by the force in the covalent bonds of the system. For packing fractions greater than 0.3, approximately 30% of the pressure in the melt arises from this source.
Quasidegenerate many-body perturbation theory of CH2
Authors: Wang Xiao-Chuan , Freed Karl F.
Published: July 2002

The effective valence shell Hamiltonian (Hν) formulation of quasidegenerate many-body perturbation theory is applied to the CH2 molecule and its positive ion to investigate the ability of the method to describe large spaces having a wide range of orbital energies and patterns of quasidegeneracy that vary greatly with molecular geometry. Sensitive tests are provided by the adiabatic singlet–triplet energy difference, by the lowest singlet excitation energy, by the ionization potentials to a series of ion states, and by the question of whether accurate energies emerge simultaneously for a whole set of valence and ion states from a single Hν computation. Computations assess the dependence of Hν calculations on the choice of orbitals and orbital energies, the only degrees of freedom available to the Hν method once the valence space has been prescribed. Third order corrections are analyzed in preparation for the computations of the following paper that utilize the present calculations as a guide in studying the theoretical basis of the pairwise additivity assumption of semiempirical all valence electron methods.
The intramolecular vibrations of the ammonia water complex. A matrix isolation study
Authors: Engdahl Anders , Nelander Bengt
Published: July 2002

The infrared spectrum of the ammonia water complex has been recorded from 10 to 4000 cm−1 for complexes trapped in neon and krypton matrices. Spectra have been observed for NH3 or ND3 complexed with H2O, HDO, or D2O. The observed intramolecular fundamentals are compared with frequencies calculated from a rigid-molecule harmonic oscillator model.
Diffusivity of the hydrogen molecule sorbed in NaA zeolite by a neutron scattering experiment
Authors: Kahn R. , Cohen De Lara E. , Viennet E.
Published: July 2002

The diffusion of hydrogen in NaA zeolite was studied by incoherent neutron scattering. An experiment was carried out on samples loaded with 1.2 to 3.4 molecules per cavity and at several temperatures from 70 to 150 K. The angular (θ) dependence of the elastic and quasielastic intensities shows that the H2 molecule has a translational motion in a nonrestricted volume. A diffusion model where the molecule undergoes isotropic jumps of mean length l̄=3.9 Å independent of temperature and is at rest for a time τ0 between two jumps accounts for the width of the quasielastic scattering in the entire (θ,T) range (τ0=10.8 ps at T=100 K). This leads to a diffusion coefficient D(cm2/s)=6×10−4 exp(E/RT) with E=2 kJ/mol for the less loaded samples. The diffusion coefficient increases slightly with the loading.
Doppler spectroscopy of chlorine atoms generated from photodissociation of hydrogen chloride and methyl chloride at 157 and 193 nm
Authors: Matsumi Yutaka , Das Puspendu Kumar , Kawasaki Masahiro
Published: July 2002

Hydrogen chloride and methyl chloride are photodissociated at 193 and 157 nm. Branching ratios for production of the resulting chlorine atoms in the 2P1/2 state relative to the 2P3/2 state are determined to be 0.20±0.02 at 193 nm and 0.35±0.05 at 157 nm for HCl and 0.23±0.02 at 193 nm and 0.26±0.05 at 157 nm for CH3Cl. The Doppler profiles of the chlorine fragments have been measured and are interpreted by assuming their anisotropy parameters and translational energies. HCl undergoes a perpendicular optical transition at each of the two wavelengths. For CH3Cl, the transition is parallel at 193 nm and a mixture of parallel and perpendicular types at 157 nm.
Picosecond fluorescence depletion spectroscopy. III. Intramolecular vibrational relaxation in the excited electronic state of p-cyclohexylaniline
Authors: Smith P. G. , McDonald J. D.
Published: July 2002

This paper presents the results of the picosecond time-resolved study of intramolecular vibrational relaxation (IVR) in the molecule p-cyclohexylaniline. The results represent the second extensive study of a molecule using the technique which we have developed known as time-resolved fluorescence depletion (TRFD). Fluorescence depletion decays and dispersed fluorescence spectra of 19 S1 vibronic features are presented. The decays show a progression of dynamic behavior, including stationary behavior at low densities of states, quantum beating at intermediate densities, and fast decay of the initially prepared state at high state densities. The data allow us to assign IVR lifetimes which range from 250 ps at 820 cm−1 of excess vibrational energy to 2.2 ps at 2362 cm−1. Even at the highest frequencies we observe, the decays do not tend toward a simple exponential, but instead exhibit weak damped quantum beats.
Dynamics of entangled linear polymer melts:  A molecular-dynamics simulation
Authors: Kremer Kurt , Grest Gary S.
Published: July 2002

We present an extensive molecular-dynamics simulation for a bead spring model of a melt of linear polymers. The number of monomers N covers the range from N=5 to N=400. Since the entanglement length Ne is found to be approximately 35, our chains cover the crossover from the nonentangled to the entangled regime. The Rouse model provides an excellent description for short chains N<Ne, while the dynamics of the long chains can be described by the reptation model. By mapping the model chains onto chemical species we give estimates of the times and distances of onset of the slowing down in motion due to reptation. Comparison to neutron spin-echo data confirm our mapping procedure, resolving a discrepancy between various experiments. By considering the primitive chain we are able to directly visualize the confinement to a tube. Analyzing the Rouse mode relaxation allows us to exclude the generalized Rouse models, while the original reptation prediction gives a good description of the data.
Fine structure branching ratios of the O(3P
j) atomic fragments from photodissociation of oxygen molecules at 157 and 193 nm
Authors: Matsumi Yutaka , Kawasaki Masahiro
Published: July 2002

Direct photodissociation and predissociation of molecular oxygen from the B 3∑−u state have been studied by photoexcitation at 157 and 193 nm, respectively. The fine structure branching ratios and Doppler profiles of O(3Pj) photofragments were measured by a resonance-enhanced multiphoton ionization technique. The branching ratios of O(3Pj) j=2,1,0 are 0.74 ±0.03:0.21 ±0.02:0.042±0.004 for photodissociation at 157 nm and 0.47±0.05:0.31 ±0.04:0.22±0.44 at 193 nm. Both distributions are not statistical. The Doppler profiles of O(3Pj) from photodissociation of O2 at 157 nm are consistent with formation of O(1D)+O(3Pj).
Structural evolution within the one-phase region of a three-component microemulsion system: Water–n-decane–sodium-bis-ethylhexylsulfosuccinate (AOT)
Authors: Chen S.-H. , Chang S.-L. , Strey R.
Published: July 2002

The continuous inversion from a water-in-oil (w/o) microemulsion at low temperatures to an oil-in-water (o/w) microemulsion at higher temperatures within the one-phase channel of water (0.6% NaCl)–n-decane–AOT microemulsion system is investigated by small angle neutron scattering (SANS). At constant AOT (surfactant) weight fraction γ of 12%, the structural evolution as a function of temperature takes place in different forms as the oil-to-water weight fraction α is varied from 15 to 90 %. At low o-w weight fractions (α=15 and 20 %) the microemulsions transform from a water-internal, oil-continuous structure at lower temperatures to an oil-internal, water-continuous droplet structure at higher temperatures jumping across an intermediate region of a lamellar phase (Lα). However, at higher o-w weight fractions (α=80 and 90 %) the evolution goes through a stage of percolation of the water droplets first into extended water clusters, then the structural inversion takes place probably through a transition of these water clusters into an entangled tubular structure. At equal oil-to-water volume ration (α=40%), the structure can be described as bicontinuous at both low and high temperatures. In this case we are able to extract two lengths characterizing the structure from SANS data using different models for the scattering length density fluctuation correlation function of a bicontinuous microemulsion.
Hydrogen bond dynamics in isotopically substituted benzoic acid dimers
Authors: Stöckli A. , Meier B. H. , Kreis R. , Meyer R. , Ernst R. R.
Published: July 2002

The hydrogen pair transfer in the hydrogen-bonded dimers of benzoic acid and its carboxyl-deuterated species is investigated in the solid. Measurements are reported for the temperature-dependent NMR relaxation time T1 in single crystals containing dimers with one or two carboxylic deuterons. Combined with previous data, the temperature dependence of the measurements is analyzed in terms of a master-equation description for a one- or two-dimensional quantum-mechanical model of the transfer motion. The description by a one-dimensional model is found to be inadequate as it yields unrealistic isotope effects in the model parameters. On the other hand, reasonable results are obtained for a two-dimensional model which includes, apart from the transfer motion of the hydrogens, a heavy atom mode with properties suggested by x-ray structural data. This model explains the thermal activation of the transfer process mainly as a result of promotion of tunneling by heavy atom rocking. Activation energies remain considerably smaller than the barrier height and indicate predominance of tunneling even at room temperature.
Photodissociation of hydrogen chloride and hydrogen bromide
Authors: Matsumi Yutaka , Tonokura Kenichi , Kawasaki Masahiro , Ibuki Toshio
Published: July 2002

The Doppler profiles of the hydrogen atom photofragments from the photodissociation of HCl and HBr at 157 and 243 nm are observed to determine the symmetries of the photoexcited states by using a resonance enhanced multiphoton ionization technique. The phototransitions are perpendicular for HCl at 157 nm and 193 nm and for HBr at 157 nm and 243 nm. Isotope effects on the branching ratios of [Cl*(2P1/2)]/[Cl(2P3/2)] are observed for the photodissociation of HCl and DCl at 157 and 193 nm. Our results indicate that in the photodissociation processes of HCl (A 1Π) the dynamical curve crossings during the breakup of the molecule play important roles in the determination of the branching ratio.
Discretized path integral method and properties of a quantum system
Authors: Sethia Ashok , Sanyal Sudip , Singh Yashwant
Published: July 2002

Using the discretized path integral formalism we develop a numerically convenient and accurate method for evaluating the finite time propagator (and density matrix) of a given system. The eigenfunctions and energies of large number of states of the system are also found. We demonstrate the accuracy of the method by computing eigenvalues, eigenfunctions, position and flux autocorrelation functions and complex time propagators for a large number of systems and comparing these results with those available from other sources. In all the cases we find very good agreement. The method thus provides a very simple and accurate procedure for the study of static and dynamic properties of a quantum system.
Intersystem crossing in the [Fe(ptz)6](BF4)2 spin crossover system (ptz=1-propyltetrazole)
Authors: Hauser Andreas
Published: July 2002

[Fe(ptz)6](BF4)2 (ptz=1-propyltetrazole) is an Fe(II) spin crossover system, which shows a light-induced low-spin (1A1)→high-spin (5T2) conversion below ∼50 K by irradiating into the spin allowed 1A1→1T1 d–d absorption band. This phenomenon, known as light-induced excited spin state trapping (LIESST), is reversible, and a subsequent irradiation into the 5T2→5E band results in a light-induced 5T2→1A1 conversion (reverse LIESST). Single crystal absorption spectra of the title compound in the region of d–d transitions are reported. In addition to the well-established spin allowed 1A1→1T1 and 1A1→1T2 transitions of the low-spin species and the 5T2→5E transition of the high-spin species two weak bands in the NIR are assigned to the spin forbidden 1A1→3T1 and 1A1→3T2 transitions. Direct irradiation into the 1A1→3T1 absorption band at 20 K results in a quantitative 1A1→5T2 conversion, proving that this low lying triplet state plays an important role in the mechanism of LIESST. A full kinetic scheme for LIESST and reverse-LIESST with the 3T1 state as intermediate state is developed, and the quantum efficiencies for the various intersystem crossing steps involved are given: they are of the order of unity for the first step from the initially excited 1T1 and 5E states to the intermediate 3T1 state, respectively. The branching ratio from the 3T1 state to the 1A1 and the 5T2 states is 1:4.
‘‘Lakes–straits’’ model of field-inversion gel electrophoresis of DNA
Authors: Zimm Bruno H.
Published: July 2002

We replace the tube in the simple reptation model of the gel electrophoresis of DNA by a chain of open spaces, ‘‘lakes,’’ connected by narrow ‘‘straits.’’ We also allow loops of DNA to ‘‘overflow’’ out of the sides of the lakes under the pull of the electric field; a method of estimating the frequency of such overflows is developed based on Kramers’ theory of diffusion over a barrier. We study this model under both steady-field and inverting-field conditions. With small fields it explicitly gives an improved form of the simple-reptation formula, distinguishing between the contour length of the chain of lakes and the contour length of the DNA chain within them. With higher fields it is necessary to use computer simulation to integrate the equations of motion. For long chains the results show a very pronounced antiresonance, that is, a minimum, in the dependence of mobility on cycle period with cyclically reversing fields, in semiquantitative agreement with recent experiments. The antiresonance arises from the development of conformations shaped like the Greek letter lambda, Λ, with two arms both pulled in the direction of the field and high tension in the chain near the vertex. Under these conditions the chain moves very slowly, but when the field inverts the lambda appears as a V, and the high tension causes the chain to move rapidly toward the vertex. The antiresonance appears when the timing of the field cycle matches the time of lambda formation, so that the fast motion in the short, backward part of the cycle nearly cancels the slow motion in the long, forward part. The period of the antiresonance is proportional to the time needed for the chain to traverse its own length in steady field; the dimensionless proportionality constant appears to have a value of 0.4±0.1 both in our simulations and in experiments from the literature over a variety of conditions.
Nearest-neighbor distributions in fluid systems
Authors: Edgal Uduzei F.
Published: July 2002

Because nearest-neighbor probability distributions have been found to be useful for an exact investigation of the statistical thermodynamics of classical fluids, their exact form (valid for fluids in which many-body interaction potentials may obtain) has therefore been developed in the present paper. For an introductory familiarization with nearest-neighbor probability density functions (NNPDF’s) their connection with the commonly employed n-body distribution functions is first given. The usefulness of NNPDF’s in the analysis of structure in classical systems is also briefly discussed. A notable development associated with the paper is the fact that while ‘‘useful’’ exact expressions are not easily determined directly for n-body distribution functions (as evidenced by the several unsuccessful attempts to derive them in the literature), they are readily obtained directly for NNPDF’s (which are closely related functions) even where the ‘‘most general’’ interparticle interaction potentials may be said to obtain.
Stabilities and structures of gas phase MgO clusters
Authors: Ziemann Paul J. , Castleman A. W.
Published: July 2002

Gas phase (MgO)+n and (MgO)nMg+ clusters (n≤90) were produced in a gas aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. The abundance maxima observed in the mass spectra indicate that the clusters form compact cubic structures similar to pieces of the MgO crystal lattice. The abundance maxima of the metal-rich clusters show an interesting dependence on the ionization wavelength that appears to be due to different fragmentation pathways for the cluster ions and neutrals, and may be indicative of excess electron behavior analogous to that observed in solid state color centers. Calculations of cluster structures and stabilities made with an ionic model were useful in obtaining qualitative information about the primary fragmentation channels and cluster electronic properties, but also indicate that covalent bonding interactions must be included to obtain quantitatively accurate results.