bluuebunny/crossref_metadata_2025 huggingface dataset:
A hybrid model for vibrational energy transfer at the gas–solid interface: Discrete surface atoms plus a continuous elastic bulk
Authors: Vilallonga Eduardo , Rabitz Herschel
Published: July 2002

We introduce a discrete-continuum hybrid treatment of solid vibrations in order to describe the collisional excitation of adsorbate and defect modes by atom impacts. The inhomogeneous surface is represented by: (a) one or more atom clusters corresponding to the defect sites and their immediate neighbors, which are harmonically coupled to (b) an elastic continuous bulk. The model thus aims at reproducing the long-wavelength spectrum of the lattice as well as the high-frequency localized modes contributed by adsorbates and surface defects. The hybrid model is tested against lattice results in one-dimensional simulations that allow for analytic solution of the surface motion (which would be unfeasible for three-dimensional imperfect lattices); hybrid and lattice results are thus compared in detail under identical conditions. The model is also evaluated under the worst possible conditions for the continuum approximation, since collinear collisions correspond to three-dimensional situations in which the transferred momentum and, therefore, short-wavelength excitations are maximal. Comprehensive tests are presented for He atoms scattering from CO chemisorbed on Pt and on Ni substrates, and from N2 adsorbed on W. The scattering dynamics is treated by time-correlation functions of the transition operator previously developed for polyatomic targets [J. Chem. Phys. 84, 3162 (1987); 85, 2300 (1987); 86, 750 (1987)]. All the energetically open states are thus incorporated, as well as the thermal average over initial states, without need of basis-set expansions. Distributions of transferred energy are presented as would be observed in a one-dimensional scattering ‘‘experiment’’ using impact energies between 0.1 and 1 eV and with the solid at nonzero temperature. The collisional spectra obtained from the hybrid models are found to be in excellent agreement with those of the corresponding inhomogeneous lattices throughout wide ranges of impact velocity, surface initial temperature (including 0 K) and transferred energy. The results indicate that discrete-continuum treatments provide a powerful tool for analyzing the transfer of energy at the gas–adsorbate–solid interface.
On the thermodynamic basis of the affinity decay rate
Authors: Garcia-Colín L. S. , Piña E. , de la Selva S. M. T.
Published: July 2002

In the past five years exhaustive studies in chemical reactions have lead to an empirical equation describing how isothermal–isometric homogeneous reactions evolve towards equilibrium independently of their particular mechanism or rate law. Such an equation expresses the time rate of change of the chemical affinity as a linear function of the inverse of time. In this paper we show that by invoking the local equilibrium hypothesis one may provide, a time evolution equation for the chemical affinity that is uniquely given by the solution of the particular rate law of the reaction considered. Consequently such an equation is not of the same functional form for all reactions. On the other hand, integration of Dalton’s law under specific initial conditions, together with the local equilibrium assumption and the ideality requirement for the reacting species, exhibits a unique inverse time decay for the chemical affinity. This explains the good fitting of the inverse in time dependence of the chemical affinity with experimental data.
A crossed laser-molecular beam study of the photodissociation dynamics of Zn(C2H5)2 and (Zn(C2H5)2)2 at 248 and 193 nm
Authors: Hou Hui-qi , Zhang Zhuangjian , Ray Urmi , Vernon Matt
Published: July 2002

A molecular beam of zinc–diethyl (ZnEt2) is photodissociated at 248 and 193 nm and the velocity distributions of the photofragments are measured by time-of-flight techniques. One and two photon processes are observed. The dominant one photon process at both wavelengths is the dissociation of (ZnEt2)2 to form two ZnEt2 monomers. The absence of secondary dissociation of the ZnEt2 photofragments at both excitation wavelengths and the small fraction of the available energy partitioned to product translation implicates dissociation to an excited electronic potential energy surface correlating to one electronically excited ZnEt*2 monomer. The mass spectrum of the ZnEt2 photofragments is the same as measured for ‘‘cold’’ ZnEt2 monomers in the molecular beam, suggesting that the electronically excited ZnEt*2 monomers have fluoresced prior to ionization in the mass spectrometer. A small photodissociation signal of uncomplexed ZnEt2 is observed only at low expansion pressures. The sensitive dependence of this monomeric photodissociation signal to the Ar pressure of the adiabatic expansion suggests that ground state vibrational excitation is required for monomeric photodissociation at 248 nm.

In contrast to the dimer single photon photodissocation channel, when ZnEt2 monomers are photodissociated, a significant fraction of the available energy appears as product translation. A qualitative molecular orbital analysis can explain the observed fast photoproduct velocity if dissociation occurs via a repulsive triplet state which correlates to electronic ground state products. The two photon process observed is assigned to single photon photodissociation of the electronically excited ZnEt*2 monomers produced in the dimer photodissociation step. The photofragment velocity distributions for the two photon channel can be quantitatively modeled by sequential ethyl eliminations on the ground state ZnEt2 and ZnEt potential energy surfaces. The product velocity distributions are consistent with a microcanonical energy distribution for both ethyl eliminations. Approximately 50% of the ethyl photofragments are created with sufficient vibrational energy to break the weak ethyl C–H bond (36 kcal/mol) forming ethylene. Implications of the ZnEt2 photodissociation mechanism for Zn film deposition using 248 and 193 nm excimer radiation are discussed.
The determination of electronic dephasing rates in time-resolved quantum-beat spectroscopy
Authors: Walmsley I. A. , Tang C. L.
Published: July 2002

Time-resolved quantum-beat spectroscopy provides a simple method for determining the amplitude relaxation times of heavily damped vibrational modes in large molecules. We report a calculation which shows that the dephasing times of the electronic transitions involved in producing the beats may also be determined from a pump–probe experiment. The method involves detecting the phase of the quantum beats at several wavelengths within the probe pulse spectrum. It requires only the addition of a spectrometer to the experimental setup; it does not require a tunable wavelength pump or probe pulse.
X-ray diffraction and depolarization ratio of ν1 Raman line in liquid carbon tetrachloride
Authors: Hyodo Shi-aki , Tatsuta Narihito , Okamoto Tokuhiko
Published: July 2002

The x-ray diffraction and the depolarization ratio of the ν1 Raman line were measured in liquid carbon tetrachloride. The previously proposed liquid structure models for an x-ray diffraction pattern were found to be difficult to distinguish from each other. It has been reported that the relation between the local structure and its fluctuation corresponds to the depolarization ratio of the ν1 Raman line in liquid carbon tetrachloride [S. Hyodo and T. Fujiyama, Bull. Chem. Soc. Jpn. 53, 2456 (1980)]. The information obtained from this correspondence suggests that those models were clearly distinguishable. The contribution of the fluctuations of the molecular center and molecular orientation and that of the distortion from the temporarily assumed ideal lattice structure were also discussed to both the x-ray diffraction curve and the depolarization ratio.
Quantum statistical theory of damped, driven Morse oscillator: Bistable features and spectral characteristics
Authors: Ray Deb Shankar
Published: July 2002

A new quantum statistical formulation of a damped classically driven Morse oscillator is presented. The theory is based on the realization of the Morse oscillator in terms of generators of an SU(2) Lie algebra, which allows us to construct the spin coherent states for the Morse oscillator. The c-number equivalents of the master equation in the form of the Fokker–Planck and Langevin equations have been derived and solved in the mean field limit to demonstrate the existence of multiple steady states and the associated molecular bistability. The nonstationary solution derived under adiabatic elimination of relevant variable and secular approximation is also presented. Some spectral characteristics such as shift and linewidth due to phase fluctuations have been calculated.
Effect of relaxation of ionic atmosphere on the short-time dynamics of diffusion-controlled reaction
Authors: Ibuki K. , Nakahara M.
Published: July 2002

The effect of the relaxation of the ionic atmosphere on the rate of diffusion-controlled reaction at short times was investigated. The generalized diffusion equation, in which the time-dependent friction is taken into account, was solved numerically together with the radiation boundary condition to predict the short-time behavior of the rate constant of an encounter reaction between a neutral target molecule at rest and migrating ions. The time-dependent friction kernel for a moving ion was assumed to be given as the sum of the δ function one and the Debye–Falkenhagen one where the effect of the relaxation of the ionic atmosphere was taken into account. The resultant rate constant is larger at short times than that for the time-independent friction; this prediction is in qualitative agreement with experiment. Other types of friction kernel examined could not account for experiment; the rate constant for the Gaussian kernel is smaller at short times than that for the time-independent friction, while the exponential kernel behaves in the same manner as that of the time-independent friction at all times. When the intrinsic reaction constant at the reaction radius becomes small, the effect of the time-dependent friction vanishes.
New electronic states of NH and ND observed from 258 to 288 nm by resonance enhanced multiphoton ionization spectroscopy
Authors: Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

Three new electronic states of NH and ND (imidogen radical) have been observed by REMPI (resonance enhanced multiphoton ionization) spectroscopy in the region of 258 through 288 nm and assigned. The NH (ND) was produced by the photolysis of HN3 (DN3) (hydrazoic acid) in the same wavelength region. The observed two-photon transitions are from the a 1Δ state to 3p Rydberg states. Transitions were also observed from the a 1Δ state to the d 1Σ+ state. The new state assignments are: f 1Π (3pσ) at 86 378 cm−1, g 1Δ (3pπ) at 88 140 cm−1, and h 1Σ (3pπ) at 89 531 cm−1. Rotational constants (B and D) and, where possible, vibrational spacings for the thirteen observed bands are also determined.
The effect of collisions in the multiphoton decomposition of chloroform-d
Authors: McRae Glenn A. , Yamashita A. Brian , Goodale J. W.
Published: July 2002

The multiphoton decomposition (MPD) of chloroform-d, CDCl3, with a pulsed 13CO2 infrared laser is reported. A new method is developed that takes into account the effect of the number of pulses required to obtain a measurable decomposition. This method relies on difference equations and replaces current methods based on differential calculus. The rate equation language of the latter is shown to be inappropriate. The decomposition probability is measured at constant fluence up to a pressure of 2 kPa and empirical representations of the pressure dependence of the MPD are reported. A collision-dependent reaction scheme is presented to explain the pressure dependence of the up-pumping through the vibrational manifold.
Stochastic analysis of a driven chemical reaction
Authors: Leonard D. , Reichl L. E.
Published: July 2002

Floquet operator techniques are applied to a chemical reaction 2X→Y, to which X molecules are supplied periodically in time. The reaction is modeled as a death process with periodic immigration of the species X. We find the Floquet transition operator for this stochastic process, and we obtain exact expressions for the time-dependent probability distribution of the number of X molecules in the system, and the first and second moments of this distribution. The Floquet spectrum reveals the dynamics of the reaction process which is regular in this case. Numerical results for small systems are provided. The first moment agrees with the predictions of a kinetic model, while, as expected for small systems, fluctuations about the mean are significant.
Stoichiometric relations of complex chemical reaction network and stoichiometric number theory: A criterion for preserving overall stoichiometry in reduced mechanisms
Authors: Masuda Makihiko
Published: July 2002

The stoichiometric relations of complex reaction networks were discussed with the stoichiometric number theory primarily developed by Horiuti and Nakamura. In the present report, the theory was applied to the stoichiometries in the reaction network of the 11 pseudoelementary reactions of the Belousov–Zhabotinski (BZ) reaction and in a reduced mechanism of the BZ reaction called the amplified Oregonator. Both of the mechanisms were developed by Noyes et al. for analysis of the oscillatory phenomena of the BZ reaction. In the present report, the conclusion was derived from the stoichiometric number theory that the amplified Oregonator was sufficient to describe the stoichiometry of the reaction network of the 11 pseudoelementary reactions. In comparison of our analysis of the stoichiometries in the above two mechanisms, a criterion for the reduced mechanism is also introduced.
Ion dip spectroscopy of van der Waals clusters
Authors: Stanley R. J. , Castleman A. W.
Published: July 2002

We report the implementation of ion dip spectroscopy in a supersonic molecular beam time-of-flight mass spectrometer as a powerful mass-selective method for observing ground-state vibrational levels in van der Waals clusters. Ion dip spectra of phenylacetylene and phenylacetylene-NH3 are demonstrated in the range of 900–1100 cm−1, showing prominent dips at 978.0, 1002.8, and 1028.0 cm−1. These dips have been tentatively assigned as 1301 3511, 1101 3510, and 3512, respectively, in phenylacetylene. Shifts in the 3512 and 11013510 vibrational bands of the complex are observed while the 13013511 band of the complex is either shifted or attenuated.
Photoelectron spectroscopic studies of polyatomic molecules: Detection-integrated cross sections for ionization in fixed T
d systems
Authors: Chandra N.
Published: July 2002

We have, in this paper, derived expressions for the partial photocurrent produced by ionization in the electric dipole approximation in an orbital of an oriented polyatomic molecule. The cross-section formulas, which are integrated over all directions of ejection of the photoelectron but differential with respect to the direction of the fixed molecular axis, are obtained in their simplest possible forms by taking full account of the transformation properties of the point symmetry group of the target and are thus applicable to photoionization in any oriented molecule belonging to one of the 32 point groups. The theory, as an example, has been applied to photoionization in a1 orbital of those oriented nonlinear systems whose point symmetry group is Td. This application shows that the singly differential, detection-integrated partial cross section for ionization by unpolarized, linearly or circularly polarized light in a1 orbital of a fixed Td molecule is (i) independent of the direction of its axis and (ii) equal to that averaged over all orientations of the target in space. Both conclusions are in agreement with recent experimental measurements on photoionization in 6a21 orbital of fixed CCl4. These results, in turn, are shown to mean that the variations with respect to the orientation of a molecule in space, found in theoretical calculations of photoelectron angular distribution for ionization in a1 valence orbitals of some of the Td targets, are due completely to the terms which stem from freezing both the molecular axis and the photoelectron detector in space. These terms completely vanish on integrating over all directions of propagation of the photoelectron, resulting in a current which is isotropic with respect to the orientation of the molecular axis in space. In such cases, it is therefore necessary to study the angular distribution of electrons ejected by photoionization in oriented molecules in order to obtain cross sections which will change with the direction of the target axis.
Tricritical phenomena in quasibinary mixtures. VIII. Calculations from the van der Waals equation for binary mixtures
Authors: Pegg Ian L. , Knobler Charles M. , Scott Robert L.
Published: July 2002

The van der Waals equation for binary mixtures has been used to calculate thermodynamic properties in the region of a tricritical point. Three different models have been used: (A) molecules of the same size, but different energies of interaction, and zero interchange energy; (B) mixtures of chain molecules using the Flory entropy of mixing; and (C) mixtures of chain molecules using the Flory entropy of mixing and the Prigogine chain entropy parameter c. The results from all three models are qualitatively similar, but model C comes closest to reproducing the experimental three-phase coexistence curves of binary and quasibinary mixtures of ethane with higher hydrocarbons (C17 to C20). However, the calculated susceptibilities yield a Griffiths first sum ∑1 that differs substantially from the experimental results; it is finite and nonzero at the tricritical point, as the extended classical theory predicts, and changes only slightly with increasing chain length. This supports our view that the discrepancies between theory and experiment are not associated with being too far from the tricritical point, and the conclusion that, since amplitude ratios are expected to be nonclassical, the experimental ∑1 is in fact divergent.
Local structure and stability in a model glass
Authors: Tsumuraya K. , Watanabe M. S.
Published: July 2002

We study the relation between the potential energy of an atom and the shape of the Voronoi polyhedron around the atom in a sodium glass produced by the molecular dynamics method. The energy of an atom decreases with an increasing number of pentagon on the polyhedra, whereas it increases with increasing numbers of tetragon and hexagon. The central atom of the 13-atom icosahedral cluster is found to have the lowest potential energy among the abundant top 13 clusters which appear in the glass. We define the local pair-distribution function (PDF) around atoms with a specified shape of the polyhedron. The splitting of the second peak in the local PDF is the sharpest for the central atoms of the icosahedral cluster. The second peak splitting which appears in the average PDF is due to the existences of the icosahedral cluster and packing order of atoms around the cluster, i.e., the first peak comes from the first shell around the central atom of the icosahedral cluster, the second main peak from the second shell, and the second subpeak from the third shell.
Density functional theory of vacancies
Authors: McRae Robin , McCoy John D. , Haymet A. D. J.
Published: July 2002

The density functional (DF) theory of freezing is extended to predict the equilibrium concentration of vacancies in the crystal. The vacancy density in the crystal at phase coexistence is predicted to be approximately 5×10−5 for the Lennard-Jones model system near the triple point, which is comparable to concentrations measured in real systems such as argon and krypton. The dependence of the vacancy density on the temperature and pressure of the system is investigated using the new DF formalism. The concentration of vacancies in the hard sphere crystal at coexistence is also calculated, and it is found to be very small. The perfect crystal approximation, used earlier by us and others, is found to be very accurate for both the hard sphere and Lennard-Jones systems.
Refractive index of polarizable fluids: Numerical results from the mean spherical approximation solution
Authors: Ho/ye J. S. , Lomba E.
Published: July 2002

The numerical solution of the MSA (mean spherical approximation) for a fluid of polarizable particles interacting via the electromagnetic dipole radiation field is presented. The effects of this interaction are studied for molecules with polarizabilities both independent and dependent on frequency. Imaginary and real parts of the refractive index are evaluated, and their behaviors in connection with the absorption bands are determined. Finally, time-correlation and response functions have been evaluated.
Configurational relaxation in dense liquids based on sequentially correlated displacements. Origin and limitations of nonexponential decay
Authors: Mazur Stephen
Published: July 2002

A model for configurational relaxation in a hard sphere liquid is described in which it is assumed that individual atomic displacements are sequentially dependent upon the displacement of other atoms, these sequences originating with atoms adjacent to a small number of vacancies (packing defects) scattered randomly throughout the sample. This results in an inhomogeneous distribution of waiting times for displacement of atoms located at different distances from the vacancies. The inhomogeneity is formalized as a normalized distribution in s, the state of interference. Solution of the master equations yields a description of the redistribution of atoms among states at equilibrium. Configurational relaxation is represented by the autocorrelation for atoms neighboring vacancies. Relaxation exhibits a time regime characterized by nonexponential decay, followed by transition to a simple exponential decay at longer times. Persistence of the nonexponential regime increases with decreasing vacancy concentration. The change in decay law can be identified with a crossover from localized to global redistribution of the vacancies. Comparisons with empirical relaxation functions and implications for macroscopic properties are noted.
Dissociative excitation of GeH4 by collisions with Ar and He active species
Authors: Tsuji Masaharu , Kobarai Kazunari , Nishimura Yukio
Published: July 2002

The product channels in reactions of metastable Ar(3P2) and He(23S) atoms and Ar+, (Ar+)*, He+, and He+2 ions with GeH4 have been studied by observing emission spectra in the flowing afterglow and beam apparatus. H*, Ge*, GeH(A 2Δ–X 2Π), and GeH+(a 3Π0+,1–X 1Σ+) emissions were observed in the 190–600 nm region. The effect of SF6 addition into the discharge flow indicated that secondary electron-ion recombination processes participate in the formation of Ge* and GeH(A) in the Ar and He afterglows. The formation of Ge* through the Ar+/GeH4 reaction followed by electron-ion recombination processes was confirmed by simultaneous formation of Ar+ and low-energy electrons through the He(2 3S)/Ar Penning ionization. The total emission rate constants were determined to be 8.6 and 0.15×10−12 cm3 s−1 for Ge* and GeH(A) in the Ar(3P2)/GeH4 reaction, and 0.068 and 5.8×10−12 cm3 s−1 and for H* and Ge* in the He(2 3S)/GeH4 reaction, respectively.
State-to-state vibrational energy transfer in DF(v=1–3)
Authors: Robinson J. M. , Muyskens M. A. , Rensberger K. J. , Crim F. F.
Published: July 2002

Overtone vibration–laser double resonance studies of DF(v=1–3) energy transfer yield self-relaxation rate constants for v=1, 2 and 3 of k1=(0.37±0.06)×10−12 cm3 mol−1 s−1, k2=(22.0±2.0)×10−12 cm3 mol−1 s−1, and k3=(17.0±1.8)×10−12 cm3 mol−1 s−1, respectively. The approach also directly measures the relative importance of vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V–T,R) energy transfer. The fraction of DF(v) molecules relaxing by V–V energy transfer is 1.1±0.1 and 0.72±0.10 for v=2 and v=3, respectively. Essentially all of the vibrational energy transfer in v=2 occurs via the V–V mechanism. The slower relaxation of DF(v=3) compared to DF(v=2), in contrast to simple scaling law predictions, reflects the decreasing influence of the V–V mechanism, even though it is still the primary relaxation pathway for DF(v=3). Comparison with HF self-relaxation qualitatively indicates that V–R energy transfer is important in V–T,R relaxation of DF(v=1).
Simple models for heterogeneous catalysis with a poisoning transition
Authors: Meakin Paul
Published: July 2002

One- and two-dimensional simulations have been used to explore the effects of the Eley–Rideal process CO+OS→CO2+S on a simple model for the catalytic oxidation of carbon monoxide by oxygen. In this model, CO and O2 are fed to the surface (lattice) to form the adsorbed species COS and 2OS. The addition of CO is assumed to require just one vacant surface site (S), while the adsorption of O2 requires two adjacent (nearest neighbor) sites. The only parameter in this model (YCO) is the relative rate at which the two reactants are fed to the surface. For small values of YCO (YCOY2), the surface becomes completely covered by COS and the surface is poisoned. In an intermediate range of values of YCO (Y1<YCO<Y2), the surface is partially occupied by OS, COS, and S sites in the steady state regime and reaction can proceed indefinitely. In two dimensions, the main effect of adding the Eley–Rideal process is to remove the ‘‘continuous’’ transition at YCO=Y1 and shift the ‘‘discontinuous’’ transition at YCO=Y2. In the one-dimensional case, there is no reactive steady state regime. The Eley–Rideal process results in the formation of a new regime (YCO<YCOC) in which a continuous reaction can be sustained. Simulations have also been carried out using a model in which A and B species are added randomly to individual sites and ASBS pairs are eliminated randomly if they occupy adjacent sites. Addition of the asymmetric A+BS→S+P (product) Eley–Rideal process again leads to the formation of a new regime (YA<YAC) in which sustained reaction can occur under steady state conditions. For both models, the nature of the poisoning transition is investigated.
Reactant segregation in a Langmuir–Hinshelwood surface reaction
Authors: Kang H. C. , Weinberg W. H. , Deem M. W.
Published: July 2002

We have performed Monte Carlo simulations of a Langmuir–Hinshelwood reaction between two species A and B adsorbed on a square lattice, with the goal of determining how spatial correlations between the species vary with reaction rate. Adsorption of each species occurs when a gas-phase molecule, either A or B, impinges upon a vacant lattice site. The probability that a molecule impinges upon and adsorbs successfully into a vacant lattice site per unit time is pa/2 for both species. Desorption is not allowed and the surface reaction is allowed to occur only between nearest-neighbor AB pairs. For each nearest-neighbor AB pair, the probability of reaction per unit time is pr. A novel feature of this investigation is that we explicitly simulate the diffusion of the particles on the lattice. The particles are allowed to migrate by hopping to vacant nearest-neighbor sites, where the probability of a hop per unit time is pm. In all these simulations we have set pm to be unity, and varied pr from 0.01 to unity. We have also set pa=pr/5 for all the simulations in order to maintain moderately low fractional surface coverages. ‘‘Islanding’’ of each type of particle occurs even for the lowest value of pr used, although the entire surface is never poisoned. For range of values of pr used, the ‘‘islands’’ grow to a finite steady-state size. We also found that the islands that are formed are consistent with a dimension of two. A nearest-neighbor correlation function φ is defined to describe the process of islanding, and the dependence of φ upon pm/pr is studied. By studying this simple model we show that quite large inhomogeneities can be reasonably expected to occur in catalytic systems even when reaction probabilities are small compared to diffusion rates, and that these inhomogeneities affect total reaction rates.
A theoretical study of electron–hole pair formation due to the collision of an atom with a solid surface
Authors: Zeiri Y. , Kosloff R.
Published: July 2002

Electronic excitation in a semiconductor induced by the collision of energetic atoms with the solid surface is investigated theoretically. The modeling has been performed for a one-dimensional independent-electron system where the solid is described by a chain of 10–20 atoms. The time evolution of the nuclei (i.e., colliding atom and chain atoms) has been described by classical mechanics while quantum mechanical description has been used for the electronic dynamics. The two systems (i.e., the atoms and the electron) were coupled to each other and the equations of motion were solved self-consistently. Energy dissipation from the chain to the rest of the solid was included via the GLE approach. This study establishes the relationship between the probability of electron–hole formation and various parameters of the system such as collider translational energy, magnitude of the band gap, and existence of impurities in the solid. In addition, two excitation mechanisms were examined, electronic excitation due to a direct coupling between the electron and the colliding atom and an indirect mechanism due to electron–phonon coupling.
Structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of CH2FCH2, CHF2CH2, and CF3CH2 radicals: An a
b i
n
i
t
i
o study
Authors: Chen Yonghua , Rauk Arvi , Tschuikow-Roux E.
Published: July 2002

The equilibrium geometries, rotational barriers, and harmonic vibrational frequencies for β-fluoroethyl (CH2FCH2), β,β-difluoroethyl (CHF2CH2), and β,β,β-trifluoroethyl (CF3CH2) radicals have been determined by ab initio molecular-orbital techniques using the gaussian 86 system of programs at the unrestricted Hartree–Fock, UHF/6-31G* level of theory. Three conformational minima in the potential-energy surface were found for the CH2FCH2 and CHF2CH2 radicals, while only one was found for CF3CH2. The radical centers for all three species are nonplanar, but the effect on the geometries of these radicals in replacing β-position hydrogen by fluorine atoms is much weaker than is the case for α-position fluorine substituted ethyl radicals. Transition structures for internal rotation were also located. Estimates for the correction of electron correlation effects were obtained by single-point calculations using second-order Moller–Plesset perturbation theory (UMP2). The rotation about the C–C bond is almost free for all three radicals, which again is much different from the situation in the α-fluorine substituted ethyl radicals. Based on calculated vibrational frequencies and moments of inertia, thermodynamic properties including heat capacities, entropies, enthalpy, and free-energy functions are tabulated as a function of temperature. Several isodesmic–homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of the β-fluoroethyl and β, β-difluoroethyl radicals for which experimental values are not available. The theoretical heats of formation thus evaluated are −10.65 and −66.26 kcal/mol for CH2FCH2 and CHF2CH2, respectively. Together with the measured ΔH0f(CF3CH2), these data are used to evaluate ΔH0f,T, ΔG0f,T, and log Kf for all three radicals as a function of temperature.
Primary events following electron injection into water and adsorbed water layers
Authors: Barnett R. N. , Landman Uzi , Nitzan Abraham
Published: July 2002

The initial stages of the evolution of an electron injected into bulk water (at 300 K) and into thin water films (1–4 monolayers) adsorbed on a Pt(111) substrate at 50 K are investigated. It is shown that for electrons injected into bulk water with an initial translational kinetic energy between 1.54 and 6.18 eV (i.e., subexcitation energies), the electron momentum time-correlation function 〈p̂(0)p̂(t)〉, decays to zero on a time scale of less than 1 fs, reflecting strong backscattering of the electron by the water molecules. On this time scale the electron propagation in the medium is dominated by elastic processes. Furthermore, during this initial stage the system is well represented by a static aqueous medium. Transmission of electrons injected into thin films of adsorbed water is also dominated by elastic scattering. The dependence of the electron transmission probability on the film thickness and the initial injection energy are in accord with recent experimental results of photoinjected electrons into adsorbed water films.
Rotational motion of SF6 inside small pores of silica glass
Authors: Nikiel L. , Zerda T. W.
Published: July 2002

Raman spectra of the ν2 and ν5 vibrational modes of SF6 confined within pores of silica gels of average diameter 12 Å are recorded at pressures varied between 84 and 1827 bars and at temperatures 296, 353, and 398 K. The experimental correlation functions for SF6 inside the pores are compared with the theoretical functions obtained from the extended diffusion model and the Fokker–Planck–Langevin model in order to find relaxation times for the angular momentum. It is shown that although experimental correlation functions of SF6 for neat liquid and inside the pores exhibit similar time and density dependence, the mechanisms of reorientational relaxation are different. At low densities molecules collide mainly with the walls and it is suggested that those collisions are less effective in changing the angular momentum.
Fluorescence study of trivalent americium in fluorozirconate glass
Authors: Valenzuela Raúl W. , Brundage R. T.
Published: July 2002

The fluorescence spectrum of tripositive americium in a fluorozirconate glass at low temperature has been measured. Six fluorescence lines were observed between 515 to 880 nm and identified as Am3+ transitions. Fluorescence decay measurements at 15 K are reported for transitions that originate from states 5D1′ (∼1.0 ms) and 5L6′ (∼17 μs). The 5D1′ state was excited directly to produce fluorescence in order to confirm the assignment of the excited state.
Path integral study of the correlated electronic states of Na4–Na6
Authors: Hall Randall W.
Published: July 2002

Feynman’s path integral formulation of quantum mechanics is used to study the correlated electronic states of Na4–Na6. Two types of simulations are performed: in the first, the nuclei are allowed to move at finite temperature in order to find the most stable geometries. In agreement with previous calculations, we find that planar structures are the most stable and that there is significant vibrational amplitude at finite temperatures, indicating that the Born–Oppenheimer surface is relatively flat. In the second type of simulation, the nuclei are held fixed at symmetric and asymmetric geometries and the correlated electron density is found. Our results show that the electrons are localized, rather than delocalized as previous workers have concluded from examination of the single-particle orbitals. We find that the best picture of these clusters is that they contain three-center, two-electron bonds.
A four component lattice model for nonaqueous microemulsions made with nonionic surfactants
Authors: Martino A. , Schick M. , Kaler E. W.
Published: July 2002

A simple four component lattice model is used to reproduce qualitatively phase behavior of nonaqueous microemulsions. In microemulsions where glycerol and propylene glycol replace water, 2_-3-2̄ phase transitions are observed as the glycerol content in the polar organic mixture increases. The model predicts such phase behavior and reproduces the effect of oil solvophobicity and the surfactant solvophilic–lipophilic balance. In addition, temperature insensitive phase behavior is observed in these nonaqueous mixtures and is predicted in the model. The results are used to propose possible molecular interaction mechanisms which would account for the microemulsion phase behavior. The model is compared with a similar model developed for water microemulsions and the differences are cast in light of the different molecular interactions which occur between propylene glycol, water, and nonionic surfactants.
The ionization energy of SF5 and the SF5–F bond dissociation energy
Authors: Sieck L. Wayne , Ausloos Pierre J.
Published: July 2002

The techniques of Fourier transform spectroscopy and high pressure mass spectrometry have been used to investigate the ionization energy (IP) of SF5 via electron exchange reactions. An IP of 9.60±0.05 eV has been derived from this data, which is in disagreement with estimates from flowing afterglow measurements (IP≥10.15 eV). In addition, no evidence has been found for a fluoride transfer equilibrium involving CF+3 and SF6, which was thought to interrelate the heats of formation of CF+3 and SF+5. Our results are consistent with the recommended (SF5–F) bond strength of ∼4.0 eV, and suggest an energy threshold for SF+5 production from SF6 of 13.6 eV.
Chemiluminescent reactions of group IV A atoms with PCl5 and SnCl4
Authors: Wang J. H. , Cheong B. S. , Fang C. C. , Parson J. M.
Published: July 2002

Earlier work on chemiluminescent (CL) reactions of group IV A atoms with halogens has been extended to two chlorine-rich molecules PCl5 and SnCl4. The CL products in these reactions were found to be group IV A dihalides formed in a one step mechanism instead of the two step mechanism proposed in the study of the reactions involving Br2, I2, and ICl. For PCl5, the CL products were found to be group IV A dichlorides, while for SnCl4, the CL products are believed to be SnCl2 for all three atoms. The overall absolute cross sections for the reactions have been estimated, as well as the relative cross sections as functions of collisional energy. The observation of different collisional energy dependence for the reactions of PCl5 and SnCl4, along with other evidence, suggests that two different mechanisms produce the CL products in the two groups of reactions.
The multiplet state distribution of O(3P
J) produced in the 193 nm photodissociation of SO2
Authors: Huang Yu-Lin , Gordon Robert J.
Published: July 2002

The multiplet state distribution of oxygen atoms produced in the photodissociation of SO2 was measured in a pump-and-probe experiment. An ArF laser was used to dissociate the parent molecule, while the O(3P) product was detected by two-photon laser-induced fluorescence under collisionless conditions. The multiplet populations were found to be 59.2%±5.0%, 29.5%±3.2%, and 11.4%±3.3% for J=2, 1, and 0, respectively. These results are indistinguishable from the statistical distribution of 5:3:1.
Molecular dynamics studies on molten alkali hydroxides. II. Rotational and translational motions of ions in molten LiOH
Authors: Okazaki Susumu , Ohtori Norikazu , Okada Isao
Published: July 2002

Dynamics of ions in molten LiOH has been investigated by molecular dynamics simulation analyzing the trajectories of the particles. Rotational autocorrelation function of the OH− ion shows rapid and large oscillatory decay at small t as well as rather slow and exponential decay at large t. The function agrees well with that obtained by polarized and depolarized Raman scattering measurements. The former decay was caused by libration of the OH− ion. The latter is based on the reorientation overlapping the relatively frequent libration. It is also found that the exponential decay has a strong correlation with the structural relaxation of the liquid. The OH− ion cannot rotate without any breakdown of the structure of the surrounding ions. Significantly oscillatory translational motions have been demonstrated in terms of various one-particle and ensemble averaged autocorrelation functions. The spectrum analysis elucidated that there exists a strong correlation between the translational motion of the Li+ ion and the rotational and translational motion of the OH− ion. This is in contrast to the case of LiNO3 melt, where Li+ and NO3− ions appear to move independently, at least for the short time oscillatory motion.
Bootstrap approach to fitting spectra of molecules with classically chaotic dynamics
Authors: Standard Jean M. , Lynch Eric D. , Kellman Michael E.
Published: July 2002

The detailed level-by-level fitting of spectra of systems with strongly chaotic classical dynamics is investigated. Fitting of these ‘‘unassignable chaotic spectra’’ is advocated as a desirable counterpart to statistical analysis, giving a detailed probe of molecular potential surfaces and intramolecular dynamics, including phase-space ‘‘bottlenecks.’’ The role of ‘‘assigning’’ levels in a spectral fit is discussed. It is concluded that what is really necessary for fitting is to ‘‘label’’ the spectrum, in a sense made precise. There is no barrier, in principle, to fitting chaotic spectra that may be unassignable in terms of the usual zero-order quantum numbers such as normal modes. The difficulties introduced into the labeling process by experimental factors of spectral resolution and sensitivity are considered. The occurrence of missing levels or features is identified as the key labeling problem. A ‘‘bootstrap’’ method is introduced to label and fit a spectrum with missing levels. The ‘‘bootstrap’’ method works by using the knowledge obtained at each energy range of the fit to successively label and fit higher energies. The bootstrap method is tested on simulated experimental spectra. A program called fitter uses the bootstrap procedure successfully to identify missing levels, label and fit the spectrum, [AV:and reproduce the molecular potential. The occurrence of doublets in highly excited spectra is noted as possible spectral evidence of a bottleneck giving rise to chaotic local modes. Fitting of large-scale ‘‘clump’’ features is discussed as a means to probe energy flow dynamics and potentials of ‘‘doorway’’ modes of molecules. Fitting of features or individual levels on successively finer energy scales is discussed as a probe of subsequent energy flow dynamics through ‘‘interior’’ modes.
The reaction of atomic fluorine with formyl fluoride: An experimental and theoretical study
Authors: Francisco J. S. , Zhao Y.
Published: July 2002

The rate coefficient for the reaction of F+HFCO has been determined over the temperature range 298–368 K by monitoring the appearance rate of HF infrared chemiluminescence using the infrared multiphoton dissociation-infrared chemiluminescence technique. Results, expressed in the Arrhenius form k(T) =4.4±2.6×10−11 exp[−1800±400/RT] cm3 molecule−1 s−1, are compared with the barrier height calculated using ab initio molecular orbital theory. The rate coefficient is theoretically discussed with the aid of essential features of the potential energy surface for the F+HFCO system and the potential importance of the reaction is considered. A rate coefficient for deactivation of excited HF by HFCO is reported as 1.2±0.2×10−12 cm3 molecule−1 s−1.
The fine and magnetic hyperfine structure of 87SrF in its X 2Σ+ state
Authors: Azuma Y. , Childs W. J. , Goodman G. L. , Steimle T. C.
Published: July 2002

A molecular beam rf–optical double resonance experiment was performed on 87SrF in its naturally occurring abundance ratio. The natural occurring abundances of the strontium isotopes are 86Sr (9.8%), 87Sr(7.02%) and 88Sr (82.5%). Numerous magnetic dipole allowed transitions between ρ-doublets in the X 2Σ+ state were measured to an accuracy of 3 kHz. The observed spectra were analyzed in terms of an effective Hamiltonian which includes the magnetic hyperfine and electric quadrupole interactions arising from the 87 Sr (I=9/2) and 19F (I=1/2) nuclei. The extracted spectroscopic hyperfine parameters were interpreted in terms of a simple molecular orbital picture for the electronic nature of the X 2Σ+ state. A comparison is made to previous results for the more abundant 86SrF and 88SrF isotopic forms.
Longwave properties of the orientation averaged Mueller scattering matrix for particles of arbitrary shape. I. Dependence on wavelength and scattering angle
Authors: Shi Yaoming , McClain W. M.
Published: July 2002

Starting from a dipole array theory of elastic light scattering by particles of arbitrary shape and composition, we develop long-wave asymptotic formulas for all sixteen elements of the Mueller scattering matrix, valid after orientation averaging. The six dipole elements have long been known to be asymptotically proportional to k4 where k is the wave vector length of the light. However, the remaining ten nondipole elements experience intricate cancellations within the averaging operation that cause the asymptotic behavior to go like k5, k8, or k9; or at certain angles, like k6. For example, we show that (M34)=Ck9 sin2(θ), where C is a constant dependent on particle properties, and θ is the scattering angle. We find analogous simple angular basis functions for all 16 of the formulas and we show that they agree with numerical results from the exact algorithm. We contrast dipole array theory with central multipole expansion theory, which fails to account correctly for six of the Mueller elements, no matter how many multipoles are used.
Collisional dynamics of the BrCl B 3Π(O+) state. I. Electronic quenching
Authors: Perram Glen P. , Davis Steven J.
Published: July 2002

Electronic quenching of BrCl B 3Π(O+) was studied in pulsed and steady-state laser induced fluorescence experiments. BrCl(B) deactivation rate coefficients were determined as a function of vibrational quantum number for the following collision partners: He, Ne, Ar, Kr, N2, O2, and Cl2. Quenching from a vibrationally thermalized, 300 K BrCl(B,v′) distribution by the noble gases N2, and Cl2 was slow with all rate coefficients less than 4.3±0.7×10−13 cm3 molecule−1 s−1 . The quenching by O2 was much faster with a rate coefficient of 6.5±0.6×10−12 cm3 molecule−1 s−1 . The quenching of BrCl(B), under single collision conditions, depends strongly on vibrational quantum number of BrCl(B), with rate coefficients ranging from 4.3±0.1×10−13 cm3 molecule−1 s−1 for v′=0 to 1.4±0.3×10−10 cm3 molecule−1 s −1 for v′=6 under collisions with Cl2 and with rate coefficients ranging from 1.5±0.3×10−13 cm3 molecule−1 s−1 for v′=0 to 1.0± 0.4×10−10 cm3 molecule−1 s−1 for v′=6 under collisions with He. Several mechanisms for the electronic deactivation of BrCl(B) are discussed. The radiative lifetime of BrCl(B) was determined to be 38.7±1.7 μs, independent of vibrational level for v′≤6.
Cation dependence of the vibrational and rotational relaxation of OH− ion in molten MOH(M=Li, Na, K, Rb, and Cs) by Raman scattering measurements
Authors: Ohtori Norikazu , Okazaki Susumu , Okada Isao
Published: July 2002

Polarized and depolarized Raman scattering measurements have been performed for the stretching vibrational mode of OH− ion in molten MOH (M= Li, Na, K, Rb, and Cs). Vibrational and rotational autocorrelation functions, Cv(t) and Cr(t), have been evaluated from Fourier transformation of the spectra. The relaxation rate of Cv(t) becomes more rapid and the asymmetry of the isotropic band becomes larger as the cation size becomes smaller. These results can be interpreted in terms of the coulombic interaction. Applying the two-parameters model based on the Kubo line shape theory to the observed spectra, it is suggested that the OH− ion apparently follows the fast modulation regime due to the libration of OH− ion. For all melts, the Cr(t) has oscillatory character caused by libration of the OH− ion at small t <0.1 ps. The function shows ordinary exponential decay at large t≳0.1 ps. The cation dependence of the libration at t< 0.1 ps and the rotational diffusion process at t≳0.1 ps were investigated separately. The larger the cation size becomes, the longer and larger the period and amplitude of the libration become, respectively. This indicates that in the libration process the OH− ion seems to have more extended space in the melt as the cation size becomes larger. With respect to the rotational diffusion process, the rate shows rather small cation dependence.