bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic phase transitions in a surface reaction model with local interactions
Authors: Bagnoli F. , Sente B. , Dumont Martine , Dagonnier R.
Published: July 2002

The steady-state properties of the CO oxidation on a catalyst surface are investigated by Monte Carlo simulations. We extend the irreversible model of Ziff, Gulari, and Barshad [Phys. Rev. Lett. 56, 2553 (1986)] by means of adsorption probabilities depending on the instantaneous local coverages. We almost follow Kaukonen and Nieminen [J. Chem. Phys. 91, 4380 (1989)] simulation procedure. It results in a qualitative change in the original phase diagram with, especially, the vanishing of the second order phase transition of the adsorbate. We also show how the addition of the diffusion of CO in our simulations shifts the transition points obtained so far.
Temporary anion states in the chloromethanes and in monochloroalkanes
Authors: Guerra Maurizio , Jones Derek , Distefano Giuseppe , Scagnolari Francesco , Modelli Alberto
Published: July 2002

The resonance energies for electron attachment to the chloromethanes are evaluated by means of bound and continuum multiple scattering Xα calculations. The results closely reproduce the experimental electron transmission spectroscopy data and confirm their previous assignment. Electron transmission and dissociative attachment spectra of monochloroalkanes are also reported, in order to obtain information on the effects of branching at the substituted carbon atom and of alkyl chain length on the resonance and chlorine anion production energies.
Electronic excitation transfer in clustered chromophore systems: Calculation of time-resolved observables for intercluster transfer
Authors: Marcus A. H. , Fayer M. D.
Published: July 2002

A theoretical description is given for electronic excitation transport among interacting clusters of chromophores. Each cluster is a finite volume system with a limited number of chromophores. At high cluster concentration, intercluster transfer will become significant. The theory is based on a first-order cumulant approximation of the solution to the transport master equation. Gs(t) the probability of finding the excitation on the initially excited chromophore is calculated. The problem is first solved for two clusters at fixed separations. This result is extended to many clusters and then to the thermodynamic limit of an infinite number of clusters in an infinite volume. An example calculation is performed of excitation transport among chromophores on the surfaces of interacting micelles. For realistic parameters characterizing the system octadecylrhodamine B (chromophores) in Triton X-100 micelles, it is found that intermicelle excitation transfer can compete with intramicelle transfer. For an isolated micelle–chromophore system (chromophores on the surface of a sphere), a new time domain expression for Gs(t) is obtained.
Chemiluminescent reactions of electronically excited alkaline earth atoms. II. Energy dependence in Ba*+O2→BaO*+O
Authors: Visticot J. P. , Alcaraz C. , Berlande J. , Cuvellier J. , Gustavsson T. , Mestdagh J. M. , Meynadier P. , de Pujo P. , Sublemontier O.
Published: July 2002

The reaction of Ba(6s 6p1P1) with O2 has been investigated in a crossed beam experiment as a function of the collision energy. The electronically excited product BaO is not statistically populated and the difference from statistics increases with the collision energy. Such a result is markedly in contrast with what was observed in the reactions of Ba(6s21S0) and Ba(6s5d1D2) with O2 where the existence of a long-lived complex results in a statistical distribution of internal energy of the BaO product. The present result is interpreted as due to the collisional system following a different channel involving excited electronic states of the intermediate, BaO2 above the ionization limit, instead of the strongly bound states of barium peroxide which are involved in reactions of less excited Ba states.
Adsorbate effects on photoemission from Ag
Authors: Kim C.-W. , Villagrán J. C. , Even U. , Thompson J. C.
Published: July 2002

We have used pyridine and a number of related molecules to lower the workfunction of thin Ag films so as to compare the effects of various molecular properties on the photoemission process and on surface electronic modes. We find that a nitrogen lone pair electronic orbital is essential for a large threshold shift. Other molecular properties, such as dipole moment and polarizability have a very small effect. The films are rough, therefore we see strong photoyields at the energy of the surface plasma mode, 3.6 eV. There is also a peak in the photoyield at 3.85 eV, the energy of the bulk plasmon.
Static and dynamic properties of a new lattice model of polypeptide chains
Authors: Kolinski Andrzej , Milik Mariusz , Skolnick Jeffrey
Published: July 2002

The equilibrium and dynamic properties of a new lattice model of proteins are explored in the athermal limit. In this model, consecutive α-carbons of the model polypeptide are connected by vectors of the type (±2,±1,0). In all cases, the chains have a finite backbone thickness which is close to that present in real proteins. Three different polypeptides are examined: polyglycine, polyalanine, and polyleucine. In the latter two cases, the side chains (whose conformations are extracted from known protein crystal structures) are included. For the equilibrium chain dimensions, with increasing side chain bulkiness, the effective chain length is smaller. The calculations suggest that these model polypeptides are in the same universality class as other polymer models. One surprising result is that although polyalanine and polyleucine have chiral sidechains, they do not induce a corresponding handedness of the main chain. For both polyleucine and polyalanine, the scaling of the self-diffusion constant and the terminal relaxation time are consistent with Rouse dynamics of excluded volume chains. Polyglycine exhibits a slightly stronger chain length dependence for these properties. This results from a finite length effect due to moderately long lived, local self-entanglements arising from the thin effective cross section of the chain backbone.
Dissociative excitation of CH4 by collisions with helium active species
Authors: Tsuji Masaharu , Kobarai Kazunari , Obase Hiroshi , Kouno Hiroyuki , Nishimura Yukio
Published: July 2002

Dissociative excitation of CH4 by collisions with He(2 3S), He+, and He+2 has been studied by observing CH(A 2Δ–X 2Πr, B 2Σ−–X 2Πr, and C 2Σ+–X 2Πr) and H (Balmer) emissions in the flowing afterglow and beam apparatus. The effect of SF6 addition into the He afterglow indicated that the formation of CH(A,B,C) in the flowing afterglow proceeds through both the primary He(2 3S)/CH4 reaction and a secondary electron–ion recombination reaction through the He+/CH4 and/or He+2/CH4 reactions. The emission rate constants of A–X, B–X, and C–X of CH, Hα, and Hβ produced from the He(2 3S)/CH4 reaction were determined to be 5.6, 1.5, 0.011, 0.46, and 0.072×10−13 cm3 s−1 in the beam experiment, respectively. The nascent vibrational distribution of CH(A) from He(2 3S)/CH4 was determined as N0:N1=100:37±5. The rotational distributions were expressed by single Boltzmann temperatures of 3200±200 and 2600±300 K for v′=0,1 of CH(A) and 3300±200 K for v′=0 of CH(B).
Time correlation function approach to vibrational energy relaxation in liquids: Revised results for monatomic solvents and a comparison with the isolated binary collision model
Authors: Adelman S. A. , Muralidhar R. , Stote R. H.
Published: July 2002

A refined version of a molecular theory of liquid phase vibrational energy relaxation (VER) [S. A. Adelman and R. H. Stote, J. Chem. Phys. 88, 4397, 4415 (1988)] is presented and compared to the isolated binary collision (IBC) model. The theory is based on the Gaussian model for the fluctuating force autocorrelation function of the solute vibrational coordinate. Within the Gaussian model, the VER rate constant may be constructed in terms of solute–solvent site–site potential energy and equilibrium pair correlation functions. In the present refined treatment, crossfrictional contributions to the fluctuating force autocorrelation function are retained and its initial value 〈F̃2〉0 is evaluated from an exact rather than an approximate formula. Applications of the theory are made to model Lennard-Jones systems designed to simulate molecular iodine dissolved in liquid xenon at T=298 K and molecular bromine dissolved in liquid argon at T=295 K and T=1500 K. The refinements, along with an improved polynomial fitting procedure for the solute–solvent pair correlation functions, are found to yield significant changes in both the absolute VER rates and in their isothermal density dependencies.

Moreover, it is found for all three solutions that the Gaussian decay time is nearly independent of density from ideal gas to the dense fluid regimes. This condition is sufficient for the emergence of an IBC-like factorization of the VER rate constant kliq(T) into a liquid phase structural contribution proportional to 〈F̃2〉0 and a dynamical contribution which is nearly density independent. The liquid structural contribution is, in general, not well-approximated by a contact collisional assumption but rather depends on a range of solute–solvent interatomic separations. For the Br2/Ar solutions, the rate constant isotherms show a superlinear deviation from the low density extrapolation kliq(T)≂ρ0kgas(T) which is qualitatively similar to that observed for a number of cryogenic and pressurized fluids. For the I2/Xe solution a qualitatively different sublinear rate isotherm is found. This ‘‘nonclassical’’ isotherm is correlated with the nonmonotonic density dependence of the magnitudes of the solute–solvent pair correlation function in the region important for the determination of 〈F̃2〉0 found for the I2/Xe system.
Theoretical approaches to reversible diffusion-influenced reactions: Monomer–excimer kinetics
Authors: Szabo Attila
Published: July 2002

Three formalisms that describe the influence of diffusion on the kinetics of the reversible reaction, A+B⇌AB, are discussed and compared. The simplest involves a modification of the irreversible rate equations of Smoluchowski theory; the second is based on a generalization of physically appealing convolution relations that hold rigorously for reversible reactions between isolated pairs, and the third can be obtained by using a superposition approximation to truncate the hierarchy of equations satisfied by the reactive reduced distribution functions. The various formalisms are developed to the point that their implementation requires knowledge only of the time-dependent irreversible association rate coefficient and the microscopic dissociation rate constant. All these approaches give the correct equilibrium concentrations at infinite time, have the same short-time behavior, reduce correctly when the dissociation rate is zero, and become equivalent in the reaction-controlled limit. However, none of them provides an exact treatment of the underlying many-particle diffusive model of the reaction. Some illustrative calculations are presented and the relative merits of these approaches are discussed. All three approaches predict that the relaxation of a small initial deviation of the concentrations from their equilibrium values is nonexponential, except, of course, in the reaction-controlled limit. With a view towards treating monomer–excimer kinetics, the formalisms are generalized to incorporate unimolecular decay pathways.
Millimeter-wave spectrum of NCS radical in the ground 2Π state
Authors: Amano Takayoshi , Amano Takako
Published: July 2002

The rotational transitions of NCS have been observed in the 1 to 3 mm wavelength region in a hollow cathode discharge in a gas mixture of CS2 (∼1 mTorr) and N2 (∼30 mTorr). The molecular constants in the ground state are determined by fitting the observed frequencies to the standard Hamiltonian for 2Π states. The accuracy of the molecular constants are greatly improved compared with the values obtained from the optical data. The lines in the excited vibronic states have also been observed. The analysis including those excited state lines is in progress and the results will be published separately.
A possible molecular structural indicator of the liquid–glass transition
Authors: Kivelson Daniel , Steffen Werner , Meier Gerhard , Patkowski Adam
Published: July 2002

In studying the VH depolarized Rayleigh light-scattering spectrum of orthoterphenyl we have separated an ‘‘intermediate’’ line with almost temperature-independent halfwidth of 50 GHz from both the narrower rotational line with strongly temperature-dependent halfwidth and the broader base line associated with short-range overlap interactions. The integrated intensity IVH of the intermediate line decreases by a factor of 25 as the temperature decreases over a range for which the viscosity increases by 11 orders of magnitude. It appears that the extrapolated value of (IVH/ρ2)1/2 for this line vanishes as T→T0, where ρ is the density and T0 is the ideal liquid–glass transition temperature established by fitting a Vogel–Fulcher relation to the viscosity. We associate this intermediate line with dipole–induced-dipole interactions; its intensity is then given by an equal-time correlation function involving two-, three-, and four-body interactions. Because dipole interactions are quite long range, this correlation function may be a good probe of local molecular order, and IVH may thus be a good molecular structural (thermodynamic) indicator (order parameter) of the liquid–glass transition, the first such indicator identified.
Direct observation of silicon (3P) following state-selected photofragmentation of Ã 1B1 silylene
Authors: McKay Ruth I. , Uichanco Angelo S. , Bradley Andrew J. , Holdsworth John R. , Francisco Joseph S. , Steinfeld Jeffrey I. , Knight Alan E. W.
Published: July 2002

The photophysics of silylene (SiH2), formed during the infrared multiphoton dissociation (IRMPD) of organosilanes, is investigated using photofragmentation excitation spectroscopy (PHOFEX). Silylene molecules are formed in the X̃ 1A1(000) ground state via IRMPD of n-butylsilane. Laser induced fluorescence (LIF) is used to detect ground state (3p2 3P0) Si atoms following rovibronically resolved photoexcitation of SiH2 to the Ã 1B1(0v20) state. Variations in Si atom production are measured simultaneously with the SiH2 excitation spectrum, allowing comparisons to be made between Si yield and the rovibronic structure in the SiH2 1B1 manifold. We have examined the correlation between the widely varying fluorescence lifetimes of the individual rovibronic states of SiH2 and the relative yields of Si production. The presence of additional Si precursors in the primary dissociation process is suggested. Mechanisms for Si release following IRMPD of n-butylsilane and electronic excitation of SiH2 are developed and discussed.
Diffusion-influenced excimer formation kinetics
Authors: Berberan-Santos M. N. , Martinho J. M. G.
Published: July 2002

A model for reversible monomer–excimer kinetics is developed, taking into account that different distributions of monomers around the excited ones are created by light absorption and excimer dissociation. The excimer formation rate coefficient departs from the Collins–Kimball equation owing to reversibility, originating significant deviations to Birks’ kinetics in the monomer and excimer decays. The contribution of the geminate pair created by excimer dissociation on the overall kinetics is significant for low monomer concentrations and high viscosities. Simultaneous analysis of the monomer and excimer decay curves according to the model developed should allow to extract all the relevant information concerning the excimer formation diffusion controlled process.
Microwave and submillimeter spectroscopy of Ar–NH3 states correlating with Ar+NH3 (j=1,‖k‖=1)
Authors: Zwart E. , Linnartz H. , Meerts W. Leo , Fraser G. T. , Nelson D. D. , Klemperer W.
Published: July 2002

Microwave and submillimeter transitions for Ar–NH3 have been observed and assigned for the ∑ and Π states correlating asymptotically with Ar+NH3 (j=1,‖k‖=1). The ∑ states are found to lie below the Π states and are separated by approximately the inversion splitting of free NH3. For the Π states the NH3 inversion tunneling is nearly quenched, being only weakly allowed through Coriolis interactions with the nearby ∑ states. The observed microwave and submillimeter spectra also allow the determination of 14N quadrupole coupling constants and relative submillimeter absorption intensities. All the above results are interpreted using a model internal–rotation inversion Hamiltonian, leading to detailed information about the anisotropy of the intermolecular potential.
Vacuum ultraviolet photodissociation and photoionization studies of CH3SH and SH
Authors: Nourbakhsh S. , Norwood K. , Yin H.-M. , Liao C.-L. , Ng C. Y.
Published: July 2002

The kinetic energy releases of the photodissociation processes, CH3SH+hν (193 nm)→CH3+SH, CH3S+H, and CH2S+H2, have been measured using the time-of-flight mass spectrometric method. These measurements allow the direct determination of the dissociation energies for the CH3–SH and CH3S–H bonds at 0 K as 72.4±1.5 and 90±2 kcal/mol, respectively. The further dissociation of SH according to the process SH+hν (193 nm)→S+H has also been observed. The appearance energy (AE) of S produced in the latter process is consistent with the formation of S(3P)+H. The photoelectron–photoion coincidence (PEPICO) spectra for CH3SH+, CH3S+ (or CH2SH+), and CH2S+ from CH3SH have been measured in the wavelength range of 925–1460 Å. The PEPICO measurements make possible the construction of the breakdown diagram for the unimolecular decomposition of internal-energy-selected CH3SH+ in the range of 0–83 kcal/mol. The AE measured for CH2S+ is consistent with the conclusion that the activation energy is negligible for 1,2-H2 elimination from CH3SH+.
Formulation of N- and v-representable density functional theory. VIII. Relationship between the density-driven and the local-scaling versions
Authors: Kryachko Eugene S. , Ludeña Eduardo V.
Published: July 2002

It is shown that Cioslowski’s density-driven construction of one-particle orbitals corresponds to a finite basis representation of local-scaling transformations. Implications of this correspondence, with respect to the formulation and implementation of a variational principle for the density, are discussed.
Structure of a colloidal suspension confined in a planar slit
Authors: González-Mozuelos P. , Alejandre J. , Medina-Noyola M.
Published: July 2002

In this paper we present a simple theoretical scheme to calculate the inhomogeneous structure of an aqueous monodisperse suspension of highly charged spherical particles confined between two parallel charged walls. The theoretical model is based on the Derjaguin–Landau–Verwey–Overbeek level of description for the particle–particle and wall–particle interactions. The main features of the theoretically predicted local-concentration profile are found to be in good agreement with our Monte Carlo simulations for the same model. In particular, in the limiting case of large wall-to-wall separation, the predictions of previous work, concerning the structure of a suspension in the neighborhood of a highly repulsive wall, are found to agree with our simulation results.
On the derivation of the Stillinger–Lovett moment conditions for neutral plasmas
Authors: Altenberger Andrzej R. , Dahler John S.
Published: July 2002

A new method of derivation of the moment conditions is presented and additional moment conditions are discovered.
Comparisons of statistical and nonstatistical behavior for bond fission reactions in 1,2-difluoroethane, disilane, and the 2-chloroethyl radical
Authors: Sewell Thomas D. , Schranz Harold W. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The unimolecular dissociation reactions of the 2-chloroethyl radical involving C–H and C–Cl bond fissions are investigated using classical trajectories and two variational transition-state theory methods on the same potential-energy surface. The transition-state theory methods employed are the efficient microcanonical sampling-transition state theory method, previously used to study the decomposition dynamics of disilane and 1,2-difluoroethane, and a J-conserving variant of this method that introduces constraining equations in the efficient microcanonical sampling procedure, such that the sampling is restricted to phase-space points associated with both a constant value of the system energy and total angular momentum.

The results demonstrate that the unimolecular dissociation of the 2-chloroethyl radical is well described by statistical theories that assume an equal weight for all energetically accessible phase-space points. The results obtained from the statistical calculations form upper bounds to the trajectory-computed rate coefficients as expected for a statistical system. In addition, there is no evidence of mode-specific dynamics present in the trajectory results. The statistical behavior of the 2-chloroethyl radical stands in sharp contrast to the dissociation dynamics of disilane and 1,2-difluoroethane which have previously been shown to exhibit pronounced nonstatistical effects. It is shown that the existence of nonstatistical behavior cannot, in general, be qualitatively predicted from energy considerations alone.

Comparison of the 2-chloroethyl radical, 1,2-difluoroethane, and disilane results again demonstrates that the existence of an energy decay rate out of a given bond that is fast relative to the unimolecular reaction rate is not a sufficient condition to guarantee statistical dynamics. It is found that the statistical behavior observed for 2-chloroethyl is due, in large part, to an increase in the potential coupling between the dissociating atom and the beta-carbon that occurs as the bond breaks. This coupling is associated with the conversion of the C–C single bond to a C■C double bond upon C–Cl or C–H bond fission in 2-chloroethyl. It is concluded that unimolecular reactions will tend to exhibit nonstatistical dynamics if (1) the internal energy is close to the dissociation threshold, (2) motion along the reaction coordinate does not produce large energetic changes in one of more bonds in the remainder of the molecule, and (3) there exists a formation coordinate for the activated reactant that is strongly coupled to the dissociation coordinate but only weakly coupled to the other internal coordinates of the molecule.
Photodissociation of Hg–Ar van der Waals molecules: Alignment effects
Authors: Chen Xiushan , Burnett Keith , Segal Daniel M.
Published: July 2002

We present a calculation of the polarization of the fluorescence from the Hg λ253.7 nm transition following photodissociation of the Hg–Ar van der Waals molecule. For this case, it is possible to produce appreciable photodissociation near to threshold directly from the ground vibrational state. This means one can study the nonadiabatic mixing of molecular states over a wide range of translational energies by varying the frequency of the dissociating light. The formalism of Singer et al. [J. Chem. Phys. 79, 6060 (1983)] is used to cast the observables in terms of reduced half-collision T matrix elements. These half-collision fragmentation matrix elements have been found using a time propagation method.
External magnetic field effects and rotational state dependence on fluorescence of pyrazine-d4
Authors: Ohta Nobuhiro , Takemura Takeshi
Published: July 2002

External magnetic field effects on intensity and decay of fluorescence of pyrazine-d4 have been examined with excitation at the individual rotational lines of the 0–0 band belonging to the S0→S1 transition. A single exponential decay modulated by the quantum beats or a pseudobiexponential decay of fluorescence observed at zero field with excitation into very low rotational levels changes to a biexponential decay, as the strength of the external magnetic field (H) increases. The intensity of the slow component effectively decreases with increasing H, whereas the intensity of the fast component increases with increasing H, though both intensities reach constant values at high fields, respectively. The field-induced change of the fast component becomes smaller with increasing J′, whereas the magnetic quenching of the slow component becomes more efficient with increasing J′. The fluorescence lifetime of the slow component of pyrazine-d4 decreases with increasing H and has a tendency to increase with increasing J′ both in the absence and in the presence of H. A field-induced mixing between T1(nπ*) and T2(ππ*) is suggested to play a significant role in magnetic field effects on fluorescence of pyrazine-d4.
Far-infrared vibration–rotation-tunneling spectroscopy of Ar–NH3: Intermolecular vibrations and effective angular potential energy surface
Authors: Schmuttenmaer C. A. , Cohen R. C. , Loeser J. G. , Saykally R. J.
Published: July 2002

Two new intermolecular vibration–rotation-tunneling (VRT) bands of Ar–NH3 have been measured using tunable far infrared laser spectroscopy. We have unambiguously assigned these and a previously measured FIR band [Gwo et al., Mol. Phys. 71, 453 (1990)] as Π(10, n=0)←Σ(00, n=0), Σ(10, n=0)←Σ(00, n=0), and Σ(00, n=1)←Σ(00, n=0). The three upper states of these are found to be strongly mixed by anisotropy and Coriolis effects. A simultaneous least squares fit of all transitions has yielded vibrational frequencies, rotational and centrifugal distortion constants, and a Coriolis parameter as well as quadrupole hyperfine coupling constants for the upper states. An effective angular potential energy surface for Ar–NH3 in its lowest stretching state has been determined from these data, after explicitly accounting for the effects of bend stretch interactions. Features of the surface include a global minimum at the near T-shaped configuration (θ=90°), a 30 cm−1 to 60 cm−1 barrier to rotation at θ=180° (or 0°), and a very low barrier or possibly a secondary minimum at θ=0° (or 180°). Both attractive and repulsive interactions are shown to contribute significantly to the anisotropic forces in the complex. Comparison with ab initio calculations are presented.
Nuclear spin relaxation in a hexagonal lyotropic liquid crystal
Authors: Quist Per-Ola , Halle Bertil , Furó István
Published: July 2002

The hexagonal (E) phase in the sodium dodecyl sulphate (SDS)/decanol/water system is investigated by 2H and 23Na nuclear magnetic resonance (NMR) of the selectively deuterated SDS and the sodium counterion. Using macroscopically oriented E phase samples, prepared from the magnetically aligned nematic (NC) phase, we measure the orientation-dependent relaxation rates R1Z and R1Q as well as the line shape of both nuclei. The orientation dependence of the lab-frame spectral densities, determined from the relaxation rates, allow us to separate contributions from different types of molecular motion. In particular, we find a dominant contribution from molecular diffusion around the cylindrical aggregate. From this contribution we determine the lateral diffusion coefficient of SDS to (1.4±0.2)×10−10 m2 s−1 at 25 °C (activation energy 26±2 kJ mol−1 ) and the counterion surface diffusion coefficient to (4.8±0.9)×10−10 m2 s−1 at 25 °C (a factor 2.8 smaller than in an infinitely dilute aqueous electrolyte solution). Furthermore, the flexibility of the cylindrical aggregates in the investigated E phase (aggregate volume fraction 0.27) is quantified in terms of an orientational order parameter SDC≊0.9.
Spectroscopic signatures of structural aufbau in (benzene)n; n=7–19
Authors: Easter David C. , Li Xiuling , Whetten Robert L.
Published: July 2002

Optical spectra of larger (benzene)n clusters, n=7–19, have been investigated in detail using the molecular B2u←A1g000 and 610 transitions. The clusters are formed in a helium jet and are detected mass selectively by two-color resonant two-photon ionization spectroscopy at moderate resolution. The association of spectral line shifts with distinct molecular sites leads to a simple picture of the building up of clusters of nonpolar molecules, as proposed earlier [Easter et al. Chem. Phys. Lett. 157, 277 (1989)]. Coarse band shapes and fingerprintlike fine structure are discussed in terms of the numbers and multiplicities of sites predicted by the icosahedral aufbau sequence, and yield a particularly clear correspondence around B13, whose quasiicosahedral structure (as determined by minimum-energy simulations) forms the sequence’s core. Distinctive multiplet structure observed in the center of the 610 band for n=12–15 is attributed to exciton interactions among equivalent sites.
Computer simulations of n-alkane melts
Authors: Winkler R. G. , Ludovice P. J. , Yoon D. Y. , Morawitz H.
Published: July 2002

Stochastic dynamics simulations of n-alkane melts (C13H28 and C28H58 ) have been performed in order to calculate their static properties. The system consists of linear chains of mass points, which are connected by fixed bond lengths. Each point of a chain is subjected to valence bond and torsional forces. Additionally, nonbonded interactions are taken into account by a truncated, Lennard-Jones potential. The mean-square end-to-end distance and the radius of gyration of the chains as well as the average fraction of trans conformations were calculated and compared with the results obtained by Monte Carlo simulations and by unperturbed rotational isomeric states model calculations. The comparison exhibits very good agreement between the results of the different approaches, and demonstrates the potential advantages of stochastic dynamics simulations for polymeric systems. An investigation of different torsional potentials exhibited the subtle effect of the curvature of the torsional potential on bulk structures.
Analysis of polarizabilities, potentials, and geometries of alkali–halide dimers
Authors: Chauhan R. S. , Sharma S. C. , Sharma S. B. , Sharma B. S.
Published: July 2002

An analysis of electronic polarizabilities, interionic potentials, and equilibrium geometries for 20 alkali–halide dimers such as Na2Cl2 is presented. The electronic polarizabilities of ions in dimers have been calculated by taking account of the electrostatic potential due to neighboring ions within the Seitz–Ruffa energy level analysis. The polarizabilities thus obtained for dimers are then used to evaluate the magnitudes of polarization energies and van der Waals dipole–dipole energies. For describing the overlap repulsive potential we have adopted two forms: (i) the revised version of the Born–Mayer potential and (ii) the Harrison form for the overlap repulsive energy. These potentials have been used to obtain the binding energies of monomers as well as dimers, spectroscopic constants of monomers, equilibrium geometries of dimers, and dimerization energies. The results have been discussed and compared with available experimental data.
Photodissociation of oxygen molecules at 226 nm in the Herzberg I system
Authors: Tonokura Kenichi , Shafer Neil , Matsumi Yutaka , Kawasaki Masahiro
Published: July 2002

Doppler profiles of O(2p 3Pj) (j=2,1,0) produced from the photodissociation of O2 in the Herzberg I system (A3Σ+u←X3Σ−g) at 226 nm are measured with a resonance-enhanced multiphoton ionization technique. The anisotropy parameter for the angular distribution of the photofragments is found to be 1.6±0.4, in quantitative agreement with theory and indicating that the Herzberg I system borrows most of its intensity from a parallel band. The branching ratios to the j levels of O(2p 3Pj) (j=2,1,0) are found to be (2.5±0.3):1:(0.26±0.03) and are compared with the values expected for the limits of adiabatic and diabatic dissociation.
Critical fluctuations and molecular dynamics at liquid-crystalline phase transitions. II. Electron spin resonance experiments
Authors: Nayeem Akbar , Rananavare S. B. , Sastry V. S. S. , Freed Jack H.
Published: July 2002

Electron spin resonance (ESR) relaxation studies at nematic–isotropic (N–I), and nematic–smectic-A (N–SA ) phase transitions in two liquid crystals, 4O,6 and 6OCB–8OCB, using the three spin probes, PD-tempone, MOTA, and P are described. In general, one finds that (i) at the N–I transition, as TNI is approached, the linewidths diverge with a critical exponent of 1/2; (ii) at the N–SA transition, the linewidths diverge with a 1/3 power law as the transition is approached from the nematic side. The nature of the critical divergences in the relaxation parameters is interpreted and analyzed in terms of fluctuations in the nematic and smectic order parameters at the respective transitions and the coupling of the orientational dynamics of the probe to these modes. Good quantitative agreement with theory for the N–I transition required the inclusion of the effects of asymmetric probe ordering. The theory developed in detail in paper I is applied to interpret the results at the N–SA transition. This theory is extended to include the effects of the measured anisotropies in (a) translational diffusion of the probe, (b) smectic correlation lengths, and (c) dynamic scaling exponents. In general, the magnitudes of the observed effects as well as their critical exponents are of the order expected, provided the averaging of the effects of density fluctuations within a smectic layer by probe diffusion is incomplete as a result of hindered diffusion.
The rotational spectrum of H2 in water ice
Authors: Taylor David G. , Strauss Herbert L.
Published: July 2002

The vibrational band of H2 and the first four rotational bands of H2 dissolved in ice have been observed by Raman spectroscopy. The presence of ice is indicated by its Raman spectrum. The H2 vibrational band is shifted to higher frequency than in the gas phase. The rotational bands show small shifts in frequency and widths that are relatively constant with changing J.
Monte Carlo and mean-field studies of successive phase transitions in rod monolayers
Authors: Kramer Diego , Ben-Shaul Avinoam , Chen Zhong-Ying , Gelbart William M.
Published: July 2002

In this paper we present a rigid-rod model (involving a restricted set of orientations) which is solved first with mean-field theory and then by Monte Carlo simulation. It is shown that both interparticle attractions and anisotropic adsorption energies are necessary in order for two successive fluid–fluid transitions to occur. The first is basically a gas–liquid condensation of ‘‘lying down’’ rods in the plane of the surface, and the second involves a ‘‘standing up’’ of the particles. A close qualitative correspondence is established between the results obtained in the mean-field and Monte Carlo descriptions. The role of biaxial states, i.e., in-plane orientational ordering, is also discussed in both contexts. To this end, we develop an analogy between our one-component rod monolayer and a bidisperse system of interconverting isotropic particles.
Time dependence of OH overtone relaxation in the hydroperoxyl radical
Authors: Chapman Douglas , Bowman Joel M. , Gazdy Bela
Published: July 2002

The time-dependent relaxation of zero-order O–H vibrational overtone states of nonrotating HO2 on the X 2A″ double many-body expansion (DMBE)-I potential surface is examined. The time dependence of these states is obtained by expanding them in a large basis of all the bound molecular eigenstates as well as L2 states above dissociation. Franck–Condon factors and survival probabilities are calculated and used to determine the relaxation time scales using techniques described by Heller [E. J. Heller, Phys. Rev. A 35, 1360 (1987)]. For low overtones, the relaxation is very slow and is due mainly to isomerization. Higher O–H overtones relax much more rapidly due to dephasing and intramolecular vibrational relaxation (IVR). The time dependence of several wave packets is depicted in coordinate space.
Orientational order in liquid crystal solutions of polyhexylisocyanate. I. Infrared dichroism
Authors: Wang Honghui , DuPré Donald B.
Published: July 2002

The concentration and molecular weight dependence of the order parameter was measured for liquid crystalline solutions of the semiflexible, extended chain polymer polyhexylisocyanate in chloroform and toluene. Order parameters were determined from the infrared dichroism of the stretching vibration of the CN bond in the helical polymer backbone. Results are compared with predictions of a version of the Onsager theory for wormlike chains, extended to higher solute densities by the use of a decoupling approximation.
Dynamics of the reaction of O− with H2O : Reactive and nonreactive decay of collision complexes
Authors: Varley D. F. , Levandier D. J. , Farrar J. M.
Published: July 2002

We present a study of reactive and nonreactive collisions between O− and H2O over the collision energy range from 0.67 to 1.07 eV. Kinetic energy analysis of the O− scattered nonreactively from H2O shows two components: The first arises from direct scattering and is nearly quasielastic, while the second occurs at significantly lower barycentric energies and corresponds to O− ejected without reaction from electrostatically bound O−⋅H2O complexes formed by approaching reagents. This latter flux is significantly more intense than the reactive OH− flux. The kinetic energy distributions for the low energy O− nonreactive flux are in qualitative agreement with statistical phase space theory, although recoil distributions that model the exit channel by an r −4 potential underestimate the kinetic energy release.

The reactive flux distributions show a strong energy dependence. At the lowest collision energy, the OH− is produced through two pathways, the first involving the participation of a complex living a fraction of a rotational period, the second producing OH− strongly backward scattered and with a much broader kinetic energy distribution. With increasing collision energy, the complex contribution to the scattering falls off rapidly, and product formation moves from the backward hemisphere to the forward direction. The angular distribution asymmetries at the lowest collision energies can be interpreted in terms of the osculating model for chemical reactions taking place in a fraction of a rotational period of the intermediate complex. This model suggests that the complex lifetime is ∼250 fs at collision energies between 0.7 and 0.8 eV, a result in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. The kinetic energy distributions at these energies are in good agreement with statistical phase space theory calculations. At the highest collision energies, still below the threshold for impulsive stripping collisions, the OH− product is scattered sharply forward with a broad kinetic energy distribution peaking near 0.3 eV. We interpret the high energy dynamics as direct, but still involving significant interaction among all four atoms. The rapid variation in dynamics over a narrow collision energy range is attributed to the heavy–light–heavy mass combination of this system.
Influencing the nonadiabatic branching in the photodissociation of ICN by sampling bent geometries at the conical intersection
Authors: Kash P. W. , Butler L. J.
Published: July 2002

This work measures the change in branching between the CN+I(2P3/2) and the CN+I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold ICN at 248.5 nm. The crossed-laser molecular beam experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries. The results provide a critical comparison between the long-standing empirical surfaces for ICN photodissociation and more recent ab initio potential energy surfaces for ICN’s first absorption band. In the experiment, we observe an increase in branching from 49% to 58% I(2P3/2) products when the temperature of the ICN parent photodissociated is increased in going from a 100 °C vs a 400 °C nozzle expansion. We analyze the angular distributions of the photofragments to eliminate the possibility that the change in branching is due to an increased contribution from direct absorption to the electronic state correlating with I(2P3/2) products. We discuss the relationship between these studies, which mediate the effective off-diagonal potential coupling by forcing the nuclear dynamics to sample more strongly coupled regions of the intersecting potential surfaces, with intramolecular electron transfer studies which examine how the coupling depends on the particular conformer.
Precursor and overlayer structural effects in the interaction of CO with Ni(110)
Authors: DeAngelis Michael A. , Glines Andrew M. , Anton A. Brad
Published: July 2002

We present isothermal measurements of CO adsorption and desorption rates on Ni(110) that reveal clearly the participation of a precursor state in both processes at all coverages. Accounting for its effect allows activation energies and preexponential factors to be determined that are specific to the coverage regimes where c(8×2), c(4×2), and (2×1) ordered structures are known to form at low temperatures. The preexponential factors include a significant contribution attributable to relaxation of the adsorbed layer from one structure to another as desorption proceeds, and the relaxation process is also manifested in the relationship between measured activation energies and integral heats of adsorption. The results have general implications for adsorption systems involving series of ordered overlayers.
Predissociation of H3 n=2 Rydberg states: Product branching and isotope effects
Authors: Peterson J. R. , Devynck P. , Hertzler Ch. , Graham W. G.
Published: July 2002

A translational spectroscopy technique is used to obtain predissociation kinetic energy release spectra from the lowest bound states of H3, 2s 2A′1 and 2p 2A″2. These H*3 states are formed in near-resonant electron capture by 3 keV H+3 in Cs vapor. Their ground rovibrational levels are energetically about 1 eV above the H+H+H dissociation limit, thus all levels can yield both H2+H and the three-body products. The spectra contain both three-body and two-body components and are deconvoluted to obtain the branching ratios. Data obtained from two different ion sources show that the three-body/two-body ratio increases with increased rovibrational energy in the H*3. The results are compared to recent theory and with previously reported ratios from dissociative recombination of H+3. The comparison suggests that the ratio increases monotonically with the total electronic and rovibrational energy in the H*3. D*3 predissocation has a similar behavior. The H- and D-atom spectra from the two-body decay of HD*2 show that the ejected H atom is strongly favored on a per-atom basis.
Test of a model for reversible excimer kinetics: Pyrene in cyclohexanol
Authors: Martinho J. M. G. , Farinha J. P. , Berberan-Santos M. N. , Duhamel J. , Winnik M.A.
Published: July 2002

A model for reversible monomer–excimer kinetics that considers the time dependence of the excimer formation rate coefficient is proposed and tested for pyrene in cyclohexanol from 25 up to 85 °C. Simultaneous analysis of the monomer and excimer experimental decay curves allows the determination of all the relevant parameters for this reaction. The diffusion coefficient follows an Arrhenius plot with activation energy of 36±1 KJ mol−1, the encounter radius varies between 7.3 and 8.9 Å, and the intrinsic rate constant for excimer formation varies between 2.4×109 and 1.1×1010 M−1 s−1. The intrinsic reciprocal lifetime of the excimer follows an Arrhenius plot with activation energy of 12±1 KJ mol−1, and the rate constant for excimer dissociation, determined for high temperatures (T>55 °C) when reversibility is important, has different values depending whether geminate pair effects are considered or not in the analysis. The binding energy of pyrene excimer obtained considering pair effects (ΔH=32±2 KJ mol−1) agrees with the published values for nonviscous solvents.
Analysis of dispersion interaction: Noble gas dimers
Authors: Cybulski Sl/awomir M.
Published: July 2002

Several approaches for the evaluation of the dispersion energy are analyzed. The two alternative derivation schemes of the dispersion energy expression use as a starting point either the Casimir–Polder formula or many-body theory. It is argued that only the second approach offers a framework in which the classification of various effects (correlation, response) is unambiguous. The cases for which the two formulations are equivalent are discussed. Calculations, some of which are more accurate than any previously published results, of the nonexpanded dispersion energies for dimers composed of noble gas atoms are reported by five different methods. The examined approaches range from the uncoupled Hartree–Fock treatment to the time-dependent Hartree–Fock (TDHF) method. Comparison with the experimental C6 coefficients indicates that for each dimer a different method offers the best approximation to the ‘‘true’’ dispersion energy. The nonexpanded TDHF dispersion energies are compared with the expanded ones evaluated by using the procedure of Tang and Toennies. The agreement is fair for He–Ne, Ne2, and Ar2 complexes, but for the He dimer the series representing the dispersion energy is found to diverge.
Stimulated emission pumping of intermolecular vibrations in OH–Ar(X 2Π)
Authors: Berry Mary T. , Loomis Richard A. , Giancarlo Leanna C. , Lester Marsha I.
Published: July 2002

Stimulated emission spectroscopy has been used to access the intermolecular bending and stretching vibrations supported by the OH(X 2Π)+Ar(1S0) potential-energy surfaces. Manifolds of OH–Ar bending levels, correlating with the j= (3)/(2) , (5)/(2) , and (7)/(2) rotational levels of OH(2Π3/2), have been observed with zero to three quanta of intermolecular stretch. OH–Ar complexes have also been prepared in intermolecular vibrational levels of the spin–orbit excited state correlating with OH(2Π1/2). The first dissociation limit, producing OH(2Π3/2) v=0, j= (3)/(2) +Ar(1S0) fragments, has been determined to lie between 93 and 103 cm−1 above the zero-point level. Complexes prepared in metastable levels, detected up to 200 cm−1 beyond the first dissociation limit, undergo predissociation by using OH rotational or spin–orbit excitation to break the OH–Ar intermolecular bond.
The low translational energy dependence of the reaction Ar+(2P3/2,1/2)+H2(D2)
Authors: Hawley Michael , Smith Mark A.
Published: July 2002

We report the measurement of the spin–orbit state specific rate coefficients for the reactions Ar+(2P3/2,1/2)+H2(D2) at translational temperatures from 0.5 to 3 K in a free jet flow reactor. The measured rate coefficient for Ar+(2P3/2) with either H2 or D2 shows a slow monotonic increase with increasing collision energy from 4×10−5 to 10 eV. The observed kinetic isotope effect at low energy is consistent with the simple mass-dependent change in the collision frequency. Below 3 K, excitation into the 2P1/2 state is found to enhance the total reaction rate by a factor of 2.7 for the reaction with H2 and by 1.6 for the reaction with D2. The full range of energy-dependent experimental results are compared to current reaction models.
The influence of vibrational and translational motion on the reaction dynamics of O(1D)+H2(1Σ+g,v)
Authors: Mikulecky Klaus , Gericke Karl-Heinz
Published: July 2002

The OH product state distribution from the reaction O(1D)+H2(v)→OH(v″,J″,Ω,Λ)+H was determined by laser-induced fluorescence (LIF) in the Δv=−3 band for v″=3 and 4 with resolution of the J″, Ω, and Λ sublevels. The rotational state population distribution is inverted strongly in v″=3, weaker in v″=4. There is a higher propensity for production of OH in the Π(A′) Λ-sublevels. Vibrationally excited H2 was used for a part of the experiments. Excitation was achieved by stimulated Raman pumping (SRP). The population ratio of the vibrational states was determined to be P(v=3)/P(v=4)=3.5 for the reaction with H2(v=0) and 3.0 when there is H2(v=1) in the reaction chamber. Higher OH product states are populated than it would be expected from the mean available energy of the reaction. The translational energy of the reactants is transferred into OH rotation.