bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser-induced fluorescence studies of jet-cooled S2O: Axis-switching and predissociation effects
Authors: Zhang Qingguo , Dupré Patrick , Grzybowski Bartosz , Vaccaro Patrick H.
Published: July 2002

Laser-induced fluorescence spectroscopy has been used to probe the intense C̃ 1A′–X̃ 1A′ (π*←π) electronic system of S2O (315–340 nm) under supersonic free-jet conditions that yield effective rotational temperatures of roughly 1 K. Least-squares analysis of high-resolution scans performed on the 2v0(v=0–5) progression, where ν2 corresponds to the S–S stretching mode, not only furnish refined band origins and rotational constants, but also provide evidence for an axis-switching effect in this asymmetric triatomic species. Based on the limited set of vibronic bands examined in the present study, the harmonic frequency and anharmonicity for S–S stretching motion in the C̃ state are determined to be ω2=415.2(4) cm−1 and x22=−2.10(6) cm−1, respectively. Predissociation of the C̃ 1A′ potential energy surface is found to become more pronounced with increasing excitation of the ν2 mode. Collision-free lifetime data, obtained either directly from time-resolved fluorescence decay profiles or indirectly from measurements of broadened spectral linewidths, permit formulation of a simple, one-dimensional tunneling model which predicts the excited state predissociation barrier to be located in the vicinity of the 26 vibrational level. These results, as well as possible candidates for the electronic manifold responsible for the predissociation process, are discussed in light of preliminary ab initio calculations.
Analytical semiclassical calculation of photodissociation of the HCl molecule
Authors: Duhoo Thierry , Pouilly Brigitte
Published: July 2002

This paper describes an analytical method to the solution of semiclassical first-order, time-dependent coupled equations in the case of a three states process. The method is applied to the study of the photodissociation of the HCl molecule. The results of the semiclassical instantaneous probabilities as function of the interparticle distance are compared with quantum–quantum flux redistribution calculations [M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752 (1993)].
Towards phase transferable potential functions: Methodology and application to nitrogen
Authors: Jordan Peter C. , van Maaren Paul J. , Mavri Janez , van der Spoel David , Berendsen Herman J. C.
Published: July 2002

We describe a generalizable approach to the development of phase transferable effective intermolecular potentials and apply the method to the study of N2. The method is based on a polarizable shell model description of the isolated molecule and uses experimental data to establish the parameters. Consideration of the Ne dimer shows this to be a conceptual advance over point polarizability descriptions of atomic interaction. Our parametrization of N2 accurately describes not only the molecule’s electrostatic field (i.e., a practical representation of the molecular charge distribution) but also its response to electrical and mechanical stress (polarization and deformation). The purely intermolecular terms in our potential reflect shell-shell interactions. These are parametrized by fitting properties of the low temperature solid phase of nitrogen. We derive a phase transferable potential able to account for the second virial coefficient of the gas phase, the pressure induced phase transition between nitrogen’s cubic and tetragonal phases, and a wide range of liquid properties (pair distribution function, heat of vaporization, self-diffusion coefficient and dielectric constant).
The Boltzmann equation and nonequilibrium ensemble method
Authors: Eu Byung Chan
Published: July 2002

By combining the phenomenological theory of irreversible processes and the results of the kinetic theory of irreversible processes obtained from the Boltzmann equation for a dilute gas mixture, a nonequilibrium ensemble method is formulated for dilute classical gases as a parallel extension to the Gibbs ensemble method in equilibrium statistical mechanics. This method is distinct from those of McLennan and Zubarev. The main distinguishing features are: the use of an irreversible kinetic equation (i.e., the Boltzmann equation) instead of the time-reversal invariant Liouville equation; the extended Gibbs relation for calortropy; generalized hydrodynamic equations consistent with the second law of thermodynamics; and thermodynamical identifications of the parameters appearing in the nonequilibrium canonical distribution function.
Orientation relaxation in glassy polymers. II. Dipole-size spectroscopy and short-time kinetics
Authors: Patashinski Alexander Z. , Ratner Mark A.
Published: July 2002

The orientational diffusion of a rodlike particle embedded in a glassy polymeric matrix is considered; the underlying kinetics is that of local rearrangements. A defining parameter of the theory is the length of the particle. The timing of steps of the random walk in orientation space is determined by rearrangements. We discuss the physical properties of the glass state in connection with the rearrangement kinetics. The orientational diffusion is influenced by the local disorder; this influence is different for dipoles of different length. For a short dipole, the resulting diffusion is of generalized Debye type. Nonexponential relaxation of physical quantities may then be caused by the distribution of rearrangement barriers. For longer dipoles and if the orientation is uniquely determined by the configuration of the embedding cluster, the motion is a random walk on a given random map on a sphere. An ensemble of random mappings is considered. For even longer dipoles, hierarchical (multiscale) relaxation is expected. We discuss the relation of the theory to the short time depoling kinetics in a system of dipoles having different length, such as are found in relaxation of electrically poled polymer materials.
Hybrid treatment of spatio-temporal behavior in surface reactions with coexisting immobile and highly mobile reactants
Authors: Tammaro M. , Sabella M. , Evans J. W.
Published: July 2002

For surface reactions on single-crystal substrates which involve highly mobile adspecies, there is a vast separation in natural time and length scales. Adspecies hop rates can be many orders of magnitude larger than rates for other processes. Strong spatial correlations or ordering can exist on the atomic scale, while spatial pattern formation occurs on a macroscopic scale due to high diffusivity. An efficient analysis of such systems is provided by a ‘‘hybrid treatment’’ which we apply here to the monomer–dimer surface reaction model in the case of coexisting immobile dimer adspecies and highly mobile monomer adspecies. Specifically, we combine a mean-field treatment of the ‘‘randomized’’ mobile adspecies, and a lattice-gas description of the immobile adspecies. Monte Carlo simulations then reveal bistability and ‘‘critical’’ bifurcation phenomena, while precisely accounting for the influence of correlations in the immobile adspecies distribution. A corresponding analysis of the evolution of macroscopic spatial inhomogeneities is achieved through parallel simulation of the distributed macroscopic points with distinct correlated states and adspecies coverages. These simulations are appropriately coupled to describe diffusive mass transport of the mobile adspecies. In this way, we examine for this model the propagation and structure of chemical waves, corresponding to interface between bistable reactive states, and thereby determine the relative stability of these states.
Transient hole burning of s-tetrazine in propylene carbonate: A comparison of mechanical and dielectric theories of solvation
Authors: Ma Jangseok , Bout David Vanden , Berg Mark
Published: July 2002

The solvation dynamics of s-tetrazine, a nonpolar solute, in propylene carbonate, a polar solvent, have been measured in the temperature range of 190–300 K and the time range of 1.5–300 ps by transient hole burning. A detailed model of the gas-phase spectrum of s-tetrazine is used to extract purely solvent-induced effects from steady-state and ultrafast spectra. Absolute measurements of the solvation response function are extracted from these spectra and are compared to dielectric and mechanical theories of solvation. Although the theories postulate very different solute–solvent interactions mechanisms, either theory can account for the available data.
Computer simulation study of Rayleigh and Raman spectra and spectral moments of fluid C2H6
Authors: Keller M. , Mueller A. , Versmold H. , Steele W. A.
Published: July 2002

Depolarized Rayleigh and Raman scattering from fluids is composed of orientational (OR), collision induced (CI), and cross (X) contributions. In order to investigate the influence of the interaction induced terms (CI and X) on allowed spectra (OR), C2H6, a molecule with a small ratio of the anisotropic part γ to the isotropic part α of the polarizability is considered. Both Rayleigh and ν3 Raman correlation functions were determined by molecular dynamics (MD) computer simulation and compared with experiment. The investigations extend over a wide range of densities (0.67ρc–2.85ρc, where ρc is the critical density, ρc=0.206 g/cm3). The different contributions to the time correlation functions (TCFs) were simulated within the frame of a first-order center–center dipole–induced-dipole polarizability model. The potential model used was a standard two-site Lennard-Jones potential. Details concerning partial OR, CI, and X correlation functions and second moments for Rayleigh and Raman scattering are presented. At the lower densities a large CI and a smaller X contribution to the total Rayleigh TCF is found. At higher densities the X and CI contributions increase but tend to cancel each other so that the OR contribution becomes more important. Concerning the Rayleigh second spectral moments an explanation of the minimum of the experimental second moment observed at intermediate densities and of the increase of the second moment with density for higher densities is presented. Both the Raman TCFs and second moments are essentially orientational due to the large polarizability anisotropy derivative γ′.
Shear flow at liquid–liquid interfaces
Authors: Padilla Paz , Toxvaerd So/ren , Stecki J.
Published: July 2002

Two nonmiscible liquids separated by planar interfaces and undergoing shear flow have been simulated with nonequilibrium molecular dynamics (NEMD) methods. A homogeneous shear scheme was used for imposing shear flow in the system. The homogeneous shear algorithm needs to be combined with a profile-unbiased thermostat (PUT) in order to assure meaningful results in our nonhomogenous system. Local values of several quantities such as viscosity, local stream velocity, temperature, shear stress, and rate of entropy production were calculated. Planar Couette flow appears in the ‘‘bulk’’ regions of the system with a slip between the two streams of bulk fluid at the interfaces. The shear stress is constant across the system (PUT results) at low strain rates but at high shear rates the shear stress at the interface is lower than in the bulk region. The shear viscosity at the interfaces is lower than in the bulk region showing that the transport of momentum in the former region is less efficient than in the bulk. At high strain rates, the differences in the local rates of viscous heat production and heat removal result in strong temperature gradients. When comparing the viscosity values in the bulk region of the inhomogeneous system with values computed in independent simulations of the bulk, no important differences are found.
Density dependence of the ionization potential of CH3I in argon and of the quasi-free electron energy in argon
Authors: Al-Omari A. K. , Reininger R.
Published: July 2002

Field ionization has been employed as a new technique to determine the ionization potential of an impurity doped in a nonpolar fluid. This has been shown for a model system, CH3I doped into argon for argon densities ranging from the gas up to the triple point liquid. The ionization potential in the medium reflects the different possible configurations of the medium atoms around the dopant at the moment of excitation and, therefore, is given by a distribution. This distribution is identified with that of the polarization energy between the positive ion and the medium. The first and second moments of the polarization distribution were calculated at the densities at which the experiments were performed. Simulated spectra (generated using the experimental results obtained in pure CH3I, the calculated moments, and a Gaussian shape for the polarization distribution) reproduce closely the experimental ionization potential. Furthermore, by combining the experimental data with the calculated distributions, we obtained the density dependence of the quasi-free electron energy in argon. These results are consistent with previous experimental data and with recent theoretical calculations.
A semiclassical surface hopping propagator for nonadiabatic problems
Authors: Herman Michael F.
Published: July 2002

A semiclassical propagator is developed for general multisurface, multidimensional nonadiabatic problems. It is demonstrated that this propagator satisfies the time-dependent Schrödinger equation through order ℏ. This is the same order satisfied by the usual semiclassical propagator in single surface problems. The zeroth-order term (in the nonadiabatic coupling) for the propagator is just the well-known single surface adiabatic propagator. The first-order terms involve single hops from the initial adiabatic state to other states. Energy is conserved in these hops and the momentum change accompanying each hop occurs in the direction parallel to the nonadiabatic coupling for the transition. Both transmitted and reflected contributions are included after a hop. The propagator expression has the zeroth-order (single surface) semiclassical form before and after the hop. The complete propagator includes terms with any number of hops and all possible hopping points. These multihop terms are defined analogously to the first-order (single hop) terms. An alternative formulation of the semiclassical propagator, which includes contributions from a broader range of hopping trajectories, is also developed.
Calculations of the spectra of rare gas dimers and trimers: Implications for additive and nonadditive intermolecular forces in Ne2–Ar, Ne2–Kr, Ne2–Xe, Ar2–Ne, Ar3, Ar2–Kr and Ar2–Xe
Authors: Ernesti Andreas , Hutson Jeremy M.
Published: July 2002

Calculations of ground-state energies and rotational constants are carried out for a variety of van der Waals dimers and trimers formed from Ne, Ar, Kr and Xe. It is found that the existing pair potentials for Ne–Ar, Ne–Kr and Ne–Xe do not adequately reproduce the measured rotational constants of the van der Waals dimers. Modified pair potentials, with equilibrium distances that differ from the originals by less than 1% but give much better rotational constants, are then proposed. Calculations of rotational constants for Ne2–Ar, Ne2–Kr and Ne2–Xe are carried out using pairwise-additive potentials constructed from both the original and the modified pair potentials. The modified pair potentials give much better agreement with experiment for the trimers as well as the dimers. The effect of an Axilrod–Teller triple-dipole term on the rotational constants is considered, and found to be significant, especially for the A rotational constant. However, the best available Ne–Ne potential is not accurate enough to allow unambiguous information on three-body forces to be extracted.
Intermolecular vibrations and spin–orbit predissociation dynamics of NeOH (X 2Π)
Authors: Chuang Cheng-Chi , Andrews Peter M. , Lester Marsha I.
Published: July 2002

Stimulated emission pumping of NeOH is used to access the bound intermolecular vibrational levels supported by the OH X 2Π3/2+Ne potential energy surface as well as predissociative levels correlating with the spin–orbit excited state of OH X 2Π1/2+Ne which lie more than 100 cm−1 above the dissociation limit. Intermolecular stretching intervals and rotor constants yield the radial dependence of the average interaction between Ne and OH X 2Π3/2,1/2, while the spacings between angular levels provide information on the anisotropy of the potentials. The lifetime of spin–orbit predissociative levels is found to increase by a factor of 3 upon intermolecular stretching excitation, from 8.5 ps to more than 27 ps. This increase in lifetime is attributed to the fall-off in the coupling between the two spin–orbit states with increasing intermolecular separation distance. The dominant coupling for spin–orbit predissociation is provided by the difference potential, the change in the intermolecular potential when the unpaired electron of OH lies in or out of the NeOH plane. The energies, rotor constants, and lifetimes of the bound and predissociative NeOH levels observed experimentally are compared with theoretical calculations of these quantities based on ab initio potentials for Ne+OH X 2Π by Yang and Alexander (accompanying paper). Remarkably good agreement is found between experiment and theory, given the weakness of the Ne+OH X 2Π interaction.
Velocity reassignment echoes in proteins
Authors: Xu Dong , Schulten Klaus
Published: July 2002

A new echo phenomenon in proteins, a generalization of so-called temperature quench echoes, is introduced and shown to reveal, through molecular dynamics simulations, periodic motions (normal modes) in proteins with phase coherence times of about one picosecond. The echoes are induced through reassignments of Cartesian velocities to protein atoms at times t=0 and t=τ (0<τ≤1 ps) and appear as two sharp (widths of about 5 fs) features in the kinetic and potential energy at t=3τ/2 and t=2τ. The velocities, assigned at t=0 and at t=τ to each atom, need to be correlated, but can otherwise be random. The echo at 3τ/2 can be induced without any change in the temperature of the protein. Skeletal motions involving angular and stretch motions contribute principally to the echo effect. Electrostatic interactions do not affect the echoes. The echoes in the temperature are described, in the framework of the harmonic approximation, in terms of the equilibrium temperature–temperature correlation function. The velocity reassignments induce the echoes through the generation of phase coherence of protein modes. Phase relaxation due to anharmonic interactions, lead to a dependence of the echo depths on the time interval τ between velocity replacements which can be accounted for by phase diffusion or by Langevin oscillators. The echo effect can provide a sensitive probe for the study of non-diffusive energy transport in proteins.
Kinetic equations for site populations from the Fokker–Planck equation
Authors: Moro Giorgio J.
Published: July 2002

A new method is proposed for the calculation of kinetic coefficients from Fokker–Planck (FP) equations. Starting from the time scale separation between the kinetic modes of the FP operator and the other faster eigenmodes associated to the local equilibration of the coordinates, a set of site-localizing functions is introduced for the ensemble of stable states of the system by means of linear combinations of the kinetic eigenfunctions. They allow the mapping of a nonequilibrium distribution onto a set of site populations which obey to rate equations of kinetic type. Such a procedure is easily implemented by using numerical eigenfunctions of the time evolution operator, so providing a set of transition rates which leads to the same decay rates of the FP kinetic modes. Several examples are considered in order to illustrate the typical results of the method. Particularly interesting is a two-dimensional model for the chain of two bistable oscillators bound to a wall. A new kind of kinetic processes is recovered, besides ordinary transitions associated to saddle point crossings. They represent displacements localized within the chain without saddle point crossing, with analogy to crank-shaft transitions of polymers.
Propensities toward C2H(Ã 2Π) in acetylene photodissociation
Authors: Zhang J. , Riehn C. W. , Dulligan M. , Wittig C.
Published: July 2002

When expansion-cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH-stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(Ã 2Π). This differs markedly from one-photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X̃ 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(Ã 2Π) exclusively.
Theory of coherent transient spectroscopy in molecular aggregates: The effects of interacting excitons
Authors: Spano Frank C. , Manas Eric S.
Published: July 2002

Using a density matrix formalism we derive a general expression for the resonant response of an ensemble of molecular aggregates to two optical pulses. The aggregates, which can be of arbitrary geometry, are described by a Frenkel exciton Hamiltonian including exciton–exciton interactions. A hierarchy of resonant coherent transient signals is generated having wave vectors sk1+(1−s)k2, where ki is the wave vector of the ith pulse and s is an integer. General expressions for the differential pump–probe absorption spectrum D(ω;tD) (s=0) and the photon echo Iecho(tD) (s=−1) are presented for arbitrary pulse intensities. Applications are made to a linear chain of interacting Frenkel excitons with nearest neighbor exciton–exciton interaction A and exciton transfer J. Analytical expressions for D(ω;tD) and Iecho(tD) are obtained which are valid to third order in the aggregate-field interaction. The influence of the optically active red (blue) biexciton which detaches from the bottom (top) of the two-exciton band when A<−2‖J‖ (A≳2‖J‖) is investigated. Biexcitons appear as extra peaks in D(ω;tD) and as oscillations in Iecho(tD). Finite size effects are responsible for quantum beat oscillations in D(ω;tD) and long time oscillations in the echo decay. Quantum beats arise predominantly from the difference between the lowest one-exciton frequencies. Our theory successfully describes the salient spectral features in the pump–probe spectra of J-aggregates.
Manipulating reactant–product distributions in electron transfer reactions with a laser field
Authors: Dakhnovskii Yuri , Coalson Rob D.
Published: July 2002

The effect of a strong cw laser field on the process of nonadiabatic electron transfer in polar solvents is considered. First-order kinetic equations are derived in which the forward and backward rate constants depend on the electric field parameters. The forward rate constant, which governs the early time dynamics, exhibits dramatic variation with field intensity in the barrierless and activationless regimes. The sum of forward and backward rate constants, which determines the long time kinetics and hence may be termed the full rate constant, is less sensitive to the field intensity in the same regime of molecular parameter space. It is shown that the asymptotic populations of the reactant and product states are in general non-Boltzmann; their ratio can be varied by many orders of magnitude as the frequency and intensity of the applied electric field are varied.
Stochastic theory of molecular radiative transport
Authors: Berberan-Santos M. N. , Nunes Pereira E. J. , Martinho J. M. G.
Published: July 2002

The radiative transport (or radiative migration) of electronic excitation energy in a molecular ensemble is analyzed from a stochastic point of view. This approach yields results considerably more general than those of previous treatments, where successive hops are (implicitly) assumed to be uncorrelated, while they constitute in fact a Markov process. The time evolution of the fluorescence intensity emitted by the molecular ensemble, in response to excitation by an external beam of linearly polarized and essentially monochromatic light is obtained. In contrast to the previous treatments, only known parameters are required. The time evolution of the fluorescence anisotropy is also obtained for the first time.
Reactive scattering of O(1D)+HD: Product speed and angle distributions
Authors: Che Dock-Chil , Liu Kopin
Published: July 2002

The reaction dynamics of O(1D)+HD → OH+D and OD+H was investigated at a collision energy of 4.55 kcal/mole in a crossed-beam apparatus. The speed and angle distributions of H(D) products were interrogated using Doppler-shift measurements in a (1+1) REMPI (resonantly enhanced multiphoton ionization) detection scheme. Both product channels exhibited marked forward–backward asymmetry in the angular distributions and the translational energy release to the OD+H channel was substantially larger, about 1.4 on the average, than that to the OH+D product.
Viscoelasticity of a fluid of dynamically disordered harmonic macromolecules
Authors: Chatterjee Avik P. , Loring Roger F.
Published: July 2002

We develop a generalization of the Rouse model for the dynamics of a flexible, linear macromolecule. This dynamically disordered Rouse (DDR) model is based on a Smoluchowski equation for bead coordinates, in which the bead mobilities are stochastic variables which fluctuate between zero and a finite value. The DDR model may be regarded as a generalization of previous extensions of the Rouse model with nonuniform but time-independent bead mobilities to the case in which the mobilities of the beads are allowed to fluctuate. We focus on the contribution of intrachain relaxation processes to the viscoelastic shear modulus, G(t), of a macromolecular fluid. In the limit of rapid medium fluctuations, we recover for G(t) the prediction of the conventional Rouse model. For a slowly relaxing medium, G(t) is characterized by an initial decay, followed by a plateau, and a terminal decay regime exhibiting renormalized Rouse behavior, in qualitative agreement with the shear modulus of dense polymer fluids at short and intermediate times. The center-of-mass diffusion constant displays a crossover from the Rouse result to behavior controlled by obstacle relaxation as the lifetime of medium fluctuations is increased.
Density dependence of the ionization potential of CH3I in krypton and of the quasi-free electron energy in krypton
Authors: Al-Omari A.K. , Reininger R.
Published: July 2002

Field ionization has been employed to determine the ionization potential of CH3I doped in krypton for krypton densities between the gas and the triple point liquid. The ionization potential is correlated to the different configurations of the host atoms around the dopant at the moment of excitation and is, therefore, represented by a distribution. The ion-medium polarization energy distribution was calculated, from the first and second moments, at the densities at which the experiments were performed. The calculated widths are in very good agreement with those of the experimentally determined ionization potentials. By combining the experimental results with those of the calculations, the density dependence of the quasi-free electron energy in Kr was determined. These results are compared to those obtained in previous experiments and with recent theoretical calculations.
Dynamics of the N(4S)+NO(X 2Π)→N2(X 1Σ+g)+O(3P) atmospheric reaction on the 3A″ ground potential energy surface. III. Quantum dynamical study and comparison with quasiclassical and experimental results
Authors: Aguilar Antonio , Gilibert Miquel , Giménez Xavier , González Miguel , Sayós R.
Published: July 2002

A detailed reactive–infinite-order sudden approximation (R-IOSA) study of the reactivity of the N+NO→N2+O system has been carried out in the 0.0038 to 1.388 eV translational energy range and the results have been compared with the existing quasiclassical trajectory (QCT) and experimental data available. The general features already observed in the previous QCT studies are reproduced qualitatively in the quantum study, even though some differences arise in the product vibrational distributions and state-to-state opacity functions in the low energy range. The observed differences have been justified in terms of the anisotropy of the potential energy surface and the vibrational barriers to reaction at fixed angles. A strong vibrational adiabaticity is observed quantally in the low translational energy range, disappearing at moderately high collision energies (around 0.3 eV), where a simple Franck–Condon type model is capable of describing the evolution of the vibrational distribution with translational energy. The vibrational distributions at fixed angles have been discussed within the context of Polanyi’s and Light’s correlation between products vibrational excitation and the features of the potential energy surface. The validity of extending the conclusions drawn from collinear to three-dimensional (3D) collisions is discussed. Finally, the detailed reaction mechanism is examined in light of the vibrational matrix elements of the close-coupling interaction matrix.
Charged dielectric spheres in electrolyte solutions: Induced dipole and counterion exclusion effects
Authors: Sushkin Nicholas V. , Phillies George D. J.
Published: July 2002

The electrostatic interaction between two charged spheres in the presence of a screening electrolyte is calculated at the level of the linearized Debye–Hückel theory. The calculation was performed analytically as a multipole expansion by applying two-center spherical harmonic expansions and computer symbolic manipulation methods. We focus on charge–charge and charge–induced dipole interactions, calculated here for two spheres of possibly unequal size. The former interaction is given to good approximation by the familiar Debye–Hückel form ∼q1q2 exp[−κ(R−2a)]/[εr(1+κa)2]. The new results are the charge-induced dipole interactions. Physically, these terms arise from two sources, namely (i) surface polarization charge at the surface of each sphere, and (ii) exclusion of the counterion cloud of each sphere from the volume occupied by the other sphere. With parameters appropriate for micelles, at small separations the charge-induced dipole interactions dominate the charge–charge interaction.
Photodynamics in oxygen doped solid deuterium
Authors: Danilychev A. V. , Bondybey V. E. , Apkarian V. A. , Tanaka Satoshi , Kajihara Hideo , Koda Seiichiro
Published: July 2002

A spectroscopic investigation of O2 isolated in D2 matrices is reported. Standard matrix isolation methods lead to only a small observable concentration of isolated molecules in D2 and fail in the case of H2. Both resonant excitation of the molecule over the A′(3Δu)←X(3Σ−g) transition, and dissociative pumping at 193 nm, lead to vibrationally relaxed A′→X emission, with a lifetime of 5.4 μs. Independent of the initial Ω component accessed, the fluorescence is polarized with a polarization ratio of 1.25±0.05; indicating that O2 does not rotate and that the transition gains its intensity by crystal field induced mixing between the 3Δu and a nearby 3Πu state. This uniquely identifies the anisotropy of the local field as Y43+Y4−3, which in turn implies that the molecule is trapped in an fcc site with its axis aligned along the [111] direction. The observed zero phonon line intensities can be explained consistently if the spin-orbit multiplet is assumed to be normal, in contrast with the accepted scheme in the gas phase. The zero phonon lines of O2 isolated in D2, show librational satellites with a 15 cm−1 spacing in the A′ state, and ∼25 cm−1 spacing in the X state. Intensity analysis of these progressions leads to the conclusion that the molecular axis in the excited state is tilted relative to the ground state. Deposits in D2 and H2 samples overcoated with Xe, after annealing, exhibited spectra of isolated O2 which are perturbed by the Xe film.
The electronic spectroscopy of jet-cooled difluorocarbene (CF2): The missing Ã-state stretching frequencies
Authors: Cameron Melanie R. , Kable Scott H. , Bacskay George B.
Published: July 2002

The Ã(1B1)←X̃(1A1) electronic transition of difluorocarbene (CF2) has been studied by laser-induced fluorescence spectroscopy in a supersonic free jet and by ab initio theoretical methods. The radical was formed by pyrolysis of C2F4 at ∼1000 °C with a heating chamber at the tip of the nozzle just prior to the gas expansion. Fairly complete rotational cooling, but incomplete vibrational cooling allowed the identification of several new hot band transitions. Ab initio calculations for the X̃(1A1), ã(3B1), and Ã(1B1) electronic states were performed using the CASSCF method in conjunction with Dunning’s cc-pVTZ basis set, as well as two smaller sets. The calculations allow us to distinguish between several possible assignments of the observed transitions, and hence determine the vibrational frequencies of the two previously unassigned Ã-state stretching frequencies: ν3′=1180±2 cm−1 and ν1′=1011±2 cm−1. The ordering of these modes is different from that in the ground state (ν1″≳ν3″) and mechanisms for the switching are discussed.
Numerical simulations of electron tunneling in water
Authors: Mosyak Alex , Nitzan Abraham , Kosloff Ronnie
Published: July 2002

Electron tunneling through molecular layers has long been under study in conjunction with electron tunneling microscopy. More recently solvent effects on the tunneling matrix elements associated with electron transfer problems and with ‘‘underwater’’ electron tunneling microscopy have come under discussion. This paper describes the results of computer simulations of electron tunneling through frozen water layers. A water layer (∼10 Å) is confined between two electrodes, and is equilibrated and evolved in time in order to generate an ensemble of barrier configurations. The electron–(classical) water interaction is represented by a suitable pseudopotential. It is assumed that the water dynamics is negligible on the time scale of the tunneling process, so tunneling is studied for the resulting group of frozen configurations. Several numerical methods for evaluating the transmission through such disordered barriers are described and compared. It is shown that tunneling probabilities as low as 10−10 can be calculated with sufficient accuracy. We find that tunneling in this system cannot be described by averaging over one-dimensional paths. Furthermore, in contrast to common practice which assumes that the barrier to tunneling may be estimated by lowering the bare (vacuum) barrier by a magnitude associated with the electronic dielectric response of water taken as a dielectric continuum, the simulations show that transmission is strongly reduced due to the fact that much of the physical barrier space is blocked by the practically impenetrable oxygen cores. The tunneling probability significantly depends on the water configuration in the barrier, in particular on the orientational distribution of the water molecules. These observations suggest that external variables such as temperature and electric field will affect the tunneling through their effect on the water density and orientation, in addition to the effect of these variables on the bare (vacuum) tunneling.
Conformational disorder of conjugated polymers: Implications for optical properties
Authors: Yaliraki Sophia N. , Silbey Robert J.
Published: July 2002

A physical picture of a conjugated chain as a collection of almost planar segments, separated by large angular breaks arises from a microscopic model which includes conjugation and steric interactions. The conjugation part of the standard phenomenological Hamiltonian for torsional motion is also derived from the model. We obtain a probability distribution of the length of segments between those breaks as the relevant factor for the behavior of the chain. We also perform numerical simulations of the structure and properties of these chains; the results of this are in agreement with our analytic predictions. In explaining experimental data for optical properties, such as the second hyperpolarizability, γ, our theory provides improved agreement over previous models.
Raman intensities from Kohn–Sham calculations
Authors: Stirling András
Published: July 2002

Raman intensity calculations have been performed for nine small main-group molecules using the Kohn–Sham density functional method. A combination of numerical and analytic derivation techniques was used as implemented in the program package DEMON. The effect of the applied functional, the basis set augmentation, and the numerical fitting of the exchange-correlation potential have been investigated along with other aspects of the computations. The results obtained at the local level using valence triple-zeta plus 2 polarization functions (VTZP+) basis sets compare well with experiment and with the results obtained from the Hartree–Fock and correlation methods using large basis sets, whereas nonlocal corrections did not yield improvements in the predicted local Raman intensities. Systematic analysis proved the sensitivity of the results obtained with the gradient corrected nonlocal functional to the numerical fitting applied in the calculations of the exchange-correlation terms. We demonstrated that omitting the fitting procedure from nonlocal calculations improves the quality of the Raman intensities while the grid used for fitting does not have an influence on the Raman intensities. Effects of the reference geometry, step size for evaluating the numerical derivatives and the threshold of energy convergence were also tested.
Optical emission spectra of Ag3 molecules in the gas evaporation technique
Authors: Okazaki Tsugio , Saito Yahachi , Kasuya Atsuo , Nishina Yuichiro
Published: July 2002

A study was conducted of optical emission spectra, taken at a region near the source of the evaporation of Ag in a low pressure helium gas (100 Torr). A new Ag3 emission band system was identified. Spectroscopic constants for an excited and a lower state (the ground state) in the gas phase were derived as follows: ν00=19 809.4±2.0 cm−1, ω′1=106.8±1.0 cm−1, x′11=−0.2±0.5 cm−1, ω″1=129.0±1.0 cm−1, and x″11=−0.1±0.5 cm−1 for the 107Ag2109Ag trimers. Spectroscopic constants for three other isotope species were also derived. The lower state was the 2B2 ground state.
Size-consistent quasiparticle representation of nonlinear optical susceptibilities in many-electron systems
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

The optical response of a many-electron system is calculated by mapping it onto a coupled set of classical oscillators representing the electron–hole pair components of the reduced single- electron-density matrix. This classical representation is rigorously established using a Poisson bracket relation. Expressions for the nonlinear optical susceptibilities obtained using a Green’s-function solution of the oscillator equations of motion are used to analyze the size scaling of the off-resonant response and the resonant structure of the response.
Calculation of nuclear shielding constants and magnetizabilities of the hydrogen fluoride molecule
Authors: Åstrand Per-Olof , Mikkelsen Kurt V.
Published: July 2002

Nuclear shielding constants and magnetizabilities of the hydrogen fluoride molecule have been calculated adopting the multiconfigurational self-consistent field method. Effects of vibrational averaging are calculated by adopting a Gaussian function with variable width and position to describe the vibrational part of the wave function. The rovibrationally averaged 19F shielding constant is calculated to 407.4 ppm and its anisotropy to 111.2 ppm, both in good agreement with experimental results. Also the isotope effect, 〈σF(DF)〉−〈σF(HF)〉, which is calculated to 3.0 ppm agrees well with the available NMR experiment. The effective geometry, dipole moment, vibrational frequencies, and rotational constants are also calculated and discussed.
Rotational analysis of bands in the 460 nm system of nickel dichloride produced in a free-jet expansion: Determination of the structure and electronic ground state of nickel dichloride
Authors: Ashworth Stephen H. , Grieman Fred J. , Brown John M.
Published: July 2002

By use of a free-jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl2 at rotational resolution. The rotational temperature in these recordings was about 12 K. Several bands have been recorded and analyzed for three isotopomers, 58Ni35Cl2, 60Ni35Cl2, and 58Ni35Cl37Cl in natural abundance. Spin components with Ω values of 0 and 1 have been identified in both the upper and lower states of the transition. Accurate values for all three vibrational intervals ν1, ν2, and ν3 have been determined for nickel dichloride in the upper state and for the bending wave number ν2 in the lower state. The results show that the molecule is linear in both states involved in the transition and that the lower (ground) state is 3Σ−g in character. Evidence is presented from the nickel isotope shifts to show that the transition is vibronically induced through the bending vibration and that the upper state is vibronically 3Πu in character; it probably derives from an electronic 3Δg state. The zero-point averaged bond lengths are determined for both states as r0′=0.209 435(13) nm and r0″=0.205 317(14) nm. The fine structure parameters for the X̃ 3Σ−g state are interpreted in terms of low lying 1Σ+g and 3Πg states, which are shown to lie a few thousand reciprocal centimeters above the ground state.
Time dependent diffusion coefficient in a disordered medium
Authors: de Swiet Thomas M. , Sen Pabitra N.
Published: July 2002

We present a closed form expression for the diffusion coefficient in a dilute suspension of reflecting spheres, which is valid for all times. Our result is in agreement with general long and short time asymptotic results, which have been used in the interpretation of nuclear magnetic resonance experiments.
Wetting/nonwetting behaviors in a ternary amphiphilic system
Authors: Chen Li-Jen , Lin Shi-Yow , Xyu Jia-Wen
Published: July 2002

We consider the three-liquid-phase, α, β, and γ, coexisting system water+n-tetradecane +diethylene glycol monohexyl ether and the densities of these three phases are in the order ρα<ρβ<ργ. An enhanced video pendant drop tensiometer is applied to measure the interfacial tensions to further reconfirm that a wetting transition occurs by the γ phase at the αβ interface upon approaching its lower critical end point. The mechanism of reappearance of suspended drops at the αβ interface is carefully observed by the enhanced video microscopy system. It is found that these suspended drops are made of the γ phase, and are formed by a very slow process of aggregation of very small drops. The contact angle θ spanned by the αγ and βγ interfaces of a γ phase suspended drop at the αβ interface is verified to be a quasiequilibrium property. In addition, the interfacial tension resulting from the profile of a suspended drop below the αβ interface is consistent with the tension of βγ interface σβγ by the pendant drop digitization method. A variety of wetting/nonwetting behaviors as well as wetting transitions is also discussed. It is found that there is a β phase wetting layer separating the air and the α phase.
Bifurcation analysis and liquid–crystal phases in Landau–Ginzburg model of microemulsion
Authors: Ciach A.
Published: July 2002

Landau–Ginzburg model for oil–water–surfactant mixtures with three order parameters is derived from the lattice vector model [Ciach, Ho/ye, and Stell, J. Phys. A 21, L777 (1988)]. In case of oil–water symmetry all the coupling constants are explicitly expressed in terms of surfactant volume fraction ρs, temperature T, and a parameter γ describing the strength of surfactant. The bifurcation line in the parameter space (ρs,T) and the positions of the Lifshitz and the tricritical points are found for different values of γ. The structure of the ordered phases below the bifurcation is determined. For growing surfactant concentration lamellar, double-diamond, simple cubic and face-centered phases are stable; the last one is followed by the tricritical point. For strong surfactants the ordered phases appear for ρs∼10%. We claim that the present model is suitable for weak as well as for strong surfactants.
On the adiabatic connection method, and scaling of electron–electron interactions in the Thomas–Fermi limit
Authors: Levy Mel , March Norman H. , Handy Nicholas C.
Published: July 2002

In this paper we examine three aspects of electron–electron scaling: (i) the electron–electron repulsions are only scaled in Thomas–Fermi theory; (ii) the electron–electron repulsions are scaled, and the one electron potential is adjusted to give a prescribed density, in Thomas–Fermi–Dirac theory; and (iii) new approaches to the adiabatic connection formulas are presented to help improve the exchange–correlation functional. A new generalized two-point expression is presented. Models (i) and (ii) are solved exactly.
Comparative studies of triplet monocyclic aromatic diazines under pressure
Authors: Chan I. Y. , Wang W.
Published: July 2002

We report a zero-field optically detected magnetic resonance study under high pressure for triplet pyrimidine in benzene, pyrazine in benzene and in p-dichlorobenzene, and s-tetramethylpyrazine in durene. Generally, the pressure sensitivity of the zero-field splitting (ZFS) parameter D, ∂D/∂P, for these compounds is much higher than that for quinoxaline. This is rationalized in terms of a smaller π-electron cloud in the monocyclics than in quinoxaline. For pyrazine and pyrimidine, the 3nπ* nature of the lowest triplet leads to a larger spin–orbit contribution to the pressure shift. We observed a larger change in the ZFS parameter E for pyrazine in benzene than in dichlorobenzene. This is explained by the difference in crystalline packing between the two host lattices. There is a large change in D and a multiplet splitting under high pressure for tetramethylpyrazine. These are ascribed to the presence of a pseudo-Jahn–Teller interaction in this molecule.
Ab initio study of electronic, structural, and vibrational properties of the Si4C cluster
Authors: Zdetsis Aristides D. , Froudakis George , Mühlhäuser Max , Thümnel Helmar
Published: July 2002

Using Möller–Plesset second-order perturbation theory (MP2) for the geometry optimizations, we have examined various structural possibilities for the Si4C cluster. The energies of the MP2-optimized structures have been calculated using singles and doubles coupled cluster (CCSD) theory and the CCSD (T) method. The structure of lowest energy is a C3V symmetric trigonal pyramid made from four silicons and one carbon atom in a face capping position. Very close in energy (around 5 kcal/mol) lies an isomer with C2V symmetry, resembling the pyramid of the previous structure but with the carbon atom in an edge capping position this time. Both of these structures are closely related to the Si5 ground state structure. Planar and linear structures analogous to C5 and C4 lie higher in energy and they are transition states in most of the cases examined. To help future experimental tests of our present results, we have computed, at the MP2-level, the harmonic frequencies, infrared intensities, and isotopic shifts for the two lowest-lying isomers. Dipole moments and 1s core electron energy shifts are also given. The building up principle we have recently suggested from a study of the Si3C3 clusters is found to be fully operative for the Si4C cluster.
Thermal nucleation and cavitation in helium-3 fluids
Authors: Zeng X. C. , Oxtoby D. W. , Cheng E.
Published: July 2002

We have investigated droplet nucleation and bubble cavitation in the quantum fluid helium-3 based on a nonlocal density-functional approach. A marked effect of droplet (or bubble) curvature on the rate of droplet nucleation or cavitation has been found. Without considering this curvature effect (as in the classical theory of nucleation) the droplet nucleation rate for helium-3 could be underestimated (i.e., near 1 K) or overestimated (i.e., near 2.5 K) by orders of magnitude, respectively; for bubble cavitation, the rate could be underestimated by more than twenty orders of magnitude (near 1 K).