bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear optical spectroscopy of the Ag(111) surface in an electrolyte and in vacuum
Authors: Bradley R. A. , Georgiadis R. , Kevan S. D. , Richmond G. L.
Published: July 2002

In this paper, we investigate the electronic structure of a metal surface in the presence of aqueous electrolyte and an applied potential by optical second harmonic generation (SHG). We have obtained the detailed wavelength dependence (λSH=300–350 nm) of the SH response from Ag(111) in both an aqueous electrolyte and in ultrahigh vacuum (UHV) and find that, when the Ag(111) electrode is biased at the potential of zero charge (PZC), the SH response can be strongly correlated with the SH response in UHV. For the surface in both environments, there is a sharp peak near 3.82 eV. Possible contributing factors to this peak are discussed. In the electrochemical environment, the effect of applied potential on the SH response at longer wavelengths (nonresonant regime) is consistent both with previous observations at fixed frequencies and predictions of the surface charge density (SCD) model. At resonant wavelengths, there is a dramatic deviation from behavior predicted by the SCD model, a result consistent with previous experiments at discrete wavelengths.
Energy transfer rate constants for highly excited rovibrational levels of I2(X)
Authors: Nowlin M. L. , Heaven M. C.
Published: July 2002

Rate constants for collisional transfer between highly excited levels of I2(X) have been measured. A single vibration–rotation state (v″=42, J″=17) was populated by stimulated emission pumping. The prepared level, and collisionally populated levels, were detected by laser excitation of the D–X system. Relaxation kinetics were determined by variation of the delay between the dump and probe laser pulses. Collisions with He, Ar, I2, and O2 were investigated. Rotational energy transfer rate constants were measured for all collision partners. Vibrational energy transfer (Δv=−1) was observed for He, and O2, but this process was immeasurably slow for Ar and I2.
Critical phenomena in mixtures. I. Thermodynamic theory for the binary critical azeotrope
Authors: Munoz F. , Chimowitz E. H.
Published: July 2002

In this series of papers we provide new results related to critical phenomena in binary mixtures particularly with regard to the phenomenon of critical azeotropy. In Part I we carefully develop a set of necessary and sufficient conditions guaranteeing the existence of critical azeotropy in these mixtures. The sufficiency conditions are particularly useful for determining whether or not a critical azeotrope exists in a system, without having to actually find this point itself, typically a difficult experimental problem. Classical critical exponents of various thermodynamic properties along various paths are determined for the critical azeotrope. These are compared to the analogous situation near a pure fluid critical point in a dilute binary mixture. Both situations display analogies as well as differences that are discussed at some length. In Part II we concentrate upon describing interesting synergistic solvation effects in these systems using many of the results presented in Part I.
Atomic energies from renormalization of the large-dimension limit
Authors: Kais Sabre , Sung Stella M. , Herschbach Dudley R.
Published: July 2002

By augmenting Hartree–Fock (HF) results for nonrelativistic ground-state energies of N-electron atoms by analytic expressions for the D→∞ limit derived by Loeser, we obtain a simple renormalization procedure which substantially enhances accuracy. A renormalized nuclear charge, Z∞, is found which renders the dimensionally scaled energy at D→∞ a good approximation to that for D=3 with the actual Z. The renormalized charge is readily evaluated by comparing the HF energy (or any other input approximation) with its D→∞ limit. For atoms with any N or Z, the computations are elementary, requiring little more than solution of a quartic equation. With only HF input in addition to the D→∞ limit, the renormalization procedure yields about 2/3 or more of the correlation energy, for neutral atoms with N=Z=2→86. Further improvements in the method seem feasible, but will require better means to incorporate shell-structure in the large-D limit.
Surfactant film bending elasticity in microemulsions: Structure and droplet polydispersity
Authors: Sicoli F. , Langevin D. , Lee L. T.
Published: July 2002

The surfactant film bending elasticity can be described by a spontaneous curvature C0 and two elastic constants K and K̄ associated with the mean curvature and the Gaussian curvature, respectively. These parameters are very important in the determination of the structure of the dispersions stabilized by the surfactant (droplets or sponge-like structures). We have studied ternary mixtures of oil, water, and nonionic surfactants of different chain lengths. Depending on the temperature, the microemulsions are in equilibrium with excess oil (o/w structure), excess water (w/o), or both excess oil and water (bicontinuous). We present neutron scattering data from which we determine the microemulsion structure and, in the case of droplet structures, of the droplet polydispersity. These results, in combination with those from earlier experiments using ellipsometry, are used to estimate K̄.
First- and second-order phase transitions in asymmetric polymer mixtures
Authors: Deutsch Hans-Peter
Published: July 2002

The critical properties of dense asymmetric binary polymer mixtures are studied by grand canonical simulations within the framework of the three-dimensional bond fluctuation lattice model. The monomers interact with each other via a potential ranging over the entire first peak of the pair distribution. An asymmetry is realized by giving the ratio of interactions λ≡εAA/εBB between monomers of the A species and of the B species a value different from 1. Using multiple histogram extrapolation and finite size scaling techniques for the data analysis, the two-phase region, which is a line of first-order transitions driven by the chemical potential difference, and the critical point are determined for a mixture of chains with 32 monomers each and various asymmetries up to λ=5. At a critical potential difference Δμc unmixing occurs below a critical temperature Tc. It is found that the quantities Δμc/(1−λ)ε and 4kBTc/(3+λ)ε are both independent of the asymmetry, consistent with the prediction of the Flory theory. But the numerical values are overestimated by Flory theory by roughly a factor 1.5 for Δμc and 3.2 for Tc. For the first time a finite size scaling at first-order transitions and a way to distinguish first- and second-order transitions are presented for polymer mixtures.
Microscopic theory of solvation dynamics in dipolar liquids
Authors: Chandra A. , Wei Dongqing , Patey G. N.
Published: July 2002

A self contained microscopic theory is described for ion solvation dynamics in dipolar fluids. The theory takes account of inertial and non-Markovian effects which are of critical importance for the fast initial phase of the relaxation of the ion–solvent energy. The theory also includes diffusional effects important at long times. The results obtained are shown to be in good agreement with recent computer simulation studies of ions in Stockmayer solvents.
Control of fragment alignment via photodissociation from different types of parent rotation
Authors: David D. , Bar I. , Rosenwaks S.
Published: July 2002

The alignment of the OH fragment following dissociation of H2O (1,0,0) at 193 nm has been measured as a function of initially prepared parent rotations. The degree of alignment is almost perfect for an in-plane rotation, but is reduced markedly for a mixture of in- and out-of-plane rotations.
Photodissociation spectroscopy of Cu3, Cu3Ar, and Cu3Kr
Authors: Knickelbein Mark B.
Published: July 2002

The photodissociation spectra of the copper trimer and its argon and krypton adducts have been recorded in the visible. For Cu3, we observe a number of resolved vibronic transitions to the blue of the Ã←X̃ origin at 539.8 nm in accord with the observations of Morse et al. [J. Chem. Phys. 79, 5316 (1983)]. By contrast, the spectra of Cu3Ar and Cu3Kr consist of continuous absorption bands with maxima at 516 and 517 nm, respectively. While the oscillator strengths of the Cu3Ar and Cu3Kr absorption bands are nearly an order of magnitude larger than that derived for Cu3 from its photodissociation spectrum, they are in much closer agreement with the oscillator strength determined from the Cu3 Ã→X̃ radiative lifetime.
Vibrational contributions to static polarizabilities and hyperpolarizabilities
Authors: Cohen Michael J. , Willetts Andrew , Amos Roger D. , Handy Nicholas C.
Published: July 2002

Calculations of the vibrational contribution to diagonal components of static polarizabilities and hyperpolarizabilities of HF and H2O are compared with the results of perturbation theory. The calculations are based on finite difference derivatives of the vibrational energy. Effects of mechanical and electrical anharmonicity are shown to be important in the calculation of these properties.
Carbonyl–water hydrogen bonding: The H2CO–H2O prototype
Authors: Ramelot Theresa A. , Hu Ching-Han , Fowler Joseph E. , DeLeeuw Bradley J. , Schaefer Henry F.
Published: July 2002

The potential energy surface (PES) of the water–formaldehyde complex has been examined using ab initio methods. Three energetically low-lying stationary points were located on the potential surface corresponding to one minimum and two transition states. All stationary points were examined using a double-ζ plus polarization (DZP) basis set at the self-consistent field (SCF), single and double excitation configuration interaction (CISD), and single and double excitation coupled-cluster (CCSD) levels of theory. In addition, the minimum was more thoroughly investigated through the use of the triple-ζ plus double polarization (TZ2P) basis set, the TZ2P plus higher angular momentum functions [TZ2P(f,d)] basis, and the TZ2P basis set augmented by a set of diffuse functions (TZ2P+diff) with those same theoretical methods. For each of the stationary points, geometrical parameters, absolute energies, classical binding energies, and zero-point vibrational energy corrections are reported. Additional information concerning the minimum includes vibrational frequencies and corresponding infrared intensities, as well as predicted vibrational frequency shifts of the intramolecular frequencies for the complex and its deuterium substituted isotopomers.
Cooperative motion in liquids: On librational dynamics of chloroform throughout its normal liquid-phase range
Authors: Rothschild Walter G. , Cavagnat Raymond M.
Published: July 2002

We have extended the Raman spectral accumulations of the ν3 mode (A1, 367 cm−1) of liquid CHCl3–Cl-35 and its simulation in terms of an orientational equilibrium renewal process [W. G. Rothschild, R. M. Cavagnat, and P. Maraval, J. Chem. Phys. 99, 8922 (1993)] to a temperature of 338 K, about the normal boiling point of the system (335 K). The values of the best-fit parameters predict that the orientational motion of liquid chloroform, even at such a relatively high kinetic energy, is described predominantly by libratory states; their lifetime (∼1 ps) is four times longer than that of the free-rotational steps. The character of the orientational motion of the system, when traversing the range of 213 to 338 K from just above its melting to near its boiling point at about atmospheric pressure, reflects the softening of the liquid-cage structure in terms of an increasing dispersion and/or a decreasing value of the mean libration frequency, a lowering of the depth of its potential well, but near-invariance of its lifetime. Simultaneously, there is an approximately twofold increase in the lifetime of the much shorter stages of free-rotational motion. In essence, the system dynamics remain that of an assembly of librators.
Percolation and diffusion in two-dimensional microporous media: Pillared clays
Authors: Chen B. Y. , Kim H. , Mahanti S. D. , Pinnavaia T. J. , Cai Z. X.
Published: July 2002

We have investigated the adsorptive and diffusive properties of N2, H2O, and rare gas atoms (Ar and He) in the pillared layered silicate clay systems [Cr(en)3+3]x[Co(en)3+2−(en)]1−x−L, where L is vermiculite (V), fluorohectorite (FHT), or montmorillonite (M), and (en) is an ethylenediamine ligand. In these mixed ion intercalates the intercalated [Cr(en)3+3] cation, where all three en ligands are coordinated to chromium, represents a laterally small pillaring agent, whereas [Co(en)3+2−en] represents a laterally large, ligand-dissociated species. Such systems are excellent models for two-dimensional microporous media. Adsorption measurements were carried out for N2, H2O, and Ar and diffusion studies were performed using simulation methods for both Ar and He. We find that the adsorptive and diffusive properties depend sensitively on the size of the diffusing species and the concentrations x and (1−x) of the intercalants. For Ar adsorption in the FHT system we observe a percolative response when x reaches 0.79. Using simple geometrical models to describe these microporous media, along with computer simulation, we can understand the x=0.79 percolation threshold. In addition, simulation studies of the relative diffusion rates of He and Ar for x=0 and 1, and comparison of these rates with experimental measurements by Zhou and Solin, suggest that He diffusion near narrow constrictions may be strongly suppressed by quantum effects.
Resonance Raman spectra and quantum chemical vibrational analysis of the C7H7⋅ and C7D7⋅ benzyl radicals
Authors: Langkilde Frans W. , Bajdor Krzysztof , Wilbrandt Robert , Negri Fabrizia , Zerbetto Francesco , Orlandi Giorgio
Published: July 2002

Time-resolved resonance Raman (RR) spectra of the benzyl radical and its perdeutero isotopomer are presented. The radicals are created by laser flash photolysis (λ=266 nm) of benzylchloride in solution. The spectra are excited at a wavelength of 315 nm, in resonance with the intense D3(2 2A2)←D0(1 2B2) transition. Twenty Raman bands, both polarized and depolarized, are observed. The RR spectra are analyzed through quantum chemical force field for π electrons (QCFF/PI) and ab initio calculations of equilibrium geometries, vibrational frequencies, Franck–Condon factors, and vibronic interactions. Polarized intense bands are assigned to totally symmetric a1 modes, and a number of depolarized bands to fundamentals of b1 modes. The observed activity of b1 modes suggests vibronic coupling, which is confirmed theoretically by calculations of vibronic interactions between the D3(2 2A2) and D5(4 2B2) states. The results from semiempirical and ab initio calculations are compared with experiment. The contributions of the different internal coordinates to the normal modes are calculated together with the changes in the molecular structure upon electronic excitation from D0 to D3 and D5.
Vibrational and rotational effects in the Cl+HOD↔HCl+OD reaction
Authors: Nyman Gunnar , Clary David C.
Published: July 2002

Quantum scattering calculations on the Cl+HODR⇌Cl+OD reaction have been performed at collisional energies up to 1.6 eV. The rotating bond approximation is used. In this method, the OD rotation and HCl vibration as well as the bending motion and OH local stretch of HOD are treated explicitly. Here, the theory is extended to account for thermal HOD reactant rotation. The potential energy surface used has accurate reactant and product rovibrational energy levels, correct bond dissociation energies, and a transition state geometry in accord with ab initio data. Mode selectivity is observed—HOD vibrational stretch energy enhances reaction more than vibrational bend energy. Translational energy enhances reaction more than vibrational stretch energy at low total energies, but not generally at higher total energies. Excitation of the local OH stretch in the reactant HOD produces vibrationally excited HCl product. The OD product rotation depends on the reactant HOD rovibrational state. The OD+HCl(v=0) reaction preferentially produces HOD in the vibrational ground state, while the OD+HCl(v=1) reaction preferentially produces HOD with one quantum of vibrational stretch energy. A calculated OH product rotational distribution for the Cl+H2O reaction agrees quite well with experiment.
Method for quasiclassical trajectory calculations on potential energy surfaces defined from gradients and Hessians, and model to constrain the energy in vibrational modes
Authors: Varandas A. J. C. , Marques J. M. C.
Published: July 2002

A method for calculating quasiclassical trajectories on potential energy surfaces defined using a sequence of model quadratic surfaces (QCT/GH) is suggested, and tested for atom–diatom collisions against the traditional quasiclassical trajectory approach. A simple model is also suggested to constrain the classical energy of a bound vibrational mode to be greater than a specified amount, namely, its zero-point energy value. Essentially the model consists of assuming that the sum of the energies in the nonrelevant vibrational modes (typically unbound modes) of the supermolecular complex acts as a pool from which energy may be taken to compensate any leak of vibrational energy in the relevant bound modes, hence preventing the latter from falling below zero-point value. Extensive QCT/GH trajectory calculations carried out for the H+H2 exchange reaction, which occurs over an energy barrier, as well as exploratory trajectories for the reaction O+OH→O2+H, which occurs on a potential energy surface with a deep chemical well, have shown that the total energy and total angular momentum are conserved within a small numerical tolerance. Correcting for the leak of zero-point vibrational energy still leaves the total energy rigorously conserved but the total angular momentum is then only approximately kept constant. For H+H2(v=0, j=0)→H2(v′, j′)+H, the calculated state-to-state QCT/GH cross sections show reasonably good agreement with those of converged quantum results reported in the literature for the same H3 potential energy surface. This agreement does not deteriorate after correction of zero-point energy leak. For both H3 and HO2, accurate global analytical potential energy surfaces based on the double many-body expansion method have been utilized. Using these prototype systems, an assessment is made of the difficulties encountered on direct reaction dynamics using the novel QCT/GH method.
Exploring complexity in some simple nonlinear chemical kinetic schemes
Authors: Chaudry A. N. , Coveney P. V. , Billingham J.
Published: July 2002

We study two nonlinear chemical kinetic schemes which are arguably the simplest that can display chaotic behavior. These schemes model situations where precursor decay is neglected and included respectively and can represent both isothermal and thermokinetic processes. We make use of a consistent nondimensionalization that has the advantage of unifying all the previously published related models. A systematic investigation of the dynamical behavior within a subspace of the full parameter space reveals clearly distinguished regions where sequences of period doubling, chaos and mixed-mode oscillations exist. We find evidence for a sequence of mixed mode oscillations convolved with chaotic attractors in an extremely complex manner; in this region of parameter space our studies confirm that bistability, and period-doubling to chaos from both simple and mixed mode oscillations can occur, and support conclusions recently reported by Petrov et al. [J. Chem. Phys. 97, 6191 (1992)]. Detailed numerical work indicates that this complexity may be associated with the presence of a tangent homoclinic orbit biasymptotic to a periodic orbit. In addition, Lyapounov spectral analysis confirms the existence of low dimensional chaotic attractors. We suggest some typical experimental scenarios where such complex behavior might be expected.
Integral equation and simulation studies of a realistic model for liquid hydrogen chloride
Authors: Martín C. , Lomba E. , Lombardero M. , Lado F. , Ho/ye J. S.
Published: July 2002

Liquid hydrogen chloride is modeled by a system of heteronuclear two-center Lennard-Jones particles with embedded point dipoles and quadrupoles. The effect of molecular polarizability is incorporated via an effective dipole approximation. The study is performed by Monte Carlo reaction field simulation and by hypernetted chain and reference hypernetted chain integral equations. Our simulation results yield dielectric properties in excellent agreement with experimental data for liquid HCl. As for the integral equation approach, we have experimented with an empirical choice of the reference system in the spirit of a recently proposed treatment which has proved extremely successful for pure and quadrupolar two-center Lennard-Jones fluids. The hypernetted chain equation performs slightly better when accounting for the multipolar contributions to the configurational energy, but as a whole the reference hypernetted chain equation, as introduced, here proves to be a more appropriate choice.
A molecular dynamics study of chain configurations in n-alkane-like liquids
Authors: Brown David , Clarke Julian H. R. , Okuda Motoi , Yamazaki Takao
Published: July 2002

We present precise measurements of the configurational properties of n-alkane-like liquids with 4, 5, 8, 20, 50, and 100 methylene groups. Quantities measured include the distributions of square radii of gyration, square end-to-end distances and proportion of conformers. To achieve high precision for n=50 and 100 a Fujitsu AP 1000 massively parallel processing machine with up to 512 processors is used. The results are compared with those obtained for the Flory model of alkane chains in which only correlation between nearest-neighbor torsion angles are considered; the latter were obtained by Monte Carlo sampling of isolated chains using the pivot algorithm. Discrepancies between the two sets of data are discussed in terms of solvent and end effects. For n=100 full configurational relaxation was not observed within 2 ns.
Electron detachment reactions of fluorinated carbanions with atomic hydrogen
Authors: Morris Robert A. , Viggiano A. A. , Paulson John F.
Published: July 2002

Rate constants were measured for electron detachment reactions of atomic hydrogen with CF−3, C2F−5, and C3F−3. The experiments were performed using a selected ion flow tube (SIFT) instrument. The density of atomic hydrogen was calibrated by studying the reaction of F− with H, and rate constants were measured relative to the known rate constant for this reaction. The reactions with H are fast, proceeding at 17%–66% of the collisional rates. No ionic products were observed for any of the reactions studied. While associative detachment is exothermic for all three anions, other reactive detachment processes are also possible. These anions were found to be unreactive with H2.
Large basis set calculations using Brueckner theory
Authors: Kobayashi Rika , Amos Roger D. , Handy Nicholas C.
Published: July 2002

Calculations are reported using the Brueckner doubles method, with and without a perturbative estimate of the effects of triple excitations, and using basis sets which are at least triple zeta with two sets of polarization functions in quality. Equilibrium geometries and harmonic vibrational frequencies are calculated for HF, N2, H2O, NH3, CH4, HCN, H2CO, NNO, and O3. Comparison with experimental data confirm the high accuracy of Brueckner methods.
Optimal control of molecular dynamics via two-photon processes
Authors: Yan YiJing
Published: July 2002

The theory of optimal control of molecular dynamics via a two-photon process is developed in terms of the density matrix in Liouville space. Applications are made to the generalized Tannor–Rice pump-dump molecular control scheme involving two and three electronic states. The solutions of two coupled eigenequations directly give the optimal pump and dump control fields with respect to the target. The globally optimal solutions are analyzed and the numerical implementation is discussed. Formal comparisons with pump–probe (χ(3)) optical processes are also given.
Metal/conjugated polymer interfaces: A local density functional study of aluminum/polyene interactions
Authors: Fredriksson C. , Lazzaroni R. , Brédas J. L. , Ouhlal A. , Selmani A.
Published: July 2002

The interactions between aluminum atoms and model molecules representing trans-polyacetylene are studied quantum chemically by a local density functional method. We focus on the chemical and electronic structure of the organoaluminum complexes. Special emphasis is put on a comparison between results at the local spin density approximation and ab initio Hartree–Fock levels. In unmetallized polyenes, the density functional method provides a very good description of the carbon–carbon bond lengths of conjugated systems; in the case of hexatriene, it reproduces the bond dimerization in very good agreement with experimental measurements. Upon metallization, a strong covalent interaction between aluminum and carbon is found. The Al–C bond formation induces an interruption of the bond alternation pattern and reduces the π-conjugation in the oligomer, in qualitative agreement with photoelectron spectroscopy data and previous theoretical results at the Hartree–Fock level. Notably, the π-electron levels in the organoaluminum complexes maintain delocalization. In contrast to Hartree–Fock results where an aluminum atom binds to a single carbon, the interactions calculated with the local spin density approximation lead to (i) formation of multicenter aluminum–carbon bonding; (ii) near planarity of the polyene molecule; and (iii) a lower degree of charge transfer from the metal atom to the polymer.
Dielectric relaxation of electrolyte solutions: Molecular dynamics and theoretical results for ions in simple dipolar solvents
Authors: Chandra A. , Patey G. N.
Published: July 2002

Computer simulation and theoretical results are given for the frequency dependent dielectric function of model electrolyte solutions. The influences of salt concentration, ionic charge, and solvent dipole moment are discussed. It is found that a substantial salt induced dielectric decrement persists up to the dispersion frequency. At higher frequencies, however, the dielectric constant of the solution is larger than that of the solvent giving a salt induced dielectric increment.
Non-Markovian optical dephasing dynamics in room temperature liquids investigated by femtosecond transient absorption spectroscopy: Theory and experiment
Authors: Cong Peijun , Yan Yi Jing , Deuel Hans P. , Simon John D.
Published: July 2002

This paper examines the electronic dephasing dynamics of the dye molecule HITCI in an ethylene glycol solution. Degenerate transient absorption data are presented as a function of laser pulse width and detuning from the absorption maximum. The dephasing dynamics revealed by the experimental results are shown to be non-Markovian. The experimental data cannot be quantitatively described by stochastic theories that assume a single relaxation time for the solvent. A model is presented that includes contributions from both fast and slow modulations to the absorption line shape. Using this approach, quantitative agreement is observed between theory and experiment. These results clearly indicate that multiple time scales for liquid dynamics contribute to the absorption line shape of solute molecules. The solvent parameters obtained are discussed in light of results from complementary experiments designed to measure characteristic relaxation times.
Chemisorption of sulfur on nickel: A study of cluster convergence in the linear combination of Gaussian-type orbitals local density functional approach
Authors: Ackermann L. , Rösch N.
Published: July 2002

Chemisorption of sulfur at the (100), (110), and (111) surface of nickel has been studied, using the linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. Employing various cluster models consisting of 11 to 29 substrate atoms, adsorption at the experimentally known sites has been considered. Besides the equilibrium distance and the force constant of the vertical adsorbate motion, the dynamical dipole moment was evaluated and it turned out to provide a sensitive probe of cluster convergence. The influence of atoms from the third substrate layer on the various observables is found to be considerable in some cases. With increasing cluster size, bond lengths are stabilized to 0.02 Å, frequencies to 20 cm−1, and dipole moments to 0.1 D. The converged results agree very well with experiment. Adsorption induced population changes are restricted to only four to six neighboring substrate atoms of the modifier atom. However, energy resolved charge density differences reveal a possible mechanism for the transmission of the long-range electronic effects caused by the adatom.
Perylene–rare-gas heteroclusters. II. Nuclear dynamics and spectral line shapes
Authors: Heidenreich Andreas , Bahatt Dar , Ben-Horin Narda , Even Uzi , Jortner Joshua
Published: July 2002

In this paper we report on molecular dynamics simulations of the line shapes of the absorption spectra of perylene⋅Arn heteroclusters (n=1–45), which rest on the spectral density method. Inhomogeneous semiclassical absorption line shapes were calculated by averaging of microcanonical spectra over the accessible phase space region. We utilized a potential modeling scheme for the excited-state atom–atom potentials, which accounts well for the spectral shifts of small (n=1–6) clusters. The size dependence and the isomer specificity of the spectral shifts and spectral linewidths were elucidated. The analysis of the power spectra of the energy gap correlation function established that the dominating contribution to the line shape and line broadening of the electronic origin at finite temperatures (T≥10 K) originates from the low-frequency diffusive soft mode. For these heteroclusters the soft mode contribution can be described within the stochastic slow modulation limit, being determined by the dispersion of the soft mode and being independent of its damping. Finite-frequency intermolecular motion gives only a minor (≤10%) contribution to the line broadening. Our analysis interrelates spectral line broadening and nuclear dynamics.
The effect of an external electric field on diffusion-controlled bulk ion recombination
Authors: Isoda K. , Kouchi N. , Hatano Y. , Tachiya M.
Published: July 2002

The effect of an external electric field on the rate of diffusion-controlled ion recombination was investigated. The Brownian motion of positive and negative ions under the influence of the mutual Coulomb attractive potential and an external electric field was described by using the diffusion equation. Both fully and partially diffusion-controlled recombinations were considered. Analytical expressions for the rate constant were obtained for low electric fields. For high electric fields, the rate constant was calculated numerically. For fully diffusion-controlled recombination, the rate constant always increases with increasing field strength. However, for partially diffusion-controlled recombination, the rate constant decreases with increasing field strength if the intrinsic rate at the encounter distance is very slow. Possible physical mechanisms were proposed to explain these results. Finally, these results were compared with available experimental data on the electric field effect on electron–ion recombination.
An improved quasistatic line-shape theory: The effects of molecular motion on the line wings
Authors: Ma Q. , Tipping R. H.
Published: July 2002

A theory is presented for the modification of the line-shape functions and absorption coefficient due to the breakdown of the quasistatic approximation. This breakdown arises from the effects of molecular motion and increases the absorption in the near wings. Numerical calculations for the high-frequency wing of the ν3 band of CO2 broadened by Ar are reported and it is shown that these effects are significant near the bandhead. The importance of such corrections in other spectral regions and for other systems is discussed briefly.
System–bath relaxation theory approach to nonadiabatic coupling effects on condensed phase electronic absorption spectra
Authors: Evans D. G. , Coalson R. D.
Published: July 2002

Separation of a many-body system into a primary system plus a bath of background modes enables approximate calculation of electronic absorption spectra and zero-temperature resonance Raman scattering cross sections in cases where there is nonadiabatic coupling between two or more Born–Oppenheimer excited-state potential surfaces. In particular, the low-resolution optical line shape theory recently developed to describe curve-crossing phenomena [D. G. Evans and R. D. Coalson, J. Chem. Phys. 99, 6264 (1993)] is extended to systems where there is a primary nuclear coordinate mode that is characterized by large excited-state displacements and an ensemble of weakly displaced bath modes. The accuracy of the resulting approximation scheme is illustrated using the spectroscopic spin-boson model, in which all surfaces are harmonic, and path integral techniques can be used to obtain exact results. Application to more complicated systems is discussed.
Visible absorption and infrared emission of CN–Xe in an argon matrix
Authors: Thoma A. , Schallmoser G. , Smith A. M. , Wurfel B. E. , Bondybey V. E.
Published: July 2002

The visible absorption of the B 2Σ←X 2Σ transition and the ground state vibrational infrared emission of the CN–Xe complex are studied in an argon matrix.
Highly concentrated salt solutions: Molecular dynamics simulations of structure and transport
Authors: Payne V. A. , Forsyth M. , Ratner M. A. , Shriver D. F. , de Leeuw S. W.
Published: July 2002

Molecular dynamics (MD) simulations in NaI solutions, where the solvent has been represented by the Stockmayer fluid, were performed as a function of temperature, salt concentration, and solvent dipole strength. At higher temperatures contact ion pairs become more prevalent, regardless of solvent strength. An examination of the temperature dependence of the potential of mean force demonstrates the entropic nature of this effect. The transport properties calculated in the simulations are dependent on the balance between solvent dielectric constant and ion charge. In systems with a large solvent dipole moment, the ions appear to be independently mobile, and deviations from Nernst–Einstein behavior are small. In systems of smaller solvent dipole moment or greater ion charge, the ions form clusters, and large deviations from Nernst–Einstein behavior are observed.
Interaction of Turing and flow-induced chemical instabilities
Authors: Dawson S. Ponce , Lawniczak A. , Kapral R.
Published: July 2002

The interaction between the Turing instability and the instability induced by a differential flow is studied in the Selkov model. Both instabilities give rise to the formation of spatial patterns, and for a range of parameter values, these patterns can compete. The effect of anisotropic diffusion on the pattern formation process is investigated. Stripes with different orientations that travel with time and the suppression of patterns due to a competition of both instabilities are observed.
The influence of preadsorbed K on the adsorption of PF3 on Ru(0001) studied by soft x-ray photoelectron spectroscopy
Authors: Chakarian Varoujan , Shuh David K. , Yarmoff Jory A. , Tao Hui-Shu , Diebold Ulrike , Maschhoff Brian L. , Madey Theodore E. , Shinn Neal D.
Published: July 2002

Soft-x-ray photoelectron spectroscopy (SXPS) is utilized to study the coadsorption of K and PF3 on Ru(0001) at 90 and 300 K. In the absence of K, PF3 adsorbs molecularly at both temperatures. In the presence of a fractional monolayer of K, initially PF3 completely dissociates resulting in the formation of adsorbed KF and P species. As the surface is further exposed to PF3, some of the PF3 molecules adsorb via partial dissociation, resulting in the formation of PF and PF2. This process continues until all the K has reacted. At 300 K, a fraction of the incoming PF3 molecules react with the adsorbed KF and form a species which is tentatively identified as KPF6. The data show that surface chemistry is different at the two temperatures, as some of the chemical reaction channels occurring at 300 K are blocked at 90 K. The reduced surface mobility of the incident PF3 molecules at 90 K adversely affects the probability of PF3 and KF interactions, which, in turn, causes the concentration of adsorbed PF3 relative to P to be larger at lower temperatures.
On the low energy limits of inelastic molecule–surface scattering
Authors: Bittner Eric R.
Published: July 2002

The zero energy scattering limit of inelastic molecule–surface scattering is studied within the context of a multiphonon expansion of the molecule–bath wave function. By assuming that at low scattering energies the expansion may be truncated at first order in the phonon operators, we derived a closed form solution to the Lippmann Schwinger equation for the scattering wave function which includes a nonlocal and energy dependent self-energy term which correctly incorporates virtual phonon transitions in the elastic channel. The closure relation results from the use of a discrete spectral (ℒ2) form of the inelastic channel Greens functions. We compute the zero energy limit of these wave functions and discuss the trapping and reflection of cold atoms from ultracold surfaces. Our results indicate that for realistic atom surface interactions the low energy limit of the sticking coefficient, s, can deviate markedly from the expected s∝E1/2 behavior and is shown to approach a constant nonzero limiting value. This trend is consistent with recent experimental work involving the sticking of spin polarized hydrogen atoms on liquid He films.
Analysis of concentration and temperature patterns on catalytic surfaces
Authors: Colin Pierre , Balakotaiah Vemuri
Published: July 2002

A simple mathematical model for pattern formation on isothermal as well as nonisothermal catalytic surfaces is developed and analyzed. The model accounts for diffusion of the species, conduction of heat, convection from the fluid phase, and a bimolecular Langmuir–Hinshelwood type kinetic expression. The isothermal model is shown to exhibit stationary concentration patterns for typical sets of parameters. The nonisothermal model exhibits stationary temperature and concentration patterns only for near stoichiometric composition of the reactants (three equation model). The calculations show that these stationary patterns exist in regions near the ignition and extinction points and are most likely to form during ignition or extinction of the surface. It is also found that moving concentration and temperature patterns exist near the Hopf bifurcation point of the ignited homogeneous branch. The moving patterns predicted for realistic values of the transport and kinetic parameters are concentration patterns with almost constant temperature distribution on the surface. The typical size of the patterns and the period of oscillation are estimated in terms of the physicochemical parameters.
The photodissociation dynamics of ICN at 304.67 nm by state-selective one-dimensional translational fragmentation spectroscopy
Authors: Griffiths Jennifer A. , El-Sayed Mostafa A.
Published: July 2002

The photodissociation dynamics of ICN to CN+I(2P3/2) are investigated by state selective one-dimensional photofragmentation translation spectroscopy at 304.67 nm. Translational energy release, laboratory anisotropy factors, and energy distributions are obtained from analysis of the velocity and spatial distributions of the photodissociated iodine atoms. Two velocity distributions peaks are deconvoluted which are found to be separated by 2000 cm−1, which is the CN stretching vibration of the CN radical. The high intensity velocity peak is assigned to dissociation to I+CN(X 2Σ+) in v=0 (channel I), while the weak lower velocity peak is attributed to dissociation to I+CN(X 2Σ+) in v=1 (channel II). More than 80% of the iodine are produced from channel I and are found to have a relatively small anisotropy parameter, β, that is independent of velocity, suggesting a mixed absorption polarization leading to rapid dissociation. The weak shoulder, representing less than 20% of the photodissociated iodine, is formed via channel II and found to have a β value that decreases with velocity and produces CN with more of the available excess energy appearing in rotation, suggesting longer dissociation time that allows for more energy redistribution prior to dissociation. The dissociation mechanisms involved in these two channels are discussed in terms of these results, the theoretically predicted properties of the 3Π+0 and 3Π1 surfaces of ICN, our previous conclusion that suggests that ICN bends prior to dissociation via channel II, the laser wavelength used, and curve crossing between the 3Π+0 and 1Π1 surfaces.
Perturbative and complete model space linked diagrammatic expansions for the canonical effective operator
Authors: Hurtubise Vincent , Freed Karl F.
Published: July 2002

Effective Hamiltonians and effective operators produce, respectively, exact energies and matrix elements of a time-independent operator A for a finite number of eigenstates of a time-independent Hamiltonian H. We obtain degenerate and quasidegenerate perturbative expressions for the particularly useful canonical effective operator ÂC through second order in perturbation theory. The corresponding ÂC diagrammatic expressions are derived for the case where ÂC acts in a complete finite space. Our first order results have been used previously for ab initio computations of dipole and transition dipole moments in diatomic hydrides and for testing the assumptions in semiempirical methods for dipole properties. A brief discussion is also provided on the computational labors required by first and second order ÂC many-body calculations, the derivation of ÂC diagrams when ÂC acts in an incomplete finite space, and on the derivation of diagrammatic rules for ÂC in arbitrary perturbation order.
A comparison of photoelectron spectroscopy and two-photon ionization spectroscopy: Excited states of Au2, Au3, and Au4
Authors: Handschuh H. , Ganteför G. , Bechthold P. S. , Eberhardt W.
Published: July 2002

Photoelectron spectra of Au−n with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two-photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag−n clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.
Floating spherical Gaussian orbitals based quantum Monte Carlo method in molecular electronic calculations
Authors: Leu Shyn-Yi , Mou Chung-Yuan
Published: July 2002

We report a new method for fixed-node diffusion quantum Monte Carlo calculation in order to improve the time step-size convergence and the fluctuation problems associated with nodes of wave functions. It adopts floating spherical Gaussian orbitals instead of Slater-type orbitals to guide random walkers (psips). They establish harmonic velocity fields under which psips undergo random walks which are simulated exactly according to the Ornstein–Uhlenbeck process. The nodes of this guiding function are used to guide psips in a novel way; as psips approach to a node, they are swept away therefrom. Explicit enforcement of a sink boundary condition is avoided to some extent, and the behavior of the random walks near nodes is efficiently simulated. The method is illustrated by applying to the ground-state energies of H2, LiH, and HF. The results are excellent compared to previous calculations.
Microwave spectrum and molecular structure of the cyclic C3H radical
Authors: Yamamoto Satoshi , Saito Shuji
Published: July 2002

The rotational spectral lines of the cyclic C3H radical in the 2B2 ground electronic state are observed by microwave spectroscopy. The radical is produced in an absorption cell by discharging a mixture of C2H2, CO, and He. The spectral lines of two 13C isotopic species are also observed. The radical is a planar molecule with C2v symmetry, and the rs structure is determined as follows: rs(C–H)=1.0760 Å, rs(C–C)=1.3771 Å, and rs(C–CH)=1.3739 Å. From the observed inertial defect, the vibrational frequency of the C–C antisymmetric vibration is estimated to be about 500 cm−1, which is fairly low for the frequency of the C–C stretching mode. The low vibrational frequency arises from the vibronic interaction between the ground electronic state and the low-lying 2A1 electronic state. On the basis of the hyperfine coupling constants obtained for the 13C species, the unpaired electron is found to be almost evenly distributed among the three carbon atoms. Such delocalization of the unpaired electron is consistent with the observed geometrical structure.
Resonance positions and widths by complex scaling and modified stabilization methods: van der Waals complex NeICl
Authors: Ryaboy Victor , Moiseyev Nimrod , Mandelshtam V. A. , Taylor H. S.
Published: July 2002

The discrete variable representation (DVR) formulation of the complex coordinate method as has been used for calculating several resonances of NeICl [J. Chem. Phys. 98, 1888 (1993)], and a modified version of the recent developed stabilization method [Phys. Rev. Lett. 70, 1932 (1993)] are used for calculating all 30 isolated narrow resonances of NeICl (B, ν=2). The two ℒ2 methods require a similar computational effort. The modified stabilization method requires the calculations of eigenvalues of real and symmetric Hamiltonian matrices in a sequence of ever larger enclosing boxes. The complex DVR method requires the use of complex arithmetic and calculations of eigenvalues of complex symmetrical matrices.
Quantum scattering calculations on the CH4+OH→CH3+H2O reaction
Authors: Nyman Gunnar , Clary David C.
Published: July 2002

Quantum scattering calculations on the CH4+OH→CH3+H2O reaction have been performed at thermal energies. The rotating bond approximation is used, treating CH3 as a pseudoatom. The OH rotation and a reactive C–H stretch of CH4 are treated explicitly as well as the bending motion and one OH local stretch vibration of H2O. Two potential energy surfaces are used. Both have accurate reactant and product rovibrational energy levels for the modes explicitly treated in the scattering calculations and incorporate the zero point energy of the other modes. They have correct bond dissociation energies and transition state geometries in reasonable accord with ab initio data. Mode selectivity is found: reactants in the ground rovibrational states produce ground state H2O, and vibrationally excited CH4 produces vibrationally excited H2O. Reactant OH rotational excitation decreases the reaction cross sections. Rate constants are obtained using an adiabatic approach to account for all degrees of freedom not explicitly treated in the scattering calculations. Large contributions due to tunneling are observed. The rate constants are in quite good agreement with previous theoretical and experimental work.