bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum Monte Carlo studies of small B(H2)n clusters
Authors: Vegiri Angeliki , Alexander Millard H. , Gregurick Susan , McCoy Anne B. , Gerber R. Benny
Published: July 2002

The structure and stability of clusters of a boron atom with one to eight H2 molecules is investigated. For the simplest BH2 clusters, the lowest ab initio adiabatic potentials for o-H2 and p-H2 interacting with a boron atom are used. For the larger clusters (n=2–8), the p-H2 is treated as a sphere, and the total potential is taken to be the sum of pairwise additive B–H2 and H2–H2 interactions which include, in the former case, an anisotropy due to the orientation of the unpaired B 2p electron. This electronic interaction is considerably more attractive when H2 approaches the B atom in a plane perpendicular to the orientation of the 2p orbital. The local and global minima on these potential surfaces were located and diffusion quantum Monte Carlo simulations were used to determine the energies and properties of the ground state wave functions for these B–(H2)n clusters. For the B(H2) cluster, a comparison is made with the results of variational calculations.
Monte Carlo simulation for chain molecules in supercritical ethane
Authors: Koga Yoshio , Iwai Yoshio , Arai Yasuhiko
Published: July 2002

Monte Carlo simulation was applied to calculate the static properties of chain molecules (C28H58, C30H62, and C32H66) in supercritical ethane at 308.15 K. Chain molecule and ethane were treated as a linear chain with many sites and single site molecules, respectively. The potential functions proposed by Jorgensen et al. were used for the interaction energy between pair sites and the rotational potential energy of valence bonds. The residual chemical potential was calculated by the isothermal–isobaric Kirkwood method in order to calculate the solubilities of chain molecules in supercritical ethane. An intermolecular interaction parameter between ethane and chain molecule was introduced to fit the calculated solubilities to the literature data. Furthermore, the mean-square end-to-end separation, the mean-square radius of gyration, the probability density distribution of rotational angles of the chain molecules, and the radial distribution function were reported as fundamental knowledge of the microstructure of the chain molecule in supercritical ethane.
Photoinduced fluorescence from the perylene cation isolated in Ne and Ar matrices
Authors: Joblin C. , Salama F. , Allamandola L.
Published: July 2002

The fluorescence and fluorescence excitation spectra of the perylene cation isolated in neon and argon matrices are reported. This is the first report of the fluorescence spectrum of a polycyclic aromatic hydrocarbon ion in any phase.
Ion desorption from H2O chemisorbed on Si(100) by O 1s electron excitation at room temperature
Authors: Sekiguchi Tetsuhiro , Ikeura Hiromi , Tanaka Kenichiro , Obi Kinichi , Ueno Nobuo , Honma Kenji
Published: July 2002

The photon stimulated ion desorption (PSID) of H+ and O+ from monolayer H2O adsorbed on the Si(100) has been studied using pulsed synchrotron radiation in the 510–620 eV range. Ions were detected and mass analyzed by means of time-of-flight spectroscopy. Relative yield spectra of these ions indicated characteristic behavior near the O K edge. The predominant H+ fragment ion shows a sharp rise at about 530 eV and two broad peaks (535 eV, 558 eV), while O+ shows a delayed threshold at about 570 eV and gradual increase up to 700 eV. Above the O K edge, these ion yields were found to deviate significantly from the O(KVV) Auger electron yield. This was interpreted by the fact that ion desorption was enhanced by multiple ionization and is strongly affected by the decay process with the surface. In order to elucidate the mechanism of the PSID, the technique of photoion–photoion coincidence between H+ and O+ ions was also applied. The results suggested the formation of multiply charged intermediates, i.e., OH4+ ions, which were incompletely reneutralized despite of strong interaction with substrate and desorbed as singly charged H+ and O+ ions.
Infrared vibrational spectra of matrix-isolated cyclic Li2F2, Li3F3, and Li4F4 isotopomers
Authors: Redington Richard L.
Published: July 2002

Infrared (IR) spectra of the 6,7Li isotopomers of the LiF species isolated in rare gas matrices are interpreted with the input of molecular geometries and vibrational spectra computed using ab initio molecular orbital (MO) theory at the MP2/6-31G* level. The MO computations, together with IR matrix-isolation observations, support firm species assignments and vibrational assignments for the IR-active modes of the title molecules. The study presents the first evidence for the existence of matrix-isolated Li4F4 cyclic tetramer, and resolves contradictory species/vibrational assignments for the smaller oligomers. Most of the matrix-isolated cyclic trimer and tetramer population is formed by coalescence of smaller species during the matrix deposition process or during subsequent matrix annealing/warming processes. There appears to be no evidence in the available matrix-isolation data for the existence of a matrix-stabilized Li2F2 linear dimer.
The rotational state dependence of the dynamics of pyrimidine in the S1 (1B1) state studied by time resolved spectroscopy
Authors: Carter Robert T. , Bitto H. , Huber J. Robert
Published: July 2002

The dynamics of pyrimidine in three vibronic levels of the S1 (1B1) state with excess energies of 1100–1300 cm−1 above the 00 level has been studied by time resolved spectroscopy. The fluorescence decays from 40 to 50 single rovibronic levels in each vibronic band were measured and were found to be biexponential due to coupling with background triplet states. Analysis of the decays yielded the amplitude ratio of the fast (coherent) and slow (incoherent) component of the fluorescence intensity, A+/A−, and the lifetime of the slow component, being τ ≊700–900 ns. Both data show a clear dependence on the S1 rotational quantum number J. The A+/A− ratio, which reflects the number of coupled states, takes the form of a linear dependence on (2J+1) implying full rovibronic state mixing in the background 3B1 state. More states were found to be coupled than could be accounted for by the calculated background state density, suggesting further state mixing, possible mechanisms for which are discussed. The lifetime results were found to converge to limiting values at high J, the lifetime of the 3B1 state at the given energy. A small but significant difference (∼10%) in the lifetimes of the asymmetry doublets with Kc=1 was observed. Finally, a comparison of the rotational state dependent dynamics in pyrimidine with those in propynal and butynal is presented, pointing to a role for nuclear hyperfine interactions in dynamics.
Correlation function approach to transition state resonances in collinear (He,H2+) collisions
Authors: Mahapatra Susanta , Sathyamurthy N.
Published: July 2002

Dynamical resonances in collinear He+H2+ → HeH++H reaction on the McLaughlin–Thompson–Joseph–Sathyamurthy potential energy surface are identified in terms of the transition state spectrum I(E), computed by Fourier transforming the autocorrelation function C(t) for the system. Eigenenergies of quasibound states corresponding to the resonances, starting from the nonreactive regime up to the dissociation threshold are reported. The nature of some of the resonance states is investigated by calculating the eigenfunctions for the corresponding eigenenergies and also their lifetimes. The relation to the corresponding resonant periodic orbits in classical mechanics at those energies is also established. In addition we predict several resonances at higher energies not reported in earlier calculations.
Gauge invariance of linear response properties using the perturbed electron propagator
Authors: Deleuze Michael , Packer Martin J. , Pickup Barry T. , Wilton David J.
Published: July 2002

The perturbed electron propagator is discussed as a means of obtaining static linear response properties. The dynamic self-energy is derived at first order in the field and second order in correlation using a diagrammatic method. We discuss the invariance properties of the first-order perturbed electron density which may be obtained from the perturbed electron propagator formalism.
Quantum chemical study on the equilibrium geometries of S3 and S−3, The electron affinity of S3 and the low lying electronic states of S−3
Authors: Koch Wolfram , Natterer Johannes , Heinemann Christoph
Published: July 2002

The geometries and relative stabilities of the open, C2v symmetric and closed, D3h symmetric forms of thiozone and its anion, the adiabatic electron affinity of S3 and the energies of the three low-lying excited electronic states of the thiozone anion (Ã 2B2,B̃ 2A1,C̃ 2A2) at the optimized geometry of the X̃ 2B1 ground state are computed employing coupled-cluster [CCSD(T)], second-order multireference perturbation theory (CASPT2), and multireference CI (MRCI and IC-MRCI) methods using large atomic natural orbital basis sets. In addition, the saddle point for the open→closed isomerization on the neutral S3 potential energy surface is being studied. Surprisingly, the calculations do not show the expected underestimation of the experimentally determined electron affinity, in sharp contrast to test calculations on the sulfur atom, the disulfur molecule, and earlier results for ozone. Apart from this, thiozone and its anion behave in many respects qualitatively similar as ozone and O−3, while quantitatively various differences are observed.
Harmonic vibrational frequencies of the water monomer and dimer: Comparison of various levels of ab initio theory
Authors: Kim Jongseob , Lee Jin Yong , Lee Sik , Mhin Byung Jin , Kim Kwang S.
Published: July 2002

Various levels of ab initio theory using various basis sets have been tested for the energy, structure, and harmonic vibrational frequencies of the water monomer. The level of the single, double, and perturbative triple excitation coupled-cluster method [CCSD(T)] using a large basis set (O:13s,8p,4d,2f/ H:8s,4p,2d) reproduced the experimental harmonic vibrational frequencies of the water monomer within the error of 0.6 cm−1. Other calculational methods seem to have inherent errors in predicting vibrational frequencies. Even with the above large basis set significant differences between symmetric and asymmetric stretching frequencies were found at various levels of calculational method including the commonly used Möller–Plesset 2nd (MP2) and 4th (MP4) order perturbation theories. The harmonic vibrational frequencies and force constants of the water dimer at various levels of ab initio theory have also been studied, and their shifts in the dimer relative to the monomer are discussed. At the Hartree–Fock (HF) level frequency shifts in the dimer relative to the monomer as well as hydrogen bonding strength in the dimer are underestimated, while at the MP2 level these are overestimated. The values at the CCSD(T) level seem to be reliable.
Molecular dynamics simulation of the photodissociation of adsorbed HCl on a MgO (001) surface
Authors: Hintenender M. , Rebentrost F. , Gerber R. B. , Kosloff R.
Published: July 2002

The photodissociation of HCl/MgO (001) is studied by classical molecular dynamics of a single adsorbate system including the substrate phonon modes. An important quantum effect is accounted for by taking the hydrogen coordinates and momenta in the initial state from a vibrational ground state wave function. In the angular distribution of the scattered photofragments characteristic structures due to rainbows, scattering shadow and resonances are found, that are already well described within the rigid surface approximation. The hydrogen kinetic energy release also shows a pronounced peak structure corresponding to different energy transfer mechanisms and is significantly affected by inclusion of energy transfer to the phonon modes. Due to multiple collisions with the surface and the chlorine, the hydrogen can lose more than 3.5 eV of its 4.7 eV excess energy. The angular resolved energy spectrum is explained by several types of trajectories connected with the above mechanisms. The results suggest further that the different mechanisms can be separated in an experiment.
Application of integral equation theories to predict the structure of diatomic fluids
Authors: Lue Leo , Blankschtein Daniel
Published: July 2002

We compare the capabilities of the site–site Ornstein–Zernike equation and the Chandler–Silbey–Ladanyi equations to predict the fluid structure for: (i) fluids composed of homonuclear diatomic Lennard-Jones molecules, and (ii) fluids composed of nonpolar or polar heteronuclear diatomic Lennard-Jones molecules. In (i), we solve the site–site Ornstein–Zernike (SSOZ) equation with the Percus–Yevick (PY) closure, and the Chandler–Silbey–Ladanyi (CSL) equations with the hypernetted-chain (HNC) closure to predict the various pair correlation functions at various bond lengths, fluid densities, and temperatures. In general, we find that the CSL equations become more accurate, when compared with computer simulation results, as the bond length increases or as the density decreases, with temperature having no significant effect. In fact, at densities below the critical density, the fluid structure predictions of the CSL equations are found to be in closer agreement with the computer simulation results than those of the SSOZ equation. We also present a general method for computing the low-order density bridge functions in the context of the CSL equations. In the case of homonuclear diatomic molecules, the zeroth-order bridge functions, B(0), are found to have little effect on the pair correlation function predictions of the CSL equations. However, the addition of the first-order bridge functions, B(1), results in a significant improvement of these predictions.

In general, the accuracy of the CSL equations, including the various bridge function corrections, is found to increase as the bond length increases or as the density decreases, similar to what we found when the HNC closure (in which the bridge functions are set equal to zero) was used. Finally, in (ii), we find that for nonpolar heteronuclear diatomic fluids, the CSL equations, with the HNC, HNC+B(0), and HNC+B(1) closures, perform very well in predicting the correlation functions between the larger interactions sites. For polar heteronuclear diatomic fluids, we find that the CSL equations seem to offer an improvement over the SSOZ equation. Once again, the CSL equations provide better predictions for the correlation function between the larger interaction sites.
One- and two-photon fluorescence excitation spectra of the 2 1A
g states of linear tetraenes in free jet expansions
Authors: Petek Hrvoje , Bell Andrew J. , Choi Young S. , Yoshihara Keitaro , Tounge Brett A. , Christensen Ronald L.
Published: July 2002

One- and two-photon fluorescence excitation spectra of the S1←S0 transitions of the all-trans isomers of 1,3,5,7-octatetraene, 1,3,5,7-nonatetraene, and 2,4,6,8-decatetraene have been obtained in free jet expansions. Comparison of the one- and two-photon spectra allows the unambiguous identification of electronic and vibronic origins and, for octatetraene and decatetraene, provides clear evidence for molecular inversion symmetry. One-photon spectra show ag progressions built on Herzberg–Teller, bu promoting modes, while two-photon spectra are built on progressions of ag modes starting from the 2 1Ag←1 1Ag electronic origins. In nonatetraene, the absence of inversion symmetry results in an allowed electronic origin in both the one- and two-photon spectra. Nevertheless, bands built on vibronic origins dominate the one-photon spectrum. The S1←S0 spectra of nonatetraene and decatetraene exhibit characteristic splittings of vibronic bands that can be quantitatively explained by the tunneling of the methyl groups through low energy, torsional barriers in the S1 states. Couplings between methyl torsions and low frequency skeletal modes further complicate the optical spectra. Fluorescence lifetimes indicate abrupt onsets of nonradiative decay processes (tentatively attributed to trans→cis isomerization) at ∼2100 cm−1 excess energy. Systematic differences in the energy dependencies of S1 nonradiative decays in the three polyenes can be explained by the higher densities of vibronic states in the methyl-substituted compounds.
Buoyant convection in the Belousov–Zhabotinsky reaction. II. Chemically driven convection and instability of the wave structure
Authors: Zhang Dongmei , Peltier William R. , Armstrong Robin L.
Published: July 2002

A numerical study of the complete system of reaction-diffusion-convection equations that govern the dynamics of Belousov–Zhabotinsky (BZ) reactive fluid has been conducted in order to understand the interaction of chemical waves with fluid convection induced by chemical reactions. In the regime studied, it was found that although the heat of reaction alone could drive convection, it was too weak to destabilize the wave. When the influence of the isothermal chemical reactions was combined with that of the heat of reaction, however, convective instability of the wave structure could result for both up-going and down-going waves. The regime in the parameter space was defined within which instability of the up-going wave and stable propagation of the down-going wave is expected, as observed in a magnetic resonance imaging (MRI) experiment. The velocity of an undistorted wave front in the presence of convection was found to be independent of the lateral scale of the system and the direction of wave propagation, as was also observed in the MRI experiment.
Direct femtosecond observation of the transient intermediate in the α-cleavage reaction of (CH3)2CO to 2CH3+CO: Resolving the issue of concertedness
Authors: Kim Sang Kyu , Pedersen Soren , Zewail Ahmed H.
Published: July 2002

When a reaction involving two equivalent bonds has sufficient energy to break both of them, it can proceed by either a concerted or a stepwise mechanism. For Norrish type-I and other reactions, this issue has been controversial since direct time resolution of the individual C–C cleavage events was not possible. Here, for the elementary α-cleavage of acetone, we report on the femtosecond resolution of the intermediates using mass spectrometry. The results show the nonconcertedness of the reaction, provide the times for the primary and secondary C–C breakage, and indicate the role of electronic structure (σ*, antibonding impulse) and the vibrational motions involved.
Gibbs adsorption and the compressibility equation
Authors: Aranovich Grigoriy L. , Donohue Marc D.
Published: July 2002

A new approach for deriving the equation of state is developed. It is shown that the integral in the compressibility equation is identical to the isotherm for Gibbs adsorption in radial coordinates. The Henry, Langmuir, and Frumkin adsorption isotherms are converted into equations of state. It is shown that using Henry’s law gives an expression for the second virial coefficient that is identical to the result from statistical mechanics. Using the Langmuir isotherm leads to a new analytic expression for the hard-sphere equation of state which can be explicit in either pressure or density. The Frumkin isotherm results in a new equation of state for the square-well potential fluid. Conversely, new adsorption isotherms can be derived from equations of state using the compressibility equation. It is shown that the van der Waals equation gives an adsorption isotherm equation that describes both polymolecular adsorption and the unusual adsorption behavior observed for supercritical fluids.
Binary nucleation rate measurements of n-nonane/methane at high pressures
Authors: Looijmans K. N. H. , Luijten C. C. M. , van Dongen M. E. H.
Published: July 2002

We present homogeneous nucleation rates in the binary mixture n-nonane/methane in the supercritical regime, measured in a new pulse expansion tube at pressures up to 40 bar, at a temperature of 240 K. It is shown that both n-nonane and methane are present in the critical cluster. The critical supersaturation of n-nonane decreases from 30 at 10 bar, down to 6 at 40 bar total pressure.
Newtonian propagation methods applied to the photodissociation dynamics of I3−
Authors: Ashkenazi Guy , Kosloff Ronnie , Ruhman Sanford , Tal-Ezer Hillel
Published: July 2002

A uniformly convergent propagation scheme designed for non-hermitian Hamiltonian operators is presented. The method is based on a Newtonian interpolation polynomial which is created by a recursive application of the Hamiltonian operator on an initial wavefunction. The interpolation points used to construct the Newtonian polynomial are located in the complex eigenvalue space of the Hamiltonian. A new algorithm is developed to construct the interpolation points. Both time dependent and time independent quantities can be obtained using the same polynomial expansion. The method is particularly useful when negative imaginary potentials are used. The photodissociation dynamics of I3− is studied as an example of the utility of the scheme to gain insight on a dynamical encounter. The bond cleavage is followed in time simultaneously with the calculation of the Raman spectra. The study addresses the role of vibrational excitation of the reactant I3− on the nascent I2− spectral modulations and Raman spectra.
Dipole–dipole coupling effects upon infrared spectroscopy of compressed electrochemical adlayers: Application to the Pt(111)/CO system
Authors: Severson Mark W. , Stuhlmann Christopher , Villegas Ignacio , Weaver Michael J.
Published: July 2002

Experimental infrared spectra for CO adlayers on Pt(111) electrodes having known real-space structures as deduced by scanning tunneling microscopy are compared with predictions extracted from conventional dipole–dipole coupling models in order to test the validity of such treatments for compressed electrochemical adlayers, especially with regard to band-intensity transfer effects. The specific structures considered are (2×2)–3CO and (√19×√19)R23.4°–13CO hexagonal adlayers; the former is especially close packed (θCO=0.75) with a pair of threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (θCO=13/19) and involves largely asymmetric binding sites. The comparisons between dipole-coupling theory and experiment include infrared spectra for various 13CO/12CO mixtures, thereby exploiting the well-known systematic alterations which are induced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. ) the conventional dipole–dipole treatment can account semiquantitatively for the marked higher intensity of the atop relative to the threefold hollow C–O stretching band in the observed infrared spectra even though the occupancy on the latter site is twofold greater and the singleton frequencies are substantially (∼280 cm−1) different. This coupling-induced intensity transfer toward the higher-frequency band component is likely to be a widespread phenomenon for densely packed adlayers.

For the (2×2) adlayer, however, the isotope composition-dependent spectral band frequencies and relative intensities deviate markedly from the experiment. While the inclusion of stochastic broadening effects associated with adlayer disorder improves the situation, a satisfactory fit between theory and experiment requires the incorporation of vibrational coupling associated with short-range intermolecular interactions. For the (√19×√19) adlayer, on the other hand, dipole–dipole coupling with stochastic broadening accounts well for the observed spectral behavior. The more pronounced limitation of the conventional theory for the (2×2) structure may well be due to the abnormally high adsorbate packing density enhancing the importance of short-range interactions.
Neutron diffraction study on chloride ion solvation in water, methanol, and N,N-dimethylformamide
Authors: Yamagami Motoyuki , Wakita Hisanobu , Yamaguchi Toshio
Published: July 2002

Pulsed neutron diffraction measurements have been carried out on 8.6, 5.8, and 1.7 molar lithium chloride (LiCl) solutions in deuterated water (D2O), methanol-d4 (MeOD), and N,N-dimethylformamide-d7 (DMF), respectively. A first-order difference method with chlorine isotopes substitution was used to derive the Cl−-dependent partial structure factors and radial distribution functions. The oscillation patterns of all Cl−-related structure factors normalized by the concentration persist up to the high momentum transfer region (∼10 Å−1), suggesting the presence of the short-range ordering around chloride ion in the three solvent systems. The normalized radial distribution functions have revealed that methanol molecules are hydrogen bonded to a chloride ion with almost linear orientation of Cl⋅⋅⋅D–O, as in the case of chloride hydration. The nearest-neighbor Cl–D distance and the solvation number for Cl− in the methanol solutions were determined as 2.21±0.03 Å and 3.6±0.5, respectively, compared with 2.29±0.01 Å and 5.8±0.5 for the aqueous solutions. The smaller solvation number for Cl− in the methanol solutions suggests that an Li+–Cl− ion association takes place in the solutions. In the DMF solutions, the first peak was observed at a much longer distance, ∼2.85 Å, and assigned to the distance between Cl− and the formyl H atoms of the DMF molecule due mostly to the ion-dipole interaction. The number of DMF molecules around the chloride ion was estimated as 6.8±0.5. The most likely conformations of the solvent molecules around the chloride ion are proposed and discussed on the basis of the solvent properties.
The stability of bicontinuous microemulsions: A molecular theory of the bending elastic properties of monolayers comprised of ionic surfactants and nonionic cosurfactants
Authors: Cantor Robert S.
Published: July 2002

In recent work, a statistical thermodynamic approach was used [J. Chem. Phys. 99, 7124 (1993)] to study curvature elastic properties of self-assembled films of nonionic surfactants at the oil/water interface in microemulsions. In the present work, this approach is extended to investigate films comprised of mixtures of ionic surfactants and nonionic cosurfactants, commonly used to form droplet and bicontinuous microemulsions. A simple description of the interactions among charged head groups is incorporated into the existing theoretical framework, yielding predictions of spontaneous curvature, mean and Gaussian elastic moduli, molecular area, and chain segment distributions as a function of monolayer composition, the strength of the electrostatic repulsion, and the lengths of the surfactant and cosurfactant chains. These predictions are found to be in good agreement with measured curvature elastic and structural properties, and with characteristic features of the phase diagrams of well-studied systems, particularly with respect to the effect of concentration of added electrolyte and the chain length and concentration of the alcohol (cosurfactant). More importantly, the results provide an unexpected explanation of the role of the alcohol in the stabilization of bicontinuous microemulsions, i.e., to form a monolayer of zero spontaneous curvature and low bending elastic moduli.
Multiple time step nonequilibrium molecular dynamics simulation of the rheological properties of liquid n-decane
Authors: Cui S. T. , Cummings P. T. , Cochran H. D.
Published: July 2002

Nonequilibrium and equilibrium molecular dynamics simulations are reported for a united-atom model of n-decane at a state point in the liquid phase. The viscosity calculated by our nonequilibrium molecular dynamics simulations is in good agreement with that obtained from our equilibrium molecular dynamics simulations via the Green–Kubo relation and with that obtained by Mundy et al. [J. Chem. Phys. 102, 3776 (1995)] using the same potential model at the same state conditions. Additionally, the viscosity calculated by nonequilibrium molecular dynamics is in very good agreement with experimental results for n-decane. The algorithm used for the equilibrium molecular dynamics simulations is an application to alkanes of the multitime step Nosé dynamics algorithm developed by Tuckerman and Berne. For the nonequilibrium molecular dynamics simulations, an extension of the multitime step method is derived for the nonequilibrium equations of motion describing planar Couette flow with Nosé thermostat. The contributions of the intramolecular interactions to the stress tensor and its relaxation have been analyzed; the bond stretching motions play a dominant role in the short-time behavior of the atomic stress–stress correlation.
Excited and ionized states of free base porphin studied by the symmetry adapted cluster-configuration interaction (SAC-CI) method
Authors: Nakatsuji Hiroshi , Hasegawa Jun-ya , Hada Masahiko
Published: July 2002

The SAC(symmetry adapted cluster)/SAC-CI method is applied to the calculations of the ground, excited, and ionized states of the free base porphin. The electronic spectrum of porphin is well reproduced and new assignments for the B (Soret), N, L, and M bands are proposed. The present result shows that the four-orbital model is strongly perturbed for the B and N bands by the excitations from the lower 4b1u MO and that the σ electron correlations are important for the description of the excited states. The absorption peaks in the ionization spectrum are assigned and the reorganization effect is found to be large especially for the n and σ electron ionizations.
Surface diffusivities and reaction rate constants: Making a quantitative experimental connection
Authors: Allen C. E. , Seebauer E. G.
Published: July 2002

For diffusion-controlled reactions in three dimensions, continuum mechanics provides a quantitative relation between the steady-state reaction rate constant k and the diffusion coefficient D. However, this approach fails in two dimensions, where no steady-state solution exists on an infinite domain. Using both Monte Carlo methods and analytical techniques, we show that previous attempts to circumvent this problem fail under real laboratory conditions, where fractional coverages often exceed 10−3. Instead, we have developed a rigorous and general relation between k and D for all coverages on a square lattice for the reaction A+A→A2. For short times or high coverages, the relation k=πD/γ holds exactly, where γ denotes the two-dimensional packing fraction. For lower coverages, however, k depends on time in both constant-coverage (adsorption allowed) and transient-coverage (adsorption forbidden) regimes. In both cases, k decreases in response to the evolution of nonrandom adsorbate configurations on the surface. These results indicate that diffusion-limited surface reactions may be identified unambiguously in the laboratory and also provide a quantitative link between diffusion parameters and experimentally determined recombination rate parameters. Practical experimental methods highlighting such effects are outlined.
Pressure dependence and metastable state formation in the photolysis of dichlorine monoxide (Cl2O)
Authors: Nickolaisen Scott L. , Miller Charles E. , Sander Stanley P. , Hand Michael R. , Williams Ian H. , Francisco Joseph S.
Published: July 2002

The photodissociation of dichlorine monoxide (Cl2O) was studied using broadband flash photolysis to investigate the influence of variations in the photolysis wavelength domain, bath gas pressure and bath gas identity on the yield and temporal dependence of the ClO product. ClO yields were independent of bath gas pressure when the photolysis spectral band extended to 200 nm (quartz cutoff) but for photolysis restricted to wavelengths longer than about 250 nm, ClO yields decreased with increasing bath gas pressure and there was a pressure-dependent delay in the formation of ClO. Under these conditions, a weak, highly structured absorption spectrum was observed in the range 16 600–26 000 cm−1 with a lifetime on the order of 500 ms. A portion of the spectrum could be analyzed (22 000–26 000 cm−1) which showed progressions having differences of 283, 443, and 505 cm−1. Ab initio calculations were performed to evaluate vertical excitation energies and oscillator strengths from the lowest-energy singlet (X 1A1) or triplet (1 3B1) states to various excited states. The calculations indicated that the 2 3A2←1 3B1 transition has an unusually large oscillator strength. The transition energy, 3.05 eV, is consistent with the observed metastable spectrum. The observed pressure dependence of ClO formation could be modeled using a mechanism which assumed that Cl2O excitation at wavelengths longer than about 300 nm leads to rapid intersystem crossing to two metastable states in the triplet manifold. These states undergo competitive dissociation to ClO+Cl and collisional relaxation to the ground state. The dynamics of Cl2O may serve as a model for other molecules of importance in the earth’s lower stratosphere such as ClONO2 where filtering of the solar spectrum by ozone restricts photolysis to the weak tail of the absorption continuum.
Vibrational analysis of the (Cs+CO)-(2×2) compound layer on Ru(0001)
Authors: He Peimo , Xu Yabo , Jacobi Karl
Published: July 2002

On a Ru(0001) surface Cs and CO form a very well ordered (Cs+CO)-(2×2) compound layer whose structure was analyzed recently (Cs on-top, CO in threefold sites). Here we present a vibrational analysis of the same system using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). The bonding of CO to Ru is both local and nonlocal. Two (C–O) stretch frequencies are observed depending on whether there are one or two CO molecules in the 2×2 cell. They change in energy between 155 and 204 meV depending on CO coverage θCO. Setting θmaxCO=1.0, the evolution of the C–O stretch intensities indicates that up to θCO=0.22 the 1-CO-(2×2) phase is formed exclusively implying some mobility of the Cs layer. For θCO≳0.22 the 2-CO-(2×2) phase grows additionally until at θCO=1.0 only the 2-CO-(2×2) phase is found. Two Ru–CO stretch modes are observed for the first time and are assigned to adsorption in the hcp and fcc hollow sites within the 2×2 unit cell. They are very weak in intensity which is attributed to the threefold-hollow site and some screening in the 2D compound. With CO adsorption a change of the electronic structure of the Cs adlayer is observed; the adlayer loses metallicity and the Cs–Ru stretch becomes visible. Strong changes of the Cs–Ru stretch energies are observed with CO coverage.
A continuum solvation theory of quadrupolar fluids
Authors: Chitanvis Shirish M.
Published: July 2002

We have derived a generalization of Poisson’s equation, a fourth-order partial differential equation, to describe the electrostatic behavior of polarizable, quadrupolar fluids. Our theory is in accord with the approach of Evangelista and Barbero. This equation was solved for the case of multipoles of arbitrary order placed at the center of a spherical cavity in a quadrupolar fluid. Our solution indicates that the quadrupolar portion of the disturbance created by an electrostatic probe in a polarizable quadrupolar fluid is localized to a distance of about a bohr, while asymptotically the fluid behaves as a polarizable medium. Internal field corrections as well as internal field gradient corrections have been computed. Fairly good agreement is found between our theory and the experimentally determined dielectric constant for carbon dioxide. The cavity model solution has been applied toward understanding the solvation of ions and dipolar molecules in supercritical carbon dioxide. We have used our theory to show that ions do not dissolve in supercritical carbon dioxide. Our theory displays improving agreement with data on the solubility of water molecules in supercritical carbon dioxide as pressure and temperature are increased. Ways to enhance agreement with experiments are suggested. We speculate on generalizations of our approach to fluids composed of higher-order multipoles, e.g., methane, whose first nonvanishing moment is an octupole.
State-to-state reaction dynamics of R+HCN (ν1νl22ν3)→RH+CN(v, J) with R=Cl, H
Authors: Kreher Christoph , Theinl Robert , Gericke Karl-Heinz
Published: July 2002

Vibrational overtone excitation of HCN in the wavelength region 6 500 cm−1–18 000 cm−1 is used to initiate the endothermic reaction of chlorine and hydrogen atoms with HCN. HCN is excited to the overtone levels (002), (004), (302), (105), and (1115). The labeling of the vibrational levels (ν1ν2l2ν3) corresponds to the normal modes ν1=CN, ν2=bend, ν3=CH, and l2=vibrational angular momentum. The product state distribution of CN(X 2Σ+) is completely analyzed by laser induced fluorescence (LIF). Excitation of the first overtone of CH-stretch leads to vibrationally excited CN in the reaction of Cl+HCN(002), implying the existence of a long living complex. The CN vibrational excitation increases with increasing H–CN stretch excitation. However, a slightly higher CN vibrational excitation is found when at the same internal energy of HCN three quanta of CN-stretch and two quanta of CH-stretch are excited. Therefore, the energy is not completely redistributed in the collision complex. The ratio of rate constants between the reactions of HCN(004) and HCN(302) with Cl is 2.8±0.6. The CN product vibrational excitation decreases again, when HCN is excited to the (105) state. At these high HCN vibrational energies the reaction mechanism seems to change toward a more direct reaction where the time left is not sufficient for energy randomization. The reaction of hydrogen with HCN(004) leads to CN-products with a similar vibrational distribution, as in the case of chlorine, but with a lower rotational excitation. The reaction H+HCN(302) shows no significant generation of CN products and a lower limit of the ratio of rate constants, k(004)/k(302)≳4, is obtained.
Thermodynamic perturbation theory for association with bond cooperativity
Authors: Sear Richard P. , Jackson George
Published: July 2002

A simple model of a hydrogen-bonding molecule is considered, each model molecule can bond to two other molecules. This bonding is not, in contrast to previous work, taken to be pairwise additive. The association energy contains a three-body term which either increases or decreases the strength of the bonds in chains of three or more molecules, i.e., it makes the bonding cooperative. Motivation for this form of the intermolecular potential is provided by the behavior of hydrogen fluoride: the bond strength of a hydrogen fluoride dimer is considerably less than the bond strength in chains of three or more molecules. An analytical theory for the free energy of this model molecule is derived via a new approach which is sufficiently general to accommodate the non-pairwise additive interaction. Calculations are performed which show the effect of the cooperative bonding on vapor–liquid coexistence.
Quantum dynamics of bond breaking in a dissipative environment: Indirect and direct photodesorption of neutrals from metals
Authors: Saalfrank Peter , Kosloff Ronnie
Published: July 2002

The dynamics of uv/visible laser-induced nonthermal desorption of neutral molecules from metal surfaces are studied by Liouville–von Neumann equations for quantum open systems. A one-dimensional, two-state Gadzuk–Antoniewicz model is adopted, representative for NO/Pt(111). Electronic quenching due to coupling of the adsorbate negative ion resonance to the metal electrons is treated within the Lindblad dynamical semigroup approach. Both indirect (hot-electron mediated) and hypothetical direct (dipole) excitation processes are considered. For the indirect pathways, DIET (single-excitation) and DIMET (multiple-excitation) limits are studied using one- and double-dissipative channel models, respectively. To reproduce experimental desorption yields and desorbate translational energies, we estimate the quenching lifetime for NO/Pt(111) to be less than 5 fs. We also extend previous quantum treatments of photodesorption processes to the case of coordinate-dependent quenching rates. Further, the characteristic scaling laws of desorption yields versus laser fluence are derived for each of the individual excitation pathways. Finally, the possibility to control photoreactivity at surfaces by different, vibration-promoted schemes (surface heating, ir+uv two-photon strategies, use of pulsed uv lasers) is examined.
Capture of vapor molecules by a realistic attraction potential of a drop
Authors: Vasil’ev Oleg V. , Reiss Howard
Published: July 2002

We derive the rate of the capture of a molecule from the surrounding dilute vapor by a drop with a realistic attraction potential. For a small drop, which is of interest in the vapor phase nucleation theory, the result is substantially different from the conventional one.
The vibrational dephasing and relaxation of CH and CD stretches on diamond surfaces: An anomaly
Authors: Lin J.-C. , Chen K.-H. , Chang H.-C. , Tsai C.-S. , Lin C.-E. , Wang J.-K.
Published: July 2002

The temperature dependence of infrared absorption spectra of CH and CD on diamond nanocrystal surfaces has been investigated. Phase relaxation was closely examined by analyzing frequency shifts and line broadening in the spectra. Based on the model of Persson and Ryberg [Phys. Rev. B 40, 10 273 (1989)], coupling phonons responsible for the pure dephasing process were found to resonate at ω0≊1200 cm−1 for the CH stretch. By including both the phase and energy relaxation in the linewidth analysis and assuming that energy relaxes via three-phonon emission, we estimate a pure dephasing time of T*2≊340 ps at room temperature. This value is one order of magnitude larger than the energy relaxation time, T1≊19 ps, measured by Chin et al. [Europhys. Lett. 30, 399 (1995)] on a C(111) single crystal surface. We interpret the anomalous observation to be the result of the high frequency of the coupling phonons. For the CD stretches, however, severe line broadening due to exceedingly rapid energy relaxation disallows accurate determination of T*2. Since CH and CD stretches have similar thermal and inhomogeneous broadening characteristics, the lifetime of the latter is estimated to be T1≊220 fs. This T1 value is one of the shortest vibrational energy relaxation times for any adsorbate on a dielectric surface. Systematic comparison of these anomalous results with the measurements of SiH stretches on Si(100) and Si(111) is given.
Fourier transform infrared and theoretical isotopic study of the ν4(σu) and ν5(σu) modes of linear C7
Authors: Kranze R. H. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared, 13C isotopic study has been made of two previously identified fundamentals, ν4(σu) and ν5(σu), of the linear C7 carbon cluster which, in the present investigation, was formed by trapping the products of the evaporation of graphite in solid Ar at ∼10 K. Measured isotopic shifts were compared with theoretical predictions in order to confirm the assignments as well as to investigate the general quality of such predictions. The shifts for the ν4(σu) mode were found to be highly sensitive to the level of calculation, whereas consistently good agreement between theory and experiment was found for the ν5(σu) mode. This difference in behavior between the two modes is predicted by the isotopic sensitivity index which is shown to be a useful guide for future identifications of vibrational modes based on comparisons between measured and theoretically calculated isotopic shifts.
Quasielastic light scattering of polystyrene in diethyl malonate in semidilute concentration
Authors: Wang C. H. , Sun Z. , Huang Q. R.
Published: July 2002

A detailed light scattering study of polystyrene (PS) in diethyl malonate (DEM) semidilute solution has been carried out. DEM is a theta solvent for PS at 32 °C. Dynamic light scattering measurements carried out at temperatures closed to the theta temperature show only a diffusion mode in the intensity autocorrelation function; no slow modes are found in the semidilute PS/DEM solution with concentration up to more than 18 times the overlap concentration. For this system, the mutual diffusion coefficient is found to be mainly determined by the osmotic modulus, with the solution stress modulus making a negligible contribution. A slowing down of the mutual diffusion coefficient as the temperature approaches the critical temperature (at the critical concentration) is observed. These results show that despite considerable chain entanglements existing in the semidilute/concentrated PS/DEM solution, no viscoelastic slow modes are found. These results are not in agreement with the existing literature data for the same system. The absence of the viscoelastic modes is interpreted as due to the fact that PS and DEM are isopycnic and the coupling of concentration fluctuations and the solution stress modulus is negligible for isopycnic polymer solutions, in accordance with a theory recently developed in this laboratory.
Numerical test of Kramers reaction rate theory in two dimensions
Authors: Berezhkovskii Alexander M. , Zitserman Vladimir Yu. , Polimeno Antonino
Published: July 2002

The Fokker–Planck–Kramers equation for a system composed by a reactive coordinate x coupled to a solvent coordinate y is employed to study the effect of additional degrees of freedom on the dynamics of reactive events. The system is studied numerically in the diffusional regimes of both coordinates, for different topologies of the bistable potential function and anisotropies of friction. The eigenvalue spectrum is evaluated by representing the time evolution operator over a basis set of orthonormal functions. A detailed analysis of the effect of the explicit consideration of the slow nonreactive mode is carried on to show that a variation of qualitative picture (scenario) of the reaction dynamics occurs when friction along different directions is strongly anisotropic, depending also on the structure of the two-dimensional potential surface. The numerical study supports both the qualitative picture of the reaction dynamics and the rate constant expressions obtained analytically. For those cases where the Langer theory has a restricted range of applicability because of the change in the reaction dynamics scenario, this fact has been numerically demonstrated. Here the Langer expression for the rate constant is replaced by the one obtained as a result of the consideration of the effective one-dimensional problem along the solvent coordinate, characterized by a smaller activation energy than that in the initial problem. All of these facts were confirmed by the numerical test, which shows a satisfactory agreement with the analytical results.
Bulk and interfacial thermodynamics of a symmetric, ternary homopolymer–copolymer mixture: A Monte Carlo study
Authors: Müller M. , Schick M.
Published: July 2002

We present results of a Monte Carlo simulation of a dense blend, comprising two incompatible homopolymers and a symmetric diblock copolymer, all of the same degree of polymerization. The simulations, in the framework of the bond fluctuation model, yield information on the phase diagram. At high temperatures the copolymer dilutes the homopolymer blend and shifts the critical temperatures of the demixing transition to lower temperatures. The line of second order transitions ends in a tricritical point, below which there is three-phase coexistence between two homopolymer-rich phases and a spatially structured copolymer-rich one. The simulations indicate that this latter phase is a microemulsion at intermediate incompatibility, and a lamellar phase at high incompatibility. Using a multimagnetic reweighting scheme, we independently determine the reduction of the interfacial tension and the copolymer excess at the interface between the coexisting homopolymer-rich phases. The bending rigidity is estimated by measuring the spectrum of interfacial fluctuations. We outline a method to determine the interaction between copolymer monolayers, and find that in the two-phase region it is attractive and its range increases upon addition of copolymers.