bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluctuations and transitions at chemical instabilities. I. A computer simulation approach
Authors: Termonia Yves
Published: February 2003

A stochastic model is designed for the study of the critical behavior of nonequilibrium chemical systems characterized by multiple homogeneous stationary states. In the model, the molecules are distributed on a lattice and reactive collisions are smeared out over small reaction cells which overlap over half their width. The model is studied using Monte Carlo techniques. The typical examples we choose for our study are the two Schlögl’s reaction mechanisms. For Schlögl’s first model, we find a transition in all physical dimensionalities (d=1, 2, and 3), as predicted by a deterministic analysis. For the second model, there is a transition for d≥2 only, in agreement with recent analytical and Monte Carlo calculations. For both models, we find that the deviations of the fluctuations from Poissonian behavior in small volumes increase as the square of the radius, as previously observed. As the critical point is approached, the fluctuations for Schlögl’s first model remain finite, as already predicted by the master equation approach. The relative fluctuations, however, diverge. For d<4, we find ‘‘nonclassical’’ values of the critical exponent, which agree with the prediction of a recent renormalization group analysis. Our results for Schlögl’s second model show a divergence of the fluctuations themselves, also in agreement with previous work. In the limit of homogeneous fluctuations, we recover the usual classical exponents for both a ‘‘magnetic field’’- and a ‘‘temperature’’-like approaches to the critical point. For d<4, nonclassical values are obtained. For the temperature-like approach, these are similar to the values found in the Ising model for equilibrium critical phenomena, as predicted by renormalization group studies.
Fluctuations and transitions at chemical instabilities. II. Effects of molecular interactions
Authors: Termonia Yves
Published: February 2003

The effect of molecular interactions on nonequilibrium multiple steady state transitions is studied, using the stochastic lattice model introduced in the previous paper (paper I). In order to deal theoretically with that model, we start by making use of a ‘‘microenvironment’’ approximation which attributes a single effective reaction cell to each type of site on the lattice. In the presence of molecular interactions, the probabilities for a transition at a particular site are then made dependent on (the Boltzmann exponent of) the instantaneous interaction energy of that site inside its microenvironment. Results obtained with the help of our approximate analytical calculation for Schlögl’s second reaction mechanism, in the absence of molecular interactions, are in good agreement with the ‘‘exact’’ values obtained by computer simulation (paper I). With the help of our analytical calculation, we then show that attractions between molecules of the noncontrolled reactant X in Schlögl’s mechanism tend to destabilize the system by favoring the autocatalytic reaction of the mechanism. At the same time, the multiple steady state transition becomes steeper and a sharp increase in the concentration of X occurs immediately past the instability point. An opposite effect is observed for repulsions between X molecules. Interactions now act to stabilize the system by shifting the threshold of instability to larger values of the pump parameter. Their effect is thus similar to that of multiplicative noise. Our results also show that the multiple steady state transition becomes smoother and ultimately disappears in the limit of very strong repulsions.
Quantum chemistry in Fock space. II. Effective Hamiltonians in Fock space
Authors: Kutzelnigg Werner , Koch Sigurd
Published: February 2003

The concept of an effective Hamiltonian in Fock space is introduced. It is based on the division of the entire one-particle space into subspaces of ‘‘active’’ and ‘‘inactive’’ orbitals. The effective Fock space Hamiltonian has—for active model states—the same eigenvalues as the full Hamiltonian. The theory outlined in this context differs from that of paper I mainly in a different definition of the ‘‘diagonal part’’ of an operator, and in the fact that the ‘‘quasidegenerate case’’ applies throughout. The separation theorem, and as a consequence the connected diagram theorem, is shown to hold, in a more limited sense though, even for those normalizations where it did not in the context of universal wave and energy operators. Unlike in the theory of the ‘‘universal’’ operators of paper I the Fock space and n-particle Hilbert space approaches with analogous normalizations are no longer equivalent. In particular, the Primas normalization with a fully Lie-algebraic structure does not lead to a connected diagram expansion if it is formulated in n-particle Hilbert space, only so in a Fock space formulation. In n-particle Hilbert space with the present definition of the diagonal part of an operator the normalizations b (‘‘canonical’’) and c (‘‘Primas’’) happen to agree. As an alternative to the construction of the wave and energy operator W and L by perturbation theory the nonperturbative approach is presented as a generalization of the coupled-cluster method, in detail both in the intermediate and in the unitary normalization. In the unitary variant only a linear system for σ (the logarithm of the wave operator) has to be solved in order to get L correct through fifth order in perturbation theory with important contributions of higher orders included. A generalization of the Hartree–Fock method to Fock space theory is outlined, which guarantees stationarity of all (active) eigenstates with respect to one-particle transformations. A generalized electron pair theory is also defined. An analysis of the necessary computational steps shows that the nonperturbative approaches do not require significantly more computational effort than perturbation theory to the corresponding order. As a numerical example the H2 molecule in a small basis is discussed.
Local mode theory for C3v molecules: CH3D, CHD3, SiH3D, and SiHD3
Authors: Halonen L. , Child M. S.
Published: February 2003

A three parameter potential model for the stretching vibrations of AH3D and AHD3 species (A=C or Si) is shown to give good agreement with experimental data thereby justifying predictions for the term values of unobserved levels. The structures of the overtone manifolds for different molecules are rationalized by means of an AH3 local mode to normal mode correlation diagram.
A theoretical study of possible benzene dimerizations under high-pressure conditions
Authors: Engelke Ray , Hay P. Jeffrey , Kleier Daniel A. , Wadt Willard R.
Published: February 2003

We offer a theoretical explanation of the rate processes observed macroscopically in materials composed of aromatic ring structures subjected to high pressure. Earlier workers have made qualitative suggestions that the origin of these processes may be due to interring pi bonding. By making quantum-mechanical calculations on a simple special case of such systems (i.e., two interacting benzene rings), we attempt to produce a quantitative microscopic foundation for the suggestions. We briefly review earlier experimental and theoretical work on the subject and thereby motivate the working hypotheses used in the calculations. The principal hypothesis is that by studying restricted parts of the two benzene-ring energy hypersurface, we can learn something about the pressure-induced rate process for all the arene structures. By use of the modified-neglect-of-diatomic-differential-overlap (MNDO) method and the generalized valence bond ‘‘perfect-pairing’’ (GVP–PP) method supplemented by configuration interaction, we found two metastable ground electronic state dimers of benzene; we suggest that one of these is the source of the observed rate process seen in benzene at high pressure. Further, we suggest that analogous dimerizations are responsible for the rate processes seen in larger arene materials subjected to very high pressures. The detailed geometries and energies of both benzene dimers are given. Suggestions for experimentally testing whether the proposed explanation is correct are given.
A functional perturbation theory for nonuniform molecular fluids. Effect of a weak dipole in a liquid–vapor interphase
Authors: Chacón E. , Tarazona P. , Navascués G.
Published: February 2003

A perturbation theory for nonuniform molecular fluids using the functional formalism is developed. Approximate expressions are written down for axial molecules using the spherical harmonic expansion. Under certain approximations Tarazona–Navascués and Haile et al. perturbation theories are recovered. Numerical calculation for the molecular orientation in liquid–vapor interphase of an axial fluid with a weak dipole is included.
A generalized van der Waals model for solvation forces between solute particles in a colloidal suspension
Authors: Freasier Ben C. , Nordholm Sture
Published: February 2003

A model for solvation forces between two solute particles in a colloidal suspension is solved within the framework of the generalized van der Waals model (GvdW) of fluids. The results for this approximation are compared to machine simulations for a subcritical solvent. Density profiles, adsorption excess, and force of interaction are studied. Particular attention is paid to the phenomenon of liquid film formation between the particles as a function of interparticle distance. Theory and simulation are found to be in good overall agreement.
Relative vibrational spectral intensities of the internal modes of phase II potassium nitrate
Authors: Ismail Mohammad A. , Jayasooriya Upali A. , Kettle Sidney F. A.
Published: February 2003

The infrared and Raman vibrational spectral intensities of the internal modes of KNO3(II) are readily understood when it is recognized that the crystal structure (Pmcn,D162h) is a distortion of a (P3̄m1,D33d) lattice. To a first approximation it is the selection rules of this latter lattice which are obeyed by the internal modes of the NO−3 group.
Homogeneous nucleation of toluene
Authors: Schmitt J. L. , Zalabsky R. A. , Adams G. W.
Published: February 2003

The authors have used a fast expansion chamber to measure the homogeneous nucleation rate of toluene as a function of temperature and supersaturation. The measured nucleation rate ranges from 102 to 105 drops/cm3 s over a temperature range of 215–267 K. The measurements are compared with the ‘‘classical’’ nucleation theory and with the RKC theory. The inclusion of the RKC replacement factor brings the data into good agreement with theory using physically realistic values of the surface tension and the sticking coefficient. An empirical curve fit to the data is presented as well as a full listing of the thermodynamic constants used for the calculations.
On the relation between self-diffusion and viscosity of liquids
Authors: Zwanzig Robert
Published: February 2003

Evaluation of the velocity and stress time correlation functions of a liquid, using a simple but speculative model for liquid state dynamics, leads to a relation between self-diffusion and viscosity. The result resembles the Stokes–Einstein formula, and agrees reasonably well with experimental data.
Deuterium site occupation in the oxygen-stabilized η-carbides Zr3V3ODx. I. Preparation and neutron powder diffraction
Authors: Rotella F. J. , Flotow H. E. , Gruen D. M. , Jorgensen J. D.
Published: February 2003

The structures of the single-phase, well-crystallized family of hydrides Zr3V3ODx for x=0, 1.86, 2.85, and 4.93 have been refined by the Rietveld method from neutron powder diffraction data taken at 298 K. The alloy and the three deuterides studied all possess the η-carbide structure (cubic Fd3m; origin at 4̄3m). Deuterium is found to occupy four different tetrahedral interstices preferentially. Sites with the largest number of Zr near neighbors appear to fill first unless occupation is hindered by nearby oxygen or simultaneously occupied deuterium sites. The volume expansion per number of deuterium atoms is nearly linear, even though the lowest stable hydride concentration at 298 K is near Zr3V3OD1.5 and markedly nonlinear distortions occur within the unit cell as different sites fill.
Dynamics of ion-induced-dipole capture: Effects of ion mass and collision energy
Authors: Babcock Lucia M. , Thompson Donald L.
Published: February 2003

Quasiclassical trajectories have been used to investigate ion–molecule complex formation and dissociation in model systems. A model ion-induced-dipole potential was used to represent the long-range attractive forces and an exponential was used for the repulsive part of the potential. The emphasis is on Cl−+Cl2 ⇄ [Cl3−]*, with some studies of the influence of ion mass on capture and lifetime. Capture cross sections as a function of complex lifetime were computed.
Collisional relaxation of vibrationally excited O2+ ions
Authors: Böhringer H. , Durup-Ferguson M. , Fahey D. W. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

Vibrationally excited O2+ ions were produced and injected into a helium flow tube and found to survive more than 105 collisions with He atoms without significant relaxation. The first, second, and higher vibrational states of O2+ were detected by their enhanced reactions with Xe, SO2, and H2O. Using these reactions as probes for vibrational excitation, the relaxation of O2+ (v=1) and O2+ (v=2) ions was studied for collisions with Ne, Ar, Kr, H2, D2, N2, CO, CO2, H2O, CH4, SO2, SF6, and O2. The resulting quenching rate constants were found to vary from 1(−9) to <2(−15) cm3 s−1. The quenching rate constant of O2+ (v=2) was approximately twice that of O2+ (v=1) in each case. The rate constants were found to correlate with the bond energy of the ion–molecule collision complex. For CO2, Kr, and O2, the energy dependence of the quenching rate constant was investigated in the range 0.04–0.3 eV. The results indicate that the relaxation process proceeds through a long-lived complex where the vibrational excitation is transferred into the weak complex bond resulting in dissociation, analogous to vibrational predissociation in van der Waals complexes. In the special case of O2+ (v)–O2 collisions, the relaxation is clearly dominated by the resonant charge transfer process and the rate constants for v=1 and v=2 are found to be equal.
Branching ratios and partition of the excess energy for the predissociation of CO+2 C̃ 2Σ+g molecular cations
Authors: Bombach R. , Dannacher J. , Stadelmann J.-P. , Lorquet J. C.
Published: February 2003

He–Iα photoelectron–photoion coincidence spectroscopy is used to study the decay of CO+2 molecular cations, initially prepared in various vibrational levels of the C̃ 2Σ+g state. The branching ratios for the energetically accessible fragmentation pathways are determined. Owing to the precisely known energetics and to an accurate measurement of the kinetic energies released, the partition of the respective excess energy among internal and external degrees of freedom of the separating fragments could be established. The formation of highly rotationally excited diatomic fragments must be proposed in order to explain the experimental results. The disappearance of O+ ions in favor of CO+ fragments as soon as the energy exceeds the dissociation limit for CO+ formation is consistent with a reinterpretation of previous ab initio calculations of potential-energy surfaces. The observed branching ratio for the population of the v=0 and v=1 levels of the CO fragment can be accounted for by an application of the statistical adiabatic channel model.
On laser−induced charge transfer in the HeH2+ quasimolecule
Authors: Errea L. F. , Méndez L. , Riera A.
Published: February 2003

In a recent publication, the charge transfer cross section for He2++H(ls) collisions through photon-assisted 2pσ–3dσ transitions was calculated; this calculation, however, contained several errors whose quantitative–even qualitative effect on the results is not obvious. We present a correct evaluation of this laser-induced cross section, which turns out to be larger, and present a maximum for longer wavelengths, than the values previously reported. In addition, we have checked the applicability of perturbation theory, of the stationary phase, uniform and Landau–Zener approximations, and the importance of potentially competitive photon-assisted reactions.
Slow and explosive decompositions of thermally unstable solids: A study with time-resolved mass spectrometry
Authors: Krishna Mohan V. , Tang Tong B.
Published: February 2003

This paper reports an investigation of the slow decomposition and explosion of solid energetic materials using continuously recorded time-resolved mass spectrometry. Five compounds in powder form have been examined, namely dilead 5,5′-azobis tetrazolate dihydroxide (LAT), 2,4,6-trinitro-1,3-benzenediol, lead salt (or ‘‘lead styphnate’’ LS), 2-azido-1,3,5-trinitrobenzene (or ‘‘picryl azide’’ PA), and two arylidene tetrazol-5-y1 hydrazones (TzTNB). A quadrupole instrument interfaced with a video-recording system was used to obtain spectra scanned at 0.3 ms per amu. Ionization voltages selected were 70 V and 14–18 V to minimize fragmentation. Steady decomposition as well as intermittent explosions of individual particles have been studied. During both kinds of events stable and unstable intermediates were found. Among the unstable species some could not be detected during explosions, a phenomenon attributed either to their comparatively short lifetimes or differences between explosion and decomposition routes for certain compounds. Of those that could be recorded the one with the highest mass was C6H2NO (m/e=104) originating from TzTNB. There were, in addition, species that appeared only during explosion, e.g., N4C–N from LAT. The spectrometric results enabled us to construct reaction schemes which reflect structural similarities of the parent molecules. A notable example concerns the tetrazole (CN4) moiety, present in both the decomposition and explosion of LAT and TzTNB. Generally, for a given compound, these two types of events share the same mechanism, but with LS a different pathway predominates in each process. The breakdown sequences for decomposition have been confirmed by parallel experiments carried out in a conventional magnetic mass spectrometer, but those for explosion could only be deduced from time-resolved data.
Experimental studies of polarization spectroscopy in dye solutions
Authors: Saikan S. , Sei J.
Published: February 2003

By changing solvent species, concentration of dye, and state of polarization of the pumping beam, experimental investigations have been made of the central thermal spike which has been frequently observed in polarization spectroscopy. From these investigations it was found that the spectrum in a dilute aqueous solution is rarely degraded by the influence of the thermal effect and it was also found that measurement of polarization dependence enables us to extract the contribution of the thermal spike. From the spectra without the thermal spike, absorption recovery times have been obtained in several dyes.
The role of long ranged forces in determining the structure and properties of liquid water
Authors: Andrea Tariq A. , Swope William C. , Andersen Hans C.
Published: February 2003

We have performed molecular dynamics calculations for liquid water using the revised central force model potential truncated at various distances and using two sizes of system, in order to study the effect of system size and range of the potential on the calculated thermodynamic and structural properties and to compare the results with those obtained by Ewald summation. All calculations were performed for a cubic system using periodic boundary conditions. Provided the side of the cube is equal to or greater than twice the range of the potential, the thermodynamic properties and distribution functions, including an orientational distribution function, are insensitive to the size of the system, for fixed range. Provided the range of the potential is equal to or greater than 6 Å, the thermodynamic energy and pressure are only slightly dependent on the range of the potential, to an extent that is, however, larger than that observed for dipolar hard spheres and the Stockmayer potential which do not have the tetrahedral structure similar to water. For potentials with ranges of 6 Å or greater, the atom–atom distribution functions are very insensitive to the range, but the orientational correlations are very sensitive to the range, as had been observed many times in studies of dielectric properties of simulated fluids. A potential with a range of 6 Å has thermodynamic and structural properties very similar to those of a longer ranged potential and similar to those obtained by Ewald summation. Use of such a model, which correctly describes interactions between nearest neighbors and next nearest neighbors but has no longer ranged forces, lead to significant increases in the speed of simulations.
Long time behavior of the Burnett transport coefficients
Authors: Brey J. J.
Published: February 2003

The long time behavior of the integrands in the expressions for the Burnett transport coefficients for a system of hard spheres or disks is studied using kinetic theory. It is shown that, in the ring approximation, the kinetic parts have hydrodynamic tails behaving as t1−d/2. It follows that the expressions for the Burnett transport coefficients are all divergent if this behavior persists for t→∞.
Molecular interpretation of relaxation in glasses: Free volume and density fluctuations
Authors: Chow T. S.
Published: February 2003

The volume relaxation and recovery in amorphous polymers near the glass transition are generally attributed to free volume collapse. In this work, a molecular kinetic theory based on equilibrium and nonequilibrium statistical thermodynamics has been developed to describe the phenomena in terms of free volume distribution. The work provides a coherent interpretation of some recent models with the successful multiparameter equations of Kovacs, Aklonis, and their co-workers being derived from the present theory. The structural contributions to relaxation times may be interpreted as the result of nonequilibrium fluctuations of the system and are related to the change of entropy. When both time and space are included in a non-Markovian stochastic process, the volume recovery of glasses has to be determined by solving multidiffusion equations in order to be successful in describing the memory effects. Finally, the present theory has been employed to analyze the isothermal relaxation of thermal density fluctuations in polymer glasses. The theory reveals that the decay of thermal density fluctuations is always slower than that of volume and its rate depends on annealing temperature and structure.
The influence of adsorbate interactions on kinetics and equilibrium for CO on Ru(001). II. Desorption kinetics and equilibrium
Authors: Pfnür H. , Feulner P. , Menzel D.
Published: February 2003

A variety of methods [temperature programmed desorption via pressure rise and via work function changes (Δφ); isothermal desorption via Δφ: quasiequilibrium measurements via isobars monitored by Δφ, in combination with sticking coefficients] has been used to obtain detailed data on the coverage dependence of the adsorption equilibrium and desorption kinetics for CO on the basal Ru(001) face. While the deviation from reversibility varies strongly over these methods, no significant influence of the degree of irreversibility on the results has been found. Desorption energies and isosteric heats are constant at 160 kJ/mol for 0<Θ<0.2, then rise slowly up to 175 kJ/mol at Θ=0.33, where they fall abruptly to 120 kJ/mol and more gradually at higher coverage. The ‘‘first order’’ frequency factor (Arrhenius preexponential normalized by the coverage) is 1016 s−1 at Θ=0, rises precipitously, especially in the range 0.2<Θ<0.33, to 1019 s−1 at Θ≊0.33, where it drops abruptly to ≊1014 s−1. The main conclusions drawn are: (1) The dependence on coverage of the desorption energies and preexponentials can be understood in terms of the equilibrium statistical mechanics of the chemisorption layer, governed by lateral adsorbate–adsorbate interactions. In particular, the high preexponentials and their strong increase close to Θ=0.33 are due to strong localization in the adlayer. (2) No strong influence of precursor kinetics exists in desorption. (3) Possible dynamic effects have constant influence throughout the range of measurements and can be described by the behavior of the sticking coefficient. These conclusions are discussed in connection with other recent findings.
Broadband population inversion by phase modulated pulses
Authors: Baum J. , Tycko R. , Pines A.
Published: February 2003

We present a class of continuously phase modulated radiation pulses that result in coherent population inversion over a large range of transition frequencies. The continuously modulated pulses can be approximated by sequences of discrete phase shifted pulses. Simulations of the inversion properties of the continuously modulated pulses and of the discrete pulse sequences are given.
About the numerical solution of the Percus–Yevick equation in the critical region for nontruncated potentials
Authors: Brey J. J. , Santos A.
Published: February 2003

Some problems related to the numerical solution of the Percus–Yevick equation in the critical region for a nontruncated potential are discussed. It is shown that the numerical method employed by the authors in a previous paper produces an abrupt decay of h(r) and an anticipation of the asymptotic behavior of c(r), when r tends to the cutoff distance R.
The Enskog theory for multicomponent mixtures. II. Mutual diffusion
Authors: Kincaid J. M. , López de Haro M. , Cohen E. G. D.
Published: February 2003

We present a detailed description of the mutual diffusion coefficients of binary and ternary dense fluid mixtures of hard spheres, as given by the revised Enskog theory (RET) of van Beijeren and Ernst and the standard Enskog theory (SET) of Tham and Gubbins. The formulas for the diffusion coefficients [see part I of this series, J. Chem. Phys. 78, 2746 (1983)] involve the contact values of the equilibrium pair distribution functions and the chemical potentials, for which the Carnahan–Starling approximation is used. The formulas, which were obtained by making an expansion in Sonine polynomials, are evaluated up to the third order and the convergence of the Sonine polynomial expansion is discussed. Except at low densities, the SET cannot be used to describe diffusion in hard-sphere mixtures. We find, using the CS approximation, that the SET gives values for the direct mutual diffusion coefficients that become negative at very high densities. A study is made of the dependence of the RET mutual diffusion coefficients on a variety of the parameters that determine the mixture (density, composition, molecular masses, and diameters) and a summary of the trends found is given. In particular, the influence of the addition of a third component on the mutual diffusion of two other components is discussed.
Electron detachment for H−(D−) in collisions with Ne
Authors: Wang T. S. , Delos J. B.
Published: February 2003

Total cross sections for electron detachment in collisions of H− and D− with Ne are calculated, using a model based on a first-order solution to close-coupled equations. Quantities needed for the calculation are the energy gap and the coupling between bound and free states. The energy gap is taken from previous calculations and the coupling is assumed to be of exponential form, with parameters adjusted to fit experimental data. Special examination is made of isotope effects in the cross sections.
A comment on the order of the wetting transition at a solid–fluid interface
Authors: Evans R. , Tarazona P.
Published: February 2003

We have repeated calculations of Teletzke et al. which describe the wetting transition at some models of a solid–gas interface. Our numerical results differ significantly from theirs in several cases. Contrary to Teletzke et al. we find that this transition is always first order for a Lennard-Jones 9-3 substrate potential when the van der Waals integral theory is employed.
The laboratory millimeter and submillimeter spectrum of HCO
Authors: Blake Geoffrey A. , Sastry K. V. L. N. , De Lucia Frank C.
Published: February 2003

The rotational absorption frequencies of 68 new lines from the HCO radical in its ground electronic state have been measured in the millimeter and submillimeter spectral region. The large zero-field data set acquired has allowed the complex spectrum of this light asymmetric rotor with unpaired electronic spin and magnetic hyperfine interactions to be completely analyzed to within experimental accuracy (<0.1 MHz) for the first time. The wide range of states observed provides a highly accurate map of the rotational frequencies of the formyl radical, which should enable the abundance and excitation of interstellar HCO to be examined in detail.
Flowing afterglow studies of the electron affinity of SO2
Authors: Grabowski Joseph J. , Van Doren Jane M. , DePuy Charles H. , Bierbaum Veronica M.
Published: February 2003

Flowing afterglow and selected ion flow tube studies of thermal energy electron transfer reactions of sulfur dioxide provide a bracket of the electron affinity of SO2: 0.744 eV<EA(SO2)<1.422 eV. These results are consistent with the previously established value of the electron affinity of SO2 (1.1±0.1 eV) but in disagreement with a recent determination (2.2±0.1 eV) using ion cyclotron resonance spectrometry.
Modified moments and continued fraction coefficients for the diatomic linear chain
Authors: Wheeler John C.
Published: February 2003

A simple, stable, and rapid procedure for the calculation of modified moments and continued fraction coefficients for the diatomic linear chain is presented. The Chebyshev modified moments are obtained stably from a simple three-term recursion relation. Stable determination of the continued fraction coefficients requires a more sophisticated approach, also based on three-term recursion relations. This procedure also allows stable calculation of the continued fraction coefficients for more complex model densities with singularities within the bands. Examples are given and the corresponding behavior of the continued fraction coefficients is discussed.
On energy transfer and catalytic reaction of 1-butene on platinum
Authors: Foner S. N. , Hudson R. L.
Published: February 2003

Single collision energy transfer catalytic reaction of 1-butene of polycrystalline platinum at temperatures up to 1040 °C has been studied with a modulated molecular beam mass spectrometer. The free radicals, C4H7 and C3H5, which has previously been reported as strong evidence for catalytic reaction by platinum, are not evolved. In collision with the heated surface, 1-butene molecules acquire considerable internal energy, which lowers the appearance potentials for the radical ions. By studying appearance potential curves for C4H+7 and C3H+5 from 1-butene heated in a molecular beam oven, as well as by the platinum ribbon, and using time-of-flight measurements to determine translational energy accommodation, several aspects of the energy transfer process have been investigated. A possible explanation for the disparate results on free radical production by 1-butene on heated platinum is presented.
The reaction of saturated and unsaturated hydrocarbons with the (110)-(1×2) and (111) surfaces of iridium
Authors: Szuromi P. D. , Engstrom J. R. , Weinberg W. H.
Published: February 2003

The dehydrogenation of propane, cyclopropane, propylene, propyne, and allene has been investigated on the reconstructed Ir(110)-(1×2) surface. Annealing adlayers of these hydrocarbons (at low coverages) leads to the formation of surface hydrogen and hydrocarbon fragments of approximate stoichiometry C3H2. The importance of the β′2 adsite of hydrogen on this surface of Ir has been demonstrated further by poisoning studies with hydrogen, CO and surface carbon. The Ir(111) surface was found to dehydrogenate propylene but neither propane nor cyclopropane under the same reaction conditions, indicating a strong effect of surface geometry for the C–H bond activation of alkanes.
Theory of Ostwald ripening: Competitive growth and its dependence on volume fraction
Authors: Marqusee J. A. , Ross John
Published: February 2003

The theory of Ostwald ripening is extended to include the dependence on the volume fraction of the minority phase. The size distribution function for droplets of the minority phase and the power laws of the time dependences are derived for the late stages of phase separation. The asymptotic distribution function is found to be independent of initial conditions but does depend on the equilibrium volume fraction associated with a given quench. We show that the average radius grows as t1/3 and the density of droplets decays as t−1. The growth law and the amplitudes for these temporal power laws derivate from their value in the limit of zero volume fraction as the square root of the volume fraction. The effect of competition among droplets causes the distribution to broaden and to increase the coarsening rate.
Factorization of the association rate coefficient in ligand rebinding to heme proteins
Authors: Young Robert D.
Published: February 2003

A stochastic theory of ligand migration in biomolecules is used to analyze the recombination of small ligands to heme proteins after flash photolysis. The stochastic theory is based on a generalized sequential barrier model in which a ligand binds by overcoming a series of barriers formed by the solvent protein interface, the protein matrix, and the heme distal histidine system. The stochastic theory shows that the association rate coefficient λon factorizes into three terms λon =γ12〈x2(0)〉Nout, where γ12 is the rate coefficient from the heme pocket to the heme binding site, 〈x2(0)〉 is the equilibrium pocket occupation factor, and Nout is the fraction of heme proteins which do not undergo geminate recombination of a flashed-off ligand. The factorization of λon holds for any number of barriers and with no assumptions regarding the various rate coefficients so long as the exponential solvent process occurs. Transitions of a single ligand are allowed between any two sites with two crucial exceptions: (i) the heme binding site acts as a trap so that thermal dissociation of a bound ligand does not occur within the time of the measurement; (ii) the final step in the rebinding process always has a ligand in the heme pocket from where the ligand binds to the heme iron.
Observation of D fluctuations in photoemission spectra and resolution of broad fluorescence bands
Authors: Chatzidimitriou-Dreismann C. A.
Published: February 2003

The detection of dynamics-induced (D) fluctuations in photoemission spectra is reported. It is achieved through a frequency-selective photon counting experiment, in connection with previous work by the present author [Int. J. Quantum Chem. 23, 1505 (1983)]. Experimental results are reported concerning the resolution of the fluorescence band of fluoranthene via measurement of D fluctuations.
Observation of the ν2 (1−←0+) inversion mode band in H3O+ by high resolution infrared spectroscopy
Authors: Haese Nathan N. , Oka Takeshi
Published: February 2003

The high resolution spectrum of the ν2 (1−←0+) inversion mode of gas phase oxonium ion H3O+ has been observed for the first time by diode laser spectroscopy. The molecular constants derived from the spectrum agree well with those predicted by Bunker, Kraemer, and Spirko.
A unified integral curve search technique for equilibrium- and transition- state geometries
Authors: McKelvey John M. , Hamilton John F.
Published: February 2003

An integral curve search technique has been developed for equilibrium and transition states. This procedure has the eigenvectors of the non-mass-weighted Hessian as the natural coordinates. Transition states can be sought analytically starting from an all-positive definite region. Equilibrium structure searches can be analytically sought, starting with a Hessian having simultaneously positive, zero, and negative curvatures.
Classical limit quantum mechanics for indistinguishable particles
Authors: Collins Michael A.
Published: February 2003

Formal expressions are derived for the classical limit of unitary matrix elements in systems of indistinguishable particles. Simple analytic examples are used to illustrate the contribution to these matrix elements from different classical paths involving exchanges of indistinguishable particles. The Pauli exclusion principle is shown to follow from the formal results and an application to molecular symmetry is discussed.
First-order polarization propagator approach to nuclear spin–spin coupling tensors in SiH4 and AlH−4
Authors: Lazzeretti P. , Rossi E. , Zanasi R.
Published: February 2003

A first-order polarization propagator approximation has been adopted to compute the full nuclear spin–spin coupling tensors 1J(X–H) and 2J(H–H) in SiH4 and AlH−4. The higher efficiency of the computational scheme related to the propagator formalism, with respect to the conventional coupled Hartree–Fock method, has been proved. The diamagnetic spin-dipolar term has been found to provide a larger contribution than the Fermi-spin-dipolar cross term to the anisotropy of 1J(X–H). Spin-dipolar contributions are negligible and the bulk of the average coupling is determined by the Fermi contact term.
The electronic structure of small zinc clusters. Resemblance of the clusters to bulk Zn
Authors: Tomonari Mutsumi , Tatewaki Hiroshi , Nakamura Takashi
Published: February 2003

The Zn clusters Zn1–Zn6 have been investigated by an ab initio SCF method. The Zn5 and Zn6 clusters are found to have a very similar electronic structure to that of the bulk Zn. For instance, the top of the d band is at −9.8 eV relative to the Fermi level for Zn6 and is at −9.6 eV for the bulk. The d bandwidth is 0.8 eV for the cluster and 1.0 eV for the bulk. For the d electron ionization, the effect of relaxation (reorganization) is very important and the resulting d hole is localized at one atom; the atomlike ionization process holds for the atom, clusters, and the bulk. The present results are compared with those of band theories where the exchange potential proportional to  ρ1/3 is employed.
Theoretical determination of the isotropic and anisotropic components of the indirect nuclear spin–spin coupling tensors of dihydrides containing B, N, and P
Authors: Galasso V.
Published: February 2003

SOS-CI calculations at ab initio level according to Nakatsuji’s formalism, considering all the singly excited configurations and at the same time introducing doubly excited configurations in a restricted way, are presented for the isotropic and anisotropic components of the indirect nuclear spin–spin coupling tensors of directly bonded nuclei in N2H4, P2H4, PH2NH2, B2H4, BH2NH2, BH2PH2, B2H6, BH3NH3, and BH3PH3. Full tensor matrices were calculated taking into account all the spin interaction operators, including the usually neglected ‘‘diamagnetic’’ orbital term. The relative importance of the various interaction mechanisms for J and ΔJ as well as the effects of dihedral angle rotation around the NN, PP, and PN bonds and of the pyramidization of nitrogen in PH2NH2 were examined.
Nonlocal polarizability densities and the effects of short-range interactions on molecular dipoles, quadrupoles, and polarizabilities
Authors: Hunt K. L. C.
Published: February 2003

Light scattering and absorption processes that are single-molecule forbidden may be observed in compressed gases and liquids, because of interaction-induced distortions of molecular charge distributions. In order to extract information about intermolecular dynamics from line shape studies of ‘‘forbidden’’ spectroscopic processes, it is necessary to determine collision-induced electrical properties as functions of the separations and relative orientations of the interacting molecules. When overlap is not negligible and series expansions valid at long range break down, a model that represents the distribution of polarizable matter in the interacting molecules by means of nonlocal polarizability densities is applicable in computing these properties. In the model the field that induces nonlocal polarization in one molecule is the sum of an external field and the field due to the polarization (permanent, fluctuating, and induced) of the collision partner. With the inclusion of nonlinear response, the model yields accurate expressions for the induction and dispersion contributions to collision-induced dipoles and polarizabilities; exchange effects are additive to lowest order. To first order in the atomic interaction, analytical results are presented for the dipole induced in one hydrogen atom by the approach of a second, the collision-induced pair quadrupole, and the pair polarizability tensor.
Repulsive interactions of closed-shell ions with He and Ne atoms: Comparison of beam and transport measurements
Authors: Viehland Larry A. , Mason E. A.
Published: February 2003

Measurements of repulsive interactions of closed-shell alkali and halogen ions with He and Ne atoms by ion-beam scattering are compared with measurements of ion mobility and diffusion in drift tubes at high electric field strengths. Both measurements probe the repulsive interaction and have a region of overlap which can be tested for consistency. As a by-product, generalized Einstein relations for calculating ion diffusion coefficients from measured mobilities are also tested. The overall agreement is good, and the few discrepancies are discussed. Their probable causes can be assigned with some confidence because of the high redundancy in the data.
Neutron diffraction structural study of pyroelectric Li2SO4⋅H2O at 293, 80, and 20 K
Authors: Lundgren J.-O. , Kvick Å. , Karppinen M. , Liminga R. , Abrahams S. C.
Published: February 2003

Lithium sulfate monohydrate has been studied at 293, 80, and 20 K by neutron diffraction using the same ground spherical crystal. Li2SO4⋅H2O is monoclinic, space group P21, with lattice dimensions at 293 K of a=5.450(3), b=4.872(3), c=8.164(4) Å, β=107.31(3)°; at 80 K: a=5.449(2), b=4.832(2), c=8.141(2) Å, β=107.21(1)°; and at 20 K: a=5.449(2), b=4.832(2), c=8.137(2) Å, β=107.19(1)°. There are two formula units in the unit cell. Least-squares refinement based on 1749 (293 K), 1712 (80 K), and 880 (20 K) reflections gave final R(F) values of 0.022, 0.016, and 0.014, respectively. All structure factors were corrected for thermal diffuse scattering. A Gram–Charlier expansion of the probability density function was used to describe the thermal vibrations of the water molecule which at 293 K are consistent with a previously postulated flipping motion. The O(W)⋅⋅⋅O(W) hydrogen bond distance is 0.091(3) Å shorter and the O(W)⋅⋅⋅O(W)⋅⋅⋅O(W) angle is 3.9(1)° larger at 20 K than at 293 K: the O(W)⋅⋅⋅O(1) distance to a sulfate oxygen is 0.041(3) Å shorter. Both kinds of hydrogen bonds are weak and bent. A slight asymmetry in the SO2−4 ion at all temperatures is related to a difference in the coordination of the four oxygen atoms: that forming the shorter S–O(1) bond (by 0.015 Å) is hydrogen bonded to O(W) and coordinated to one Li+ ion whereas the remaining three with equal S–O distance of 1.475(1) Å at 293 K are each coordinated to two Li+ions.
Amplitude equations for the onset of convection in a reactive mixture in a porous medium
Authors: Steinberg Victor , Brand Helmut R.
Published: February 2003

Amplitude equations near the onset of both the stationary and the oscillatory instability in a binary mixture (of miscible fluids) with fast chemical reaction in a porous medium are derived. A forward bifurcation for stationary convection and both forward and inverse bifurcations for oscillatory convection, depending on the value of some characteristic parameters, are predicted. The system also studied exhibits, in a certain parameter range for the oscillatory onset, nonlinear focusing phenomena which lead, together with the always present dissipation, to spatially and temporally irregular behavior near threshold. The competition between dispersive tendencies and damping can give rise to this complex structure even for initially small perturbations.
On the coupling of electron and vibrational energy distributions in carbon monoxide post dischargesa)
Authors: Gorse C. , Capitelli M. , Ricard A.
Published: February 2003

The electron energy distribution and the vibrational population of CO have been numerically calculated in discharge and post-discharge conditions. The calculations show that the electronic and vibrational energies are strongly coupled in both cases. In the post-discharge this coupling which reflects the balance of vibrational inelastic losses and superelastic gains can last up to times of 10−3–10−2 s (p=5 Torr, Tg=500 K) until electrons disappear by recombination and diffusion to the tube walls (R∼1 cm).
Stability of As–S clusters
Authors: Martin T. P.
Published: February 2003

Mass spectra are reported for clusters formed by quenching hot As–S, P–S, and As–O vapor in He gas. The composition of the As–S clusters observed varied from S60 to arsenic rich As16S16. Unusually strong peaks in the mass spectra are interpreted to indicate clusters of high stability. Such clusters could not only serve as building blocks for new cluster compounds but could also reflect the local structure in As–S glass.
Two-photon absorption spectra of single crystalline anthracene by CARS and CSRS and observation of a charge-transfer band
Authors: Shimizu M. , Igarashi R. , Adachi Y. , Maeda S.
Published: February 2003

Two-photon absorption spectra of single crystalline anthracene were obtained by analyzing the CARS and CSRS line shapes of 1262 cm−1 Raman mode in three different polarization schemes. Five two-photon peaks were observed in the energy range 2ω=31 000–41 500 cm−1 and were reasonably assigned to even-parity Frenkel excitons corresponding to the solution data, except a remarkable one at 31 400 cm−1. It was concluded that the last band is most probably assigned to a charge-transfer state of the crystal, by showing that such a two-photon transition can have rather large cross section in contrast with the small value expected for the one-photon transition to the conjugate CT state.
Molecular jet study of aniline–helium van der Waals molecules and aniline radiationless relaxation in the 1B2 excited electronic state
Authors: Bernstein E. R. , Law K. , Schauer Mark
Published: February 2003

Fluorescence excitation (FE), dispersed emission (DE), and time of flight mass spectroscopy (TOFMS) techniques are employed to study the van der Waals molecules formed in a molecular supersonic jet of aniline and helium (AnHex). It is found that AnHe1, AnHe2, and AnHe3 have absorption bands that fall under all the An (ν′) vibronic features in the 1B2 electronic state of An. The features to the high energy side of the An vibronic transitions are tentatively associated with vibrations of the above van der Waals molecules. The DE from both An and AnHex can be characterized as either ‘‘relaxed’’ or ‘‘unrelaxed’’ depending on whether or not the aniline vibronic state emitting is the same as or different from that which was pumped. All ‘‘relaxed’’ An emission can be associated with vibrational predissociation of the AnHex species. All ‘‘unrelaxed’’ emission from AnHex (ν′+v), with ν′=An mode and v=vdW bond mode, can be associated with Δv=0 transitions from AnHex. A number of other relaxation mechanisms are presented and discussed including collisional ones. Some time estimates for the major processes can be discussed.