bluuebunny/crossref_metadata_2025 huggingface dataset:
Volumetric properties of 1–1 aqueous electrolyte solutions and ionic hydration
Authors: Onori G. , Santucci A.
Published: July 2002

A simple model based on the hypothesis of existence of hydration spheres and their overlap with increasing the salt concentration is proposed. A expression for the volumetric properties of 1–1 aqueous electrolyte solutions is derived in terms of a consistent set of parameters related to the volumetric properties of the hydration spheres. Such a model is applied to study the concentration dependence of the molar volume, compressibility and expansivity of aqueous sodium chloride solutions at different values of temperature and pressure. The concentration dependence of the molar volume of some aqueous alkali halides at 20 °C is also discussed.
Comparison of size effects in aluminum and silicon clusters
Authors: Nolte Hans Jürgen , Jug Karl
Published: July 2002

Semiempirical molecular orbital calculations by the SINDO1 method are used as the basis for a comparative study of medium size Aln and Sin clusters (n=13,14,19,23,35,43). Particular emphasis is placed on the differences between metallic and semiconductor properties in dependence on cluster size. Optimized structures of crystalline and noncrystalline forms are compared with respect to average bondlengths, binding energies per atom, ionization potentials, energy gaps between highest occupied and lowest unoccupied molecular orbitals, densities of states, and charge distributions. Solid state properties begin to show up at cluster size of n=43, but more clearly for aluminum than for silicon.
Scaled particle theory for mixtures of nonadditive hard disks or hard spheres: An alternative scaling
Authors: Mazo R. M. , Bearman R. J.
Published: July 2002

The scaled particle theory of nonadditive hard disks or spheres of Tenne and Bergman, studied numerically by us in two previous papers, has been modified by relaxing the requirement that the particle radius scaling respect the nonadditivity out to arbitrarily large radii. The resulting theory is much simpler to implement numerically, while the results are comparable to those of the previous theory. A comparison is made between the predictions of phase separation of the present theory and those of numerical simulations.
Efficient microcanonical sampling for a preselected total angular momentum
Authors: Nyman Gunnar , Nordholm Sture , Schranz Harold W.
Published: July 2002

Expressions for the molecular momentum density of states as a function of spatial configuration in an angular momentum resolved microcanonical ensemble are derived. These expressions are then used to formulate an efficient sampling scheme for the generation of spatial configurations or full phase space vectors in an ensemble where both energy and angular momentum are predetermined. Applications to simple diatomic (OH) and triatomic (H2O) molecular models are presented.
Space–time coherences induced by ultrashort electromagnetic pulses
Authors: Fain Benjamin , Lin Sheng H.
Published: July 2002

In femtosecond laser-pulse experiments the pump pulse, with duration comparable or shorter than a typical period of intra- or intermolecular vibrations, creates a nonstationary wave packet. In this paper we use the density-matrix method to analyze creation of space–time coherences by the pump pulse and their effect on the probe pulse. Expressions for the density-matrix jumps, induced by the probe pulse, have been obtained in a general case. The material equations, determining propagation of the probe pulse, have been derived.
The effect of density variation on the structure of liquid hydrogen chloride. A Monte Carlo study
Authors: Steinhauser O. , Boresch S. , Bertagnolli H.
Published: July 2002

An intermolecular potential for liquid hydrogen chloride is derived from ab initio calculations and is further refined by using solid state properties. The potential function includes a two-center Lennard-Jones term, a six-center point charge model and many-body polarization forces. Monte Carlo calculations are performed for two densities (ρ=0.85 g/cm3, 0.50 g/cm3) at two temperatures (T=25 °C, 100 °C). The relative importance of the various contributions to the intermolecular potential are elucidated by comparison to neutron diffraction experiments.
Kinetically balanced geometric Gaussian basis sets for relativistic atoms
Authors: Mohanty A. K. , Clementi E.
Published: July 2002

We present Dirac–Hartree–Fock (DHF) atomic structure results for many-electron atoms (Z=2–86) using kinetically balanced geometric basis sets of Gaussian functions. There is no ‘‘variational collapse’’ or bound failure if we impose the condition of kinetic balance between the large and the small component spinors along with proper boundary conditions for bound state orbitals. Furthermore, with a finite-size nucleus model, the DHF total energy for atoms converges more rapidly to the numerical Dirac–Fock (DF) limit with a smaller number of basis functions than those calculations where a point nucleus is assumed. By systematically extending the geometric basis sets, we have obtained the DHF total energies for all atoms(up to Z=86) within a few millihartrees of the numerical (DF) limit, with smooth convergence from above.
An interpretation of the bifurcation of orientational relaxation processes in a supercooled liquid
Authors: Bagchi Biman , Chandra Amalendu , Rice Stuart A.
Published: July 2002

The orientational relaxation of molecules in a supercooled liquid is known to exhibit interesting dynamical behavior. As the temperature of the liquid is lowered towards its glass transition temperature, there is a bifurcation of the relaxation dynamics into primary (α) and secondary (β) processes; the former is associated with the collective motion responsible for the glass transition, while the latter is associated with single particle motion. In this paper we present a theory of orientational relaxation in a supercooled liquid. This theory provides both qualitative and quantitative descriptions of the (αβ) bifurcation phenomenon at a molecular level. The theory exploits the properties of a time dependent free energy functional which explicitly includes the effects of the collective motions in the liquid on the orientational motion of a solute (or a tagged) molecule. In the overdamped limit, this analysis leads to two coupled Smoluchowski equations for the orientation distribution function. These equations, when solved, reveal the essential features of the (αβ) bifurcation phenomenon. Explicit calculations are presented for orientational relaxation in a liquid of dipolar hard spheres, a liquid of nonpolar ellipsoids, and an orientationally disordered solid. Our calculations demonstrate the ubiquity of the (αβ) bifurcation phenomenon and they reveal many of its aspects. The relevance of the present work to current theories of glass transition is discussed briefly.
Accurate three-dimensional quantum scattering studies of long-lived resonances for the reaction He+H+2→HeH++H
Authors: Kress J. D. , Walker R. B. , Hayes E. F.
Published: July 2002

The adiabatically adjusting principal-axis hyperspherical (APH) formulation of Pack and Parker for quantum reactive scattering in three dimensions (3D) is used to obtain converged results for the reaction of helium with H+2 (v=1–4) for total angular momentum J=0. The ab initio potential energy surface computed by McLaughlin and Thompson and fitted by Joseph and Sathyamurthy is utilized for the HeH+2 interaction potential. The predicted energy dependence of the accurate 3D state-to-state reaction probabilities show clear evidence for quantum resonances. These resonances are even more numerous than those reported earlier for reduced dimensionality studies of this reaction. The calculated time delays for several of these resonances are found to be over 1 ps. Bending corrected rotating linear model (BCRLM) studies of this same reaction are also reported. These results provide useful insight in sorting out the nature and contribution of the resonances found in the 3D studies.
Layering transitions in cylindrical pores
Authors: Peterson Brian K. , Heffelfinger Grant S. , Gubbins Keith E. , van Swol Frank
Published: July 2002

We report grand canonical Monte Carlo simulations and nonlocal density functional theory results for layering transitions that take place in a smooth cylindrical pore with strong fluid–wall interactions. For a pore with a radius of seven molecular diameters and a temperature below the bulk fluid triple point temperature the adsorption isotherms exhibit three distinct layering transitions ending in capillary condensation. The growth of the first layering in the simulation appears supercritical with respect to the layering critical point, but the subsequent layers appear to be true (first-order) transitions. In addition to the layering transitions we also present evidence for a quasi-two-dimensional freezing transition. That is, we observed the first adsorbed layer undergo a transition to a frustrated crystalline state prior to the next layering transition. Finally, for an adsorption isotherm at the bulk triple point there is no evidence for any first-order transitions, indicating that for our system the critical temperature associated with each layering transition does not exceed the bulk triple point temperature.
On the origin of metastable decay in Ar+2
Authors: Whitaker B. J. , Woodward C. A. , Knowles P. J. , Stace A. J.
Published: July 2002

Metastable Ar+2 (Ar+2 →Ar++Ar) has been observed in a double-focusing mass spectrometer from ions created by 70 eV electron bombardment of an Ar cluster beam. New ground and excited state potential energy curves have been calculated for Ar+2, and these have been used to show that metastability is due to radiative decay from the II(1/2)u state of the ion. It is shown that vertical (FC) ionization from neutral Ar2, with a vibrational temperature of approximately 30 K, results in a significant fraction of the ions occupying the II(1/2)u state. Detailed pressure dependent measurements show that collision-induced dissociation does not contribute to the observed Ar+ signal. The mean kinetic energy released to the Ar+ has been measured as 44 cm−1 in the center-of-mass frame, and calculations show that this value is consistent with the proposed mechanism.
The influence of the structure of the surface of a liquid on the properties of a supported monolayer
Authors: Shin Seokmin , Rice Stuart A.
Published: July 2002

This paper describes a simple lattice model of a liquid supported monolayer of long chain amphiphile molecules. The new feature which this model possesses is incorporation of the structure of the surface of the support liquid. The liquid–monolayer interface is represented as a multilayer region, and its structure is described in terms of the associated inhomogeneous density distributions of the support liquid molecules and of the segments of the chain molecules. The properties of this diffuse surface–amphiphile monolayer model are compared with the properties of the smooth surface–amphiphile monolayer model described in our previous work [J. Chem. Phys. 92, 1427 (1990)]. The general behavior of the monolayer equation of state, including the prediction of two successive fluid–fluid phase transitions, is preserved in the present model. We find that the existence of the diffuse surface structure of the supporting liquid leads to an increase in the number of the amphiphile chain segments that lie in the surface region, particularly at low surface densities. We also find that the multilayer character of the diffuse surface region can alter the equation of state of the monolayer at high surface densities. The two layer diffuse surface–amphiphile monolayer model supports a change in the slope of the surface pressure versus area isotherm in the very high surface density region, which can be ascribed to a conformational ‘‘freezing’’ of the chain molecules. The theory illustrates the qualitative features of the system which define the close interplay between the structure and properties of the monolayer and the structure of the surface of the support liquid.
On using shaped light pulses to control the selectivity of product formation in a chemical reaction: An application to a multiple level system
Authors: Tersigni Samuel H. , Gaspard Pierre , Rice Stuart A.
Published: July 2002

A previously reported method for shaping electromagnetic field pulses to achieve chemical selectivity is extended and applied to a simple multiple level model system. The pulse shaping approach is based on optimal control theory, where both the time-dependent Schrödinger equation and the constant pulse energy are used as constraints on the variational scheme. A conjugate gradient direction method is used to direct the convergence of the iterative process used to calculate the optimum pulse shape. The method is applied to a five-level system interacting with an optical (laser) field. Results demonstrating selectivity and stability are compared to those of other recent related investigations.
Reaction path power series analysis of NH3 inversion
Authors: Steckler Rozeanne , Truhlar Donald G.
Published: July 2002

A new tool for reaction-path analysis of chemical dynamics is introduced, namely, series expansion of reaction-path potentials. This analysis involves the expansion of vibrational frequencies in powers of a reaction coordinate measuring progress along a suitably defined reaction path. The method is applied to generalized-normal-mode vibrational frequencies along two choices of reaction path for the inversion of ammonia. The leading nonzero terms in the power series are calculated by Mo/ller–Plesset second-order perturbation theory and are fully converged with respect to basis set and the step size used for calculating the minimum energy path and the Hessian.
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o model potential study of the equilibrium geometry of alkaline earth dihalides: MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I)
Authors: Seijo Luis , Barandiarán Zoila , Huzinaga Sigeru
Published: July 2002

The ground state equilibrium geometry of alkaline earth dihalides MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I) has been optimized at the Hartree–Fock (HF) level using the Cowan–Griffin relativistic ab initio model potential method and a uniformly good, extended, spd valence basis set. The results show that, according to the method, all magnesium dihalides and CaCl2, CaBr2, and CaI2 are linear, SrF2 and all barium dihalides are bent, and CaF2 and SrCl2, SrBr2, and SrI2 are quasilinear molecules. The alkaline earth (n−1)d orbitals are shown to be responsible for the bending of the heavier molecules while their (n−1)p orbitals contribute considerably to the final quantitative prediction of the apex angle and the relative stability of the bent structures. Relativistic effects are shown to be very small on the bond distances and vibrational frequencies; they are important on the size of the bending barrier of the bent molecules. The results obtained are compared to previous theoretical studies and provide some insight in the interpretation of the contradictory experimental conclusions.
Characterization of cluster ions produced by the sputtering or direct laser vaporization of group 13 metal (Al, Ga, and In) oxides
Authors: King Fred L. , Dunlap Brett I. , Parent Denise C.
Published: July 2002

The stabilities and reactivities of cluster ions generated from the fast-atom bombardment (FAB) or the direct laser vaporization (DLV) of the Group 13 metal oxides (Al2O3, Ga2O3, and In2O3) were examined by mass spectrometry. The nascent cluster ion distributions, fragmentations, and reactions were studied. The observed patterns of stability and reactivity were compared with the structures and heats of formation calculated from theoretical studies of aluminum oxide cluster ions using MNDO, Xα, and Born–Mayer pair potentials. The method of production of the metal oxide cluster ions, whether by FAB, DLV, or through the reaction of sputtered bare metal cluster ions with oxygen, had little influence on the abundance distribution observed. In agreement with the known M–O binding energies, a trend of increasing cluster oxidation state was observed in the abundance distributions of the cluster ions for decreasing metal atom z value. Dissociation of the oxide cluster ions occurred through the loss of particularly stable neutral fragments which together with theoretical considerations suggest a preference for particular parent cluster stoichiometries. Although gallium oxide and indium oxide cluster ions exhibited little reactivity toward oxidation agents, the aluminum oxide ions reacted readily with most oxidants. ‘‘Oxygen saturation’’ effects were observed for the ions with 2 and 3 aluminum atoms. These saturation effects could be explained by the formation of structures in which the Al atoms are three-fold planar coordinated.
Mutual polarization of monomer charge distribution in (HCN)2, (HCN)3, and (HCN)∞
Authors: Ruoff R. S.
Published: July 2002

A simple classical mutual polarization model accurately predicts the induced dipole moments of linear HCN dimer and trimer. The model employs the first four nonzero electrical molecular moments of each HCN, and both the molecular polarizability and the C–H and C–N bond dipole polarizabilities. The model is extended to linear H-bonded oligomers up to the pentamer, and also to an infinite linear H-bonded chain, appropriate for comparison with the H-bonded chains present in the HCN crystal. For the dimer and trimer, a relation is seen between the change in the electric field gradient at the N of HCN and the calculated induced C–N bond dipole moment. A semiquantitative proportionality constant, derived based on this relation, is used to enhance understanding of the underlying cause of the large increase in the 14N quadrupole coupling constant in the HCN crystal relative to the free HCN monomer. This increase results from the strong polarization of the C–N bond by the local field at each HCN caused by the rest of the HCN crystal lattice.
Diffusion disallowed crystal growth. I. Landau–Ginzburg model
Authors: Schofield Sarah A. , Oxtoby David W.
Published: July 2002

We present an analysis of crystal growth of a pure material from its melt, using a phase field model. This model for a first-order phase transition couples nonconserved order parameter kinetics to thermal diffusion. It has been shown to give unique one-dimensional solutions, with velocity selection. We demonstrate that these steady-state solutions cease to exist for the thermal diffusivity above a critical value, which depends on the parameter coupling the kinetics to the temperature. We suggest a scaling which leads to agreement between numerical integration of the time-dependent equations and a perturbation analysis of the coupling. For small velocity, our model reduces to a standard model commonly applied to dendritic growth, which replaces the kinetic equation with one of interfacial equilibrium. Because the velocity vanishes at the critical point, the thermal diffusion length and dendritic wavelength diverge at this point. We relate this critical point to bifurcations in the phase space of a mechanical equivalent to the steady-state growth equations. We conclude that solutions cease because thermal diffusion drives the system to local kinetic equilibrium, and we discuss experimental accessibility and means of avoiding equilibrium.
Nonadiabatic dynamics of excited excess electrons in simple fluids
Authors: Space B. , Coker D. F.
Published: July 2002

We present a surface hopping trajectory method for studying nonadiabatic excess electronic relaxation in condensed systems. This approach is used to explore the nonadiabatic relaxation after photoexciting an equilibrated excess electron in dense fluid helium. We survey the different types of nonadiabatic phenomena which are important in excess electronic relaxation. Very rapid diabatic processes are common when the nuclear dynamics provides only weak couplings between the adiabatic states. This is generally the case when the states are localized in different regions of space. We find that the nuclear dynamics provides a mechanism for strong coupling between s- and p-like states localized in the same solvent cavity. These strong nonadiabatic interactions can persist over a wide range of nuclear configurations and for many hundreds of femtoseconds.
Benchmark calculations of thermal reaction rates. II. Direct calculation of the flux autocorrelation function for a canonical ensemble
Authors: Day Paul N. , Truhlar Donald G.
Published: July 2002

The thermal rate coefficient for the benchmark reaction H+H2→H2+H with zero total angular momentum is calculated by an ℒ2 basis-set evaluation of the flux autocorrelation function. This involves the direct evaluation of the thermally averaged flux operator without calculating any state-to-state reaction probabilities. The results are internally converged to 0.1% and agree with the totally independent results of the previous paper to within 0.03%. This confirms that the correlation-function approach is stable enough to converge practical calculations to three or more significant figures, which is adequate for all purposes, and that the calculations of this and the previous paper may be used to test the convergence and accuracy of other methods for calculating canonical-ensemble reaction rates.
Calculations on the rate of the ion–molecule reaction between NH+3 and H2
Authors: Herbst Eric , DeFrees D. J. , Talbi D. , Pauzat F. , Koch W. , McLean A. D.
Published: July 2002

The rate coefficient for the ion–molecule reaction NH+3 +H2→NH+4+H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50–100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH+3+H2+He→NH+4+H+He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.
Differential translation energy-loss distributions at 1 eV energies: K–C2H2
Authors: Velegrakis M. , Beck D.
Published: July 2002

In K–C2H2 collisions at collision energies 0.4≲E≲1.3 eV and scattering angles 40°≲ϑ≲180° surprisingly little transfer of energy from translation into the internal degrees of freedom of the acetylene molecule is found. The observations are consistent with the assumptions of electronically covalent encounters, insignificant activation of the acetylene vibrations, and rotational excitation produced by spindle-shaped equipotentials, nearly spherical in a broad vicinity of the C 3/4 C bond.
Dynamics of recombinative desorption: Angular distributions of H2, HD, and D2 desorbing from Cu(111)
Authors: Rettner C. T. , Michelsen H. A. , Auerbach D. J. , Mullins C. B.
Published: July 2002

We have determined the angular distributions of H2, HD, and D2 desorbing from Cu(111) for surface temperatures in the range 370–800 K. These are found to be strongly peaked and symmetric about the surface normal in every case. Results for all three isotopes are found to be indistinguishable, being close to a cos 12θf distribution at 600 K, slightly narrower at 370 K, and slightly broader at 800 K. Results are discussed in terms of other previous desorption measurements and related to adsorption data via detailed balance.
Parity propensities in rotational energy transfer of OH X 2Πi with helium
Authors: Wysong Ingrid J. , Jeffries Jay B. , Crosley David R.
Published: July 2002

Rotational energy transfer in OH (X 2Πi, v=2) induced by collisions with He, has been studied using a two-laser technique. Strong propensities are found for conservation of total parity in collisions which change the rotational level and spin orbit component. These results have implications concerning the shape of the OH–He potential.
The length scale dependence of viscosity approaching the glass transition in glycerol
Authors: Dixon Paul K. , Nagel Sidney R. , Weitz David A.
Published: July 2002

We measure the viscosity of glycerol by using photon correlation spectroscopy to observe the Brownian motion of small (∼100 nm) polystyrene spheres. By comparing the viscosity measured with different size spheres, we can measure the length scale dependence of the viscosity as the liquid is cooled towards the glass. In contrast to published reports, we observe no spatial dispersion in the viscosity.
Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. I. The naphthalene cation (C1H+8)
Authors: Salama F. , Allamandola L. J.
Published: July 2002

The ultraviolet, visible, and near infrared absorption spectra of naphthalene (C10H8 ) and its radical ion (C10H+8 ), formed by vacuum ultraviolet irradiation, were measured in argon and neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing ion production in the solid phase. The absorption coefficients were calculated for the ion and found lower than previous values, presumably due to the low polarizability of the neon matrix. This study presents the first spectroscopic data for naphthalene trapped in a neon matrix, where the perturbation of the isolated species by its environment is minimum; a condition crucial to astrophysical applications.
The van der Waals vibrations of aniline–(argon)2 in the S1 electronic state
Authors: Bieske Evan J. , Rainbird Mark W. , Knight Alan E. W.
Published: July 2002

Vibrational structure associated with van der Waals modes of the aniline–(argon)2 complex has been observed in the region near the origin of the S1←S0 electronic transition of the complex using resonance enhanced, multiphoton ionization (REMPI) spectroscopy. The aniline–Ar2 spectrum in this region displays only a few discrete bands built on an intense electronic origin. The dominant vibrational band, associated principally with the symmetric van der Waals stretching motion of the two argon atoms against the aromatic frame, occurs at 38.5 cm−1 displacement relative to the 000 band, with weaker transitions at 15 and 30 cm−1 displacement. A simple model for the van der Waals vibrations of aromatics bound to one and two rare gas atoms is developed and allows allow us to explain the aniline–Ar2 spectrum, using van der Waals bond parameters determined from the previously measured An–Ar1 spectrum. The agreement between the predicted and observed aniline–Ar2 spectrum confirms the view that van der Waals stretching vibrations are coupled anharmonically to near resonant bending vibrations in aniline–Ar1 and aniline–Ar2.
Dielectric relaxation of liquid mixtures
Authors: Wei Dongqing , Patey G. N.
Published: July 2002

General expressions in terms of van Hove time correlation functions are given for the wave vector frequency-dependent dielectric function of multicomponent mixtures. The van Hove functions are obtained by applying the Kerr approximation and the dielectric relaxation at zero wave vector is considered in detail. At this level of theory, the frequency-dependent dielectric constant depends upon the self-reorientational correlation times of the various species involved and upon the equilibrium pair correlation functions. It is shown that if the self-correlation times are assumed to be given by the Stokes–Debye relationship, and if the equilibrium direct correlation functions obey certain relatively weak conditions, then for particles of equal size (i.e., the self-correlation times are the same for all species) the dielectric relaxation behavior can be described by a simple Debye formula with a single concentration-dependent relaxation time. This observation is independent of the number of components, of the concentration, and of the molecular dipole moments of the different species present. It may help explain why for some binary mixtures of polar molecules experimental measurements indicate only a single relaxation channel. The exact Kerr result for binary mixtures is expressed explicitly as the sum of two Lorentzians, and some numerical results are given for solutions of dipolar hard spheres of different diameter.
Equations of state for hard spheres fluid in the metastable fluid region
Authors: Maeso M. J. , Solana J. R. , Amorós J. , Villar E.
Published: July 2002

A number of existing approximants are reviewed and tested for the equation of state of the hard spheres fluid in the metastable fluid region, namely, at densities higher than the normal freezing density: ρ*=0.943. This is a region which is particularly sensitive to the quality of a given equation of state; however, it is frequently ignored in the study of analytical equations of state. A new set of approximants is also discussed, including as particular cases several other currently used equations of state for the hard spheres fluid.
Quantum dynamics of the H+D2→D+HD reaction: Comparison with experiment
Authors: D’Mello Michael , Manolopoulos David E. , Wyatt Robert E.
Published: July 2002

The Kohn variational principle for the log-derivative matrix is used to calculate integral cross sections for H+D2 (v=0, j=0) to D+HD (v′=0,1,2, all j′) at the experimentally accessible collision energies of 0.55 and 1.3 eV. Comparison is made with experimental and theoretical studies in the literature. Product state relative rotational distributions, vibrational branching ratios, and energy partitioning fractions are all in good agreement with the recent experimental results of Rinnen, Kliner, and Zare. Absolute cross sections are compared with the experimental work of Levene et al. and Johnson et al. Our results agree very well with their experiments. It is found that the quasiclassical results of Blais and Truhlar compare well with the present exact quantum mechanical predictions in many respects, however, the product rotational distributions are ‘‘hotter’’ than the quantal ones.
Theoretical study of the spectra of CuH and CuD
Authors: Marian C. M.
Published: July 2002

The electronic states of CuH correlating with the two lowest dissociation channels have been studied by ab initio methods. Potential energy curves, electric dipole (transition) moments, and spin–orbit coupling matrix elements are evaluated using correlated wave functions determined from the (relativistic) no-pair equation. The excited states of CuH and CuD cannot be characterized as pure spin states. By including spin–orbit coupling in the calculations, a one to one correspondence can be established between experimentally known term energies and computed level positions. The A 1Σ+ and B 3Π0+ states results from different linear combinations of 1 1Σ+ and 1 3Π0+. The 3Π state is found to have an inverted fine-structure pattern in contradiction to earlier assumptions. The experimentally known C1 and c1 states originate from admixtures of 1 3Δ and 1 1Π. The large coupling between LS states explains the intensity of the otherwise forbidden transitions from 3Π and 3Δ to the ground state. Among the states predicted from the present analysis is a low-lying 3Σ+ state which may be responsible for the bands observed in the red region.
Two and three dimensional laser induced fluorescence: Application to photodissociation of LiI
Authors: Shafer Neil , Bersohn Richard
Published: July 2002

A new method is proposed for the measurement of translational energy distributions in which a species with three energy levels is excited by two successive transitions with perpendicular laser beams. Simulations of the method are carried out for the photodissociation of H2 O and a pilot experiment was performed on Li atoms dissociated from LiI vapor.
On the use of Feynman–Hibbs effective potentials to calculate quantum mechanical free energies of activation
Authors: Voth Gregory A.
Published: July 2002

The use of Feynman–Hibbs effective potentials to estimate quantum mechanical free energies of activation is shown to be a well defined approximation to a more accurate version of quantum activated rate theory. The potential pitfalls of employing such an approximation are also discussed.
Rovibrational spectra of open-shell van der Waals complexes: Ar–OH(X 2Π)
Authors: Chakravarty C. , Clary D. C.
Published: July 2002

The infrared spectrum of Ar(1S0)–OH(X 2Π) has been predicted using an ab initio potential-energy surface. We assume that the electronic spin and orbital angular momenta remain coupled to the rotational motion of the diatom in a van der Waals complex. The rovibrational energies and wave functions are calculated using a basis-set method involving expansion of the rovibrational wave function in terms of a product basis of optimized radial and angular wave functions. Unusual features are observed due to the nonzero electronic angular momentum of the diatom in the rovibrational levels. The effect of deuterium substitution on the predicted spectra has also been studied.
Theoretical stabilization and scattering studies of resonances in the addition reaction H+CO = HCO
Authors: Gazdy Bela , Bowman Joel M. , Cho Seon-Woog , Wagner Albert F.
Published: July 2002

We report the first coordinated stabilization and coupled channel scattering calculations of resonances for a realistic, triatomic molecule, HCO, using a global, ab initio potential-energy surface. The two set of calculations are in excellent agreement with each other for the nine resonance energies reported.
Electronic transition moment variation and Einstein coefficients for the NO(B 2Π–X 2Π) system
Authors: Piper Lawrence G. , Tucker Thomas R. , Cummings William P.
Published: July 2002

This paper details an investigation of the variation in the electronic transition moment with internuclear separation for the NO(B2Π–X2Π) transition. Measurements of the relative intensities of a number of NO B–X vibronic transitions having a common upper level were used to construct a relative transition-moment function between 1.27 and 1.60 Å. After normalizing this relative function by experimentally determined radiative lifetimes, the transition-moment function was extended down to 1.23 Å by incorporating data from oscillator strength measurements. In contrast to empirical transition-moment functions that have been proposed previously, the function in this paper decreases with increasing internuclear separation. Unlike these other functions, however, this one is consistent with theoretical predictions, with most available oscillator strength data, and with the observed trend in B-state radiative lifetimes as a function of vibrational level.
A multichannel quantum defect half-collision analysis of K2 photodissociation through the B 1Πu state
Authors: Dubs Richard L. , Julienne Paul S.
Published: July 2002

Recent experiments by Zafiropulos et al. [Phys. Rev. Lett. 61, 1485 (1988)] indicate that K2 photodissociation through the B 1Πu state results in fluorescence polarization which is strongly dependent on excitation wavelength. To understand these results, we have studied the K2 system quantum mechanically using a half-collision analysis derived from the generalized form of multichannel quantum defect theory. This analysis factors the transition amplitudes into separate terms representing absorption and final state interactions. An approximation called the adiabatic(a→c)/recoil(c→e) approximation is developed for the half-collision matrix which reproduces quantitatively the exact half-collision results projected from the close-coupled wave function. This specific approximation applies to the homonuclear molecule K2 because of the very long range of the excited state potential, which varies as 1/R3. The quantum mechanical expression for the polarization as a function of initial rotational quantum number J0 and total energy E is found to be extremely simple in this approximation, depending only on J0 and the single-channel P, Q, and R phase shifts for the adiabatic reference states corresponding to the B 1Πu state at short distance. In the high J limit, the quantum expression is in exact agreement with a semiclassical expression which is derived in the accompanying paper by Kleiber et al.
Calculation of infrared intensities of highly excited vibrational states of HCN using Van Vleck perturbation theory
Authors: McCoy Anne B. , Sibert Edwin L.
Published: July 2002

Canonical Van Vleck perturbation theory (CVPT) is used to calculate electric dipole intensities for one-, two-, and three-dimensional models of HCN and a six-dimensional model of H2CO. Lehmann and Smith [J. Chem. Phys. 93, 6140 (1990)] have shown that the intensities of overtone transitions are sensitive to the details of the inner wall of the potential. Dipole intensities calculated for several, similar one-dimensional CH stretch potentials demonstrate that perturbation theory correctly predicts this sensitivity. The perturbation intensities of a two-dimensional ab initio dipole surface indicate the importance of selected stretch–stretch resonance interactions in interpreting the CH stretch overtone spectra of HCN. The inclusion of the bend confirms that this degree of freedom plays a significant role in weakening the intensity of the CN overtones. The CH stretch overtone spectra of H2CO is calculated to illustrate the utility of the perturbative approach for predicting the transition intensities for a system in which there are multiple Fermi interactions.
The free energy and the chemical potential of molecular fluids
Authors: Chen X. S. , Forstmann F. , Kasch M.
Published: July 2002

Expressions for the Helmholtz free energy and the chemical potential are derived for fluid mixtures with nonspherical interactions. For the important reference hypernetted chain (RHNC) approximation we can derive explicit formulas. We evaluate the free energy formula for a hard sphere dipole system. The results for the Helmholtz free energy agree well with Monte Carlo simulation.
Mechanism and temperature dependence of polymer electrolyte conductivity in terms of dynamic disorder
Authors: Druger Stephen D.
Published: July 2002

The dynamic disorder theory previously developed for ionic transport in polymer electrolytes is extended to the case of two distinct sequential steps in the transport. It is shown how this can describe in greater detail than before a specific microscopic mechanism usually assumed for cation transport. Expressions are obtained for the frequency and temperature dependence of the charge transport, and various limits are delineated in which the model leads to the same temperature dependence for the cationic conductivity as applies also for the viscosity on a microscopic scale, thereby providing an improved justification within the context of dynamic disorder theory for application of the Vogel–Tamman–Fulcher equation to ionic conduction when it applies to appropriate mechanical properties. An analogous model based directly on the earlier dynamic disorder approach is presented, is shown to be a special case of that considered here, and is shown capable of fitting dc conductivity data for polymer electrolytes in various limiting cases, thereby clarifying the relation between the two models and showing the present theory to be similarly capable of explaining the dc conductivity data considered.
Simulation of highly stretched chains using long-range Monte Carlo
Authors: Deutsch J. M. , Reger J. D.
Published: July 2002

The behavior of highly stretched chains is not well described by computer simulations using the usual short-range Monte Carlo algorithms. Results from these have been shown to produce unphysical slowing down due to artificial energy barriers imposed by the dynamics. Furthermore, relaxation in the absence of a field from a highly stretched state is shown to be improperly described by the usual Monte Carlo algorithms. We propose a novel model employing long-range moves that should describe the motion in such situations more correctly. We implement this model on a square lattice to study gel electrophoresis. Using a highly efficient numerical code, the time for one long-range move scales as log(N), where N is the number of monomers in the chain. This allows us to conduct simulations of DNA molecules in a regime of molecular weights much greater than previously possible. Our results are in surprisingly good agreement with previous work for short chains using more realistic dynamics, and for longer chains, interesting data is obtained.
Algebraic approach to molecular rotation–vibration spectra. III. Infrared intensities
Authors: Iachello F. , Leviatan A. , Mengoni A.
Published: July 2002

We suggest a form of the dipole operator to be used in the analysis of intensities of infrared transitions within the framework of the vibron model. We give the corresponding matrix elements in explicit form and discuss their qualitative features.
Photoelectron spectroscopy of krypton and xenon clusters
Authors: Carnovale Frank , Peel J. Barrie , Rothwell Richard G.
Published: July 2002

Ultraviolet photoelectron spectra have been measured for pulsed supersonic beams of krypton and xenon as dilute mixtures in helium. The spectra exhibit broad bands which are located at lower ionization energies relative to the monomer ion states, 2P3/2 and 2P1/2, with which they correlate. The structural features of the spectra, particularly in the first band group, become more complex with increasing condensation, apparently relating to the mean cluster size associated with each spectrum. The spectra are interpreted using the cluster-size dependent core-ion model developed to explain the analogous spectra of the argon clusters. The argon clusters spectra were interpreted as showing the presence of Ar+3, Ar+7, and Ar+13 core ions, with Ar+3 involved in the ionization of small neutral clusters, and Ar+13 produced by the dominant ionization mechanism in large clusters as well as in solid argon. The krypton and xenon clusters show variations of this behavior. The relatively large spin-orbit splitting of the Kr+ and Xe+ p-hole states is reflected in the two band groups observed in the respective Kr and Xe clusters spectra. The lower ionization energy band group in each case exhibits structural features similar to those observed for the argon clusters spectra. The krypton spectra indicate, that for the largest clusters, all three core-ion mechanisms are operative; whereas, for xenon, the largest clusters show only the triatomic core ion, Xe+3, as involved in the ionization mechanism. The high pressure clusters spectra of argon, krypton, and xenon are effectively identical with those reported for their respective condensed thin films, indicating that these variations in the core-ion mechanism of ionization are also responsible for the differences observed in the ultraviolet photoemission spectra of the rare-gas solids.
Normal mode analysis of van der Waals vibrations
Authors: Li S. , Bernstein E. R.
Published: July 2002

Two algorithms are presented for calculation of the van der Waals modes of weakly bound clusters. Both methods rely on the harmonic normal mode approximation and a chosen intermolecular potential. These calculational techniques differ specifically in the form of the force field employed: one method uses the total (both intra- and intermolecular) force field for the cluster and the other uses only the intermolecular force field. Both methods require rather elaborate coordinate transformation and their first and second partial derivatives; these are provided in detail. The two calculations generate eigenvalues and eigenvectors that are in complete agreement with one another for a given potential. The methods insure that the van der Waals modes are calculated at the proper cluster equilibrium configuration for which all torques and forces on molecules and/or atoms are effectively zero. Examples are presented for (H2O)n (n=2,...,7), benzene clustered with water, methane, and ammonia, and a number of different intermolecular potentials. Some of the observed heterogeneous cluster van der Waals modes are reassigned in light of these new results.
A theoretical study of the low-lying states of Ti2 and Zr2
Authors: Bauschlicher Charles W. , Partridge Harry , Langhoff Stephen R. , Rosi Marzio
Published: July 2002

The 1Σ+g, 3Δg, 3Σ+u, and 7Σ+u states of Ti2 and Zr2 have been studied using a multireference configuration-interaction (MRCI) approach. Although our best calculation produces a 1Σ+g ground state for Zr2, the 3Σ+u and 3Δg states are found to be very low lying. Additional support for a 1Σ+g ground state assignment comes from the fact that the resulting vertical excitation spectrum is consistent with the optical spectrum of Zr2 observed in noble gas matrices. For Ti2, it proved more difficult to make a definitive assignment of the ground state, because of the many low-lying states and the large effect of inner-shell (3s and 3p) correlation. With only valence correlation included, the ground state is predicted to be 7Σ+u at both the MRCI and averaged coupled-pair functional (ACPF) levels of correlation treatment. However, inner-shell correlation effects, estimated based on modified coupled-pair functional (MCPF) and contracted configuration-interaction (CCI) calculations, preferentially lower the 1Σ+g and 3Δg states, resulting in a 3Δg ground state and a very low-lying 1Σ+g state. Since the 3Δg state lies below the 1Σ+g state at all levels of correlation treatment and both states are derived from the same atomic asymptote, we prefer a 3Δg assignment for the ground state of Ti2. This is consistent with the failure to observe an electron-spin resonance (ESR) signal, but such an assignment requires an explanation for the absence of subcomponents on the lines observed in the resonance Raman spectrum of Ti2.
Coordinate transformations of cubic force constants and transferability of anharmonic force constants in internal coordinates
Authors: Challacombe M. , Cioslowski J.
Published: July 2002

A general approach to transforming cubic anharmonic force constants from the Cartesian coordinates to an arbitrary set of nondegenerate internal coordinates is presented and a computer program performing the transformation is described. It allows one to study the transferability of the cubic anharmonic force constants between similar molecules. The harmonic and anharmonic force constants are calculated at the Hartree–Fock level for the C2H6–nFn series. The diagonal cubic force constants for the bond stretching are found to be transferable to the same extent as the quadratic (harmonic) ones. The force constants for the bendings are not directly transferable. The present approach makes it possible to construct anharmonic force fields by fitting parametric functions to the ab initio anharmonic force constants in the internal coordinates.