bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer simulation of ion recombination in irradiated nonpolar liquids
Authors: Bartczak Witold M. , Hummel Andries
Published: July 2002

The problem of the diffusion controlled recombination of ions for the case where initially two or more pairs of oppositely charged ions are found in each other’s Coulomb field, as occurs in the track of a high-energy electron in a nonpolar liquid, is treated by means of computer simulation. Results are presented on the probability of survival as a function of time for ion pairs in clusters containing up to four ion pairs initially. The effect of an external electric field on the probability of escape from recombination is investigated. The multipair results are compared with the calculations based on the single-pair theory. The consequences of the consideration of multipair effects in addition to the single ion-pair contribution in the nonhomogeneous kinetics of the track of high-energy electron in nonpolar liquids are discussed briefly.
Br2 ion pair state formation by electron beam excitation
Authors: Shannon D. C. , Killeen K. P. , Eden J. G.
Published: July 2002

The emission spectrum of Br2 in the ultraviolet (180≤λ≤400nm) has been examined in electron beam pumped rare gas–hydrogen bromide (HBr) gas mixtures. Temporally resolved and rare gas pressure-dependent studies of the Br2 fluorescence at 292 (D′→A′), 312, 334, and 358 nm indicate that the formation processes for Br2 ion pair states differ for Ne and Ar diluents. Experimental evidence suggests that Br2 excited states in the first tier [correlated with Br−(1S0)+Br+(3P2)] in Ar/HBr mixtures are formed by ion–ion recombination in which Br+(3P2) is produced by charge transfer between Ar+(2P) and HBr, followed by predissociation of HBr+(A 2Σ+). In Ne buffer, Br2 states well above D′ are apparently produced by Penning ionization of HBr by Ne 3s 3P followed by recombination of Br+(3P0,1,2 ) ions with Br−(1S0). Also, Ar is effective in collisionally relaxing the lowest manifold of six Br2 ion pair states but both Ne and Ar are poor quenchers of the D′(2g) level.
Improved potential energy surfaces for the reaction O(3P)+H2→OH+H
Authors: Joseph Tomi , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

We present global 3A′ and 3A″ potential energy surfaces for the reaction O(3P)+H2→OH+H and its isotopic analogs. The new surfaces are based in part on the surface of Lee et al. [J. Chem. Phys. 76, 3563 (1982)] for collinear O–H–H, which is modified to give accurate properties for reactants and products. The potentials for noncollinear O–H–H geometries are based on bending energies of Bowman et al. [J. Chem. Phys. 81, 1739 (1984)] as fit previously and called surface set M2 by two of the authors [B. C. Garrett and D. G. Truhlar, Int. J. Quantum Chem. 29, 1463 (1986)], and the potentials for H–O–H geometries are based on a new interpolation scheme. The new surfaces treat the approach of an O to either end of H2 equivalently. We used improved canonical variational transition state theory calculations with the least-action ground-state tunneling approximation (ICVT/LAG) to recalibrate the classical barrier height to 13.0 kcal/mol. The ICVT/LAG rate constants calculated using the new surfaces are in excellent agreement with experimental values over the temperature range 298–2400 K. Rate constants and kinetic isotope effects for reactions involving D2 and HD also are presented and analyzed. Here again, the theoretical results are in very good agreement with experiments.
MNDO calculations of stilbene potential energy properties relevant for the photoisomerization dynamics
Authors: Troe J. , Weitzel K.-M.
Published: July 2002

Modified neglect of differential overlap-Configuration interaction (MNDO-CI) calculations of the low-lying electronically excited states and the electronic ground state of stilbene have been performed. The dependences of the potential energy on the angle of rotation around the central ethylene bond, on the length of this bond, and on the twist angle of the two phenyl groups are explored. There is no evidence for a nonadiabatic participation of the doubly excited ‘‘phantom state’’ in the photoisomerization dynamics. Instead, the calculated properties of the singly excited 1 1B state support a mechanism with adiabatic rotation around the central ethylene bond to the perpendicular conformation, followed by internal conversion. There appears to be a small ‘‘CI-induced’’ energy barrier along this pathway which will be overcome by the combined motion in several coordinates. Therefore, a complicated activated complex structure arises.
Experimental and theoretical studies of the doubly charged NO2+2 ion
Authors: Fournier P. G. , Eland J. H. D. , Millie P. , Svensson S. , Price S. D. , Fournier J. , Comtet G. , Wannberg B. , Karlsson L. , Baltzer P. , Kaddouri A. , Gelius U.
Published: July 2002

Six features observed in the energy distribution of H− ions arising from the single collision double charge transfer of 3–5 keV protons on nitrogen dioxide are interpreted as vertical double ionization energies of NO2 of 35.1, 38.1, 39.5, 41.0, 43.2, and 46.8 eV to form doublet states. Theoretical energies of the first 13 doublet and quartet electronic states of NO2+2 have been calculated by the ab initio CIPSI method at a fixed N–O distance and for several O–N–O bending angles. Thresholds and kinetic energy releases observed in dissociative double photoionization of NO2 are interpreted by comparison with the calculations and charge transfer results. The N- and O-Auger spectra of NO2 have been recorded and some of the 40 features observed have been assigned, confirming and improving the spectrum of NO2+2.
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o study of electronic energy transfer in the quenching of N2(A 3Σ+u) and N2(B 3Πg) by H2
Authors: Sperlein Robert F. , Golde Michael F.
Published: July 2002

The interactions of N2(A 3Σ+u) and N2(B 3Πg) with H2(X 1Σ+g) have been characterized through potential energy calculations, which were performed using a combination of MCSCF and multiple-reference SDCI methods. In C2v geometry with the N2 and H2 bond axes parallel, at the highest level of theory used, energy transfer from N2(A 3Σ+u) to H2(b 3Σ+u) and dissociation of the latter into H atoms is found to proceed through an adiabatic reaction path with a barrier of 0.513 eV. In C2v geometry with perpendicular orientation of N2 and H2, the two lowest 3B2 surfaces are shown to exhibit a strongly avoided crossing; the lower surface shows a favorable pathway for energy transfer from N2(B 3Πg) to H2(b 3Σ+u). In each case, energy transfer occurs via a two-electron exchange mechanism as a result of mixing between orbitals with the proper energy and symmetry. Consistent with the isoconfigurational electronic structure of N2(B) and CO(a 3Π), the results for N2(B) are similar to those which we found previously for quenching of CO(a) by H2. The overall results are shown to be consistent with available experimental kinetics data, which show quenching of N2(A) by H2 to be inefficient. Application of these results to electronic quenching and vibrational relaxation of N2(A) by other small molecules is discussed.
On the ground state of the hydrogen molecule–ion H+2 enclosed in hard and soft spherical boxes
Authors: Gorecki J. , Byers Brown W.
Published: July 2002

Iterative boundary perturbation theory is applied to the hydrogen molecule–ion H+2 enclosed in spheroidal boxes with finite and infinite walls. The mechanical pressure dependence of ground state properties such as the equilibrium bond length, the change of electronic kinetic energy, quadrupole moment, and the vibrational force constant are presented and discussed. For the hard box, our results are compared with those calculated by the variational method and it is clear that in this case the boundary perturbation theory gives a better description of ground state properties. The behavior of the new more realistic model of H+2 at high pressures based on the concept of finite potential wall (soft or padded box) is discussed.
On the singlet–triplet splitting in SiH2, GeH2, and SnH2. Local-spin-density calculations
Authors: Selmani A. , Salahub D. R.
Published: July 2002

We have calculated the 1A1–3B1 splitting in SiH2, GeH2, and SnH2 using the LCGTO-LSD-VWN method. A model potential was used for Si, Ge, and Sn to describe the core electrons. The results are in good agreement with the available experimental data and compare favorably with the best CI calculations on these systems.
Rotational tunneling of the NH+4 ion in phase II of (NH4)2 PtBr6 by inelastic neutron scattering spectroscopy
Authors: Kearley Gordon J. , Blank Herma
Published: July 2002

The observed inelastic neutron scattering (INS) spectrum of rotational tunneling of the NH+4 ion in (NH4)2PtBr6 is shown to be in good agreement with the calculated INS spectrum for this ion at a site of C3 rotational symmetry. Previous studies of (NH4)2PtBr6 have suggested different site symmetries for the ammonium ion in this crystal.
Adsorption hysteresis in narrow pores
Authors: Heffelfinger Grant S. , van Swol Frank , Gubbins Keith E.
Published: July 2002

We present molecular dynamics computer simulation results for adsorption hysteresis in a cylindrical pore. The adsorption is examined by inducing pore filling and emptying by changing the pore length at constant number of particles. By calculating the full density profile of the fluid in the pore, one can observe both processes. Hysteresis, overshooting the thermodynamic transition, is observed for both pore filling and emptying. That is, we observe metastable one-phase states on either side of the transition. The end of the gas branch (pore filling) is signaled by the growth of an unduloid, resulting in the formation of a biconcave lens of liquid, while the end of the liquid branch (pore emptying) occurs by the formation of a bubble. However, the latter appears to be an artifact of the lack of pore ends in the model. In the presence of pore ends, the bubble formation is expected to be preempted by the receding of the meniscus.
The relationship between recombination, chemical activation and unimolecular dissociation rate coefficients
Authors: Smith Sean C. , McEwan Murray J. , Gilbert Robert G.
Published: July 2002

A new solution to the master equation relating the rate coefficients for unimolecular, recombination (association) and chemical activation reactions, incorporating weak collision effects, is presented. The solution establishes conditions for the validity of the commonly used procedure of relating the recombination rate coefficient, throughout the falloff regime, to the reverse single-channel unimolecular rate coefficient via the equilibrium constant. In addition, a relationship between the rate coefficient for stabilization in a chemical activation reaction and the reverse multichannel unimolecular dissociation rate coefficient is derived. This result, in conjunction with recently developed methods for fully incorporating angular momentum conservation into the solution of the master equation for unimolecular dissociation, enables both angular momentum and weak collision effects to be accurately incorporated into the solution of the master equation for chemical activation reactions in the falloff regime. Application of this method to a typical ion/molecule chemical activation reaction, that of CH+3 with NH3, illustrates the importance of weak collision and angular momentum effects in this system.
Experimental study of the dynamics of D+H2 reactive and inelastic collisions below 1.0 eV relative energy
Authors: Phillips David L. , Levene Harold B. , Valentini James J.
Published: July 2002

We report the results of state-to-state dynamics experiments on the D+H2→HD+H reaction as well as D+H2→H°2+D energy transfer at relative energies of 0.67 and 0.79 eV. Both product state distributions and absolute partial cross sections have been determined, from coherent anti-Stokes Raman scattering (CARS) spectra of the HD and H°2 products recorded under single-collision conditions following pulsed-laser photolysis of DI to generate the D atom reactant. At both energies and for both reactive and inelastic collisions there is a strong dynamical bias against rotational and vibrational excitation of the product. However, at 0.67 eV there is an enhancement of both the relative and absolute yield of HD (v′=1), and to a lesser extent H2(v′=1), the only energetically accessible vibrationally excited product states. This may be the result of a Feshbach resonance at ≈0.65 eV, just above the v′=1 threshold energy. Product quantum state distributions from quasiclassical trajectory calculations are in fairly good agreement with the experimental results, except that they do not show the v′=1 enhancement at 0.67 eV. However, the partial cross sections from the trajectory calculations are systematically larger than those measured.
An investigation of the trimethylammonium chloride molecule in the vapor phase by pulsed-nozzle, Fourier-transform microwave spectroscopy
Authors: Legon A. C. , Rego C. A.
Published: July 2002

The ground-state rotational spectra of the symmetric-top isotopomers [(CH3)314N, H35Cl], [(CH3)314N, D35Cl], [(CH3)315N, H35Cl], and [(CH3)315N, H37Cl] of a dimer formed between trimethylamine and hydrogen chloride have been observed in the vapor above heated samples of solid trimethylammonium chloride by pulsed-nozzle, Fourier-transform, microwave spectroscopy. Spectroscopic constants have been determined in each case and for [(CH3)314N, H35Cl] the values are B0=1800.4605(2) MHz, DJ=0.320(10) kHz, DJK=13.59(11) kHz, χ(35Cl)=−21.625(5) MHz, and χ(14N)=−3.504(5) MHz. The observed rotational constants B0 indicate that the nuclei N, H, and Cl lie along the C3 symmetry axis of the molecule in the order N⋅⋅⋅H⋅⋅⋅Cl with the distance r(N⋅⋅⋅Cl)=2.8164(3) Å. A comparison of the values of the nuclear quadrupole coupling constants χ(35Cl) and χ(14N) with those in related dimers and molecules leads to the evocation of some ionic character (CH3)3N +H C̄l arising from a significant extent of proton transfer from Cl to N. This conclusion is reinforced when the values of χ(35Cl) and χ(14N) predicted on the basis of a recent electrostatic hydrogen-bonded model (CH3)3N⋅⋅⋅HCl of the dimer are considered. The intermolecular stretching force constants kσ=84(3) N m−1 determined from DJ has a magnitude closer to that expected in the ion-pair limit than in the hydrogen-bond limit.
Viscosity expansions in reactive diffusion processes
Authors: Agmon Noam
Published: July 2002

Properties of chemical reactions in systems undergoing diffusional motion depend on the ratio of chemical to diffusional rates. The present work deals with perturbation expansions in this quantity. For bounded diffusion, the Laplace transformed survival probability, mean lifetime, eigenvalues, and eigenfunctions are expanded in this ratio. The theory is developed mainly in the fast diffusion limit. In this limit, the survival probability for an initial equilibrium state is shown to be exponential up to linear order. For unbounded diffusion, expansions are derived for the steady-state concentration profile and rate coefficient. By inverting the series one obtains Padé-like approximations for rate coefficients with much improved convergence. Several examples are worked out in detail. These include the ‘‘radiation’’ boundary condition, barrierless isomerization, steady-state binding, and Förster quenching.
(e,2e) momentum spectroscopic study of the C=C π orbitals of the vinyl halides
Authors: Gorunganthu R. R. , Coplan M. A. , Leung K. T. , Tossell J. A. , Moore J. H.
Published: July 2002

The distribution of electron momentum density has been measured for the outermost occupied orbitals of the vinyl halides and ethylene using the (e,2e) technique. In contrast to the ionization potentials for these π orbitals which decrease monotonically from vinyl fluoride to vinyl iodide, the values of the momenta where the distributions are a maximum, pmax, increase from the fluoride through the bromide and then shift back to a lower value for the iodide. This observation can be analyzed in terms of B(r), the Fourier transform of the observed momentum distribution, and ΔB(r), the difference between B(r) functions. The shape of ΔB(r) for the fluoride, chloride, and bromide in comparison to ethylene reflects the effect of the carbon–halogen antibonding interaction in these vinyl halides. On the other hand, in vinyl iodide the antibonding interaction is compensated for by the diffuse iodine 5p character of the molecular orbital. The relation of these observations to chemical properties of the vinyl halides is discussed along with differences between experiment and calculations at low momentum.
On the structure of solid Ca(ND3)x; with x≈6
Authors: Press W. , Damay P. , Leclercq F. , Chieux P.
Published: July 2002

The bcc structure of Ca(ND3)5.7 is found to be stable down to He temperatures. The crystal is characterized by frozen-in orientational disorder and also positional disorder of the ND3 molecules. The latter may be explained by a distribution of octahedrally coordinated and tetrahedrally coordinated Ca ions. The relation of occupation numbers with stoichiometry cannot be made quantitatively, however. There are no indications of a significant departure from the geometry of a free ammonia molecule in the solid.
A molecular-dynamics simulation study of the β phase of nitrogen
Authors: Powell B. M. , Pawley G. S.
Published: July 2002

The structural characteristics of the β phase of solid nitrogen have been studied for large samples by the molecular-dynamics simulation technique. An effective interatomic potential appropriate to this phase was developed including nonbonded and quadrupole terms. Several properties of the crystal structure have been studied as functions of both temperature and pressure. In particular, a detailed investigation of the molecular orientational order has been made as a function of these parameters. At high temperatures the simulation sample is found to have a hexagonal structure with randomly oriented molecules. As the temperature decreases an orientational ordering transition is found to occur at ≊35 K. The effects of increasing pressure are found to be analogous to those of reducing the temperature. The nature of the local transition in the sample is investigated by calculating correlation parameters which specify twinning and tetrahedral ordering. The variation of the latter indicates that tetrahedral ordering increases strongly with decreasing temperature or increasing pressure, suggesting that the low-temperature phase is a condensation of this arrangement.
Formation of thermodynamically stable dications in the gas phase by thermal ion–molecule reactions: Nb2+ with small alkanes
Authors: Gord James R. , Freiser Ben S. , Buckner Steven W.
Published: July 2002

The gas-phase reactions of Nb2+ with small alkanes at thermal energies are reported. For methane and ethane, dehydrogenation is a prominent reaction pathway. For propane and butane, charge transfer is virtually the only reaction pathway observed (>99%). NbCH2+2 and NbC2H2+2 formed in the reactions of Nb2+ with methane and ethane are thermodynamically stable with D(Nb2+–CH2)=197±10 kcal/mol, D(Nb+–CH+2)=107±10 kcal/mol, D(Nb2+–C2H2)≥74 kcal/mol, and D(Nb+–C2H+2)≥7 kcal/mol. The stability of these ions is most likely due to the charge-stabilizing effect of the metal center. Collision-induced dissociation of these ions results in charge-splitting reactions as well as reactions in which both charges remain on the metal center. Hydride transfer is observed to be competitive in the primary reactions of Nb2+ with alkanes. The hydride- and charge-transfer results are in qualitative agreement with a simple curve-crossing model.
Nonadditive effects in HF and HCl trimers
Authors: Chal/asiński G. , Cybulski S. M. , Szcze̢śniak M. M. , Scheiner S.
Published: July 2002

Nonadditive effects are calculated for (HF)3 and (HCl)3 complexes and analyzed via the combination of perturbation theory of intermolecular forces with Mo/ller–Plesset perturbation theory (MPPT). In both systems the nonadditivity is dominated by the self-consistent field (SCF) deformation effect, i.e., mutual polarization of the monomer wavefunctions. Heitler–London exchange and correlation effects are of secondary importance. Three-body terms exhibit much lesser basis set dependence than the two-body effects and even quite moderate basis sets which are not accurate enough for treatment of two-body forces can yield three-body effects of quantitative quality. This is due in large measure to the additivity of strongly basis set dependent components such as uncorrelated and correlated electrostatics and dispersion. Various approximate models for the three-body potentials and total interaction in the (HF)3 cluster are analyzed from the point of view of their ability to predict the orientation dependence of interaction energy.
Vibrational spectra of Mg(OH)2 and Ca(OH)2 under pressure
Authors: Kruger M. B. , Williams Q. , Jeanloz R.
Published: July 2002

The effects of compression on the structure and bonding of Mg(OH)2 brucite and Ca(OH)2 portlandite are documented to pressures of 34 and 24 GPa, respectively, by way of infrared spectroscopy of the O–H stretching vibration at 300 K. The frequency of the infrared-active (A2u) mode of both hydroxides decreases with pressure, by −0.6 cm−1/GPa for Mg(OH)2 over the pressure range studied, and by −3.5 cm−1/GPa for Ca(OH)2 to 10.6 GPa. An additional mode observed in Mg(OH)2 is due to an optically induced hot band (A2u X A2u overtone): with increasing pressure, its intensity grows and its frequency decreases (−3.7 cm−1/GPa). The negative pressure dependencies of these modes (negative Grüneisen parameters) are explained in terms of compression increasing the hydrogen bond strength within the hydroxide structure. On increasing pressure above 12.8 GPa, the full width at half-maximum of the O–H vibration in Ca(OH)2 rapidly broadens by a factor of 4. The infrared data suggest that Ca(OH)2 undergoes pressure-induced amorphization at 11.7(±1.1) GPa at 300 K, consistent with independently collected x-ray diffraction patterns. In the amorphous phase, a distribution of O–O bond lengths ranging between 2.8 and 3.3 Å is inferred from the breadth of the vibrational band. On decompression, the infrared spectra show a full return to the crystalline form at 2.8 (±1.4) GPa. Neither upon amorphization with increasing pressure nor upon recrystallization with decreasing pressure are there any prescursory phenomena evident in the spectra. In contrast with Ca(OH)2, the isomorphous Mg(OH)2 appears to be stable throughout the pressure range of this study.
Polar molecules in planar interfaces. Role of the triplet direct correlation function in the asymptotic behavior of the profile
Authors: Zhang Q. , Badiali J. P. , Su W. H.
Published: July 2002

We investigate the density-orientation profile of a polar liquid in planar interfaces. This liquid, which represents one side of the interface, is coupled with a second medium which can be an ideal solid characterized only by its dielectric constant or another polar liquid. We derive an expression for the asymptotic behavior of the profile which is exact provided the molecules interact by an additive pairwise potential. As a consequence of the renormalization of bonds, we show that the result does not depend explicitly on the particular form of the short range part of the pair potential. Moreover, the asymptotic behavior of the profile does not depend on the physical origin of the second medium provided it behaves as a dielectric. In addition, we show that the result remains exact in presence of a soft wall and can be extended to the liquid–vapor interface. As established in a previous paper, we show that the isotropic part of the profile is intimately connected with the electrostriction phenomena observed in bulk phase when a strong electrical field is applied. In order to clarify this point, we derive the expression of the electrostriction constant from the relation density-one body direct correlation function which has been used for studying the profile. We point out that the electrostriction and the long tail of the profile represent two ways for investigating some orientational components of the triplet direct correlation function in bulk phase. More generally, we show that the triplet direct correlation function largely determines the orientational structure of the profile.
A general solution of the molecular Ornstein–Zernike equation for spheres with anisotropic adhesion and electric multipoles
Authors: Blum L. , Cummings P. T. , Bratko D.
Published: July 2002

We present the analytic solution of the molecular Ornstein–Zernike equation for a very general closure in which the direct correlation function is of the form suggested by the mean-spherical approximation for arbitrary multipolar interactions and the total correlation function contains terms that arise in the Percus–Yevick approximation for spheres with anisotropic surface adhesion. In addition to generalizing several earlier analyses of special cases of this closure, the solution presented here contains new simplifying insights that reduce the complexity of the resulting algebraic equations. A special case of the analysis is described to illustrate the method of solution.
Polymer deformation in strong high-frequency flows
Authors: O’Shaughnessy Ben , Durning Chris , Tabor M.
Published: July 2002

The conformation of a polymer chain subjected to periodic straining fields of arbitrary amplitude Ω and modulation frequency ω is studied in the Rouse model of polymer dynamics in the high-frequency limit ωτR≫1 where τR is the Rouse relaxation time. We specialize to the case of sinusoidal time dependence, but our results are expected to be general. We calculate the dimensionless mean square extension μ of a polymer segment containing s monomers, defined as the ratio of the mean square size to the equilibrium value. For simple shear we find μ=3+λ2f1(φ) for large segments, ωτs≫1, where τs is the segment relaxation time, λ≡Ω/ω, and f1 is a nonuniversal function of the phase, φ≡ωt, of the straining field. For small segments, ωτs≪1, we find μ=3+λ2(ωτs)1/2 f2(φ) with nonuniversal f2. In extensional flow the extension along the stretching axis is derived: μ=f3(φ, λ) for ωτs≫1 and μ=1+(ωτs)1/2 f4(φ, λ) for ωτs≪1 (again f3 and f4 are nonuniversal). These results are interpreted in terms of blobs of relaxation time ∼ω−1: the chain of blobs deforms affinely in the flow, but within a blob the polymer has time to relax. In the nonlinear régime (λ≳1) the blobs are strongly distorted and the polymer within a blob relaxes to an elongation well beyond its equilibrium size such that its dimensions vary linearly with number of monomers. In the case of elongational flow, the fluctuations in the velocity field entirely suppress the ‘‘yo–yo’’ instability that has been conjectured to play an important role in the phenomenon of drag reduction.
Static and dynamic properties of the Widom–Rowlinson model mixture. I
Authors: Borgelt P. , Hoheisel C. , Stell G.
Published: July 2002

The simple and interesting Widom–Rowlinson (WR) mixture model is investigated by molecular-dynamics calculations in comparison with equilibrium and kinetic theories. In the present paper only static properties of the WR model are considered. Particularly, the phase separation of the mixture is studied. The exact molecular-dynamics results are found to be in reasonable agreement with predictions of Percus–Yevick and mean-field approximations.
Glass formation in continuous cooling processes: A molecular dynamic study of a monatomic metal system
Authors: Kondo Toshiharu , Tsumuraya K. , Watanabe M. S.
Published: July 2002

We simulate the continuous cooling processes of a single-component metallic model liquid with the molecular dynamics method in order to reexamine the claim [K. Shinjo, J. Chem. Phys. 90, 6627 (1989)] that the liquid–glass transition cannot be simulated with the method, i.e., the volume versus temperature relationship does not show a sharp or broad break but shows a meniscus curve in the continuous cooling processes. A sharp break means that the volume decreases linearly on both sides of the glass transition temperature. A broad break means that the volume decreases continuously in the intermediate temperature region although it changes linearly at both the sides. A meniscus curve means that the volume decreases continuously taking the shape of a bow. We analyze the structure of the continuously cooled state with the pair-distribution function and Voronoi polyhedron analysis. We find that the glass state can be formed even in the continuously cooled processes and even when the potential energy versus temperature curve shows a meniscus shape. We also find that a sharp break appears in the potential versus temperature curve in the continuous cooling process when the liquid is cooled slowly enough. The number of icosahedral clusters increases from just below the melting temperature and appears to saturate at a temperature below the break that corresponds to the conventional glass transition. The number of such clusters
Corrected effective medium method. V. Simplifications for molecular dynamics and Monte Carlo simulations
Authors: Stave Mark S. , Sanders David E. , Raeker Todd J. , DePristo Andrew E.
Published: July 2002

We present the conceptual and formal simplifications of the recently developed corrected effective medium (CEM) theory that enable this theory to be used directly in molecular dynamics (MD) and Monte Carlo (MC) simulations of large systems, hence the acronym MD/MC-CEM. The essential idea involves adjustment of the CEM embedding functions to include approximately the original explicit correction for kinetic-exchange-correlation energy differences between the real system and the many atom–jellium systems used as the zeroth order model. Examples of this construction are provided for the Ni, Pd, Ar, and H/Pd(111) systems. Finally, a few brief applications of this method to large systems are provided. These include relaxation of metal surfaces, structure of pure Ni and mixed NiCu clusters, sticking of Cu on Cu(100), and the scattering of Ar from H covered Pd(111).
The emission spectrum of helium hydride. III. Bands near 5200, 5300, and 6000 Å
Authors: Ketterle Wolfgang
Published: July 2002

The emission bands of helium hydride near 5200, 5300, and 6000 Å were analyzed for 4HeH and 3HeD. They are assigned to the emission of nine (HeD) or eight (HeH) coupled electronic states to the A 2Σ+ (5200 and 5300 Å) and B 2Π (6000 Å) states. Because of the high rotational temperature of 2500 – 3500 K and several perturbations, a very complex rotational structure was observed. The strongest band near 6000 Å is emitted by the five components of the 3d states, which show strong uncoupling of the 3d electron from the symmetry axis. The D 2Σ+ and 3pE 2Π states interact by Λ-type doubling interaction and with the 3d states since interactions with Δl=±1 are allowed in a strongly polar molecule as HeH. The line intensities are affected by interference effects due to the interactions of the electronic states, by different lifetimes of the upper states and by predissociation, which is only strong for the A′ parity component of 4HeH.
Tracer diffusion of interacting particles on incomplete lattices: Effective medium approximation
Authors: Silverberg Marvin , Ratner Mark A. , Granek Rony , Nitzan Abraham
Published: July 2002

Dynamic percolation theory is adapted to obtain diffusion coefficients for particles with blocking interactions on incomplete lattices, within an effective medium approximation (EMA). The substrate lattices have static bond disorder. The motion of a tracer particle among identical background particles is regarded as particle motion in a fluctuating random environment superimposed on the statically disordered lattice; the fluctuations results from the motion of the background particles. Several schemes for incorporating the effect of the background particles are discussed, all relating their motion in different ways to the macroscopic diffusion. Comparisons with Monte Carlo simulations are performed for two-dimensional simple square and three-dimensional simple cubic lattices. In the range where single bond EMA is thought to be reliable, good agreement with the simulation is achieved.
The viscosity dependence and reaction coordinate for isomerization of cis-stilbene
Authors: Abrash S. , Repinec S. , Hochstrasser R. M.
Published: July 2002

Femtosecond spectroscopic methods are used to study the dynamics following optical excitation of cis-stilbene molecules in hydrocarbon solvents. Transient absorption spectra of cis-stilbene over the range 320–1100 nm are reported. The anisotropies of these transients permit the assignment of the various excited electronic states in this region to A type in C2 symmetry. The excited state absorptions disappear at rates that are weakly dependent on solvent friction and comparisons with simple statistical mechanical theories and various potentials indicate that there is likely to be a barrier crossing process responsible for the observed decay times being in the range 0.7–1.4 ps. For observation times longer than ca. 100 fs an exponential decay of the cis population is observed and the transient spectrum does not appear to change in shape. A Kramers model fits the frictional dependence of the decay rates. Direct observation of what appears to be trans isomer ground states at 335 nm is reported following excitation of cis at 312 nm. The appearence time of this species is indistinguishable from the cis-disappearence time and any intermediate in the process cis→product (trans) is found to have a lifetime of less than 150 fs. This result implies that hot trans-stilbene molecules are produced in the isomerization. Previous studies probing at 312 nm may have detected cooling of these hot molecules. By means of polarized light excitation of cis and detection of the photoproduct it is found that the reactant and product of the isomerization are aligned as if the reaction coordinate were a twisting about the double bond by 180° while maintaining the orientation of the twofold axis in the laboratory frame. This high alignment, in view of obvious possible depolarizing influences in the solution, suggests a more intricate reaction coordinate involving the motion of the ethylene carbons.
Laser spectroscopy of crossed molecular beams: The dissociation energy of BaI from energy-balance measurements
Authors: Vaccaro P. H. , Zhao D. , Tsekouras A. A. , Leach C. A. , Ernst W. E. , Zare R. N.
Published: July 2002

Through application of energy-balance arguments to the crossed-beam reaction Ba(1S0)+HI(X 1Σ+) →BaI(X 2Σ+) +H(2S1/2), a lower limit for the BaI bond dissociation energy is determined to be D00(BaI) ≳76.8±1.7 kcal/mol (3.33±0.07 eV). Based on the upper bound of D00(BaI) ≲78.5±0.5 kcal/mol, as determined from earlier predissociation studies [M. A. Johnson, J. Allison, and R. N. Zare, J. Chem. Phys. 85, 5723 (1986)], we recommend a BaI bond strength of 77.7±2.0 kcal/mol (3.37±0.09 eV). This dissociation energy is more than 5 kcal/mol higher than the previously accepted value of D00(BaI) as derived from mass spectrometric measurements.
Interaction universality and scaling laws for interaction potentials between closed-shell atoms and ions
Authors: Koutselos A. D. , Mason E. A. , Viehland L. A.
Published: July 2002

A universal scaling scheme is developed for closed-shell interactions. The exchange energies (total energies minus the Coulombic energies) are found to scale with two parameters to universal interaction curves for noble gas–noble gas, alkali ion–noble gas, and halogen ion–noble gas interactions. The interaction potentials constructed from the universal interaction curves agree well with experimentally determined potentials, and also successfully reproduce measured ion mobilities and diffusion coefficients. The universal interactions can be viewed not just as a correlation scheme, but also as operating to extend the range of the potentials for a number of ion–atom systems to both larger and smaller distances than are presently probed by direct measurements. They also provide the basis for predictions of potentials for systems lacking experimental measurements. In the case of the noble gases, they reduce by two the number of parameters required for the formulation of an accurate extended principle of corresponding states.
Selected dissociation- and correlation-consistent configuration interaction by a perturbative criterion
Authors: Merchán M. , González-Luque R. , Nebot-Gil I.
Published: July 2002

We propose a perturbative criterion to select the most important dissociation- or correlation-consistent type of contributions to perform generalized valence bond-configuration interaction (GVB-CI) calculations, dissociation-consistent configuration interaction (DCCI) or correlation-consistent configuration interaction (CCCI) approach, respectively. The procedure presented is computationally less demanding than the CCCI proposed by Goddard and co-workers. To ensure the distance consistency of the MOs used, the nonvalence virtual orbitals are obtained by a projection technique. The results obtained for a few test calculations show the ability of the suggested approach to get close results to full CI, DCCI, and CCCI values using a small CI expansion. It seems to be a promising way to treat correlation changes in large molecular systems which would be inaccessible by other methods.
Computer simulation of fluid–fluid phase coexistence in mixtures of nonadditive soft disks
Authors: Mountain Raymond D. , Harvey Allan H.
Published: July 2002

Fluid–fluid phase equilibrium in binary mixtures of positively nonadditive soft disks is studied using both molecular dynamics and Gibbs ensemble Monte Carlo simulations. The molecular dynamics simulations are unable to provide quantitative results for the phase transition, but they do indicate that some sort of ordering takes place as the temperature is lowered. In contrast, the Gibbs ensemble simulations demonstrate unambiguously the presence of a first-order transition and give quantitative results for the coexistence curve; however, convergence is very slow for mixtures that are not symmetric. Comparison with the Gibbs ensemble results indicates that a simple first-order perturbation theory provides good results far from the consolute point. Closer to the consolute point, the theory does not reproduce the flatness of the coexistence curve; this limitation is intrinsic in any mean-field theory. Comments are made regarding the relative merits of molecular dynamics and Gibbs ensemble simulations for studying fluid–fluid phase coexistence.
Time-dependent Hartree–Fock second-order molecular properties with a moderately sized basis set. II. Dispersion coefficients
Authors: Spackman Mark A.
Published: July 2002

Time-dependent coupled Hartree–Fock frequency-dependent polarizabilities are used to obtain ab initio dipole–dipole C6, dipole–quadrupole C8, and triple–dipole ν, dispersion coefficients. The moderately sized 6-31G (+sd+sp) basis set optimized for calculation of molecular static dipole polarizabilities has enabled the computation of dispersion coefficients for a wide variety of molecules containing atoms up to chlorine. Results are presented for 48 molecules including benzene, cyclohexane, SF6, and CCl4. Appropriate scaling of 6-31G(+sd+sp) results for C6 and ν enables the prediction of these coefficients to within 3% of experimental estimates that are based on dipole oscillator strength distributions. Results for the mean static quadrupole polarizability C̄(0) and C8 dispersion coefficient are presented for nondipolar molecules and the origin dependence of C8 is investigated.
Theory of vibrationally mediated photodissociation of HOOH: Delocalized tails in a localized wave function
Authors: Colbert Daniel T. , Sibert Edwin L.
Published: July 2002

The vibrationally mediated photodissociation (VMP) of hydrogen peroxide [T. M. Ticich et al., J. Chem. Phys. 87, 5820 (1987)] is modeled. The two-photon VMP process proceeds via a highly vibrationally excited state on the ground electronic surface, and affords a unique view of the dynamics in the high energy region of the potential energy surface. We calculate, in a local mode basis set, the third OH stretch overtone wave function using the ground potential energy surface of Harding [L. B. Harding, J. Phys. Chem. 93, 8004 (1989)]. Although the overtone wave function is highly localized in the excitation mode, the delocalized tails which extend into the wide amplitude OO stretch region are crucial to the VMP mechanism. Interferences between zero-order states comprising these tails are shown to lead to a single state’s being excited, the latter suggested by the VMP spectrum. The low frequency torsional mode is seen to play an important role in the makeup of the wave function.
Verification of the linear, carbon bonded structure for HCCH–CO from vibration–rotation spectra of the 12CO and 13CO containing species
Authors: Marshall Mark D. , Kim James , Hu T. A. , Sun Ling Hong , Muenter J. S.
Published: July 2002

The 3μ vibration–rotation spectrum of the HCCH–13CO van der Waals molecule is obtained and analyzed. Comparison with the results from an earlier study of the 12CO containing species provides experimental verification of the linear, carbon bonded structure proposed in the original work. Perturbations of the upper vibrational state in the 13CO isotopomer similar to those in the 12CO containing species are observed suggesting that a common state is responsible.
Molecular reorientation dynamics of polar dye probes in tertiary-butyl alcohol–water mixtures
Authors: Dutt G. B. , Doraiswamy S. , Periasamy N.
Published: July 2002

The rotational motion of the four dye probe molecules—nile blue and thionine (monocations), resorufin (monoanion), and nile red (polar but neutral) have been investigated in aqueous mixtures of tertiary-butyl alcohol using picosecond fluorescence depolarization spectroscopy. The simple hydrodynamic theory due to Stokes–Einstein–Debye is unable to describe the entire profile of the plot of rotational reorientation time vs viscosity, but is adequate in the water rich region (low viscosity). If the dielectric friction theory proposed for pure solvents is considered, then it is possible to mimic the bivalued profile seen in the alcohol rich region (high viscosity). The inability to produce a satisfactory fit over the entire composition range possibly demands a suitable dielectric friction theory for binary mixtures.
A test of continuum models for dielectric friction. Rotational diffusion of phenoxazine dyes in dimethylsulfoxide
Authors: Alavi D. S. , Hartman R. S. , Waldeck D. H.
Published: July 2002

Rotational diffusion times for three mechanically similar phenoxazine dyes with differing electronic properties were measured. These studies probe the importance of dipole/dipole and ion/dipole couplings to the friction. The experimental results are compared with the predictions of available continuum theories of dielectric friction. Future directions for a more realistic model of dielectric friction are discussed.
The C 3Π–X 3Π band system of the SiC radical
Authors: Butenhoff Thomas J. , Rohlfing Eric A.
Published: July 2002

We report a study of the C 3Π–X 3Π band system of the SiC radical. Jet-cooled SiC is produced by a laser vaporization/pulsed beam source and is probed by pulsed laser-induced fluorescence excitation spectroscopy. Vibrational term energies and rotational constants are determined for v′=0–8 of the C state and for v″=0–5 of the X state. The spectroscopic constants for the C state are Te=22 830.4(9) cm−1, ωe=615.7(8) cm−1, and re=1.919(3) Å; for the X state, ωe=965.16(24) cm−1 and re=1.7182(2) Å.
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o calculations of the fundamental OH frequency of bound OH− ions
Authors: Hermansson Kersti
Published: July 2002

In contrast to the OH stretching frequencies of bound H2O molecules, which are always found at lower wave numbers compared to the free molecule, the experimentally determined frequency of the OH− ion can be either lower or higher than the free-ion value. Optimized geometries and fundamental stretching frequency of OH− have been calculated here by ab initio methods at the Hartree–Fock and second-order Mo/ller–Plesset levels for a number of cation–OH−, HOH⋅⋅⋅OH−, cation–OH−⋅q−, and cation–OH−⋅OH2 complexes for Li+, Mg2+, and Al3+. The importance of electrostatic effects on the OH− frequency has been assessed by comparison with calculations of different point-charge and homogenous-field OH− systems. As long as the interaction is not dominated by electronic overlap, the frequency shift is found to be largely determined by electrostatic forces: with increasing field strength the OH− frequency rises to a maximum and then decreases. The OH− dipole moment and Mulliken charges vary monotonically with the field strength, whereas the equilibrium OH distance goes through a minimum and the bond electron density through a maximum. In strongly polarizing fields, such as in the optimized Al3+⋅OH− and Mg2+⋅OH−⋅⋅⋅OH2 systems, the OH− frequency falls below the free-ion value. Ar experimentally observed frequency downshift for an OH− ion in the condensed phase cannot be used as a criterion for the existence of H bonding. The OH− ion acts as an H-bond donor only when strongly polarized by a neighbor on its oxygen side.
Low-frequency Raman scattering and structure of amorphous polymers: Stretching effect
Authors: Achibat T. , Boukenter A. , Duval E. , Lorentz G. , Etienne S.
Published: July 2002

Low-frequency Raman scattering of amorphous polymers [polyethyleneterephthalate (PET) and polymethylmethacrylate] is investigated. The low-frequency Raman band called ‘‘Boson peak’’ is interpreted in terms of a noncontinuous structure, similar to that of inorganic glasses, i.e., solid amorphous polymers are composed of 50 Å blobs. These are related blobs to transient entanglements in the polymer melt. Modifications induced by stretching are described for a PET film. The nature of the blob structure is discussed.
Square-well chains: Bulk equation of state using perturbation theory and Monte Carlo simulations of the bulk pressure and of the density profiles near walls
Authors: Yethiraj Arun , Hall Carol K.
Published: July 2002

Equations of state are obtained for square-well chains via perturbation theory about the hard chain system. The molecules are modeled as a pearl necklace of freely jointed spheres that interact via a site–site square-well intermolecular potential. The local structure of the reference fluid (required in perturbation theory) is obtained from polymer reference interaction site model (polymer–RISM) theory. The theory is compared to Monte Carlo simulation data for the pressure of square-well chains obtained from simulations of the fluid between walls. At high temperatures the density profiles are characterized by the packing/entropic effects observed in hard chains; as the temperature is lowered the attractions cause a severe depletion of sites from the region near the walls. The agreement of the theory with the simulation data for the compressibility factor is good for square-well 4-mers and 8-mers but not as good for square-well 16-mers.
Two-vector correlations and microscopic branching in chemical dynamics: Alignment and orientation effects for the Mg+HF→MgF+H reaction
Authors: Alvariño José María , Laganà Antonio
Published: July 2002

Correlations between vector parameters characterizing the Mg+HF→MgF+H reaction have been investigated by carrying out extended trajectory calculations. For this study, use has been made not only of the traditional trajectory output, but also of vector distributions and related Legendre moments. Particular attention has been paid to the correlation of the direction of the final rotational angular momentum to that of the reagents’ relative velocity. For reactive events, the final rotational angular momentum was found to be perpendicularly polarized. The degree of alignment is high for trajectories taking a direct path from the saddle to the product asymptote, while the alignment is partially disrupted for those sampling the intermediate HMgF well. These results are compared with predictions from the constrained product orbital angular momentum (CPOAM) model.