bluuebunny/crossref_metadata_2025 huggingface dataset:
Reduced dimensionality calculations of quantum reactive scattering for the H+CH4→H2+CH3 reaction
Authors: Takayanagi Toshiyuki
Published: July 2002

The dynamics for the H+CH4→H2+CH3 reaction has been studied using reduced dimensionality quantum-mechanical theory. The system is treated as a linear four-atom chemical reaction, reducing the system to a three-dimensional scattering problem. The vibrational modes of ν1 and ν4 of CH4, the stretching vibration of H2, and the umbrella ν2 mode of CH3 are taken into consideration in the reaction dynamics based on the vibrational analysis along the reaction path. The semiempirical potential energy surface which has recently been developed by Jordan and Gilbert [J. Chem. Phys. 102, 5669 (1995)] is employed. Rotationally averaged cross sections and thermal rate constants are calculated using an energy-shifting approximation in order to take into account the effect of all the degrees of freedom. It is shown that excitation of the ν1 mode of CH4 significantly enhances the reactivity, indicating that there is a strong coupling between the ν1 mode of CH4 and the reaction coordinate. The vibrational state distributions for the products H2 and CH3 have also been studied. In the energy range considered here, the population of vibrationally excited H2 is found to be very small, while the umbrella ν2 mode of CH3 is found to be excited.
Hydrogen transfer in vibrationally relaxing benzoic acid dimers: Time-dependent density matrix dynamics and infrared spectra
Authors: Scheurer Christoph , Saalfrank Peter
Published: July 2002

We employ time-dependent density matrix theory to characterize the concerted double-hydrogen transfer in benzoic acid dimers—the ‘‘system’’—embedded in their crystalline environment—the ‘‘bath.’’ The Liouville–von Neumann equation for the time evolution of the reduced nuclear density matrix is solved numerically, employing one- and two-dimensional models [R. Meyer and R. R. Ernst, J. Chem. Phys. 93, 5528 (1990)], the state representation for all operators and a matrix propagator based on Newton’s polynomials [M. Berman, R. Kosloff, and H. Tal-Ezer, J. Phys. A 25, 1283 (1992)]. Dissipative processes such as environment-induced vibrational energy and phase relaxation, are accounted for within the Lindblad dynamical semigroup approach. The calculation of temperature-dependent relaxation matrix elements is based on a microscopic, perturbative theory proposed earlier [R. Meyer and R. R. Ernst, J. Chem. Phys. 93, 5528 (1990)]. For the evaluation of the dissipative system dynamics, we compute (i) time-dependent state populations, (ii) energy and entropy flow between system and bath, (iii) expectation values for the hydrogen transfer coordinate, (iv) characteristic dephasing times and (v) temperature-dependent infrared spectra, determined with a recently proposed method by Neugebauer et al. Various ‘‘pure’’ and ‘‘thermal’’ nonequilibrium initial states are considered, and their equilibration with the bath followed in time.
A molecular dynamics analysis of resonance emission: Optical dephasing and inhomogeneous broadening of CH3I in CH4 and Ar
Authors: Fan R. , Kalbfleisch T. , Ziegler L. D.
Published: July 2002

The spontaneous resonance emission of CH3I in high pressures (800–1600 psi) of CH4 and Ar excited in the region of the Rydberg B-state origin (∼201 nm) are reported. These emission spectra consist of narrow Raman-like (RL) and broad fluorescence-like (FL) spectral features. The observed ratio of the Raman/fluorescence intensity in these high pressure solutions is a function of the excitation wavelength as the incident radiation is tuned through the pressure broadened electronic origin band. Molecular dynamics simulations are implemented for the analysis of the observed emission spectral shapes and their excitation frequency dependence. The four-time dipole correlation functions required for the calculation of this nonlinear polarization derived signal are successfully approximated by a product of two two-time dipole correlation functions for these chromophore-bath systems (factorization approximation). The complex emission band shapes and their excitation frequency dependence are captured by this approach. The dispersion in the RL/FL emission redistribution is due to the multiple time scales inherent to the decay of the resonant optical coherence of these pressure-broadened absorptions. The wavelength dependent pure-dephasing rate is determined by the nonlinear shape of the solute–solvent difference potential. The observational time scale dependence of the spectroscopic homogeneous and inhomogeneous line broadening labels is clearly demonstrated and contrasted here for absorption and Raman scattering.
Sequence fluctuation-dependent adsorption–depletion transition for random heteropolymers
Authors: Gutman Lorin , Chakraborty Arup K.
Published: July 2002

We develop a model for the adsorption of random heteropolymers onto solid surfaces from solutions that have a finite concentration of polymer. Previous studies that properly average over the quenched sequence distribution have been concerned with isolated chains near surfaces. Our self-consistent-field theory predicts a transition from situations where the surface segment density is enhanced compared with bulk solution concentration to one wherein the surface segment density is depleted. For a specific chemical identity of the random heteropolymer segments and the surface, this adsorption–depletion transition occurs above a threshold value of the strength of the sequence fluctuations. This intriguing finding can be tested directly via neutron scattering experiments (in the reflection mode), and offers opportunities for manipulating interfacial properties. The variation of the excess surface density of segments with polymer concentration in solution near the adsorption–depletion transition is also elucidated.
High pressure solid phases of benzene. III. Molecular packing analysis of the crystalline structures of C6H6
Authors: Thiéry M-M. , Rérat C.
Published: July 2002

With a computer simulated mechanical model for molecular packing analysis, the reconstitution and identification of the intermediate pressure-induced phase II of solid benzene C6H6 at 293 K, has been undertaken. The atom-atom intermolecular potential of the Buckingham type was generalized to account for short interatomic distances, especially under pressure. The model includes thermal motion and molecular deformation effects. Various crystal structures calculated in the pressure range of phase II and checked by their reticular distances and structure factors, are compared with the structure IIo proposed for this phase. Among them two possible monoclinic structures IIc and IIc′ have been evidenced by the calculation. Structure IIc has energy and enthalpy levels lower than that of phases Ic and IIIc, in the pressure range 0.5<P<1.0 GPa. This allows to suggest this stable monoclinic structure IIc for the real structure of the experimentally observed phase II. This structure IIc corresponds to the structure previously determined as metastable by Dzyabchenko and Bazilevskii [J. Struct. Chem. 26, 553 (1985)].
Free energy simulations: Correcting for electrostatic cutoffs by use of the Poisson equation
Authors: Resat Haluk , McCammon J. Andrew
Published: July 2002

The use of electrostatic cutoffs in calculations of free energy changes by molecular dynamics or Monte Carlo simulation is known to introduce errors, which can be quite large when the net charge of the system is changed. The Born equation has often been used to correct for such errors, but this and other analytical methods cannot be used for many systems with complicated structures. Here, we show that numerical methods for solving the Poisson equation, which have been extensively developed recently for studies of solvation thermodynamics, provide a more generally applicable alternative to the traditional Born-type corrections.
Quantum mechanical three-dimensional wavepacket study of the Li+HF→LiF+H reaction
Authors: Gög̃tas Fahrettin , Balint-Kurti Gabriel G. , Offer Alison R.
Published: July 2002

A three-dimensional time-dependent quantum mechanical wavepacket method is used to calculate the state-to-state reaction probabilities at zero total angular momentum for the Li + HF → LiF +H reaction. Reaction probabilities starting from several different initial HF vibrational–rotational states (v=0,j=0,1,2) and going to all possible open channels are computed over a wide range of energies. A single computation of the wavepacket dynamics yields reaction probabilities from a specific initial quantum state of the reactants to all possible final states over a wide range of energies. The energy dependence of the reaction probabilities shows a broad background structure on which resonances of varying widths are superimposed. Sharp resonance features seem to dominate particularly at low product translational energies. There are marked changes in the energy dependence of the reaction probabilities for different initial or final diatom rotational quantum numbers, but it is noticeable that, for both reactants and products, odd and even rotational quantum numbers give rise to similar features. Our results clearly identify some resonance features which are present in the reaction probability plots for all product and initial states, though they appear in the form of sharp peaks in some plots and sharp dips in others. We speculate that these features arise from reactive scattering resonances which serve to redistribute the flux preferentially to particular product quantum states. The present calculations extend to higher energies than previously published time-independent reactive scattering calculations for this system.
Auger decay of the dissociating core-excited states in the HCl and DCl molecules
Authors: Kukk E. , Aksela H. , Sairanen O.-P. , Aksela S. , Kivimäki A. , Nõmmiste E. , Ausmees A. , Kikas A. , Osborne S. J. , Svensson S.
Published: July 2002

High-resolution Auger electron spectra from the decay of the Cl 2p→σ* excitation in HCl and DCl have been measured. The spectra are analyzed, separating molecular and atomic features, which are assigned to transitions to the HCl (5σ2π)4σ* and Cl (3s3p)6 states, respectively. Auger line shapes, as affected by the molecular dissociation, are studied by comparing the experiment with the results of Monte-Carlo computer simulations based on a semiclassical model.
Calculation of Lennard-Jones energies of molecular fluids
Authors: Matyushov Dmitry V. , Schmid Roland
Published: July 2002

In view of the ever increasing awareness of the importance of dispersion forces to chemical solvent effects, reliable liquid Lennard-Jones (LJ) energies are eagerly required in order to assess the dispersion component of nonionic solvation. For this purpose two major methods of calculating LJ energies—one based on nonpolar gases solubilities and the other on the generalized van der Waals (GvdW) equation of state—are critically reexamined and updated by applying modern liquid state theories. The former method is improved over previous evaluations by including the cavity formation term according to the Boublik–Mansoori–Carnahan–Starling–Leland equation and by a molecular-based calculation of the solute solvation energy due to both dispersion and induction forces. For the second approach, the attraction parameter of the GvdW equation of state is separated into the contributions of (i) dipole–dipole (permanent and induced) and (ii) dispersion interactions. The first part (i) is treated in the Wertheim theory of polar polarizable liquids. Liquid LJ energies are extracted from the second part (ii) by utilizing Weeks–Chandler–Andersen theory. The dispersion part of the compressibility factor is treated by two routes: (I) in the mean-field approximation and (II) by employing experimental liquid state data. Except for strongly polar liquids, route I appears to be presently the best method of calculating LJ energies as tested by two independent ways. The first is the principle of corresponding states. LJ energies of nonpolar liquids, calculated from route I, demonstrate a universal linear correlation with the logarithm of the vapor pressure at T = 298 K. The other way is calculation of the solvent-induced shift of the absorption line of a model chromophore through molecularly defined solute–solvent interactions. The dispersion component, assessed by using LJ energies from route I, adds up nicely with the induction component, calculated by an extended Wertheim theory, to produce the overall solvent effect showing a linear trend with the polarity function ψ=(ε∞−1)/(ε∞+2) of the liquid high-frequency dielectric constant ε∞ for both nonpolar and polar liquids, just as is typically found by experiment. The delicate compensatory influence of dispersion and induction forces stresses the importance of a rigorous parametrization of liquid properties in describing solvent effects.
A new approach to the dynamics of hydrogen bond network in liquid water
Authors: Matsumoto Masakazu , Ohmine Iwao
Published: July 2002

The relation between topology and rearrangement dynamics of the hydrogen bond network (HBN) in the supercooled liquid water is investigated by using molecular dynamics (MD) calculation and examining topological indices. We have found that there is very strong correlation among certain pairs of hydrogen bonds. HBN is shown to be represented by an ‘‘undirected’’ graph. Topology and rearrangement dynamics of HBN are then simply described in terms of the network defects and their motions. Based on this fact, a new lattice dynamic model is proposed. The model shows that spontaneous heterogeneous hydrogen bond rearrangement occurs even when the network structure is homogeneous.
Coupled cluster calculations of the potential energy surfaces and spectroscopic constants of SiF2, PF+2, SO2, PO−2, and ClO+2
Authors: Pak Youngshang , Woods R. Claude
Published: July 2002

Three dimensional near-equilibrium potential energy surfaces for the 32-electron C2ν triatomics SiF2, PF+2, SO2, PO−2, and ClO+2 have been calculated using the coupled cluster method with single and double substitutions augmented by perturbative treatment of triple excitations [CCSD(T)] with a basis set of 169 contracted Gaussian-type orbitals (cGTOs). A complete set of rotation–vibrational spectroscopic constants for each species has been calculated using second-order perturbation theory formulas. The CCSD(T) equilibrium geometries of PF+2, PO−2, and ClO+2 are re=1.505 Å, θe=102.6°, re=1.506 Å, θe=118.9°, and re=1.425 Å, θe=120.8°, respectively. The calculated fundamental frequencies (v1,v2,v3) are 1017.8, 411.3, 1058.4 cm−1 (PF+2), 1059.7, 460.5, 1212.7 cm−1 (PO−2), and 1005.1, 496.1, 1271.7 cm−1 (ClO+2). Dipole moments of these species have been calculated at each of the CCSD(T) equilibrium geometries to predict microwave intensities.
Chain length dependence of the polymer–solvent critical point parameters
Authors: Wilding N. B. , Müller M. , Binder K.
Published: July 2002

We report grand canonical Monte Carlo simulations of the critical point properties of homopolymers within the bond fluctuation model. By employing configurational bias Monte Carlo methods, chain lengths of up to N=60 monomers could be studied. For each chain length investigated, the critical point parameters were determined by matching the ordering operator distribution function to its universal fixed-point Ising form. Histogram reweighting methods were employed to increase the efficiency of this procedure. The results indicate that the scaling of the critical temperature with chain length is relatively well described by Flory theory, i.e., Θ−Tc∼N−0.5. The critical volume fraction, on the other hand, was found to scale like φc∼N−0.37, in clear disagreement with the Flory theory prediction φc∼N−0.5, but in good agreement with experiment. Measurements of the chain length dependence of the end-to-end distance indicate that the chains are not collapsed at the critical point.
Kinetics and dynamics of the initial dissociative chemisorption of oxygen on Ru(001)
Authors: Wheeler M. C. , Seets D. C. , Mullins C. B.
Published: July 2002

We have used supersonic molecular beam techniques to measure the initial dissociative chemisorption probability S0 of O2 on Ru(001) as a function of incident kinetic energy Ei, surface temperature Ts, and angle of incidence θi. We observe different behavior in the adsorption dynamics in two separate kinetic energy regimes: the value of S0 decreases with incident energy in the low kinetic energy regime, and the value increases with incident energy in a higher kinetic energy regime. In the low energy regime, we observe a large inverse dependence of S0 on surface temperature which is consistent with a trapping-mediated mechanism. Moreover, adsorption in the low energy regime can be accurately modeled by a trapping-mediated mechanism, with a surface temperature independent trapping probability α into a physically adsorbed state followed by a temperature dependent kinetic competition between desorption and dissociation. The barrier to dissociation from the physically adsorbed state is ∼28 meV below the barrier to desorption from this state as determined by analysis of kinetic data. In the high kinetic energy regime, values of the initial adsorption probability scale with normal kinetic energy, and S0 approaches a value of unity for the highest incident energies studied. However, we report an unusual surface temperature dependence of S0 in the high energy regime that is inconsistent with a simple direct mechanism. Indeed, in this higher energy regime the value of S0 rises as the surface temperature is increased. We suggest a mechanism involving electron transfer from the ruthenium surface to account for this phenomena.
Autodetachment from vibrational levels of the O−2 A 2Πu resonance across its dissociation limit by photoexcitation from O−2 X 2Πg
Authors: Bailey Christopher G. , Lavrich David J. , Serxner David , Johnson Mark A.
Published: July 2002

We report the observation of resonance structure in the photodetachment spectrum of O−2 in the 4 eV range, which results from the excitation of autodetaching vibrational levels of the O−2 A–X transition near the dissociation limit. The evolution of the resonances with increasing vibration is simply explained using continuity of the inner part of the vibrational wave functions across the dissociation threshold. This affords the possibility of investigating the DA process at the half-collision, in a kind of ‘‘correspondence limit’’ where the outer turning point slowly recedes and the vibrational wave function incrementally adopts the character of the dissociation continuum. Photoexcitation near one of the resonances results in the population of significantly higher vibrational levels in the O2 a1Δg state (which are cleanly resolved) than the typical ‘‘Franck–Condon’’ pattern observed for nonresonant photodetachment. Finally, hot-band structure is also observed in the detachment spectrum, allowing us to extract a more accurate value of the O−2 vibrational quantum (ΔG=134.4±0.8 meV) by about an order of magnitude over previous determinations.
Crystalline structures of pristine and hydrated mercaptohexanol self-assembled monolayers on Au(111)
Authors: Poirier G. E. , Pylant E. D. , White J. M.
Published: July 2002

The crystalline structures of hydroxy-functionalized hexanethiol self-assembled monolayers on Au(111) were characterized using gas-phase transport of molecular precursors and an ultrahigh vacuum scanning tunneling microscope. In the pristine state the monolayer exhibits a commensurate lattice with an oblique primitive unit-cell of dimensions a=3, b=√13, α=a tan(2∛); a structure that is fundamentally different than the hexagonal lattice observed for methyl-terminated monolayers. Hydration of the monolayer results in conversion to a polymorphic phase. These results demonstrate the importance of end-group chemical bonding in the molecular packing of this widely studied class of materials.
Vibrational relaxation of HgI in ethanol: Equilibrium molecular dynamics simulations
Authors: Gnanakaran S. , Hochstrasser R. M.
Published: July 2002

When HgI2 is photodissociated, it produces a vibrationally hot ground state HgI molecule. Pugliano et al. have reported a T1 time of ∼3 ps for dipolar HgI in ethanol [J. Chem. Phys. 103, 6498 (1995)]. We have carried out equilibrium molecular dynamics simulation for the above system to calculate the T1 time theoretically and to understand the nature of solvent forces giving rise to the relaxation. The solvent friction on the vibrational coordinate was calculated for dipolar and nonpolar HgI. The vibrational decay rate obtained from the simulation is in quantitative agreement with the experimentally determined value. The dissipation of excess vibrational energy in HgI is mainly facilitated by the Lennard-Jones force fluctuations. The structure and nature of the solvent molecules giving rise to these interactions are discussed in detail. The effects of anharmonicity, the similarity between the friction spectra and the optical Kerr spectra and the validity of the Landau–Teller formula are discussed.
Supersonic molecular beam studies of the dissociative chemisorption of GeH4 and Ge2H6 on the Ge(100) and Ge(111) surfaces
Authors: Jones M. E. , Roadman S. E. , Lam A. M. , Eres G. , Engstrom J. R.
Published: July 2002

The reaction probabilities of GeH4 and Ge2H6 on the Ge(100) and Ge(111) surfaces have been measured as a function of substrate temperature, incident kinetic energy, and angle of incidence employing supersonic molecular beam scattering techniques. At sufficiently large incident kinetic energies (Ei≳1 eV) both GeH4 and Ge2H6 react by direct dissociative chemisorption on both surfaces examined, with the reaction probability increasing approximately exponentially with increasing (scaled) incident kinetic energy. At moderate kinetic energies (Ei∼0.4 eV), however, Ge2H6 reacts by a precursor-mediated mechanism on Ge(100), as demonstrated by a decrease in the reaction probability with either increasing substrate temperature or incident kinetic energy. Interestingly, under similar conditions, no evidence is found for precursor-mediated adsorption of Ge2H6 on the Ge(111) surface. The reaction of Ge2H6 does not exhibit a GeH4 production channel on either Ge(100) or Ge(111) for the conditions examined here. The results obtained at high incident kinetic energies (≳1 eV) are well described by a statistical model based upon a Rice–Ramsperger–Kassel–Marcus (RRKM) framework. The moderate incident kinetic energy results for Ge2H6 on Ge(100) are well described by a model that assumes reaction via a trapping, precursor-mediated mechanism.
High resolution photoelectron spectroscopy of sulfur 2p electrons in H2S, SO2, CS2, and OCS
Authors: Siggel Michele R. F. , Field Christian , Sæthre Leif J. , Bo/rve Knut J. , Thomas T. Darrah
Published: July 2002

High-resolution photoelectron spectra for the 2p electrons in H2S, SO2, CS2, and OCS show the effects of vibrational excitation in the core-excited species as well as the splitting of the 2p3/2 hole state by the molecular field. Theoretical calculations at the Hartree–Fock level account reasonably well for the vibrational structure. The molecular-field splitting is calculated with a configuration interaction-based method using large basis sets. This produces values for the 2p3/2 splitting of 108, 96, 129, and 144 meV for the title molecules, to be compared with experimental values of 110, 105, 140, and 150 meV. Thus all observed features in the spectra are quantitatively accounted for by theoretical modeling.
Direct dissociative chemisorption of propane on Ir(110)
Authors: Kelly D. , Weinberg W. H.
Published: July 2002

We have employed molecular beam techniques to investigate the initial probability of direct dissociative chemisorption, Pd, and the intrinsic trapping probability, ξ, of C3H8, C3D8, and (CH3)2CD2 on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 59 kcal/mol. For C3H8 and (CH3)2CD2, a measurable (≥ 0.02) initial probability of direct dissociative chemisorption is observed above a beam energy of approximately 7 kcal/mol. For C3D8 this energy is roughly 10 kcal/mol. Above these energies the initial probability of direct chemisorption of each of the isotopomers of propane increases nearly linearly with Ei, approaching a value of approximately Pd=0.48 at Ei=52 kcal/mol for C3H8 and (CH3)2CD2, and Pd=0.44 at Ei=59 kcal/mol for C3D8. This kinetic isotope effect for the direct chemisorption of C3D8 relative to C3H8 is smaller than that expected for a mechanism of H (or D) abstraction by tunneling through an Eckart barrier, suggesting a contribution of C–C bond cleavage to direct chemisorption. The lack of a kinetic isotope effect for the direct chemisorption of (CH3)2CD2 relative to C3H8 indicates that 1° C–H bond cleavage dominates over 2° C–H bond cleavage during the direct chemisorption of propane on Ir(110). The trapping behavior of each of these isotopomers of propane is approximately identical as a function of Ei, with ξ >0.9 at Ei=1.5 kcal/mol, ξ = 0.3 at Ei=20 kcal/mol, and ξ < 0.1 above Ei= 40 kcal/mol.
A parity function for studying the molecular electronic structure
Authors: Schmider Hartmut
Published: July 2002

Sections through the molecular Wigner function with zero momentum variable are shown to provide important information about the off-diagonal regions of the spinless one-particle reduced density matrix. Since these regions are characteristic for the bonding situation in molecules, the sections are qualitatively even more affected by the presence of chemical bonds than a complementary projection, the reciprocal form factor. In this paper we discuss, on the grounds of a variety of examples, how this rather simple function may aid the understanding of the chemical bond on a one-particle level.
Cumulant expansion of the free energy: Application to free energy derivatives and component analysis
Authors: Archontis G. , Karplus M.
Published: July 2002

The free-energy change in a system represented by an empirical energy function, when calculated by thermodynamic integration from the initial to the final state, can be expressed as a sum of ‘‘free-energy components,’’ that correspond to contributions from different energy terms or different parts of the system. Although the overall free energy is path independent, the values of the components depend on the integration path, which determines the physical processes involved in the change. A cumulant expansion is used to express the total free-energy change as a sum of terms that contain the cumulants of a single energy term plus a coupling term involving cumulants of more than one energy term. The dependence of the free-energy components on the thermodynamic integration path is shown to arise from the different partitionings of the latter term. A concerted linear integration path, along which the total energy function changes from the initial to the final state as a linear function of an external parameter λ, as well as piecewise-linear paths, along which individual energy terms are changed linearly, are examined. Along the concerted linear path the partitioning of the coupling term preserves permutation symmetry with respect to energy components to all orders. This contrasts to piecewise-linear paths, where the total coupling is projected onto one component. To illustrate the analysis, calculations are made for the transformation. Cl(0)→Br(−) (a neutral particle with the van der Waals parameters of Cl(−) to a charged particle with the van der Waals parameters of Br(−)) in aqueous solution along various paths. Terms through third order in the cumulant expansion for the change in the solvation free energy are evaluated by use of the reference interaction site model with the hypernetted chain closure, and the values obtained along the linear and piecewise-linear paths are compared and analyzed. The utility of cumulants for the prediction of the effect of specific changes in the energy terms, and for interpreting free-energy components is discussed. In most cases a first-order approximation is not accurate enough to be useful, but results through third order are accurate to 5% or better. In the Appendix a general expression for the free-energy derivatives of a Hamiltonian with a general functional dependence on a parameter λ is derived.
Vibrational excitation of ammonia clusters by helium atom scattering
Authors: Buck U. , Krohne R. , Schütte S.
Published: July 2002

In a crossed molecular beam arrangement helium atoms are scattered from ammonia clusters of the averaged sizes n̄=18, 745, and 1040 which are generated by isentropic expansions with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of less than 5 meV at a collision energy of 95 meV. The energy transfer increases with increasing deflection angle and extends to 65 meV. Intensity maxima are observed between 11 and 16 meV, around 27 meV, and at 33 meV. The one in the middle is attributed to the vibration of a specific cluster network, while the other two occur in the energy regime of the translational or librational modes of the solid with a preference for small or large clusters, respectively.
Structure H clathrate unit cell coordinates and simulation of the structure H crystal interface with water
Authors: Pratt Ronald M. , Mei Dong-Hai , Guo Tian-Min , Sloan, Jr. E. Dendy
Published: July 2002

The Cartesian coordinates for the structure H clathrate unit cell have been determined to within 5% error. The unit cell, consisting of 34 water molecules and 6 hydrocarbon molecules, was replicated to create a system amenable to molecular dynamics simulation. Radial distribution functions were determined for this crystal. The method of molecular dynamics was employed to model the interface between the crystal and liquid water. It was found that the interfacial behavior spanned a region of 15 Å on both sides of the interface. Both orientational and translational motion in the liquid phase became more vibrational as the interface was approached, while the temperature profile remained constant within about 15° across the interface. In addition, there was a small tendency for the molecules in the liquid phase of the interfacial region to align themselves in an ordered, solidlike configuration. The solid region within about 10 Å of the interface demonstrated persistent steady-state disorder compared with the solid bulk phase.
Universal binary theory of photochemical charge separation and distribution
Authors: Burshtein A. I. , Frantsuzov P. A.
Published: July 2002

Using integral formalism we developed the encounter theory of reversible photoionization followed by charge recombination. This is a problem that can not be approached with conventional (differential) formalism, unless ionization is highly exothermic and thus irreversible. In this limit, the integral theory supplemented by the recipe for calculating the ion distribution may be successfully reduced to the differential theory used in our previous work. However, there is no alternative to integral theory when ionization is quasiresonant and the back electron transfer to the excited state should be accounted for. Using the contact approximation we calculated the free-energy dependence of the Stern-Volmer constant of reversible photoionization accompanied by charge recombination.
A comparison of two methods for direct tunneling dynamics: Hydrogen exchange in the glycolate anion as a test case
Authors: Smedarchina Zorka , Fernandez-Ramos Antonio , Rios Miguel A.
Published: July 2002

Two methods for studying tunneling dynamics are compared, namely the instanton model and the approach of Truhlar and co-workers, which are based on the direct output of electronic structure calculations and thus are parameter free. They are employed to evaluate the zero-level tunneling splitting due to intramolecular hydrogen exchange in the glycolate anion. The first method was developed in a series of recent studies and presents a combination of the instanton theory with quantum-chemically computed potentials and force fields. For the compound at hand, which has 21 internal degrees of freedom, a complete potential-energy surface is generated in terms of the normal modes of the transition-state configuration. It is made up of the potential-energy curve along the tunneling coordinate and harmonic force fields at the stationary points. The level of theory used is HF/6–31++G**. All modes that are displaced between the equilibrium configuration and the transition state are linearly coupled to the tunneling mode, the couplings being proportional to the displacements in dimensionless units. These couplings affect the instanton trajectory profoundly and, depending on the symmetry of the skeletal modes, can enhance or suppress the tunneling. In the glycolate anion all modes have such displacements and thus are included in the calculation. Based on the similarity with malonaldehyde, it is argued that tunneling prevails in the studied process, and the zero-level tunneling splitting is predicted. The latter is found within the computational scheme developed earlier, which avoids explicit evaluation of the instanton path and thus greatly simpli-fies the tunneling dynamics. These results are tested by the method of large-curvature tunneling of Truhlar and co-workers implemented in a dual-level scheme. The potential energy surface needed for the dynamics calculations is generated at the semiempirical PM3 level of theory and then corrected by interpolation with high-level HF/6–31++G** results for the stationary points. The code corresponding to this approximation is in the package MORATE 6.5. The tunneling splittings found by the two approaches are in quantitative agreement. We have found that the computational scheme based on the instanton model is much less time consuming both in the static and dynamics part. This computational efficiency, also demonstrated in a number of earlier studies, merits future application of the method to fairly large systems of practical interest, such as clusters and organic compounds with excited-state proton transfer.
Homogeneous nucleation rates for n-pentanol from expansion wave tube experiments
Authors: Luijten C. C. M. , Baas O. D. E. , van Dongen M. E. H.
Published: July 2002

Within the scope of joint experiments by the international Nucleation Workshop Group, nucleation experiments on n-pentanol were carried out using a pulse-expansion wave tube. Data were obtained for nucleation at temperatures between 240 K and 260 K. Total pressures of the carrier gas (helium) during nucleation varied from 89 to 109 kPa. The results are presented in tabular form, to facilitate future comparison. Our results are consistent with existing data by Hrubý et al. Comparisons are made to the Kinetic Classical Theory (KCT) as well as to the semiphenomenological theory by Kalikmanov and Van Dongen (KvD–SPT). Although both theories predict nucleation rates that are apparently too low in the temperature range of interest, the KvD–SPT is approximately two orders of magnitude closer to the experimental results.
Site symmetry dependence of repulsive interactions between chemisorbed oxygen atoms on Pt{100}-(1×1)
Authors: Ge Q. , Hu P. , King D. A. , Lee M.-H. , White J. A. , Payne M. C.
Published: July 2002

Ab initio total energy calculations using density functional theory with the generalized gradient approximation have been performed for the chemisorption of oxygen atoms on a Pt{100}-(1×1) slab. Binding energies for the adsorption of oxygen on different high-symmetry sites are presented. The bridge site is the most stable at a coverage of 0.5 ML, followed by the fourfold hollow site. The atop site is the least stable. This finding is rationalized by analyzing the “local structures” formed upon oxygen chemisorption. The binding energies and heats of adsorption at different oxygen coverages show that pairwise repulsive interactions are considerably stronger between oxygen atoms occupying fourfold sites than those occupying bridge sites. Analysis of the partial charge densities associated with Bloch states demonstrates that the O–Pt bond is considerably more localized at the bridge site. These effects cause a sharp drop in the heats of adsorption for oxygen on hollow sites when the coverage is increased from 0.25 to 0.5 ML. Mixing between oxygen p orbitals and Pt d orbitals can be observed over the whole metal d-band energy range.
Femtosecond studies of exciton dynamics in a novel main chain chiral conjugated poly(arylenevinylene)
Authors: Zhang J. Z. , Kreger M. A. , Hu Q.-S. , Vitharana D. , Pu L. , Brock P. J. , Scott J. C.
Published: July 2002

The formation and decay dynamics of photogenerated excitons in an optically active poly(arylenevinylene), PAV, in solution have been studied using femtosecond transient absorption spectroscopy. Photoexcitation initially creates hot excitons which quickly (<200 fs) relax geometrically towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 6.5 and 420 ps in toluene. The decays become faster (5 and 250 ps) in pyridine, indicating a dependence of the relaxation process on the solvent environment. The fast decay is attributed to vibrational relaxation and internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and nonradiative pathways. Anisotropy decay shows a fast component (6 ps in toluene and 10 ps in pyridine) and an offset which persists up to 650 ps. Possible explanations for the fast decay include internal conversion, vibrational relaxation, conformational change, and exciton migration. The offset may decay on a longer time scale through local reorientation of the conjugation segments, exciton migration, or rotational diffusion of the polymer. Comparison to a well-studied system, MEH-PPV [poly(2-methoxy, 5-(2-ethylhexoxy)-p-phenylenevinylene], provides further insight into the relaxation mechanism of photoexcitations in this PAV polymer.
Tunneling splitting due to weak coupling between methyl rotators in acetylacetone
Authors: Choi Changho , Pintar M. M.
Published: July 2002

In acetylacetone the energy level matching spectrum of nuclear Zeeman and tunneling energies of CH3 groups in the rotating frame of their proton spins demonstrates resonant transfers of population characteristic of a 100 KHz range tunneling splitting. However, in this material two A to E tunneling splittings, of 10.4 GHz and 1 GHz, were observed by inelastic neutron scattering. Therefore the small energy splitting must be a result of a weak torsion–torsion interaction, which has not been resolved in neutron spectroscopy. This small splitting is investigated through the dependencies of its energy level matching spectra on mixing time, tilt angle in the magnetic field, temperature, and the strength of the main dc field H0. Upon heating, the level matching resonances begin to broaden at 10 K and average out to zero at 25 K. The spectrum does not depend on H0. These observations are explained by Zeeman-tunneling level matching transitions within manifolds of EEE-symmetry states and AEE-symmetry states, both of which are split by the interaction among three CH3 groups. Based on resonances detected at νz=nΔ, where n=1/4, 1/3, 1/2, and 1, this splitting Δ is determined to be 116±2 KHz at 10 K.
The νCC+3νCH rovibrational manifold of acetylene. I. Collision-induced state-to-state transfer kinetics
Authors: Milce Angela P. , Orr Brian J.
Published: July 2002

Infrared–ultraviolet double resonance (IR–UV DR) spectroscopy is used to measure colli-sion-induced, rotationally resolved state-to-state energy transfer in the νCC+3νCH vibrational manifold of gas-phase acetylene, C2H2. Attention focuses on three sets of vibrational eigenstates spectroscopically labeled (0 1 3 0 0)I0, (0 1 3 0 0)II0, and (4 0 3 3)+0, with vibrational term energies in the region 11 585–11 600 cm−1. IR–UV DR spectroscopy identifies the channels of J-changing rotational energy transfer (RET) and inter-mode vibrational (V–V) transfer. The second-order state-to-state kinetics of these channels is measured by scanning the IR–UV pulse delay with the IR PUMP and UV PROBE lasers tuned to particular spectroscopic features. There is a clear propensity for even-numbered changes ΔJ of rotational quantum number in the observed RET and V–V transfer, consistent with conservation of ortho or para nuclear-spin symmetry in the state-selected C2H2 molecule, but there are some notable exceptions as unusual symmetry-breaking processes result in odd-ΔJ V–V transfer. Anomalies of this type have been investigated in a preceding paper [A. P. Milce and B. J. Orr, J. Chem. Phys. 104, 6423 (1996)]. A detailed IR-UV DR study is made with the UV PROBE laser monitoring the (0 4 0 3 3)+0, J=12 rovibrational level; RET is measured when the IR PUMP laser prepares molecules in various J-states of (0 4 0 3 3)+0, while V–V transfer is monitored when initial J-states of (0 1 3 0 0)I0 or (0 1 3 0 0)II0 are prepared by the IR PUMP. The corresponding IR-UV DR kinetic curves are fit to a detailed rate-equation model in which empirical exponential-gap fitting laws are used to describe even-ΔJ channels of RET and V–V transfer. It is remarkable that the kinetics of symmetry-breaking odd-ΔJ V–V transfer between the (0 1 3 0 0)I0, J=5 and (0 4 0 3 3)+0, J=12 rovibrational levels is well fit by the same model. The dynamical implications of these results are discussed.
Density functional theory investigation of hyperfine coupling constants in peroxyl radicals
Authors: Wetmore Stacey D. , Boyd Russell J. , Eriksson Leif A.
Published: July 2002

The geometries and O17 hyperfine coupling constants in several peroxyl radicals have been determined through the use of density functional theory. Becke’s three-parameter hybrid exchange functional (B3) together with the correlation functional of Lee, Yang, and Parr (LYP) in combination with a variety of basis sets was used to study basis set effects. Subsequently, the effects of different gradient-correlated functionals were also examined. Results comparable to experimental values are obtained for all of the alkyl peroxyl radicals at the B3LYP level with IGLO-III or s-shell decontracted IGLO-III, 6-311G(d,p), 6-311+G(2df,p), and the augmented correlation-consistent polarized-valence triple-zeta basis set of D. E. Woon and T. H. Dunning [J. Chem. Phys. 98, 1358 (1993)], R. E. Kendall, T. H. Dunning, and R. J. Harrison [J. Chem. Phys. 96, 6796 (1992)], and T. H. Dunning [J. Chem. Phys. 90, 1007 (1989)]. Calculations imply that the spin density ratio between the inner and outer oxygens is 0.3:0.7, supporting earlier theoretical work [S. L. Boyd, R. J. Boyd, and L. R. C. Barclay, J. Am. Chem. Soc. 112, 5724 (1990)]. Erratic and strongly fluctuating results are exhibited for the fluoroperoxyl radical. Geometries close to the experimental values can be obtained at the B3LYP level, but at the expense of considerable spin contamination. A high degree of spin contamination can also be observed in calculations of the hyperfine coupling constants for this molecule. Possible explanations for the apparent failure to obtain converged results for FOO, apart from the considerable spin contamination, include vibrational, multireference, and matrix effects.
Chemical association of spherical particles
Authors: Evans G. T.
Published: July 2002

Aggregation equilibria of dilute fluids of spherical classical particles are considered using a formalism derived by Lockett and Wertheim. The present theory is a simple application of the Lockett–Wertheim theory, and uses a set of ring, chain, and star Mayer graphs in the grand canonical ensemble partition function. This analysis is applied to a fluid of hard spheres with a thin exterior square well. Calculated here are the equilibrium constants for n-mer association, the n-mer distribution, and the fluid pressure. The shape of the n-mer aggregates is inferred from their individual structure factors.
Quantum simulations of energy transfer and state-to-state transitions in collision of an atom with a large anharmonic cluster: He+Ar13
Authors: Rom A. Y. , Gerber R. B.
Published: July 2002

A time-dependent self-consistent field approach is used to simulate a He atom colliding with an Ar13 cluster. Direct energy transfer during the collision, and energy redistribution among the vibrational degrees of freedom of the anharmonic cluster following the collision, are studied. An important advantage of the method used is that quantum state-to-state transition cross sections can be computed for large systems. The following main results are obtained: (1) The process can be interpreted in terms of a direct collision, followed by post-collision energy redistribution in Ar13 , a description that appears only when the cluster vibrations are not described by the eigenstates of this system. A time scale of one picosecond is found for the post-collision intracluster energy distribution. (2) The long-time final state distribution of Ar13 is less state selective than the distribution immediately after the impact, but it is also not completely statistical. (3) There are state-to-state transitions having cross sections of observable magnitude. (4) The dominant transitions are those involving zero, one, and two “phonon” excitations. Some of the “two phonon” excitations have cross sections comparable to strong “single phonon” transitions. (5) Different types of modes show different propensities for excitations in the collision, in close relation to the geometric character of the modes. The results show that the TDSCF approximation is a powerful tool for treating both direct collision dynamics and collision-induced dynamics in scattering of large anharmonic systems.
Interaction of hydrogen atoms with coadsorbed D/CH3I adlayers on Ni (100) surfaces: Evidence for hot atom mediated reactions
Authors: Kammler Th. , Küppers J.
Published: July 2002

Adlayers of coadsorbed hydrogen (deuterium) atoms and methyliodide, H/CH3I, D/CH3I, H/CD3I, D/CD3I, were prepared on Ni(100) surfaces by adsorption of methyliodide on hydrogen (deuterium) saturated surfaces at 120 K. Upon directing thermal D (H) atoms at these adlayers as gaseous products hydrogen and methane molecules were observed which constitute of surface species only, e.g., H2 and CH4 in the combination: D→H/CH3I. The rates of formation of these products give evidence for reaction mechanisms in which adsorbed atoms, e.g., H, activated by collisions with incoming atoms, e.g., D, react with surface species, either H or CH3I to form H2 and CH4. The reaction cross sections and isotope effects in H→D/CD(H)3I or D→H/CH(D)3I reactions support a collision model in which hot atoms act as mediators in the reaction pathways.
Laser-induced fluorescence spectroscopy of the ketenyl radical
Authors: Brock L. R. , Mischler B. , Rohlfing Eric A. , Bise Ryan T. , Neumark Daniel M.
Published: July 2002

We report the first laser-induced fluorescence (LIF) excitation spectrum of the ketenyl radical, HCCO, which is produced by the 193 nm photolysis of ketene in a free jet expansion. A series of vibronic bands in the B̃ 2Π–X̃ 2A′′ system are observed. The LIF band positions and rotational structures are in excellent agreement with those of a recent photofragment yield (PFY) spectrum [D. L. Osborn et al., J. Chem. Phys. (to be published)] from the origin at 33 424 cm−1 to 35 100 cm−1 . At higher energies the LIF spectrum breaks off sharply due to a rapid increase in the rate of predissociation. The lifetime of the vibrationless level of the B̃ state is less than 1 ns; the estimated quantum yield of fluorescence is ∼10−3.