bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo calculations of free energy in the solid phase
Authors: Jhung Kyu Soo , Kum Oyeon , Lee Hi Won
Published: July 2002

The method of direct Monte Carlo measurement of free energy in the solid phase developed by Jhung and Jhung (J. Chem. Phys. 87, 5403 (1987)] has been applied to the inverse power potential systems and Lennard-Jones system. The calculated results show that the method provides an accurate and practical means for the calculation of the solid-phase Helmholtz free energy.
Spectroscopy and photodissociation of molecular chlorine in argon matrices
Authors: Kunz H. , McCaffrey J. G. , Schriever R. , Schwentner N.
Published: July 2002

A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10−18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge-transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the molecular A→X emission. The dissociation efficiency of this band was found however to be significantly less than in the 130 nm band and very sample preparation dependent, viz., sample crystallinity. In crystalline samples dissociation efficiencies were typically two orders of magnitude less than in noncrystalline samples. No dissociation occurs following excitation into the lower energy 1Π(1u) band at 300 nm irrespective of sample preparation conditions.
Effects of translational, rotational, and vibrational energy on the dynamics of the D+H2 exchange reaction. A classical trajectory study
Authors: Aoiz F. J. , Herrero V. J. , Sáez Rábanos V.
Published: July 2002

Quasiclassical trajectory (QCT) calculations for the D+H2(v,j)→HD+H system have been performed on the Liu, Siegbahn, Truhlar, Horowitz (LSTH) potential energy surface in order to study the combined effects of translation, rotation, and vibration on the reactivity. The range of initial conditions covered has been ET =0.25–1 eV, v=0, 1, and 2 and j=0–12. Integral cross sections, opacity functions, solid angle differential cross sections, and the energy partitioning among the products’ degrees of freedom have been obtained. The minimum in the dependence of the total cross section with rotational excitation observed in previous QCT calculations for v=0 and v=1 at low collision energies is here verified also for v=2. The center-of-mass (c.m.) angular distributions of the scattered HD product are predominantly backward with respect to the direction of the D incoming atom, at low energies, but they broaden markedly and become more forward with increasing total energy. Translational and vibrational excitation in the reactants are largely adiabatic and tend to remain as translation and vibration in the products. Where they can be compared, present results are in good agreement with recent quantum mechanical calculations and with experimental measurements.
Energy distribution in aniline scattered from various low energy surfaces
Authors: Paz Y. , Naaman Ron
Published: July 2002

Aniline has been scattered from three organic substrates and a LiF single crystal surface. Applying multiphoton ionization and time of flight measurements the vibrational, rotational, and translational energy distributions were measured simultaneously. The NH2 ‘‘umbrella’’ like mode was found to be a very efficient accepting mode in the energy transfer process. The less rigid the surface, the greater the efficiency with which this mode is populated. The mode specificity does not exist for the rigid LiF surface. A model is presented, which explains all observations based on the collision time and kinematics.
Collinear reactive scattering of state-selected H+2+He→HeH++H: A method for probing dynamical resonances
Authors: Pollard J. E. , Syage J. A. , Johnson L. K. , Cohen R. B.
Published: July 2002

We report on an experimental geometry for studying dynamical resonances in bimolecular collisions. The method is sensitive to atom–diatom reactions at low angular momentum and low impact parameter corresponding to the collinear coordinate. State-selected reaction probabilities for H+2+He→HeH++H were measured at c.m. scattering angles θ=0° and 180°; the collision energy was scanned from 0.35–1.90 eV.
Gradient theory applied to the Brueckner doubles method
Authors: Kobayashi Rika , Handy Nicholas C. , Amos Roger D. , Trucks Gary W. , Frisch Michael J. , Pople John A.
Published: July 2002

The Brueckner doubles variant of coupled cluster theory has recently been reintroduced by the authors. The use of Brueckner orbitals means that the governing equations for T̂2 take a particularly simple form. Here we give the details for the evaluation of the gradient of the Brueckner doubles energy for (a) the unrestricted spin–orbital formalism and (b) the closed-shell restricted formalism. Applications are presented for H2O, NH3, CH4, H2CO, C2H2, HCN, and CO2 and comparisons are made with the Hartree–Fock, second order Mo/ller–Plesset and quadratic configuration interaction models and with experiment.
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o study of the reaction of atomic hydrogen with sulfur dioxide
Authors: Binns Derk , Marshall Paul
Published: July 2002

The potential energy surface for H(1 2S)+SO2 has been investigated computationally in order to study the catalytic removal of atomic hydrogen in flames by sulfur dioxide. HF/3–21G(*) and MP2/3–21G(*) levels of theory were employed to locate stationary points, which were then characterized by calculation of the vibrational frequencies. Some geometries were also optimized with the 6–31G* basis set. Two adducts HOSO and HSO2, with H bonded to O or S, respectively, were studied. Energies were estimated at the optimized geometries using spin-projected MP4/6–31G* calculations, which show that planar cis HOSO is more stable than Cs HSO2. An H–OSO bond energy of 109 kJ mol−1 is predicted. By contrast HSO2 is predicted to be 25 kJ mol−1 endothermic with respect to H+SO2, and is insufficiently stable to be significant in combustion chemistry. Transition states were located and the information used to derive the kinetics of H+SO2+Ar⇄HOSO+Ar from 298 to 2000 K. An unusually large energy barrier to recombination, of about 95 kJ mol−1 relative to H+SO2, is proposed. The results are compared with available kinetic measurements. Other potential decomposition channels for HOSO, to SO+OH and isomerization to HSO2, were also analyzed.
Photoelectron spectroscopy of the halogen oxide anions FO−, ClO−, BrO−, IO−, OClO−, and OIO−
Authors: Gilles Mary K. , Polak Mark L. , Lineberger W. C.
Published: July 2002

The 351 nm photoelectron spectra of FO−, ClO−, BrO−, IO−, OClO−, and OIO− are reported. The spectra of the halogen monoxides display transitions to both spin–orbit states of the 2Πi ground state neutrals. Anion vibrational frequencies are observed in the spectra and bond lengths are obtained for the anions from Franck–Condon simulations. Spectra of the halogen dioxides display two active vibrational modes—the symmetric stretch and the bend. Anion symmetric stretching frequencies and normal coordinate displacements from the corresponding neutral are reported. Adiabatic electron affinities found for the halogen oxides are 2.272(6) eV (FO), 2.276(6) eV (ClO), 2.353(6) eV (BrO), 2.378(6) eV (IO), 2.140(8) eV (OClO), and 2.577(8) eV (OIO). The difference between the neutral and anion dissociation energies [D0(XO)−D0(XO−)] is reported for each of the halogen monoxides. Anion heats of formation (298 K) are also determined.
Study of the LiZn excimer: Blue–green bands
Authors: Milošević Slobodan , Li Xinghua , Azinović Davorka , Pichler Goran , van Hemert Marc C. , Stehouwer Aafke , Düren Rudolf
Published: July 2002

We performed a study of the LiZn excimer formed in the photochemical reaction of Li2(C 1Πu) with Zn(4 1S0). The Li2(C 1Πu) molecules were prepared by excitation with either a dye laser in the region from 348 to 377 nm or an excimer laser at 308 nm. We observed bound–free emission of LiZn at 477 nm. Nonrelativistic ab initio calculations of the potential energy curves and the relevant dipole transition moments have been performed. Using these results in spectral simulations the observed emission is interpreted as belonging to the 2 2Π–X 2Σ+ transition of the LiZn molecule.
Raman-ultraviolet double resonance in acetylene: Rovibrational state preparation and spectroscopy
Authors: Chadwick B. L. , Orr B. J.
Published: July 2002

We report time-resolved optical double resonance spectroscopic experiments in which gas-phase acetylene molecules are selectively prepared and monitored in discrete rotational states of the v2=1 (C≡C stretch, 1974 cm−1) vibrational level. This is achieved by pulsed coherent Raman excitation and laser-induced fluorescence detection. State-selective spectra of single rovibrational states are presented under effectively collision-free conditions. Several new rovibronic bands in the Ã←X̃ absorption system of acetylene are identified in this way, owing to the enhanced sensitivity and spectral simplification of our Raman-optical double resonance technique. Investigations of C2H2(g) concentrate on rotationally resolved vibronic bands of the form 21030x (where x=1,2,3,...), exploring spectroscopic subtleties such as axis switching. The method has also been extended to the 21030x410 vibronic bands of C2H2(g), by Raman excitation in the (ν2+ν4−ν4) hot band, and to studies of the deuterated isotopomers, C2HD(g) and C2D2(g). Two distinct experimental strategies are demonstrated, in terms of their utility for spectroscopic assignment and energy transfer applications. One such approach comprises a rovibronic fluorescence excitation spectrum, recorded with fixed Raman excitation frequency. The alternative approach yields state-selected Raman spectra, with the Raman excitation frequency varied and the rovibronic excitation wavelength fixed.
An infrared study of monomeric and oligomeric (n=2, 3, and 4) hydrogen chloride in liquified noble gases
Authors: van der Veken B. J. , De Munck F. R.
Published: July 2002

Infrared spectra (3100–2600 cm−1) of HCl dissolved in liquid argon (94–124 K), liquid krypton (117–167 K), and liquid xenon (161–221 K) at concentrations varying from 0.8×10−3 to 2.8×10−2 M are reported. At low concentrations in all three solvents, only the spectrum due to monomeric species is observed. For solutions in liquid argon, the observed rotational fine structure was assigned. For all solvents, the monomeric stretching frequency shows a linear relation to the relative density of the solvent, extrapolating to the dilute vapor phase frequency. At higher concentrations in liquid argon and liquid krypton, bands due to oligomeric species are found. Factor analysis shows at least three oligomeric species are present. The band profile analysis of the oligomer absorptions allows the assignment of observed bands to dimer, trimer, and tetramer. From the temperature dependence of the oligomer band intensities, the enthalpy difference for dimerization is found to be 3.78±0.33 kJ mol−1 in liquid argon and 5.09±0.55 kJ mol−1 in liquid krypton. The trimerization and tetramerization enthalpy differences in liquid argon were measured to be 12.6±1.9 and 18.7±3.2 kJ mol−1, respectively.
The structure of N2⋅SO2 from diode laser and molecular-beam electric resonance spectroscopy
Authors: Juang Y. D. , Walsh M. A. , Lewin A. K. , Dyke T. R.
Published: July 2002

The b-type vibration–rotation band of N2⋅SO2 near the SO2 ν3 band origin was observed in a molecular-beam, diode laser direct absorption experiment. Rotational transitions and Stark effect data for this complex were additionally measured using molecular-beam electric resonance methods. The vibrational band origin was 1361.1440(2) cm−1, shifted by 0.9167(2) cm−1 from that of the SO2 monomer. Rotational constants were measured for the upper and lower vibrational states with A″=8875.3(22) MHz, B″=1620.3(22) MHz, C″=1426.1(24) MHz, A′=8832.4(26) MHz, B′=1617.3(28) MHz, and C′=1431.6(15) MHz. The electric dipole moment components were determined, with μa = 0.0441(16) D and μc = 1.5884(29) D. The c component of the nitrogen quadrupole coupling component was found to be eqccQ = 1.30(21) MHz. A structure analysis gave the separation between the centers of mass of the monomers as 3.8925(28) Å. The angles between the symmetry axes of the SO2 and N2 units and the line connecting these monomers were calculated as 61.35° and 24.54°, respectively. Additionally, the SO2 monomer a axis was found to lie along the b axis of the complex. The electric dipole moment data indicate that the equilibrium angle for the SO2 is much closer to 90° than the rms result. These structural results were compared to model calculations of the binding energy of the complex.
Recombinative desorption of H2 on Si(100)-(2×1) and Si(111)-(7×7): Comparison of internal state distributions
Authors: Shane Stacey F. , Kolasinski Kurt W. , Zare Richard N.
Published: July 2002

The dynamics of recombinative hydrogen desorption from the Si(100)-(2×1) and Si(111)-(7×7) surfaces have been compared using (2+1) resonance-enhanced multiphoton ionization to probe the desorbed H2. After dosing the surface with disilane (Si2H6), we performed temperature programmed desorption in a quantum-state-specific manner. The rovibrational-state distributions of H2 desorbed from both Si(100)-(2×1) and Si(111)-(7×7) are found to be the same within experimental accuracy. The rotational distribution is non-Boltzmann and has an average energy significantly lower than kTs, where Ts is the surface temperature. In contrast, superthermal energy is observed in the vibrational degree of freedom, and the v=1 to v=0 population ratio is approximately 20 times higher than that predicted by Boltzmann statistics. Our results imply that the details of the recombinative desorption process that affect the product state distribution are remarkably insensitive to the structural differences between the surfaces. We suggest that the transition-state geometry is similar on both surfaces and propose a model for hydrogen recombinative desorption localized at a single silicon atom.
Proton–donor properties of water and ammonia in van der Waals complexes with rare-gas atoms. Kr–H2O and Kr–NH3
Authors: Chal/asiński G. , Szczȩśniak M. M. , Scheiner S.
Published: July 2002

The perturbation theory of intermolecular forces in conjunction with the supermolecular Mo/ller–Plesset perturbation theory is applied to the analysis of the potential-energy surfaces of Kr–H2O and Kr–NH3 complexes. The valleylike minimum region on the potential-energy surface of Kr–H2O ranges from the coplanar geometry with the C2 axis of H2O nearly perpendicular to the O–Kr axis (T structure) to the H-bond structure in which Kr faces the H atom of H2O. Compared to the previously studied Ar–H2O [J. Chem. Phys. 94, 2807 (1991)] the minimum has more of the H-bond character. The minimum for Kr–NH3 corresponds to the T structure only, in accordance to the result for Ar–NH3 [J. Chem. Phys. 91, 7809 (1989)]. The minima in Kr–H2O and Kr–NH3 are roughly 27% and 19%, respectively, deeper than for the analogous Ar complexes. To examine the proton–donor abilities of O–H and N–H bonds the ratios of the deformation energy to dispersion energy are considered. They reflect fundamental differences between the two bonds and explain why NH3 is not capable of forming the H-bond structures to rare-gas atoms.
Vibrational predissociation of the Ar⋅⋅⋅Cl2 van der Waals complex: The small molecule limit for intramolecular vibrational redistribution
Authors: Halberstadt Nadine , Serna Santiago , Roncero Octavio , Janda Kenneth C.
Published: July 2002

A converged three-dimensional quantum treatment of vibrational predissociation in the Ar⋅⋅⋅Cl2(BΠ0u+3,υ′) van der Waals complex is presented. The potential energy surface used is a sum of pairwise Morse atom–atom interactions adjusted asymptotically to a C6/R6+C8/R8 anisotropic van der Waals form. Calculations have been performed in the energy region of Ar⋅⋅⋅Cl2(B,υ′=6, 10, and 11) excited levels. In agreement with the experimental findings, the final rotational distribution of Cl2 is found to be strongly dependent on the initial υ′ state being excited, as well as on the number of vibrational quanta lost in the vibrational predissociation process. The role of intramolecular vibrational redistribution for υ′=10 and 11 for which the Δυ=−1 channel is closed is also studied. It is found that the vibrational predissociation (VP) dynamics are dominated by the coupling of the zero-order ‘‘bright’’ state with a single ‘‘dark’’ state from the υ′−1 manifold of van der Waals vibrationally excited states which then decays to the continuum, and that the product state distribution is determined by the dissociation of the dark state. This is characteristic of the sparse limit for intramolecular vibrational redistribution. It also implies that the dissociation rate is not governed by a simple function of the initial quantum numbers such as the one given by the energy gap law. The golden rule approximation gives surprisingly accurate results for Ar⋅⋅⋅Cl2 dynamics. This will be very useful for fitting a potential energy surface to experimental results.
A first approach to a simple intermolecular potential model for the condensed phases of C60
Authors: Gamba Z.
Published: July 2002

In this paper we search for a simple model to describe the intermolecular interactions in the condensed phases of C60. Although several and very complete works have been published on its condensed phases, a simple model that could allow the study of large samples of molecules, and then, the statistical properties of this compound is still lacking. Here we propose a first approximation to the real intermolecular potential model.
The effect of amplified spontaneous emission on the measurement of the multiplet state distribution of ground state oxygen atoms
Authors: Huang Yu-Lin , Gordon Robert J.
Published: July 2002

Amplified spontaneous emission (ASE) is a process which competes with laser-induced fluorescence (LIF) in three level systems. This was demonstrated in the case of oxygen atoms, for which both ASE and LIF were observed at 845 nm. Ground state oxygen atoms were generated by photodissociating O2, SO2, and NO2 in a pump and probe experiment. Both nascent and thermalized O(3Pj″) atoms were measured using two-photon LIF at 226 nm. The fine structure populations deduced from the 845 nm (3p3Pj′→3s3S) emission were considerably hotter than the distributions obtained from the 130 nm (3s3S→2p3P) transition. A kinetic model is developed showing that the distributions observed at 845 are distorted by ASE, which selectively depletes the populations of atoms originating from more highly populated multiplet levels.
Terraces in the cholesteric phase of DNA liquid crystals
Authors: Van Winkle David H. , Davidson Michael W. , Rill Randolph L.
Published: July 2002

Near the transition to the columnar phase, the cholesteric liquid crystal phase in an aqueous solution of DNA fragments with contour lengths approximating the persistence length undergoes an unwinding of the cholesteric pitch. Unwinding of the cholesteric with planar alignment of the fragments was studied by polarized light microscopy. Terraces or ‘‘Grandjean planes’’ of cholesteric are seen as uniformly birefringent fields of distinct hues (typically blue), bounded by lines which moved as the local concentration of DNA increased. These lines are interpreted as disclination lines, bounding regions of different total twist, which move as the intrinsic pitch of the cholesteric varies with concentration.
Compact formulas for vibrational dynamic dipole polarizabilities and hyperpolarizabilities
Authors: Bishop David M. , Kirtman Bernard
Published: July 2002

Compact, readily-programmable formulas are given for the vibrational dynamic dipole polarizability (αv) and hyperpolarizabilities (β v and γv) for polyatomic molecules. They are based on perturbation theory and should replace those presented by us previously.
Ab initio model potential study of pressure effects on K2NaGaF6:Cr3+
Authors: Seijo Luis , Barandiarán Zoila , Pettersson Lars G. M.
Published: July 2002

In this paper we present the results of an ab initio study of the pressure effects on the 4A2g ground state and the 4T2g and 2Eg excited states of Cr3+-doped K2NaGaF6. Complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on a CrF63− cluster embedded in an ab initio model potential (AIMP) representation of the K2NaGaF6 lattice at ambient and high pressure. The results are in close agreement with the experimentally measured pressure shifts of vibrational frequencies and emission spectra, which demonstrates the ability of the ab initio embedding model potential method to accurately model hydrostatic pressure experiments in this type of defect crystals. It is also shown that if only the standard Madelung embedding potential is used, none of the observed effects of pressure are reproduced.
Analysis of homogeneous nucleation data of polar molecules: Vapor association, dipole orientation, and elongated clusters
Authors: Wright D. , El-Shall M. S.
Published: July 2002

In this paper we present the results of several theoretical models examined in order to explain the observed systematic deviations of the homogeneous nucleation of highly polar substances from the predictions of the classical nucleation theory (CNT). It is found that molecular association in acetronitrile or nitromethane vapor cannot explain the observed higher supersaturations. Similarly, the use of virial coefficient data to obtain a size dependent surface energy correction factor does not reproduce the experimental results. It is shown that orientation of the surface dipoles within the spherical droplets results in higher supersaturations in agreement with the experimental findings. We also generalize the CNT for elongated clusters and show that these clusters can allow for favorable end-to-end and antiparallel alignments of the dipoles. It is found that for reasonable agreement with experiment, acetonitrile, and benzonitrile clusters require a larger degree of elongation compared to nitromethane and nitrobenzene clusters. These results are discussed in relation to molecular simulations of bulk liquids and clusters.
Photochemistry near a semiconductor surface
Authors: Moini Shohreh , Puri Ashok , Das Purna C.
Published: July 2002

A numerical study of photochemistry near a rough InSb surface, whose roughness is modeled as a spheroidal bump protruding out of the substrate plane, has been carried out. The semiconducting nature of the substrate is taken into account via a model dielectric function. ‘‘Shape resonance’’ effects through the resonant coupling of the bump excitations to photons and various substrate modes (plasmons, phonons, etc.) are calculated and included in the study. Unlike in the case of a metal surface, radiation damping as well as damping due to adsorbate–substrate coupling are seen to play a minor role.
Pulse radiolysis studies of ion-electron recombination in gaseous argon
Authors: Cooper Ronald , van Sonsbeek Roger J. , Bhave R. N.
Published: July 2002

Using pulse radiolysis techniques, rates of recombination of Ar2+ ions with electrons in bulk argon were measured over a wide pressure range (150 to 1065 torr) at bulk gas temperatures of 295, 335, and 375 K. No pressure dependence for the total recombination rate constant was observed at any temperature studied. This observation is consistent with the model, whereby the encounter pair is stabilized by electron energy loss while in the Coulomb field of the cation. The presence of a Ramsauer minimum in the electron/argon atom momentum-transfer cross section versus energy profile means that, at low energies, energy loss is very slow. The neutral-assisted three-body recombination mechanism in argon does not significantly enhance the total rate of recombination above the two-body rate.
Adsorbed chain-stretched coil: An unusual phase transition
Authors: Gorbunov A. A. , Skvortsov A. M.
Published: July 2002

An exact expression is obtained for the partition function of a Gaussian chain of arbitrary length adsorbed on a plane with stretching force acting on the chain’s free end. It is found that the increase in the stretching force makes the macromolecule undergo an unusual phase transition from the adsorbed state into a stretched chain state. In the thermodynamic limit, the free energy has a discontinuity in the slope at the transition point, the average number of chain segments contacting with the plane changes abruptly, and finite latent heat is absorbed. However, metastable states are completely absent and the dependence of the thermodynamic potential on the order parameter always has only one minimum. In the vicinity of the transition point, the anomalous slowing down of large scale relaxation takes place. Thus the transition under consideration has simultaneously some features of both first- and second-order phase transitions. Behavior of the finite systems under the transition is discussed.
Properties of fluorobenzene⋅⋅⋅Ar and p-difluorobenzene⋅⋅⋅Ar complexes: Ab initio study
Authors: Hobza P. , Selzle H. L. , Schlag E. W.
Published: July 2002

The potential energy surfaces of fluorobenzene...Ar and p-difluorobenzene...Ar were studied by ab initio calculations with inclusion of second-order Mo/ller–Plesset correlation energy. The optimal structures of both complexes agree nicely with experimental data. The theoretical stabilization enthalpy for the second complex (294 cm−1) agrees well with the experimental data (190–242 cm−1). The fluorine substitution of the benzene molecule does not influence the characteristics of the respective complex. The dipole moment of both complexes was calculated; in both cases the electron transfer from fluorobenzene to Ar was found.
Collisional energy transfer in bimolecular ion–molecule dynamics M++(H2; D2; or HD)→(MH++H; MD++D; MH++D; or MD++H)
Authors: Gutowski Maciej , Roberson Mark , Rusho Jon , Nichols Jeff , Simons Jack
Published: July 2002

Guided ion beam kinetic energy thresholds in the ion–molecule reactions M++H2→MH++H, where M+=B+, Al+, and Ga+ exceed by 0.4–∼5 eV the thermodynamic energy requirements or theoretically computed barrier heights of these reactions. In addition, the formation of MD+ occurs at a significantly lower threshold than MH+ when M+ reacts with HD. Moreover, the measured reaction cross sections for production of MH+ product ions are very small (10−17–10−20 cm2). These facts suggest that a ‘‘dynamical bottleneck’’ may be operative in these reactions. In this work, the eigenvalues of the mass-weighted Hessian matrix, which provide local normal-mode frequencies, are used to identify locations on the ground-state MH2+ potential energy surfaces where collisional-to-internal energy transfer can readily take place. In particular, the potential energies at geometries where eigenvalues corresponding to interfragment and to internal motions undergo avoided crossings are related to the kinetic energies of apparent reaction thresholds. This near-resonance energy transfer model, applied to M++HD reactions, displays the experimentally observed preference to form MD+ at lower collision energies than MH+ as well as the fact that reaction thresholds may greatly exceed thermodynamic energy requirements. This model explains the small reaction cross sections in terms of high energy content and subsequent dissociation of nascent MH+ (or MD+) ions. Although the mass-weighted Hessian matrix is used as a tool in this analysis, the model put forth here is not equivalent to a reaction-path Hamiltonian dynamics approach.
Spin–orbit branching in the photofragmentation of HCl
Authors: Alexander Millard H. , Pouilly Brigitte , Duhoo Thierry
Published: July 2002

The dynamics of the photofragmentation of HCl and DCl, subsequent to A 1Π←X 1Σ+ electronic excitation, is treated exactly based on new multireference, configuration-interaction ab initio calculations of the relevant electronic potential energy curves and off-diagonal matrix elements. The calculated total cross section agrees well with both earlier calculations and experiment. By contrast, the relative cross sections for formation of the two accessible fine-structure channels [Cl(2P1/2) and Cl(2P3/2)] disagree with the most recent experimental results, and, more dramatically, with the results of prior theoretical predictions. Analysis of the redistribution of the photofragment flux, as a function of the H–Cl separation, reveals that the product branching is determined at relatively large HCl distances, considerably beyond the Franck–Condon region, and is governed by the spin–orbit coupling between the initially excited A 1Π state and the Ω=1 components of the a 3Π and 1 3Σ+ states.
Solvent and polymer dynamics in concentrated polystyrene/toluene solutions
Authors: Floudas G. , Steffen W. , Fischer E. W. , Brown W.
Published: July 2002

Depolarized Rayleigh scattering (DRS) and dielectric spectroscopy (DS) are employed to study the solvent and polymer dynamics in the polystyrene (PS)/toluene system and for PS concentration up to 0.6 g/ml. The DRS measurements were made in the temperature range from 255 to 373 K with different interferometers making possible the separation of the ‘‘fast’’ solvent (in the ps time scale) from the ‘‘slow’’ polymer dynamics (in the ns time scale). The DS measurements were made in the temperature range from 128 to 310 K and two processes could also be observed. With DRS and DS we investigate the very different mobilities in the PS/toluene solutions over a frequency range of ∼10 decades. The prominent features of the experimental data are (i) the existence of two primary (α−) relaxations in the macroscopically homogeneous solutions which have been analyzed using both techniques and (ii) that these relaxations reflect the polymer and solvent dynamics and display the usual Vogel–Fulcher–Tammann behavior, however, with a broad distribution of relaxation times, especially at low temperatures. The existence of two time scales in the solutions is discussed in terms of concentration heterogeneities composed of PS-rich and toluene-rich domains, large enough to exhibit two glass transitions. As a result of the strong temperature dependence for the fast solvent reorientation times the rate of change of solvent friction with polymer concentration becomes temperature dependent at low temperatures.
Resonance enhancement effects in coherent two-photon ionization of CH3I
Authors: Strobel Andreas , Lochschmidt Andreas , Fischer Ingo , Niedner-Schatteburg Gereon , Bondybey Vladimir E.
Published: July 2002

One-color two-photon zero kinetic energy (ZEKE)-spectra of CH3I were obtained via an intermediate resonant state. In contrast to one-photon spectra an intense progression in the totally symmetric C–I stretching mode ν3+ is observed. The observation can be explained by a resonant-Raman like enhancement in the intermediate dissociative state.
An efficient new method for calculating eigenvalues and spectra of van der Waals complexes
Authors: Slee Tom , Roy Robert J. Le
Published: July 2002

We present a new method of solving the Schrödinger equation for vibration–rotation levels of van der Waals complexes, which exploits a novel way of avoiding problems associated with poor convergence of radial basis sets. This ‘‘iterative secular equation’’ or ISE method is based on the secular equation/perturbation theory approach of Hutson and Le Roy [J. Chem. Phys. 83, 1197 (1985)]. It begins with a secular equation built from a small radial basis set which provides an initial approximation to the state of interest, and then uses a perturbation theory approach to determine optimal improvements to this initial basis set, iterating until the solution is converged. While it effectively solves the close coupled equations for the system of interest, the growth of computational effort with the number of coupled channels N is distinctly slower than the N3 behavior associated with conventional close coupling calculations. The present implementation also obtains solutions for a few states at a time, a feature that makes it particularly efficient when only a small number of states are of interest. The new method is illustrated by application to truly bound levels of the Ar–HCl complex and to predissociating levels of the He–HF and He–C2H2 complexes.
Correlations between macroions in mixtures of charged and uncharged macroparticles
Authors: Vlachy V.
Published: July 2002

Addition of uncharged or charged macroparticles to a solution of macroions and counterions significantly affects the macroion–macroion correlation function. This effect is studied using the hypernetted-chain integral equation. Two simple models are examined: (i) a multicomponent primitive model treats a solution as a mixture of charged and uncharged hard spheres, (ii) a one-component model where the electrostatic interactions are accounted for by the screened Coulomb potential and the effect of neutral macroparticles is given by an approximate ‘‘volume exclusion’’ potential. Both models ignore the molecular nature of solvent. The calculations presented here indicate that the one-component model is a poor representation of the actual interactions between macroions in the presence of the neutral macroparticles represented as hard spheres. In the second part of the work we study the mixtures of macroions (two positively charged macroions that differ in size) with a common counterion. The radial distribution functions are presented for systems containing mono- or divalent counterions for a range of macroion concentrations.
Ab initio molecular orbital study of the potential energy surface for the HF...ClF binary complex
Authors: De Almeida Wagner B. , Barker David A. , Hinchliffe Alan
Published: July 2002

The potential energy surface for the complex between the HF and ClF monomer units has been comprehensively investigated at the Hartree–Fock self-consistent-field (SCF) level with the 4–31G basis set. Pointwise calculation of a tridimensional surface revealed that several stationary points, characterized as minima and maxima, are present on the minimum energy path. These structures were further fully optimized at the correlated Mo/ller–Plesset second-order perturbation theory (MP2) level of theory employing the 6–31+G** basis set. Zero-point energy corrections were also evaluated, which, together with a treatment of electron correlation, are found to be needed for the correct determination of the global minimum energy structure.
Unusual angular dependence of trapping probability at highly corrugated surfaces
Authors: Wang Y. R. , DeLouise L. A. , Orlowski T. E.
Published: July 2002

We show that the kinematics of gas–surface scattering has two distinctively different regions which bear consequence on the incident angle dependence of the gas/surface trapping probability. The two regions are characterized by 1−μ−2λ≳0 and 1−μ−2λ<0, where λ represents the degree of surface corrugation, and μ is the mass ratio of the gas particle to that of the effective surface scattering unit. When 1−μ−2λ≳0, the gas particles after initial scattering have a high probability of reflection. In this case, the trapping probability exhibits normal behavior, i.e., increases with an increase of the scattering angle. For the other case, the gas particles maintain a mean velocity towards the surface after the initial scattering event, and the trapping probability exhibits unusual behavior, i.e., decreases with increasing the scattering angle. A theory constructed for the latter case shows good agreement with experimental data for Cl2 molecule scattering on the Cl covered GaAs (110) surface.
Transport properties of the hard ellipsoid fluid
Authors: Bereolos Peter , Talbot Julian , Allen Michael P. , Evans Glenn T.
Published: July 2002

Transport properties of isotropic fluids composed of hard ellipsoids of revolution are studied using molecular dynamics simulation. The self-diffusion coefficient, the shear viscosity, and the thermal conductivity are evaluated for a range of densities and elongations and are compared with the results from an Enskog kinetic theory for nonspherical bodies. The full anisotropic pair correlation function, which is required input in an Enskog kinetic theory, can be obtained from simulation or can be approximated. If the pair correlation function is taken as isotropic on the contact surface, with a contact value derived from an accurate equation of state, the resulting kinetic theory transport properties agree to within a few percent of those calculated on the basis of the exact pair correlation function. The simulation and the kinetic theory values for the shear viscosity and the thermal conductivity show the same qualitative behavior, i.e., increasing with density and with particle nonsphericity. Quantitatively, there is good agreement at low densities (up to 30% of closest packing); at higher densities (60% of closest packing), deviations from Enskog theory are larger than and in the opposite direction to those seen for hard spheres. The Stokes–Einstein and Debye relations are tested and indicate a transition from a kinetic theory region towards the hydrodynamic limit as density increases.
Individual spectral densities and molecular motion in polycrystalline hexamethylbenzene-d18
Authors: Hoatson Gina L. , Vold Robert L. , Tse Tak Y.
Published: July 2002

Methods are described for obtaining the orientation dependence of individual motional spectral densities, J1(ω0) and J2(2ω0), from deuterium spin relaxation experiments on polycrystalline materials. Spectral density measurements provide detailed information in a motional regime too fast to be studied by the two-dimensional (2D) exchange method. Their potential as a source of detailed kinetic and geometric information is illustrated for hexamethylbenzene-d18 (HMB). The relaxation behavior of HMB cannot be explained exclusively by six-site jumps around the C6v axis. Agreement between the experimentally determined spectral densities and simulations is improved if the methyl rotation is explicitly included. At ambient temperature the experimental data are best fitted with the simultaneous jump rates, k6=3.85×108 s−1 and k3=5.0×1011 s−1. This is significantly different from the rate determined using a simple six-site jump model, k6=3.9×109 s−1. Geometric distortions of the methyl rotation axes can account for the observed motionally averaged electric field gradient tensor. When these distortions are included in analysis of the spectral density data, there is a small, but significant, improvement in the fit. k3 is unchanged and the best fit k6 is reduced to 2.2×108 s−1, with distortions out of plane by δ=2.5° and in plane ε=ε′=1.202.
The σ* absorption peak at the oxygen 1s edge of O2: Exchange splitting, ultrafast dissociation, and atomiclike Auger spectra
Authors: Kuiper Pieter , Dunlap Brett I.
Published: July 2002

The x-ray absorption spectrum of solid O2 at the O 1s absorption edge is analyzed, using its polarization dependence and the Auger de-excitation spectra. Rydberg states are quenched in the solid, and the exchange splitting of the 1s ionization threshold (1.1 eV) can be observed. Below the ionization threshold, core electrons can be excited into unoccupied antibonding π* and σ* orbitals. We conclude from the relative intensities and from the polarization dependence, that the exchange splitting of the σ* final state is small (<1 eV). This is confirmed by LDA calculations of core-excited states. The calculated σ* potential surfaces are repulsive, which explains the large width of this absorption peak. Because of ultrafast dissociation (∼5 fs), core hole decay is likely to occur at large interatomic distances. Auger spectra at the σ* absorption of O2 do indeed show peaks that would be expected for free core-excited oxygen atoms.
Distance dependence of electronic energy transfer between donor and acceptor adlayers: p-terphenyl and 9,10-diphenylanthracene
Authors: Haynes D. R. , Tokmakoff A. , George S. M.
Published: July 2002

Investigations of energy transfer between adlayers on single-crystal surfaces provide a unique opportunity to explore electronic energy transfer in restricted geometries. In this study, laser induced fluorescence techniques and donor quantum yield measurements were used to examine the distance dependence of electronic energy transfer between donor and acceptor adlayers on Al2O3(0001). The donor adlayer was p-terphenyl, the acceptor adlayer was 9,10-diphenylanthracene, and n-butane was the variable spacer adlayer. The electronic energy transfer rates vs spacer thickness were determined at both 30 and 85 K in ultra high vacuum. The butane spacer experiments showed that the donor energy transfer rate decreased with a 1/d3 dependence, where d is the thickness of the spacer adlayer. Given a Förster quantum mechanical or a Kuhn classical energy transfer mechanism with randomly oriented dipoles, a 1/d3 distance dependence is consistent with resonance electronic energy transfer from a two-dimensional donor adlayer to a three-dimensional array of acceptors. The spacer measurements yielded a critical transfer distance of d0=44 ±4 Å at 30 K and d0=33 ±6 Å at 85 K. The differences in the critical transfer distance at 30 and 85 K could be explained by the redshift in the p-terphenyl fluorescence spectrum at 85 K that reduces the overlap between the donor fluorescence and acceptor absorption spectra. Values of d0=44 Å at 30 K and d0=35 Å at 85 K were calculated theoretically from a 1/d3 analysis and were in excellent agreement with the experimental measurements. The rate of donor–donor intralayer energy migration was also determined by measuring the electronic energy transfer rate versus donor coverage on the acceptor adlayer. The donor quantum yield measurements versus donor adlayer coverage were consistent with the spacer results and indicated that electronic energy migration does not occur within the p-terphenyl adlayer. These results vs spacer thickness and donor coverage reveal that electronic energy transfer in spatially confined geometries can be described using a modified Kuhn energy transfer mechanism.
Pressure dependencies of rotational, translational, and viscous friction coefficients in water-d2, acetonitrile-d3, acetonitrile, chloroform, and benzene
Authors: Wakai Chihiro , Nakahara Masaru
Published: July 2002

Molecular rotational friction coefficients (ζ) were determined for neat water-d2, neat acetonitrile-d3, neat acetonitrile, a 15% solution of chloroform-d1 in chloroform, and a 3% solution of benzene-d6 in benzene by measuring 2H and 14N nuclear magnetic resonance spin–lattice relaxation times as a function of pressure (0.1–300 MPa). The pressure dependencies of the rotational ζ values were obtained from the single-body rotational correlation times for deuterated molecules in each liquid. The pressure dependencies were compared with those of the translational and viscous ζ values derived, respectively, from the known self-diffusion coefficients and viscosities. In such simple molecular liquids as chloroform and benzene, the translational and viscous ζ values had almost the same pressure coefficient or activation volume, whereas the rotational ζ values had considerably smaller pressure coefficients. The fractional viscosity (η) exponent α in the phenomenological linear relation between ζ and ηα was 0.9 for the translational ζ in acetonitrile and 0.4–0.6 for the rotational ζ in acetonitrile (tumbling motion), chloroform, and benzene. Water was found to be exceptional because the pressure dependence of ζ depended more strongly on the modes of molecular motions. The deviation of the viscosity exponent from unity clearly indicates a breakdown of the Stokes–Einstein–Debye law with respect to pressure variations. The viscosity exponent is not universal, but specific to intermolecular interactions and therefore dependent on the liquid structure.
Equilibrium structures and approximate HF vibrational red shifts for ArnHF (n=1–14) van der Waals clusters
Authors: Liu Suyan , Bačić Zlatko , Moskowitz Jules W. , Schmidt Kevin E.
Published: July 2002

This paper presents a theoretical study of the size evolution of equilibrium structures and approximate HF vibrational red shifts for ArnHF van der Waals clusters, with n=1–14. Pairwise additive ArnHF intermolecular potential energy surfaces were constructed from spectroscopically accurate Ar–Ar and anisotropic Ar–HF potentials. The latter depend on vibrational excitation of the HF monomer. The global and energetically close-lying local minima of ArnHF, n=1–14, for HF v=0 and v=1, were determined using simulated annealing followed by a direct minimization scheme. For ArnHF clusters with n≤8, the lowest-energy structure always has HF bound to the surface of the Arn subunit. In contrast, for n≥9, the global minimum of ArnHF corresponds to HF inside a cage. Ar12HF has the minimum-energy configuration of an HF-centered icosahedron, which appears to be unusually stable. Size dependence of the HF vibrational red shift in ArnHF (n=1–14) clusters was investigated by means of a simple approximation, where the red shift was represented by the energy difference between the global minima of a cluster obtained for HF v=0 and v=1, respectively. The approximation reproduced rather accurately the experimentally determined variation of the ArnHF red shift with the number of Ar atoms, for n=1–4, although it overestimated their magnitude. For larger ArnHF clusters, 4<n≤14, a nonmonotonic, step-like dependence of the red shift on the cluster size is predicted, which can be interpreted in terms of changes in the minimum-energy cluster geometries. The predicted red shift for the icosahedral Ar12HF, where the first solvation shell is full, is 44.70 cm−1, which is only 5.4% higher than the experimental HF vibrational red shift in an Ar matrix, of 42.4 cm−1.
Semiclassical treatment of tunneling effects in HONO cis–trans isomerization
Authors: Qin Yue , Thompson Donald L.
Published: July 2002

The classical-plus-tunneling method of Waite and Miller [J. Chem. Phys. 73, 3713 (1980)] has been used to study tunneling effects in the cis–trans isomerization of HONO. The calculated classical-plus-tunneling rate coefficients are two and three times larger than the corresponding purely classical values for trans→cis and cis→trans isomerizations, respectively. The tunneling rate is governed by the rate of energy flow into the tunneling coordinate, the effective mass, and the tunneling frequency. An approximation to the actual torsional potential used in the classical dynamics calculations was developed to give an analytical solution for the Jeffreys–Wentzel–Kramers–Brillouin (JWKB) barrier-penetration integral. The approximation yields accurate results (compared to those obtained by solving the JWKB integral numerically using the true torsional potential) and a considerable reduction in computational expense.