bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical characterization of the minimum energy path for hydrogen atom addition to N2: Implications for the unimolecular lifetime of HN2
Authors: Walch Stephen P. , Duchovic Ronald J. , Rohlfing Celeste McMichael
Published: July 2002

The minimum energy path (MEP) for the addition of a hydrogen atom to N2 is characterized in CASSCF/externally contracted CI calculations using a [4s3p2d1f/3s2p1d] ANO basis set, with additional single point calculations at the stationary points of the potential energy surface using a [5s4p3d2f/4s3p2s] ANO basis set. These calculations represent the most extensive set of ab initio calculations on HN2 completed to date, yielding a zero-point corrected barrier for HN2 dissociation of ≊8.5 kcal mol−1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional transition state theory and a method which utilizes an Eckart barrier to compute one-dimensional quantum mechanical tunneling effects. This study concludes that the lifetime of the HN2 species is very short, greatly limiting its role in combustion processes.
The electronic and molecular structure of C6: Complete active space self-consistent-field and multireference configuration interaction
Authors: Parasuk V. , Almlöf J.
Published: July 2002

Energies and equilibrium geometries have been calculated for different structures of the C6 molecule, using multiconfiguration self-consistent-field (MCSCF) and multireference configuration interaction (MRCI) methods with large basis sets of ANO type. The ground state is linear with a cumulene-like electronic structure of 3Σ−g symmetry. This result is consistent with recent experimental findings, but disagrees with other recent ab initio calculations using perturbation methods.
Formyl fluoride photodissociation: Potential energy surface features of singlet HFCO
Authors: Goddard John D. , Schaefer Henry F.
Published: July 2002

The unimolecular dissociation of formyl fluoride to hydrogen fluoride and carbon monoxide and its unimolecular rearrangement to fluorohydroxymethylene have been examined theoretically. Methods ranged from double zeta plus polarization self-consistent-field to triple zeta plus double polarization coupled cluster singles and doubles. The coupled cluster method including single, double, and linearized triple excitations (CCSDT-1) was applied as well with the double zeta plus polarization basis. The present studies predict a barrier height of ∼47 kcal mol−1 to the molecular dissociation which is ∼11 kcal mol−1 lower than previous theoretical work and in reasonable agreement with experiments that indicate this barrier lies in the range 40 to 43 kcal mol−1. The existence of singlet trans- and cis-fluorohydroxymethylenes at ∼43 kcal mol−1 above formyl fluoride is further confirmed theoretically. The 1,2-hydrogen shift transition state connecting formyl fluoride to the trans-fluorohydroxymethylene lies at ∼77 kcal mol−1. The trans- and cis-fluorohydroxymethylenes are separated by an internal rotation barrier of ∼17 kcal mol−1.
Geometries and energies of electronic states of AsH3, SbH3, and BiH3 and their positive ions
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

Geometries, potential energy surfaces, bond energies, dipole moments, ionization potentials and other properties of MH3 and MH+3 species (M=As, Sb and Bi) are obtained using complete active space self-consistent field (CASSCF) followed by full second-order configuration interaction (SOCI) and relativistic configuration interaction (RCI) methods. Potential energy surfaces of six electronic states of AsH3 and SbH3 and three states of BiH3 are obtained. The ground states of MH3 species are found to be of X 1A1 symmetries (C3v) with pyramidal geometries. The geometrical and other parameters of MH3 species are AsH3[re=1.517 Å, θe=92.2°, D(H2As–H)=74.6 kcal/mol, IP=9.5 eV], SbH3[re=1.719 Å, θe=91.5°, D(H2Sb–H)=63.3 kcal/mol, IP=8.9 eV], BiH3[re=1.865 Å, θe=90.3°, D(H2Bi–H)=51.8 kcal/mol, IP=9.0 eV]. The equilibrium geometries of MH+3 ions are found to be close to planar. For AsH3 the agreement in IP and bond energy between the present theoretical results and the experimental values obtained recently by Berkowitz [J. Chem. Phys. 89, 7065 (1988)] is excellent.
Theoretical characterization of the 5Π and 3Π potential energy surfaces for NH+O→N+OH
Authors: Walch Stephen P.
Published: July 2002

The reactant, product, and saddle point regions of the 5Π and 3Π potential energy surfaces for the reaction NH+O→N+OH have been characterized using complete active space self consistent field/externally contracted configuration interaction (CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets. The computed barrier heights are 5.6 and 11.7 kcal/mol on the 5Π and 3Π surfaces, respectively. Transition state theory with an Eckart tunneling correction is used to estimate the rate constant on the 5Π surface.
Rotational spectra and structures of the OC– and H3N–HCN–HF trimers: Coaxial mixing nozzle for reactive species
Authors: Emilsson T. , Klots T. D. , Ruoff R. S. , Gutowsky H. S.
Published: July 2002

Rotational spectra are reported for several isotopic species of the OC–and H3N–HCN–HF heterotrimers, detected with a pulsed nozzle, Fourier transform, Balle/Flygare microwave spectrometer. Rotational constants for the main isotopic species of the OC trimer are a B0 of 615.574 MHz and DJ of 251 Hz, and for H3N, a symmetric top, a B0 of 1067.161 MHz and DJ and DJK of 0.40 and 63 kHz. Their structures are composites of those reported for the X–HCN and HCN–HF dimers. They are effectively axially symmetric but have some shrinkage from the distances in the dimers. The shrinkages found in r1, the c.m. to c.m. distance for X–HCN are 0.070 and 0.098 Å for X=OC and H3N, respectively, and in r2 for HCN–HF, 0.033 and 0.027 Å. The 14N and H–F hyperfine interactions in OC–HCN–HF are the same as those reported for the HCN–HF dimer. Detection of the X=OC and H3N trimers out of the many species possible required care in their generation. Both were favored by the strongly bonded HCN–HF subunit. The OC–HCN–HF was further enhanced by using a high concentration of CO in the gas expansion. For H3N–HCN–HF a coaxial mixing nozzle was developed to avoid the formation of NH4F(s). The selectivity and simplicity of the nozzle should be helpful in extending the range of species observable with pulsed nozzles.
A split interaction representation for quantum correlation functions of dissociative systems
Authors: Founargiotakis M. , Light J. C.
Published: July 2002

We propose and test a new general approach to the ℒ 2 evaluation of quantum correlation functions for dissociative systems. This method introduces a split interaction representation (SIR) for the correlation function based on formally exact manipulations, and greatly enhances convergence of the correlation function with respect to the range of the ℒ 2 basis, by suppressing the spurious ‘‘reflections’’ from the ‘‘edges’’ of the basis. The method is tested by applying it for the evaluation of flux autocorrelation function from which the quantal thermal rate constant is obtained by integrating it over time. Computations for the one-dimensional Eckart barrier show remarkable convergence of the flux autocorrelation function with respect to both the size and the range of the basis.
Energy dependence of the inelastic process Ba(6s6p1P1)+Ar,He→Ba(6s6p3P1,2)+Ar,He
Authors: Visticot J. P. , Berlande J. , Cuvellier J. , Mestdagh J. M. , Meynadier P. , de Pujo P. , Sublemontier O. , Bell A. J. , Frey J. G.
Published: July 2002

The cross section of the inelastic transfer from Ba(6s6p 1P1) to Ba(6s6p 3P1,2) induced by collision with argon or helium is measured as a function of the collision energy. The existence of a 0.15 eV energy barrier in the formation of Ba(6s6p 3P1) is observed and allows to explain why Ba(6s6p 3P2) was the only product formed in a previous cell experiment.
A potential energy surface for the process H2+H2O→H+H+H2O : A
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o calculations and analytical representation
Authors: Schwenke David W. , Walch Stephen P. , Taylor Peter R.
Published: July 2002

We have performed extensive ab initio calculations on the ground state potential energy surface of H2+H2O, using a large contracted Gaussian basis set and a high level of correlation treatment. An analytical representation of the potential energy surface was then obtained which reproduces the calculated energies with an overall root-mean-square error of only 0.64 mEh. The analytic representation explicitly includes all nine internal degrees of freedom and is also well behaved as the H2 dissociates; it thus can be used to study collision-induced dissociation or recombination of H2. The strategy used to minimize the number of energy calculations is discussed as well as other advantages of the present method for determining the analytical representation.
The electronic and molecular structure of C4: Multireference configuration-interaction calculations
Authors: Parasuk V. , Almlöf J.
Published: July 2002

Multireference configuration-interaction methods have been used to calculate equilibrium geometries for different C4 structures, using large basis sets of atomic natural orbitals. The ground state of C4 is found to be linear, cumulene-like with a 3∑−g electronic state and with an energy 4.1 kcal/mol below the rhombic structure. We show that the choice of basis set plays a crucial role in the determination of the ground-state conformation.
Molecular dynamics simulation of local chain motion in bulk amorphous polymers. II. Dynamics at glass transition
Authors: Takeuchi Hisao , Roe Ryong-Joon
Published: July 2002

The local chain dynamics of bulk amorphous polymers has been studied by means of molecular dynamics simulation in the vicinity of the glass transition temperature Tg. Two models of polymers are used, one mimicking a polyethylene chain and the other a hypothetical freely-rotating chain, both of infinite chain length. The structural relaxations are examined by means of the time-correlation function of vectors embedded in the polymer backbone and of the distribution of the angles by which these vectors reorient after a time interval t. Some degree of chain mobility is seen to persist even as the temperature is lowered to Tg and below. In addition to the rotational diffusion process that takes place as a result of a series of small step motions, some large-angle jump motions are also seen to occur in both models. The jump motions, which are obscured by prevalent faster modes of motions at high temperatures, become clearly revealed in the reorientation angle distributions as the temperature is lowered. In the polyethylene model, the large-angle jump is directly associated with conformational transitions, while in the freely-rotating chain model, in which no torsional barrier exists, the jump probably arises because of the local potential minima created by the surrounding chains. The conformational transitions in the polyethylene model are highly cooperative among the segments neighboring along the chain, especially so as the temperature is lowered through Tg.
Gas–liquid nucleation in Lennard-Jones fluids
Authors: Zeng X. C. , Oxtoby David W.
Published: July 2002

We have applied a nonclassical density functional theory of nucleation to the gas–liquid and liquid–gas transitions of a Lennard-Jones fluid. For the liquid-to-gas transition (cavitation) deviations from classical theory are extremely large: 15 orders of magnitude in rates. For the gas-to-liquid transition (condensation) the deviations are smaller in magnitude but still systematic. Our nonclassical theory agrees with classical theory in its prediction of the dependence of nucleation rates on supersaturation, but it differs in its prediction of temperature dependence. Good agreement is found between our theory and experiments on condensation nucleation of nonane.
On the heat of formation of C5 and higher carbon clusters
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The heat of formation of C5 has been computed ab initio using a variation on G1 theory. The computed ΔH0f = 251.7 ± 4 kcal/mol disagrees quite appreciably with the third-law measurements of Drowart et al.; however, the cause of this is shown to be in the use of very approximate free energy functions by these authors. Using better free energy functions leads to a revised experimental value of 253.3±5.8 kcal/mol, in excellent agreement with our ab initio value. A revision of previous estimates for C6 to C10 by Raghavachari and Binkley is presented.
The energy of N2H2 and related compounds
Authors: Pople J. A. , Curtiss L. A.
Published: July 2002

A b initio molecular orbital theory at the G2 level has been used to study the energy of N2H2 and related compounds. Overall, the agreement between theory and experiment is good. The G2 enthalpy of formation ΔH0f0(N2H2) of 49.6 kcal/mol supports the experimental estimate of ≥46.6 kcal/mol derived by Ruscic and Berkowitz [J. Chem. Phys. 95, xxx4 (1991)] in a recent photoionization study. Predicted dissociation energies are D0(HN■NH)=122.8 kcal/mol, D0(HNNH–H)=43.6 kcal/mol, and D0(H2N■NH–H)=82.1 kcal/mol. The G2 value for the proton affinity (PA) of N2 at 298 K is PA298=118.1 kcal/mol. The G2 ionization potential of N2H3 of 7.54 eV is in agreement with the new value of ≤7.61 eV reported by Ruscic and Berkowitz. The G2 results for the ionization potential of N2H4 and the appearance potential of N+2 from N2H2 are in disagreement with experimental results suggesting that detection of the origins in these cases are thwarted by large geometry changes and significant Franck–Condon effects.
Infrared vibrational studies of CO adsorption on Cu/Pt(111) and CuPt(111) surfaces
Authors: Rodriguez José A. , Truong Charles M. , Goodman D. Wayne
Published: July 2002

Adsorption of CO on ultrathin Cu films supported on Pt(111) has been studied using infrared reflection absorption spectroscopy (IRAS). Our results indicate that the infrared intensities of adsorbed CO are not representative of the relative composition of the Cu<1.0/Pt(111) surfaces. The Cu-bonded CO molecules screen CO molecules bonded to Pt, making them invisible in the infrared spectrum. The ‘‘screening’’ effect depends on the morphology and polarizability of the Cu overlayer. Changes in the morphology of the Cu adlayer produce large variations in the position and line shape of the Cu–CO signal in the infrared spectrum. CO molecules bonded to small Cu clusters show a higher (∼40 cm−1) C–O stretch frequency than CO molecules adsorbed on large Cu islands. The present results were compared with those reported in the literature for the CO/Cu/Ru(0001) and CO/Cu/Rh(100) systems. For CO adsorbed on supported monolayers of Cu, a correlation was found between the strength of the Cu–CO bond, the amount of π backdonation, and the C–O stretch frequency. This correlation cannot be explained using simple models of molecular orbital theory. On the bimetallic surfaces, the C–O stretch frequency is more sensitive to the charge on the metal center to which CO is bonded, than to the electron population of the CO(2π) orbitals.
Ion size effects on the dynamic and static dielectric properties of aqueous alkali solutions
Authors: Wei Yan-Zhen , Chiang Ping , Sridhar S.
Published: July 2002

Dielectric spectroscopy studies of aqueous ionic solutions ACl/H2O (A=Li, Rb, Cs) were carried out at frequencies between 45 MHz and 20 GHz, using recently developed coaxial measurement techniques. The behavior of the static dielectric constant εs0 and the dielectric relaxation time τD of the solutions were studied as functions of ion size and concentration. For moderate concentrations both εs0 and τD decrease linearly with solution conductivity. While the behavior of εs0 can be understood in terms of either static or Hubbard–Onsager kinetic polarization models, the experimental results for τD are at present not understood quantitatively in terms of these models. However we point out the good correlation of the τD data with empirical viscosity results, which suggests an alternative explanation based upon the solution viscosity, modified by ion size effects, which play an important role in the dielectric response. We also discuss the various length scales relevant to dielectric and conductivity processes in the solutions.
Self-consistent polymer integral equation theory: Comparisons with Monte Carlo simulations and alternative closure approximations
Authors: Yethiraj Arun , Schweizer Kenneth S.
Published: July 2002

Recently, Schweizer, Honnell, and Curro reported a self-consistent formulation of the polymer reference interaction site model (PRISM) theory for polymer melts. The purpose of this paper is to compare the predictions of this theory to Monte Carlo simulations of hard chains thereby allowing an independent test of the various approximations in the PRISM theory. We find that the self-consistent PRISM theory with the Percus–Yevick (PY) closure is very accurate for both the intramolecular and intermolecular correlations in hard 20-mers for a wide range of densities. The accuracy of the theory for longer chains is somewhat diminished. We also compare the PRISM theory predictions using the PY, hypernetted chain (HNC), and Martynov–Sarkisov (MS) atomiclike closures. All the closures give equally good predictions at high densities, but the HNC and MS closures exhibit unphysical features, and ultimately fail to converge, at lower densities and/or large chain lengths.
Nonadiabaticity and the competition between alpha and beta bond fission upon 1[n,π*(C=O)] excitation in acetyl- and bromoacetyl chloride
Authors: Person M. D. , Kash P. W. , Butler L. J.
Published: July 2002

This work investigates how molecular dissociation induced by local 1[n(O),π*(C=O)] electronic excitation at a carbonyl functional group can result in preferential fission of an alpha bond over a weaker bond beta to the functional group and how nonadiabaticity in the dynamics drives the selectivity. The experiment measures the photofragment velocity and angular distributions from the photodissociation of acetyl chloride and bromoacetyl chloride at 248 nm, identifying the branching between bond fission channels and the mechanism for the selectivity. The anisotropic angular distributions measured shows dissociation occurs on a time scale of less than a rotational period, resulting in primary C–X (X=Cl, Br) bond fission, but no significant C–C bond fission. While the selective fission of the C–Cl over the C–C alpha bond can be predicted from the adiabatic correlation diagram for this special class of Norrish type I cleavage, the preferential fission of the C–Cl alpha bond over the C–Br bond beta to the carbonyl group would not be predicted on the adiabatic potential energy surface. In bromoacetyl chloride, fission of the C–Cl and C–Br bonds occurs with a branching of 1.0:1.1 (approximately 1.0:0.5 from the 1nπ* transition) compared with a predicted statistical branching ratio of 1:30. This preferential α-bond fission is attributed to a dissociation mechanism on the coupled [n,π*(C=O)] and [n(X),σ *(C–X)] electronic states, a model consistent with the lack of C–C fission and the measured kinetic energy and angular distributions.

The selectivity results from the relative strengths of the electronic coupling between the initially excited [n,π*(C=O)] bound configuration and the two [n(X),σ *(C–X)] states, the weaker coupling inhibiting the adiabatic crossing over the barrier to C–Br bond fission. The results demonstrate the need to go beyond the Born–Oppenheimer approximation to gain predictive ability in any reactive system where the electronic configuration changes along the reaction coordinate, particularly at barriers due to configuration crossings. In addition, the Cl product angular distribution determines the orientation of the 1[n(O),π*(C=O)] transition dipole moment and shows it is governed by the C2v symmetry of the localized carbonyl electronic orbitals and not by the asymmetric substitution at the carbonyl group. Spectra of the Br atoms from direct dissociation at 193 nm help separate the contribution from the overlapping nσ *(C–Br) transition at 248 nm.
The geometry of small fullerene cages: C20 to C70
Authors: Zhang B. L. , Wang C. Z. , Ho K. M. , Xu C. H. , Chan C. T.
Published: July 2002

The ground-state structures of small fullerenes below C70 were determined by tight-binding molecular-dynamics total energy optimization. An efficient simulated annealing scheme was used to generate closed, hollow, spheroidal cage structures for all even-numbered carbon clusters from C20 to C70. As a general trend, fullerenes prefer geometries which separate the pentagonal rings as far apart as possible. Except for C60, C70, and C50, most fullerenes have relatively low symmetries.
Dynamics of third-order nonlinearity of canthaxanthin carotenoid by the optically heterodyned phase-tuned femtosecond optical Kerr gate
Authors: Orczyk Maciek E. , Samoc Marek , Swiatkiewicz Jacek , Prasad Paras N.
Published: July 2002

It is shown that polarization sensitive (optically heterodyned) detection combined with π/2 optical phase biasing between the optical Kerr gate signal and the local oscillator signal provides an effective method for the investigation of dynamics of third-order nonlinear optical processes. The method permits selective enhancement of either the real or the imaginary component of the nonlinear optical response of the medium under study. Both the magnitudes and the signs of the real and the imaginary components of the complex third-order optical susceptibility, contributing to the Kerr gate phenomenon, can thus be determined independently. The method is used to investigate the third-order nonlinear optical response of solutions of canthaxanthin (4,4′-dioxo-β-carotene) in tetrahydrofuran. With the use of ultrafast 60 fs laser pulses at 620 nm, the instantaneous, coherent part of the response is separated from an incoherent, time-delayed response. The real and the imaginary components of the instantaneous complex second hyperpolarizability γ are derived. The analysis of the delayed part of the signal suggests that it originates in an effective χ(5) nonlinear process due to the change of linear susceptibility after population of a two-photon excited state. However, an unusual feature observed is that the two-photon pumped excited state does not produce a significant change of linear susceptibility, but it is a lower state, most probably the 2 1Ag state, subsequently populated by nonradiative relaxation which produces a dominant change in linear susceptibility responsible for the incoherent nonlinear optical response. From a theoretical fit of the temporal behavior of the delayed nonlinear response, the rates of nonradiative population and subsequent decay of the 2 1Ag state are derived. These rates are in qualitative agreement with published spectroscopic results for this class of compounds.
Efficiency and mechanism of electronic predissociation of B state I2–Ar
Authors: Burke M. L. , Klemperer William
Published: July 2002

The isomer of the I2–Ar complex which yields discrete bands in the B←X spectrum is shown, as expected, to be T shaped on the basis of rotational structure observed in the vibronic bands. Precise fluorescence quantum yields for I2–Ar relative to I2 were measured via simultaneous acquisition of absorption and fluorescence excitation spectra in a slit nozzle expansion. These fluorescence quantum yields provide vibrational predissociation efficiencies for B state I2–Ar as a function of vibrational state from v′ of 15 to 26. This is an oscillating function with local maxima at v′ of 16, 22, and 26. For v′=22 and 26, 73%±3% of the complexes undergo vibrational, rather than electronic predissociation. Fluorescence intensities of combination bands with excitation in the van der Waals modes were also found to have oscillating v′ dependencies with patterns nearly identical to that for the bands without van der Waals mode excitations. Thus, these oscillations must arise from the electronic predissociation channel, rather than the vibrational channel. Deconvolution of the lifetime of B state I2–Ar into vibrational and electronic lifetimes indicates that the similar overall lifetimes at v′ of 18 and 21 result from a twofold increase in the electronic lifetime at v′=21, which compensates for a decrease in the vibrational lifetime. Assumption of a smooth v′ dependence for the vibrational lifetime leads to oscillatory predicted overall lifetimes of 35, 77, 82, 51, and 30 ps over the v′ range of 20–24, respectively. Based on symmetry arguments, as well as the observed vibrational predissociation efficiencies, the electronic predissociation of I2–Ar must arise from coupling of the B state to the Πg state. This coupling may also be the dominant channel for collisional quenching of B state I2.
The ultraviolet photochemistry of diacetylene: Direct detection of primary products of the metastable C4H*2 + C4H2 reaction
Authors: Bandy Ralph E. , Lakshminarayan Chitra , Frost Rex K. , Zwier Timothy S.
Published: July 2002

The products of diacetylene’s ultraviolet photochemistry over the 245–220 nm region are directly determined for the first time. At these wavelengths, the photochemistry is thought to proceed from a metastable excited state of C4H2 rather than by direct photolysis. The experimental method employs a small reaction tube attached to a pulsed nozzle. C4H2 is excited within the reaction tube where collisions of C4H*2 with C4H2 form products which are subsequently ionized by vacuum ultraviolet radiation (118 nm) in the ion source of a time-of-flight mass spectrometer. The C4H*2 + C4H2 reaction produces C6H2 (+C2H2), C8H2 (+2H,H2), and C8H3 (+H), all in good yield. An extensive set of experiments is carried out to ensure that the products observed are initial products formed by single-photon excitation of gas phase C4H2. Under certain conditions, secondary products formed by subsequent reaction of the initially formed products with C4H2 are also observed. These are dominated by C10H3 and C12H3. Thermochemical arguments are made which point to C8H3+C4H2 as the source of these secondary products. Collisional deactivation of C4H*2 from its initially excited energy (∼120 kcal/mol above the ground state) to the lower levels of the metastable state (∼74 kcal/mol) is important in determining the relative amounts of C8H2 and C8H3 products. In cases where C4H*2 undergoes extensive deactivation prior to reaction, C8H3+H products dominate. When collisional deactivation is minimized, much of the C8H3 products are formed with enough energy to subsequently dissociate further to form C8H2+2H. Mechanisms are postulated for the observed reactions and some suggestions for further work to assess the importance of these reactions in planetary atmospheres are given.
Ultraviolet absorption and photoconduction spectra of polyimide films fabricated at various curing temperatures
Authors: Kan L. , Kao K. C.
Published: July 2002

The absorption and photoconduction spectra in the ultraviolet region for PMDA-ODA polyimide films fabricated under various curing temperatures have been measured. There are four absorption peaks located at 3.65, 4.35, 5.65, and 6.40 eV in the fully cured polyimide films. The peak at 6.40 eV is practically independent of the curing temperature, but the other three peaks increase with increasing curing temperature. These four peaks are due to intra- and intermolecular optical transitions. In the photoconduction quantum efficiency spectra, there are also four peaks in the fully cured polyimide films but they are located at 3.55, 4.05, 4.90, and 5.80 eV. Analysis of the photoconduction quantum efficiency spectra yields values of 4.17 and 4.37 eV as the potential barrier heights for photoemission from Au and Ag electrodes to polyimide, respectively. The quantum efficiency increases with increasing applied electric field, following closely the Onsager theory with a thermalization separation 30 Å in the uv region. The primary yield increases with increasing curing temperature. All experimental results are briefly discussed.
Combined classical trajectory and quantum scattering analysis of collision-induced vibrational energy transfer. Deactivation of HCl(v=1) in an Ar gas
Authors: Sewell Thomas D. , Nordholm Sture , Miklavc Adolf
Published: July 2002

A simple method for calculating vibrational transition probabilities in three-dimensional atom–diatomic collisions, which we call the classical approach quantum encounter (CAQE) method, is presented. It consists of using suitably initiated classical trajectories as a means of generating ensembles of phase-space points corresponding to the hard part of collisions in which the repulsive core of the interaction potential is sampled; these are then used as input to standard expressions for computing quantum vibrational transition probabilities. The method is applied to study the temperature dependence of the vibrational deactivation of HCl(v=1) in collisions with Ar atoms. It can be generalized, however, to treat vibrational energy exchange in encounters between two polyatomic molecules. Strengths and weaknesses of the method are discussed and comparisons are made with a preceding simpler theory wherein the classical dynamics of the approach was replaced by statistical approximations.
Theoretical characterization of the reaction NH2+NO→products
Authors: Walch Stephen P.
Published: July 2002

The potential energy surface for NH2+NO has been characterized using complete active space self-consistent-field (CASSCF)/derivative calculations to determine the stationary point geometries and frequencies followed by internally contracted configuration interaction (ICCI) calculations to determine the energetics. Production of N2+H2O is found to involve a complex mechanism. The initially formed NH2NO undergoes a 1,3-hydrogen shift to give an HNNOH isomer (with the substituents trans about the NN bond and cis about the NO bond) which undergoes subsequent cis–trans isomerizations about the NN and NO bonds before decomposing to N2+H2O. The saddle point for production of N2+H2O has an approximately rectangular arrangement of one H atom, the two N atoms, and the O atom. This process does not involve a barrier with respect to NH2+NO. Formation of HN2+OH can occur from any of the isomers of HNNOH with no barrier, but the overall process is endothermic by 0.7 kcal/mol [based on the computed Δ Hf0 (0 K) of HN2]. The results obtained in this work are qualitatively the same as previous work, but both the stationary point geometries and energies should be more reliable due to the use of larger basis sets and more extensive inclusion of electron correlation effects.
Ionic transport in polymer electrolytes based on renewing environments
Authors: Druger Stephen D.
Published: July 2002

A theory of ionic transport in polymer electrolytes is presented based on ionic environments reorganizing randomly into one another. This differs from previous work by allowing for Langevin like motion rather than instantaneous hops, by including possible carrier motion attached to moving segments, and by providing for direct relation to free-volume theory, leading to a Vogel–Tammann–Fulcher (VTF) equation for the dc conductivity. Application to the long-range component of the frequency-dependent conductivity shows that the statistical distribution of carrier probability within a confining environment is attained much more rapidly than change from one confining environment into another. Conditions are obtained under which an increased rate of environmental reorganization generally increases conductivity.
A comparison of the structure and dynamics of liquid water at hydrophobic and hydrophilic surfaces—a molecular dynamics simulation study
Authors: Lee Song Hi , Rossky Peter J.
Published: July 2002

The structure and dynamics of liquid water at the interface with three solid surfaces has been investigated via molecular dynamics simulation. The three surfaces include a flat hydrophobic surface, an atomically rough hydrophobic surface, and a contrasting, hydrophilic, fully hydroxylated silica surface. The results of analysis show that, as expected, the solvent near each of the two hydrophobic surfaces behaves essentially equivalently, with loss of hydrogen bonding at the interface. For the hydroxylated surface, surface–solvent hydrogen bonding is stronger than interactions in the bulk solvent, with the nearest solvent layer interacting specifically with up to three surface hydroxyl groups. Nevertheless, distinct structural perturbation of the solvent extends in every case no more than about 10 Å from the surface, and the perturbation is only strong in the immediate solvation layer. Furthermore, the corresponding dynamical perturbation of the solvent, as measured by the diffusion rates and reorientation times in comparison to the bulk, is always relatively small. For the hydrophilic case, it is largest, but even here it is less than a factor of 5 at the immediate interface and less than a factor of 2 in the second hydration layer. The residence time for solvent at the interface is found to be insensitive to the hydrophilicity of the surface. Calculated nuclear magnetic resonance (NMR) order parameters for the solvent are found to reflect solvent orientational ordering, but are shown not to be distinctive of the nature of that order.
Benchmark calculations with correlated molecular wave functions. V. The determination of accurate ab initio intermolecular potentials for He2, Ne2, and Ar2
Authors: Woon David E.
Published: July 2002

Dimer interactions of helium, neon, and argon have been studied using the augmented correlation consistent basis sets of Dunning and co-workers. Two correlation methods have been employed throughout; Mo/ller–Plesset perturbation theory through fourth-order (MP4) and single and double excitation coupled-cluster theory with perturbative treatment of triple excitations [CCSD(T)]. Full configuration interaction (FCI) calculations were performed on He2 for some basis sets. In general, only valence electrons were correlated, although some calculations which also correlated the n=2 shell of Ar2 were performed. Dimer potential energy curves were determined using the supermolecule method with and without the counterpoise correction. A series of additional basis sets beyond the augmented correlation consistent sets were explored in which the diffuse region of the radial function space has been systematically saturated. In combination with the systematic expansion across angular function space which is inherent to the correlation consistent prescription, this approach guarantees very accurate atomic polarizabilities and hyperpolarizabilities and should lead to an accurate description of dispersion forces. The best counterpoise-corrected MP4 values for the well depths of the three dimers are (in microhartrees, empirical values in parentheses) He2, 31.9 (34.6); Ne2, 123 (134); and Ar2, 430 (454). The corresponding CCSD(T) values are He2, 33.1; Ne2, 128; and Ar2, 417. Although these values are very good, the nearly exponential convergence of well depth as a function of basis quality afforded by using the various series of correlation consistent basis sets allows estimation of complete basis set (CBS) limiting values. The MP4 estimated CBS limits are He2, 32.2; Ne2, 126; and Ar2, 447. The corresponding CCSD(T) values are He2, 33.4; Ne2, 130; and Ar2, 430. Equilibrium separations are also reproduced with similar accuracy.
High-field hopping mobility in disordered molecular solids: A Monte Carlo study of off-diagonal disorder effects
Authors: Gartstein Yu. N. , Conwell E. M.
Published: July 2002

In the model of high-field hopping studied extensively by Bässler and collaborators, disorder arising from the varying intersite wave function overlap (off-diagonal disorder) was incorporated by random assignment of an overlap decay rate to each site independently, admittedly an arguable procedure. We have carried out Monte Carlo simulations incorporating assignment of a random overlap to each bond. The assumptions of a Gaussian density of states and a Miller–Abrahams type of elementary jump rate were retained. We find the value and field variation of the mobility to be quite sensitive to the model of off-diagonal disorder considered. However, the results of our simulations may also be fitted by an exponential dependence of mobility on the square root of field for some ranges of field and parameters of the disorder.
The solution of the time dependent Schrödinger equation by the (t,t′) method: The use of global polynomial propagators for time dependent Hamiltonians
Authors: Peskin Uri , Kosloff Ronnie , Moiseyev Nimrod
Published: July 2002

Using the (t,t′) method as introduced in Ref. [J. Chem. Phys. 99, 4590 (1993)] computational techniques which originally were developed for time independent Hamiltonians can be used for propagating an initial state for explicitly time dependent Hamiltonians. The present paper presents a time dependent integrator of the Schrödinger equation based on a Chebychev expansion, of the operator Û(x,t′,t0→t), and the Fourier pseudospectral method for calculating spatial derivatives [(∂2/∂x2),(∂/∂t′)]. Illustrative numerical examples for harmonic and Morse oscillators interacting with CW and short pulsed laser fields are given.
Spectra and relaxation of Hg atoms and molecules in low temperature matrices. III. Hgm and HgmXn (X=H2O and NH3) systems in rare gas matrices
Authors: Crépin C. , Tramer A.
Published: July 2002

Absorption, fluorescence-excitation, and fluorescence spectra as well as fluorescence yields and decay times are recorded for Hg2 molecules in argon matrices pure and doped with small quantities of water or ammonia. All excited electronic states of Hg2 decay rapidly to the lowest metastable A0±g state inducing a long-lived (τ≂200 μs) luminescence with quantum yield close to unity. The gas-to-matrix shifts and the effects of Hg–NH3 and Hg–H2O interactions are discussed in relation to the spectroscopic properties of free, jet-cooled Hg2Arn and Hg2Xn (where X=H2O or NH3) [Chem. Phys. Lett. 170, 153 (1990); Z. Phys. D 20, 73 (1991); J. Chem. Phys. 98, 5281 (1993); Laser Chem. (in press); S. Matrenchard–Barra thesis, Orsay, 1993].
Velocity dependence of free ion production in K(np)–C2Cl4, CS2, and O2 collisions: Internal-to-translational energy transfer
Authors: Popple R. A. , Dionne M. A. , Smith K. A. , Dunning F. B.
Published: July 2002

The rates for free ion production through electron transfer in collisions between K(np) Rydberg atoms and C2Cl4, CS2, and O2 are measured as a function of Rydberg atom velocity for intermediate values of n, 13≤n≤21. The data show that postattachment interactions between the product positive and negative ions are important and can lead to conversion of internal energy from the negative ion, which is formed in an excited state, into translational energy of the product ion pair. This energy conversion occurs without ion–ion neutralization and can stabilize the negative ion against dissociation or autodetachment.
A density functional theory for pair correlation functions in molecular liquids
Authors: Donley James P. , Curro John G. , McCoy John D.
Published: July 2002

We employ density functional methods to derive an integral equation for the two-point intermolecular correlation function in molecular liquids. This radial distribution function is expressed as a two molecule average over a Boltzmann factor involving a ‘‘bare’’ site–site interaction, plus a pairwise additive, intermolecular, medium induced potential which mimics the remaining molecules in the system. This theory is formally exact in the low density limit. While the theory is valid in general for large molecule and polymer liquids, we demonstrate its use here for the case of the simple diatomic liquid. In this application, good agreement is found at all densities for the radial distribution function and equation-of-state when compared with computer simulations. Furthermore, the theory appears to give pressures that are more thermodynamically consistent than those obtained with reference interaction site model (RISM) theory.
Photodissociation dynamics of NH2OH from the first absorption band
Authors: Gericke K.-H. , Lock M. , Schmidt F. , Comes F. J.
Published: July 2002

The dynamics of the photofragmentation of hydroxylamine from its lowest excited electronic state, Ã 1A′, have been investigated. The main dissociation channel leads to H+H+HNO with a quantum efficiency of 1.7 for hydrogen atoms. The H atoms have been analyzed by laser induced fluorescence using a frequency tripled dye laser with sub-Doppler resolution. A sequential decay process is proposed where the first ejected H fragment leaves a highly vibrationally excited intermediate which dissociates after intramolecular vibrational redistribution into H+HNO. Another photodissociation channel leads to OH(X 2Π) and NH2(Ã 2A1). NH2(Ã) has been detected by its emission spectrum, Ã 2A1→X̃ 2B1, indicating strong vibrational excitation of the ν2 bending mode. The OH product shows no vibrational excitation, whereas rotational states up to N=20 have been observed. Observation of the product state distributions and of the 〈μ⋅v〉 and 〈v⋅J〉 correlations yield a qualitative picture of the upper potential energy surface (PES). Out of the nine coordinates characterizing the normal vibrational modes of H2NOH only the NO distance, the NOH bending angle (responsible for OH rotation), and the NH2 bending angle (responsible for NH2 bending motion) are involved in the NH2+OH fragmentation channel.
A self-adaptive multilevel finite element method for the stationary Schrödinger equation in three space dimensions
Authors: Ackermann J. , Erdmann B. , Roitzsch R.
Published: July 2002

An error controlled finite element method (FEM) for solving stationary Schrödinger equations in three space dimensions is proposed. The method is based on an adaptive space discretization into tetrahedra and local polynomial basis functions of order p=1–5 defined on these tetrahedra. According to a local error estimator, the triangulation is automatically adapted to the solution. Numerical results for standard problems appearing in vibrational motion and molecular structure calculations are presented and discussed. Relative precisions better than 1e-8 are obtained. For equilateral H++3, the adaptive FEM turns out to be superior to global basis set expansions in the literature. Our precise FEM results exclude in a definite manner the stability or metastability of equilateral H++3 in its ground state.
Atomic-site-specific nonoptical excitation spectroscopy of chlorofluoromethanes CF4−nCln (n=0–4): Absolute generalized oscillator strength measurement of below-edge electronic excitations of C 1s and Cl 2p shells
Authors: Ying J. F. , Leung K. T.
Published: July 2002

The absolute transition probabilities, more commonly known as absolute generalized oscillator strengths (GOSs), have been determined for selected electronic excitations from the C 1s or Cl 2p shell to the lowest-lying σ* orbitals for chlorofluoromethanes CF4−nCln (n=1–4) using angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy. The GOS profiles of these inner-shell transitions were found to have shapes characteristic of predominant dipole-allowed transitions. Furthermore, the proportions of dipole-allowed components of these GOS profiles appeared to increase with successive chlorination for these molecules. In particular, the GOS profile of the C 1s→σC–Cl* [lowest unoccupied molecular orbital (LUMO)] transition of CF3Cl was found to contain relatively more intense higher momentum-transfer (K) components than those of the corresponding features in CF2Cl2 and CFCl3 (and CCl4). The GOS profiles for the Cl 2p3/2,1/2→σC–Cl* transitions of CF3Cl were found to contain a well-defined secondary maximum at K2∼1.2 a.u., characteristic of predominant nondipole interaction, in addition to the primary dipole-dominated peak at zero momentum transfer. Within the present statistics, our GOS measurements have not, however, revealed any discernible difference between the Cl 2p3/2→11a1 and Cl 2p1/2→11a1 features that could be attributed to spin effects. Furthermore, the dipole-dominated GOS profile was found to become more concentrated in the lower momentum transfer region for the Cl 2p3/2→σC–Cl* transition in CCl4 than that in CFCl3. The present work illustrates the possibilities of atomic-site-specific investigation of the excited states by means of GOS measurements of electronic excitations from localized inner-shell initial states.
Accurate ab initio potential energy surfaces of Ar–HF, Ar–H2O, and Ar–NH3
Authors: Tao Fu-Ming , Klemperer William
Published: July 2002

We present accurate potential energy surfaces for Ar–HF, Ar–H2O, and Ar–NH3 from the supermolecular calculations using Mo/ller–Plesset perturbation theory up to the complete fourth-order (MP4) and efficient basis sets containing bond functions. Preliminary calculations on Ar–HF are given to show the usefulness of bond functions and the stability of the results with respect to the change of the basis set. Detailed MP4 calculations on Ar–HF with a fixed HF bond length of r=〈r〉v=0 give a global potential minimum with a well depth of 200.0 cm−1 at the position R=3.470 Å, θ=0° (linear Ar–H–F), a secondary minimum with a well depth of 88.1 cm−1 at R=3.430 Å, θ=180° (linear Ar–F–H), and a potential barrier of 128.3 cm−1 that separats the two minima near R=3.555 Å, θ=90° (T shaped). Further calculations on the three main configurations of Ar–HF with varying HF bond length are performed to obtain vibrationally averaged well depths for v=0, 1, 2, and 3.

Our primary wells are about 15 cm−1 higher than those of Hutson’s H6(4,3,2) potential for v=0, 1, 2, and 3, and our minimum distances are about 0.05 Å longer. Extensive MP2 calculations (R=3.1–5.0 Å) and brief MP4 calculations (near the radial minimum) are performed for the intermolecular potentials of Ar–H2O and Ar–NH3 with the monomers held fixed at equilibrium geometry. For Ar–H2O, MP4 calculations give a single global minimum with a well depth of 130.2 cm−1 at R=3.603 Å, θ=75°, φ=0°, along with barriers of 22.6 and 26.6 cm−1 for in-plane rotation at θ=0° and 180° respectively, and a barrier of 52.6 cm−1 for out-of-plane rotation at θ=90°, φ=90°. All these are in good agreement with experiment, especially with Cohen and Saykally’s AW2 potential. The dependence of the Ar–H2O potential on an OH bond length is calculated to study the effect from excitation of the bond stretching vibration and the result agrees well with the red shift observed. For Ar–NH3, MP4 calculations give a single global minimum with a well depth of 130.1 cm−1 at R=3.628 Å, θ=90°, φ=60°, along with barriers of 55.2 and 38.0 cm−1 for end-over-end rotation at θ=0° and 180°, respectively, and a barrier of 26.6 cm−1 for rotation about NH3 symmetry axis at θ=90°, φ=0°. All these are in good agreement with experiment and Schmuttenmaer et al. AA1 potential. The effects on potential from the change of the normal NH3 pyramidal geometry to the planar geometry are calculated and the results indicate that the Σ states with tunneling motion perpendicular to the radial coordinate remain virtually unchanged from free NH3 whereas the Π states with tunneling motion parallel to the radial coordinate have the tunneling motion nearly quenched. Comparisons of the potentials for the systems from Ar–HF, Ar–H2O, to Ar–NH3 are made to reveal the periodic trends of bonding and structure in the van der Waals complexes.