bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics study of pressure-induced transition in MgF2
Authors: Nga Y. A. , Ong C. K.
Published: July 2002

The pressure-induced structural phase transition of MgF2 is studied for the first time by constant-pressure molecular dynamics. Our results demonstrate the transformation from low symmetry rutile structure to CaF2-type structure under high pressure, which is in agreement with the experimental observation. Details of atomic configurations evolving with pressure have been examined and a mechanism for the rutile-flourite transition has been proposed.
Scaling behavior near glass instability in mode-coupling model for dense fluids
Authors: Das Shankar P.
Published: July 2002

The scaling relations close to the ideal glass transition point in a self-consistent mode coupling model with realistic structural properties are studied. The behavior of the long time limit of the density autocorrelation function or the nonergodicity parameter over a reasonable density range around the transition point follows a scaling relation with an exponent that is q dependent. Next the solution of the model equations for decay of density correlation over the so-called β-relaxation regime is studied. The results for different wave vectors indicate a substantial q dependence in the exponents of the power law relaxations.
Homogeneous nucleation in supersaturated vapors of polar molecules: Acetonitrile, benzonitrile, nitromethane, and nitrobenzene
Authors: Wright D. , Caldwell R. , Moxely C. , El-Shall M. S.
Published: July 2002

The critical supersaturations (Sc) required for the homogeneous nucleation of acetonitrile, benzonitrile, nitromethane, and nitrobenzene vapors have been measured over wide temperature ranges using a thermal diffusion cloud chamber. At Tr=0.44, the experimental results are higher than the predictions of the classical nucleation theory by 20% (CH3NO2), 50% (CH3CN), 57% (C6H5NO2), and 112% (C6H5CN). This trend correlates well with the dipole moments and to a lesser extent with the polarizabilities of these substances. Corresponding states of simple fluids and scaled nucleation law show that CH3CN and CH3NO2 have similar nucleation behavior which is different from that of C6H5CN and C6H5NO2. Correlations with other less polar substances are examined. The scaled law, with Ω (the excess surface entropy per molecule) determined from the temperature dependence of the bulk surface tension, gives a better description of the experimental results than the classical theory with the exception of nitromethane. The results suggest that the classical theory does not take proper account of the dipole–dipole interaction in calculating the free energy of the embryonic droplets.
A selection rule in SiHn(n=1–4)+F reactions: An ab initio molecular orbital study
Authors: Sato Kota , Yamada Hiroshi , Iwabuchi Susumu , Hirano Tsuneo , Koinuma Hideomi
Published: July 2002

Reactions of SiHn(n=1–4) with a fluorine atom were studied by using an ab initio molecular orbital method [fourth-order Mo/ller–Plesset (MP4/6-21G**//second-order Mo/ller–Plesset MP2/6-21G**)]. Structures of the transition states were determined. In the cases of SiH3+F and SiH+F, fluorine atom addition to a silicon atom was a favorable reaction at the singlet state and abstraction of a hydrogen by a fluorine atom was a favorable route at the triplet state. Abstraction of a hydrogen was also favorable at the doublet state in the case of SiH2+F. A selection rule of these reactions on the basis of the atomic spin density on each atom was proposed. The selection rule proposed was proved to also be applicable to the reaction CH3+F.
A study of the products of the gas-phase reactions M+N2O and M+O3, where M=Na or K, with ultraviolet photoelectron spectroscopy
Authors: Wright Timothy G. , Ellis Andrew M. , Dyke John M.
Published: July 2002

Products of the gas-phase reactions M+N2O and M+O3, where M=Na or K, have been investigated with UV photoelectron spectroscopy and bands have been assigned with the assistance of results from ab initio molecular orbital calculations. For the M+N2O reactions, the observed products were MO+N2. Measurement of the photoelectron bands associated with the metal monoxide MO allowed determination of the first adiabatic ionization energies of NaO and KO. The values obtained were AIE[NaO(X 2Π)]=(7.1±0.1) eV and AIE[KO(X 2Π)]=(6.9±0.1) eV. A similar study of the Li+N2O reaction gave AIE[LiO(X 2Π)]=(7.6±0.2) eV. The reactions M+O3, with M=Na or K, were observed to give MO+O2 as the major reaction products. However, for each reaction, a band was observed which was assigned to the first ionization energy of the secondary reaction product MO2. From the spectra obtained, the first adiabatic ionization energies of NaO2 and KO2 were measured as AIE[NaO2(X 2A2)]=(6.2±0.2) eV and AIE[KO2(X 2A2)]=(5.7±0.1) eV. For both the M+N2O and M+O3 reactions, production of MO A 2Σ+ was found to be favored relative to production of the MO X 2Π state, a result which has important implications in understanding the sodium night glow in the mesosphere.
Computed barrier heights for H+CH2O↔CH3O↔CH2OH
Authors: Walch Stephen P.
Published: July 2002

The barrier heights (including zero-point effects) for H+CH2O→CH3O and CH3O→CH2OH have been computed using complete active space self-consistent field (CASSCF)/gradient calculations to define the stationary point geometries and harmonic frequencies and internally contracted configuration-interaction (CCI) to refine the energetics. The computed barrier heights are 5.6 and 30.1 kcal/mol, respectively. The former barrier height compares favorably to an experimental activation energy of 5.2 kcal/mol.
Photoionization mass spectrometry of CH2S and HCS
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The transient species CH2S and HCS were studied by photoionization mass spectrometry. They were prepared in situ from CH3SH by sequential hydrogen abstraction with fluorine atoms. CH2S was also prepared by pyrolysis of CH3SCl and CH3SSCH3. The photoion yield curve of CH2S displays an abrupt threshold, and is similar in overall shape to that of the homologue CH2O. The adiabatic ionization potential of CH2S is found to be 9.376±0.003 eV. Evidence has been found for nd and/or ns and np Rydberg states converging to the first excited state of CH2S+. In addition, the HCS+ fragment from CH2S has been determined to appear at ≤11.533±0.021 eV at 0 K. In contrast to CH2S, the photoion yield curve of HCS+ from HCS displays a very broad Franck–Condon envelope, consistent with a transition from bent HCS to linear HCS+. A Poisson fit to the experimental Franck–Condon factors indicates that the adiabatic ionization potential of HCS is ≤7.499±0.005 eV, and perhaps as low as 7.412±0.007 eV. The fragment curves at m/e=46, 47, 48, and 49 from CH3SSCH3 have also been examined, and their relative shifts in energy determined. Together with measurements on CH2S and HCS, and the previously reported ΔH○f0 (CH2SH+)=211.5±2.0 kcal/mol (≤213.1±0.2 kcal/mol), this is sufficient to establish ΔH○f0 (CH2S)=28.3±2.0 kcal/mol (≤29.9±0.9 kcal/mol) and ΔH○f0 (HCS)=71.7±2.0 kcal/mol (≤73.3±1.0 kcal/mol), ≥69.7±2.0 kcal/mol). These values are in very good agreement with recent ab initio calculations. The implications for various bond energies within the CHnS system are also discussed.
State-to-state vibrational predissociation of H2–HF and D2–HF. Direct comparisons between theory and experiment
Authors: Bohac E. J. , Miller R. E.
Published: July 2002

The optothermal detection method has been used to measure photofragment angular distributions resulting from the vibrational predissociation of both H2–HF and D2–HF. These angular distributions show resolvable structure which can be related to the final rotational state distributions, including the scalar intermolecular rotational correlations. H2–HF dissociates via a predominantly V–R process, while in D2–HF the open D2 vibrational channel is the main depository for the excess energy. The experimental results are compared directly with the recent time-independent close-coupling calculations of Clary [J. Chem. Phys. 96, 90 (1992)] and the time-dependent golden rule calculations of Zhang, Zhang, and Bačić [J. Chem. Phys. 97, 927; 3149 (1992); Chem. Phys. Lett. 194, 313 (1992)]. The fact that these two systems have very different dynamics, while the potential-energy surfaces are the same, is particularly useful in providing new insights into the nature of the coupling responsible for predissociation.
High resolution spectroscopy of the Cs2 D 1Σu+–X 1Σg+ transition and hyperfine structure
Authors: Kobayashi Tooru , Usui Takashi , Kumauchi Takahiro , Baba Masaaki , Ishikawa Kiyoshi , Katô Hajime
Published: July 2002

The Doppler-free high resolution laser spectroscopy of Cs2 D 1Σu+–X 1Σg+ transition is extended up to v′=65. By comparing the spectral linewidth and the time-resolved fluorescence intensity, the line broadening observed for transitions to the D 1Σu+(v′=63,J′≤70) levels is identified as the lifetime broadening originating from the predissociation. Line splittings are observed for the D 1Σu+(v′=46,J′≥95)–X 1Σg+(v″= 1,J″) transitions and are identified as the hyperfine splitting due to a magnetic dipole interaction between nuclear spin and electron. The hyperfine splitting is attributed to mixing of the (2) 3Πu state, whose wave function changes from Hund’s case (a) to case (b) at large J. The dependence of the electric dipole transition moment on the internuclear distance for the D 1Σu+–X 1Σg+ transition is determined by comparing the observed and calculated line intensities of the dispersed fluorescence.
Quantum calculation of vibrational states in the aniline–argon van der Waals cluster
Authors: Parneix P. , Halberstadt N. , Bréchignac Ph. , Amar F. G. , van der Avoird A. , van Bladel J. W. I.
Published: July 2002

Theoretical calculations of vibrational intermolecular states of the aniline–argon van der Waals complex for J=0 are reported. A fully-quantum method (LCHOP) was used in order to describe the van der Waals cluster. Results in the first two electronic states S0 (X̃ 1A1) and S1 (Ã 1B2) are presented; in the S1 state a comparison with available experimental data is made. We introduce an additive repulsive interaction between N and Ar in the S1 state in order to account for the spectral features observed in larger clusters. Several parametrizations of this term in the potential are discussed with a view to applications to semiclassical simulation of the spectra of the larger An–Arn clusters.
Intramolecular coupling enhanced predissociation in HCCCN–HCN
Authors: Yang X. , Kerstel E. R. Th. , Scoles G.
Published: July 2002

The effect of intramolecular couplings on the dynamics of predissociation in the HCCCN–HCN complex has been studied using optothermal high resolution IR spectroscopy. Two perturbations have been observed in the ν2 band. In both cases, predissociation is enhanced by the perturbations. Because of the particular values of the rotational level spacing and the unperturbed linewidth (lifetime) of this band, perturbations are seen to affect several levels as the perturbing states tune in and out of resonance with the infrared active state. An analysis of the line intensities indicates that one of the perturbations is anharmonic in nature. A simple Fermi’s golden rule two level model for the perturbation around J=10 of the v2=1 state (the one measured with better signal to noise ratio) can not explain the experimental linewidths. Other models involving more than two levels are discussed, but firm interpretation of the data must await for a more complete theoretical analysis which goes beyond the scope of this article.
Liquid-state properties of electrorheological fluids
Authors: Xu B. C. , Hass K. C.
Published: July 2002

We examine a simple model of the liquid state of an electrorheological fluid, with emphasis on the role of many-particle (local-field) effects. The induced dipole moments are treated as internal degrees of freedom. The distribution function for these dipole moments is calculated self-consistently within the mean-spherical approximation (MSA). The present MSA formalism is more rigorous and slightly more general than the ‘‘mean-field’’ treatment by Adriani and Gast [Phys. Fluids 31, 2757 (1988)]. We also use a more accurate MSA correlation function which leads to numerical results suggestive of a phase transition out of the liquid state at a critical value of the dimensionless dipole interaction strength.
Calculation of enthalpy and entropy differences of near-critical binary mixtures with the modified Leung–Griffiths model
Authors: Rainwater James C. , Friend Daniel G.
Published: July 2002

In previous applications of the Leung–Griffiths model as modified by Moldover and Rainwater, many near-critical vapor–liquid equilibrium surfaces have been described successfully in the space of pressure, temperature, density, and composition, but calorimetric properties such as entropy and enthalpy have not been examined. Such calculations are difficult in general because of the need to determine a parameter that is usually unknown in the definition of an independent field variable (the fugacity fraction). However, a generalization for mixtures of the Clapeyron equation is available which allows for the calculation of differences in entropy, enthalpy, and free energies between a pair of (noncoexisting) dew and bubble points at the same temperature and composition. A calculation of entropy differences from a correlation of carbon dioxide+propylene agrees well with results inferred indirectly from experimental data. As a more direct check, calculation of enthalpy differences from a correlation of n-pentane+benzene agrees with an interpolation of enthalpy data provided by calorimetric measurements of Lenoir and Hipkin. The generalized Clapeyron equation appears to be singular at the maxcondentherm point, but it is shown that the singular terms cancel one another and that the leading-order finite term is consistent with a thermodynamic Maxwell relation.
Resonance enhanced multiphoton ionization photoelectron spectra of CO2. III. Autoionization dominates direct ionization
Authors: Taylor David P. , Johnson Philip M.
Published: July 2002

In (3+1) resonance enhanced multiphoton ionization photoelectron spectra (REMPI-PES) of CO2, photoionization competes with dissociation. In addition to direct photoionization, autoionization is possible through accidental resonances embedded in the continuum at the four-photon level. Photoabsorption from these long-lived autoionizing states leads to resonance enhanced above threshold absorption (REATA). REATA produces photoelectron terminations on the C̃ state of CO2+. Previous experiments did not indicate whether the dissociation occurred at the three-photon level or four-photon level. REMPI-PES of CO2 via several Rydberg states have been collected at a number of laser intensities, and it was found that the photoelectron spectra terminating on each individual ionic state do not change over the range of experimentally available laser intensities. This indicates that the dissociation of CO2 occurs at the four-photon level. The long vibrational progressions in the PES indicate that the dominant ionization process is autoionization rather than direct ionization. Relative intensities of the X̃ and C̃ state components of the PES do change with intensity, confirming the C̃ state assignment and its five-photon mechanism.
Dynamics of barrier crossing in solution: Simulations and a hard-sphere model
Authors: Charutz D. M. , Levine R. D.
Published: July 2002

Molecular-dynamics simulations for the Cl+Cl2 activated exchange reaction are reported for all the rare-gas solvents. For all except Xe, the dynamics of barrier crossing are rather similar to that in the gas phase. The role of the solvent being confined to providing the necessary activation at the foothills of the barrier and to deactivating the separating products. In Xe another mechanism becomes operative: a rescaling of the barrier (either by nonsuccessful reactants or by departing products). This caging of the reactants does not, however, lead to successful recrossings of the barrier to reaction. A simple collinear hard-sphere model is able to provide a qualitative and even semiquantitative account of this caging phenomenon including the role of the solvent mass, the number of collisions between the reactants within the cage, and also of the temperature. The use of ‘‘dressed variables,’’ in which the solvent motion is implicitly taken into account, proves very convenient in developing simple models for reactions in solution.
Multipolar electrolyte solution models. II. Monte Carlo convergence and size dependence
Authors: Eggebrecht John , Peters Günther H.
Published: July 2002

Monte Carlo simulations of mixtures of charged and multipolar hard spheres with equivalent diameters are presented. A solution with dipolar solvent at a state point considered earlier, with greatly different results, by Chan et al. [Mol. Phys. 66, 299 (1989)] and by Caillol et al. [Mol. Phys. 69, 199 (1990)] is re-examined. Structural, thermodynamic, and dielectric properties are determined and particle number dependence in these averages is examined. The rate of convergence is shown to be satisfactory for correlation functions, other than that of repulsive ion pairs, and for thermodynamic properties. These are in good agreement with hypernetted chain theory. The reliability of the Metropolis Monte Carlo technique for the evaluation of thermodynamic properties in conveniently small systems of this solution model is established. We also consider a solution in which the solvent has both dipolar and linear quadrupolar symmetry. Two smaller systems give structural and thermodynamic properties in good agreement with hypernetted chain theory. In a larger system ionic clustering occurs to produce very different solution structure.
Brownian dynamics simulation of uniform comb polymers in three dimensions
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

Brownian dynamics simulations are made for continuum, excluded, and nonexcluded volume, uniform H-comb polymers in three dimensions. It is found that the end-to-end distance and radius of gyration of the inner branches are expanded more than those of the outer branches for excluded volume combs. The ratio of the end-to-end distance scaling amplitudes appears to be a universal quantity in agreement with the conjecture of Gaunt et al. [J. Phys. A 19, L811 (1986)]. Moreover, the ratio of the radii of gyration scaling amplitudes seems to be universal. The expansion effect is smaller in three dimensions than in two dimensions.
Microscopic equations of state of polyethylene: Hard-chain contribution to the pressure
Authors: Yethiraj Arun , Curro John G. , Schweizer Kenneth S. , McCoy John D.
Published: July 2002

The athermal contribution to the pressure of polyethylene is investigated via integral equations and mean field generalized Flory-type theories. The molecules are modeled as fused-hard-sphere chains with fixed bond lengths and bond angles; torsional rotations are treated via the rotational isomeric state approximation with literature values for the trans–gauche energies. The hard sphere diameter is obtained by matching structure factor predictions of the polymer reference interaction site model (PRISM) theory for hard chains to data from wide-angle scattering experiments. In all, five hard chain equations of state are investigated: three via different thermodynamic routes in the PRISM theory, and two via different extensions (to fused-sphere chains) of the generalized Flory-dimer (GFD) theory. The integral equation approaches consist of a free energy ‘‘charging’’ route, the compressibility route, and the ‘‘wall’’ route (where the pressure is obtained from the density profile of the fluid at a hard wall). The two GFD approaches correspond to different choices for the reference monomer and dimer fluids required in the theory. Each of the five equations of state results in significantly different predictions for the pressure. The predictions of the various equations relative to each other are nearly independent of chain length, and this allows us to draw conclusions for polymeric fluids (where simulation results are not available) by testing the performance of the equations for diatomics (where simulation results are available). We thus speculate that the charging route overestimates the pressure, the compressibility route underestimates the pressure, and the GFD and wall equations of state are the most accurate.
An accurate short-time small-wave vector approximation for the dynamic structure factor of a Rouse polymer
Authors: Grayce Christopher
Published: July 2002

A nonperturbative approximation and a fortuitous cancellation of errors lead to an accurate, simple approximation for the dynamic single-chain structure factor—or coherent intermediate scattering function—of a Rouse polymer fluid in the short to intermediate time and wave vector region. With this approximation the breakdown at small wave vector of the wave vector to the fourth scaling of the ‘‘Rouse frequency’’ is illustrated, and compared to recent neutron spin-echo measurements on polydimethylsiloxane. Experimental study of this breakdown might provide information about the relation between static structure and dynamic behavior in polymer melts.
Picosecond measurement of substrate-to-adsorbate energy transfer: The frustrated translation of CO/Pt(111)
Authors: Germer T. A. , Stephenson J. C. , Heilweil E. J. , Cavanagh R. R.
Published: July 2002

The transient infrared response of CO/Pt(111) following picosecond visible excitation is reported. A spectrally broad decrease in reflectivity correlates with heating of the Pt lattice, and an observed shift in the CO(v=0→1) transition is interpreted as heating of the 60 cm−1 in-plane frustrated translational mode. A phenomenological three temperature model that assumes the adsorbate vibrational temperature Tads exclusively couples to either the electronic temperature Te (with a time constant τe) or to the lattice temperature Tlat (with a time constant τlat) describes the temporal response of the adsorbate vibrations. The lattice phonon temperature Tlat(z,t) and measured temperature dependence of the optical constants predict the observed spectrally broad reflectivity change. Density matrix methods model the infrared response of the transiently heated molecule. Limits of τe=2±1 ps or τlat<1 ps are established by comparison of predicted spectra and the data.
Charge transfer and collision-induced dissociation reactions of OCS2+ and CO22+ with the rare gases at a laboratory collision energy of 49 eV
Authors: Price Stephen D. , Rogers Steven A. , Leone Stephen R.
Published: July 2002

Product channels for the reactions of OCS2+ and CO22+ with each of the rare gases are determined at a laboratory collision energy of 49 eV. A beam of dications is generated using electron impact ionization and mass selection by a quadrupole mass spectrometer. The dication beam is focused into a collision region and reaction products are monitored using a time-of-flight mass spectrometer. In addition to rare gas ions, we observe S+, CO+, and OCS+ as products from the reactions of OCS2+; O+, CO+, and CO2+ are detected as products from reactions of CO22+. The relative yields of these product ions are measured directly. For both dications, the total reaction cross section increases dramatically as the collision partner is varied from He to Xe. OCS2+ reacts with He and Ne almost exclusively by collision-induced dissociation, while Ar, Kr, and Xe react predominantly by charge transfer. The charge transfer reaction of OCS2+ with Ar populates the stable ground state of the OCS+ ion, while reactions with Kr and Xe populate dissociative electronic states of OCS+ resulting in the formation of S+ ions. CO22+ reacts with He principally by collision-induced dissociation. Charge transfer reactions occur when CO22+ reacts with Ne and Ar, and these reactions populate stable states of CO2+. Kr and Xe react with CO22+ principally by charge transfer, forming unstable states of CO2+ ion which dissociate to give O+ or CO+ ions. The variations in charge transfer reactivity are modeled successfully using Landau–Zener theory.
Single frequency laser probing of velocity component correlations and transport properties of Ba+ drifting in Ar
Authors: Bastian Michael J. , Lauenstein Christian P. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Velocity distributions for Ba+ ions drifting in argon under the influence of an external electric field are measured at directions of 0°, 45°, and 90° with respect to the electric field using single frequency laser-induced fluorescence probing. Values for the reduced mobility, translational temperatures (second central moments), skewness (third central moment), and excess (fourth central moment) are presented as a function of field strength up to E/N values of 201 Td, which corresponds to a mean Ba+/Ar center-of-mass collision energy of 0.371±0.038 eV. Maxima are observed in both the reduced mobility, 2.40±0.05 cm2 V−1 s−1 at 160 Td, and in the skewness, 0.95±0.03 at 119 Td. The dimensionless skewness parameter characterizes the asymmetry of the velocity distributions and is the ratio of the cube root of the third central moment to the square root of the second central moment. A comparison of the moments of the measured 45° velocity distribution to a distribution synthesized without correlation from the 0° and 90° distributions shows that a positive correlation exists between velocity components parallel and perpendicular to the electric field. This is the first experimental verification of velocity component correlation in drifting ions.
On the Meyer–Botschwina–Burton potential energy surface for H3+
Authors: Bramley Matthew J. , Henderson James R. , Tennyson Jonathan , Sutcliffe Brian T.
Published: July 2002

The 87GTO/corrected H3+ potential energy surface of Meyer, Botschwina, and Burton [J. Chem. Phys. 84, 891 (1986)] has been widely used for studies of the H3+ system. It transpires that two slightly different implementations of this surface are in common use. This observation accounts for a number of small discrepancies noted in studies of the higher vibrational states of the ion.
Protein side chain rotational isomerization: A minimum perturbation mapping study
Authors: Haydock Christopher
Published: July 2002

A theory of the rotational isomerization of the indole side chain of tryptophan-47 of variant-3 scorpion neurotoxin is presented. The isomerization potential energy, entropic part of the isomerization free energy, isomer probabilities, transition state theory reaction rates, and indole order parameters are calculated from a minimum perturbation mapping over tryptophan-47 χ1×χ2 torsion space. A new method for calculating the fluorescence anisotropy from molecular dynamics simulations is proposed. The method is based on an expansion that separates transition dipole orientation from chromophore dynamics. The minimum perturbation potential energy map is inverted and applied as a bias potential for a 100 ns umbrella sampling simulation. The entropic part of the isomerization free energy as calculated by minimum perturbation mapping and umbrella sampling are in fairly close agreement. Throughout, the approximation is made that two glutamine and three tyrosine side chains neighboring tryptophan-47 are truncated at the Cβ atom. Comparison with the previous combination thermodynamic perturbation and umbrella sampling study suggests that this truncated neighbor side chain approximation leads to at least a qualitatively correct theory of tryptophan-47 rotational isomerization in the wild type variant-3 scorpion neurotoxin. Analysis of van der Waals interactions in a transition state region indicates that for the simulation of barrier crossing trajectories a linear combination of three specially defined dihedral angles will be superior to a simple side chain dihedral reaction coordinate.
The structure and dynamics of solid benzene. II. Molecular dynamics studies
Authors: Craven C. J. , Hatton P. D. , Pawley G. S.
Published: July 2002

Extensive molecular dynamics (MD) simulations are reported for benzene-d6, based on the potential of Karlström et al. [J. Am. Chem. Soc. 105, 3777 (1983)], linearly rescaled. Three phenomena have been identified as a function of temperature, a solid state structural phase transition, a lattice instability, and melting. The simulations clarify that the first two phenomena are distinct from premelting; they rationalize some of our recent experimental data, and lead to a suggestion for further experiments to substantiate these phenomena. Rescaling the ab initio potential gave lattice parameters and an internal energy in very good accord with experiment across a wide temperature range. The MD lattice instability at 335 K was shown not to be true melting, the melting point being found at 272 K using a system with free surfaces. This result, the latent heat and volume discontinuity at melting, and the thermal motion amplitudes were all found to be in very good agreement with experiment. The first high-pressure phase of benzene was also successfully modeled by the potential, giving excellent values for the structural parameters. A MD result not found in experiment is a disordering transition in which the distinction between the orientations of pairs of molecules in the unit cell is lost. Experimental data appears to show that the same effect would be seen in real benzene were the solid not to melt first. It has also been shown that the application of moderate pressures may make the transition more apparent experimentally.
Ultrafast photodissociation of I3− in ethanol: A molecular dynamics study
Authors: Benjamin Ilan , Banin Uri , Ruhman Sanford
Published: July 2002

A detailed molecular model of I3− photodissociation in liquid ethanol is developed. Extensive molecular dynamics trajectory calculations are used to determine product energy distribution, time-dependent spectra, and reorientation dynamics in semiquantitative agreement with experimental data.
Calculation of vibrational dynamic hyperpolarizabilities for H2O, CO2, and NH3
Authors: Bishop David M. , Kirtman Bernard , Kurtz Henry A. , Rice Julia E.
Published: July 2002

Dynamic vibrational first and second hyperpolarizabilities, applicable to a number of nonlinear optical processes, have been calculated for H2O, CO2, and NH3 at a typical laser frequency. As a percentage of their electronic counterparts, they range from being very small to being on the order of 20–30 %. The static values have also been calculated and are much larger. The calculations were based on perturbation theory and required knowledge of (a) first and second derivatives of the electrical properties (dipole moment, polarizability, and first hyperpolarizability functions) with respect to the normal coordinates, and (b) the harmonic and first anharmonic force constants. These were found at both the SCF and MP2 levels of approximation. In most cases electron correlation has a marked effect on the vibrational hyperpolarizabilities. Anharmonicity contributions are relatively small except for the static case and for the dc-Kerr effect.
Connectedness and clustering of two-phase disordered media for adhesive sphere model
Authors: Lee Sang Bub
Published: July 2002

We present computer simulation results of the pair-connectedness function and the two-point cluster function for random media, consisted of equisized particles of adhesive sphere model. The pair-connectedness function P(r1,r2) is defined as that the quantity ρ2 P(r1,r2)dr1 dr2 represents the probability of finding two particles centered in the volume elements dr1 and dr2 about r1 and r2, respectively, and are physically connected. On the other hand, the two-point cluster function C2(r1,r2) gives the probability of finding two points at positions r1 and r2, in the same cluster of one of the phases. Data are compared with the analytical results from the Percus–Yevick (PY) approximation. In low densities, Monte Carlo data reasonably agree with the PY approximation results, while in high densities near percolation thresholds, data significantly deviate from the analytical results.
Single-photon threshold photoionization of NO
Authors: Wiedmann Ralph T. , White Michael G. , Wang Kwanghsi , McKoy V.
Published: July 2002

Single-photon threshold photoionization spectra for jet-cooled NO have been measured for the v+=0 and 1 vibrational levels of the X 1Σ+ ground state of NO+. The NO+ rotational state distribution for the v+=0 level is shown to be perturbed by nearby autoionizing levels, whereas the v+=1 level exhibits a cation rotational distribution which is in near quantitative agreement with calculated spectra near threshold. Only small changes in total angular momentum are observed (‖ΔJ‖=‖J+−J″‖≤5/2) even though a wide range of photoelectron angular momenta (l=0–3) are predicted to contribute to the near-threshold photoelectron continua. The present results are also discussed in light of recently published two-photon threshold photoionization spectra of NO which exhibit nearly identical NO+ rotational state distributions.
Optimal control of the electric susceptibility of a molecular gas by designed nonresonant laser pulses of limited amplitude
Authors: Shen Liyang , Shi S. , Rabitz H. , Lin C. , Littman M. , Heritage J. P. , Weiner A. M.
Published: July 2002

We present a theoretical study on optimal control of the electric susceptibility change of a homogeneous molecular gas resulting from orientational anisotropy induced by nonresonant lasers with limited intensity. It is assumed that the molecular gas is initially in thermal equilibrium. Two types of optimal control objectives have been considered: terminal control and temporal profile control (i.e., trajectory control). A step function is introduced into the cost functionals which successfully helps to realize the restriction on the magnitude of the field amplitude in numerical optimization, as demonstrated by the examples. Calculations are carried out for CS2 which has a small rotational constant (B=0.1091 cm−1) and a quite large polarizability anisotropy (Δα=9.6 Å3). For terminal control of a maximal susceptibility change at a target time T, it is found that the optimal control field is composed of a series of rectangular pulses with identical amplitudes equal to a preassigned bound value. All of the optimal fields for terminal control are functions of (T−t) over the time interval [0,T] with characteristic time 1/8B and period 1/2B. For temporal profile control, the degree of control is strongly dependent on the length of time interval over which a target profile is defined. Usually, if a time interval is shorter than 1/8B and a target profile is a smooth and non-negative function with a reasonable maximal value, the control can be achieved perfectly. In other cases the detailed assignment of the weight function in the cost functional plays an important role in determining how to make an optimally controlled susceptibility change profile approach the target profile. Furthermore, we have also examined the temperature effects on optimal control in this paper. It can be shown that the general optimal control properties observed by CS2 will also be valid for other linear molecular gases with small rotational constants.
State-to-state photodissociation of the fundamental symmetric stretch vibration of water prepared by stimulated Raman excitation
Authors: David D. , Strugano A. , Bar I. , Rosenwaks S.
Published: July 2002

The state-to-state photodissociation at 193 nm of the fundamental symmetric stretch vibration of water, H2O (1,0,0), is studied. Stimulated Raman excitation and coherent anti-Stokes Raman scattering are used to prepare and detect, respectively, particular rotational states of H2O (1,0,0). Laser induced fluorescence is used for monitoring the OH species which are formed from particularly selected rotational states of the H2O (1,0,0) and also from photodissociation of all occupied rotational states of the ground vibrational state, H2O (0,0,0), at room temperature. The cross section for photodissociation from a particular rotation of H2O (1,0,0) at 193 nm is found to be ∼550 times greater than that for H2O (0,0,0). The formation of the OH product in different rotational, Λ-doublet and spin–orbit states is analyzed for the photodissociation of H2O (0,0,0) and for the photodissociation of the 101, 110+111, 212+211, and 303 rotational states of H2O (1,0,0). The rotational distribution of the OH resulting from photodissociation of H2O (1,0,0) shows a structured distribution that is dependent on the particular rotation of the vibrationally excited state, while that resulting from photodissociation of H2O (0,0,0) presents a smooth distribution. The Λ-doublet ratio in the two spin–orbit states shows preference of the A″ component for photodissociation from the above rotational states of H2O (1,0,0), while only a small preference at high N is observed for photodissociation from the ground vibrational state. The results are compared to available theoretical calculations based on the Franck–Condon model and show reasonable agreement between experiment and theory.
Hybrid quantum/semiclassical wave packet method for molecular dynamics: Application to photolysis of Ar...HCl
Authors: García-Vela A. , Gerber R. B.
Published: July 2002

A hybrid quantum/semiclassical method is proposed and applied to study realistically the dynamics of the three-fragment photodissociation process Ar...HCl+hν→Ar+H+Cl. In the method the hydrogen motion is treated by exact quantum mechanics, while the heavy atoms are described by semiclassical Gaussian wave packets. This treatment is expected to reproduce the main quantum features of the dynamics. Part of the wave packet is found to describe resonance events in which the light particle is temporarily trapped inside the Ar...Cl cage and oscillates periodically between the heavy atoms before it dissociates. Interference between frequency components of the H wave function that populate different resonance levels give rise to interesting quantum effects. Such effects appear in the angular distribution of the hydrogen fragment, which shows some diffraction oscillations, and scattering into classically forbidden regions. Quantum interferences between the resonances are also the cause of a pronounced structure of peaks in the H photofragment kinetic energy distribution (KED). Time-correlation functions of the wave functions involved are computed, and the implications for the absorption spectrum and its relation to the KED of the H atom are discussed. The results demonstrate the power and applicability of quantum/semiclassical time-dependent self-consistent-field (TDSCF) as a tool for studying the dynamics and spectroscopy of realistic molecular systems.
On the vibrational autoionization in NO2
Authors: Châteauneuf F. , Lefebvre-Brion H.
Published: July 2002

The vibrational autoionization for NO2 is evaluated by ab initio calculations and compared to recent experimental results of Grant et al. It is concluded that the resonances are broader for symmetric stretching than for asymmetric stretching as in the case observed, but this conclusion depends of the nature of the Rydberg orbital.
The possibility of superconductivity in quasi-one-dimensional polymers with a carbon skeleton
Authors: Hu Chengzheng
Published: July 2002

The possible ordered phases of quasi-one-dimensional polymers with a carbon skeleton are studied by means of a temperature dependent Green’s function approach. The transition temperature obeys a power law for such polymers. This unusual property is due to the presence of a divergent density of states at the Fermi level. It follows that they may represent a new class of organic superconductors.
Surface tension of C60 solutions in toluene
Authors: Xu Qihua , Liu Xiaohua , Frisch Harry L. , Broderick William E.
Published: July 2002

The surface tensions of toluene and benzene solutions of C60 were measured as a function of temperature and concentration. C60 was found to be surprisingly surface active; at the largest concentration studied C60 occupied nearly a monolayer as indicated by the ‘‘occupied’’ surface diameter of 14 Å.
Electronic structure of diradical and dicarbene intermediates in short-chain polydiacetylene oligomers
Authors: Kollmar Christian
Published: July 2002

Semiempirical molecular orbital (MO) calculations on short-chain (n=2 to n=8 monomer units) diradical and dicarbene intermediates obtained in the solid state polymerization reaction of diacetylene monomer crystals have been performed. With increasing chain length a transition from the diradicals with two unpaired electrons to the dicarbenes with four unpaired electrons is found in agreement with experimental results. The energetic ordering of the low-lying multiplets (singlet and triplet for the diradicals; singlet, triplet, and quintet for the dicarbenes) is discussed in terms of a model Hamiltonian containing a Hubbard term which takes into account the electron transfer between the radical electrons on different chain ends, and a Heisenberg term describing the exchange interaction of radical electrons on the same chain end in the case of the dicarbenes. All intermediates have a singlet ground state. The small energy gaps to the higher triplet and quintet states decrease rapidly with increasing chain length due to a decrease in the ratio t/U, with t and U being the transfer integral and the on-site Coulomb repulsion of the Hubbard model, respectively. The fine structure arising from the magnetic dipolar interaction of the unpaired electrons is discussed.
The orientation of adsorbed methyl halides on a LiF(001) surface: A Monte Carlo study
Authors: Huang Zhi-Hong , Guo Hua
Published: July 2002

Monte Carlo simulations have been carried out to study the adsorption structure of methyl halide molecules (CH3I and CH3Br) on a LiF(001) surface. The ionic substrate is treated as a three-dimensional stationary mesh of point charges and the adsorbed molecules are taken as rigid bodies. The adsorption at different coverages in the monolayer and submonolayer regimes is investigated. The interaction potential between the surface and adsorbate includes electrostatic contributions due to ion–dipole (both permanent and induced) interactions and Lennard-Jones terms that are introduced to account for the remaining interactions. The adsorbate–adsorbate interaction is assumed to be the sum of the permanent dipole–dipole and Lennard-Jones potentials. All the interaction potentials are assumed to be pairwise. The adsorbate–adsorbate interaction is dominated by the Lennard-Jones component, largely due to the halogen–halogen interaction, while Coulomb forces account for a few percent of the total adsorption energy. As a result, the adsorbates tend to form close-packed structures on the surface. The optimal adsorbate structure on the surface is determined by the relative strength of the adsorbate–adsorbate vs adsorbate–substrate interactions. For high coverages close to monolayer, the system has the lowest energy when the molecular axes of the adsorbates are parallel to the surface normal with alternating methyl-up and methyl-down configurations. For an isolated molecule, however, the preferred orientation is perpendicular to the surface normal.
Vibrational relaxation in the lowest electronically excited state of N2 in solid Kr and Xe
Authors: Kuszner D. , Schwentner N.
Published: July 2002

Vibrational relaxation in the A 3Σu+ state of N2 in rare gas matrices has been investigated by selective excitation with an excimer pumped and frequency doubled dye laser system combined with stimulated anti-Stokes Raman scattering. The relaxation is dominated at high N2 concentrations of some percent by nonresonant electronic energy transfer to other randomly distributed N2 molecules and at low concentrations by multiphonon processes. The relaxation by energy transfer can be described with the Förster–Dexter model and exchange interaction. Time resolved data are compared with calculated configuration averaged rate constants. The multiphonon processes can compete with energy transfer only if the N2–N2 spacings exceed several lattice constants. The multiphonon rate constants are comparable to the radiative ones in Kr but are much smaller than the radiative ones in Xe. The increase in radiative rate constants with vibrational quantum number is explained by a stronger mixing with the host excitons due to a better energy resonance.
Dissociation dynamics of doubly- and triply-charged molecules studied by the triple photoelectron–photoion–photoion coincidence method
Authors: Masuoka Toshio
Published: July 2002

The photoelectron–photoion–photoion coincidence (PEPIPICO) technique has been used together with a source of synchrotron radiation to elucidate the dissociation dynamics of doubly- and triply-charged molecules, i.e., carbonyl sulfide (OCS), sulfur dioxide (SO2), and methyl fluoride (CH3F). Using the flight time distributions of the lighter and heavier ions produced in pairs, we determined whether the dissociation occur simultaneously or in sequential stages. It was found that for the three-body dissociation of the type ABC2+→A++B++C, the dissociation mechanism is not dependent on the excitation energies over the range of 46–80 eV. This phenomenon suggests the presence of two site-specific positive holes each of which is related to a particular dissociation pathway. It is additionally concluded that atomization of the precursors is a dominant process in OCS and SO2 at higher excitation energies.
Density functional investigation on the electron affinity of the CFnClm series, n+m=3 and 4
Authors: Gutsev G. L.
Published: July 2002

The results of calculations on the electronic and geometrical structure of the CFnClm and CFnClm− series, n+m=3 and 4, are used for estimating the vertical (EAvert) and adiabatic (EAad) electron affinities of the neutral species and fragmentation energies of the upper decay channels. The calculations were carried out within a local spin density approximation supplemented with a nonlocal gradient correction to the exchange. All the anions considered are found to be stable toward the loss of an extra electron and dissociation, except for CF4− which possesses the negative EAad. All the neutral fluorochloromethanes are shown to have the negative EAvert, whereas the EAvert of CF3 and CFCl2 is positive and that of CF2Cl and CCl3 is close to zero. The EAad value of the CFkCl3−k radicals is found to be closely related to the presence of a CF2 or CCl2 fragment inside a given radical.
Charge and intracule densities in singly excited heliumlike ions
Authors: Cann Natalie Mary , Boyd Russell J. , Thakkar Ajit J.
Published: July 2002

The variation of the charge and intracule densities of singly excited states of two-electron ions with respect to spin multiplicity, nuclear charge, degree of excitation, and angular momentum quantum number is studied systematically. The n 1S, n 3S, n 1P, n 3P, n 1D, and n 3D states with n=3–6 are considered for all the ions from He through Ne8+ using highly accurate explicitly correlated wave functions. Special attention is paid to spin multiplicity differences, that is differences between the densities of a pair of states arising from the same electron configuration of the same ion.