bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionic association in electrolyte solutions: A Voronoi polyhedra analysis
Authors: Gil Montoro J. C. , Bresme F. , Abascal J. L. F.
Published: July 2002

Voronoi polyhedra (VP) analysis of ionic solutions generated via Monte Carlo simulations for a wide range of concentrations and ionic charges are reported. The properties investigated are the VP volumes, VP surface shared with unlike/like-ions and VP nonsphericity configurational mean values distributions. The study shows that high concentrations favor a molten salt like structure while low concentrations tend to disrupt such ordering so small aggregates are more likely. The degree of separation between these forms is strongly determined by the charge of the ions. In the limit of low concentrations/high ionic charge, small clusters with some chainlike character are present which anticipates several features characteristic of the low density liquid-gas transition of the restricted primitive model.
The absorption spectrum of the solvated electron in fluid helium by maximum entropy inversion of imaginary time correlation functions from path integral Monte Carlo simulations
Authors: Gallicchio E. , Berne B. J.
Published: July 2002

The dipole absorption spectrum of an electron in fluid helium is calculated by the maximum entropy method (MEM) numerical inversion of quantum Monte Carlo data obtained from a path integral Monte Carlo (PIMC) simulation at 309 K at the reduced densities ρ*=0.1, 0.3, 0.5, 0.7, and 0.9. Our results agree with the RISM-polaron theory results of Nichols and Chandler [A. L. Nichols III and D. Chandler, J. Chem. Phys. 87, 6671 (1987)] and the grid wave function calculation of Coker and Berne [D. F. Coker and B. J. Berne, J. Chem. Phys. 89, 2128 (1988)]. The method generated the expected long high frequency tail and the low density zero-frequency intensity caused by high conductivity. The method has also been tested by comparing the MEM absorption spectrum to the analytical spectrum of an electron confined in a spherical cavity of fluctuating radius, a model for a solvated electron in a localized state.
Thermal properties of proteins and the random energy model
Authors: Young Robert D. , Powell S. W.
Published: July 2002

We treat the thermal properties of proteins using the Gaussian random energy model (REM) [J. Onuchic and P. G. Wolynes, J. Chem. Phys. 98, 2218 (1993)] as a simple model for the rugged energy landscape of a protein molecule. The REM implies a temperature-dependent entropy and internal energy. The temperature dependence is determined by the width of the internal energy distribution. We show that the REM provides an explanation of the temperature dependence of the equilibrium populations of major conformers of carbonmonoxy-myoglobin.
Time-dependent quantum mechanical study of the photodissociation of HOCl and DOCl
Authors: Offer Alison R. , Balint-Kurti Gabriel G.
Published: July 2002

The results of time-dependent quantum mechanical calculations on the photodissociation of initially nonrotating HOCl and DOCl molecules are presented. Two photodissociation processes, 1 1A″←X̃ 1A′ and 2 1A′←X̃ 1A′, are considered. The dynamics are treated in a two dimensional model with a fixed O–H distance and the dissociation process is followed in a rotating body-fixed frame with total angular momentum of J′=1. The total photodissociation spectra and product rotational distributions resulting from dissociation of HOCl and DOCl originating in a number of different vibrational states are presented over a continuous range of incident photon wavelengths in the range 140–440 nm. It is found that, for HOCl, the physically more correct treatment with J′=1 leads to a qualitatively different OH product rotational distribution than that resulting from the more usual model treatment in which both initial and excited states are assumed to possess zero total angular momentum.
The influence of translational excitation on the dynamics of the reaction between OH and HCN
Authors: Mikulecky Klaus , Gericke Karl-Heinz
Published: July 2002

The dynamics of the reaction OH(2Π)+HCN→CN(2Σ)+H2O has been investigated at different collision energies between 55.7 and 109.5 kJ/mol with the laser pump-and-probe technique. CN is formed in its electronic and vibrational ground state. Product rotational state distributions and line profiles have been obtained. We determined the partition of the available energy to the degrees of freedom of the products. At low collision energies most of the energy is released as translation, while at high collision energies the internal excitation of the newly formed H2O molecule dominates the energy balance. The fraction of CN rotational energy remains nearly constant. The effective transfer of reactant translation to product internal excitation indicates that the reaction surmounts an early barrier. Further, absolute reaction cross sections have been determined at two collision energies.
Homogeneity and isotropy of space requisites for kinetic-energy functional derivatives
Authors: Joubert Daniel P.
Published: July 2002

Homogeneity and isotropy of space lead to two hierarchies of equations for the independent Fermion kinetic-energy functional Ts[n]. The hierarchies link the mth functional derivative of Ts[n] to the (m+1)th functional derivative. The constraints imposed on Ts[n] are different for nonlocal functionals but equivalent for local functionals. As a consequence of the first hierarchy, Ts[n] can be expressed in terms of any of its functional derivatives.
Spatial and temporal decay of localized electrons in solids: One-dimensional model
Authors: Vekhter B. G. , Ratner Mark A.
Published: July 2002

The competing effects of transfer integral (bandwidth) and local vibronic coupling on space- and time-dependent decay of initially localized electrons in anharmonic chains are investigated. The dynamics of electronic localization, caused by electron–vibrational interaction, and the energy redistribution between the electron and lattice is examined and shown to be changed drastically by the appearance of vibrations, localized by vibronic coupling. We consider effects of dynamical disorder, caused by thermal fluctuations, and find that increase in temperature produces diffuse (incoherent) transfer with the conductivity dropping off with temperature increase, as expected in a metal.
The electric field gradient at the N nuclei and the topology of the charge distribution in the protonation of urea
Authors: Aray Yosslen , Gatti Carlo , Murgich Juan
Published: July 2002

A relationship between some of the critical points of the Laplacian of the charge density of the N valence shell and the electric field gradient (EFG) present at the nucleus of the three-coordinated N atom was found for the amine group in urea and some of its adducts and salts. The qzz component of the EFG was shown to be determined by two nonbonded charge distribution local maxima present below and above the molecular plane in the N valence shell. The asymmetry parameter of the EFG was found to be related also to this nonbonded charge concentration. The effect of increasing protonation in urea produced a decrease in the N nonbonded charge concentrations, an increase in bonded concentration along the N–H bond directions and complex fluctuations in the bonded concentration along the N–C bond. The combined effect of the additional H bonds involving the amine group of urea and of the O atom protonation on the N valence shell concentration, is also discussed.
Line shape analysis of the depolarized Rayleigh spectra of CS2/PS mixtures
Authors: Pesce da Silveira N. , Stassen H. , Dorfmüller Th.
Published: July 2002

Mixtures of carbon disulfide and polystyrene with the composition 0, 5%, 20%, 40%, 60%, 80%, 100% by weight of CS2 have been studied using depolarized Rayleigh scattering experiments within the temperature range of 263–313 K. The spectral ranges between 20 and 150 cm−1 were described in the reduced representation by the 3-parameter-Mori-function, indicating that most of the spectral features depend more on the CS2 compound than on the polymer matrix. The concept of analyzing the line shapes of liquid systems by separating the logarithmic spectrum into various distinct regions with characteristic slope parameters was successfully applied to the CS2/polystyrene mixtures and indicates the existence of liquid CS2 microstructures in the mixtures and the applicability of the dipole-induced dipole mechanism to the investigated system.
Infrared laser-induced photofragmentation of the positive and negative ions of size-selected SF6 (NO)n clusters
Authors: Winkel J. F. , Jones A. B. , Woodward C. A. , Kirkwood D. A. , Stace A. J.
Published: July 2002

A detailed study has been undertaken on the infrared photofragmentation of SF6(NO)±n cluster ions. A line-tuneable CO2 laser has been used to excite the ν3 vibrational mode of the SF6 molecule which is followed by the observation of three separate fragmentation channels: –SF6, –NO, and –2NO. The relative intensities of the fragments are found to be sensitive to the sign of the charge on the ion, the cluster size, and whether n is either odd or even. Within clusters of the same charge, the most marked transitions in fragmentation pattern are found between odd- and even-sized cluster ions, with the decay channels favoring those processes which lead to the formation of even electron ions. There are also large differences in fragmentation pattern between the negatively and positively charged ions. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and their shapes confirm a pronounced difference in behavior between SF6 (NO)−n and SF6 (NO)+n. The results for the positively charged ions are interpreted in terms of a central (NO)+3 core which serves to fix the position of the SF6 molecule via an attractive ion-induced dipole interaction. In contrast, observations on the negatively charged ions, suggest that the interaction between the excess electron and the SF6 is predominantly repulsive and also sufficiently diffuse as to keep the molecule comparatively mobile even in large cluster ions. It is suggested that the excess electron in SF6 (NO)−n may occupy a surface state and that the state has associated with it approximately 15 NO molecules.
Experimental and theoretical velocity profiles for pure rotational scattering: CO–hot hydrogen atom collisions
Authors: Ni Chi-Kung , Flynn George W. , Green Sheldon
Published: July 2002

Time dependent diode laser spectroscopy was used to measure Doppler profiles of P-branch lines in the fundamental vibrational band of CO shortly after collisions with H atoms having a translational energy of 2.3 eV. Observed spectral line profiles reflect two Doppler components, a negative room temperature component from molecules scattered out of a given v=0, J state, and a hot component from molecules scattered into the same state. Observed profiles are explained qualitatively using state-to-state differential cross sections calculated from the ab initio potential energy surface of Bowman, Bittman, and Harding and a rigid rotor coupled states quantum scattering approximation.
Gas phase reactions of the sulfur-34 anion with CS2, OCS, and H2S as a function of kinetic energy
Authors: Lee Hack Sung , Bierbaum Veronica M.
Published: July 2002

We have measured rate coefficients and product branching ratios for the reactions of labeled and unlabeled sulfur anions with CS2, OCS, and H2S. These studies were carried out as a function of kinetic energy in the range between 298 K and ∼1 eV using a tandem flowing afterglow-selected ion flow drift tube instrument. For the reaction of S−+CS2 as a function of increasing kinetic energy, the total rate coefficient initially decreases, reaches a minimum at about 0.4 eV, and then increases. Thermoneutral isotope exchange is responsible for the negative energy dependence at lower energies and endothermic sulfur abstraction is responsible for the positive energy dependence at higher energies. For the reaction of S−+OCS vs kinetic energy, the total rate coefficient initially decreases, reaches a plateau, and then increases slightly. Thermoneutral isotope exchange and exothermic sulfur atom abstraction are responsible for the negative energy dependence. For the reaction of S−+H2S, the total rate coefficient has a very weak energy dependence. The strong negative energy dependence of the thermoneutral isotope exchange is counterbalanced by the positive energy dependence of the slightly endothermic hydrogen and proton transfer channels. For these reactions isotope exchange may proceed by formation of an intermediate collision complex, whereas sulfur abstraction may proceed by two different mechanisms, complex formation at lower energies and direct attack on sulfur at higher energies.
Spatiotemporal patterns in an isothermal heterogeneous model of a fixed-bed reactor
Authors: Shvartsman Stanislav , Sheintuch Moshe
Published: July 2002

This work analyzes spatiotemporal patterns that exist in a one-dimensional isothermal fixed-bed reactor with bistable or oscillatory kinetics. The model accounts for an oscillator with a diffusing activator and immobile inhibitor. Patterns emerge due to self-induced gradients of the fluid phase and the patterns are different from those reported for uniform excitable or oscillatory media. Pattern selection is determined by the phase planes spanned by the reactor and the ratio of the two slowest time scales: front residence time and period of oscillations. The main sustained spatiotemporal patterns in the bed were classified as parallel bands, oscillatory fronts (or sticking fronts), and split bands. Parallel bands represent a periodic sequence of pulses and appear when the bed spans the oscillatory domain. Split-band patterns appear when new fronts are generated inside the reactor but they cannot propagate due to refractory trail of the previous pulse.
Lattice structures and molecular tilts in Langmuir monolayers of saturated fatty acid–alcohol mixtures
Authors: Shih M. C. , Durbin M. K. , Malik A. , Zschack P. , Dutta P.
Published: July 2002

We have studied Langmuir monolayers composed of mixtures of heneicosanoic acid and heneicosanol using π-A isotherms and x-ray diffraction. We find that the two materials are miscible in monolayer form. We have determined how the phase diagram of the acid evolves to that of the alcohol as the composition varies. X-ray diffraction data along isotherms and isobars show that the head–head interactions, like the tail–tail interactions, are anisotropic. The ‘‘swiveling transition’’ between phases with different tilt directions is accompanied by a rotation of the molecules about their long axes.
A first principles theoretical determination of the rate constant for the dissociation of singlet ketene
Authors: Klippenstein Stephen J. , East Allan L. L. , Allen Wesley D.
Published: July 2002

State-of-the-art ab initio quantum chemical techniques have been employed in the determination of the reaction path and attendant energetics for the singlet dissociation of CH2CO. Variational RRKM calculations implementing these results provide first principles predictions for the dissociation kinetics which are in quantitative agreement with the corresponding experimental data.
Coverage dependence of sticking coefficient of O2 on Ag(110)
Authors: Vattuone L. , Rocca M. , Boragno C. , Valbusa U.
Published: July 2002

We report on the dependence of the sticking coefficient S of O2 on Ag(110) on oxygen coverage, for molecular and dissociative adsorption. S is found to diminish strongly with increasing coverage. The initial dependence is linear and for dissociative adsorption the slope depends on crystal temperature. The data are indicative of a coverage dependence of the adsorption barrier height, which is caused by the change of the surface work function with coverage. The analysis of the temperature variation of the dependence of S on coverage allows to extract the variation of the ratio of the pre-exponential factors and of the height of the barriers for desorption and dissociation from the molecularly chemisorbed state.
General equations for describing temperature-programmed surface segregation
Authors: Zhdanov V. P. , Norton P. R.
Published: July 2002

Temperature-programmed segregation on the surface and diffusion from the surface into the bulk are shown to be described by an integral equation. This equation is integrated by taking into account that the diffusion coefficient rapidly increases with increasing temperature. The results obtained are used to study the effect of saturation and adsorbate–adsorbate interactions on the transient kinetics with two types of the initial conditions. In the first case, the surface and bulk are in equilibrium in the beginning, and then the surface coverage decreases with increasing temperature due to diffusion from the surface into the bulk. The second class of kinetics corresponds to the situation when in the beginning the surface coverage is much lower than the equilibrium one. In the latter case, the surface coverage first increases with increasing temperature due to segregation on the surface, then reaches maximum, and finally again becomes low.
Large permittivity of computer simulated liquid cyanogen
Authors: Kita Yasuo , Kiyohara Kenji , Oobatake Motohisa , Hayashi Soichi , Machida Katsunosuke
Published: July 2002

In order to explain the large permittivity of liquid cyanogen consisting of centrosymmetric molecules, a molecular dynamics simulation which includes all degrees of freedom of vibration, rotation, and translation has been performed. The intramolecular potential energy is expressed in terms of the internal coordinates with quadratic, cubic, and quartic force constants. The intermolecular potential consists of an atom–atom pair potential with Coulombic and Buckingham terms. The simulation has given an adequate value of self-diffusion coefficient and a typical form of radial distribution function for the liquid state. The simulation has reproduced well the large value of the permittivity observed in experiments, suggesting that the value results from the extraordinarily large atomic polarization brought by the intramolecular bending motion in an antisymmetric mode. The simulation has also suggested that the nitrogen atoms collide with each other more frequently than with carbon atoms and the collision shifts the normal frequencies related to the N≡C bond to higher frequency. The infrared and Raman spectra together with the real and imaginary parts of the refractive index have also been calculated.
Self-consistent Monte Carlo simulation of the electron and ion distributions in the liquid–vapor interface of magnesium
Authors: Gomez M. A. , Rice Stuart A.
Published: July 2002

We present the results of a self-consistent Monte Carlo simulation of the liquid–vapor interface of magnesium. The longitudinal density profile shows stratification in the liquid–vapor transition zone, as has also been found in previous simulations of the liquid–vapor interfaces of alkali metals and mercury.
Saturation predissociation spectroscopy: Vibrational transition moments of HF dimer
Authors: Laush Curtis , Lisy James M.
Published: July 2002

By saturating specific rovibrational transitions in (HF)2 using a high-resolution, single-mode color center laser, the vibrational transition moments for two F–H stretches have been measured regardless of the internal state distribution of the clusters in the molecular beam. Details of the experimental procedure and data analysis are presented. A slight enhancement (∼10%) of the transition moment over the noncomplexed monomer value was observed for the proton acceptor. The proton donor transition moment was substantially enhanced by ∼100% as a result of hydrogen bonding. These results are in excellent agreement with recent theoretical studies. This method is shown to serve as a useful gauge for estimating hydrogen bond interactions within molecular clusters.
Symmetry adapted cluster-configuration interaction study on the excited and ionized states of TiBr4 and TiI4
Authors: Nakatsuji Hiroshi , Ehara Masahiro
Published: July 2002

The symmetry adapted cluster-configuration interaction (SAC-CI) method is briefly reviewed and applied to the excitation and ionization spectra of TiX4 (X=Br, I). The valence excited states of these molecules are investigated systematically and compared with the previous study on TiCl4. The experimental spectra are well reproduced and assigned by the SAC-CI calculation including spin–orbit interaction of the ligand p atomic orbital (AO) and Ti d AO. Nine A1, ten A2, 20 E, and 30 T1 and T2 states are calculated for the excited states, and the oscillator strengths are distributed among the transitions to 30 T2 states, which cause the excitation spectra to be very complicated, especially for the TiI4 molecule. The ordering of the ionized states in the outer valence region is (1t1)−1<(3t2)−1<(1e)−1<(2t2)−1< (2a1)−1, which is the same as that of TiCl4. The spin–orbit splittings in the 2T2 states of TiBr4 and TiI4 are estimated to be smaller than those of the previous studies, and we propose a new assignment for the experimental photoelectron (PE) spectra.
Semiclassical quantization of a nonintegrable system: Pushing the Fourier method into the chaotic regime
Authors: Sohlberg Karl , Shirts Randall B.
Published: July 2002

Semiclassical Einstein–Brillouin–Keller (EBK) quantization of the nonintegrable Hénon–Heiles Hamiltonian succeeds using the Fourier transform method of Martens and Ezra. Two innovations are required for this success: (1) the use of tunneling corrected quantizing actions obtained from an approximate, one-dimensional Hamiltonian and (2) exploitation of intermediate-time approximate quasiperiodicity or ‘‘vague tori’’ wherein the Fourier transform of chaotic motion over 10–100 vibrational periods allows the determination of frequencies and amplitudes which approximate motion during the time interval. Approximate tori, actions, and EBK energy levels are then straightforward. We use an interpolation method to smooth over small resonance zones that are not expected to be important quantum mechanically.
The microwave spectrum, structure, and large amplitude motions of the methylacetylene⋅SO2 complex
Authors: Tan Xue-Qing , Xu Li-Wei , Tubergen Michael J. , Kuczkowski Robert L.
Published: July 2002

Rotational spectra of five isotopomers of the methylacetylene⋅SO2 (MA⋅SO2) van der Waals complex have been observed with a Fourier transform microwave spectrometer. Each species showed two sets of rotational transitions, one associated with the A (m=0) and the other with the E (m=±1) methyl group internal rotation states. The rotational transitions of the isotopomers with S 16O2 and the doubly substituted S 18O2 also showed inversion splitting ranging from tens of kHz to a few MHz. This splitting was absent in the S 16O 18O isotopomers. The spectra of these species have been assigned and fit, yielding rotational constants, which allowed a complete determination of the structure of the complex. The SO2 was found to sit above the carbon–carbon triple bond, with one of the S–O bonds roughly parallel to the symmetry axis of methylacetylene. The centers-of-mass distance between the two monomers was determined to be 3.382(10) Å. The center frequencies of the inversion doublets (or quartets) were used in a fit of both the A and the E transitions; the barrier hindering the internal rotation of the methyl group was determined to be 62.8(5) cm−1. Based on the dependence of the inversion splitting on the transition dipole direction and isotopic substitution, the inversion motion was identified as an ‘‘in plane’’ wagging of the SO2 relative to methylacetylene. A pure inversion splitting of 3.11 MHz (free from rotation) was extracted from the A-state spectrum of the normal species, from which an inversion barrier height of about 63 cm−1 was estimated.
Ab initio study of nonadditive interactions in the Ar2HF and Ar2HCl clusters. II. Analysis of exchange and induction effects
Authors: Cybulski S. M. , Szcze̢śniak M. M. , Cha /asiński G.
Published: July 2002

The nonadditive effects are studied in Ar2HX (X=F, Cl) clusters using the supermolecular Mo/ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. The range of geometrical variations include the rotation of HX within the plane of the cluster and perpendicular to it. The three-body effect is rigorously dissected into the three fundamental nonadditive components: exchange, induction, and dispersion. The exchange nonadditivity is further divided into two terms: ES3, the electrostatic interaction between the HX and Ar2 subunits due to the appearance of exchange-induced quadrupole moment on Ar2 [Cooper and Hutson, J. Chem. Phys. 98, 5337 (1993)], and its exchange counterpart X3. ES3 is physically interpreted as originating from the interatomic electron exchanges within the Ar2 part. The X3 term originates from triple exchanges among all three monomers. The induction nonadditivity is dominated by the third-order effect describing the interaction of moments induced on the Ar atoms by the field of HX. This effect is faithfully represented by the multipole approximation.
CH stretching local mode behavior in alkanes: A global analysis of mono- and fully hydrogenated ethanes, propanes, and butanes up to 6 quanta of CH stretching
Authors: Duncan J. L. , New C. A. , Leavitt B.
Published: July 2002

Existing vibrational data for monohydrogenated C2 to C4 alkanes up to 4 quanta in CH stretching, and data at 6 quanta of excitation in the fully hydrogenated species, are combined in this study, along with CH deformation overtones and CD2 (or CD) stretching fundamentals, to enable a global local mode refinement of CH stretching to be made, to include the effects of both Fermi resonance and interbond coupling effects. Refinement to data over 14 isotopic ethanes, propanes and isobutanes is achieved in terms of consistent CH stretching vibration and anharmonicity parameters and stretch–bend resonance parameters. Data available on n-butanes fully support the findings. CH stretching vibration wave numbers correlate very closely with the structural characteristics in CH3, CH2, and CH groups. The associated CH stretching anharmonicities, however, are found to correlate with the type of CH bond present, primary (−59.6 cm−1), secondary (−62.1 cm−1), or tertiary (−64.5 cm−1), rather than with the individual CH stretching vibration wave numbers, as previously thought. A vibrational dependence of the Fermi resonance parameter is required to reproduce the isolated CH stretching manifolds of CH(CD3)3 and CH(CH3)3 up to 6 quanta in CH stretching.
Rotations and local modes in stannane, 116SnH4: Photoacoustic overtone spectra with a titanium:sapphire ring laser
Authors: Zhan Xiaowei , Halonen Marjo , Halonen Lauri , Bürger Hans , Polanz Oliver
Published: July 2002

The fifth and the seventh stretching vibrational overtone bands of a monoisotopic stannane sample, 116SnH4, have been recorded with Doppler limited resolution using intracavity photoacoustic technique with a titanium:sapphire ring laser. Both band systems resemble closely symmetric top parallel bands in agreement with local mode predictions. The rotational fine structures of these bands have been analyzed with the symmetric top energy level formula and with a spherical top Hamiltonian which treats the problems as interacting rotational levels of A1 and F2 vibrations. Altogether about 200 vibration–rotation transitions have been assigned for both band systems. The fifth overtone band system shows some minor local perturbations due to interactions with bending states. The seventh overtone band is completely unperturbed. The observed vibration–rotation parameters are in good agreement with the ones calculated from a simple vibrational model.
Evaluation of the vibration-rotation-tunneling dynamics at the basis set superposition error corrected global minimum geometry of the ammonia dimer
Authors: Muguet Francis F. , Robinson G. Wilse , Bassez-Muguet M. Palmyre
Published: July 2002

With the help of a new scheme to correct for the basis set superposition error (BSSE), we find that an eclipsed nonlinear geometry becomes energetically favored over the eclipsed linear hydrogen-bonded geometry. From a normal mode analysis of the potential energy surface (PES) in the vicinity of the nonlinear geometry, we suggest that several dynamical interchange pathways must be taken into account. The minimal molecular symmetry group to be considered should be the double group of G36, but still larger multiple groups may be required. An interpretation of experimental vibration-rotation-tunneling (VRT) data in terms of the G144 group, which implies monomer inversions, may not be the only alternative. It appears that group theoretical considerations alone are insufficient for understanding the complex VRT dynamics of the ammonia dimer.
Artificial boundary inhomogeneity method for quantum scattering solutions in an ℒ𝓉2  basis
Authors: Jang Hyo Weon , Light John C.
Published: July 2002

A novel method for quantum reactive scattering calculations is introduced and tested for simple model problems. It is shown to be efficient and quite accurate. The method is based on a simple modification to the time independent Schrödinger equation, (H−E)Ψ=0. It is obtained by setting (H−E)Ψ=B where B is a localized boundary inhomogeneity. A necessary and sufficient number of arbitrary linearly independent wave functions represented by a real ℒ 2 basis set over a finite range of scattering coordinate are generated. The subsequent analysis of the wave functions using a point fitting technique or flux amplitude evaluations produces the full S matrix. The real matrix representation of Green’s operator and energy independent integrals involved promise an efficient calculational method. Even for multiarrangement reactive scattering, only an ℒ 2 basis defined on a single coordinate system is needed.
Quantum state-selected photodissociation of K2(B 1Πu←X 1Σ+g): A case study of final state alignment in all-optical multiple resonance photodissociation
Authors: Ji Bing , Kleiber Paul D. , Stwalley William C. , Yiannopoulou Alexandra , Lyyra A. Marjatta , Julienne Paul S.
Published: July 2002

Polarized emission resulting from the quantum state-selected photodissociation of K2 through quasibound levels of the B 1Πu state has been studied experimentally by high-resolution all-optical triple resonance techniques, and theoretically by generalized quantum defect theory using the adiabatic/recoil model for the dissociation dynamics. Observation of alignment in the initial state (from the photopreparation steps) and in the photofragment (from the photodissociation step) provides detailed insight into the dynamical evolution. Excellent agreement between the theoretical calculations and the experimental results is shown.
Atomization enthalpy and enthalpy of formation of gaseous Si2 and Si3 from mass spectrometric equilibrium measurements
Authors: Schmude R. W. , Ran Q. , Gingerich K. A. , Kingcade J. E.
Published: July 2002

Recent experimental and theoretical studies have allowed us to calculate new thermal functions for the Si2 and Si3 molecules. We have determined the atomization enthalpies and enthalpies of formation of Si2 and Si3 from mass spectrometric data and have combined them with reevaluated previously published mass spectrometric data using a consistent set of ionization cross sections and thermal functions. The selected atomization enthalpy and enthalpy of formation for the Si2 molecule are ΔHa,0=319±7 and ΔHf,0=584±7 kJ mol−1, the corresponding values for Si3 are 705±16 and 649±16 kJ mol−1. Reevaluated values for Si4 are 1151±22 and 654±22 kJ mol−1. Experimental literature values (Neumark and co-workers) for the electron affinities of Si2–Si4 have been combined with the present results to obtain enthalpies of formation and binding energies for the anions Si−2–Si−4. The experimental atomization enthalpies are compared to recent theoretical studies.
Direct-inelastic scattering of chlorine molecules from cesiated Si(100)
Authors: Namiki A. , Hayashi H. , Ukai Y. , Geuzebroek F. H.
Published: July 2002

The interaction of Cl2 molecules with the cesiated Si(100) has been investigated by probing the dynamics of scattered Cl2 molecules escaping from sticking. The observed time of flight distributions for the zero Cl coverage regime are ascribed to direct-inelastic scattering even for the very low incident energy of 0.058 eV. This fact suggests that the interaction potential of the Cl2 molecules with the Cs-covered surface is repulsive up to the top of the barrier for sticking. An explanation in terms of Pauli repulsion is proposed.
Energy relaxation of localized excitations in solid argon
Authors: Cenian A. , Hennig S. , Gabriel H.
Published: July 2002

The mechanisms of energy transfer from a single excited lattice particle (an energy spike) to the bulk crystal are investigated in detail. An argon matrix built up by nearly 3000 atoms serves as a model system and the molecular dynamics method is used to study the energy flow through the lattice on a femtosecond time scale. Excitation energies up to 4 eV have been used and the energy transport has been found to proceed most effectively via shock waves along 〈110〉 crystalline directions. A theoretical model is employed, which confirms the shock wave character of energy transport and shows even quantitative agreement with the numerical results.
Stimulated emission pumping spectroscopy via two-color resonant four-wave mixing
Authors: Williams Skip , Tobiason Joseph D. , Dunlop James R. , Rohlfing Eric A.
Published: July 2002

We present a combined theoretical and experimental study of the application of two-color resonant four-wave mixing (RFWM) to stimulated emission pumping (SEP) spectroscopy. The theoretical approach employs time-independent, diagrammatic perturbation theory and a spherical tensor analysis in an extension of a recent treatment of degenerate four-wave mixing [Williams, Zare, and Rahn, J. Chem. Phys. 101, 1072 (1994)]. The resulting signal expression for two-color RFWM separates the molecular properties from purely laboratory-frame factors determined by the polarizations of the input beams and the rotational branch types of the SEP PUMP and DUMP transitions. This expression is valid in the limit of weak fields and for molecules in which the total angular momentum (omitting nuclear spin) is a good quantum number. In addition, we demonstrate that the spectral response for tuning the DUMP laser is a simple Lorentzian in free-jet experiments. We test our theoretical results and demonstrate the applicability of RFWM-SEP to jet-cooled, transient species in experiments on C3 and HCO. Using the well-studied Ã 1Πu–X̃ 1Σ+g system of C3, we illustrate and compare the two possible schemes for RFWM-SEP. These are defined as ω1=ω2 (PUMP) and ω3=ω4 (DUMP) or ω1=ω4 (PUMP) and ω2=ω3 (DUMP), where ω1, ω2, and ω3 are the input frequencies and ω4 is the signal frequency.

Using the B̃ 2A′–X̃ 2A′ system of HCO, we obtain RFWM-SEP spectra that probe ground-state vibrational resonances lying above the low threshold for dissociation to H+CO. Varying the polarization of the input beams or PUMP rotational branch produce dramatic effects in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well-described by our theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation are consistent with the theoretically predicted Lorentzian line shape; the full widths for these levels are in good agreement with those determined via unsaturated fluorescence depletion SEP.
Rydberg state dynamics of rotating, vibrating H3 and the Jahn–Teller effect
Authors: Stephens J. A. , Greene Chris H.
Published: July 2002

We extend a recently developed multichannel quantum defect theory (MQDT) of electron half-collisions with a triatomic ion. The calculations reproduce much of the observed photoabsorption spectrum of H3 near its lowest ionization thresholds. The method utilizes a rovibronic frame transformation that accounts simultaneously for vibrationally and rotationally inelastic collisions (preionization) and for l-uncoupling effects. Jahn–Teller interactions among degenerate Rydberg states play a crucial role in the formulation. Although H3 is a weak Jahn–Teller molecule, this interaction is responsible for major resonance features seen in the experimental photoabsorption spectra. Calculations over an extended photon energy range for photoabsorption from the H3 (1s23s 2A1′, vi=0, Ni=1, Ki=0) initial state into final states with total angular momentum Nf=0,1,2 are given for parallel and perpendicular (Mi=0, ‖Mf‖=0,1) polarization schemes for the excitation. In spectral regions where experiments have been carried out, semiquantitative agreement with experiment is obtained. The limitations of the present theory are briefly discussed.
Study of excited states of polyethylene in the Hartree–Fock, Tamm–Dancoff, and random-phase approximations
Authors: Vrac̆ko Marjan , Champagne Benoît , Mosley David H. , André Jean-Marie
Published: July 2002

We report calculations of the excitonic spectra of all-trans polyethylene in the Hartree–Fock, Tamm–Dancoff, and random-phase approximations. For the formulation of approximations beyond Hartree–Fock level, we have used the particle–hole propagator technique. In the first case the interaction between the excited electron and hole is neglected and in the latter two cases it is taken into account at first order in electron–electron interaction. Using the 3–21G atomic basis set, the elementary gap is reduced from about 17 eV at the Hartree–Fock level to 13 eV in the Tamm–Dancoff approximation. The spectra in both approximations are also different. The difference between the results obtained in the Tamm–Dancoff and the random-phase approximations is very small. We discuss the densities of exciton states for different exciton quasi momenta (K=0 or K=π/a), different band interactions and different spins.
Experimental transverse diffusion and Monte Carlo simulation studies of Rb+ swarm drifting in He gas
Authors: Ong P. P. , Tan T. L.
Published: July 2002

Experimental measurements have been made of the transverse diffusion of Rb+ swarms drifting in He gas at E/N (drift field to gas concentration ratio) values ranging from 5 to 500 Td, and at gas temperatures between 302 and 304 K. Results with typical errors of ±3% are reported. In addition, Monte Carlo simulation (MCS) calculations were made of the mobility, transverse, and longitudinal diffusion coefficients of Rb+ in He using two recently proposed interaction potentials. Comparisons of the MCS results with available experimental data give an indication of the validity and accuracy of the respective assumed interaction potentials at different interatomic distances. Further calculations of the transverse and longitudinal diffusion coefficients from known mobility values using the generalized Einstein relations based on the three-temperature theory suggest that the theory is fairly accurate at all E/N values despite the high ion-neutral mass ratio.
Spectroscopy of benzene dimers in the algebraic model
Authors: Iachello Francesco , Oss Stefano
Published: July 2002

We study the spectroscopy of benzene dimers within the algebraic model. We report results of calculations of the infrared and Raman spectrum in the region of the CH stretching fundamentals ν7, ν13, ν20 and ν2. On the basis of these calculations, we suggest that the precise geometric form of the dimer can be inferred from detailed vibrational spectroscopy.
Photodissociation dynamics of the methyl radical 3s Rydberg state
Authors: North Simon W. , Blank David A. , Chu Pamela M. , Lee Yuan T.
Published: July 2002

The photodissociation dynamics of methyl radical have been investigated at 193.3 nm using photofragment translational spectroscopy. The formation of CH2 and H(2S) was the only dissociation pathway observed. Although it is not possible to assign the spin state of the methylene unambiguously, we believe that methylene is produced predominately in the ã 1A1 excited state. The translational energy distribution of the products is peaked at ∼13 kcal/mole which is consistent with the magnitude of the exit barrier on the excited state potential energy surface. The breadth of the distribution suggests that the methyl radicals dissociate from a wide range of geometries. From the photofragment angular distribution an anisotropy parameter of β=−0.9±0.1 was determined.
Global phase behavior based on the simplified-perturbed hard-chain equation of state
Authors: van Pelt A. , Peters C. J. , de Swaan Arons J. , Deiters U. K.
Published: July 2002

The equation of state that results from the simplified-perturbed hard-chain theory (SPHCT) has been used to calculate phase diagrams for binary fluid mixtures and to classify these phase diagrams in accordance with the system of van Konynenburg and Scott. For molecules with equal or similar sizes, the global phase diagrams are similar to the ones obtained with the van der Waals, Redlich–Kwong, and Carnahan–Starling–Redlich–Kwong equation of state. In addition to the types I–V, one can calculate also types VI, VII, and VIII with the SPHCT equation. For molecules with large size differences two new, main types of phase behavior have been discovered. We propose to call them type IX and X.
The effect of incident energy upon adsorbate structure for Cl2 chemisorption onto Si(111)-7×7 surfaces
Authors: Yan Chun , Jensen John A. , Kummel Andrew C.
Published: July 2002

Scanning tunneling microscopy (STM) and molecular beam techniques are used to investigate the effect of adsorption mechanism upon adsorbate structures for Cl2 chemisorption onto Si(111)-7×7 surfaces. At incident energies less than 0.11 eV, the SiCl island formation as well as isolated-site reaction are observed. STM images of the internal structure of the islands show that the SiCl islands nucleate at sublimation defects and not at step edges. Conversely, for 0.44 eV dosing, only the isolated-site reaction occurs. It is proposed that the island formation results from precursor mediated chemisorption and that the isolated-site reaction results from direct activated chemisorption. For direct activated chemisorption, the competition between Cl-atom abstraction and dissociative chemisorption was also observed. The abstraction probability decreases with increasing Cl2 incident translational energy.
Direct ab initio dynamics studies of vibrational-state selected reaction rate of the OH+H2→H+H2O reaction
Authors: Truong Thanh N.
Published: July 2002

We present direct ab initio dynamics studies of vibrational-state selected reaction rates of the OH+H2→H+H2O reaction. Rate constants for both the OH+H2(v=1) and OH(v=1)+H2 reactions were calculated based on a full variational transition state theory plus multidimensional semiclassical tunneling approximations within a statistical diabatic model. The potential energy surface information was calculated at an accurate level of molecular orbital theory. In particular, geometries and frequencies along the minimum energy path were calculated at the quadratic configuration interaction level including all single and double excitations (QCISD) with the 6-311+G(d,p) basis set. Energies along the minimum energy path were further improved by a series of single point projected fourth-order Möller–Plesset perturbation theory (PMP4) calculations using the 6-311++G(2df,2pd) basis set. Our present results of vibrational excited state rate enhancements agree very well with previous experimental data. In view of these results, we also discuss the accuracy of the Schatz–Elgersma potential energy function in more detail.
Low frequency Raman spectroscopy of supercooled fragile liquids analyzed with schematic mode coupling models
Authors: Alba-Simionesco C. , Krauzman M.
Published: July 2002

Recent experimental studies of the glass transition of molecular liquids have exploited light scattering techniques in order to support the dynamical model proposed by the mode coupling theory. In the framework of the dipole-induced-dipole (DID) formalism and the Stephen’s approximation, we have checked this theory with several memory functions in the microscopic region, where phononlike excitations dominate, i.e., in the frequency window of 5–130 cm−1 accessible by a classical Raman spectrometer. The fitting procedure compares the experimental susceptibility spectra of one of the simplest fragile molecular liquid, m-toluidine, to the theoretical ones and estimates, in each case, the T dependence of the different control parameters as well as the crossing point of the transition line of type B. The agreement observed for spectra from a temperatures above the melting point down to the glass transition temperature Tg suggests, on the one hand, that information about the dynamical behavior of the supercooled liquid are contained in this frequency region and, on the other hand, that vibrational contributions are incorporated in this formalism, independently of the form of the relaxation kernel. Finally, the two-peak shape in the microscopic range of the susceptibility spectra is related to the relaxation of a linear combination of the Fourier components of the two density correlators.
Matrix isolation study of the interaction of excited neon atoms with NF3. Infrared spectra of NF+3, NF+2, and NF−2
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:NF3 sample is codeposited at approximately 5 K with a beam of pure neon that has been passed through a microwave discharge, the infrared spectrum of the resulting deposit includes prominent absorptions of NF2 and a structured absorption assigned to NF. In addition, two groups of absorptions contributed by previously unassigned products can be distinguished, based on their behavior on subsequent filtered mercury-arc irradiation of the deposit. Three absorptions which disappear readily on near-ultraviolet irradiation are assigned to NF−2. The remaining absorptions are contributed by NF+3 and probably also by NF+2. Vibrational assignments for these absorptions are proposed, and processes which occur on mercury-arc irradiation of the deposit are considered.
Electronic structure via the auxiliary-field Monte Carlo algorithm
Authors: Charutz D. M. , Neuhauser Daniel
Published: July 2002

Auxiliary-field Monte Carlo (AFMC) is an exact approach for calculating the ground state of a system of fermions (or bosons) interacting by pair-potentials. The method uses the Hubbard–Stratonovich transformation to replace the exact imaginary-time propagator by an average over an ensemble of propagators for independent particles in the presence of a varying external field, so that the calculation of the exact energy is reduced to multiple independent calculations, each of which costs essentially the same as one Hartree–Fock iteration. Here we consider the application of AFMC to calculate molecular structure, and present preliminary simulations on He and Be. We develop two simple methods to partially alleviate a ‘‘sign-problem’’ in AFMC through restriction of the length of the imaginary-time propagation, by either a simultaneous propagation of several initial states followed by subspace-diagonalization or by incorporation of information from all propagated time steps. The first method is tested and found to yield significant improvement in accuracy. For the present simulations, the single-particle orbitals are expanded in a given set of primitive orbitals. The resulting spectral-AFMC method yields, for sufficiently converged ensembles, the full-CI energy associated with a given basis. The developments reported here, and in particular the demonstration of subspace-diagonalization, have however general validity independent of whether a basis set or a grid representation is used for the single-particle orbitals (in the first case a full-CI result is obtained in the given basis, while a converged grid representation would yield the exact result).