bluuebunny/crossref_metadata_2025 huggingface dataset:
The kinetics of infrared laser photodissociation of n-butylbenzene ions at low pressure
Authors: Uechi Guy T. , Dunbar Robert C.
Published: July 2002

Molecular ions of n-butylbenzene trapped in the ion cyclotron resonance ion trap were dissociated by cw CO2 -laser irradiation at pressures near 1×10−8 Torr where collisions play a negligible role in the kinetics. The dissociation as a function of time after laser onset showed a well-defined induction time τind, followed by exponential decay of a parent ion with first-order rate constant kdiss. Data were collected over a range of laser intensities giving kdiss values from 0.3 to 10.5 s−1. Computer simulation of the dissociation curves was carried out using a random-walk method to solve a master equation for the kinetics, and assuming infrared radiative relaxation as the mechanism of ion deactivation. The simulations yielded curves also showing well-defined induction times and dissociation rate constants. It was necessary to consider both the induction times and the dissociation rates to constrain the fit of experiment and computer simulation. The resulting fit was good. The thermal picture of low-pressure infrared multiphoton dissociation kinetics developed in previous work was applied to the data, and thermal kinetics were found to give an excellent description of the observed kinetics. The observed activation energies were successfully interpreted in terms of the modified Tolman’s theorem.
Implementation of analytical first derivatives for evaluation of the many-body nonadiabatic wave function with explicitly correlated Gaussian functions
Authors: Kozlowski Pawel M. , Adamowicz Ludwik
Published: July 2002

A nonadiabatic many-particle wave function is generated using an expansion in terms of explicitly correlated Gaussian-type basis functions. In this approach, motions of all particles are correlated at the same time, and electrons and nuclei are distinguished via permutational symmetry. We utilize our newly proposed nonadiabatic variational approach [P. M. Kozlowski and L. Adamowicz, J. Chem. Phys. 95, 6681 (1991)], which does not require the separation of the internal and external motions. The analytical first derivative of the variational functional with respect to the nonlinear parameters appearing in the basis functions are derived and implemented to find the minimum. Numerical examples for the ground state of the hydrogen molecule are presented.
Giant satellite in the x-ray photoelectron spectroscopy core-hole spectra of adsorbates
Authors: Decleva P. , Ohno M.
Published: July 2002

To investigate the dependence of ligand core-hole spectra on the electronic structure of the metal, we performed the ab initio 2h2p/3h2p configuration interaction (CI) calculation of the core-hole spectra of PdCO and Pd2CO molecules, using an extended basis set. The main line is the one-hole state and takes a much larger intensity than for NiCO and NiN2 but still smaller than for free CO. As in the case of NiCO and NiN2, for both PdCO and Pd2CO the π charge-transfer shakeup satellite of a small intensity is obtained. The most striking spectral feature of PdCO and Pd2CO which differs from NiCO and NiN2 is the absence of the 5 eV giant σ shakeup satellite in the carbon spectra of PdCO and Pd2CO. For PdCO the corresponding oxygen satellite of small intensity is shifted toward the higher energy (around 8 eV). This oxygen satellite is missing in Pd2CO. For the core-hole spectra of the CO/Pd(100) system which are not available, we expect satellite lines of small intensity around 10 eV. The absence and enhancement of the giant satellite in the core-hole spectra for the CO/Ni, N2/Ni, and CO/Pd systems are explained in terms of the different degree of the dσ-s hybrid and s-d promotion in the ground and ionized states.
Molecular rattling in two-dimensional fluids: Simulations and theory
Authors: Variyar Jayasankar E. , Kivelson Daniel , Tarjus Gilles , Talbot Julian
Published: July 2002

We have carried out molecular dynamic simulations over a range of densities for two-dimensional fluids consisting of hard, soft, and Lennard-Jones disks. For comparison we have also carried out simulations for the corresponding systems in which all but one particle are frozen in position. We have studied the velocity autocorrelation functions and the closely related velocity-sign autocorrelation functions, and have examined the probabilities per unit time that a particle will undergo a first velocity sign reversal after an elapsed time t measured alternately from the last velocity reversal or from a given arbitrary time. At all densities studied, the first of these probabilities per unit time is zero at t=0 and rises to a maximum at a later time, but as the hardness of the disks is increased, the maximum moves in toward t→0. This maximum can be correlated with the ‘‘negative’’ dip observed in the velocity correlation functions when plotted versus time. Our conclusion is that all these phenomena can be explained qualitatively on the basis of a model where memory does not extend back beyond the last velocity reversal. However, at high density, the velocity-sign-autocorrelation function not only shows a negative dip (which is explained by the model) but also a second ‘‘oscillation’’ which is not described, even qualitatively, by the model. We conclude that the first dip in the velocity and velocity-sign correlation functions can occur even if there are no correlated or coherent librations, but the existence of a ‘‘second’’ oscillation is a better indication of such correlations.
Dynamics of trapping and localization of excess electrons in simple fluids
Authors: Space B. , Coker D. F.
Published: July 2002

We employ a surface hopping trajectory method to study the rapid nonadiabatic relaxation after an excess electron is injected in unperturbed fluid helium. Several distinctively different relaxation processes, characterized by their relative importance at different times during the relaxation to a localized equilibrium state are detailed. These processes include: Short time nonadiabatic leakage from cavity to cavity, exploring the fluctuating unperturbed solvent structure. This relaxation involves slow drifting of the occupied state through a continuum of levels. This is followed by rapid diabatic expansion of a particular solvent cavity once the electron–solvent forces begin to take effect on the solvent atoms in a particular region of the fluid. We also study the importance of nonadiabatic hang up trajectories in which the excess electron gets caught in the first excited state of a bistable well potential offered by a pair of closely coupled cavities in the solvent. We study the density dependence of the time scales and relative importance of these different processes and their influence on the transient absorption spectrum after electron injection into an unperturbed fluid. Though the dynamical properties of excess electrons under the conditions considered here have never been studied before, the behavior is remarkably similar to that observed in both experimental and theoretical studies of electron hydration dynamics, indicating that the processes we describe in this paper may be very general relaxation mechanisms for localization and trapping in fluids.
Spectroscopy and dynamics of resonance Raman scattering by iodobenzene excited in the B continuum
Authors: O’Brien Sean C. , Kittrell Carter , Kinsey James L. , Johnson Bruce R.
Published: July 2002

Resonance Raman scattering in iodobenzene is studied for excitation in the region 219–233 nm (B 1A1←X 1A1). There is a rich Raman spectrum containing strong fundamentals of ring-based modes and the CI stretch. On resonance, overtones and combination bands of most modes are strongly enhanced. The modulation of the CI stretch activity as a function of wavelength is interpreted in terms of the initial dynamical evolution of the molecule in the excited state. A normal mode analysis is also made of the ground-state planar modes of iodobenzene in order to correlate the observations with the resonance Raman spectra in benzene.
The exponential distorted-wave approach for systems for which the S matrix of the distorted wave is not diagonal
Authors: Sisak Michael , Secrest Don
Published: July 2002

The recoupled states approximation is a distorted-wave approximation and as such leads to a nonunitary S matrix. Several unitarization approaches are known for systems for which the S matrix of the distorted wave is diagonal. In particular, the exponential distorted-wave method has been extremely successful. We develop in this paper a general unitarization technique, similar to the exponential distorted-wave method, useful when the S matrix of the distorted wave is nondiagonal. This approach is applied to the case of the recoupled states for which the distorted waves are the coupled states wave functions. For this particular example, unitarization does not improve the results.
Two-color double resonance spectroscopy via Ã 1A
u state of acetylene: 3p Rydberg state and its Renner–Teller effect
Authors: Takahashi Makoto , Fujii Masaaki , Ito Mitsuo
Published: July 2002

The two-color double resonance multiphoton ionization spectra due to the transitions from various rovibrational levels of the Ã 1Au state of acetylene to the 3p Rydberg state have been measured. The observed spectra showed large dependence on the quantum number K. It was concluded from the spectral analysis that the geometry of the 3p Rydberg state is linear. The spectra also showed a long progression of the trans-bending vibration, supporting the linear structure. The trans-bending progression belonging to the 1Δg electronic state showed the Renner–Teller splittings. The observed splittings were reproduced by the calculation using the formulas for a linear triatomic molecule in a Δ electronic state.
Spectroscopic determination of the state-to-state differential cross section for inelastic collisions
Authors: Reid Katharine L. , McCaffery Anthony J.
Published: July 2002

Information on the differential scattering cross section is contained in high resolution double resonance line shapes measured following collision. Here we demonstrate how this relationship may be exploited and present predictions of double resonance line shapes for model differential cross sections. We show the dependence of line shape on peak position and width of the differential cross section and find the shape to be very sensitive to these parameters in certain conditions. Finally we discuss some of the experimental limitations of the proposed method and how, in practice, these may be overcome to yield the differential cross section from a spectroscopic measurement.
A grazing incidence x-ray diffraction study of the transverse structure function of the liquid–vapor interface of Ga
Authors: Flom Erik B. , Cai Zhong-hou , Acero Anibal , Lin Bin-hua , Maskil Nissan , Liu Li , Rice Stuart A.
Published: July 2002

We report the results of a grazing incidence x-ray diffraction study of the liquid–vapor interface of Ga. It is shown that the transverse structure function in the liquid–vapor interface of Ga is indistinguishable from the structure function of bulk liquid Ga. When compared with the transverse structure functions obtained from calculations of models of the liquid–vapor interface of a similar system [J. Chem. Phys. 86, 1036 (1987)], this result implies that the widely used point local density approximation to the properties of an inhomogeneous liquid is inaccurate and must be replaced by a representation which includes, at least to first order, the effect of the force which generates the density inhomogeneity in the liquid.
Dephasing processes in glasses with strong strain interactions
Authors: Zürcher U. , Silbey R.
Published: July 2002

Spectral diffusion decay is calculated for a glass modeled by two level systems which are strongly coupled to phonons. The spin-phonon interaction induces an effective spin–spin interaction which dominates the energy scale. We show that spectral diffusion is a property of macroscopic local fields which fluctuate on time scales that are much longer than the spin-phonon relaxation time T1. We assume for the spectral diffusion a Gaussian distribution and derive a self-consistent equation for its variance which is nonlocal in time. At high temperatures, the variance grows linearly with time while at low temperatures, we find strong deviations from simple diffusive decay. In a particular case, the growth of the variance is steplike. For very long times, we find an asymptotic sublinear behavior w∝t2/3. A heuristic argument shows that this law is determined by the form of the distribution.
Quantum dynamics of overtone relaxation in benzene. II. 16 mode model for relaxation from CH(v=3)
Authors: Wyatt Robert E. , Iung Christophe , Leforestier Claude
Published: July 2002

This series is concerned with the quantum dynamics of overtone relaxation in benzene and in reduced mode benzene fragments. In part II of this series, emphasis is placed upon time-dependent studies of v=3 CH overtone relaxation in a 16 mode planar benzene model (5 CH stretch modes are inactive out of the 21 planar modes). Through use of the wave operator contraction algorithm, primitive vibrational basis sets containing up to 1.1×109 states are contracted to active spaces of dimension about 4000 states. The exact dynamics within the active space is developed with the recursive residue generation method. The time-dependent survival probability of the CH(v=3) overtone is compared with results from classical mechanics and with the Fourier transform of the experimental overtone spectra. The classical and quantum results are in good agreement only at short times (t<0.1 ps). At longer times multiple small recurrences are present in the quantum results. The quantum and experimental results share common features (rapid falloff at short times, followed by multiple small recurrences), but there are quantitative differences. The sensitivity of the quantum survival to alterations in the anharmonic potential is also studied. The short-time dynamics (t<0.5 ps) is shown to be accurately reproduced by reduced 4 and 5 mode models. Only a small subset of states in these carefully selected modes are significantly populated during the early decay of the v=3 overtone, even though the total density of states is very large.
Laser vaporization generation of the diatomic radicals PdB, 105PdB, PdAl, and 105PdAl: Electron spin resonance investigation in neon matrices at 4 K
Authors: Knight Lon B. , Babb Robert , Hill Devon W. , McKinley Allan J.
Published: July 2002

The new diatomic radicals PdB and PdAl have been generated by depositing the products produced from the pulsed laser vaporization of the elemental mixtures into neon matrices at 4 K. ESR (electron spin resonance) studies of these matrix isolated radicals, including an analysis of the 105Pd(I=5/2), 27Al(I=5/2), and 11B(I=3/2) nuclear hyperfine interactions, show that both have X 2Σ electronic ground states. These new results are compared with previous ESR measurements for PdH, PdCH3, YPd, ScPd, and PdH2+ to reveal electronic structure information and bonding trends as the complexity of the ligand increases 1s, 2p, 3p, 3d, and 4d. Ab initio UHF (unrestricted Hartree–Fock) theoretical calculations were also conducted on four of these small palladium radicals as part of this experimental investigation. A simple interpretation of the 105Pd hyperfine interactions and molecular g tensors based on the degree of charge transfer to palladium is presented. The observed magnetic parameters (MHz) for 105Pd11B in a neon matrix at 4 K are g∥=2.009(2), g⊥=2.042(2), A∥=−1483(15), and A⊥=−1483(2) for 105Pd, and A∥=197(4) and A⊥=140(1) for 11B. The parameters for 105Pd27Al are g∥=2.010(1), g⊥=2.0343(5), A∥=−1283(10) and A⊥=−1268(2) for 105Pd, and A∥=182(2) and A⊥=84.6(5) for 27Al.
The structures of small methyl fluoride clusters from infrared dissociation experiments
Authors: Ehbrecht M. , de Meijere A. , Stemmler M. , Huisken F.
Published: July 2002

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methyl fluoride clusters upon excitation of the ν3 C–F stretch vibration at 1048.6 cm−1. Size selection has been achieved by dispersing the (CH3F)n cluster beam by a secondary rare gas beam. For the methyl fluoride dimer only very weak dissociation signals could be observed. The corresponding spectrum features a single, 13.4 cm−1 broad absorption line. This observation is explained with a symmetric dimer structure, in which both monomer units reside at equivalent positions, and an inefficient coupling of the molecular vibration to the intermolecular bond. For the trimer and tetramer very strong dissociation yields are observed. Whereas the trimer shows a complicated spectrum which is attributed to its nonsymmetric structure, the tetramer spectrum is again characterized by a single peak. In order to obtain supplementary information, dissociation spectra have also been measured for small methyl fluoride clusters residing inside or on the surface of large Arx host clusters. These matrixlike spectra are consistent with the free gas-phase cluster data. Finally, in a computational approach, the structures of the methyl fluoride dimer, trimer, and tetramer have been determined by total energy minimization. The theoretical results are in perfect agreement with the experimental findings.
A parametric model for the global thermodynamic behavior of fluids in the critical region
Authors: Luettmer-Strathmann J. , Tang S. , Sengers J. V.
Published: July 2002

The asymptotic thermodynamic behavior of fluids near the critical point is described by scaling laws with universal scaling functions that can be represented by parametric equations. In this paper, we derive a more general parametric model that incorporates the crossover from singular thermodynamic behavior near the critical point to regular classical thermodynamic behavior far away from the critical point. Using ethane as an example, we show that such a parametric crossover model yields an accurate representation of the thermodynamic properties of fluids in a large region around the critical point.
Quantum flux redistribution during molecular photodissociation
Authors: Manolopoulos David E. , Alexander Millard H.
Published: July 2002

A new method proposed recently by Alexander for studying the mechanisms of inelastic collisions is adapted to the study of molecular photodissociation. This adaptation involves the determination of the current density associated with a driven photodissociation wavefunction at each fixed scattering energy, and yields a picture of how, as a function of the dissociation coordinate, the outgoing photofragment flux rises in the Franck–Condon region on absorption of the photon and how it redistributes between the available internal channels as the photofragments move apart. This picture complements the usual time-dependent picture of photodissociation, allowing one in particular to analyze the mechanism of the photodissociation in detail at each individual excitation laser frequency. A study of flux redistribution in a simple two-state model for the electronically nonadiabatic photodissociation of methyl iodide is presented as a first illustration of the approach.
High resolution laser spectroscopy of the C̃ 2Δ–X̃ 2Σ+ transition of CaOH and CaOD: Vibronic coupling and the Renner–Teller effect
Authors: Jarman C. N. , Bernath P. F.
Published: July 2002

The C̃ 2Δ–X̃ 2Σ+ transition of CaOH and CaOD has been observed in the blue region of the spectrum. This electronically forbidden transition becomes allowed through vibronic coupling with the bending mode. The (010)–(000) band for CaOH and the (010)–(000), (020)–(010), (010)–(010), (000)–(010), and (010)–(0200) bands for CaOD have been recorded and analyzed at high resolution. The C̃ 2Δ(000) state of CaOD has a band origin at 21 907.128(1) cm−1, a rotational constant B of 0.293 536(7) cm−1, and a spin–orbit constant A of 11.4944(5) cm−1. A rotational analysis of the C̃ 2Δ(020)μΣ and the C̃ 2Δ(020)κΣ states of CaOD has also led to the first experimental determination of the Renner–Teller parameter ĝ4 for a 2Δ state at ĝ4=−0.038 62(9) cm−1.
Multiple-band theory of dynamics for interacting adsorbates coupled to phonons. II. Single adsorbate dynamics
Authors: Reilly Philip D. , Harris Robert A. , Whaley K. Birgitta
Published: July 2002

The dynamics of an adsorbate described by a two-band generalization of the small-polaron model [J. Chem. Phys. 95, 8599 (1991)] are investigated. Lattice induced mixing of the bands can result in the adsorbates becoming self-trapped at one end of an adsorption site or the other. Tunneling between adjacent sites can either preserve or change the end of the site in which the adsorbate resides. The associated rates have very different temperature and mass dependences, and contribute to the overall diffusion in very different ways. The observed equilibrium and transport properties of the H/W(110) system can be consistently explained within our model.
Potential energy surfaces describing ion complexes containing molecular hydrogen
Authors: Ischtwan Josef , Smith Brian J. , Collins Michael A. , Radom Leo
Published: July 2002

High-level ab initio molecular orbital calculations have been carried out to describe the potential energy surfaces for a series of systems corresponding formally to dimer ions of molecular hydrogen with an inert-gas atom or a first- or second-row hydride [X⋅⋅⋅H2]̇ +, where X=He, Ne, Ar, Kr, HF, HCl, H2O, H2S, NH3, and PH3. Of the [XH2]̇ + ion complexes, [Ne⋅⋅⋅H2]̇ +, [Ar⋅⋅⋅H2]̇ +, [Kr⋅⋅⋅H2]̇ +, and [HFH⋅⋅⋅H]̇ + lie in wells of significant depth and should be experimentally observable. The energetics of ion–molecule reactions associated with the [XH2]̇ + systems have been examined in detail. The reactions include proton- and hydrogen-transfer reactions and hydrogen-exchange reactions. Pathways and transition structures for scrambling of hydrogen atoms within the [XH2]̇ + complexes, some of which are relevant to the exchange reactions, have also been determined.
Dynamics of bimolecular reactions in solution: A nonadiabatic activation model
Authors: Ben-Nun M. , Levine R. D.
Published: July 2002

A simple Hamiltonian model which accounts for the main features of the role of the solvent in activated bimolecular exchange reactions is discussed. The properties of the potential energy along the reaction coordinate of the solute enter in an essential way and explain the different roles of the solvent near the barrier and at the foothills of the potential and the corresponding separation of time scales. The activation energy necessary to surmount the barrier is provided by a localized, vibrationally nonadiabatic, energy exchange between the solvent and solute. Caging with and without recrossing of the barrier is discussed. The predictions of the model are compared with exact trajectory results for the given Hamiltonian and with full molecular dynamics simulations. The influence of the physical parameters such as masses, barrier height strength of solvent–solute coupling, etc., is well accounted for by the model and is summarized by two dimensionless coupling parameters. In particular, the efficiency of solvent solute energy exchange is governed by a vibrational adiabaticity parameter.
Vibrational relaxation of N2+(X 2Σg+,v=1) in collisions with 4He. II. Classical path calculations
Authors: Zenevich V. A. , Lindinger W. , Billing G. D.
Published: July 2002

Classical path calculations have been carried out for vibrational relaxation of N2+ colliding with He. The potential energy surfaces used for the dynamical investigations were obtained as analytical fits to recent 3D ab initio data.
Spectroscopy and dynamics of jet-cooled hydrazines and ammonia. I. Single-photon absorption and ionization spectra
Authors: Syage Jack A. , Cohen Ronald B. , Steadman Jhobe
Published: July 2002

Electronic-state properties of hydrazine (N2H4), monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH), and ammonia were investigated by single-photon vacuum ultraviolet (vuv) absorption and photoionization spectroscopy in a molecular-beam apparatus. The photoionization spectrum of NH3 shows a sharp threshold at a value of 10.16 eV in excellent agreement with previous measurements. The ionization thresholds of the hydrazines are very broad reflecting the large (nearly 2 eV) structural relaxation energy for conversion from a C2 pyramidal gauche neutral geometry to a D2h planar ion geometry. The measured threshold photoionization potentials are 8.32 eV (N2H4), 8.05 eV (MMH), and 7.87 eV (UDMH). The vuv absorption spectra of the hydrazines are also broad, which is to be contrasted with the well-resolved spectrum of NH3. The band shapes can be attributed to planarization of the Rydberg excited states in analogy to the ion structure. A molecular-orbital model is developed to provide understanding of the excited states in terms of their symmetries, energy ordering, and selection rules, as a function of geometry.
Molecular dynamics study of the coverage dependence of Xe desorption from Pt(111)
Authors: Jansen A. P. J.
Published: July 2002

Molecular dynamics simulations with periodic boundary conditions are used to calculate temperature-programmed desorption spectra of Xe/Pt(111). The activation barrier is overcome using the compensating Hamiltonian method. Monte Carlo simulations are used to correct for the finite size of the simulated system. A comparison between calculated and experimental temperature-programmed desorption spectra shows that there is a strong reduction in the Xe–Xe attraction due to substrate-mediated effects. Still, strong island formation occurs. In spite of the fact that Xe atoms in islands desorb less easily than isolated Xe atoms, the desorption from islands dominates at low temperature, and the desorption of isolated Xe atoms at high temperature. This is caused by the variation in the number of isolated Xe atoms and Xe atoms in islands as a function of temperature and coverage.
A study of binary hard core Yukawa mixtures by molecular dynamics and the mean spherical approximation
Authors: Rey C. , Gallego L. J. , González L. E. , González D. J.
Published: July 2002

The static bulk properties of several types of binary mixtures of hard spheres interacting through an attractive Yukawa tail are studied by molecular dynamics (MD) simulation, and the results compared with the predictions of the mean spherical approximation (MSA) using the energy, compressibility, and virial routes. For mixtures with components of equal diameter, our MD results differ significantly, under certain working conditions, from recently reported results of Monte Carlo calculations. For both these mixtures and mixtures with appreciable size mismatch, comparison with the MSA results shows that the ‘‘exact’’ MD data are always more closely approximated by the energy route. These results, which are all consistent with those of a previous study of pure Yukawa fluids, show that, using the energy route, the MSA allows good description of the thermodynamic properties of binary Yukawa mixtures, even in situations departing considerably from ideality.
Gibbs free-energy changes for the growth of argon clusters adsorbed on graphite
Authors: Strozak Mary Ann , Lopez Gustavo E. , Freeman David L.
Published: July 2002

Changes in Gibbs free energies for the process Arn−1,(a) + Ar(a) → Arn,(a) are calculated by Monte Carlo simulations for Ar clusters physisorbed on a graphite substrate. Calculations are performed for clusters Ar2 through Ar12 at 10 K. Using a simulated annealing procedure, the minimum-energy configurations at 0 K are obtained. In all cases studied minimal-energy two-dimensional structures are found at a distance above the surface identical to that determined for an argon monomer. Some cluster sizes exhibit isomerization between several low-energy configurations during the simulations. This isomerization leads to sampling difficulties that are significantly reduced by using the J-walking method. Minima in the Gibbs free energy are found for cluster sizes 7, 10, and 12. An explanation for the location of the minima in the free-energy function is given in terms of cluster structure and energetics.
The general treatment of dynamic solvent effects in electron transfer at high temperature
Authors: Wang Zhiyu , Tang Jau , Norris James R.
Published: July 2002

A general treatment of dynamic solvent effects for outer-sphere electron transfer reactions based on the interactions between the electron transfer system and the solvent bath is presented. The electron transfer rate constant as a function of the solvent longitudinal relaxation time is categorized into the three solvent relaxation regions of adiabatic, normal, and fast. New nonadiabatic effects in the very fast solvent relaxation region are identified. A unified picture for adiabatic and nonadiabatic reactions, as well as a criterion for adiabaticity is given.
Self-relaxation of vibrationally excited H2O molecules
Authors: Shin H. K.
Published: July 2002

This study on the self-relaxation of vibrationally excited H2O molecules is based on a model in which an excited molecule and its collision partner undergo a long-lived collision in the presence of strong molecular attraction. Transition probabilities are obtained by semiclassical procedures using ladder operators. The study shows that large-impact parameter collisions are responsible for the negative temperature dependence of the VV transition probabilities in H2O(001)→H2O(020), H2O(020)→H2O(010), and H2O(010)→H2O(000). These energy transfer processes occur intramolecularly, and the transfer of the vibrational energy mismatch that is released by the VV process to the low frequency oxygen–oxygen motion in the complex is found to be very efficient, whereas energy transfer to the hindered rotational motions of the excited molecule is inefficient. The model predicts VV probabilities to decrease on deuteration.
Nuclear exchange spin couplings in metal trihydrides: A tight binding approximation
Authors: Hiller Eric M. , Harris Robert A.
Published: July 2002

A multiband tight binding model of the metal trihydrides, including on-site repulsion and exchange, is examined. By linearly coupling the system to a harmonic bath we show that the multiband model yields an effective temperature dependent Heisenberg spin Hamiltonian. This system exhibits a temperature dependent competition between ferro- and antiferromagnetic exchange spin couplings. The resulting spin spectrum is consistent with observed anomalous isotope dependent metal trihydride H-N.M.R. spectra.
The determination of electron affinities of the open shell systems C and CH2 using the multiconfigurational spin tensor electron propagator method
Authors: Yeager Danny L. , Nichols Jeffrey A. , Golab Joseph T.
Published: July 2002

The multiconfigurational spin tensor electron propagator method (MCSTEP) is a Green’s function approach for accurately predicting and analyzing ionization potentials (IPs) and electron affinities (EAs). Unlike more traditional Green’s function approaches, MCSTEP is applicable to highly correlated and open shell systems as well as to closed shell systems with small correlation effects. We apply MCSTEP for the determination of EAs for C and CH2. This is the first time that MCSTEP has been used to determine the EAs for systems which have both open shell neutral and anionic ground states. Our best MCSTEP results for the EA of C and the adiabatic EA of CH2 are 1.2904 and 0.6356 eV, respectively, compared to 1.2607 and 0.6306 eV, respectively, obtained with large scale multireference configuration interaction (MRCI) using the same basis sets. Experimental values are 1.268 eV for C and 0.628±0.031 eV for CH2. We also show that accurate EAs for these systems can be obtained both with MRCI and especially with MCSTEP using moderately sized basis sets composed of a good, standard core–valence basis set supplemented with some additional diffuse functions.
Solubilization in spherical block copolymer micelles: Scaling analysis based on star model
Authors: Nagarajan R. , Ganesh K.
Published: July 2002

We develop a theory of solubilization of low molecular weight species in block copolymer micelles, employing the scaling approach and building on the analogy between micelles and star polymers. Specifically, we consider spherical micelles of AB-diblock copolymers formed in selective solvents, in which the core and the shell regions are assumed to have radial concentration variations similar to those occurring in the solution state of a star polymer. Invoking the results for the conformation of a star polymer derived by Daoud and Cotton, expressions are developed for the free energy per molecule of the micelle ΔG, the core radius R, the shell thickness D, and the overall volume fraction of the polymer within the core φA as functions of the micellar aggregation number g. From the minimization of the free energy of an isolated micelle, the equilibrium values for all the micellar structural parameters and for the extent of solubilization are obtained. The results expressed as scaling relations for g, R, D, and φA are derived for various situations of interest including those of the solvent S being a good or theta solvent for the shell block B, and the solubilizate J being a good or theta solvent for the core block A.
Binding energies of positronium fluoride and positronium bromide by the model potential quantum Monte Carlo method
Authors: Schrader D. M. , Yoshida Takashi , Iguchi Kaoru
Published: July 2002

A method previously used by the authors in an accurate calculation of the binding energy of positronium chloride [Phys. Rev. Lett. 68, 3281 (1992)] is applied to positronium fluoride and positronium bromide. The binding energies obtained with this method are PsF, 1.98±0.17 eV; PsCl, 1.91±0.16 eV; PsBr, 1.14±0.11 eV.
Antisymmetry in the quantum Monte Carlo method with the A-function technique: H2 b 3Σu+, H2 c 3Πu, He 1 3S
Authors: Bianchi R. , Bressanini D. , Cremaschi P. , Morosi G.
Published: July 2002

The diffusion Monte Carlo method for the solution of the Schrödinger equation for atomic and molecular systems is extended to incorporate the antisymmetry constraint. The sign problem is treated constraining the diffusion process of signed walkers within the fluctuating nodes of a function A, defined as sum of Gaussians centered on the psips. The function A is able to build up the full nodal surface in the 3N dimensional space. The algorithm is shown to scale as Nα3+Nβ3 where Nα and Nβ are the number of α and β electrons. Results are given for the b 3Σu+ and c 3Πu states of H2 and the 1 3S state of He.
On a phenomenological model for nonequilibrium interaction between thermally activated chemical reactions
Authors: Gorecki Jerzy
Published: July 2002

A simple phenomenological model for time dependent nonequilibrium effects associated with a thermally activated chemical reaction is applied to describe the nonequilibrium coupling between many competing reactions. A good agreement with the results of molecular dynamics simulations for a system with two competing reactions has been obtained. An analytical formula, which estimates the strength of nonequilibrium effects, is derived.
On the calculation of single ion activity coefficients in homogeneous ionic systems by application of the grand canonical ensemble
Authors: Sloth Peter
Published: July 2002

The grand canonical ensemble has been used to study the evaluation of single ion activity coefficients in homogeneous ionic fluids. In this work, the Coulombic interactions are truncated according to the minimum image approximation, and the ions are assumed to be placed in a structureless, homogeneous dielectric continuum. Grand canonical ensemble Monte Carlo calculation results for two primitive model electrolyte solutions are presented. Also, a formula involving the second moments of the total correlation functions is derived from fluctuation theory, which applies for the derivatives of the individual ionic activity coefficients with respect to the total ionic concentration. This formula has previously been proposed on the basis of somewhat different considerations.
Numerical Laplace transform density of states calculation for medium and large molecules
Authors: Romanini D. , Lehmann K. K.
Published: July 2002

A novel implementation of the Laplace transform method for the calculation of the density of states of molecules, for which the partition function can be explicitly given is described. It consists of doing the inverse Laplace transform numerically after multiplying the integrand by a smoothing factor. This evaluation is more accurate than the method of steepest descent, and the computation can still be done on a PC in a few minutes. By first order expansion of the partition function in the anharmonic parameters we have been able to calculate the density of states for a model molecule composed by anharmonically coupled anharmonic oscillators, which cannot be treated by the well known Beyer–Swinehart algorithm.
The infrared and microwave spectroscopy of the argon–propyne dimer
Authors: Blake Thomas A. , Eggers David F. , Tseng Shao-Hui , Lewerenz Marius , Swift R. Paul , Beck Rainer D. , Watts R. O. , Lovas Frank
Published: July 2002

Microwave and infrared measurements are reported for the van der Waals complex Ar–CH3CCH and its isotopomers. The structure is T shaped with equilibrium center-of-mass separation of 3.73 Å and an angle of 82° between the molecule symmetry axis and the van der Waals bond. The infrared and microwave spectra are complex due to the effects of a slightly hindered internal rotor. Analysis of the spectral data shows that the dipole moment is almost parallel to the dimer b axis. A modified vibration/rotation Hamiltonian that includes an internal rotor potential is used to show that the barrier to internal rotation is near 10.8 cm−1.
Analytical approach to molecular liquids. V. Symmetric dissociative dipolar dumbbells with the bonding length σ/3≤L≤σ/2 and related systems
Authors: Zhou Yaoqi , Stell George
Published: July 2002

The exact asymptotic behavior of the particle–particle direct correlation function for dissociative dipolar dumbbells is discussed. It reveals the sense in which the complete association limit is like a critical point and suggests several approximations as well as the conditions under which they can be expected to be useful. The simplest of these is an extended mean spherical approximation (EMSA) that can be solved analytically for a model liquid of symmetric dissociative dipolar dumbbells with two centers (each bearing a point charge of opposite sign) a distance L apart, when σ/3≤L≤σ/2, where σ is the diameter of the spheres that consistute the dumbbells. The analytical expressions for the Born solvation free energy of a symmetric dipolar dumbbell in a symmetric dipolar dumbbell solvent and in a dipolar hard-sphere solvent are also obtained. Such expressions can be expected to be useful in investigating intramolecular electron-transfer reactions. Results for σ/2≤L≤σ that have a somewhat different conceptual status are obtained as well. They suggest a new interpretation of the Percus–Yevick solution to the sticky-sphere model considered by Baxter.
Effect of three-body forces on the statics and dynamics of SF6–(Rg)n and (Rg)13 clusters
Authors: Chartrand Darryl J. , LeRoy Robert J. , Kumar Ashok , Meath William J.
Published: July 2002

Molecular dynamics and Monte Carlo simulations are used to examine the effect on the structural properties of heterogeneous SF6–(Ar)n and SF6–(Kr)n clusters, and on the melting behavior of heterogeneous SF6–(Ar)n and homogeneous (Ar)13 and (Kr)13 clusters, of including the three-body Axilrod–Teller–Muto triple–dipole dispersion energies in the total potential energy surface governing the dynamics of the system. The behavior of these systems is governed by potentials constructed from the best available two-body interactions, and from accurate constrained dipole oscillator strength values for the triple–dipole dispersion energy coefficients reported here for the first time. The structural studies show that (virtually) all isomers are destablized by inclusion of the three-body terms, with the ‘‘stacked’’ or ‘‘nonwetting’’ structures being destablized relatively more than isomeric ‘‘monolayer’’ or ‘‘wetting’’ structures. However, the qualitative trends in relative stability are unchanged; in particular, the preference for the SF6 to be fully solvated in larger clusters formed with Ar, but to lie on the surface of larger clusters formed with Kr, remains unchanged. In contrast, the melting temperatures of the stacked and monolayer isomers of the heterogeneous SF6–(Ar)12 cluster undergo substantial change on inclusion of the three-body terms, the former dropping from ca. 30 to 22 K and the latter from 10 to 6 K. The melting temperatures of the homogeneous (Ar)13 and (Kr)13 icosahedral isomers also decreased on inclusion of the three-body interactions, and the resulting values are quite different than those obtained using the commonly accepted ‘‘effective’’ two-body LJ(12,6) pair potentials for these systems.
Quantum dynamics of overtone relaxation in benzene. III. Spectra and dynamics for relaxation from CH(v=3)
Authors: Wyatt Robert E. , Iung Christophe
Published: July 2002

This series is concerned with the quantum dynamics of overtone relaxation in benzene and in reduced mode benzene fragments. In part III of this series, emphasis is placed on the CH(v=3) overtone spectrum and the survival probability for both 16 mode (five CH stretch modes are forced to remain inactive out of 21 planar modes) and 21 mode planar benzene models. Through use of the wave operator contraction algorithm, primitive vibrational basis sets containing up to 9×109 states are contracted to active spaces containing 2000–7000 states. The exact dynamics within the active space is developed with the recursive residue generation method (RRGM). Specific results reported in this study include the following: (a) a comparison is made between dipole spectra and residue spectra, the former explicitly involving the CH dipole function; (b) for the 16 mode model, the following quantities are displayed: overtone spectrum, survival probability of the initial state, complex-valued autocorrelation function, number of phase space cells explored as a function of time, and the rate of exploring phase space cells; (c) sensitivity of the overtone spectrum to the size and composition of the active space built with the wave operator contraction algorithm; (d) for 21 mode benzene, we consider the sensitivity of the overtone spectrum to ‘‘softening’’ the anharmonic potential; (e) comparisons are made with recent jet cooled beam experimental overtone spectra.
Random copolymers with long range interaction in the equilibrium state
Authors: Schulz Michael , Stepanow Semjon
Published: July 2002

Macromolecules with quenched random disorder along the chain and long range interaction between the monomers are studied. Averaging over the quenched disorder results in a collapse regime for the macromolecule. The mean-square radius depends on the polymer length L as a power law <R2≳∼L1/3 for large length, whereas for short length a repulsion regime <R2≳∼L2/3 was predicted. The thermodynamic equation of state for a diluted solution of these macromolecules is determined.
Statistical mechanics of helical wormlike chains. XVI. Excluded-volume effect on the mean-square electric dipole moment
Authors: Yoshizaki Takenao , Yamakawa Hiromi
Published: July 2002

The expansion factor αμ for the mean-square electric dipole moment is studied on the basis of the helical wormlike chain with the excluded-volume effect incorporated in the Yamakawa–Stockmayer–Shimada scheme. A general expression is formulated for the first-order perturbation coefficient Kμ(L) for the chain of total contour length L. The asymptotic solution for Kμ(L) in the limit of L→∞ is evaluated analytically in the Daniels approximation by an application of the operational method. In contradiction to the common notion, it is found that, in the case of κ0τ0≠0 with κ0 and τ0 being the constant curvature and torsion, respectively, of the characteristic helix, Kμ(∞) does not vanish even for the chain having a local electric dipole moment vector perpendicular to the chain contour, indicating that αμ diverges with increasing molecular weight.
The ultraviolet photodissociation dynamics of d1-vinyl chloride
Authors: Huang Yibo , Yang Yung-an , He Guo-xin , Gordon Robert J.
Published: July 2002

We have used partially deuterated vinyl chloride to elucidate the photodissociation mechanism of this molecule. We have found that 75% of the HCl fragments are produced by three-center α,α elimination. Surprisingly, the rotational energy distributions of HCl are the same for both elimination pathways. One possible explanation is that H atom migration in the parent molecule results in similar transition states for the two pathways. Alternatively, dissociation and fragment isomerization may be strongly coupled so that the product angular momentum distribution is determined late in the reaction coordinate. Very different rotational state distributions were observed for v″=0 and v″≳0. We speculate that this dichotomy is caused by formation of a hydrogen-bonded π complex between HCl(v″≳0) and acetylene. We have also determined that HCl+ ions are produced primarily by an α,β mechanism. This mechanism consists of electronic excitation of vinyl chloride, followed by photoelimination and photoionization of electronically excited HCl*. Finally, we determined that H atoms are preferentially produced by detachment from the β carbon, as predicted from the relative stabilities of the α- and β-chlorovinyl radicals.
Electronic quenching rate constants of KrF(B,C) and Kr2F*
Authors: Xu Jie , Gadomski W. , Setser D. W.
Published: July 2002

The laser-assisted reaction between Kr and F2 was used to generate KrF(B,C) molecules during a 7 ns laser pulse in a stainless steel cell. The first-order decay rates of the collisionally coupled KrF(B) and KrF(C) states in ≥50 Torr of Kr buffer gas were monitored by the KrF(B–X) fluorescence in the presence of added reagents at 300 K. Two-body quenching rate constants for KrF(B,C) by diatomic and polyatomic molecules and the rare gases were determined from the pressure dependence of the first-order decay constants of KrF(B,C). The mixed three-body quenching rate constants (with Kr) for added He, Ne, and Ar were also measured. For Kr pressure above 200 Torr, Kr2F(4 2Γ) molecules were formed via three-body quenching of KrF(B,C), and the quenching rate constants for Kr2F(4 2Γ) by the same set of reagents were determined. The general trends in the quenching constants for KrF(B,C) and Kr2F(4 2Γ) are similar to those for XeCl(B,C) and Xe2Cl(4 2Γ), respectively; however, the rate constants for KrF(B,C) and Kr2F(4 2Γ) are generally larger than for the corresponding xenon chloride molecules. Quenching mechanisms for KrF(B,C) and Kr2F(4 2Γ) are discussed.
Structure and dynamics of simulated (SF6)N clusters in the size range N=7–55
Authors: Boutin Anne , Maillet Jean-Bernard , Fuchs Alain H.
Published: July 2002

Isolated (SF6)N clusters have been studied by molecular dynamics simulations in order to complete the temperature-size phase diagram for small clusters sizes (N≤55). The transition between noncrystalline and crystalline ‘‘bulk-like’’ properties occurs in the size range between 13 and 19 molecules. This is two orders of magnitude below the range in which the same transition occurs in argon. Some evidence is provided of the existence of a triple point in the temperature-size phase diagram of SF6 at T∼60 K and N∼34 molecules. In very small clusters such as (SF6)7 and (SF6)13, the centers of masses of the octahedral sulfur hexafluoride molecules can accommodate to a fivefold symmetry with some distortions. Contrary to what had been observed in Lennard-Jones (LJ)13, no sign of a sharp ‘‘transition’’ with coexistence between rigid and nonrigid states was found here. This has been explained in terms of the existence of many quasidegenerate low energy states in the potential energy surface of small SF6 clusters.