bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy partitioning in the dissociation reaction Ar3+→Ar2++Ar
Authors: Furuya Kenji , Kimura Katsumi , Hirayama Takato
Published: July 2002

Threshold-photoelectron photoion coincidence (TPEPICO) spectra of argon clusters ionized with synchrotron radiation have been observed, and the average kinetic energy of Ar2+, which originates from Ar3+, has been estimated. All the line shapes of the Ar2+ ion observed in the TPEPICO spectra have been appropriately simulated in terms of superposition of a sharp and a broad Gaussian function, where the sharp Gaussian function corresponds to ionization of the argon dimer Ar2. The observed line shapes agree very well with the simulated ones, implying that the excess energy in the Ar3 ionization is thermally or quasithermally partitioned to translational energy of the fragment species. From the average kinetic energies of Ar2+, it has been found that the percentage of excess energy in Ar3+, that is partitioned into the total translational energy of the fragment species, is dependent on the excitation energy. The percentage is slightly less than the value of 40% expected from statistical theory. It has also been found that complete dissociation of Ar3+ takes place in the excess energy range 0.3–1.1 eV.
An empirical many-body potential-energy function for aluminum. Application to solid phases and microclusters
Authors: Johnston Roy L. , Fang Jian-Yun
Published: July 2002

An empirical two-plus-three-body potential, developed by Murrell and co-workers [J. N. Murrell and R. A. Rodriguez-Ruiz, Mol. Phys. 71, 823 (1990)], is applied to the study of fcc aluminum. The parameters in the potential are derived by fitting the experimental phonon-dispersion curves and elastic constants. Calculations, using this potential, on a number of one-, two- and three-dimensional extended systems give results which are in quantitative agreement with recent ab initio calculations [I. J. Robertson, M. C. Payne, and V. Heine, Europhys. Lett. 15, 301 (1991)]. Calculations on small- and medium-sized aluminum clusters give cluster geometries and growth patterns which agree qualitatively with previous ab initio molecular-orbital and density-functional studies.
On the rate of radiationless intermolecular energy transfer
Authors: Scholes Gregory D. , Clayton Andrew H. A. , Ghiggino Kenneth P.
Published: July 2002

The effects of higher multipole interactions (up to electric quadrupole–quadrupole) and small exchange contributions on the rate of intermolecular energy transfer are examined. Second-order Coulombic interactions are described within a molecular quantum electrodynamical framework. A correction due to a small first-order exchange mechanism is then proposed. It is concluded that use of the multipole expansion in the interaction Hamiltonian is not always a good approximation at interchromophore separations of less than about 10 Å. This is attributed to a combination of large molecular dimensions compared to intermolecular separation, and wave function overlap effects. For larger separations, the interaction is described well by the usual dipolar coupling formalism. The inclusion of small exchange effects in a simplistic model at small to intermediate separations demonstrates the likelihood of a substantially greater rate of energy transfer than that predicted by either a Förster-type (dipole–dipole) or a purely exchange-type mechanism. The relevance to the photophysics of multichromophoric systems, such as aromatic polymers, is discussed.
The decay dynamics of photoexcited argon cluster ions
Authors: Smith J. A. , Gotts N. G. , Winkel J. F. , Hallett R. , Woodward C. A. , Stace A. J. , Whitaker B. J.
Published: July 2002

Following the photoexcitation of argon cluster ions, Ar+n for n in the range 4–25, kinetic energy release measurements have been undertaken on the fragments using two quite separate techniques. For Ar+4–Ar+6, fragment ion kinetic energy spectra were recorded at 532 nm in a crossed beam apparatus as a function of the angle of polarization of the laser radiation with respect to the incident ion beam. Only Ar+ from Ar+4 was observed to exhibit a polarization dependence together with a comparatively high kinetic energy release. The principal fragment ion Ar+2 was found both to emerge with a low kinetic energy release and to display no dependence on the angle of polarization of the radiation. In a second series of experiments, mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time-of-flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption of a photon, all cluster ions up to Ar+25 were found to eject one/two neutral atoms with comparatively high kinetic energies. Any remaining internal energy appears to be dissipated through the loss of further neutral atoms with low kinetic energies. An analysis of the laser polarization dependence of these events, shows that those atoms identified as having high kinetic energies are ejected on a time scale which is short compared with the rotation period of a cluster (≂10 ps).

These experimental observations are consistent with the results of recent molecular dynamics simulations of excited states in rare gas clusters by Landman, Jortner, and co-workers [J. Phys. Chem. 91, 4890 (1987); J. Chem. Phys. 88, 4273 (1988)]. Kinetic energy releases calculated from the TOF spectra exhibit marked fluctuations as a function of cluster size, with Ar+15 showing a minimum and Ar+19 a maximum. It is suggested that such behavior is part of a dynamic response to changes in structure as the cluster ions increase in size. A qualitative explanation is provided through the assumption that the cluster ions take the form of solvated Ar+2 structures. Considerations of the energy available from the photon and the relative contribution each TOF feature makes to the total signal, places an upper limit of two as the number of high kinetic energy atoms ejected by the larger cluster ions.
Liquid-state integral equations at high density: On the mathematical origin of infinite-range oscillatory solutions
Authors: Henderson J. R. , Sabeur Z. A.
Published: July 2002

Analytic asymptotic analysis and finite element numerical procedures are used to elucidate the mathematical reasons for the appearance of infinite-range oscillatory solutions to certain integral equation theories of wall–fluid interfacial structure and liquid state radial distribution functions. The results contribute to two issues of recent debate: (i) what physical significance (if any) can be attributed to the apparent ‘‘solidlike’’ structure that is often (but not always) seen in high density solutions to liquid state integral equation theories and (ii) is the same mathematical structure present in density functional theories (i.e., in the presence of a variational condition arising from a free energy functional)?
Restricted open-shell Hartree–Fock-based many-body perturbation theory: Theory and application of energy and gradient calculations
Authors: Lauderdale Walter J. , Stanton John F. , Gauss Jürgen , Watts John D. , Bartlett Rodney J.
Published: July 2002

A new approach for many-body perturbation theory (MBPT) built upon a restricted open-shell Hartree–Fock (ROHF) reference function is presented. ROHF-MBPT is shown to give much improved results compared to unrestricted Hartree–Fock (UHF) MBPT in cases where there is large spin contamination of the UHF reference function, and to converge much more rapidly to the infinite-order coupled-cluster result. Equations for analytical gradients at the MBPT(2) level are described and implemented. ROHF-MBPT and restricted open-shell Hartree–Fock single- and double-excitation coupled cluster (ROHF-CCSD) applications are presented for several difficult cases. These include the structure and electron affinity of the CN radical; structure, binding energy, and vibrational frequencies of Li3; the structure and vibrational frequencies for the unobserved FCS molecule; and the multiplet structure of the Ni atom.
Sticking of rare gas atoms on the clean Ru(001) surface
Authors: Schlichting H. , Menzel D. , Brunner T. , Brenig W.
Published: July 2002

Absolute values of the initial sticking coefficients of rare gas atoms (Ne, Ar, Kr, Xe) on a flat, clean Ru(001) surface have been determined with thermal beams and a highly sensitive thermal desorption method. The sticking coefficients increase with increasing mass of the atoms. Their decrease with increasing gas temperature is stronger the lighter the atom; different surface temperatures within the accessible range do not measurably affect the sticking efficiency. At a gas temperature of 300 K and a surface temperature of 6.5 K the initial sticking coefficients are 0.004 for Ne, 0.13 for Ar, 0.25 for Kr, and 0.71 for Xe. Forced oscillator calculations treating the substrate phonons quantum mechanically have been performed. With the well depths derived from experiment, and other reasonable input parameters, absolute values and functional forms of the sticking coefficients can be reproduced. The low values are due to the high elastic reflection probability which is a consequence of the inefficient energy transfer and the phonon quantization. The calculated Debye–Waller factors at zero gas and surface temperature are 0.92 for Ne, 0.36 for Ar, 0.14 for Kr, and 0.01 for Xe. A classical interpretation of the sticking data is impossible at least for Ne and Ar.
The geometric and electronic structures of Arn+ (n=3–27)
Authors: Ikegami Tsutomu , Kondow Tamotsu , Iwata Suehiro
Published: July 2002

The most stable structures of Arn+, n=3–27, are determined with the analytical gradient method for the diatomics-in-molecules Hamiltonian. The oscillator strength distribution is evaluated. The charge is found to be localized on the central three atoms, which form the trimeric ion core. The first solvation shell evolves around the ion core and is completed at n=25. The calculation shows that the photoabsorption band is in the visible region, which originates from the 2Σu+→ 2Σg+ transition of the Ar3+ ion core, and is red-shifted with the increase of the cluster size, reproducing the experimental results. The red-shift is explained in terms of the solvated ion core model, in which the excited state of the ion core interacts strongly with the surrounding solvent atoms.
Photodissociation dynamics of CO2 at 157.6 nm by photofragment-translational spectroscopy
Authors: Stolow Albert , Lee Yuan T.
Published: July 2002

The photodissociation of CO2 at 157 nm was studied by the photofragment-translational spectroscopy technique. Product time-of-flight spectra were recorded and center-of-mass translational energy distributions were determined. Two electronic channels were observed—one forming O(1D) and the other O(3P). With previously determined anisotropy parameters of β=2 for the O(3P) channel and β=0 for the O(1D) channel, an electronic branching ratio of 6%±2%O(3P) was obtained, consistent with previous results. The translational energy distribution for the CO(v)+O(3P) channel was very broad (over 30 kcal/mol) and appeared to peak near CO(v=0). The value of β=2 for the O(3P) channel was confirmed by comparing Doppler profiles, derived from our measured translational energy distribution, with previously measured Doppler profiles. This suggests that the O(3P) channel arises from a direct transition to an excited triplet state. The O(1D) channel had a structured time-of-flight which related to rovibrational distributions of the CO product. The influence of the excitation of the CO2(ν2) bending mode was investigated and shown to have a small but not negligible contribution. Based upon a comparison of our data with a previous vacuum-ultraviolet (VUV) laser induced fluorescence study, we obtain as our best estimate of the vibrational branching ratio, CO(v=0)/CO(v=1)=1.9, for the CO(v)+O(1D) channel.
Nonequilibrium solvation: An ab initio quantum-mechanical method in the continuum cavity model approximation
Authors: Aguilar M. A. , Olivares del Valle F. J. , Tomasi J.
Published: July 2002

The electrostatic relationships necessary for the quantum-mechanical evaluation of the properties of a solute experiencing sudden changes in its internal charge distribution are here presented in a form suitable to perform accurate quantum-mechanical calculations of the solute properties. Attention has been paid to express the boundary conditions in the most convenient form and to avoid further constraints on the elaboration of the computational procedures. The approach exploits the separation of orientational (inertial) and electronic (inertialess) components of the polarization and complements the polarizable continuum method [Chem. Phys. 65, 239 (1982)], usually employed for static descriptions. Examples of application of the method to photoionization and electronic transitions processes are shown.
Finite element multiconfiguration Hartree–Fock determination of the nuclear quadrupole moments of chlorine, potassium, and calcium isotopes
Authors: Sundholm Dage , Olsen Jeppe
Published: July 2002

The nuclear quadrupole moments of chlorine, potassium, and calcium nuclei are determined by combining calculated electric field gradients at the nucleus with the experimental nuclear quadrupole coupling constants. The electric field gradients are obtained using large-scale finite-element multiconfiguration Hartree–Fock calculations on Cl(3p5;2P3/2), K(4p; 2P3/2), K(3p53d4s;4F9/2), and Ca(3d4s;1D2). The calculated nuclear quadrupole moments are Q(35Cl)=−81.65(80) mb, Q(37Cl)=−64.35(64) mb, Q(39K)=60.1(15) mb, Q(40K) =−74.9(19) mb, Q(41K)=73.3(18) mb, Q(41Ca)=−66.5(18) mb, and Q(43Ca) =−40.8(8) mb with the estimated uncertainty given within parentheses.
Laser vaporization generation of the SiB and SiAl radicals for matrix isolation electron spin resonance studies; comparison with theoretical calculations and assignment of their electronic ground states as X 4Σ
Authors: Knight Lon B. , McKinley Allan J. , Babb Robert M. , Morse Michael D. , Arrington Caleb A.
Published: July 2002

The first experimental spectroscopic study of the SiB and SiAl diatomic radicals is reported. Electron spin resonance results indicate that both molecules have X 4Σ ground electronic states, in agreement with earlier theoretical calculations. The SiB and SiAl radicals were generated in neon matrices at 4 K by trapping the products produced from the pulsed laser vaporization of their alloys. Electronic structure information for these radicals is especially interesting given the utilization of silicon doped materials in semiconductor applications. The observed nuclear hyperfine interactions (A tensors) for 10B, 11B, and 27Al in these molecular radicals were compared with the results of ab initio configuration-interaction theoretical calculations which were conducted as part of this experimental study. The neon matrix magnetic parameters (MHz) for Si 11B are D=800(2), g∥=2.0014(8), g⊥=2.0005(4), A⊥=92.4(5), and A∥=111(2). For Si 27Al the results (MHz) are D=9710(2), g∥=1.9994(8), and g⊥=1.9978(4), ‖A⊥‖=10.3(6), and ‖A∥‖=43.5(8).
Outer-sphere electron transfer in polar solvents: Quantum scaling of strongly interacting systems
Authors: Song Xueyu , Stuchebrukhov Alexei A.
Published: July 2002

The spin–boson Hamiltonian model is used to study electron transfer (ET) reactions of strongly interacting systems in polar solvents in the limit of fast dielectric relaxation of the solvent. The spectrum of polarization modes consists of low frequency modes which are treated classically, and high frequency modes which are treated quantum mechanically. A general explicit formula for the rate valid in all orders of perturbation theory in electronic coupling is derived. The rate formula is applicable in a wide range of parameters, including the inverted region of the reaction where the quantum tunneling corrections give the main contribution to the rate. It is found that the quantum degrees of freedom can be effectively eliminated from the model by renormalizing the electronic coupling matrix element. This renormalization results in the following scaling property of the electron transfer systems: a system containing both classical and quantum degrees of freedom is equivalent to a system of lower dimensionality, containing only classical degrees of freedom, with renormalized electronic coupling matrix element. An explicit formula for the renormalization is obtained.
Time-resolved small-angle neutron scattering study of spinodal decomposition in deuterated and protonated polybutadiene blends. II. Q-dependence of Onsager kinetic coefficient
Authors: Jinnai Hiroshi , Hasegawa Hirokazu , Hashimoto Takeji , Han Charles C.
Published: July 2002

The spinodal decomposition (SD) of a critical mixture of deuterated and protonated polybutadiene of nearly equal chain lengths was investigated. This mixture has an upper critical solution temperature type phase diagram and the spinodal temperature at the critical point is 99.2 °C. Phase separation was induced by quenching a single-phase specimen at an initial temperature, Ti (=102.3, 123.9, and 171.6 °C), to a final temperature, Tf (=−7.5, 1.1, and 10.5 °C). The subsequent SD was followed by time-resolved small-angle neutron scattering. The Onsager coefficient, Λ(q;Tf), as a function of wave number q and Tf, derived from experimental growth rates, R(q;Tf), of the Fourier mode of concentration fluctuations and estimation of ST(q;Tf), was compared to the reptation model theories of Pincus and Binder. Experimental Λ(q;Tf) was found to give a q-dependence greater than that given by the theories. Here, ST(q;Tf) denotes the virtual structure factor at Tf inside the spinodal region. The reduced wave number Qm(τ) and intensity S̃m(τ) at the peak of the scattering structure factor in the early and intermediate stages of SD were found to be scalable in terms of a reduced time τ when Ti was fixed and Tf was varied, but not when Tf was fixed and Ti was varied. The failure of the scaling law in the latter instance may be attributed to the fact that the concentration fluctuation at the onset of SD has a different memory of the thermal concentration fluctuation in the single-phase region depending on Ti, which affects the subsequent SD over an extended period of time.
Water clusters: Contributions of binding energy and entropy to stability
Authors: Shi Z. , Ford J. V. , Wei S. , Castleman A. W.
Published: July 2002

The binding energies of water cluster cations are obtained by measuring decay fractions of metastable dissociation and employing Klots’ model of evaporative dissociation. Their variation with degree of solvation shows the commonly observed decrease, followed by a slow rise in magnitude, which typifies the trend found for solvated cations. There is no observed abrupt change in the vicinity of the well-known magic number (H2O)21⋅H+ corresponding to (H2O)20⋅H3O+. Other data are used to deduce free energies for water clusters up to size n=28, allowing a determination of entropy changes with size. All of the thermochemical data, including prior literature values, are assessed in terms of calculations made using the liquid drop model and standard statistical mechanical equations. It is concluded that entropic rather than energetic effects give rise to the referred to magic number.
Normal modes analysis of Ã-state acetylene based on directly observed fundamental vibrations
Authors: Tobiason J. D. , Utz A. L. , Sibert E. L. , Crim F. F.
Published: July 2002

Recent experimental results permit a detailed normal modes analysis of Ã-state acetylene (C2H2) and its isotopomers (C2HD and C2D2). Using only experimentally determined frequencies and measured or estimated anharmonicities, we determine harmonic frequencies for the 11 directly observed and unambiguously assigned vibrational fundamentals. The normal modes calculation varies force constants to fit the 11 harmonic frequencies and yields a complete set of harmonic frequencies, force constants, and Coriolis coefficients for the three isotopomers. A complete set of fundamental frequencies calculated from the set of harmonic frequencies allows a comparison to and, in some cases, suggests a reassessment of frequencies for tentatively assigned fundamental vibrations.
Dynamics of Penning ionization reactions involving He(2 3S) metastable atoms
Authors: Rutherford G. H. , Soletsky P. A. , Bryan W. H. , Dunning F. B. , Walters G. K.
Published: July 2002

Spin-labeling techniques, specifically the use of electron-spin-polarized He(2 3S) metastable atoms coupled with energy-resolved spin analysis of the product electrons, are used to investigate the dynamics of Penning ionization in collisions between He(2 3S) atoms and H2O, SO2, NO, and NO2. The data complement earlier studies of the reaction dynamics based on analysis of the energy distribution of the product electrons and confirm that ionization can occur via a number of different reaction channels. For example, the present results show that in collisions with targets having a positive electron affinity, ionization via the ionic channel is important and that effects due to spin-orbit coupling must be considered. The data also provide evidence that exchange may be important in collisions with open-shell targets.
Competitive internal transfers in metastable decay of cluster ions
Authors: Buonomo E. , Gianturco F. A. , Delgado-Barrio G. , Miret-Artés S. , Villarreal P.
Published: July 2002

In a previous study of fragmentation patterns of (Ar)+3 clusters [G. Delgado-Barrio, S. Miret-Artés, P. Villarreal, and F. A. Gianturco, Z. Phys. D 27, 354 (1993)] it was found that overall rotations control the lifetimes of the occupied metastable states of the cluster and that a spherical, effective interaction was sufficient to describe the dynamical process. In the present study, the strong anisotropy of a more realistic three-particle interaction is introduced and its effects on metastable decay are examined. By separating internal rotations from internal vibrations of the diatomic ion, it is possible to show that internal predissociation pathways are very efficient and lead to very short lifetimes. The latter can be lengthened only when overall rotational states are directly included, thus confirming the physical picture of the earlier work.
Molecular hyperfine structure in the photoassociation spectroscopy of laser cooled atoms
Authors: Williams Carl J. , Julienne Paul S.
Published: July 2002

The photoassociation of colliding ultracold trapped atoms permits the observation of high resolution molecular spectra of high lying vibrational levels with low total angular momentum and outer turning points greater than 50 Å. This paper describes how the hyperfine structure of alkali dimer molecules at large internuclear separation can be used to assign the electronic character of the observed photoassociation spectra and how hyperfine structure helps to explain the observed photoassociation spectrum of the attractive electronic states of the Na2 molecule that correlate with Na(3 2S)+Na(3 2P3/2) atoms.
Determination of diabatic coupling potentials by the inversion of inelastic atom–atom scattering data: Case studies for He++Ne and Li+I
Authors: Boyd Robert , Ho Tak-San , Rabitz Herschel , Padmavathi D. A. , Mishra Manoj K.
Published: July 2002

A general iterative inversion algorithm based on first-order functional sensitivity analysis and Tikhonov regularization is extended for the determination of diabatic coupling potentials from inelastic scattering data. For simplicity, the two-state case is presented here, and it is assumed that the (diagonal) diabatic potentials are known. ‘‘Noisy’’ and ‘‘noise-free’’ numerically simulated data, calculated from model potentials for He++Ne and Li+I, are used to illustrate the method. Various coupling potential trial functions are used, ranging from a constant multiple of the model to a step function. For most cases, the important regions of the coupling potential (i.e., those regions which are most sensitive to the inelastic scattering data, including the region of crossing) are recovered to high precision within four to seven iterations. Those cases which show a small range for convergence may indicate a limit of the present algorithm, based solely on first-order functional derivatives, and the need to extend it to include higher-order terms.
Chemical dimerization of crystalline anthracene produced by transient high pressure
Authors: Engelke Ray , Blais Normand C.
Published: July 2002

We report the production of a chemically bound dimer of anthracene produced by the shock compression of crystalline anthracene. The experimental probe used to detect the dimer structure was time-of-flight (TOF) mass spectrometry. The principal method used to produce the shock compression was the impact of electrically accelerated flyers (‘‘slappers’’) with the surface of the anthracene crystals. Our work correlates well with earlier experimental and theoretical work concerning chemical processes that can occur in pressurized anthracene and other aromatic materials. We briefly review the earlier work. The anthracene crystals were shocked to various pressures in the interval 9 to 22 GPa. Also various crystal thicknesses were employed; this allowed us to control the time interval over which various anthracene ‘‘particles’’ in the crystal were held at high pressure and temperature. At a pressure of ∼22.1 GPa we observe dimer formation in 20 ns or less. For a shock pressure of 9 GPa no dimer is produced, whereas for shock pressures of 18.4 GPa or higher dimer production is always observed. Under some conditions significant conversion of monomer to dimer is seen (up to a conversion of over 50%). A remarkably simple experimental observation is that the only new chemical species observed in the experiments at 18.4 GPa and higher is the dimer species. We present evidence that the dimerization is the result of a pressure-driven Diels–Alder reaction and that such reactions are characteristic of materials containing aromatic rings when they are shocked to high pressure.
Initial sticking coefficient of O2 on Ag(110)
Authors: Vattuone L. , Rocca M. , Boragno C. , Valbusa U.
Published: July 2002

We investigated the dynamics of the adsorption of O2 on Ag(110) with the molecular beam technique combined with EEL spectroscopy and with the method of King and Wells. The initial sticking coefficient S0 is reported for molecules impinging along both high symmetry azimuthal directions 〈001〉 and 〈11̄0〉 as a function of total energy and angle of incidence of the molecules and crystal temperature. The initial sticking coefficient is anisotropic for low as well as for room temperature adsorption. The dependence of the sticking coefficient on the temperature of the sample, Ts, indicates that adsorption takes place directly into the molecular well. Dissociation is eventually induced thermally for Ts≳150 K. The physisorbed state plays no role in the chemisorption process in the investigated impact energy range.
The intermolecular hydrogen–hydrogen structure of chain–molecule liquids from neutron diffraction
Authors: Londono J. D. , Annis B. K. , Turner J. Z. , Soper A. K.
Published: July 2002

Neutron diffraction isotopic substitution experiments on liquid n-decane (C10H22) and n-eicosane (C20H42) are described. The intermolecular H–H structure function hHH(Q) and the intermolecular H–H correlation function ginterHH(r) are obtained without recourse to models of the intramolecular structure. The structure of the ginterHH(r) found at 2.5, 5.0, and 7.0 Å corresponds to different shells in the H–H pair correlation function. In addition, ginterHH(r)<1 for a considerable range, due to the screening of intermolecular correlations by intramolecular correlations. This ‘‘correlation hole’’ effect is accentuated by extrapolation of the structure functions to the expected infinite wavelength limit, and shows good agreement with values determined from small-angle neutron scattering (SANS) data. All of these features are in good agreement with the results of molecular dynamics simulations for the closely related system C13H28.
Conformational dynamics and relaxation in bulk polybutadienes: A molecular dynamics simulation study
Authors: Gee Richard Hayes , Boyd Richard Hays
Published: July 2002

Conformational dynamics and relaxation of the torsional angle autocorrelation (ACF) function in bulk polybutadienes (PBD) have been studied over a wide temperature range via molecular dynamics simulations. All trans, all cis and 70 mol % trans were the configurations of the systems studied. Companion studies of these systems in the phantom state were also carried out to illuminate the effects of packing in the bulk. Conformational transitions at the –CH2–CH= bond alpha to the double bond and at the –CH2–CH2– beta bond were monitored. The activation energies for the overall transition rates for the α bond and for the β bond were found to correspond in both bulk and phantom states to one barrier height in their respective torsional potentials. A considerable degree of correlation was found between second neighbor bonds. At the α bond in trans-PBD correlated conformational changes across the double bond predominated but changes across the β bond when in the trans conformation were common also. In cis-PBD, due to steric hindrance at the α bond when in the cis conformation, correlated changes across the β bond were relatively more common although exchanges across the double bond were common also. In all of the above correlations, the direction of rotation in the two bonds are of opposite sign. In cis-PBD a correlated transition with same sign rotation was also found. The activation energies for relaxation times for the torsional ACF in phantom PBD were also found to correspond to single torsional barrier heights. In bulk PBD, however, the activation energies were significantly higher than single barrier. This behavior is the result of the conformational transitions becoming more heterogeneously distributed spatially over the bonds in the bulk as temperature is lowered. This phenomenon has also been found recently in simulations on polyethylene.
The breaking and remaking of a bond: Caging of I2 in solid Kr
Authors: Zadoyan R. , Li Z. , Martens C. C. , Apkarian V. A.
Published: July 2002

The caging of I2 in solid Kr is followed in real-time following its dissociative excitation on the A(3Π1u) surface. The experiments involve pump–probe measurements with a time resolution of ≥150 fs. The experimental signals are reproduced using classical molecular dynamics simulations, and the classical Franck approximation. The comparison between experiment and simulation allows an unambiguous interpretation of features in the observed signal as being due to the initial impulsive stretch of the I–I bond, collision of the atoms with the cage wall, recoil and recombination, and the subsequent coherent oscillations of the nascent I2 molecule. These detailed observations are possible due to retention of coherence along the I–I coordinate throughout the caging process. The extent of coherence is dictated mainly by the initial impact parameters of the molecule-cage collision, which in turn is controlled by the thermal and zero-point amplitudes of lattice vibrations. The caging is well-described as a sudden process, involving a binary collision between I and Kr atoms with nearly complete energy loss of the I atom upon completion of the first collision. Vibrational relaxation of the bound molecule proceeds on the time scale of 12 ps. The nontrivial relation between this relaxation time and decay rates that may be extracted from experimental transients is discussed. Although the interplay between the nested A and A′ potentials is not detectable, it is clear that in the studied range of initial excess energies, 1000–1700 cm−1, the recombination remains effectively adiabatic, and does not involve the ground state.
Criticality of charged systems. II. The binary mixture of hard spheres and ions
Authors: Zhou Yaoqi , Stell George
Published: July 2002

The pairing mean spherical approximation (PMSA) developed in the previous paper of this series is applied to a binary mixture of hard spheres and ions of the restricted primitive model. The resulting equation of state is used to investigate the phase equilibrium for the binary mixture. It is found that the model exhibits type-III phase behavior. This work serves as the basis for more realistic models.
Intracavity absorption spectroscopy of the third stretching overtone transition of jet cooled methane
Authors: Campargue Alain , Permogorov Dmitri , Jost Rémy
Published: July 2002

The third stretching overtone transition of jet cooled methane has been recorded between 11 170 and 11 315 cm−1 by intracavity laser absorption spectroscopy associated with a supersonic slit jet expansion. By varying the methane percentage seeded in the carrier gas, we have studied the evolution of the spectrum vs the rotational temperature. The rotational cooling drastically reduces the congestion of the spectrum and only a few lines are observed in the spectrum corresponding to the lowest rotational temperature. In the 11 245–11 315 cm−1 range, the rotational structure of two superimposed bands is clearly observed. The lowest rotational levels of each band are assigned and the corresponding band origins, rotational constants and Coriolis parameters are obtained. The two bands are assigned to the two components of a Fermi dyad corresponding to the interaction of the 3ν1+ν3 (F2) stretching level carrying the oscillator strength with a near resonant vibrational state of the background. The high mixing of the vibrational wave functions is reflected by the close values of the rotational constants obtained for the two bands. In the lower energy range of the spectrum (11 170–11 245 cm−1), two weaker vibrational bands are tentatively assigned.
Chemical reaction jet spectroscopy, molecular structure, and the bending potential of the Ã 1A″ state of monofluorosilylene (HSiF)
Authors: Harper Warren W. , Karolczak J. , Clouthier Dennis J. , Ross Stephen C.
Published: July 2002

The jet-cooled laser induced fluorescence excitation spectrum of the Ã 1A″–X̃ 1A′ band system of HSiF has been observed with the chemical reaction jet technique. Vibrational analysis of the spectrum gave upper state fundamental vibrational frequencies of ν1=1547 cm−1, ν2=558 cm−1, and ν3=857 cm−1. Seven bands in the spectrum were recorded at high resolution and rotationally analyzed, providing excited state molecular constants. The upper state vibrational and rotational bending levels were fitted to a semirigid bender model to obtain the equilibrium geometry and the potential energy barrier to linearity. Due to correlations in the parameters, it was necessary to fix the bond angle at the ab initio value of 114.5°. The resulting fitted model yielded re(Si–F)=1.602 Å, re(Si–H)=1.548 Å with a potential energy barrier to linearity of 9130 cm−1.
How accurate must potentials be for successful modeling of protein folding?
Authors: Pande Vijay S. , Grosberg Alexander Yu. , Tanaka Toyoichi
Published: July 2002

Protein sequences are believed to have been selected to provide the stability of, and reliable renaturation to, an encoded unique spatial fold. In recently proposed theoretical schemes, this selection is modeled as ‘‘minimal frustration,’’ or ‘‘optimal energy,’’ of the desirable target conformation over all possible sequences, such that the ‘‘design’’ of the sequence is governed by the interactions between monomers. With replica mean field theory, we examine the possibility to reconstruct the renaturation, or freezing transition, of the ‘‘designed’’ heteropolymer given the inevitable errors in the determination of interaction energies, that is, the difference between sets (matrices) of interactions governing chain design and conformations, respectively. We find that the possibility of folding to the designed conformation is controlled by the correlations of the elements of the design and renaturation interaction matrices; unlike random heteropolymers, the ground state of designed heteropolymers is sufficiently stable, such that even a substantial error in the interaction energy should still yield correct renaturation.
Global optimization using ab initio quantum mechanical potentials and simulated annealing of the classical Liouville equation
Authors: Field Martin J.
Published: July 2002

The routine location of the global minimum energy structures of a molecular system remains a challenging problem in chemical physics. Recently Straub and co-workers [J. Ma et al., J. Chem. Phys. 99, 4024 (1993) and 101, 533 (1994)] have introduced a number of novel dynamic simulated annealing algorithms for global optimization. Instead of using Newton’s equations, they are based on Liouville’s equation and the particles are described with phase space distributions. The methods were shown to have significantly enhanced effectiveness over traditional methods when used to locate the global minima of Lennard-Jones clusters. In this paper the extensions necessary to use these methods with ab initio quantum mechanical potentials are presented and their application is illustrated by the simulated annealing of a small lithium atom cluster.
Vibrational analyses of trans-polyacetylene based on ab initio second-order Mo/ller–Plesset perturbation calculations of trans-oligoenes
Authors: Hirata So , Torii Hajime , Tasumi Mitsuo
Published: July 2002

Structures and vibrational force fields of all-trans-oligoenes of various chain lengths are studied by ab initio molecular orbital (MO) calculations at the second-order Mo/ller–Plesset perturbation (MP2) level with the 3-21G and 6-31G* basis sets. Dependencies of the structure parameters and the force constants of trans-oligoenes on the chain length and the position in the chain are analyzed quantitatively. The structure and the force field of trans-polyacetylene are extrapolated from those of trans-oligoenes. Normal coordinate calculations are performed for trans-polyacetylene on this basis. The following results show the reliability of the structure and the vibrational force field of trans-polyacetylene obtained in the present study. (1) The calculated C=C and C–C bond lengths of trans-polyacetylene are in satisfactory agreement with the observed. (2) The calculated δ=0 (null phase difference) frequencies are in good agreement with the observed infrared and Raman frequencies. (3) The calculated phonon dispersion curves are consistent with some criteria obtained experimentally. (4) Most of the observed infrared bands which do not correspond to the δ=0 frequencies of trans-polyacetylene are assignable to peaks in the profile of the calculated density of vibrational states. (5) The calculated hydrogen-amplitude-weighted density of states is in reasonable agreement with the observed inelastic neutron scattering spectrum.
Single-electron transfer in collisions of He2+ with NH3 and H2S: Vibrational state populations of NH+3 and H2S+
Authors: Fárník Michal , Herman Zdenek , Ruhaltinger Thomas , Toennies J. Peter
Published: July 2002

Single-electron charge transfer between He2+ and NH3 and H2S was investigated at the projectile energy of 70 eV by the translational energy spectroscopy method with a resolution of 120 meV. The products were He+*(2P) and the ground state molecular ion. Populations of the vibrational states of the molecular ion were derived from the spectra. The vibrational state distributions differ only slightly from the distributions obtained by photoelectron spectroscopy and the differences can be qualitatively accounted for by distortions of the equilibrium configuration of the target molecule by the approaching ion projectile.
Anisotropic anharmonicity of lattice and molecular vibrations of 1,2,4,5-tetrabromobenzene determined by piezomodulated Raman spectroscopy
Authors: White K. M. , Eckhardt C. J.
Published: July 2002

Strain-induced coupling constants for the anharmonicity of Raman-active lattice and certain molecular modes of vibration in 1,2,4,5-tetrabromobenzene (TBB) crystals have been determined using piezomodulated Raman spectroscopy. These constants, which are directly related to the first anharmonic term in the potential energy expansion for lattice dynamical calculations, are a quantitative measurement of the modal anharmonicities in the TBB molecular crystal. Application of uniaxial stress in the experiments permits the anisotropy of the anharmonicity to be determined as well as its magnitude. The TBB lattice modes are significantly coupled by the induced strains and the effects of coupling were observed to be dependent on the direction and symmetry of the strains. The molecular modes investigated were, by comparison, less coupled by the acoustic phonons and generally exhibited less anharmonic response with increasing frequency.
Generalized sum rules for optical nonlinearities of many-electron systems
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

Sum rules which connect spectral moments of linear and nonlinear optical susceptibilities of nonrelativistic many-electron systems to their ground state properties are derived. These sum rules provide a direct connection between chemical bonding and optical properties and may assist in constructing tight-binding Hamiltonians for the effective modeling of electronic spectroscopy. In the time-dependent Hartree–Fock approximation, the only relevant ground state information is the Hartree–Fock ground stated reduced single-electron density matrix.
Modeling intermolecular exchange integrals between nonorthogonal molecular orbitals
Authors: Jensen Jan H.
Published: July 2002

The intermolecular exchange integrals between nonorthogonal molecular orbitals can be approximated by representing the region of overlap as a spherical Gaussian. This spherical Gaussian overlap approximation is shown to perform better, especially if localized orbitals are used, than the Mulliken approximation for a variety of water dimer conformations.
Kinetic evidence for the dependence of surface reaction rates on the distribution of reactants on the surface
Authors: Xu Mingde , Liu Jinyao , Zaera Francisco
Published: July 2002

Two kinetically distinct types of oxygen atoms can be identified during the isothermal oxidation of CO on Pt(111) even though they all sit in identical sites at the start of the reaction. This is explained by a lowering in the reaction activation barrier with oxygen islanding.
Computer simulation study of the approximations associated with the generalized Flory theories
Authors: Kumar Sanat K. , Szleifer Igal , Hall Carol K. , Wichert John M.
Published: July 2002

The chain increment method and configurational bias Monte Carlo methods are used to test the approximations made in the derivation of the generalized Flory-Dimer (GF-D) theory for tangent hard sphere chains. Insertion probabilities and residual chemical potentials are calculated for hard chain fluids containing chains of length n=4, 8, 16, and 32 at monomer densities, ρM, up to 0.8. We find that the largest errors in the GF-D theory are those associated with assuming that the probability of inserting a monomer into a chain fluid is approximately equal to the probability of inserting a monomer into a monomer fluid, as predicted by the Carnahan–Starling equation of state. The errors in the incremental compressibility factor of the second segment associated with assuming that the conditional probability of inserting a second bead next to the first bead in a chain fluid is approximately equal to the probability of inserting a second bead next to the first bead in a dimer fluid as predicted by combining the Carnahan–Starling and Tildesley–Streett equations of state are relatively small. Consistent with the findings of Mooij and Frenkel, we find that these two approximations lead to an overprediction of the incremental contributions to the compressibility factor. Despite the overprediction of the incremental contributions to the compressibility factor of the first segment, the GF-D equation of state accurately predicts the compressibility of hard chains; this accuracy is traced to (1) the insensitivity of the compressibility factor to errors in the insertion probability and (2) cancellation of errors in the incremental compressibility factor of the first segment with small cumulative errors in the incremental compressibility factors of the third and subsequent segments.
High resolution single-photon ionization of HBr in the spin–orbit autoionization region
Authors: Irrgang R. , Böwering N. , Drescher M. , Spieweck M. , Heinzmann U.
Published: July 2002

The rotationally resolved photoelectron yield in the ionization of jet-cooled HBr was measured in the energy range between the spin–orbit split 2Π ionic thresholds. For single-photon excitation, narrow-band VUV radiation was generated by resonant frequency mixing. The spectrum is complex due to interaction of autoionizing resonances belonging to several series converging to different rotational states of the ion core. For low Rydberg orders, n≤11, a description of angular momentum coupling according to Hund’s case (c) is found to be appropriate. An assignment of higher Rydberg series in the range of 17≤n≤20 was made in Hund’s case (e) coupling scheme by comparison with the corresponding photoionization spectra of Kr, HI, and HCl.
Molecular structure of the linear C3H radical: Microwave spectrum of the 13C substituted species
Authors: Kanada Mitsuka , Yamamoto Satoshi , Saito Shuji , Osamura Yoshihiro
Published: July 2002

The rotational spectra of three 13C isotopic species of the linear C3H radical in the 2Πr ground vibronic state and the 2Σμ vibronic state of the ν4 (HCC bending) vibrational mode were observed by using a source-modulated spectrometer. The radical was produced in a free-space cell by a glow discharge in a mixture of C2H2, He, and 13CO. The rotational, spin-rotation interaction, Λ-type doubling, and hyperfine interaction constants were determined from the analysis of the observed spectra, where vibronic interaction between the 2Πr and 2Σμ states was taken into account. The rs structure of C3H, denoted by CαCβCγH, was derived from the rotational constants for the normal, deuterated, and three 13C species: rs(Cα–Cβ)=1.3263(1) Å, rs(Cβ–Cγ)=1.2539(2) Å, and rs(Cγ–H)=1.0171(1) Å. The short C–H distance is interpreted in terms of a large amplitude motion of the ν4 mode, whose vibrational energy is very low due to the Renner–Teller effect. An ab initio molecular orbital calculation was carried out, and the structure in which the HCC angle is bent was found to be slightly more stable than the linear structure. The spin densities of three carbon atoms are derived from the hyperfine interaction constants, and they are discussed in connection to the quasilinear nature of the molecule.
Moderately dense gas quantum kinetic theory: Aspects of pair correlations
Authors: Snider R. F. , Wei G. W. , Muga J. G.
Published: July 2002

A recently formulated density corrected quantum Boltzmann equation emphasizes the need to explicitly include pair correlations and the conversion of kinetic energy to potential energy as important effects in the kinetic theory of moderately dense gases. This paper first considers an appropriate evolution equation for the pair correlations which includes their decay via interactions with other particles in the gas. The molecular description is given of such a gas close to local thermal equilibrium, together with expressions for the associated hydrodynamic variables. Wigner functions are used to uniquely separate macroscopic and microscopic properties. An accompanying paper solves the combination of linearized Boltzmann and correlated pair equations to obtain expressions for the transport coefficients.