bluuebunny/crossref_metadata_2025 huggingface dataset:
Probing the dynamics of hydrogen recombination on Si(100)
Authors: Kolasinski Kurt W. , Shane Stacey F. , Zare Richard N.
Published: July 2002

We have measured rotational distributions for H2 and D2 thermally desorbed from Si(100) surfaces using resonance-enhanced multiphoton ionization (REMPI) for detection. These distributions are described by average rotational energies that are significantly lower than kTs (Ts=surface temperature) and exhibit slight, if any, isotopic dependence, i.e, 〈Erot(H2)〉=368±67 K and 〈Erot(D2)〉=348±65 K. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high-impact-parameter collisions. The rotational distributions indicate that some dynamical constraint causes very little torque to be applied to molecular hydrogen during recombination. Our data may be interpreted as resulting from an exaggerated preference for reactive trajectories that are characterized by low-impact parameters and/or a high degree of symmetry of the bond axis relative to the potential energy surface, followed by prompt desorption of the newly formed molecular hydrogen from Si(100).
Quasielastic neutron scattering measurements of n-butane in its crystalline, plastic, and liquid phases
Authors: Bradley K. F. , Chen S.-H. , Brun T. O.
Published: July 2002

We report here measurements of quasielastic neutron scattering from n-butane at temperatures of 90, 115, 125 and 190 K and in a momentum transfer range of 0.8–2.4 Å−1. These measurements confirm that between 115 and 125 K butane forms a plastic crystal in which the centers of mass of the butane molecules form a crystalline structure, but the individual molecules are free to rotate. At these two intermediate temperatures, there exists both an elastic peak, characteristic of a solid structure, and quasielastic components arising from the rotational motions of the butane molecules. At 90 K, the butane scatters neutrons only elastically, while at 190 K, the butane scatters neutrons only quasielastically. In both the plastic and the liquid phases, the presence of at least two quasielastic processes must be assumed in order to explain the measurements. In the plastic crystal, we associate a broad Lorentzian component with intramolecular reorientations about the central carbon–carbon bond and a second, relatively narrow, Lorentzian component with whole molecule rotations.

The latter process gives rise to a rotational quasielastic peak having a width of 400 μeV, which is constant to within the instrumental resolution of 70 μeV at both temperatures and at all measured momentum transfers. In a continuous diffusion model, this width corresponds to a rotational diffusion constant of 0.277 rad2/ps, a value which is about 3.5 times larger than one extracted from a molecular dynamics simulation of n-butane in the plastic phase recently published by Refson and Pawley [Mol. Phys. 61, 669 (1987); 61, 693 (1987)]. On the other hand, the first process, which corresponds to the carbon–carbon reorientation peak, is about 16 meV wide, indicating that this reorientation occurs on a time scale of about 0.1 ps. The absence of this broad peak in the solid butane at 90 K indicates that this fast carbon–carbon reorientation is coupled to an aspect of the structure or dynamics of the plastic phase. In the scattering from liquid butane at 190 K, there is a third, narrow quasielastic peak which also has a Lorentzian energy distribution. This peak width is about 200 μeV and corresponds to a translational diffusion constant of 0.23 Å2/ps, a result in rough agreement with a recent molecular dynamics simulation of liquid butane by Ullo and Yip. [J. Chem. Phys. 85, 4056 (1986)].
Solid state proton spin relaxation and t-butyl and methyl group reorientation in 1-bromo-2,4,6-tri-t-butylbenzene
Authors: Fry Anne M. , Beckmann Peter A. , Fry Albert J. , Fox Peter C. , Isenstadt Ari
Published: July 2002

We have used the solid state proton spin relaxation technique to investigate the barriers for methyl and t-butyl group reorientation in polycrystalline 1-bromo-2,4,6-tri-t-butylbenzene. The barriers in the range of 15–19 kJ/mol (3–5 kcal/mol) are compared with those found in related molecules. It is shown that the neighboring ring bromine atom has an effect on the barrier for t-butyl group reorientation similar to that of a neighboring hydrogen atom despite the significantly larger van der Waals’ radius of a bromine atom. This most likely occurs because of the relatively long carbon–bromine bond, the distorted ring geometry, and the relatively high polarizability of bromine. In addition, the barriers for methyl group reorientation, about 16 kJ/mol, seem to be largely intra-t-butyl in origin.
Electronic structure of thiophene and pyrrole dimers: 2,2′-bithiophene, 2,2′-thienylpyrrole, and 2,2′-bipyrrole
Authors: Birnbaum Duane , Kohler Bryan E.
Published: July 2002

Vibrationally resolved fluorescence and fluorescence excitation spectra have been measured for solid solutions of 2,2′-bithiophene, 2,2′-thienylpyrrole, and 2,2′-bipyrrole in n-hexane at 4.2 K. The vibronic development of the spectra of all three molecules is very similar to what has been reported previously for longer polythiophene oligomers. Bithiophene and thienylpyrrole have similar S0 to S1 excitation energies: 29 603 and 30 006 cm−1, respectively. The S0 to S1 excitation energy of bipyrrole is significantly higher, 32 720 cm−1, indicating nonplanarity of the pyrrole rings. The vibrational development of the spectra and coincident origin energies in excitation and emission establish that the S0 to S1 transition is symmetry allowed for all three molecules, reasonably described in simple molecular orbital theory as the promotion of one electron from the highest energy occupied molecular orbital (HOMO) to the lowest energy unoccupied molecular orbital (LUMO).
Fluid–fluid phase separation of nonadditive hard-sphere mixtures as predicted by integral-equation theories
Authors: Gazzillo Domenico
Published: July 2002

We investigate the existence of a fluid–fluid phase separation in binary mixtures of equal-size hard spheres with positively nonadditive diameters [i.e., d11=d22≡d, d12=(1+Δ)d with Δ>0]. An integral-equation approach is used to evaluate both thermodynamics and structure of many symmetric (equal to equimolar) mixtures (with Δ=0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1) and some asymmetric cases. We present the results obtained via the Percus–Yevick, the Martynov–Sarkisov, and the Ballone–Pastore–Galli–Gazzillo closures; the thermodynamic consistency of these approximations is discussed and some possible ways to get further improvements are proposed too. The integral-equation results are then compared with the available ‘‘exact’’ simulation data, a first-order perturbation approach, and a scaled particle theory. Our study predicts that there exists a demixing for each considered value of the nonadditivity parameter Δ.
Simulation of the band structure of liquids: Some calculation considerations and a test of the mean-spherical approximation
Authors: Ganguly Kishor , Stratt Richard M.
Published: July 2002

The discovery that simple classical liquid theories such as the mean-spherical approximation (MSA) provide a route for calculating electronic band structure opens up some interesting possibilities for investigating the interplay between liquid structure and electronic behavior. However, the MSA is better known for providing qualitative insights than it is for the accuracy of its quantitative predictions. In this paper we compare the MSA-computed band structure of a hard-sphere liquid with the exact band structure obtained from direct numerical diagonalization of the Hamiltonian at Monte Carlo–generated liquid configurations. In the medium dense to dense liquid range, the agreement is qualitatively reasonable with both s and p orbital basis sets, regardless of whether one includes nonorthogonality effects. Lower densities, as expected, are not as well treated. In the course of this comparison we also found it necessary to be aware of the limitations on computer-simulated band structures which are imposed by finite system size.
The shape of two-dimensional linear and star polymers with and without excluded volume
Authors: Bishop Marvin , Clarke Julian H. R. , Rey Antonio , Freire Juan J.
Published: July 2002

The shape of isolated two-dimensional linear and star polymers, with and without excluded volume, have been determined by Monte Carlo simulations using a pivot algorithm. Stars with 3, 4, 5, and 6 arms are studied. The high efficiency of the pivot algorithm has allowed us to obtain accurate data for long chains. Good agreement is found with existing analytical expressions for nonexcluded volume chains and with other computer simulations for excluded volume chains.
Modeling reaction–diffusion pattern formation in the Couette flow reactor
Authors: Elezgaray J. , Arneodo A.
Published: July 2002

We report on a numerical and theoretical study of spatio–temporal pattern forming phenomena in a one-dimensional reaction–diffusion system with equal diffusion coefficients. When imposing a concentration gradient through the system, this model mimics the sustained stationary and periodically oscillating ‘‘front structures’’ observed in a recent experiment conducted in the Couette flow reactor. Conditions are also found under which oscillations of the nontrivial spatial patterns become chaotic. Singular perturbation techniques are used to study the existence and the linear stability of single-front and multi-front patterns. A nonlinear analysis of bifurcating patterns is carried out using a center manifold/normal form approach. The theoretical predictions of the normal form calculations are found in quantitative agreement with direct simulations of the Hopf bifurcation from steady to oscillating front patterns. The remarkable feature of these sustained spatio–temporal phenomena is the fact that they organize due to the interaction of the diffusion process with a chemical reaction which itself would proceed in a stationary manner if diffusion was negligible. This study clearly demonstrates that complex spatio–temporal patterns do not necessarily result from the coupling of oscillators or nonlinear transport.
Temporary negative ions and vibrational excitation in CH3Cl and CD3Cl
Authors: Shi Xueying , Stephen T. M. , Burrow P. D.
Published: July 2002

Electron energy loss spectroscopy is used to study the vibrational modes of CH3Cl and CD3Cl excited through the formation of a temporary negative ion state centered at 3.5 eV. Using symmetry and charge distribution arguments together with the experimentally determined excitation functions for several vibrational modes, we confirm that this resonance results from filling of the 8a1(C–Cl) σ* molecular orbital. Evidence for a broad higher-lying resonance which we attribute to the unfilled 4e orbital is also presented.
The microwave spectrum of the CH2N radical in the X̃ 2B2 ground electronic state
Authors: Yamamoto Satoshi , Saito Shuji
Published: July 2002

The rotational spectral lines of the CH2N radical in the 2uBd2d ground electronic state are detected by using a source-modulation microwave spectrometer. The radical is produced in a glow discharge plasma of a gaseous mixture of CH4 and N2. In total, 181 fine and hyperfine components of the rotational transitions with N=1–0 to N=5–4 are observed in the frequency region between 73 and 370 GHz. They are analyzed to obtain the molecular constants including hyperfine interaction constants for the H and N nuclei. The molecular structure and the spin density of CH2N are determined from the obtained molecular constants.
Quantum functional sensitivity analysis for the collinear H+H2 reaction rate coefficient
Authors: Chang Johnny , Brown Nancy J. , D’Mello Michael , Wyatt Robert E. , Rabitz Herschel
Published: July 2002

The effects of features in the potential energy surface on the collinear H+H2 reaction rate coefficient are investigated by the method of quantum functional sensitivity analysis (QFSA). The calculations use QFSA to connect features in the microscopic realm, with their response upon macroscopic quantities of chemical interest, via the intermediary sensitivities of the reactive transition probabilities. While the sensitivities of the individual transition probabilities show considerable structure, there is an attendant loss of structure in the rate coefficient sensitivities because of the thermal averaging. For the range of temperatures used in our study (200–2400 K), the most important region of the potential energy surface is found to be not at the top of the barrier, but rather at the lower energy shoulders of the barrier. There are also regions near the barrier where an increase in the potential surface actually increases the reaction rate! The effects of using different underlying potentials [the Porter–Karplus (PK2), Liu–Siegbahn–Truhlar–Horowitz (LSTH), and double many-body expansion (DMBE) surfaces] on the nature of the results were also compared. The absolute sensitivity magnitudes on the PK2 surface vary considerably from the other two, but the relative change in the rate coefficient is about the same on all three surfaces. Furthermore, the identified regions of importance on the potential surfaces remain essentially the same. The reactive scattering calculations were performed with the log-derivative version of the Kohn variational principle.
Rotational coherence spectroscopy and structure of phenol dimer
Authors: Connell L. L. , Ohline S. M. , Joireman P. W. , Corcoran T. C. , Felker P. M.
Published: July 2002

Rotational coherence spectroscopy has been used to measure the rotational constants of four isotopomers of phenol dimer and a single isotopomer of p-cresol dimer. From the results of these measurements, together with spectroscopic results reported by others, a geometry for phenol dimer is deduced. The species is found to be bound by an O–H⋅⋅⋅O hydrogen bond. The orientation of the phenyl moieties is such that they make maximal contact consistent with the constraints imposed by the hydrogen bond and by the van der Waals radii of the atoms. This geometric feature is cited as evidence for the significance of aromatic–aromatic attraction in the intermolecular interaction between the phenols.
Location of a 1A
g state in bithiophene
Authors: Birnbaum Duane , Kohler Bryan E.
Published: July 2002

The 0–0 excitation energy of a 1Ag state of 2,2′-bithiophene has been determined by measuring the two-photon fluorescence excitation spectrum of a dilute solution of this molecule in crystalline n-hexane at 77 K. Because the 0–0 energy is what would have been predicted by extrapolating previously measured α,ω-dithienylpolyene 2 1Ag 0–0 energies to zero polyene chain length, it is assigned to the 2 1Ag state. The 0–0 band is centered at 36 173 cm−1, approximately 6570 cm−1 above the 0–0 of the 1 1Ag to 1 1Bu transition. This order of bithiophene excited singlet states is opposite to that of the linear polyene with the same number of double bond but may reverse for chains longer than six repeat units.
Laser induced stabilization and alignment in multiphoton dissociation of diatomic molecules
Authors: McCann James F. , Bandrauk André D.
Published: July 2002

The two step photodissociation (1 1Σ+g→1 1Σ+u→1 1Πg) transition in Li2 is studied by a coupled equations method. The first transition is assumed weak and selects particular rovibrational states as intermediate states. The second transition is induced by a high intensity infrared laser. The photodissociation yield for different isotopes is investigated as well as the coherence effects in the excited intermediate states produced by the intense field. Stabilization of dressed molecular states and laser induced alignment effects are shown to occur in the photodissociation at high intensities of the second laser. The effect of the polarization of the lasers on the differential cross sections is also discussed.
Equation of state of two-dimensional lattice chains at the theta point
Authors: Dickman Ronald
Published: July 2002

Systems of two-dimensional lattice self-avoiding walks with nearest-neighbor attractive interactions are studied in Monte Carlo simulations, focusing on the θ point, where the second virial coefficient vanishes. The equation of state is determined for the first time, for chains of 40 and 80 segments over a wide range of densities. The results are consistent with des Cloizeaux’ scaling law, and yield a value for the tricritical exponent νt0.57(3), in close agreement with recent estimates. The simulations also provide information on the the density profile at a wall, and on the variation of chain dimensions with density at the θ point.
Comment on: Is multiple quantum nuclear magnetic resonance spectroscopy of liquid water real?
Authors: Abergel Daniel , Delsuc Marc A. , Lallemand Jean-Yves
Published: July 2002

Recently, two-dimensional NMR experiments have been performed, which were likely to show that a coupling between otherwise uncoupled or equivalent spins was observable, yielding signals originating from multiple-spin multiple-quantum operators in the density matrix. According to the authors, such surprising results could be interpreted as a consequence of the radiation damping phenomenon. For this purpose, a fully quantum-mechanical frame was used, thus treating the radiation field in a quantized manner. Our analysis, based on the same theoretical grounds, as well as on new experimental arguments, lead us to diverging conclusions.
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s-butadiene π-valence states using the effective valence shell Hamiltonian method
Authors: Graham Richard L. , Freed Karl F.
Published: July 2002

Low-lying π-electron vertical excitation energies of trans-butadiene are calculated using the effective valence shell Hamiltonian method. The results are compared with previous experimental and theoretical analyses of this molecule’s congested electronic spectra. The computations employ a large basis set (126 functions) which includes both diffuse functions on the carbon atoms and polarization functions on all atoms. Good agreement is obtained with the experimentally well known vertical excitation energies to the 1 3Bu, 1 3Ag, and 1 1Bu states where deviations from experiment are only 0.01, 0.01, and 0.22 eV, respectively. We confirm the experimental assignment of a valence like 1Ag state around 7.4 eV (calculated at 7.49 eV). Likewise, a member of a symmetry allowed 3p Rydberg series (of Au or Bu symmetry) in the electron impact spectrum with origin at 7.07 eV is assigned as the 2 1Bu state (with calculated vertical excitation energy of 7.00 eV). Most experiments place the 2 1Ag state above the 1 1Bu state; however, a resonance Raman assignment places it below. Our calculated excitation to the 2 1Ag state is 0.05 eV above the 2 1Bu state, about 0.5 eV lower than previous ab initio determinations. The computed vertical excitation energies are in good agreement with the interpretation of experimental electronic spectra, are in much better agreement with experiment than previously published ab initio calculations, provide the first definitive assignment of the 2 1Bu state at 7.08 eV, and conclusively assign the 3 1Ag state at 7.4 eV. The accuracy of the large basis effective valence shell Hamiltonian is, in part, due to retention of both valence and Rydberg orbitals in the valence space, a feature which has a bearing on intruder state problems and on current semiempirical π-electron theories.
Structural transitions in a monolayer of fluorinated amphiphile molecules
Authors: Barton Scott W. , Goudot Anne , Bouloussa Othman , Rondelez Francis , Lin Binhua , Novak Frank , Acero Anibal , Rice Stuart A.
Published: July 2002

We present results of an extensive x-ray diffraction study of a monolayer of C10F21CH2COOH spread on water (pH2) at 19.1 °C. Lever-rule analysis of the in-plane scattering is used to show that there is a coexistence region between ordered condensed islands and a dilute disordered phase. The coexistence region is found to be bounded by molecular areas of 29 Å2 and 2000±600 Å2, in agreement with the pressure-area isotherm. The molecular tilt of the ordered phase remains unchanged from closest packing (near collapse of the monolayer) throughout the coexistence region, and has a value of 2±3 degrees with respect to the normal to the liquid surface. These results are contrasted with those for hydrocarbon monolayers in which the onset of order in the coexistence region is close to the condensed phase boundary, and the ordered phase is compressible with a continuously variable tilt angle ranging from 30 to 0 deg at closest packing. The differences are attributed to the enhanced chain stiffness of the fluorinated chain. To illustrate this interpretation, preliminary scattering results are presented for a monolayer of C8F17(CH2)4COOH on water, which has some of the features of the aliphatic lipid monolayers. Recent molecular dynamics simulations have been found to reproduce all the important qualitative features of these systems [S. Shin, N. Collazo, and S. A. Rice, J. Chem. Phys. 96, xxxx (1992)].
Emission cross sections of excited fragments produced by electron impact on BCl3
Authors: Tokue Ikuo , Kudo Mikiko , Kusakabe Masanobu , Honda Tomohisa , Ito Yoshio
Published: July 2002

Emission spectra in the 190–600 nm region produced by electron impact on BCl3 have been studied up to 110 eV. Emission cross sections of the B(2s2p2 2D–2p 2P0) and B(3s 2S–2p 2P0) lines and the BCl(A 1Π–X 1Σ+) band are evaluated to be 4.9±1.0, 4.5±0.7, and (1.9±0.3)×10−18 cm2, respectively, at 100 eV. Formation cross sections of these species have been determined from the analysis of their fluorescence decaying curves. Two continuous emissions observed in the 230–380 and 400–580 nm regions are attributed to the BCl*2 band. The fluorescence lifetime of BCl*2 in the 300–342 nm region is obtained to be 1.65±0.2 μs, which is nearly independent of the wavelength.
Differential cross sections for state-selected products by direct imaging: Ar+NO
Authors: Suits A. G. , Bontuyan L. S. , Houston P. L. , Whitaker B. J.
Published: July 2002

State-selected differential cross sections have been obtained by directly imaging the products of collisions in crossed molecular beams. The new technique allows final state resolution and simultaneous detection of all scattering angles. The method has been used to study inelastic collisions between Ar and NO(2Π1/2, υ=0, J=0.5) at a collision energy of 0.21 eV. Rotational rainbows in the product angular distribution are directly observed to change in position as a function of the final rotational state; the peak of the angular distribution moves toward the backward hemisphere and the angular distribution broadens with an increase in final rotational quantum number. The method relies on multiphoton ionization of the product but is otherwise generally applicable to reactive as well as inelastic collisions.
Electronic spectra in polar fluids: Reference hypernetted chain theory
Authors: Ladanyi Branka M. , Shemetulskis Norah E. , Loring Roger F.
Published: July 2002

We treat the statically broadened absorption spectrum of a polar chromophore, whose dipole moment is assumed to change upon electronic excitation, at infinite dilution in a polar solvent. Calculations based on the reference hypernetted chain (RHNC) equation are compared to results from the mean spherical approximation (MSA) and to Monte Carlo simulations. The RHNC calculations are in quantitative agreement with simulation results for a strongly coupled solute–solvent system.
Coupled diabatic configuration interaction treatment of the O2 B′–X transition including computation of predissociation linewidths, optical f values, and generalized oscillator strengths
Authors: Li Yan , Honigmann Michael , Bhanuprakash K. , Hirsch Gerhard , Buenker Robert J. , Dillon Michael A. , Kimura Mineo
Published: July 2002

A b initio CI (configuration interaction) calculations are carried out for O2 in various 3Σ−u excited states in order to study the avoided crossing that is responsible for the discrete B′–X bands found in the high-energy region of the Schumann–Runge continuum. A diabatic transformation is carried out which allows for the description of nonadiabatic effects in this spectral range. A coupled vibrational treatment is employed in conjunction with the complex scaling technique in order to compute the predissociation linewidths of the B′ levels, and these results are found to be in good agreement with experiment, particularly for trends observed between different isotopomers of this system. Optical f values and generalized oscillator strengths (GOS) are calculated for a series of vibrational transitions and are also found to compare well with measured data. The observed finding that the GOS varies quite slowly with K2 for the 0–0 transition, but decreases rather sharply for the 1–0 species is reproduced in the calculations and the behavior of this property for other transitions is predicted.
Nonlinear optical response of one-dimensional molecular crystals: Breakdown of the local field approximation
Authors: Spano Frank C.
Published: July 2002

An analytical expression for the third order hyperpolarizability of an infinite one-dimensional molecular aggregate is derived, and compared with the same quantity calculated under the local field approximation. It is found that the peak nonlinear absorption is greatly overestimated in this approximation by a factor of (‖J‖/Γ)1/2, where J is the near neighbor dipole–dipole coupling and Γ is the molecular excited state lifetime.
The interaction of rotation and local mode tunneling in the overtone spectra of symmetric hydrides. II. The spatial properties of the XH4 eigenstates
Authors: Lehmann Kevin K.
Published: July 2002

The matrix elements of the XH4 rotation-tunneling Hamiltonian are given for basis states with all of the vibrational excitation in a single bond. A basis set of symmetrized rovibrational wave functions is given that are eigenfunctions in the limit in which the local mode tunneling rate is much smaller than the rotational precession rate. Using this basis, it is demonstrated that, in this limit, vibrational tunneling occurs with a simultaneous reorientation of the molecule, such that the spatial properties of the excited bond are the same as if tunneling did not take place at all. A resonant two-photon dissociation experiment is proposed that could observe this effect. It is found that as one moves away from this limit, the polarization properties of the F2 symmetry vibrational state retain much of the symmetric top structure.
Vibrational predissociation in D2HF
Authors: Clary David C.
Published: July 2002

Calculations are presented of linewidths for the vibrational predissociation of D2HF induced by excitation of the HF stretching vibration in the weakly bound molecule. A potential energy surface based on ab initio computations with the correct leading multipole terms has been used. The vibrational predissociation linewidths are obtained from accurate close-coupling computations of scattering resonances for the D2+HF collision. The calculated linewidths are in excellent agreement with those obtained from infrared absorption experiments and confirm that the mechanism for vibrational predissociation is D2HF(v=1) →D2(v1=1) +HF(v2=0). It is found that the vibrational predissociation linewidths associated with the Σ vibration in para-D2HF are about five times narrower than those for the Π vibration. This implies that it should be possible to observe the D2HF (v=0, Π→v=1,Σ) transition in infrared spectroscopy measurements. The rotational distributions of the D2+HF products of the vibrational predissociation process are predicted and should be observable.
Light scattering from disorder and glass-transition-dynamics in GeSBr2
Authors: Krüger M. , Soltwisch M. , Petscherizin I. , Quitmann D.
Published: July 2002

Low frequency Raman spectra (5–50 cm−1) show very obvious effects of disorder in the glass, and of the changes in the system which occur at the liquid–glass transition. Yet this frequency range has been considered in only very few glass formers. Here we report on this frequency range for the glass former system GeSBr2 which constitutes a statistically interrupted network. The temperature range studied was from far below the glass transition temperature (Tg=−30 °C) to well above it (T=−168 °C to +71 °C). In the glass, the loss of q conservation for phonon observation (due to static disorder) leads to a broad inelastic scattering peak around 10–15 cm−1, the so-called boson peak. The Raman spectrum is interpreted in terms of a correlation length Rc of phonon propagation and of phonon–photon-coupling; Rc is a measure of intermediate range order and amounts to ∼10 Å, independent of temperature. As T is raised, the minimum between this peak and the elastic peak is gradually filled up due to fast dynamic processes which increase rapidly above Tg. This scattering is interpreted in terms of fast dynamical processes, and the connection is made to the dynamical aspects of the liquid–glass transition. The strength and the time constants of fast processes are discussed in terms of two phenomenological models; both models describe the observed spectra well using a small number of parameters. The relaxation time found for the fast dynamical process (10−11–10−12 s) is in rough agreement with the value found independently in Brillouin scattering.
The lowest energy singlet state of tetrathiophene, an oligomer of polythiophene
Authors: Birnbaum Duane , Fichou Denis , Kohler Bryan E.
Published: July 2002

Fluorescence and fluorescence excitation spectra have been measured for solid solutions of tetrathiophene in tetradecane at 12 and 4.2 K. At 4.2 K, the spectra exhibit full vibrational resolution {inhomogeneous origin [full width at half-maximum, (FWHM)] approximately 5 cm−1 }. Narrow band excitation and detection establish that there are four independent, but nearly identical excitation/emission pairs with dipole allowed origins at 22 248, 22 241, 22 214, and 22 187 cm−1. The data are consistent with the idea that these multiple spectra come from a single tetrathiophene confomer which can occupy four different sites in the tetradecane lattice. The vibrational frequencies of the modes in the ground (excited) state that couple strongly to the electronic excitation are 162, 327, 703, 1478, and 1531 cm−1 (161, 333, 688, 1235, and 1551 cm−1 ). The overlap of the excitation and emission origins and vibrational development of the spectra establish that the S0 to S1 transition is symmetry allowed (1 1Ag to 1 1Bu for C2h all-trans tetrathiophene) with the excited 1 1Bu state described reasonably as being derived from the ground state by the promotion of one electron from the highest energy occupied molecular orbital to the lowest energy unoccupied molecular orbital (HOMO to LUMO).
Asymptotic behavior of the pair-correlation function of a polar liquid
Authors: Caillol J. M.
Published: July 2002

We derive the asymptotic behavior of the pair-correlation function of a polar liquid in a finite geometry in the frame of linear-response theory. The case of periodic and hyperspherical geometries is discussed in detail and our predictions are compared with the results of numerical simulations. A new method for the determination of the dielectric constant of a polar fluid confined to the surface of a hypersphere is proposed.
The viscosity tensor of a ferrofluid under flow
Authors: Salueña C. , Perez-Madrid A. , Rubi J. M.
Published: July 2002

We obtain the expression for the viscosity tensor of a ferrofluid under external flow. The system is modeled as a suspension of rigid dumbbells with a dipole moment in the direction of their bond vector. Our starting point is the Kramers–Kirkwood expression for the stationary viscous pressure tensor whose average is computed with the stationary solution of the corresponding diffusion equation. We obtain the pressure tensor up to second order in the velocity gradient and for any value of the magnetic field. From the pressure tensor, we derive the viscosity tensor including non-Newtonian effects. The components of the pressure tensor are related to different viscosity coefficients which are analyzed for flow through a pore and for planar elongational flow.
Monte Carlo simulation of the hard-sphere fluid with a high-temperature quantum correction in the region of the fluid–solid phase transition
Authors: Yoon Byoung Jip , Jhon Mu Shik , Scheraga Harold A.
Published: July 2002

Monte Carlo simulations have been carried out for the hard-sphere system in the region of the fluid–solid phase transition with a quantum mechanical correction. The two-body Slater sum has been used as the quantum mechanical probability in the configurational space rather than the classical Boltzmann distribution function. The internal energy and the pressure depend not only on the density, but also on the thermal wavelength which is a function of the mass and temperature, while the classical results depend only on the density. These thermodynamic quantities are greater, in general, in the quantum mechanical treatment than in the classical one. The correction term in the compressibility factor is about 40% of the classical value in the phase transition region when the ratio of the thermal wavelength to the hard-sphere diameter is 0.1; as the quantum effect increases, the region of the phase transition appears at lower density. Two kinds of equation of state are presented, one for the solid branch and one for the fluid branch of the quantum mechanical hard-sphere system, by using the classical expressions but replacing the classical hard-sphere diameter by the effective diameter.
Molecular-dynamics simulations of interfaces between water and crystalline urea
Authors: Boek E. S. , Briels W. J. , van Eerden J. , Feil D.
Published: July 2002

Molecular-dynamics simulations of several water-crystalline urea interfaces have been performed. The structure and dynamics of water close to the urea crystal surface are discussed in terms of density profiles, positional and orientational distribution functions, and diffusion coefficients. The water structure close to the interface is strongly determined by the structure of the crystal surface: the (001) and (111) interfaces reveal strong adsorption of water while the (110) and (1̄1̄1̄) interfaces do so to a lesser extent. Assuming that the growth rate of a specific crystal face decreases with increasing solvent adsorption, the appearance of only (111) on the urea growth form is predicted. We argue that on the other hand the dominance of (110) over (001) cannot be explained using a simple layer growth model.
Franck–Condon overlap integrals between the 3sσ 3Πg Rydberg and the 1 3Πg valence states of O2
Authors: Sur Abha , Nguyen Loane , Nikoi Nana
Published: July 2002

The (2+1) resonance-enhanced multiphoton ionization spectrum of the 3sσ 3Πg←←3Σ−g transitions in O2 shows diffuse structure for the 0–0, 1–0, 3–0, and 4–0 vibrational bands with linewidths ranging from about <50 cm−1 to >150 cm−1. The spectrum of the 2–0 band, however, is rotationally resolved with a fullwidth at half maximum of 6 cm−1. The 3sσ 3Πg Rydberg state predissociates via the dissociative 1 3Πg valence state. The observed variations in the linewidths of different vibrations in the two-photon transition are a consequence of the variations in the Franck–Condon overlap integrals between the bound and dissociative states. We have constructed an empirical representation of the 3Πg valence potential by calculating trial-and-error Franck–Condon (FC) overlap between the Rydberg and valence states. The theoretical potential curve of the 3Πg valence state is varied systematically to obtain FC densities compatible with the experimentally determined linewidths for the two-photon transition in 16O2, 17O2, 18O2 and 18O 17O. Our study stresses the importance of using isotopic data in order to obtain an unambiguous and accurate representation of the state.
Accurate specific molecular state densities by phase space integration. II. Comparison with quantum calculations on H+3 and HD+2
Authors: Berblinger Michael , Schlier Christoph , Tennyson Jonathan , Miller Steven
Published: July 2002

The semiclassical determination of N(E;J) and ρ(E;J), the specific number and density of quantum states at energy E, and fixed total angular momentum J, by Monte Carlo integration of phase space is compared to recent exact quantum calculations on H+3 and HD+2, which yielded lists of up to 900 quantum states for single values of J. This allows for the first time tests of such a procedure to be made without assuming anything about separability or harmonicity of the potentials. The excellent agreement between semiclassical and quantum state counts shows that the semiclassical numerical computation is a viable and simple method for the determination of state numbers and densities in small molecules with a precision of the order of 1%. For J=0, the procedure has been extended to state numbers for the different symmetry species occuring in H+3 and HD+2.
Binding energies and electron affinities of small silicon clusters (n=2–5)
Authors: Curtiss Larry A. , Deutsch Peter W. , Raghavachari Krishnan
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of Sin and Si−n (n=1–5) clusters. The G2 energies are used to derive accurate binding energies and electron affinities of these clusters. The calculated electron affinities of Si2–Si4 are in agreement to within 0.1 eV with results from recent photoelectron spectroscopic measurements.
Variational principles for reactive collisions based on the generalized Lagrange multiplier method
Authors: Baer Michael , Nakamura Hiroki
Published: July 2002

In this study is discussed a general approach to derive variational principles for physical magnitudes Q(φ) which are dependent on functions φ known to be solutions of physical equations [see Gerjuoy et al., Rev. Mod. Phys. 55, 725 (1983)]. The approach is based on Lagrange multipliers which are incorporated into the expression for calculating Q by demanding that the functions φ fulfill their equations. This approach is first applied to derive a general variational principle for the T-matrix elements based on the Lippmann–Schwinger equation. As special cases we obtained the bilinear forms of the Schwinger and the Newton variational principles. Next this approach is employed to derive a new Kohn-type variational principle for the S-matrix elements for reactive collisions based on the Schrödinger equation which contains complex potentials. Finally, this approach is further utilized to derive a novel variational principle for the flux function.
Theoretical study of the Rydberg excited electronic states of Ar+2
Authors: Cachoncinlle C. , Pouvesle J. M. , Durand G. , Spiegelmann F.
Published: July 2002

The first Rydberg states of the Ar+2 molecular ion dissociating into Ar+(3p5,2P) +Ar*(3p54s,3,1P) are calculated using multireference configuration interaction and effective core pseudopotentials. At long internuclear distance, their electronic structure is shown to be determined by resonance interaction and the magnitude of the Rydberg electron-transfer integral, inducing long-distance wells with De in the range 0.6–0.8 eV around Re≂9.0a0. In the short distance range, the electronic structure can be understood as resulting from an ionic Ar++ core with an outer Rydberg electron, and bound or quasibound Rydberg states are obtained around Re∼4a0. The fluorescence spectra of those states are simulated and the possible intervention of the Ar+2 system in the third continuum spectrum is considered.
Effective Feynman propagators and Schrödinger equations for processes coupled to many degrees of freedom
Authors: Schwartz Steven D.
Published: July 2002

This paper presents a new approach to quantum evolution in the presence of a quantum bath. We develop an equation of motion for an observed system evolving under the influence of an unobserved quantum bath. The methodology we follow uses operator expansions of the Feynman propagator. Corrections to the zeroth order approximation are corrections to an adiabatic approximation. In this paper we explicitly develop an approximation which is infinite order in bath and system coupling, but first order in system degree of freedom. This infinite order approximation is found through a resummation of an infinite class of terms in the operator expansion. We first present a simplified single time (Markovian) version of the theory. We then present a derivation for including the effects of memory. The approach developed in this paper also has the potential for systematic improvement. In other words, while the bath and system coupling in this calculation is treated to infinite order, the system itself is only treated to first order. We will briefly discuss how these higher order corrections can be found. Finally, we present a test calculation of the our approach with comparison to exact results. For a two-dimensional test problem with potential much like that for collinear H+H2 the effective one-dimensional approximation we apply produces essentially exact results.
Molecular-dynamics studies of the structure and properties of monolayers of perfluorinated amphiphiles
Authors: Collazo Nancy , Shin Seokmin , Rice Stuart A.
Published: July 2002

We report the results of molecular-dynamics simulations of monolayers of CF3(CF2)nCOOH supported on water, using a pseudoatom model for the amphiphile and a continuum model for the water surface. The calculations explore the influence of temperature, chain length and surface density on the packing structure of the monolayer. These fluorinated amphiphiles support a much lower gauche configuration concentration than do their corresponding hydrocarbon analogs and, of course, have different intermolecular interactions. The most striking results of the simulations are as follows. (i) Even and odd members of the series CF3(CF2)nCOOH, under the same external constraints (T=300 K, π=0.16 dyn cm−1), pack in monolayers in which the tilt is, respectively, towards next-nearest neighbors and towards nearest neighbors. (ii) At constant surface pressure a monolayer of CF3(CF2)10COOH is predicted to undergo a transition between 325 and 350 K, in which the direction of the tilt changes from towards next-nearest neighbors to towards nearest neighbors, not unlike a pseudorotation. (iii) Between 275 and 325 K a monolayer of CF3(CF2)10COOH, held at constant surface pressure (π=0.16 dyn cm−1), is predicted to have negative thermal expansion. A number of other aspects of the structures of these monolayers, and the correspondence with the structures of monolayers of hydrocarbon amphiphiles, are briefly discussed.
Deexcitation cross sections of Ar(3P2, 3P1, 3P, and 1P1) by CH4, SiH4, and GeH4
Authors: Yoshida Hiroaki , Kawamura Hideo , Ukai Masatoshi , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

Deexcitation cross sections of Ar(3P2), Ar(3P1), Ar(3P0), and Ar(1P1) by CH4, SiH4, and GeH4 have been measured. The cross section values obtained are largely dependent on both the electronic states of excited argon atoms and the target molecules. The magnitude of these values is discussed in terms of energy transfer mechanisms or interactions responsible for the deexcitation transition. In the deexcitation of the metastable atoms, Ar(3P2) and Ar(3P0), the magnitude of the cross sections is mainly ascribed to the adiabatic transition from the interaction potentials for Ar*–M (M=CH4, SiH4, and GeH4 ) to those for Ar–M* due to a long-range potential curve crossing, while in the deexcitation of the resonant atoms, Ar(3P1) and Ar(1P1), the magnitude is mainly ascribed to resonant energy transfer due to a long-range dipole–dipole interaction.
Rotational relaxation in a free expansion of HCl
Authors: Rudich Y. , Gordon R. J. , Nikitin E. E. , Naaman R.
Published: July 2002

The rotational relaxation of HCl in a free jet expansion was studied in a pump-and-probe experiment using infrared laser excitation and resonant multiphoton ionization detection. Rate constants were determined for various J to J′ energy transfer processes in the vibrationally excited molecule. There is a strong indication that the rotational energy transfer occurs via a near-resonant dipole–dipole interaction between vibrationally excited and vibrationally cold HCl molecules.
Equilibrium geometry of the HCCN triplet ground state: Carbene or allene? An open-shell coupled cluster study including connected triple excitations
Authors: Seidl Edward T. , Schaefer Henry F.
Published: July 2002

The molecular structure of the HCCN molecule has been investigated using open-shell coupled cluster methods. The full single and double excitation procedure (CCSD) is used initially. Connected triple excitations were then included perturbatively via the CCSD(T) method. A standard double zeta plus polarization (DZP) basis set, designated H(4s1p/2s1p), C,N(9s5p1d/4s2p1d), was used initially. The predicted carbene H–C–C equilibrium bond angles are 133.8° (CISD), 138.4° (CCSD), and 139.8° with the CCSD(T) method. The corresponding bent-linear energy differences are 5.30, 2.70, and 2.15 kcal/mol, respectively. A much larger basis set was chosen to consider the importance of d functions on hydrogen and f functions on the carbon and nitrogen atoms. Using this H(5s2p1d/3s2p1d), C,N(10s6p2d1f/5s3p2d1f) basis, the H–C–C equilibrium bond angle is 136.3° with the CISD method and the bent-linear energy difference 3.8 kcal/mol. Best estimates of the HCC equilibrium bond angle and singlet–triplet separation are 142° and 0.8 kcal/mol, respectively. The relationship of these theoretical results to several experiments is discussed.
Relativistic effect on the interatomic platinum–oxygen potential and its consequences in high energy O2/Pt(111) scattering
Authors: Kirchner E. J. J. , Baerends E. J. , van Slooten U. , Kleyn A. W.
Published: July 2002

The influence of relativity on the binding in the Pt–O molecule is investigated using density functional calculations and proves to be quite important, not only for the potential well but also for the repulsive wall. Using a Born–Mayer fit to this interatomic gas phase potential, we perform a classical trajectory study on high energy O2/Pt(111) scattering (Ei=80 eV). The Born–Mayer form of the interatomic potential leads to a higher degree of dissociation for O2/Pt(111) than for O2/Ag(111) which is also experimentally found. The role of relativity turns out to be significant. The dissociation mechanism, however, does not change when going from O2/Ag(111) to O2/Pt(111). The molecules were found to first gain primarily rotational energy, which is largely transfered to vibration at the turning point of the second atom, in the case of finally dissociating molecules. Since the calculated dissociation in the case of platinum is still less than found experimentally, we investigate the influence of better fits to the interatomic potential, as well as inclusion of the potential well. It is possible to improve agreement with the experimental results by directly reducing the long range of the Born–Mayer potential.