bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of collisional broadening and shift parameters in X 1Σ+g→A 1Σ+u Li2 using Doppler scanning techniques
Authors: Fell C. P. , McCaffery A. J. , Ticktin A.
Published: July 2002

Line shift and broadening rates have been measured for three v′ j′ levels of the X→A manifold in Li2 with Xe as perturber gas. The lines are shifted and asymmetrically broadened and have been fitted using a speed dependent Voigt function that correlates the effect of velocity and broadening and shift rates. This asymmetric speed dependent Voigt profile (ASDVP) permits resolution of both effects at low foreign gas pressures, thus a shift corresponding to 11 MHz at line center was measured at 10 Torr xenon. Results are presented for the levels (v′,j′) 18,6; 20,20; 32,15 of A 1Σ+u and a reduction of shift and broadening rates is observed as j′ increases. This is attributed to the effective anisotropic contribution as rotor speed increases.
A note on the application of the semiclassical wave packet dynamics
Authors: Kolář M. , Ali M. K. , O’Shea S. F.
Published: July 2002

The optimal choice of the initial parameters of the Gaussian wave packets in the time dependent quantum mechanical calculations using a Gaussian wave packet representation of the wave function is discussed. The proposed procedure is illustrated with the simple case of one dimensional scattering and the propagation of the ground state of the Morse potential.
Continuum theory of critical phenomena in polymer solutions: Formalism and mean field approximation
Authors: Goldstein Raymond E. , Cherayil Binny J.
Published: July 2002

A theoretical description of the critical point of a polymer solution is formulated directly from the Edwards continuum model of polymers with two- and three-body excluded-volume interactions. A Hubbard–Stratonovich transformation analogous to that used in recent work on the liquid–vapor critical point of simple fluids is used to recast the grand partition function of the polymer solution as a functional integral over continuous fields. The resulting Landau–Ginzburg–Wilson (LGW) Hamiltonian is of the form of a generalized nonsymmetric n=1 component vector model, with operators directly related to certain connected correlation functions of a reference system. The latter is taken to be an ensemble of Gaussian chains with three-body excluded-volume repulsions, and the operators are computed in three dimensions by means of a perturbation theory that is rapidly convergent for long chains. A mean field theory of the functional integral yields a description of the critical point in which the power-law variations of the critical polymer volume fraction φc, critical temperature Tc, and critical amplitudes on polymerization index N are essentially identical to those found in the Flory–Huggins theory. In particular, we find φc ∼N−1/2, Tθ−Tc∼N−1/2 with (Tθ the theta temperature), and that the composition difference between coexisting phases varies with reduced temperature t as N−1/4t1/2. The mean field theory of the interfacial tension σ between coexisting phases near the critical point, developed by considering the LGW Hamiltonian for a weakly inhomogeneous solution, yields σ∼N−1/4t3/2, with the correlation length diverging as ξ∼N1/4t−1/2 within the same approximation, consistent with the mean field limit of de Gennes’ scaling form. Generalizations to polydisperse systems are discussed.
Trajectory calculations of high temperature and kinetic energy dependent ion–polar molecule collision rate constants
Authors: Bei Hu Cheng , Bhowmik Pradip K. , Su Timothy
Published: July 2002

The trajectory method is used to calculate high temperature and kinetic energy dependent ion–polar molecule collision rate constants. A new ion–polar molecule interaction potential energy surface is derived which takes into account the average hard sphere diameter of the colliding partners and the dimensions of the permanent dipole and the induced dipole vectors. A model system, H+3+NH3 is chosen for the calculation. The calculated results show that at low temperatures and kinetic energies, the effects of the size of the ion–molecule partners and the dimensions of the dipole vectors on the collision rate constant are small. At high temperature and high relative kinetic energies, however, these effects are significant. The calculated results are in good agreement with existing experimental kinetic energy dependent reaction rate constants.
On the structure and dynamics of lithium counterions in polyelectrolyte solutions: A nuclear magnetic resonance and neutron scattering study
Authors: van der Maarel J. R. C. , Powell D. H. , Jawahier A. K. , Leyte-Zuiderweg L. H. , Neilson G. W. , Bellissent-Funel M. C.
Published: July 2002

The structure around lithium counterions in polyacrylate solutions has been investigated by neutron diffraction, and it is concluded that small cations remain hydrated when they accumulate around a highly charged polymer. Interactions between the polyion and the counterions over a range outside the hydration shell are also observed. The relaxation rate of 7Li due to dipolar coupling to 17O was obtained by nuclear magnetic resonance. The results indicate that the polyion exerts a moderate influence on the reorientational dynamics of the lithium aqua-ion.
Dynamics of ferroelectric phase transition in vinylidene fluoride/trifluoroethylene (VF2/F3E) copolymers. I. Acoustic study
Authors: Tanaka Hajime , Yukawa Hideyuki , Nishi Toshio
Published: July 2002

We report the behavior of ultrasonic absorption and velocity in the ferroelectric copolymers of vinylidene fluoride (VF2) and trifluoroethylene (F3E) near the Curie point. The ferroelectric transition of VF2/F3E is strictly of the first order, however, it has the second-order character especially in the copolymers with low VF2 content. A large critical absorption due to cooperative fluctuation has been observed near the Curie point. From the temperature dependence of the critical absorption, we have found that the critical anomaly of the absorption is weak and the specific heat has logarithmic divergence at the Curie point if we ignore the effect of the distribution of the Curie point which may come from a distribution of lamellar thickness. This weak critical anomaly can be ascribed to the long-range nature of the bare interaction between dipoles due to connectivity of polymer chain or the electric dipolar interaction, the randomness of the interaction due to random copolymerization, and/or the dimensionality of the order parameter. The rotation of dipole is always accompanied by cooperative conformational change of the backbone, which is unique to a polymer chain. We have also found that a type of the dominant interaction between acoustic phonon and the order parameter depends on VF2 content.
Dynamical solvent effects on conformational isomerization of cyclohexane and 1,1-difluorocyclohexane
Authors: Ashcroft Joseph , Xie Chuan-Liang , Jonas Jiri
Published: July 2002

The effect of pressure on the conformational isomerization of cyclohexane in n-pentane-d12 solvent has been investigated by using the high-resolution, high-pressure nuclear magnetic resonance (NMR) technique. The isomerization rate of cyclohexane increases with increasing pressure at 218 K. The experimental data are interpreted in terms of stochastic models for isomerization reactions, and the results show that cyclohexane isomerization in n-pentane-d12 exhibits behavior typical for low-friction, energy controlled regime. The effect of temperature and pressure on the conformational isomerization of 1,1-difluorocyclohexane in solution has been investigated with the use of 19F Fourier transform (FT) NMR spectroscopy. The solvents used in this study are n-pentane, acetone-d6, and methylcyclohexane. Similarly, as in the case of cyclohexane, the ring inversion in 1,1-difluorocyclohexane is accelerated when pressure is increased, and this pressure dependence is nonlinear. The interpretation of the experimental data for 1,1-difluorocyclohexane depends critically on the estimate of ΔV‡TST which value is not known at the present time. Therefore, it is impossible to determine whether isomerization of 1,1-difluorocyclohexane exhibits the Kramers turnover.
Reinvestigation of the Raman spectra of dihydrogen trapped in rare gas solids. I. H2, HD, and D2 monomeric species
Authors: Alikhani M. E. , Silvi B. , Perchard J. P. , Chandrasekharan V.
Published: July 2002

The vibration–rotation and pure rotational Raman spectra of H2, HD, and D2 trapped in Ar, Kr, and Xe matrices have been recorded at 9 K. The frequencies which have been measured within an accuracy of 0.3 cm−1 are compared to the results of recent calculations. Except for the S0(0) line of HD the agreement between observed and calculated matrix shifts is satisfactory. The anomalous blue shift observed for the S0(0) line of HD is well interpreted within the rotation translation coupling (RTC) framework.
Intermediate representation formulation: An exact treatment for multiphoton absorption and dissociation in multilaser fields
Authors: Jolicard Georges , Billing Gert Due
Published: July 2002

The multiphoton dissociation of a diatomic molecule under multilaser fields is studied using a semiclassical description of the system. An L2 discretization of the dissociation channel is made in the framework of the optical model method. The time displacement propagator is calculated by a multistep first order Magnus treatment using an intermediate representation with respect to the laser field–molecule interaction. An application is presented with a six bound state HF model potential perturbed by two strong laser fields. Large nonlinear effects appear in this simple system revealing the high efficiency of the two mode interaction in the dissociation process from the ground state.
Z-transition state calculations of energy changes and electrostatic potentials in isoelectronic atoms and molecules
Authors: Sen K. D. , Seminario Jorge M. , Politzer Peter
Published: July 2002

A Z-transition state (ZTS) method based on nonintegral nuclear charges is tested numerically for calculating isoelectronic energy differences and electrostatic potentials. A self-interaction-corrected local spin density functional approximation is used for atoms, and an ab initio SCF-MO procedure with a modified STO-5G basis set for molecules. It is shown that the ZTS method permits accurate isoelectronic energy differences to be obtained through a single computation at an intermediate nuclear charge. The electrostatic potentials are found to vary nearly linearly with nuclear charge over small intervals.
The temperature dependence of the direct current electrical conductivity in quasi-one-dimensional organic metals
Authors: Staib Arnulf G. , Böhm Michael C.
Published: July 2002

The dc electrical conductivity above the Peierls-transition temperature Tc of quasi-one-dimensional (1D) organic metals is calculated by an ab initio single-particle theory based on the Kubo–Mori formalism. The respective inverse relaxation time in this approach is calculated numerically. The theory allows for a reliable reproduction of experimentally derived normalized conductivity curves of highly anisotropic organic metals. The applicability of the model is reduced with decreasing anisotropy. Phenomenologically it can be shown that a T−2 law of decay of the dc electrical conductivity above Tc is conventionally connected with large anisotropies. Decreasing anisotropy leads to T−n curves, where n is sizeably smaller than 2. The theoretical approach reproduces a T−n, n≊2, law of decay in the framework of one-phonon electron scattering processes. Therefore it is suggested that the deviations from the T−1 behavior of conventional three-dimensional metals is caused by the strong Kohn anomaly in 1D systems. The influence of the electron–phonon coupling and the Debye temperature on the analytic structure of the normalized dc electrical conductivity curves has been studied. A general theoretical description is suggested to account for the influence of electronic correlations on the magnitude of the electron–phonon coupling as well as on the Peierls-transition temperature. It is shown qualitatively that the organic metals belong to a class of valence fluctuating systems where charge fluctuations are conserved also in the limit of strong electronic correlations.
Electron densities in momentum and position spaces. II. Application of density mapping to the helium ground state
Authors: Koga Toshikatsu , Yamamoto Yoshiaki , Kryachko Eugene S.
Published: July 2002

The recently proposed method of density mapping between momentum and position spaces is applied to the ground state of the helium atom. The Kellner densities and wave functions are used as reference, and the Eckart densities γ(p) and ρ(r) are employed as test densities keeping their parent wave functions in blind. From the momentum density γ(p), the corresponding position density ργ(r) and the position moments 〈rn〉γ (−2≤n≤4) are generated within a few percent error. The inverse procedure generates the momentum density γρ(p) and the momentum moments 〈pn〉ρ (−2≤n≤4) from the position density ρ(r) with a better accuracy. Density-functional calculations are performed in a simplified manner and the energies close to the Hartree–Fock limit value are obtained.
Theoretical study of the ionization of B2H5
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

A b initio molecular orbital calculations at the G1 level of theory have been carried out on neutral B2H5 radical, doubly bridged B2H+5 cation, and the first triplet excited state of B2H+5. Singly bridged B2H5 is 4.0 kcal/mol (without zero-point energies) more stable than doubly bridged B2H5. Based on this work and previous theoretical work on triply bridged B2H+5, ionization potentials (vertical and adiabatic) are determined for B2H5. The adiabatic ionization potentials of the two B2H5 structures are 6.94 eV (singly bridged) and 7.53 eV (doubly bridged). A very large difference is found between the vertical and adiabatic ionization potentials (3.37 eV) of the singly bridged B2H5 structure. The first triplet state of B2H+5 is found to be 4.55 eV higher in energy than the lowest energy B2H+5 cation (triply bridged). The results of this theoretical study support the interpretation of Ruščić, Schwarz, and Berkowitz of their recent photoionization measurements on B2H5.
On the direct energy transfer from donors to acceptors in chainlike polymer systems
Authors: Roy A. K. , Blumen A.
Published: July 2002

In this work we study the direct, incoherent energy transfer from excited donors to acceptors, which are attached to chainlike polymers. We consider both multipolar and exchange-type interactions between the donors and the acceptors. The polymers are modeled through random walks (Gaussian or self-avoiding, depending on the solvent). With the use of the end-to-end distribution function of the walks, we calculate the time dependence of the ensemble averaged decay of the excitation of the donor. For multipolar and exchange-type interactions, we find Kohlrausch–Williams–Watts (KWW) stretched exponential and exponential–logarithmic decay patterns, respectively. The decay forms reflect the quality (good or poor) of the solvent directly and they may be used to test transitions in the polymer conformation.
Observation of collisionally transferred spikes in 13CH3F by infrared–infrared double resonance
Authors: Matsuo Yukari , Schwendeman R. H.
Published: July 2002

The method of infrared–infrared double resonance has been used to record the line shapes of a number of transitions in the 2ν3←ν3 band of 13CH3F by means of an infrared microwave sideband laser spectrometer while the QR(4,3) transition in the ν3 band was being pumped by constant-frequency radiation from a CO2 laser operating on the 9P(32) transition. A double modulation technique was used to eliminate single-resonance effects. The double-resonance line shapes for a series of transitions whose lower state is the (J,3) level in the v3=1 vibrational state show a transferred spike whose width is narrow for J values near 5, but slowly increases as J increases. The transferred spike is superimposed on a component of the line shape that is Gaussian with the expected Doppler width. Evidence for a transferred spike still remains for J=22. By contrast, for K≠3, there is only a Gaussian component for any J, except for J≊5, K=6, for which some evidence for a transferred spike can be seen. If the dominant selection rule for collisionally induced transitions is ΔJ=0,±1, then observation of a transferred spike for J=22 indicates some retention of velocity after as many as 17 collisions. The transferred spikes have been analyzed by a line shape function based on the Keilson–Storer collision kernel and the results have been used to estimate the root-mean-square value of the change in velocity upon collision.
The liquid–gas transition and the polymer–magnet analogy
Authors: Kholodenko A. L. , Qian C.
Published: July 2002

We propose a new unified field-theoretic path integral treatment of liquid–gas transition occurring in simple neutral fluids and neutral monodisperse polymer solutions. Obtained theoretical results indicate that both systems belong to the same Ising-type universality class which is strongly supported by the most recent experimental results on the liquid–gas transitions.
The generation and trapping of 28SiH+2 and 29SiH+2 in neon matrices at 4 K: Electron spin resonance and theoretical investigations
Authors: Knight Lon B. , Winiski Michael , Kudelko Paul , Arrington C. A.
Published: July 2002

The 28SiH+2 29SiH+2 cation radicals have been generated under neon matrix deposition conditions at 4 K by the photoionization (16.8 eV) of SiH4(g). A resolution of the 29Si and hydrogen A tensors was possible which enables a complete characterization of the unpaired electron in this highly reactive ionic species for its X 2A1 ground electronic state. The A tensor assignments were facilitated by the occurrence of preferential orientation of the isolated SiH+2 molecule in the neon lattice. Ab initio theoretical calculations of the nuclear hyperfine tensors were conducted which showed good agreement with the experimental results. The experimental magnetic parameters in neon at 4 K are: gx=2.0043(3), gy=1.9970(3), gz =2.0018(3); Ax(H) =110.4(3), Ay (H)=124.1(3), Az (H)=116.6(3); for 29Si, Ax =−762.9(3), Ay =−760.8(3) and Az=−1005.0(3) MHz where X is perpendicular to the molecular plane and Z lies along the C2 axis of the C2v radical. No experimental evidence for SiH+4 or the complex, SiH+2 ⋅⋅⋅H2 was obtained. Theoretical calculations indicate that hyperfine structure from the ‘‘complexed’’ H2 should be resolvable if this species was present. However, rotational averaging of H2 above the SiH+2 plane would reduce the hfi of H2 and might prevent its detection.
A Fulton–Gouterman approach to exciton localization and excitonic solitons
Authors: Wagner Max , Köngeter Andreas
Published: July 2002

A one dimensional model for Frenkel excitons interacting with acoustical phonons is considered. The generalized Fulton–Gouterman transcription lays open a peculiar topological property which is very suggestive for variational calculations. A very simple Ansatz is shown to yield already lower groundstate energies than previous dressed exciton or soliton theories, containing both latter as special limits.
A new technique for the determination of surface adsorbate geometries utilizing second harmonic generation and absorption band shifts
Authors: Peterson E. S. , Harris C. B.
Published: July 2002

A combination of second harmonic generation (SHG) and a simple dipole–dipole interaction model is presented as a new technique for determining adsorbate geometries on surfaces. The polarization dependence of SHG is used to define possible geometries of the adsorbate about the surface normal. General formulas for absorption band shifts using the geometry constraints imposed by the SHG data are derived for a dimer constructed from two arbitrarily placed monomers on the surface using the dipole–dipole interaction potential. These formulas can be used to determine the orientation of the two monomers relative to each other. A simplified version of this formalism is used to interpret absorption band shifts for a system of rhodamine B adsorbed on quartz.
Potential energy surfaces for YH+2 and ZrH+2
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Electronic structures, potential energy surfaces, and one-electron properties of 16 electronic states of two transition metal dihydride ions YH+2 and ZrH+2 are calculated using the complete active space multiconfiguration self-consistent field (MCSCF) followed by a full second-order configuration interaction (SOCI) method. The MCSCF/SOCI method yields 1A1 as the ground state for YH+2 with re=1.833 Å and θe=116° and a 2A1 ground state for ZrH+2 with re=1.75 Å and θe=113.5°. The SOCI atomization energy [YH+2(1A1) →Y+(1S)+2H] is calculated to be 5.22 eV in good agreement with an experimental value of 5.52 eV at 298 K for this process. The De(Y+–H) for YH+ is calculated to be 58.9 kcal/mol in excellent agreement with an experimental value of 58±3 kcal/mol. The first adiabatic ionization potentials of YH2 and ZrH2 are calculated as 6.18 and 6.95 eV, respectively. The excited Zr+(2D) ion inserts spontaneously into H2 leading to the bent ground state of ZrH+2, while the ground state of Zr+ does not insert into H2. The 1S0 ground state of Y+ has to surmount a small barrier of about 10 kcal/mol for insertion into H2. The addition of f type diffuse functions does not change the geometrical parameters much.
Resonant response of a driven chemical system: Effects on efficiency
Authors: Hjelmfelt Allen , Ross John
Published: July 2002

Periodic variations are applied to the influxes of oxygen and methane entering a reaction vessel in which takes place a combustion reaction and, in the absence of these variations, the system is in a stable focus. We measure the reaction of the system to these variations and find a resonant response, and changes of phase relations between the forcing and response of the system, near the autonomous frequency. We calculate the enthalpy content of the gases and using a simple model of a Carnot engine we study the power output of the system and find increases (∼7% in power) in these quantities near the autonomous frequency. The experimental results are compared to the predictions of a numerical model specific to our system and to the analytic solution of a linear set of differential equations with a stable focus. We find good agreement with both, but there is an aspect of the experimental results which requires additional hypotheses.
Gaussian functions in Hylleraas-configuration interaction calculations. III. Wave functions for the X 1ξ+g state of hydrogen
Authors: Frye D. , Lie G. C. , Clementi E.
Published: July 2002

The Hylleraas-CI method with Cartesian Gaussian basis functions of various sizes has been used to calculate the ground state energies and wave functions for the hydrogen molecule at internuclear distances of 1.0, 1.4011, 4.0, and 8.0 bohr. The best basis sets reproduce energies obtained with the confocal elliptical coordinates by Kolos et al. to within 0.3 cm−1. A few cross sections of the wave functions are presented graphically, showing clearly that the improvement of Hylleraas-CI over conventional CI is due to the presence of a slightly deeper Coulomb hole around the electrons.
The permanent electric dipole moment of chromium monoxide
Authors: Steimle Timothy C. , Nachman David F. , Shirley Jeffrey E. , Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

The permanent electric dipole moments for the X 5Π and B 5Π states of gas-phase chromium monoxide, CrO, have been experimentally determined using the sub-Doppler optical technique of intermodulated fluorescence spectroscopy in conjunction with the Stark effect. The measured values are 3.88±0.13 and 4.1±1.8 D for the X 5Π and B 5Π states, respectively. The theoretical values determined for the X 5Π state, using multireference configuration interaction iterative-natural-orbital and finite-field calculations, are in excellent agreement with the experimental value.
Solid state polymerization of cyanoacetylene into conjugated linear chains under pressure
Authors: Aoki K. , Kakudate Y. , Yoshida M. , Usuba S. , Fujiwara S.
Published: July 2002

Cyanoacetylene underwent polymerization reaction in a solid phase at pressures above 1.5 GPa. The Raman study of the reaction product showed that the polymer had a conjugated linear backbone with CN pendant groups. The Raman spectra for this substituted polyacetylene demonstrated a resonance behavior similar to that reported for trans-polyacetylene. The optical gap associated with the π–π* transition in the conjugated system was smaller than that of trans-polyacetylene, probably due to the resonance interactions between the CN triple bonds and the conjugated double bonds.
Prediction of the infrared spectrum for the neon–ethylene van der Waals complex
Authors: Tiller A. R. , Peet A. C. , Clary D. C.
Published: July 2002

The van der Waals states of Ne–C2H4 up to J=10 are calculated with the ethylene molecule in both its ground and ν7 excited vibrational states. The method involves separation of the van der Waals bending motions from the stretching motions and ethylene vibrations. The infrared spectrum of the complex is predicted in the region of the ethylene ν7 vibrational mode, and the results are compared with the calculated spectrum of free ethylene. The effect of ignoring off-diagonal Coriolis terms (which mix together different K states) in the Ne–C2H4 Hamiltonian is investigated.
Diffusion effects of hydrogen bond fluctuations. II. From the short to the long time regime in the translational dynamics of water
Authors: Bertolini Davide , Grigolini Paolo , Tani Alessandro
Published: July 2002

To improve the quantitative agreement with the deviations from the predictions of Fick’s law, revealed by computer simulation, we develop a new theoretical model for translational diffusion of water. The time scale of hydrogen bond fluctuations can be comparable to that of the dynamics of translation velocity, and a tagged molecule of water might not have enough time to reach the diffusional regime before the establishment of new environmental conditions determined by hydrogen bond fluctuations. Thus we base our description on the modulation of the physical parameters determining the velocity time evolution rather than on the modulation of the diffusion coefficients. Computer simulation of water dynamics shows that the correlation function Fs(k,t)≡〈e−ik⋅r(0) eik⋅r(t)〉 is characterized by a fast decay at short times followed by a slower decay process. We show that this mainly depends on the velocity time evolution of molecules with two or three hydrogen bonds. We also show that the fitting parameters, within our theoretical modeling, lead to a satisfactory agreement with the time evolution of the velocity autocorrelation functions and their spectra, and also result in a remarkable quantitative agreement with the deviations from Fick’s law determined through computer simulation, except at high temperature.
Restricted rotational diffusion of rigid bodies
Authors: Kumar Anil
Published: July 2002

Analytical solution to the restricted-diffusion-in-a-cone model for a symmetric top is obtained using a set of orthogonal functions of nonintegral order. It is shown that the case of the restricted-diffusion-in-a-cone model for an asymmetric top cannot be solved in analytical closed form: Series solution is, however, possible. The correlation functions for arbitrary functions of the orientation of a rigid body are also obtained. These functions are required in obtaining the correlation functions for the restricted rotational diffusion of articulated bodies and are useful in interpreting the data from relaxation experiments for determining the diffusion coefficients and conal restriction angles.
Torque constraints for modeling the behavior of rigid and semirigid macromolecules
Authors: Ying Ruoxian , Peters Michael H.
Published: July 2002

This work investigates the rotational dynamics of rigid and partially flexible macromolecules in solution by means of Brownian dynamics simulation techniques. In a previous study by Diaz et al. [J. Chem. Phys. 87, 6021 (1987)] on the rotational correlation functions of rigid models, the effect on rotational dynamics from pure rotations of the model’s elements has been neglected due to the difficulty of introducing rotational constraints in the simulations. We present expressions for rotational constraints in a rigid dimer system, which makes it possible to completely describe the translational, rotational, and coupled translational–rotational motions for interacting spherical Brownian particles by employing the generalized algorithm of Dickinson et al. [J. Chem. Soc. Faraday Trans. 2 81, 591 (1985)]. Theoretical studies indicate, and our simulation calculations confirm, that the pure rotational effects are appreciable. The torque constraint expressions are verified by comparison of the simulation results to the exact analytical results for the rigid dimer system. The torque constraint expressions also allow us to examine the effects of flexibility on the rotational correlation functions for a dimer system.
A theoretical study of transition state spectroscopy: Laser dressed potential energy surface and surface hopping trajectory calculations on K+NaCl and Na+KCl
Authors: Yamashita Koichi , Morokuma Keiji
Published: July 2002

Spectroscopy during the chemical reactions, K+NaCl and its reverse, has been studied by surface hopping trajectory calculations. Laser absorption and emission processes are modeled as the transitions between the laser-dressed ground and excited state potential energy surfaces (PESs), which are constructed from ab initio potential energy and transition dipole functions. The theoretical excitation spectrum measured by Na–D emission intensity as a function of laser wavelength agrees qualitatively with the experiment by Maguire et al. [J. Chem. Phys. 85, 844 (1986)]. The excitation spectrum is found to be very different from the absorption spectrum, because only a small portion of excited trajectories reach the Na* product due to the endothermicity of the excited state reaction. Therefore the excitation spectrum reflects only the excited state dynamics but not the transition state spectroscopy. The laser wavelength dependence of the spectra is well explained by a characteristic shift of the crossing seam accompanied with changes in laser wavelength. We have also predicted the absorption and excitation spectra for the reverse reaction and found that in this case the intensity of the product emission as a function of laser wavelength reflects to a large extent the true transition state spectroscopy.
Conformational transitions of long chain molecules in frozen solutions
Authors: Hirschmann R. , Friedrich J. , Daltrozzo E.
Published: July 2002

We show by temperature-cycling-hole-burning experiments that the conformational barriers of one-dimensional long chain aggregates of pseudoisocyanine iodide in frozen solutions follow a distribution of the form Pv(V)∼1/V . This is the same distribution as has been measured for three-dimensional glasses.
Mechanism of collisionally induced transitions among fine-structure levels: Semiclassical calculations of alignment effects in the Na–He system
Authors: Kovalenko Laurie J. , Leone Stephen R. , Delos John B.
Published: July 2002

To gain insight into the mechanism of Na(3p)2P3/2→2P1/2 fine-structure transitions induced by collision with He, we monitor the expectation values of the orbital- and spin-angular momentum vectors, l and s, as a function of time along the trajectory, using a semiclassical formalism. In a typical collision, 〈s〉 remains nearly space-fixed while 〈l〉 precesses about the rotating internuclear axis. Thus, in the interaction region, the projection of 〈l〉 onto the internuclear axis, 〈λ〉, remains nearly constant, and the molecular alignment of the orbital is preserved. We show how equations of motion for the classical analogues of these expectation values agree qualitatively with the quantum equations of motion. A qualitative comparison is also made with the Cs–He system for which the spin–orbit coupling is much stronger. We calculate cross sections for Na(2P3/2)+He→Na(2P1/2)+He as a function of the alignment of the excitation laser polarization with respect to the asymptotic relative velocity vector. For stationary pumping of the excited F=3 hyperfine level, this calculation predicts that the perpendicular alignment gives a cross section which is larger by a factor of 1.8 than that obtained by parallel alignment.
The quasi-two-dimensional regime of fluids absorbed in porous media
Authors: Adams Phil , Henderson J. R. , Walton J. P. R. B.
Published: July 2002

We investigate the one-body structure of fluids confined within model pores when the confinement is sufficient to force the fluid into a quasi lower-dimensional regime. The concept of the quasi-two-dimensional regime of atomic fluids confined to symmetric planar slit geometry is introduced and statistical mechanical arguments are shown to predict a universal form for the density profile; or to be more precise, to imply that the many-body contribution to the one-body structure is a particular universal function of position. Computer simulation data of Lennard-Jones fluid confined to planar slit pores are presented, which clearly illustrate the statistical mechanical prediction. Generalizations of this phenomenon to include fluid mixtures, molecular fluids and nonplanar geometries are discussed.
Collisions of excited alkali atoms with O2. III. Visualization of the collision path
Authors: Paillard D. , Mestdagh J. M.
Published: July 2002

Inelastic scattering of electronically excited Na atoms by ground state O2 molecules was studied theoretically using a multiple-curve-crossing model. The movement of the collisional system within the potential grid describing the Na–O2 and Na+– O−2 pairs was visualized for two initial electronic states of Na (5S and 4D) at collision energies ranging between 0.2 and 1.0 eV. Visualization of the collision path was used to better understand prominent features of the redistribution of energy by collision within the system. Knowledge of the collision path for inelastic scattering also helped in discussing a competing reactive channel.
Thermodynamic and transport properties of cyclohexane computed by molecular dynamics with use of a six-center Lennard-Jones potential
Authors: Hoheisel C. , Würflinger A.
Published: July 2002

A six-center Lennard-Jones potential is proposed for cyclohexane and tested by molecular dynamics (MD) calculations. The thermodynamic and transport properties of experimental C6H12 in liquid and solid states were reproduced in acceptable approximation. In particular, the phase transition to the high-temperature rotator phase is correctly predicted by the MD calculation, though an amorphous phase rather than a crystalline one was studied. The following properties have been investigated in detail: Radial pair correlation function and pressure, self-diffusion, shear-viscosity, and thermal conduction coefficient, furthermore, the first two reorientational correlation functions of three vectors coinciding with different symmetry axis of the molecule. The latter functions allowed valuable insight into the rotational behavior of the occurring liquid and solid phases.
Separation of classical equations of motion based on symmetry
Authors: Banerjee Ajit , Adams Noah P.
Published: July 2002

We show that an invariance of a system of N particles simplifies the solution of the classical equations of motion.A transformation can be constructed in a straightforward manner to yield a set of active and redundant coordinates in which Hamilton’s equations are separable. That is, the redundant coordinates are cyclic and the kinetic energy is block diagonal between the two sets. Furthermore, the active coordinates have properties very similar to the parent Cartesian coordinates; e.g., the derivatives of the Hamiltonian remain unchanged. The numerical application of the procedure is very straightforward; we have performed trajectories of symmetric motions of an A2B-type molecule within the subspace of the C2v configurations.
Analysis of the vibrational bandwidths of alkane–urea clathrates
Authors: Wood Kurt A. , Snyder Robert G. , Strauss Herbert L.
Published: July 2002

The only large amplitude motion possible for an n-alkane molecule in urea-inclusion compounds is libration–torsion about the long axis of the chain. We present a quantitative model that incorporates the effect of this motion on the widths of the alkane vibrational bands. This model explains the difference in the widths of the different vibrations of the alkanes and their temperature dependence. Two effects are combined: (1) a modulation of the angles between the components of the polarizability in the space and the molecule-fixed frames for Raman spectra or between the components of the dipole moment for the infrared spectra, and (2) a modulation of the frequency of the alkane vibration via anharmonic coupling terms with the libration–torsion. The first effect gives rise to a distinctly non-Lorentzian band shape, which is convoluted with the approximately Lorentzian band of the second effect to produce the final result. The libration–torsional motion is modeled as that of a Brownian harmonic oscillator. Most of the parameters that enter the calculation are obtained from data other than that involving the bandwidths themselves. The libration–torsion relaxation time of about 1 ps obtained from fitting the observed bandwidths agrees with the value obtained from recent quasielastic neutron scattering experiments. Other bandwidth mechanisms that have been proposed are evaluated and it is shown that site hopping is too slow to account for the observations.
Dissociation of O2 induced by selective K-shell excitation
Authors: Saito Norio , Suzuki Isao H.
Published: July 2002

Dissociation of O2 stimulated by soft x-ray absorption has been studied by using a monochromatized synchrotron radiation and a time-of-flight mass spectrometer. The parent molecular ion O+2 was formed only at 531 eV (excitation of 1s→1πg), and a fragment ion O+ had the highest intensity in all the energies giving core–hole states. The measured time-of-flight spectra were reproduced by a simulation calculation, which provided kinetic energy distributions of O+ and O++. The dissociation pathways from the core–hole states of O2 were discussed using the obtained kinetic energy distribution and ion intensity ratios as well as Auger electron spectra in the literature.
Computational studies of heterogeneous reactions of SiH2 on reconstructed Si(111)–(7×7) and Si(111)–(1×1) surfaces
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The dynamics of chemisorption and decomposition of SiH2 on Si(111)–(1×1) and recontructed Si(111)–(7×7) surfaces have been investigated using classical trajectories on a previously described [Surf. Sci. 195, 283 (1988)] potential-energy surface modified to yield the experimental bending frequencies for chemisorbed hydrogen atoms and to incorporate the results of ab initio calculations of the repulsive interaction between SiH2 and closed-shell lattice atoms. The Binnig et al. model is employed for the (7×7) reconstruction. Sticking probabilities are found to be unity on the (1×1) surface and near unity on Si(111)–(7×7). The major mode of surface decomposition on the (7×7) surface is by direct molecular elimination of H2 into the gas phase. Hydrogen atom dissociation to adjacent lattice sites is a much slower process and the chemisorbed hydrogen atoms thus formed exhibit very short lifetimes on the order of (1.13–10.6)×10−13 s. The calculated rate coefficients for these two decomposition modes are 3.4×1010 and 0.79×1010 s−1 , respectively. The rate coefficients for the corresponding reactions on the (1×1) surface are 6.6×1010 and 5.3×1010 s−1 , respectively. The rates on the (1×1) surface are faster due to the increased exothermicity released by the formation of two tetrahedral Si–Si bonds upon chemisorption compared to a single Si–Si bond on the (7×7) surface. Molecular beam deposition/decomposition experiments of SiH4 on Si(111)– (7×7) surfaces reported by Farnaam and Olander [Surf. Sci. 145, 390 (1984)] indicate that chemisorbed hydrogen atoms are not formed in the SiH4 decomposition process whereas the present calculations suggest that such a reaction, although slow, does occur subsequent to SiH2 chemisorption. It is suggested that energetic differences between SiH4 and SiH2 chemisorption are responsible for these differences.
Coriolis-induced vibrational energy transfer in D2CO–D2CO collisions: A classical perspective
Authors: Parson Robert
Published: July 2002

Recent experimental studies of vibrational energy transfer in D2CO–D2CO collisions have demonstrated extraordinarily high cross sections for an intramolecular V–V process in which a vibrational quantum is transferred between the two low-frequency modes ν4 and ν6 . The key to understanding this process appears to be the strong Coriolis interaction between these modes; an approximate quantum treatment has demonstrated, however, that rotor asymmetry is also critical. In this paper a purely classical description of the energy-transfer mechanism is put forward. This description draws heavily upon two recent classical studies of the intramolecular dynamics of formaldehyde. It is argued that the energy transfer arises from a resonant interaction between the Coriolis-coupled vibrations and the nutation of an asymmetric top; this induces a slow flopping motion of the molecule’s dipole moment (superimposed upon end-over-end rotation) which is coupled to the vibrational dynamics. The torque on this dipole produced by a passing molecule can thus pump energy into the vibrations. Classical perturbation theory is used to obtain rough estimates of the energy-transfer cross sections, which agree reasonably well with the experimental values.
Simulation of the electronic structure of an atom dissolved in a hard-sphere liquid
Authors: Dobrosavljević Vladimir , Henebry Charles W. , Stratt Richard M.
Published: July 2002

Some of the differences between the electronic structure of an atom situated in a liquid and that in the gas phase have been long appreciated, but it is only relatively recent that methods became available for studying these differences microscopically—i.e., in a way that takes full cognizance of the fluctuating positions of the neighboring solvent molecules. We show in this paper that it is now possible to treat the electronic structure of the simplest nontrivial model for a bound state in a liquid, a hydrogenic atom in a hard-sphere fluid—(numerically) exactly by using a quantum Monte Carlo simulation. The average ground-state energy of such an atom is found to increase monotonically with liquid density, but the corresponding wave function displays a much more interesting behavior; the fluid’s randomness is more efficient in hybridizing p character into the ground state at intermediate densities than it is in either a dense liquid or a low pressure gas.
Quenching and vibrational energy transfer in the B 2Π state of the NS molecule
Authors: Wysong Ingrid J. , Jeffries Jay B. , Crosley David R.
Published: July 2002

Quenching and vibrational energy transfer in the B2Π state of the NS free radical have been studied using temporally and spectrally resolved laser-induced fluorescence in a low-pressure discharge flow reactor. The collision partners were He, Ar, H2, N2, O2, SF6, CO2, and N2O. Total removal cross sections show an oscillatory behavior with v′ in the range of unperturbed levels, v′=4–7, for all colliders studied save O2 where a nearly monotonic increase is seen. Vibrational transfer occurs for H2 and the polyatomics; the rates vary little with v′. Δv=2 transfer occurs with the polyatomic colliders for v′=5. Fluorescence decay traces from the perturbed v′=3 and 8 levels differ from the unperturbed levels and from each other. v′=3 is perturbed by quartets and shows ‘‘gateway’’ level behavior whereas v′=8 is perturbed by a doublet and shows efficient interelectronic transfer for all rotational levels.
Effects of initial electronic and translational energy on chemiluminescent reactions of Cu with F2
Authors: Parson J. M. , Fang C. C.
Published: July 2002

Chemiluminescent reactions of the ground 2S and metastable 2D states of Cu with F2 have been studied at two collision energies. Spectral simulations show that the 2D states lead to a highly inverted distribution of vibrational energy in CuF formed in the b3Π state and a bimodal distribution in the C1Π state. The effect of added translational energy is to broaden the CuF vibrational distribution formed from both the 2S and 2D states. These results, as well as the collision energy dependence of the cross section for chemiluminescence, may be attributed to differing electronic configurations accessed by the Cu states, with the 2S state reaching the FCuF ground state and the 2D states undergoing direct reaction by way of excited intermediates.
An ab initio study of the vibrational frequencies and infrared intensities of CH2F2
Authors: Fox Gary L. , Schlegel H. Bernhard
Published: July 2002

The infrared vibrational frequencies and intensities were calculated for methylene fluoride using ab initio molecular orbital theory. Calculations were carried out using several split valance basis sets (3-21G, 4-31G, 6-31G, 6-311G) augmented with diffuse functions and several sets of polarization functions. The CID/6-31G(d) optimized geometry was found to agree well with the experimental Re structure and was used for all of the frequency calculations. The atomic polar tensors were calculated by differentiation of the dipole moment at the Hartree–Fock and MP2 levels. The HF/6-311++G(3d,p) dipole moment is 2.20 D; the dipole moment is 2.12 D at the MP2/6-311+G(d,p) level and is estimated to be 1.90 D at the MP2/6-311++G(3d,3p) level, compared to the experimental value of 1.9785±.0021 D. The average error in the calculated harmonic frequencies is 5% at HF/6-311++G(3d,p), 1.9% at MP2/6-311+G(d,p), and 1.3% at the estimated MP2/6-311++G(3d,3p) level (compared to the experimental harmonic frequencies). The infrared intensities are more sensitive to the size of the basis set than the frequencies. The average error in the calculated double harmonic IR intensities is 26 km/mol at HF/6-311++G(3d,p), 7.8 km/mol at MP2/6-311+G(d,p) and 3.1 km/mol at the estimated MP2/6-311++G(3d,3p) level (compared to the observed values for A2, A4, A8, A1+A6, and A3+A7+A9 ).
Photon-induced desorption of CO chemisorbed on the oxidized Ni(111) surface
Authors: Guo Xingcai , Yoshinobu Jun , Yates John T.
Published: July 2002

Photon-induced desorption studies of chemisorbed CO on Ni(111), Ni(111) with chemisorbed oxygen, and oxidized Ni(111) surfaces have been carried out. Only the oxidized Ni(111) surface exhibits measurable CO photodesorption in the energy range from 1.8 to 4.1 eV. Both direct observation of CO photodesorption and post-irradiation thermal desorption of CO have been performed. The photodesorption process is first order in photon flux and in CO coverage. At a photon energy of 4.1 eV, the cross section for CO photodesorption is measured to be 5×10−18 cm2. The measured threshold energy, 2.7±0.5 eV, is in good agreement with a model involving the O2−2p→Ni2+3d interband transition in NiO as the excitation process leading to photodesorption.