bluuebunny/crossref_metadata_2025 huggingface dataset:
A Monte Carlo trajectory study of angular distributions in inelastic scattering of NO from graphite surfaces
Authors: Nyman Gunnar , Pettersson Jan B. C.
Published: July 2002

A Monte Carlo classical trajectory study of NO/graphite surface scattering was carried out. NO was modeled as a rigid rotor interacting with a smooth vibrating surface through two Lennard-Jones 12-6 potential terms. The study simulated the experiments performed on the same system, first published by Frenckel et al. (1982). Angular distributions were calculated and found to generally reproduce the measured scattering lobes best at a simulated surface mass of 18 amu. Two potential well depths were used and at the larger of these all measured lobes could be quite well reproduced.
Relativistic calculations of electronic states of TeH
Authors: Balasubramanian K. , Han Ming , Liao M. Z.
Published: July 2002

Relativistic configuration interaction calculations of five ω–ω states [3/2, 1/2, 1/2(II), 3/2(II), 5/2] of TeH are carried out. Comparison calculations of the 2Π and 4Π λ–s states are also carried out with the objective of understanding the effect of spin–orbit interaction on the electronic states of TeH. These calculations enable assignment of some of the experimentally observed spectra. The 1/2(II), 3/2(II), and 5/2 states exhibit interesting avoided crossings.
Cl-Kβ emission of chloromethanes and comparison with semiempirical and a
b i
n
i
t
i
o MO calculations
Authors: Perera Rupert C. C. , LaVilla Robert E. , Gibbs G. V.
Published: July 2002

The chlorine Kβ emission in fluorescence from gas/vapor phase CCl4, CHCl3, CH2Cl2, and CH3Cl were measured with ‘‘good statistics’’ and previously undetected weak x-ray emission features were observed. Results from ground state minimal and extended basis set ab initio (STO-3G) and MNDO MO calculations for these molecules are presented. The chlorine Kβ emission spectra were analyzed by comparison with complementary spectral data and the MO calculations. In addition, weak features in Cl-Kβ emission spectra resulting from self-absorption in the sample were identified.
Generalized Langevin equation approach for the rotational relaxation of a molecule trapped in a 3D crystal. II. Application to CO and CH3F in argon
Authors: Delgado V. , Breton J. , Hardisson A. , Girardet C.
Published: July 2002

A numerical integration of the Langevin equations connected to the motions of a diatomic molecule trapped in a rare gas matrix is performed using a Runge–Kutta procedure and a Monte Carlo–Metropolis sampling for the initial configurations of the so-called primary system (cf. paper I). The rotational energy transfer from the molecule to the crystal is shown to strongly depend on the coupling between the molecule and the nearest-neighbor (NN) atoms and also on the ability for these NN atoms to dissipate their energy into the bath. Several cases are discussed according to the values of the viscous terms describing the damping of the molecule rotation and translation and of the NN atom vibrations. The prolate CH3F molecule trapped in an argon matrix seems to relax more quickly its rotational energy than the nearly isotropic CO molecule. Special trajectory calculations, when the molecule is rotationally excited or in thermal equilibrium, are considered in order to study the well jump and the librational motion of the CO molecule.
Pure rotational transitions of allene in excited vibrational states observed by microwave Fourier transform spectroscopy using Stark and double resonance techniques
Authors: Vogelsanger B. , Bauder A.
Published: July 2002

Pure rotational transitions of allene in the degenerate vibrational states v9=1, v10=1, and v11=1 have been observed with a pulsed microwave Fourier transform spectrometer. The assignment of the J=0→1 transitions was confirmed by microwave–microwave double resonance experiments. From the latter and from Stark effect measurements the magnitude of the vibrationally induced dipole moments have been determined. The rotational constants known from high-resolution infrared spectra have been improved.
Investigations of the resonance Raman excitation profiles of cytochrome P450cam
Authors: Bangcharoenpaurpong O. , Champion P. M. , Martinis Susan A. , Sligar Stephen G.
Published: July 2002

We have measured the resonance Raman excitation profiles (REP’s) of several vibrational modes associated with the heme active site of cytochrome P450cam. The important Fe–S axial ligand mode (351 cm−1) of the substrate bound (high-spin ferric) complex is found to have structure in its blue shifted REP. Inverse transform techniques allow the line shape of the resonant charge transfer absorption to be reconstructed directly from the REP data. The observed splitting (3200 cm−1) is associated with an inequivalence in the resonant S→Fe charge transfer excitations. The position of the high-energy component (∼323 nm) is found to be in excellent agreement with z-polarized single crystal measurements, while the low-energy component (∼360 nm) is not clearly observed in the single crystal analysis. The ‘‘spin-marker’’ band ν3 is found to have a REP that is significantly red shifted with respect to the theoretical predictions. A variety of perturbations including non-Condon and multiple state coupling, as well as state dependent damping, are unable to account for the observed red shift. Studies of other ferric heme protein systems, including substrate free cytochrome P450cam (low-spin) and aquomet myoglobin (high-spin) reveal ‘‘normal’’ behavior of their REP’s. The electron–nuclear coupling strengths are directly extracted from the measured absolute cross sections in these cases.
Effects of chain length on atom/chain interaction dynamics
Authors: Shin H. K.
Published: July 2002

Effects of chain length on linear atom/chain interaction dynamics have been studied at collision energies below, at, and above the adsorption threshold by solving the equations of motion numerically. Near the threshold, the collision dynamics depends strongly on chain length, and a physically reasonable model should consist of many more than a ‘‘few’’ atoms in its lattice chain. Adsorption trajectories are markedly different from those for which the incident particle is only briefly trapped. Lattice defects in the first few atoms can have serious effects on the extent of energy transfer. Numerical results are obtained for Ar/W.
Effect of charge transfer on chain dimension in t
r
a
n
s-polyacetylene
Authors: Murthy N. S. , Shacklette L. W. , Baughman R. H.
Published: July 2002

X-ray diffraction measurements on all-trans-polyacetylene are consistent with a chain-axis length elongation upon donor doping (+0.026 Å for lithium and +0.04 Å for potassium) and a chain-axis length contraction upon acceptor doping (−0.010 Å for iodine), where the changes refer to the length L of a C2H2 unit (2.457 Å in the undoped polymer). These new experimentally derived results for heavily doped compositions, which ignore possible corrections for cell nonorthogonality in the lithium and iodine complexes, are similar to experimental results for graphite intercalation complexes and are consistent with theoretical predictions for doped polyacetylene. The meridional diffraction lines observed at L and L/2 for potassium-doped polyacetylene indicate that there is no lattice symmetry element which includes a translation operation of L/2 in the chain-axis direction. The observations are consistent with a structural model in which alkali–metal ions with an intracolumn spacing of 4.96 Å are commensurate with the polymer chains for the composition (CHM0.125)x. The likely polymer chain-axis repeat length is 2L (i.e., C4H4) and a lattice symmetry element which includes a translation of L is expected.
Third-order susceptibility tensors of partially ordered systems
Authors: Mazely T. L. , Hetherington W. M.
Published: July 2002

The general problem of averaging a microscopic third-order susceptibility tensor over a distribution of orientations is discussed. For the common case in which the distribution is uniform under rotation about one axis, the patterns of the macroscopic tensors for degenerate four-wave mixing, third harmonic generation, CARS, and nondegenerate four-wave mixing are given. Comparison of measurements involving variations in the polarizations and propagation directions of the fields can lead to the determination of the orientation distribution if the microscopic tensor is known.
Ag7 cluster: Pentagonal bipyramid
Authors: Bach S. B. H. , Garland D. A. , Van Zee R. J. , Weltner W.
Published: July 2002

The Ag7 metal cluster was formed in a solid neon matrix at 4 K. The identity of the cluster was established via X-band electron-spin-resonance (ESR) spectroscopy, and its structure was found to be a pentagonal bipyramid (D5h symmetry) with a 2A″2 ground state. Thus its properties are completely analogous to its Group IA alkali-metal counterparts.
An analysis of normal and local mode dynamics based on periodic orbits. I. Symmetric ABA triatomic molecules
Authors: Stefanski Krzysztof , Pollak Eli
Published: July 2002

This paper shows how to quantize Hamiltonians of symmetric ABA molecules using energies and stability frequencies of simple normal and local mode periodic orbits. It is shown that the quantization can be based either on the idea of adiabatic dynamical potentials or on the hindered rotor representation. In the former case, the stable periodic orbits correspond to the wells and the unstable ones to the barriers of the adiabatic potentials. In the latter case the normal mode periodic orbits correspond to the equilibria of the hindered rotor Hamiltonian, and the local mode periodic orbits correspond to the rotor’s orbits with the maximal allowed ‘‘angular momentum.’’ Results of extensive numerical testing of both approaches are presented for the H2O model Hamiltonian used by Sibert et al., and for the Davis–Heller potential.
Vibrational population distributions of the product of the chemiluminescent charge transfer reaction: O+(2D)+HCl→O+HCl+(A 2Σ+)
Authors: Shiraishi Yasushi , Kusunoki Isao
Published: July 2002

The chemiluminescent charge transfer reaction of O+ (2 D)ions with HCl has been studied in the energy range of 7 to 100 eVc.m. . The vibrational population distributions of the HCl+(A 2 Σ+ ,v′) product have been analyzed from the emission spectra of the A→X transition. At low collision energy the distribution has a peak at v′=3. The partial reaction cross section σ(3) for this level increases with decreasing collision energy. The features of the reaction are very similar to the F+ +CO→ F+CO+ (A) reaction reported previously. The mechanism has been discussed on the basis of curve crossing between the initial and final states along the HCl vibrational coordinate in the asymptotic region of the [O–HCl]+ system. The projectile ions C+ and N+ have also been tested for the charge transfer reaction of HCl(X)→HCl+(A).
Numerical simulation of the adsorption of hard spheres on a lattice of attractive sites
Authors: Caillol J. M. , Levesque D. , Weis J. J.
Published: July 2002

The adsorption of a dense fluid of hard spheres on a lattice of attractive sites disposed on a planar interface is studied by numerical simulations. These show that the adsorbed spheres associate into clusters and that the adsorption occurs continuously, apparently without order–disorder transition.
The analytic configuration interaction gradient method: The calculation of one electron properties
Authors: Rendell Alistair P. L. , Bacskay George B. , Hush Noel S. , Handy Nicholas C.
Published: July 2002

The implementation of the analytical energy gradient method for the calculation of first order one electron properties using closed shell configuration interaction wave functions of the single reference plus single and double substitutions (CI-SD) type is discussed. The method used is based on the Z vector formalism of Handy and Schaefer which is readily extended to allow a full orbital optimization to be carried out for a given CI-SD wave function. The results of comparative test calculations are reported for the HF, H2O, CO, HCN, and O3 molecules, for which the dipole and quadrupole moments and the electric field gradients at the nuclei have been calculated by both the expectation value and the energy derivative formalisms using several standard basis sets. The effects of orbital optimization on the above properties at the equilibrium geometries as well as at a range of distorted geometries for HF, are also discussed. It is found that agreement between the different formalisms is best when the reference state in the CI expansion is strongly dominant. The DIIS scheme of Pulay has been incorporated into the orbital optimization method and has been found to be efficient in generating the fully optimized CI-SD/MCSCF wave functions.
Vibrationally induced nuclear quadrupole coupling in the v3=1 state of 189OsO4
Authors: Scappini Flavio , Kreiner Welf A. , Frye Joan M. , Oka Takeshi
Published: July 2002

Electric nuclear quadrupole hyperfine structure arising from a quadrupolar nucleus at the center of tetrahedral molecules, such as 189OsO4, is symmetry forbidden. However, through vibration–rotation distortion a small nuclear quadrupole coupling is induced. The hyperfine structure due to the vibrationally induced eqQ has been measured for a number of P- and R-branch transitions in the ν3 fundamental of 189OsO4, by using inverse Lamb dip spectroscopy. Microwave modulation sidebands of CO2 laser lines have been used as the tunable infrared radiation. From the analysis of the observed hyperfine structure patterns, the values of the scalar and tensor coupling constants have been determined to be χVs=−4.103±0.048 MHz and χVt=−3.090±0.059 MHz.
Angular distributions of CH+3 photofragments from CH3I+ prepared by multiphoton ionization
Authors: Kawasaki Masahiro , Sato Hiroyasu , Kikuchi Toshiyuki , Kobayashi Shuichiro , Arikawa Tatsuo
Published: July 2002

Methyl iodide ions prepared by two-photon resonant three-photon ionization via Rydberg states are photodissociated by a one-photon process to result in the formation of methyl fragment ions by laser irradiation at 335–403 nm. The parallel angular distributions of these fragment ions have rather small anisotropy parameters, suggesting that dissociation is predissociative via the Ã(2E1/2) state of CH3I+.
Resonance Raman depolarization ratios as a probe of excited state dephasing times
Authors: Myers Anne B. , Hochstrasser Robin M.
Published: July 2002

The effect of the finite time for resonance Raman scattering on Raman depolarization ratios is evaluated. Partial depolarization of the resonance Raman emission will occur if the scatterers can rotate during the time interval between the absorption and emission steps. Equations are derived which express the depolarization ratio in terms of Fourier transforms of the usual time-dependent vibrational overlap multiplied by a rotational correlation function. Particularly simple results are obtained in the limit of severe inhomogeneous broadening of the electronic transition. The effect of the finite resonance scattering time on the line shape of the resonance Raman transition is also discussed.
Continuous ultraviolet emissions produced by electron impact on SF6 and NF3
Authors: Jabbour Z. J. , Blanks K. A. , Martus K. E. , Becker K.
Published: July 2002

We studied the continuous emissions in the wavelength range 2000 to 3400 Å produced by low energy electron impact on SF6 and NF3. In both cases two different features contribute to the emission spectrum. In NF3, we found a structured emission with maxima at 2880, 3005, and 3130 Å and an onset energy of 8.5±2 eV superimposed on a continuous feature from 2500 to 3500 Å with an onset at about 30 eV. We attribute the structured emission, which has an emission cross section of 2.2×10−19 cm2 at 25 eV, to the NF2 fragment. In SF6, we observed a broad emission feature from 2000 to 3300 Å with maximum intensity at 3010 and 3145 Å with a comparatively weak shoulder at 3160 Å. The 3160 Å feature, which has an emission cross section of about 2×10−20 cm2 at 25 eV, has a single onset at 13.5±1.5 eV, whereas at least two different processes with onsets at 28±2 eV and 42±3 eV, respectively, contribute to the main SF6 emission. The potential relevance of these continuous emissions for diagnostic purposes of NF3 and SF6 processing plasmas utilizing plasma-induced emission spectroscopy and laser-induced fluorescence techniques is discussed.
Density dependence of the depolarized light scattering spectrum of xenon
Authors: van der Elsken J. , Huijts R. A.
Published: July 2002

Measurements of the depolarized light scattering spectra of xenon at densities between 0.2 and 2.6 times the critical density are presented. The density dependences of the first three even spectral moments are compared with the predictions by the theory under several simplifying assumptions where use is made of data for the static structure factor from the literature. Comparison is also possible between some characteristics of the experimental spectra and thermodynamic data and transport properties. It is clearly shown that the theory hitherto used is valid up to densities slightly above the critical density. At higher densities the density dependence of the spectral moments as well as the spectral distribution seems to follow the density dependence of the fluctuations with wave vector kσ≊π. That such fluctuations would render the dominant contributions to the spectrum in contrast with the theoretical predictions is attributed to the incorrectness of the substitution of density fluctuations for dielectric fluctuations with corresponding wave vectors.
Study of two-body and three-body channels for the reaction of metastable helium atoms with selected atomic and molecular species
Authors: Pouvesle J. M. , Khacef A. , Stevefelt J. , Jahani H. , Gylys V. T. , Collins C. B.
Published: July 2002

This work reports the measurements of rate coefficients for excitation transfer reactions of metastable He(2 3S) atoms, produced in 0.3–4 atm of helium, with various reactants presenting a wide range of characteristics. In all cases studied, three-body reaction channels were identified with most probable values of rate coefficients lying in the range from 0.2 to 6.7×10−31 cm6 s−1, for Ne and N2O, respectively. These are generally more than one order of magnitude smaller than previously reported. The interpretation of this discrepancy in terms of a more reactive intermediate He2(Σ*) complex in dynamic equilibrium with the He(2 3S) population seems to be confirmed, and is analyzed in some detail.
Temperature-quench phase separation and thermal convection in polystyrene solutions: Ultrasound and optical results
Authors: Fenner D. B.
Published: July 2002

Nonequilibrium phenomena in solutions of polystyrene (PS) in the poor solvent diethyl malonate are studied with pulsed ultrasound at 5 MHz and polychromatic light. The time evolution of the attenuation and velocity of ultrasound and the attenuation of transmitted light were measured simultaneously during phase separation after rapid temperature changes (T quench) through the coexistence curve. Solution concentrations both below and near the critical values were investigated, and the monodisperse-PS sample molecular weights were 35 and 900 k. All of the parameters measured changed dramatically during phase separation and showed some unusual features. Up to an hour after a T quench, the samples were reversible upon T jump. For substantially longer times, only the acoustic velocity recovered, indicating that network softening (and swelling) occurs first in polymer mixing. The acoustic attenuation was higher during the opalescent time after T quench due to sound scattering from the larger nucleating droplets, whose growth and dissipation rates could thus be monitored. Under a temperature gradient and far from phase separation, solutions experienced instabilities in the convective flow which was sensitively detected as slow modulations of the acoustic attenuation. For smaller driving heat flux, the large modulations were nearly sinusoidal with ∼5 min period, and drops of dye traced out a single toroidal convective cell in this small-aspect-ratio sample. At higher heat flow, the oscillatory convection became very irregular.
Dual Lanczos transformation theory: Spectral and temporal properties of the Brownian harmonic oscillator embedded in the dynamically invariant subspaces for first and second moment coordinate and momentum fluctuations
Authors: Wassam W. A. , Torres-Vega Go.
Published: July 2002

The utility of dual Lanczos transformation theory in elucidating and solving time evolution and spectral density problems is demonstrated in a theoretical treatment of those spectral and temporal properties of the Brownian harmonic oscillator embedded in the dynamically invariant subspaces for first and second moment coordinate and momentum fluctuations. By coupling the mathematical apparatus of dual Lanczos transformation theory with symbolic manipulation techniques, some new results along with known results are obtained in a rather trivial fashion as compared with the methods usually adopted in classical Brownian motion theory. Aside from the theoretical treatment of the Brownian harmonic oscillator, the present work illustrates a number of important concepts and the use of the theory, including: (i) the mechanics of its implementation; (ii) the notion of dynamically invariant subspaces embedded with certain spectral and temporal properties; (iii) the investigation of generalized Langevin equations and memory function hierarchies; (iv) the use of spectral domains in the complex plane to make rigorous statements about expected temporal character, including the possible existence of separations in time scales and the role of memory effects; (v) the extraction and utilization of dynamically embedded information to determine the time evolution or spectral densities of dynamical variables and their ensemble averages, autocorrelation functions, and cross correlation functions; and (vi) the handling of time evolution and spectral density problems that are inaccessible by matrix diagonalization techniques and those that may not be given the usual continued fraction representation.
New approach to orientationally disordered molecular crystals
Authors: Jansen A. P. J.
Published: July 2002

A new theory is presented for the description of orientationally disordered molecular crystals. The theory is based on the thermodynamic variation principle with a generalized Ising Hamiltonian. The optimized single-molecule states are calculated, and the occupation of these states and the correlation in the occupation for pairs of molecules is determined via the cluster variation method. The theory is applied to the β phase of solid nitrogen. A delocalized orientational probability distribution is found that is formed by six localized states which are equally occupied. Correlation functions for the orientations of nearest neighbors are given.
Dynamics of helical worm-like chains. IX. Dynamic intrinsic viscosity
Authors: Yoshizaki Takenao , Yamakawa Hiromi
Published: July 2002

The dynamic intrinsic viscosity of flexible chain polymers in dilute solution is studied on the basis of the discrete helical worm-like chain. The correlation function formalism of the complex intrinsic viscosity [η] is given, taking account of the effect of the finite hydrodynamic volumes of subbodies of the chain. A new basis set, which is a hybrid of the one- and two-body excitation basis functions, is introduced. Then the eigenvalue problem for the representation of the diffusion operator may be reduced to N six-dimensional eigenvalue problems with N the number of subbodies. Among the six branches of the eigenvalue spectrum, one global and two local branches make contribution to the dynamic intrinsic viscosity. The theory predicts the existence of the high-frequency plateau which is distinguished from the infinitely high-frequency viscosity. The plateau arises from the interaction between the global and local chain motions (caused by the helical nature of the local chain contour), the constraints, and the finite hydrodynamic volumes of the subbodies. The interaction above vanishes for the Kratky–Porod worm-like chain possessing no helical nature. A comparison of theory with experiment is made with respect to the plateau value, and it is found that its dependence on the chemical structure of the chain may well be explained.
The equilibrium interfaces of simple molecules
Authors: McMullen William E. , Oxtoby David W.
Published: July 2002

We employ the density functional formalism of inhomogeneous systems in conjunction with square-gradient methods to discuss interfaces between coexisting isotropic and ordered phases of uniaxial molecules. The theory suggests that the surface free energy is most anisotropic for interfaces between the isotropic and molecular crystal, especially when the latter is highly noncubic. We provide criteria for the application of the square-gradient approximation, and describe ways to improve the theory when it otherwise fails for long-ranged interaction potentials.
Self-diffusion in fluids: A molecular model
Authors: Sceats Mark G.
Published: July 2002

A molecular model of self-diffusion in an atomic fluid is developed and tested against hard sphere simulation results. The model is based on a variation of Kirkwood’s theory of friction and a theory of binary encounters of Sceats. The model uses characteristics of the potential of mean force to characterize the deviation of D from Enskog theory DE as the density increases. At intermediate density D/DE exceeds unity because of the reduction of flux of binary encounters caused by third body collisions and at high density this effect is overwhelmed by the enhancement of the zero frequency value of the force power spectrum due to three center force correlations. Fluctuations in local structure are important at densities where caging becomes important.
Stochastic models for solution dynamics: The friction and diffusion coefficients
Authors: Rodger P. Mark , Sceats Mark G. , Gilbert Robert G.
Published: July 2002

Molecular dynamics simulations are reported for a solute immersed in a monatomic solvent; systems modeled represent monatomic and diatomic solute species (whose atoms are larger and heavier than the solvent), with varying force constant and bond length for the diatomic. From these simulations, autocorrelation functions, diffusion coefficients (D), and friction coefficients (ξ) are determined; for the diatomic, these are found for both the center-of-mass and relative coordinates. These results are used to develop simple models for D and ξ, including (for the diatomic relative coordinate) their frequency dependence. The models enable D and ξ to be readily determined from properties such as bulk viscosity, potential parameters, etc. These D and ξ can be used to interpret and predict picosecond time scale data for solute dynamics using stochastic models (e.g., the Kramers or Langevin equations) at the molecular level; their theoretical basis is such that they should apply to many types of solute moieties (e.g., aromatic rings) as well as to the large atoms used in the simulations.
Solubility of xenon in amino-acid solutions. II. Nine less-soluble amino acids
Authors: Kennan Richard P. , Himm Jeffrey F. , Pollack Gerald L.
Published: July 2002

Ostwald solubility (L) of xenon gas, as the radioisotope 133Xe, has been measured as a function of solute concentration, at 25.0 °C, in aqueous solutions of nine amino acids. The amino-acid concentrations investigated covered much of their solubility ranges in water, viz., asparagine monohydrate (0–0.19 M), cysteine (0–1.16 M), glutamine (0–0.22 M), histidine (0–0.26 M), isoleucine (0–0.19 M), methionine (0–0.22 M), serine (0–0.38 M), threonine (0–1.4 M), and valine (0–0.34 M). We have previously reported solubility results for aqueous solutions of six other, generally more soluble, amino acids (alanine, arginine, glycine, hydroxyproline, lysine, and proline), of sucrose and sodium chloride. In general, L decreases approximately linearly with increasing solute concentration in these solutions. If we postulate that the observed decreases in gas solubility are due to hydration, the results under some assumptions can be used to calculate hydration numbers (H), i.e., the number of H2O molecules associated with each amino-acid solute molecule. The average values of hydration number (H̄) obtained at 25.0 °C are 15.3±1.5 for asparagine, 6.8±0.3 for cysteine, 11.5±1.1 for glutamine, 7.3±0.7 for histidine, 5.9±0.4 for isoleucine, 10.6±0.8 for methionine, 11.2±1.3 for serine, 7.7± 1.0 for threonine, and 6.6±0.6 for valine. We have also measured the temperature dependence of solubility L(T) from 5–40 °C for arginine, glycine, and proline, and obtained hydration numbers H̄(T) in this range. Between 25–40 °C, arginine has an H̄ near zero. This may be evidence for an attractive interaction between xenon and arginine molecules in aqueous solution.
Molecular interactions and interface properties of nematic liquid crystals
Authors: Tjipto-Margo B. , Sullivan D. E.
Published: July 2002

Molecular perturbation methods are used to derive an approximate free-energy functional for nonuniform nematic liquids with anisotropic repulsive and attractive intermolecular forces. The free energy is simplified to a Landau–de Gennes form, providing expressions for the elastic constants and surface fields of the latter in terms of microscopic interactions. These expressions are evaluated using the Gay–Berne model potential (GB) between rigid rod-like molecules. Short-range attractive and repulsive components of that potential are found to give similar contributions to the Landau–de Gennes parameters, but these are strongly counterbalanced by long-range attractive contributions. The theory is applied to orientational alignment and nematic wetting at the free liquid–vapor interface. The preferred nematic alignment is predicted to be perpendicular to the interface, due to a weak dominance by repulsive and short-range attractive forces. It is found that wetting by the nematic phase changes from complete to incomplete as the molecular elongation increases.
Calculation of the effect of the Debye–Waller factor on the intensities of molecular modes measured by neutron inelastic scattering. Application to hexamethylenetetramine
Authors: Jobic H. , Lauter H. J.
Published: July 2002

The neutron inelastic scattering spectra (NIS) of hexamethylenetetramine (HMT) have been measured at several temperatures: 5, 60, 150, 250, and 310 K. A method for simulating these NIS spectra is presented. It includes the contributions from the fundamentals and from the multiphonon due to the lattice modes. The effect of the lattice Debye–Waller factor on the NIS spectra of HMT is calculated as a function of temperature and neutron momentum transfer. A partial reassignment and a new force field is proposed for this molecule, they are based on the NIS frequencies and intensities of HMT-h12 and HMT-d12.
The energy of the resonance soliton
Authors: Collins Michael A.
Published: July 2002

The resonance soliton, originally derived by Davydov and Kislukha, is a localized mobile exciton–phonon state whose energy lies below the bottom of the exciton band in a one-dimensional lattice. The soliton energy, as reported hereto, is proportional to the square of a dimensionless exciton–phonon coupling constant α2. It is shown that the soliton energy has a correction proportional to α, due to a redistribution of the phonon density of states. This term can dominate the soliton energy in the regime of validity of the theory α≪1. The applications of this model and its relation to other theories are significantly affected.
The microwave spectrum of difluoromethanimine, CF2=NH
Authors: Groner P. , Nanaie H. , Durig J. R. , DesMarteau D. D.
Published: July 2002

The microwave spectrum of a common decomposition product of CF2=NCl and CF2=NBr has been analyzed. The observed hyperfine structure requires the presence of 14N and the absence of Cl and Br in the molecule. The inertial defect establishes the planarity of the molecule. The observed rotational constants are A=11 464.024(4), B=11 151.892(4), and C=5643.146(4) MHz, which are very similar to those reported for CF2=O. The molecule is identified to be difluoromethanimine, CF2=NH. The dipole moment components were determined from the Stark effect to be |μa|=0.415(1), |μb|=1.330(1), and |μt|=1.393(1) D. The 14N quadrupole coupling constants are χaa=1.029(20), χbb=−2.560(17), and χcc =1.531(22) MHz. These data are compared to similar quantities for some corresponding molecules.
A crossed laser-molecular beam study of the photodissociation dynamics of Fe(CO)5 at 193 nm
Authors: Ray Urmi , Brandow Susan L. , Bandukwalla Gulnar , Venkataraman Bhawani K. , Zhang Zhuangjian , Vernon Matt
Published: July 2002

A crossed laser-molecular beam study of the photodissociation mechanism of Fe(CO)5 has been performed at 193 nm where time-of-flight measurements of the primary iron containing photofragments have been recorded under collision free conditions. The center-of-mass velocity distributions derived from the TOF data by the method of moments show that Fe(CO)2 accounts for >99% of all photoproducts formed after absorption of one photon. The only mechanism which quantitatively reproduces the measured velocity distributions is a sequence of three, uncorrelated, statistical CO eliminations. At high photon flux, a second photon can be absorbed by the Fe(CO)2 photofragment which decomposes by an uncorrelated sequential elimination of the remaining two CO ligands.
Electron density distributions in hydrogen bonds: A local density-functional study of α-oxalic acid dihydrate and comparison with experiment
Authors: Krijn M. P. C. M. , Feil D.
Published: July 2002

Accurate local density-functional calculations of the electron density distribution in α-oxalic acid dihydrate, C2H2O4⋅2H2O, are performed and compared with experimental densities obtained from high-resolution x-ray diffraction measurements at 100 K. Results are corrected for basis set superposition errors. The agreement between the vibrationally averaged theoretical and experimental density improves significantly when taking into account the effects of hydrogen bonding and crystal environment. Electron density redistributions due to strong hydrogen bonds are found to outweigh the experimental standard deviation only in the bond region of the molecules involved as donor of the hydrogen bond.
Tricritical phenomena in quasi-binary systems. VII. Light scattering from ternary ethane systems
Authors: Kumar Anil , Cannell David S. , Scott Robert L. , Knobler Charles M.
Published: July 2002

The tricritical region of quasibinary mixtures of ethane with higher hydrocarbons (n-heptadecane, n-octadecane, n-nonadecane, and their mixtures) has been studied by light scattering from the three phases. The individual susceptibilities, as determined from intensities and correlation lengths, diverge as predicted by the asymptotic mean-field theory, but the Griffiths first sum is neither zero nor constant; instead it increases sharply as the tricritical point is approached. This behavior of the first sum is consistent with a divergence predicted by nonclassical theory or with an approach to the constant limiting value at the tricritical point predicted by an extended classical theory. The Griffiths second sum approaches zero in the way expected from theory (both classical and nonclassical). The ratio of the light-scattering intensities of the outer phases (α and γ) and the corresponding ratios for the correlation lengths do not appear to approach unity as the tricritical point is approached, in disagreement with all present theories, classical or nonclassical.
Photon angular momentum approximations for molecular collisions in an intense nonresonant laser field
Authors: Peploski James C. , Eno Larry
Published: July 2002

We consider the problem of photon angular momentum approximation for molecular collisions in an intense nonresonant laser field. A couple of orientation averaging schemes are proposed for dealing with the absence of rotational invariance in the full (inclusive of photon angular momentum) dynamical equations. Application of the schemes is made for reactive laser switching between a pair of one-dimensional potential curves which are free to rotate in space. The preferred scheme of the two depends upon there being only a single initial or final state of interest but both are very effective over the range of intensities examined. A third approximation scheme which essentially blots out photon angular momentum effects is less satisfactory.
Orientational ordering of rod- and disk-like mesogens with flexible alkyl chains
Authors: Cheng G. Q. , Dong Ronald Y.
Published: July 2002

A model has been developed by Emsley, Luckhurst, and Stockley to calculate the potential of mean torque for flexible molecules and to account for the observed variations in the segmental order profiles of the chain deuterons in the rod-like liquid crystals 4-n-alkyl-4′-cyanobiphenyl by deuterium NMR spectroscopy. In this paper we have applied this model to two other homologous series, one with a rod-like core (p-alkyloxybenzylidene-p-alkylaniline) and one with a disk-like core (hexaalkyloxytriphenylene), by considering the fragment which contains the deuterated alkyloxy chain. The subtle variations in the segmental order profiles are due to the geometry of the bond that governs the orientation of the chain and the symmetry of the core. The latter point is examined by using discotogens. The model involves segmental interaction parameters Xa and Xcc, and the excess energy of gauche over a trans state Etg. These values are obtained by fitting the theory to the experimental quadrupolar and/or dipolar splittings in mesophases of liquid crystals.
Spectroscopy and photophysics of refractory molecules at low temperature. II. The green systems of ZrS
Authors: Simard B. , Mitchell S. A. , Hackett P. A.
Published: July 2002

The diatomic molecule ZrS has been observed by laser induced fluorescence in a supersonic molecular beam following reaction of laser-vaporized zirconium atoms with ≈2% carbonyl sulfide (OCS) added into a flow of He carrier gas. Two band systems have been observed in the 413–700 nm region. The R heads of their band origins lie at 20 218.9 and 20 208.3 cm−1. The former is tentatively ascribed to the E 1Σ+–X 1Σ+ transition; the latter remains unassigned but is shown to borrow all of its intensity from the former through a simple intensity borrowing mechanism. A vibrational analysis leads to the following molecular constants for the ground state: ωe=548.4 cm−1, ωexe=1.6 cm−1. Franck–Condon factors for several vibronic transitions are also reported. Intrinsic radiative lifetimes of several vibrational levels of both upper states have been measured, and confirm that the two states perturb each other. A limited deperturbation analysis has been carried out. Vibrational constants, mixing coefficients, interaction matrix elements, and other spectroscopic parameters are reported.
Accurate transition moments between the A 3Σ+u, B 3Πg, and B′ 3Σ−u of N2 using multiconfigurational linear response
Authors: Rizzo Antonio , Graham Richard L. , Yeager Danny L.
Published: July 2002

Accurate values of the electronic transition moment function for the first positive (A 3∑+u−B 3Πg) and infrared afterglow (B 3Πg −B′3∑−u) systems of N2 were obtained through multiconfigurational linear response (also known as the multiconfigurational time-dependent Hartree–Fock, MCLR/MCTDHF). MCLR/MCTDHF furnished both excitation energies and transition moments. MCLR/MCTDHF calculations on the first positive system demonstrate the importance of configurations involving the 2σg orbital. To obtain accurate values, configurations were chosen with a complete active space of all valence orbitals (2σg, 2σu, 1πu, 3σg, 1πg, 3σu). This is the first MCLR/MCTDHF calculation for molecules as large as first row diatomics in which a complete active space including the entire valence shell is employed. The results are in substantial agreement with accurate theoretical calculations by others and, hence, disagree in their overall behavior with the previously published empirical values. One of our conclusions is that the experimental values for the first positive system far from the equilibrium region are in error.
Some identities and relations for transport coefficients of dense fluids
Authors: Eu Byung Chan
Published: July 2002

The transport coefficients appearing in the dense fluid kinetic theory by the author are cast in the forms of time correlation functions suitable for numerical simulation methods. The Einstein relation for the diffusion coefficient and friction constant is found to follow exactly from the collision integral for the diffusion coefficient in the dense fluid kinetic theory. By assuming that the momentum and the force (or configuration) relax at different rates in the dilute and dense regimes of density, it is shown that the self-diffusion coefficient and the viscosity have two different relations, i.e., the one predicted by the mean free path theory and the Stokes–Einstein-type relation. The intrinsic viscosity of a polymer solution is shown to scale like mν/2a, where ν is an index and ma is the molecular weight, if the momentum relaxation time of the polymer scales like (∼(l−2a))ν/2 with ∼(l−2a) denoting the mean square end-to-end distance. If ν=1, the scaling follows Flory’s prediction for instrinsic viscosity.
Quantitative absorption and fluorescence spectroscopy of Cd(CH3)2 in vacuum ultraviolet
Authors: Suto Masako , Ye Chao , Lee L. C.
Published: July 2002

Photoabsorption and fluorescence cross sections of Cd(CH3)2 were measured in the 115–270 nm region using synchrotron radiation as a light source. Vibrational structures superimposing on absorption continua were observed in the 140–230 nm region. Fluorescence appears at excitation wavelengths shorter than 184 nm. Fluorescence produced at 123.6, 147, and 157.5 nm was dispersed to identify the emission to be mainly from excited Cd (326.1 nm). Emissions from excitation of Cd(CH3)2 at 157, 193, and 248 nm via a multiphoton excitation process were investigated.
Position moments linearly averaged over the wave function
Authors: Koga Toshikatsu
Published: July 2002

It is shown that there exists a set of relations between position moments rk (with k being a nonnegative integer) linearly averaged over the wave function. The true wave function must satisfy all of these relations, and therefore they can be used as criteria to assess the accuracy of approximate wave functions. The zero momentum energy formula proposed previously is the simplest case of the present results.
Theory and application of time-resolved optoacoustics in gases
Authors: Beck Kenneth M. , Gordon Robert J.
Published: July 2002

The acoustic waves generated by a relaxing gas have a shape which depends on the ratio of the relaxation rate to the acoustic propagation rate. It is shown how the relaxation time constant can be extracted from this waveform for a variety of relaxation rate laws, using a Green’s function to solve the linearized wave equation. The accuracy of the method is confirmed by measuring the relaxation rate constants for OCS diluted in Ar and He. These, plus a previous measurement of the relaxation rates of highly excited SF6 and C6F5H in Ar, are in good agreement with the literature.
Adsorption of Cu and Ag atoms on Si(111) surfaces: Local density functional determination of geometries and electronic structures
Authors: Chou Shih-Hung , Freeman A. J. , Grigoras S. , Gentle T. M. , Delley B. , Wimmer E.
Published: July 2002

The electronic structures, adsorption geometries, chemisorption energies, and vibrational frequencies of single Cu and Ag atoms on Si(111) surfaces are determined by self-consistent total energy calculations using first principles, local density functional theory, with a numerical basis for a cluster of 20 Si atoms. The binding energy results reveal that both Cu and Ag adsorb in threefold hollow sites with equilibrium heights of 0.74 Å (Cu) and 1.48 Å (Ag) above the plane of the surface Si atoms. The adsorption energies are found to be 92 kcal/mol for Cu and 72 kcal/mol for Ag. Assuming a rigid substrate, the calculated frequencies of the perpendicular vibrational modes are 58 cm−1 for Cu and 90 cm−1 for Ag. The lateral diffusion barriers, assuming an unreconstructed rigid Si(111) surface, are found to be 12 and 8 kcal/mol for Cu and Ag, respectively. Calculations for Cu and Ag atoms being moved towards the interior of the cluster, including geometric relaxation of the nearest-neighbor Si atoms, demonstrate that Cu has a much lower vertical penetration barrier than Ag (4 vs 53 kcal/mol). Therefore, at elevated temperatures, Cu can be expected to penetrate through the silicon surface, whereas Ag should remain above the surface Si atoms. Adsorbate-induced electron density differences indicate that Cu weakens the bonds between surface and subsurface silicon atoms, while Ag has a significantly smaller effect. Contour maps of eigenfunctions, which are associated with surface states, show that the dangling bonds of the silicon atoms at the surface interact with the metal s and d orbitals. The Cu 3d orbitals interact stronger than the Ag 4d electrons. The results suggest that the catalytic activity of Cu and the absence of activity of Ag in the syntheses of methylchlorosilanes (‘‘direct process’’) is possibly due to the ability of Cu to penetrate into the surface thus forming the initial stages of a copper-silicide, whereas Ag stays at the surface and desorbs at higher temperatures.
Note on standard free energy of transfer and partitioning of ionic species between two fluid phases
Authors: Zhou Yaoqi , Stell George , Friedman Harold L.
Published: July 2002

We investigate the partition of ionic species in a system of two phases that are immiscible even in the limit of vanishing ionic concentrations. An analytic expression for the solvation free energy and for the standard free energy of transfer is obtained for dipolar hard-sphere solvents in the mean spherical approximation. We find that both dipolar hard-sphere solvents and continuum solvent models yield an electric potential difference (equilibrium junction potential) between two pure solvents in contact that is identically zero. Simple analytic expressions for partition coefficients and junction potentials are obtained in the ideal-solution limit for these solvent models. We note that for the junction potential the ideal solution limits differ in general from the pure-solvent values. For nonideal solutions, approximations based on the mean spherical approximation can be applied conveniently to calculate the activities of ionic species and to obtain the partition coefficients.
Adsorption–desorption transition of polymer chains interacting with surfaces
Authors: Forgacs G. , Privman V. , Frisch H. L.
Published: July 2002

We report exact results for two and three dimensional directed models of polymer chain adsorption–desorption transition in the presence of attracting substrate. A number of properties of this transition are evaluated explicitly. These include the phase diagram, the adsorption fraction and the monomer-density profile. Our results are obtained by a novel application of the transfer matrix method particularly suitable for studies of polymer chain conformations.