bluuebunny/crossref_metadata_2025 huggingface dataset:
Simplifications in the Determination of the Wavefunction and of the Band Structure for a Semi-infinite Crystal in the Mixed Representation
Authors: Torrini M. , Zanazzi E.
Published: February 2004

The one-electron wavefunctions near the surface in a solid may be set up by means of a mixed representation. This representation uses a coordinate normal to the surface, and Fourier components on the bidimensional reciprocal lattice of the surface. In this way the Schrödinger equation may be transformed into a first order system of differential equations with periodic coefficients in only one independent variable. The heart of the method is the determination of a fundamental matrix for the system, and the eigenproblem relevant to this matrix. The matrix must be generally determined by numerical integration of the system through a period. Remarkable simplifications in both the determination of the fundamental matrix and the search for its eigenvalues and eigenvectors can be found by applying symmetry operations. These simplifications may consist of reducing the integration interval or of halving the size of the matrix for which the eigenproblem must be resolved. We examine in detail several cases which occur very often in practical calculations.
ESR Study of the Pyrolysis of Hydrogen on Tungsten
Authors: Cochran Edward L. , Bowers Vernon A. , Adrian Frank J.
Published: February 2004

Electron spin resonance (ESR) techniques have been adapted to the study of the atomization of hydrogen occurring on the surface of heated filaments. The method involves heating an axial metal filament in a circular cavity to temperatures of 1000–2000°K and measuring the steady-state atom concentration produced in the gas. Heating is accomplished by DC current pulsed at 6000 pulses/sec with a 15% duty cycle. Under these conditions, the sample space is free a relatively high proportion of the time from current-induced field inhomogeneities. The sensitivity and spectral line shapes observed compare favorably with those observed in control experiments in the same cavity in the absence of the heated filament. The atomization of hydrogen on tungsten filaments can be conveniently studied by ESR at hydrogen pressures between 0.1 and 100 torr; thus, this method complements previously developed techniques that are most useful at lower pressures (10−6-10−2 torr). The observed pressure and temperature dependences of the H atom concentration are interpreted in terms of a mechanism involving the production of hydrogen atoms on the filament, their diffusion into the gas, and recombination on the cavity wall. The surface reaction appears to be complex, changing character at an H2 pressure somewhere around 0.5 torr. Extrapolation of the ESR data to very low H2 pressures yields results consistent with previous low pressure studies. At higher H2 pressures the rate of H atom production decreases due to surface saturation effects.
Magnetic and Crystal Structures of ErC2 at 297–2°K
Authors: Atoji Masao
Published: February 2004

Neutron-powder diffraction measurements show that the tetragonal body-centered ErC2 becomes antiferromagnetic below 19 ± 2° K. The moment alignment is governed by several narrowly dispersed, incommensurate transverse magnetization waves. The major modulation waves propagate along the a axis and are polarized in the b axis direction. The magnetization wavelengths are 5.78, 5.4, 6.7, and 7.1 Å, and the amplitudes are respectively 6.55, 2.2, 2.1, and 1.6 μB at 4.2°K. The ferromagnetic alignment in the plane perpendicular to a is slightly altered by several long-periodicity modulations propagating along the b and c axes. A commensurate, antiferromagnetic wave also participates in the moment alignment below 10 ± 3° K. The commensurate wave is represented by setting the wavelength equal to the a spacing in the incommensurate wave mode. The commensurate moment is 2.5 μB at 4.2°K. The root-mean-square value of all the moment amplitudes is 7.8 μB at 4.2°K and increases to 7.9 μB at 2°K. These features are likely caused by a strong perturbation between the RKKY coupling and the crystal-field effect. The proposed structure is by no means unique, although an exhaustive search for other probable models was fruitless. The crystal structure is characterized by the C–C bonding distance of 1.288 ± 0.006 Å and the Er–C distances of 2.403 ± 0.003 and 2.640 ± 0.001 Å. These distances are indicative of a large 5d-πg2p contribution in the conduction-band wavefunction.
Nuclear Dynamic Polarization and Spain-Lattice Relaxation in Plastic Cyclohexane Doped with a Nitroxide Free Radical in Low Concentration
Authors: Verdun H. , Meerovici B. , Volino F.
Published: February 2004

In order to check the theory of spin diffusion with a new kind of sample, we present measurements of dynamic polarization and spin-lattice relaxation, at 3300 G, on the plastic phase of cyclohexane doped with a nitroxide free radical. The EPR lines are sufficiently narrow to allow an ideal ``solid effect.'' The enhancement of the proton resonance was measured as a function of microwave power for various temperatures. The decay rate of these enhanced signals both as a function of temperature and radical concentration was studied, in order to determine the proton spin-lattice relaxation time. The electronic spin-lattice relaxation time was measured by the saturation method. The data were found to be fairly consistent with a rapid diffusion regime between −10 and −60°C. The critical radius (i.e., the radius of a sphere, centered on the electronic spin, inside which the diffusion coefficient is zero) was found to be ≃ 1.7 Å. This value is much smaller than the distance between the free radical and the nearest cyclohexane molecules. The meaning of this fact is not yet clear.
Infrared Absorption Spectrum of KN3 Crystal: Selection Rules and Analysis of Internal ± Internal and Internal ±External Mode Absorptions
Authors: Iqbal Zafar
Published: February 2004

In this paper we report the measurement of the polarized infrared spectrum between 400 and 5000 cm−1 of single crystal KN3, at 80° and 298°K. A large number of absorptions due to internal, internal± internal, and internal±external mode transitions were observed. Selection rules for internal±internal and internal±external phonon pairs along different symmetry sections of the Brillouin zone have been worked out. The observed band structure is assigned in many cases to critical points along symmetry lines and at points in the Brillouin zone, in terms of these selection rules. In the Γ, Σ, M, Λ, and H regions of the zone the spectral observations concerning the external translatory modes are compared with the rigid ion model calculation and neutron data of Rao et al.
Spectrum of Exchange Coupled Cr3+ Pairs in YAlO3
Authors: van der Ziel J. P.
Published: February 2004

The optical spectrum and exchange splittings of the two types of nearest neighbor Cr3+ pairs in orthorhombic YAlO3 have been studied. The emission spectrum from 7200 to 7350 Å yields the ground state exchange −JS.S+j(S.S)2 with Ja=−24.7, ja=−0.66 cm−1 and Jb=−26.2 and jb=−0.91 cm−1. From the shift of the lines under uniaxial compression along the crystal z axis we conclude these values correspond, respectively, to pairs whose axes are in the xy plane and along the z axis. The energies of several pair levels with one ion in 2E and the other in 4A2, having S=1 and 2 have been obtained. Selective enhancement of nn pair luminescence is observed when the excitation energy is at about twice the single ion 2E energy. The excitation and absorption spectra indicate these are pair levels corresponding to both ions in the 2E state and have S=0 and 1. The single ion transitions are magnetic dipole in agreement with Cr3+ being at a center of inversion. The pair lines are electric dipole, and the strongest transitions in the infrared, and the pair lines in the ultraviolet obey the selection rule ΔS=0 indicating the pair transitions obtain their intensity from the spin dependent dipole moment mechanism.
Initial Distribution of CO† from the Reaction O+CS→ CO†+S
Authors: Foster K. D.
Published: February 2004

The CS2/O2 cw chemical laser has been studied in a transverse flow system. From a determination of the temperature and the upper state J value for which maximum gain occurred, in several CO vibrational bands, an estimate of part of the initial vibrational distribution of CO† from the reaction O+CS→CO†+Swas obtained. This distribution indicated that the ``most probable'' level of formation of newly born CO† is v=12± 1, which represents approximately 90% of the energy librated from the reaction.
Transformation Properties of Antisymmetric Spin Eigenfunctions under Linear Mixing of the Orbitals
Authors: Wormer Paul E. S. , van der Avoird Ad
Published: February 2004

After recalling the duality between the general linear group GL(m), represented by its N-fold inner product, and the permutation group SN, we have given a survey of its quantum chemical consequences. It causes the one-to-one correspondence between the total spin quantum number and the permutation symmetry of N-electron spin functions, and, via the Pauli principle which imposes permutation symmetry on the spatial part also, it leads to specific properties of antisymmetric spin eigenfunctions under orbital transformations. Such functions can be classified according to the irreducible representations of GL(m). For special orbital transformations, often occurring in quantum chemistry, which mix only orbitals in different subsets among each other, we have derived how the transformation of the N-electron wavefunctions simplifies, by a reduction of the representations of GL(m). The theory is illustrated by an example and some applications are discussed.
Angular Relaxation of the Symmetrical Top. II. The Rough Sphere
Authors: Rider K. L. , Fixman M.
Published: February 2004

We investigate the reorientation of molecules in a bath of atoms without internal degrees of freedom. A Markoffian kinetic equation for the rough sphere model is formulated. A collision operator is derived and the appropriate kinetic equation for the two time orientational autocorrelation functions is solved for a variety of molecular parameters. It is found that the finite collision rough sphere model allows the retainment of greater dynamic coherence than does the Langevin model of the spherical top.
Optimized Cluster Expansions for Classical Fluids. III. Applications to Ionic Solutions and Simple Liquids
Authors: Andersen Hans C. , Chandler David , Weeks John D.
Published: February 2004

In a previous paper we have derived a number of simple approximations for the thermodynamic and structural properties of liquids. Here we test these approximations by applying them to models for ionic solutions and atomic liquids and comparing the results with those obtained from Monte Carlo and molecular dynamics simulations. In particular, the pair-correlation functions for 1–1 primitive model aqueous ionic solutions are calculated using the EXP approximation. The internal energy for 2–2 salts is obtained from the ORPA+B2 approximation. The pair-correlation function for the Lennard-Jones fluid is calculated with the EXP approximation. In all cases the results obtained agree closely with those obtained from computer experiments. Thus, the ORPA+B2 approximation for the free energy and the EXP approximation for the pair correlation function provide a theory that is both very accurate and applicable to a wide variety of classical fluids.
Low Temperature Phase Transition in TlN3. Infrared Absorption and Raman Scattering Study
Authors: Iqbal Zafar , Malhotra M. L.
Published: February 2004

The long wavelength modes of tetragonal (Phase I) TlN3 in the frequency range 30–2500 cm−1, have been observed and assigned. The low frequency spectrum becomes fairly complex at low temperatures due to an orthorhombic distortion of the Phase I lattice. The possibility of a non-centrosymmetric structure is indicated by an apparent breakdown of the rule of mutual exclusion for some of the lattice frequencies. The phase transition has been found to involve a distortion of the unit cell at ≈ 278° K followed by a distortion of the azide ions at ≈ 225 ° K. The latter temperature is close to the value observed by differential thermal analysis. The low frequency Raman spectrum has been measured at different low temperatures and the temperature dependence of the frequencies are discussed in terms of microscopic approaches to the phase transition problem.
Kinetic Energy Release in Unimolecular Ionic Reactions. Thermochemical Aspects
Authors: Jones E. G. , Beynon J. H. , Cooks R. G.
Published: February 2004

The kinetic energy (T) released in the unimolecular reactions of metastable gaseous ions has been compared with literature values obtained by several methods for the same reactions occurring in the ion source. The internal energy difference between the long-lived and the short-lived reactant ions has a relatively small effect upon the kinetic energy release when rearrangement and elimination reactions are studied, large kinetic energy releases being observed by both methods. Simple cleavages, which are typically accompanied by far smaller energy releases, show large relative variations when the time-scale of the reaction is changed with the values measured in the field-free region being much lower (e.g., 0.002 eV vs 0.18 eV in the case of H loss from propane). These and other results provide strong evidence that metastable reactions occur from ions whose average internal energies are only slightly in excess of the activation energy for the process in question. The consequences of this result for studies on energy partitioning by mass spectrometry and for improving thermochemical data on ions are stressed and illustrated. Franklin has emphasized the importance of T values, extrapolated to ions of threshold internal energy, as a means of correcting for the excess energy of the activated complex and so improving the quality of thermochemical measurements obtained by mass spectrometry. Since the metastable technique is directly applicable to ions of approximately threshold energy, no extrapolation is necessary, and a rapid and simple method of obtaining improved thermochemical data is therefore available. It is further suggested that these results should also provide a basis for correcting measured appearance potentials for the excess energy term arising from the presence of a reverse activation energy.
Rotational Diffusion Model with a Variable Collision Distribution
Authors: Frenkel D. , Wegdam G. H. , van der Elsken J.
Published: February 2004

Starting with an m-diffusion model a matrix description is given of the rotational motion of a dipole molecule undergoing frequent collisions. This treatment gives rise to an analytical expression for the dipole correlation function and for the angular momentum correlation function in which a limited number of parameters from the model appear. It is argued that the collision distribution which determines the rotational diffusion process need not necessarily be a Poisson distribution. In liquids with strong interactions the distribution is governed by the frequency distribution of the medium. This leads to the inclusion of a librational motion in the rotational diffusion model. A comparison of simulations with different collision distributions and experimental data is given.
Dissociative Attachment of Thermal Electrons to N2O and Subsequent Electron Detachment
Authors: Warman J. M. , Fessenden Richard W. , Bakale G.
Published: February 2004

The attachment of electrons to nitrous oxide has been studied, following pulse ionization, by a microwave conductivity technique. For pure N2O, and N2O diluted with N2, extrapolation of the experimentally observed attachment rate constant to zero pressure gives an upper limit of 1 × 10−15 cm3 molecule−1 · sec−1 for the rate constant for two-body attachment at 298°K. When C2H6 or C2H8 are used as diluents a higher two-body rate constant of 6× 10−15 cm3 molecule−1 · sec−1 is found which is believed to represent the rate constant of the dissociative attachment reaction e−+N2O→N2+O−. The much lower value for pure N2O or N2O–N2 mixtures is explained by the regeneration of electrons, via the reactions O−+N2O→NO+NO− and NO−+N2O→NO+N2O+e−, which largely nullifies the primary attachment step. The presence of a hydrocarbon prevents electron regeneration by irreversibly reacting with O− and allows observation of the uncomplicated rate constant for dissociative attachment. The rate constant for dissociative attachment increases markedly with increasing temperature and in the range studied, 278 to 355°K, has an activation energy of 0.42± 0.04 eV. Confirming evidence for the proposed mechanism is obtained from the effect, on the attachment rate constant, of H2 added to C2H6–N2O mixtures. The H2 reacts with O− by the associative detachment reaction O−+H2→H2O+e− to nullify the primary attachment step and to reduce the rate constant by an appropriate amount. In the steady-state radiolysis of N2O, the yields of the products N2 and NO are found to increase markedly with decreasing pressure below 50 torr. These increases in yields may be quantitatively related to the chain reaction resulting from dissociative attachment followed by electron regeneration.
Dependence of Reactivity on Internal and Translational Energy in K+SF6, CCl4, and SnCl4
Authors: Sloane Thompson M. , Tang S. Y. , Ross John
Published: February 2004

The dependences of the nonreactive scattering of potassium atoms at thermal energies from SF6, CCl4, and SnCl4 on internal energy of the molecules and initial relative kinetic energy are studied as a function of scattering angle. A collimated, velocity-selected beam of potassium atoms is crossed at right angle with a molecular beam whose source temperature is varied over the range of 300°–580°K. The angular dependence of the nonreactive cross sections is analyzed with an optical model of chemical reactions to obtain information about potential parameters in an assumed form of two-body interaction, the threshold conditions for reaction, the probability of reaction as a function of the distance of closest approach and internal energy, as well as an estimate of the total reaction cross section. The threshold conditions, energy and distance, in the reaction K + SF6 are independent of the initial relative kinetic energy at both low and high SF6 temperatures; the threshold distance increases by about 6% from T(SF6) = 300°K to 580°K. The variation of the nonreactive scattering with the SF6 beam source temperature, or SF6 internal excitation, can best be explained by a model in which only the stretching vibrational modes of SF6 are assumed to have an effect on the reactivity with K. The calculated differential cross sections based on this model fit very well to the measured results over the experimental ranges of SF6 temperature and of initial relative kinetic energy (2.05–5.18 kcal/mole). The effect of internal energy in CCl4 on its reactivity with K is similar to that of SF6 but smaller. The effect in SnCl4 is very small and uncertain; the interpretation is complicated by the lack of knowledge about the interaction potential.
Raman Spectra of Gases. II. Monosubstituted Cyclobutanes
Authors: Durig J. R. , Carreira L. A. , Willis J. N.
Published: February 2004

The low frequency Raman spectra of chloro-, fluoro-, and methylcyclobutane have been recorded for these molecules in the gaseous state. Only the Δv=1 transitions were observed for the ring-puckering vibrations of these molecules of Cs symmetry. From the observed data, potential functions governing these low frequency vibrations have been calculated.
Classical Trajectory Studies on Planar Reactive H + H2
Authors: Saxon R. P. , Light J. C.
Published: February 2004

Classical trajectory calculations on planar reactive collisions of H+H2 are presented and compared with previously reported quantum calculations. Probability of reaction versus impact parameter compares reasonably well for the two calculations. Classical angular distributions match the back peak of the quantum results but show no forward peak. The classical distribution of product rotational states is sharply peaked at the initial state while the quantum distribution is a very smooth function of rotational state.
Theory of Translational-Vibrational Energy Transfer for Reactive Collisions
Authors: Fischer Sighart F. , Ratner Mark A.
Published: February 2004

A quantum mechanical theory for polyatomic molecules undergoing bimolecular reactions is presented which allows prediction of the vibrational excitation of the products as a function of (a) the incident translational energy, (b) the curvature of the reaction path, (c) the frequency changes for degrees of freedom perpendicular to the reaction coordinate, and (d) the position of the saddle point of the potential hypersurface.
Vibrations of Glasslike Disordered Systems. II. The A2B3 Network in Two Dimensions—Application to Vitreous As2Se3 and As2S3
Authors: Bermudez Victor M.
Published: February 2004

A computer-generated two-dimensional A2B3 glasslike disordered network having a Gaussian distribution of A–B–A bond angles has been formulated as an approximate model of the vitreous layer-type chalcogenides As2Se3 and As2S3. A two-parameter potential function involving nearest-neighbor central and noncentral interactions has been used to generate the equations of motion which have been solved by application of the negative eigenvalue theorem. Under the appropriate choice of boundary conditions, the bands in the computed densities of states adequately reproduce those in the experimental spectra in number and position. Eigenvectors have been computed for selected frequencies throughout the spectra, and assignments of observed spectral bands to specific normal modes have been formulated.
High-Resolution Calculations on Atom-Surface Scattering: Method and Comparison with Experiment for Ne/LiF
Authors: McClure J. Doyle
Published: February 2004

Calculated scattering distributions for Ne on (001) LiF are compared with corresponding experimental distributions. For calculational purposes, the surface is effectively modeled by a monolayer of 3D harmonic oscillators representing F− ions. The atom-surface potential at close range is assumed to arise from two-body dispersion-exchange interactions which are represented by superposed Lennard-Jones (12, 6) model potentials. The foregoing potential is shown to be large with respect to that due to induced-dipole interaction. The long-range van der Waal's potential, for which the strength is independently specified, is taken to be one-dimensional in nature. The classical trajectory method combined with Monte Carlo distribution of surface impact coordinates and thermal states of gas/surface atoms is used to determine angular distributions of scattered beam intensity. Physical concepts underlying an efficient calculational algorithm required for high resolution scattering calculations are described. For a variety of incident beam conditions, including variation of beam ``temperature,'' angle of incidence, and azimuth of the plane of incidence, calculated angular positions of dominant intensity peaks in the scattered beam typically agree with experimental determinations to within 3°–5°. Predictions of high intensity ``ridge'' structure, which is quite different in form for the <010> and the <110> planes of incidence, are confirmed by experimental studies undertaken elsewhere. Calculated values of absolute scattering probability in the plane of incidence typically agree with prior experimental determinations to within 10%–15%. Statistical fluctuations in the calculated intensities (∼5%–10% of the calculated value) preclude a rigorous determination of whether or not the model reproduces all observed substructure on the high-intensity ridges. The parameters defining the semiempirical Ne/LiF potential used in the present study correspond to a heat of adsorption of ∼0.016 eV, and to a two-body range parameter of 3.9 Å. Means for determination of the potential parameters from a small part of the available experimental data and means for assessing the accuracy of these values are reported in a companion paper.
Determination of Atom-Surface Properties for Ne/LiF by Scattering Studies
Authors: McClure J. Doyle
Published: February 2004

The preceding paper contains a quantitative comparison of calculated and experimental intensity distributions for atomic beams of Ne scattered from the (001) LiF surface. Those results imply self-consistency of both the scattering model and the semiempirical atom-surface potential derived from the study, since a large number of observables are predicted on the basis of a semiempirical potential determined by appropriate choice of three disposable parameters. A systematic trials study used both to determine these potential parameters from a limited part of the experimental data, and to assess the significance of uncertain values of other parameters associated with the scattering model is reported here. The trials study clarifies mechanisms associated with scattering phenomenology, and permits one to identify characteristic features of the colorful intensity structure (surface rainbows) with specific physical parameters that influence the atom-surface interaction. The use of scattering theory to extract atom-surface properties from experimental observables is dependent on the latter relationship.
Electric Dipole Moment of Diatomic Molecules by Configuration Interaction. IV. Basis Set Dependence in CO a3Φ
Authors: Green Sheldon
Published: February 2004

The effect of basis set quality on configuration interaction wavefunctions is considered theoretically. In particular, one-electron operator properties and basis sets not too far from the Hartree-Fock limit are considered. It is then shown that a CI function will contain essentially the same error due to basis set as the SCF function upon which it is based. This result is used to account for the error in a previously reported dipole moment calculation on CO a3Φ; starting from an SCF function with an energy 0.009 a.u. and a dipole 0.16 D above the Hartree-Fock limit, a CI dipole of 1.55 D was obtained. Using a larger basis set, an improved SCF function is presented which has an energy only 0.001 a.u. and a dipole 0.04 D above the HF limit. A CI function based on this SCF gives a dipole moment of 1.43 D, which may be compared with the experimental value of 1.37 D.
Librational Mode Linewidths of Thiourea
Authors: Laulicht I. , Pellach E. , Brith M.
Published: February 2004

Raman spectra of polycrystalline thiourea were measured at various temperatures above and below the 169°K ferroelectric phase transition. No ``soft'' mode was found. The temperature dependence of two libration linewidths was analyzed by using a theory which is a combination of Ivanov et al.'s theory for Raman linewidths in molecular crystals and Brot's theory for the preexponential factor in activated reorientational processes. Agreement with theory is good as far as the reorientational contribution to the linewidth is concerned.
Resonance Fluorescence from Large Molecules
Authors: Nitzan Abraham , Jortner Joshua
Published: February 2004

In this paper we apply the T matrix formalism of scattering theory to derive general expressions for the absorption cross sections, the cross sections for resonance fluorescence and the emission quantum yields from large molecules in the statistical limit. In the simple case of an isolated molecular resonance both the absorption line shape and the photon scattering cross section exhibit a Lorentzian distribution on the photon energy, the emission quantum yields are distributed among the ground state vibronic levels according to their radiative widths and, most important, the emission quantum yields are independent of the photon energy and of the spectral width of the exciting light. We were able to derive general expressions for the resonance scattering from a pair of overlapping resonances, including radiative corrections to infinite order. The absorption cross section does not vanish in the region of destructive interference but assumes a finite value which depends on the radiative widths. A sharp maximum in the partial and in the total emission quantum yields is exhibited in the destructive interference regions. This general scheme was applied to a pair of zero order discrete states, one of which is optically active, which interact with an optically inactive quasicontinuum. The energy dependent quantum yield depends on the total width of the radiatively impotent state and may exhibit a minimum. We have demonstrated that when interference effects are involved the decay characteristics of this system will differ for coherent and for narrow band excitation. The general formalism was utilized to derive approximate relations for the resonance fluorescence cross sections and for the quantum yield in the case of a Fano absorption line shape which are valid away from the interference region. Finally, we have applied the general theoretical scheme to the case of the direct photodissociation spectrum of molecules. We have demonstrated that a finite energy dependent emission quantum yield will be observed when a molecule is optically pumped into a dissociative continuum.
EPR Study of Copper(II) Ion Pairs in Y-Type Zeolites
Authors: Chao Chien-Chung , Lunsford Jack H.
Published: February 2004

The structures of identical and nonlinear Cu2+ pairs in Y-type zeolites have been studied by EPR spectroscopy. The identical pairs are formed by exchange-coupled Cu2+ ions which have the same symmetry axis and magnetic parameters. They are formed by coexchanging Cu2+ with Mg2+, Ca2+, or La3+ ions into a NaY zeolite. The spectrum is characterized by a zero field splitting of D=0.0476 cm−1. The nonlinear pairs which are formed upon more extensive Cu2+ exchange have symmetry axes that form an angle of 110° with respect to each other. These models compare favorably with the available x-ray diffraction data on CuY zeolites.
Thermal Creep in Gases
Authors: Annis B. K.
Published: February 2004

The development of a pressure gradient in a gas-filled capillary due to the application of a temperature gradient is known as thermal transpiration. In the near-continuum limit (small Knudsen number) the mechanism for the transpiration effect is a creeping motion of the gas in a thin layer adjacent to the surface. Measurements performed in the near-continuum limit are compared with a number of theoretical calculations for the thermally induced creep velocity. The experimental data for He, Ne, Ar, Kr, and N2 agree best with the recent theoretical work of Loyalka and indicate little or no dependence on gas type, in contrast with another recent theory which indicates a marked dependence on the thermal accommodation coefficients. In addition, the effect of the thermal creep coefficient upon the evaluation of rotational collision numbers from thermal transpiration measurements is discussed.
Density Fluctuations in Water and in Simple Fluids
Authors: Chay Teresa Ree , Frank Henry S.
Published: February 2004

The method of Fisher and Adamovich for obtaining the second order fluctuation of the particle number in a microscopic volume, and the correlation between such fluctuations in adjacent regions, has been modified to permit integration directly over x-ray scattering intensities instead of over the derived radial distribution function. With the new method and the more extensive x-ray data which are now available computations have been carried out for water at room temperature and at 200°C. In the former case the correlations are found to be essentially zero at all distances greater than two molecular diameters, in contrast to the results of Fisher and Adamovich, in which sizeable positive correlations appeared at distances up to and exceeding 30 Å. On comparing the density fluctuations in cold water with those calculated for Percus-Yevick liquids of appropriate hard-sphere diameters it is found that the two curves can be made to fit well except in the range 2.5 Å<D<8 Å, where no fit is possible (D = diameter of volume element). It is also found that for the Percus-Yevick liquid for which σ3ρ = 0.633 the fluctuations exceed the thermo-dynamic-limit value (〈 δ NG2/ N̄G 〉 = ρ k T κT) for all volumes less than (75 σ)3 where ρ is the number density, σ the hard-sphere diameter, and κT the isothermal compressibility. The method of integrating over x-ray scattering intensities has also been used with data for argon and krypton at several temperatures, with results in good agreement with expectation. The fluctuations in cold water are discussed from the point of view of their bearing on the problem of water structure. It is concluded that they could in principle be accounted for in terms either of an interstitial or of a ``continuum'' model.
Potential Function for the Ring Puckering Vibration of B2H6 and B2D6
Authors: Pringle W. C. , Meinzer A. L.
Published: February 2004

The v = 1 → 2 transition of the ring puckering mode of B2H6 has been observed at 389.5±0.5 cm−1 and identified on the basis of relative intensity measurements of the rotational fine structure of the overlapped v = 0 → 1 and v = 1 → 2 bands. The band center for the ring puckering fundamental has been determined to be 369.3 cm−1 utilizing an asymmetric rotor calculation. The potential function for the ring puckering vibration based on the v = 0 → 1 and v = 1 → 2 transitions is V(X) = 25.25(X4+11.78X2), where X is a reduced coordinate. The band centers for the v = 0 → 1 and v = 1 → 2 bands for B2D6 were assigned to the ``holes'' in the band envelope at 261.3 and 272.6 cm−1, respectively. The rotational fine structure in the bands of the deuterated molecule was not resolved. The potential for the deuterated molecule predicted from a reduced mass model is V(X) = 15.93(X4+14.71X2).
Microwave Spectrum of Compressed O2-Foreign Gas Mixtures in the 48–81 GHz Region
Authors: Mingelgrin U. , Gordon Roy G. , Frenkel L. , Sullivan T. E.
Published: February 2004

A Fabry-Perot interferometer with one spherical mirror was used to measure the absorption of pure O2 and O2–Ar and O2–N2 gas mixtures. The experimental pressure range was 3.7–51 atm and the frequency range 48–81 GHz. The measured spectrum was successfully reproduced by a calculated line shape using Gordon's pressure broadening theory and 3- and 4-body classical scattering calculations.
Molecular Theory of Diffusion Constants in Liquids
Authors: Ahn Myong-Ku , Jensen S. J. Knak , Kivelson Daniel
Published: February 2004

A molecular theory of diffusion constants in liquids, independent of model, is developed in the language of time-dependent correlation functions and statistical projection operators. Diffusion constants are related to friction constants which in turn depend upon force autocorrelation functions with a time dependence arising from a Liouville operator projected orthogonally to the momentum. The resulting expressions are related to those previously obtained and the ``projected'' friction constants are related to those connected with the dissipation of momentum, i.e., those considered by Kirkwood. The influence of molecular translations on spin relaxation in liquids is analyzed from the same molecular-statistical viewpoint, and the effect of diffusion on both spin-echo experiments and on intermolecular magnetic dipolar relaxation are discussed. The behavior of diffusion phenomena at critical points is also considered, and emphasis is placed on the distinction between particle (self-) and concentration (mutual) diffusion. The theory, it is hoped, represents a simple, coherent molecular-statistical treatment of most of the commonly encountered diffusion phenomena.
Magnetic Quenching of Iodine Fluorescence Excited by a 6328 Å He/Ne Laser
Authors: Chapman G. D. , Bunker P. R.
Published: February 2004

The extent of the magnetic field induced quenching of the fluorescence from the (v′ = 6, J′ = 32) and (v′ = 11, J′ = 128) levels of the B 3Π (Ou+) state of I2 has been measured. A He/Ne laser operating at 6328 Å has been used to excite the fluorescence. Combining these measurements with earlier results obtained for the higher vibrational levels allows us to determine the shape of an Ou− repulsive potential energy curve which crosses the B-state curve. Since the v′ = 6 level is observed to be quenched much more strongly than the levels of higher v′, this Ou− curve crosses the B-state curve at a low v′ value.
Absorption Spectra of Ne2 and HeNe Molecules in the Vacuum-uv Region
Authors: Tanaka Y. , Yoshino K.
Published: February 2004

The absorption spectra of the Ne2 and HeNe molecules have been investigated in the 585–760 Å region with a 6.65 m vacuum spectrograph using the helium continuum as a background source. Twelve discrete band systems are identified in the Ne2 spectrum. The ground state is stable, with a potential depth De=30.2 cm−1, supporting two stable vibrational levels, v=0 and 1, and with B0=0.17 cm−1, giving r0=3.1 Å. A number of existing intermolecular potential calculations agree with these results. State designations, dissociation products, stabilities, and observed transitions are discussed, and analogies are drawn with a previous study of the Ar2 molecule. It is suggested that the observed band systems represent members of Rydberg series converging to some of the Ne2+ states derived from atom combinations Ne (2p6 1S0)+ Ne+ (2p5 2P3/2,1/20). In addition, six band groups are attributed to the HeNe molecule. Here the ground state potential supports only one vibrational level, v=0.
The Relationship between Stochastic and Deterministic Models for Chemical Reactions
Authors: Kurtz Thomas G.
Published: February 2004

The Markov chain and ordinary differential equation models for chemical reaction systems are compared. It is shown that if the volume of the reaction system is taken into account in an appropriate way in the formulation of the Markov chain model, then the o.d.e. model is the infinite volume limit of the Markov chain model. A central limit theorem is also given for the deviation of the Markov chain model from the o.d.e. model.
Photoelectronic Properties of Acid-Doped Poly (N-Vinylcarbazole): o-Dinitrobenzene Charge Transfer Complex Films
Authors: Pfister G. , Abkowitz M. , Williams D. J.
Published: February 2004

The light intensity dependence of the buildup and the dark decay of the current through the poly(N-vinylcarbazole):o-dinitrobenzene charge transfer complex doped with trichloroacetic acid has been studied at room temperature. The weight ratio of these components was (24:1:1). We find that the buildup and decay of the current can be described by Is tanhβt and the second order rate law, Is/(αt+1), respectively, where Is is the saturation current and t the time. α and β depend upon the square root of the light intensity whereas Is increases in a manner roughly proportional to it. The acid greatly enhances Is, and reduces the second order decay constant α by orders of magnitude as compared to acid free systems. It is shown that the slow dark decay of the current is closely associated with the presence of a photogenerated radical species which gives rise to an ESR signal. Experiments on multilayer films indicate that the current measured following illumination arises predominantly from hole flow. The carrier generation rate is found to depend only weakly upon the applied field. A kinetic model is presented which successfully accounts for the experimental findings.
Diffusive and Collective Motion in Classical Fluids
Authors: Hassan M. , Lado F.
Published: February 2004

A ``perturbation'' technique previously developed is used to generate a systematic sequence of approximate relations between the coherent and incoherent scattering functions of classical fluids measured by inelastic neutron scattering. This sequence, recently obtained independently by Kim and Nelkin, has as its first member the Vineyard convolution approximation, while the second member is Kerr's generalization of this result. We show that the third member leads to a relationship between diffusion and viscosity coefficients of a form previously obtained by very different theories.
Excited-State Dipole Moments, Polarizabilities, and Local Fields in Molecular Crystals; Polarizabilities of Benzophenone in its Lowest Singlet and Triplet nπ Excited States
Authors: Barker J. W. , Noe L. J.
Published: February 2004

A refined calculation that accounts for molecular polarizabilities, crystal reaction fields and resulting field induced moments in polar molecular crystals allows excited-state dipole moments and polarizabilities to be derived from optically determined Stark splitting versus applied electric field results. The determination of these excited-state properties in a dipolar lattice is particularly dependent on the evaluation of the local field at a molecular site. For this case, the local field is evaluated by introduction of an ellipsoidal cavity approximation and by using the Ewald-Kornfeld lattice summation method for dipolar arrays. The dipolar benzophenone crystal was chosen to demonstrate this method. The dipole moments of the lowest 1nπ* and 3nπ* states of benzophenone have been determined and are 1.23 D and 1.72 D, respectively; these are the zero field values and refer to the benzophenone molecules in the absence of the crystal environment. The polarizabilities of benzophenone in its ground, 1nπ* and 3nπ* states are, in units of 10−23 cm3, 0.532, 0.898, and 0.56, respectively. The excited-state moments are nearly parallel to the ground state moment in the crystal environment and the benzophenone dipole does not reverse direction in going from the ground to the lowest singlet and triplet states.
Reorientation and Vibrational Relaxation as Line Broadening Factors in Vibrational Spectroscopy
Authors: Nafie Laurence A. , Peticolas Warner L.
Published: February 2004

The theory of Gordon which relates the shape of the depolarized components of certain Raman bands to a molecular reorientation correlation function is extended to a number of types of molecular vibrations not previously covered. In addition, the contribution to the line shape for vibrational relaxation and reorientation is obtained for the three types of vibrational spectroscopy, infrared, Raman, and hyper-Raman, and it is shown that the form of the vibrational relaxation contribution is the same for each spectroscopy. Conditions and assumptions are given under which the line broadening contributions of reorientation and vibrational relaxation can be separated and experimentally determined.
On Moment Conserving Decoupling Techniques for Electron Propagators
Authors: Babu S. V. , Ratner M. A.
Published: February 2004

Moment-conserving decoupling procedures for the one-electron Green's function (electron propagator) are investigated. By analysis of one particular system (Hubbard model) it becomes clear that the moment-conserving techniques so far proposed for interacting systems are all special cases of [1, 0], [2, 1], or [3, 2] Padé approximants differing only in which terms they omit, how they define self-consistency, and which operator algebra they utilize. The number of formally conserved moments appears not to be a particularly useful criterion of the accuracy of a decoupling procedure, since a two-moment scheme (Hartree-Fock) may be numerically closer to the exact answer than some three-moment procedures. Use of the complete Padé approximant, as suggested by Lukman and Goscinski for the particle-hole propagator, appears to be the most consistent method.
Dissociative Attachment in Carbon Dioxide
Authors: Chantry P. J.
Published: February 2004

The production of O− from CO2 by dissociative attachment has been studied with electrons having energies in the range 0–20 eV. In addition to the two already well established peaks in the 0–10 eV range, a third peak has been substantiated, at 13 eV. At room temperature the 13 eV peak cross section is approximately 6 × 10−21 cm2, rising to approximately 2 × 10−20 cm2 at 1000°K. Measured O− ion kinetic energy distributions corresponding to the three processes peaking at 4.4, 8.2, and 13.0 eV are presented and their relevance to previous work is discussed. In all three processes only the high energy tails of the ion energy distributions are found to be sensitive to electron energy, with a decreasingly small fraction of the available energy appearing as kinetic energy of separation of the fragments. In this respect the results are similar to previous measurements of O− production from N2O.
Anomalous Effects Owing to Electric Fields in the Nematic Phase of 4,4′-di-n-heptyloxyazoxybenzene
Authors: McLemore D. P. , Carr E. F.
Published: February 2004

The nematic phase of 4, 4′-di-n-heptyloxyazoxybenzene exhibits a negative low frequency dielectric anisotropy, therefore, one would expect an anomalous alignment in a dc electric field with the long molecular axes preferring a direction parallel to the electric field. If the sample becomes slightly decomposed this is what is observed. In the very pure sample the preferred direction is perpendicular to the field for both dc and ac fields and it does not appear to exhibit domains or dynamic scattering. When a pure sample is doped to increase its conductivity, a dc or 25 Hz electric field appears to be more effective than a field of higher frequency in producing an ordering with the long molecular axes preferring a direction perpendicular to the field. The existence of another anomalous behavior with the long molecular axes preferring a direction perpendicular to the field is suggested. Since the dc conductivity for this particular sample was greatest in a direction perpendicular to the long molecular axes, when ordered by a high magnetic field, a mechanism similar to that suggested earlier for anomalous alignment is proposed.
Electron Scattering by Molecules with and without Vibrational Excitation. V. Elastic Scattering and Nonresonant Vibrational Excitation of N2 at 30–83 eV
Authors: Truhlar Donald G.
Published: February 2004

Model polarized Born approximation calculations are compared to all the available intermediate energy differential cross section data for elastic scattering by N2 and for electron impact excitation of vibration in N2. The various models predict the angle dependence of the elastic scattering differential cross sections accurately for scattering angles less than 40°–70° at 30 and 83 eV. Vibrational excitation is found to be dominated by the polarization interaction at 33–50 eV.
General Description of the Mechanical Anisotropy in Drawn Polymers in Terms of Spherical Tensors
Authors: McBrierty Vincent J.
Published: February 2004

The paper presents a general theoretical description of the elastic behavior of oriented polycrystalline polymers in terms of the spherical components of the compliance tensor. The use of the spherical representation has a number of advantages. For example, the analysis of Roe and Kringbaum [J. Chem. Phys. 40, 2608 (1964)] which describes the crystallite orientation distributions in partially oriented systems in terms of experimental x-ray data can be incorporated directly into the theory. A second advantage relates to the simple way in which different crystal symmetries can be treated. The analysis may be readily extended to include two-phase systems.
Elasticity in Polymers. II. The Directional Properties of the Compliance Constants in Uniaxially and Biaxially Drawn Polymers
Authors: McBrierty Vincent J.
Published: February 2004

The theoretical analysis of elasticity in oriented polymers in terms of the spherical components of the compliance tensor presented earlier has been extended to include biaxial orientation and also the case of a segment cut at an arbitrary angle in a drawn polymer. The significance of the results is discussed.