bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of triplet state EPR spectra from dimers: g and D tensors not coaxial
Authors: Lund T. , Hatfield William E.
Published: February 2004

Expressions for the EPR transition fields and probabilities as a function of angle for pairs of S = 1/2 paramagnetic ions, coupled by exchange and anisotropic exchange/dipolar interactions, have been calculated for situations where the g and D tensors do not have the same axes. The nuclear hyperfine interaction has been included and the formulas derived have been used to produce simulated EPR spectra for comparison with experimental results.
Stochastic and analytical investigation of tritium hot-atom reactions
Authors: Wotzak Gregory P.
Published: February 2004

Detailed models for reactive and scattering phenomena have been successfully employed in the investigation of hot-atom reactions by stochastic and analytical methods. These models include an energy dependence for differential scattering cross sections and energy loss parameters. Analytical and stochastic yield results for the T–H2, T–D2, and T–CH2 hot tritium systems exhibit satisfactory agreement with experimental values.
Infrared absorption spectra of doped and undoped lead azide
Authors: Varma S. P. , Williams Ferd
Published: February 2004

The far and near infrared absorption of thin films of pure and doped lead azide are reported. The effects of thallium and bismuth doping were investigated. The far ir lattice modes are essentially unaffected by doping; the near ir intra-azide ion modes are markedly affected by Tl doping. These effects are related to changes in the electronic properties with doping and are attributed to azide vacancies which compensate for, and pair with, substitutional Tl acceptors.
Precision neutron diffraction structure determination of protein and nucleic acid components. XII. A study of hydrogen bonding in the purine-pyrimidine base pair 9-methyladenine · 1-methylthymine
Authors: Frey Michel N. , Koetzle Thomas F. , Lehmann Mogens S. , Hamilton Walter C.
Published: February 2004

A neutron diffraction study of the 1 : 1 complex between 9-methyladenine and 1-methylthymine, C6H7N5· C6H8N2O2, has been carried out. The structure is monoclinic, space group P21/m, with two base pairs per unit cell; cell parameters a =8.304(2), b =6.552(2), c =12.837(3)Å, and β=106.83(5)°. The structure has been refined by full-matrix least squares techniques starting from the x-ray results of Hoogsteen [K. Hoogsteen, Acta Crystallogr. 12, 822 (1959); Acta Crystallogr. 16, 907 (1963)]. All hydrogen atoms have been located with a precision better than 0.01 Å, with the exception of methyl group hydrogens. The thymine molecules appear to be slightly disordered by means of a 180° rotation about N3 ··· C6, which has the effect of interchanging N1 and C5 while leaving the positions of all other atoms approximately unchanged. Between 10% and 13% of the thymine molecules in the structure are disordered in this way. Average refined neutron scattering lengths for nitrogen and carbon are b̄N=0.910(3) and b̄C=0.657(3) × 10−12 cm; oxygen and hydrogen scattering lengths were fixed at bH=−0.372 and b0=0.575 × 10−12 cm. The purine and pyrimidine bases are joined with N3 of thymine hydrogen bonded to N7 of adenine. Because of the disorder present in the structure, 87%–90% of the adenine · thymine pairs have the Hoogsteen configuration and the remainder have the reversed Hoogsteen configuration. These geometries differ from that of a Watson-Crick adenine · thymine pair in which N3 of thymine donates a proton to N1 of adenine. The hydrogen bonding scheme in the crystalline complex consists of one N–H ··· N and two N–H ··· 0 bonds and is quite normal; there is no evidence for proton transfer in any of the hydrogen bonds which are formed.
Evaluation of matrix elements of spin-dependent operators for N -electron systems. II Two-body operators
Authors: Cooper Ian L. , Musher Jeremy I.
Published: February 2004

The procedure for evaluation of matrix elements of spin-dependent operators developed in Paper I is extended to the case of two-body operators, such as those arising in spin-spin dipolar coupling and spin-other-orbit coupling effects, and necessitating the consideration of both symmetric and antisymmetric two-body operators. Reduced spin matrix elements and coupling coefficients are evaluated for N = 2,3,4 and the corresponding reduced spatial matrix elements are evaluated.
Absolute electron impact ionization cross sections of Li in the energy range from 100 to 2000 eV
Authors: Jalin R. , Hagemann R. , Botter R.
Published: February 2004

The absolute electron impact ionization cross sections for lithium in the energy range between 100 and 2000 eV were measured by the nonmodulated crossed beam technique. The neutral lithium beam is produced by a Knudsen cell and crossed at right angles with the electron beam. The ions formed are collected on a plate and their intensity determined with a dc amplifier. The neutral beam is condensed on a cold trap cooled with liquid nitrogen, this temperature being much lower than that required to obtain total condensation. The amount of metal deposited is measured by the isotopic dilution technique and by atomic absorption, and the density of the atoms in the neutral beam is calculated. The total absolute ionization cross sections can then be determined. All possible errors have been carefully analyzed and their magnitudes estimated. The absolute ionization cross section for Li at an energy of 500 eV is QLi=0.358 × 10−16 cm2. This value is half of that obtained by McFarland and Kinney. The partial ionization cross sections for the singly and doubly charged ions, is determined with a mass spectrometer attached to this apparatus. For the singly charged ions, the variation of the cross section with the energy of the ionizing electrons is in agreement with the optically allowed transition law Q = A log BE/E. From the variation of Q with E, the squared matrix element of the transition moment |Mi|2 is determined. New calculations of the ionization cross section of Li were performed in the framework of the Born-Bethe approximation as modified by Gaudin and Botter to take into account collisions with large momentum variation of the incident electron. Hartree-Fock type wavefunctions for the ground state atom (tabulated by Clementi) were used. The calculated values are in good agreement with our experimental results and with the former theoretical results calculated by various methods. This work also indicates that the experimental results of McFarland and Kinney for Li are too large.
Deuterium, methyl, and solvent effects on the phosphorescence lifetime of benzene
Authors: Haaland D. M. , Nieman G. C.
Published: February 2004

The phosphorescence lifetimes of benzene and a number of its methyl and deutero derivatives have been investigated in a wide variety of solvents at temperatures below 65 °K. Deuterium effects indicate a dominant role for the aryl hydrogens in the radiationless process. Methylation increases the triplet decay rate by approximately 25% per methyl group in a nearly linear fashion. Intensity data indicate that this is not due to ``allowed'' character introduced by any reduction in molecular symmetry as is theoretically predicted to be the case. Perfluorocarbons act as inert solvents for benzene derivatives whereas typical hydrocarbons produce 50% or greater changes in the lifetimes even at very low temperatures. The data suggest a common mechanistic pathway (or perturbed state) for both the effects of solvent and methyl groups
Far infrared and Raman spectra of gaseous carbon suboxide and the potential function for the low frequency bending mode
Authors: Carreira L. A. , Carter R. O. , Durig J. R. , Lord R. C. , Milionis C. C.
Published: February 2004

The far infrared spectrum of gaseous carbon suboxide under high resolution (0.3 cm−1) has been investigated (20–80 cm−1). Two series of absorption maxima were observed resulting from the anharmonicty of the low frequency bending vibration, ν7. In one series the maxima, found to be relatively sharp, were assigned to P-branch bandheads, and in the other the more diffuse peaks were assigned to Q branches for excited state transitions of ν7. The Raman spectra of gaseous and solid carbon suboxide were obtained under moderately high resolution (1.0 cm−1). Several lines were found to have striking fine structure. More than ten Q branches were observed in the Raman spectrum of the gas in the region of 2ν7. The observed Q branches in the infrared and Raman spectra were assigned with the help of a potential function of the form V(cm−1)=0.728 ± 0.010 q4−6.40 ± 0.36 q2, where q is one of the reduced polar coordinates q and φ. This function leads to a barrier to the linear configuration of 14 ± 2 cm−1 with the lowest energy level at 19.7 cm−1. Thus, the carbon suboxide molecule is linear in the ground vibrational state. The 0→ 1 transition was found to be at 22.2 cm−1. The potential function transformed to the dimensioned form by using the reduced mass gives an angle of 10.2° at the shallow potential minimum. Many of the other Raman bands showed extensive fine structure due to ``hot band'' transitions associated with ν7 and it was only possible to locate the ground state transition reliably by observing the corresponding frequency in the solid. The gas-phase entropy of carbon suboxide, calculated with the help of the new assignment for ν7 agrees reasonably well with the experimental third-law entropy by there is indication that a small amount of additional entropy may be required from a solid-state transition near 120 °K with Δ H ≃ 50 cal/mole.
Recombination of iodine atoms by flash photolysis over a wide temperature range. IV Kinetic isotope effect
Authors: Chang Heh Won , Burns George
Published: February 2004

Recombination of I atoms was studied, by flash photolysis, in the presence of an excess of H2, HD, and D2, between 300 and about 580 °K. The recombination rate constants were summarized by the following equations: logkrH2=9.686 − 2.054 log(T/300),logkrHD=9.617 − 1.737 log(T/300),logkrD2=9.547 − 1.772 log(T/300).The variation in rate constants with isotopic substitution of the third body suggests that the recombination proceeds via the energy transfer mechanism. It is also consistent with earlier trajectory studies on the third body mass effect in recombination reactions.
Thiouridine spectroscopy. Temperature dependences and decay kinetics in the deactivation of excited states
Authors: Shalitin Nurit , Feitelson Jehuda
Published: February 2004

Aqueous 4-thiouridine, when excited at room temperature in its last absorption band at 335 nm, shows a weak emission at 550 nm. The intensity of this emission increases with decreasing temperature and below the freezing point (of ethylene glycol-water 1 : 1) an additional peak, centered at ∼ 475 nm, appears. Further lowering of the temperature causes a very drastic increase in emission with the 475 nm peak becoming more prominent. Decay measurements show that the 550 nm emission has a lifetime of ∼ 250 nsec at room temperature which increases to about 380 μsec at 77 °K. The 475 nm emission has at all temperatures a lifetime 2–3 times longer than that at 550 nm. It is suggested that both emissions are due to phosphorescence the 475 nm peak representing the triplet of thiouridine while the 550 nm emission stems from either the triplet whose solvent sheath has undergone a rearrangement or from the triplet of a species formed by a monomolecular process from excited thiouridine.
Fluorescence-absorption energy gap in the stable anthracene dimer spectra
Authors: Zgierski Marek Z.
Published: February 2004

A possible explanation of the existence of an energy gap between absorption and fluorescence spectra of the anthracene stable dimer is given. It is consistent with the existing experimental data.
Electric dipole transitions in the carbon isoelectronic sequence calculated with symmetric states obtained from superposition of configurations
Authors: Shrivastava Surendra K. , Westhaus Paul A.
Published: February 2004

Sinanoğlu's nonclosed shell many electron theory (NCMET) is used to determine and analyze symmetry-adapted correlated wavefunctions for states of the F(IV) ion belonging to the 1s 22s 22p2 and 1s 22s 2p3 configurations. We argue that these wavefunctions contain those specific correlation effects important in calculating accurate oscillator strengths for allowed transitions between states of these two configurations. The types of correlations included, the reasons for their importance, the methods of approximating them, and the necessity and techniques for introducing symmetry into these calculations are discussed. Comparison of results using Hartree-Fock as opposed to correlated wavefunctions clearly shows the necessity of including correlation effects in calculating the electric-dipole oscillator strengths in F(IV). The conclusion follows for both the dipole-velocity and the dipole-length formulations. These new results for the six electric-dipole transitions in F(IV) are combined with previously published results for the same transitions in the isoelectronic species C(I), N(II), O(III) in order to extrapolate the Z dependence of these transitions and thereby make reasonable predictions of the oscillator strengths for the heavier ions. We conclude with a formalism, based on plausible but as yet unproven speculations, which shows in a transparent way what correlation effects are of significance in the general problem of computing oscillator strengths.
Coulomb holes and expectation values. II. Configuration interaction wavefunctions
Authors: Seddon G. J. , Banyard K. E.
Published: February 2004

The Coulomb hole and several two-electron expectation values are calculated using a series of configuration interaction (CI) wavefunctions of differing accuracy for heliumlike ions. The behavior of these properties as the basis set is increased is examined. A comparison with the values obtained using explicitly correlated (EC) wavefunctions, see Paper I, allows us to comment upon the relative performance of the CI and EC treatments with regard to two-electron properties.
NMR static and rotating frame relaxation studies in solid Cl3CC(CH3)2Cl
Authors: Albert Shmuel , Ripmeester John A.
Published: February 2004

Cl3CC(CH3)2Cl (1,1,1,2-tetrachloro-2-methylpropane or, briefly, TCMP) was investigated by means of temperature dependent static and rotating frame spin-lattice relaxation times, T1 and T1ρ, respectively, between 102 and 423°K. Rotation of the methyl group about its triad axis with an activation energy of 2.5 ± 0.2 kcal/mole and a relatively slow motion such as rotation of the molecule about its C–C axis or, over-all molecular tumbling, with activation energy of ∼ 9 kcal/mole takes place in the low temperature phase. A discontinuity in T1 and T1ρ is observed at the phase transition temperature of 210.8°K. Over-all molecular tumbling as well as self-diffusion with activation energies of 2.8 ± 0.2 and 18 ± 2 kcal/mole, respectively, are determined from the respective T1 and T1ρ data in the high temperature cubic phase. The presence of chlorine nuclei in Cl3CC(CH3)2Cl seems to affect the T1ρ proton relaxation just above the transition. Proton-chlorine dipolar relaxation contributions are observable because of short chlorine spin-spin and spin-lattice relaxation times. An estimate of the chlorine T1 values and correlation times in the high temperature phase is given. The influence of impurities on our results is dealt with also, and an attempt to separate the impurities' contribution to T1 was carried out. The TCMP data are discussed and compared with those of hexamethylethane and hexamethyldisilane.
Structure and bond shift kinetics of cyclooctatetraene studied by NMR in nematic solvents
Authors: Luz Zeev , Meiboom Saul
Published: February 2004

A nuclear magnetic resonance study of cyclooctatetraene (COT) in a number of different nematic solvents covering a wide temperature range (− 35 to 170°C) is reported. Proton spectra for both normal COT and partially deuterated COT under conditions of deuterium decoupling were studied. From an analysis of the spectra in terms of direct dipolar interactions, it is concluded that the COT molecule has a symmetrical tub (D2d) conformation, and coordinates for the protons, within a scale factor, are derived. The spectrum undergoes profound changes with temperature, which are interpreted in terms of a bond shift process. The activation energy is found to be ΔE = 10.9 kcal mole−1 and the rate at 0 °C: 1/τ = 2.5 × 102 sec−1. In an appendix, symmetry considerations are used to identify those spectral peaks which are not affected by the bond shift process. In another appendix, the exchange broadening in a system consisting of two equivalent protons with direct dipolar interaction is treated quantitatively.
Absorption and circular dichroism spectra of ethylenic chromophores-trans-cyclooctene, α- and β-pinene
Authors: Mason M. G. , Schnepp O.
Published: February 2004

The absorption and circular dichroism spectra of trans-cyclooctene, α-pinene and β-pinene were measured in the gas phase in the vacuum ultraviolet. The absorptions were recorded to 110 nm and the CD spectra to 150 nm. Consistent assignments were made for these three ethylene chromophore molecules using the measured anisotropy factors (g = Δε/ε) to characterize transitions. Also, available theoretical work was used as a guide. In particular, magnetic dipole allowed transitions in the parent molecule, ethylene, have been characterized and assigned.
Depolarized Rayleigh-wing scattering in water and aqueous electrolytic solutions
Authors: Gray Michael A. , Loehr Thomas M. , Pincus Philip A.
Published: February 2004

Depolarized Rayleigh-wing scattering has been studied in the region 20–110 cm−1 in pure H2O and aqueous KI and MgSO4 solutions, for temperatures between 0 and 90°C and concentrations in the 0–3M range (0–1M for MgSO4). The integrated intensity increases faster with temperature in pure H2O than would be expected from the collision-induced polarizability theory of Bucaro and Litovitz. We ascribe this excess to temperature-induced structural changes (the breaking of hydrogen bonds). The integrated intensity in KI solutions increases with concentration and in MgSO4 solutions decreases slightly with concentration. This is also discussed in terms of structural enhancement or breakdown.
Magnetic properties of concentrated metal-ammonia solution. II. Nuclear magnetic resonance
Authors: Lelieur J. P. , Rigny P.
Published: February 2004

The 133Cs and 14N Knight shifts have been measured in the cesium-ammonia solutions versus metal concentration and temperature. Using our results of magnetic susceptibility, the electron density at the nucleus has been calculated.
Collision of a diatomic molecule with a solid surface
Authors: Wolken George
Published: February 2004

The collision of a diatomic molecule with a solid surface (neglecting phonon excitation and molecular fragmentation) is formulated as a set of coupled linear, second-order differential equations. A model potential for the collision of H2 with the (001) face of LiF is proposed. Scattering intensities are computed including 29 diffraction processes for the ν = 0, J = 0 internal state and 5 diffraction processes for the ν = 0, J = 2 internal state (34 channels in all). By comparing with a calculation omitting rotationally excited states (that is, the 29 channels for ν = 0, J = 0), it is observed that most of the 0 → 2 rotational excitation occurs at the expense of the 0 → 0 specular reflection. The 29 channel basis set includes all diffractions through third order and some fourth-order diffractions as well. Based on previous calculations of He+LiF scattering, it is concluded that this basis is sufficiently large to provide an accurate description of all rotationally elastic diffraction processes, but must be augmented to take proper account of rotation excitation. Calculations are also carried out for D2+LiF collisions, and several of the trends observed experimentally for these two isotopes are also observed here.
Conformational comparison of aromatic molecules and their anions. II. Tetraphenylethylene by NMR
Authors: Geraldes Carlos F. G. C. , Gil Victor M. S. , Eargle Dolan H.
Published: February 2004

The conformation of the phenyl rings of tetraphenylethylene is shown by calculation of ring-A-ring-B-ring-C NMR proton interactions to be nearly perpendicular to the plane of the C=C ethylenic bonds. There is, however, some contribution from other geometries. Calculations are made using John-Bovey tables and the results are compared with the experimental finding of a single sharp peak near 2.95 τ. The diamagnetic dianion is also studied by NMR and through the Fraenkel relation (electron density to chemical shift); the conformation is found to lie somewhere between ``twisted'' and totally planar.
NMR conformational analysis of chain molecules using the local interaction model. I. 2,4,-disubstituted pentanes
Authors: Moritani Tohei , Fujiwara Yuzuru
Published: February 2004

The practical usefulness of the local interaction model in the NMR conformational analysis of chain molecules is demonstrated for 2,4-disubstituted pentanes, CH3CHRCH2CHRCH3, which are model compounds of vinyl polymers. In the present treatment based on this model, the relative energies of all the possible conformers are expressed as sums of the five kinds of intramolecular interaction energy parameters, which are determined to reproduce the observed temperature dependence of the vicinal spin-spin coupling constants between the methine and methylene protons. The method was applied to the analyses of pentane-2,4-diol, 2,4-dichloropentane and 2,4-diphenylpentane on the basis of the NMR data in the literature. The energy parameters, the conformer energies, and the populations were determined. The observed solvent effect in pentane-2,4-diol can be interpreted directly as the variation in the intramolecular interaction energy between the two hydroxyl groups, −1.0 to 0.8 kcal mol−1. In the analysis of this solvent effect, it is shown that the trans and gauche coupling constants, Jt and Jg, which are essential in NMR conformational analysis, can be obtained experimentally as Jt=11.5 ± 1.0 and Jg=2.1 ± 1.0 Hz. The energy parameters obtained in the present method may be used in calculations of the configurational statistics for polymer chains.
Isotopic thermal diffusion factors for argon and krypton
Authors: Taylor W. L. , Weissman Stanley
Published: February 2004

We have measured the temperature dependence of the thermal diffusion factor αT of argon and krypton isotopes from approximately 200 to 900 °K thereby extending experimental determinations from 250 to 300 ° higher than heretofore. Furthermore, we observed that αT for krypton at low temperatures was nearly twice as large as previously reported. The range of the available data for these species is now sufficiently extensive to provide a rather sensitive test of the intermolecular potential and consequently we fitted these data to empirical equations within the experimental uncertainties. Numerous intermolecular potentials were used to evaluate αT and comparison with the empirical equations resulted in the ``elimination'' of many of those proposed.
Repeated flashes in radical-containing solid nitrogen crystals at low temperatures
Authors: Seshadri K. , Berlad A. L.
Published: February 2004

Alternative glows and flashes have been observed repeatedly in N-atom-containing N2 crystals at low temperatures. The analysis presented attempts to explain these phenoma on the basis of thermal stability theory.
Diffusivity of helium in fused silica
Authors: Masaryk Joseph S. , Fulrath Richard M.
Published: February 2004

A statistical mechanical model of diffusion is applied to helium diffusion in fused silica. The number of gas atoms moving solubility sites is related to the number of atoms in those sites. The number of atoms in the excited state, between two adjacent solubility sites, relative to the number occupying solubility sites can be described by their partition functions. The process of diffusion is treated as a random walk process which results in a diffusion equation in terms of the temperature, fundamental constants, and material parameters. The model is compared to data reported in the literature on He diffusion in fused silica in order to determine the unknown parameters. The model also shows that absolute rate theory is not able to account for the nonlinearity observed in logD vs 1/T for this system.
Raman study of crystalline ethylenes and the structure determination through model calculations of the lattice spectra
Authors: Elliott G. R. , Leroi G. E.
Published: February 2004

Complete Raman spectra of the intra- and intermolecular regions of pure ethylene, ethylene-d4, and their dilute mixed crystals have been obtained. The observations agree with group theoretical predictions based on a C2h5 space group. Model calculations of the Raman lattice spectra for the two possible molecular orientations consistent with this space group were undertaken to evaluate which of these two structures more satisfactorily reproduces the experimental results. Various sets of carbon-carbon, carbon-hydrogen, and hydrogen-hydrogen interaction parameters were utilized in the calculation of crystal energies, librational frequencies, and relative Raman scattering intensities. It is noteworthy that the predicted relative intensities are only very slightly dependent on the nature of the intermolecular potential chosen. This investigation provides strong evidence that the crystalline ethylene structure with P 121/n 1 (C2h5) space symmetry, the so-called ``b structure,'' is correct.
Photoelectron spectrum of pentaborane
Authors: Jones R. W. , Koski W. S.
Published: February 2004

The photoelectron spectrum of pentaborane (B5H9) was studied using a hemispherical electron spectrometer with a He I (21.22 eV) photon source. Good agreement was obtained with existing ab initio SCF molecular orbital calculations permitting an assignment of all the observed bands.
3ν3 band of 12CH4 at 1.1 μ
Authors: Pugh Larry A. , Owen Tobias , Rao K. Narahari
Published: February 2004

The 3ν3 absorption band of 12CH4 has been measured using a high resolution vacuum infrared spectrometer. A technique is described which allows an extension of the use of the absorption standards available at 4.7 μ to the photomultiplier region of the spectrum. The new measurements indicate a shift of about 0.18 cm−1 from the most recently published measurements of this band.
Condensation in CO2 free jet expansions. II. Growth of small clusters
Authors: Dorfeld William G. , Hudson John B.
Published: February 2004

An investigation into the kinetics of molecular clustering processes which occur in the highly supersaturated vapor of a CO2 free jet is described. Terminal cluster concentrations in expansions from room temperature and reservoir pressures in the range 400–900 torr were determined for trimeric CO2 for nozzle diameters of 7.6× 10−3 cm, 1.5× 10−2 cm, and 2.3× 10−2 cm and in the case of CO2 tetramer and pentamer for 1.5× 10−2 cm nozzle diameter. The measurements were made mass spectrometrically in a supersonic molecular beam apparatus. A kinetic analysis was developed from an extension of the critical nucleus concept of nucleation theory and rate equations based on intermolecular collisions. The observed cluster populations could be accounted for by the theory by adopting sticking probabilities of approximately 3× 10−2 for monomer impinging on dimer, trimer, and tetramer.
Excitation and electron capture in collisions of Li+ with Li
Authors: Chang Chi-Ming , Rapp Donald
Published: February 2004

Coupled-state calculations have been performed for the collision processes Li++Li(2s)→Li++Li(nl),→Li(nl)+Li+, in which 1, 3, 4, or 6 of the states 2s, 2p0, 2p1, 3s, 3p0, 3p1 were included on each nuclear center as traveling-wave atomic orbitals. The rectilinear path impact parameter method was used, and cross sections for populating the various channels were determined over the energy range 0.4–400 keV. The atomic wavefunctions were obtained by solving the time-independent Schrödinger equation for a single valence electron in the field of the nucleus plus an exponential inner charge cloud. This pseudopotential was carried over into the dynamical calculation as the potential holding the single electron to the effective Li+ center. Therefore, our atomic wavefunctions are essentially exact solutions of the asymptotic Hamiltonian at large nuclear separations. The same numerical methods used previously for (H+, H) and (He++, H) collisions were used here. The inclusion of the 2p states had a strong effect on the calculated cross section for total electron capture, but higher states did not. When the 2p states are included, we find that the large oscillations in the curve of total capture cross sections vs energy are almost completely damped out. This is in disagreement with experiment, where oscillations are observed. However, the oscillations derive mostly from small impact parameters, where our method is least reliable. The over-all magnitude and energy dependence are in good agreement with experiment. Our calculations also predict a very large cross section [∼ 60 Å2 at 100 keV] for direct excitation of Li by Li+ at high collision energies. No data are available for comparison at present.
Stress fluctuations and Brownian motion in incompressible visco-elastic fluids
Authors: Chow T. S. , Hermans J. J.
Published: February 2004

It is assumed that the visco-elastic behavior is linear and can be described by integral relations between local stresses and strain rates. When stress fluctuations are added, the equations describe non-Markovian random processes. To find the correlation functions for the fluctuating stresses, use is made of an analogy with the generalized Langevin equation. An application to the Brownian motion of a spherical particle in a Maxwell fluid shows that the random force on the particle has a memory which extends over times of the order of the relaxation time of the Maxwell model. The asymptotic behavior at large times is exactly the same as in a Newtonian liquid.
Avoided intersection of potential energy surfaces: The (H+ + H2, H + H2+) system
Authors: Bauschlicher Charles W. , O'Neil Stephen V. , Preston Richard K. , Schaefer Henry F. , Bender Charles F.
Published: February 2004

Nonempirical electronic structure calculations have been carried out on the two lowest 1A1 states of H 3+. When one proton is infinitely separated from the other two, these 1A1 potential surfaces cross each other. The nature of this avoided intersection is examined by means of potential curves, contour diagrams, and perspective plots. Surface hopping is discussed within a Landau-Zener-Stuckelberg (LZS) framework and the LZS assumptions concerning the surfaces are shown to be reasonable near the avoided intersection. Ab initio LZS parameters are compared with those obtained from the semiempirical diatomics-in-molecules surfaces of Preston and Tully. The agreement is good, better than might have been anticipated.
Electron mobilities in liquid olefins: Structure effects
Authors: Dodelet J. -P. , Shinsaka K. , Freeman G. R.
Published: February 2004

Electron mobilities (square centimeters per volt second) at 293°K and their activation energies (kilocalories per mole) in several liquid olefins are, respectively, cis-butene-2, 2.2 and 3.7; trans-butene-2, 0.029 and 5.4; 2, 3-dimethylbutene-2, 5.8 and 2.0; cyclohexene, 1.0 and 3.6. Mobilities in butene-1 and isobutene at 293°K are 0.064 and 1.44 cm2/v· sec, respectively. The mobilities in all of the liquids are independent of electric field strength in the range 1–14 kV/cm. Molecular dipole moments up to 0.5 D have negligible effects on the magnitudes of the mobilities. A plot of the thermal electron mobilities against epithermal electron ranges in the olefins falls on the same curve as that for electrons in saturated hydrocarbons. This implies that molecular negative ion states are not involved in the scattering processes that control the thermal electron mobilities, nor in those that limit the epithermal electron ranges. Attempts to measure electron mobilities in conjugated dienes failed, presumably due to capture of the thermalized electrons by the dienes to form molecular negative ions.
Electron scavenging in the radiolysis of supercritical propane and propylene: Effect of density
Authors: Yamaguchi Yoh-ichi , Nishikawa Masaru
Published: February 2004

The nitrogen yield observed upon electron scavenging by N2O in the γ radiolysis of supercritical propane and propylene at 120°C and at various densities can be quantitatively described by the square root model, G(N2)=G(N2)fi+G(N2)gi(A[N2O])1/2, in the lower concentration range. G(N2)fi decreases smoothly with increase in density in both hydrocarbons. At a given density, G(N2)fi in propane is always larger than that in propylene, e.g., at d = 0.35 g/ml, G(N2)fi = 1.13 and 0.36, respectively. The analysis of G(N2)fi on the assumption G(N2)fi = G(free ion) indicates that (i) the density-normalized range of electrons is constant over the density range of 0.24–0.42 g/ml in propane and 0.12–0.44 g/ml in propylene, and (ii) the efficiency of electron thermalization in propylene is about twice as good as in propane. The results are related to the thermalization mechanisms of electrons in hydrocarbons. An interpretation of the change in values of A with density is given in terms of the change in the distribution function of ion-electron separation distances and the diffusion coefficient of electrons.
Far-infrared and Raman spectra of the crystal KNCS
Authors: Dao Nguyen Quy , Wilkinson G. R.
Published: February 2004

Polarized far-infrared and Raman spectra of KNCS single crystals were recorded between 10 and 400 cm−1 at room and liquid nitrogen temperatures. Most of the lattice vibrations of the crystal were observed and an assignment of the bands, based on group theory, was attempted.
Gas-phase EPR linewidths and intermolecular potentials. II. Experimental results for the F–He–F2 system
Authors: Cook Thomas J. , Miller Terry A.
Published: February 2004

Detailed experimental results are reported for F-atom EPR linewidths in both the 2P3/2 and 2P1/2 states in the presence of the perturbing species He, F2, and F atoms. Line broadening cross sections for He and F2 show similar features. The 2P3/2 EPR transitions involving a Zeeman level with projection quantum number of absolute value 3/2 have larger cross sections than ones involving only 1/2 levels. However, both the ratio and magnitude differ between He and F2. For both F2 and He, 2P1/2 cross sections are in the range 50–100 times smaller. On the other hand F-atom perturber cross sections all are about equal in the 2P3/2 state and only slightly larger than the cross section in the 2P1/2 state. The He–F cross sections are subjected to detailed comparison with the theory of the preceding paper, with the conclusion that the model second-order dipole-dipole potential does an excellent qualitative job of predicting the observed cross sections, though quantitative discrepancies are noted. Possible explanations for these discrepancies are advanced. Likewise qualitative explanations are set forth to explain the F2 and F-atom cross sections. It is speculated that additional theoretical and experimental work may demonstrate that EPR line broadening cross sections are effective probes of both long- and short-range potentials.
Vibrational force fields and mean amplitudes. Specific imposition of potential parameters on nonbonded distances
Authors: Cyvin S. J.
Published: February 2004

Expressions for Jacobian elements of mean amplitudes with respect to the force constants are given. An alternative method (not using these Jacobians) for studying variations in mean amplitudes is proposed and referred to as specific imposition of potential parameters on nonbonded distances. Using tetrachloroethylene as example it is shown that potential parameters may be introduced so as to affect in a fairly selective way the different nonbonded mean amplitudes.
Hot electron injection into dense argon, nitrogen, and hydrogen
Authors: Smejtek P. , Silver M. , Dy K. S. , Onn D. G.
Published: February 2004

Hot electrons have been injected into very dense argon, nitrogen, and hydrogen gases and liquids. The current-voltage characteristics are experimentally determined for densities (N) of argon, nitrogen, and hydrogen ranging from about 1020 to 1022 cm−3 and applied fields (E) ranging from about 10 to 104 V cm−1. The argon data show a square root E/N dependence of the current. The nitrogen and hydrogen data show a complicated dependence of the current on E/N due to the rapid thermalization in the region of the image potential of the injected electrons through inelastic collision processes not present in argon. A hydrodynamic-two-fluid model is developed to analyze the nitrogen and hydrogen data. From the analysis of our data, we obtain the density dependence of the momentum exchange scattering cross section and the energy relaxation time for the injected hot electrons.
Determination of vibrational anisotropy for M 1 or E 1 gamma resonance transitions
Authors: Bowen L. H. , Heimbach C. L. , Dunlap B. D.
Published: February 2004

For an M 1 or E 1 nuclear gamma resonance transition with quadrupole splitting, the intensity of a given component in the powder absorption spectrum can be written as Ij=Bj2+(Cj2+Dj2)F/2+CjDjG, where Bj, Cj, Dj are functions only of η, the asymmetry parameter, and F and G are integrals, depending on the vibrational anisotropy at the nuclear site. The integrals F and G have been evaluated numerically over a range of vibrational anisotropy. The use of the resulting tables to obtain quantitative information on vibrational anisotropy from a Mössbauer absorption spectrum is considered for both the 3/2–1/2 and 7/2–5/2 transitions.
Chemical shifts and fine structure of the cobalt K -absorption edge
Authors: Dey A. K. , Agarwal B. K.
Published: February 2004

We have measured the chemical shifts of the x-ray K -absorption edge of cobalt in the compounds CoF2, CoO, Co2O3, Co(CH3COO)2· 4H2O, CoSO4· 7H2O, CoC2O4· 2H2O, CoCl2· 6H2O, CoBr2, CoS, CoCO3, and Co3(PO4)2 8H2O. The chemical shifts have been found to vary linearly with the product i m2, where i is the ionicity and m the valence. We have also measured the fine structure observed on the high energy side of the edge in the metal and in the first six compounds. The positions of maxima and minima have been compared with theoretical predictions. The first absorption maximum in the metal has been interpreted as a plasma absorption. The bond lengths have been estimated from the fine structure.
Reactions of F atoms and aromatic and heterocyclic molecules: Energy distribution in the reaction complex
Authors: Shobatake Kosuke , Lee Yuan T. , Rice Stuart A.
Published: February 2004

This paper reports studies of the reactions of fluorine atoms and several aromatic and heterocyclic molecules. As in earlier work we have used the method of crossed molecular beams to create long-lived reaction intermediates and to deduce from the angular distribution of the product molecules information about the energy distribution in the reaction complex. The data reported in this paper complement those collected earlier. They show that when a light group is emitted from a reaction complex randomization of the internal energy of the complex is not more rapid than reaction. When a heavy atom is emitted from the reaction complex the limited data available indicate that almost all vibrational degrees of freedom of the complex participate in the reaction. This latter observation provides an interesting connection between vibration-translation energy exchange and the mechanism of chemical reaction.
General SCF operator satisfying correct variational condition
Authors: Hirao K. , Nakatsuji H.
Published: February 2004

We have shown that the correct variational equations for the general SCF orbitals are [Fi−Σj |ψj〉 〈 ψj |Gji] |ψi〉=0, where Gji=λjiFj+(1−λji)Fi; λji≠ 0 and how these may be combined into simple eigenvalue problems. In the course of discussions, we re-examined whether the coupling operators suggested previously are based on the correct variational conditions.
Bond susceptibilities and ionicities in complex crystal structures
Authors: Levine B. F.
Published: February 2004

The concept of crystal ionicity has proved to be a useful unifying concept for understanding chemical trends in diverse problems in chemistry and solid state physics. In particular, the dielectric description of ionicity developed by Phillips and Van Vechten (PV) has been successfully employed in a wide variety of areas. However, the applicability of this PV dielectric analysis has been limited to only the simple AN B8-N compounds which contain only one type of bond. We have therefore generalized this approach to include such diverse structures as CsCl, PbS, GeTe, MgF2, SnO2, CaF2, Li2O, Al2O3, Ga2Se3, Cd3As2, Si3N4, HgS, Se, Te, LiGaO2, ZnGeP2, NaClO3, and AIPO4. Our approach is to decompose the bulk crystal properties into parameters which can be associated with individual bonds such as the polarizability, the ionicity, and the average number of electrons per bond. In order to determine the functional dependence of these bond properties on the fundamental variables such as the bond length, we use the basic dielectric description of PV for each type of bond, attempting to make the smallest number of additional assumptions. For example, we use the same relationship between the homopolar potential (Eh) and bond length (d) that PV do (i.e, Eh ∝ d−2.48). In addition to obtaining the individual bond properties we also present some striking empirical relationships between them. For example, we find that the Thomas-Fermi prescreening factor, which plays a fundamental role in the PV theory, is simply related to the average coordination number. Hopefully, this systemization of the dielectric properties will prove useful for understanding a variety of problems in physics and chemistry.
Cholesteric twist in a model system
Authors: Wulf A.
Published: February 2004

A highly idealized model is studied to gain some understanding of the structure of cholesteric liquid crystals. Asymmetric molecules, characterized by two perpendicular axes, the ``long'' and the ``short'' molecular axes, are considered; the centers of mass of the molecules lie on a rectangular lattice. An effective interaction that takes account of left-right asymmetry is assumed between nearest neighbors; also, an external field, correcting for a deficiency of the simple interaction, is chosen so that in the absence of a cholesteric twist the system is purely ``nematic.'' The results of the model (treated in mean field approximation) are (i) there is no cholesteric-nematic phase transition, an experimentally well-established fact; (ii) S∼ γ θ where S is a measure of asymmetry in the orientations of the ``short'' molecular axes, θ is the rotation (in radians) of the cholesteric helix from one layer of molecules to the next, and γ =O(10−2); (iii) the pitch of the helix is insensitive to temperature changes. An argument showing the reason for result (iii) in the present model is given.
Vibrational relaxation of HBr (V = 1) state in methane, water, helium, and hydrogen gaseous mixtures
Authors: Hopkins Barbara M. , Chen Hao- Lin
Published: February 2004

The rates of vibrational relaxation of HBr(V = 1) in CH4, CD4, H2O, 3He, and HD mixtures have been measured by the laser-excited vibrational fluorescence technique. The relaxation rates of HBr–3He and HBr–HD mixtures at 296 °K show cross sections of V→ T, R energy transfer as σHBr–3He=3.2± 0.2× 10−21 cm2, σHBr–HD=1.3± 0.1× 10−20 cm2. The rates of HBr in methane and water gaseous mixtures show σHBr–CH4=6.4± 0.5× 10−-18 cm2; σHBr–CD4=4.6± 0.6× 10−17 cm2; σHBr–H2O=1.5± 0.2× 10−18 cm2. The predominance of vibration-to-vibration energy transfer, and not vibration-to-translation or rotation in HBr-methane collisions, is suggested by the larger cross section of the deuterated compound collision. Comparison of the relative efficiencies of vibrational deactivation of HBr by collisions with hydrogen isotopes and helium isotopes reveals that intramolecular vibration-to-rotational energy transfer seems to be an important process involved.
Vibrational energy transfer in NO
Authors: Stephenson John C.
Published: February 2004

Laser-excited vibrational fluorescence measurements have been used to obtain rate constants at room temperature for vibrational relaxation of the V = 1 state of NO in collisions with He, Ar, H2, CO, NO, N2, and CO2. Pulses from a CO2 laser, frequency doubled in a tellurium crystal, provided the excitation source. The rate for the V-V exchange NO(1)+NO(1)→NO(0)+NO(2) was obtained.
Vibrational relaxation of HF in the temperature range 600–2400 °K
Authors: Blair L. S. , Breshears W. D. , Schott G. L.
Published: February 2004

Hydrogen fluoride vibrational deactivation by HF, Ar, and F atoms has been studied in the temperature range 600–2400 °K using the shock tube-laser-induced fluorescence method. Mixtures of HF–Ar and F2–HF–Ar were heated by reflected shock waves; following establishment of thermal equilibrium, HF was vibrationally excited by pulsed radiation from an HF pin laser. Relaxation times were obtained from the time-resolved decay of the laser-induced vibrational fluorescence. Measurements of relaxation by F atoms were limited to temperatures > 1500 °K, where F2 dissociates completely behind the reflected shock wave. The HF self-relaxation time, p τHF–HF, exhibits a broad maximum with temperature, peaking at ≈ 0.10 μsec · atm near 1400 °K. Fluorine atoms were found to be 2–5 times more efficient than HF molecules for HF vibrational deactivation over the range 1500–2400 °K. Argon is a very inefficient collision partner; values of p τHF–Ar over the range 800–2400 °K are reported. This is the first study in which the maximum of pτHF–HF with temperature has been fully described; the present results both confirm and unify those of previous high (> 1400 °K) and low (< 1000 °K) temperature studies.
Oxidation and reduction of barbituric acid dihydrate by ionizing radiation at 4.2 °K
Authors: Box Harold C. , Budzinski Edwin E.
Published: February 2004

The primary oxidation and reduction products produced by ionizing radiation in single crystals of barbituric acid irradiated at 4.2 °K have been identified from their ESR and ENDOR spectra. The primary reduction product is an anion formed by electron addition. The primary oxidation product is a neutral radical resulting from the loss of hydrogen from an /\ NH group.
Vibrational relaxation of oxygen in an unsteady expansion wave
Authors: Cleaver J. W. , Crow I. G.
Published: February 2004

The time-resolved population density of the fifth vibrational level of the ground electronic state of oxygen has been obtained in an unsteady expansion wave by measuring the absorption of light at a wavelength of 2283 Å. By assuming that a Boltzmann distribution exists among the vibrational degrees of freedom the vibrational temperature has been inferred and compared with those calculated for an ideally centered expansion wave based on relaxation times obtained from shock wave studies. The results suggest that the vibrational de-excitation of oxygen proceeded at a rate agreeing with that found behind normal shock waves.
Phase transitions of the anion radical salts of [(C6H5)3PCH3]1−x+  [(C6H5)3AsCH3]x+  (TCNQ)2− (0 ≤ x ≤ 1) as studied by the electrical conductivity measurements
Authors: Iida Yôichi
Published: February 2004

The phase transitions of the anion radical salts containing the mixed cations represented by [(C6H5)3PCH3]1−x;+ [(C6H5)3AsCH3]x+ (TCNQ)2−, (0 ≤ x ≤ 1), were investigated by measuring the electrical conductivity and its variation with the temperature. All the salts except for that with x = 1.00 were found to show a sharp discontinuity in the temperature dependence of the electrical conductivity. At the temperature where the anomaly occurred, the electrical conductivity increased abruptly in the higher temperature range. The transition temperature, Tc, and the magnitude of the discontinuity of the electrical conductivity at Tc, were determined as a function of the composition parameter. These experimental results were useful for investigating the mechanism of the phase transitions of these anion radical salts.