bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of the 13 to 100 eV electron impact excitation cross section for the X 1Σ+g→a 1Πg transition in N2
Authors: Finn T. G. , Doering J. P.
Published: February 2003

The electron impact excitation cross section for the X 1Σ+g→ a 1Πg transition in N2 has been measured from 13 to 100 eV by an inelastic electron scattering technique. The cross section varies from a maximum value of 3.6×10−17 cm2 between 15 and 18 eV to 5.2×10−18 cm2 at 100 eV. The incident electron impact energy was calibrated by an optical technique. The agreement between the measured intensities of the v′=0–6 vibrational bands of the transition and the intensities predicted by the Franck–Condon factors was found to be within experimental error. The cross section results of the present experiment are in excellent agreement with those of previous investigators obtained with a variety of techniques up to 30 eV. At the higher energies, the present values agree within experimental error with the results of Holland at 100 eV and those of Aarts between 60 and 100 eV.
A neutron diffraction study of structure and thermal motion in several monovalent metal azides
Authors: Choi C. S. , Prince E.
Published: February 2003

The crystal structures of the monovalent metal azides β-NaN3, KN3, TlN3, and RbN3 were reinvestigated by means of neutron diffraction. All except β-NaN3 were studied at low temperature as well as at room temperature. These compounds form ionic crystals with spherical metal cations and linear azide anions. Because the azide ions in each case occupy centrosymmetric sites, their thermal motions can be treated, except for the small amplitude N–N stretching and the internal bending motions, as the motions of rigid bodies without screw correlations. All structures were, therefore, refined using rigid-body constraints, leading to final R values, both weighted and unweighted, ranging from 0.020 to 0.045. KN3, TlN3, and RbN3 all crystallize in the tetragonal space group I4/mcm, while β-NaN3 crystallizes in the space group R3̄m. The uncorrected N–N distances range from 1.159 to 1.183 Å, but when corrections for thermal motions are applied all distances except for β-NaN3 lie in the range 1.181–1.187 Å, with a weighted mean of 1.186 Å. In the β-NaN3 the corrected N–N distance is 1.180(1) Å. This apparently unusually short distance may be attributable to disorder, with the axis of the N3 group making a small angle with the c axis. In the tetragonal crystals, where the azide ions occupy sites with mmm symmetry, the anisotropy of libration is consistent with the steric hindrance in the structure. The librational amplitudes determined from the diffraction studies were compared with those determined by Raman scattering, and the agreement is satisfactory.
Optical properties of N-vinylcarbazole crystals
Authors: Sano Reiji , Matsuda Shunsuke
Published: February 2003

Optical measurements were made on a N-vinylcarbazole single crystal. An Urbach tail was observed in the absorption spectrum and the determination of refractive index in the transparent region was made from the reflectivity measurement. A three-line Gaussian structure was found from the emission spectrum, which seems to be associated with quantum states of the N–C stretching vibrational mode. This interpretation is basically supported by both charge distribution mapping of NVC calculated by HMO and oscillator strength calculated from the overlap integral of vibrational modes. A proposed molecular arrangement of NVC crystal is presented through an analysis of the luminescence spectrum.
A method of calculating magnetic properties for a modified free-electron model
Authors: Walnut Thomas H.
Published: February 2003

A new method of calculating the magnetic susceptibilities for simple model systems is demonstrated. The model, consisting of a particle on a circular track having an arbitrary potential function and subject to a uniform magnetic field, is used as a basis for calculation. The second-order perturbation equation is solved exactly to yield the magnetic susceptibility and the ring current. As an illustration, the method is applied to a simple system with a single square potential well.
The electron propagator and superoperator resolvent
Authors: Simons Jack
Published: February 2003

In this paper we demonstrate how the definition of the one-particle Green’s function, or electron propagator, can be extended to include averages over nonexact reference states without affecting the exact nature of the poles of this function. We also make connections between this Green’s function and an average value of the superoperator resolvent.
Mobilities of various mass-identified positive and negative ions in helium and argon
Authors: Dotan I. , Lindinger W. , Albritton D. L.
Published: February 2003

The mobilities of the ions CH3O2+, NO+⋅H2O, C2H2−, Cl−, O2−, O3−, NO2−, CO3−, SO3−, SO2F−, SF5−, and SF6− in helium and 0+, O2+, O3−, and CO3− in argon have been measured at room temperature with a flow–drift tube. The measurements were obtained over the range 5⩽E/N⩽120 Td for helium and 5⩽E/N ⩽270 Td for argon (1 Td=10−17 V⋅cm2). The E/N dependence of these data fits the general pattern observed previously for a variety of ions in helium and argon, with the exception that the O+ mobilities in argon exhibit a rare minimum. Several of the polyatomic ions, notably CH3O2+, NO+⋅H2O, and O3−, were observed to break up at elevated E/N.
A semicontinuum model for solvated electrons in alcohol–alkane solutions and glasses
Authors: Fueki K. , Narayana P. A. , Kevan Larry
Published: February 2003

A semicontinuum model is developed for solvated electrons in alcohol–alkane solutions and glasses. Two geometrical models are considered. In model I a discrete number (4 or 6) of alcohol molecules forms the first solvation shell for the electron and the alkane forms the continuum. In model II the first solvation shell contains discrete alcohol molecules, the second solvation shell contains a continuum of alcohol molecules, and the alkane forms the continuum beyond the second solvation shell. The calculated optical absorption maxima agree reasonably with the limited experimental data available. The most interesting prediction is that in solution, but not in the glass, a bound–free transition dominates the optical spectrum for model I while a bound–bound transition dominates the optical spectrum for model II. The second alcohol solvation shell appears to play a large role in localizing the excited state wavefunction.
Fast excited state formation and decay in the pulse radiolysis of gaseous argon–iodine systems
Authors: Sauer Myran C. , Mulac William A. , Cooper Ronald , Grieser Franz
Published: February 2003

Processes responsible for the formation and decay of excited states of I2 emitting in the 320–345 nm region in the pulse radiolysis of gaseous argon–iodine systems are investigated using an electron pulse of half-width less than 40 psec and an analyzing system with a time resolution slightly better than 1 nsec. The observed emission behavior is interpreted on the basis of a competition between light emission from vibrationally excited levels of an excited electronic state and vibrational deactivation of these levels by collision with argon. Vibrational deactivation rate constants in the range of 2×10−11 cm3 sec−1 are observed. The lifetime τ (=1/k) for the emission at the longer wavelength end of the emission region is 6.7 nsec; the pressure independence of this value indicates that it is the radiative lifetime. Possible processes responsible for the formation of the excited I2 are discussed. We conclude that an excited state of I2 is produced on a subnanosecond time scale via a direct interaction of energetic electrons with iodine molecules in their ground electronic state. Collisions of these excited I2 molecules with argon are necessary to reach the excited states of I2 which emit in the 320–345 nm region. The processes investigated in this study occur too rapidly to be connected with ion-recombination processes or collisional transfer of energy to I2 from excited argon atoms or Ar2 excited dimers.
Equilibrium properties of high-density fluids
Authors: Ree Francis H.
Published: February 2003

A recipe is given extending the theory of Weeks, Chandler, and Anderson to the calculation of high-density equilibrium properties of a fluid near its freezing point. Thermodynamic properties for a Lennard-Jones system and a system whose particles obey an inverse 12th-power repulsive potential are computed using this recipe. The calculation shows that the present formulation predicts physically reasonable values for the hard-sphere diameter over the entire dense fluid region and gives thermodynamic data which agree satisfactorily with the available Monte Carlo data.
Reactive scattering calculations on a splinefitted a
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o surface: The He+H+2(v=0,1,2) →HeH++H reaction
Authors: Sathyamurthy N. , Rangarajan R. , Raff L. M.
Published: February 2003

The reaction probabilities for He+H+2(v=0,1,2,3) →HeH++H have been computed as a function of total energy using quasiclassical trajectory procedures. The potential-energy surface was obtained by employing 2D cubic spline interpolation of previously reported ab initio SCF results. The calculated reaction probabilities are found to be significantly increased whenever the total energy is selectively partitioned into H+2 vibration. These results are in accord with previously reported experimental findings but are in contrast to previous quantum mechanical calculations on a DIM analytic fit to the ab initio surface points. It is shown that this discrepancy is a result of small differences between the splinefitted and DIM surfaces and not to either the collinear nature of the computations or the differences between quantum and quasiclassical procedures.
One-dimensional random walks of linear clusters
Authors: Reed David A. , Ehrlich Gert
Published: February 2003

A stochastic formalism is developed for the one-dimensional surface diffusion of atom clusters, with component atoms located in adjacent channels, by representing the diffusion as a random walk of the center of mass (c.m.). Relations between the mean square displacement of the center of mass and the rate constants characterizing c.m. motion are derived for dimers and trimers, starting from the Kolmogorov equation. For dimers in the limit of long diffusion intervals, c.m. rate constants and individual atomic jump rates can be deduced knowing the mean square displacement and the frequency of occurrence of different dimer configurations. This analysis is feasible for trimers only under special conditions; even then, separation into the individual atomic rate processes is not, in general, possible.
Calculation of the energies, geometries, and vibration frequencies of alkali halide dimers
Authors: Trugman Stuart , Gordon Roy G.
Published: February 2003

The potential energy surfaces for a number of alkali halide dimer molecules were calculated from an electron gas model. These results were used to predict binding energies, equilibrium geometries, and vibration frequencies for M2X2 molecules, with M=Li, Na, K, and Rb, and X=F, Cl, and Br. The accuracy of these a priori results (no adjustable parameters) is comparable to that of previous semiempirical models with several adjustable parameters.
Distribution of vibrational populations of CO electronic states produced in CO2 photodissociation
Authors: Band Yehuda B. , Freed Karl F.
Published: February 2003

The vibrational distributions of CO(d 3Δ), CO(a′ 3Σ+), and CO(a 3Π) states produced from CO2 photodissociation at various incident photon energies are calculated using a quantum mechanical theory of poyatomic molecule dissociation and are compared with experiment. It is assumed that CO(d 3Δ) and CO(a′ 3Σ+) are produced via direct photodissociation from the CO2 ground state to a repulsive electronic state. Agreement between theoretical and experimental distribution is obtained. The CO(a 3Π) vibrational distribution calculated using the direct photodissociation assumption could not be made to agree with experiment. Evidence that the CO(a 3Π) dissociation in the photon energy range considered proceeds via a predissociative state of CO2 is given. Comparisons are made with other models of dissociation.
Comparison of the energy dependences and branching ratios of the reactions of Ar+ and N2+ with SO2
Authors: Dotan I. , Albritton D. L. , Fehsenfeld F. C.
Published: February 2003

The reactions of Ar+ and N2+ with SO2, which have similar energetics due to the nearly equal ionization potentials of Ar and N2, have been studied in a flow–drift tube from 0.04 to 3 eV relative kinetic energy. The energy dependences and product branching ratios of the two reactions are found to be almost identical. With increasing relative kinetic energy, the rate constants show a rather steep initial decline, followed by an equally steep increase. At the lower energies, both reactions yield only the charge-transfer product SO2+, and at the higher energies, both reactions yield predominately the dissociative charge-transfer product SO+.
Polarization of laser-excited fluorescent lines from 85Rb2 and 87Rb2 molecules
Authors: Tam A. C. , Happer W.
Published: February 2003

Polarization studies are made of the various fluorescent lines from the excited 85Rb2(C) state or the 87Rb2(C) state populated by the 4765 or the 4880 Å Ar ion laser lines. We conclude that the previous hypothesis that all these fluorescent lines are Q branches is incorrect.
Species enrichment due to Mach-number focusing in a molecular-beam mass-spectrometer sampling system
Authors: Sharma P. K. , Knuth E. L. , Young W. S.
Published: February 2003

The analysis and measurements reported here emphasize the need to take into account species enrichments due to Mach-number focusing when sampling gas mixtures using a molecular-beam mass-spectrometer sampling system. Depending upon source conditions and system geometry, the enrichment of a given species may vary from unity to a value which is of the order of magnitude of the ratio of the given-species mass to the major-species mass. Procedures for handling effects of Mach-number focusing on species enrichments are presented. These procedures are more general than previous procedures in that they accommodate, as needed, (a) flow divergence upstream from the skimmer and (b) transition to free-molecule flow (for one or more of the several species) upstream from the skimmer. The measurements support a procedure for handling effects of pressure diffusion near the sampling source which was suggested earlier. An empirical curve for handling modest effects of skimmer interference is included.
The effect of an impurity on the critical point of a binary liquid system as a surface phenomenon
Authors: Rice O. K.
Published: February 2003

An impurity which is considerably more soluble in one of the components of a binary liquid system than the other raises the temperature of an upper critical (consolute) point. Since the interfacial tension vanishes at the critical temperature, this effect can be described as a surface effect: The impurity raises the interfacial tension and is thus negatively adsorbed at the interface. The amount of impurity in the interface is assumed to be calculable by summing the dissolved amounts at all points of the interface, and can be seen to depend on the second derivative of the solubility as a function of concentration. These phenomena were treated thermodynamically with the help of the Gibbs adsorption equation in earlier papers, before the nature of the singularities at a critical point were well understood. These ideas are now brought up to date by incorporating the more recent developments about critical points, and scaling laws are found for the solubility and the adsorption at the interface. Renormalization of exponents has been considered. Information about the interface thickness can be obtained by applying the theory to experimental data. Application is made to the cyclohexane–aniline system. The thickness has been found to be given as a function of the temperature T and the critical temperature Tc by the formula 2.1 [(Tc−T)/ Tc]−0.62 Å. This is 4 or 5 times less than the thickness of the interface in the cyclohexane–methanol system determined by optical measurements. Part of the difference might be removed by use of better measurements of the solubility of water in cyclohexane–aniline.
Rate constants for specific product channels from metastable Ar(3P2,0) reactions and spectrometer calibration in the vacuum ultraviolet
Authors: Gundel L. A. , Setser D. W. , Clyne M. A. A. , Coxon J. A. , Nip W.
Published: February 2003

The product channels from interaction of metastable argon atoms (3P2,0) with a series of chlorine containing molecules (Cl2, NOCl, ClO, HCl, CCl4, PCl3, and SnCl4), as well as some other molecules (Br2, N2O, NO2, H2S, and COS), have been investigated by emission spectroscopy. The rate constants for product channels were established by comparison of the emission intensities from the excited state products to the emission intensities from excited state Kr atoms, which have known rate constants for excitation by metastable argon atoms. Comparison of the individual product rate constants with previously determined total quenching rate constants of Ar(3P2,0) gave the branching ratios for emitting product channels. Although our measurements include the wavelength range from 120–800 nm, emphasis is placed upon the vacuum ultraviolet region and upon the ArCl*, ArBr*, and ArO* product channels. The highest ArCl* yield, ∼50%, was obtained for Cl2 and ClO. Quenching mechanisms for the halogen containing reagents are discussed with an emphasis on analogy to reactions of alkali metal atoms with halogen containing molecules. The nature of the bound-free ArCl* emission also is discussed. Since the spectral response of the monochromator in the vacuum ultraviolet is of critical importance for assignment of rate constant values, the molecular branching ratio method employed for the calibration of the optical system is carefully described.
The transferability of molecular fragment canonical orbitals
Authors: Lichtenberger Dennis L. , Fenske Richard F.
Published: February 2003

The ability of certain canonical orbitals of isolated molecular fragments to transfer largely unchanged to the molecular environment is examined. The separation of fragment canonical orbitals from the total molecular electronic problem is compared with the more familiar separations of atomic core orbitals and fragment localized orbitals. The specific example of the carbonyl functional group in formaldehyde is examined in detail. These studies lead to a new concept of valence electron only calculations in which the molecular valence electrons are assumed to move in an effective field provided by frozen molecular fragment canonical cores. In addition, for the case of assumed fragment canonical orbital transfer, perturbation theory analysis is found to be an efficient method of assessing the quality of the approximate wavefunction, thus eliminating much of the uncertainty as new systems are studied. The methods developed in the course of these studies offer certain practical advantages for the construction of approximate wavefunctions for large molecules. The details of application of these concepts to existing molecular orbtial methods are also presented.
Evolution of the Clausius–Mossotti function of noble gases and nitrogen, at moderate and high density, near room temperature
Authors: Vidal D. , Lallemand M.
Published: February 2003

The static dielectric constant of noble gases (excepting xenon) and nitrogen has been measured near room temperature in a large range of pressures (up to 12 kbar for He, Ne, Ar, and N2). The Clausius–Mossotti function (C.M.) has been deduced, when density data are known, with a precision of 10−3 in the moderate pressure range and 3×10−3 above 1.0 kbar. Values of the first, second, and in some cases third dielectric virial coefficient have been determined. We have found, in agreement with previous results obtained at low pressure, that the light gases and the heaviest ones show a different behavior of their C.M. function in terms of the density. For He and Ne, the second dielectric virial coefficient is negative; for Ar, Kr, and N2 it is positive, but the C.M. function goes through a maximum and then decreases. The effect of density on the C.M. function of Ar and N2 is approximately a drop of 3.5% at 12 kbar. These results have been compared with the predictions of molecular theories of dielectric properties of nonpolar fluids in the case of a hard spheres model. We have found that the Kirkwood–Yvon theory based upon the DID approximation does not reproduce completely the results of experiments. By contrast, we have been able to show that the semiquantal theory of Heller and Gelbart which takes into account the influence of the electronic distortion is more in agreement for a wide range of densities. This is the case for Ne and Ar when the excess of the pair polarizability tensor of a diatom is computed within the electron–gas approximation, and for He only if it is deduced from the Hartree–Fock scheme.
Trigonal ligand field and zero field splitting diagrams for the d5 configuration
Authors: Hempel Judith C.
Published: February 2003

Complete trigonal ligand field, Coulombic repulsion, and spin–orbit interaction matrices have been computed for the d5 configuration. The ligand field potential for D3d, D3, C3v and a special case of C3 symmetry is expressed as a normalized spherical harmonic (NSH) potential. Ligand field energy level diagrams for the quartet states, as well as zero field (spin–orbit) splitting diagrams for the 6A1(S) ground state of a six-coordinate system, are presented. The advantages and limitations of the NSH formulation are discussed. The sign of the zero field parameter D is correlated with the sense of the trigonal distortion for six coordinate d5 systems of D3d symmetry, and two manganese(II) systems characterized by D3d symmetry are specifically considered.
Powder magnetic resonance spectra in the presence of planar rotational jumps
Authors: Baram A. , Luz Z. , Alexander S.
Published: February 2003

The effect on magnetic resonance line shapes of powder due to inplane jumps and planar rotational diffusion is discussed. Two approaches are used to calculate the line shapes: (i) A group theoretical method developed previously is applied to describe the effect of discrete jumps about a principal axis of a nonaxial Hamiltonian. (ii) A formalism is developed which considers a continuous range of jump angles, distributed about a preferred angle. This formalism reduces to the discrete jump case or the planar diffusion case in the proper limits. Numerical results for some specific cases in which the jump process involves rotations by π/2 and π/3 are presented. It is shown that the discrete jumps lead in the intermediate rate region to the appearance of conspicuous features in the spectra typical of the jump process. When the jump angles are diffused these peaks broaden out and disappear completely in the rotational diffusion limit. Experimental results on the radical AsO4−4 in powder samples of KH2AsO4 which exhibit these predicted features are presented.
Enthalpy of formation of methyl and methylene radicals of photoionization studies of methane and ketene
Authors: McCulloh K. E. , Dibeler Vernon H.
Published: February 2003

Photoion yield curves for CH3+ and CH2+ from methane have been measured near threshold at 295 and 115 °K, and the curves for CH2+ from ketene have been obtained at 295 and 130 °K. Although the detection efficiences for positive and negative ions were nearly equal, a search for the ion-pair process yielding CH3++H− gave negative results. The methane data are successfully fitted on the assumption that the full rotational energy is available for formation of CH3+, but that only two rotational degrees of freedom contribute to the available energy for the process yielding CH2+. Neglecting excess energy at threshold, the values ΔHf°0(CH3) =149.4±0.5 kJ/mole (35.70±0.12 kcal/mole and ΔHf°0(CH2) =392.5±2.1 kJ/mole(93.8±0.5 kcal/mole) from methane. Correction of the threshold for CH2+ from ketene for rotational energy results in the concordant value ΔHf°0(CH2) =390.8±1.7 kJ/mole (93.4±0.4 kcal/mole) on the assumption that excess energy can be neglected at threshold. The mean of the two determinations is selected as the preferred value ΔHf°0(CH2) =391.6±1.7 kJ/mole (93.6±0.4 kcal/mole).
31P NMR chemical shielding tensors of α-Ca2P2O7
Authors: Kohler Susan J. , Ellett J. David , Klein Melvin P.
Published: February 2003

The 31P NMR chemical shielding tensors were measured in a crystal of α-Ca2P2O7; the principal values, relative to 85% H3PO4, were found to be −42, 32, 65 ppm for P(1) and −48, 44, 67 ppm for P(2) with an average relative standard deviation of 7%. The most probable orientations of the tensors in the molecule were determined, and in both cases the most downfield component of the shielding tensor is along the P–O(P) bond direction; this orientation is correlated with the electronic distribution around the phosphorous nuclei. The high resolution spectra used in determing the shielding tensors were taken using the multiple pulse WAHUHA technique which removes homonuclear dipolar interactions in solids [J. S. Waugh, L. M. Huber, and U. Haeberlin, Phys. Rev. Lett. 20, 180 (1968)]. The linewidths were reduced from 1 KHz to 100 Hz by this technique. Details of the spectrometer design and the tuneup procedure for the WAHUHA experiment are also given.
Normal vibrational modes resulting from the lifting of degeneracies in anisotropic crystals: The internal optic modes of K2SO4
Authors: Wu Guo-Jen , Frech Roger
Published: February 2003

The vibrational normal coordinates of internal optic modes originating in molecular normal modes whose degeneracies have been lifted through site group effects are discussed. The normal mode frequencies are calculated in a molecular dipole formalism. The coupling between different Cartesian components of a phonon mode is described with particular attention to the symmetry based sufficiency condition for that coupling to occur.
Theoretical investigations on the light scattering of spheres. XVII. Angular and spectral location of intensity maxima and minima
Authors: Heller W. , Nakagaki M.
Published: February 2003

Comprehensive data were collected on the angular location of the various types of maxima and minima of Mie scatterers within the m range 1.050–1.333. Detailed comparison with the extrema calculated for the simple Rayleigh–Gans–Debye case of (m−1) →0 led to a relatively simple correction equation which allows one to calculate with fairly good accuracy the angular location of Mie extrema quickly and easily and within a wide range of α and m values the limits of which are defined. Application of the results is discussed for fast particle size determinations in monodisperse systems of isotropic homogeneous spheres.
Graph theory and molecular orbitals. XV. The Hückel rule
Authors: Gutman I. , Trinajstić N.
Published: February 2003

The topological analysis of the Hückel (4m+2) rule is carried out and shows its dependence on the structural features of conjugated systems. The proof of the validity of the Hückel rule for a given molecule is also presented.
Electrical properties of KF-doped hexagonal ice
Authors: Inchackal Joseph J. , Weber A. H.
Published: February 2003

KF doped ice single crystals were grown between platinum electrodes from water of about 5 MΩ cm resistivity and their electrical ac conductance Gac and capacitance C measured over a temperature range of −10 to −90 °C and a frequency range of 50 to 20×103 Hz using a Wayne–Kerr B 641 ac bridge. The activation energy Eσ for ac conduction was determined from the plot lnG vs 1000/T (concentration range of KF, 10−6 to 10−3 N). The dielectric relaxation time τ was obtained from the Debye dispersion relation and the activation energy Eτ of dielectric relaxation found from the plot lnτ vs 1000/T. For the same KF ice, Eσ was 5.9 kcal/mole and Eτ was 4.06 kcal/mole. In KF ice Eσ was always found to be greater than Eτ. Direct current conductivity Gdc obtained from plots of εm″ω vs f2 also showed a temperature dependence and hence an activation energy Eσ (determined from the plot lnσdc vs 1000/T) implying that it is unnecessary to invoke the mechanism of quantum mechanical tunneling for dc conduction. The static dielectric constant εs for KF ice was found to decrease with decreasing temperature, the values ranging from 15 to 50×102 for the temperature range −90 to −13 °C. The experiments indicate that the predominant mechanism for ac conduction is that of propagation of Bjerrum defects and that the dopant (F− ions, KOH, or HF) is incorporated into the ice lattice substitutionally.
Temperature and pressure dependence of the NMR transition frequencies of 27Al in LiAl(SiO3)2, spodumene
Authors: Harley S. F. , Williams C. D. , Tipsword R. F.
Published: February 2003

Equations have been developed, correct to second order in the normal-mode amplitudes for lattice vibration, which describe the constant-volume temperature dependence of the NMR transition frequencies for a spin 5/2 nucleus located at a site of arbitrary asymmetry parameter. These equations indicate that an analysis of the temperature dependence of the transition frequencies will not yield rigorously the temperature dependence of the quadrupole coupling constant or asymmetry parameter unless all the transition frequencies are measured. The temperature and pressure dependences of two of the NMR transition frequencies of 27Al in LiAl(SiO3)2 have been measured over a temperature range of 4.2–300 K at atmospheric pressure, and a pressure range of 0–7 kbar at 195, 273, and 298 K. From these measurements, as well as from measurements of the volume expansion coefficient and the compressibility, the temperature dependence of the transition frequencies at constant volume was calculated. An analysis of these results indicates that, with the assumption of three vibrational modes of characteristic temperatures of approximately 150, 675, and 2600 K, there is reasonable agreement between experiment and a simplified form of the theory.
Effective Hamiltonian methods for the semiclassical treatment of molecular collisions
Authors: Augustin Stuart , Rabitz Herschel
Published: February 2003

Effective Hamiltonian theory has previously been applied in a quantum mechanical framework, where the computational savings resulted from the reduced number of coupled equations. The present paper shows how effective Hamiltonian theory can be combined with classical S-matrix theory. In this case the computational difficulty is reduced by lowering the number of degrees of freedom that must be semiclassically ’’quantized’’ via root-searching techniques. It is shown, for example, that a full classical S-matrix calculation for collisions of two rigid diatoms would require root searches in a four-dimensional space while an effective potential calculation would need only two-dimensional root searches. This can represent a substantial decrease in computational effort. A modified effective potential and the centrifugal decoupling method are formulated for application to classical S-matrix theory. Also included is a description of the rotational coordinates and momenta needed for the semiclassical treatment of an arbitrary, nonreactive bimolecular collision.
Brillouin–Rayleigh scattering studies of polypropylene glycol. III
Authors: Wang C. H. , Huang Y. Y.
Published: February 2003

A comprehensive Brillouin scattering experimental study of polypropylene glycol (PPG), average molecular weight of 1025, 2025, and 4000, has been carried out. The hypersonic velocity, attenuation coefficient, the modulus of elasticity, and the Landau–Placzek ratio have been obtained as a function of temperature for each polymer. The PPG 425 data reported earlier are also included for comparison. For all the quantities measured, only the Laudau–Placzek ratio (below ambient temperature) is molecular weight dependent; the hypersonic velocity, attenuation coefficient, and elastic modulus are found to be independent of molecular weight in the present 425–4000 weight range. The differences in the Landau–Placzek ratio between the different polymers are attributed to different degree of frozen-in density fluctuations due to the difference in the medium’s viscosity; the mechanism of structural relaxation is not sufficient to explain the increase of the Landau–Placzek ratio of each polymer below ambient temperature. All polymers display the velocity dispersion and sound attenuation maximum. However, contrary to that observed in the ultrasonic frequency region, the maximum of the hypersonic attenuation coefficient appears to lie close to the beginning of the velocity dispersion curve and not its middle. The present results on the sound velocity and attenuation can be satisfactorily fit to a theory developed by Isakovich and Chaban, who have modeled the highly viscous liquid as a microinhomogeneous medium.
Temperature dependent activation energy for electron transfer between biological molecules
Authors: Jortner Joshua
Published: February 2003

This paper considers electron transfer between biological molecules in terms of a nonadiabatic multiphonon nonradiative decay process in a dense medium. This theoretical approach is analogous to an extended quantum mechanical theory of outer sphere electron transfer processes, incorporating the effects of both low-frequency medium phonon modes and the high-frequency molecular modes. An explicit, compact and useful expression for the electron transfer probability is derived, which is valid throughout the entire temperature range, exhibiting a continuous transition from temperature independent tunneling between nuclear potential surfaces at low temperatures to an activated rate expression at high temperatures. This result drastically differs at low temperatures from the common, semiclassical, Gaussian approximation for the transition probability. The experimental data of De Vault and Chance [Biophys. J. 6, 825 (1966)] on the temperature dependence of the rate of electron transfer from cytochrome to the chlorophyll reaction center in the photosynthetic bacterium Chromatium are properly accounted for in terms of the present theory.
Lamb dip studies of the nuclear quadrupole coupling and dipole moment in an excited vibrational state of NH2D
Authors: Van Stryland E. W. , Shoemaker R. L.
Published: February 2003

The hyperfine structure of two transitions in the ν2=0→1 vibrational band of the asymmetric rotor NH2D is resolved. These spectra represent the first direct observation of 14N nuclear quadrupole splittings in the optical region. The transitions were observed using Lamb dip spectroscopy in which a Stark field is used to tune the spectral components through resonance with a CO2 laser. Splittings of less than 1 MHz are resolved, and the nuclear quadrupole coupling constants in the excited vibrational state are determined to be χaa=2.52±0.24, χbb=1.46±0.36, χcc=−3.98±0.29 MHz. In addition, we find μc=1.09±0.04 D for the excited state permanent dipole moment. These values differ significantly from the ground state values, and some discussion of their physical implications is presented.
Variational time-dependent Hartree–Fock calculations. II. Six-electron molecules
Authors: Watson D. K. , Stewart R. F. , Dalgarno A.
Published: February 2003

The time-dependent Hartree–Fock equations are solved variationally for the six-electron molecules Li2, BH, and CH+. The values obtained for excitation energies, oscillator strengths, and other properties are in harmony with the available accurate comparison data. The calculations demonstrate the presence of bound excited states and resonances due to transitions from inner shells.
Chemical trapping and crystalline amorphous transition accompanying energetic proton and deuteron bombardment of silicon and germanium
Authors: Gruen D. M. , Varma R. , Wright R. B.
Published: February 2003

Undoped single and polycrystalline silicon and germanium disks and internal reflection plates were bombarded with 15–38 keV H+1, D+1, H+2, and D+2 to fluences up to 2×1019 ions/cm2. The infrared and Raman local-mode frequencies of Si–H, Si–D, Ge–H, and Ge–D were observed in the ion implanted targets. The chemical trapping efficiencies of Si and Ge for the incident ions have been determined from infrared integrated absorption intensities of the hydride and deuteride bands. The chemical trapping efficiencies for D+1 and D+2 were found to be higher than those observed for H+1 and H+2 under equal ion energy and fluence conditions. The H or D/Si and H or D/Ge atom ratios approach 2 for high fluence bombardments. A trapping mechanism to account for these observations is presented. Laser Raman scattering data showed that the surface amorphization of Si and Ge occurred to a greater extent for D+1 compared to H+1 bombardments at equal fluence and incident ion energy. Annealing of H+1, D+1, H+2, and D+2 ion implanted surface regions was monitored by the laser Raman scattering technique. The amorphous-to-crystalline transition was shown to occur at 630 and 480 °C for Si and Ge, respectively.
Rayleigh–Brillouin scattering in gaseous mixtures He–Kr and Ar–Kr
Authors: Lao Q. H. , Schoen P. E. , Chu B. , Jackson D. A.
Published: February 2003

Measurements of Rayleigh–Brillouin spectra of gaseous mixtures helium–krypton and argon–krypton have been performed over a range of concentrations and temperatures. The data have been compared successfully with the theoretical spectra using the approximate equation of Cohen, Sutherland, and Deutch. We have considered, for the first time, the effects of temperature changes upon the spectra and have also successfully employed the analytical expressions of Kestin et al. to determine the thermodynamic constants of our mixtures.
The measurement and interpretation of T1 and T2 in the inversion doublets of 15NH3 and the rotational transitions in OCS
Authors: Hoke W. E. , Bauer D. R. , Ekkers J. , Flygare W. H.
Published: February 2003

Transient experiments have been completed to measure T1 and T2 as a function of pressure for the inversion transitions in several (J,K,‖M‖ =J) states in 15NH3 and in the J=1→J=2, M=±1 transitions in OCS. In OCS, T1=T2 for both the J=0→J=1, M=0 and J=1→J=2, M=±1 transitions. In NH3, however, the ratio T2/T1 varies over the range 1.0?T2/T1?2. These results are interpreted using a modified form for the Anderson theory of line broadening. Comparisons are made between the results of this work and earlier steady state double resonance work.
Vacancy migration and void formation in γ-irradiated ice
Authors: Eldrup M.
Published: February 2003

Positron annihilation techniques (PAT) have been used to study the effects in ice of γ irradiation at −196°C and of subsequent heating, both in poly- and monocrystalline samples. The main effects of irradiation are (1) the appearance in positron lifetime spectra of two long-lived components (1.2 and 2.3 nsec) ascribed to ortho-positronium (ortho-Ps) trapping in radiation created mono- and divacancies, (2) a strong decrease of the intensity of a short-lived component in the spectra, associated with para-Ps, and (3) an increase in the width of Doppler broadening spectra. The two latter effects are ascribed to Ps inhibition and conversion by radiation created OH radicals. Heating above −165°C makes both effects disappear in agreement with radiation chemistry results on OH. Heating also increases the longest lifetime up to 11 nsec at −130°C. This is explained as vacancy migration leading to void formation. A vacancy migration energy is obtained, Em=0.34±0.07 eV, around three times higher than the previously assumed value. The advantage in this kind of study of using PAT, which are specifically sensitive to vacancy type defects, is pointed out.
Theoretical analysis of collision characteristics
Authors: Tarr Susan M. , Sampson John , Rabitz Herschel
Published: February 2003

A very simple program is presented for the evaluation of total cross sections for a wide range of molecular systems. The theoretical framework and the resulting computer program were designed to be flexible so that improvements could be made as desired. This led to the choice of effective Hamiltonian techniques combined with exponential forms for the dynamics. The present paper utilized the simplest of these procedures, an effective potential with the exponential Born approximation. In addition, an analytic smoothing technique was employed to facilitate evaluation of the partial wave sums. It is shown that simple programs of this type can provide useful qualitative information. This is done through a discussion of the physical effects and general trends observable in the following model collision systems: atom–rigid rotor, atom–breathing sphere, atom–vibrotor, atom–symmetric top, two rigid rotors, two breathing spheres, two vibrotors, and two symmetric tops.
Observation of orbiting resonances in H2–rare gas scattering
Authors: Toennies J. P. , Welz W. , Wolf G.
Published: February 2003

Measurements of the orbiting resonances of H2–Ar, H2–Kr, and H2–Xe in the collision range 8–500 cm−1 are reported.(AIP)
Absorption in the infrared of surface electromagnetic waves by adsorbed molecules on a copper surface
Authors: Bhasin K. , Bryan D. , Alexander R. W. , Bell R. J.
Published: February 2003

Experimental observations have been made for the first time that surface electromagnetic wave (SEW) spectroscopy can be used to measure the infrared spectra of adsorbed molecules and very thin films on metal surfaces. Using a tunable CO2 laser (9.2–10.8 μm wavelength), the 975 and the 1037 cm−1 resonances of 5–25 Å thick physisorbed benzene on copper were observed. Broadening of these resonances for the thinnest layers of benzene was observed. Also SEW spectroscopy and ellipsometry were used to study thin films of cellulose acetate (15–75 Å) on copper. Further, it was shown that film thicknesses determined by SEW spectroscopy are in agreement with the ellipsometric thicknesses and that the SEW method is more sensitive for very thin (<25 Å) films.
Monte Carlo and hypernetted chain equation of state for the square-well fluid
Authors: Henderson Douglas , Madden William G. , Fitts Donald D.
Published: February 2003

Results of Monte Carlo and hypernetted chain theory calculations are reported for both the radial distribution function and the equation of state of the square-well fluid for the particular case where the potential is cut off at 1.5σ, where σ is the hard sphere diameter. At high densities, substantial errors are found in the hypernetted chain equation results at all temperatures. However, these errors are very nearly independent of temperature. The hypernetted chain theory accounts for the perturbing effects of the attractive forces with good accuracy despite appreciable errors in the treatment of the repulsive forces.
Microwave spectrum, molecular structure, barrier to internal rotation, and dipole moment of methyl monofluorogermane
Authors: Roberts Ronald F. , Varma Ravi , Nelson James F.
Published: February 2003

Microwave rotational spectra of twelve isotopic species of methyl monofluorogermane (CH3GeH2F, 13CH3GeH2F, and CD3GeH2F with Ge=70Ge, 72Ge, 74Ge, and 76Ge) have been assigned in the 8 to 32 GHz frequency range. Differences in the effective moments of inertia of the 70Ge, 72Ge, and 74Ge species and information obtained from internal rotation analysis concerning the orientation of the −CH3 group with respect to the C–Ge bond axis have been utilized with the assumptions r (Ge–H) =1.525 Å and &HGeH=110° to calculate the following rs structural parameters for methyl monofluorogermane: r (C–Ge) =1.925±0.002 Å, r (C–H) =1.094±0.005 Å, r (Ge–F) =1.751±0.005 Å, &HCH=108°56′±20′, &FGeC=106°20′±20′. Analysis of ground state rotational transition splittings yield potential barriers to internal rotation of 941±20 and 921±20 cal/mole for the −CH3 and −CD3 species of the molecule, respectively. Internal rotation analyses indicate that the symmetry axes of the −CH3 and −CD3 groups are not coincident with the respective C–Ge bond axes. It has been determined that these groups are tilted toward the Ge–F bond by 1°54′±15′ and 1°51′±15′ in CH3 74GeH2F and CD3 74GeH2F, respectively. Stark effect measurements on CD3 74GeH2F yield principal axis dipole components of ‖μa‖=2.42±0.12 D, ‖μb‖=0.93±0.14 D, and a total dipole moment ‖μ‖=2.59±0.16 D, which is inclined at an angle of 23° with the Ge–F bond and an angle of 51° with the C–Ge bond.
Molecular beam chemistry: Reactions of Cs+ with benzyl chloride
Authors: Safron S. A. , Miller G. D. , Rideout F. A. , Horvat R. C.
Published: February 2003

Angular and translational energy distributions for the C7H7+ and C5H+5 products from the reactions of Cs+ with benzyl chloride are reported. The kinematic contour diagrams for the C7H7+ at collision energies of 7.5 eV and 9.5 eV show that there are two direct mechanisms operative: a strippinglike process that is the more important at the lower energy and a repulsive, backward-scattering process that seems to involve electronic excitation of the benzyl chloride and that dominates at the higher energy. The C5H5+ product is found to be scattered nearly isotropically at 9.5 eV, but essentially entirely backward at 14.4 eV. This is shown to be consistent with the mechanism that requires electronic excitation of the benzyl chloride, followed by the decomposition of an excited benzyl ion. A ’’propensity rule’’ is also observed in that of all the possible energetically allowed products of the reaction, only those with a singlet ground state are found.
Polarization CI wavefunctions: the valence states of the NH radical
Authors: Hay P. Jeffrey , Dunning Thom. H.
Published: February 2003

The polarization configuration interaction (POL–CI) wavefunction is described. Calculations are reported for the X 3Σ−, a 1Δ, b 1Σ+, A 3Π, and c 1Π states of the NH molecules in a double zeta plus polarization Gaussian basis. Calculated excitation energies and spectroscopic constants are compared with results from more extensive calculations in this basis and with previously reported calculations. The starting point for POL–CI calculations is a generalized valence bond (GVB) calculation to insure proper dissociation. Results of using both ground state and excited state GVB orbitals show that a single set of (ground state) orbitals may be used for all states without loss of accuracy. Dipole moments and electronic transition moments as a function of R are also given.
The anisotropic interaction between hydrogen molecules
Authors: Raich J. C. , Anderson A. B. , England W.
Published: February 2003

The extent to which it is possible to estimate the anisotropic potential between two hydrogen molecules directly from ab initio calculations of the short range repulsive part of the interaction potential and the available semiempirical van der Waals and quadrupolar potentials is investigated. The anisotropic potential parameters ε0(R), ε2(R), ε4(R), and B (R) due to nonquadrupolar anisotropic interactions are estimated for hydrogen in the range of intermolecular separations from 2.5 to 5 Å. At the nearest neighbor distance in solid hydrogen, R0=3.756 Å, we recommend the following values ε0=−0.002 cm−1, ε2=−0.027 cm−1, ε4=−0.016 cm−1, and B=−0.310 cm−1. At the next nearest neighbor distance of √2 R0, we suggest ε0=−0.0011 cm−1, ε2=−0.0017 cm−1, ε4=−0.0029 cm−1, and B=−0.131 cm−1.