bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic and magnetic properties of metal diiodides MI2 (M=V, Cr, Mn, Fe, Co, Ni, and Cd) from 129I Mössbauer spectroscopy
Authors: Friedt J. M. , Sanchez J. P. , Shenoy G. K.
Published: February 2003

The hyperfine interaction parameters at the iodine site in various metal diiodides (M=V, Cr, Mn, Fe, Co, Ni, and Cd) were measured using the 27.8 keV Mössbauer transition in 129I. Charge densities in the valence orbitals are deduced from an interpretation of isomer shift and quadrupole coupling constant values. Magnetic hyperfine interactions transferred at the originally diamagnetic iodine ion in the magnetically ordered phases of the compounds allow one to determine the unpaired spin densities in the valence orbitals. Information relative to the magnetic structures is obtained from the number and intensities of magnetically nonequivalent sites and from the relative orientation between the transferred field and the electric field gradient axes. The Mössbauer spectra of 129Xe impurities created by β− decay of 129I in the NiI2 lattice reveal the absence of bonding of xenon atoms in this matrix.
Multiphoton molecular dissociation in intense laser fields
Authors: Mukamel Shaul , Jortner Joshua
Published: February 2003

In this paper we advance a model for multiphoton photofragmentation of an ’’isolated,’’ collision-free, polyatomic molecule on the ground state potential surface. The molecular energy levels are separated into three regions. In the low energy range the level structure is sparse and only dynamic Stark shifts will be exhibited. In the intermediate energy range the density of bound vibrational states is high and the level structure can be described in terms of mixed (zero-order) states. We argue that in this energy range intrastate anharmonic scrambling may be of central importance in the excitation process, but that intrastate vibrational relaxation and energy redistribution is not encountered for medium-sized molecules. In the high energy region dissociative channels open up and reactive intramolecular decay is handled in terms of resonance theory. The time evolution of a multilevel system, whose highest energy levels are metastable, and which is driven by an intense laser field, is handled by the effective Hamiltonian formalism. Explicit expressions are derived for the photofragmentation yields and their dependence on the molecular parameters and on the field parameters. Specific applications to two distinct model systems are presented. First, we treat the quasidiatomic model, which disregards level scrambling in the intermediate energy range, whereupon near-resonant radiative coupling prevails between the states of a truncated anharmonic oscillator. Second, we have considered the two-ladder model where in the low energy range near-resonant radiative coupling occurs within an anharmonic ladder, while in the intermediate energy range resonant radiative coupling between mixed states prevails. We present numerical simulations of the photofragmentation yields and their dependence on the molecular parameters, such as the diagonal anharmonicity, the molecular dissociation energy, the predissociative widths, and the isotopic shift. We have also explored the dependence of the photofragmentation yields on the pulse parameters, such as the off-resonance energy, the field intensity, and the pulse duration. The quasidiatomic model seems to overestimate the power onset for photodissociation and the power required for the onset of saturation effects, while the two-ladder model is quite adequate to account for the gross features of coherent multiphoton molecular photofragmentation.
Exciton dynamics in solid neon
Authors: Pudewill D. , Himpsel F.-J. , Saile V. , Schwentner N. , Skibowski M. , Koch E. E. , Jortner Joshua
Published: February 2003

In this paper we report the results of an experimental study of the photoelectric yield of pure and doped solid Ne in the extreme ultraviolet (h/ω=8–30 eV) by use of synchrotron radiation from the DESY synchrotron. Results for the range of impurity excitations, exciton states, and interband transitions were obtained for Xe in Ne, Kr in Ne, and Ar in Ne. The photoemission data where utilized to extract new information concerning (a) energetics, (b) nonradiative relaxation phenomena, and (c) exciton dynamics in solid Ne. Concerning (a) a definite value for the electron affinity V0 in solid Ne was established experimentally. Concerning (b) we were able to show that the n=2 (1/2) exciton decays in an Auger-type process into an ionized impurity in the (3/2) state and a free electron on a time scale τ (Auger) ?10−13 sec, which is comparable or even shorter than the n=2→n=1 nonradiative relaxation process. Concerning (c) the analysis of the energy dependent photoemission line shape at different film thicknesses resulted in information concerning the diffusion length of excitons and escape length of electrons in solid Ne.
Composition of partially metastable rare gas beams
Authors: Neynaber R. H. , Magnuson G. D.
Published: February 2003

Ion–rare gas atom reactions which proceed for atoms in the ground but not metastable state are studied in a merging-beams apparatus to measure the fraction of ground-state and metastable atoms in rare gas beams produced by charge transfer of the rare gas ions in an alkali vapor. Experimental fractions are compared with values calculated using a statistical weight theory.
Conformational contribution to the entropy of melting. II. Comparison between theory and experiment
Authors: Bianchi U. , Turturro A.
Published: February 2003

Experimental and theoretical values of the conformational entropy contribution ΔSc to the melting entropy of linear hydrocarbons agree fairly well; however, the chain model in which each bond has three possible conformations (g+, g−, and t) seems to be probably oversimplified. However, ratios of ΔSc to the actual number of bonds, which are effective in increasing the number of conformations, increase theoretically with chain length, whereas experimental values decrease. This latter behavior can be due to the increasing intra- and intermolecular packing difficulties as the hydrocarbon chain length increases.
The interaction of a large amplitude internal motion with other vibrations in molecules. The effective Hamiltonian for the large amplitude motion
Authors: Quade C. Richard
Published: February 2003

A theory for the interaction of a large amplitude internal motion (LAM) with the small amplitude vibrations in molecules is formulated. Valence internal coordinates are used for the development of the kinetic and potential energies in terms of the usual G−1, G, and F matrices with all three being functions of the internal coordinates. A coordinate transformation is defined that separates the large amplitude internal motion from the other vibrations in zeroth order in the kinetic energy but at the same time modifies the force constants for the 3N-7 vibrations. The higher order momentum coupling between the large and small amplitude vibrations is developed. Expressions are obtained for the small amplitude coordinate dependence of the kinetic energy coefficient for the large amplitude motion. A Van Vleck transformation is used to obtain the effective Hamiltonian of the large amplitude motion for the vth vibrational state of the other vibrations. The effective Hamiltonian is suitable as a parametric equation for analysis of experimental data or as the starting point for quantitative theoretical calculations. No attempt has been made to formulate the interaction of the large amplitude internal motions and other vibrations with the overall rotational motion, which is taken as the zero angular momentum state.
Analytical theory of crystal growth
Authors: Weeks J. D. , Gilmer G. H. , Jackson K. A.
Published: February 2003

The motion of the crystal–vapor interface is examined by means of the kinetic solid-on-solid model, a restricted version of the kinetic Ising model. We formulate an exact kinetic equation for the model and discuss some of the implications of the mean field and random distribution approximations. These methods require numerical integration of a differential-difference equation and are accurate only in the limit of high temperatures or high deposition rates. We then introduce a new set of approximations which permit the kinetic equations to be solved analytically and discuss their validity. We obtain simple analytic expressions for both the interface width and the growth rate under steady-state conditions. The qualitative dependence of the interface width on both temperature and deposition rate is correctly described by the theory and the predicted growth rates compare favorably with recent Monte Carlo calculations over a very wide range of temperatures and deposition rates.
Attenuation and single electron charge-exchange cross sections of He+ on He in the energy range 500–5000 eV
Authors: Eisele Fred L. , Nagy Stephen W.
Published: February 2003

A He+(1s), monoenergetic ion beam was used to bombard He gas in a dual purpose scattering chamber. The total charge-exchange cross section was measured using the parallel plate collection method for low energy target ions, while a lower bound to the total cross section was obtained via a beam attenuation measurement. Both cross sections decreased with increasing incident ion beam energy, but they did not coincide until approximately 2500 eV. Similar cross sections obtained in previous measurements suggested a much closer correspondence between these two cross sections and thus a near-zero non-charge-exchange cross section at much lower energies. Further comparison of the present charge-exchange data with both experiment and theory is also displayed, and relatively good agreement is observed.
Formaldehyde photochemistry: Appearance rate, vibrational relaxation, and energy distribution of the CO product
Authors: Houston Paul L. , Moore C. Bradley
Published: February 2003

The mechanism of formaldehyde photochemistry has been investigated by monitoring the appearance rate, relative yield, and vibrational distribution of the CO photochemical product detected either by its infrared fluorescence or by its absorption of a cw CO laser. In the limit of low formaldehyde pressures, the CO product appears with a rate more than 100 times slower than the decay rate of the formaldehyde first excited singlet state. This fact indicates the presence of a long-lived intermediate state between S1 and the molecular products. Collision-induced CO production following 337.1 nm formaldehyde excitation occurs with appearance rates of 2.7×10−11 cm3 molecule−1⋅sec−1 for D2CO and 4.7×10−11 cm3 molecule−1⋅sec−1 for H2CO. After its production, CO(v=1) relaxes to the ground vibrational state in collisions with D2CO at a rate of 3.3×10−12 cm3 molecule−1⋅sec−1 and in collisions with H2CO at a rate of 3.7×10−14 cm3 molecule−1⋅sec−1. These rates have been confirmed by a separate measurement which monitors CO(v=1) fluorescence following excitation with a Q-switched CO laser. The CO photolysis yield decreases with the addition of argon, but increases dramatically for 305.5 nm photolysis upon the addition of NO or O2. Vibrational distributions of the CO product have been measured as a function of energy and vibrational level of the formaldehyde singlet state. Although the amount of energy appearing in CO vibrational modes increases with increasing excitation energy, the CO vibrational energy accounts for only between 0.7% and 4.5% of the energy available to the products at the measured dissociation wavelengths.
Molecular orbital study of the ground and excited states of ozone
Authors: Messmer R. P. , Salahub D. R.
Published: February 2003

Molecular orbital results obtained by the self-consistent-field Xα scattered wave (SCF-Xα-SW) method employing overlapping spheres are presented for the ground state and 15 excited states of ozone (O3). The results are compared with those of ab initio calculations which used the Hartree–Fock (HF), the generalized valence bond (GVB), and GVB with configuration interaction (CI) methods. The Xα–SW method yields the correct gound state (1A1) as do the GVB and GVB–CI methods which explicitly include electronic correlation effects. This is in contrast to the HF method, which predicts the ground state to be a 3B2 state. The Xα–SW molecular orbital (MO) results for the lower, dominantly covalent electronic states considered represent a substantial improvement over the HF–MO results and are consistent with the accurate GVB and GVB–CI results. The results are much less satisfactory for the higher states which are more ionic in character.
Effects of the Pauli principle on electron scattering by open-shell targets
Authors: Riley Merle E. , Truhlar Donald G.
Published: February 2003

The semiclassical exchange approximation and two free-electron–gas exchange approximations are applied to electron scattering from the hydrogen atom. In the triplet spin state the results are similar to our previous findings [M. E. Riley and D. G. Truhlar, J. Chem. Phys. 63, 2182 (1975)] for electron scattering by closed-shell atoms. However these results, already of useful accuracy, are improved by the inclusion of one-electron exchange terms or an orthogonality constraint. For the singlet spin state it is essential to include the one-electron exchange terms except at high impact energies (greater than about 50 eV). The approximation of Lippmann, Schey, Burke, and Chandra for treating exchange in closed-shell systems is tested and found to be less accurate than the present approximations.
Intramolecular 13C kinetic isotope effects in the decarboxylation of oxalic acid in dioxane solution
Authors: Kidd Richard W. , Huang Thomas T.-S. , Yankwich Peter E.
Published: February 2003

Intramolecular 13C kinetic isotope effects in the decomposition in dioxane solution of oxalic acid to carbon dioxide and formic acid were determined at several temperatures between 102 and 141°C. The isotope effect is small (approximately 0.5%) and any temperature dependence is masked by the experimental imprecision (which averages about 0.09%). The method of three-element reaction coordinates (with ν1‡=0) was employed to expose possible properties of the transition state. Relatively good definition of transition state properties is obtained with several models. But, when one requires simultaneous fit of the model calculations to the results recorded here and to the intermolecular isotope effects reported earlier (’’double correspondence’’), the parameters are severly confined. The best double correspondence is obtained when: (a) transition state force constants are set to correspond to bond orders (near the C atom which will appear in carbon dioxide product) of C–C near 0.1, of C–O near 1.9, with that of O–H set at 0.5; (b) the relative internal coordinate displacements comprising the reaction coordinate eigenvector are C–C extension, 0.50 to 0.75; C–O contraction, 0.15 to 0.18; and O–H extension, 0.35 to 0.07. Anharmonicity and other small corrections are neglected without substantial effect on the conclusions described.
Macromolecular configurations simulated by random walks with limited orders of non-self-intersections
Authors: Wall Frederick T. , White R. Allen
Published: February 2003

Computer computations have been carried out for various orders of self-avoiding random walks used to simulate flexible macromolecules. (The order of a walk is defined as the number of steps in the longest loop excluded by reason of the self-avoiding feature.) Calculations have been carried out for walks of orders up through 10 on a simple square lattice in two dimensions and for walks of orders up through 12 on a tetrahedral lattice in three dimensions. The calculations established precisely the parameters associated with the asymptotic behavior of long chains subject to the model assumed. The results are in agreement with earlier theoretical predictions as to form, and they serve to identify the factors contributing to increasing chain dimensions with length of walk. Asymptotically, the mean square end-to-end separation is linear in the number of steps for any self-avoiding walk of finite order. The behavior of completely self-avoiding walks (i.e., those of unlimited order) is represented very closely by the envelope of asymptotes corresponding to walks of different orders. In addition, the largest symmetric and antisymmetric eigenvalues associated with the transition matrix governing unit increments in contour chain length have been evaluated. Those eigenvalues determine an upper limit to the asymptotic behavior of long model chains.
The adsorption of hard spheres near a curved surface
Authors: Mandell M. J.
Published: February 2003

The adsorption of hard spheres near a curved surface is calculated from scaled particle theory and from the Percus–Yevick theory of hard sphere mixtures.
Deactivation of Hg(3P2) in crossed molecular beams
Authors: Liu K. , Parson J. M.
Published: February 2003

Experimental studies of electronic energy transfer in collisions of metastable Hg(3P2) with molecules are reported. Product molecules were detected by spectrally resolved light emission from the region of intersection of crossed molecular beams. In the case of NO scattering, emission was observed from the A 2Σ+ and B 2Π states of NO, as well as from the 3P1 state of Hg. The collision energy dependence of these processes, and for formation of Hg(3P1) in collisions with N2, CO, H2, D2, O2, CO2, NH3, CH4, and C2H6, was studied in the range 2–9 kcal/mole. The initial energy dependence and final state distribution (for NO scattering) are shown to be highly nonstatistical. The simple functional form σ∼1−V (Rc)/E is found to be adequate for fitting the collision energy dependence of many of the systems.
Optical spectra of orientationally disordered crystals. VI. The Raman spectrum of the translational lattice vibrations of ice Ih
Authors: Wong P. T. T. , Whalley E.
Published: February 2003

The Raman spectrum of the translational vibrations of polycrystalline ice Ih has been investigated in the range 350–20 cm−1. All the vibrations are Raman active, and there is much fine structure, presumably due to particular points in the Brillouin zone. Tentative assignments are suggested for some of the features. The theory of the Raman scattering by the translational vibrations of orientationally disordered crystals has been discussed and it is shown that the origin of the Raman intensity lies mainly in the disorder in the location of the hydrogen atoms surrounding an O–H⋅⋅⋅O bond. The intensity of scattering below ∼200 cm−1 is much weaker than that above ∼240 cm−1, which suggests that while the vibrations below ∼200 cm−1 are disorder allowed, those above ∼240 cm−1 might be connected with the order-allowed A1g, E1g, and E2g vibrations of the hypothetical crystal with real hexagonal symmetry. The strong Raman scattering near 310 cm−1 is not caused by a splitting of the transverse and longitudinal optic vibrations, such as occurs in ionic crystals, but by significant electrostatic interactions, particularly of the transition moments of neighboring hydrogen bonds. They cause the hydrogen-bond stretching constant to depend on the relative orientation of the two water molecules, and the coupling constant for the stretching of neighboring hydrogen bonds to differ for the symmetric, O–H⋅⋅⋅O⋅⋅⋅H–O and O⋅⋅⋅H–O–H⋅⋅⋅O, and asymmetric, O–H⋅⋅⋅O–H⋅⋅⋅O, configurations. The differing coupling constants cause significant differences in the forces restricting the motion of molecules parallel and perpendicular to their polar axes, and probably causes the peaks in the spectrum at ∼310 and 228 cm−1.
The structure of the liquid–vapor interface
Authors: Lovett Ronald , Mou Chung Yuan , Buff Frank P.
Published: February 2003

A formal derivation is presented for the equilibrium relation between the singlet density in a fluid and the direct correlation function and for the equivalent relation involving the pair number density. It is shown that this relation is equivalent to the macroscopic condition for hydrostatic equilibrium when the singlet density varies sufficiently slowly to permit the introduction of local thermodynamics. Some aspects of the usage of this in the determination of the singlet density in the liquid–vapor transition region are discussed.
Molecular motions in compressed liquid water
Authors: Jonas J. , DeFries T. , Wilbur D. J.
Published: February 2003

Proton and deuteron NMR spin–lattice relaxation times in liquid water and heavy water were measured as a function of pressure and temperature in the range 10–90°C and 1 bar–9 kbar. D2O was also studied at 150 and 200°C. Availability of density and viscosity data under these experimental conditions enabled us to separate the effects of temperature and density on the spin–lattice relaxation times, T1, and viscosities. Under the assumption that the intermolecular dipolar contribution to the proton T1 follows the changes in shear viscosity with temperature and density, we separated the intramolecular and intermolecular dipolar contributions to the proton T1. We found that at a temperature of 10°C the initial increase in density leads to faster reorientation of the water molecules. The effect was much smaller at 30°C. Analysis of the experimental data on H2O and D2O leads to the conclusion that compression diminishes the coupling between the rotational and translational motions of water molecules. The change in the nature of the rotation–translation coupling with increasing density is mainly responsible for the failure of the Debye equation to describe the density effects on the reorientation of water molecules. In the case of D2O we find a relatively small variation in the deuteron quadrupole coupling constant with increasing density. Its average value is approximately 230 kHz over the range of our experimental conditions. Another experimental finding of this study is the decrease in the activation energies for relaxation and shear viscosity with increasing density. All the experimental evidence indicates that compression of water leads to significant distortion and/or disruption of the hydrogen bond network with the important consequence that the dynamic behavior of water under high compression resembles more that of a ’’normal’’ molecular liquid of comparable molecular size. At high densities the hard core repulsive interactions begin to dominate over the directional interactions which are mainly responsible for the open structure of liquid water at low temperatures and pressures.
New values of the light scattering depolarization and anisotropy of water
Authors: Farinato Raymond S. , Rowell Robert L.
Published: February 2003

A new value for the depolarization ratio of pure water has been measured at 514.5 nm using an argon ion laser light source and photon counting detection. This depolarization ratio is lower than any previous literature value by a substantial amount. Stray light and photometer geometry are shown to be responsible for overestimated values reported previously. The depolarization ratio is also shown to be a function of the portion of the spectrum being investigated. With no filters we find ρv=0.026. The magnitude of error due to finite acceptance angle is calculated and shown to be substantial. Depolarization ratios using 0.46 and 22.5 nm filters (half-peak bandwidth) are reported.
One-electron properties as variational parameters
Authors: Henderson George A. , Zimmerman Robert K.
Published: February 2003

The algorithm of Clinton et al. [Phys. Rev. 177, 7 (1969)] for the direct determination of an idempotent density matrix from one-electron expectation values is adapted to perform energy-variational calculations in which the expectation values serve as variational parameters. As an illustration, constrained variational calculations are performed on lithium hydride. A technique is presented to insure that from virtually any idempotent initial guess density matrix, the algorithm will converge to that idempotent density matrix having the desired expectation values which is ’’closest’’ to the initial guess.
Kinetic theory of NMR in dilute molecular gases. Existence of a minimum in the transverse relaxation time
Authors: Tenti G. , McCourt F. R.
Published: February 2003

A kinetic theory of nuclear magnetic relaxation in dilute molecular gases is proposed. The theory is based on the linearized Waldmann–Snider equation and developed from the standpoint of studying the fluctuation spectrum with the well-established methods of linear response theory. The main result of this work is the prediction of a minimum for the transverse relaxation time T2, in contrast to previously proposed theories.
Molecular oxygen effects on the electron spin relaxation in a spin-labeled polystyrene
Authors: Brown I. M.
Published: February 2003

The effects of molecular oxygen on the phase memory time TM and the electron spin–lattice relaxation time T1 of a spin-labeled polystyrene have been investigated over the temperature range 77–295 K. Two mechanisms whereby molecular motions can influence the phase memory times of free radical molecules have been identified. In a type 1 mechanism, molecular motions involving neighboring B spins contribute directly by dipolar interactions to the local field fluctuations at the A spin sites. In a type 2 mechanism, which is operative at fast motional frequencies close to the Larmor frequency, the molecular motions influence the spin–lattice relaxation rates and indirectly determine TM by spin-lattice induced radical B spin flips. With oxygen present in the samples, the temperature dependences of TM and T1 exhibit a minimum value at the same temperature. The minimum in T1 is attributed to a time dependent dipolar interaction between the molecular oxygen and the radical spin, whereas the minimum in TM is the result of a type 2 mechanism with the radical spins acting as B spins. Although the temperature dependence of the correlation time for the local field fluctuations at the radical site, τc, has not been evaluated, the TM and T1 values indicate that it is in the range τc<10−8 s at 77 K to τc<2×10−11 s for temperatures above the T1 minimum. The correlation times are determined either by localized motions of the oxygen molecules when they are restricted to the pre-existing holes present in the polymer or by the oxygen electron spin–lattice relaxation time. With the samples in a vacuum, the values of T1 obey a monotonically decreasing function with increasing temperature, whereas the TM values show a minimum at 140 K. We interpret the latter to indicate the onset of the molecular motions associated with the δ relaxation.
Rayleigh linewidth and turbidity measurements near a multicomponent fluid tricritical point
Authors: Gollub J. P. , Koenig Albert A. , Huang John S.
Published: February 2003

Rayleigh linewidth measurements have been performed near the tricritical point of an ethanol–water–benzene–ammonium sulfate mixture. We approached the tricritical temperature Tt along various paths of constant composition using several different samples. The linewidth always varies linearly with temperature in both two phase and three phase regions near Tt (but in the hydrodynamic regime). Since the linewidth is inversely proportional to the correlation length ξ in this regime, we find an exponent ν=1.0 in both two and three phase regions, as expected from mean field theory for thermodynamic paths of constant composition. Because of the strong temperature dependence of ξ, the hydrodynamic regime extends only to T−Tt?1 K. Measurements of the turbidity τ indicate that the product Γτ1/2 is constant within our experimental uncertainties, from which we infer that the susceptibility exponent γ?2 along paths of constant composition near the tricritical point.
Electron impact excitation of the Meinel band system of N+2
Authors: Mandelbaum D. , Feldman P. D.
Published: February 2003

The emission spectrum of the Meinel band system (A 2Πu–X 2Σ+g) of N+2 was obtained in the region between 0.72 and 1.6 μm by electron impact excitation of N2. The relative excitation function is presented for the (0, 0) band from 22 to 120 eV. Cross sections of the Meinel bands that fall in this region were measured relative to the (2, 0) Meinel band. The results obtained are in good agreement with previously published values below 1.1 μm except for the (0, 0) band which is 30% higher. The electronic transition moment at large internuclear distances is found to agree with theoretical estimates.
The effect of vibrational and translational energies of ionic reactant on the reaction NO++iso-C4H10→HNO+C4H+9
Authors: Honma K. , Tanaka K. , Koyano I. , Tanaka I.
Published: February 2003

The effect of vibrational energy of the ionic reactant on the rate of the reaction NO++iso-C4H10→HNO+C4H+9 has been examined by directly determining the relative reaction cross sections for the individual vibrational states from v=0 to 4. The reactant ions were produced by photoionization at suitable wavelengths and the relative abundances of the ions in various vibrational states were determined at each wavelength by photoelectron spectrometry. The translational energy dependence of the individual cross sections has also been studied by changing the repeller voltage in the ionization chamber. The cross section slightly decreased with increasing vibrational energy at lower relative kinetic energies, but it was almost constant within the experimental error at higher relative kinetic energies. The result has been explained in terms of the two-mechanism model, viz., complex formation predominating at lower relative kinetic energies and stripping predominating at higher relative kinetic energies. Photoelectron spectra of NO taken at 133.6, 127.7, 122.9, 120.5, and 117.6 nm are also presented.
Charge transfer in electrostatically coupled chains: Effects of the nearest neighbor Coulomb correlations
Authors: Chang Jhy-Jiun , Jafarey S. , Scalapino D. J.
Published: February 2003

The effects of the nearest neighbor Coulomb correlations on the character of the charge transfer between donor and acceptor molecular chains are investigated. We use a modified Hubbard model for the individual donor and acceptor chains. In this model the intramolecular Coulomb interactions are taken to be infinitely large, and the transfer integrals are considered to be infinitely small. All the intermolecular interactions except those between the nearest neighbors along the chains are treated within the mean field approximation. The free energy of this model can be calculated for an arbitrary strength of the nearest neighbor interaction and results are given for the temperature dependences of the charge transfer and the zero field paramagnetic spin susceptibility.
Argon–helium mixture viscosities from independent intermolecular potentials
Authors: Reissner John E. , Riehl John W.
Published: February 2003

The viscosity of gaseous argon–helium mixtures at 298 K is calculated in terms of quantum collision integrals for many-parameter potentials that were previously obtained without the use of measured values of nonequilibrium bulk properties. The Chapman–Enskog first approximation expression and the Sutherland expression are used to evaluate the viscosity, with the Chapman–Enskog expression proving superior, in fact fitting experiment to within the uncertainty of the potential parameters for all but the most helium-rich compositions. To confirm this result, the collision integrals are obtained by least-squares fits of the two viscosity expressions to the data itself. The Chapman–Enskog expression accommodates the data within the experimental tolerance of ±0.1%, while the Sutherland fit deviates from the data as much as ±0.5% and does so with an argon–helium collision integral significantly different from that of the ab initio calculation.
Metric geometry of equilibrium thermodynamics. V. Aspects of heterogeneous equilibrium
Authors: Weinhold F.
Published: February 2003

The general analysis of phase equilibrium in heterogeneous systems is considered from an abstract geometric point of view. Particular attention is drawn to the thermodynamic ’’invariants’’ (or ’’symmetries’’), which arise as null eigenvectors of the thermodynamic metric matrix and can be associated with variations which leave the thermodynamic state unchanged. The analysis of these invariants leads to conditions connecting the thermodynamic field vectors, including Gibbs–Duhem relations, Clausius–Clapeyron equations, Gibbs–Konowalow laws, and systematic generalizations thereof.
Molecular constants of sulfur dichloride from microwave spectrum analysis
Authors: Murray J. T. , Little W. A. , Williams Quitman , Weatherly T. L.
Published: February 2003

The microwave spectrum of 32S35Cl2 has been investigated in the frequency region 9000–36 000 MHz. An analysis of the rotational spectrum and hyperfine structure yields the following molecular constants: rotational constants: A=14 613.36 MHz, B=2920.93 MHz, C=2430.74 MHz; asymmetry parameter: κ=−0.919526; quadrupole coupling constants: χaa=−38.98±0.10, χbb=−8.87±0.10, χcc=47.95±0.10, ‖χab‖=64±10 MHz. The structural parameters obtained from these are d (S–Cl) =2.014±0.005 Å, Cl–S–Cl angle=102.8°±0.2°. The electric dipole moment obtained from the Stark effect of the 00→10 transition is μb=0.36±0.01 D.
Ion velocity distributions in an ICR spectrometer and their effect on measured rate constants
Authors: Chesnavich Walter J. , Su Timothy , Bowers Michael T.
Published: February 2003

The translational velocity distributions of ions formed by electron impact in both square and flat ion cyclotron resonance (ICR) cells are theoretically determined. These velocity distributions result from acceleration of the ions in the trapping field of the spectrometer. ICR rate constants are computed from these distributions assuming the reaction cross section is proportional to v−n. The computations are compared to thermal values for a range of n and for a range of trapping voltages. In the most severe cases the ICR rate constants are predicted to deviate from thermal values by ∼20%. The analysis considers only reactions occurring immediately following ion formation in the electron beam. Collisions with unreactive species will, of course, rapidly transform the ICR distribution to the thermal distribution. The effect of ion mass on the velocity distribution functions and rate constants was explicitly studied. The effects on the velocity distributions may be appreciable in extreme cases but the effect on the predicted ICR rate constants is small.
A measurement of the rate constant for the reaction O + O2 + O2→O3 + O2
Authors: Hogan L. G. , Burch D. S.
Published: February 2003

The three-body heterogeneous and wall recombination coefficients for atomic oxygen were measured using an electron paramagnetic resonance spectrometer to monitor the time dependence of the atomic oxygen concentration. This measurements was made for a pressure range from 0.224 to 1.10 torr using pyrolysis of ozone as a source of oxygen atoms. The three-body heterogeneous rate constant (with molecular oxygen as the third body), resulting from a computer fit of the data to the usual recombination model, was determined independent of ozone concentration to be (6.26±0.94) ×10−34 cc2 molecule −2 sec−1. The wall recombination efficiency γ for a fused metaphosphoric acid–sodium metaphosphate mixture (the wall coating) was found to be 9.1×10−6.
Isothermal compressibility of supercooled water and evidence for a thermodynamic singularity at −45°C
Authors: Speedy R. J. , Angell C. A.
Published: February 2003

Using a capillary technique for small samples, the isothermal compressibility κT of water has been measured to −26°C. Accelerating increases of κT at the lower temperatures can be described by an expression of the form κT=Aεγ [where ε= (T−Ts)/Ts], which is known to describe anomalies encountered in the vicinity of a thermodynamic singularity located at Ts. The implication that the thermodynamic and certain other properties of water at lower temperatures may be decomposed into a normal component and an anomalous component which diverges at Ts=−45°C is supported by analysis of numerous other thermodynamic and relaxation data which extend into the supercooled regime. The anomalous characteristics are shown to originate primarily in the sensitivity of the volume to temperature changes, suggesting a geometrical basis for the cooperative behavior. The singularity at Ts=−45°C may be a lambda transition associated with the cooperative formation of an open hydrogen-bonded network, but the near coincidence of Ts with the experimental homogeneous nucleation temperature suggests, as an alternative, that Ts may correspond to the limit of mechanical stability for the supercooled liquid phase.
Teratomic recombination in alkali metal–noble gas vapors
Authors: Scheps Richard , Gallagher Alan
Published: February 2003

The teratomic recombination rates for the reaction M*+2X→MX* (A 2Π)+X have been studied for Rb–Xe, Na–Ar, Kr, Xe, and Li–Ar, Kr, Xe. This analysis employs data and molecular potential determinations from earlier measurements where free alkali M was optically excited to M* in the presence of noble gas X and the MX* (A 2Π) →MX(X 2Σ) fluorescence resulting from teratomic recombination was measured. Collisional processes compete with radiative decay, so that in the low [X] limit where dissociation collisions are negligible the ratio of bound molecular to atomic photon emission gives the branching ratio between the molecular formation rate and the known atomic radiative rate. The molecular spectrum is observed as a continuum whose intensity profile may be used to infer a bound state vibrational distribution at each [X]. Both bound and quasibound states are included in this distribution, as both contribute to the observed molecular spectra. The low pressure limit of this distribution yields the teratomic recombination rate as a function of binding energy, and the total recombination rate constant kf. It is generally found that this distribution per quantum state is almost independent of binding energy; i.e., the probability of molecular formation appears to be about the same for all bound states, it does not favor the highest bound levels. When the density of states is considered, weakly bound molecules are formed more frequently, but this weighting depends on the shape of the potential rather than the gas temperature. Comparison with theoretical models is made, and reasons for the discrepancies are discussed.
Determination of collision induced polarizability in Ar, Kr, and Xe by means of collision induced scattering analysis and empirical pair potentials
Authors: Barocchi F. , Zoppi M.
Published: February 2003

We show that for the determination of the collision induced polarizability form in Ar, Kr, and Xe from the moment analysis of collision induced scattering it is of primary importance to choose a correct form for the pair interaction potential. We chose the empirical potentials determined by Barker and co-workers for our investigation, and we determined the polarizability forms from the available experimental data. We compare our argon results with theoretical many-body calculation of the polarizability.
Further NMR studies of fluoride ion motion in doped β-PbF2
Authors: Hwang T. Y. , Lowe I. J. , Lau K. F. , Vaughan R. W.
Published: February 2003

The fluoride ion motion in β-PbF2 doped with NaF was studied by measurement of the 19F transverse relaxation time (T2), spin lattice relaxation time (T1), and the spin lattice relaxation time in the rotating frame (T1r). Measurements over the temperature range of −50 to 160°C lead to activation energies for T1, T1r, and T2 of 0.205±0.01, 0.29±0.02, and 0.27±0.01 eV/ion, and a T1r minimum at 56°C yields a correlation time of 0.74 μsec. Pressure dependence of T1 and T2 yields activation volumes of ?0.2 cm3/g-mole and 1.76±0.05 cm3/g-mole, respectively. These data along with the measured magnetic field independence of T1 suggest that the measured T1’s are not caused by 19F motion, but by thermally excited carriers.
The transition state, the electron propagator, and the equation of of motion method
Authors: Purvis George D. , Öhrn Yngve
Published: February 2003

The transition state idea is extended beyond the independent particle model and applied to the electron propagator theory and the so-called EOM method for calculation of electron binding energies. Application to the second order propagator equations for the neon atom yields excellent results for ionization energies of 1s, 2s, and 2p electrons.
Orientational order parameters and conformation of nematic p-ethoxybenzylidene-p-n-butylaniline
Authors: Shashidhara Prasad J.
Published: February 2003

Orientational order parameters for nematic liquid crystalline compound p-ethoxybenzylidene-p-n-butylaniline (EBBA) have been estimated by studying the dipole–dipole splitings in the wide line NMR absorption spectra. An attempt has been made to determine the conformation of the same by the NMR studies of one specifically deuterated derivative of the compound.
Saturation effects on collision broadened rotational lines
Authors: Malo Otieno
Published: February 2003

Saturation effects on a three-dimensional rigid rotator are studied as an extension to several previous works by Gross and the author. The solutions which are obtained for the Karplus–Schwinger (K–S) and the Gross–Lebowitz (G–L) models exhibit a further broadening of the absorption lines.
The ligand hyperfine interaction with rare earth ions. I. A revised covalent model
Authors: McGarvey Bruce R.
Published: February 2003

It is shown that a reformulation of the covalent model can explain the 19F hyperfine interactions observed for Yb3+ and Tm2+ in cubic and tetragonal sites of alkaline earth fluorides. The same model does not explain data for Ce3+, Gd3+, or Eu2+ indicating that the spin polarization model is the dominant mechanism in the first part of the rare earth series but that the covalent model becomes dominant in the latter half of the series. The analysis shows that the distortion from cubic symmetry in the tetragonal site is not as severe as an earlier NMR analysis suggested but is still large. It further suggests that the interstitial ion is closer to the rare earth ion than most observers have assumed.
The predicted infrared spectrum and the structure of the isolated UF5 molecule
Authors: Krohn Burton J. , Person Willis B. , Overend John
Published: February 2003

The infrared absorption spectrum by the fundamental modes of UF5 is calculated using force constants transferred from UF6 to predict the wavenumbers and using the F atom polar tensor from CH3F to predict the intensities. Calculations are made for several assumed geometrical configurations of UF5. Comparison of the predicted spectra with the recently observed spectrum, in the UF-stretching region, of a photolysis product of UF6 isolated in an Ar matrix suggests strongly that the structure is square pyramidal (C4v) with the U atom above the F atom equatorial plane. Although the model for the intensities is simple, the calculated spectrum is expected to predict the correct order of magnitude of the intensities of the fundamentals, first overtones, and binary combination bands of UF5, and the approximate wavenumbers for the expected absorption. Some comparisons are given between the calculations for the several structures of UF5 and observed spectra of other XF5 molecules, including PF5, BrF5, and IF5.
Nonclassical terms in the true effective valence shell Hamiltonian: A second quantized formalism
Authors: Iwata Suehiro , Freed Karl F.
Published: February 2003

A formulation is presented for the exact effective valence shell Hamiltonian ℳv to enable the extraction of individual matrix elements of ℳv within the valence orbital basis in a fashion where these matrix elements are independent of the remaining valence electrons so the same ℳv applies to the neutral and ions of a given molecule. Particular attention is placed upon the isolation of nonclassical terms which are absent in traditional semiempirical theories, such as Pariser–Parr–Pople theory, MINDO, etc. These nonclassical terms include those that are neglected in zero differential overlap approximations as well as ones involving three, four,... electron operators. The latter are shown to correspond to dynamical variable electronegativity corrections, and particular experimental consequences are noted. A second quantization formalism is employed to enable the isolation of these terms, and the analysis utilizes a generalized perturbation expansion wherein the lowest order corresponds to an open-shell sum-of-the-pairs theory, obviating the cumbersome diagram summation procedures necessitated by the use of ordinary perturbation expansions. This work represents an advancement over previous theories of ℳv in the use of generalized perturbation theory, the removal of all other valence orbital dependences, and the simplicity for extensions to nonorthogonal orbitals and pseudopotentials.
Calculation of vibrational level populations in multiphoton absorption processes
Authors: Hougen Jon T.
Published: February 2003

A simple procedure is described for computing steady-state excited vibrational state populations for multiphoton absorption processes in many-level systems. The procedure utilizes techniques already in the literature, i.e., expansion of the density matrix as a Fourier series in the frequency of the monochromatic radiation field, and introduction of linewidths and thermal relaxation by the addition of empirical terms to the density matrix time-development equation. Numerical computations can be carried out quickly on idealized systems, using commercially available band matrix inversion routines. A sample calculation for a 10-level, nine-photon process is presented.
Comments on exciton phonon coupling: Temperature dependence
Authors: Yarkony David , Silbey Robert
Published: February 2003

We discuss upper bounds to the free energy of an exciton interacting with a lattice of phonons with a linear coupling. From these upper bounds, we find the effective number of phonons clothing an exciton to vary with exciton bandwidth and temperature. The possibility of an abrupt transition from a delocalized to a localized exciton with an increase in temperature is discussed.
Fixed-molecule photoelectron angular distributions
Authors: Dill Dan
Published: February 2003

Expressions are obtained for the angular distribution of electrons ejected by electric dipole interaction from molecular targets fixed in the laboratory coordinate frame. The analysis is geometrical and independent of any particular dynamical description of the photoionization process. Thus the results will serve as the framework for study of the dynamics of particular processes, such as photoionization of molecules oriented on surfaces or by molecular beam techniques.
Transport of hydrogen in liquid lithium
Authors: Alire R. M.
Published: February 2003

The solubilities and diffusion coefficients of H2 in liquid Li contained in cylinders of Nb and Nb/1%Zr were determined in the temperature range 625–905°C and pressure range 13–1333 Pa. Simultaneous radial and axial diffusion into a finite solid cylinder adequately described the transport of H2 into liquid Li. The diffusion coefficient was expressed by D (cm2/sec) =13±1 exp(−2.5±0.2×104/RT).