bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation processes in the OH molecule
Authors: van Dishoeck Ewine F. , Dalgarno A.
Published: February 2003

A systematic investigation of the excited states that may participate in the photodissociation of the OH molecule has been carried out by ab initio calculations which use two different self-consistent field with configuration-interaction methods. Potential energy curves for states of 2Σ+, 2Σ−, 2Π, and 2Δ symmetries have been computed and the transition dipole moments connecting the states have been obtained. Photodissociation cross sections for absorption from the v″=0 vibrational level of the ground X 2Π state into the repulsive 1 2Δ, B 2Σ+, and 2 2Π states are reported. It is shown that the 1 2Δ and B 2Σ+ states, together with the 1 2Σ− state considered in an earlier study, are important channels for photodissociation. Photodissociation may take place also following absorption into bound electronic states and absorption oscillator strengths for these transitions have been determined. The mechanisms by which the bound states can be dissociated are discussed and estimates of the dissociation efficiencies of the mechanisms are made. Absorption into the bound 3 2Π state may be an important dissociation channel. Calculations of the interstellar photodissociation rate have assumed that photodissociation proceeds by absorption into the C 2Σ+ state for which a large oscillator strength had been adopted. We find that the C 2Σ+ channel is not significant in photodissociation and we present new estimates of the interstellar photodissociation rates. Our calculations demonstrate that OH is dissociated by the absorption of Lyman alpha radiation, a channel of particular significance in shocked interstellar gas and in cometary atmospheres.
Infrared multiphoton dissociation of 1-chloro-1-fluoro ethylene: Competitive reactions and pressure dependence
Authors: Jalenak W. A. , Nogar N. S.
Published: February 2003

We have examined the infrared multiphoton dissociation of CH2CFCl as a function of pressure and laser fluence. The pressure dependence could be interpreted in terms of a competition between rotation hole filling and vibrational relaxation. The fluence-dependent studies showed that the competitive hydrogen halide elimination reactions could be influenced by optical pumping rates near the dissociation threshold. Numerical modeling of the results using the energy-grained master equation allowed us to obtain numerical values for the vibrational relaxation efficiencies of a number of bath gases. The numerical modeling suggests that while our numerical approach is adequate to describe the population changes near the dissociation threshold, and under the influence of collisions, it is (not surprisingly) inadequate to describe potential coherent effects at low levels of internal excitation.
Saturation behavior of a far-wing satellite
Authors: Molander William , Belsley Michael , Burnett Keith , Farrelly David
Published: February 2003

We use a saddle point method to derive an asymptotic approximation to the adiabatic perturbation theory transition amplitude. This result is used to discuss the saturation of an ‘‘optical’’ collision when the field is resonant at an extremum of the potential energy difference. We point out that although the onset of saturation in this satellite occurs for lower field strengths than those needed to saturate a linear crossing, the satellite saturates more slowly than a linear crossing as the field strength is increased.
Chemiluminescent reaction between alkali dimers and oxygen molecules
Authors: Figger H. , Schrepp W. , Zhu Xu-hui
Published: February 2003

The reaction of alkali dimers with oxygen molecules was studied in a crossed-beam experiment. Weak luminescence from the crossing zone was observed. It was found that it was predominantly emitted by the electronically excited products of the reactions M2+O2→MO2+M* and M2+O2→MO*2+M (*=electronic excitation). Lower limits for the dissociation energies between M and O2 in the product molecules were derived from the cut-off wavelengths of the continuous chemiluminescence spectra observed. They are found to be consistent with the values calculated using an ionic model for the product molecule MO2.
A new semiclassical approach to the molecular dynamics: Label variable classical mechanics
Authors: Shi Shenghua
Published: February 2003

A new semiclassical approach to molecular collision dynamics is developed. In this approach the state of a system is described by a unit vector in Hilbert space. By choosing a reference unit vector and a continuous mapping, a correspondence between the vector in Hilbert space and the point in a label space is established, i.e., the vector in Hilbert space is parametrized or labeled by complex variables. It is shown that the label variables formally obey classical mechanics. Thus, the time evolution of the vectors in Hilbert space, i.e., the evolution of the states of the system, can be determined by calculating the evolution of the label variables classically. To illustrate this idea, the formalism for calculating the vibrational transition probability in the collinear collision A+BC is presented, and a demonstrative calculation for the collinear Secrest–Johnson model of He+H2 vibrationally inelastic scattering has been carried out. The comparison with the exact quantum results shows that the agreement is encouragingly good.
Studies of the photoionization cross section of the 2π level of nitric oxide
Authors: Smith Maile E. , Lucchese R. R. , McKoy V.
Published: February 2003

We present photoionization cross sections and asymmetry parameters for the 2π level of nitric oxide which are obtained from the direct solution of the e+NO+ collisional equations at the static-exchange level. These cross sections differ significantly from those obtained previously using a moment theory approach [J. J. Delaney, I. H. Hillier, and V. R. Saunders, J. Phys. B 15, 1477 (1982)]. The calculated cross sections show a broad nonresonant feature at a photon energy of 29 eV which is not as pronounced as observed experimentally. The σ shape resonance in our cross section occurs at 14 eV which is about 5 eV below the feature in the measured cross sections attributed to this shape resonance. The probable role of autoionization and vibrational averaging on these cross sections is also discussed.
Theoretical study of Jahn–Teller distortions in C6H+6 and C6F+6
Authors: Raghavachari Krishnan , Haddon R. C. , Miller Terry A. , Bondybey V. E.
Published: February 2003

Ab initio molecular orbital theory with the 6–31G basis set is used to study Jahn–Teller distortion effects in the benzene cation (C6H+6) and the perfluorobenzene cation (C6F+6). π-electron correlation is included in these calculations. Completely optimized Jahn–Teller distorted geometries are obtained for the ground electronic states of both C6H+6 and C6F+6. The stabilization energies resulting from these distortions are calculated for both systems. The stabilization is partitioned into contributions from C–C stretch, C–H or C–F stretch, C–C–C bend, and C–C–H or C–C–F bend coordinates. Detailed comparison is made between C6H+6 and C6F+6 to consider the effects of substitution. The calculated geometries and stabilization energies for C6F+6 are compared to values derived from laser-induced fluorescence experiments. The overall calculated distortion parameters for C6F+6 are in good agreement with the experimentally derived distortions, though the calculated C–C bond length change is somewhat larger than the experimentally derived value. The total calculated stabilization energy (≂1000 cm−1) is in good agreement with the experimental value (820±≂100 cm−1). The individual stabilization energy contribution for the C–C stretch mode is calculated to be larger than the experimental estimate, paralleling the overestimation of the C–C bond length change for this mode. The other stabilization parameters are in reasonably good agreement with the experimental values.
The 〈110〉 oriented FCl− molecule–ion in KFa)
Authors: Delbecq Charles J. , Hutchinson Elson , Yuster Philip H. , Marshall S. A.
Published: February 2003

The heteronuclear diatomic molecule–ion FC1− with its molecular axis oriented along a crystal face diagonal, i.e., FC1− 〈110〉, has been produced by ionizing radiation in single crystal KF doped with PbCl2 and KC1. Evidence to support this species identification as well as its orientation relative to the lattice is obtained from anisotropic optical absorption spectral measurements as well as from electron spin resonance (ESR) absorption spectra. Optical absorption spectral data, estimates of the molecular reorientation temperature, as well as ESR absorption spectral parameters are given for the FCl− 〈110〉. A qualitative explanation of the differences of the optical and ESR parameters of FCl− 〈111〉 and FCl− 〈110〉 is discussed.
Concentration dependence of diffusion in liquid crystals. A new experimental approach
Authors: Hakemi H.
Published: February 2003

The impurity diffusion of a chiral species into an aligned nematic film displays a unique textural behavior. A dynamical analysis of this type of texture provides a new approach to study the concentration dependence of the diffusion coefficients in liquid crystals. By a simple variable rearrangement in the solution to the Fick’s Law, we evaluated the diffusion coefficients from the linear portion of the typical x2k/t - ln t plot at various measurable concentrations (pitches). The results showed an inverse law between the diffusion coefficient and the concentration of the diffusion profile. Extrapolation to zero concentration would give a good approximation to the self-diffusion coefficient of the nematic solvent.
The quantal collision integrals and transport coefficients of dilute gaseous spin-polarized deuterium with spin-polarized hydrogen and the isotopes of helium
Authors: Lim T. K. , Maurone P. A.
Published: February 2003

The collision integrals and transport coefficients of spin-polarized deuterium D↑ and its admixtures with spin-polarized hydrogen H↑ and the isotopes of helium are calculated with representative Lennard-Jones interatomic potentials at temperatures below 2 K. Diffractive features previously predicted to be significant in H↑ are absent here in D↑ and its admixtures.
Laser spectroscopy of the 1A′′–X̃ 1A′ system of CuOH and CuOD
Authors: Trkula Mitchell , Harris David O.
Published: February 2003

The green bands of CuOH and CuOD have been studied by chemiluminescence and laser-excitation spectroscopies. CuOH and CuOD were produced by means of a recently developed hollow-cathode sputtering source. The transitions probed by laser-excitation spectroscopy were found to arise from the (0,0,0)–(0,0,0) band of a 1A′′–X̃ 1A′ electronic transition. The rotational analyses of the (1,0) and (0,1) rotational subbands of this system is presented. The molecules are well described as near prolate asymmetric rotors and are bent in both the ground and excited electronic states with bond angles of 111(1)° and 119(1)°, respectively.
The electronic structure and the He(I) photoelectron spectra of biferrocenylene and biferrocene
Authors: Böhm Michael C. , Gleiter Rolf , Delgado-Pena Federico , Cowan Dwaine O.
Published: February 2003

The electronic structure of biferrocenylene (1) and biferrocene (2) has been studied by means of He(I) photoelectron (PE) spectroscopy, and by means of semiempirical MO calculations of the INDO type. Large reorganization effects are predicted for ionization events of strongly localized MO’s with pronounced Fe 3d amplitudes. ΔSCF and ‘‘transition operator’’ (TO) calculations converged into hole states with localized wave functions; the delocalized Hartree–Fock (HF) orbitals represent not the true minimum on the HF surface. The instabilities of the symmetry-adapted orbitals are analyzed in the framework of the one-particle Green’s function formalism and are compared with core hole-state calculations on O+2 and N+2. The formal correspondence between the weakly coupled Fe dimers and O+2/N+2 is demonstrated. The nature of the metal–metal interaction is discussed. The estimation for the time of hole exchange in 1+ and 2+ leads to a classification for the Fe dimers which is in line with other theoretical models for mixed valence species.
Monte Carlo calculation of the intermolecular dipolar spin relaxation in a liquid solution
Authors: Fries P. H. , Belorizky E.
Published: February 2003

The time correlation function characteristic of the intermolecular dipolar spin relaxation in a liquid solution is calculated by a Monte Carlo diffusive simulation. The validity of the Monte Carlo method for modeling molecular dynamics is examined referring to the careful analysis by Hilhorst and Deutch of the Monte Carlo work on polymer kinetics of Verdier et al. The Monte Carlo estimates of the time correlation function are compared with those obtained by analytical solutions of translational diffusion equations. The normalized solutions of the Smoluchowski diffusion equation, including the effects of the nonuniform radial molecular distribution, are appropriate for describing the relative random walks of two given molecules in the liquid.
EPR study of europium ions in type A zeolite. The general classification of the EPR spectra of S-state rare-earth ions in disordered polycrystalline or glassy matrices
Authors: Iton L. E. , Brodbeck C. M. , Suib S. L. , Stucky G. D.
Published: February 2003

A study of polycrystalline A zeolite exchanged with Eu(OH)2 solution has been performed using EPR spectroscopy. Detailed computer simulation techniques have been used to simulate EPR spectra from first principles, with the explicit incorporation of random distributions in the crystal field interaction parameters. This method is equally well applicable to the analysis of the spectra of glasses. Two components in the spectrum of the dehydrated material have been distinguished. The first corresponds to three- coordinate Eu2+ ions at regular six-ring sites in the zeolite, giving rise to an axial EPR spectrum with a large (b02)avg value of 0.32 cm−1. The second corresponds to an orthorhombic EPR spectrum with a (b02)avg value of similar magnitude, and a rhombic component b22≂0.34 (b02)avg. This latter feature is attributed to three-coordinate Eu2+ ions located at six-ring sites, where one of the oxide ions in the ring has been protonated. The magnitudes of the axial components of these spin Hamiltonian crystal field terms are the largest reported to date for Eu2+ or Gd3+. A general classification into four types has been made for the S-state rare-earth ion EPR spectra in zeolites. The same classification is valid for other glassy or disordered polycrystalline hosts. The spectra types are distinguished by the average magnitude of the b02 component in the spin Hamiltonian, and the dominant symmetry as defined by the ratio λ′=‖ b22/b02 ‖. The use of the superposition model theory and the literature of structural information permit the correlation of each type of spectrum with a model of the local coordination of the cations in the host material.
Resonance Raman scattering of ethylene: Evidence for a twisted geometry in the V state
Authors: Ziegler L. D. , Hudson B. S.
Published: February 2003

The resonance Raman spectra of ethylene obtained with 212.8 and 193.4 nm excitation are presented. With these excitation frequencies, on the red edge of the N → V absorption system, strong enhancement of the even overtones of the torsional coordinate is found. In addition, scattering to the carbon–carbon double bond stretching and the symmetric CH2 scissors modes show enhancement. These results demonstrate that the CH2 groups lie in mutually perpendicular planes in the nuclear equilibrium configuration of the V state, while the C–C bond distance has been lengthened and the HCH angles reduced relative to those of the ground state. These results are discussed within the context of the G16(2) dynamic point group and recent calculations.
The evaluation of photochemical quantum yields by holography
Authors: Deeg F. W. , Pinsl J. , Bräuchle Chr. , Voitländer J.
Published: February 2003

The recently developed holographic technique for the investigation of solid-state photochemical reactions is applied to the determination of photochemical quantum yields. An explicit formula is derived for the case of a simple one-photon reaction which can be easily modified to handle more complicated reaction schemes. The only input parameters needed are the temporal growth curve of the photochemically formed hologram and the change in molar refraction. The last parameter can be obtained from the absorption spectra of educt and product species or from direct measurements of the index of refraction. The theory is applied to the phototransformation of o-nitrobenzaldehyde in a polymethylmethacrylate matrix. The resulting quantum yield is compared to that obtained by a conventional absorption spectroscopic procedure and found to be in good agreement. The advantages of the holographic technique compared to conventional absorption spectroscopy are discussed.
Ion–water interaction potentials for alkali metal cations and halide anions
Authors: Gowda B. Thimme , Benson Sidney W.
Published: February 2003

Empirical potential parameters for ion–water interactions have been constructed by three simple methods and successfully used for describing ion hydration. In the first two methods, we have computed the range parameter (ρij) by fixing the energy parameter at a particular value, in a potential of the form EREP =Aij exp(−Rij/ρij). For this purpose, we used either the experimental ion–water distances or one of the experimental enthalpies (ΔH0, 1). In the third method, we have developed a simple correlation scheme by which both the parameters (Aij and ρij) can be computed. Although the parameters obtained by all the three methods seem to describe the gas phase hydration energies equally well, the last method seems to be the best as it allows the computation of both the parameters. These procedures are useful in reproducing the experimental energetics from the experimental ion–water distances and vice versa without requiring parameters taken from other sources. The consistency of one can be checked by the other, independently. The range parameter for the repulsive interaction has been shown to be the most sensitive of all the parameters and hence plays a significant role in deciding the magnitudes of the energies and geometries for the hydration of the ions. Parameters derived from scattering experiments have also been used to describe the gas phase hydration energies but they are found to be unsuccessful in producing consistent results. The major result of the present investigations is that a simple correlation scheme can be developed for describing all interactions. This type of analogy is extremely helpful especially for explaining ion–ion, ion–atom, atom–atom, atom–molecule, molecule–molecule, or ion–molecule interactions.
The phase diagram of charged colloidal suspensions
Authors: Hone Daniel , Alexander S. , Chaikin P. M. , Pincus P.
Published: February 2003

The thermodynamics of a system of identical electrically charged spheres in colloidal suspension in a polar fluid (typically, H2O), are calculated within a model of point particles interacting via a screened Coulomb (Yukawa) potential of suitably renormalized strength. The particles are taken to be sufficiently massive that a classical theory is appropriate. The free energies of the crystalline phases are calculated within a mean field theory which is an extension of the self-consistent harmonic approximation. The melting curve is estimated from the semiempirical Lindemann rule, with mean squared displacements again calculated self-consistently. As functions of the relevant parameters: electrical charge of each sphere, density of spheres and added salt concentration, stable predicted phases include fcc and bcc crystals and a disordered fluid. Under suitable circumstances a reentrant bcc→fcc→bcc transition is predicted as salt is added, and bcc is predicted always likely to be the stable phase just below the melting transition.
Magnetic properties of Cr, Mo, and W-modified Mn3B4
Authors: Ishii T. , Shimada M. , Koizumi M.
Published: February 2003

Magnetic properties of Cr, Mo, and W-modified Mn3B4 were examined, and it was found that the ferromagnetic moment of (Mn1−xMx)3B4 (M=Cr, Mo, W) increased with increasing x (0≤x≤0.1), but the value of the effective Bohr magneton μeff was almost constant for the present compositional range studied and μeff was independent of the kind of substituted elements. This result suggests that ferromagnetism of the present solid solutions is due to canting of the antiferromagnetic sublattice of Mn 2(c) atoms of Mn3B4.
A note about a systematic study of the electrostatic potential at the nucleus and the interelectronic interaction in the atoms and ions
Authors: Castro Eduardo A. , Fernández Francisco M.
Published: February 2003

The change of the difference on the interelectronic interaction energy and the electrostatic potential at the nucleus obtained from Clementi’s near Hartree–Fock calculations on isoelectronic atomic series is discussed in base to the Linderberg’s Z-perturbation theory. Results agree with those derived from the alternative study made from the introduction of the concept of effective nuclear charge.
Infrared intensities of H3O+, H2DO+, HD2O+, and D3O+
Authors: Colvin Michael E. , Raine Gwendolyn P. , Schaefer Henry F. , Dupuis Michel
Published: February 2003

Infrared intensities of isotopic molecular ions H3O+, H2DO+, and D3O+ are reported.(AIP)
Metal–nonmetal transition in copper–xenon system
Authors: Eatah A. I. , Ghani Awad A. A.
Published: February 2003

Measurements of the electrical conductivity of the Cu-Xe system are reported. Met-nonmetal transition in the binary system Cu-Xe is studied with change of composition. The samples were prepared in a He crystat in vacuum by condensing simulatneously Cu-Xe gas and metal vapor. (AIP)
A Fourier method solution for the time dependent Schrödinger equation: A study of the reaction H++H2, D++HD, and D++H2
Authors: Kosloff R. , Kosloff D.
Published: February 2003

A new quantum mechanical time dependent integrator was used in the study of wave packet dynamics on potentials which include a deep well. The purpose of the study was to find the conditions, if any, for complex formation. The integrator, which is stable, conserves energy and norm and was used on the H++H2 system whose classical trajectory had been previously worked out. Almost no complex formation is found for the H++H2 system and its isotopic analogs. For high translational energies there was a good correspondence with the classical trajectory results, while for low translational energies where the classical trajectories become complex, the quantum calculations still show nonstatistical behavior. For even lower energies, a quantum effect took place leading to zero reactivity.
Rotational predissociation of H+.2 ions of different precursor origins
Authors: Brenton A. G. , Richard E. G. , Fournier P. G. , Beynon J. H.
Published: February 2003

The ro-vibronic states of ground state (1s σg)H+.2 that can fragment by the mechanism of rotational predissociation have been predicted theoretically and so have the lifetimes of these states. Six of these predicted quasibound states have previously been detected using a mass spectrometric method in which states are characterized by determining the translational energy released during the fragmentation process. In the present investigation, H+.2 has been prepared, not from hydrogen gas itself, but from a number of organic molecules by electron bombardment. It is shown that by this method, H+.2 appears in highly rotationally excited states and that the relative population densities of the states varies from compound to compound. The relative lifetimes of the fragmenting states have been investigated by varying the time after formation of the H+.2 ions in which observations of fragmentation are made. A total of nine quasibound states in H+.2 from methane were observed and assigned from theoretical calculations. Bound and quasibound states of H+.2 molecular ions have been calculated in the framework of the Born–Oppenheimer approximation. The energy levels and lifetimes with respect to predissociation of these quasibound states are determined. The effect of adiabatic corrections are estimated and discussed.
State-to-state differential and integral cross sections for vibrational-rotational excitation and elastic scattering of electrons by N2 at 5–50 eV: Calculations using extended-basis-set Hartree–Fock wave functions
Authors: Rumble John R. , Truhlar Donald G. , Morrison Michael A.
Published: February 2003

We have calculated differential, integral, and momentum-transfer cross sections for vibrational-rotational excitation as well as for pure rotational excitation, pure vibrational excitation, and elastic scattering for electron collisions with N2 at 5–50 eV impact energy. Scattering calculations were carried out in the fixed-nuclei approximation and excitation cross sections obtained using the sudden approximation. The interaction potential used in these calculations consists of three terms: static and local-exchange potentials, calculated from extended-basis-set Hartree–Fock wave functions as functions of internuclear distance, and a semiempirical polarization potential. Two different local exchange potentials were employed. The results are compared to each other, to previous calculations, and to experiment.
Density functional calculations for H2O, NH3, and CO2 using localized muffin-tin orbitals
Authors: Müller J. E. , Jones R. O. , Harris J.
Published: February 2003

Density functional calculations of forces and total energies have been performed for the ground states of H2O, NH3, and CO2 using the localized muffin-tin orbital (MTO) basis of Harris and Painter. Exchange-correlation effects are described using the local density (LD) approximation. It is shown that: (i) the localized MTO basis yields results of useful accuracy with a small number of functions; (ii) the LD approximation gives a remarkably accurate equilibrium geometry in each case; and (iii) details of the electronic structure, such as the inversion barrier in NH3, are reproduced with reasonable accuracy. The bonding in all three molecules is discussed in terms of the geometry dependence of the one-electron eigenvalues (Walsh diagrams).
Density functional calculations for low-lying states of CO2
Authors: Jones R. O.
Published: February 2003

Density functional calculations have been performed for the potential energy surfaces of the 1A1, 3B2, and 3A2 states of CO2, and for the related singlet–triplet splittings. The geometry (rCO=2.38 a0, αOCO=118°) and excitation energy (6.1 eV) of the lowest-lying 1B2 state agree well with experimental estimates. The energy surfaces are in accord with the qualitative predictions of Mulliken and Walsh.
The classical dynamics of three particles in hyperspherical coordinates
Authors: Johnson B. R.
Published: February 2003

Classical dynamics of the three body problem, formulated in hyperspherical coordinates, is investigated. Hamilton’s equations of motion are derived and then reduced from 12th order to eighth order. In addition to the general case in which the system moves in three-dimensional space, the special cases of planar motion, collinear motion, and zero angular momentum motion are studied and related. A brief description of the theory of small amplitude vibrations and normal modes in hyperspherical coordinates is also presented.
The quantum dynamics of three particles in hyperspherical coordinates
Authors: Johnson B. R.
Published: February 2003

A derivation of the quantum mechanical wave equation for the three body problem expressed in hyperspherical coordinates is presented. The coordinates, due to Smith and Whitten and later modified by Johnson [B. R. Johnson, J. Chem. Phys. 73, 5051 (1980)], are used in this study. The analysis is presented from a point of view that emphasizes the role of the three-dimensional configuration space that is associated with these coordinates. Three types of motion are analyzed: full three dimensional; motion restricted to a plane surface; and zero angular momentum motion.
Non-nearest neighbor random walks in reaction-diffusion processes
Authors: Musho Matthew K. , Kozak John J.
Published: February 2003

In this paper, we consider reaction-diffusion processes in which it is assumed that a migrating species (A) is subject to an interaction potential and may undertake nonnearest neighbor jumps in its diffusion towards a reaction center (B). For the case that the species A and B react irreversibly upon first encounter, we show how the efficiency of the process A+B → C is influenced by the nature of the interaction potential [a power law of the form V (r) =r−s with r≳0 and 1≤s≤12] and the geometry (dimensionality and spatial extent) of the reaction space assumed. Our approach is based on a recently introduced, exact theory of d-dimensional walks on finite and infinite (periodic) lattices with traps and, accordingly, the results presented in this study provide an exact quantitation of the interplay between potential interactions and system geometry for the reaction-diffusion problems considered. The results reported here may have considerable relevance to the problems of exciton migration in crystals, photosynthesis, and the surface diffusion of adatoms.
Anisotropic magnetic susceptibility of single crystal UCl4
Authors: Gamp E. , Edelstein N. , Malek C. Khan , Hubert S. , Genet M.
Published: February 2003

The anisotropic magnetic susceptibilities χ∥ and χ⊥ of UCl4 have been measured in single crystals between 1.6 and 350 K. Powder measurements of the average susceptibility were also performed but were found to be unreliable due to orientation effects. The single crystal results were interpreted in terms of the parametrized crystal field model with the following parameters: ζ=1819 cm−1; F2=42 632 cm−1, F4=38 680 cm−1; F6=23 320 cm−1; B20=−1008 cm−1; B40=1730 cm−1; B44=−2704 cm−1; B60=−2705 cm−1; B64=346 cm−1; α=29 cm−1; β=−638 cm−1; and γ=1617 cm−1. These parameters yield a Γ4(3H4) ground state and a paramagnetic first excited state Γ5(3H4) at 110 cm−1 and are in good agreement with the optical spectrum.
Statistics of the entanglement of polymers: Unentangled loops and primitive paths
Authors: Helfand Eugene , Pearson Dale S.
Published: February 2003

Attempts to apply the reptation and tube model to describe polymer disentanglement and associated relaxations have had wide success. However, questions have arisen in various applications as to the statistics of entanglement of a random walk with an obstacle net. Calculations are presented of a number of probabilities which characterize the degree to which a polymer, represented by a lattice walk, entangles with an array of obstacles. In particular, we have calculated the number of ways that such a walk can form unentangled closed loops of various types. If one reels in a general random walk from its ends, pulling out unentangled loops, one is left with the so-called primitive path, which is taken to represent the path of the tube. The probability that an N step random walk has a K step primitive path has been determined. Asymptotic formulas for this probability are presented.
Electron scattering from supersonic jet-cooled molecules
Authors: Doering John P.
Published: February 2003

First results are reported from an experiment designed to measure the electron energy loss spectrum of vibrationally and rotationally cooled molecules. Spectra were obtained for N2 and 2,3-dimethyl-1,-butadiene (DMB) seeded in helium and expanded in a free supersonic jet. Rotational cooling is demonstrated for several transitions in N2 whose optical absorption bands fall in the 972–954 Å region. Cooling is observed in a number of DMB bands in the 5.0–8.0 eV energy loss region. The use of electron scattering techniques to study further cooled molecular systems is discussed. (AIP)
Measurement of the J=0→1 rotational transitions of three isotopes of ArD+
Authors: Bowman Wayne C. , Plummer Grant M. , Herbst Eric , De Lucia Frank C.
Published: February 2003

Microwave spectra −ArD+ are reported. Rotational transitions in 40ArD+ and 38ArD+ are reported and analyzed. (AIP)
Relativistic effects in Cu2 bonding
Authors: Pélissier Michel
Published: February 2003

The use of effective core potentials derived from relativistic and nonrelativisitic atomic calculations exhibits an important relativisitic bond shortening in the case of Cu2.
Scattered light intensity cross correlation. I. Theory
Authors: Dhont J. K. G. , de Kruif C. G.
Published: February 2003

It is shown that in a so-called cross-correlation set up, an experimental technique where two laser beams and two detectors are used, double scattering contributions to the measured intensity correlation function are negligibly small compared to the first order contribution. Furthermore, it is shown that the theoretical work of Phillies concerning this subject is incomplete, due to the use of an incomplete Siegert relation. In doing this, an important second order term is overlooked. The cross-correlation set up is best suited for experiments at 90° scattering angle. At other scattering angles experimental difficulties arise, due to the necessity, as is shown in this paper, to position the two detectors at different angles with respect to the laser beams.
Absorption spectra, energy levels, and crystal-field analysis of trivalent neodymium in the γ phase of neodymium sesquisulfide (γ-Nd2S3)
Authors: Gruber John B. , Leavitt Richard P. , Morrison Clyde A.
Published: February 2003

We report absorption spectra of Nd3+ in the γ phase of Nd2S3. Energy levels of all manifolds from the ground state (4I9/2) through 2H11/2 (15 616 cm−1) were determined from the spectra. A best least-squares fit of the experimental levels was made with a crystal-field Hamiltonian of S4 symmetry, resulting in the following crystal-field parameters Bkm (in cm−1): B20=−209, B40=402, B44=1236, B60=−774, RB64=−423, and IB64=103. The rms deviation between calculated and experimental levels is 8.7 cm−1. A total of 50 levels was included in the fit (out of a possible 53 within the manifolds considered). The results are shown to be consistent with an effective point-charge model for the γ-Nd2S3 lattice in which Nd3+ vacancies are distributed randomly.
Molecular dynamical studies of the dissociation of a diatomic molecular crystal. I. Energy exchange in rapid exothermic reactions
Authors: Tsai D. H. , Trevino S. F.
Published: February 2003

We discuss the results of a study of the exothermic dissociation of a model diatomic molecular crystal. Our main purpose is to investigate the dynamics of energy transport and energy sharing in this system during the dissociation process. The crystal was prepared in a metastable molecular form, in thermal equilibrium at a low initial temperature and pressure. When we heated the system to a higher temperature, at constant volume, we observed thermally initiated dissociations which progressed rapidly to completion. During the dissociation process, we found that the sharing of the potential energy released by the metastable molecules with the rest of the system, and the sharing of the kinetic energy of the dissociated fragments with the kinetic energy of the molecules in the translational degrees of freedom, were rather efficient. But the intra- and intermolecular exchange of the kinetic energy among the various degrees of freedom, viz., translation-rotation, translation-vibration, and rotation-vibration was inefficient. Dissociation would occur in one of the regions of high local kinetic energy density, after a molecule in that region had acquired sufficient vibrational energy to break apart, and when the ‘‘caging’’ effect was favorable to allow the molecule to dissociate. From such a reaction site, and there may be others, the reaction would spread to a neighboring site, and continue this way until all the molecules became dissociated. The induction time showed an approximately logarithmic dependence on the inverse of the temperature of the system after heating. But during the process of rapid dissociation, both the potential energy and the kinetic energy of the system underwent rapid changes, and thermal equilibrium was not reached until the end of the process.
Two-photon-ionization mass spectroscopy of ammonia clusters in a pulsed supersonic nozzle beam
Authors: Shinohara Hisanori
Published: February 2003

Two-photon-ionization (TPI) with an ArF excimer laser at 193 nm, when combined with a mass spectrometer, is found to be a very sensitive method for the detection of ammonia clusters. Ammonia cluster distributions in the pulsed beam expansion have been measured via TPI by varying the delay time between the pulsed beam and the laser firing. The relative intensity of the clusters as observed by the TPI is a sensitive function of the delay time. Furthermore, seeding effects on the ammonia cluster distribution with He, Ne, Ar, and Kr carrier gases are examined. The distribution shifts toward high mass region with increasing mass of the seeding gases. Finally, an effective competition between the one-photon predissociation and two-photon ionization at the Ã 1A″2 state of the ammonia monomer (which is insignificant in the clusters) is found and discussed in terms of a simple rate-equation treatment valid in this system.
A classical model for the low-energy e±-H(1s) elastic scattering
Authors: Grujić P. , Tomić A. , Vučić S.
Published: February 2003

A new one-electron atom classical semistochastic model has been proposed and used for calculations of an electron (positron)-ground state hydrogen elastic scattering in the low-energy region. Differential cross sections are evaluated numerically for E=6.7 eV, within the classical trajectory, extended Monte Carlo method. The results for the electron impact appear reasonably good for θ≳90 °, but suffer from the forward scattering divergence. Except for the backward scattering, positron results turned out to be consistently higher than for the electron impacts. A thorough discussion of the classical approach and its perspectives in e±-H scattering is given and further possible elaborations are proposed.
The effective reaction rate and diffusion coefficients for a two-phase medium
Authors: Bedeaux D. , Kapral R.
Published: February 2003

The transport properties of a system composed of an ensemble of small particles moving in a two-phase medium are calculated. One phase is assumed to be finely dispersed in the other so that the homogeneous regions are small compared to the size of the system. The particles may diffuse and react (disappear) in either phase. The coarse-grained (effective) reaction rate and diffusion coefficients are calculated for arbitrary volume fractions of the two phases for various cases. The calculation utilizes the introduction of background rate and diffusion coefficients to motivate the definition of self- and local density fields. The introduction of these fields in conjunction with the assumption of the random distribution of the inclusions of one phase in the other leads to reasonable approximations for the effective transport coefficient for all volume fractions. The result of model calculations are presented.
Correlation effects in vibrational hole burning
Authors: Friedrich J. , Haarer D.
Published: February 2003

This paper presents a theoretical model which describes correlation effects in nonresonant photochemical holes. The model calculations are compared with experiments of photochemical hole burning (PHB) in electronically excited vibrational states of 1,4-dihydroxyanthraquinone (DAQ) in alcohol glass matrices. Vibrational hole burning shows some specific features such as the occurrence of several nonresonant holes in the electronic origin which differ considerably in their widths and integrated optical density from the resonant hole. The difference in linewidth is related to a vibrational inhomogeneity which is superimposed on the observed relaxation broadening. The experiments show that the vibrational inhomogeneity is not uniform but varies with the vibrational state.
1H NMR study of hydrogen motion in the β phase of the Mg2NiHx system
Authors: Hayashi Shigenobu , Hayamizu Kikuko , Yamamoto Osamu
Published: February 2003

The motion of hydrogen atoms in the β phase of Mg2NiHx was studied by the 1H pulsed NMR spectroscopy. Fourier-transformed spectra and spin-lattice relaxation times in the laboratory frame (T1) and in the rotating frame (T1ρ) for Mg2NiH3.85 were measured in the temperature range between 170 and 460 K. The 1H magnetization in the hydride relaxes by the dipolar modulation caused by the hydrogen motion and paramagnetic impurities. It was confirmed that all that hydrogen atoms in the hydride are equivalent with respect to their motion. The correlation frequencies were determined by the temperature dependences of the linewidth and the two relaxation times T1 and T1ρ, and they are in good agreement among them, giving a single activation energy 12 kcal/mol in the temperature range between 210 and 460 K. The 1H–1H dipole–dipole interaction exists in the motional narrowing region, which supports that the hydrogen motion is not a diffusion but a kind of rotation.
Static dipole moment of two coupled systems
Authors: Vigoureux J. M.
Published: February 2003

We present the expression for the dipole moment of a pair of nonidentical systems in terms of the frequency dependent polarizabilities and hyperpolarizabilities. Our calculations are quite general and include the two particular cases of the near-zone (London) and of the far-zone (Casimir) limits. We first consider the case oftwo disssimilar centrosymmetric systems and then, the more particular one of two spherical ones.
Thermodynamics and quantum corrections from molecular dynamics for liquid water
Authors: Berens Peter H. , Mackay Donald H. J. , White Gary M. , Wilson Kent R.
Published: February 2003

In principle, given the potential energy function, the values of thermodynamic variables can be computed from statistical mechanics for a system of molecules. In practice for the liquid state, however, two barriers must be overcome. This paper treats the first problem, how to quantum correct the classical mechanical thermodynamic values available from molecular dynamics, Monte Carlo, perturbation, or integral methods in order to compare with experimental quantum reality. A subsequent paper will focus on the second difficulty, the effective computation of free energy and entropy. A simple technique, derived from spectral analysis of the atomic velocity time histories, is presented here for the frequency domain quantum correction of classical thermodynamic values. This technique is based on the approximation that potential anharmonicities mainly affect the lower frequencies in the velocity spectrum where the system behaves essentially classically, while the higher spectral frequencies, where the deviation from classical mechanics is most pronounced, involve sufficiently harmonic atomic motions that harmonic quantum corrections apply. Thus, a harmonic quantum correction can be applied at all frequencies: at low frequencies where it is inaccurate it will be small, while at high frequencies where it is large it will also be relatively accurate. The approach is demonstrated by computation of the energy and constant volume heat capacity for water from classical molecular dynamics followed by quantum correction. The potential used to describe the interactions of the system of water molecules includes internal vibrational degrees of freedom and thus strong quantum effects. Comparison of the quantum corrected theoretical values with experimental measurements shows good agreement. The quantum corrections to classical thermodynamics (which are also derived for free energy and entropy) are shown to be important not only for internal vibrational motion, but also for intermolecular hindered rotational and translational motions in liquid water. They are presumably also important for other strongly associated molecules, including bimolecules, and thus should be included when comparing calculated and measured thermodynamic quantities. The approach illustrated here allows the calculation of thermodynamic quantum corrections for liquids, solutions, and large molecules such as polymers (including proteins and nucleic acids) with full inclusion of both intra- and intermolecular degrees of freedom.
Hydrogen diffusion and hydride formation at the metal–hydride interface
Authors: Butera R. A. , Weaver J. H. , Peterman D. J. , Franciosi A. , Peterson D. T.
Published: February 2003

Synchrotron radiation photoemission has been used to examine interface reaction of overlayers of V and Ca deposited onto clean surfaces of the bulk hydride YH2 and the deuteride NbD0.75. Changes in the hydrogen-induced bonding bands and the d bands near EF and variations in the intensities of the substrate core level emission as a function of metal coverage indicate that hydrogen diffuses from the substrate into the overlayer. The results are discussed in terms of the mechanism for hydride formation.
The freely jointed chain in expanded form
Authors: Galli Anthony J. , Brumage William H.
Published: February 2003

The distribution function for an exact treatment of an n-link freely jointed chain is obtained in expanded form to order 1/n3. Comparisons are made with modified Gaussian distribution functions of the same order.
Analysis of small angle neutron scattering spectra from polydisperse interacting colloids
Authors: Kotlarchyk Michael , Chen Sow-Hsin
Published: February 2003

In this paper, we outline a simple procedure for analyzing small angle neutron scattering (SANS) spectra from interacting colloids containing a continuous distribution of particle sizes. In particular, the effects of polydispersity on the apparent interparticle structure factor S′(Q) observed by SANS is investigated. We find that the oscillations in S′(Q) are significantly damped in comparison to those of the true structure factor S(Q). When our procedure is extended to the analysis of SANS spectra from nonspherical particles, we find that orientational averaging affects S′(Q) in a qualitatively similar way. The applicability of the procedure to the analysis of real data is demonstrated with spectra taken from water-in-oil microemulsions, ionic micelles, and interacting proteins.
Rotational R-branch Raman spectroscopy in CO2
Authors: Altmann K. , Klöckner W. , Strey G. , Finsterhölzl H. , Schrötter H. W.
Published: February 2003

Godfried and Silvera1; claim regarding the rotational Raman spectra of CO2, is proved incorrect. Several citations are given in support of the argument that these rotational Raman spectra of CO2 and other linear molecules have been observed before. (AIP)
Angle-resolved photoelectron spectroscopy of HCl from a photon energy of 16 to 80 eV
Authors: Carlson Thomas A. , Krause Manfred O. , Fahlman Anders , Keller Paul R. , Taylor James W. , Whitley Timothy , Grimm Frederick A.
Published: February 2003

Angle-resolved photoelectron spectroscopy was performed on HCl using synchrotron radiation over a photon energy from 16 to 80 eV. The partial cross sections and angular distribution parameters β were obtained for photoionization of both the 2π and 5σ orbitals. Multiple scattering Xα calculations were also carried out for the cross sections and β values. The calculations, together with previously published results on the cross section using a Hartree–Fock model, were compared with experiment and gave reasonable qualitative agreement. Both experimental and theoretical results were examined with particular regard to the nature of the Cooper minimum, and the differences between the behavior of the minima for the two orbitals are discussed in detail.