bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystal field effects on oxygen in solid methane and the catalysis of spin-species conversion of methane
Authors: Kim Janice J. , Pitzer Kenneth S.
Published: February 2003

The energy states are calculated for an oxygen molecule in a crystal field of cubic symmetry that would be appropriate for a substitutional site in solid methane. The resulting heat capacity is also calculated and discussed in relation to data for oxygen-doped methane. The catalytic effect of oxygen on the spin-species conversion of methane is treated by consideration of the dipole–dipole interaction of the electron spin of oxygen with the proton spin of methane. The matrix elements are calculated for the low-energy states of methane and of oxygen. It is found that oxygen should be an effective catalyst for spin-species conversion above about 0.3°K but that its effectiveness may decrease rapidly below that temperature.
The SPF–Dunham expansion for the potential well: A regression model for systematic analysis of differential elastic beam scattering cross sections
Authors: Bickes R. W. , Bernstein R. B.
Published: February 2003

The Simons–Parr–Finlan (SPF) modified Dunham expansion of the interatomic potential is utilized for a systematic least squares (nonlinear regression) analysis of thermal energy differential cross section data to yield a reliable determination of the interaction potential well V (r). The procedure provides potentials which produce excellent fits to both computer-generated differential cross sections and experimental angular distributions (from the literature). As an additional check, all eigenvalues calculated from the deduced potential wells are found to agree (within ?0.5 cm−1) with the known values, further indicating that the SPF potential well is nearly indistinguishable from that of the true V (r). In contrast to many ’’inversion’’ procedures in the literature, the present iterative method represents a straightforward, practical way to deduce V (r) from angular distribution data. In addition, the SPF expansion provides quite accurate analytical representations of the interatomic potentials for strongly bound diatomics (e.g., H2, CO, and I2) valid over a wide range of r, reproducing the exact eigenvalues to an accuracy within ?0.1% (of the well depth).
Quantum mechanical study of electronic transitions in coplanar atom–diatom collisions: Quenching of fluorine by H2
Authors: DeVries Paul L. , George Thomas F.
Published: February 2003

A coplanar quantum mechanical study of rotational energy transfer in the quenching of the fluorine atom by H2 is carried out. While constrained to a fixed plane, the internal nuclear degrees of freedom of the system include rotation as well as vibration. The electronic degrees of freedom are represented by three interacting semiempirical potential energy surfaces. The study indicates that the primary factors in the relative ordering of the magnitudes of state-selected cross lengths are the magnitude and sign of the energy defect, and the existence (or nonexistence) of nearby thresholds.
Influence of the potential function on the determination of multipole moments from pressure-induced far-infrared spectra
Authors: Cohen E. Richard , Birnbaum George
Published: February 2003

The effect of the choice of the potential function on the evaluation of molecular multipole moments from pressure-induced far-infrared spectra is investigated. The analysis is restricted to low densities where collisions are predominantly bimolecular. The quadrupole and hexadecapole moments of N2 and O2, and the octupole and hexadecapole moments of CH4 and CF4 are evaluated from far-infrared data using potential models consisting of a spherically symmetric part represented by the Lennard-Jones, Kihara, and m–6–8 potentials and an anisotropic part representing the electrostatic interactions of the permanent multipole moments. In general, although different values of the multipole moments are obtained with different potentials, for similar molecular diameters the variation is not great.
Electron tunneling, charge transfer, and the intermolecular forces between two H2 molecules
Authors: Gallup G. A.
Published: February 2003

The spherically averaged van der Waals potential for the system H2–H2 has been calculated using a nonorthogonal CI method based on SCF orbitals for the individual molecules. With suitable restrictions on the CI calculation we have isolated various contributions to the van der Waals potential, and have found that charge transfer and dispersion effects are comparable to distances corresponding to the potential minimum. The charge transfer contribution is further discussed in terms of a mechanism involving electron tunneling through the intervening space between the molecules. Our calculation gives a spherically averaged van der Waals well depth of 2.96 meV at 3.49 Å. Experimental values are 3.00 meV at 3.49 Å.
The semiconductor model of electric conductivity applied to the supercritical mercury vapor
Authors: Popielawski Jan , Gryko Jan
Published: February 2003

The semiconductor model of electric conductivity in dense metallic vapors is modified and applied to the hard-sphere fluid with hard spheres preserving electronic properties of mercury atoms. This fluid models the supercritical mercury vapor in low density limit of the semiconductor regime. Supplementary calculations of the total scattering cross section of slow electrons on mercury atoms are described. The calculated activation energy of the electric conductivity and the values of electric conductivity are in a qualitative accord with the existing experimental data for supercritical mercury vapors.
Thermotransport of 85Sr and 54Mn in potassium chloride single crystals
Authors: Williams S. J. C. Rushbrook , Allnatt A. R.
Published: February 2003

The thermotransport of 85Sr and 54Mn tracers in single crystals of KCl was measured in the temperature range 950–1025 K, by means of Soret effect measurements. The mean value of the impurity heat of transport was −3.18±0.09 eV for Sr in pure or Sr-doped crystals, and was −5.45±0.83 eV for Mn in pure or Mn-doped crystals. There was no significant difference between values from pure and doped crystals for Sr, but for Mn this may not be the case. A discussion of the experimental and theoretical limitations of such measurements as a source of information about the heats of transport for individual jumps was given, and a comparison made with earlier thermotransport measurements in simple ionic solids.
The Raman spectrum of the ordered phase of NH4Cl and ND4Cl: Dipole and polarizability derivatives
Authors: Fredrickson L. R. , Decius J. C.
Published: February 2003

The polarized Raman spectra of NH4Cl and ND4Cl have been observed at temperatures of the order of 25–35 K such that the crystal is in the ordered Td phase. The transverse, longitudinal frequency separations for modes ν3, ν4, ν5 (the lattice translation) and the Fermi resonant combination ν2+ν4 in NH4Cl have been used to deduce the infrared intensity parameters ∂μ/∂Q. Then, using these quantities together with an estimate of the electronic contribution to the nonlinear susceptibility, and measurements of the relative intensities of longitudinal, transverse Raman line pairs, we calculate ∂α/∂Qj for j=3, 4, 5. Since only the mode j=5 is well determined by this procedure, the final values of the polarizability derivatives are based upon mode 5 as an internal standard along with the intensities of the other modes relative to 5. The theory is developed on the assumption of additive molecular polarizabilities, and the ∂α/∂Qj values reported are those for vanishing local field. Both the ∂μ/∂Q and ∂α/∂Qj parameters appear to satisfy simple isotopic intensity rules. The value found for ∂α/∂Q1 is about 0.4 of the corresponding value in gaseous CH4. Several combination and overtone bands are partially analyzed using polarization, and the components of 2ν6 are shown to be in excellent agreement with theory.
Statistical phase space theory of polyatomic systems: Rigorous energy and angular momentum conservation in reactions involving symmetric polyatomic species
Authors: Chesnavich Walter J. , Bowers Michael T.
Published: February 2003

Classical formulas for the sum and density of rotational and rotational–orbital states pairs of polyatomic molecules are derived using a geometrical approach that rigorously conserves energy and angular momentum. The pair combinations considered include a spherical top in combination with either a linear, spherical, or symmetric top molecule. With the formulas presented here it is possible to perform exact classical statistical phase space theory calculations for bimolecular reactions involving polyatomic molecules. In suitable limits our results reduce to well-known classical expressions for rotational state densities and sums or to equations previously derived by Klots using an aprroximate phase space approach. Calculations are given that demonstrate the distortions produced in the system phase space by molecular oblateness and prolateness are usually small.
Matrix reactions of oxygen atoms with H2S molecules
Authors: Smardzewski R. R. , Lin M. C.
Published: February 2003

Ultraviolet and laser photolysis of dilute argon matrix samples containing O3 and H2S molecules at 8°K produced new infrared absorptions at 3425.0, 1177.0, 763.0, and 444.8 cm−1 which were assigned to the previously unobserved HSOH intermediate species, hydrogen thioperoxide. The HSOH molecule is believed to be formed by the following reaction in the argon matrix cage; {O+H2S→[H2S=O]→HSO+H→HSOH}cage. Extensive 18O and deuterium isotopic studies had shown this species to contain one oxygen atom and two nonequivalent hydrogen atoms, while the observation of the S–O stretching vibration at 763.0 cm−1 had confirmed the presence of a sulfur–oxygen single bond. The intense blue-violet chemiluminescence which appeared when the photolyzed matrix samples were warmed to 20°K was subsequently identified as SO2 phosphorescence (3B1→1A1). Fluorescence from 1B1→1A1 was not observed.
The Raman spectra of liquid and plastic CF4 and transition dipole–transition dipole interaction
Authors: Gilbert M. , Drifford M.
Published: February 2003

The liquid and plastic phases of CF4 are investigated between 77 and 185 K. Little difference is found between the Raman spectrum of the liquid at the melting point and that of the plastic crystal. In the liquid, the strong infrared line ν3(f2) exhibits a complex Raman profile due to a large transition dipole–transition dipole interaction, similar to the LO–TO structure usually encountered in the Raman spectra of ionic crystals. An analysis of the various factors influencing the ν3 profile is outlined, and other possible cases of the occurrence of strong transition dipole–transition dipole interaction in liquids are cited. The other depolarized lines ν2(e) and ν4(f2) appear symmetrical, temperature sensitive, and are subject to weak vibrational relaxation. They provide precise information on rotations which is used in the analysis of previous NMR results of Rugheimer and Hubbard.
Excluded volume effects in the stabilization of colloids by polymers
Authors: Middlemiss Keith M. , Torrie Glenn M. , Whittington Stuart G.
Published: February 2003

A self-avoiding walk on a lattice, confined between two planes, is examined as a model of the stabilization of a colloidal system by adsorbed polymer. If the walk interacts with neither plane, there is a repulsive force on the planes due to the decrease in the volume of configuration space available to the walk. This effect is larger for a self-avoiding walk than for a random walk. If the walk interacts with only one of the planes, with an attractive potential, there is a decrease in the magnitude of the stabilizing effect as the temperature decreases, but the force on the planes remains repulsive. If the walk interacts with both planes, with an attractive potential, there is a stabilizing effect at high temperatures but a destabilization (sensitized flocculation) at low temperatures.
Phase transitions in small clusters of atoms
Authors: Kaelberer J. B. , Etters R. D.
Published: February 2003

The Monte Carlo method is used to calculate the average thermodynamic properties of small (N?13) clusters of atoms. All cluster sizes studied exhibit fairly sharp solid–liquid, as well as liquid–gas, transitions. In addition, some of the larger clusters also undergo structural transitions between different isomeric forms. The solid–liquid transition temperature is determined by four independent tests. The melting points predicted by these tests do not differ by more than 5%.
Negative ions in CO2
Authors: Fansler T. D. , Colonna-Romano L. M. , Varney R. N.
Published: February 2003

Reaction rate coefficients for negative ions in CO2 according to the reaction O−+2CO2⇄kCBkBCCO3 − +CO2 have been evaluated experimentally over a range of 45⩽E/N⩽400 Td at 320±2 °K. Most of the measurements were made with a drift-tube mass spectrometer operated in a mode to gate electrons as well as negative ions. The attachment of electrons in the drift tube introduces details into the arrival time spectra of the ions that enhance the resolving power for evaluation of rate coefficients. At low E/N, kBC approaches (5±0.5) ×10−28 cm6 s−1, and declines slowly with increasing E/N. The value of kCB is negligible below E/N=300 Td, then rises steeply to ≃1×10−11 cm3 s−1 at 400 Td. Changes of gas temperature have been shown to act to shift the reaction equilibrium in the same sense that changes in E/N do. At E/N⩾200 Td, collisional detachment of electrons from O− occurs according to the reaction O−+CO2kBA→e−+CO2+O and the rate rises steeply with E/N. At 400 Td, kBA reaches 9×10−12 cm3 s−1.
HTFFR kinetic studies of the fate of excited BaO formed in the Ba/N2O chemiluminescent reaction
Authors: Felder William , Gould Robert K. , Fontijn Arthur
Published: February 2003

Rate coefficients for quenching kQ and intersystem conversion kT of excited BaO formed in the reaction Ba+N2O(1)→BaO+N2 have been measured for several collision partners. Methods are developed and applied here for obtaining quenching rate coefficients in chemiluminescent reactions involving a reservoir state. Experiments were performed between 450 and 1000 K at 1–120 Torr in a modified high-temperature fast-flow reactor (HTFFR). The rate coefficients (or their upper limits) are, in ml molecule−1 s−1 units: kArQ (600–1000 K) ?3×10−13; kHeQ (460 K) <3×10−13; kN2Q (600 K) = (4.8±2.0) ×10−12; kN2OQ (600 K) = (4.2±1.0) ×10−10; 2.5×10−10?kO2Q (600 K) ?7.0×10−10; kArT (600 K) ?1.5×10−11. Incidental data at ?1000 K indicate that k1?5×10−11, and that the Ba/O2 reaction has about the same rate coefficient. In addition to BaO emission, strong Ba atomic emission arising from energy transfer processes was observed for [Ba]≳1×1013 ml−1 at T?1000 K. Measurements of the spectral distributions of the Ba/N2O chemiluminescence as a function of [Ar], [He], and [N2] yield relations among vibrational relaxation rate coefficients: kv, kT, and kQ for specific BaO(A 1Σ) vibrational levels. The set of reactions currently in use to describe the Ba/N2O chemiluminescence is shown to be inadequate to account for the observed changes in BaO(A 1Σ−X 1Σ) spectral distribution with pressure.
Electronic and vibrational states of biphenyl crystals using two-photon excitation spectroscopy
Authors: Hochstrasser Robin M. , Sung H. N.
Published: February 2003

The two-photon excitation spectra are reported for biphenyl-h10 and biphenyl-d10 neat and mixed crystals at 77 and 1.8 K. The region studied involves the low energy electronic states in the 3000–2850 Å region. We have presented background in regard to the theory of the TPE anisotropy for crystals containing more than one molecule in the unit cell, and for k=0 exciton levels having g total symmetry. An important conclusion is that previous theoretical studies of two-photon anisotropy in crystals are shown to have neglected the effects due to birefringence in using real representations for the incident fields. Expressions valid for anisotropic crystals are given here. The 0–0 transition is the strongest band seen in two-photon excitation and is located at 33128 cm−1, coincident with the energy of a previously observed mixed magnetic–electric dipole transition. The anisotropy of the TPE was measured in all possible polarizations involving principal faces for the 0–0 transition of crystals at 77 K. The results showed that the state at 33128 cm−1 is of the 1B3g type in the idealized symmetry group D2h. Evidence for crystal field mixing was observed from studies on the ab face, where the oriented gas predicts zero intensity. A number of new vibrational states of 1B3g were identified and attributed to particular species on the basis of their two-photon fluorescence excitation anisotropy. These were 578 (b3g), 1049, 1451, and 1483 (Ag total symmetry). Our results confirm the existence of two electronic states of biphenyl in the 3000 Å region.
Electron paramagnetic resonance of divalent nickel in zinc fluosilicate
Authors: Rubins R. S. , Jani Shirish K.
Published: February 2003

EPR in single crystals of nickel doped ZnSiF6⋅6H2O has been studied at Ku band. The crystal field parameter D was found to vary from (−0.1325±0.0010)cm−1 at 4.2 °K to (−0.64±0.01)cm−1 at 302 °K. The magnitude of ∂D/∂T had a maximum of (0.0027±0.0001)cm−1/°K, decreasing to about 0.0015 cm−1/°K at room temperature. The spin–lattice relaxation rate was determined to vary as T2 from linewidth measurements between 77 and 325 °K. From the expression for the Raman process in the high temperature limit, the Debye temperature was deduced to be of the order of 100 °K.
A photochemical study of rotational state dependence by laser excitation of formaldehyde (? 1A2). I. Coriolis and singlet–triplet perturbation
Authors: Tang Kenneth Y. , Fairchild Paul W. , Lee Edward K. C.
Published: February 2003

A study of rotational dependence of radiationless transition rate and radical yield in the photodecomposition of formaldehyde, H2CO(? 1S2), was made with laser-induced fluorescence excitation spectroscopy and chemiluminescence excitation spectroscopy [Lewis, Tang and Lee, J. Chem. Phys., 65, 2910 (1976)]. The effect of perturbations on some rovibronic levels excited by the 220410 and 230410 transitions is examined.
Brownian dynamics: Its application to ionic solutions
Authors: Turq Pierre , Lantelme Frédéric , Friedman Harold L.
Published: February 2003

The method of molecular dynamics is applied to a solvent-averaged model of electrolyte solutions, described by a generalized Langevin equation. For Brownian motion without solute–solute interactions, we recover the characteristic features of an infinitely dilute solution. For interacting brownons, the results exhibit a noticeable dependence of the calculated self-diffusion coefficients on the influence of the Coulomb forces. The model system exhibits ion association when the Coulomb forces are made strong enough.
Injection and migration of protons in single crystals of KHSO4
Authors: Sharon M. , Kalia Anjan K.
Published: February 2003

KHSO4 single crystals are found to be pure ionic conductors with no electronic contribution towards the electrical conduction. A new dc electrolysis cell is designed to measure the transport number of protons, which is found to be 1 (TH+?1). The current–voltage characteristics along the c axis are interpreted in terms of a space-charge-limiting current. The number of thermally produced carriers n0 and the number of trap centers nt in pure KHSO4 crystal at 35 °C are calculated to be 8.70×1013 and 2.70×1014 cm−3, respectively. The effective mobility of protons at 35 °C is found to be 4.07×10−8 cm2 V−1⋅sec−1. On the basis of thermogravimetric studies it is confirmed that a KHSO4 single crystal undergoes an irreversible phase transition around 170 °C. The dc conductance of KHSO4 single crystals doped with cobalt II is studied in the temperature range of 25 to 165 °C. A break in the conductivity plot is observed suggesting the presence of extrinsic and intrinsic regions. The activation energies for these regions are 0.49±0.01 and 1.21±0.01 eV, respectively. The values for the activation entropy for these regions are −2.2×10−3 and 6.04×10−4 eV K−1, respectively.
Solvation processes of the electron in polar liquids. II. Continuum model
Authors: Tachiya M. , Watanabe H.
Published: February 2003

The solvation process of the electron in polar liquids is investigated within the framework of the continuum model. The medium is assumed to be a continuous dielectric characterized by the static dielectric constant εs, the optical dielectric constant εop, and the relaxation time τ. The motion of the excess electron and the relaxation of the medium are treated consistently. The former is treated quantum mechanically and the latter classically. The time evolution of the wavefunctions, polarization, energy levels, and transition energy is calculated.
Validity of approximate methods in molecular scattering. III. Effective potential and coupled states approximations for differential and gas kinetic cross sections
Authors: Monchick Louis , Green Sheldon
Published: February 2003

Two dimensionality-reducing approximations, the jz-conserving coupled states (sometimes called the centrifugal decoupling) method and the effective potential method, were applied to collision calculations of He with CO and with HCl. The coupled states method was found to be sensitive to the interpretation of the centrifugal angular momentum quantum number in the body-fixed frame, but that choice leading to the original McGuire–Kouri expression for the scattering amplitude—and to the simplest formulas—proved to be quite successful in reproducing differential and gas kinetic cross sections. The computationally cheaper effective potential method was much less accurate.
Liquid ammonia: Molecular correlation functions from x-ray diffraction
Authors: Narten A. H.
Published: February 2003

For nearly spherical molecules the x-ray scattering from liquids yields structure and correlation functions for molecular centers. The distribution of electron density in an ammonia molecular is very nearly spherical, and orientational correlation between molecules in the liquid is not ’’seen’’ by x rays. Structure and correlation functions for molecular centers (nitrogen atoms) are derived from x-ray data on liquid NH3 at 4 °C and tabulated. They provide a sensitive test for future work on a molecular theory of liquid ammonia.
Two-dimensional oscillators with C4v potential functions
Authors: Mathur S. N. , Harmony M. D.
Published: February 2003

A quantum-mechanical investigation of two-dimensional oscillators having C4v potential functions has been performed. The symmetry properties of such systems have been elucidated, and the state symmetries have been correlated with C∞v and C2v cases. Numerical calculations are presented to illustrate the behavior of the energy levels as a function of the potential energy coupling term. The results show clearly how degeneracies arise and are broken as the coupling term varies between limiting cases. The C4v case that has been numerically treated shows both double-minimum and fourfold tunnelling phenomena, and would be an appropriate zero-order model for describing the amino wagging motions of a molecule such as hydrazine.
Phase behavior of multicomponent fluids
Authors: Fleming P. D. , Vinatieri J. E.
Published: February 2003

The phase behavior of a system of five or more components cannot be totally represented in three dimensional space. The most common method of projection representing the phase behavior of such systems employs a smaller number of ’’pseudocomponents.’’ Such a representation must be chosen with care because the multiphase region is less well defined than that of the corresponding true component system. Several examples are discussed in which ambiguous results are obtained when a system is treated as if it consisted of an incomplete set of pseudocomponents. The ordinary lever rule has been generalized to apply to three-phase systems, making it possible to calculate the mass fractions of three phases from their compositions. For true ternary systems this lever rule can be inverted to yield the compositions of the three phases in terms of the mass fractions at three observation points. For more than three observations on pseudoternary systems the deviations from true component behavior can be estimated from the spread in the calculated phase compositions obtained from different triplets. The method is illustrated for a true ternary system and two pseudoternary systems. The lever rule and its inverse may also facilitate an approach to a tricritical point where three fluid phases become identical.
Conformation maps of some saturated six and seven membered rings
Authors: Graveron-Demilly D.
Published: February 2003

Using a new semirigid model, conformation maps of cyclohexane, p-dioxane, m-dioxane, and cycloheptane are recalculated. The ideal out-of-plane coordinates are used together with in-plane coordinates and are found to give the correct energies of all the significant conformations of six membered rings and of the chair–twist–chair form of cycloheptane. Conformation maps display higher symmetry than maps previously reported.
A search for the infrared fundamental of matrix-isolated XeF
Authors: Ault Bruce S. , Andrews Lester , Green David W. , Reedy Gerald T.
Published: February 2003

Various xenon/fluorine mixtures were photolyzed in solid argon matrices and examined in the infrared spectral region with either a Fourier transform spectrometer or with a grating spectrometer. After photolysis, a strong peak was observed at 215 cm−1 in addition to the known XeF2(ν3) peak at 549 cm−1. The appearance of the same two bands in Ar matrices containing the vapors over XeF2 indicated the assignment of the 215 cm−1 band to XeF2(ν2). The fundamental of XeF was not observed either because it is masked by the XeF2(ν2) peak or because it has a small infrared absorption coefficient.
Relaxation processes and spectra in liquids and dense gases
Authors: Talin Bernard , Galatry Louis , Klein Lewis
Published: February 2003

The spectral response of a molecule dissolved in a liquid or dense gas is calculated using a correlation function approach. It is shown that the characteristic spectra of the molecule can be used as a probe of the molecular motions and the dynamic structure effects. A method developed by Blume and Kubo which assumes the solute–solvent interaction parameter to be a stochastic process is employed. The induced Q branch in the vibration–rotation spectra of polar molecules is discussed as an example and the cases in which the rotational relaxation of the molecule is determined by either the dynamic structure or by collisional processes are obtained in the appropriate limits.
Semiclassical theory of electronic transitions in molecular collisions: Combined effects of tunneling and energetically inaccessible electronic states
Authors: Laing John R. , Yuan Jian-Min , Zimmerman I. Harold , DeVries Paul L. , George Thomas F.
Published: February 2003

The semiclassical theory of collision-induced electronic transitions is extended to describe tunneling and the effect of energetically inaccessible states. The powerful technique of analytic continuation is utilized to yield very good agreement with exact quantum mechanical results on some model systems. The inaccessible states have a significant effect on transmission coefficients and thus can be important in semiclassical studies of molecular reactions.
Analysis of the matrix induced changes in the optical spectra of matrix isolated noble metal atoms
Authors: Forstmann F. , Kolb D. M. , Leutloff D. , Schulze W.
Published: February 2003

The absorption spectra of the noble metal atoms Cu, Ag, and Au isolated in Ar, Kr, and Xe matrices have been measured as a function of matrix temperature in the range between 3 K and about 0.4 of the respective matrix triple point temperature. The spectra show a pronounced energy shift of the absorption peaks with temperature which is entirely reversible in well annealed matrices in the above-given temperature range. These spectra were evaluated by crystal field arguments to obtain more direct information on the matrix influence, which is discussed in terms of difference in level shifts, change in spin–orbit splitting, and crystal field splitting. The results are then explained by a model in which the matrix atoms are treated as clouds of negative charge which introduce a repulsive potential in the region of the metal electrons when the metal atom is squeezed into the matrix cage. The change with temperature in the interaction between metal atom and matrix is discussed in detail. In addition, an unambiguous assignment of the optical transitions under consideration has been made for the metal atoms in the crystal field of the matrix.
Molecular dynamics of the rough sphere fluid. II. Kinetic models of partially sticky spheres, structured spheres, and rough screwballs
Authors: Berne Bruce J.
Published: February 2003

The rough sphere model is extended and suitably generalized to collisions between partially rough spheres, ’’spheres’’ with anisotropic mass distributions, and spheres with domains of roughness and smoothness distributed in such a way as to model polyatomic models. These models are used to compute various relaxation and correlation times. In addition, a model is devised for the interaction of a chiral molecule and nonchiral solvent molecules. This model is used to compute the couplings between translations and rotations. In treating the chiral systems we extend the independent binary collision model to the multivariate case.
Properties of helium isotopes adsorbed on uniform surfaces. I. Isolated atoms
Authors: Derderian Edmond J. , Steele William A.
Published: February 2003

Experimental data are reported for the submonolayer adsorption of 4He and 3He on the basal plane of a graphitized carbon black and on this substrate after coating with one monolayer of solidified xenon. The results obtained for these four systems include heats of adsorption as a function of coverage, Henry’s law constants, and derived properties of an isolated atom interacting with the solid, such as the average energy, the Slater sum, and the nonconfigurational entropy. Theoretical calculations of the gas–solid potential energy surfaces are reported and compared with previous calculations. The eigenstates for 4He and 3He that have been obtained by other workers for these systems were then used in direct theoretical calculations of the experimental quantities. Comparison of theory and experiment is used to comment upon the choice of parameters in the calculated gas–solid potentials for these systems.
Raman spectroscopic studies of yttrium (III) chloride–alkali metal chloride melts and of Cs2NaYCl6 and YCl3 solid compounds
Authors: Papatheodorou G. N.
Published: February 2003

The Raman spectra of molten YCl3–A Cl (A=Cs, K, Li) mixtures have been measured at different compositions and temperatures up to 890 °C. The Raman spectra of polycrystalline Cs2NaYCl6 and YCl3 were also measured from 25 °C to temperatures above melting. The factor group analysis of crystalline Cs2NaYCl6 was used to identify the three Raman active modes (ν1, ν2, ν5) of the YCl6−3 octahedra. For liquid mixtures rich in alkali halide, the predominant features of the spectra are characterized by a polarized and a depolarized band with frequencies near the ν1 and ν5 frequencies of the YCl6−3 octahedron and thus indicates the existence of such species in the melt. In melts containing above 25% YCl3 a new polarized band D appears in the spectra which shifts continuously and rapidly to higher energies with increasing YCl3 concentration. The frequency shift is attributed to a continuous distortion mechanism of the YCl6−3 octahedra by the neighboring yttrium ions. The continuous frequency shift of the D band and a comparison of the liquid and solid Raman spectra of yttrium chloride suggest the existence of lattice-type modes in these melts.
Surface structures of normal paraffins and cyclohexane monolayers and thin crystals grown on the (111) crystal face of platinum. A low-energy electron diffraction study
Authors: Firment L. E. , Somorjai G. A.
Published: February 2003

The surfaces of the normal paraffins (C3–C8) and cyclohexane have been studied using low-energy electron diffraction (LEED). The samples were prepared by vapor deposition on the (111) face of a platinum single crystal in ultrahigh vacuum, and were studied both as thick films and as adsorbed monolayers. These molecules form ordered monolayers on the clean metal surface in the temperature range 100–220 K and at a vapor flux corresponding to 10−7 Torr. In the adsorbed monolayers of the normal paraffins (C4–C8), the molecules lie with their chain axes parallel to the Pt surface and Pt[11̄0]. The paraffin monolayer structures undergo order–disorder transitions as a function of temperature. Multilayers condensed upon the ordered monolayers maintained the same orientation and packing as found in the monolayers. The surface structures of the growing organic crystals do not corresond to planes in their reported bulk crystal structures and are evidence for epitaxial growth of pseudomorphic crystal forms. Multilayers of n-octane and n-heptane condensed upon disordered monolayers have also grown with the (001) plane of the triclinic bulk crystal structures parallel to the surface. n-Butane has three monolayer structures on Pt(111) and one of the three is maintained during growth of the crystal. Cyclohexane forms an ordered monolayer, upon which a multilayer of cyclohexane grows exhibiting the (001) surface orientation of the monoclinic bulk crystal structure. Surface structures of saturated hydrocarbons are found to be very susceptible to electron beam induced damage. Surface charging interferes with LEED only at sample thicknesses greater than 200 Å.
Classical dynamics of triatomic systems: Energized harmonic molecules
Authors: Parr C. A. , Kuppermann A. , Porter R. N.
Published: February 2003

The dynamical assumptions underlying the Slater and RRK classical-mechanical theories of unimolecular reaction rates are investigated. The predictions of these theories for several nonlinear, triatomic, harmonically bonded molecular models are compared with the results obtained from the integration of the classical equations of motion. The accuracy of the small-vibration and weak-coupling assumptions are found to break down at energies above about one-quarter of a bond dissociation energy. Nonetheless, the small-vibration approximation predicts reaction frequencies in good agreement with the exact results for the models. The effects of rotation on intramolecular energy exchange are examined and found to be significant.
Franck–Condon factors in studies of the dynamics of chemical reactions. III. Analysis of information theory for vibration–rotation distributions and isotopic branching ratios
Authors: Schatz George C. , Ross John
Published: February 2003

A previously developed approximate theory of chemical dynamics based on generalized Franck–Condon factors is used to study the information theoretic analysis of vibration–rotation distributions and of isotopic branching ratios. We begin by examining the surprisal function I obtained from the Franck–Condon factors for rotational and vibrational distributions. For rotational distributions we find linear surprisal behavior for low rotational excitation in the limit of strong potential and weak kinematic coupling, but nonlinear surprisals for high rotational excitation in that limit. In addition, nonlinear rotational surprisals are generally obtained for any degree of rotational excitation in the limit of strong kinematic and weak potential coupling. We find these generalizations from the Franck–Condon factors and their applications to the H+H2, F+H2(D2), and H+Cl2 reactions. For F+H2(D2), nearly microcanonical rotational distributions are obtained (for low j′), due to the cancellation of contributions from the angular coordinate overlap factor [which leads to a positive slope (temperature) parameter ϑ] and centrifugal stretching effects (which lead to negative ϑ). For vibrational distributions linear surprisals are obtained for F+H2(D2), where little of the reaction exoergicity is released in the exit channel and the region of maximum overlap of reagent and product wavefunctions is highly localized, but not for H(D)+Cl2, which has a higher repulsive energy release (in the terminology of Polanyi and co-workers) and a more delocalized overlap. For both rotational and vibrational surprisals, we find that linearity occurs when the potential constrains the reaction to occur through a highly localized set of nuclear configurations (and hence in the limit of strong potential coupling and of highly localized overlaps). In our study of branching ratios, we consider the isotopic branching in F+HD→FH(FD)+D(H). We first show that the purely dynamical Franck–Condon factor provides a correct qualitative description of the branching ratio (especially its dependence on reagent rotational excitation). We then use information theory to predict the same ratio, and find some points of similarity to the purely dynamical result (such as the dependence on parameters of the product state distributions), but also certain important points of difference (such as dependence on degree of reagent rotational excitation). These points of similarity and difference may be reinterpreted in terms of the relative contribution of strongly coupled potential and kinematic effects, respectively, and the success of simple information theoretic branching ratio predictions depends on the relative importance of these factors.
Energy transfer in the 1A
u state of glyoxal. III. Deuterium effects
Authors: Zittel P. F. , Lineberger W. C.
Published: February 2003

Collision-free lifetimes and collisional quenching rate constants are reported for a number of single vibrational levels of glyoxal-d2 (1Au) and glyoxal-hd (1A″) and are compared with previous results for glyoxal-h2. There was some success in correlating the dependences of collision-free decay rates on specific vibrational level and on isotopic substitution with the predictions of intramolecular, nonradiative decay theory. There are strong isotope effects on the collision-free lifetimes, but the rate constants for collision depopulation of a vibronic level, which increase sharply with energy in the excited state, are strikingly isotope independent. The increase is believed to be primarily a result of increased intersystem crossing rate and is compared with a mixed state model proposed by Freed.
The temperature dependence of ’’single collision’’ bimolecular beam–gas chemiluminescent reactions. I. Theory
Authors: Preuss D. R. , Gole James L.
Published: February 2003

We consider a ’’single collision’’ bimolecular beam–gas chemiluminescent reaction in which a metal beam formed effusively intersects a tenuous atmosphere of oxidant gas (10−4 to 10−6 torr) resulting in the emission of visible radiation from excited electronic states of the reaction products. The kinetics of this beam–gas chemiluminescent reaction are studied in order to derive the relationship between the chemiluminescent intensity and the parameters of the beam–gas reaction. A formula is derived which relates the relative single collision chemiluminescent intensity to reactant mass and temperature, the enthalpy of vaporization or sublimation of the (metal) beam source material (ΔHsub, ΔHvap), and the Arrhenius activation energy (Eexp) for formation of products in particular, chemiluminescing, excited electronic states. The determination of the temperature dependence of the observed chemiluminescence provides an upper bound to ΔH vaporization (ΔHsub). If ΔH is accurately known through independent studies, and heat capacity data is available, Eexp may be evaluated. The significance of Eexp determined from chemiluminescence experiments is considered for metal beams with and without thermally populated internal states.
Sedimentation and density gradients near the liquid–liquid critical solution point in high gravitational fields
Authors: Block Thomas E. , Dickinson Eric , Knobler Charles M. , Schumaker Verne N. , Scott Robert L.
Published: February 2003

Experiments in the ultracentrifuge and theoretical analysis both strongly suggest that, in a binary liquid mixture near its liquid–liquid critical solution point, the sedimentation rate diverges approximately as (T−Tc)−0.6. As a result of the divergence of the sedimentation rate, a density inversion (a negative value of dρ/dh) can occur at temperaures substantially above the equilibrium critical solution temperature. The turbulence resulting from the instability can give the appearance of phase separation.
Vibronic states in fluorobenzene crystal spectrum
Authors: Pierre M.
Published: February 2003

A complete assignment of the vibronic transitions corresponding to the 2643 Å 1A1→1B1 absorption of the fluorobenzene crystal is given. The details of the spectrum are explained within a resonance model between two-particle states and phonon-sideband states of vibronic levels. Temperature studies of the pure electronic transition show an unusual change of integrated intensity, attributed to localized excitation effects.
Silver atom solvation and desolvation in ice matrices: Electron spin resonance studies of radiation-produced silver atoms formed at 4 K
Authors: Kevan Larry , Hase Hirotomo , Kawabata Keisuke
Published: February 2003

Silver ions rd by electrons at 4 K in aqueous matrices produce silver atoms with an Ag+ solvation shell. Brief warming to 77 K causes complete solvation of Ag0 which appears to correspond to water molecule rotation around an OH bond to hydrogen bond with Ag0. This atom solvation process can be reversed by optical excitation at 400 nm in the Ag0 absorption band.(AIP)
Relativistic corrections for H2+ and its isotopes
Authors: Bishop David M.
Published: February 2003

The relativistic correction to the potential energy curve of HD+ and H2+ is calculated. The corrections are an order of magnitude more accurate than these reported earlier.(AIP)
The effects of optimization and scaling of AO exponents on molecular properties
Authors: Goddard John D. , Csizmadia Imre G. , Mezey Paul G. , Kari Roy E.
Published: February 2003

The combined effects of direct optimization and various scaling techniques of Gaussian AO basis sets on the calculated molecular wavefunctions were analyzed for two examples, the hydrogen fluoride and ammonia molecules. An 8s4p Gaussian basis set for the fluorine atom which had been rigorously optimized for the total atomic energy by the conjugate gradient method was employed. For nitrogen, a similar atom optimized 9s5p basis set was employed. Three basis sets obtained in the course of the exponent optimization, the final optimized basis set, and basis sets derived from the optimum AO basis sets by various scalings of the exponents were employed in molecular calculations. The total energies and numerous one-electron properties were analyzed. The convergence of molecular one-electron properties to their limiting values is much slower than the energy convergence, and basis sets optimized for energy may be improved in terms of their property predictions by rigorous optimization. The simple scaling procedure slightly improved the total molecular energy but not necessarily the one-electron properties relative to the molecular calculation which employed the fully optimized atomic bases. Though it was generally assumed that with proper scaling of optimum AO basis sets, significant improvement can be achieved in a molecular wavefunction, the generality of this assumption is brought into question by the present examples of the hydrogen fluoride and ammonia molecules. Basis sets optimized directly by the conjugate gradient method for atoms appear to be a good compromise if the overall quality of the molecular basis is of interest and not just the total energy.
The structure of molecular effective potentials in compounds of heavy elements, with application to I2
Authors: Ewig Carl S. , Osman Roman , Van Wazer John R.
Published: February 2003

A new expression of the Phillips–Kleinman pseudopotential for many-valence-electron molecules is proposed. All remaining terms in the valence Hamiltonian, represented by a local potential, are evaluated from ab initio expressions based on the form of the atomic Fock operator. Sample calculations are reported for the orbital energies, equilibrium bond length, and vibrational frequency of the ground state of the I2 molecule.
The pairwise correlated generalized valence bond model of electronic structure. IV. Low lying S states of two-electron atoms and ions
Authors: Light Stuart L. , Petersson G. A.
Published: February 2003

Quantitative tests for theories of electron correlation are rare, since exact nonrelativistic energies are generally unknown. An important exception to this dilemma is found in the case of two-electron atoms. Accurate nonrelativistic energies are available for ground and excited states of two-electron atoms and ions. Comparison with these exact results provides an ideal testing ground for a theory of electron correlation. We have, therefore, carried out SCF calculations on the (1s,1s)1S ground state and on the (1s,2s)1S and (1s,2s)3S excited states of two-electron atoms for a wide range of nuclear charges. The resulting correlation energies (i.e., EEXACT−ESCF) are then compared to the overlap approximation we developed in an earlier paper. The observed correlation energies agree with the overlap approximation to within 2% for the singlet states. Hence, combining the overlap approximation for the correlation energy with the calculated SCF energy (i.e., the pairwise correlated generalized valence bond method) gives total energies two orders of magnitude more accurate than the SCF calculations, with only a trivial increase in computing effort. The agreement for triplet states is equally good for the pair of orbitals for which the 2s eigenvalue equals the ionization potential. However, discrepancies of 10% are found for the pair of orbitals for which the one-electron Hamiltonian is diagonal.
Resonant energy transfer cross sections formulated in terms of form factors: An appraisal of the first Born approximation
Authors: Huo Winifred M.
Published: February 2003

In the Born approximation without exchange the differential cross section of energy transfer between two molecules is expressible in terms of the generalized oscillator strengths (f) for the excitation of individual molecules by electron impact. Thus, the Born cross section can be unambiguously calculated using electron impact data. In the absence of such data, an approximate expression of f can still be obtained from experimental values of transition moments, ionization potentials, and excitation energies based on the first term of Lassettre’s series representation of f. It is shown, in the case of dipole–dipole energy transfer, that the cross section calculated with the approximate f corresponds to scattering from a shielded dipole–dipole intermolecular potential, with the correct asymptotic behavior at both large and small r. Hence, it appears that even when approximate f’s are used, the present method is superior to the dipole–dipole theory of Cross and Gordon. The validity of the Born approximation at thermal energies is discussed. It is argued that the Born approximation should work best in the calculation of near resonant dipole–dipole energy transfer. Two numerical examples are given to test the above argument. In the case of energy transfer between Ar 3,1P1 and H2, HD, or D2, the Born results are in reasonable agreement with the data of Fink, Wallach, and Moore when the energy gap is ≲30 cm−1. In the second example, energy transfer between p-difluorobenzene and ketones, due to the absence of electron impact data, approximate f’s are used and the calculated cross section is a lower bound of the Born value. Exchange effects on the Born approximation is studied. It is concluded that the exchange amplitude has the same symmetry property as the direct term. Thus, contrary to the Dexter approximation, the cross term between direct and exchange contributions do not vanish in the expression of the energy transfer cross section.