bluuebunny/crossref_metadata_2025 huggingface dataset:
A temperature dependent study of the spin exchange reactions of muonium with O2 and NO in the range 295 to 478 K
Authors: Mikula Randall J. , Garner David M. , Fleming Donald G.
Published: February 2003

The temperature dependence of the gas phase spin exchange reaction rates for muonium collisions with O2 and NO has been measured over the temperature range 295 to 478 K. In both cases the bimolecular rate constant k(T) was found to vary essentially as T1/2, leading to the following temperature independent spin exchange cross sections: σSE(Mu+NO) = (10.5±0.6)×10−16 cm2 and σSE(Mu+O2) = (9.1±0.7)×10−16 cm2. These results are in quantitative agreement with the H+O2 and H+NO results reported by Gordon et al., demonstrating that there is no isotope effect in spin exchange in this temperature range.
Brillouin light scattering from polymer gels
Authors: Marqusee J. A. , Deutch J. M.
Published: February 2003

This paper is devoted to the theory of laser light scattering from polymer gels. The principal purpose of the inquiry is the structure of the Brillouin peaks that arise from the damped sound modes that are supported by the polymer network–fluid system. Our analysis includes coupling between sound waves in the fluid and elastic waves in the polymer network that is present even in the absence of any dissipative mechanisms. Explicit expressions are given for the eigenmodes of the gel in the presence of viscous damping and strong friction between the fluid motion and the polymer network. A comparison is made between the theory and available experimental evidence. The main point is that the shift and width of the Brillouin light scattering peaks (and by analogy sound attenuation) may usefully be employed to study both the static and dynamical properties of polymer gels.
Relaxation parameters for power-broadened microwave transitions. III. Numerical calculations
Authors: Peterson Keith L. , Schwendeman R. H.
Published: February 2003

The collisional relaxation parameters needed for the calculation of power-broadened line shapes of the j = 2←1 transition in OCS and the (j, k) = (3, 3) transition in NH3 have been calculated by the modified Anderson theory and the sudden approximation described in papers I and II of this series. Relaxation parameters of all tensor orders have been obtained, enabling a comparison of the Karplus–Schwinger–Townes line shape and the recently-derived continued-fraction line shape for power-broadened microwave transitions. It is shown, for the two transitions studied and the relaxation parameters calculated, that the Karplus–Schwinger–Townes equation is a very good representation of the continued-fraction line shape. However, the (T1/T2)0 obtained is 5–10% larger than the true T1/T2.
On the ESR detection of the σ-radical RNHĊO in irradiated crystals of 5,5-dihydroxybarbituric acid (alloxan) trihydrate
Authors: Sagstuen Einar , Skjærvo/ Halvard
Published: February 2003

In single crystals of 5,5-dihydroxybarbituric acid (alloxan) trihydrate, an ESR absorption was observed after irradiation at 77 K which is ascribed to a σ-radical RNHĊ = O. Its spectral parameters are described and compared to those from the structurally similar σ-acyl radical in malonic and succinic acid. It is suggested that the radical is formed from the primary oxidation product RCȮ(OH) through electronic rearrangements leading to an opening of the pyrimidine ring.
On the separation of resonance Raman scattering into orders in the time correlator theory
Authors: Page John B. , Tonks D. L.
Published: February 2003

A detailed study is carried out of the separation of resonance Raman scattering into orders within the time-correlator formulation of Hizhnyakov and Tehver (HT). This formulation is exact for a multimode system at all temperatures and for all electron–phonon coupling strengths within well-defined ’’standard assumptions.’’ As in any Raman theory, the full m-phonon Raman scattering involves the electron–phonon coupling to infinite order, owing to virtual phonon transitions accompanying the m real final state phonon transitions. The HT separation into orders is seen to correspond to a particularly convenient grouping of the contributions, such that the full m-phonon scattering is expressed as a power series in explicit orders of the electron–phonon coupling, but with each term of the series also containing factors involving the electron–phonon coupling implicitly to infinite order. Each term of this series except the first vanishes for T→ 0, with the result that the (explicitly) mth order scattering and the full m-phonon scattering are identical at T = 0. Most importantly, at any temperature the contributions from each order to the resonance Raman excitation profile line shapes are obtainable from the optical absorption in a direct and simple way. We have previously stressed the usefulness of the first-order version of this feature for the analysis of experimental data, and here that work is contrasted with recent work of Hassing and Mortensen, who expressed the full one-phonon series for a single-mode system in a different form. For completeness, a detailed derivation of the entire theory is given, including proofs via phonon many-body techniques of the two fundamental identities upon which the HT approach rests and which were not proven in HT’s rather condensed presentation of their time-correlator formulation.
Classical, diatomic molecule, kinetic theory cross sections. II. Collision dynamics
Authors: Curtiss C. F.
Published: February 2003

The dynamics of collisions between nonvibrating diatomic molecules is discussed in terms of a system of collision coordinates introduced earlier in the development of classical expressions for the generalized or kinetic theory cross sections. The expressions developed earlier are then rewritten in terms of precollision values of functions introduced in the discussion of the dynamics, which may be evaluated either through a perturbation solution of the equations or a stepwise numerical solution.
Decay kinetics of NCl(b 1Σ+, v′)
Authors: Pritt A. T. , Patel D. , Coombe R. D.
Published: February 2003

Measurements have been made of the time decay of NCl(b 1Σ+,v′) generated by the laser photolysis (193 nm) of ClN3 diluted in helium. Rate constants for collisional quenching of the v′ = 0 level of the b 1Σ+ state are 47.7±1.3(ClN3), 0.54±0.25 (Cl2), 7.1±0.6 (F2), 38±3 (ClF), 7.4±0.3 (H2), 59±3 (HF), 33±3 (HCl), and 1.71±0.06 (CO2) in units of 103 s−1 Torr−1. These results are interpreted in terms of possible reactive and energy transfer mechanisms for electronic quenching. With the exception of ClN3, quenching within the vibrational manifold of the b 1Σ+ state is faster than electronic quenching for each of the chaperone gases tested. Analysis of the b 1Σ+ decay rate for quenching by ClN3 indicates that 13±2% of the vibrational energy contained in NCl(b 1Σ+) is utilized in enhancing the rate of the quenching process.
On application of 0s orbitals in SCF calculations
Authors: Szalewicz Krzysztof , Monkhorst Hendrik J.
Published: February 2003

The 0s orbitals, i.e., functions of the form [exp(−ar)−exp(−br)]/r, appear in the iterative solution of the Hartree–Fock equations. The inclusion of these functions in the basis set for the standard SCF calculation for helium is shown to give much better values for the SCF energy than those obtained by using the 1s functions alone, especially in smaller basis sets. This result is rather contradictory to a previous work of Zung and Parr. Using a five term basis set we have obtained the SCF energy of the He atom to be equal to −2.861 679 995 612 a.u. Our results indicate that the disagreement between the Roothaan–Soukup and Gázquez–Silverstone ’’SCF limits’’ for the He atom should be resolved in favor of the latter, which is identical with our result to all digits quoted. A routine use of the 0s functions in SCF calculations for atoms and molecules is advocated.
The role of the Criegee intermediate in the matrix thermoluminescence study of the CH2+O2 reaction
Authors: Martinez Richard I. , Huie Robert E. , Herron John T.
Published: February 2003

The identification by Lee and Pimentel [J. Chem. Phys., 74, 4851 (1981)] of two new progressions of formic acid arising from the reaction of CH2 with O2 in an argon matrix provides the first direct identification of excited formic acid as an intermediate product of this reaction. Their observations provide further evidence that the reaction of CH2 with O2 probably proceeds to products through the initial formation of the dioxmethylene adduct H2ĊOȮ (a Criegee intermediate) which then rearranges to the excited formic acid through the CH2O2 intermediates dioxirane and methylenebis (oxy). (AIP)
Generalizations of Newton–Raphson and multiplicity independent Newton–Raphson approaches in multiconfigurational Hartree–Fock theory
Authors: Jo/rgensen Poul , Olsen Jeppe , Yeager Danny L.
Published: February 2003

The application of Newton–Raphson (second order) approaches in multiconfigurational Hartree–Fock (MCSCF) can significantly improve convergence over other MCSCF procedures. When the Hessian (second derivative) matrix has small eigenvalues, convergence of second order procedures may be slowed significantly both far from and closer to convergence. We derive techniques related to the multiplicity independent Newton–Raphson approach which are less affected by these convergence problems. We also formulate and derive generalized Newton–Raphson approaches which show quadratic, cubic, quartic, etc. convergence. We prove that certain fixed Hessian-type Newton–Raphson iterations will show quadratic, cubic, quartic, etc. convergence and demonstrate how these approaches may be advantageously used only for a few iterations. The approaches we describe in both the energy and generalized Brillouin’s theorem formulation have about the same complexity in structure and in actual implementation on a computer as the Newton–Raphson approach. Finally, we examine and discuss conditions for an MCSCF state to be a good approximation to the electronic state.
A
b i
n
i
t
i
o (STO-3G) dipole moment derivative properties of the CHn series
Authors: Kornblum Zvi C.
Published: February 2003

The atomic and bond dipole moment derivative properties associated with the total infrared absorption intensity of the methylidyne, methylene, and methyl free radicals, and of methane, have been calculated by means of the ab initio STO-3G algorithm. These properties include the coordinate-dependent atomic polar tensors and the following invariant quantities: the dipole derivatives for a bond stretch, mean atomic dipole moment derivatives, atomic anisotropies, and the effective atomic charges. Three sets of calculations were conducted for each species by combining the experimental or theoretically determined equilibrium molecular geometry with the standard or optimum orbital exponent scaling factors. The relationship between each of these dipole derivative properties and, in turn, the equilibrium geometry, the carbon and hydrogen scaling factors, and the number of bonded hydrogen atoms is analyzed. According to any calculation, it is found that the calculated dipole derivative value for a C–H stretch in methane is negative, which agrees with the experimentally deduced results, but is positive for all the radicals, except the doublet state of CH. Furthermore, it is shown that the optimum scaling factors must be used in the calculations in order to consistently obtain good agreement between theory and experiment. Based on both the theoretical equilibrium geometry and the optimum scaling factors of a species, the following trends are exhibited by this homologous series of hydrocarbons: (1) The dipole derivative magnitude for a C–H bond stretch is a monotonic increasing function of the product of the carbon and hydrogen scaling factors. (2) The effective charge of carbon increases as the optimum size of the carbon valence shell increases, and decreases as additional hydrogens are bonded to the carbon. (3) The hydrogen effective charge generally decreases as the number of hydrogens increases. (4) The mean dipole derivative of hydrogen is a monotonic decreasing function of the number of C–H bonds, and is negative only for methane. (5) Calculations on the doublet and quartet electronic states of CH yield different values for the dipole moment, the effective charge, and the anisotropy, the dipole derivatives for a bond stretch have opposite directions.
Study of liquid water by computer simulations. I. Static properties of a 2D model
Authors: Okazaki Keiji , Nosé Shūichi , Kataoka Yosuke , Yamamoto Tsunenobu
Published: February 2003

A computer-simulation study of a water-like system is carried out by making use of a two-dimensional version of the Ben-Naim and Stillinger potential. The pair potential is set up such that at 0 K it yields a square net structure at low pressures and an interpretation of two square nets at high pressures. The liquid state is surveyed over a wide range of temperature and pressure. Various kinds of molecular distribution functions are derived to see how the hydrogen-bond network structure depends on temperature and density. The pressure and thermal equations of state are ’’experimentally’’ determined by a least square fitting to the pressures and energies calculated for about 200 different state points. The well-known anomalous behavior of liquid water is reproduced at least in a semiquantitative way. The singular properties of supercooled water also are reproduced and their origin is ascribed to the thermodynamical instability. New anomalies are predicted at high temperatures and pressures.
Recombination kinetics: Langevin dynamics with a space dependent friction coefficient
Authors: Schell Mark , Kapral Raymond , Cukier Robert I.
Published: February 2003

A Langevin equation describing the relative motion of a pair of potential reactants in a fluid is numerically solved to obtain the time dependent probability of recombination. The pair friction coefficient that appears in the Langevin equation depends on the pair separation and is obtained from a kinetic theory. We assess the effect of this space dependence on the recombination probability by comparison with the constant friction results.
Studies of hydrogen bonding in the vapor phase by measurement of thermal conductivity and molecular orbital calculations: Methanol–water binary mixtures
Authors: Curtiss L. A. , Frurip D. J. , Blander M.
Published: February 2003

The thermal conductivities of serveral methanol–water binary vapor mixtures were measured in a modified thick hot-wire cell between 352 and 375 K at pressures ranging from 100 to 1400 Torr. Analysis of the data indicates that molecular association to form a 2:1 methanol–water trimeric species is a major source of enhancement of the thermal conductivity. The enthalpy and entropy of association of this trimer are −10.48 kcal mol−1 and −38.24 cal mol−1 deg−1, respectively. Theoretical calculations, based on ab initio molecular orbital calculations, of the enthalpies and entropies of association for various open chain and cyclic methanol–water trimers indicate that the 2:1 methanol–water trimeric species is more stable than the 1:2 trimeric species which is consistent with the results of the analysis of the thermal conductivity data. The theoretical calculations indicate that it may be an entropic effect which makes the 2:1 trimeric species more stable than the 1:2 species.
On the mobility of an ion near a conducting boundary
Authors: Mazo Robert M.
Published: February 2003

When a charged particle moves near a conducting wall, currents are induced which lead to dissipation and an additional resistance to the motion. The magnitude of this effect is calculated by the adaptation of results for the propagation of radio waves from a dipole antenna. The effect is negligibly small for good conductors, but may be nonnegligible for semiconductors.
Extinction coefficients and integrated intensities for linear- and bridged-bonded CO on platinum
Authors: Vannice M. A. , Twu C. C.
Published: February 2003

For the first time, infrared integrated absorption intensities and extinction coefficients have been determined for both the linearly adsorbed (high frequency-HF) and the bridged-bonded (low frequency-LF) forms of CO on Pt. Platinum dispersed on η-Al2O3, SiO2, SiO2–Al2O3, and TiO2 was characterized by both hydrogen and CO chemisorption which allowed the determination of individual surface coverages of the two CO species. The integrated intensities A for the HF species were 9–19×107 cm mol−1 for typical Pt catalysts (using SiO2, Al2O3, and SiO2–Al2O3) while A values over 30×107 cm mol−1 were measured for the TiO2-supported catalysts. Integrated intensities for the LF form were always lower on a given catalyst and ranged from 2 to 25×107 cm mol−1. Similarly, extinction coefficients ε at the HF band maxima were always higher than those for the LF species. Values for εHF fell between 0.6 and 2.1×106 cm2 mol−1 while εLF values ranged from 0.1 to 0.5×106 cm2 mol−1 for the typical catalysts. The higher values for the titania-supported catalysts may be a consequence of SMSI (strong metal-support interaction) behavior which alters the electronic state of the Pt and, as a consequence, the CO surface coverage.
Hyperfine and spin–rotational structure of CaBr X 2Σ (v = 0) by molecular-beam laser-rf double resonance
Authors: Childs W. J. , Cok David R. , Goodman G. L. , Goodman L. S.
Published: February 2003

The molecular-beam, laser–rf, double-resonance technique has been used to make high-precision measurements of the spin–rotation and hyperfine interactions in the X 2Σ (v = 0) electronic ground state of Ca79Br and Ca81Br. The spin–rotation interaction is found to have a strong N dependence. The Frosch–Foley magnetic hyperfine parameters b and c and the electric–quadrupole hfs parameter eqQ are determined for both molecules.
HF rotational laser emission through photoelimination from vinyl fluoride and 1,1-difluoroethene
Authors: Sirkin Eric R. , Pimentel George C.
Published: February 2003

Rotational laser emission by HF has been observed at 33 frequencies between 325 and 1250 cm−1 from the flash photolysis (1.2 μsec FWHM) of vinyl fluoride and of 1,1-difluorethylene. The transitions lie within the v = 0 to v = 5 manifolds and range from J″→J′ = 8→7 to 31→30. Increasing the atomic weight or the partial pressure of the inert buffer gas (He, Ne, or Ar) raises the gain of nearly all transitions, showing that collisional relaxation processes are active in pumping the laser emission. The high gains displayed by both precursors in the J = 14→13 transitions for the v = 0,1,2, and 3 manifolds indicate that V→R energy transfer is pumping molecules into the v′, J = 14 state from the near-resonant v′+1, J = 2, 3, and 4 states. In a similar way, the highest J transitions J = 31→30 to 28→27 with v = 0 and/or 1, are best explained by V→R energy transfer from near-resonant low-J states from much higher vibrational manifolds v′ = 4, 5, and 6. This would imply collision-induced multiquantum energy transfer with large Δv (up to Δv = 5) and large ΔJ (up to ΔJ = 26) or a rapid succession of steps with smaller Δv and ΔJ. In contrast, the high gains displayed by the J = 10→9 transitions in the v = 0, 1, and 2 manifolds are best explained in terms of R→T relaxation from a uniform nascent population. While there are indications that the nascent rotational distributions provided by these photoeliminations probably furnish population to high J states, the gain patterns indicate that the V→R and R→T energy relaxation processes are strongly influential, the former surely involving multiquantum steps with large ΔJ and probably with Δv>1 as well.
Energy transfer as a random walk on regular lattices
Authors: Blumen A. , Zumofen G.
Published: February 2003

In this paper we study the incoherent energy transfer in molecular crystals, where the intermolecular interactions are due to electrostatic forces. We analyze the random walk of the excitation both analytically and numerically and we take into account the occurence of long-range transfer steps. For five lattices (simple-cubic, body-centered-cubic, face-centered-cubic, diamond, and a sparse-cubic lattice) we present the average number Mn of returns to the origin and the mean number Sn of distinct sites visited on an n-step walk, where the steps are due to different multipolar interactions. The computations are carried out using both simulation and matrix inversion methods and they are checked against analytical asymptotic forms. We give the coefficients of these analytical expressions obtained from the first four terms in the expansion of the generating functions for nearest-neighbor walks. The results obtained behave smoothly with respect to changes in the range of the interaction and in the coordination number of the lattice (number of nearest neighbors). This allows us to use the continuous description of the random walk in terms of a diffusion model to obtain convenient semiquantitative expressions for Mn and Sn for different lattices. Since the methods presented vary greatly in their degree of sophistication, we are also able to demonstrate the advantages and drawbacks of the different approaches under realistic conditions.
Sound propagation in moderately supercooled liquids. A comparison with water
Authors: D’Arrigo Giovanni
Published: February 2003

Ultrasonic investigations in benzophenone, diphenylamine, and a solution of these two liquids on going from the stable down to the supercooled region confirm the peculiar behavior of sound propagation previously found in other systems. A comparison with ultrasonic absorption and velocity data in supercooled water shows that both kinds of systems exhibit similar anomalies. This suggests that the same physical interpretation may explain such findings. The large absorption in the supercooled region is attributed to structural relaxation processes whose characteristic times largely increase in the metastable state. An evaluation of such times in water indicates that dispersion in both absorption and velocity would not be detectable down to ∼−20 °C. Such findings agree with recent experimental observations and eliminates some inconsistencies present in the existing experimental data. A comparison of the structural relaxation times with those found in other transport properties in supercooled water shows a close connection with a nearly equal activation energy. Sound velocity anomalies at lower temperatures are explained on the grounds of ’’heterophase’’ contributions.
Self-diffusion of bulky molecules in solution: 5-α-Cholestane in C6F6 or c
i
s-polybutadiene
Authors: von Meerwall E. , Ferguson R. D.
Published: February 2003

We have measured the self-diffusion of both components in the system 5-α-cholestane + hexafluorobenzene, and in 5-α-cholestane dissolved in cis-polybutadiene. Diffusion coefficients and activation energies follow free-volume models, but the bulk, shape, and rigidity of the cholestane molecule leads to anomalous free-volume and collision-dynamic parameters and nonideal behavior in solutions.
The qualitative behavior of the ground state wave function of H2+
Authors: Hoffmann-Ostenhof T. , Morgan John D.
Published: February 2003

We use differential inequality techniques to study the qualitative behavior of the ground state wave function of H2+. Our results include proofs that the ground state eigenfuction is cylindrically decreasing and that it has exactly two maxima at the nuclear positions and no local minimum.
Polarization studies in multiphoton absorption spectroscopy
Authors: Andrews D. L. , Ghoul W. A.
Published: February 2003

Using the principles of quantum electrodynamics, the theory of two-, three-, and four-photon absorption in polyatomic gases and liquids is developed. Expressions are derived for the rates of single-frequency absorption from plane polarized, circularly polarized, and unpolarized light. It is shown that for n-photon absorption with n?3, the rate for unpolarized radiation is in each case expressible as a linear combination of the rates for plane polarized and circularly polarized light; no such relationship exists for four-photon absorption. For each multiphoton process, it is demonstrated how the fullest information about the symmetry properties of excited states can be derived by a simple linear processing of the results from experiments with different polarizations. A detailed examination of the selection rules is made, based on a reduction of the molecular transition tensor into irreducible components, and a new classification scheme is introduced to assist with the interpretation of experimental results. Finally, it is shown that the theory may also be applied to resonance-enhanced multiphoton ionization spectroscopy.
Laser-collision induced chemical reactions: A comparison of quantum mechanical and classical model results
Authors: Kulander K. C. , Orel A. E.
Published: February 2003

We compare the results of a classical model for the laser enhancement of the H+LiF→Li+HF reaction to accurate quantum mechanical results. Structure in the reaction probability as a function of collision energy below the fieldfree threshold predicted classically by Orel and Miller is found to be less pronounced in the quantum results. A much better understanding of the various causes of the observed maxima and also of the reliability of this classical model are obtained as a result of these calculations. A fundamental inconsistency in the classical model introduced when the Langer modification is made which can lead to unphysical results is discussed and a procedure for correcting this inconsistency is presented. Improved results are obtained.
Ion-molecule reactions of N3+, N4+, O2+, and NO2+ in nitrogen containing traces of oxygen
Authors: Matsuoka Shingo , Nakamura Hirone , Tamura Takaaki
Published: February 2003

A new sort of stationary pulsed afterglow method with a mass spectrometric analysis is presented to study quantitative ion behavior at atmospheric pressure. Repetitive x-ray pulses from an electron linear accelerator are used to ionize gases. Experiments were carred out in a system of nitrogen containing traces of oxygen. The ion–molecule reaction rate constants for N3++O2 and N4++O2 were measured as (7.1±0.7)×10−11 and (2.86±0.3)×10−10 cm3 s−1, respectively. A main cause for ±10% errors in the rate constants is attributed to errors of measurements of oxygen contents. An error due to ion sampling through the reactor wall aperture is evaluated to be less than ±0.5%. Thus, the presented method is very useful for accurate quantitative study. Product ion NO+2 is shown to be lost in nitrogen via successive reaction. The endothermic reaction NO+2+N2→NO++N2O with a rate constant of 1.2×10−15 cm3 s−1 is postulated. The rate constant of the exothermic reaction O+2+N2→NO++NO was found to be less than 2×10−18 cm3 s−1.
Laser studies of the collisional quenching of electronically excited Mg(3s3p 1P1) atoms
Authors: Breckenridge W. H. , Umemoto H.
Published: February 2003

Quenching of the Mg(3s3p 1P1) state by several simple gases has been studied using pulsed laser techniques. Absolute quenching cross sections were determined by pulsed laser excitation of Mg(1P1) and time-resolved measurements of 2852 Å fluorescence. Qualitative identification of major exit channels was accomplished using laser-induced fluorescence at short time delays. Except for the inert gases and the perfluoroalkanes, the cross sections were approximately gas kinetic and correlated well with the C6 long-range forces coefficient to the power 0.48±0.05. In contrast to the analogous Cd(5s5p 1P1) level, production of Mg(3PJ) by collision-induced intersystem crossing was negligible for all quenching molecules studied. Production of MgH was observed for H2, CH4, C3H6, C3H8, and n-C4H10, but not for C2H2 or C2H4.
Dynamics of ion solvation. Li++H2O→Li+(H2O)*
Authors: Hase William L. , Feng Da-Fei
Published: February 2003

The classical trajectory method has been used to calculate the primary cross section for Li++H2O recombination versus relative translational energy. The criterion used for the formation of a vibrationally and rotationally excited Li+(H2O)* cluster is the presence of more than one inner turning point in the Li++H2O relative distance. As the relative translational energy is increased, there is a dramatic decrease in the primary cross section. It is 542 Å2 at Erel = 0.5 kcal/mole and essentially zero at Erel = 5.0 kcal/mole. The results are discussed in terms of the inefficiency of intramolecular energy transfer from the Li++H2O relative motions to the H2O vibrational and rotational degrees of freedom.
Theoretical investigation of complex dynamic behavior of the CO oxidation on a Pt catalyst
Authors: Rathauský J. , Hlaváček V.
Published: February 2003

Two mathematical models are used to illustrate the fact that the influence of adsorbed species on the rate of an isothermal catalytic reaction may lead to a complex dynamic pattern, including multiplicity of steady and oscillatory states. Direct numerical techniques for calculation of limit points and Hopf’s bifurcation points, connected with the continuation algorithm, were used for calculations. Multiple oscillations and chaotic behavior cannot be calculated from the models. A comparison of theoretical predictions with experimental data was made.
Calculations of the deuterium electric field gradient from simple wave functions in the HD molecule
Authors: Zamani-Khamiri O. , Hameka H. F.
Published: February 2003

We attempt to find a simple procedure for evaluating the electric field gradient at the position of a deuteron in a molecule by performing a set of sample calculations on the HD molecule. Starting from a set of simple molecular orbital or valence bond functions, we calculate the expectation values of the DEFG (deuterium electric field gradient) values with an accuracy of about 10%. By making use of a variation–perturbation procedure we evaluate correction terms which reduce the errors to 1% or 2%. An essential feature in the variation-perturbation method is the use of 3d atomic orbitals with unusually large orbital exponents (q = 7). We feel that our methods are applicable to larger molecules also.
Reduced local energy as a criterion for the accuracy of Hartree–Fock wave functions
Authors: King Frederick W. , LeGore Mary A. , Kelly Michael K.
Published: February 2003

The reduced local energy concept is applied to assess the local accuracy of Hartree–Fock wave functions. The method is illustrated by application to all the Clementi and Roetti’s wave functions for the helium atom and to the first row of the helium isoelectronic series. The use of the reduced local energy as a means for improvement of expectation values is discussed. Application to the five term helium wave functions of Clementi and Clementi and Roetti is considered.
The reduced linear equation method in coupled cluster theory.
Authors: Purvis George D. , Bartlett Rodney J.
Published: February 2003

A numerical procedure for efficiently solving large systems of linear equations is presented. The approach, termed the reduced linear equation (RLE) method, is illustrated by solving the systems of linear equations that arise in linearized versions of coupled-cluster theory. The nonlinear coupled-cluster equations are also treated with the RLE by assuming an approximate linearization of the nonlinear terms. Very efficient convergence for linear systems and good convergence for nonlinear equations are found for a number of examples that manifest some degeneracy. These include the Be atom, H2 at large separation, and the N2 molecule. The RLE method is compared to the conventional iterative procedure and to Padé approximants. The relationship between the projection method and least square methods for reducing systems of equations is discussed.
New developments in the model potential method
Authors: Sakai Y.
Published: February 2003

Significant improvements are introduced in the process of determining the model potential parameters and truncating the basis sets in Bonifacic and Huzinaga’s model potential. The pseudovalence orbitals can simulate closely the shapes of the reference atomic valence orbitals given by all-electron calculations including the inner nodes. Test molecular calculations are carried out for CO, HCl, P2, and Cl2. The model potential developed by the present author yields molecular orbital energies, atomization energies, geometries, and spectroscopic constants in near perfect agreement with the result of all-electron reference calculations.
On the dielectric and Kerr-effect relaxation processes caused by time-varying electric fields
Authors: Morita Akio , Watanabe Hiroshi
Published: February 2003

Expressions for the electric polarization and birefringence are obtained for extremely low time-varying electric field E(t), i.e., (i) continuous reversing square pulses, (ii) E(T) = E0e−kt, and (iii) E(t) = E0(1−e−kt), where t is time, E0 is the strength of the field, and k is a decay constant. Also, the time-dependent behavior of the electric birefringenece is extensively discussed.
Classical, diatomic molecule, kinetic theory cross sections
Authors: Curtiss C. F.
Published: February 2003

A linearized form of the classical Boltzmann collision integral generalized to a gas of nonvibrating diatomic molecules which was recently developed is used to define a set of kinetic theory cross sections which are the classical analog of those which are defined in terms of the matrix elements of the quantum mechanical Waldmann–Snider collision operator. The integrations over the velocity of the center of mass are carried out explicitly. This leads to expressions for the cross sections which are generalizations of the ’’Ω integrals’’ of the kinetic theory of spherical molecules.
Vibrational study of and conduction mechanism in β alumina. II. Nonstoichiometric β alumina
Authors: Colomban Ph. , Mercier R. , Lucazeau G.
Published: February 2003

Single crystals of nonstoichiometric β alumina 11 Al2O3, (1+x)M2O3(x = 0.25 to 0.6) with M = Na, K, Tl, Ag have been studied by Raman spectroscopy between 2 and 1000 cm−1 in the 20–1400 K temperature range. Far infrared measurements have been performed between 10 and 250 cm−1. Ag(Ag)3 and (Tl)3 clusters of coupled cations have been evidenced in the low temperature spectra of Ag+ and Tl+ compounds. Ag–Ag and Tl–Tl ’’bond stretching’’ force constants were found equal, respectively, to 0.10 and 0.23 mdyn Å−1. In the low temperature state, potential barriers separating sites available for cations have been found to increase at a small extent from x = 0 to x = 0.6 compounds. The temperature dependence of the low frequency Raman spectra is reported and shows that vibrational correlations between silver ions are partly responsible for the decrease of the energy necessary for the cation hopping mechanism.
Neutron-diffraction studies of Tb2C and Dy2C in the temperature range 4–296 K
Authors: Atoji Masao
Published: February 2003

Neutron power-diffraction data of the trigonal Tb2C have been obtained and compared with our previous results (given in parentheses) for a Tb-rich Tb2C. Significant differences were observed in the c spacing and the ferromagnetic moment, e.g., the saturation value of 7.6(8.6) Bohr magnetons, which is 84(96)% of the free-ion moment, indicating a considerable crystal-field effect. The Curie temperature (266 K) and an unusual magnetostriction along c were unchanged. The ordered-moment direction (parallel to c) has now been established unambiguously using a magnetically induced, preferential crystallite orientation in Tb2C. A short-range ferromagnetic ordering, which coexists with the long-range ordering in Tb2C, was analyzed anew. Dy2C becomes ferromagnetic below 168 K, and the saturation ordered moment is 6.8 Bohr magnetons, which is 68% of the free-ion moment. A magnetically induced, preferential crystallite orientation in Dy2C indicated that the ordered-moment direction is parallel to the [104] axis, which corresponds to the [100] axis of the high-temperature cubic modification.
Isotope shifts in EPR spectra
Authors: Morton J. R. , Preston K. F.
Published: February 2003

Small changes observed in the 19F isotropic hyperfine interaction in F2NO upon isotopic substitution of the N or O atom are shown to be statistically significant. The effect is attributed to a modulation of the hyperfine interaction by the ν2 bending mode of the free radical.
Spectroscopic properties of molecules interacting with small dielectric particles
Authors: Gersten Joel , Nitzan Abraham
Published: February 2003

Optical properties of small dielectric spheroids with or without adsorbed molecules are studied theoretically. Expressions for the absorption line shapes, the radiative and nonradiative decay rates, and quantum yields are derived. In the case of a molecule near a spheroid the magnitudes differ dramatically from the corresponding case of a molecule near a plane.
Dominant formation and quenching kinetics of electron beam pumped Xe2Cl
Authors: Marowsky G. , Glass G. P. , Smayling M. , Tittel F. K. , Wilson W. L.
Published: February 2003

Processes leading to the production and removal of the triatomic excimer Xe2Cl* subsequent to short duration electron beam excitation of Ar/Xe/CCl4 mixtures have been investigated. The radiative lifetime of Xe2Cl* has been measured to be 135 (+70−60) ns. Formation of the excimer has been shown to occur principally via a termolecular reaction involving XeCl*. Rate constants for the formation and collisional quenching of Xe2Cl* by CCl4, Xe, and Ar have been determined.
Time-dependent moment theory of hot-atom reactions
Authors: Knierim K. D. , Lin S. L. , Mason E. A.
Published: February 2003

A two-temperature moment method of solving the Boltzman equation is used to describe hot-atom relaxation and reaction. The moment equations are written in terms of standard kinetic-theory collision integrals and integrals over the reaction cross section. The theory is tested on a model system, and calculated hot yields are compared with Monte Carlo results. Convergence of the hot yield in increasing order of approximation is good for disparate masses of hot atoms and reservoir gas molecules, but severe difficulties occur with nearly equal masses (within a factor of about 3). A bimodal velocity distribution is introduced for these cases. The resulting moment equations give yields that are in much better agreement with the Monte Carlo calculations.
Temperature dependence of the triplet exciton yield in fission and fusion in tetracene
Authors: Arnold S. , Whitten W. B.
Published: February 2003

The increase in quantum efficiency for luminescence with increasing excitation intensity is measured as a function of temperature in tetracene. The results indicate that the quantum efficiency for the production of triplets from an excited singlets decreases markedly below 300 °K. This decrease is accounted for by a process which competes with exciton fission with a rate K = (1.3±0.2)×108 sec−1. This value is shown to be inconsistent with a singlet trapping interpretation for the nonexponential decays found by others in the prompt fluorescence. The results are consistent with a nearly temperature independent value for the fraction of exciton fusion events which lead to excited singlets f. The relative constancy of f above 300 °K implies that fusion resulting in a triplet proceeds via the first excited triplet. This in turn provides a qualitative explanation for the apparent lack of a relationship between the measured value of f and the rudimentary value inferred from spin statistics.
Semiempirical potential surfaces for the alkali–halogen molecule reactions
Authors: Zeiri Y. , Shapiro M.
Published: February 2003

Potential surfaces for the alkali (Li,Na,K,Rb), halogen–molecule (F2,Cl2,Br2,I2) reactions are presented. The surfaces are constructed by a semiempirical method based on valence bond formalism developed previously by the authors. The method, contrary to diatomics in molecules, uses as input only the ground diatomic potentials. A simplified one-covalent–two-ionic configuration model is used to produce the three lowest adiabatic surfaces. Alternatively, the separate covalent ionic and off-diagonal terms, which are useful to charge exchange colisions, can be given. Comparison with ab initio results for the Li+F2 are very encouraging. All surfaces are found to have wells in the T-shape configuration and except for the F2 case, to have a barrier of a few kcal/mol in the entrance channel. The barrier is minimal in the collinear geometries. The effect of a collinear transition state and a T-shape well is shown to lead to a systematic change in the differential cross sections of the K+X2 and Rb+X2 reactions, as the halogen molecule becomes heavier.
NMR investigations of N, N, N′, N′ -tetramethyl-p-phenylenediamine and its oxidized tetrafluoroborate salts
Authors: Yamauchi Jun , McDowell Charles A.
Published: February 2003

Proton and fluorine magnetic resonance absorption and spin–lattice relaxation time measurements have been carried out on N, N, N′,N′-tetramethyl-p-phenylenediamine and its two oxidized compounds containing the BF4 anion moiety. The second moments and the spin–lattice relaxation times are analyzed in terms of the methyl and BF4 group motions. The methyl group motion is greatly influenced by the oxidation states, and the BF4 isotropic motion is found to be remarkably mobile in the environments of these organic cation molecules. The 1H and 19F relaxation times in the paramagnetic state showed two characteristic temperature dependences above and below the phase transition temperature. It is found that, in the high temperature region, the hyperfine interaction is responsible for the 1H relaxation, while the dipolar electron–nuclear interaction is responsible for the 19F relaxation. In the low temperature region, both the 1H and 19F relaxations are controlled by the triplet exciton motions, in addition to the isotropic BF4 rotational motion in the case of the 19F relaxation.
Infrared and Raman studies of pressure effects on the vibrational modes of solid CO2
Authors: Hanson R. C. , Jones L. H.
Published: February 2003

Infrared and Raman studies have been made on the librational and internal modes of solid CO2 in the pressure range of 6 to 120 kbar at ambient temperature. The Fermi resonance has been analyzed in detail. Both of the stretching modes stiffen with pressure while the bending mode softens with pressure. The magnitude of the Fermi interaction parameter W decreases somewhat with pressure. A simple model using gas phase anharmonic constants predicts much of the observed behavior.
Accurate van der Waals coefficients from absolute total cross sections for the Ar–noble gas systems
Authors: Beijerinck H. C. W. , van der Kam P. M. A. , Cottaar E. J. E. , Verster N. F.
Published: February 2003

Using a method based on the absolute calibration of the density–length product of a supersonic secondary beam in a 20 K cryoexpansion chamber, total cross sections are found for Ar–Ar, Ar–Xe, and Ar–Kr collisions. (AIP)
Response to the comment on ‘‘Self-nucleation in the sulfuric acid–water system’’
Authors: Doyle George J.
Published: February 2003

The specific basis for a criticism by Renninger et al. [J. Chem. Phys. 75, 1584 (1981)] of Doyle’s technique for finding the saddle point on the free energy surface of a two-component liquid droplet [J. Chem. Phys. 35, 795 (1961)] is refuted. However, extensions of this technique to modify the Kelvin and Laplace relationships seems invalid as argued by Renninger et al. The technique’s validity is probably confined to capillary nucleation theory for a two component system.
Effect of vibration on rotational energy transfer. A quasiclassical trajectory study of He+H2
Authors: Thompson Donald L.
Published: February 2003

Quasiclassical trajectories have been used to compute state-to-state energy transfer cross sections in collisions of He with H2 initially in high vibrational–rotational states at 0.5 eV collison energy. The results show that for high rotational states (J?8) there is significant enhancement of the cross section for rotational energy transfer due to initial vibrational excitation. There is also enhancement of vibrational energy transfer by high initial rotational excitation. Rotational–vibrational energy transfer is the dominant mechanism for relaxation from initial high v, J states.
Orthorhombic phase of nickel bromine boracite Ni3B7O13Br: Room temperature ferroelectric–ferroelastic crystal structure
Authors: Abrahams S. C. , Bernstein J. L. , Svensson C.
Published: February 2003

Nickel bromine boracite Ni3B7O13Br is a fully coupled ferroelectric–ferroelastic crystal with a Curie temperature of 398 K that crystallizes in the orthorhombic system with space group Pca21 and four formula units per unit cell. The lattice constants at 298 K are a = 8.5218±0.0002, b = 8.5127±0.0002, and c = 12.0408±0.0002 Å. The integrated intensities of 11 682 reflections were measured with an Enraf-Nonius CAD-4 diffractometer using monochromatic Mo Kα radiation and a small single-domain crystal. The crystal structure was refined by the method of least squares, using the coordinates of isomorphous Mg3B7O13Cl as the initial model, with 1531 symmetry-independent Fmeas. The final agreement factor is 0.0492, based on anisotropic thermal vibrations for all atoms. Six independent boron atoms form tetrahedra with nearest-neighbor oxygen atoms at an average B–O distance of 1.478 Å. The remaining B(3) is 0.140 Å from a plane formed by three oxygen atoms, wth average B–O = 1.378 Å, and is 2.198 Å from a fourth oxygen. Each Ni has four O neighbors at 2.024 Å, one Br at 2.667 Å, and a more distant Br at 3.370 Å, on average. Two ferroelectric–ferroelastic tranformation mechanisms are applicable, each fully coupled. In the first, the sense of the spontaneous polarization is reversed as the spontaneous strain is reoriented: it has a magnitude calculated to be about 0.9×10−2 C m−2, based on a point charge model. The largest atomic displacement in this transformation is 0.72 Å for a Ni2+ ion: All other displacements are less than 0.5 Å. In the second mechanism, the spontaneous polarization is rotated through 90° as the spontaneous strain is also reoriented: it has a calculated magnitude of 2.6×10−2 C m−2. The largest corresponding atomic displacement is 0.60 Å for a Ni2+ ion. As-grown crystals could be only partially detwinned in an electric or a stress field. Ni3B7O13Br is closely isomorphous with Mg3B7O13Cl, the largest difference between coordinates of corresponding atoms being only 0.175 Å, with an average difference of 0.055 Å.
Generalized van der Waals theory. VI. Application to adsorption
Authors: Johnson Mark , Nordholm Sture
Published: February 2003

A recently developed free energy density functional theory (GvdW theory) is here applied to adsorption of inert gases at a flat wall. This application illustrates the need for the fine-grained version of the theory wherein the entropy functional is nonlocal. Calculation of density profiles for argon adsorbed on an idealized carbon dioxide surface and krypton adsorbed to graphite indicate that the GvdW theory somewhat underestimates the amount of adsorption and the amount of nonuniform structure. Nevertheless, a qualitatively correct and quantitatively useful description of adsorption is obtained.