bluuebunny/crossref_metadata_2025 huggingface dataset:
The electronic structure of tris(octamethylpyrophosphoramide)manganese(II) ion. Electronic absorption, emission, and excitation spectra of Mn(OMPA)3(ClO4)2
Authors: Hempel Judith C. , Palmer Richard Alan , Yang Mark Chin-Lan
Published: February 2003

Single crystal absorption, emission, and excitation spectra of tris(octamethylpyrophosphoramide) - manganese(II) perchlorate are reported. Crystal field calculations for D3 symmetry support assignments of the observed bands. Interelectron repulsion, normalized spherical harmonic (NSH) trigonal potential, and spin–orbit interaction parameters are fitted, with B=670, C=3620, DQ=26 291, DSIG=−2000, DTAU=2000, and λ=60 cm−1. (The DQ, DSIG, and DTAU values correspond to the conventional crystal field parameter values of Dq=836, Dσ=−239, and Dτ=−93 cm−1.) Intensity borrowing mechanisms are discussed in light of the polarizations and splittings observed. The results are correlated with the previously reported zero field parameters.
Ion–polar neutral momentum transfer collision frequencies: A theoretical approach
Authors: Barker R. A. , Ridge D. P.
Published: February 2003

A simple statistical theory is proposed which gives quantitative values for ion–polar neutral momentum transfer collision frequencies. The calculated collision frequencies depend on the rotational temperature of the neutrals, the translational temperature of the ions and neutrals, the masses of the ion and neutral, and the ratio of the neutral dipole moment to the square root of the neutral polarizability. Agreement with available experimental collision frequencies is very good. The results are consistent with available experimental rate constants for exothermic proton transfer reactions. The effects of nonorbiting collisions and finite ion–neutral hard sphere radius are examined.
Correlation effects in the spectra of polyenes
Authors: Schulten K. , Ohmine I. , Karplus M.
Published: February 2003

A Hamiltonian of the Pariser–Parr–Pople form is employed to investigate the effect of correlation on the π-electron spectrum of polyenes. Two limiting cases for the electron–electron interaction (short- and long-range limit) are considered, and it is shown that they yield descriptions corresponding to the standard valence-bond (Dirac–Heisenberg) and molecular-orbital models, respectively. The intermediate, chemically most interesting, range is examined in detail by means of a full configuration-interaction treatment with an exponential model potential that includes a variable effective range parameter. It is shown that correlation effects become more important as the effective range of the interaction decreases. The states of polyenes are classified as covalent or noncovalent, and it is found that the former are much more sensitive to correlation than the latter. Configuration interaction through double excitations yields a qualitatively correct ordering for all states in the chemical range, but triple and quadruple excitations are required for quantitative results. Applications to butadiene, hexatriene, and benzene demonstrate that correlation effects in these molecules lead to an important lowering in energy of the manifold of covalent states relative to that of the noncovalent states; most important, the first covalent (1A−g) state of the polyenes is found to be near degenerate with the strongly allowed noncovalent (1B+u) state. Density correlation functions and the fluctuation potential are obtained for the polyenes and used to clarify the nature of the correlation correction. Configuration interaction including double excitations is performed for polyenes through C12H14 to exhibit the length dependence of the correlation effects. It is shown that with increasing chain length, an increasing number of covalent states appears in the energy range of the two usually observed excited 1B+u and 1A+g (cis peak) states.
Quenching of N(2D) by N2 and H2O
Authors: Slanger T. G. , Black G.
Published: February 2003

Quenching rate coefficients have been measured for interaction of N(2D) with N2 and with H2O. For N2, data were taken between 198 and 372 °K, and the rate expression is kN2=1.0×10−13 exp(−1020/RT) cm3 molec −1 ⋅sec−1. Reaction with H2O was measured only at 300 °K, and a rate coefficient of 2.5±0.5×10−10 cm3 molec−1 ⋅sec−1 was found. Earlier temperature-dependent measurements of N(2D) quenching by N2O were checked and confirmed.
MCSCF potential energy surface for photodissociation of formaldehyde
Authors: Jaffe Richard L. , Morokuma Keiji
Published: February 2003

The ground state potential energy surface for the dissociation of formaldehyde H2CO→H2+CO is calculated with the ab initio MCSCF method with an extended (4–31G) basis set. The location, barrier height, and force constants of the transition state are determined and the normal coordinate analysis is carried out. The calculated barrier height is 4.5 eV. Based on the calculated quantities, the detailed mechanism of the photochemical dissociation is discussed.
Absolute rate parameters for the reaction of atomic hydrogen with hydrazine
Authors: Stief L. J. , Payne W. A.
Published: February 2003

Absolute rate parameters for the reaction of atomic hydrogen with hydrazine H+N2H4→H2+N2H3 have been determined in a direct manner using flash photolysis of dilute mixtures of hydrazine in helium and time dependent observation of H via resonance fluorescence. By measuring the H-atom decay under pseudo-first-order conditions, the bimolecular rate constant k1 was obtained over the temperature range 228–400°K. The data were fitted with good linearity to the Arrhenius expression k1= (9.87±1.17) ×10−12 exp(−2380±100/RT) cm3 molecule−1⋅sec−1. The data were shown to be free of any contributions from secondary reactions involving H as a reactant or product. A comparison of the present results with those from previous studies employing discharge flow methods is presented.
The adsorption of neon on graphitized carbon in the submonolayer and multilayer region between 1.5 and 30°K
Authors: Antoniou A. A.
Published: February 2003

Heat capacities and heats of adsorption (calorimetrically measured) together with equilibrium pressures are reported for coverages up to three monolayers. The data indicate that neon remains localized up to 30°K in the submonolayer region. Phase transitions of the adsorbed neon were observed in the temperature and coverage range of this investigation. The regions in which these transitions take place are shown in a coverage vs temperature plane. In region II of this plane, a phase transition takes place which results in excess heat capacities in the submonolayer region. In region III, a transition of a different nature takes place which resembles the ’’melting’’ process. The ’’two-dimensional gaslike’’ phase could only appear at temperatures higher than those attained in this work. In region I, the adatoms behave as highly localized oscillators, and those at the high energy sites contribute a major portion to the heat capacity below 3°K. The residual entropy was calculated for the two submonolayer coverages and was found to be equal to zero within the experimental error. The Ne–solid interaction energy is 2990 J/mole at 24°K and 2770 J/mole at 29°K. The integral heat of adsorption at 0°K was calculated in the submonolayer and multilayer region, and the difference between the first and subsequent monolayers is found to be more than 700 J/mole. The integral adatom–adatom interaction energy of adsorbed neon in the submonolayer region at 24°K varies almost linearly with coverage and is approximately equal to 290 J/mole at the completion of the first monolayer. The monolayer capacity of the adsorbent for neon depends on temperature and the coefficient of the thermal expansion of the first monolayer in the xy plane is estimated to be about 0.002 deg−1 in the temperature range 24–29°K. For higher coverages a substantially higher coefficient is observed, and this is attributed to a phase change which takes place in the structure of the film. Finally, the degree of homogeneity of graphitized carbon, Spheron 6, i.e., the ratio of the high energy sites to the planar sites of graphitized carbon, is found to be approximately equal to 0.1.
Chemistry of nuclear recoil 18F atoms. VIII. Mechanisms and yields of caging reactions in liquid phase 1,1-difluoroethane and 1,1,1-trifluoroethane
Authors: Manning Ronald G. , Root John W.
Published: February 2003

New procedures are reported for the specification of caging yields in nuclear recoil chemistry experiments. All five hot 18F substitution channels in CH3CF3 and CH3CHF2 exhibit caging at large density. The respective total caged yields at 195 °K are 4.0%±0.6% and 5.6%±0.6%, and the total yields of stabilized substitution products are 8.9%±0.4% and 8.6%±0.6%. The simplest plausible caging mechanism involves primary Franck–Rabinowitsch radical recombination of 18F atoms with aliphatic radicals. Density-variation results cannot be used for the qualitative detection of caging reactions unless excitation-stabilization complications have been shown to be unimportant.
Electronic structure of the luminescent centers in rare earth oxysalts doped by bismuth
Authors: Jacquier Bernard
Published: February 2003

A cluster model is assumed for the luminescent centers in Y2O3:Bi, BiOCl, and LnOCl:Bi (Ln=La,Y). A semiempirical self-consistent charge and configuration molecular orbital theory has been used to determine valence MO energies and populations. Including spin orbit and configuration interactions, we compute the pattern of the excited levels. Changing the number and the nature of the ligands in the cluster, one can see the variation of the localized charge with respect to the constitution and symmetry of the several environments. Dependence of the electronic levels on distance indicates a large substitution effect comparing with presently available experimental data. The results confirm the presence of allowed electronic excited states with strong oscillator strength in the near uv range, and trap levels. One of the latter always is the lowest electronic level and thus suggests an interpretation of the luminescent processes similar to that proposed for the La2O3:Bi phosphor.
Microwave spectrum, structure, and dipole moment of 3,3-difluorocyclopropene
Authors: Ramaprasad K. R. , Laurie Victor W. , Craig Norman C.
Published: February 2003

The microwave spectra of four isotopic species of 3,3-difluorocyclopropene have been investigated in the region from 18–40 GHz. Structural parameters obtained are r (C1C2) =1.321±0.001, r (C1C3) =1.438±0.007, r (CH) =1.075±0.001 , r (CF) =1.365±0.005 Å, &FCF=105.5±0.5°, &C1C3C2=54.6±0.4°, &HC1C2=148.4±0.1°. From the Stark effect the dipole moment has been determined to be 2.98±0.02 D.
Infrared and Raman spectra of the M+Cl3− ion pairs and their chlorine–bromine counterparts isolated in argon matrices
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

The argon matrix reactions of alkali chloride salts with Cl2 have been investigated through infrared and Raman spectra. In each infrared experiment, an intense product band was observed between 327 and 375 cm−1 and assigned to the antisymmetric stretching vibration ν3 of the Cl3− anion in the ion pair M+Cl3−. Raman spectra of these reaction mixtures yielded the same ν3 bands and, in addition, new signals between 253 and 276 cm−1 which are assigned to the symmetric stretching vibration ν1 of Cl3− in the M+Cl3− species. The observation of two stretching modes in the Raman spectra indicates that the Cl3− ions are asymmetric in these ion pairs. Bromine counterparts and the mixed chlorine–bromine anions were also formed, and used to verify that the anion in the M+X3− species contains three halogen atoms, with the metal ion beside the anion in an asymmetric ’’T’’ shaped structure.
Variational localized-site cluster expansions. I. General theory
Authors: Klein D. J.
Published: February 2003

Cluster expansions in terms of general sites (which may be spin orbitals, spin-free orbitals, shells, component atoms in a molecule, etc.) are developed for electronic wavefunctions. Emphasis is placed upon the potential simplification of such expansions in the case that the sites are localized. Formulas for both diagonal and off-diagonal matrix elements for such cluster expanded wavefunctions are given, and approximations to these general formulas are discussed.
Variational localized-site cluster expansions. II. Trees and near trees
Authors: Klein D. J. , Garcia-Bach M. A.
Published: February 2003

Cluster expanded wavefunctions involving localized-site states and only pair excitations between nearest neighbor sites are considered. In the case that the bonds connecting these nearest neighbor sites form a tree graph (without rings), exact matrix element formulas are obtained in terms of the inverse of a modified type of topological matrix associated with this tree graph. The treatment of some near-tree cases, without any touching rings, is also discussed.
Electron scattering by nitrogen molecules: Theory and application to elastic scattering and rotational excitation at 30–75 eV
Authors: Brandt Maynard A. , Truhlar Donald G. , Van-Catledge F. A.
Published: February 2003

An effective potential model applicable to electron scattering by small or large molecules, including rotational and vibrational motions, is developed for electron scattering by N2. The INDO/1s method is used to calculate the static potential at a large number of distances and a simple model is used for the polarization potential. The rotational and vibrational matrix elements of these potentials are accurately calculated and the scattering is treated by a truncated close-coupling expansion. With no adjustable parameters, calculated electronically and vibrationally elastic scattering cross sections are in reasonable agreement with experiment for angles 40°–50° and less at energies 30–75 eV. The rotational excitation cross sections have never been measured for N2 but are predicted to be large (as large as half the pure elastic cross sections when integrated over scattering angles and larger than the pure elastic differential cross sections at a range of large scattering angles at each energy).
Mode to mode energy transfer in OCS directly pumped by a CO2 laser
Authors: Siebert Donald R. , Flynn George W.
Published: February 2003

Laser-induced fluorescence has been observed from all the modes of OCS after pumping the 0→2ν2 overtone transition with the P (22), 9.6 μ line of the Q-switched CO2 laser. In pure OCS approximately 145 collisions are required to equilibrate the laser pumped bending modes (ν2=520 cm−1) with the antisymmetric stretching mode (ν3=2062 cm−1), while approximately 915 collisions are required for ν2–ν1 (ν1=859 cm−1) equilibration. The overall equilibration of the vibrational degrees of freedom with the translational/rotational degrees of freedom (deactivation of ν2) determined in these experiments corresponds to 7800 collisions for pure OCS, in excellent agreement with ultrasound results. The behavior of this relaxation process in OCS–rare gas mixtures suggests that very little vibrational energy is transferred directly into rotational degrees of freedom during deactivation of ν2 as might be expected for a molecule like OCS with no light atoms such as H or D. Simple calculations of the V–T relaxation probability using a short range repulsive force model are in unexpectedly good agreement with the experimental data, particularly for pure OCS. On the other hand, a long range attractive potential, dipole–dipole calculation which emphasizes the large electrical anharmonicity of the OCS bending modes appears to be in good agreement with the experimentally observed ν2–ν3 V–V energy transfer probability.
Solution for bound state wavefunctions and matrix elements by the piecewise analytic method
Authors: Dunker Alan M. , Gordon Roy G.
Published: February 2003

The solution of multidimensional bound state problems in quantum mechanics by the piecewise analytic method introduced by Gordon is discussed in detail. The numerical techniques necessary to calculate the wavefunctions and matrix elements are presented. An efficient least squares method for fitting the potential energy function to spectroscopic frequencies is described. Tests of the accuracy of the piecewise analytic method have been performed and the results are given. The techniques presented here are especially advantageous for systems in which the motion along the several coordinates is strongly coupled (beyond the perturbation theory limit).
Product magnetic deflection-slotted disk velocity analysis molecular beams kinetics: LiO(X 2Π) and LiO(A 2Σ) from Li+NO2
Authors: Sholeen Charlotte M. , Herm Ronald R.
Published: February 2003

Product magnetic deflection-slotted disk velocity analysis establishes both LiO(X 2Π) and LiO(A 2Σ) as products of the Li+NO2 bimolecular reaction. Velocity distributions of LiO(X 2Π) reactively scattered from crossed thermal beams of Li and NO2 are reported. Transformation of these data into the center-of-mass coordinate system provides a contour map of the differential reactive cross section which indicates that (a) the reaction mechanism is direct, i.e., the lifetime of the LiNO2 intermediate is less than its rotational period; (b) the LiO(X 2Π) product is preferentially scattered forward, i.e., in the direction defined by the Li velocity; (c) an appreciable fraction of the reaction exothermicity (∼45%) appears as product recoil energy; and (d) distributions in product recoil angle and energy are weakly coupled with forward scattering favoring higher recoil energy.
The effect of nonspherical interactions on the collision integrals for He–H2, He–HD, and He–HT systems
Authors: Dutta S. , Barua A. K.
Published: February 2003

Collisions integrals for the systems He–H2, He–HD, and He–HT are evaluated using nonspherical interactions in addition to the spherical ones. The effect of nonspherical interactions on the averaged values of the collision integrals is found to be not prominent. (AIP)
Molecular beam chemistry: Reactions of Cs+ with o-, m-, and p-chlorotoluenes
Authors: Miller G. D. , Safron S. A.
Published: February 2003

The reactive scattering of the C7H7+ and C5H5+ products from the collision of Cs+ with o-, m-, and p-chlorotoluenes at 9.5 eV is reported and compared with the results of the similar experiments with α-chlorotoluene. The analogous ’’stripping’’ and ’’rebound’’ mechanisms appear to be operative, with the following principal differences: (1) The C7H7+ ion is probably tropylium in these reactions rather than benzyl; (2) there is a significant inhibitive ’’steric’’ effect due to the neighboring methyl group in the o-chlorotoluene reaction; and (3) the C5H5+ ion from m- and p-chlorotoluenes is produced via an excited tropylium intermediate, while that from o-chlorotoluene seems to come from the decomposition of an excited benzyl ion. The implications for the reaction pathway of the observed ’’propensity rule,’’ that only products with singlet ground states are found at low collision energies, are also discussed.
Excited state bromine atom and molecule reactions
Authors: McAfee K. B. , Lum R. M. , Hozack R. S.
Published: February 2003

Excitation of bromine at wavelengths near the dissociation limit of the B 3Π+0u electronic state is shown to produce substitution and extraction reactions of excited bromine with olefins. These reactions are produced by drastically shortening free-radical chains in contrast to more conventional photochemical experiments which yield addition products only. Shortening of radical chains is accomplished by irradiating a flowing gas mixture in a capillary optical reactor. Wavelength selective photochemical reactions of excited 2P1/2 bromine atoms are found to occur simultaneously with gas phase ground state 2P3/2 atom reactions and with surface reactions. Evidence for reactions of electronically excited molecules of bromine in the B 3Π+0u state is also found.
On different forms for the binary transition superoperator
Authors: Olmsted R. D. , Rainwater J. C. , Snider R. F.
Published: February 2003

Eu [J. Chem. Phys. 63, 298 (1975)] has recently derived a formula relating three superoperators. On comparison with a related equation of Snider and Sanctuary [J. Chem. Phys. 55, 1555 (1971)], Eu questions the validity of the latter. This paper points out distinctions between quantities which Eu associates as being identical, or whose differences are of no import. It is shown that these distinctions appear sufficient to account for the difference between Eu’s formula and that of Snider and Sanctuary.
Triplet exciton caging in two dimensions
Authors: Arnold S. , Alfano R. R. , Pope M. , Yu W. , Ho P. , Selsby R. , Tharrats J. , Swenberg C. E.
Published: February 2003

It is proposed that the bimolecular process of triplet exciton fusion to form singlet excitons can be enhanced by reducing the size of the domain in which the triplet exciton pair is free to move. These small domains, or exciton cages, are much more effective when the host material is highly anisotropic, and the triplet excitons are constrained to move in one or two dimensions. In the present case, the host material is tetracene crystal in which triplet exciton diffusion is essentially two dimensional. The exciton caging is produced by introducing relatively high concentrations of a guest molecule that can reflect the triplet exciton rather than trap it; in the present case, 2,3-benzocarbazole (BC) is used. Polycrystalline mixtures of BC in tetracene were prepared in the mol fraction range 0%–50%. In tetracene, the mode of decay of the first excited singlet state is by fission into two neighboring triplet excitons that can undergo geminate recombination. The introduction of BC is found to increase the tetracene fluorescence lifetime at room temperature from 100 psec at 0% BC to 360 psec at 50% BC, while the lifetime at 77 °K is relatively constant. In addition, the characteristic increase in the tetracene fluorescence quantum efficiency with increasing excitation intensity is found to diminish as the BC concentration is increased. The lifetime and the relative fluorescence efficiency experiments are interpreted in part by invoking the concept of exciton caging, in which the geminate recombination of the original triplet exciton fission pair is enhanced by the presence of exciton reflectors that reduce the size of the domain in which the excitons can freely move. A computer simulation is made of the fission and fusion process in an ideal mixed crystal containing randomly distributed exciton reflectors, and the response of the model agrees well with the observed results.
Self-consistent phonon calculation of the elastic constants of the β phase of solid N2
Authors: Goldman Victor V. , Klein Michael L.
Published: February 2003

The self-consistent phonon approximation of anharmonic lattice dynamics is used to calculate the elastic constants of solid hcp β-N2. We have assumed completely disordered molecular orientations and a (12–6) Lennard–Jones intermolecular potential. Unlike previous work our calculations take explicit account of certain cubic anharmonic terms. Commonly accepted values of the intermolecular potential parameters give rise to triple point elastic constants that are in poor agreement with recent experimental values derived from Brillouin scattering in oriented single crystals. Calculations using intermolecular potentials derived from sphericalization of atom–atom potentials to be found in the recent literature yield equally poor results. Possible explanations for this are discussed.
Excluded volume effect on polymer dynamics in concentrated solutions
Authors: Freed Karl F.
Published: February 2003

We consider the dynamics of polymers in concentrated solutions under the combined effects of excluded volume interactions and hydrodynamical interactions (including all inter- and intrachain effects). Exact dynamical equations are derived which describe the motion of a single chain with screened hydrodynamics in the presence of the instantaneous excluded volume field of all other chains, an exact equation representing the precursor to all reptationlike models. Analogous equations are also obtained for the dynamics of a pair of correlated chains, e.g., ones involved in a temporary entanglement in a concentrated system. For the uncorrelated case of phantom chains, Edwards’s results for the excluded volume effect on equilibrium properties are incorporated into the averaged dynamics to determine the excluded volume effect on the polymer viscosity and the effective bead friction coefficient in concentrated solutions. Experimental viscoelastic data for concentrated solutions and melts are shown to provide a basis for the use of phantom chain models when the polymer chain lengths do not become too large. The effective bead friction coefficient is used in an accompanying paper, along with the dynamical equations derived here, to study quasielastic neutron scattering cross sections from concentrated polymer solutions.
Excluded volume effect on quasielastic neutron scattering from concentrated polymer solutions
Authors: Freed Karl F. , Edwards S. F. , Warner Mark
Published: February 2003

A study is made of the excluded volume dependence of quasielastic neutron scattering from polymer solutions which are concentrated enough that excluded volume interactions are screened for distances comparable to the mean-square-end-to-end vector distance. Both incoherent and coherent scattering are considered as well as the scattering by individual isotopically labeled chains. The theory is developed by employing two different formulations as certain further generalizations will be facilitated from the availability of the two viewpoints. One approach uses the Mori–Zwanzig theory of generalized Brownian motion with the recognition that at low frequencies the memory terms are unimportant for chain dynamics. The hydrodynamic interactions are fully incorporated through the use of a concentration dependent effective bead friction coefficient ν as derived in an accompanying paper, and the screened excluded volume interactions are included for the high concentration limit following Edwards. The coherent scattering cross section contains an additional vρ0k2ν−1 term in the ω−k dispersion relation beyond the usual Rouse (k)4 term, where v is the excluded volume and ρ0 is the monomer number density. The incoherent scattering cross section and the coherent one for scattering by individual labeled chains display the same k4 Rouse dependence, described by de Gennes, but with coefficients that are altered by the excluded volume. The second approach uses a self-consistent Fokker–Planck formalism that was previously developed for the study of turbulence. The theory ignores the effects of entanglements, and the experimental justification for this approximation is provided for chains with molecular weights below a certain critical value.
Perturbation theory and ionic models for alkali halide systems. II. Dimers
Authors: Brumer Paul , Karplus Martin
Published: February 2003

Contributions to the potential energy of alkali halide dimer systems (MXM′X′) and the assumptions inherent in ionic models commonly used to describe dimer structure are analyzed using exchange perturbation theory. Numerical results demonstrate that no simple ionic model satisfactorily accounts for all available experimental data. The perturbation theory approach suggests the incorporation of many-body terms, first order in the potential and second order in the overlap, as the first correction necessary to account for the experimental results. The need for further theoretical work on many-body contributions and experimental work on dimer properties is emphasized.
Statistical mechanics of helical wormlike chains. I. Differential equations and moments
Authors: Yamakawa Hiromi , Fujii Motoharu
Published: February 2003

A statistical-mechanical theory of wormlike chains with helical conformations arising from bending and torsional energies is developed. A differential equation for the trivariate distribution function of the end-to-end distance, the unit tangent vector, and the unit curvature vector is derived from the path integral formulation, and several moments are evaluated. The results show that the characteristic ratio for the end-to-end distance or the radius of gyration as a function of chain length t exhibits a maximum with a swelling at some t greater than the maximum point under certain conditions. It is applied to atactic and syndiotactic poly(methylmethacrylate) chains, and their helix parameters are determined reasonably. The theory is also applied to the helix–coil transition in polypeptide chains.
Comparison of three numerical techniques for calculating eigenvalues of an unsymmetrical double minimum oscillator
Authors: Wicke Brian G. , Harris David O.
Published: February 2003

Disagreement between calculated vibrational eigenvalues for the E, F 1Σ+g state of H2 obtained by three different numerical techniques has been interpreted in the literature in terms of inherent inadequacies of these techniques. We have tested these three numerical procedures by using each to obtain the eigenvalues of an unsymmetrical double minimum potential. An analytical potential was used to eliminate uncertainties introduced by interpolation. All three techniques are shown to give accurate results when properly applied.
Br2 B 3Π (0u+) excitation spectra and radiative lifetimes in rare gas solids
Authors: Bondybey V. E. , Bearder S. S. , Fletcher Christine
Published: February 2003

Following excitation into either the bound B 3Π (0u+) vibrational levels or into the continuum, Br2 emits from the v=0 level of this state. Study of the excitation spectrum in the continuous region permits direct measurement of the B state contribution to the visible absorption and its separation from the stronger overlying 1Π1 continuum and confirms the assignment of the long wavelength shoulder on the visible absorption spectrum to the B state. A value of 7.3 μs is found for the B (0u+) v=0 emission lifetime in solid argon. Extrapolation based on the matrix data leads to an estimate of 12±2 μs for the radiative lifetime of the gas phase molecule.
Response to the Comment: ’’Concerning the effect of an applied electric field on the dissociation of ion pairs into free ions’’
Authors: Mozumder A.
Published: February 2003

A reply is made to comments [G. R. Freeman, J. Chem. Phys. 64, 911 (1976)] made on a recent paper [A. Mozumder, J. Chem. Phys. 60, 4305 (1974)] concerning Onsager’s theory of the effect of an applied field on the dissociation of ion pairs in the free ions. (AIP)
Infrared and Raman spectra and structures of matrix isolated magnesium dihalides: MgF2, MgCl2, MgBr2, and MgI2
Authors: Lesiecki Michael L. , Nibler Joseph W.
Published: February 2003

Infrared and Raman spectra are presented for the magnesium dihalides isolated in matrices. Of particular interest is the Ramen observation of the ν1 symmetric stretch in argon matrices at 550.0, 326.5, 197.9, and 147.6 cm−1 for MgF2, MgCl2, MgBr2, and MgI2, respectively. For MgF2, ν1 had previously been assigned to a strong infrared feature near 480 cm−1, and this activity (along with isotopic frequency shifts) has been cited as evidence for a nonlinear structure for MgF2. The present results undermine this argument and a linear structure is favored, a result consistent with the failure to detect a dipole moment in gas phase electric deflection measurements. Force constants calculated for a linear structure are compared with values obtained from several theoretical models and the results favor an ion–ion interaction model in which the bonding is essentially ionic. Finally, features due to MgX2 dimers are assigned on the basis of a bridged D2h model, and some features attributed to weak complexes between MgX2 and impurities in the matrix are discussed.
Velocity dependence of electronic excitation transfer reactions: (CO*,N2*,Kr*)+Hg(6 1S) → (CO,N2,Kr)+Hg(6 3P1)
Authors: Lee Wellington , Martin Richard M.
Published: February 2003

The velocity dependences of the sensitized fluorescence of mercury by metastable CO(a 3Π, N2(A 3Σ+u), and Kr(5 3P2,0) were determined using crossed molecular beams with the metastable beams time-of-flight velocity selected. With CO(a) and N2(A) excitation is directly to Hg(6 3P1), whereas with Kr(3P) higher triplet states of mercury are also excited. The collisional energy ranges studied with CO, N2, and Kr were 32–516, 30–468, and 31–394 meV, respectively. The cross section collisional velocity dependence was found to be quite similar for the three systems, with a negative v−n velocity dependence in the low velocity region becoming neutral or positive at higher velocities. The values of n were 0.750±0.014 for CO(a) + Hg up to 242 meV, 1.009±0.016 for N2(A) + Hg up to 202 meV, and 0.810±0.024 for Kr(3P) + Hg up to 140 meV. The results are compared with velocity dependences of other nonadiabatic reactions, and discussed in terms of electronic coupling vs curve crossing theories for such processes.
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o calculations of the barrier for internal rotation in nitrous acid
Authors: Benioff Paul , Das Gurupada , Wahl Arnold C.
Published: February 2003

A b initio calculations of the trans–cis energy difference and the barrier for internal rotation in nitrous acid were made using a double zeta quality basis set and MCSCF+CI methods. The geometry parameters used were the optimized ones obtained by Skaarup and Boggs. The trans–gauche barrier and trans–cis energy difference were calculated here to be 11.4 and 2.5 kcal/mole in good agreement with the experimental values of 11.57±0.2 and 0.4±1.0 kcal/mole, respectively. It was found that exclusion of only one configuration involving excitation of a lone pair on the NOH oxygen from the MCSCF and CI lists reduced the trans–gauche barrier to 6.4 kcal/mole. This shows the importance of lone pair electrons on the oxygen, and not on the nitrogen as previously suggested, to the high barrier.
Properties and reactions of uranium hexafluoride by ion cyclotron resonance spectroscopy
Authors: Beauchamp J. L.
Published: February 2003

Ion cyclotron resonance spectroscopy has been employed to investigate the formation and reactions of both positive and negative ions derived from uranium hexafluoride. Positive ion reactions are dominated by processes whereby the various UF+n fragments (n=1–3,5) react to form UF+4, and by clustering of UF+n (n=3–5) with UF6. Trapped negative ion studies indicate that UF−6 can be generated both by electron attachment, which is unusually slow, and by electron transfer from other species, including UF−5, SF−6, and Cl−. In addition, UF−7 is observed, both in UF6 alone, and in higher abundance when fluoride donors such as SF−5 are present. Certain limits on the thermochemical properties of ions derived from UF6 have been determined, and a high electron affinity is indicated for UF6. The species UF−6 is especially stable and much less reactive than SF−6 in mixtures with acidic neutrals. The propensity of negative ions derived from UF6 to form clusters is substantially less than observed for positive ions.
Kinetic and thermodynamic properties of INO and INO2 intermediate complexes in iodine recombination
Authors: van den Bergh H. , Troe J.
Published: February 2003

The recombination of iodine atoms after laser flash photolysis has been studied in the presence of NO or NO2 at 320–450 K and 0–200 atm of He. The reaction proceeds in two stages. The first stage is governed by the recombination I+NO(+He) →INO(+He) or I+NO2(+He) →INO2(+He), for which the full falloff curves are derived. The second stage is governed either by reaction between two INO (or INO2) molecules, or, at higher temperatures, by thermal decomposition of INO (or INO2). The reaction involves the elementary steps of a typical intermediate complex mechanism of atom recombination. The rates of all of these elementary steps can be separated. From the I+NO⇄INO and I+NO2⇄INO2 equilibrium constants the enthalpies of formation of INO and INO2 of ΔHfo298(INO) =29.0±1 kcal mol−1 and ΔHfo298(INO2) =14.4±1 kcal mol−1, respectively, are derived.
Spatial distribution of energy deposited in nitrogen by electrons
Authors: Barrett John L. , Hays Paul B.
Published: February 2003

The spatial distribution of the energy deposited by kilovolt electrons moving through gaseous molecular nitrogen was measured. The range of electrons of initial energy 300 eV to 5 keV was obtained and can be expressed by the formula R=K1+K2EA1−K3EA2, where R is the range, E is the initial energy of the electrons, and Ki,Ai are constants. The range, in this energy interval, is greater than that determined by previous measurements. A source of error, not previously discussed, is considered. The energy region (above 1 keV) where the simpler expression R=KEA holds is discussed. It is shown that this is the energy region where the energy and range dependence of the energy deposition curve can be removed and a normalized, characteristic energy deposition curve λ can be obtained. The efficiency of conversion of electron energy at 1 keV and 280 μ pressure to energy of photons at 3914 Å was measured to be (0.28±0.03) %.
Quantum mechanical streamlines. III. Idealized reactive atom–diatomic molecule collision
Authors: Hirschfelder Joseph O. , Tang K. T.
Published: February 2003

An atom–diatomic molecule collision is simulated by considering an idealized potential energy surface which is a two-dimensional duct with an adjustable potential in the corner region. This potential is symmetric with respect to an interchange of the x and y Cartesian coordinates. Explicit expressions for the wavefunctions are obtained which make use of this symmetry. Also analytical relations are obtained between the transmission and reflection coefficients and their phases. Quantum mechanical streamlines are computer graphed for a large number of energies and positive, negative, and zero values of the potential energy in the corner region. Special attention is given to the quantized vortices (surrounding wavefunction nodes) which appear in the streamlines. When only one energy channel is open, the streamlines are symmetric and the flux is antisymmetric. This occurs because the wavefunction is a linear combination (with complex coefficients) of two real solutions, one symmetric, the other antisymmetric. When two energy channels are open, the streamlines are no longer symmetric. This is because the two wavefunctions are now linear combinations (with complex coefficients) of four real solutions. Although the second wavefunction is expressed in terms of the first, their streamlines have quite different patterns. From the Argand diagrams, it is clear that the reflection, transmission, and Feshbach-type resonances which occur correspond to the Breit–Wigner type. At a reflection resonance energy all of the streamlines disappear; at slightly higher energy, the streamlines reappear with the same vortices as before but rotating in the opposite direction. The probability density is only symmetric at transmission resonance energies. At energies higher than the Feshbach-type resonance energy (where a second energy channel becomes open), the streamlines pattern ceases to be symmetric and the flux antisymmetric. The exact quantum mechanical wavefunction for this idealized model is the same as the exact semiclassical wavefunction. These wavefunctions are infinite series. It is found that for the low energies which we consider, an excellent approximation is obtained by truncating the series after the first two terms. This corresponds to a semiclassical wavefunction formed from two manifolds of classical trajectories.
Surprisal analysis and probability matrices for rotational energy transfer
Authors: Levine R. D. , Bernstein R. B. , Kahana P. , Procaccia I. , Upchurch E. T.
Published: February 2003

The information-theoretic approach is applied to the analysis of state-to-state rotational energy transfer cross sections. The rotational surprisal is evaluated in the usual way, in terms of the deviance of the cross sections from their reference (’’prior’’) values. The surprisal is found to be an essentially linear function of the energy transferred. This behavior accounts for the experimentally observed exponential gap law for the hydrogen halide systems. The data base here analyzed (taken from the literature) is largely computational in origin: quantal calculations for the hydrogenic systems H2+H, He, Li+; HD+He; D2+H and for the N2+Ar system; and classical trajectory results for H2+Li+; D2+Li+ and N2+Ar. The surprisal analysis not only serves to compact a large body of data but also aids in the interpretation of the results. A single surprisal parameter ϑR suffices to account for the (relative) magnitude of all state-to-state inelastic cross sections at a given energy.
Comparison of quantum mechanical and quasiclassical scattering as a function of surface topology
Authors: Kellerhals G. E. , Sathyamurthy N. , Raff L. M.
Published: February 2003

All-channel, time-dependent quantum mechanically computed total reaction probabilities for collinear A+BC→AB+C reactions have been compared with quasiclassical trajectory results on three hypothetical LEPS-type potential-energy surfaces. Each surface had identical reactant and product limits and barrier height but differed in the location of the saddle point. In all calculations the atomic masses were taken to be 1 amu to maximize possible quantum effects. Quantum and quasiclassically computed total reaction probabilities are found to be in reasonable accord, except near threshold, for reactions taking place on symmetric potential-energy surfaces analogous to that for the (H+H2) exchange reaction. If the surface symmetry is removed by displacing the saddle point into the exit channel, however, the extent of agreement between quantal and quasiclassical results is drastically reduced. At an energy of 1.5 kcal/mole above threshold, the quasiclassical reaction probability is over five times the quantal result. Even at energies 7.5 kcal/mole in excess of threshold, the quasiclassical to quantum mechanical ratio is 2. The extent of agreement between the two calculations is also reduced if the surface symmetry is removed by moving the saddle point into the entrance channel, although the effect is smaller than for displacement into the exit channel. Both quasiclassical and quantal calculations predict a large increase in reaction probability for asymmetric surfaces with the barrier in the exit channel whenever the total energy is preferentially partitioned in BC vibration. These results indicate that approximate scattering theories are best tested by investigation of systems with low reduced mass moving on asymmetric potential-energy surfaces with the saddle point located in the product channel.
Infinite-order sudden approximation for rotational excitation of hydrogen molecules by electrons in the energy range 10–40 eV
Authors: Truhlar Donald G. , Poling Ruth E. , Brandt Maynard A.
Published: February 2003

Electron scattering by H2 is treated using a noncentral interaction potential including short-range and long-range static contributions, exchange, and polarization effects. The molecule is treated as a rigid rotator and the scattering is treated in the infinite-order sudden approximation. The results show that the rotational excitation cross section exceeds the elastic scattering cross section at large scattering angles at intermediate energies but not at small angles at low energies.
Characterization of the methylene 13C chemical shift tensor in the normal alkane n-C20H42
Authors: VanderHart D. L.
Published: February 2003

13C NMR chemical shifts in a single crystal of n-eicosane (n-C20H42) have been measured using the method of high power proton decoupling and the chemical shift tensors were characterized. Principal values in ppm relative to CS2 are: (a) For CH3: σ11=166.7±2, <22=171.2±2.0, σ33=189.8±2; (b) For the α-methylene: σ11=156.0±2.5, σ22=163.1±2.5, σ33=178.0±2.5; and (c) For the interior methylenes: σ11=142.6±2.0, σ22= 154.6±2.0, σ33=175.6±2.0. From x-ray studies, only the unit cell parameters are known. On the basis of isolated molecule symmetry considerations, cross-polarization rate studies, and oriented polyethylene spectra, the interior methylene chemical shift tensor is assigned with respect to molecular orientation. The crystallographic axes are also related to the interior methylene chemical shift tensor and a prediction is made for the C–CH3 bond direction relative to these crystallographic axes. The principal axes of the α-CH2 and the CH3 chemical shift tensors do not coincide with any bond directions. The difference (5.6 ppm) between the interior methylene average chemical shielding in solid n-eicosane and the isotropic liquid value is also discussed.
Alkali halide molecules: Configurations and molecular characteristics of dimers and trimers
Authors: Welch D. O. , Lazareth O. W. , Dienes G. J. , Hatcher R. D.
Published: February 2003

The equilibrium configurations, equilibrium distances, binding energies, and vibrational frequencies have been calculated for dimer and trimer molecules of the alkali halides. The treatment is based on an earlier application of a simple shell model to monomer molecules and crystals of the alkali halides in which all characteristic interaction parameters have been assigned. Thus, there are no adjustable parameters in the present calculation of the dimer and trimer characteristics. The stable dimer configuration was found to be the rhombus with the halogen–metal–halogen angle varying systematically from greater than 90° to less than 90° as (r−−r+), the difference in the ionic radii, varies from positive to negative [i.e., at (r−−r+) ?0 the configuration is essentially a perfect square, e.g., Na2F2, Rb2Cl2]. The linear dimers are unstable relative to the rhombus. The perfect hexagon was found to be the stable configuration of the trimer molecules. There is good agreement with the meager experimental data available.
On the dilute gas two paticle density matrices of p-H2 and He4
Authors: Weres Oleh
Published: February 2003

In the preceding paper we demonstrated thar the reduced two particle density matrix of simple quantum liquids could profitably be re-expressed in terms of a Taylor expansion of its logarithm about the diagonal. In the present publication we examine the Taylor coefficients which arise when the dilute gas two particle density matrix is expanded in this way. In particular, we evaluate the leading coefficients of p-H2 and He4 exactly and extend the Wigner-Kirkwood approximation to provided approximate expressions for them. We demonstrate how these approximate expressions may be applied to yield results superior to those yielded by the ordinary Wigner-Kirkwood approximation. IN an appendix we demonstrate how the Block equation for the dilute gas two particle density matrix may be reduced to an equivalent closed set of equations for the leading Taylor coefficients.
Paramagnetic resonance study of photolyzed crystals of dimethylnitramine
Authors: Owens F. J. , Vogel Veronica L.
Published: February 2003

The uv-irradiation of single crystals of dimethylnitramine, (CH3)2–N–NO2, produces an EPR spectrum arising from an unpaired electron with hyperfine coupling to six hydrogen and one nitrogen nuclei. The parameters of the axial spin Hamiltonian are g∥=2.0017, g⊥=2.0091, A∥(N) =33.0 G, A⊥(N) =6.5 G, and Aiso(H) =14.0 G. The spectrum is assigned to a (CH3)2–N radical. INDO molecular orbital calculations of hyperfine splittings in the (CH3)2–N radical support the assignment when allowance is made for the strong dependence of the hydrogen isotropic hyperfine splitting on the C–N bond length.
Dipole moments of KI, RbBr, RbI, CsBr, and CsI by the electric deflection method
Authors: Story T. L. , Hebert A. J.
Published: February 2003

Electric deflection experiments using a velocity selected molecular beam apparatus have given the following electric dipole moments [measured in μe (Debye)]: KI, 10.82 (10); RbBr, 10.86 (10); RbI, 11.48 (10); CsBr, 10.82 (10); CsI, 11.69 (10).