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Volume 70, Issue 11, 01 June 1979

Electron scavenging in glasses
View Description Hide DescriptionElectron scavenging reactions in amorphous media are analyzed using a random walkhopping model, which can yield a time dependent rate constant. The underlying waiting time distribution for the time between hops is shown to be able to lead to an exponential of a fractional power of time decay of the reactants, as opposed to simple exponential decay. This type of kinetic behavior has been found by Hamill and Funabashi when replotting electron scavenging data in an appropriate manner. A different waiting time distribution which has been used for the analysis of charge transfer in amorphous xerographic films is shown to lead to an algebraic decay law, as has been observed in various phosphors.

PMR spectroscopy of the tunneling ammonium ion: Generalized line shape theory and some experimental examples
View Description Hide DescriptionThe site symmetry of the NH_{4} ^{+} ion is low in many crystal structures of ammonium compounds. This renders inadequate the commonly considered theoretical models of the tunnel‐split ground librational state of NH_{4} ^{+}, which are based on a perfectly tetrahedral crystal field due to assumption of equivalent threefold, and neglect of twofold, axes of symmetry. A general treatment of the NMRline shapetheory is therefore considered and Hamiltonian matrix elements for tunneling are explicitly calculated in terms of the seven overlap‐integral parameters related to the four C_{3}– and three C_{2}–symmetry axes of a tetrahedron. The model allows us to discuss all possible ground torsional level structure of the tunneling ammonium ion. The proton NMRline shapes are calculated for all representative cases, and the results are used in comparison with experimental data to elucidate the ground torsional level structures of NH_{4}VO_{3}, (NH_{4})_{2}ZrF_{6}, (HN_{4})_{2}TiF_{6}, (NH_{4})_{2}TeO_{4}, and (NH_{4}) IO_{4}.

Ionic transitions hypersensitive to environment
View Description Hide DescriptionIt is shown that the so‐called dynamic‐coupling mechanism of Mason, Peacock and Stewart, which was introduced to account for the peculiar hypersensitivity of the optical transitions of rare‐earth ions with respect to their environment, is formally identical to an earlier proposed mechanism based on an inhomogeneousdielectric. Errors in the derivation of the dynamic‐coupling mechanism are corrected, and examples of the rare‐earth trihalides are considered.

Clustering distances critical to metal dimer formation in the secondary ion mass spectra (SIMS) of cesium chloride
View Description Hide DescriptionCs^{+} _{2} has been measured in SIMS of KCl–CsCl solid solutions in which [Cs]/[K] varies from 10^{−1} to 10^{−5}. The data indicate that there is a maximum clustering distance, i.e., Cs–Cs distance, above which Cs^{+} _{2} is not formed. This distance is ∼200 and ∼400 Å for He^{+} and Ar^{+} primary ions, respectively, and is independent of primary energy over the range 0.2–3.0 keV. The results are consistent with a mechanism in which bound secondary clusters consisting of atoms from contiguous as well as noncontiguous lattice sites are formed through potential interactions during irreversible adiabatic expansion of an activated region near the surface surrounding the impact site.

Effect of pressure on resistivity and Mössbauer spectra of the mixed valence compound Sn_{2}S_{3}
View Description Hide DescriptionA continuous decrease of the resistivity by five orders of magnitude has been observed on Sn_{2}S_{3} at room temperature under pressures up to 120 kbar. PbSnS_{3} shows only a decrease by three orders of magnitude. SnGeS_{3} and PbGeS_{3} remain insulators in the same pressure range. Mössbauer measurements on ^{119}Sn in Sn_{2}S_{3} at room temperature and pressures up to 55 kbar indicate that Sn_{2}S_{3} remains a type I mixed valence compound in this pressure range. The two crystallographic positions of Sn exhibit significantly different Debye temperatures, Θ_{ D }(Sn^{II}) =207 K, Θ_{ D }(Sn^{IV}) =429 K. Moreover, distinct ^{119}Sn pressure shifts ∂δ (Sn^{II})/∂P?+2.1 mm/sec kbar and ∂δ (Sn^{IV})/∂p?+1.2 mm/sec kbar are observed.

Monte Carlo quasiclassical trajectory study of the collision‐induced dissociation of hydrogen by neon
View Description Hide DescriptionCross sections for dissociation from particular vibration–rotation states of H_{2} on collision with Ne were calculated using the quasiclassical trajectory(QCT) method. An accurate a b i n i t i opotential energy surface was used for the Ne–H_{2} interaction. Two different forms were used for the H_{2} potential: the a b i n i t i o results of Kolos and Wolniewicz and a simple Morse function. Cross sections were similar for both cases and indicated that dissociation occurs from all v states. Both vibrational and rotational energy were shown to enhance the cross sections, and the probability of dissociation was shown to be strongly dependent on the orientation of the collision.

Effect of chemical environment on magnitude of x‐ray absorption resonance at L _{ I I I } edges. Studies on metallic elements, compounds, and catalysts
View Description Hide DescriptionWe report spectra isolating the L _{ I I I } x‐ray absorption threshold resonance of the elements iridium,platinum, and gold in the pure metallic state and in a variety of compounds. When normalized spectra obtained on the metals are subtracted from those obtained on the compounds, the resulting difference spectra are related to differences in the electronic structure of the absorber atom in the two types of environment. The change in area of a threshold resonance line obtained from such a difference spectrum can be related to the ionicity of the bonds of the absorber atom in its compounds. Measurements on supported platinum and iridium catalysts provide information on electronic changes in the metal due to the small size of the metal clusters or to interaction with the support material.Information on electronic changes due to interaction of the catalysts with gas molecules may also be obtained.

Study of the Ar–N_{2} interaction. II. Modification of the electron gas model potential at intermediate and large distances
View Description Hide DescriptionThe failure of the electron gas model calculation of the Ar–N_{2}interaction potential at large intermolecular separations [J. Chem. Phys. 68, 5001 (1978)] is remedied by using a model developed earlier by one of the present authors and Gordon. The results give predictions of the Ar–N_{2}interaction potential for the entire portion of physically interesting region of the potential surface. The spherically averaged potential is compared with the available experimental potentials and shows the improved agreement.

Low temperature vibrational spectra, lattice dynamics, and phase transitions in some potassium hexahalometallates: K_{2}[XY_{6}] with X=Sn or Te and Y=Cl or Br
View Description Hide DescriptionThis paper deals with the observation and identification of phonon frequencies resulting from the low temperature phase transitions in K_{2}XY_{6} crystals. By means of a simple lattice dynamical model, the vibrational Raman and IR data available in the literature and obtained here have been analyzed. The model used is an extension of one previously used to explain the vibronic spectra of several related compounds.

Approximate scaling formula for ion–ion mutual neutralization rates
View Description Hide DescriptionA complex potential model is used to treat the mutual neutralization of positive and negative ions. The model is used to examine neutralization by the charge transfer mechanism and also by internal excitation leading to capture. It is found that electron transfer is the dominant process for simple ions and small hydrated ions. The numerical results of the theory have been parameterized in terms of the reduced mass of the collision and the electron affinity of the electron donor. This procedure yields an approximate scaling formula that fits a wide range of experimental data to an accuracy of about ±30%.

Amorphous state studies with the Gaussian core model
View Description Hide DescriptionProperties of the supercooled amorphous state have been studied for the Gaussian core model at reduced density ρ*=1. By monitoring the self‐diffusion constant and mean energy during a slow cooling to absolute zero, a glass transition was observed separating supercooled fluid from a rigid amorphous packing. At this glass transition the Wendt–Abraham criterion for the pair correlation function (g ^{(2)} _{min}/g ^{(2)} _{max}=0.14) is satisfied. However, former suggestions that second‐peak splitting in g ^{(2)} universally signifies the glassy state for monatomic substances are not supported.

Molecular beam electric deflection study of ammonia polymers
View Description Hide DescriptionThe molecular beam electric deflection behavior of (NH_{3})_{ n }, n=1 to 6, has been determined. The ammonia dimer is found to be polar and presumably has a single hydrogen‐bond structure. The higher polymers are nonpolar, compatible with cyclic, hydrogen‐bonded ring structures.

On the theory of chemically induced electron polarization (CIDEP). II. Potential forced diffusion
View Description Hide DescriptionA formula is developed for the spin polarization developed by radicals simultaneously diffusing in a spherically symmetric potential field and precessing in a magnetic field; general initial conditions are included, and two boundary conditions: zero rate and infinite rate of reaction at the point of contact. The special case of simple diffusion and infinite rate of reaction is also discussed and compared to the unreacting case treated in the first paper of this series. Asymptotic formulas are given that allow a simplification of the formulas when the exchange forces are very large. This takes the form of multiplying the spin polarization for simple diffusion by simple functions of the potential, allowing a very easy estimate of the effect of potential forced diffusion.

Trajectory study of O+H_{2} reactions on fitted a b i n i t i o surfaces I: Triplet case
View Description Hide DescriptionClassical trajectory calculations of cross sections for the reaction 0(^{3} P)+H_{2}(^{1}Σ^{+} _{ g }; ν, j) →OH(^{2}Π; ν′, j′)+H(^{2} S) have been performed for collision energies 1 kcal/mol⩽E⩽40 kcal/mol using an analytical fit to a recent a b i n i t opotential energy surface. Three initial vibrational states of H_{2}, ν=0, 1, and 2, are considered in order to study the influence of vibrational reactant energy on the OH production. With increasing vibrational quantum number, (a) the threshold shifts to lower energies, and (b) the cross sections rise more steeply with collision energy. Rotational excitation of H_{2} enhances the total reaction cross section for each vibrational state over the range of H_{2}(ν,j) states studied. The cross sections have been used to calculate reaction rate constants for temperatures 300°K⩽T⩽1000°K and the three lowest vibrational states. The ratio k (T,ν=0):k (T,ν=1):k (T,ν=2) is found to be 1:1.13×10^{4}:1.42×10^{6} at 300°K and 1:2.12×10^{1}:1.24×10^{2} at 1000°K, demonstrating that vibrational energy strongly enhances the reaction rate. At low temperatures, T≲500°K, the calculated rate constants for ν=0 are considerably smaller than the experimental results indicating that the barrier in the potential surface is probably too high. Agreement with experiments is improved for intermediate temperatures and is best for T≳1500°K where threshold effects are least important. The present calculations are in very good agreement with a recent rate measurement at T∼302°K by Light for reactions of vibrationally excited H_{2}(ν=1) molecules.

The Xα valence bond theory of weak electronic coupling. Application to the low‐lying states of Mo_{2}Cl_{8} ^{4−}
View Description Hide DescriptionWe show that valence bond (VB) concepts can be introduced into Xα theory. The resulting Xα–VB model yields energy states which either are pure multiplets or can be combined by straightforward projection to give pure multiplets. The new theory should be more computationally efficient than Hartree–Fock‐based CI models. A preliminary study of the δ→δ* transition in Mo_{2}Cl_{8} ^{4−} yields an excitation energy closer to experiment than previous theoretical values, including those obtained to date from GVB–CI calculations.

The high temperature vaporization and thermodynamics in the lutetium–sulfur system
View Description Hide DescriptionThe high temperature vaporizations of Lu_{3}S_{4} and nonstoichiometric LuS_{ x } (0.75<x<1.30) compositions in the lutetium–sulfur system were investigated by mass spectrometric and target collection Knudsen effusion techniques. Lu_{3}S_{4} vaporizes congruently according to: (1) LuS_{1.33}(s) =Lu(g)+1.33S(g) and (2) LuS_{1.33}(s) =LuS(g)+0.33S(g). The equilibrium partial pressures are: log P _{Lu}(atm) = −(29162±239)/T+(7.117±0.112); log P _{S}(atm) =−(29513±338)/T+ (7.104±0.159); and log P _{LuS}(atm) =−(31366±675)/T+(8.052±0.317). The second‐law thermodynamic values are: Reaction (1) ΔH°_{298}= 317.6±2.5 kcal mol^{−1}; ΔS°_{298}=79.6±1.5 eu; Reaction (2) ΔH°_{298}=196.9±3.3 kcal mol^{−1}; ΔS°_{298}=55.9±1.8 eu; and for LuS_{1.33}(s), ΔH°_{ f,298}=−126.5±2.5 kcal mol^{−1}; S°_{298}= 18.0±1.5 eu. The dissociation energy of LuS(g) is computed as D°_{0}= 120.6±3.2 kcal mol^{−1} from the mass spectrometric slopes for log I ^{+} _{Lu} T and log I ^{+} _{LuS} T vs 1/T, and with an iterated slope for logI ^{+} _{S} T consistent with the congruency of the vaporizationreactions. The activity of Lu across the homogeneity range of the monosulfide was measured. Together with the Gibbs–Duhem derived values for the activity of sulfur, the thermodynamic properties of stoichiometric LuS_{1.0} were calculated. These are: (in kcal mol^{−1}) ΔH°_{atom,298}=268.5±3.0; ΔH°_{subl,298}= 147.7±3.0; and ΔH°_{ f,298}=−99.6±3.1.

Temperature dependence of resonant isotopic vibrational energy transfer in H^{35}Cl–H^{37} Cl
View Description Hide DescriptionThe resonant vibration‐to‐vibration energy transfer process, H^{35}Cl(v=1)+H^{37}Cl(v=0) ?H^{35}Cl(v=0) +H^{37}Cl(v=1)+ΔE=2.1 cm^{−1} has been investigated over the temperature range 192–632 K by use of an isotopically selective transverse discharge chemical laser and infrared fluorescence techniques. The transfer probability shows a temperature dependence of P _{ V V }∼T ^{−1.6} over the range studied. This dependence is similar to those observed for a number of other near‐resonant processes, but differs from current theoretical predictions for resonantvibrational energy transfer. It is shown that a model based on a statistical distribution of rotational states resulting after a long‐lived collision is useful in understanding the parameters which must be important in resonantvibrational energy transfer.

Vibrational–translational energy exchange in liquid chloroform as a function of temperature as determined by Brillouin scattering
View Description Hide DescriptionThe temperature dependence of the vibrational relaxation time τ_{vib} of pure liquid CHCl_{3} was determined by Brillouin scattering between −5 and +55 °C. From the experimental results the number of intermolecular collisions Z _{10} necessary for the deactivation of the lowest vibrational mode has been calculated using the moving wall cellmodel. In contrast to other liquids no linear relationship between lnZ _{10} and (1/T)^{1/3} could be found. Different reasons for this behavior, which possibly is due to the weak dipole moment of CHCl_{3}, are discussed.

Laser catalyzed translational to vibrational energy conversion in CH_{3}F–O_{2} mixtures
View Description Hide DescriptionTranslational energy changes accompanying laser induced V–V equilibration processes in CH_{3}F/O_{2} and CH_{3}F/Ar mixtures have been studied using the time resolved thermal lensing technique. Enhanced translational cooling in CH_{3}F/O_{2} mixtures over equivalent CH_{3}F/Ar mixtures was observed. The cooling amplitude varied directly with O_{2} mole fraction while remaining independent of Ar mole fraction. The above observations suggest strongly the presence of a large endothermic V–V energy transfer pathway in pure CH_{3}F. A knowledge of the CH_{3}F equilibration pathway yields a complete description of the vibrational and translational energy distribution at V–V steady state, and suggests that the ν_{3}manifold is the predominant reservoir for vibrational energy. In mixtures with a large excess of O_{2}, each absorbed laser photon leads to a vibrational excitation approximately one and one half times the initial photonenergy. This vibrational energy increase is brought about by ’’stealing’’ energy from the translational/rotational degrees of freedom. The techniques used here to derive the relative heating and cooling amplitudes for CH_{3}F/O_{2} mixtures undergoing a specified energy transfer path are general and can be applied to any molecular gas or gas mixture.

Molecular beam studies of benzene dimer, hexafluorobenzene dimer, and benzene–hexafluorobenzene
View Description Hide DescriptionA detailed study of the electric deflection of molecular beams of (C_{6}H_{6})_{2}, (C_{6}F_{6})_{2}, and C_{6}H_{6}–C_{6}F_{6} is reported. Although no resolved microwave or radio frequency transitions were observable, examination of unresolved beam transitions at radio frequencies were useful in establishing that the homomolecular dimers (C_{6}H_{6})_{2} and (C_{6}F_{6})_{2} are asymmetric rotors while the heteromolecular dimer C_{6}H_{6}–C_{6}F_{6} is a symmetric top. From analysis of the quantitative electric deflection the dipole moment of C_{6}H_{6}–C_{6}F_{6} is 0.44±0.04 D.