bluuebunny/crossref_metadata_2025 huggingface dataset:
Rate constants and quenching mechanisms for the metastable states of argon, krypton, and xenon
Authors: Velazco J. E. , Kolts J. H. , Setser D. W.
Published: February 2003

Rate constants have been measured by the flowing afterglow technique at 300 °K for the quenching of Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2) by a large number of small molecules. For the same reagent, the magnitudes of the cross-sections usually increase in the series Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2). The Ar(3P2) and Ar(3P0) data are compared to results in the literature for these states and to data for Ar(3P1) and Ar(1P1). The set of thermal quenching cross sections are used to test the correlations between the magnitudes of the cross sections and properties of the reagents as predicted by the orbiting, absorbing-sphere, golden rule, and curve-crossing mechanisms for quenching. The best correlation is between the cross sections and the C6 coefficient. The analysis supports the proposition that the orbiting-controlled, curve-crossing model is the general mechanism governing the magnitude of the thermal cross sections for quenching of the metastable states. This model explains the very large quenching cross sections of F2 and OF2 (relative to other molecules composed of first row elements) because covalent–ionic curve crossing occurs outside the conventional orbiting radius. The validity of the simple van der Waals dispersion forces as being the dominant entrance channel interaction between the excited state rare gas atoms and the reagents is discussed.
Resonance Raman spectra of octaethylporphyrinato-Ni(II) and m
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o-deuterated and 15N substituted derivatives. II. A normal coordinate analysis
Authors: Abe M. , Kitagawa T. , Kyogoku Y.
Published: February 2003

Normal coordinate calculations are carried out for all the in-plane modes of octaethylporphyrinato-Ni (II) and its meso-deuterated and 15N substituted derivatives. With 37 constants of a modified Urey–Bradley force field and a structural model with D4h symmetry, 59 resonance Raman lines (A1g+B1g +A2g+B2g) and 38 infrared bands (Eu) of these three molecules are assigned. The vibrational modes of the Raman active species are represented in terms of the Cartesian atomic displacement vectors. Based on the present results, some important resonance Raman lines of hemoproteins are interpreted. The so-called ’’oxidation state maker’’ (Band IV) is due to an in-phase breathing-like mode of four pyrrole rings although being somewhat deformed by the large contribution of the Cα–N symmetric stretching term. The spin state sensitive Raman lines, namely, Band I and III, are associated mainly with methine bridge (Cα–Cm) stretching modes. Two prominent anomalously-polarized Raman lines of hemoproteins around 1580 and 1300 cm−1 are primarily due to the Cα–Cm stretching and Cm–H bending modes, respectively.
Low pressure infrared multiple-photon photochemistry of acetonitrile
Authors: Lesiecki Michael L. , Guillory William A.
Published: February 2003

The infrared(ir) multiple-photon photolysis of CH3CN has been performed over the pressure range 10 mtorr to 5 torr. The rotational and vibrational distributions of CH(X 2Π) and CN(X 2Σ+) as well as their corresponding temperatures, Tr and Tv for CN(X) and Tr for CH(X), are reported as a function of initial CH3CN pressure. Experiments involving the time evolution of CH, CN, and C2 along with ir photolysis of a 93.0% 13CH312CH sample have provided the basis for possible mechanisms by which these species as well as other unidentified species are formed.
The role of Pt2+ and Pt4+ impurity ions in the photochemistry of single-crystal silver chloride
Authors: Eachus R. S. , Graves R. E. , Olm M. T.
Published: February 2003

Both divalent and tetravalent platinum ions are substitutionally incorporated into silver chloride during the melt growth of single crystals. Pt2+ is believed to occupy sites of octahedral or tetragonal symmetry which are formulated as (PtCl6)4− and { (PtCl6)4−⋅Ag+vac}, respectively. The former complex functions as an electron trap, whereas the charge-compensated center has a significant capture cross section for holes. The predominant products of these reactions, (PtCl6)5− and { (PtCl6)3−⋅nAg+vac}, where n=1 to 3, have been identified by EPR spectroscopy. Pt4+ ions are fully charge compensated in AgCl, and the resultant complex, { (PtCl6)2−⋅3Ag+vac}, is a deep electron trap. The most stable photoproduct from both AgCl‖Pt2+‖ and AgCl‖Pt4+‖ was { (PtCl6)3−⋅2Ag+vac}. The lifetime at room temperature and the trap depth for this complex have been determined.
The far ultraviolet and HeI photoelectron spectra of alkyl and fluorine substituted silane derivatives
Authors: Roberge R. , Sandorfy C. , Matthews J. I. , Strausz O. P.
Published: February 2003

The HeI photoelectron and vacuum uv absorption spectra of a number of methyl, ethyl, methylfluoro, and ethylfluoro derivatives of silane are presented. The spectra can be interpreted on the basis of their similarity to those of the related methane or ethane derivatives. The lowest lying excited states are related to transitions to 4s, 4p, and 3d orbitals having mixed Rydberg–valence shell character. The spectra can be divided into two categories: methane (’’round field’’) or ethane (’’long field’’) types. Fluorine substitution influences these spectra in a characteristic way.
Rate processes in condensed media: The role of anharmonicity
Authors: Efrima Shlomo , Metiu Horia
Published: February 2003

We discuss the properties of a model which describes a variety of phenomena such as excitation migration in solids (exciton), electronic conductivity of organic crystals (polaron), electron transfer reactions, isomerization reactions, radiationless transitions, and motion of chemisorbed atoms. It is shown that, by taking into account the anharmonicity of the medium, the temperature dependence of the rate of the processes enumerated above can change substantially. In the case of the polaron and exciton, the Arrhenius temperature dependence is changed by the anharmonicity into a weak temperature dependence, which is in agreement with the experimental observations. For radiationless transitions and electron transfer reactions, the equations obtained by considering harmonic vibrations are in agreement with the experimental data and we conclude that the anharmonic effects in these cases are small. In the case of the isomerization reactions or migration of chemisorbed atoms, the anharmonicity does not change the form of the equation enough to allow one to distinguish experimentally between the two cases. The interpretation of the activation energy and the temperature dependence of the pre-exponential factor are, however, changed by strong anharmonicity.
Collective effects in physical adsorption
Authors: Delanaye F. , Schmeits M. , Lucas A. A.
Published: February 2003

The collective electronic polarization waves occurring in a regular two-dimensional lattice of rare gas atoms adsorbed on a metallic surface are studied, including interactions mediated by the metallic substrate. The atoms are represented as three-dimensional isotropic harmonic oscillators interacting via electrostatic dipolar forces. The polarization eigenmodes of an isolated monolayer are first determined, from which the cohesive energy of the layer can be deduced from the total zero-point energy of the modes. Then the role of the substrate is introduced by treating the metal first as a perfectly conducting medium (ε=−∞) and then, more appropriately, as a substrate of dynamical dielectric function ε (ω) =1−ω2p/ω2. The monolayer–metal interaction produces a shift of the collective atomic frequencies and of the metal surface plasmon frequencies. From the zero-point energy shift, one obtains an expression of the total interaction energy. Numerical applications are made for a square lattice of Kr adsorbed on metals of various plasmon frequencies.
Quenching of Ne, F, and F2 in Ne/Xe/NF3 and Ne/Xe/F2 mixtures
Authors: Huestis D. L. , Hill R. M. , Nakano H. H. , Lorents D. C.
Published: February 2003

As part of a kinetic study of XeF laser media, we have followed the time evolution of the Ne*, F*, and F*2 densities in e-beam excited Ne/Xe/NF3(F2) mixtures. The Ne* and F* densities were monitored by laser absorption while the F2* was observed in emission at 158 nm. Rate coefficients for the quenching of these excited states by Ne, Xe, NF3, and F2 have been determined. The radiative lifetime of the F2* was measured to be 41±4×10−9 sec.
Absorption spectra of metal oxides using optogalvanic spectroscopy
Authors: Schenck Peter K. , Mallard W. Gary , Travis John C. , Smyth Kermit C.
Published: February 2003

The absorption spectra of neutral flame species may be detected by measuring current changes induced by photon irradiation at wavelengths corresponding to electronic transitions. This paper presents the first results using this optogalvanic spectroscopy to study molecular absorption. Numerous transitions have been detected for ScO, YO, and LaO in the 360–630 nm wavelength region. The most extensive data have been obtained for LaO, where we observe a total of at least four electronic states and 18 sequences. Five of the sequences have not been seen in previous emission studies.
The Green’s function method of sensitivity analysis in chemical kinetics
Authors: Hwang Jenn-Tai , Dougherty Eugene P. , Rabitz Suzanne , Rabitz Herschel
Published: February 2003

A new approach to the sensitivity analysis of large computational models is presented. The basis for the method is the well-known Green’s function technique. By working with the same differential equations as the conventional direct method, this new approach reduces the number of differential equations to be solved and replaces them by a set of integrals. Altogether, there is only one set of differential equations in the Green’s function method, regardless of the number of system parameters m. Sensitivity coefficients of all orders are expressed in integral form and evaluated in a recursive manner. Since evaluating well-behaved integrals is usually much easier than solving stiff differential equations, substantial savings can be achieved by the method when the number of system parameters is large. It is estimated that if only linear sensitivity coefficients are desired, then the Green’s function method could be advantageous for the case m≳n, where n is the number of dependent variables. However, if both linear and higher order sensitivity coefficients are to be computed, the method could be competitive with other approaches even when m<n. A numerical calculation on a simple linear system is presented to provide a brief illustration of the method.
Recombination and local equilibrium in growing oxide films
Authors: Fromhold A. T.
Published: February 2003

The possibility of a recombination of certain diffusing point defects in growing oxide films is examined within the framework of a simple collision model. Local equilibrium between diffusing cation interstitials and cation vacancies (or between diffusing anion interstitials and anion vacancies, or between electrons and electron holes) can represent a viable approximation for 100 Å oxide films when the average diffusing point defect density exceeds 1021/cm3; for a 100 μ thick oxide layer, the minimum defect density for such reduces to 1017/cm3. On the other hand, arguments are presented on the basis of physical considerations which indicate that local equilibrium between the diffusing cation and anion point defect species within the growing oxide film is not to be expected; likewise, local equilibrium between the diffusing electronic and ionic species giving rise to new oxide growth is not to be expected.
Shape resonances in e–SF6 scattering
Authors: Dehmer J. L. , Siegel Jon , Dill Dan
Published: February 2003

Elastic scattering cross sections for e-SF6 are calculated. The spectrum exhibits resonances of ag, t1u, t2g and eg symmetries between 0 and 40 eV.(AIP)
Proton chemical shift tensors of alkaline earth hydrides
Authors: Nicol A. T. , Vaughan R. W.
Published: February 2003

The proton chemical shifts for solid state CaH2, SrH2, and BaH2 obtained by multiple pulse NMR techniques are reported. A trend in the isotropic value to shift toward higher field as a function of cation electronegativity is noted.
Visible emission spectra of Ga–Zn and Ga–Cd excimers
Authors: Chilukuri Santaram , Vaidyan V. K.
Published: February 2003

A spectroscopic study of Ga–Zn and Ga–Cd excimers have been carried out for the first time. Both the mixtures, excited by an rf discharge, show new excimer spectra in the visible region. The emission is centered at 5190 Å for the Ga–Zn mixture and 5380 Å for the Ga–Cd mixture. The density of Zn or Cd in the study is in the range of 1018–1019 atoms/cc and that of Ga about 1011 atoms/cc. Spectra have been studied with a dispersion of 5 Å/mm. Vibrational structures have been resolved and analyzed on the basis of triatomic species. The emitters are most likely ZnGa2 for the 5190 Å system and CdGa2 for the 5380 Å system.
On difficulties with a total angular momentum labeled CS approximation
Authors: Khare V. , Kouri D. J.
Published: February 2003

The J-labeled version of the CS approximation for atom–diatom collisions is carefully examined. It is shown that a consistent treatment leads to unphysical and unrealistic features of the scattering amplitude and definite polarization cross sections. The most serious of these are (i) infinite total cross sections and (ii) rotationally inelastic transitions for spherically symmetric potentials.
Analytical evaluation of three- and four-center electron-repulsion integrals for Slater-type orbitals
Authors: Guseinov I. I.
Published: February 2003

A particular method has been introduced for obtaining a spherical harmonic expansion of a Slater-type orbital about a point displaced from the orbital center. Utilizing this method, a general analytical formula has been established for the radial coefficients of the expansion which contains terms only of the product of the overlap integrals and the radial parts of Slater-type orbitals. By the use of the derived expression for a spherical harmonic expansion, three- and four-center electron-repulsion integrals with the arbitrary location of Slater-type orbitals are expressed in terms of overlap integrals and one of the one-center Coulomb, and two-center Coulomb, hybrid and exchange integrals over Slater-type orbitals for the evaluation of which, recently, the analytical formulas have been established by the author. The convergence for the expansion has been tested by calculating concrete cases.
Infrared reflectivity from surface damaged crystals
Authors: Nichols Henry , Frech Roger
Published: February 2003

Polarized infrared reflection spectra are obtained from the faces of mechanically damaged single crystals of Na14NO3,Na15NO3,and KBr03. The reflectivity spectra depend on the method of surface preparation. Deviations from the reflectivity observed with highly polished crystal faces are shown to originate in reflection from microcrystalline fragments with essentially random orientation with respect to the incident light. Several new spectral features are observed in the damaged crystals which may result from the size dependence of the optical constants of the microcrystalline fragments.
Deviations from the Stark effect of the rigid rotator measured for the CsF molecule
Authors: Charifi M. , Gräff G. , Ley R. , Wolf M.
Published: February 2003

An electric molecular beam resonance apparatus has been used to observe rotational transitions in the CsF molecule in electric fields up to 12 000 V/cm. Stark shifts up to 3.8 GHz have been measured with a natural linewidth of 4 kHz at minimum values of the transition frequencies, allowing the determination of the anisotropy of an effective polarizability (γ∥−γ⊥) ′=2.09±0.04 Å3 for the vibrational states v=0 and v=1. This value describes the difference between the Stark effect of a rigid rotator and that of the actual molecule. It is shown that all deviations from the Stark effect of the rigid rotator can be brought to a form where they have exactly the same dependence on the rotational quantum numbers J and m as a second rank tensor. This fact confirms the impossibility of a separate determination of the individual polarization contributions by spectroscopic methods involving only a change in J and m.
Solution of the time-dependent Onsager problem
Authors: Hong K. M. , Noolandi J.
Published: February 2003

We obtain the Green’s function solution of the Smoluchowski equation with a Coulomb potential and an electric field, corresponding to a general boundary condition at the origin. In the low-field limit the solution exhibits diffusive behavior for long times, but above a critical field the long-time decay becomes purely exponential. We derive expressions for the time evolution of quantities such as the rate of geminate recombination and the survival probability, and for t→∞, a→0 (where a is the radius of a perfectly absorbing sphere at the origin) we recover Onsager’s results. We consider a variety of initial conditions, such as a δ function, isotropic and constant distributions, as well as a more complicated distribution applicable to fluorescence quenching in the presence of an electric field. For small values of the electric field and the Laplace transform variable we obtain an expression for the probability of a scavenging reaction taking place in competition with geminate recombination. Using the prescribed diffusion approximation, we also obtain a simple expression for the time dependence of the survival probability, valid for an isotropic initial distribution, and we discuss the results of the numerical evaluation of various quantities of physical interest. Throughout the paper we have obtained parallel results for both Coulomb attraction and repulsion between the pair of charges.
The incomplete basis set problem. II. Perturbative corrections to coupled perturbed Hartree–Fock theory
Authors: McDowell Keith
Published: February 2003

CIBS (corrections to an incomplete basis set) theory is formally extended to encompass coupled perturbed Hartree–Fock theory. Calculations of the helium static dipole polarizability are carried out for the first and second correction terms using systematically increasing basis sets. The correction terms are found to converge and the corrected polarizabilities are significantly improved.
Mode coupling and 79Br–81Br isotope effects in the Raman spectra of NaClO3–NaBrO3 mixed crystals
Authors: Engstrom Herbert , Frech Roger , Bates J. B.
Published: February 2003

Raman scattering measurements have shown that interionic coupling causes a mixing of the ν1 and ν3 modes of the BrO3− ion in mixed NaClO3–NaBrO3 crystals. A 79Br16O−3–81Br16O3− isotopic splitting of approximately 1 cm−1 was observed in the ν1 mode in crystals containing low concentrations (?1 mole%) of bromate ions.
The interactions between excited and ground state rare gas atoms
Authors: Vallée O. , Glasser J. , Ranson P. , Chapelle J.
Published: February 2003

In this paper, we report on a two component potential model. A general formula for the electrostatic part is derived, giving the second order perturbation terms of the multipolar expansion. The first of these terms is the well-known van der Waals potential. In the second section, we study the repulsive part of the potential using the Smirnov–Roueff potential: this is based on the scattering properties of the optical electron of an excited state atom on a second atom in its ground state. We introduce a correction term which enables one to find a more realistic potential well, even if the scattering length of the atom in its ground state is negative. In a third section, we apply the model to the calculation of the interaction potentials of rare gases in the first excited states, and the Ar, Kr, and Xe atoms in their ground states.
Conducting polymers: Halogen doped polyacetylene
Authors: Chiang C. K. , Park Y. W. , Heeger A. J. , Shirakawa H. , Louis E. J. , MacDiarmid Alan G.
Published: February 2003

A study of the electrical conductivity of the halogen doped transpolyacetylene system, (CH)x, is reported. When films of trans- (CH)x are exposed to chlorine, bromine, or iodine vapor, uptake of halogen occurs; and the conductivity increases markedly, over eleven orders of magnitude in the case of iodine. The behavior of the halogenated polyacetylene is like that of a series of semiconductors with activation energies which vary with halogen content. The results are discussed in terms of a model of the doping process based on charge transfer.
Generation of O2(c 1Σu−, C 3Δu, A 3Σu+) from oxygen atom recombination
Authors: Slanger T. G.
Published: February 2003

The spectrum produced in the afterglow of an O2–He discharge has been studied between 4000 and 8000 Å. Until recently, only the O2(A 3Σu+→X 3Σg−) emission bands had been observed in this region. The work of Lawrence et al. established that, under the appropriate conditions, the O2(c 1Σu− →X 3Σg−) system could also be observed. We were able to duplicate their results, and in addition we have discovered three O2 band systems not previously seen in gas phase laboratory spectra. These systems are C 3Δu→a 1Δg, C 3Δu →X 3Σg−, and c 1Σu−→a 1Δg and were positively identified by isotopic substitution experiments. The 0–v″ progression in the C–a system is the first gas phase spectrum involving a large range of the higher vibrational levels in the a 1Δg state. Analysis of the system establishes a new set of a 1Δg vibrational constants : ωe=1510.23 ±0.34 cm−1, and ωexe=13.368±0.12 cm−1. The C–a system involves only the Ω=1 spin component of the C 3Δu state. The C 3Δu→X 3Σg− system, on the other hand, radiates mainly through the Ω=2 component. By combining the present data on the C 3Δu(v=0) level, the absorption measurements of Herzberg on the v=5 and v=6 levels, the known dissociation energy, and the high-pressure bands measured by Herman and by Finkelnburg and Steiner, we have established vibrational constants for the C 3Δu state. These are ωe=803.5±1.0 cm−1, ωexe=8.18±0.13 cm−1, and ωeye =−0.872±0.006 cm−1. From data existing in the literature, combined where necessary with the present measurements, we have estimated radiative lifetimes for the three O2 metastable states that we observed. From the band strength data of Hasson et al., one may calculate τ=250–160 msec for A 3Σu+(v=0–6), and deduce an estimate of τ=25–50 sec for c 1Σu−(v= 0–10). For C 3Δu(Ω=1, v=0–6), the estimated lifetime is 5–50 sec, and for C 3Δu(Ω=2, v=6), the estimated lifetime is 10–100 sec. The O2 spectra have been produced not only in O2-He afterglows, but also in NO-titrated N2–He afterglows (i.e., in an O2-free environment), proving that the source of excited O2 molecules is oxygen atom recombination. The results are pertinent to O2 emissions in the terrestrial and Venusian air glows, and to combustion processes.
The hot-atom collisional energy density function
Authors: Prisant M. G. , Ollison W. M. , Cross R. J.
Published: February 2003

Using stochastic methods we have computed the hot-atom collisional energy density functional, n (E) dE, the number of collisions suffered by a translationally hot atom between E and E + dE. We examine the dependence of n (E) on several variables such as the mass and energy of the hot atom and the parameters of the intermolecular potential.
Rayleigh–Brillouin scattering from acetic acid
Authors: Patterson G. D. , Alms G. R. , Lindsey C. P.
Published: February 2003

Rayleigh–Brillouin scattering is studied in acetic acid as a function of temperature over the normal liquid range. The Brillouin linewidth displays a maximum at 30°C. The spectra are analyzed in terms of the general viscoelastic theory of light scattering developed by Rytov. Relaxation times for reorientation and structural relaxation are derived from the data.
Raman depolarization ratios and order parameters of a nematic liquid crystal
Authors: Miyano K.
Published: February 2003

The depolarization ratios of the Raman vibrational bands from the terminal C≡N and the central C–C biphenyl link of an oriented nematic liquid crystal [p-n-pentyl-p′-cyanobiphenyl (5CB)] have been measured. The orientational order parameters 〈P2〉 and 〈P4〉 were calculated. The parameter 〈P4〉 remained far less than the mean-field theory prediction although a systematic discrepancy between the order parameters obtained from the two bands was noted. The Raman tensor of C≡N band was found not to be uniaxial, contrary to the previous assumption, and a strong association of molecules is suggested. Problems associated with this technique are discussed.
Extended basis set calculations of nonlinear susceptibilities of conjugated hydrocarbons
Authors: McIntyre Eugene F. , Hameka Hendrik F.
Published: February 2003

We reconsidered perturbation treatments of molecular eigenstates by expanding in terms of other molecular eigenstates. This constitutes an improvement over a previously reported treatment where we considered molecular orbitals which were expanded in terms of other molecular orbitals. In the present paper, we take the ground state molecular wavefunction as a single determinant SCF function. Fourth order energy perturbations are then obtained as a sum of contributions from singly and doubly excited molecular configurations. We evaluated the various fourth-order energy perburbations and corresponding third-order electric susceptibilities for a group of about 30 aromatic hydrocarbons and for a group of linear conjugated hydrocarbons. Our results are roughly consistent with the limited experimental information that is available for these molecules.
Kramers’s theory of chemical kinetics: Eigenvalue and eigenfunction analysis
Authors: Larson R. S. , Kostin M. D.
Published: February 2003

The Smoluchowski equation for a double-minimum potential is studied as a means of calculating rate constants for chemical reactions. In the one-dimensional case, a symmetric fourth-degree potential is used, and the solution is obtained in terms of eigenvalues and eigenfunctions. Asymptotic expressions for the lower eigenfunctions are found by means of singular perturbation theory, and the corresponding eigenvalues are obtained via a variational principle. A quantum-mechanical analogy is used to generate the higher eigenvalues and eigenfunctions. The expression for the rate constant is seen to be a generalization of earlier results, and its range of validity is determined by solving the appropriate equations numerically. It is found that the new formula is accurate over a rather wide range of barrier heights. In the three-dimensional case, the rate constant is again found by means of a variational calculation, using as a trial function the eigenfunction for a separable potential. The result is seen to reduce to the more familiar expression in the appropriate limit, and its range of validity is investigated through direct numerical computations of the eigenvalues.
Application of saturation spectroscopy for measurement of atomic Na and MgO in acetylene flames
Authors: Pasternack Louise , Baronavski A. P. , McDonald J. R.
Published: February 2003

The technique of laser induced fluorescence saturation spectroscopy is evaluated for determination of absolute atomic and molecular concentrations in flame sources. The combustor is an aspirating slot burner using premixed acetylene and air to create a stable homogeneous flame source. Concentration measurements of atomic sodium and MgO (X 1Σ+ and A 1Π) are made by saturation spectroscopy and laser induced fluorescence techniques. Independent atomic and molecular absorption measurements are made for Na and MgO (X 1Σ+) which are in agreement with saturation spectroscopy concentration determinations. The effect of using various laser beam profiles (rectangular, Gaussian, and truncated Gaussian) are evaluated. It is found that MgO exists in the flame in a significant inversion in the (A 1π) state and possible production mechanisms for MgO in the flame are considered.
Atomic and molecular model potentials
Authors: Laurenzi Bernard J.
Published: February 2003

A constructive theory of atomic and molecular model potentials is developed using the work of Bottcher and Dalgarno as a point of departure. In this theory, all of the terms required by successful, empirical, model potential calculations are obtained from a perturbation theory in which exchange is neglected. We find that valence electrons must be allowed to penetrate into the core regions even in systems with well defined cores and distant valence electrons. Within this theory, exact model potentials for hydrogenlike atoms and H2-like molecules are obtained and specific, short- and long-range forms of the model potentials are proposed for other atoms and molecules.
Penning and associative ionization in the metastable helium–atomic deuterium system
Authors: Neynaber R. H. , Tang S. Y.
Published: February 2003

Measurements of Penning and associative ionization cross sections (QPI and QAI) and branching ratios (R) for the He*–D system have been repeated in this laboratory. The studies were made using a merging-beams technique for a relative kinetic energy W of the reactants from nominally 0.01 to 10 eV. This is a wider energy range than covered previously. The He* represents a composite of about 15% He(2 1S) and 85% He(2 3S). The new results are more acurate than the old due to improvements in the apparatus. From the new studies, a well depth of approximately 2.1 eV was obtained for the He(2 3S) –D potential and between 0.25 and 0.4 eV for the He(2 1S) –D potential. These well depths agree with values obtained by others. Laboratory-energy distributions of Penning ions indicate differences in the kinetics of reaction for He(2 1S) and He(2 3S). The new QAI, QPI, and R agree with those of several other experimenters although there are discrepancies in QPI and R with the calculations of Hickman and Morgner.
The dynamic interchange and relationship between germanium centers in α-quartz
Authors: Isoya J. , Weil J. A. , Claridge R. F. C.
Published: February 2003

The Ge3+ centers [GeO−4(I)]− ≡Ge(I), [GeO4−(II)]− ≡Ge(II), and B, which are produced by x irradiation of α-quartz containing germanium, have been studied by EPR. The spin-Hamiltonian parameters ḡ, Ā73Ge, Ā29Si(1)⋅⋅ ⋅Ā29Si(4) for Ge(I), and ḡ, Ā73Ge for Ge(II), both sets measured at ≈15 K, and ḡ, Ā73Ge for B at ≈220 K are reported. The direction of the sp hybridized germanium orbital of the unpaired electron is found to coincide with the bisector of one of the O–Si–O angles [106.3° from the crystal c axis for Ge(I); crystal twofold axis for Ge(II)]. Centers Ge(I) and Ge(II) are found to be different, energetically inequivalent versions of the same basic center. The B center appears as a result of rapid electron jumping between Ge(I) and Ge(II) above ≈150 K. The observed line positions of B coincide with ones calculated from motional averaging among three states: two of Ge(I) and one of Ge(II). The relative Ge(I) and Ge(II) concentrations are obtained from signal intensity ratios at low temperatures (14:1 at ≈15 K, 4.4:1 at ≈80 K) and from the line positions of center B at sufficiently high temperatures (3.0:1 at ≈220 K, 2.7:1 at ≈300 K).
Electric field quenching of recombination fluorescence in photoionized nonpolar solutions. II. Electron range and the quantum yield for geminate pair creation
Authors: Bullot J. , Cordier P. , Gauthier M.
Published: February 2003

Data on the photocurrent and the electric field quenching of fluorescence in photoionized solutions of TMPD in nonpolar liquids are used to compute the field dependent escape probability by means of the Onsager theory. A discriminating procedure between possible initial separation distances distribution functions is suggested. The primary quantum yield for ion pair creation φip is calculated and shown to depend upon the type of distribution function chosen. Both the distance parameter and φip are shown to vary with the excitation energy.
Transmission function vs energy splitting in tunneling calculations
Authors: Weiner J. H.
Published: February 2003

Two methods have been widely employed for computing tunneling rates in a double-well potential, one based on a transmission function, the second on energy splitting. Semiclassical calculations (Brickmann and Zimmermann) show that the transmission method leads to lower rates than the splitting method. It is shown here without employing the semiclassical approximation that a more accurate relation between the energy splitting and the transmittion function may be obtained by using a decomposition of the stationary states into scattering states. This relation is then used to provide an analytical basis for the usual heuristic picture in which the particle oscillates with classical frequency in one well and has a probability of tunneling to the other well equal to the transmission function in each classical period of oscillation. It is concluded that the transmission method should give more meaningful results, particularly in situations where interactions of the system with a heat bath have significant effects in the tunneling times predicted by the splitting method. This is in accord with previous work in which both methods were used in an attempt to fit experimental data for hydrogen diffusion in niobium.
Absorption bands in a CsHe system associated with forbidden Cs atomic transitions
Authors: Tam A. C.
Published: February 2003

We have made direct absorption measurements of the Cs(7S)He←Cs(6S)He absorption band (5250–5700 Å) and the prominent part of the Cs(5D5/2)He←Cs(6S)He absorption band (5800–6850 Å). The spectrum of the reduced absorption coefficients we obtained is in reasonable agreement in shape with that recently reported by Sayer et al, who used a laser induced fluorescence technique, but our absolute magnitudes are bigger than those of Sayer et al by approximately a constant factor of 5.
ir photolysis of SeF6: Isotope separation and dissociation enhancement using NH3 and CO2 lasers
Authors: Tiee Joe J. , Wittig Curt
Published: February 2003

The isotopically selective ir photodissociation of SeF6 using two lasers is reported. The output from an NH3 laser (780.5 cm−1, 0.75 J, 0.5 μsec FWHM) and a line tunable CO2 TEA laser (2.5 J, 150 nsec FWHM) are combined in order to enhance the photodissociation rate. Although the CO2 laser output by itself is ineffective in dissociating SeF6, the CO2 laser output can enhance the NH3 laser photolysis of SeF6 by as much as a factor of 50. We find that the isotopic selectivity is independent of the CO2 laser energy, and the rate of dissociation is unaffected by changing the CO2 laser frequency.
Piezo- and elasto-optic properties of liquids under high pressure. II. Refractive index vs density
Authors: Vedam K. , Limsuwan Pichet
Published: February 2003

In Paper I of this series it was shown that high pressure interferometric measurements on a number of liquids reveal that the refractive index of liquids increases linearly with Eulerian strain even though the volume strain involved is as high as 33%. Further, it was also found that this linearity criterion can be used to discriminate between the various equations of state for liquids and that Keane’s equation appears to be universally valid for all liquids. Having thus established the P–V relationship of liquids, the next problem of the refractive index–density relationship of liquids is considered in this article. The various equations prevalent in the literature—namely, Lorentz–Lorenz, Drude, Eykman, Onsager–Bottcher, Kirkwood–Brown, and Omini equations— are analyzed. The last two equations have been derived using a detailed statistical mechanical approach. It is found that none of these equations can predict the elasto-optic behavior of liquids at high pressure quantitatively even though the latter two equations appear to yield values in reasonably good agreement with experimentally observed values.
Mössbauer effect study of the magnetic properties of S=1 ferrous tetraphenylporphyrin
Authors: Lang G. , Spartalian K. , Reed Christopher A. , Collman James P.
Published: February 2003

Mössbauer spectra of the polycrystalline form of the square planar compound α,β,γ,δ-tetraphenylporphinato–iron (II) have been observed at a variety of temperatures. Analysis of the resulting spectra yield a positive electric field gradient interaction with quadrupole splitting ΔE=1.51 mm/s (4.2 K) and 1.52 mm/s (300 K). At low temperature, an external magnetic field induces a negligible internal field along the symmetry axis of the molecule. In the transverse direction, it gives rise to a positive internal field with little temperature dependence up to 30 K. The data can be interpreted in terms of a crystal field model involving a low-lying 3A2g state which is split by spin–orbit coupling into a ground singlet and a doublet lying 80 cm−1 above it.
Homonuclear dipolar-modulated chemical shift spectra in polycrystalline solids: The proton chemical shift tensor in CCl3COOH
Authors: Stoll M. E. , Vega A. J. , Vaughan R. W.
Published: February 2003

A single resonance, multiple pulse nuclear magnetic resonance experiment is proposed and demonstrated which produces a homonuclear dipolar modulation of a chemical shift powder pattern. The correlation generated between the homonuclear dipolar Hamiltonian and the chemical shift Hamiltonian can be used to determine the relative orientation of the chemical shift principal axis frame to the molecular frame. The experimental scheme is applied to determine the orientation of the proton–proton vector in a CCl3COOH dimer relative to the chemical shift tensor of the hydrogen-bonded proton.
Raman study of molecular reorientation in liquid chloroform and chloroform-d under high pressure
Authors: Schroeder John , Schiemann Valeria H. , Jonas Jiri
Published: February 2003

The molecular reorientation in liquid chloroform and chloroform-d has been studied as a function of temperature and pressure by analysis of the Raman line shapes for the ν1(A1) C–H and C–D stretching modes at 3020 and 2256 cm−1, respectively. The measurements cover the temperature range 303–363 °K and the pressure range 0.03–4.5 kbar. Analysis of the Raman bands yields the reorientational correlation times τϑ, and the reorientational correlation functions under isobaric, isothermal, and isochoric conditions. The τϑ’s show a good agreement with the correlation times obtained from NMR relaxation measurements. The results on rotational second moments MR(2) provide experimental evidence that the rotational second moments are density dependent. In addition, the experiments indicate that collisional induced scattering contributes in a major way to the observed MR(2). Comments on the different peak positions for the IVV and IVH bands are also included.
Stochastic theory of intramolecular energy transfer
Authors: Ramaswamy R. , Augustin S. , Rabitz H.
Published: February 2003

The problem of internal energy redistribution in an isolated polyatomic molecule is treated by a stochastic theory approach. The fundamental assumption of this work is that a random phase approximation is valid at specific time intervals. This results in the replacement of the Schrödinger equation by a master equation that governs the evolution of a probability distribution in the quantum levels of the molecule. No assumptions regarding the strength of the coupling are made, and the problem of energy conservation is specifically considered. A stochastic variable is introduced in order to satisfy the requirement that the total energy remain fixed. The further approximation of the master equation by a Fokker–Planck diffusionlike equation is outlined; the latter approach is particularly attractive for treating large molecules. Finally, the master-equation theory is applied to a model problem representing a linearly constrained triatomic molecule, and the time evolution of an initially localized excitation is discussed.
Vibrational excitation of ethylene by electron impact: 1–11 eV
Authors: Walker Isobel C. , Stamatovic A. , Wong S. F.
Published: February 2003

Differential cross sections for vibrational excitation of ground state C2H4 and C2D4 by electron impact have been studied with a crossed-beam apparatus for electron energies between 1 and 11 eV. The scattering is dominated by two resonance regions in which the vibrational cross sections of totally symmetric modes are preferentially enhanced to the order of 10−16 cm2. The first resonance region is centered near 1.8 eV. Here all energy-loss spectra and energy and angular dependences of cross sections can be accounted for by a 2B2g shape resonance of an intermediate lifetime. The second resonance region centered near 7.5 eV is very broad. The dominant vibrational modes and the corresponding angular distributions are distinctly different from those in the lower region. We interpret this second region in terms of short-lived shape resonances, the dominant one being a 2Ag compound state comprising the target molecule plus an electron in the 4ag orbital.
Electronic spectrocopy in condensed media: Spectra of styrene and cyclo-octatetraene anions
Authors: Banerjee Ajit , Simons Jack
Published: February 2003

A general theoretical formalism developed in earlier publications for treating the electronic spectra of solvated anions is found to successfully describe the spectra of styrene and cyclooctatetraene anions in MTHF glass at 77 °K. The analysis provided supports the conclusions that the first two excited states of the styrene anion and the first excited state of the cyclo-oclatetraene anion in solution have sufficiently long lifetimes (≳10−14 sec) so as to have negligible lifetime broadening of their spectra. The entire widths of the observed spectra can be accounted for via Franck-Condon progressions alone; the degree of structure in the experimental spectra provide information about solvent inhomogeneous broadening.
Solvent dependence of the 5D3→5D4 transition of Tb3+ in POCl3:SnCl4
Authors: Chrysochoos John
Published: February 2003

Light excitation of Tb3+ in POCl3:SnCl4 at 368±1 and 353±1 nm leads to fluorescence emission arising from both the 5D3 and the 5D4 states of Tb3+, respectively. The 5D4 state is populated via the 5D3 state exclusively, by means of the 5D3→5D4 transition. The appropriate transition probability depends upon adjacent Tb3+ ions in their ground state (self-quenching, kq) and upon interactions between the 5D3 state and the solvent (ks). The ratio of the intensities of the fluorescence emission arising from the 5D3 and 5D4 states, monitored at 437±1 nm (5D3→7F4), 491±1 nm (5D4 →7F6) and 545±1 nm (5D4→7F5), respectively, varies linearly with [Tb3+]. Such fluorescence intensity ratios were employed to determine kq/ks and subsequently to obtain the value of ks. At room temperature ks is equal to 630±10 sec−1. The depopulation process of the 5D4 state of Tb3+ is independent of the solvent.
Rotational dependence of the fluorescence quantum yields of H2CO and D2CO (? 1A2): single rovibronic level values and their average values for the 41 level
Authors: Shibuya Kazuhiko , Lee Edward K. C.
Published: February 2003

The fluorescence quantum yields, ΦF(K′, J′), of single rovibronic levels (SRVL’s) of the 41 (i.e., ν4′=1) vibrational state of H2CO and D2CO (? 1A2) have been measured at low pressures by laser-induced fluorescence using a multiple path cell. From these data, the ’’absorption intensity weighted’’ averages of ΦF are obtained for several K′ rotational manifolds as <ΦF(K′) 〉 and for the 41 level as 〈ΦF〉. The latter value of 0.031 confirms the validity of the SVL value of 0.0344 obtained from broad-band optical excitation by Miller and Lee. A random variation of ΦF (K′, J′) with J′ is observed, and its implication to the question of radiationless transitions in formaldehyde molecules is discussed.
Angular distribution of product ions in the ion-molecule reaction H+2(He,H) HeH+ by quantum mechanical impulse approximation
Authors: Guha Madhuri , Dastidar T. K. Rai
Published: February 2003

Quantum mechanical impulse approximative is used to obtain the angular distribution of product ions for the reaction H2++He→HeH++H.(AIP)
Lattice dynamics of the ethylene crystal with interaction potentials from a
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o calculations
Authors: Wasiutynski T. , van der Avoird A. , Berns R. M.
Published: February 2003

The long range (electrostatic, dispersion, induction) and short range (exchange and penetration) interaction energy between ethylene molecules has been calculated by ab initio methods as a function of the molecular orientations and distances. The results, when fitted with an exp-6-1 atom–atom potential and used in a harmonic lattice dynamics calculation on the ethylene crystal, yield fair agreement with the experimental structure data, ir and Raman phonon frequencies. Although the fit with the atom–atom potential is reasonably good, some specific deviations from the ab initio results indicate the importance of the effects of chemical bonding on the intermolecular potential (leading to noncentral and nonpairwise additive atom–atom forces). The usual empirical atom–atom potentials are grossly corroborated, their main defect being the neglect or underestimate of electrostatic (quadrupole–quadrupole) interactions.
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o configuration interaction study of the low-lying electronic states of MgH
Authors: Saxon R. P. , Kirby K. , Liu B.
Published: February 2003

A b initio configuration interaction calculations have been performed on the X 2Σ+, A 2Π, and B′ 2Σ+ states of MgH. Numerical values for the potential curves have been obtained for 23 internuclear distances between 2.2 and 9.5 a0. Dipole and transition moment functions have also been calculated. Spectroscopic constants obtained from the calculated potential curves are found to be in good agreement with experimental data; the maximum errors in Re, De, Te, and ωe are 0.003 nm, 0.09 eV, 0.05 eV, and 38 cm−1, respectively. The calculated band absorption oscillator strength for the X–A transition of 0.161 is believed to have ±20% accuracy.
Nuclear corrections to molecular properties. VI. Vibrational transition moments in asymmetric-top molecules
Authors: Krohn Burton J. , Kern C. William
Published: February 2003

A general expansion is derived for the transition moment between two nondegenerate vibrational states of a polyatomic molecule. This analysis is combined with ab initio surfaces for the potential energy and the dipole moment of water vapor to predict integrated band absorption intensities. Calculations are compared with experiment for several transitions which originate in the zero-point vibrational state.
Laser-excited resonant isotopic V→V energy transfer: H35Cl–H37Cl, H79Br–H81Br, D35Cl–D37Cl, and D79Br–D81Br
Authors: Horwitz Alexander B. , Leone Stephen R.
Published: February 2003

An isotopically selective transverse discharge chemical laser coupled with infrared fluorescence techniques is used to measure vibration-to-vibration energy transfer rates between v=1 levels of the isotopic species H 35Cl?H37Cl, H 79Br?H 81Br, D 35Cl?D 37Cl, and D 79Br?D 81Br. The measured rate constants are kHCl= (1.91±0.04) ×10−11, kHBr= (1.50±0.06) ×10−11, kDCl= (1.18±0.08) ×10−11, and kDBr= (8.34±0.17) ×10−12 cm3 molecule−1 sec−1. All of the processes are nearly resonant and the rates for the hydrogenated species are on the order of one-tenth gas-kinetic. The rates for DCl and DBr are found to be 1.6 and 1.8 times slower than the corresponding hydrogen halides. Direct comparison of the results with theoretical predictions for such resonant energy transfer processes shows disagreement, indicating that further development of theoretical methods for these systems is desirable.