bluuebunny/crossref_metadata_2025 huggingface dataset:
Distribution of final states resulting from the autoionization of the Σg1(2pσu2) states of H2 and D2
Authors: Hazi Andrew U.
Published: February 2004

Using a simple classical model, we calculate the distribution of final states resulting from the autoionization of the Σg1(2pσu2) state of H2(D2) to the 2Σg(1sσg) state of H2+(D2+). The population of vibrational levels differs markedly from that predicted by the Franck-Condon principle for direct ionization. The calculated kinetic energy distributions of H+ and D+ have maxima at threshold (Ekin=0). The probability of forming dissociated products is smaller for D2 than H2.
Rate coefficients for the clustering of CO2, N2, and O2 to NO+
Authors: Heimerl J. M. , Vanderhoff J. A.
Published: February 2004

The rate coefficients for three-body, ion-neutral association have been measured at room temperature for the clustering of the atmospheric gases CO2, N2, and O2 to NO+. The rate coefficients are 2.4×10−29 cm6/sec for CO2 with CO2 as the major third body (M≈CO2), 2×10−31 cm6/sec for N2 (M≈N2), and 9×10−32 cm6/sec for O2(M≈O2). Estimated experimental error is ±30%, and the effect of possible additional uncertainties is indicated. The rate coefficients for collisional dissociation of the cluster ions have upper limits of ∼2×10−11 cm3/sec for NO+(N2) and ∼1×10−11 cm3/sec for both NO+(O2) and NO+(CO2).
Photoelectron spectra of polynuclear aromatics. III. The effect of nonplanarity in sterically overcrowded aromatic hydrocarbons
Authors: Boschi R. , Clar E. , Schmidt W.
Published: February 2004

The high-resolution photoelectron spectra of 65 polycyclic aromatic hydrocarbons containing up to 11 condensed benzene rings have been obtained in the gas phase at temperatures between 20 and 450°C. Vibrational analysis of the spectra coupled with semiempirical MO calculations permitted positive identification of the π ionizations in the 6–11 eV range. Irrespective of size and symmetry of the hydrocarbons, two-thirds of all π bands are found to precede the onset of σ ionization, which occurs at 10.6±0.2 eV in the molecules beyond naphthalene. The possibility of Jahn-Teller forces operating in the degenerate ionic ground states of D3h and D6h molecules is discussed, and comments are made on the origin of the 1350 cm−1 vibrational progression that prevails in the first band of all planar hydrocarbons. In sterically overcrowded systems, excitation of low-frequency twisting and out-of-plane bending modes results in a blurring of the vibrational structure. In cases where reliable x-ray data are unavailable (e.g., dibiphenylene-ethylene), this observation allows a distinction to be made between planar and nonplanar hydrocarbons. The present data are of sufficiently high quality to permit the question as to the planarity of phenanthrene to be settled with some certainty.
Nonexponential decays in single vibronic level fluorescence: A comparison between kinetics and quantum mechanical treatment
Authors: Lahmani F. , Tramer A. , Tric C.
Published: February 2004

We study the decay of a system resulting from the strong coupling (pv≫1) of a radiative state |s〉 with a coarse manifold of nonradiative levels {l} (``intermediate case'' molecule in the gas phase) excited by a δ-pulse (coherent) excitation. By the ``effective Hamiltonian'' formalism we show that a necessary consequence of the coherent excitation is a short quasiexponential decay very similar to the decay obtained in the ``statistical limit.'' In the absence of efficient deactivation processes, this decay is followed by a longer one (the ``anomalously long fluorescence'') due to the incoherent decay of the mixed states which result from the s-l coupling. The form of this longer decay- quasiexponential in some well-defined cases-is sensitive to the width of the zero order levels: Collision effects are accounted for by a linear dependence of these widths on the gas pressure. The statistical limit is achieved in the case of efficient deactivation processes. This model is compared to a kinetic scheme including a reversible intersystem crossing, and a good correspondence between the two models is obtained, except for some finer details that only appear in the quantum mechanical treatment. Pressure effects on the luminescence lifetimes and yields are discussed.
Fluorescence and quenching of O2(1Δg) and [O2(1Δg)]2 in liquid oxygen
Authors: Huestis D. L. , Black G. , Edelstein S. A. , Sharpless R. L.
Published: February 2004

Liquid oxygen, oxygen/nitrogen, and oxygen/argon mixtures were excited by 1.060 μ radiation from a Nd:glass laser. The fluorescence decay was monitored at 6340 Å, 7030 Å, 1.27 μ, and 1.58μ. The quenching rate coefficient of O2(a 1Δg) by O2(X 3Σg−) was found to be 1.0±0.1×10−18 cm3/sec at 77°K. The temperature variation between 77 and 322°K is consistent with 2.2×10−18 (T/300)1/2 cm3/sec. Quenching by N2 and argon was at least an order of magnitude slower. In every case the decay of [O2(1Δg)]2 was twice as fast as that of O2(1Δg) alone, indicating that even at liquid nitrogen temperature [O2(1Δg)]2 is in rapid collisional equilibrium with O2(1Δg).
Two-dimensional free-bound Franck-Condon factors. Application to intramolecular isotope effect in predissociation
Authors: Abgrall Hervé , Fiquet-Fayard Florence
Published: February 2004

We calculate free-bound two-dimensional Franck-Condon factors for a molecule HXD with constant angle HXD. We assume separability into normal modes, but the normal modes are different for the attractive and the repulsive potential surfaces. These Franck-Condon factors can be related to the linewidth of a predissociated level and to the relative probability of dissociation into the channel H+XD or the channel D+XH. Classically, we can predict that the Franck-Condon factor will be a maximum as a function of energy if one of the turning points (for vibration in the bound state) happens to be on the repulsive surface at that energy; furthermore, the location of this turning point on the repulsive surface determines the isotope effect, which can be favorable to either D+XH or H+XD. Our two-dimensional study of the O-ν and ν-0 levels shows that these classical predictions are correct, in contrast to the results obtained using a one-dimensional model; in the latter case, strong enhancement of the Franck-Condon factors can occur through interference effects that cannot be predicted classically; interference effects do not show up in the two-dimensional calculation. For the 3–3, 2–2, and 3–2 levels, classical predictions are in rough agreement with the calculations, but we observe strong enhancement of the Franck-Condon factors for certain relative positions of the two surfaces. In conclusion, interference effects do exist for two-dimensional Franck-Condon factors, though they disappear for 0-ν and ν-0 levels.
Neutron quasielastic scattering study of the ammonium ion reorientations in a single crystal of NH4Br at 373 °K
Authors: Livingston R. C. , Rowe J. M. , Rush J. J.
Published: February 2004

The ammonium ion reorientations in a single crystal of NH4Br in its disordered CsCl phase have been investigated by quasielastic neutron scattering at 373°K. Neutron spectra were measured at four different crystal orientations (with the [110] planes of the crystal in the scattering plane) and data were recorded simultaneously at a variety of scattering angles providing a range of momentum transfers for elastic scattering, 0.5 Å−1 ≤ Q ≤ 2.5 Å−1. The experimental results were fit to models allowing instantaneous random jumps around the C2 and C3 axes of the (NH4)+ ions using a variety of analytical procedures that are described in some detail. The results of these fits establish conclusively that the ammonium ion reorientations are dominated by 90° jumps around the C2 axes with an average time τ between jumps of 3.2±0.4 psec at 373°K. The inelastic part of the measured neutron spectra shows a torsional vibration peak at 305 cm−1, in good agreement with previous values. The results and analysis demonstrate the value of single crystal measurements in neutron studies of dynamic orientational disorder in solids.
New experimental tests of existing interpretations of electronic transitions of ethylene
Authors: Foo P. D. , Innes K. K.
Published: February 2004

The absorption spectra of the gases ethylene-d0, d1, 1,1-d2, cis-d2, trans-d2, d3, and d4 have been photographed using the first order of a 21-ft vacuum spectrograph. Analyses of both the Rydberg system at 1750 Å and the π*-π system between 1500 and 2100 Å have been extended. Band contour analysis has confirmed the assignment of the Rydberg system as 1B3u-1Ag. The resulting rotational constants of the 42 level of the excited state of C2D4 lead to an inertial defect indicative of a nonplanar molecule. Assuming point group D2, we have combined the constants with Franck-Condon calculations in a complete structure determination for the 42 level of the Rydberg state: rCC = 1.41 Å, rCH = 1.08 Å, [right parenthesis lessthan] HCH = 124.4° and an azimuthal angle of 37°. Linewidths in the successful simulation of the C2D4 0–0 band indicate a lifetime greater than 10−13 sec for molecules in the zero level of the Rydberg state. For the π*-π system, all observed bands for all molecules studied have been assigned. Most striking is the relative inactivity of the C–C stretching vibration; except in C2D4, only the torsional mode is active. It is concluded that rCC for the π*-π state is not very different than it is for the Rydberg state. Finally, the extrapolation to the (unobserved) origin of the π*-π system is revised to give 5.5 eV (2285 Å).
Analysis of multiple pulse NMR in solids. II
Authors: Rhim W.-K. , Elleman D. D. , Schreiber L. B. , Vaughan R. W.
Published: February 2004

A systematic method, an extension of the average Hamiltonian formalism, is presented for calculating the effects of pulse errors and imperfections in the multiple pulse nuclear magnetic resonance experiments. Application of this method to account for effects of pulse nonidealities such as phase errors, phase transient effects, pulse size errors, and rf inhomogeneity is found to agree with experimental observation, and the results furnish a basis for understanding the complex couplings between the pulse errors and other interactions such as the dipolar and the chemical shift Hamiltonians.
``Forbidden'' rotational spectra of phosphine and arsine
Authors: Chu F. Y. , Oka T.
Published: February 2004

``Forbidden'' rotational transitions with the selection rules ΔJ =0, Δk = ± 3 (k = ± 1 ← ± 2) caused by centrifugal distortion of molecules have been observed in PH3, PD3, and AsH3 by using a conventional Stark modulation spectrometer. The observation confirms the existence of Δk = ± 3 transitions that were first considered for rotational equilibration of interstellar NH3. The distinction between the Δk = ± 3 transitions and the Δk = ± 1 (ortho-para) transitions was made by observing Stark shifts of the spectral lines. The observed spectra provide accurate information on rotational parameters that are complementary to those previously obtained. The C constants for PH3, PD3, and AsH3 were determined to be 117 488.09±2.2 MHz, 58 972.20±3.0 MHz, and 104 882.584±0.03 MHz, respectively. From these constants the molecular structures were determined.
Time autocorrelation functions for harmonic solids from Gaussian quadratures
Authors: Cukier Robert I. , Wheeler John C.
Published: February 2004

Gaussian quadratures determine time autocorrelation functions (TAF's) of harmonic solids from modified moments of the phonon density of states. The quadratures determine the TAF's with great precision out to times more than twice those at which the Taylor series expansion in time ceases to be accurate. The quadratures provide rigorous bounds to the TAF's over most of this range in time. Moreover, a single set of abscissas and weights determines the quadratures for both position and momentum TAF's. The method can determine TAF's to long times, so that the asymptotic behavior can be examined for a simple model solid.
Vacuum ultraviolet absorption spectra of chlorofluoromethanes from 120 to 65 nm
Authors: Gilbert R. , Sauvageau P. , Sandorfy C.
Published: February 2004

The far ultraviolet absorption spectra of CF3Cl, CF2Cl2, CFCl3, CF2HCl, CFH2Cl, and CFHCl2 have been measured from 120 to 65 nm (∼80 000 to 160 000 cm−1). The observed bands can be accounted for as Rydberg bands related to the first and higher ionization potentials. Several discrete bands are superimposed on the ionization continuum. Between 70 000 and 90 000 cm−1 the bands are due to Rydberg transitions departing from orbitals of the chlorine lone pair type, between 90 000 and 105 000 cm−1 to transitions from orbitals of the C–Cl bonding type, and at higher frequencies to transitions from orbitals of the fluorine lone pair and C–F bonding type.
Relative vibrational intensities for the B1Σ+←X1Σ+ transition in carbon monoxide
Authors: Klump Kirby N. , Lassettre Edwin N.
Published: February 2004

In the electron impact spectrum of carbon monoxide two vibrational peaks (v′=0, 1) are found in excitation of the B1Σ+ state. (Higher vibrational states are predissociated.) According to the Franck-Condon principle, relative intensities of vibrational progressions in a single electronic transition should not change with scattering angle and this has been verified, in past research, for many such progressions. However, the intensity ratio for the two peaks, associated with the B1Σ+ state, changes markedly with angle. For example, with incident electrons of 400 eV energy the ratio changes by a factor of ten in passing from a scattering angle of 0 to 20°. This is the typical behavior of excitations associated with different electronic transitions. Conceivably, strong interaction of two electronic states (due to breakdown of the Born-Oppenheimer approximation) could be involved. If we accept the assignment of the two peaks as due to transitions to the v′=0 and v′=1 vibrational levels of B1Σ+ (as spectroscopists do) then breakdown of the Born-Oppenheimer approximation is the only acceptable alternative.
Integral-transform trial functions: A numerical study of generalized scaling, iterated transforms, and the scale space integration
Authors: Somorjai R. L. , Grimaldi F.
Published: February 2004

Two conceptually important and computationally useful aspects of the integral-transform (IT) approach have been tested on the He sequence. Both generalized scaling and the iteration approach enable one to optimize the shape function; a priori discretization of the IT trial functions by M-point integration rules is computationally expedient; coupled with extrapolation procedures it can alleviate the integration difficulties inherent in the IT approach. By optimizing a mapping (generalized scaling) the initial choice of a shape function becomes less crucial; for discretized IT functions it results in the reduction of the number of simultaneous parameter optimizations. The iteration approach transforms the nonlinear parameter optimization into a multistage process, with its attendant advantages. It also minimizes the importance of the initial choice of the cubature rule (discretization). A judicious blend of mapping and iteration, coupled with a priori discretization and subsequent extrapolation seems to be the way to combine the conceptual advantages of the IT approach with computational feasibility. The IT orbitals were of the form φ1(r)= ∫0∞ dt tνexp(−q t) exp(−tλr2), they were discretized by Gauss-Laguerre-type M-point integration rules, and extrapolation studies indicated that as M → ∞, λ(∞)≈−1.5 and E(∞)≈EH.F. for He.
Kinetic model for dissociative adsorption of a diatomic gas
Authors: Vette K. J. , Orent T. W. , Hoffman D. K. , Hansen Robert S.
Published: February 2004

A general kinetic formulation is given for the adsorption, desorption, and skating of adatoms formed by the dissociative adsorption of a diatom onto a general, two-dimensional lattice. This formulation is then specialized to include only irreversible adsorption without skating or desorption. This problem is solved exactly for the linear array and is solved by successive approximations for primitive lattices of three, four, and sixfold symmetry. For practical purposes, results, which are in satisfactory agreement with limiting results, could be obtained by an analysis limited to the first and second shells of sites around a given site. Limiting fractional surface coverages, which are due to isolated, empty sites surrounded by filled sites, are given for all lattices. Results agree well with Monte Carlo experiments on square lattices. Results for irreversible adsorption without skating are readily adaptable to the problem of irreversible desorption without skating.
Computer study of a classical quadrupole solid. II
Authors: Mandell M. J.
Published: February 2004

The Monte Carlo method is used to study the properties of classical free rotors with first- and second-neighbor electric quadrupole interactions. The study is carried out for 108 particles on an fcc lattice and for 96 particles on an hcp lattice. Good agreement is found with transition temperature, latent heat, Raman scattering, and NQR results for N2, taking the interaction constant Γ/kB=3.1°K. In particular, the transition temperature for the classical system is found to be kBT0 = (12.7±0.3) Γ. The hcp structure appears to undergo a two-dimensional ordering with no thermal anomaly. Susceptibilities and frequencies for quadrupole fields with certain wavevectors, which may be measurable by neutron scattering, are presented for both lattices.
Quantum mechanical scattering of asymmetric molecules. III Calculations or total molecular beam cross sections for the systems He–D2 and He–HT
Authors: Heukels W. F. , van Montfort J. Th. , van de Ree J.
Published: February 2004

Exact quantum mechanical coupled channels calculations including closed channels have been performed for the systems He–D2 as an example of an interaction potential with electronic anisotropy, and for the system He–HT in order to compare for the effect of the shift of the center of interaction from the center of mass. Total integrated collision cross sections have been evaluated for an energy range which is experimentally accessible. The effects of elastic and inelastic rotational contributions necessitate a reconsideration of the law of conservation of cross section. Feshbach resonances, undulating structures due to metastable orbiting resonances, and Newton-Fonda anomalies at thresholds for rotational excitation are discussed. The magnitude of the deviations for asymmetric molecules in different rotational states forbid the use of the customary approximation calculations. These deviations will be responsible for systematic errors in experiments if one tries to analyze the data with spherical symmetric potentials. The influence of the source temperature is mentioned. The determination of the ratio of polarized beam cross sections yields interesting results about alignment of spins. Whereas the determination of the repulsive anisotropy in the high energy limit appears to be possible within 20% error with the shift approximation, the attractive anisotropy is only accessible with exact coupled channels calculations because of the presence of metastable orbiting influence in the intermediate energe range.
Energy requirements and energy disposal: Reaction probability matrices and a computational study of a model system
Authors: Rebick C. , Levine R. D. , Bernstein R. B.
Published: February 2004

The joint distribution of reactant and product energy states (a reaction probability matrix) is considered for the quantal case and the corresponding classical limit, and the implications of microscopic reversibility are noted. Measures of the selectivity of energy consumption, the specificity of energy release, and their interdependence (the relevance) are defined in terms of the joint distribution. To illustrate these principles, a computational study has been carried out for a model problem, namely rearrangement collisions (A+BC−AB+C) for simple square-well potential energy surfaces. Classical trajectory calculations and coarse-grained joint probability matrices are presented for systems moving in one and three dimensions. The model, while simplistic, illustrates the method for constructing P matrices and shows several interesting systematic features. The probability matrices are strongly dependent on the Eyring—Polanyi skewing angle β [where cos2β = mAmC/(mA+mB) (mB+mC)], and for a given β, are much less sensitive to variations in the individual atomic masses. The entropy defects and relevance are calculated for three different three-dimensional systems and the relevance is shown to be a compact measure of the dependence of the product energy partitioning on the energy distribution of the reactants.
Mechanisms of x-ray-induced excitation. Effect of scavengers on the radioluminescence of glassy solutions
Authors: Steen Harald B. , Strand Odd
Published: February 2004

The luminescence of indole and tryptophan in EG/H2O glass at 77 K during irradiation with x rays and uv light has been studied with various concentrations of two electron scavengers (acrylamide and trichloroacetate). The two types of radiation gave identical emission spectra except for a considerable enhancement of the phosphorescence to fluorescence ratio with x irradiation. The effect of the scavengers on the yields of x-ray-induced singlet (S) and triplet (T) levels were calculated from the results. It was found that the yield versus scavenger concentration curves could be decomposed into two components, one being independent of the scavenger, the other one decreasing exponentially with increasing scavenger concentration. Associating the former component with excitation directly from the ground state and the latter with excitation resulting from recombination between solute cations and electrons, it was found that about 80% of the solute excitation is due to recombination. In the case of indole the ratio of triplets to singlets T/S formed by direct excitation was T/S=0.2, whereas for the recombination T/S=1.2. The latter value was attributed primarily to spin relaxation of the recombining ion pairs, whereas intersystem crossing between highly excited levels and recombination in spurs appeared to be of minor importance in this respect. The spin relaxation of the recombining ions, the large scavenger concentrations needed to quench recombination, and the observation that T/S was independent of the LET of the radiation indicated that the recombining ions are closely associated so that the ion pair may be comparable to the CTTS states involved in photoionization of solutes rather than to pairs of free separate ions.
Studies of fluorite and related divalent fluoride systems at high pressure by Raman spectroscopy
Authors: Kessler Jane R. , Monberg Eric , Nicol Malcolm
Published: February 2004

Raman spectra of CaF2, BaF2, CdF2, and PbF2 have been measured at pressures as high as 35 kbar at room and liquid-nitrogen temperatures. The mode Grüneisen constants for the Raman-active modes of the fluorite phases of the Ca, Ba, and Cd salts are 1.8, 2.0, and 1.4, respectively. BaF2 (at 23 kbar) and PbF2 (at 4 kbar) undergo phase transformations to Vh16 (α-PbCl2-like) structures. Raman spectra of the high-pressure phases have been obtained. The spectra of these high-pressure phases have been assigned by comparison with the recently assigned spectrum of the corresponding phase of BaBr2. These observations are compared with predictions derived from relatively simple models of the lattice dynamics of ionic materials.
Upper bound to the collision diameter from HNC and PY effective pair potentials
Authors: Present R. D.
Published: February 2004

Extensions of the HNC and PY theories to take account of three-body forces lead to formulas for the effective pair potential in terms of the true pair potential. Molecular dynamics calculations with an effective (12, 6) potential agree well with recent neutron diffraction measurements in liquid Ar near the triple point. It is shown that the slow-collision diameter σ* of the effective pair potential is an upper bound for the slow-collision diameter σ of the true pair potential, and that in the case of the Ar–Ar interaction σ < 3.405 Å. We also observe that the PY and HNC equations with the (12, 6) potential lead to radial distribution functions g(r) at the triple point of Ar that are in good agreement with molecular dynamics and neutron diffraction g(r) data except for a discrepancy in the depth of the first minimum.
On a variety of wave phenomena in chemical reactions
Authors: Ortoleva Peter , Ross John
Published: February 2004

A variety of wave phenomena are analyzed and discussed for systems in which chemical reactions and transport take place. Certain families of wave solutions of reaction-transport equations arise owing to the weak stability of a reference state to a class of perturbations. We consider both wave induction by heterogeneities and autonomous waves and seek perturbation solutions which provide the dispersion relation and the wave vector dependence of the amplitude for one-parameter families of waves characterized by the wave vector. For the case of an arbitrary reaction mechanism possessing a homogeneous steady state we derive, by use of bifurcation theory and frequency renormalization, small amplitude autonomous plane waves and standing and rotating waves. We find solutions corresponding to long wavelength waves, static structures, and phenomena existing only at intermediate frequencies and wavelengths. The theory is found to have a nonuniformity in convergence in the core region of pacemaker and spiral-like solutions. Various types of behavior are illustrated with a model for which we find analytic wave solutions. We investigate next wave extensions from a homogeneous limit cycle. For heterogeneous systems we show the relation of phase diffusion waves to kinematic waves. For homogeneous systems with arbitrary nonlinear kinetics we derive by use of singular perturbation theory long wavelength extensions for plane waves, centers, and other nonplanar autonomous phenomena and give an example of threshold excitation waves. The problem of bifurcation with symmetry breaking in a homogeneous limit cycle is analyzed with a small amplitude perturbation method.
Dipole moments of the lowest excited singlet and triplet states of 2,4,5-trimethylbenzaldehyde in a durene host crystal
Authors: Sheng S. J. , Hanson D. M.
Published: February 2004

From Stark spectra, the differences in dipole moments between the ground state and the lowest singlet and triplet excited states of 2,4,5-trimethylbenzaldehyde isolated in a durene host crystal are determined within a precise Lorentz local field approximation to be 1.65 ± 0.09 D and 1.05 ± 0.06 D, respectively, for the most prominent sites. Neither dipole moment change is parallel to the carbonyl axis, but the change for the singlet state has the largest component along this axis while that for the triplet state does not. These data are consistent with the assignment of these states as primarily 1nπ* and 3ππ*. The dipole moment of the ground state was determined to be 3.53 ± 0.05 D from dielectric constants of benzene solutions.
Exciton dynamics in molecular solids from line shape analysis: An assessment of the extent of line shape distortion resulting from use of real crystals
Authors: Morris G. C. , Sceats M. G.
Published: February 2004

For crystal absorption systems, the line profile of the frequency dependence of the dielectric permittivity ε(ω) contains information about the exciton dynamics that may be studied by the autocorrelation function generated by the Fourier transformation of ε(ω) into the time domain. However, ε(ω) obtained through transforming normal incidence reflectance data R (ω) of a real crystal when the photon-crystal eigenmodes are strongly coupled may be considerably distorted from ε(ω) of a perfect infinite crystal. In this paper, we consider the ways by which such distortions may arise and, by using a model for ε(ω) that might reasonably correspond to the 4000 Å b-polarized 0–0 absorption system at low temperatures of crystalline anthracene probed on the (001) face, we illustrate the dependence of the extent of distortions on the line profile of ε(ω) upon the following number of factors, viz., (i) spatial dispersion of the exciton bands; (ii) use of an oblique angle of incidence as an approximation to normal incidence in determining R (ω); (iii) thickness of the crystal slab used to determine R (ω); (iv) extent of roughness on the crystal surface; (v) mole fraction of defects in the crystal; and (vi) mole fraction of impurities in the crystal. The treatment allows definition of the condition [real ε(ω) < 0] under which various quasiparticles (longitudinal excitons, surface excitons, excitons bound to impurities) may be excited in a particular crystal system. The methods employed in this paper are of general applicability to strongly absorbing crystal systems and will be of use in understanding exciton dynamics in such systems. The data provide a firm foundation for interpreting reflectance data of a strongly absorbing crystal system, and thus we are able to discuss existing spectral data for anthracene crystals, especially narrow structure observed in low temperature reflection spectra, as well as suggest areas for both theoretical and experimental work.
Intermode vibrational energy transfer in laser excited CH3Cl
Authors: Grabiner Frederick R. , Flynn George W.
Published: February 2004

The infrared fluorescence risetimes of the (ν1, ν4) and ν2 modes of CH3Cl have been measured after pumping the ν6 mode with a Q-switched CO2 laser. The rates of rise were found to be 214 ± 60 msec−1 · torr−1 for (ν1, ν4) and 160 ± 45 msec−1 · torr−1 for ν2. Possible filling pathways for these modes are discussed, and infrared fluorescence rates for all modes in CH3Cl are summarized. The results are compared with similar intermode vibrational transfer rates in CH3F.
Direct correlation function between charges and the equation of state of simple fluids
Authors: Baer S.
Published: February 2004

A parallel drawn between classical correlation functions of molecular densities and correlation functions of charge densities arising in the quantum mechanical treatment of molecular fluids enables us to introduce the notion of a direct correlation function between charges, which can be used to derive an accurate equation of state that incorporates both the effect of the short-range repulsion and the effect of additive and nonadditive many-molecule dispersion forces.
Electronic-to-vibrational energy transfer efficiency in the O(1D)–N2 and O(1D)–CO systems
Authors: Slanger T. G. , Black Graham
Published: February 2004

With the aid of a molecular resonance fluorescence technique, which utilizes optical pumping from the v=1 level of the ground state of CO by A1Π-X1Σ+ radiation, we have investigated the efficiency of E-V transfer from O(1D) to CO. O(1D) is generated at a known rate by O2 photodissociation at 1470 Å in an intermittent mode, and the small modulation of the fluorescent signal associated with CO(v=1) above the normal thermal background is interpreted in terms of the E-V transfer efficiency. The CO(v=1) lifetime in this system is determined mainly by resonance trapping of their fundamental band, and is found to be up to ten times longer than the natural radiative lifetime. For CO, (40 ± 8)% of the O(1D) energy is converted into vibrational energy. By observing the effect of N2 on the CO(v=1) fluorescent intensity and lifetime, it is possible to obtain the E-V transfer efficiency for the system O(1D)–N2 relative to that for O(1D)–CO. The results indicate that the efficiency for N2 is (83 ± 10)% of that for CO. In both cases the initial vibrational distribution remains unspecified. The relatively high efficiency of the O(1D)–N2 reaction implies that it is the sole source of the N2 vibrational temperature in the earth's upper atmosphere.
Molecular beam reactions of potassium atoms with ZnCl2, ZnI2, CdI2, HgBr2, and HgI2
Authors: Bullitt Marion K. , Fisher Charles H. , Kinsey James L.
Published: February 2004

The reactions of K atoms with ZnCl2, ZnI2, CdI2, HgBr2, and HgI2 have been studied in crossed molecular beams. Angular and velocity distributions of the potassium halide products were obtained over wide ranges. The angular distributions indicate that the reactions proceed through long-lived complexes for some cases and through ``osculating complexes'' for others. The velocity distributions are compared to approximate results from phase space theories.
Effect of magnetic and electric fields on the transport properties of polyatomic gases. II. The second order effect for rigid rotors in a magnetic field
Authors: Viehland Larry A. , Curtiss C. F.
Published: February 2004

The formal theory previously developed is applied to rotating, nonvibrating, diatomic molecules (i.e., rigid rotors) in a homogeneous magnetic field. It is shown that the approximations made in the formal theory are quite good. Nevertheless, a ``straightforward'' calculation of the Senftleben-Beenakker effect which is of second order in the potential anisotropy does not agree with experiment. This effect is shown to be of some mixed or higher order in the anisotropy.
Electronic spectrum and magnetic properties of CsMnBr3
Authors: McPherson G. L. , Aldrich H. S. , Chang Jin Rong
Published: February 2004

The polarized crystal spectrum of CsMnBr3 and manganous ion doped into the isostructural magnesium salt, CsMgBr3, have been investigated in the 15 000–30 000 cm−1 region. The general spectral features are very similar to those reported for (CH3)4NMnCl3. The intensities of the ligand field transitions appear to be strongly affected by exchange interactions, since these intensities are quite dependent on the manganese ion concentration. The magnetic susceptibilities of powdered samples of CsMnBr3 and CsMg0.92Mn0.08Br3 have been investigated in the 77–300°K range. The CsMnBr3 obeys the Curie&Weiss law above 190°K, but deviates significantly at lower temperatures. The dilute sample, CsMg0.92Mn0.08Br3, shows near Curie law behavior. The magnetic properties indicate the presence of strong antiferromagnetic coupling in CsMnBr3.
Charge and excitation migration in DNA chains
Authors: Dee Diana , Baur M. E.
Published: February 2004

A theoretical treatment of migration of electrons, holes, and singlet and triplet electronic excitations in ideal DNA helical chains is presented. A localized model is adopted, in which charge and excitation migration is considered to take place through discrete site to neighbor site transfer neglecting long-range quantum mechanical coherence effects. Accordingly a probability function to describe the location of a charge or excitation is introduced. A stochastic equation is developed to describe the time evolution of this function and hence the average behavior of the migrating species. The formalism permits the introduction of base sequencing information through the use of probabilities for occurrence of each of the 16 distinct kinds of nearest-neighbor pairs of bases in a single strand. Migration in a nonuniform but nonrandom DNA chain can then be treated to within a perturbation approximation. The fundamental base-to-base charge and excitation transfer parameters which enter the stochastic equation are calculated for each migrating species and possible pair of neighbor bases using SCF LCAO molecular orbital methods without configuration interaction. A noteworthy feature is the much larger magnitude of the triplet excitation exchange term in this work compared to previous work, leading to relatively large triplet transfer coefficients. Implications of the calculated transfer coefficients for excitation trapping are examined and found to be consistent with experiment. Numerical results for the time dependence of the probability function and for diffusion coefficients for migrating species in DNA's of biophysical interest are presented. The general aspect of these results is similar to that for a uniform DNA chain, but some important differences of detail are present. The mobility of electrons and holes moving along the DNA chain and the resultant electrical conductivity along the chain axis are estimated and found to be considerably larger than in organic crystals.
Electrical conductivity and viscosity of supercritical isobutyric acid + water solutions
Authors: Gammell P. M. , Angell C. A.
Published: February 2004

The electrical conductivity of the isobutyric acid + water critical system has been measured for several concentrations including the critical. Measurements were made using a hybrid conductance/viscosity cell-a modified Ubbelhode viscometer-which permitted almost simultaneous measurements of viscosity and conductance. Absence of a conductivity anomaly was verified for T – Tc ≥ 0.03°C. The results of additional measurements made on a critical solution in which 2% of the protons were replaced by K+ ions clearly show that the absence of an anomaly is not simply due to the fact that, in this binary system, most of the current is carried by the proton. The possibility of an anomaly in the ac conductance quite unrelated to the viscosity anomaly and strongly dependent on bridge frequency, occurring (for audio frequencies) at smaller T – Tc values than those reached in this work, is discussed.
Electronic structure of polyethylene: Theory and ESCA measurements
Authors: Delhalle J. , Andre J. M. , Delhalle S. , Pireaux J. J. , Caudano R. , Verbist J. J.
Published: February 2004

Theoretical and experimental results for the band structure of polyethylene are presented. Theoretical results are obtained from both semiempirical (CNDO/2 and extended Hückel) and nonempirical methods. The density of states is measured by the ESCA method. An energy gap of 7.5 ± 0.5 eV and a C1s line at 284.6 eV are deduced. Although CNDO/2 results appear to be unreliable, the correlation between theory and observed density of states is found to be very good for extended Hückel and ab initio results.
Space-fixed vs body-fixed axes in atom-diatomic molecule scattering. Sudden approximations
Authors: Pack Russell T
Published: February 2004

The Arthurs and Dalgarno space-fixed (SF) axes formulation of the quantum theory of atom-diatom scattering is compared with the body-fixed (BF) axes formulation of Curtiss using consistent notation to facilitate the comparison. While equivalent, the two theories are not always equally convenient. When rotation is treated in a sudden approximation, the BF formulation has a tremendous conceptual and computational advantage: It allows an infinite-order sudden approximation, independent of the form of the potential energy, which should be very helpful in vibrationally inelastic and reactive scattering problems. Also, a rapid procedure for calculating WKB phase shifts is presented.
Free radicals in pyrimidines: ESR of x-irradiated 5-ethylbarbituric acid and its potassium salt
Authors: Gutierrez Peter , Benson Brent
Published: February 2004

Irradiation of 5-ethylbarbituric acid yields a free radical formed by hydrogen abstraction from C(5) of the pyrimidine ring. The characteristics of this radical are similar to those of the same radical formed by abstraction of an ethyl group from 5, 5-diethylbarbituric acid. There is no evidence of an ethyl abstraction radical in this compound, indicating that the abstraction of the hydrogen rather than the ethyl is highly preferred. Irradiation of the potassium salt of 5-ethylbarbituric acid yields a free radical formed by hydrogen abstraction from the ethyl group. Principal values are as follows: g1=2.0032, g2=2.0031, g3=2.0025, A1H = 26.4 G, A2H = 16.5 G, A3H = 10.9 G, A1CH3 = 21.0 G, A2CH3 = 20.5 G, A3CH3 = 19.6 G. Only one molecule per unit cell was observed magnetically, although the crystal structure would indicate that two or three should be seen. The formation of this hydrogen abstraction radical is surprising because a simple ionization of the parent molecule would yield the very stable radical observed in 5-ethylbarbituric acid and 5, 5-diethylbarbituric acid.
Formation of CN from CO and its excitation in active nitrogen
Authors: Young Robert A. , Morrow William
Published: February 2004

It is shown that CN is formed from CO in active nitrogen by reaction with an excited state of N2, probably the ν′ = 0 level of the W3Δu state, and excited by energy transfer from an excited state of N2, probably the 5Σ state. The A 3Σu+ state of N2 is not involved in either the formation or excitation of CN. The three-body association of C and N to form CN, although important in maintaining a fixed ratio between [C] and [CN], is unimportant in forming electronically excited CN. These statements may apply only to conditions where [Ar] ≫ [N2] in the discharge as well as in the observation region and for small additions of CO.
Adsorption of ethylene and hydrogen on tungsten single crystals: Flash desorption, adsorption kinetics, and work function changes
Authors: Barford B. D. , Rye R. R.
Published: February 2004

Adsorption kinetics and coverage dependence of work function changes and decomposition spectra are determined for ethylene on W(110), W(100), W(112), and W(111) planes using flash desorption and Kelvin probe work function techniques. The appearance of gaseous H2 from ethylene decomposition was found to be rate limited by the ethylene decomposition reaction and in some cases by desorption from separate hydrogen binding sites present on the surface. Adsorption was found to occur on all four faces with near unity sticking probabilities with adsorption kinetics proportional to the number of available sites at any given coverage. Work function measurements show that the work function decreased monotonically over the coverage range with saturation values of −1.1, −0.48, −0.46, and −0.44 eV for the (110), (100), (112), and (111) planes, respectively. These results are explained in terms of the chemical and physical implications drawn from the structures of the ethylene surface complexes. For di-σ adsorbed ethylene, the structures of these complexes, assuming minimal strain in the molecule, are determined by the metal-metal spacings available at the surface. In the course of this investigation it was necessary to study hydrogen adsorption on the four planes using the same techniques. These results are discussed in light of values reported in the literature where available. From the work function data and information in the literature the interaction of hydrogen with these four planes can be divided into two cases: induced heterogeneity on the (110) and (100) planes, and site heterogeneity on the (211) and (111) planes. The assumption of one hydrogen atom adsorbed per two missing nearest neighbor tungsten atoms is shown to be in quantitative agreement with the maximum coverages of hydrogen on all four planes.
Superposition approximation and natural iteration calculation in cluster-variation method
Authors: Kikuchi Ryoichi
Published: February 2004

In the cluster-variation method of cooperative phenomena and also in the quasichemical method, the bottleneck step has been to solve simultaneous equations. This paper proposes a new iterative procedure for solving the equations. This iteration does not use differentiation nor matrix inversion and may be called the natural iteration method. The free energy always decreases as the iteration proceeds, with a consequence that the iteration always converges to a stable solution (a local minimum of free energy) as long as the initial state is a physically acceptable one. The method derives in its introductory step a superposition approximation which writes the distribution variables of the basic cluster as a product of those of subclusters. The method is first explained with the pair approximation of the Ising ferromagnet, and then is applied to the fcc binary alloys to derive a phase diagram which is compared with the one reported recently by van Baal.
Comparison between square, triangular, or one dimensional lattice of distortions in smectic A and cholesteric crystals for superposed strain and magnetic field of any directions
Authors: Delrieu J. M.
Published: February 2004

We study the conditions of observations of a static periodic square grid or lattice of distortions in cholesteric of smectic A liquid crystals under a static magnetic field or fast tensile strain. We show that a square lattice is energetically favorable with respect to other lattices. With a tilted magnetic field H we expect two threshold fields: (a) a one dimensional distortion threshold field H1 and (b) a two dimensional square distortion field H2>H1. H1 and H2 are function of the angle θ of H with the normal to the layers. There exists only a one dimensional solution for θ>30°.
Grüneisen functions of polymer polycrystals
Authors: Gibbons T. G.
Published: February 2004

The relationship between the bulk Grüneisen function of a polycrystalline polymer and the Grüneisen functions of the individual anisotropic crystallites is examined. The approximations of uniform strain and uniform stress are shown to yield markedly different predictions for the bulk behavior. The role of the internal degrees of freedom in determining the main features of the temperature dependence of γ is discussed and can be understood qualitatively in terms of the behavior of a simple one-dimensional model. The analysis reveals the similarities and basic assumptions underlying previous theoretical work on simple polymer models. The neeed for single crystal data is emphasized. It is shown to be essential for a more quantitative understanding of the nature of the interactions in polymers.
Viscosities of polar gases by relaxation of capillary flow
Authors: Casparian Armen S. , Cole Robert H.
Published: February 2004

A simple method is described in which capillary flow is measured by the exponential decay of capacitance difference of gases in two reservoirs connected to the ends of the capillary and viscosity is determined from the time constant. Results for Ar, CH3F, CHF3, and CH3CF3 in the temperature range 20–150°C are given, together with 12–6 potential parameters derived using the theory of Monchick and Mason. These parameters for the polar gases are believed to give a much better description of the central forces than parameters derived from pressure virial coefficients.
High temperature mass spectrometry, vaporization, and thermodynamics of vanadium monosulfide
Authors: Owzarski Thomas P. , Franzen Hugo F.
Published: February 2004

A high temperature mass spectrometric investigation of the congruent vaporization of vanadium monosulfide, VS(s), from 1700 to 2000°K has shown that the two principal vaporization reactions produce VS(g) and V(g) + S(g) with a minor contribution from the reaction to produce VS2(g) + V(g). Thermodynamic results were obtained from three investigations of the congruent vaporization of the stoichiometric monosulfide. The second law enthalpy changes for the vaporization reactions at absolute zero were calculated from the experimental data and by use of a measured low temperature heat capacity for VS(s), estimated heat capacities for the solid above room temperature and for the vapor molecules, and literature values for the heat capacities of the atomic species. The Δ H0° values obtained and the estimated uncertainties are: 143±3 kcal for the reaction to form VS(g), 259±5 kcal for the reaction to form V(g) + S(g), and 144±5 kcal for the reaction to form ½ VS2(g) + ½ V(g). Ionic fragmentation of VS+ is considered and shown to be unimportant. Approximate third-law calculations of partial pressures of the important species are reported and the results are shown to be in acceptable agreement with the observed ion intensities.