bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic beam diffraction and scattering from A (3±5) W (110)–carbide surface
Authors: Bush Philip S. , Raff L. M.
Published: February 2003

Atomic beams of helium, neon, and argon generated from a nozzle source have been scattered from a tungsten surface which x-ray, LEED, and Auger spectroscopic measurements show to be a (3×5) W (110)–carbide structure superimposed upon the (110) plane of tungsten and rotated 35° from the principal axes. Helium scattering from this surface at 298 K produces a prominent bimodal structure characteristic of the classical surface rainbow maxima described by McClure. Superimposed upon this structure are at least five distinct peaks or shoulders corresponding to the (2̄,0), (1̄,0), (0,0), (1,0), and (2,0) diffraction directions along the 5-spacing of the superstructure. Neon scattering is observed to produce a broad, bimodal rainbow pattern but no features that can clearly be identified as surface diffraction. As the incident neon-beam energy is increased by seeding the beam with helium, the rainbow scattering becomes more prominent and some evidence of diffraction is seen. This result suggests that the strength of the periodic interaction felt by the incident gaseous atom is responsible for the presence or absence of rainbow and diffractive scattering. Argon spatial distributions exhibit no structure and are trapping dominated. The variation of scattered intensities with surface temperature suggests that for helium and neon a significant fraction of the incident beam is preferentially scattered in directions corresponding to out-of-plane diffraction maxima. In general, the results corroborate the observation of helium diffraction from the (3×5) W (110)–carbide surface previously reported by Weinberg and Merrill. However, several significant differences between their observations and those obtained in the present work are noted and discussed.
The room temperature oxidation of Cu (111): Pressure effects
Authors: Boggio Joseph E.
Published: February 2003

Ellipsometry has been used to follow the oxidation of the (111) face of a copper single crystal at 21 °C for oxygen pressures from 0.03–760 Torr. The pressure dependence is complex, with the short term oxide film thickness increasing to a limiting value as pressure increases, whereas the long term thickness goes through a maximum as pressure increases. The low pressure growth kinetics, as well as previously published contact potential measurements, can be described using the Cabrera–Mott theory for the formation of very thin oxide films modified to include the effects of space charge in the oxide. The rate determining step is taken to be the formation of cation vacancies at the gas–oxide interface. It is suggested that a monolayer of CuO forms on top of the Cu2O film at high oxygen pressures with a concomitant chance in mechanism.
On angular momentum decoupling approximations and factorization in diatom–diatom scattering
Authors: Goldflam Rudolf , Kouri Donald J.
Published: February 2003

Angular momentum decoupling approximations for rotor–rotor collisions are investigated. The coupled states and infinite order sudden approximations are derived from a novel point of view. In addition to ordinary cross section, the CS and IOS results for relaxation cross sections in molecular gaseous mixtures are obtained. It is shown that the full IOS leads to factorization of the scattering amplitude, integral and differential cross sections, and the nonresonant relaxation cross sections. Limited test calculations of the factorization for degeneracy averaged integral cross sections are reported for the o-H2+p-H2 system.
The graphical unitary group approach to the electron correlation problem. Methods and preliminary applications
Authors: Brooks Bernard R. , Schaefer Henry F.
Published: February 2003

Recent theoretical research by Paldus and by Shavitt has strongly suggested that the unitary group approach to the many body problem may be useful in molecular electronic problems. The graphical unitary group approach (GUGA) has now been developed into an extraordinarily powerful theoretical method. The theoretical/methodological contributions made here include a solution of the upper walk problem, the restriction of configuration space employed to the multireference interacting space, and the restructuring of the Hamiltonian in terms of loop types. Several test calculations are examined in detail to illustrate the unique features of the method. For large general multireference configuration interaction (CI) problems, computation times are typically only 15% of those reported using state-of-the-art conventional techniques. Finally, these methods are applied to the vertical electronic spectrum of ketene, and excellent agreement with experiment is found.
The photodissociation of formaldehyde: Potential energy surface features
Authors: Goddard John D. , Schaefer Henry F.
Published: February 2003

Features of the S0 potential energy surface of formaldehyde relevant to its dissociation to molecular products, H2+CO, to radical formation, H + HCO, and to rearrangement to hydroxycarbene, HCOH, have been studied by means of ab initio calculations. A gradient procedure was used to locate and to characterize both equilibrium and transition state geometries. Basis sets of at least double zeta (DZ) quality were employed throughout and many calculations involved more flexible basis sets including polarization functions. Force constants, normal modes and vibrational frequencies were calculated at the SCF level for stationary points on the surface. Extensive configuration interaction (CI) calculations were also carried out. For the molecular dissociation the energy barrier including the effects of polarization functions and electron correlation was 4.06 eV (93.6 kcal mole−1, 32 700cm−1). Correcting for changes in zero point vibrational energy gave an approximate activation energy of 3.76 eV (87 kcal mole−1, 30 300cm−1) with an estimated error of ±0.2 eV (±5 kcal mole−1, ±1700cm−1). The energy required for the rearrangement of formaldehyde to trans-hydroxycarbene was calculated to be 3.85 eV (89 kcal mole−1, 31 000cm−1) at the DZ + polarization + CI level with the inclusion of zero point corrections. The large imaginary frequencies associated with the reactive motion imply sharp and thin barriers through which tunneling is estimated to be of considerable importance. Based on the calculated features of the potential energy surface the mechanism of the photodissociation of formaldehyde is discussed.
Dissociative photoionization of carbon disulphide and carbonyl sulphide
Authors: Eland J. H. D. , Berkowitz J.
Published: February 2003

High resolution photoionization mass spectrometry shows that S+ from CS2 and COS, and CS+ from CS2 appear at the thermochemical dissociation limits with no kinetic or internal energy release to the fragments, accompanied by zero energy electrons. S2+ is formed from CS2 by a bimolecular reaction of CS2+(?) of very high cross section, as well as by unimolecular fragmentation. Five new Rydberg series of COS leading to the ? and ? states are identified, and empirical observations on resonance peak shapes which differ in different channels are presented.
Microwave and radio frequency spectrum of XeHCl
Authors: Chance K. V. , Bowen K. H. , Winn J. S. , Klemperer W.
Published: February 2003

Microwave and radio frequency spectra of 132XeH35Cl are measured by molecular beam electric resonance spectroscopy. The spectroscopic constants determined for this complex are B=989.262 (5) Mhz, DJ=3.84 (32) kHz, (eqQ)a 35Cl=−34.640 (20) MHz, μa=1.0901 (30) D. Only a single vibrational level, with zero angular momentum along the heavy atom axis, has been observed. The results are interpreted in terms of a linear complex with large amplitude oscillation. The structure is assumed to have atomic arrangement XeHCl. The average Xe–Cl distance is 4.258 Å. The average angle of the HCl subunit with the heavy atom axis is 34.8°. The stretching frequency of the weak bond is 33.5 cm−1.
Electronic to vibrational energy transfer from Br(4 2P1/2) to H2, HD, and D2
Authors: Grimley A. J. , Houston P. L.
Published: February 2003

Electronic to vibrational energy transfer from Br* (*=2P1/2) to H2, HD, and D2 has been monitored indirectly by observing subsequent vibrational to vibrational energy transfer from excited hydrogen to CO. From the decay of the Br* fluorescence at 2.71 μm following laser photolysis of Br2 we find the total deactivation rates of Br* by H2, HD, D2, and CO to be (8.66±1.00) ×104, (2.07±0.10) ×105, (2.20±0.14) ×104, and 348±18 sec−1 Torr−1, respectively. For mixtures of hydrogen and CO the amplitude of the CO fluorescence has been compared to the amplitude of the Br* fluorescence. The results indicate that the lower limit to the number of vibrational quanta excited per Br* deactivation is 0.60±0.11 for H2, 0.78±0.22 for HD, and 0.094±0.033 for D2.
Electron mean-free pathlengths through monolayers of cadmium arachidate
Authors: Brundle C. R. , Hopster H. , Swalen J. D.
Published: February 2003

The inelastic mean-free pathlengths λ, of 1042–1402 eV electrons through cadmium arachidate (C19H39COO)2Cd, monolayers have been determined from attenuation measurements of the core level photoemission (XPS) signals from gold, silver and indium substrates. Both fixed angle-variable overlayer thickness experiments and variable angle-fixed overlayer thickness experiments were performed. We derive a preferred value of 3.6 nm at 1117 eV and an E0.5 energy dependence over the narrow energy range considered. Values determined for bulk cadmium arachidate and arachidic acid from absolute intensity comparisons with gold and silver are close to the monolayer values. The results are discussed in the light of the controversy existing concerning other values of λ’s determined for organic and polymeric materials.
Neutron diffraction study on formation and structure of DxTaS2 and HxNbS2
Authors: Riekel C. , Reznik H. G. , Schöllhorn R. , Wright C. J.
Published: February 2003

Cathodic reduction of 2H–TaS2 in D2SO4 was investigated by transient neutron powder diffractometry. Deuterium was found to enter the lattice in a regular way. The structural model proposed implies a strong Ta–D bond. Single crystal neutron diffraction data on H0.76NbS2 confirm the position of hydrogen in a three-center metal bond inside the dichalcogenide layers. Structural data are in accordance with inelastic neutron scattering data.
Approximate scaling formula for ion–ion mutual neutralization rates
Authors: Hickman A. P.
Published: February 2003

A complex potential model is used to treat the mutual neutralization of positive and negative ions. The model is used to examine neutralization by the charge transfer mechanism and also by internal excitation leading to capture. It is found that electron transfer is the dominant process for simple ions and small hydrated ions. The numerical results of the theory have been parameterized in terms of the reduced mass of the collision and the electron affinity of the electron donor. This procedure yields an approximate scaling formula that fits a wide range of experimental data to an accuracy of about ±30%.
Molecular beam electric deflection study of ammonia polymers
Authors: Odutola J. A. , Dyke T. R. , Howard B. J. , Muenter J. S.
Published: February 2003

The molecular beam electric deflection behavior of (NH3)n, n=1 to 6, has been determined. The ammonia dimer is found to be polar and presumably has a single hydrogen-bond structure. The higher polymers are nonpolar, compatible with cyclic, hydrogen-bonded ring structures.
On the theory of chemically induced electron polarization (CIDEP). II. Potential forced diffusion
Authors: Monchick L.
Published: February 2003

A formula is developed for the spin polarization developed by radicals simultaneously diffusing in a spherically symmetric potential field and precessing in a magnetic field; general initial conditions are included, and two boundary conditions: zero rate and infinite rate of reaction at the point of contact. The special case of simple diffusion and infinite rate of reaction is also discussed and compared to the unreacting case treated in the first paper of this series. Asymptotic formulas are given that allow a simplification of the formulas when the exchange forces are very large. This takes the form of multiplying the spin polarization for simple diffusion by simple functions of the potential, allowing a very easy estimate of the effect of potential forced diffusion.
Trajectory study of O+H2 reactions on fitted a
b i
n
i
t
i
o surfaces I: Triplet case
Authors: Schinke Reinhard , Lester William A.
Published: February 2003

Classical trajectory calculations of cross sections for the reaction 0(3P)+H2(1Σ+g; ν, j) →OH(2Π; ν′, j′)+H(2S) have been performed for collision energies 1 kcal/mol⩽E⩽40 kcal/mol using an analytical fit to a recent ab inito potential energy surface. Three initial vibrational states of H2, ν=0, 1, and 2, are considered in order to study the influence of vibrational reactant energy on the OH production. With increasing vibrational quantum number, (a) the threshold shifts to lower energies, and (b) the cross sections rise more steeply with collision energy. Rotational excitation of H2 enhances the total reaction cross section for each vibrational state over the range of H2(ν,j) states studied. The cross sections have been used to calculate reaction rate constants for temperatures 300°K⩽T⩽1000°K and the three lowest vibrational states. The ratio k (T,ν=0):k (T,ν=1):k (T,ν=2) is found to be 1:1.13×104:1.42×106 at 300°K and 1:2.12×101:1.24×102 at 1000°K, demonstrating that vibrational energy strongly enhances the reaction rate. At low temperatures, T≲500°K, the calculated rate constants for ν=0 are considerably smaller than the experimental results indicating that the barrier in the potential surface is probably too high. Agreement with experiments is improved for intermediate temperatures and is best for T≳1500°K where threshold effects are least important. The present calculations are in very good agreement with a recent rate measurement at T∼302°K by Light for reactions of vibrationally excited H2(ν=1) molecules.
Ionic transitions hypersensitive to environment
Authors: Judd B. R.
Published: February 2003

It is shown that the so-called dynamic-coupling mechanism of Mason, Peacock and Stewart, which was introduced to account for the peculiar hypersensitivity of the optical transitions of rare-earth ions with respect to their environment, is formally identical to an earlier proposed mechanism based on an inhomogeneous dielectric. Errors in the derivation of the dynamic-coupling mechanism are corrected, and examples of the rare-earth trihalides are considered.
Clustering distances critical to metal dimer formation in the secondary ion mass spectra (SIMS) of cesium chloride
Authors: Honda Fumihiro , Fukuda Yasuo , Rabalais J. Wayne
Published: February 2003

Cs+2 has been measured in SIMS of KCl–CsCl solid solutions in which [Cs]/[K] varies from 10−1 to 10−5. The data indicate that there is a maximum clustering distance, i.e., Cs–Cs distance, above which Cs+2 is not formed. This distance is ∼200 and ∼400 Å for He+ and Ar+ primary ions, respectively, and is independent of primary energy over the range 0.2–3.0 keV. The results are consistent with a mechanism in which bound secondary clusters consisting of atoms from contiguous as well as noncontiguous lattice sites are formed through potential interactions during irreversible adiabatic expansion of an activated region near the surface surrounding the impact site.
Effect of pressure on resistivity and Mössbauer spectra of the mixed valence compound Sn2S3
Authors: Amthauer G. , Fenner J. , Hafner S. , Holzapfel W. B. , Keller R.
Published: February 2003

A continuous decrease of the resistivity by five orders of magnitude has been observed on Sn2S3 at room temperature under pressures up to 120 kbar. PbSnS3 shows only a decrease by three orders of magnitude. SnGeS3 and PbGeS3 remain insulators in the same pressure range. Mössbauer measurements on 119Sn in Sn2S3 at room temperature and pressures up to 55 kbar indicate that Sn2S3 remains a type I mixed valence compound in this pressure range. The two crystallographic positions of Sn exhibit significantly different Debye temperatures, ΘD(SnII) =207 K, ΘD(SnIV) =429 K. Moreover, distinct 119Sn pressure shifts ∂δ (SnII)/∂P?+2.1 mm/sec kbar and ∂δ (SnIV)/∂p?+1.2 mm/sec kbar are observed.
Effect of chemical environment on magnitude of x-ray absorption resonance at L
I
I
I edges. Studies on metallic elements, compounds, and catalysts
Authors: Lytle F. W. , Wei P. S. P. , Greegor R. B. , Via G. H. , Sinfelt J. H.
Published: February 2003

We report spectra isolating the LIII x-ray absorption threshold resonance of the elements iridium, platinum, and gold in the pure metallic state and in a variety of compounds. When normalized spectra obtained on the metals are subtracted from those obtained on the compounds, the resulting difference spectra are related to differences in the electronic structure of the absorber atom in the two types of environment. The change in area of a threshold resonance line obtained from such a difference spectrum can be related to the ionicity of the bonds of the absorber atom in its compounds. Measurements on supported platinum and iridium catalysts provide information on electronic changes in the metal due to the small size of the metal clusters or to interaction with the support material. Information on electronic changes due to interaction of the catalysts with gas molecules may also be obtained.
Low temperature vibrational spectra, lattice dynamics, and phase transitions in some potassium hexahalometallates: K2[XY6] with X=Sn or Te and Y=Cl or Br
Authors: Chodos Steven L. , Berg Rolf W.
Published: February 2003

This paper deals with the observation and identification of phonon frequencies resulting from the low temperature phase transitions in K2XY6 crystals. By means of a simple lattice dynamical model, the vibrational Raman and IR data available in the literature and obtained here have been analyzed. The model used is an extension of one previously used to explain the vibronic spectra of several related compounds.
The high temperature vaporization and thermodynamics in the lutetium–sulfur system
Authors: Franzen Hugo F. , Hariharan Alleppey V.
Published: February 2003

The high temperature vaporizations of Lu3S4 and nonstoichiometric LuSx (0.75<x<1.30) compositions in the lutetium–sulfur system were investigated by mass spectrometric and target collection Knudsen effusion techniques. Lu3S4 vaporizes congruently according to: (1) LuS1.33(s) =Lu(g)+1.33S(g) and (2) LuS1.33(s) =LuS(g)+0.33S(g). The equilibrium partial pressures are: log PLu(atm) = −(29162±239)/T+(7.117±0.112); log PS(atm) =−(29513±338)/T+ (7.104±0.159); and log PLuS(atm) =−(31366±675)/T+(8.052±0.317). The second-law thermodynamic values are: Reaction (1) ΔH°298= 317.6±2.5 kcal mol−1; ΔS°298=79.6±1.5 eu; Reaction (2) ΔH°298=196.9±3.3 kcal mol−1; ΔS°298=55.9±1.8 eu; and for LuS1.33(s), ΔH°f,298=−126.5±2.5 kcal mol−1; S°298= 18.0±1.5 eu. The dissociation energy of LuS(g) is computed as D°0= 120.6±3.2 kcal mol−1 from the mass spectrometric slopes for log I+LuT and log I+LuST vs 1/T, and with an iterated slope for logI+ST consistent with the congruency of the vaporization reactions. The activity of Lu across the homogeneity range of the monosulfide was measured. Together with the Gibbs–Duhem derived values for the activity of sulfur, the thermodynamic properties of stoichiometric LuS1.0 were calculated. These are: (in kcal mol−1) ΔH°atom,298=268.5±3.0; ΔH°subl,298= 147.7±3.0; and ΔH°f,298=−99.6±3.1.
Temperature dependence of resonant isotopic vibrational energy transfer in H35Cl–H37 Cl
Authors: Horwitz Alexander B. , Leone Stephen R.
Published: February 2003

The resonant vibration-to-vibration energy transfer process, H35Cl(v=1)+H37Cl(v=0) ?H35Cl(v=0) +H37Cl(v=1)+ΔE=2.1 cm−1 has been investigated over the temperature range 192–632 K by use of an isotopically selective transverse discharge chemical laser and infrared fluorescence techniques. The transfer probability shows a temperature dependence of PVV∼T−1.6 over the range studied. This dependence is similar to those observed for a number of other near-resonant processes, but differs from current theoretical predictions for resonant vibrational energy transfer. It is shown that a model based on a statistical distribution of rotational states resulting after a long-lived collision is useful in understanding the parameters which must be important in resonant vibrational energy transfer.
Vibrational–translational energy exchange in liquid chloroform as a function of temperature as determined by Brillouin scattering
Authors: Huf-Desoyer G. , Asenbaum A. , Sedlacek M.
Published: February 2003

The temperature dependence of the vibrational relaxation time τvib of pure liquid CHCl3 was determined by Brillouin scattering between −5 and +55 °C. From the experimental results the number of intermolecular collisions Z10 necessary for the deactivation of the lowest vibrational mode has been calculated using the moving wall cell model. In contrast to other liquids no linear relationship between lnZ10 and (1/T)1/3 could be found. Different reasons for this behavior, which possibly is due to the weak dipole moment of CHCl3, are discussed.
Laser catalyzed translational to vibrational energy conversion in CH3F–O2 mixtures
Authors: Shamah Irwin , Flynn George
Published: February 2003

Translational energy changes accompanying laser induced V–V equilibration processes in CH3F/O2 and CH3F/Ar mixtures have been studied using the time resolved thermal lensing technique. Enhanced translational cooling in CH3F/O2 mixtures over equivalent CH3F/Ar mixtures was observed. The cooling amplitude varied directly with O2 mole fraction while remaining independent of Ar mole fraction. The above observations suggest strongly the presence of a large endothermic V–V energy transfer pathway in pure CH3F. A knowledge of the CH3F equilibration pathway yields a complete description of the vibrational and translational energy distribution at V–V steady state, and suggests that the ν3 manifold is the predominant reservoir for vibrational energy. In mixtures with a large excess of O2, each absorbed laser photon leads to a vibrational excitation approximately one and one half times the initial photon energy. This vibrational energy increase is brought about by ’’stealing’’ energy from the translational/rotational degrees of freedom. The techniques used here to derive the relative heating and cooling amplitudes for CH3F/O2 mixtures undergoing a specified energy transfer path are general and can be applied to any molecular gas or gas mixture.
Molecular beam studies of benzene dimer, hexafluorobenzene dimer, and benzene–hexafluorobenzene
Authors: Steed Joseph M. , Dixon Thomas A. , Klemperer William
Published: February 2003

A detailed study of the electric deflection of molecular beams of (C6H6)2, (C6F6)2, and C6H6–C6F6 is reported. Although no resolved microwave or radio frequency transitions were observable, examination of unresolved beam transitions at radio frequencies were useful in establishing that the homomolecular dimers (C6H6)2 and (C6F6)2 are asymmetric rotors while the heteromolecular dimer C6H6–C6F6 is a symmetric top. From analysis of the quantitative electric deflection the dipole moment of C6H6–C6F6 is 0.44±0.04 D.
Molecular orientation at a vapor–liquid interface: Theoretical and computer simulation results for a model of chlorine
Authors: Thompson S. M. , Gubbins K. E.
Published: February 2003

A first order perturbation theory is used to calculate the density-orientation profile ρ (z1ω1) for the vapor-liquid interfacial region for a fluid of homonuclear diatomic molecules. The intermolecular potential used is a site–site Lennard–Jones model, with the parameters chosen to approximate chlorine. The theoretical results are compared with molecular dynamics results. The molecular dynamics calculations show that the molecules favor an alignment perpendicular to the surface on the liquid side, whereas an alignment parallel to the surface is preferred on the gas side of the dividing surface. The theory predicts both of these qualitative features. The quantitative agreement between theory and simulation is quite good on the liquid side, but is poorer on the gas side of the dividing surface.
Angular correlation in linear copolymers from the compositional dependence of their dipole moments. VI. Time dependent correlations in poly(4-chlorostyrene, 4-methylstyrene) copolymers
Authors: Work Richard N.
Published: February 2003

The compositional dependence of the shape of the map in the complex plane of the dielectric constant of undiluted poly(4-chlorostyrene, 4-methylstyrene) copolymers, P(4CS,4MS), has been investigated at temperatures above the glass transition. The copolymer method, which was developed for the study of equilibrium dipole–dipole angular correlations, has been adapted to include time dependence for this investigation. Results are given in terms of differences of the shapes of contributions of 000, 010, 011, and 111 sequences, where 0 and 1 represent 4MS and 4CS units, respectively. The short range (next neighbor) of the compositional dependence of the relaxation characteristics relative to the longer range already established for the equilibrium angular correlations suggests that the latter represent averages over rotational states of backbone bonds that are undergoing fast and nearly independent transitions from one state to another.
The dipole moment of a 4Π NO
Authors: Lisy James M. , Klemperer William
Published: February 2003

Molecular beams of the metastable a 4Π state of NO are produced by electron impact excitation. The average dipole moment of the a 4Π vibrational distribution is determined to be μ=0.20(2)D by quantitative analysis of the beam focused by an electrostatic quadrupole lens. Analysis of the vibrational population produced by the excitation process indicates that the electric dipole moment of the v=3 level of a 4Π NO is μ=0.20(4)D.
Upper and lower bounds on the F5S–F bond energy
Authors: Kiang T. , Estler R. C. , Zare R. N.
Published: February 2003

The upper and lower bounds for the F5S-F bond energy based on the chemiluminescent reaction of SF6 with metastable Ca and Sr atoms are reported. These observations were made under single-collision conditions. AIP
Experimental and theoretical study of exciton surface polaritons on organic crystals. I. (010) face of TCNQ ° single crystals
Authors: Brillante A. , Philpott M. R. , Pockrand I.
Published: February 2003

An exciton surface polariton has been observed at room temperature by the technique of attenuated total reflection (ATR) spectroscopy on the (010) face of tetracyanoquinodimethane (TCNQ °) single crystals. From the p-polarized reflectivity as a function of the angle of incidence at constant wavelength, the experimental dispersion of the surface mode and the real and imaginary part of the dielectric function were determined in the wavelength region 515–425 nm. Supporting theoretical studies also reported include a calculation of the dispersion curve of a free exciton surface polariton, the calculation of perspective figures representing the ATR at fixed prism-crystal gaps for variable frequency and angle of incidence, and the calculation of the intensity of the electromagnetic field in the prism, gap and the crystal.
Application of nucleation to the study of the gas phase photo-oxidation of sulfur dioxide
Authors: Mirabel P. , Clavelin J. L.
Published: February 2003

The photo-oxidation of SO2 by disproportionation has been investigated by excitation in the singlet absorption band (λ=305 nm). The experiments were conducted in a diffusion cloud chamber in which the water, used as the working fluid, was maintained at a maximum activity of 2.65. The rate of SO3 production was directly related to the observed rate of nucleation through our previous study on binary nucleation in H2SO4–H2O mixtures. Our results lead to a rate constant of SO2 disproportionation k1a= (8.4±2) ×109 l mole−1s−1, value which agrees with that of the recent study by Marvin and Reiss (if one corrects their results to take into account the pressure saturation quenching effect), but is 200 times larger than the previous values determined in the absence of water vapor.
The methyl torsion in toluene (h8, d8) and nitromethane (h3, d3) in the polycrystalline state
Authors: Kanesaka Isao , Naka Haruko , Kawai Kiyoyasu
Published: February 2003

The Raman spectra of toluene and nitromethane at about −150 °C were observed in the region of 10–250 cm−1. In toluene-h8 two weak and sharp bands observed at 77 and 115 cm−1 were assigned to the CH3 torsion. In toluene-d8 the CD3 torsion was not observed distinctly. The assignment was confirmed by the calculation of the torsional energy levels on the basis of the C2v symmetry by the finite element method. The potential function was assumed to be Σ1/2Vn(1−cosnβ) (n=2, 6, and 12). The best fit was obtained when V2, V6, and V12 were 28, 99, and −42 cm−1, respectively, and further confirmed that the other structures observed at 106 and 94 cm−1 could be also assigned to the CH3 torsion. In nitromethane-h3 or -d3 a very weak and broad band observed at 148 or 132 cm−1, respectively, was assigned to the methyl torsion. In the process of the adjustment of the potential constants it was found that V3 was important. The best fit was obtained when V3, V6, and V12 were 210, 150, and 30 cm−1, respectively. This shows that a sort of association takes place between a hydrogen atom of a molecule and an oxygen atom of the other.
A stochastic basis for isothermal equilibrium and nonequilibrium chemical thermodynamics
Authors: Gardiner C. W.
Published: February 2003

The equilibrium and nonequilibrium thermodynamics of diffusing chemically reacting systems is shown to be derivable from a stochastic master equation basis. The simple combinatorial form of transition probabilities is shown to give the ideal system limit of chemical thermodynamics in equilibrium situation, and thus provide the natural generalization of these to nonequilibrium situations.
Effects of rotationally inelastic collisions on gaseous transport coefficients
Authors: Wood Robert R. , Curtiss C. F.
Published: February 2003

The Monchick, Yun, and Mason generalization of the Wang Chang–Uhlenbeck–deBoer formulation of kinetic theory is the starting point for the generalized phase shift (GPS) evaluation of the transport coefficients. It is shown that the expressions for these properties can be written in terms of the rotational energy moments of the degeneracy averaged cross sections. The appropriate thermal averages of these ’’cross sections’’, referred to as collision integrals, are numerically evaluated. These quantities are then used to determine the values of the coefficients of binary diffusion, viscosity, thermal conductivity, and thermal diffusion. Calculations are made for gaseous systems consisting of a dilute mixture of a diatomic species in an excess of an atomic species. The diatomic is treated as a rigid rotor, and the effects of rotational inelasticity on the transport coefficients are analyzed.
Fluorescence transient and optical free induction decay spectroscopy of pentacene in mixed crystals at 2 K. Determination of intersystem crossing and internal conversion rates
Authors: de Vries Harmen , Wiersma Douwe A.
Published: February 2003

In this paper the recently reported fluorescence transient, appearing during the onset of cw dye laser excitation of the energetically lower sites of pentacene in p-terphenyl at 2 K, is analyzed quantitatively. The role of the lowest triplet state in causing the long (compared to the fluorescence lifetime) transient, is firmly established. For the lower sites a kinetic analysis yields a T1←S1 intersystem crossing (ISC) yield of about 0.4% and an S0←T1 ISC rate of (2.2±0.1) × 104 s−1. Using these data in calculating the S1←S0 transition dipole moment from our earlier OFID experiments yields for the O1 site (proto) μ=0.71±0.24 D. The S0→S1 internal conversion yield is estimated to be 22%. The fluorescence transients of the energetically higher sites of pentacene in p-terphenyl exhibit a much faster decay, which indicates a drastic increase in ISC yield from S1. Optical free induction decay (OFID) experiments performed on these sites confirm this and imply that the T1←S1 ISC yield for these sites is ≳60%. We further discuss results of an OFID experiment on pentacene in naphthalene and derive in this case a T1←S1 ISC yield of ≲2.8% and an S0←S1 internal conversion yield of 31.5±1.5%. The off-resonance pumping effect in a 3-level system was also studied, using the parameter values for pentacene in p-terphenyl. For the lower sites, with the excitation intensities used (∼1 MW/m2) the part of the inhomogeneous absorption line that is pumped has a width of about 2.5 GHz (effective excitation bandwidth <1 MHz).
Wavelength-selective bleaching of the optical spectra of trapped electrons in organic glasses. II.
Authors: Paraszczak Jurij , Willard John E.
Published: February 2003

Further resolution of the inhomogeneous optical spectra of trapped electrons (e−t) in organic glasses has been obtained from wavelength selective bleaching and thermal decay studies on 3-methylpentane-d14(3MP-d14) and 2-methyltetrahydrofuran (MTHF) following γ irradiation in the temperature region of 20 K, and limits on the degree of resolution achievable have been indicated. Exposure of 3MP-d14 to light of wavelengths ≳2100 nm (from a tunable laser) reduces the optical densities at the bleaching wavelength and longer to zero, while ’’peeling off’’ a portion of the O.D. at all shorter wavelengths but leaving the remainder of the spectrum unaffected. The fraction of the integrated optical spectrum, ℱOD d (eV), removed by bleaching at each wavelength tested, and also by thermal decay, is equivalent to the fraction of the total e−t spins removed and measured by ESR. 1064 nm light bleaches the spectrum nearly uniformly, confirming that the spectra of all of the e−t have blue tails with similar ease of bleaching. Heretofore unobserved low temperature thermal decay of e−t occurs at 20 and 40 K (20% of the spin concentration in 30 min, 35% in 3h). The rate of decay of the optical spectrum decreases with decreasing wavelength of observation (2.5, 2.2, 1.8, and 1.5 μ), but at each wavelength is the same at 40 K as at 20 K, consistent with a tunneling mechanism. Unique inhomogeneities in the spectrum of e−t produced in MTHF at 20 K are revealed by bleaching at 1338 and 1556 nm. The effects of biphenyl solute and of exhaustive bleaching at these wavelengths are reported and discussed. The initial quantum yields of bleaching e−t in MTHF at 77 K with 1338 nm or 1064 nm light are less than 10−4 of the yields for e−t produced and bleached at 20 K.
On the Stokes problem for a suspension of spheres at nonzero concentrations. II. Calculations for effective medium theory
Authors: Muthukumar M. , Freed Karl F.
Published: February 2003

We present the detailed calculations for the effective medium approximation to the description of the hydrodynamics of a stationary suspension of randomly distributed, penetrable spheres where the fluid motion is generated by the motion of a single sphere with uniform velocity through the suspension. The formal theory, given in Part I, is reduced to a set of nonlinear integral equations for the concentration dependent effective hydrodynamic interactions in the suspension. The effective medium theory equations are solved approximately by use of two methods. The first is a ’’hydrodynamic’’ approximation which is utilized to evaluate the shear viscosity η, the translational friction coefficient, ζ, and the hydrodynamic screening as expansions in cζ/η. A more general approximation scheme includes the effects of ’’microscopic viscosity’’ and is shown to slightly alter the analytic form of this concentration dependence. Because of this alteration, the method is also applied to suspensions of Brownian spheres, the theory of which is presented elsewhere, to demonstrate that in the mobile case the analytic form is not altered by the inclusion of microscopic viscosity from that generated by the hydrodynamic approximation.
Basis set dependence of the structure and properties of liquid hydrogen fluoride
Authors: Jorgensen William L.
Published: February 2003

An intermolecular potential function for the hydrogen fluoride dimer has been obtained from ab initio molecular orbital calculations with a minimal basis set (STO-3G). Dimerization energies for 285 orientations of (HF)2 selected using the enery distributed random geometries method were fit to a 12–6–3–1 potential with standard deviations of ∼0.3 kcal/mol. The function was used in a Monte Carlo simulation of liquid hydrogen fluoride at 0 °C. Structural and thermodynamic results are compared with those based on an earlier potential derived from double zeta basis set (6-31G) calculations. The structural predictions are affected by the local density which is too high from the 6-31G potential. Significant improvement is obtained with the STO-3G function. A detailed analysis of the structure of liquid HF is presented including coordination number and energy distribution functions. Each HF monomer has an average of two hydrogen bonded neighbors with an additional five peripheral contacts within 3.6 Å. The hydrogen bonded monomers form winding chains with significantly bent hydrogens bonds in contrast to the linear hydrogen bonds found in the solid and in the gas phase dimer. As in the case for water, it is apparent that reasonable intermolecular potential functions for hydrogen bonded dimers can be obtained from minimal basis set calculations. In the absence of CI level calculations, this appears to be the method of choice.
The ionization of PCl6− in the solid state under pressure. II. Raman spectrum of phosphorus pentachloride III
Authors: Cahay R. , Wong P. T. T. , Whalley E.
Published: February 2003

The Raman spectrum of phosphorus pentachloride III, which is obtained by squeezing the ordinary phase II to a few kilobars (see preceding paper), has been recorded at ∼100 K and zero pressure. The ratio of the integrated intensities of the PCl4+ and PCl6− ions is much smaller in phase III than in phase II, showing that the ionization PCl6−→PCl4++2Cl− occurs in the solid at the transition. It appears to be the first example of a pressure-induced ionizing dissociation in a solid. Phase III is similar to, but apparently not identical with, a phase obtained by subliming phase II and condensing the vapor at, apparently, room temperature. The implications of this transformation for the synthesis of inorganic compounds is briefly discussed.
The ionization of PCl6− in the solid state under pressure. III. Phosphorus-31 NMR spectra of three phases of PCl5
Authors: Ripmeester J. A. , Wong P. T. T. , Davidson D. W. , Whalley E.
Published: February 2003

The 31P resonance lines of PCl6− and PCl4+ in three solid forms of PCl5 were studied at 72.8 MHz between 140 and 395 K. The high-pressure phase III has the composition 2PCl4+⋅PCl6−⋅Cl−, in contrast to phases I and II which consist of equal numbers of PCl4+ and PCl6− ions. In normal tetragonal phase II reorientational narrowing of the PCl6− and PCl4+ lines is centered at 202 and 240 K, respectively. Between 280 and 380 K an axially symmetric chemical-shift tensor, with Δσ=17.8 ppm, is found for PCl6−, while no asymmetry is seen for PCl4+. Transformation to phase I at 385 K is accompanied by a fall of Δσ for PCl6− to 9.2 ppm.
The thermodynamic potential for homogeneous nucleation in a two-dimensional lattice gas
Authors: Christensen Roderick A. , Woodbury George
Published: February 2003

The Bethe or quasichemical approximation, is applied to the two-dimensional, triangular lattice gas in order to compute the probability of liquid drop formation in a supersaturated vapor. Comparison is made with the classical theory of homogeneous nucleation of Becker and Doring. The probability is of the form ρl exp(−Δψ/kT), where ρl is the density of bulk liquid (instead of the density of supersaturated vapor, which appears in the classical theory) and Δψ is a potential which approaches the classical form in the large drop limit. The potential for well-characterized, compact drops does not exist for sufficiently small drops, which we interpret to mean that such drops do not exist in the supersaturated vapor. When the limiting compact drop size is smaller than the classically predicted critical drop, the classical theory holds; however, in the opposite case large discrepancies occur. The error increases with increasing degree of supersaturation. Surface tensions were calculated in the large drop limit, showing a significant drop in surface tension with increasing supersaturation.
Temperature effect on the quantum yield for production of S(1S) from OCS
Authors: Black G. , Sharpless R. L.
Published: February 2003

The production of S(1S) from OCS, over the wavelength range 142–170 nm, has been investigated at temperatures of 232, 296, and 425°K. Changing the temperature produces no measurable effect on the S(1S) yield in the region of high quantum yield (142–157.7 nm). In the fall off region (157.7–170 nm), raising the temperature increases the quantum yield with the fractional increase growing to longer wavelengths—to a factor of 1.9±0.4 at 170 nm (for a temperature change from 232 to 425°K). Both rotational and vibrational energy may contribute to the effect but, based on considerations of the photodissociation mechanism in this region, vibrational energy (in the bending mode) is thought to play a more important role.
ArF laser photolysis of OCSe. I. Se(1S) quenching processes
Authors: Bischel W. K. , Black G. , Hawkins R. T. , Kligler D. J. , Rhodes C. K.
Published: February 2003

Quenching rates for Se(1S) and Se*2 by OCSe have been measured using ArF laser photolysis of OCSe at 193 nm for the generation of Se(1S) atoms and the reactive mechanism Se(1S)+OCSe→Se*2+ CO as the source of Se*2. The experimentally determined quenching rates are (1.6±0.4) ×10−10 and 4.6×10−11 cm3 molecule−1 sec−1 for the processes OCSe+Se(1S) and OCSe+Se*2, respectively. Evidence is presented which demonstrates that the Se*2 is formed initially in a state other than the strongly radiating B 3Σ−u level.
Resonance absorption measurements of atom concentrations in reacting gas mixtures. I. Shapes of H and D Lyman-α lines from microwave sources
Authors: Lifshitz Assa , Skinner Gordon B. , Wood David R.
Published: February 2003

Profiles of the Lyman-α lines of H and D emitted from microwave discharges in H2–D2–He mixtures, using 0.01, 0.1 and 1% H2 and D2 in lamps of three configurations, have been measured at high spectral resolution. Under most of the conditions studied, substantial self-absorption and line reversal were observed. Representative plasma temperatures have been measured from the profiles of the Balmer-α and -β lines. Lyman-α profiles have been modeled using two parameters (taking into account self-absorption in the plasma and between the plasma and the lamp window) along with the plasma temperature. This information is valuable for calibrations and in the choice of operating conditions in resonance absorption and fluorescence measurements of H and D atoms.
Resonance absorption measurements of atom concentrations in reacting gas mixtures. II. Calibration of microwave sources over a wide temperature range
Authors: Chiang Chi-Chang , Lifshitz Assa , Skinner Gordon B. , Wood David R.
Published: February 2003

A series of experiments was carried out to calibrate three different microwave discharge lamps for analysis for D or H atoms, using Lyman-α absorption. Known concentrations of D atoms were produced in a shock tube by the reaction of 0.05–4 ppm D2 with N2O in argon at 1800–3000 K. H atoms were produced by dissociation of 2,2,3,3-tetramethylbutane (10 ppm in argon) at 980–1140 K. These absorption data were compared with the absorption calculated from Lyman-α line shapes reported in an earlier paper, good agreement being found. These experiments provide a sound basis for obtaining the temperature and concentration dependence of the absorption coefficient over a wide temperature range, for H and D concentrations between 10−12 and 10−10 mole/cc (for a 7.5 cm light path).
Quantum theory of exchange reactions: Use of nonorthogonal bases and coordinates
Authors: Stechel E. B. , Schmalz T. G. , Light J. C.
Published: February 2003

A general approach to quantum scattering theory of exchange reactions utilizing nonorthogonal (’’over-complete’’) basis sets and nonorthogonal coordinates is presented. The method is shown to resolve many of the formal and practical difficulties attending earlier theories. Although the inspiration came from the early and accurate work on the collinear H+H2 reaction by Diestler possible applications include electron transfer processes as well as chemical exchange reactions. The mathematics is formulated in detail and the solution is presented in terms of the R-matrix propagation method preserving all the symmetries of the physical process, i.e., conservation of flux and microscopic reversibility.
Time independent quantum theory of electron transfer collisions using a nonorthogonal basis and R-matrix propagation
Authors: Schmalz T. G. , Stechel E. B. , Light J. C.
Published: February 2003

We briefly review the difficulties associated with the traditional adiabatic quantum treatment of low energy charge transfer collisions between atoms and suggest that the charge transfer problem can be most conveniently treated by using atomic-centered basis functions and expressing the Hamiltonian simultaneously with respect to two separate atomic origins. As a result, we must solve the quantum nuclear equations of motion in a nonorthogonal basis. We demonstrate that this may be efficiently done by building the R-matrix propagator in this basis. Flux is automatically conserved even though the R matrix is built up from a series of nonunitary transformations. A simple model problem designed to represent the radial coupling in the reaction Li++Na→Li+Na+ is solved to illustrate the technique.
Optical absorption of matrix isolated Li, Na, and Ag clusters and microcrystals
Authors: Welker T. , Martin T. P.
Published: February 2003

Optical absorption spectra are reported for Li, Na, and Ag in various stages of aggregation ranging from atom to microcrystal. The samples were prepared by the simultaneous condensation of rare gas and metal vapor onto a liquid helium temperature substrate. The dependence of the strength of the individual absorption lines on metal concentration and on selective light induced diffusion allowed the tentative assignment of spectral features to one, two, three, and four atom clusters. Comparison of the observed transition energies with an empirical, extended Hückel calculation seems to indicate that the trimers are triangular and the four atom clusters have tetrahedral shape. For metal concentrations higher than 10% the sharp molecular lines abruptly disappear and a single broad plasmon absorption, characteristic of microcrystals, appears. The shape of this absorption band can be explained using a continuum theory of dielectric response if microcrystal size, shape, and mutual interaction are properly taken into account.
Abrupt density dependence of attachment of thermal electrons in high density c
i
s-butene-2 vapor
Authors: Wada Toshinori , Freeman Gordon R.
Published: February 2003

In highly purified cis-butene-2 vapor, electron attachment by a trace impurity occurs under a narrow range of conditions near the critical region. When the cell contains an amount of material that gives the critical density when it is all in the gas phase, the attachment efficiency increases by an order of magnitude between 430 and 433.5 K, then decreases again such that the value at 440 K is similar to that at 430 K. The critical temperature is 433 K. The behavior is similar to that reported earlier for electron attachment to oxygen in dense helium gas. The interpretation of the earlier results has been reopened.