bluuebunny/crossref_metadata_2025 huggingface dataset:
Collisions in a noncentral field: A variational and trajectory investigation of ion–dipole capture
Authors: Chesnavich Walter J. , Su Timothy , Bowers Michael T.
Published: February 2003

Variational rate theory and trajectory methods are used to investigate the classical dynamics of capture on a noncentral long range potential. The ion–dipole surface is investigated in detail. An upper bound to the true thermal capture rate constant is derived using the variational approach. The upper bound is independent of the moment of inertia of the rotor and has only a multiplicative inverse square root dependence on the system reduced mass. For the ion–dipole surface the upper bound is in excellent agreement with trajectory calculations of the true capture rate constant and with experimental data on thermal heavy particle transfer rate constants between ions and polar neutrals. The trajectory calculations are done using an angular momentum conserved coordinate system which eliminates the need to solve two sets of Hamilton’s equations. A precise criterion for capture is derived and is used in the trajectory calculations. Reduced variables are introduced for the ion–dipole surface and it is shown that the ratio of the true thermal capture rate constant to the Langevin rate constant depends at most on only two reduced variables. An upper bound to the values of the impact parameter that can lead to capture is also derived and is compared to some earlier trajectory results of Dugan and Magee. It is shown that the upper bound arises from a noncentral field periodic orbit.
Asymptotic solutions of the FKN chemical wave equation
Authors: Schmidt S. , Ortoleva P.
Published: February 2003

A new matched asymptotic expansion technique is used to obtain the approximate velocity and profile of chemical waves in the Belousov–Zaikin–Zhabotinskii (BZZ) mixture as described by Murray’s reduction of the Field–Koros–Noyes (FKN) equations. It is shown that the wave dynamics reduces, in a physically interesting limit, to the solution of a Stefan (moving boundary) problem with a Fisher nonlinearity.
A molecular dynamics simulation of dephasing in liquid nitrogen. II. Effect of the pair potential on dephasing
Authors: Levesque D. , Weis J.-J. , Oxtoby David W.
Published: February 2003

We carry out a computer simulation of vibrational dephasing in liquid nitrogen near its boiling point using a multiparameter potential which includes short range valence, dispersion, and quadrupole interactions. The effect of vibration–rotation coupling on the line shape is also included in the calculation. The isotropic Raman line shift obtained is −5 cm−1 while the dephasing time T2 (inversely proportional to the linewidth) is 100±10 psec. It is shown that there is substantial cancellation between repulsive and attractive contributions to the linewidths, so that the Raman spectrum is very sensitive to details of the molecular interactions.
Analysis of the LMR spectra of methoxy, CH3O
Authors: Russell D. K. , Radford H. E.
Published: February 2003

The rotation spectrum of the free methoxy radical (CH3O) has been measured at 18 discrete wavelengths between 0.25 and 0.65 mm by the laser magnetic resonance method. A least-squares fit to the measured spectra yields precise values for the molecular parameters of the 2E ground state of CH3O.
Experimental and theoretical study of exciton surface polaritons on organic crystals. II. Highly reflecting faces of CTIP and PTS
Authors: Philpott Michael R. , Pockrand Iven , Brillante Aldo , Swalen J. D.
Published: February 2003

Exciton surface polaritons have been observed at room temperature on the highly reflecting crystal faces of CTIP [γ-cyclopropyl-bis  (1,3,3-trimethylindolenine-2-yl)pentamethinium fluoroborate] and PTS [poly-2,4-hexadiyne-1,6-diol bis  (p-toluene sulfonate)]. These surface modes, which cannot be excited by light incident from air, were detected by measuring the attenuated total reflection (ATR) of a focused laser beam inside a prism separated from the crystal face by a gap of the order of the wavelength of the exciting light. The experimental dispersion curves of the surface modes were obtained by plotting the photon energy against the angle of the reflectance minimum. The surface active component of the complex dielectric tensor was calculated for both cyrstals and found to agree quite well with values obtained from normal incidence reflectivity studies. Several supporting theoretical studies are also reported, including: (i) the calculation of the free exciton surface polariton dispersion curves for the natural faces of CTIP, (ii) model calculations representing the ATR for all energies and angles as a perspective figure, and (iii) an investigation of the stop-point region where the virtual exciton surface polariton intersects the volume polariton modes of the CTIP crystal.
A thermodynamic study of the gaseous thorium carbides, ThC, ThC2, ThC3, ThC4, ThC5, and ThC6
Authors: Gupta Satish K. , Gingerich Karl A.
Published: February 2003

Six gaseous carbides of thorium, ThCn(n=1–6), have been identified in a Knudsen effusion mass spectrometric investigation of the vapor phase above a thorium–uranium–rhodium–graphite system at high temperatures. The partial pressures of the thorium containing species were measured as a function of temperature in the 2300–2700 °K range. Third law enthalpies for the reactions Th(g)+nC(graphite) =ThCn, n=1 to 6, and of various other homogeneous and heterogeneous reactions were evaluated. By combining the experimental enthalpies with appropriate thermodynamic data taken from literature, the following values for the atomization energies ΔH °at,298, and standard heats of formation ΔH °f,298 of thorium carbides have been derived:
Molecular dynamics studies of orientational and collision-induced light scattering in molecular fluids
Authors: Frenkel D. , McTague J. P.
Published: February 2003

Molecular dynamics calculations of the Rayleigh and Raman light scattering properties of atom–atom Lennard-Jones fluids (’’N2’’, ’’O2’’, ’’Cl2’’, and ’’CO2’’) have been made, including collision-induced (CI) effects to lowest order in the dipole-induced dipole approximation. CI effects are shown to result in deviations from the Lorentz–Lorenz (Claus–Mosotti) relation, changes in the spectral moments of both Rayleigh an Raman bands and in their depolarization ratios. Where possible, the predicted magnitudes are compared with experiment. Calculated CI influences on the effective molecular polarizability are compared with theoretical estimates based on the superposition approximation. The importance of isotropic CI light scattering in molecular fluids is indicated. Finally, we discuss the separability of orientational and CI contributions to depolarized spectra.
Direct measurement of velocity distributions in argon beam–tungsten surface scattering
Authors: Janda Kenneth C. , Hurst Jerry E. , Becker Charles A. , Cowin James P. , Auerbach Daniel J. , Wharton Lennard
Published: February 2003

Velocity distributions for Ar atoms scattering from a clean, polycrystalline tungsten surface have been measured for a wide range of incident supersonic beam energies 300 K < (1/2) m〈v2〉/k<2000 K, and surface temperatures 350 K<Ts<1900 K. This work studies directly the nature of the scattering process of an intermediate mass atom on a clean metal surface over a very wide range of conditions. Direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process. No distinct elastic or quasielastic scattering occurs. Only at the lowest temperatures is a trapping–desorption scattering process observed. The direct inelastic scattering process is characterized by the linear proportional relationship 〈KEe〉=0.83 〈KEi〉+0.20 〈KETs〉 over the entire range of energies and temperatures for 45° angle of incidence and observation in the specular direction (KEe, KEi, and KETs are the kinetic energy of the exiting Ar, the incident Ar, and the Ar in equilibrium at the surface temperature, respectively). The proportionality relationship is nearly the same for observation in the normal direction. The collision-caused spread in velocities is linearly proportional to the surface temperature, and substantially independent of incident energy. Tangential momentum accommodation is also discussed.
Dielectric friction and polar molecule rotational relaxation
Authors: McMahon D. R. A.
Published: February 2003

Using the Onsager cavity model the frequency dependent torque due to the long range dipole–dipole interaction is derived for an electric dipole rotating in a polar liquid. This generalizes to all orders the result first order in the angular velocity derived by Fatuzzo and Mason and by Nee and Zwanzig. For a constant angular velocity the dielectric frictional torque on a rotor is shown to depend upon the complex permittivity only at the frequency of rotation and has no zero frequency contribution as given by the first order theory. The effect of dielectric friction upon the rotational Einstein relation and the second fluctuation–dissipation theorem is derived. Unlike the first order theory and consistent with the suggestion of Hubbard and Wolynes this theory invalidates the rotational Einstein relation when long range dipolar coupling effects are included in the theory of rotational relaxation. The first order theory is valid only for high angular frequencies above (2kT/I⊥)1/2. The formulation presented in this report is most conveniently applicable when significant inertial effects are present. In a sample calculation for highly compressed polar gases it is shown that dielectric friction produces a contribution to the angular momentum relaxation time second order in the gas density. This contribution is significant for rapidly rotating polar molecules of small moment of inertia at number densities above 2×1021 cm−3.
The reactivity–selectivity principle: The derivation of bounds and a computational study
Authors: Pollak E. , Levine R. D.
Published: February 2003

The decline of the reaction probability due to selectivity of the reaction with respect to the initial, reagents, states or with respect to the final, products, states is demonstrated analytically and by computational results for the H+H2, F+H2, and H+Cl2 collinear reactive collisions. Upper and lower bounds on the reaction probability in terms of the entropy deficiency of the observed or computed products state distribution (or of the relative rates for state selected reagents) are derived and applied to the results of the collinear computations. For very selective processes, the maximal suprisal provides a tight bound for the (logarithm of the) reaction probability.
Optical rotatory behavior of polypeptide solutions in the liquid crystal and pretransitional regions
Authors: Patel Dahyabhai L. , DuPré Donald B.
Published: February 2003

The optical rotatory behavior of solutions of the synthetic polypeptides poly-γ-benzyl-l-glutamate (PBLG) and poly-γ-benzyl-d-glutamate (PBDG) was studied in various solvents as a function of wavelength, concentration of polymer and temperature. The specific rotation in the liquid crystal was found to be inversely proportional to the pitch of the cholesteric structure and therefore does not follow the deVries equation previously thought to describe the optical rotatory dispersion of these liquid crystals. Solutions of PBLG and PBDG were also examined in the pretransitional region at concentrations just below the critical point for the formation of cholesteric spherulites. An enhancement of the optical rotatory power was observed which is attributable to short range chiral ordering of the long axes of the macromolecules. The sense of the emergent chiral ordering is strongly solvent dependent but the effects may be separated into three solvent classifications. Residual form optical activity was also found in the pretransition region of racemic mixtures of PBLG and PBDG in each solvent class.
Quantum–stochastic approach to laser-stimulated desorption dynamics and population distributions of chemisorbed species on solid surfaces
Authors: Lin Jui-teng , George Thomas F.
Published: February 2003

A quantum–stochastic treatment is presented for multiphoton processes of chemisorbed species on solid surfaces. The average excitation is evaluated for different stochastic processes. In the Markovian fast modulation limit, the average excitation takes a Lorentzian form, while taking Gaussian in the slow modulation limit. The addition of the memory effects is calculated by Mori’s projection operator technique and is found to be a function of the correlation time and the frequency modulation of the stochastic processes. For low excitations, the average excitation is found to be linearly proportional to the laser intensity, and is saturated by the red shift of the anharmonicity for high excitations. The overall broadening of the infrared line shape is caused by the fluctuation of the adspecies-field interaction energy, the excitation-induced surface migration, the phonon-dispersion-induced dephasing, and the energy relaxation due to the adspecies–phonon coupling. The population distributions for the slow T1 relaxation rate are characterized by a Poisson-type distribution (governed by the average excitation), and are characterized by a Boltzmann thermal distribution for the fast T1 relaxation rate (governed by both the average excitation and the vibrational degrees of freedom of the adspecies). A comparison of laser-stimulated desorption and the effective thermal desorption is quantitatively shown by introducing an effective temperature. Finally, a chemisorbed–physisorbed state–state transition channel with lower desorption energy (and hence enhancing the desorption probability) is proposed.
Diatomic molecules as perturbed Morse oscillators. V. Centrifugal distortion
Authors: Huffaker J. N.
Published: February 2003

The perturbed-Morse-oscillator model is extended to include computation of Dunham parameters Yjk for k⩽3 and for arbitrary j. A computer code incorporating this extension is used to compute energy eigenvalues Ev,J for the ground state of CO for v⩽28 and for J⩽160. Results are compared with eigenvalues obtained by other methods and analyzed to show the various problems of convergence encountered.
Thermally stimulated depolarization of dipolar defects in MgO single crystals
Authors: Krokoszinski H.-J. , Bärner K. , Freund F.
Published: February 2003

For the first time, the TSD technique has been applied for the detection of dipolar defects in MgO single crystals. Three electrical depolarization bands at 200, 214, and 240 K were found and are ascribed to localized defect dipoles. The band at 214 K changed in height as long as 30 h after the crystals had been exposed to UV radiation of wavelengths between 250 and 300 nm. This indicates the formation of dipolar defects by solid state reactions between photogenerated space charge carriers and localized defect centers. The relations of the TSD bands to the V− and VAl centers are discussed. Another broad band was observed at 400 K which seems to arise from the depolarization of electronic space charge layers.
Trinickel
Authors: Moskovits Martin , DiLella Daniel P.
Published: February 2003

The resonance Raman spectrum of a nickel-containing species isolated in solid argon is reported and assigned to Ni3. On the basis of isotopic fine structure analysis its geometry is determined to be C2v with an apex angle between 90° and 100°. This structure is postulated to come about as a result of two effects: the high flexibility of Ni3 along the bending coordinate about the linear form and the geometry of available trivacancies within the face centered cubic solid support. The molecular ground state is suggested to be Δu of unknown spin multiplicity. Consequently, it is argued that the molecular lability comes about as a result of the Renner–Teller effect.
Analysis and extension of some recently proposed classical models for electronic degrees of freedom
Authors: Meyer Hans-Dieter , Miller William H.
Published: February 2003

This paper analyzes various methods that have been developed recently for constructing a classical model for a finite set of quantum mechanical states (electronic states for our applications) and also shows how one of them, the spin matrix mapping method of Meyer and Miller, can be generalized in two aspects. First, it is shown how the methodology can be modified to obtain a classical model of any desired number of degrees of freedom, rather than only one degree of freedom as before. Second, it is shown how the method can be applied in the adiabatic representation, so as to be able to use directly the adiabatic potential energy surfaces and nonadiabatic coupling elements produced by a quantum chemistry calculation.
Raman spectra of the O–H and O–D stretching vibrations of ices II and IX to 25 °K at atmospheric pressure
Authors: Bertie John E. , Francis Bernard F.
Published: February 2003

The Raman scattering by the stretching modes of H2O and D2O in the ordered phases ice II and ice IX is reported for temperatures between 100 and 25 °K. The observed features are assigned to k=0 modes with the aid of polarization data from oriented crystals of D2O ice II and the normal coordinate calculations which have previously explained the infrared spectra [J. E. Bertie and F. E. Bates, J. Chem. Phys. 67, 1511 (1977)]. It is shown that normal coordinate, bond moment, and bond polarizability, calculations satisfactorily explain the frequencies and relative intensities in both the infrared and Raman bands due to the D2O stretching modes in these phases. These Raman bands are nearly insensitive to the order or disorder of the water molecules in phases of ice. The O–D stretching modes of ice IX that have large infrared intensities have weak Raman intensities, and no longitudinal optic modes can be clearly identified in the Raman spectra. The frequencies of the LO modes have been estimated for ices II and IX. The magnitudes of the LO–TO splittings suggest that the infrared transmission spectra may be broadened by reflection effects resulting from the dispersion of the refractive index, and the sharpness of the Raman bands suggests that the breadth of the infrared bands is not primarily due to Fermi resonance with high-order overtone and combination levels, as was previously proposed.
Experimental studies on the weak coupling of oscillatory chemical reaction systems
Authors: Nakajima K. , Sawada Y.
Published: February 2003

The interactions of two coupled oscillating systems of the Belousov–Zhabotinsky chemical reaction were investigated experimentally. A mapping diagram of coupling was experimentally obtained in the plane of ω2/ω1 and S1–2 where ω1 and ω2 were the natural frequencies of reactor 1 and 2, S1–2 was the area of the window that coupled the two reactors. The irregular synchronization region in this diagram may correspond to Tomita and Kai’s region χ where a chaotic response has been predicted.
The effect of quenching on the defect structure of calcium fluoride doped with erbium
Authors: Fontanella John , Treacy Donald J. , Andeen Carl
Published: February 2003

Audio frequency dielectric relaxation measurements over the temperature range 5.5–390 K and optical absorption studies at 8 K have been performed on various samples of rare-earth doped calcium fluoride. Erbium was the principal dopant and the studies were carried out for a series of concentrations before and after vacuum anneal at 1125 K followed by a quick quench. Strong effects were observed on all spectra. A similar anneal followed by a slow cool returned the samples to their initial state. It is shown that RI and RII correlate with optical signals associated with the A and B sites of Tallant and Wright. The annealing results are consistent with the identification of these sites as monomer (single rare-earth) sites of C4V and C3V symmetry, respectively. RIV, which has recently been shown to be associated with a dimer (two-rare earths), correlates with optical signals associated with the C site of Tallant and Wright. This suggests that the dimer exhibits approximately trigonal symmetry. In addition, RIII is found to be associated with optical signals which correlate with the D sites of Tallant and Wright. The annealing results suggest that RIII may be due to a higher order cluster. Finally, the annealing results suggest that RV is associated with a relatively simple site.
Percolation model of electron and hole mobility in liquid mixtures
Authors: Schiller Robert , Nyikos Lajos
Published: February 2003

The theory of charge percolation, originally developed for inhomogeneous mixtures of metals, is transformed to describe mobility of electrons and holes in homogeneous liquid mixtures. Systems in which the energies of the charge carriers are independent of composition are expected to obey the predictions of this formalism. Here concentration fluctuations act as microscopic inhomogeneities. As examples, hole mobility in trans-decaline–cyclohexane, and electron mobility in hexafluorobenzene–benzene and in n-hexane–ethanol mixtures are treated. The number of molecules interacting with a charge carrier appears as an important parameter. Mobility versus concentration curves are calculated in the entire concentration range and are in good agreement with observations. Finally a method is proposed for estimating electron mobility in hexafluorobenzene.
Model networks of end-linked polydimethylsiloxane chains. VII. Networks designed to demonstrate non-Gaussian effects related to limited chain extensibility
Authors: Andrady A. L. , Llorente M. A. , Mark J. E.
Published: February 2003

End-linking polymer chains by means of a multifunctional cross-linking agent provides an ideal way for obtaining elastomeric networks of any desired distribution of chain lengths. In the present investigation, this technique was employed to give polydimethylsiloxane networks consisting of various proportions of relatively long and very short network chains. The stress–strain isotherms of these networks generally showed an anomalous increase in the modulus at high elongation, and the increase persisted at high temperatures and high degrees of swelling. This non-Gaussian effect was quantitatively correlated with the limited extensibility of the network chains; specifcally, the increase in modulus was found to begin at approximately 60%–70% of the maximum extensibility of the network chains, and network rupture at 80%–90%. The elongation at which the increase becomes evident increases with decrease in the proportion of the very short chains, thus verifying the nonaffine nature of the deformation at high elongation. In addition, the elasticity constants characterizing the Gaussian regions of the isotherms, and the values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity, particularly with regard to the structure factor relating the modulus of an elastomer to the average length or molecular weight of the network chains.
Electron spin echo modulation studies of tetracyanoethylene anion solvation in methanol, acetone, and dimethylsulfoxide: Evidence for molecular dipole oriented solvation shells around large anions
Authors: Ichikawa Tsuneki , Kevan Larry
Published: February 2003

Electron spin echo modulation studies have been carried out for the tetracyanoethylene anion (TCNE−) in CD3OH, CH3OD, (CD3)2CO, and (CD3)2SO at 4.2 K to deduce details of the anion solvation geometry in these solvents. The deuterium modulation patterns have been analyzed using ratio analysis and complete simulation. The results indicate that TCNE− is solvated by four methanol molecules with the methanol molecular dipole oriented toward the anion; the distances from the anion to the hydroxyl and methyl deuterons are 5.9 and 3.8 Å, respectively. TCNE− is solvated by two molecules of acetone or dimethylsulfoxide; it is suggested that these solvating molecules are above and below the TCNE− plane with their molecular dipoles oriented toward the C=C bond in TCNE−. The anion–deuteron distances are 4.2 Å in acetone and 3.6 Å in dimethylsulfoxide; the larger distance correlates with the smaller of the molecular dipole moments which are 2.6 and 4.0 D, respectively.
Raman intensities of overtones. Analysis of 2ν4 band of cyanogen
Authors: Montero S.
Published: February 2003

Formulas for the absolute differential Raman scattering cross section of overtones of doubly and triply degenerate fundamentals as a function of molecular invariants are reported. The absolute cross section and the depolarization ratio of 2ν4(Σ+g+Δg) of cyanogen are interpreted in terms of the bond polarizability theory and of the ℒ-tensor formulation of Hoy, Mills and Strey. It is shown also, how the analysis of Raman intensities of overtones and/or combination bands supplies valuable information to solve for most ambiguities on signs and multiplicity of solutions of bond polarizability parameters arising from the analysis of the Raman intensities of vibrational fundamentals.
Studies of reactions of importance in the stratosphere. III. Rate constant and products of the reaction between ClO and HO2 radicals at 298 K
Authors: Leck Thomas J. , Cook Jac-E L. , Birks John W.
Published: February 2003

The rate constant for the radical–radical reaction ClO+HO2→HOCl+O2 was measured at 298 K by the discharge flow technique using mass spectrometry for detection of the HOCl product at m/e=52. The ClO radical was generated by reacting ozone with chlorine atoms produced in a microwave discharge, and the concentration of ClO determined by measuring the decrease in ion current due to Cl2+ at m/e=70 upon activation of the discharge. This method was found to be in agreement with a nitric oxide titration of ClO and with the stochiometric conversion of ClO to NO2 by reaction with a large excess of NO followed by absolute calibration for NO2 at m/e=46. Two reactions were used to generate the hydroperoxyl radical: (1) H+O2+M→HO2+M, and (2) Cl+H2O2→HCl +HO2. The rate constant k1 was found to be independent of pressure over the range 2–6 Torr, the result being k1= (4.5±0.9) ×10−12 cm3 molecule−1 s−1, where the error includes our estimate of the maximum possible systematic error. An upper limit of 2% for the branching ratio to the alternative products of this reaction, HCl+O3, was established by attempting to detect ozone as a reaction product. For these measurements the reactions Cl+ClOCl→Cl2+ClO and Cl+OClO →2ClO were used to generate the ClO radical in the absence of ozone. No other reaction products could be identified in the mass spectrum.
Molecular dynamics simulations at constant pressure and/or temperature
Authors: Andersen Hans C.
Published: February 2003

In the molecular dynamics simulation method for fluids, the equations of motion for a collection of particles in a fixed volume are solved numerically. The energy, volume, and number of particles are constant for a particular simulation, and it is assumed that time averages of properties of the simulated fluid are equal to microcanonical ensemble averages of the same properties. In some situations, it is desirable to perform simulations of a fluid for particular values of temperature and/or pressure or under conditions in which the energy and volume of the fluid can fluctuate. This paper proposes and discusses three methods for performing molecular dynamics simulations under conditions of constant temperature and/or pressure, rather than constant energy and volume. For these three methods, it is shown that time averages of properties of the simulated fluid are equal to averages over the isoenthalpic–isobaric, canonical, and isothermal–isobaric ensembles. Each method is a way of describing the dynamics of a certain number of particles in a volume element of a fluid while taking into account the influence of surrounding particles in changing the energy and/or density of the simulated volume element. The influence of the surroundings is taken into account without introducing unwanted surface effects. Examples of situations where these methods may be useful are discussed.
A Monte Carlo simulation of the diffusion of gases in porous solids
Authors: Evans J. W. , Abbasi M. H. , Sarin A.
Published: February 2003

A Monte Carlo simulation of the Knudsen diffusion of gases in a porous solid is described and results of such simulations presented. The solid was represented in the computer as an assemblage of spheres. Equations are presented which enable the prediction of a Knudsen diffusivity from the mean pore size (or, preferably, the mean size and standard deviation) without resort to a tortuosity factor.
UV laser photochemistry of CCl4 and CCl3F
Authors: Tiee J. J. , Wampler F. B. , Rice W. W.
Published: February 2003

The ArF (193.3 nm) irradiation of CCl4 and CCl3F has been investigated. The laser photolysis of CCl4 yields emission from CCl (A 2Δ–X 2Π), CCl2(1B1–X 1A1), Cl2[A 3Π (0u+)–X 1Σg+], and other systems. A dye laser was used to obtain the fluorescence excitation spectrum of the CCl radical near the 0–0 band center (A 2Δ–X 2Π). The CCl (A 2Δ) radicals are formed by at least a three photon absorption process and laser fluence dependences are reported for various photofragments. The ArF laser photolysis of CCl3F yields results very similar to those of CCl4. Possible mechanistic channels are presented for both CCl4 and CCl3F photolysis which are consistent with the observed photofragments, laser fluence dependences, and known thermochemical data.
Kinetic study of e-beam excited Ar–CO2 mixture
Authors: Gauyacq J. P. , Collier F. , Leblond J. B. , Cottin P.
Published: February 2003

Dissociation of the CO2 molecule by collision with metastable argon (3P2) was shown a few years ago to lead to the excitation of the first vibrational levels of the metastable (a3π) state of CO and the aim of the present work is to investigate the possibility of using this transfer reaction to obtain population inversions of the a 3Π→X 1Σ transition of the CO molecule. Because of the low CO* fluorescence intensity level, we only performed an indirect kinetic study of e-beam excited Ar–CO2 mixture. Using N2(c→B) (λ=337 nm) fluorescence emission as an Ar* population tracer in Ar–CO2–N2 mixture, we measured the kinetic constants relevant to Ar–CO2 mixture. It is shown that for certain Ar–CO2 mixtures, 50% of the initial excitation created by the electron gun is transferred to Ar* atoms and then disappears in Ar*+CO2 collisions. A comparative study of the instantaneous fluorescence intensity of CO* (a3π) in Ar–CO2 mixture with that of N2(C) in Ar*–N2 mixture showed that the branching ratio of the Ar*+CO2→CO* (a3π) reaction was, at most, 10%. Consequently, the largest population efficiency (5%) does not permit laser oscillation in our experiment. Furthermore, the feasibility of such a laser depends on eventual direct population of ground state level by Ar*+CO2 reaction.
Statistical theories for molecular collisions: A maximum entropy derivation
Authors: Pollak E. , Levine R. D.
Published: February 2003

Statistical theories are particularly appropriate when one can define a strong interaction regime. We consider the distribution of classical trajectories which enter or exit from this regime. That distribution of trajectories which is of maximal entropy subject only to total conservation of flux is shown to lead to the familiar ’’phase–space’’ expression for the reaction probability. By including more refined conservation conditions as constraints one obtains improved statistical theories. As an example the ’’unified’’ statistical theory of Miller and the Hirschfelder–Wigner expression for the reaction probability are derived by imposing one more conservation constraint. Transition state theory is derived as a special case corresponding to a particular, extreme, numerical value of the constraint. Phase–space theory is obtained when the value of the constraint is at the other extreme (in which case the constraint is not informative). Essentially, exact results for the reaction probability in the collinear H+H2 reactive collision are obtained using two conservation conditions (beside the conservation of total flux). In general, it is shown that the procedure is variational, i.e., that including additional constraints can only improve the results.
Rotational spectra and molecular structures of ArHBr and KrHBr
Authors: Keenan M. R. , Campbell E. J. , Balle T. J. , Buxton L. W. , Minton T. K. , Soper P. D. , Flygare W. H.
Published: February 2003

Rotational spectra have been observed for four isotopes of ArHBr and ArDBr and eight isotopes of KrHBr and KrDBr using a Fabry–Perot Fourier transform spectrometer with a pulsed supersonic nozzle as the molecular source. The rotational constants in the ground vibrational state ?0 with their centrifugal distortions DJ, as well as Br nuclear quadrupole coupling constants χa, are given. In addition, an important centrifugal distortion of the Br quadrupole coupling constant, Dχ, an indicator of the coupling between the radial and angular potentials, is given for ArHBr and KrHBr. The Br spin–rotation interaction c in ArHBr is also obtained. The results are: The molecular structures are consistent with a linear equilibrium geometry with the H(D) atom located between Br and the rare gas atoms. The complexes undergo large amplitude vibrations and estimates of the bending and stretching force constants and frequencies are given. By combining the bending, stretching, and their coupling, we have obtained the harmonic bending and stretching force constants (ks1 and kb1) and 6/12 Lennard-Jones potential parameters (ε and Re) for ArHCl, KrHCl, ArHBr, KrHBr.
Dissociative charge transfer of He+ ions with H2 and D2 molecules from 78 to 330 K
Authors: Johnsen Rainer , Chen Aikwo , Biondi Manfred A.
Published: February 2003

The dissociative charge transfer of He+ ions with H2 and D2 molecules has been studied using a temperature-variable drift-tube mass-spectrometer apparatus over the temperature range 78 to 330 K. The binary rate coefficients are small at 300 K, ∼10−13–10−14 cm3/sec, and only slightly larger at 78 K. Termolecular contributions to the binary rate coefficients are found to be small at 330 K but increase substantially with decreasing temperature. Two-body charge transfer with D2 is found to be slower than with H2 by a factor of 10, in good agreement with recent theoretical predictions, although the measured values of the rate coefficients are larger by a factor of ∼4 than the predicted values.
Diatomic transition operators: Results of L2 basis expansions
Authors: Kuruoglu Zeki C. , Micha David A.
Published: February 2003

We study several expansion methods for two-atom transition operators T in terms of square-integrable functions. Methods were chosen such that (i) they apply to general potentials of interest in atomic systems; (ii) their numerical accuracy can be systematically improved; and (iii) they provide T operators useful in the solution of three- and many-atom problems. We describe the Hilbert–Schmidt and unitary pole expansion in terms of variationally calculated Weinberg states, and an implementation of the Schwinger variational method for diatomic potentials. Results are presented for the lowest 1Σ and 3Σ potentials of H2, and include partially and completely off-energy-shell T matrix elements for several angular momenta, energies and wave numbers, calculated within the variational, Hilbert–Schmidt, and unitary pole expansions.
Monte Carlo studies of porphyrin in the Shpolskii matrix n-octane
Authors: Koehler T. R.
Published: February 2003

A Monte Carlo study is made of the properties of the defect created by the substitution of porphyrin molecules in the Shpolskii matrix n-octane. The atom–atom potential method is used to model the intermolecular interactions. The theoretical orientational and several other properties of the two energetically most favorable defect sites agree quite well with experimental information. In addition, a possible explanation of the Shpolskii effect in this system is found: The octane lattice is packed in a very interlocking structure and seems to be capable of supporting two or three molecule vacancy cages which do not collapse. These are large enough to hold a porphyrin molecule in such a way that its π-orbital system will be less perturbed by the surroundings than is possible in typical solid environments.
EPR powder spectrum of SrCl2⋅6H2O:Gd3+
Authors: Rappaz M.
Published: February 2003

The EPR spectrum of powdered Srcl⋅6H2o⋅Gd3+ is reported. The powder samples used in the experiments were obtained by total hydration of an hydrous Srcl:Gd3+ powder sample at room temperature in a humid atmosphere.(AIP)
B 3Σu− predissociation and relaxation processes in matrix isolated S2
Authors: Bondybey V. E. , English J. H.
Published: February 2003

Spectroscopy of S2 is rare gas solids in re-examined. Excitation of the B 3Σu− state populates by predissociation–recombination sequence several excited electronic states, which can then be characterized by their fluorescence spectra. The observed spectra are quite analogous to those observed in ’’thermoluminescence’’ experiments. Analysis of the emission and laser excitation spectra yields the spectroscopic constants of several electronic states of S2 in solid Ne. In particular, the data provide direct evidence that the lowest a 1Δg state is located in this matrix 4395 cm−1 above the ground state. Vibrational relaxation within the B 3Σu− state was examined. It exhibits a very steep rate dependence on temperature and enhanced relaxation efficiency with decreasing mass of the host atoms. It is proposed that relaxation in S2 is due to a direct multiphonon process, and that unlike relaxation in light hydride molecules, the molecular rotation is unimportant.
The metric geometry of near equilibrium irreversible thermodynamics
Authors: Nathanson Gilbert , Sinanoğlu Oktay
Published: February 2003

We present a geometrical representation of irreversible thermodynamics in the near equilibrium domain, developed in the spirit of Weinhold’s method for equilibrium thermodynamics. The Onsager reciprocity relation defines a scalar product for a linear metric space when the first differentials of thermodynamic forces are associated with elements of the abstract space. The values of the scalar product correspond to the Onsager phenomenological coefficients, which relate forces and flows in the linear region.
High-temperature thermal expansion characteristics of solid argon and krypton
Authors: Schwalbe L. A. , Wilkins R. W.
Published: February 2003

An empirical approach is used to derive a three-parameter closed-form lattice parameter equation which is then applied as a fitting function for solid argon at high temperatures along the vapor pressure curve. Previously unpublished high-temperature argon lattice parameter data are also presented. The function is shown to fit both these data well as those of earlier work within the expected uncertainties of the measurements; the variation parameters determined are compared with results obtained from independent experiments. A similar comparison, applied to corresponding solid krypton data, points up some discrepancies which may ultimately provide indirect evidence for the existence of the reported anomalously large vacancy concentrations in this system.
Investigation of the assumptions of the multiple-scattering method for electron–molecule scattering cross sections
Authors: Rumble John R. , Truhlar Donald G.
Published: February 2003

We present a comparative test of three assumptions critical to the applicability of the multiple scattering (MS) method as it has been applied to the scattering of electrons by molecules. The MS method divides space into three regions: (I) atomic spheres, (II) an interatomic region between the atomic spheres and an outer boundary, and (III) an extramolecular region outside that boundary. The assumptions tested are (i) neglect of charge polarization except in region III, (ii) constancy of the potential in region II, and (iii) spherical symmetry of the potential in region III. We consider electron–N2 scattering at impact energies 5–50 eV. We show that the use of assumptions (i) – (iii) results in cross sections that differ by about 30% from those obtained from reasonably well converged close coupling calculations without such assumptions. The differences are up to a factor of two or three for the partial integral cross sections. We have not tested whether the multiple-scattering results are improved by using a more consistent treatment of region II.
Vibrational relaxation of HF(v=1 and 3) in H2, N2, and D2 at 200 and 295 K
Authors: Bott J. F. , Heidner R. F.
Published: February 2003

The vibrational relaxation rates of HF(v=1) and HF(v=3) have been measured in H2,N2, and D2 at 200 and 295 K. The v dependence of the relaxation rates is essentially the same for N2,D2, and several other diatomic molecules that deactivate HF via exothermic processes. The rates for HF(v=3) deactivation are larger than those for HF(v=1) by a factor of ∼18 at both 200 and 295 K.
Molecular librations of solid CO2 under high pressure based on Kihara core potentials
Authors: Kobashi Koji , Kihara Taro
Published: February 2003

Molecular librations at k=0 of solid α-CO2 under high pressure at T=0 °K have been studied in terms of Kihara models consisting of both 6-n type potential (n=12 or 9) and the electrostatic quadrupole–quadrupole interaction. A rod-shaped molecular core was utilized to describe the molecular shape. The potential parameters were determined from the observed lattice constant and heat of sublimation at P=0 bar. The calculated libration frequencies at P=0 bar for either n=12 or n=9 models were in good agreement with observed Raman and neutron data. The crystal energies and the libration frequencies at high pressures were computed to obtain, respectively, the P–V relation and the Grüneisen parameters associated with the k=0 modes. Good agreement of the calculated Grüneisen parameters with observed data was attained for the n=9 models. The calculated P–V relation should be compared with experimental data which is not currently available.
The enthalpy and heat capacity of liquid water and the ice polymorphs from a random network model
Authors: Sceats Mark G. , Rice Stuart A.
Published: February 2003

The enthalpy and heat capacity of liquid water are calculated from the distributions of static hydrogen bond angles and lengths which characterize the random network model (RNM) previously introduced. As a test of consistency the energies of ices II, III, V, and VI relative to ice I are also computed; the average deviation between experiment and theory is only 0.7 kJ/mol. The enthalpy of liquid water, calculated in the same fashion, agrees with the observed values over the temperature range 0–100 °C. Three calculations of the heat capacity, from the entropy, the enthalpy, and fluctuations in the enthalpy, yield internally consistent values which are in good agreement with experimental data. Our results indicate a large contribution to the heat capacity from the strong coupling between vibrational and configurational contributions to Cp, which coupling is a reflection of the quantized molecular dynamics.
MCSCF potential energy surface for the high barrier adiabatic 1 4Σ− pathway of the O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction
Authors: Hopper Darrel G.
Published: February 2003

The collinear 4Σ− pathway for the state-specific O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction has been surveyed with ab initio calculations. A ninety-nine configuration, fifteen orbital multiconfiguration self-consistent field (MCSCF) wave function, involving the use of a double-zeta plus polarization one-electron basis, was developed for the long range 4Σ− state. This long range 4Σ− state has the character of O++N2 for long RNO, or of N+NO+ for long RNN, and is for most geometries, the lowest, or 1 4Σ−, state. The ab initio exothermicity computed with the present wave function is 0.93 eV, compared to an accurate experimental value of 1.10 eV. The saddle-point in the energy surface is 8.0 eV above O++N2, with critical values of R*NN=1.48±0.02 Å and R*NO =1.38±0.02 Å. These values are 0.38 and 0.32 Å greater than the equilibrium bond lengths of N2(X 1Σg+) and NO+(X 1Σ+). The present wave function reproduces the experimental bond lengths of these two diatomics to within 0.01 Å when the third atom is removed to 500 a.u. (264.6 Å). Preceding the barrier on the O++N2 side is an appreciable polarization well. With the present wave function the attributes of this well are D° (N2–O+) =0.38 eV, RNNE=1.108 Å, RNOe =2.350 Å, ϑeNNO=180°, keNN=24.679 md/Å, keNO=0.295 md/Å, kϑ=0.0366 md/Å. There is also a smaller polarization well on the N+NO+ side. Although the ion–atom interaction is more stable at Cs the latter well is bound by 0.02 eV at 180° with ReNN=3.34 Å, and ReNO=1.07 Å.
Repulsive potentials derived from the integral elastic scattering of Ar+ and Ne+ ions in collision with He atoms
Authors: Inouye H. , Noda K.
Published: February 2003

Integral elastic scattering cross sections have been measured for Ar+ and Ne+ with kinetic energies in the range 0.4–3.0 keV scattered by stationary He atoms through effective laboratory angles greater than 5×10−3 rad. The effective repulsive potentials in the range 0.7–7 eV have been derived from the cross sections. The potential values are compared with the statistically weighted mean of the potentials of the quasimolecular states X 2Σ and A 2Π obtained in configuration interaction calculation by Olson and Liu for Ar+–He and by Blint for Ne+–He. Agreement between the experimental and the theoretical values is satisfactory.
H2-laser photochemical study of the temperature dependent quenching of O2(b 1Σg+)
Authors: Kohse-Höinghaus K. , Stuhl F.
Published: February 2003

An H2–VUV-laser was used to generate metastable O2(b1Σg+) molecules in the photolysis of O2. The metastable molecules were monitored using the emission of their forbidden transition O2(b 1Σg+→X 3Σg−). From the decays of this emission rate constants for the quenching of O2(b 1Σg+) by H2, D2, N2, HCl, DCl, CH4, CD4, NH3, and ND3 were determined as a function of temperature. Significant isotope effects were observed for the activation energies.
A study of the electronic structure of the actinide tetrahalides UF4, ThF4, UCl4 and ThCl4 using vacuum ultraviolet photoelectron spectroscopy and SCF-Xα scattered wave calculations
Authors: Dyke J. M. , Fayad N. K. , Morris A. , Trickle I. R. , Allen G. C.
Published: February 2003

The gas phase helium (i) photoelectron spectra of the actinide tetrahalides UF4, ThF4, UCl4, and ThCl4 have been recorded. The spectra have been interpreted with the aid of nonrelativistic multiple-scattering SCF-Xα calculations and are consistent with an effective tetrahedral geometry for these molecules in the gas phase. Both the calculations and the spectra indicate that the 5f electrons in UF4 and UCl4 are essentially localized on the central metal.
A simple model for examining mixing of pumped stationary states
Authors: Noyes Richard M.
Published: February 2003

The oxidation of cerous ion by bromate has been approximated by four irreversible bimolecular processes. When this mechanism is modeled numerically in a continuously stirred tank reactor (CSTR), wide ranges of conditions generate two different stationary states that are stable indefinitely for identical values of the pumping parameters. By simultaneous variation of the residence time and reactant input, it is possible to move from one to the other of these states without any discontinuity in stationary state composition. If equal volumes of the two states are mixed while pumping is continued, one or the other will become dominant. The value of the residence time generating states of equal ’’dynamic stability’’ is grossly different whether the mixing is performed instantaneously by removing a barrier between those states or gradually by increasing sizes of holes in that barrier. Because relative ’’dynamic stability’’ is extremely dependent upon path even for a process as a simple as ’’ mixing’’, it appears doubtful that useful state functions can be developed to predict the direction of evolution of open systems pumped far from equilibrium.