bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct determination of molecular weight distributions of polystyrene in cyclohexane with photon correlation spectroscopy
Authors: Provencher S. W. , Hendrix J. , De Maeyer L. , Paulussen N.
Published: February 2003

A recently proposed method for analyzing polydispersity with photon correlation spectroscopy is verified experimentally by directly estimating the molecular weight distributions (MWD’s) of two well characterized National Bureau of Standards (NBS) polystyrene samples in cyclohexane at 35.0 °C. The MWD’s were reproducible and in good agreement with the NBS molecular weight averages. The method finds the smoothest nonnegative MWD that is consistent with the data without requiring initial estimates or subjective judgements. It is shown to work at dust levels where the method of cumulants fails. Minimum necessary precautions against dust and other sources of error are discussed.
Theoretical thermodynamic functions for a Lennard-Jones system in the liquid and vapor region
Authors: Carley David D.
Published: February 2003

Integral equation and perturbation theory are combined to calculate the theoretical equilibrium thermodynamic properties of a simple classical fluid in the liquid and vapor region. We assume a simple system of classical particles interacting pairwise according to the Lennard-Jones 6–12 potential function. The potential is separated into the sum of a reference potential and a perturbing potential and a parametric integral equation is used to compute the radial distribution function corresponding to the reference potential. From the radial distribution function the pressure of the reference system is computed as is the perturbation correction for the Helmholtz free energy (using first order perturbation theory). These calculations are performed for several densities along seven isotherms and from these pressures and Helmholtz energies other thermodynamic functions (internal energy, entropy, enthalpy, and Gibbs free energy) are obtained. Applying the conditions of phase equilibrium the fluid phase diagram for the system is obtained. The range of temperatures studied extends approximately from the triple point to the critical point.
An accurate analytic method for the calculation of the Franck–Condon factors: Its application to the study of rotational dependence
Authors: Dwivedi Parmesh H. , Lin C. S.
Published: February 2003

We incorporated the perturbed Morse oscillator function and Manneback’s formulas into the matrix method to calculate the Franck–Condon factors. The method is exact in the context of the variational principle and is presented as a viable alternative to the numerical integration method where computational facilities are limited. Its accuracy is demonstrated by computing the eigenvalues of CO and Franck–Condon factors of the (B–X) band system of Na2. It is then applied to the study of rotational dependence of the Franck–Condon factors in the (A–X) band system of MgH.
Lattice model studies of the effect of chain flexibility on the nematic-isotropic transition. IV. Semiflexible chain solutes in solvent molecules composed of rigid cores and semiflexible tails
Authors: Dowell F.
Published: February 2003

A steric, mean field, simple cubic lattice model, with and without attractive forces, for mixtures of semiflexible chain solutes in solvent molecules composed of rigid cores and semiflexible tails is presented. The calculated values and absolute trends of the following properties at the nematic–isotropic transitions of the mixtures are compared with relevant experimental data: coexistence curve slopes, solvent and solute order parameters, and solute excess properties as a function of the length and flexibility of the solute and of the solvent. With results intermediate between those for the completely flexible chain solute model and the rigid rod solute model, this semiflexible chain solute model is adequate to describe the general qualitative features and trends observed experimentally in mixtures of n-alkanes in liquid crystal solvents at the nematic–isotropic transition. This model predicts four general curvature types for the coexistence curves as a function of solute length and flexibility. It also predicts maxima in coexistence curve slopes as a function of solute length for sufficiently long and sufficiently stiff chains. While steric repulsions alone are sufficient in this model to generate the general features and trends, attractive forces permit some adjustments in trends and produce significant improvements in energetics.
Self-consistent field molecular orbital wavefunctions for the ground and excited states of the PMDA molecule
Authors: Keijzers C. P. , Bagus P. S. , Worth J. P.
Published: February 2003

We report the results of ab initio molecular orbital self-consistent field calculations on the ground state and low lying excited states of the PMDA molecule. Emphasis is placed on the consequences of localization of the molecular orbitals and on the relative positions of the n→π* and π→π* excited states. It is shown that reduction of the symmetry constraint from D2h to C2v or Cs has a large effect on the n→π* excitation but only a small one on π→π*. A study of the effect of using basis sets of different size is also presented.
Electron-impact excitation of low-lying electronic states in CS2, OCS, and SO2
Authors: Flicker Wayne M. , Mosher Oren A. , Kuppermann Aron
Published: February 2003

Variable angle, electron-impact energy-loss spectra of CS2, OCS, and SO2 have been obtained at incident electron energies of 25, 40, and 70 eV for CS2, and 30 and 55 eV for OCS and SO2. Singlet→triplet excitations are observed with peaks at transition energies of 3.36 eV in CS2, 4.94 eV in OCS, and 3.40 eV in SO2. A feature which peaks at 3.65 eV in CS2 is observed to have singlet→triplet character. The CS2 and OCS spectra do not confirm the existence of several spin-forbidden transitions reported in solid phase ultraviolet absorption studies of these molecules. In SO2, no evidence is found of transitions to the 1 3A2 and 1 3B2 states, believed to lie near the well-known ? 3B1 state.
Electron energy loss spectroscopy and the optical properties of polymethylmethacrylate from 1 to 300 eV
Authors: Ritsko J. J. , Brillson L. J. , Bigelow R. W. , Fabish T. J.
Published: February 2003

Energy loss spectra of 80 keV electrons transmitted through thin films polymethylmethacrylate (PMMA) were measured with a resolution of 0.1 eV for energy losses from 1 to 300 eV. From the loss spectra, the dielectric response function of PMMA was obtained from 1 to 100 eV and compared with recent synchrotron radiation results. The spectrum of valence excitations from 5 to 13 eV is shown to be characteristic of the pendant group and is compared to experimental gas phase spectra and molecular orbital (CNDO/S) calculations of model molecules. The spectrum of core electron excitations above 285 eV provides a measure of the distribution of empty molecular orbitals and, when the carbon 1s binding energies are taken into account, a qualitatively accurate description of the observed spectrum is made from the CNDO ground state calculation. The energy loss spectra of 20, 40, and 100 eV electrons reflected from the surface of PMMA exhibit a triplet excitation at 4.2 eV and indicate the extreme sensitivity of this material to radiation damage. Finally, the spectrum of electron damage is shown to be similar to that reported for uv photolysis but with a strong previously unreported peak at 6.3 eV assigned here to excitation of radiation induced isolated unsaturated bonds.
Low-energy electron diffraction study of the surface of thin crystals and monolayers of normal paraffins and cyclohexane on the Ag(111) crystal surface
Authors: Firment L. E. , Somorjai G. A.
Published: February 2003

The surfaces of the normal paraffins (C4–C8) and cyclohexane in the form of thin single crystal films (102 Å) condensed upon an Ag(111) single crystal surface have been studied using low-energy electron diffraction (LEED). The physical adsorption of monolayers of these molecules was also studied on the Ag(111) surface. The data were compared with similar data obtained from experiments on the Pt(111) surface (Ref. 1) to yield the following results : n-Octane condenses on either substrate with its unreconstructed (101̄) plane parallel to the substrate. Cyclohexane similarly condenses exposing its (001). n-Heptane multilayers on either substrate give the same LEED pattern, but the surface unit mesh is different from any of the expected planes of the n-heptane crystal structure. n-Hexane and n-pentane multilayers form pseudomorphic crystal structures, that are dependent upon the structure of the substrate. Physically adsorbed monolayers of n-octane, n-heptane, and n-hexane adsorb in ordered structures with their molecular axes parallel to each other and the Ag<11̄0≳. Cyclohexane adsorbs in a (9×9) structure in a hexagonal array with its molecular plane parallel to the Ag(111) surface. The generally lower temperatures necessary to order monolayers on Ag(111) indicate that unsaturated hydrocarbons are less strongly adsorbed on this surface than on the Pt(111).
Energy dependence of the differential photoelectron cross sections of molecular nitrogen
Authors: Mintz Donald M. , Kuppermann Aron
Published: February 2003

The angular distribution of photoelectron intensity for molecular nitrogen was studied using He i and Ne i resonance line discharge light sources. Studies of photoelectron angular distributions covering a range of photon energies, and thus a range of photoelectron energies, are possible using the weaker high order lines in each discharge as well as the principal lines. Peaks in three photoelectron bands of N2 were studied at the photon energies 16.85, 19.78, 21.22, 23.09, and 23.74 eV, where possible. We find that the v′=0 peak of the X 2Σ+g band has abnormally high intensity and, at the higher photon energies, an abnormally low angular distribution asymmetry parameter, β. Several mechanisms for this anomaly are discussed, including autoionization, the variation of electric dipole transition moments with internuclear distance, and possible shape resonance phenomena. None of these explanations is completely in agreement with all theoretical and experimental evidence.
The inelastic scattering of alkali halides by rare gases
Authors: McGinnis R. P. , Greene E. F.
Published: February 2003

Measurements from three laboratories of the inelastic scattering of alkali halides by rare gases are consistent with the formation of a short-lived complex. The results from transfer of large or small amounts of translational energy to or from the internal modes can be fitted by calculations based on a simple statistical model.
On the amount of information in rotational relaxation experiments with application to microwave transient T1 and T2 rates
Authors: Green Sheldon
Published: February 2003

A variety of gas phase rotational relaxation phenomena are related at the microscopic level by intermolecular forces and binary collision dynamics. By making the infinite order sudden (IOS) approximation to collision dynamics, the different phenomena can be expressed in terms of a few fundamental ’’dynamical’’ parameters and these can be chosen conveniently as the state-to-state rates out of the lowest level. Microwave transient T1 and T2 rates for a linear rigid rotor perturbed by an atom are considered explicitly. For systems where the IOS approximation is valid, i.e., where many rotational levels are energetically accessible, it is found that T1 and T2 measurements are expected to give rather similar rates, regardless of the spectral transition and in the presence or absence of an external (Stark) field; further, these measurements basically reflect the total rate of collisional excitation out of the lowest level. In principle, the small changes in T2 between different spectral lines or between Stark split sublevels provide information about specific, even Δj state-to-state rates. Similar T1 data provide, in addition, information about odd Δj transitions. As an example, IOS calculations are presented for OCS perturbed by Ar and He; these give relaxation cross sections in excellent agreement with the earlier semiclassical calculations of Liu and Marcus. Finally, methods for inverting experimental data to obtain state-to-state rates are considered.
Importance sampling and histogrammic representations of reactivity functions and product distributions in Monte Carlo quasiclassical trajectory calculations
Authors: Faist M. B. , Muckerman J. T. , Schubert F. E.
Published: February 2003

The application of importance sampling as a variance reduction technique in Monte Carlo quasiclassical trajectory calculations is discussed. Two measures are proposed which quantify the quality of the importance sampling used, and indicate whether further improvements may be obtained by some other choice of importance sampling function. A general procedure for constructing standardized histogrammic representations of differential functions which integrate to the appropriate integral value obtained from a trajectory calculation is presented. Two criteria for ’’optimum’’ binning of these histogrammic representations of differential functions are suggested. These are (1) that each bin makes an equal contribution to the integral value, and (2) each bin has the same relative error. Numerical examples illustrating these sampling and binning concepts are provided.
Signal modeling for ion cyclotron resonance
Authors: Comisarow Melvin B.
Published: February 2003

A new signal model, called the rotating electric monopole signal model, is developed for ion cyclotron resonance (ICR) experiments. Unlike prior signal models, the model is applicable to all forms of ICR spectroscopy which use radio-frequency detection methods. The signal model consists of a rotating electric monopole which is shown to be equivalent for signal generation purposes to a rotating electric dipole and for a macroscopic number of ions to a rotating electric polarization. The rotating polarization induces an alternating charge in the plates of the ICR cell and an alternating current between and an alternating voltage across the plates of the ICR cell. A noise analysis is performed, and if amplifier noise is neglected, the ICR sensitivity according to the model is found to be inversely proportional to the ion mass. This is the same sensitivity dependence as is found with conventional ICR power absorption experiments. The ICR voltage signal strength is inversely proportional to ion mass if the monitoring circuit is predominantly resistive and is independent of ion mass if the circuit is predominantly capacitive. The model clearly illustrates the parallelism between signal generation in ICR spectroscopy and signal generation in other coherent radiation spectroscopies such as NMR and microwave spectroscopy.
Applications of field theoretical methods to the calculation of infrared band shapes of molecules in strongly interacting solvents. III
Authors: Paul R.
Published: February 2003

The Green’s function theory as applied to spectroscopic lines shapes is applied to a specific problem involving a diatomic species interacting with a solvent through a Gaussian potential. The solvent interaction excites multipoles within the diatomic, and in the present calculation we consider no more than the octupoles. The projection angular momentum is summed exactly using a diagrammatic technique. The numerical calculations show that the band shapes are very sensitive to changes in the strength of the intermolecular potentials and to the temperature. There appears to be a rather surprising insensitivity of the spectrum to changes in the potential range. An increase in temperature causes a remarkable decrease in the fine structure; this can be attributed to increased thermal motion in the solvent resulting in enhanced collision rates and damping.
Corresponding states for ionic fluids
Authors: Friedman Harold L. , Larsen Bjorn
Published: February 2003

Corresponding states ideas are applied to correlate a number of equilibrium properties of ionic fluids. Consideration is given to the thermodynamic excess functions used to characterize dilute ionic fluids, to ion pairing and higher association, and to the characteristic coexistence curve and critical point found in many ionic fluids. The correlation applies to simple ionic fluids such as liquid or gaseous salts, to plasmas, and to ionic solutions as well as to the primitive model to the degree that the properties of all these systems are known. New HNC calculations are reported that were made to elucidate the structural changes in an ionic system as it is compressed along an isotherm corresponding to 2–2 aqueous electrolytes. Finally an evaluation is given of the usefulness of a number of approximation methods for calculating the equilibrium properties of the primitive model, and presumeably other Hamiltonian models, in various regions of the corresponding states diagram. Among these methods is the theory of Bjerrum (1926) which is surprisingly accurate, although some of its limitations are illustrated in the present work.
HCl rotational excitation by Ar impact: Quasiclassical close coupling approximation for atom–diatomic molecule scattering
Authors: Band Yehuda B.
Published: February 2003

We apply quasiclassical approximations for the relation motion degree of freedom to full quantum mechanical space fixed close coupling equations. This approximation is carried through in the context of atom–diatomic molecule rotational excitation collisions. Criteria for the validity of these approximations are described. We apply the method to treat Ar HCl collisions using previously suggested potential energy surfaces. The importance of diffraction, rotational inelasticity, and various features of the potential energy surface for state to state differential and integral cross sections are discussed.
Neutron scattering study of the methyl-group reorientations and the low temperature phase transition in (CH3)2 SnF2
Authors: Steenbergen Chr. , Rush J. J.
Published: February 2003

The reorientation of the methyl groups in solid (CH3)2SnF2 has been investigated by neutron quasielastic scattering. It is found that a phase transition occurs in the solid at about 70 K, which might be of second order. The shape of the measured quasielastic lines can be described by instantaneous threefold jumps of the methyl groups. From the derived residence times as a function of temperature below the phase transition, an activation energy Ea/kB=250 K and a residence time at infinite temperatures τ0=0.10 ps have been determined. Above the phase transition the activation energy is at least a factor 2 smaller.
Low-lying excited states of N3−
Authors: Rossi Angelo R. , Bartram Ralph H.
Published: February 2003

A b initio SCF molecular-orbital calculations on low-lying states of linear, symmetric N3− were performed with the Hunt–Hay–Goddard open-shell program. Both a [3s, 2p] basis contracted from a primitive (9s, 5p) Gaussian basis, and the same basis augmented by polarization functions [3s, 2p]+d, were employed at a fixed internuclear distance rN–N=2.225 a.u. in calculating vertical excitation energies for the following states: ...4σg2 3σu2 1πg4, 1Σ+g; ...1πg3 2π1u, 1,3Σ+u, 1,3Σ−u, and 1,3Δu; ⋅⋅⋅3σu1 ⋅⋅⋅2π1u, 1,3Πg; and ⋅⋅⋅4σ1g ⋅⋅⋅2πu1, 1,3Πu. The smaller basis [3s, 2p] was used in calculating energies of the 1Σ+g ground state and the 3Σ+u, 1,3Σ−u, and 1,3Δu states of linear, symmetric N−3 as a function of internuclear distance. The [3s, 2p] basis was also used in the ibmol program of Clementi and Mehl to calculate energies of the 1A1(1Σ+g) ground state and the following low-lying triplet states of bent N3−: 3B2(3Σu+), 3A2(3Δu), 3B2(3Δu), and 3A2(3Σ−u). The triplet-state energies of bent N3− were corrected by incorporating approximate configuration mixing. Comparison of calculated energies with absorption spectra of azide compounds shows only qualitative agreement, but supports the interpretation of McDonald, Rabalais, and McGlynn.
Fluorescence of potassium azide
Authors: Kemmey Peter J. , Bartram Ralph H. , Rossi Angelo R. , Levy Paul W.
Published: February 2003

The fluorescence spectrum of KN3 at 30 K consists of two components. One component, which consists of seven resolved vibrational lines separated by 720 cm−1 with the 0–0 transition at 22 620 cm−1 (2.8 eV), is identified with a 3A2→3B2 transition of bent N3−, derived from 3Σ−u→3Σ+u in the linear configuration. Analysis of the relative intensities of vibrational components shows that the change in bending angle is 19°. A second broad, featureless component with a maximum at 2.2 eV cannot be identified with certainty.
Atom–polyatomic collisions: A many-body approach
Authors: Micha David A.
Published: February 2003

We present an approach to atom–polyatomic collisions that takes into account the quantum nature of the polyatomic motion, but avoids expansions in target states by instead describing the target as a many-atom system. Following a discussion of interaction potentials, kinematics, and the general scattering formalism, we introduce two basic assumptions that lead to useful expressions for differential cross sections. The treatment is specialized to quasielastic scattering, of interest in the interpretation of experiments. This study shows that the atomic-pair correlation functions of the target play a central role in determining collisional energy and momentum transfer.
Variance reduction for lattice walks grown with Markov chain sampling
Authors: Fraser Simon J. , Winnik Mitchell A.
Published: February 2003

A Markov chain procedure for growing self- and neighbor-avoiding walks is described. By making the probabilities of all walks nearly equal, the variance in estimates of walk statistics is greatly reduced. A detailed analysis is given of the important example of next-neighbor-avoiding walks on the tetrahedral lattice: Such walks serve as a model for polymethylene chains. This case demonstrates that, in general, a similar variance reduction can be achieved at finite temperature for walks with conformationally dependent internal energy. The algorithm for constructing the sampling distributions for the steps of these finite temperature walks is summarized.
Noble gas halide ions: KrCl+, KrF+, ArCl+, ArI+
Authors: Kuen Ingrid , Howorka Franz
Published: February 2003

Noble gas halide ions are observed mass spectrometrically in low-pressure discharges of noble gas halide mixtures like Kr+Cl2 and Ar+Cl2. Under similar discharge conditions, KrCl+ forms more readily than ArCl+. The most probable process of producing KrCl+ is three-body association of Kr to Cl+ with an estimated rate coefficient of 4×10−28cm6s−1. ArCl+ cannot be formed in this way because the recombination energy of Cl+ is too low by ≳1 eV. KrF+ is observed weakly in Kr+CF4 or Kr+SF6 mixtures. ArI+ can only be observed when Ar+I2 mixtures are traversed by accelerated electrons and the ArI+ ions are not allowed to collide with neutrals or electrons. A negative peak of mass number 59 observed in Ar+CF4 could be ArF−, but is probably an impurity like C2OF−.
Theoretical studies of H2–H2 collisions. I. Elastic scattering of ground state para- and ortho-H2 in the rigid rotor approximation
Authors: Schaefer Joachim , Meyer Wilfried
Published: February 2003

Close coupling calculations of integral and differential elastic cross sections of hydrogen ground state molecule collisions have been performed, for c.m. energies below 0.5 eV. It is shown that the isotropic part of the potential, determined by the consistent ab initio potentials of five geometries, provides a very accurate (1%–2%) agreement with measured p-H2/p-H2 integral cross sections in the range of 900–2300 m/sec relative velocity. Detailed analysis of p-H2/p-H2 scattering results and the determination of a series of orbiting resonances provide a set of (virtual) quasi-bound-state energy levels. The fit formula of those levels gives two bound states of the (H2)2 system, for J=0 and 1, at −0.340×10−3 and −0.179×10−3 eV, respectively, which is close to the two binding energies found for the isotropic potential. However, the more attractive plane T configuration of the (H2)2 system gives larger binding energies when the zero point vibrations are neglected. Lifetimes and the periods of orbiting have been evaluated for the resonances. Since the resonaces are due to pure orbiting, we have compared the spacing of the p-H2/p-H2 partial integral cross section peaks with peaks of the opacities for p-H2/o-H2 and o-H2/o-H2 and found quantitative agreement almost everywhere, while the exceptions found for o-H2/o-H2 at very small energies can be explained by slightly different effective moments of inertia. The undulatory structure found for the ground state p-H2/p-H2 integral cross section is clearly due to symmetry restrictions which only allow even partial waves. For o-H2/o-H2, the cross sections of solely the symmetric or the asymmetric problem show undulatory structure as well because the even or odd partial wave contributions, respectively, dominate, while in the averaged curve these structures are completely damped. A few examples of differential cross sections and differential helicity transition cross sections at resonance energies are presented. The jz conservation in the body-fixed frame used in the approximation of McGuire has been found to be not quantitatively valid in the energy range of the strong resonances while for increasing energies jz conserving cross sections become dominant. Generally for o-H2/p-H2 collisions jz is better conserved than for o-H2/o-H2.
The origin of vibrational dephasing of polyatomic molecules in condensed phases
Authors: Shelby R. M. , Harris C. B. , Cornelius P. A.
Published: February 2003

The vibrational dephasing of polyatomic molecules in condensed phases by intermolecular vibrational energy exchange is treated theoretically. In the exchange model, dephasing arises from random modulation of the vibrational frequency caused by intramolecular anharmonic coupling to low frequency modes which are undergoing intermolecular energy exchange with the bath. The exchange rates are temperature dependent and as a consequence manifest themselves experimentally as a temperature dependent broadening and shift of the Raman spectral line shape. Using a reduced density matrix technique within the constraints of a Markoff approximation, the theory allows the time dependence introduced by the exchange process to be properly accounted for, and allows explicit expressions for the vibrational correlation function and corresponding spectral line shape functions to be derived and related to molecular parameters. Application of the theory can have important consequences experimentally, since an analysis of the temperature dependence of the line shape function allows the determination of the parameters which characterize the dephasing. These include the intermolecular energy exchange rates, the amplitude of the modulation which is in turn determined by the strength of intramolecular anharmonic coupling, and the effective activation energy for exchange which is related to the specific low frequency modes which participate in the dephasing process.
Thermodynamics of charged hard spheres
Authors: Stell G. , Larsen B.
Published: February 2003

A new series representation of the free energy is given for a symmetrically charged system of hard spheres of equal diameter (the restricted primitive model). The representation has two versions, one of which exhibits the detailed structure of the coefficients appearing in the other and suggests systematic ways of approximating those coefficients. The terms that appear to be the dominant ones are quantitatively evaluated and closed-form approximations are developed for them.
Normal coordinate treatment for propane: Force constant selection through a backward elimination regression–refinement procedure
Authors: Pearce Robert A. R. , Levin Ira W.
Published: February 2003

A 26 parameter internal valence force field for propane and four deuterium isotopic species was determined by applying a backward elimination regression–refinement technique for selecting and adjusting statistically significant force constants. In comparison to computations based upon fits to harmonic frequencies, this regression–adjustment procedure generated a more satisfactory set of force constants in a fit to observed vibrational frequencies. The regression analysis for the final minimum parameter force field retains a long range methyl-methyl interaction force constant involving H–C–C in-plane bending displacement coordinates for each methyl rotor. The general valence force field for propane, whose methyl groups are tilted such that the out-of-plane CH3 hydrogen atoms approach one another, is compared to force fields for other two rotor systems in which the methyl groups are tilted in an oppostie manner.
Diffusion and thermal diffusion phenomena of neutral species in the high temperature range (2000–4000 K). II. Theoretical calculations of the thermal diffusion factor αT of an argon/helium mixture in the high temperature range
Authors: Gouesbet G. , Valentin P.
Published: February 2003

In paper I of this series, we reported measurements of thermal diffusion factors αT in a rf argon and helium plasma jet. The assumption of a binary mixture was made to derive the values of αT although the fluid also contained ions and electrons. In this paper, we report in the temperature range under interest theoretical calculations in the binary mixture hypothesis, namely we shall calculate the Chapman–Cowling first and second approximations of the thermal diffusion factor αT, then the first and second approximations of αT in the Kihara scheme and finally we shall introduce a new scheme of approximations. We shall find that the experimental results fit very well with the theoretical calculations. The assumption of a binary mixture will be checked in Paper III.
Diffusion and thermal diffusion phenomena of neutral species in the high temperature range (2000–4000 K). III. Ranges of validity of the binary mixture hypothesis in an argon/helium plasma flow
Authors: Gouesbet G. , Valentin P.
Published: February 2003

In the paper I of this series, thermal diffusion factor measurements in a rf argon and helium plasma flow were reported. The values of the thermal diffusion factors αT have been obtained from the experimental results assuming that the plasma mixture was a binary one. In Paper II, theoretical calculations of the thermal diffusion factors in the case of an argon/helium binary mixture in the high temperature range were reported. Then we compared the theoretical calculations and the experimental results. In this paper, we have now to check the binary mixture assumption. For that purpose, electron temperatures and electron number densities were measured by means of spectrometric diagnosis. Then theoretical claculations using the molecular theory of gases (Hirschfelder–Curtiss–Bird) in order to examine the influence of charged particles on the diffusion properties of the neutral species were carried out.
The molecular structure of cycloheptane: An electron diffraction study
Authors: Dillen J. , Geise H. J.
Published: February 2003

The molecular structure of cycloheptane (C7H14) has been investigated in the gas phase by electron diffraction. Diffraction intensities can be explained by a twist–chair/chair mixture with an abundance of 76(6) % for the twist–chair conformation at 310°K. Using the nomenclature of J. Cremer and J. A. Pople [J. Am. Chem. Soc. 97, 1354 (1975)] the puckering coordinates were found to be q2=0.529 Å, q3=0.645 Å for the twist–chair and q2=0.487 Å, q3=0.715 Å for the chair form. Other geometrical parameters are: C–C=1.538 Å, C–H=1.112 Å, &C–C–C aver.=115.3° (twist–chair) and 113.5° (chair). Experimental geometries and vibrational amplitudes agree well with those obtained from force field calculations.
Intermodulated fluorescence spectroscopy of BO2 using a stabilized dye laser
Authors: Lowe R. S. , Gerhardt H. , Dillenschneider W. , Curl R. F. , Tittel F. K.
Published: February 2003

The hyper fine structure of 37 R-branch transitions involving several vibrionic bands in the ground (? 2Πg) and excited (? 2Πu) electronic states of 11BO2 has been observed by the technique of saturation spectroscopy with intermodulated fluorescence detection. Stabilization of the cw dye laser together with the elimination of spectral power broadening has permitted the achievement of linewidths of better than 10 MHz for a single hyperfine component. A set of plausible hyperfine constants for both electronic states determined from the observed splittings is also reported.
Arival-time detection in drift mobility experiments: Applications to anthracene and azulene
Authors: Young Ralph H. , Walker E. Irene P. , Marchetti Alfred P.
Published: February 2003

The time-of-flight technique for studying charge transport in a slab or film of photoconductive insulator is modified by using an interdigital electrode to monitor both the average current density in the sample and the rate at which charges arrive at its surface. The latter is a sensitive and graphic indicator of what charges are doing in the sample. The theory of the device and several tests and illustrative applications are presented. Among the applications are (a) observation, in anthracene, of electrons and holes separately when both contribute to the total photocurrent and (b) determination of a mobility for holes in azulene, a case in which the results of conventional time-of-flight experiments are inconclusive. Appendices contain a general discussion of induced charges and currents and present a variational principle for solving a mixed boundary-condition problem in electrostatics.
Theory of vibrational overtone line shapes of polyatomic molecules
Authors: Heller Donald F. , Mukamel S.
Published: February 2003

We present a general theory for the overtone absorption line shapes of isolated polyatomic molecules. The theory is based on a separation of the molecular vibrational degrees of freedom into: (1) a few relevant CH local modes which interact directly with the radiation field, and (2) the remaining ’’bath’’ modes which induce spectral broadening and determine the line shapes. Line shapes are evaluated using the standard Fano–Zwanzig scattering formalism. A simple yet general model is put forth to illustrate some of the features exhibited by polyatomic line shapes. Specific application is made to benzene. The model adequately accounts for the overtone line shapes recently observed by Bray and Berry. Within its context we are able to understand the observed vibrational state and isotopic dependence of the line shape and to predict line shapes for related but as yet unstudied systems. To our knowledge this is the first dynamical application of local mode theories. We discuss their advantages and the extent to which polyatomic line shapes can provide information about intramolecular energy redistribution processes.
Scattering of thermal He beams by crossed atomic and molecular beams. II. The He–Ar van der Waals potential
Authors: Keilb) Mark , Slankas John T. , Kuppermann Aron
Published: February 2003

Differential cross sections for He–Ar scattering at room temperature have been measured. The experimental consistency of these measurements with others performed in different laboratories is demonstrated. Despite this consistency, the present van der Waals well depth of 1.78 meV, accurate to 10%, is smaller by 20% to 50% than the experimental values obtained previously. These discrepancies are caused by differences between the assumed mathematical forms or between the assumed dispersion coefficients of the potentials used in the present paper and those of previous studies. Independent investigations have shown that the previous assumptions are inappropriate for providing accurate potentials from fits to experimental differential cross section data for He–Ar. We use two forms free of this inadequacy in the present analysis: a modified version of the Simons–Parr–Finlan–Dunham (SPFD) potential, and a double Morse–van der Waals (M2SV) type of parameterization. The resulting He–Ar potentials are shown to be equal to with experimental error, throughout the range of interatomic distances to which the scattering data are sensitive. The SPFD or M2SV potentials are combined with a repulsive potential previously determined exclusively from fits to gas phase bulk properties. The resulting potentials, valid over the extended range of interatomic distances r≳2.4 Å, are able to reproduce all these bulk properties quite well, without adversely affecting the quality of the fits to the DCS.
Hydrodynamic friction on a moving molecule
Authors: Peralta-Fabi Ramón , Zwanzig Robert
Published: February 2003

We present some calculations of the hydrodynamic drag on a molecule moving in a viscous fluid. The interaction of the molecule with the fluid is described by an intermolecular potential energy, rather than by imposing boundary conditions on a sharply defined surface. Our procedure requires solution of the Navier–Stokes equations for slow stationary flow of a viscous compressible fluid past a fixed force field. This is done here for two very simple models for the intermolecular potential: a ramp potential and a step potential. The results show that it is possible to calculate a drag force without imposing boundary conditions on a surface.
A theoretical analysis of the two-photon properties of linear polyenes and the visual chromophores
Authors: Birge Robert R. , Pierce Brian M.
Published: February 2003

A self-consistent field molecular orbital formalism for calculating molecular two-photon absorptivities is presented based on the combined use of the Pariser–Parr–Pople π-electron method including full single and double excitation configuration interaction and Monson and McClain’s two-photon orientational averaging procedures. The formalism is applied to a series of linear, nonlinear, and retinyl polyenes to study the effect of chain length, conformation, and polarity on the calculated two-photon absorptivities for various photon polarization and propagation relationships. The calculations indicate that the low-lying ’’1Ag*−’’ covalent state should be strongly two-photon allowed in virtually all polyenes, whether polar, nonpolar, linear, or nonlinear, provided a strongly one-photon allowed ’’1Bu*+’’ state is nearby. The two-photon absorptivity of the ’’1Ag*−’’ state for two linearly polarized photons is predicted to increase with increasing polyene chain length. Linearly polarized light produces the strongest two-photon absorption for polyenes with four or more conjugated double bonds. The two-photon absorptivity and polarization ratio of the ’’1Bu*−’’ state is predicted to be proportional to the degree of nonlinearity introduced into the polyene chain as a result of cis linkages. Accordingly, this state is responsible for the ’’cis band’’ in two-photon spectroscopy.
Semiclassical statistical mechanics of hard wall potentials via the transformed potential approach
Authors: Stratt Richard M.
Published: February 2003

A nonperturbative technique for treating the semiclassical statistical mechanics of hard wall systems based on the WKB quantization condition modified for a noninfinite interval is proposed. Application to some simple cases shows that the method is an improvement over a previous nonperturbative approach.
Comparison of photoconductivity and optical spectra for trapped electrons in aqueous and alcoholic glasses at 4.2 K
Authors: Rice Stephen A. , Dolivo Gilles , Kevan Larry
Published: February 2003

The photoconductivity and optical spectra of localized electrons generated at 4 K by radiolysis have been measured and compared with previous results at 85 K for a variety of polar aqueous and alcoholic matrices. At 85 K the localized electrons are largely solvated, characterized by an equilibrium distribution of solvent molecules around them. At 4 K the localized electrons have often not attained an equilibrium first solvation shell and are described as presolvated. The current results are discussed in terms of the binding energy and nature of the optical transitions associated with presolvated electrons vs solvated electrons in ethanol, ethylene glycol–water, 10 M NaOD/D2O, 7 M NaClO4/D2O and 5 M K2CO3/D2O glassy matrices.
Fluorescence excitation spectrum of the 1A
u (nπ) ←1A
g (0–0) band of oxalyl fluoride in a pulsed supersonic free jet
Authors: Liverman M. G. , Beck S. M. , Monts D. L. , Smalley R. E.
Published: February 2003

Oxalyl fluoride has been cooled to a rotational temperature of 0.17 K in a pulsed, supersonic jet. Individual rotational lines in the (0–0) band of the fluorescence excitation spectrum have been observed and assigned, and the rotational constants for both the 1Ag and 1Au states have been deduced. The pulsed nozzle used in these experiments can be operated with a far larger orifice diameter and at lower pressures than typically used in continuously operated nozzles, allowing greater cooling and higher jet densities while maintaining a low level of condensation.
Vibrationally inelastic integral cross sections for the scattering of He from H2
Authors: Lin C. S. , Secrest Don
Published: February 2003

Integral cross sections are presented for the scattering of He from H2 on the Gordon–Secrest potential. Results are reported at four energies, two above the first excited vibrational state and two above the second excited vibrational state. The energies are high enough that the vibrational transition from the ground to the first vibrational state is significant. Enough channels were included in the calculation to ensure that the integral cross sections are correct to two or three figures for the model potential used. A discussion of convergence is included. These accurate cross sections serve as test points for approximate calculations of vibrational energy transfer. The results are compared with the coupled states approximation, effective potential calculations, the semiclassical strong-coupling correspondence principle, and classical trajectory calculations which had been reported earlier for this potential model by other workers. Results of the comparisons are discussed.
Photoionization mass spectrometry of neon using synchrotron radiation: Anomalous variation of resonance widths in the noble gases
Authors: Radler K. , Berkowitz J.
Published: February 2003

Autoionization structure between the 2P3/2 and 2P1/2 ion thresholds of Ne has been measured. The spectrum appears qualitatively different from the corresponding spectra of the heavier noble gases. This is partly due to a progressive narrowing of the d-resonance width in the lighter noble gases. However, when analyzed in terms of the parameter (n*)3Γ, an anomalous behavior is noted, this parameter being larger for xenon than krypton, larger for argon than neon, but distinctly larger for argon than krypton. An analysis of the factors contributing to the resonance widths is presented. Variations in the quantum defects of the s and d series of the noble gases are presented for future analysis.
Analysis of the CsNpF6 optical spectrum
Authors: Hecht Harry G. , Varga L. P. , Lewis W. Burton , Boring A. Michael
Published: February 2003

A new analysis is given of the CsNpF6 optical spectrum, which removes several features of a previous analysis which were questionable. Possible vibronic structure, SCF–Xα–SW calculations, and analyses of related compounds have been used as a basis for the present investigation. It was found that configuration interaction effects are significant and cannot be neglected. By inclusion of linear CI terms, a fit to 33 assigned levels gave an rms deviation of 140.6 cm−1 in a least-squares refinement. The parameters determined are F2=202.2, F4=14.26, F6=1.517, ζ=2137, A4〈r4〉=1431, A6〈r6〉=112.6, α=195.2, β=−5798, and γ=9582 cm−1.
Molecular dynamics of nitric acid intercalated in graphite
Authors: Avogadro A. , Villa M.
Published: February 2003

This paper presents measurements of the NMR line shape and spin–lattice relaxation time of 2H in deuterated nitric acid intercalated in graphite. Evidence is obtained that a regular network of hydrogen bonds is established in the nitric acid layers. By comparing NMR data for both 1H and 2H in the free and intercalated nitric acid, the role of the decomposition process of HNO3 in the −25°C transition is put into evidence. Diffusional and rotational motions in the HNO3 layers, both below and above the transition, are characterized. The decomposition process markedly influences the molecular dynamics in the layers above the transition. Moreover, it seems related to the charge transfer process between carbon layers and intercalated acid, i.e., with the basal conductivity of graphite. We also discuss the problem of assigning a stoichiometric formula to the nitric acid intercalation compounds of graphite.
Kinetics and mechanisms for the decay of Paschen (1s) resonance state argon atoms
Authors: Firestone R. F. , Oka T. , Takao S.
Published: February 2003

Measurements of effects of pressure and temperature on the decay constants of excited Ar atoms (3p54s1) have been made in electron beam pulse irradiated Ar gas (100–700 Torr) by means of fast absorption spectrophotometry. Interpretation of the pressure and temperature dependence of the decay constants leads to the conclusion that the resonance state Ar (1s) species decay via formation of electronically excited molecular intermediates in this pressure range. The resonance state atoms are shown to decay in a manner consistent with a sequence of elementary steps involving collisional stabilization and radiative decay of excited molecules. Observed small positive temperature coefficients of all Ar (1s) species are attributed to effects of temperature on collision frequencies with virtually zero energies of activation at 153 to 300 K.
Intermultiplet energy transfer in BaCl(C 2Π)
Authors: Edelstein S. A. , Huestis D. L. , Wyss J.
Published: February 2003

The cross sections for collision induced intramultiplet energy transfer from BaCl (C 2Π3/2v′=0) and final vibrational state distribution in the C 2Π1/2 state were measured. The cross sections are σAr=14.0±2.8 Å2, σHe=3.1±0.6 Å2, σN2=4.7±1.9 Å2. The measurements also show that most of the transfer is through the Δv=0 channel for Ar and He as collision partners. The relevant triatomic electronic potential surfaces are discussed, but no single, simple mechanism has been found that explains the observed cross sections and product distributions.
Energetics of gas phase proton solvation by NH3
Authors: Ceyer S. T. , Tiedemann P. W. , Mahan B. H. , Lee Y. T.
Published: February 2003

The absolute proton affinity of NH3 (203.6±1.3 kcal/mole at 298 K) and the proton solvation energies by more than one NH3 have been determined by the molecular beam–photoionization method. In addition, the NH3+–NH3 interaction energy (0.79±0.05 eV) has been measured by photoionization of the neutral van der Waals dimer. These experiments have shown that photoionization of van der Waals clusters is a very powerful method for determining the energetics of gas phase proton solvation.
EPR study of central peak dynamics around the AsO44− center in KDA–KDP and ADA–ADP mixed crystals
Authors: Blinc R. , Cevc P. , Cevc G.
Published: February 2003

Using the lifetime effects in the domain splitting of the ESP spectra as a detector of the polarization fluctuations around an AsO4−4 center, we have studied the dependence of the temperature T—where the local tetragonal symmetry of the high temperature paraelectric phase is broken on the time scale of the ESP experiment—on Tc in mixed KH2AsO4–KH2PO4 (KDA–KDP) and NH4H2AsO4–NH4H2PO4 (ADA–ADP) crystals. T decreases with Tc in KDA–KDP and increases with Tc in ADA–ADP crystals though the domain splitting is nearly constant. This can be understood only if the central peak width depends on the host lattice and thus varies with Tc. The width of the central peak in the polarization fluctuation spectrum, which changes in KDA from 280 MHz at 45 °C to 6 MHz at −90 °C does not depend on the concentration of the paramagnetic defect centers within the range studied. The results can be explained by a model where in the high temperature phase the mobile clusters of the low temperature phase are trapped by the defect centers.