bluuebunny/crossref_metadata_2025 huggingface dataset:
The multiphoton ionization spectrum of xenon: Interatomic effects in multiphoton transitions
Authors: Aron Kenneth , Johnson Philip M.
Published: February 2003

The multiphoton ionization spectrum of xenon was taken in the vicinity of the laser wavelengths corresponding to one-third the energy of transitions to the 5p5(2P3/2)6s, 5p5(2P1/2)6s, and 5p5(2P3/2)5d atomic levels. Many atomic resonances were observed, the most prominent being third photon resonances with atomic 5d levels and fourth photon resonances with atomic 4f levels. However, peaks corresponding to ionization through the 6s levels were weak or absent. Resonances in these regions showed extreme pressure dependence. At higher pressures (≳50 torr) a broadening of the atomic lines as well as the existence of continuum backgrounds and strong broad resonances peaking at the energy of transitions between excited states were seen. At low pressure (40 torr) the atomic lines were narrow and the broad peaks and background were no longer present. These high pressure features were interpreted to be a consequency of resonances with vibrational continua of excited state diatomic xenon molecules and illustrate the necessity of inclusion of molecular Franck–Condon factors into the multiphoton ionization probability of atomic gases at moderate pressures.
Energy loss of electrons in random motion. Analytical result
Authors: Tachiya Masanori , Sano Hisatake
Published: February 2003

The average rate of energy loss of an electron moving in a dielectric medium can be expressed in terms of the power spectrum of the electric displacement generated by the electron. The power spectrum for an electron in random motion was analytically calculated from the autocorrelation function of the electric displacement by use of the Wiener–Khinchin theorem. Agreement between the present analytical results and numerical results by Magee and Helman [J. Chem. Phys. 66, 310 (1977)] is found to be good except at higher frequenices. It is also found that as far as the energy loss to dipolar relaxation is concerned, the random track has the same loss rate as the linear track.
Reactions of carbynes. Potential energy surfaces for the doublet and quartet methylidyne (CH) reactions with molecular hydrogen
Authors: Brooks Bernard R. , Schaefer Henry F.
Published: February 2003

The CH(2Π)+H2→CH3 and CH(4Σ−)+H2→CH2(3B1)+H reactions have been studied using ab initio molecular electronic structure theory. Basis sets used were of double zeta (DZ) quality and DZ augmented by polarization functions. For the insertion reaction, the least motion barrier height is quite high, ∼75 kcal/mole. However, nonleast motion pathways are much more favorable, involving little or no barrier. In both respects this potential energy surface is analogous to that for the methylene insertion CH2(1A1)+H2→CH4. The quartet abstraction has a sizeable barrier, ∼11 kcal, associated with it, in analogy with the triplet methylene abstraction CH2(3B1)+H2→CH3+H.
Ion–molecule reactions of N+ with CO: Integral reactive cross sections in the collision energy range 0.2–13 eV
Authors: Frobin W. , Schlier Ch. , Strein K. , Teloy E.
Published: February 2003

The guided beam technique, which allows the precise measurement of integral reactive cross sections of ions, has been used to measure the products of the collision of N+ with CO in the energy range 0.2–20 eV (lab). All five possible ionic reaction products CO+, NO+, C+, CN+, and O+ have been observed. The cross sections of the exothermic CO+ and NO+ channels show the usual decrease with E at low E (below 3 eV lab, i.e., 1 eV c.m.), then increase again to a flat maximum (near 12 eV lab, i.e., 5 eV c.m.). The CN+ and O+ channels show the expected thresholds, then a maximum (near 8 eV lab, i.e., 6 eV c.m.), whereas the C+ channel seems to have an activation energy and is still increasing at 13 eV c.m. There is no indication, in the form of kinks, of excited product channels except for dissociation. In addition, time-of-flight measurements have been done, giving some information on the forward component of the product velocity. The results are discussed in the light of simple theories, and of correlation diagrams supported by partial knowledge of the potential energy surface of (NCO)+ from CI calculations by A. A. Wu. This molecule has three stable linear isomers NCO+, NOC+, and CNO+, which have also linear low lying excited states. So the discussion of the reaction as a collinear collision seems justified. The well in the 3Π states can probably explain complex formation at low energies. The products CO+ and O+ cannot be formed adiabatically (even in Cs symmetry). This is in accord with the small cross section for O+ formation, but not with the large one for CO+, where one has to postulate the existence of nonadiabatic surface ’’jumps’’. Additional experiments, especially differential cross sections, are needed for a more comprehensive discussion of the molecular dynamics, which, after all, is not singular but rather prototypical for triatomic chemical reactions.
Picosecond dynamics of conformational changes in 1,1′ binaphthyl
Authors: Shank C. V. , Ippen E. P. , Teschke O. , Eisenthal K. B.
Published: February 2003

Optically induced conformational changes in 1,1′ binaphthyl in different liquids have been studied on a picosecond time scale with subpicosecond optical pulses from a passively modelocked cw dye laser. The technique used was to measure the risetime of an excited state absorption induced by a 307.5 nm pulse and monitored with a 615 nm probing pulse. The risetime was found to be nonexponential with a 2.5±0.5 psec rapid initial rise followed by a slower component which varied from 11 to 22 psec depending on solvent viscosity. The orientational relaxation of the molecule in its equilibrium excited state configuration was also obtained.
Diagrammatic perturbation theory applied to the ground state of the water molecule
Authors: Silver David M. , Wilson Stephen
Published: February 2003

The diagrammatic many-body perturbation theory is applied to the ground state of the water molecule within the algebraic approximation. Using four different basis sets, the total energy, the equilibrium OH bond length, and the equilibrium HOH bond angle are examined. The latter is found to be a particularly sensitive test of the convergence of perturbation expansions. Certain third-order results, which incorporate all two-, three-, and four-body effects, show evidence of good convergence properties.
Model perturbation theoretic calculations with finite continuum basis sets
Authors: Rossky Peter J. , Karplus Martin
Published: February 2003

To examine various aspects of the methods employed in diagrammatic perturbation theory applications the model problem of a hydrogenic atom of nuclear charge Z perturbed by the potential −Z′/r is analyzed. The use of a discrete quadrature for the continuous energy spectrum and a finite coordinate space integration cutoff are tested by comparison with analytic methods. It is found that accurate results are obtained if a physically reasonable choice for the spatial cutoff is used, namely, the maximum significant extent of the zero order wavefunction. For larger values the use of the discrete continuum basis results in a spurious logarithmically divergent contribution to the energy which can be of practical significance. A detailed examination is made of the summation techniques commonly employed in diagrammatic perturbation theory. For the physical choice of cutoff the geometric summation of higher order terms involving excited states in the continuum, as well as bound states, is shown to yield meaningful and accurate results. To illustrate the analysis numerical results are given for Z=1, Z′=1 and Z=4, Z′=1.
One-dimensional density-potential relation for independent Fermions in a local density approximation
Authors: March N. H.
Published: February 2003

Combining a differential form of the virial theorem with the Euler equation for the particle density, in a local density framework leads to a differential equation for the density, given the potential. This equation has the Thomas–Fermi density as a solution when the third derivative of the particle density is neglected. The arguments are restricted to one-dimensional motion of independent Fermions.
On the products’ translational distribution for the reactions of CH3I and CF3I with K and Ba
Authors: Bernstein R. B. , Wilcomb B. E.
Published: February 2003

A reanalysis is presented of the available literature data on the recoil velocity distribution of the KI product in the crossed beam reaction of K with CH3I (and with CF3I) and the resulting recoil distribution function P (Etr′) of final relative translational energy E′tr. For the CF3I reaction (at an average initial translational energy ?tr=2.9 kcal mol−1) the average c.m. recoil energy ?′tr is 7.6 kcal mol−1, corresponding to only 27% conversion of the total available energy E into product translation, compared to a value of 57% for the CH3I reaction (of about the same E, at the same ?tr). The probability density function for the KI velocity (i.e., linear momentum) in the backward (’’rebounding’’) direction is essentially Gaussian. These data are used as a test of several distinguishable impulsive models, e.g., those of Herschbach and of Levine et al. The results for the alkali reactions are compared to those of the analogous Ba reactions; qualitative differences are noted. Much of the apparent diversity can be readily understood in terms of the unifying variable p′, the products’ recoil momentum. The most probable value of p′ is not very different for all the reactions considered, in accord with the so-called photodissociation model of Herschbach and Harris. The near-Gaussian shapes of the recoil momentum distributions are in qualitative accord with the information-theoretic form of Levine et al.
Chemi-ionization in collisions of metastable helium with metastable neon
Authors: Neynaber R. H. , Tang S. Y.
Published: February 2003

Absolute and relative cross sections were obtained for the Penning-ionization (PI) reaction He*+Ne*→He+Ne++e. The studies were made by a merging-beams technique for a relative kinetic energy W of the reactants from nominally 0.01–10 eV. The He* represents a composite of He(21S) and He(23S), and Ne* a composite of Ne(3s 3P2) and Ne(3s 3P0). The data indicate that the reaction is directed and that the relative cross section is proportional to W−0.41 for W<0.1 eV. The He*–Ne* system is found to be attractive with a well depth of approximately 0.25 eV. Cross sections for the process are compared to previously determined cross sections for the associative-ionization reaction He*+Ne*→HeNe++e.
Calculation of the mean force constants of the rare gases and the rectilinear law of mean force
Authors: Lee Myung W. , Bigeleisen Jacob
Published: February 2003

The mean energies 〈U〉 and the mean force constants 〈∇2U〉 have been calculated for liquid argon and liquid krypton using the WCA perturbation theory, and for gaseous argon and krypton along the coexistence line by solution of the PY equation for the radial distribution function. Calculations have been made for the Lennard-Jones, Barker–Henderson, and Maitland–Smith potentials. There is little difference in the values of 〈U〉 and 〈∇2U〉 calculated for the three potentials. The calculated values are in good agreement with experimental data. Correlation of the calculated values of 〈∇2U〉 for the liquid and gas leads to an empirical relationship between 〈∇2U〉 and the density of the phase along the coexistence line. When the latter is combined with the law of rectilinear diameters of the density, one obtains the rectilinear law of mean force, which is in good agreement with experimental data on argon. It is shown that the scaling exponent for the mean force constant 〈∇2U〉l-〈∇2U〉g is larger than the scaling exponent of the density below the critical temperature and becomes equal to it at the critical temperature. The rational for the rectilinear law of mean force is provided by an expansion of the radial distribution function of the liquid in powers of the density and by the use of the WCA approximation to the radial distribution function.
Thermodynamic processes induced by coherent radiation
Authors: Garbuny M.
Published: February 2003

It is shown by quantum statistics that under certain stated conditions the entropy of coherent radiation is zero and it is still negligible for multimode laser operation. This makes possible gas kinetic processes which, to a small extent, have already been observed or even utilized, but which can be greatly enhanced by an optimized choice of molecular structures and radiation conditions. Radiative cooling of gases is discussed in detail. The conditions for maximum heat withdrawal are derived, and it is proposed that the processes of cooling and relaxation heating can be sufficiently separated in time to achieve certain effects and thermodynamic cycles. One of these is the complete conversion, possible in principle, of coherent radiation into work. This concept is based on a heat pump process followed by heat-to-work conversion, the heat rejected being just equal to that withdrawn by radiation. The conditions for complete conversion turn out to be the same as for maximum heat withdrawal. The feasibility of these processes depends on the degree to which practical conditions can be met, and on the validity of certain assumptions which have to await experimental verification.
The thermal conductivity of polymer melts
Authors: Fesciyan S. , Frisch H. L.
Published: February 2003

The method of time correlation functions is used to calculate the thermal conductivity for high molecular weight polymer melts. The result is independent of the molecular weight in agreement with experiment and also has qualitatively the right high temperature behavior.
Theoretical radiative lifetime of the metastable c3Πu state of H2
Authors: Bhattacharyya Dilip K. , Chiu Lue-Yung Chow
Published: February 2003

Magnetic dipole and electric quadrupole interactions are considered here as the mechanism for the forbidden radiative transition from the metastable c3Πu state of H2 to the dissociative b3Σu+ state. The discrete vibrational wavefuncion for the c3Πu state is generated from Browne’s potential. The continuum vibrational wavefunction for the b3Σu+ state is generated from an exponential type pontential function, which fits closely with Kolos and Wolniewicz’s potential. By virtue of this exponential type potential function, the phase integral and the exponential integral of the continuum vibrational wavefunction become integrable and an analytical expression is given for the vibrational wavefunction of b3Σu+. The electronic wavefunctions, used here to calculate the transition moments, are two-center wavefunctions previously derived by Chiu (for c3Πu) and by Hirschfelder and Linnett (for b3Σu+). It is found that the translational expansion of the magnetic dipole operator (originally at the center of mass) at the atomic centers generates an additional gradient operator, which contributes significantly towards the magnetic dipole transition probability. From the total transition probability which is a sum of that due to magnetic dipole transition and that due to electric quadrupole transition, a lifetime of τ=1.76 msec is calculated for the lowest vibrational level of the c3Πu state. For a comparison, the δ-function approximation is used for the continuum vibrational wavefunction. This approximation gives rather good agreement with the accurate calculation.
Subpicosecond lifetimes by infrared line broadening measurements
Authors: Weiss Shmuel
Published: February 2003

Correlation times computed from collision-induced spectra of rare gases, hydrogen, and CO2 are presented and compared to duration of collision times. It is suggested that ir line broadening could be used for the estimation of lifetimes of species of chemical interest.
Selective uptake of counterions of different size by ion-exchange gels: Effect of swelling pressure and Coulombic interactions
Authors: Gregor Harry P. , Greff Richard J.
Published: February 2003

The selective uptake of potassium ions versus tetramethylethyl-, ethyl-,and propylammonium ions by fully neutralized, crosslinked polymethacrylate gels was measured in aqueous, mixed aqueous, and nonaqueous solvents. Swelling pressures in water were calculated from isopiestic data, so the Coulombic contribution to the total free energy of the exchange process could be calculated and compared with theory. A good fit between experiment and theory was possible only by assigning values for the dielectric constant of water in these gels of about 30 in the K–TMA state and 15 in the K–TEA state.
High pressure study of the effect of viscosity on the quenching of naphthalene fluorescence by biacetyl
Authors: Brey L. A. , Schuster G. B. , Drickamer H. G.
Published: February 2003

The effect of solvent viscosity on the rate of quenching of naphthalene fluorescence by biacetyl in degassed methylcyclohexane at room temperature has been investigated using pressure to alter the viscosity from 10−2 poise to about 10 poise. Rates obtained in this work are in good agreement with those of Birks and Leite [J. Phys. B 3, 417 (1970)] who measured the quenching rates in several solvents and solvent mixtures of varying viscosities at atmospheric pressure.
Molecular dissociation by electron impact: High Rydberg fragments from methane, ethylene, and ethane
Authors: Schiavone James A. , Donohue David E. , Freund Robert S.
Published: February 2003

Electron impact dissociation of CH4, C2H4, and C2H6 produces high Rydberg H and C fragments. Time-of-flight measurements and excitation functions help to characterize the molecular processes involved. Although details differ, the three molecules display similar dissociation behavior; relatively low kinetic energy fragments (≲8 eV) arise from states between 19 and 30 eV, higher kinetic energy fragments from states between 30 and 40 eV (states most likely with doubly ionized cores), and even higher kinetic energy fragments (up to 18 eV) from states above 40 eV. Comparisons of high Rydberg kinetic energy spectra with available ion kinetic energy spectra show general agreement confirming the utility of the core ion model for polyatomic molecules.
Molecular dissociation by electron impact: Optical emission from fragments of methane, ethylene, and methanol
Authors: Donohue David E. , Schiavone James A. , Freund Robert S.
Published: February 2003

We have measured relative excitation functions for the production of electronically excited fragments by electron impact dissociation of methane, ethylene and methanol from 0 to 2000 eV. At low impact energies, 10–50 eV, the measured appearance potentials are correlated to specific dissociation limits and, when possible, to specific excited states of the parent molecules. It is found that below ∼25 eV superexcited states play the dominant role in producing these fragments. Fano plots have been constructed to determine the types of molecular excitation involved. In general, most of these excited fragments are produced through optically forbidden transitions.
Energy separation between the open (C2v) and closed (D3h) forms of ozone
Authors: Lucchese Robert R. , Schaefer III Henry F.
Published: February 2003

Configuration interaction calculations were performed using the currently developed BERKELEY system of programs on ozone, and the energy difference between closed and open isomers was determined. (AIP)
Orientational correlation times for a dilute Fokker–Planck fluid
Authors: Evans G. T. , Keyes T.
Published: February 2003

Low density orientational correlation times are calculated for a Fokker–Planck fluid using a technique taken from kinetic theory. It is found that τl=τJ/[2l (l+1)(2l+1)].
Fused hard sphere equation of state near the close packed limit
Authors: Ausloos M. , Clippe P.
Published: February 2003

An approximate equation of state for nonspherical hard molecules is derived starting from a recurrence relation in the close packed limit. An ansatz consists of generalizing a Tonks result for the probability distribution of close packing hard rods in one dimension, in order to take into account the effect of overlapping exclusion volumes in three dimensions.
Spectroscopy and vibrational relaxation of CNN in rare gas solids
Authors: Bondybey V. E. , English J. H.
Published: February 2003

The CNN free radical was prepared by the reaction of C atoms with N2 and its emission and laser excitation spectra were studied. The upper state is probably 3Π, and it shows a moderate Renner splitting. All three upper state vibrational fundamentals were determined. Vibrationally unrelaxed emission from several levels has been observed. Vibrational relaxation from the upper-state stretching modes is shown to proceed via intramolecular mode-to-mode energy transfer to the bending frequency ν2.
Collision-induced I(2P1/2) emission in argon and xenon
Authors: Krauss M. , Julienne P. S.
Published: February 2003

Long range perturbation theory is used to estimate the magnitude of the dipole transition moment leading to iodine 2P1/2 → 2P3/2 emission during a collision with a rare gas atom. The leading term in the induced transition moment varies as R−4 owing to the dipole–quadrupole term in the interaction potential. The two-body rate coefficient for collision-induced radiative decay of I 2P1/2 is calculated using approximate model potentials for XeI and ArI. The coefficients are estimated to be 2×10−19 cm3 sec−1 and 6×10−19 cm3 sec−1 for I 2P1/2 in 300 °K Ar and Xe, respectively.
On the use of the Weeks–Chandler–Andersen theory for interpreting the solubilities of gases in liquids
Authors: Goldman Saul
Published: February 2003

The Weeks–Chandler–Andersen liquid state perturbation theory was corrected for translational quantum effects in the liquid, and was then used as the basis for the derivation of a fundamental solubility theory. Equations were derived for the Henry’s law constant K, the heat of solution of a solute in a solvent at infinite dilution ΔHs⋅ and the partial molar volume of a solute in a solvent of infinite dilution, ?2⋅. The equations were applied, over a range of temperatures, to the following systems: H2 in each of Ar, N2, CH4; He in each of Ar, N2, CH4; Ne in each of Ar, N2. Lennard-Jones 6–12 pair potentials, taken from second virial coefficient data were used. The results obtained were compared with experimental data, and with previous calculations on these systems that were based on the Leonard–Henerson–Barker and the Mansoori–Canfield–Leland theories. The Weeks–Chandler–Andersen results were, for most of the systems, in very good agreement with the experimental results and, for most of the systems, the Weeks–Chandler–Andersen predictions for K were better than those that came from either the Leonard–Henderson–Barker theory or the Mansoori–Canfield–Leland theory.
Fluctuations of polarization and magnetization in dielectric and magnetic media
Authors: Felderhof B. U.
Published: February 2003

We consider thermal fluctuations of polarization, magnetization, and the associated electromagnetic fields in dielectric magnetic media which are allowed to be anisotropic and spatially nonhomogeneous. The theory is purely macroscopic and is based on an application of the fluctuation–dissipation theorem. It is shown that the correlation functions can be elegantly expressed in terms of the Green functions for Maxwell’s equations.
Deactivation of I(52P1/2) by chemical reaction with HBr and DBr
Authors: Wiesenfeld J. R. , Wolk G. L.
Published: February 2003

The collisional deactivation of electronically excited iodine atoms, I (5P1/2), by CH4, C3H8, HBr, and DBr has been investigated using time-resolved atomic resonance spectroscopy. This technique permits direct determination of the yield of I (5 2P3/2) following the deactivation of the excited atoms by the added gas. The experiment demonstrated that the deactivation of I (5 2P1/2) by CH4 and C3H8 proceeds by physical quenching, but deactivation by HBr and DBr occurs via metathetical chemical reaction to form HI+Br(4 2P3/2). This nonadiabatic reaction is discussed in the context of a curve-crossing model involving vibrationally excited virtual states of the deactivating molecule.
A nonempirical molecular orbital study on the acidity of the carbon–hydrogen bond
Authors: Hopkinson Alan C. , Lien Min H. , Yates Keith , Mezey Paul G. , Csizmadia Imre G.
Published: February 2003

The geometries of acetylene, ethylene, methane, ethane, ketene, allene, propyne, acetonitrile, ketenimine, aminoacetylene, hydroxyacetylene, and all the possible anions formed by removal of a proton from these molecules have been carefully optimized using nonempirical molecular orbital calculations employing the split-valence shell 4-31G basis set. These geometries have then been used in 6-31G basis set calculations. The computed order of gas phase acidity found for these molecules is hydroxyacetylene≳ketenimine≳ketene ≳acetonitrile≳aminoacetylene≳acetylene≳propyne≳allene ≳ethylene≳ethane≳methane.
YbH and YbD molecules: ESR and optical spectroscopy in argon matrices at 4 °K
Authors: Van Zee Richard J. , Seely Mikell L. , Weltner W.
Published: February 2003

YbH and YbD molecules have been prepared by the reaction of Yb and H(D) atoms during the formation of an argon matrix at 4 °K. Yb atom and YbH absorption and emission spectra were observed. The magnetic parameters of YbH were determined from the ESR spectrum of the 2Σ molecules (with Yb nuclear spin I=0 and I=1/2) to be g∥=1.9953(4), g⊥=1.9402(2), A∥(H) =226 MHz, A⊥(H) =224 MHz, A⊥[171Yb (I=1/2)]=5266 MHz, A∥[171Yb (I=1/2)]=5724 MHz. The hyperfine parameters indicate that the spin density is less than 20% on the hydrogen and that the bonding is largely Yb+H−. By comparison of experimental parameters with calculated Yb and Yb+ data it is deduced that the unpaired electron occupies predominately the 6sσ orbital on Yb+ with smaller contributions of 6pσ (Yb+) and 1sσ (H−).
High spin–low spin crossover and antiferromagnetic interactions in tris(1-pyrrolidinecarbodithioato)iron (III) and the 4-morpholine (FeM) and dibutyl analogs, effect of recrystallization solvent, and crystal structure of FeM⋅nitrobenzene
Authors: Cukauskas E. J. , Deaver B. S. , Sinn E.
Published: February 2003

High sensitivity magnetic susceptibility determinations, especially in the range 1.2–4.2 K on pure and dilute tris(pyrrolidinecarbodithioato)iron(III) (FeP) in its high spin form, show that a maximum at about 2 K is caused by antiferromagnetic interactions. The analogous chromium(III) complex does not exhibit significant antiferromagnetism compared to that of the iron complex, and it is likely that the upper e electrons possessed by the iron and not by the chromium are responsible for the bulk of the antiferromagnetism. As the iron atoms are about 9 Å apart in discrete molecules, the antiferromagnetic interactions presumably occur between unpaired spins delocalized on to the ligands of adjacent molecules. This is in keeping with NMR evidence that spin delocalization is greater in the iron(III) than in the chromium(III) complex. When diluted with large amounts of the cobalt(III) analog (CoP), FeP exhibits a spin state equilibrium. Thus, the structure of the FeP molecule is modified slightly (presumably with shortening of the Fe–S bond) to approach that of the CoP host lattice, which has a shorter metal–sulfur bond. The previous history of the samples of ferric dithiocarbamate complexes is shown to be far more important than had previously been suspected: When crystallized from benzene, FeP exhibits a high spin–low spin equilibrium, in constrast with the pure high spin behavior of the complex when not crystallized from benzene. The effect of adding 7% of benzene to the lattice is much greater than that of adding 50% of CoP. The dibutyl analog shows similar effects. The tris(4-morpholinecarbodithioato-S,S’)iron(III) complex FeM is shown, by single crystal x-ray data, to contain short Fe–S bond lengths (average 2.353 Å) when recrystallized from nitrobenzene. This indicates that the complex is principally low spin, in keeping with the observed magnetism and with the general strong solvent effect on the spin state. It is now proposed that the difference in Fe–S bond lengths between FeP crystallized from chloroform and FeP from benzene (the reverse of the expected differences) is due to experimental error. Crystal data for FeM⋅nitrobenzene: space group P21/c, Z=4, a=9.713(3) Å, b=31.419(8) Å, c =9.718(2) Å, β=105.04(2) °, V=2864 Å3, R=3.3%, 2712 reflections.
Equations of state for a square-well gas from a parametric integral equation
Authors: Carley David D.
Published: February 2003

A parametric integral equation is used to compute radial distribution functions for a classical gas whose particles interact pairwise with a square-well potential function. The parameter is fixed at one value for an isotherm by adjusting it to provide good agreement with available ’’exact’’ (Monte Carlo or molecular dynamics) pressure at a high density. Six isotherms in the gas region are studied and thermodynamic functions (pressure, internal energy, Helmholtz free energy, Gibbs free energy, entropy, and enthalpy) are computed from the radial distribution functions. Comparisons with available ’’exact’’ calculations indicate that these results are good for the higher isotherms but that for temperatures nearer the critical temperature, larger errors appear. These calculations, when considered with previous results, indicate that the parametric integral equation gives excellent results for a single value of the parameter along an isotherm for high temperature gases, but that a more complex determination of the parameter is needed for low temperature gases and for liquids.
Vibrational spectra, potential functions, and conformations of cycloheptanone
Authors: Bocian David F. , Strauss Herbert L.
Published: February 2003

The vibrational spectra of cycloheptanone are presented in detail. Analysis of the spectra shows that cycloheptanone is a nonrigid pseudorotating molecule with a twist-chair average structure. The pseudorotation energy surface is calculated and indicates that the symmetric twist-chair conformer and the two adjacent twist chairs are at practically the same energy. The other twist-chair forms are separated by barriers of different heights. This complicated picture fits the observed pseudorotation spectrum in detail, giving about the correct shape to the observed band below 30 cm−1 and accounting for the curious observed band at about 90 cm−1. The 90 cm−1 band is assigned as a superposition of Δn=7, 9, and 11 transitions. The agreement between calculation and experiment provides support for our dynamical model of pseudorotation in seven-membered rings.
A vibronic coupling in a degenerate electronic state via a nuclear momentum and an antisymmetric Raman scattering tensor
Authors: Nishimura Yoshifumi , Hirakawa Akiko Y. , Tsuboi Masamichi
Published: February 2003

A possibility has been pointed out of a significant vibronic coupling within a degenerate electronic state of a molecule via the nuclear momentum of a certain vibration of the molecule. As an example, a possible vibronic coupling through an a2g vibration in a molecule with D4h symmetry was discussed. It has been shown that this type of vibronic coupling should cause an inversely polarized Raman scattering. The values of the coupling constants have been estimated for some a2g vibrations in a model of the metalloporphyrin molecule.
Positive ion processes in the positive column of CO2 laser electrical discharges
Authors: Smith A. L. S. , Shields H.
Published: February 2003

A mass spectrometric study of the positive ions in the positive column of dc glow discharges in pure CO2 and CO2 gas laser mixtures has been undertaken to determine the nature of the ions and to elucidate the ion kinetics. Results are presented over the ranges 2–20 torr, 3–15 mA cm−2 and for reaction times of 0.1–1.0 s. The nature of the dominant ion species is dependent on the minority neutral species produced in the discharge. O2 + is the major species in pure CO2, but NO+ accounts for ≳70% of the positive ions in a 6% CO2–12% N2–82% He laser mixture and ions of the H3O+. (H2O)n type (n=0, 1, 2) become important in CO2–N2–He–H2 laser mixtures. A semiquantiative reaction kinetic model is developed to explain the formation of these ions and to aid predictive modelling of other CO2 laser systems.
Absorption and emission spectra of U4+ diluted in ThBr4 single crystals
Authors: Genet M. , Delamoye P. , Edelstein N. , Conway J.
Published: February 2003

The absorption and fluorescence spectra of U4+ diluted in single crystals of ThBr4 have been measured at room temperature (77 °K) and 4.2 °K. The crystal symmetry site of the U4+ ion is D2d. Twenty three levels have been assigned and a least-squares fit varying nine parameters (and fixing three parameters) gave a mean energy deviation of 89 cm−1. The values of the parameters obtained from the best fit are (in cm−1) F2=43 770±797, F4=31 302 ±4 200, F6=22 248±2 425, ζ=1719±44, α=36, β=−600, γ=1000, B20=−919±85, B40=−305±70, B44=−825 ±137, B60=636±917, and B64=−1937±202. There does not appear to be a systematic correlation between the crystal field parameters obtained for U4+ in ThBr4 and those reported earlier for U4+ in UCl4 or in ZrSiO4.
Explicit expressions for molecular integrals of a certain type
Authors: Memory J. D.
Published: February 2003

Explicit expressions are given for all molecular integrals of the type I (i,j,k) =ℱxiyjzkωsωtdτ and I′ (i,j,k) =ℱ (xiyjzk/rt) ωsωtdτ, where ωs and ωt are 2p orbitals on adjoining carbon atoms.
Photoabsorption in formaldehyde
Authors: Langhoff P. W. , Langhoff S. R. , Corcoran C. T.
Published: February 2003

Theoretical investigations employing configuration-interaction calculations and recently devised moment-theory techniques are reported of the vertical electronic dipole excitation and ionization spectra in molecular formaldehyde. A double-zeta basis of contracted Gaussian-lobe functions, supplemented with appropriate polarization, diffuse, and bond functions, is employed in the construction of Fock spectra in C2v symmetry for X 1A1 and (n→π*)3A2 states near the ground-state equilibrium geometry. The 50 occupied and virtual Fock orbitals obtained in each case are used in configuration-interaction calculations of 200-term eigenvectors of appropriate symmetry for each of the principle-axis polarization directions, and for the lowest-lying molecular ionic states. The ionization energies, discrete vertical transition frequencies and oscillator strengths, and associated approximate configurational assignments obtained are in general accord with experimental determinations and with the results of previously reported more elaborate state-specific theoretical calculations. Spectral moments calculated from 200-term pseudospectra of transition frequencies and oscillator strengths provide Stieltjes and Tchebycheff vertical electronic photoionization profiles in good agreement with appropriately averaged photoionization–mass-spectrometric measurements of the cross section for parent H2CO+ ion production. Discrepancies between the total and partial photoionization cross-section measurements in formaldehyde are indicated. Approximate configurational assignments above the first ionization threshold are made on basis of the calculated eigenvectors, and comparisons with experimental assignments are provided. It is suggested that vibronically preionized π–π* excitation gives rise to a feature at ∼13 eV in the measured photoionization spectrum for H2CO+ production not accounted for by the calculated vertical photoionization cross section.
The rotational sudden approximation at low energies
Authors: Pfeffer George , Secrest Don
Published: February 2003

The infinite-order sudden approximation is studied at low energies for a system with widely spaced energy levels to determine the limits of its validity. It is found that even under these extreme conditions it gives highly accurate results for cross sections summed over final rotational states. The elastic cross sections are also reasonably accurate in this approximation, though, for this worst case situation, the state to state cross sections for inelastic scattering are not even qualitatively correct for high Δj transitions. The high accuracy of the cross sections summed over final rotational states commends the sudden approximation for use in reducing the complexity of carrying rotational channels in the calculation of vibrational transitions. The use of the sudden approximation in studying expansions of interaction potentials is discussed.
Concentration dependence of the zero-field splittings of triplet phenanthrene-d10 oriented in biphenyl single crystals
Authors: Cullick A. S. , Gerkin Roger E.
Published: February 2003

Zero-field splittings and splitting parameters of phosphorescent phenanthrene oriented in single crystal biphenyl are reported as functions of guest content in the range 10−6 to 2×10−2 mole fraction guest from measurements at ∼77 K. The dependences, which have been fit to equations of the form y=axb, are attributed to molecular crystal field effects. In addition, EPR absorption linewidths have been measured as a function of guest content and are discussed in terms of spin–spin interactions. The liquid–solid phase behavior of the two component phenanthrene–biphenyl system is also reported.
Quantum exchange effects in trimer ground states
Authors: Cramer M. L. , Bruch L. W. , Cabral F.
Published: February 2003

The ground state of molecular trimers is studied in three dimensions for these pair potential models: the square well, the exponential, the Yukawa, the Gaussian, and the Lennard-Jones 12–6. Three spin cases are considered: three spin-zero bosons and the spin- (1/2) and spin- (3/2) states of three spin- (1/2) fermions. Variational wavefunctions are constructed which satisfy the exchange symmetry requirements for these cases. Bounds are obtained for the threshold coupling constants at which self-bound trimers occur. Consequences for possible self-bound trimers of 3He are discussed.
Infrared and Raman intensities of polyethylene and perdeuteropolyethylene by electro-optical parameters. Single chain
Authors: Abbate Sergio , Gussoni Mariangela , Masetti Giorgio , Zerbi Giuseppe
Published: February 2003

An extension of the electro-optical theory of valence to the calculation of infrared and Raman intensities of the fundamentals of polymers is presented. The method is applied to the calculation of the infrared and Raman intensities of polyethylene and perdeuteropolyethylene. Parameters from small model molecules have been used. The results are satisfactory, thus providing further evidence that the electro-optical parameters are transferable. New infrared and Raman spectra of polyethylene and perdeuteropolyethylene have been recorded and are reported. Some implications of intensity calculations in the vibrational assignments of these two polymers are discussed.
Spin trapping of radicals produced in benzene radiolysis. Evidence for the phenyl radical as an intermediate
Authors: Sargent F. P. , Gardy E. M.
Published: February 2003

The spin trapping technique has been used to show that phenyl radicals are produced during the radiolysis of benzene. (AIP)
Use of a non-pair-additive intermolecular potential function to fit quantum-mechanical data on water molecule interactions
Authors: Campbell Edwin S. , Mezei Mihaly
Published: February 2003

Permanent and induced multipole, repulsion, and dispersion terms are proposed as a model for the nonadditive potential of water molecules. 229 quantum-mechanical dimer energies of near Hartree–Fock accuracy which have been published by Popkie, Kistenmacher, and Clementi and by Kistenmacher, Lee, Popkie, and Clementi have been fit to an approximation to this model. The standard deviation 0.0019 of calculated dimerization energies for one simple form of the present model compares with a standard deviation 0.0023 given by an additive analytical fit developed by the above group. Nonadditive contributions calculated for the present model are compared with those given by the HF calculations of Hankins, Moskowitz, and Stillinger and by Kistenmacher, Lee, Popkie, and Clementi. The proposed model is compared with other models, and results to aid in the choice of multipole expansion orders in applications are given.
Nodal structure of the three-body nonadditive interaction
Authors: Bruch L. W. , Novaro O. , Flores A.
Published: February 2003

A discussion of qualitative aspects of the three-body non-pairwise-additive interaction V3 for three identical closed shell atoms and especially of its nodal structure is given. The rare-gas trimers He3, Ne3, and Ar3 and also the Be3 system are analyzed reviewing information coming from perturbation theory results for triatomic dispersion energies V3d, and of the three-body exchange–overlap energies coming from SCF and modified-effective-electron model studies. New SCF results, especially for the Be3 system, are also reported.
Surface dipolar plasmons in a polar liquid
Authors: Banville M. , Caillé A. , Zuckermann M. J.
Published: February 2003

Surface nonradiative dipolar plasmons are shown to be collective excitations of the polarization charge density in a polar liquid. The dispersion relations of the tangential and normal modes are obtained for a polar liquid layer of finite thickness using the classical complex dielectric constant of Lobo et al. In contrast with the bulk dipolar plasmon the surface dipolar plasmons are highly dispersive in the long wavelength limit when retardation effects are taken into account. Such surface modes can be excited by resonance using the method of attenuated surface reflection. This method should, therefore, provide an experimental means of differentiating between the collective mode and the single-particle libration mode.
The analysis of variationally calculated avoided-crossings
Authors: Hatton G. J. , Lichten W. L. , Ostrove N.
Published: February 2003

Very narrow avoided-crossings of energy curves play an important role in many physical systems. This paper discusses the production of spurious avoided-crossings by variational calculations and examines methods for determining the validity of calculated avoided-crossings. It is shown that the perturbation theory arguments conventionally advanced to support the von Neumann–Wigner noncrossing rule are invalid and that standard calculational techniques will, in general, yield avoided-crossings regardless of whether the exact energy levels cross or avoid crossing. A spurious avoided-crossing, produced by the incomplete basis set of a model variational calculation, is analyzed. The characteristics of the model are interpreted to suggest methods for the analysis of variationally calculated avoided-crossings. These methods are applied to approximate calculations of the one-electron systems H+2 and BH5+ and are sufficient to distinguish a spurious from a real avoided-crossing.
Calculation of sum rule properties for H2O
Authors: Chipman Daniel M. , Kirtman Bernard , Palke William E.
Published: February 2003

Accurate Hartree–Fock and valence-bond calculations on the sum rule moments Sk(r) of the electric dipole photoabsorption cross section, with k=−1, 1, and 2, are presented for the water molecule. Our values averaged over all molecular orientations are in good agreement with experiment, the discrepancies being on the order of 10% for S−1, 5% for S1, and within experimental error for S2. The errors in S−1 and S1 are due to the effect of electron correlation on the two-electron terms in the sum rule formulas.

The sum rule moments are used in conjunction with moment theory to calculate parameters characterizing a variety of optical properties, long-range intermolecular attractions, and the energy loss of fast charged particles passing through the vapor. Although the results are generally not as good as those obtained for most of the noble gases, they are semiquantitative. The agreement with experiment is, typically, within 10%. Of particular interest are the pure rotational Rayleigh plus Raman scattering and the orientation dependence of the van der Waals dispersion energy, since these depend upon the anisotropy of the moments which is (otherwise) experimentally inaccessible.
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o study of the phosphorescence and intersystem crossing of the lowest triplet state of pyrazine
Authors: Bendazzoli G. L. , Orlandi G. , Palmieri P.
Published: February 2003

Singlet–triplet spin–orbit matrix elements have been estimated for pyrazine using molecular wavefunctions obtained by an ab initio configuration-interaction method and minimal basis sets of Gaussian atomic orbitals. The computed radiative lifetime (10.2 ms) of the short lived component of the lowest 3B3u state is in good agreement with the experimental value. For the next excited triplet state (3B1u) a radiative lifetime is found similar to that of azine molecules in which the lowest triplet state is of ππ* nature. The spin–orbit matrix elements governing the rate constants of the T1→S0 and S1→T1 nonradiative processes have been evaluated.
Positron annihilation in unsaturated normal aliphatic hydrocarbons
Authors: Huang D. C. , Huang W. F.
Published: February 2003

Positron annihilation time spectra in n-hexane, 1-hexene, 1,5-hexadiene, 1,3-hexadiene, and 1-hexyne have been studied. It is found that the cross sections of triplet positroniums in a series of unsaturated normal aliphatic hydrocarbons with the same carbon number have a negative linear dependence on polarizability. The low triplet positronium intensities of 1,3-hexadiene and 1-hexyne are related to resonance and fast chemical quenching, respectively.
An improved line shape model for the matrix ENDOR response in disordered systems
Authors: Narayana P. A. , Bowman Michael K. , Becker David , Kevan Larry , Schwartz Robert N.
Published: February 2003

An improved line shape model for the matrix ENDOR response near the free-nucleus precession frequency in disordered systems is developed and applied to a spin system with S=1/2 and I=1/2 using a density matrix formalism. The model includes the radiofrequency and microwave field intensities as well as different spin relaxation paths explicitly. Application of this model to trapped electrons in 2-methyltetrahydrofuran glass gives geometrical and magnetic data which are in excellent agreement with existing data from electron spin echo experiments. The effects of the various parameters in the theory of the matrix ENDOR line shape are discussed.
Frequency-dependent multipole polarizabilities and reduced Green’s functions. I. The ground state hydrogen atom
Authors: McDowell Keith
Published: February 2003

The reduced free-particle Green’s function formalism is applied to the calculation of frequency-dependent multipole polarizabilities for the ground state hydrogen atom. A new modified polynomial fitting procedure is developed which yields satisfactory results for the polarizabilities well into the anomalous dispersion region and locates a rather large number of the excitation frequencies to high accuracy.
Theoretical study of the anion of lithium hydride
Authors: Liu B. , O-Ohata Kiyosi , Kirby–Docken K.
Published: February 2003

The potential energy curves of the ground state of LiH and LiH− have been obtained by accurate ab initio configuration interaction calculations. The bond length, dissociation energy, and spectroscopic constants obtained from the potential curve of LiH are within 1% of corresponding experimental values. The results for LiH− are Re =3.15±0.03a0, De=2.10±0.02 eV, D0=2.03±0.02 eV, and ωe=1179±11 cm−1. The calculated adiabatic electron affinity of LiH is 0.32 eV with an estimated error limit of ±0.02 eV. A semiempirical method for calculating diatomic molecular electron affinities is proposed. A detailed comparison of the current CI calculation with a previous equations-of-motion calculation is presented.