bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Test of the Franck-Condon Principle: Double Ionization of Molecular Hydrogen
Authors: McCulloh K. E. , Rosenstock H. M.
Published: February 2004

The double ionization of molecular hydrogen by electron impact has been observed by coincidence detection of the resulting proton pairs in a system especially designed for absolute kinetic energy measurements. Monte Carlo calculations provided an accurate knowledge of the resolution with which the kinetic energy measurements were made. Experimental results agree closely with predictions based on accurate Franck-Condon overlap integrals for a model involving Morse oscillator wavefunctions for the molecule and Coulomb wavefunctions for the ion. In contrast, the reflection approximation and the harmonic oscillator approximation for molecular vibration, two approximations frequently made in such Franck-Condon calculations, do not agree well either with experiment or accurate calculations. Observations at electron energies of 0.5, 0.75, and 1 keV show that the kinetic energy distribution is independent of electron energy and suggest that the double ionization cross section is inversely proportional to electron energy in this range. These results are to be expected for a direct two-electron ejection mechanism for double ionization of H2.
Analysis of Some Perturbations in the ν4 Band of Methyl Chloride
Authors: Maki Arthur G. , Thibault Robert
Published: February 2004

The ν4 band of CH3Cl has been analyzed in detail. It is found that there is a weak, but important, vibrational resonance between ν41 and 3ν61. The resonance is quantitatively taken into account in order to determine the unperturbed rotational constants for both ν41 and 3ν61. Both chlorine isotopes are resolved and treated separately. The interaction parameter is found to be given by 3.519( ± 0.006) + 0.028kl( ± 0.003) cm − 1 where the uncertainties are 3 times the computed standard errors. The k-dependent term is evidently due to a z-axis Coriolis-like interaction which is also possible between ν41(E) and 3ν61(E).
Kinetic Equations for a Gas Under the Action of Nonconservative Forces
Authors: Karamcheti Krishnamurty , Sentman Lee H.
Published: February 2004

When solid surfaces are included as nonconservative external forces acting on a gas, the determination of the behavior of the gas is a problem in the statistical mechanics of nonconservative systems. The kinetic equations for a gas under the action of such forces are obtained. It is shown that the nonconservative forces give rise to additional terms in the Liouville, S-particle, and Boltzmann equations. These terms appear because the nonconservative forces may depend upon the molecular velocity. It is shown that Liouville's theorem, which is usually derived for only conservative forces, is valid when certain types of nonconservative forces are present. It is also shown that a function of a constant of the motion will be an equilibrium distribution only for certain types of nonconservative forces. The effect of the representation of solid surfaces as external forces on the assumptions made in deriving the Boltzmann equation is examined. The conditions under which the usual forms of the H theorem remain valid are given. The resulting moment equations are obtained, and their relation to the usual equations of fluid mechanics is discussed.
Vapor-Phase Photochemistry of Furfural
Authors: Hiraoka H. , Srinivasan R.
Published: February 2004

The photochemistry of furfural (furan-2-carboxaldehyde) has been studied in the vapor phase by direct irradiation at a number of wavelengths from 3130 to 2225 Å as well as by sensitization by Hg(3P1) atoms. The products that were obtained were furan, methylacetylene, allene, cyclopropene, and carbon monoxide. The quantum yield for decomposition was ∼5 × 10−3 at 2225 and 2654 Å and was equally small in the first absorption region at 3130 Å. Two primary processes are proposed, one of which may lead to furan and CO by a hydrogen migration, and the other to C3H4 + 2CO. The possibility exists that the latter process proceeds through an excited molecule of furan as an intermediate. Evidence is presented which suggests that much of the observed photochemistry involves a low energy triplet of furfural.
Electronic Absorption Spectra of Methanal Azine and the Methyleniminyl Free Radical
Authors: Ogilvie J. F. , Horne D. G.
Published: February 2004

The methyleniminyl free radicals, H2CN, HDCN, and D2CN have been produced from flash photolysis of their stable dimers, the methanal azines H2CNNCH2, HDCNNCHD, and D2CNNCD2. The electronic absorption spectra of both radicals and parent substances are presented and discussed in terms of simple molecular orbital descriptions of the transitions. The photochemical processes are related to these transitions, and applications to the corresponding hydrocarbon diene, butadiene-1,3, are noted.
Spectrum and Structure of the He2 Molecule. V. Characterization of the Triplet States Associated with the UAO's 6− 17pπ and 7− 12pσ
Authors: Ginter Marshall L. , Ginter Dorothy S.
Published: February 2004

The electronic transitions 6 − 17pπ 3Πg→2s a 3Σu+ and 7 − 12pσ 3Σg+→2s a 3Σu+ of He2 are described and the previously unreported npσ 3Σg+ and 11 − 17pπ 3Πg states characterized. Existing data for the triplet states associated with the UAO's npπ and npπ are summarized and discussed. The N = 0, v = 0 level of the 2s a 3Σu+ state (the lowest stable electronic state for He2) is found to lie 34,302.2 ± 0.1 cm−1 below the v = 0 level of the X 2Σu+ state of He2. From extrapolation of the molecular constants for the npπ 3Πg− series, approximate values for the ground state of He2+ are ωe ≅ 1698.5cm−1, xωe ≅ 35.25cm−1, Be ≅ 7.211 cm−1, and re ≅ 1.0806 Å. “Accidental pure pecession” between the state pairs npΠ 3Πg+ and n + 1 pσ 3Σg+ is identified as the major (although not exclusive) perturbation systematically affecting the rotational levels of the npλ components with plus systmmery.
Light Scattering from a Chemically Reactive Fluid. I. Spectral Distribution
Authors: Blum L. , Salsburg Z. W.
Published: February 2004

The spectral distribution of light scattered by fluctuations about equilibrium in a chemically reactive fluid is calculated from the time dependence of the fluctuations predicted by the linearized hydrodynamic equations of irreversible thermodynamics. The theory treats a system with an arbitrary number, n, of independent chemical reactions which are coupled to the hydrodynamic processes involving viscosity and thermal conductivity. However, diffusion and any frequency dependence of the transport coefficients are neglected. The scattered light as a function of frequency has a central component and two side or Brillouin components. The central component is analyzed as a sum of n + 1 Lorentzian lines, one associated with a thermal diffusion relaxation process and n lines associated with the relaxation of the chemical reactions. In the limit of zero angle of scattering the widths of all Lorentzian lines associated with transport properties (thermal conductivity, viscosity, etc.,) vanish, and the entire width of the central component is related to the chemical relaxation processes. In fact, in this limit each chemical process contributes a Lorentzian line whose half-width is the reciprocal of the relaxation time for that process. The analysis of the widths for all the Lorentzian lines in the central component is given in detail for small but nonzero scattering angles and for the case of reactions having small enthalpy and volume changes for all scattering angles. The effect of chemical reaction rates on the position and width of a Brillouin or phonon component is also analyzed and parallels the treatment of ultrasonic dispersion and attenuation of sound waves.
Mechanism of Radiolysis of Gaseous HBr
Authors: Chen J. D. , Armstrong D. A.
Published: February 2004

The ion-pair yield for the decomposition of gaseous HBr by 120-kV x rays has been determined from measurements of hydrogen yields and saturation ionization currents in an ionization chamber. At HBr pressures in the range 200 to 305 torr, M-HBr / N is 4.8 ± 0.1. Up to one-half of this yield may be due to the excitation of HBr molecules to neutral excited states, and their subsequent dissociation to Br and H atoms. Under the conditions employed, all H atoms are expected to undergo Reaction (7):H + HBr→H2+Br. (7) A study of the effects of SF6 indicated that the yield of scavengeable thermal electrons, which form hydrogen, is 3.1 per 100 eV. The results of competition experiments with this scavenger were incompatible with electron-positive-ion combination and the dissociative capture reaction, e + HBr→H + Br−, (3) as sources of hydrogen molecules or atoms. They may be explained in terms of thermal electron capture reactions which are second order in HBr. Physical evidence for at least one such process, viz., e + 2HBr→H + Br — H — Br−, (14a) has recently been obtained.
Mass-Spectrometric Study of the Photoionization of Methane
Authors: Chupka William A.
Published: February 2004

Photoionization efficiency curves for the parent and fragment ions of CH4 have been measured with photon bandwidths of 1.6 and 0.4 Å. For the fragment ions, the effect of thermal rotational energy at room temperature is apparent in the threshold region and was taken into account to obtain accurate thresholds. The bond energy D°0(CH3 − H) was determined to be 4.477 ± 0.005 eV and the limit ΔH°f0(CH2) ≤ 4.10 eV was found. Ion-pair formation yielding CH3+ and H− was found to occur with low probability.
Ice IX: An Antiferroelectric Phase Related to Ice III
Authors: Whalley E. , Heath J. B. R. , Davidson D. W.
Published: February 2004

The dielectric properties of ice III in the frequency range 10−1 to 105 cps have been measured down to − 160°C. There is a gradual transition from the orientationally disordered III to an orientationally ordered and probably antiferroelectric phase, which is designated IX, starting at about − 65°C and reaching completion at about − 108°C. An arrangement of the hydrogen atoms in ice IX is proposed. The amplitude of the orientational polarization decreases continuously through the transformation region although the relaxation time is close to the value extrapolated from previous measurements in the disordered phase. The limiting high-frequency dielectric constant of the dispersion decreases with decreasing temperature. The cause of this behavior, which is unusual for molecular crystals, is undoubtedly that the polarization of the lattice vibrations contributes a large part of the high-frequency dielectric constant. This contribution decreases with decreasing temperature because the decreasing anharmonic interaction increases the absorption frequencies. The high-frequency dielectric constant decreases by about 5.7% at the III-IX transition although there is little volume change; this decrease might be a valuable way of detecting order-disorder transitions, particularly if they proceed slowly. The infrared polarizabilities of the various phases are briefly discussed. Ice IX could not be warmed into III because it always transforms to the more stable ice II.
Theoretical Analysis of the Role of Diffusion in Chemical Reactions, Fluorescence Quenching, and Nonradiative Energy Transfer
Authors: Steinberg Izchak Z. , Katchalski Ephraim
Published: February 2004

The kinetics of diffusion-controlled chemical reactions in solution are analyzed by a statistical treatment. To start with, the probability of interaction of two molecules A and B, separated by a given distance at zero time and undergoing Brownian motion subsequently, is determined. The probability of interaction of an A molecule with one of many surrounding B molecules is then deduced. Finally, the course of reaction between A and B molecules distributed at random at zero time in a system containing a large number of molecules of both species, is calculated. The treatment outlined is readily extended to systems in which factors other than random diffusion are operative, as well as to systems in which the distribution of A and B molecules is not random at zero time. A few examples are discussed in detail. The theoretical treatment presented is applied to the kinetics of quenching of electronically excited molecules by collision with quencher molecules in solution, and to the calculation of the extent of nonradiative transfer of electronic excitation energy between molecules which are free to undergo Brownian motion.
Theory of Excitons in Liquids. IV. A Simplified Treatment of the Shift and Damping of Polarization Waves
Authors: Rice Stuart A. , Nicolis Gregoire , Jortner Joshua
Published: February 2004

A simplified theory of the scattering of polarization waves in a liquid arising from the thermal motion of the atoms is presented. The treatment departs somewhat from the general lines of the formal exciton theory in disordered systems set up in the previous papers of this series. Most important, some additional approximations designed to simplify the analysis are introduced. The frequency-dependent lifetime of the transitions is calculated using second-order perturbation theory. From this, an expression is deduced for the damping coefficient of excitation waves, both in the case of an impurity atom in a host liquid and in the case of a pure liquid. In both calculations, the thermal motion of the atoms is represented as a small step diffusion or by using a linear-trajectory approximation. A rough numerical estimation of the level broadening produces the expected orders of magnitude for the lifetimes of states in Ar. Finally, an alternative approach to the exciton problems is discussed in much the same spirit as the Zwanzig treatment of elementary excitations in classical liquids. In the absence of scattering, we demonstrate that this approach leads to the correct form of the exciton dispersion relation.
Gaseous Ion Recombination. V
Authors: Mahan Bruce H.
Published: February 2004

Pitaevski's formulation of the three-body ion-electron recombination rate constant based on the Fokker-Planck equation is modified so as to apply to the termolecular association of gaseous ions. The theory is expected to be valid when the collisional energy changes are small compared to the kinetic energy of the recombining ions, which occurs when the mass of the neutral is much less than the masses of the ions. The calculated and measured rate constants for such systems are in substantial agreement.
Virial Coefficients of Methane, Ethane, and Their Mixtures at Low Temperatures
Authors: Hoover Alan E. , Nagata I. , Leland Thomas W. , Kobayashi Riki
Published: February 2004

A Burnett apparatus designed specifically for the measurement of virial coefficients was constructed and used to establish the second and some third virial coefficients of methane, ethane, and five equally spaced compositions at 273.15°, 240.00°, and 215.00°K. Second and third virial coefficients for methane were also determined at 200°, 191.06°, and 131.93°K. By using a gas-lubricated dead-weight gage located in a vacuum of approximately 25 μ Hg, absolute pressures could be read directly. The gage located in an ambient vacuum also allowed the determination of accurate compressibility factors from 40 atm down to the neighborhood of one atmosphere pressure. A recently developed method for the determination of virial coefficients directly from Burnett pressure measurements was applied. The range of the experimental apparatus enabled the determination of several negative values of the third virial coefficients at low reduced temperatures along with positive values at the higher temperatures. The experimental second and third virial coefficients of this and past studies were used to determine the Kihara, Lennard-Jones, and square well potential parameters. The accuracy of the representation of the virial data by the parameters over a wide range of temperatures is given.
Nuclear Spin Relaxation and the Collision Frequency in Dense Gases
Authors: Gordon R. G. , Armstrong R. L. , Tward E.
Published: February 2004

The relation between nuclear spin-lattice relaxation (T1) measurements and the collision frequency in dense gases is discussed. It is shown that the density dependence of T1 in compressed H2 gas can be accounted for by assuming a hard-sphere potential model and evaluating the Enskog correction to the collision frequency in a dense fluid. This comparison represents the first direct test of the collision frequency in a dense fluid.
Internal Compression Effects. II. Frequency and Intensity Changes in Charge-Transfer and Hydrogen-Bonded Complex Spectra
Authors: Trotter Philip J.
Published: February 2004

Some effects on spectra of weak molecular complexes due to compression of the complex bond were investigated. Changes in the charge-transfer (CT) frequency and intensity with changing bond length in model tetracyanoethylene-aromatic complexes have been estimated. These changes are related to a reduction of the complex bond length by internal compression forces of solvent (or by high external pressure). Variation of CT band frequency with compressional forces in various nonpolar solvents was studied, as was the stretching frequency of H bonds. It is concluded that solvent forces can contribute importantly to vapor-to-liquid spectral changes in weak complexes.
Atomic and Molecular Calculations with the Pseudopotential Method. III. The Theory of Li2, Na2, K2, LiH, NaH, and KH
Authors: Szasz Levente , McGinn George
Published: February 2004

The pseudopotential theory of diatomic molecules developed in the first part of this series is applied to the calculation of molecular constants for Li2, Na2, K2, LiH, NaH, and KH. Treating these molecules as two-electron problems the core–valence interaction potential is the Hartree–Fock potential plus the exact Phillips–Kleinmann pseudopotential computed in the second part of the series. The wavefunction of the valence electrons is a Heitler–London ansatz for the A2 types and an H–L ansatz augmented by an ionic term containing a correlation factor (1 + cr12) for the hydrides. The computed constants are dissociation energies, equilibrium distances, vibrational frequencies, and dipole moments (for the hydrides). The computated dissociation energies are about 13 of the empirical for the A2 types and 23 of the empirical for the hydrides. The computed equilibrium distances, obtained as the position of the energy minimum, are reasonably close to the empirical in all cases. The results are analyzed, and it is concluded that in the hydrides the charge distribution around the H+ nucleus resembles the electron distribution of an H− atom. A generalization of the pseudopotential method for polyatomic molecules is suggested and its application to large organic molecules is outlined.
Collision Density and Chemical Kinetics of Energetic Atoms. III
Authors: Chapin D. M. , Kostin M. D.
Published: February 2004

The gas-phase bimolecular reaction H + D2→HD + D of energetic hydrogen atoms scavenged by molecular iodine and moderated by argon at 1000°K is studied by stochastic and analytical methods. A differential form of the Boltzmann equation for this system is discussed and coefficients of the differential equation are derived. Solutions for the collision density obtained from the differential equation and from an independent stochastic computer program are found to be in satisfactory agreement. The effect on the collision density of changing the moderator species from argon to helium is investigated and rate constants for both the argon- and helium-moderated systems are given.
Experimental and Theoretical Kinetics of High-Temperature Fluorocarbon Chemistry
Authors: Modica Anthony P. , Sillers Sylvia J.
Published: February 2004

Ultraviolet absorption was used to measure the rate of formation of difluoromethylene (CF2) from decomposed CF3I, C2F6, and CF4 in excess argon behind shock waves. In some experiments pure CF4 was shocked Data were taken over a temperature range from 1700° to 3000°K at total concentrations between 2(10−5) and 5(10− 6) mole cm− 3. A chemical, nonequilibrium shock-tube computer program was developed to analyze the CF2 kinetic profiles. By curve fitting the data, rate constants for a number of fluorocarbon reactions are obtained.
Evidence for Structural Transformation in Liquid Octyl Alcohols from PVT Studies
Authors: Johari Gyan P. , Dannhauser Walter
Published: February 2004

The densities of 2- and 3-octanol and 2- and 5-methyl-3-heptanol have been measured over a wide range of temperature and pressure. The isochoric P–T loci of 2- and 3-octanol show definite inflections; those of the latter two compounds are linear in the range of measurement. The data are discussed in terms of a structural transformation involving H-bonded linear chains, dimer rings, and monomer and it is proposed that the inflections characterize the transition from chains to rings and monomer. The proposition is supported by reference to dielectric data and the difference between the two sets of compounds is discussed.
Electron Correlation and Separated-Pair Approximation. An Application to Berylliumlike Atomic Systems
Authors: Miller Kenneth J. , Ruedenberg Klaus
Published: February 2004

A method is developed for determining, with the help of the variation principle, the best antisymmetrized product of separated geminals for a polyatomic system. The strongly orthogonal geminals are determined directly in natural form and the natural orbitals are expressed in terms of Slater-type atomic orbitals. An application to the berylliumlike atomic systems uniformly recovers 90% of the correlation energy. Correction for defects in the K shell of the calculated wavefunctions gives an estimate of 91% for the correlation energy recovered by the best possible separated-pair approximation, corresponding to an absolute error of about 5 kcal in beryllium. The correlation energy recovered is analyzed by partitioning the energy expression into intra- and intergeminal contributions, and both are further resolved into natural orbital contributions. The analysis makes it possible to assess the roles played by the various natural orbitals in lowering the energy and shows certain types of intergeminal as well as intrageminal interactions to be negligible. It also permits the comparison with calculations by other methods. The introduction of additional terms containing interelectronic distances is found to be less efficient than the further addition of natural orbitals.
Electron Correlation and Augmented Separated-Pair Expansion
Authors: Miller Kenneth J. , Ruedenberg Klaus
Published: February 2004

As a refinement over the separated-pair approximation, the augmented separated-pair expansion is developed. The wavefunction is expanded in a series of augmented separated-pair configurations. The leading term is the separated-pair approximation. Each of the higher terms is an antisymmetrized product of separated space geminals and spin harmonics. The spin harmonics are eigenfunctions of S2 and Sz and, moreover, span irreducible one-dimensional representations of the geminal subgroups. The separated space geminals are constructed from the natural orbitals of the separated-pair approximation. It is expected that only relatively few augmented separated-pair configurations contribute significantly and that they can be readily identified. The additional correlation energy recovered by the augmented separated-pair expansion beyond the separated-pair approximation can be classified as correlation between different strongly orthogonal geminals. It can be analyzed by a partitioning of the energy in terms of contributions from the various separated-pair configurations.
Theory of Spin–Lattice Relaxation in Classical Liquids
Authors: Sillescu Hans , Kivelson Daniel
Published: February 2004

A set of coupled differential equations is obtained which represents an exact solution for the high-temperature spin autocorrelation function for spins in a liquid whose motion is governed by a classical isotropic rotational diffusion equation with a single rotational diffusion constant, D. If the diffusion is rapid, i.e., if D is large compared to the spin–lattice interaction, H1, then these equations can be solved by means of a perturbation expansion in (H1 / D). In this case, the dominant terms correspond to those in the well-known Redfield theory; in the absence of spin degeneracy the spectrum consists of Lorentzian lines whose widths T2− 1 are of the order of H12τ2 where τ2 = (6D)− 1, and whose frequencies are shifted by an amount of the order of ω0τ2T2− 1(1 + ω02τ22)− 1 from the Zeeman frequency, where ω0 is a characteristic spectral frequency difference. The present theory introduces a number of corrections: The linewidth should be corrected by terms of the order of ω0(τ2 / T2)3 / 2 (1 + ω02τ22)− 1 and T2− 1(τ2 / T2); the frequency shift should be corrected by terms of the order of T2− 1(τ2 / T2)1 / 2. Furthermore, a number of weak auxiliary Lorentzian lines at frequencies of the order of H1 from the Zeeman frequencies must be included; these lines have intensities which are of the order of (τ2 / T2) below that of the principal “Redfield lines” and their widths are τ2− 1. The superposition of these auxiliary lines on the Redfield lines gives rise to unsymmetrical, non-Lorentzian lines, but in the region (τ2 / T2) < 0.3, where this perturbation expansion is valid, the auxiliary lines contribute little to the central part of the composite lines, but they play a significant role in the wings. The coupled differential equations have been reformulated in order to treat the problem of slow diffusion, (D / H1) ≪ 1. In this case the spin Hamiltonian is diagonalized at each molecular orientation and the diffusion jumps between orientations are treated as a perturbation.
Effects of Anisotropic Molecular Rotational Diffusion on Nuclear Magnetic Relaxation in Liquids
Authors: Huntress Wesley T.
Published: February 2004

The equations for the relaxation time of a nuclear spin in a molecule undergoing diffusion reorientation have been extended to include fully anisotropic reorientation for a number of relaxation mechanisms. The full rotational diffusion tensor—both principal diffusion constants and orientation of the principal rotation axes—can be found by the measurement of the NMR relation times of an appropriate set of nuclei in the molecule.
Vibrational and Rotational Energies for the B 1Σu + , C 1Πu, and a 3Σg +  States of the Hydrogen Molecule
Authors: Kolos W. , Wolniewicz L.
Published: February 2004

An accurate potential-energy curve, including the diagonal corrections for nuclear motion, has been computed for the a 3Σg +  state of H2, and the previously computed potential-energy curve for the B 1Σu +  state has been supplemented with additional energy values for several internuclear distances. For both states, as well as for the C 1Πu state, the Schrödinger equation for nuclear motion has been integrated to yield the vibrational and rotational energies, as well as the rotational constants for all six isotopes of the hydrogen molecule. The reported theoretical results are in a good agreement with experimental values.
Proton Magnetic Resonance Coordination Number Study of Al(III), Be(II), Ga(III), In(III), and Mg(II) in Water and Aqueous Solvent Mixtures
Authors: Fratiello Anthony , Lee Robert E. , Nishida Vone M. , Schuster Ronald E.
Published: February 2004

Cation coordination number measurements of AlCl3, AlBr3, Al(ClO4)3, Al(NO3)3, BeCl2, GaCl3, GA(ClO4), InCl3, In(Cl)4)3, and Mg(ClO4)2, in water, and in aqueous acetone mixtures have been carried out by a proton magnetic resonance method. At low temperatures, solutions of these electrolytes exhibit separate proton resonance signals for bulk water, and water molecules in the first solvation shell of the cation. Integration of these signals provides a direct measure of the coordination number of the cation. The Al(III) and Ga(III) are solvated by six water molecules in all solutions, regardless of the anion present. The Be(II) is coordinated by four water molecules in water and in aqueous acetone mixtures. The In(III) and Mg(II) could only be studied in aqueous acetone mixtures, wherein they are each coordinated by six water molecules. With the possible exception of Be(II) no evidence for contact ion pairing was found.
Elastic Scattering of Light by Binary Mixtures above and below the Critical Solution Temperature. I. Methanol Cyclohexane
Authors: Balta Y. , Gravatt C. C.
Published: February 2004

An investigation was made of the light-scattering properties of the methanol–cyclohexane binary system in the one-phase region above the critical solution temperature Tc and in the two-phase region below Tc. Measurements were made down the critical isochore for T > Tc and along the methanol-rich side of the coexistence curve for T < Tc. A modification of the Ornstein–Zernike–Debye theory of light scattering was found to be in reasonable agreement with the results for T < Tc. The Debye parameter “l” was 14.5 ± 0.5 Å above Tc and 14.0 ± 1.3 Å below Tc. The ratio of the extrapolated zero-angle inverse intensity at any ΔT below Tc to that quantity at the same ΔT above Tc was 3.6. The persistence length for fluctuations below Tc as determined by the Debye theory was of the same order of magnitude as the thickness of the binary interface.
Ion-Pair Theory of Concentrated Electrolytes. II. Approximate Dielectric Response Calculation
Authors: Lovett Ronald , Stillinger Frank H.
Published: February 2004

The theoretical program initiated in the preceding paper is continued. The wavelength-dependent dielectric response function for the “primitive-model” electrolyte, ε(k), is evaluated by means of a self-consistent torque calculation for ion pairs in an external field. The result is used to obtain the longestranged component of the ion-atmosphere charge density. The rigorous second-moment condition on the ion atmosphere then is used to compute crudely the short-ranged components. Besides the usual lowconcentration terms, the implied free energy [Eq. (76)] and activity coefficient [Eq. (77)] contain negative terms varying with the four-thirds power of concentration.
Error Bounds for the Long-Range Forces between Atoms
Authors: Gordon Roy G.
Published: February 2004

A calculation of the long-range (van der Waals-London) forces between atoms is presented. Error bounds on the interaction constant are given when the following properties of the atom are known: (1) the energy of the first excited state, (2) the electron density at the atomic nucleus, (3) the number of electrons in the atom, and (4) the refractive index (polarizability) in the optical region. The error bounds are shown to be the most precise possible, given only the above information. In practice, the accuracy is limited mainly by that of the refractive-index data. The results for rare-gas atoms are in agreement with recent molecular-beam and gas-kinetic measurements.
Shock-Tube Study of the Radiative Combination of Oxygen Atoms by Inverse Predissociation
Authors: Myers B. F. , Bartle E. R.
Published: February 2004

Absolute emission-intensity measurements of the radiative combination of oxygen atoms O(3P) in the temperature range between 2500° and 3800°K are presented. The emission intensity was recorded simultaneously in six spectral intervals in the wavelength range between 2300 and 4511 Å. The absolute emission intensity was found to be proportional to the square of the oxygen atom concentration and to be characterized by an activation energy of 28.9 ± 2.2 kcal mole− 1. These results are interpreted in terms of an inverse predissociation mechanism in which the oxygen atoms combine along a repulsive potential energy surface with a transition to the B 3Σu− state of molecular oxygen and a subsequent radiative transition to the ground electronic state of oxygen. The rate constant for the over-all radiative combination of oxygen atoms was found to be 4.3 × 106exp(− 28 900 ± 2200 / RT) cm3mole− 1·sec− 1 for the wavelength range 2300–5000 Å.
Permeation of Hydrogen through Tungsten and Molybdenum
Authors: Frauenfelder R.
Published: February 2004

Using ultrahigh-vacuum and mass-spectroscopic techniques, permeation of hydrogen through wrought-and vapor-deposited W tubes and through a drawn Mo tube has been measured for pressures between 10−6 and 200 torr and temperatures between 1050° and 2400°K. Permeation rates varied with pressure and temperature in a complex manner. Expressions for the permeation rate are derived on the assumptions that (a) the rate determining step of permeation is diffusion of hydrogen in the bulk of the sample, and (b) the rate of desorption of hydrogen atoms Da, the rate of recombinative desorption of hydrogen molecules Dm, and the concentration of dissolved hydrogen atoms are each determined by the concentration of hydrogen atoms adsorbed on the metallic surface. For the hydrogen–tungsten system, this analysis agrees with measured rates over the entire range of temperature and pressure, and the experimental data is consistent with Hickmott's values for: the rates Da and Dm; the sticking probability for hydrogen atoms; and the probability that an incident hydrogen molecule adsorbs as two atoms. For the hydrogen–molybdenum system, permeation is controlled by phase-boundary processes at low driving pressures. Measured permeation constants follow the equation P = P0exp (− QP / RT) above 1 torr. The results P0 = (1.5 ± 0.5) × 10−3torr·litercm−1·sec−1·torr−1 / 2 and QP = (31.5 ± 1) kcal / mole are found for W. For Mo, the corresponding numbers are (7.1 ± 2) × 10−4 and (21.5 ± 1).
Selection and Focusing of Polar Diatomics in States of Positive Induced Electric Dipole Moment Using a Ten-Pole Field
Authors: Waech T. G. , Kramer K. H. , Bernstein R. B.
Published: February 2004

Using a ten-pole electric field (with two-pole symmetry) it has been possible to select and focus the rotational states J, M = 0, 0; 1, ±1; and 2, ±2 for velocity-selected thermal molecular beams of CsF and RbCl. However, because of the nonideality of this focusing field, the enhancement in intensity of the separated states (focusing effect) over the thermal population is only about a factor of 2.
Use of the CNDO Method in Spectroscopy. II. Five-Membered Rings
Authors: Bene Janet Del , Jaffé Hans H.
Published: February 2004

The modified CNDO method previously reported has been used to calculate the electronic spectra of cyclopentadiene, the cyclopentadienide ion, pyrrole, furan, pyrazole, imidazole, 2-pyrrolecarboxaldehyde, and furfural. In general, the results obtained agree quite well with experimental data. Because the CNDO method treats explicitly all σ and π valence electrons of a molecule, the results of the calculations are used to discuss some of the σ–π interactions which previously could not be treated. The calculations are successful in reflecting changes in the electronic spectra of compounds as a result of extending conjugation or addition of a substituent.
Correlation of Excess Energies of Electron-Impact Dissociations with the Translational Energies of the Products
Authors: Haney Max A. , Franklin J. L.
Published: February 2004

Translational energies of products of unimolecular dissociations resulting from electron impact have been measured at the appearance potentials of the fragment ions. Excess energies for each process were determined from the measured appearance potentials and the known heats of formation of ground-state products and reactants. Results are compared with the predictions of the statistical theory and found to be in agreement if the classical density of states is used. Application of the method to accurate determinations of heats of formation of radicals and radical ions is demonstrated.
Electron–Electron Double Resonance of Free Radicals in Solution
Authors: Hyde James S. , Chien James C. W. , Freed Jack H.
Published: February 2004

The technique of electron–electron double resonance, in which one part of the EPR spectrum of a paramagnetic sample is irradiated with an intense microwave field and the effect of this intense field on other parts of the spectrum is determined utilizing a second weak microwave field, has been applied to free radicals in solution. EPR signals detected by the weak microwave source are reduced in intensity when the two frequencies are separated by an integral number of hyperfine intervals. In some cases this reduction is as much as 40%. A bimodal cavity of novel design capable of supporting the two resonant microwave modes is described. A nitroxide radical has been investigated in greatest detail. Two mechanisms have been identified: rapid nuclear relaxation induced by electron–nuclear dipolar (END) interaction, which is dominant at low concentrations and temperatures, and Heisenberg exchange (HE), dominant at high concentrations and temperatures. A theoretical analysis of the relaxation processes is presented which permits extracting the critical relaxation parameters from the experimental data. The main theme of the paper is a description of the basic effects, but there would appear to be a number of analytical or structural applications of this technique.
Variational Corrections to Decoupling Approximations in Molecular Collision Theory
Authors: Levine R. D.
Published: February 2004

A simple correction to the decoupling approximation is introduced in order to account for the possible occurrence of energy-conserving transitions. The correction, derived from a variational principle for effective potentials, uses only matrix elements and wavefunctions available from decoupling procedure computations to introduce a nonlocal, imaginary part, for the effective potential. The imaginary part is used to compute transition rates for energetically allowed transitions, which are neglected in the decoupling approximation. Explicit results (and error terms) are obtained in the two-state approximation.
One-Center Molecular Calculations with Unlimited Radial Basis. H2, HeH+, and LiH Centered on One Nucleus
Authors: Keefer Jane A. , Fu Julia K. Su , Belford R. Linn
Published: February 2004

As a prelude to one-center studies of the coordination center in ligand-field problems, this paper gives results for LiH, HeH+, and H2 in their ground states. In each case the Hamiltonian and the orbital functions are expanded in series of spherical harmonics about the largest nucleus, the orbital expansions are truncated, and the resulting coupled integro-differential equations are solved numerically for the radial function associated with each orbital harmonic. The result is a Hartree–Fock solution with limited angular basis but unlimited radial basis. The results for energy, quadrupole coupling constant, and other properties are reasonably close to the limiting Hartree–Fock results, and certain properties determined by the wave-function at the nucleus are exact within the accuracy of the numerical method. For H2 at R = 1.4 bohr, the best calculated total energy is − 1.104 hartree for an expansion truncated after four spherical harmonics, s + p + d + f(− 1.128, extrapolated to many harmonics; − 1.133, Hartree–Fock limit); the calculated quadrupole coupling is 247 kHz (242, extrapolated; ∼ 232, experimental). For HeH+, a Morse curve fitted to energies at eight internuclear distances yields Req = 1.480 bohr with E = − 2.919 hartree; for R = 1.4 bohr, E = − 2.917 (− 2.932, Hartree–Fock limit). For LiH, the results with an s + p inner shell and s + p + d + f + g + h outer shell are significantly better than for s + p + d + f inner and outer shells. The best values calculated (Hartree–Fock limit in parentheses) for energy, dipole moment, and electronic field gradient at the Li nucleus are − 7.936 (− 7.987) hartree, − 5.79 (− 5.9) D, and − 0.112 (− 0.116) a.u.
Magnetic Circular Dichroism of IrCl62− in Crystalline (CH3NH3)2SnCl6
Authors: McCaffery A. J. , Schatz P. N. , Lester T. E.
Published: February 2004

The magnetic circular dichroism (MCD) of IrCl62− doped into crystalline (CH3NH3)2 SnCl6 has been studied from liquid helium to room temperature over the spectral region 6000–3000 Å. The results confirm the new assignment (based on solution MCD data) wihch places the 2T1u charge-transfer state at lower energy than the corresponding 2T2u state. The spin–orbit splitting of the 2T2u state which is not apparent in the absorption spectrum or solution MCD shows up strikingly in the crystal MCD illustrating the potentialities of such studies, particularly at low temperatures. The approximate 1 / T variation in the MCD from room temperature to 6°K shows conclusively that the allowed charge-transfer bands in MCD arise primarily from C terms and is the first experimental demonstration over a very wide range of temperature of this theoretically expected temperature dependence. There is no indication of antiferromagnetic coupling, at least down to 6°K.
Stochastic and Deterministic Formulation of Chemical Rate Equations
Authors: Oppenheim I. , Shuler K. E. , Weiss G. H.
Published: February 2004

It is shown, on the basis of some examples, that the commonly used stochastic theory of gas-phase chemical rate equations reduces to the deterministic formulation in the thermodynamic limit, N → ∞, V → ∞, N / V fixed. Since the commonly used deterministic collision theory of chemical kinetics is derivable from the Boltzmann equation with reactive scattering terms, the stochastic formulation of chemical kinetics is thus shown to be equivalent to the results of the Boltzmann equation in the thermodynamic limit. However, since the stochastic theory has not been derived from the Liouville equation for finite systems, the validity of the calculations of the deviation of the stochastic mean from the deterministric results and the fluctuations about that mean for finite N has not been established.
Multistructure Valence-Bond and Atoms-in-Molecules Calculations for LiF, F2, and F2−
Authors: Balint-Kurti G. G. , Karplus M.
Published: February 2004

Multistructure valence-bond calculations with nonorthogonal atomic orbitals have been performed for LiF, F2, and F2−. The basis set consisted of approximate Hartree–Fock functions for the neutral atoms constructed from linear combinations of Gaussian-type orbitals. Several empirical approaches for estimating an intra-atomic correction are examined, and a new method is found which yields reasonable results. The new scheme, which is a modification of Moffit's “atoms-in-molecules” method, is thought to be somewhat preferable to the previously proposed ICC scheme of Hurley, as it gives more reasonable results in the limits of both small and large internuclear distances. The attributes of the various methods are discussed and illustrated by means of a simple H2 calculation, as well as by application to LiF, F2, and F2−. A dissociation energy for F2− slightly larger than that for F2 is obtained; this surprising result, which clearly requires further investigation, is briefly discussed.
Simple Bond-Charge Model for Potential-Energy Curves of Heteronuclear Diatomic Molecules
Authors: Borkman Raymond F. , Simons Gary , Parr Robert G.
Published: February 2004

A model for the vibrational potential-energy functions of diatomic molecules, previously applied to homonuclear molecules only, is extended to 54 heteronuclear diatomic molecules in 93 different electronic states. Experimental Re and ke values are used to determine for each species the empirical bond-charge and bond-length parameters, q and ν, in the homopolar model potential W = W0 + (e2 / R) (Z2 − 4Zq) + (1 / R2)[(h2 / 8m) (q / ν2]. This potential is known to be reasonably accurate for R near Re. The molecular virial theorem requires that the term proportional to R−1 represent electronic potential energy; this is modeled by supposing that an electronic charge −qe is at the bond center, and a net charge of +Ze is at each nucleus, with q = 2Z for neutral molecules. The virial theorem also requires that the term proportional to R−2 represent electronic kinetic energy; this is modeled by supposing that the q electrons move free-electron-like in a one-dimensional box of length νR. As in the homonuclear case, the parameters ν are found to vary little from state to state in a given molecule, or through a given row in the periodic table. It is shown that heteronuclear νAB values can be estimated from homonuclear ν values using formulas like νAB(RAA + RBB) = νAARAA + νBBRBB, and it is shown further that νAB values are measures of the core (ionic) radii of atoms A and B in a molecule. Values of the parameters q = (4Re3ke / 7e2)1/2 correlate nicely with other, more conventional measures of bond population or bond order. Alternative, more complicated, heteropolar models for a heteronuclear diatomic molecule are examined. These models involve different charge parameters ZA and ZB on the two nuclei, and location of the bond charge q elsewhere than at the bond center. Predicted q and ν values, and hence the vibrational force constants, are shown to be insensitive to choice of the model. Values for the molecular electric dipole and quadrupole moments are more sensitive to the model, however. Arguments are presented for preferring the homopolar model, with ZA = ZB and q at the bond center, as the zero-order model for describing molecular vibrations.
Calculation of Lower Bounds to Energies of Molecular Systems. I. Mathematical Methods and Energy Variances for Simple Systems
Authors: Keaveny Ian T. , Christoffersen Ralph E.
Published: February 2004

The energy variance, which provides a sensitive test for any proposed wavefunction, has been calculated for a series of Gaussian-type wavefunctions for the systems H2+, H2, HeH+, He2++, and H3+. The integrals that arise over the square of the Hamiltonian can be expressed either in closed form, or involving a single numerical integration. The magnitude of the values obtained for the energy variance, which should be zero for an exact solution, are consistent with other suggestions that the operator H2 magnifies the poor features of trial functions. Moreover, for the two-electron systems, an evaluation of the three components of 〈H2〉, 〈T2〉, 〈TV + VT〉, and 〈V2〉, shows by their lack of convergence that they all contribute to this result. A further examination of these wavefunctions is made by a calculation of the net forces acting on the nuclei.
Matrix Infrared Spectrum and Bonding in the Dichloromethyl Radical
Authors: Carver T. Granville , Andrews Lester
Published: February 2004

Simultaneous deposition of chloroform and lithium atoms at high dilution in argon on a CsI window at 15°K produces new infrared absorptions which are attributed to lithium chloride and the dichloromethyl radical. The identity of dichloromethyl is provided by comparison of spectra recorded after reactions of HCCl3, DCCl3, and HCCl2Br with lithium and sodium and the detection of (HCCl2)2 following diffusion and the decrease in HCCl2 absorptions. The new absorptions are assigned to the antisymmetric H–C–Cl bending and C–Cl stretching vibrations. These vibrational assignments are supported by product-rule and normal-coordinate calculations which give the potential constants F55 = 3.60 ± 0.10 mdyn/Å, F56 = 0.35 ± 0.01 mdyn/rad, and F66 = 0.56 ± 0.05 mdyn·Å/rad2. The carbon–chlorine valence force constant deduced here exceeds normal C–Cl values, while bond dissociation energies indicate that HCCl2 is electronically stabilized. Similar results for bromomethyl radicals, in contrast to fluoromethyl radicals, suggest that (p–d)π bonding between the free-radical carbon orbital and the d orbitals on chlorine and bromine may account for the stabilization of chloromethyl and bromomethyl radicals, which cannot occur for fluoromethyl radicals or molecules with completely satisfied valence.
Photoionization of High-Temperature Vapors. VI. S2, Se2, and Te2
Authors: Berkowitz J. , Chupka W. A.
Published: February 2004

From the photoionization-efficiency curves of S2, Se2, and Te2, the thresholds for formation of parent and fragment ions have been determined. The ionization potentials of Se2 and Te2 obtained from these measurements are 8.88 ± 0.03 and 8.29 ± 0.03 eV, respectively. For D00(S2), the value 101.0 ± 0.2 kcal/mole is deduced, in general agreement with other techniques. The result D00(Te2) = 62.3 ± 0.2 kcal/mole distinctly favors one of two proposed thermochemical values. The experimental result D00(Se2) = 78.6 – 83.2 kcal/mole has considerable uncertainty but tends to favor the highest of three spectroscopic possibilities. Evidence is presented for a 0.35 ± 0.04-eV splitting between the 0g+ ground state of Te2 and the 1g state.