bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Excluded Volume on Light Scattered from Flexible Macromolecules
Authors: McIntyre D. , Mazur J. , Wims A. M.
Published: February 2004

The effect of the non-Gaussian behavior of the chain-segment distribution in flexible polymer coils on the angular distribution of scattered light is discussed. The explicit form of a general spherical-segment distribution function, W(r, N)dr = tΓ(3/t)(Γ(5 / t)Γ(3 / t) 〈RN2〉)3/2exp−(Γ(5 / t)r2Γ(3 / t) 〈RN2〉|t/2 r2dr, with different values of t, is used to evaluate the general scattering equation for the particle scattering factor P(θ). P(θ) = (1 / N2) ∑ i ∑ j (sinksrij / ksrij). In addition, the effects of excluded volume on the mean end-to-end chain separation 〈RN2〉 is taken to be of the form 〈RN2〉≈N1+ε, where N is the number of chain steps and ε is a parameter which measures the excluded volume effect on the 〈RN2〉. In this paper, an expansion of the P(θ) function to the first few terms is carried out and numerically evaluated for the dependence of P(θ) on the variable x, where x = ks〈Rexptl,〉 k is the wavenumber of the incident light, s is related to the scattering angle, and 〈Rexptl〉 is the radius of gyration determined from experimental data of the scattering cross section. The computations were carried out for the polymer in good solvent, for which data for ε and t were adopted, based on the lattice model of nonself-intersecting chain, and for polymer in theta solvent, for which Gaussian chain model was adopted. The computations were carried out for various degrees of polydispersity. A comparison was made between the theoretical and the experimental results on a polystyrene sample of molecular weight of 4 × 106 in cyclohexane at 35°, 45°, 55°C and in benzene at 40°C.
Asymptotic Behavior of the Light-Scattering Function of Coiled Molecules
Authors: Mazur J. , McIntyre D. , Wims A. M.
Published: February 2004

This paper discusses the asymptotic solution of the scattering function P(θ) for large x, where x is proportional to the scattering angle, radius of gyration, and the wavenumber. The theoretical model employed for the calculation of the P(θ)-vs-x curves was presented in the preceding paper. The results are discussed with respect to the experimental data, after the results for P(θ) are corrected for various values of polydispersity. A general analysis of the theoretical P(θ)-vs-x curves is presented, and the effect of the shape of the chain-end distribution function on the asymptotic behavior of the light-scattering function is discussed.
Symmetric Gaseous Diffusion Coefficients
Authors: Curtiss C. F.
Published: February 2004

The kinetic theory development of expressions for the diffusion velocities in multicomponent gaseous mixtures is reconsidered. A set of equations for multicomponent diffusion coefficients which are symmetric in the indices is developed. The special cases of two-and three-component systems are considered explicitly.
Temperature Dependence of F-Center Absorption in KBr and NaBr
Authors: Jacobs P. W. M. , Menon A. Krishna
Published: February 2004

The absorption due to F centers in x-irradiated KBr and NaBr has been measured over a wide range of temperatures from 4 to 370°K. In NaBr (which is exceedingly difficult to color) there is also formed a well-isolated M band without the usual difficulty of overlapping R bands and so the temperature dependence of the shape of this M band could also be determined accurately. The difficulty of the overlapping K band has been dealt with in two ways: by Gaussian plots of the untreated data (after background subtraction) and by a computer resolution of the F and K bands. The latter calculations confirm the asymmetry of the K band. The half-widths of the F and M bands obey the usual formula H(T) = H(0)[coth ℏωg / 2kT]1/2 and from the temperature dependence of the half-widths, values of the effective ground-state frequency νg were obtained. To what extent this represents a genuine local mode remains unresolved. There is some evidence for structure in the K band at low temperatures.
Environmental Effects on Phosphorescence. III. Oxygen Quenching of Naphthalene Triplets in Compressed Polymethylmethacrylate
Authors: Baldwin B. A. , Offen H. W.
Published: February 2004

Bimolecular, diffusion-controlled oxygen quenching shortens triplet lifetimes and reduces phosphorescence intensities of organic solutes embedded in polymethylmethacrylate at room temperature. Matrix compression reduces the quenching of naphthalene and naphthalene-d8 triplets until at pressures of about 12 kbar the presence of oxygen is no longer felt and the pressure dependence of the lifetime in the 12–30-kbar range is indistinguishable from measurements on thoroughly deaerated samples. The activation volume for oxygen diffusion in this polymer is approximately +12 cm3/mole, as measured from the phosphorescence lifetimes of naphthalene-d8. The high-pressure results confirm that diffusional quenching is not responsible for the effects of pressure on phosphorescence lifetimes in deaerated polymer matrices.
Self-Consistent Perturbation Theory. I. Finite Perturbation Methods
Authors: Pople J. A. , McIver J. W. , Ostlund N. S.
Published: February 2004

A general method is proposed for quantum-mechanical study of physical properties of molecules involving polarization or distortion of the electronic structure. This consists of the calculation of self-consistent molecular orbital wavefunctions (single determinants) in the presence of small but finite perturbations. The general theory of such methods is presented together with a preliminary discussion of numerical error.
Self-Consistent Perturbation Theory. II. Nuclear-Spin Coupling Constants
Authors: Pople J. A. , McIver J. W. , Ostlund N. S.
Published: February 2004

The finite pertubation method developed in the first paper of this series is applied to isotropic nuclear-spin coupling constants, assuming that only a Fermi contact mechanism couples the electron and nuclear spins. Results for some simple systems are calculated using self-consistent molecular orbital methods involving the neglect of differential overlap, and on the basis of these results, certain points regarding the mechanisms of spin coupling are discussed. A detailed discussion of the sources and magnitudes of the errors is also presented.
Indirect Spin Saturation. III. Solutions of Spin-Density Matrix
Authors: Fung B. M.
Published: February 2004

For homonuclear spin one-half ABn + C systems, in which A and C are in chemical exchange, the effect of a strong irradiation at C on the intensity of the ABn group is studied. Alexander's treatment on spin-density matrix is applied to the double-resonance systems. The matrix equations are solved. The results are compared with experimental data for an AB2 + C system. A new quantity T3 which is dependent upon J, δ, T2, and τ3, is introduced to characterize the spin exchange due to spin–spin interaction.
Ionization of Strong Electrolytes. XI. The Molecular States of Nitric Acid and Perchloric Acid
Authors: Redlich O. , Duerst R. W. , Merbach A.
Published: February 2004

Recent proton magnetic measurements in the ternary systems HNO3–DNO3–water and HClO4–DClO4–water partly confirm older data and furnish new information on the molecular states of the acids, on hydration and solvation of the hydrogen ion, and on the distribution of H in deuterated acids between the ion and water. The dissociation constants of HNO3 and DNO3 between 0° and 65° are derived from the measurements.
Crystal Structures of Anhydrous Rare-Earth Chlorides
Authors: Morosin B.
Published: February 2004

The crystal structures at 298°K of LaCl3, NdCl3, EuCl3, and GdCl3 have been refined by full-matrix least-squares methods using three-dimensional MoKα intensity data. These isomorphous compounds belong to the UCl3-P63 / m structure type; metal atoms occupy symmetry sites at (13, 23, 14) and chloride ions lie on mirror sites at (x, y, 14) with x≅0.39 and y≅0.30. Standard deviations of the interatomic separations do not exceed 1 part in 1000.
Microwave Spectrum, Ring-Puckering Potential Function, and Dipole Moment of Methylenecyclobutane
Authors: Scharpen LeRoy H. , Laurie Victor W.
Published: February 2004

The microwave spectra of methylenecyclobutane in the ground state and in the first six excited states of the ring-puckering vibration have been assigned. The υ = 0 state and υ = 1 state are very close in energy and are strongly coupled giving rise to nonrigid-rotor spectra. Analysis shows the energy difference to be 33 592 MHz. This energy difference together with the vibrational dependence of the rotational constants gives the ring-puckering potential function to be 7.65(X4 − 9.15X2) cm−1 where X is a reduced ring-puckering coordinate. This potential function contains a barrier to ring inversion of 160 ± 40 cm−1. The a component of the dipole moment is found to be 0.514 D in the ground state and to have the ring-puckering vibrational dependence μa = 0.496 + 0.0051 〈X2〉υυ (debye).
Dynamics of the Reaction of N2+ with H2, D2, and HD
Authors: Gentry W. R. , Gislason E. A. , Mahan B. H. , Tsao Chi-wing
Published: February 2004

Product-velocity-vector distributions have been determined for the reactive and inelastic scattering of N2+ by H2, D2, and HD. These distributions show that the reaction proceeds by a direct short-lived interaction rather than by a long-lived collision complex. Most products are scattered in the original direction of the N2+ projectile at a speed somewhat greater than calculated from the ideal stripping model. The internal excitation of N2D+ and N2H+ is very high and decreases somewhat with increasing scattering angle. For HD there is an isotope effect that favors N2H+ by large factors at small scattering angles, and N2D+ by smaller factors at large angles. The N2+ scattered from D2 shows very little elastic component, but does reveal an inelastic process which is probably the collisional dissociation of D2.
One-Center Perturbation Approach to Molecular Electronic Energies. IV. Ten-Electron Molecules of Type MHk
Authors: Kim Hojing , Parr Robert G.
Published: February 2004

Taking the spherically symmetric molecular puff as the zero-order problem, we carry out a perturbation calculation of the energies of the HF, H2O, NH3, and CH4 molecules. The first-order correction function is obtained through the diagonal Sternheimer approximation in which each orbital is perturbed independently by the one-electron perturbing potential. For the energy increment between puff and molecule, the integral Hellmann–Feynman formula is employed. At several M–H distances energies are computed, and for H2O and NH3 the energy also is examined as a function of the H–M–H bond angle. Stretching and bending force constants in H2O, NH3, and CH4 are found to have the right order of magnitude, although computed equilibrium internuclear separations are not so good. An extremely flat potential curve near the minimum of the energy gives a poor equilibrium angular geometry in the case of H2O. The inversion barrier of NH3 is calculated to be 0.0135 a.u. compared with the experimental value of 0.0093 a.u. It is argued that the integral Hellmann–Feynman formula combined with the Sternheimer approximation form a consistent many-electron perturbation method.
Positron Annihilation in Metal–Ammonia Solutions
Authors: Varlashkin Paul G.
Published: February 2004

The momentum distribution of photons from positrons annihilating in liquid sodium–ammonia solutions of several concentrations was found to be concentration independent and the same as that of previously measured LiNH3 liquid solutions. Single-concentration angular correlation measurements made on KNH3 and RbNH3 liquid solutions showed the same momentum distribution as the LiNH3 and NaNH3 liquid solutions. In the case of LiNH3 and CaNH3 solutions, which retain their metallic characteristics even when frozen, freezing leaves the general shape of the momentum distribution essentially unaltered. When a dilute NaNH3 solution was frozen, the angular correlation of photons from positrons annihilating in the solid was found to be identical with that obtained from annihilations in pure solid NH3. This is indicative of precipitation of the sodium at the time of freezing. The angular correlations of photons from positrons annihilating in saturated liquid LiNH3, solid LiNH3, and solid CaNH3 show the same distribution at high momenta together with a concentration-dependent narrow component. As the solvated electron concentration is increased, the narrow component broadens slightly and the percentage narrow component decreases. Interpretation of the data suggests a large percentage of positronium formation.
Variational Approach to Perturbation Theory. II. An Extension of the Minimum Principle
Authors: Kolker H. J.
Published: February 2004

The variational principle used to calculate the interaction of an atom or molecule with a weak electro-magnetic field is cast in a form analogous to the Kato identity of atomic scattering theory. A theorem from the literature of scattering theory is then used to extend the conditions under which the variational approach to perturbation theory becomes a true minimum.
Euler-Angle Reduction for Two-Electron Diatomic Molecules
Authors: Lloyd M. H. , Kalotas T. M.
Published: February 2004

We give a complete Euler-angle reduction of the Schrödinger equation for two-electron diatomic molecules. Explicit account is taken of the electron and nuclear symmetries, and the motion of the nuclei is accounted for exactly. This work is an extension of a previous analysis for three-electron atoms.
RRKM Theory Calculation of the Unimolecular Decomposition of Hexafluoroethane: Thermal Activation
Authors: Tschuikow-Roux E.
Published: February 2004

The thermal decomposition of hexafluoroethane is discussed in terms of the quantum-statistical RRKM theory of unimolecular reactions. The falloff behavior is calculated at several temperatures using the approximate rotation–vibration energy-level sums of Whitten and Rabinovitch. A very loose, Gorin-type activated complex model with five active internal rotations is found to be consistent with earlier shock-tube results. Centrifugal effects, associated with a significant extension of the critical bond length in the activated complex, are found to be important and must be taken into account. The approximate method is also applied to C2H6 and satisfactory agreement is found with previous exact computations.
Calculation of Diamagnetic Shielding in Molecules
Authors: Flygare W. H. , Goodisman Jerry
Published: February 2004

The average diamagnetic shielding at a nucleus can be computed from the free-atom shieldings plus a knowledge of the molecular structure. The validity of computing this molecular quantity from free-atom values is related to the isoelectronic principle or the negligible change in dissociation energy with change in nuclear charge (total numbers of electrons and internuclear distances remain constant). The theory is also applied to the determination of the signs of nuclear spin–rotation constants. The signs of the proton spin–rotation constants in methane and acetylene are shown to be positive.
Molecular Reorientation in Liquids and Gases
Authors: Berne B. J. , Pechukas P. , Harp G. D.
Published: February 2004

In this paper two simple criteria for guessing the time-dependent orientational distribution function from the experimentally measured dipolar autocorrelation function 〈u(0)·u(t) 〉 are examined. The first maximizes the information entropy of the distribution under the constraint imposed by the known 〈u(0)·u(t) 〉, and the second minimizes the mean-square difference between the distribution function and the equilibrium (t → ∞) distribution under the same constraint. The accuracy of these criteria is tested on the results of computer studies of the rotational relaxation of diatomic liquids. The same general method is applied to the Van Hove self-correlation function, and to the center-of-mass velocity transition probability.
Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Trichlorosilane. The Infrared Spectrum of the Free Radical SiCl3
Authors: Jacox Marilyn E. , Milligan Dolphus E.
Published: February 2004

In studies of the vacuum-ultraviolet photolysis of HSiCl3 in argon and nitrogen matrices at 14°K, absorptions appearing near 470 and 582 cm−2 have been assigned to the stretching fundamentals of SiCl3. Evidence supporting primary photodecomposition by detachment of the H atom has been obtained in CO matrix experiments, in which HCO is a major product. The observed vibrational fundamentals are consistent with an angle of 72° ± 5° between the threefold axis and each of the Si–Cl bonds. Force constants also have been estimated. The FG-matrix treatment is presented for the stretching modes of a nonplanar X2YZ species with equal XYX and XYZ valence angles. The calculated frequency and intensity pattern for the mixed chlorine isotopic species is in excellent correspondence with the observed pattern, providing strong support for the identification and proposed vibrational assignment. Unstructured absorptions between 2900 and 3300 Å and between 2150 and 2450 Å are tentatively attributed to SiCl3.
Microwave Studies of Collision-Induced Transitions between Rotational Levels. V. “Selection Rules” in NH3–Rare-Gas Collisions
Authors: Oka Takeshi
Published: February 2004

The technique of high-power microwave double resonance has been applied to the study of transitions between rotational levels of ammonia induced by NH3–He, NH3–Ar, and NH3–Xe collisions. Altogether 127 four-level systems have been examined for each of the three collisions. From the analysis of the observed results it is confirmed that the A ↔ A and E ↔ E transitions are “allowed” while the A ↔ E transitions are “forbidden” in accordance with the spin considerations. In addition, the rule Δk = 3n (n; integer) is found where k is a signed value of K. No obvious J or parity selection rules are found. A study of the symmetry of the collisional Hamiltonian leads to an understanding of these rules. It is shown that the Δk = 3 transitions can be caused by the octopole moment Ωxxx − 3Ωxyy of NH3. These considerations lead to the conclusion that NH3–rare-gas collisions are fairly short-range (strong) collisions which cannot be interpreted by a single term in the multipole expansion as has often been done.
Low-Lying Excited States of Mono-olefins
Authors: Yaris Mary , Moscowitz Albert , Berry R. Stephen
Published: February 2004

Semiempirical calculations of energies, oscillator strengths, and rotational strengths for a number of singlet–singlet transitions of ethylene and of the twisted olefin trans-cyclo-octene are compared with experiments. The analysis shows that a minimum of three low-lying singlet states are required to explain the data considered: a B1xu(πz → πz*) responsible for the strong, broad olefinic absorption, a B1zu(πz → σ*)3s Rydberg state responsible for the olefin band with ε≅1000, and a B1xg(πz → πy*)3py Rydberg like state accounting for the low-lying electric quadrupole-allowed transition of ethylene and dominating the optical rotatory properties of the twisted double bond.
Mixed Magnetic Ordering: Magnetic Susceptibility and Mössbauer Studies on Iron(III) Tellurate (Fe2TeO6)
Authors: Dehn J. T. , Newnham R. E. , Mulay L. N.
Published: February 2004

The susceptibility curve of Fe2TeO6 exhibits two peaks, a sharp one at 201°K and a broad one at 233°K. Mössbauer measurements have established the onset of long-range order at 201°K. The broad peak is thought to be due to some form of short-range ordering and two models which might be referred to as layering and pairing are considered as possible explanations. These results are also compared with the evidence of neutron-diffraction experiments. The correlation of these various experiments shows that iron tellurate provides an example of a material with magnetically distinguishable short-range and long-range order. Some aspects of this ordering have been compared with “intramolecular antiferromagnetism,” otherwise known as “isolated clustering” in polymeric coordination compounds.
Solubility and Diffusion of Hydrogen and Deuterium in Platinum
Authors: Ebisuzaki Y. , Kass W. J. , O'Keeffe M.
Published: February 2004

The diffusion and solubility of hydrogen and deuterium in monocrystalline platinum have been studied by a permeation method. The diffusion coefficient of hydrogen is D = (6.0 ± 3.5) × 10−3exp ( − 5.9 ± 1.0 kcal/RT) cm2sec−1, and the solubility in atom fraction H/Pt is given by S = 1.2 × 10−3exp (−11 kcal/RT). The isotope effects in permeation and diffusion (D1 / D2 = 1.16 ± 0.04 at 750°K) are analyzed in terms of an earlier theory. An Appendix discusses quantitatively errors due to absorption of gas by the walls of the apparatus in this type of experiment.
Absorption Spectrum of Carbazole in a Fluorene Matrix
Authors: Bree A. , Zwarich R.
Published: February 2004

The polarized absorption spectrum of carbazole in a single-crystal matrix of fluorene at about 15°K is reported. Only the lowest-energy transition of carbazole was studied, transitions to more highly excited states being obscured by the absorption bands of the matrix. The low-energy transition of carbazole was assigned 1A1←1A1 in agreement with earlier theoretical work. A vibrational analysis of the excited state is reported, and the assigned fundamentals are compared with those of the ground electronic state.
ESR Linewidths in Solution. V. Studies of Spin–Rotational Effects Not Described by Rotational Diffusion Theory
Authors: McClung R. E. D. , Kivelson Daniel
Published: February 2004

We have measured the ESR linewidths of ClO2 as a function of temperature in a number of solvents. The dominant spin relaxation mechanism is the motional modulation of the spin–rotational interaction; therefore, the measurements yield the autocorrelation times for angular momentum. The experimental data can be correlated with the aid of the relation τJα−1 = (8πr03ηκ / Iα) + Δα2[τJβτJγ / (τJβ + τJγ)], where τJα is the autocorrelation time for the component of angular momentum along the αth molecular axis (α = x, y,z), Iα is the moment of inertia about the αth axis, η is the coefficient of viscosity for the solvent, r0 is a translational hydrodynamic radius which is determined experimentally by means of diffusion experiments, Δα is the frequency of precession of the αth component of angular momentum, a frequency which vanishes for spherical tops and linear molecules and can be calculated entirely in terms of known components of the moment of inertia, and κ is an adjustable parameter which is assumed to be independent of temperature and viscosity but dependent upon solvent. There are three such equations for the three correlation times τJα, and they can be combined with linewidth formulas to yield the ESR linewidth as a function of T, η, and κ. The linewidth-versus-(T / η) curves can then be fitted by adjusting the value of the single parameter κ for each solvent. The parameter κ1/2 is a measure of the anisotropy of the intermolecular potentials; it approaches unity for very anisotropic interactions and zero for hard-sphere interactions. A derivation of the equation above is given along with an analysis of the relation between κ and the intermolecular potential. Various corrections to the results are presented together with a critique of the present theory and a comparison with other work.
Vibration–Translational Energy Transfer According to the Morse Potential
Authors: Thompson Samuel L.
Published: February 2004

A quantum-mechanical analysis of molecular vibration–translational motion energy transfer is obtained using the Morse potential, a one-dimensional model, and the method of distorted waves. All wavefunctions and transition matrix elements are obtained in closed form. In the limit of high temperatures, the results are identical to the Landau–Teller expression. At low temperatures, resonances are found to result from the attractive part of the molecular interaction which can lead to transition probabilities that increase with decreasing temperature. The theory is compared with experimental data for the deactivation of O2 by various collision partners. The agreement is excellent, though some uncertainty in the interaction well depths is found due to a lack of very low-temperature data. A comparison with the results of classical and semiclassical theories is given.
Vibrational Spectra and Structure of Organogermanes. IV. Normal Vibrations and Free Rotation in Phenylgermane
Authors: Durig J. R. , Sink C. W. , Turner J. B.
Published: February 2004

The infrared spectra of liquid phenylgermane, phenylgermane-d3, phenylgermane-d5, and phenylgermane-d8 have been recorded from 4000–33 cm−1. The Raman spectra of the liquids have also been recorded and depolarization values have been measured. The vapor-phase spectra show that the germyl group is freely rotating for these molecules. Thus, the local symmetry of the phenyl ring is C2υ and the vibrations have been assigned according to this symmetry. The effective symmetry of the GeH3 and GeD3 groups is essentially C3υ. All spectra have been interpreted in detail, and the 39 fundamental vibrations have been assigned based on previous assignments of benzene and its isotopic derivatives, depolarization ratios, and vapor-phase infrared band contours. The assignments are shown to be consistent by means of the Teller–Redlich product rule and by application of the sum rule. The free rotation of the germyl group shows that the sixfold barrier to internal rotation around the C–Ge bond is negligibly small.
Magnetic-Rotation Spectra of the 1–0 Vibration–Rotation Band of Nitric Oxide
Authors: Keck D. B. , Hause C. D.
Published: February 2004

Magnetic-rotation spectra of the 1–0 band of 14N16O and 15N16O as a function of pressure, path length, and field intensity have been obtained. A phenomenological theory of magnetic-rotation spectra applicable to this case is outlined. Computer predictions made on the basis of this theory, including the effects of finite resolution, explain the major features of the observed magnetic-rotation signals. These features include the doublet structure of the lines at high pressure and the intensity contour of the 2 subband.
Electrical Transport Processes in Beryllium Oxide
Authors: Cline Carl F. , Newkirk Herbert W.
Published: February 2004

The electrical resistivity was determined for polycrystalline and single-crystal BeO between 1200°–1700°C. Analysis was made of the effects of oxygen partial pressure, aliovalent impurities, and microstructure. Conductivity and relevant diffusion coefficients were calculated with these data. The results show the BeO samples to be impurity-controlled ionic conductors, and establish the need for higher-purity material to study intrinsic properties. Electrical migration is independent of grain boundaries and crystallographic direction. Creation of cation defects by varying the oxygen activity in the specimens is not an important process in this material. At high temperatures, above a critical roll-over temperature determined by the impurity content, electrical transport processes occur by a vacancy mechanism with an activation energy of 65 ± 5 kcal/mole. At lower temperatures, the electrical transport process is more complex and no simple theory is applicable. It is suggested that precipitation of impurities may be a more important process than the association of vacancy–impurity complexes in this system. The activation energy for this process depends inversely on the amount of aliovalent impurities present in the sample, and has a value of 73 kcal/mole at the lowest concentration studied.
Statistical Two-Particle Density Matrix. Some Considerations Relating to the Choice of Basis of Representation
Authors: Handler George S.
Published: February 2004

A preliminary examination of the problem of the statistical theory of electronic energies from the view-point of the 2 matrix is made. For the two-particle, one-dimensional harmonic oscillator, a good representation of the Fermi hole is found for the triplet state. The employment of a discrete basis for heliumlike systems leads to problems with the virial theorem.
Perturbed Morse-Oscillator Approximation in Reactive Collisions. I. An Attractive Potential
Authors: Pirkle J. C. , McGee H. A.
Published: February 2004

Reactive collisions in three-center reactions of the type A + BC → AB + C are considered quantum mechanically in terms of a model based upon a perturbed Morse oscillator and a linear-collision complex. The developed formalism is then applied to the reaction H+Br2→HBr+Br using the diatomics-in-molecules method to obtain a potential-energy surface for the reaction. This potential is attractive, that is, releasing the heat of reaction upon the approach of the hydrogen atom. Computed cross sections are larger both for the higher allowed vibrational quantum numbers of the product molecule as well as for the higher rotational states possible for each given vibrational state. These results are in agreement with the trend of semiclassical calculations using a similarly attractive potential. To establish a clear dependence of the cross sections on the vibrational state of HBr, it was necessary to treat the relative translation between HBr and Br as a potential scattering problem rather than assuming free relative motion. The calculated total cross sections were essentially independent of the initial conditions of the collision due to the neglect of activation energy in the Morse basis functions of the perturbed stationary-state expansion.
Modulation Effects in Magnetic Resonance Dispersion-Mode Curves
Authors: Evans Thomas E. , Brey Wallace S.
Published: February 2004

The dependence of the unsaturated Lorentzian dispersion-mode line-shape function on the amplitude of low-frequency sinusoidal modulation is determined in closed form. The results for signal broadening and shape distortion are plotted and tabulated. A comparison is made with experimental points determined using a Mn++-doped water sample.
Forbidden Vibrational Transitions in the Electron-Impact Spectra of Molecular Oxygen and Nitrogen
Authors: Skerbele Ausma , Dillon Michael A. , Lassettre Edwin N.
Published: February 2004

The excitation of forbidden vibrational transitions by electron impact has been studied at 45-eV electron kinetic energy. The relative intensities of these transitions in N2 and O2 referred to the elastic peak, are independent of the scattering angle between 3° and 14°. For comparison we have also included data on vibrational transitions in CO and NO at one angle (4°). These are allowed by electric-dipole selection rules. In molecular oxygen we have also observed the forbidden electronic transitions a 1Δg ← X 3Σg− and b 1Σg+ ← X 3Σg− although the latter can be barely detected above background.
Theory of the Stimulated Raman Effect
Authors: Philpott M. R.
Published: February 2004

The matrix element describing the fourth-order process in which two photons are simultaneously scattered by a molecule is calculated with the help of time-ordered graphs. The two incident and two scattered photons are assumed to have different frequencies; only dipole allowed transitions are considered; and the initial and final molecular levels are taken to be the same. In the special case where the incident photons have the same frequency, the result of Buckingham [J. Chem. Phys. 43, 25 (1965)] is recovered. The scattered photons have a continuous spectrum sharply peaked around the Stokes and anti-Stokes frequencies. If two incident photons excite a higher molecular level, the continuum is enhanced, leading to the possibility of antiresonances near the Stokes and anti-Stokes frequencies.
Self-Consistent Ionic Potentials, Fields, and Field Gradients at the Lattice Sites of Corundum (α-Al2O3)
Authors: Hafner Stefan , Raymond Michael
Published: February 2004

The electrostatic potentials, fields, and field gradients at the lattice sites of corundum (α-Al2O3) have been computed self-consistently, taking into account the ionic dipole and quadrupole moments of the oxygen ions. The dipolar and quadrupolar polarizabilities were used as parameters to fit the nuclear quadrupolar coupling data of 27Al. It is found that the induced quadrupoles make a substantial contribution. The ionic polarizabilities needed for the fit are within reason. The electrostatic field and field gradient at the aluminum sites are predominantly described by a point charge and a quadrupolar lattice, the most probable oxygen quadrupolar polarizability being α = 0.2–0.3 Å. The contribution of the dipolar lattice to the field gradient at the aluminum sites is small due to the accidentally nearly vanishing field at the oxygen positions. Consequently, the field gradient at the aluminum sites is insensitive to the O2− dipolar polarizability. As the potential distribution appears to be almost entirely determined by the particular electronic density distribution of the oxygen ions and as the ionic polarizabilities of O2− are not accurately known, an estimate of the covalent participation in the predominantly ionic bonds of Al2O3 is not possible. The nuclear quadrupole coupling tensor of 17O is estimated by use of the computed field gradient at the oxygen sites.
Melting Transition and Communal Entropy for Hard Spheres
Authors: Hoover William G. , Ree Francis H.
Published: February 2004

In order to confirm the existence of a first-order melting transition for a classical many-body system of hard spheres and to discover the densities of the coexisting phases, we have made a Monte Carlo determination of the pressure and absolute entropy of the hard-sphere solid. We use these solid-phase thermodynamic properties, coupled with known fluid-phase data, to show that the hard-sphere solid, at a density of 0.74 relative to close packing, and the hard-sphere fluid, at a density of 0.67 relative to close packing, satisfy the thermodynamic equilibrium conditions of equal pressure and chemical potential at constant temperature. To get the solid-phase entropy, we integrated the Monte Carlo pressure–volume equation of state for a “single-occupancy” system in which the center of each hard sphere was constrained to occupy its own private cell. Such a system is no different from the ordinary solid at high density, but at low density its entropy and pressure are both lower. The difference in entropy between an unconstrained system of particles and a constrained one, with one particle per cell, is the so-called “communal entropy,” the determination of which has been a fundamental problem in the theory of liquids. Our Monte Carlo measurements show that communal entropy is nearly a linear function of density.
Pressure-Induced Microwave Absorption in N2
Authors: Ho W. , Kaufman I. A. , Thaddeus P.
Published: February 2004

Pressure-induced absorption in nitrogen has been studied at a frequency of 9260 MHz over the temperature range 238°–495°K and to pressures as high as 135 atm. Expressing the dielectric loss ε″ in the form, ε″ / ν̄ = Aρ2 where ν̄ is the frequency in cm−1, and ρ is the density, we find A = 1.7(1) × 10−10(T / 273)−2.5(2)cm amagat−2. The absorption coefficient α, in cm−1 is then α = 1.07(6) × 10−9(T / 273)−2.5(2)ν̄2ρ2. The molecular quadrupole moment for N2, calculated via the Kramers–Kronig integral, using the result of the present investigation as the low-frequency limit of α(ν̄) / ν̄2 and the far-infrared result of Bosomworth and Gush for N2 at 300°K, is then 1.5 × 10−26 esu.
Interactions between Molecules in the Statistical Treatment of Real Gases. II. Derivation of van der Waals Interactions
Authors: Baer Shalom
Published: February 2004

The current perturbation theory for classical real gases, which views interaction energies between pairs, triplets, etc., of molecules, given in advance, as a perturbation to the ideal-gas behavior, is replaced by a systematic perturbation expansion in terms of the Coulomb interaction between the electrons and nuclei in the different molecules and the intramolecular correlations of these particles in the isolated molecules. The latter refer to correlations at different imaginary times t = − iτ, where 0 ≤ τ ≤ β with β = 1 / kT the reciprocal temperature. The Fourier transform of the pair correlation, expressed as a function of τ, is directly related to the susceptibility of the molecule at imaginary frequencies. In the expansion of the grand partition function the terms containing only particle pair correlations represent the contributions of molecular interactions due to the familiar additive and nonadditive van der Waals forces.
Applications of Diatomics-in-Molecules Theory. II. Prediction of a Stable Li3 Molecule
Authors: Companion Audrey L. , Steible Daniel J. , Starshak Albert J.
Published: February 2004

Diatomics-in-molecules theory applied to the Li3 molecule leads to the prediction of two forms stable with respect to Li2(1Σg+) + Li(2S) : a bent 2A1 state with an atomization energy at 0°K of 32.74 kcal/mole and an ionization energy of about 111 kcal/mole; a strongly bent 2B2 state with an atomization energy at 0°K of 30.34 kcal/mole. Potential-energy surfaces and fundamental vibrational frequencies are predicted for both states. Characteristics of the less stable linear conformation are discussed and compared with H3.
Electron Spin Resonance of Mn2+ in α-Sr2P2O7
Authors: Calvo C. , Leung K. Y. , Stager C. V.
Published: February 2004

Single crystals of Mn2+-doped α-Sr2P2O7 have been studied by electron spin resonance at 35 Gc/sec. The space-group ambiguity has been resolved because the Mn2+ ions lie on a mirror plane, and therefore the unit cell must be centric. The spin-Hamilitonian parameters have been determined to be gx = 2.0021(10), gy = 2.0015(4), gz = 2.0012(4), b20 = − 177.1(10) G, b22 = − 118.5(46) G, Ax = − 91.9(10) G, Ay = − 92.8(13) G, and Az = − 90.8(7) G. The z principal axis lies in the ab plane 17° from the b crystallographic axis. Only one EPR active site has been found rather than the two anticipated on crystallographic grounds. No evidence of a crystallographic phase transformation was found in the temperature range 100°–300°K.
Photolysis of CO2 in Far Ultraviolet
Authors: Sethi Dhanwant S. , Taylor H. Austin
Published: February 2004

The photolysis of CO2 in both static and flow systems at 1470 and 1236 Å, from room temperature up to 100°C, shows CO/O2 ratios decreasing with increasing pressure of CO2, leveling off at a value ∼5, which is then independent of both temperature and pressure. The data, obtained by mass-spectral analysis of the products after unreacted CO2 had been condensed, show no evidence of masses greater than 44. Infrared spectra show no evidence of CO3 in the gas phase even when the products were condensed on solid CO2. It is shown that CO2 can be slowly desorbed by pumping from a reaction vessel in which CO2 has been photolyzed and, if heated, H2O. It is proposed that the reaction: O(1D)+H2O(ads)→2OH(ads) occurs on the walls and that CO2 is adsorbed on OH(ads) yielding a surface bicarbonate radical HCO3(ads). It is suggested that this species could be that currently considered to be CO3.
Equilibrium Properties of a System of Charged Particles
Authors: Stell G. , Lebowitz J. L.
Published: February 2004

Motivated by the formal identification that can be made between the electronic charge e and a range parameter γ that we have previously studied, we have developed high-temperature, high-density expansions of the Helmholtz free energy and potentials of average force for systems of ions with repulsive cores. It appears that the natural ordering parameter for our free energy expansion is κR, where κ is the inverse Debye length and R an effective core diameter, and as a result the expansion is useful for either sufficiently high temperatures at any density or sufficiently low densities at any temperature. We find the terms in this expansion through order (κR)7. We have also found effects that will serve to modify the shielding length at high densities so that it is no longer just κ−1. One such effect depends upon the disparity in the core sizes of the positive and negative ions, and it vanishes only when the disparity does. A higher-order modification of the shielding length that remains in this symmetric case is also discussed.
Solutions of Boltzmann Equation and Transport Processes
Authors: Desai Rashmi C. , Ross John
Published: February 2004

An integral approximation (IA) method is proposed for the solution of certain integro-differential equations of which the linearized Boltzmann equation is one example. The lowest-order solution in this method consists of replacing the integral operator of the equation by a known function such that the solution has the correct initial value, correct initial slope in time, and correct behavior at large times. The deviation of the integral operator from the function is treated as a perturbation in higher orders. The method is applied as an example to the calculation of time correlation functions and thermal transport coefficients. Deviations from the exponential behavior of the correlation functions are explicitly evaluated. Another method of solution which involves a cumulant expansion (CU) is also used for the evaluation of these quantities. Both methods are then compared with the Chapman–Enskog (CE) method. The IA method provides a better physical approximation and better numerical estimates for the thermal transport coefficients than the CU or CE methods.
Gaussian Basis Sets for Model Anharmonic Oscillator Systems
Authors: Chesick John P.
Published: February 2004

Basis sets of equally spaced Gaussian functions are used to obtain diatomic Morse and harmonic-oscillator wavefunctions for CH and CN species. These functions are then used to obtain energy levels and eigen-functions for model linear HCN and CH2 species with both harmonic and Morse potentials for two internal coordinates.
High-Pressure Mössbauer Studies of 57Fe in Silicate and Phosphate Glasses
Authors: Lewis G. K. , Drickamer H. G.
Published: February 2004

The effect of pressure to 200 kbars has been measured on the Mössbauer resonance of 57Fe in a sodium silicate and a sodium phosphate glass. In the silicate glass, the Fe(III) ion is in a tetrahedral site. The isomer shift goes through a maximum (i.e., a minimum in the electron density at the nucleus) with increasing pressure. This is interpreted in terms of competition between the decrease in shielding of the 3s electrons by the 3d electrons and changes in orbital occupation. The octahedral Fe(II) sites also show a combination of site compression, site distortion, and change in orbital occupation with pressure. Both Fe(III) and Fe(II) sites in phosphate glass are octahedral. Both in regard to change of isomer shift and change of quadrupole splitting, these sites behave qualitatively like those in crystalline FePO4 and Fe3(PO4)2, which were previously studied. The Fe(III) ions tend to reduce to Fe(II) with pressure, just as occurs in a large number of crystalline ferric compounds.
Dissociation and Two-Body Emission in Shock-Heated Bromine. I. Br2 in Argon
Authors: Boyd R. K. , Burns George , Lawrence T. R. , Lippiatt J. H.
Published: February 2004

The dissociation of Br2 in the presence of an excess of argon was studied in shock waves using the two-body emission (2Br → Br2(1Σg+) + hν) to monitor the Br atom concentration. At 5980 Å, this emission was found to be due to 1Πu−1Σg+ transition which is also the most intense transition observed in the visible absorption spectrum of Br2. This new method of dissociation rate constant measurements is particularly insensitive to the state of internal excitation of Br2. Moreover, the method is advantageous since it involves direct determination of [Br] as a function of time; in all previous studies of Br2 dissociation, the concentration of Br atoms was calculated as a small difference between two large concentrations of Br2. The dissociation rate constants, kd's, were calculated from the initial slopes of the square-root intensity versus time plots. These constants, when related to the kr's, recombination rate constants, using the equilibrium constant Kc, may be expressed by an equation log10[kr(liter2mole−2·sec−1)] = − 104.027 + 71.507 logT − 11.378 (logT)2, which is valid between 1350° and 2400°K. Above 1500°K, these results agree with six independent published measurements of kd obtained from the optical absorption versus time plots. The agreement is gratifying, and the analysis of our experimental procedure suggests that it is not accidental. However, below 1500°K, the “emission” rate constants are smaller than the “absorption” rate constants. On the other hand, the “emission” rate constants at low shock temperatures are in agreement with kr's obtained recently by flash photolysis between 1000° and 1273°K [J. K. K. Ip and G. Burns, Discussions Faraday Soc. 44, 241 (1967)]. The discrepancy between “emission” and “absorption” rates appears to be real, and cannot be explained by the presence of impurities, boundary layer effects, or other experimental difficulties. It may be explained, however, if it is assumed that the internal degrees of freedom of Br2 were not completely relaxed in shock-wave experiments below 1500°K. Although this effect has not been observed with other diatomic molecules, it is suggested that this is possible, and that indirect evidence for the existence of this phenomenon exists for chlorine and iodine. A computer program was developed to describe the whole reaction profile behind the shock front for emission experiments. Using this program, kd,Br, the dissociation rate constant in the presence of Br atom as a collision partner, has been calculated. The value of kd,Br was found to be extremely sensitive to even minute traces of impurities, but the most reliable data based on measurements yield kd,Br / Kc(liter2mole−2·sec−1) = 8.8 × 1032T−7.3. This equation is valid between 1600° and 2240°K.
Dissociation and Two-Body Emission in Shock-Heated Bromine. II. Pure Br2
Authors: Boyd R. K. , Brown J. D. , Burns George , Lippiatt J. H.
Published: February 2004

Two-body emission from bromine atoms has been used to measure the rate of dissociation of molecular bromine behind shock waves in pure Br2 between 1000° and 2985°. Initial slope rate constants, kd,Br2, agree with those obtained previously [H. B. Palmer and D. F. Hornig, J. Chem. Phys. 26, 98 (1957)] by a different method of measurement. Present experiments extend the temperature range over which kd,Br2 was measured by a factor of 5. If present results are combined with those of Palmer and Hornig, the rate constant may be expressed by the equation kd,Br2 / Kc = 3.6 × 1026 × T−5.6liters2moles−2·sec−1, where Kc is the equilibrium constant for the reaction Br2⇋Br. Values of kd,Br, the dissociation rate constant of Br2 with Br as a collision partner, obtained from studies of complete emission profiles, have been measured between 1000° and 2985°K, and agree well with the most reliable values obtained previously [R. K. Boyd, George Burns, T. R. Lawrence, and J. H. Lippiatt (preceding paper)] from experiments using bromine–argon mixtures between 1600° and 2240°K. Thus, present experiments extend the temperature range over which kd,Br was measured by a factor of 3. The combined results for kd,Br are expressed by the equation kd,Br / Kc = 1.7 × 1036 × T−8.4liters2moles−2·sec−1. The effects of the boundary layer on rate measurements were studied using the theory of Mirels [Phys. Fluids 6, 1201 (1963); 9, 1907 (1966)]. The uncertainties in the rate constants, associated with dissipative effects at the shock-tube wall, were found to be small in the present apparatus. It has also been possible to show that nonideal behavior, occasionally observed in the present work, was due to anomalously high attenuations of the shock velocity.