bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical Opalescence of the Polystyrene—Cyclohexane System: Small-Angle X-Ray Scattering
Authors: Debye P. , Bashaw J. , Chu B. , Creti D. M. Tan
Published: January 2005

The critical opalescence of polystyrene—cyclohexane mixtures has been studied by means of small-angle x-ray scattering for polystyrene samples of molecular weight 118 000 and 147 000. The upper limit of s/λ (s = 2 sin½θ, where θ is the scattering angle, and λ is the wavelength in the medium) was extended to 0.025 Å−1, approximately 50 times greater than the upper limit accessible to visible light. At large values of s/λ, plots of the reciprocal of the intensity vs (s/λ)2 showed downward curvature. The experimental results can be represented by I = [A+B(s/λ)2]−1+C. The magnitude of the ratio of the two terms in the expression representing the intensity is consistent with the conclusion that the constant term C represents the scattering from density fluctuations, and the angular distribution of the scattering caused by concentration fluctuations is accurately represented by [A+B(s/λ)2]−1 over the range of s/λ investigated for the polystyrene—cyclohexane system. However, this is not true for mixtures of ordinary liquids for which the constant term is much too large to be interpreted as a density scattering term.
On the Bonding Energy in Cesium—Graphite Compounds
Authors: Salzano F. J. , Aronson S.
Published: January 2005

A model is proposed for the bonding of cesium in the cesium—graphite lamellar compounds. The model assumes that the compounds consist of layers of cesium ions situated between metallike sheets corresponding to the graphitic layer planes. The bonding energy of the cesium to the graphitic layers results from the electrostatic interaction of the cesium ions with the metallike sheets and can be calculated by means of image force theory. An independent value for the bonding energy is also derived from thermodynamic data on the basis of a semiempirical reaction mechanism. This mechanism resolves the heats of formation into three elementary energy parameters; the bonding energy of cesium to the adjacent graphitic layer planes; the energy to separate graphitic layer planes; and the energy of interaction of the cesium ions with neighboring layers of cesium ions. The values of the bonding energy of cesium obtained by the two methods are in good agreement. The energy to separate graphitic layer planes is compared to values obtained in other studies.
Hydrodynamics of Linear Macromolecules. I. Finite Segment Length
Authors: Reinhold Chr. , Peterlin A.
Published: January 2005

According to the ideal necklace model for the linear macromolecule, as introduced by Rouse, the macromolecule and its segments expand to infinite length if subjected to larger and larger shear stress. A more realistic model has to take into account that the segment has a finite maximum length b∞ which cannot be exceeded even at largest stress. Therefore, the elastic restoring forces transmitted by the segment and acting on the ends of the segment have to be assumed to increase more than linearly with the distance b and to become infinitely large at b∞, yielding a modification of the eigenvalues of the corresponding restoring-force matrix. The eigenfunctions are considered to be the same as in the original Rouse model. With the new set of eigenvalues the segment deformation of the linear molecule in laminar flow is claculated as a function of the flow parameter β. Saturation effects show up which are missing in the original model. The results obtained are formally identical with those of the original Rouse model if β is replaced by a modified β*=β/(E0*+½R). E0* is the nonlinearity factor of restoring force in solution at rest depending on the b∞/b0 ratio, and R is the maximum increase of this factor taking place in the central segment of the model and depending on β. Intrinsic viscosity and birefringence extinction angle are calculated for some b∞/b0 ratios and compared with the corresponding values of the dumbbell model.
Molecular Ions in Radiation Chemistry. III. Absorption Spectra of Aromatic-Hydrocarbon Cations and Anions in Organic Glasses
Authors: Shida Tadamasa , Hamill William H.
Published: January 2005

Aromatic hydrocarbons (ArH) in γ-irradiated, glassy butyl chloride at 77°K are efficient traps for migrating positive holes, forming the radical cations of 40 compounds in good yield. No ArH− ions appeared to form. Photobleaching is very inefficient, or nil. Resonance charge transfer and simple competition for holes by ArH, solvent, and anions are consistent with observation. In glassy methyltetrahydrofuran, ArH gave ArH− in good yields with very little ArH+. In 3-methylpentane, both ArH+ and ArH− were formed. Addition of propyl chloride enhanced ArH+ and suppressed ArH−. Addition of ethanol had the reverse effect.
Structure in the Photoionization Continuum of SF6 below 630 Å
Authors: Codling K.
Published: January 2005

The absorption spectrum of neutral SF6 has been studied in the far-ultraviolet spectral region (100 to 630 Å) using the pure continuum radiated by the NBS 180-MeV electron synchrotron as a background source. A well-developed vibrational progression was observed in the 550–600-Å region. A Rydberg series of neutral SF6 was observed in the 450–500-Å region, converging to a limit at 26.83 (±0.04) eV. Additional absorption features were evident with peaks at 435 and 350 Å. Absolute absorption cross-section measurements were made at 539, 584, and 704 Å. The values obtained were 170, 70 and 100 Mb, respectively; the error in these measurements is thought to be no more than 25%.
Photoionization Efficiency Curve for SF6 in the Wavelength Region 1050 to 600 Å
Authors: Dibeler Vernon H. , Walker James A.
Published: January 2005

The threshold energy of SF5+ at 811 Å (15.29 eV) has been determined by photoionization. An electronically excited state with a threshold at 750 Å (16.53 eV) resulting from partially resolved autoionizing Rydberg levels in the region of 775 Å has been observed also.
Diffusion in Organic Crystals. II. Lattice and Subgrain-Boundary Diffusion
Authors: Reucroft P. J. , Kevorkian H. K. , Labes M. M.
Published: January 2005

Self-diffusion studies at extended diffusion anneal times and impurity (phenanthrene) diffusion studies in melt-grown anthracene crystals have shown evidence for a fast subgrain-boundary-diffusion process in addition to a much slower lattice diffusion. Subgrain boundary diffusion is faster by a factor of 106−107, the activation energy being 25% to 66% the value for lattice diffusion.

Phenanthrene diffusion is very similar to self-diffusion in terms of diffusivity magnitude and activation energy. Diffusion anisotropy is more marked for phenanthrene diffusion, diffusivity perpendicular to the ac plane being greater than that perpendicular to the ab plane.
Do Exciton States Exist in the Liquid Phase?
Authors: Rice Stuart A. , Jortner Joshua
Published: January 2005

In this note we conjecture that a sufficient condition for the existence of Wannier exciton states in a dense liquid is that an excess electron be accurately describable as a plane wave. Scattering processes then lead to a broadening of the exciton level. A theory of electron scattering in simple liquids is used to estimate linewidths, and the predictions are consistent with the data available. A number of predictions resulting from, and implications of, the proposed mechanism are considered.
Sudden Approximation Applied to Rotational Excitation of Molecules by Atoms. II. Scattering of Polar Diatomics
Authors: Bernstein R. B. , Kramer K. H.
Published: January 2005

The sudden approximation is applied to the computation of rotational transition probabilities and inelastic total cross sections for the scattering of polar and nonpolar diatomic molecules (rigid rotors) by atoms. The calculations are based upon an interaction potential which includes both short- and long-range anisotropies. At thermal energies the long-range attractive parts of the potential are of principal importance in determining the inelasticity, with a significant contribution arising from the quadrupole interaction term (varying as R−7 cos3Θ). The computations show the rapid onset of ``dominant-coupling'' behavior (randomization of transition probability among all close-coupled states) for impact parameters below about (S/π)½, where S is the total inelastic cross section.
Steady-State Studies in the Conventional Verneuil Process—the Trajection (Melting) Zone
Authors: Pastor R. C. , Pastor A. C. , Kimura H. , Arita K.
Published: January 2005

In the conventional Verneuil growth of ruby, it is shown that certain parameters pertinent to the final concentration of the additive in the crystal product (solid solution) are: starting concentration, surface distribution of additive in the powder, trajection distance, and growth rate. The effects of these factors are studied at the trajection (or melting) zone.
Neutron-Diffraction Study of the Structures of Ferroelectric and Paraelectric Ammonium Sulfate
Authors: Schlemper Elmer O. , Hamilton Walter C.
Published: January 2005

Three-dimensional neutron-diffraction data have been collected for both the room-temperature (paraelectric) and the low-temperature (ferroelectric) phases of ammonium sulfate. Both phases are ordered. The transition, occurring at 223°K and involving a change from Space Group Pnam at room temperature to Space Group Pna21 in the ferroelectric phase, results in somewhat stronger hydrogen bonds in the ferroelectric phase, although there is no dramatic change in the barrier to rotation of the ammonium ions as a result of the transition. In both phases the two crystallographically independent ammonium ions have quite similar rotational barriers and hydrogen-bond strengths. The transition also results in less distorted ammonium ions in the ferroelectric phase. At room temperature, there are only two hydrogen bonds with O···H distances less than 2.0 Å. In the ferroelectric phase there are six. The mean distance is decreased by 0.1 Å in the ferroelectric phase, although the range of distances remains the same. The principal effect is thus a strengthening of the hydrogen bonds of intermediate length; this is accomplished by a reorientation of the ammonium groups. The shortest bonds at room temperature are not necessarily the shortest at low temperature.
Rotational States of a Tetrahedron in a Cubic Crystal Field
Authors: King Harry F. , Hornig Donald F.
Published: January 2005

The Schrödinger equation is solved for the rotational states of a rigid cube whose center of mass is fixed at a point of symmetry in an external field. This is shown to be equivalent to the equation of motion for a regular tetrahedron in a field with symmetry Oh. The quantum states are classified under the direct product group Ō×O, where Ō is an octahedral group of rotations about body-fixed axes and O is a similar but distinct group of rotations about space-fixed axes. The wavefunctions are obtained by an expansion in a series of symmetry-adapted linear combinations of spherical-top functions. Projection operators are defined which generate the various linear combinations. Closed-form expressions are obtained for the matrix elements of the Hamiltonian and of the projection operators. Upper and lower bounds are computed for representative energy levels. The results demonstrate that using basis sets of practical size, high accuracy can be obtained. This analysis forms the basis of a more general theory of rotation of small molecules and polyatomic ions in crystals.
Thermal Energy Ion—Neutral Reaction Rates. IV. Nitrogen Ion Charge-Transfer Reactions with CO and CO2
Authors: Fehsenfeld F. C. , Schmeltekopf A. L. , Ferguson E. E.
Published: January 2005

Charge-transfer reaction rates have been measured for N+ and N2+ ions with CO and CO2 in a pulsed flowing afterglow system. The rates are all very close to 10−9 cm3/sec at 300°K, corresponding to essentially a charge transfer on every momentum-transfer collision.
Magneto-Optical Rotatory Dispersion of Porphyrins and Phthalocyanines
Authors: Stephens P. J. , Suëtaak W. , Schatz P. N.
Published: January 2005

Recent MORD data in the region of the visible and near-uv absorption bands of some porphyrins and phthalocyanines are analysed. New computer programs are used to fit general formulas to the experimental data and to extract molecular parameters. Theoretical expressions for the latter are derived, adopting the current interpretation of the observed transitions. The over-all agreement with experiment is good, supporting the assignments. The theory also shows that the magnetic moments of the excited states can be obtained from the data. Values obtained agree with a priori calculations in order of magnitude. Further experimental and theoretical work is suggested.
Reaction of HD+ with Rare Gases
Authors: Berta M. A. , Ellis B. Y. , Koski W. S.
Published: January 2005

The reactions of HD+ ions with He, Ne, Ar, and Kr have been studied in a tandem mass spectrometer over an energy range from 2 to 25 eV. The observed isotope effect was interpreted in terms of complex formation using the polarization model at low energies and the stripping model of ion—molecule reactions at higher energies.
Van der Waals Coefficients for the Alkali Halides from Optical Data
Authors: Hajj Fuad
Published: January 2005

The van der Waals coefficients that arise from the dipole—dipole interaction of ions in the alkali halides are discussed and computed for the negative—negative, positive—positive, and negative—positive interactions of the 20 salts. Also the polarizabilities of the negative ions and the optical strength per ion are computed. The computation was mainly based on data of the absorption coefficient for frequencies corresponding up to 11.4 eV published by Eby et al. The van der Waals coefficients are compared with those of Mayer (1933) and the polarizabilities with the values given by Ruffa (1962), Tessman et al. (1953), and Pauling (1927); agreement was found to be satisfactory for both sets of values.
Electron Spin Resonance Linewidths—Magnetic-Field Correction
Authors: Kaplan Jerome I.
Published: January 2005

A method is given to modify the expression obtained by Van Vleck for the moments of the electron spin resonance linewidth of a system of spins arranged on a lattice and coupled by dipole—dipole interactions so as to include a finite magnetic-field dependence. His original expression was for the case H0→∞. An explicit expression is derived for the lowest-order non H0→∞ correction. As an example the zero-order moment is evaluated. Finally it is shown how the method can be generalized to the case of a non-Zeeman local spin Hamiltonian.
Force Fields in KrF2 and XeF2
Authors: Coulson C. A.
Published: January 2005

A possible simple explanation is given of the different signs of the bond—bond interaction constants frr for the vibrations of KrF2 and XeF2. The magnitude and sign of frr are related to the ionization potential of the central rare-gas atom and the net charge distribution in the molecule. In KrF2 the negative sign may be due to the greater weight of a no-bond structure in a resonance description.
Inelastic Collisions of H2+ and N2+ Ions with Hydrogen Molecules
Authors: Vance Dennis W. , Bailey T. L.
Published: January 2005

Absolute total cross sections have been measured for some inelastic processes occurring in collisions of N2+ and H2+ ions with H2 molecules. The reactions which were studied are: H2++H2→H2+H2+,H2++H2→H2+H+H+,N2++H2→N2+H2+,H2++H2→H3++H,N2++H2→N2+H+H+.The cross sections have been determined over the range of kinetic energy of the incident ions, 3–100 eV. The experimental method consists of energy and mass analyzing the products of collisions occurring when a beam of ions of known kinetic energy passes through a scattering chamber containing target gas at low pressure. The energy analysis provides considerable new information regarding the mechanisms of the reactions. The results are in reasonable agreement with previous measurements. Comparisons of the experimental results with existing theories are made. In the case of charge transfer, the symmetric atomic-resonance theory of Rapp and Francis is found to be a good qualitative description of the H2+–H2 charge-transfer process.
Dissociation Energy of F2(g) Obtained from the Study of Gas Flow of Intermediate Pressures Through an Orifice
Authors: Yates Robert E. , Blauer Jay A. , Greenbaum Michael A. , Farber Milton
Published: January 2005

A new treatment for gas flows at intermediate pressures through an orifice has been developed and combined with a new experimental technique to obtain a value for the dissociation energy of F2(g). The value determined is 41.8±0.2 kcal/mole.
Approximations to Hartree—Fock Perturbation Theory
Authors: Langhoff P. W. , Karplus M. , Hurst R. P.
Published: January 2005

A detailed examination is made of several approximations to the first-order Hartree—Fock perturbation equation. Four distinct methods are considered: the coupled (Method a) and the uncoupled (Method c) approximation of Dalgarno, a new alternative uncoupled approximation (Method b), and the simplified uncoupled approximation of Karplus and Kolker (Method d). By a comparison of the pertinent equations, it is shown that Methods a, b, and d correspond to each other in the use of a core potential analogous to that appearing in the zeroth-order Hartree—Fock equation, while Method c differs due to the inclusion of an extraneous self-potential term. An alternative analysis based on an orbital basis set expansion of the perturbed function demonstrates that Method c has an energy denominator of the simple Hückel type, while both Methods a and b include important two-electron correction terms. Also, it is found that of the four approximations, only Method c can be obtained as the first-order correction with a zeroth-order many-electron Hamiltonian that has the Hartree—Fock determinant as its eigenfunction; the other techniques are one electron in character, as is the Hartree—Fock method itself.

The significance of the difference in the core potential is demonstrated by test calculations of dipole and quadrupole polarizabilities and shielding factors for the two-, three-, and four-electron isoelectronic series. A variational technique is used with a trial function that has the form of a polynomial times the unperturbed orbital. For the polarizabilities, it is found that Method b is an excellent approximation to Method a, indicating that the self-consistency condition on the Method a solutions has a very small effect. The shielding factors, however, appear to be more sensitive to the self-consistency requirement. Both Method c and Method d introduce larger errors than Method b, with Method c particularly poor for three-electron atoms and ions. The constraint introduced by choice of trial function is shown to be unimportant for polarizabilities, but quite severe for the four-electron atom shielding factors.

A comparison of the complexity of the various techniques shows that the relative computing times for Methods a, b, c, and d are in the ratio of 300 to 60 to 75 to 1. Thus, Method d is simplest by far, although its speed is achieved by some loss of accuracy with respect to Methods a and b.
H2S as a Free-Radical Interceptor in the Gas-Phase Radiolysis and Photolysis of Propane
Authors: Ausloos P. , Lias Sharon G.
Published: January 2005

The gas-phase photolysis (1236 Å) and the γ-ray radiolysis of C3D8 has been investigated in the presence of varying concentrations of H2S. When 10% or more H2S is added to C3D8, the majority of the D, CD3, C2D3, and C2D5 radicals abstract an H atom from H2S to form HD, CD3H, C2D3H, and C2D5H, respectively. The fully deuterated molecules formed in these mixtures result from the unimolecular elimination of a stable molecule from C3D8 or C3D8+ and from fast bimolecular processes such as ion—molecule reactions

The mechanisms of the radiolysis and the photolysis proposed in earlier studies have been re-examined in the light of the information derived from the C3D8—H2S experiments and of some additional photolysis experiments on CD3CH2CD3—NO mixtures. The results indicate that the modes of decomposition of the neutral excited propane molecule are as follows: C3D8*→C2D6+CD2,→CD4+C2D4,→D2+C3D6,→CD3+C2D5,→D+C3D7.The internally excited C2D4, C2D5, C3D6, and C3D7 species formed in these primary processes decompose to form D, CD3, C2D2, C2D3, and C2D4 unless they are collisionally stabilized.

In the radiolysis of C3D8—H2S mixtures, yields of the free radicals can be adequately accounted for by taking into account the modes of fragmentation of the parent ion and of the excited propane. The C2D5+ ions are shown to react with H2S in part by the deuteron-transfer reaction C2D5++H2S→C2D4+DH2S+.The effect of pressure and the effectiveness of HI as a free-radical interceptor in the radiolysis have been examined.
Infrared and Raman Studies of Crystalline HCl, DCl, HBr, and DBr
Authors: Savoie R. , Anderson A.
Published: January 2005

The infrared and Raman spectra of crystalline samples of HCl, DCl, HBr, and DBr have been recorded in the temperature range 77°—10°K. The Raman results are most interesting in that four components are clearly resolved in the stretching region at low temperature. This indicates that the zigzag chains forming these crystals are probably not planar as was previously thought. Force-constant calculations based on the observed frequencies show that although the main interaction is between molecules in the same chain, there is also appreciable interaction between molecules in adjacent chains. The asymmetry of the two bands observed in the infrared persists at low temperatures. Certain anomalous features in the latter spectra are believed to originate from metastable phases.
Onsager Relations and the Spectrum of Critical Opalescence
Authors: Felderhof B. U.
Published: January 2005

The necessity of extending the hydrodynamic equations near the critical point is investigated on the basis of Onsager's assumption concerning the regression of fluctuations. It is shown that it follows from the Onsager relations that in general the new equations are nonlocal. The most important coefficient corresponding to the inverse compressibility in the original equations is obtained from the Onsager relations and expressed in terms of the equilibrium pair correlation function. Fixman's extension of hydrodynamics follows when the pair correlation is given by the Ornstein—Zernike formula. The consequences for the spectrum of light scattering near the critical point are studied and the width and relative intensity of the central scattering peak are expressed in terms of the equilibrium pair correlation function.
Electronic Bases of Molecular Vibrations. II. Generalized Energy Theorems and the Force Constants of Polyatomic Molecules
Authors: Phillipson Paul
Published: January 2005

Molecular electronic energy derivatives with respect to nuclear displacement coordinates are investigated in detail. By suitably scaling all electronic and nuclear position coordinates, hypervirial operators are introduced which (1) permit generalization of the virial theorem for polyatomic molecules so as to include non-Coulombic and external field contributions and (2) generate expressions relating the force constants of polyatomic molecules to integrals over the electronic-charge distribution. It is shown that the molecular force constants can be defined as a type of derivative of the Hellmann—Feynman force, provided several conditions are satisfied by molecular electronic wavefunctions. These conditions are always met for exact solutions of the electronic Hamiltonian, and may serve as rules for the construction of reliable approximate wavefunctions in addition to the usual energy criterion. Suggestions are made for applications of these procedures to other molecular properties.
Possible Ferroelectric Effects in Liquid Crystals and Related Liquids
Authors: Williams Richard , Heilmeier George
Published: January 2005

It has been found that p-azoxyanisole and other nematic liquids have several of the electrical properties which characterize ferroelectric crystals. These are: hysteresis loops of polarization as a function of field, switching current transients, and large dielectric constants at low frequencies. Typical values for spontaneous polarization and coercive field obtained from analysis of hysteresis loops are 0.03 μC/cm2 and 1×104 V/cm. The appearance of switching transients and their dependence on applied field are similar to those reported for ferroelectrics. We find similar effects for both anisotropic and isotropic phases, in disagreement with an earlier result reported by others. Possible interpretations of the results are discussed.
Electron Paramagnetic Resonance Studies of Photoselected Triplet Molecules. I. Phenoxazine
Authors: Lhoste Jean-Marc , Haug Alfred , Ptak Marius
Published: January 2005

Paramagnetic resonance of the phosphorescent triplet state was carried out for phenoxazine molecules in rigid glasses at 77°K. The line shapes, in Δm=2 and Δm=1 regions, depend upon the incident light polarization. With complementary results from mixed crystals of diphenylphenoxazine, it was possible to relate the principal-axis system of the tensor describing the zero-field splitting to the molecular-axis system. The zero-field energy levels of the triplet state are of the same order of magnitude as those of aromatic molecules.

Selective excitation of the families of molecules corresponding to stationary values of the resonance field showed that:

(1) the resonant triplet state is mainly populated via one singlet—singlet transition S1←S0,

(2) the dipolar electric transition moment of this excitation is approximately parallel to the nitrogen—oxygen axis of the phenoxazine molecule.

Different aspects of photoselection of magnetoselected phosphorescent molecules are discussed.
Electron Paramagnetic Resonance Studies of Photoselected Triplet Molecules. II. Coronene and Phenanthrene
Authors: Lhoste Jean-Marc , Haug Alfred , Ptak Marius
Published: January 2005

Photoselective excitation of magnetoselected triplet molecules in glassy rigid matrix at 77°K was used to determine the orientation of the molecular transition moments. Two aromatic molecules were thus selected: coronene which is highly symmetric and phenanthrene which has two different transition moments in its near-ultraviolet absorption spectrum.

Because of the high symmetry of coronene, it was only possible to show that the singlet—singlet transition moments involved in the population of the phosphorescent triplet state lie in the molecular plane.

For phenanthrene, the long-wavelength singlet—singlet excitation is known to have a transition moment parallel to the short molecular axis, the short-wavelength one a transition moment parallel to the long axis of the molecule. Paramagnetic resonance of triplet molecules photoselectively excited in the two bands with convenient filtered light confirmed these orientations.
Energy Transfer in Radiolysis of Rare-Gas—C2F6 Mixtures
Authors: Kevan Larry
Published: January 2005

The gas-phase gamma radiolysis of C2F6 with He, Ne, Ar, Kr, and Xe has been examined as a function of the fraction of energy deposition in the rare gas. For 90% energy deposition in the rare-gas energy-transfer efficiencies of 5%, 47%, 92%, 78%, and 36% in the respective mixtures of He to Xe were observed. The transfer efficiency is shown to depend on the difference between the recombination energies of C2F6 and the rare gas. This dependence indicates the existence of charge-transfer processes although some excitation transfer also takes place. In Xe–C2F6 mixtures charge transfer is endothermic and the change in product distribution supports the expectation that only excitation transfer occurs.
Magnetic-Dipole Intensities and Zeeman Effect in Cs2UCl6
Authors: Johnston D. , Satten R. , Wong E.
Published: January 2005

The oscillator strengths of several magnetic-dipole transitions for U4+ in Cs2UCl6 have been measured, and compared with calculated values using the eigenfunctions previously determined. The Zeeman effect of the only line showing a splitting in a 30-kg field in the photographic region of the spectrum was observed and g∥ and g⊥ were compared with theoretical values. The second-degree term of the D3d crystalline-field distortion from the large cubic field was obtained.
Electron Spin Resonance Studies of γ-Irradiated Ferroelectric Methylammonium Alum Crystals
Authors: Kohin Roger P. , Nadeau Paul G.
Published: January 2005

60Co γ irradiation of methylammonium alum crystals at room temperature produces the ·CH2NH3+ radical ion which was observed by ESR for the first time. Measurements were made at temperatures between 100° and 300°K. The radical, which undergoes hindered rotation at room temperature, displays isotropic hyperfine splittings of 70 Mc/sec for the two α protons, 53.8 Mc/sec for the three β protons and 11.6 Mc/sec for the β nitrogen. These values are approximately 20% higher than those measured previously in the glycine radical ·CH(CO2−)NH3+ in which the CO2− group extracts π-electron spin density from the remainder of the molecule. The isoelectronic ethyl radical ·CH2CH3 shows a much larger β-proton splitting and a slightly smaller α-proton splitting, indicating that hyperconjugation is approximately 60% more effective for the CH3 group. ESR measurements below the ferroelectric Curie temperature (177°K) indicate that below ≈ 140°K the molecule is rigidly held, but that it begins gradual motion between 140°K and the Curie point. Just at the Curie temperature a transition occurs which liberates additional degrees of freedom and allows virtually free rotation at higher temperatures.
Spin—Lattice Relaxation in Some Rare-Earth Trichlorides
Authors: Mangum B. W. , Hudson R. P.
Published: January 2005

Studies of spin—lattice relaxation of the ions Ce3+, Nd3+, Sm3+, Ho3+, and Er3+ present in low concentrations (0.2% to 2%) in the host lattice of LaCl3 have been made in the liquid-helium region by the pulsed-microwave method. The variation of the relaxation time τ with temperature permits separation of the contributions to the relaxation from the two-phonon (resonant and nonresonant) and single-phonon processes.

The substances studied are not ``well behaved''; features such as nonexponential decays and variability of the derived values for the splitting between the ground and first excited levels are discussed. Some of the ions exhibit a significant variation of τ with concentration and Nd3+ shows an interestingly large anisotropy of τ in the Raman process.
Quenching of NO2 Fluorescence
Authors: Myers G. H. , Silver D. M. , Kaufman F.
Published: January 2005

Quenching constants for NO2 fluorescence were obtained for 13 different inert gases. Quenching cross sections range from 1/25 to 1/2 of gas kinetic and increase with increasing molecular complexity, and dipole moment. No unusual resonance effect for quenching by ground-state NO2 was found. The present data are consistent with a single radiative lifetime of 4.4×10−5 sec for excited NO2. The apparent Stern—Volmer quenching ratios are shown to be functions of both the excitation and fluorescence frequency, and the fluorescence spectrum shows the corresponding red shift at high quenching pressure. This indicates that quenching is a multistep process, although qualitative considerations suggest that a large fraction of the excess internal energy of NO2* is likely to be transferred in a single effective collision.
Preparation of Highly Enriched Carbon−13 by Thermal Diffusion of Methane
Authors: Rutherford W. M. , Keller J. M.
Published: January 2005

Methane, enriched to 93% in 13C, was prepared by thermal diffusion in an 11-column, nine-stage isotope separation cascade. The experimental thermal-diffusion cascade was successfully operated at essentially steady-state conditions after a transient period of 292 days. Behavior of the cascade during steady operation was found to be in good agreement with the predictions of binary cascade theory.
Hydrogen-Molecule Excited States: 1Πu
Authors: Rothenberg Stephen , Davidson Ernest R.
Published: January 2005

The four lowest-lying electronic states of molecular hydrogen of 1Πu symmetry have been calculated theoretically within the Born—Oppenheimer framework. The potential curves are quite regular, exhibiting maxima in the lowest state (2pΠu) at 8.85 bohr of about 120 cm−1 and the third state (4fΠu) at 5.9 bohr of about 715 cm−1. The potential curves have been used to calculate the vibrational—rotational energy levels for those states.
Excess Ultrasonic Attenuation and Intrinsic-Volume Viscosity in Liquid Argon
Authors: Naugle Donald G.
Published: January 2005

The ultrasonic attenuation has been measured in liquid argon by pulse techniques with a single quartz transducer, variable-path-length apparatus. Measurements have been made at temperatures between 84° and 112°K with pressures up to 10 atm. The measured attenuation depends linearly on the square of the frequency over the range 30–70 Mc/sec; however, the measured values exceed the values calculated from the classical Stokes—Kirchhoff expression.

The excess attenuation has been interpreted as evidence for the existence of the volume viscosity in liquid argon. The volume viscosity coefficient has been calculated and the results compared with those of the current theories of liquid-transport phenomena.
Statistical Theory of Chemical Kinetics : Application to Neutral-Atom—Molecule Reactions
Authors: Pechukas Philip , Light John C. , Rankin Charles
Published: January 2005

Several processes involving uncharged species are discussed in terms of a statistical theory of strong-coupling collisions. Specifically, we study the Mies—Shuler—Zwanzig model for vibrational excitation of a diatomic by impulsive collisions; the reactions of K with HBr, Cl with KH, Cl with Na2, and the subsequent reaction of vibrationally excited NaCl with Na; and the breakup of electronically excited H2O to H atom and OH (2Σ+) radical (as an example of a bad failure of the statistical model). Except in the last case, rotational and vibrational distributions in the product diatomics predicted by the statistical theory agree well with experiment (where the experiment has been done); the total reactive cross sections, although a little high, compare reasonably well with the measured cross sections.
Mass-Spectrometric Studies of the Rare Gases Using an Electron Velocity Selector
Authors: Brion C. E. , Frost D. C. , McDowell C. A.
Published: January 2005

A 127° electron velocity selector and analyzer have been combined with the ion source of a mass spectrometer to study ionization-efficiency curves. Interesting structure in the curves for argon and krypton has been observed.
Atomic View of Surface Self-Diffusion: Tungsten on Tungsten
Authors: Ehrlich Gert , Hudda F. G.
Published: January 2005

Surface diffusion of tungsten adatoms on several smooth, low-index planes of the tungsten lattice has for the first time been followed by direct observation of individual atoms in the field-ion microscope. Contrary to expectation, the mobility at room temperature is found to increase in the order (211) > (321) ∼ (110) > (310) ∼ (111). Migrating atoms are reflected at the boundaries of the (110), (211), and (321) planes; on the latter two, motion along atomic rows is favored over diffusion across lattice steps. From quantitative determinations of the rate of change of the mean-square displacement, diffusion coefficients are obtained as follows: (110), D=3×10−2exp(−22 000/RT)cm2/sec; (321), 1×10−3exp(−20 000/RT); (211), 2×10−7exp(−13 000/RT). Differences in diffusion on the (211) and (321), planes of very similar structure, suggest a weakening of interatomic forces at lattice edges.
Atomic Displacements in One- and Two-Dimensional Diffusion
Authors: Ehrlich Gert
Published: January 2005

The statistics of surface diffusion phenomena are developed for the conditions appropriate to measurements in the field-ion microscope. Examined in detail are: (1) The effect of boundaries (lattice steps) on the mean-square displacement in a random walk; (2) the probability density for distances traversed in a two-dimensional walk along nonorthogonal surface channels; (3) the distribution of distances for random walks in which the number of jumps is subject to fluctuation, as it is in real systems. Both reflecting and adsorbing boundaries are found to diminish the mean-square displacement in the same fashion. The effect of nonorthogonality, and even more so of fluctuations, is to enhance the frequency of small displacements when compared with a Rayleigh distribution.
Shock Compression of Argon
Authors: van Thiel M. , Alder B. J.
Published: January 2005

Liquid argon has been shock compressed from two initial states at 86°K and 2 bar, and 148.2°K and 70 bar. The highest pressure of 700 kbar and temperature of 13 000°K were obtained by reflecting a shock wave with a pressure of 270 kbar from a tungsten wall. These shock loci allow a test of the interatomic potential over a wide region and show that the potential can be represented by a sum of pair interactions up to a pressure of 400 kbar. The pair potential is furthermore found to be considerably less repulsive at small interatomic distances than the 12-power repulsive law. In fact, an exponential form yields good results up to 360 kbar. Above this pressure range the reflected-shock data require a large change in the potential form which is interpreted to be due to a pressure-induced electronic transition to the conduction band. Calculations show that a discontinuity in the Hugoniot data at 50 kbar and 1200°K could be identified with the melting line. This temperature corresponds to three times the highest reduced melting temperature observed so far.
Adsorption of Carbon Monoxide on Tungsten. Abundances, Dipole Moments, and Sticking Coefficients
Authors: Bell Anthony A. , Gomer Robert
Published: January 2005

The relative abundances, relative dipole moments, and absolute sticking coefficients of virgin, alpha, and beta CO adsorbed on tungsten have been determined by a combination of work function vs coverage measurements with step-desorption spectra and sticking-coefficient measurements. The work-function data were obtained in a field-emission tube with an annular CO source, permitting uniform dosing of the tip. The other measurements were performed in a tube in which a CO source illuminated a tungsten ribbon. A movable field emitter serving as detector could be raised in front of this ribbon to receive CO from the latter in desorption or reflection. This arrangement represents a new method of determining sticking coefficients, free from the ambiguities of surface roughness and wall ad- or desorption, usable to 20°K or less.

The dipole-moment ratios were Pα/Pv = −0.68; Pβ/Pv = 0.68 for the initial, and Pβ/Pv = 1.0 for the terminal portion of the beta layer. The relative abundances were total beta/total virgin = 0.55; virgin converted to beta/beta from virgin conversion = 2.1; alpha/total beta = 1.2.

Initial sticking coefficients were close to unity up to 700°K, and decreased rapidly at coverages (relative to the maximum amount of adsorbate retained at that temperature) of 0.6–0.4.
Theory of Formation of Very Thin Oxide Films on Metals
Authors: Boggio Joseph E. , Plumb Robert C.
Published: January 2005

The growth of oxide on initially oxide-free evaporated aluminum films at room temperature and atmospheric pressure has been studied in parallel experiments, using elliptically polarized light to measure the film thickness and vibrating capacitor measurements to measure changes in the Volta potential. The film thickness reaches 21 Å in 106 sec and varies essentially logarithmically with time in the time range from 102 to 106 sec. The Volta potential is almost constant during the initial stages of growth and then varies approximately logarithmically with time. The observed change of Volta potential, interpreted as resulting from changes in the Galvani potential, is approximately 350 mV in 106 sec of oxidation.

A logical extension of the Mott theory is presented which includes (1) the effect of electronic space charge resulting from equilibration of electrons between the metal and oxide and (2) the electric field in the oxide, both in its role as a barrier to electron penetration of the oxide and in its control over the migration of interstitial aluminum ions. The extended theory predicts a linear relation between V½X (V is Galvani potential difference across the oxide and X is oxide thickness) and ln (oxidation time) whose slope is 3ℏ/(32me)½, m and e being the mass and charge of the electron and ℏ being Planck's constant divided by 2π. The experimentally determined slope has the value 1.32×10−8 V½·cm as compared to the theoretical slope of 1.46×10−8 V½·cm.
Nonequilibrium Effects in the Kinetics of Gas-Phase Reactions
Authors: Snider Neil S. , Ross John
Published: January 2005

A theoretical rate equation derived for exchange reactions in the gas phase from an assumed Boltzmann equation yields statistical rate expressions for theoretical rate coefficients which in general are functions of concentration with a ratio not equal to the equilibrium constant K. However, the theoretical rate equation may be rearranged to yield rate coefficients defined to obey the relation κf,c/κr,c = K. The rate coefficients κf,c and κr,c are also functions of concentration except in three linear cases: (1) a reaction with one of the components present in large excess; (2) a chemical reaction proceeding unaffected by inelastic processes; (3) an isomerization reaction proceeding in a large excess of inert gas. Thus for these three cases the usual phenomenological rate equation [Eq. (2)] applies. In Cases 1 and 3 the rate constants are determined not only by chemical-reaction cross sections but also by inelastic cross sections for at least some and possibly all reactants and products.
Trimethylene Oxide. III. Far-Infrared Spectrum and Double-Minimum Vibration
Authors: Chan Sunney I. , Borgers T. R. , Russell J. W. , Strauss H. L. , Gwinn W. D.
Published: January 2005

The 0–1 transition of the out-of-plane bending vibration in trimethylene oxide has been observed at 53.4 cm−1. Previously reported bands by Lord and his co-workers [J. Chem. Phys. 33, 294 (1960)] at 89.8, 105.2, 118.3, 128.9, 139.0, 147.6, 154.9, and 161.8 cm−1 have also been confirmed. In addition, six higher members of this series as well as three members of the Δv = 3 series have been observed. These transitions are readily interpreted in terms of the hot-band series of a slightly perturbed quartic oscillator. A double-minimum potential function of the form az4—bz2 predicted all the observed transitions to within experimental error (0.5 cm−1 or better). The central barrier in the potential-energy function, while low, is finite and has a height of 15.3±0.5 cm−1. It had previously been concluded, from the variation of the rotational constants with vibrational state obtained from microwave spectroscopy that the potential energy contained a small barrier of the order of zero-point energy at the planar configuration of the molecule [J. Chem. Phys. 33, 1643 (1960)]. The present far-infrared results are shown to be consistent with the microwave results. Thus an existing discrepancy between the far-infrared and microwave results on the nature of the potential function for this unique out-of-plane bending vibration is finally resolved.
Order—Disorder Phenomena. II. Elastic Constants of a Two-Dimensional Ising Model
Authors: Renard Rémi , Garland Carl W.
Published: January 2005

The mechanical behavior of an Ising model in which the spins are located on the particles of a compressible lattice has been investigated. Analytical expressions are derived for the spin contribution to all three independent elastic constants of a square ferromagnetic system. It is found that the constant area temperature dependence of c11 (the compressional stiffness constant) is dominated by a term proportional to the configurational specific heat. In contrast, the angle shear constant (denoted by c44) has a spin contribution which is directly proportional to the Ising internal energy. The other shear constant C′ is a more complicated function of temperature, but it is similar to c44 in that both constant-area shear constants have an inflection point of infinite slope at Tc.
Order—Disorder Phenomena. III. Effect of Temperature and Pressure on the Elastic Constants of Ammonium Chloride
Authors: Garland Carl W. , Renard Rémi
Published: January 2005

The adiabatic elastic constants of single-crystal ammonium chloride have been measured at 20 Mc/sec as functions of temperature and pressure in the region of the lambda transition. At atmospheric pressure, data were obtained over the range of temperature from 150° to 320°K. At five temperatures, evenly spaced between 250° and 310°K, measurements were made as the pressure was varied from 0 to 12 kbar. The values at 300°K and atmospheric pressure are: c11 = 3.815, c44 = 0.8878, C′ = 1.4698, in units of 1011 dyn cm−2. A hysteresis of the critical temperature, equal to ∼0.9°K at atmospheric pressure, was observed; this hysteresis disappears rapidly as the hydrostatic pressure is increased. The results are in good agreement with those predicted for a compressible Ising model. In particular, it is concluded that the order—disorder transition in ammonium chloride is not of a simple lambda type. It is proposed that NH4Cl is unstable in the immediate vicinity of its critical point and undergoes a first-order transition.
Infrared-Reflection Spectra of Molten Fluoride Solutions: Tantalum (V) in Alkali Fluorides
Authors: Fordyce J. Stuart , Baum Ruth L.
Published: January 2005

Infrared-absorption spectra derived from reflectivity measurements over the range 4000 to 200 cm−1 have been obtained for molten fluoride solutions of electrochemical interest. The results for alkali fluoride melts containing tantalum (V) over the composition range 21.3 KF:21.3 LiF:TaF5 to 2 KF:2 LiF:TaF5 show two strong bands which increase in intensity with increasing tantalum concentration. One band is found at 535 cm−1 and the other is a split (17-cm−1) band centered at 300 cm−1. The band positions are independent of concentration and temperature from the melting point to 800°C. Comparison of these spectra with those for well-characterized solid compounds such as K2TaF7 and CsTaF6 demonstrate that the TaF72— complex ion is the predominant species in these melts.

A study of the influence of cations on the spectra of these melts show that when KF is present TaF72— is the only species observed. In the NaF:LiF solvent, TaF6− also appears.

The spectra of vapors condensed from the various melts studied show interesting contrasts which are related to the vapor—liquid equilibria.
Thermal Transpiration. Rotational Relaxation Numbers for Nitrogen and Carbon Dioxide
Authors: Malinauskas A. P.
Published: January 2005

Thermal transpiration studies were conducted with the gases Ar, Xe, N2, and CO2 in an effort to test a recently proposed relation between the translational component of the thermal-conductivity coefficient and the maximum transpiration effect that is observed for a particular gas under a given set of temperature and apparatus conditions. The phenomenon was generated by maintaining temperatures of about 290° and 545°K on the opposite ends of 14 0.1-mm i.d. Pyrex glass capillaries arranged in parallel.

The rotational collision numbers for N2 and CO2 which were obtained from the thermal transpiration data were found to be 4.4 and 2.4, respectively, in good agreement with values determined by the more common methods.