bluuebunny/crossref_metadata_2025 huggingface dataset:
Normal Modes of Branched Polymers. II. Complex Branched Molecules and Ring Systems
Authors: Chompff Alfred J.
Published: February 2004

The normal mode analysis developed in the first part of this series to calculate the viscoelastic properties of star and ring-shaped macromolecules has been extended to more complex structures. For free-draining, symmetrically branched molecules, it is shown that their theoretically calculated viscosities, or equivalently, their radii of gyration, are consistent with those obtained by other methods. It is assumed that this provides a sufficient test of the completeness of the set of relaxation times obtained by the decoupling procedures. The relaxation spectra and other viscoelastic properties then follow directly from the characteristic equation for the decoupled configurations. The method can be shown to apply to micronetworks or cyclic systems which may be formed during the initial stages of network formation. Furthermore, for randomly branched molecules which occur in practical systems, the present method gives a reasonable estimate of the viscoelastic properties which could not have been given in the framework of previous theories.
Comparison of the Hartree–Fock and LCAO–MO Solutions for the Interaction of Rare-Gas Atoms in the Region of Small Overlap
Authors: Butler David , Kestner Neil R.
Published: February 2004

From an analysis of accurate homonuclear Hartree–Fock solutions for the interaction of two rare-gas atoms, we have found that the major difference between these results and the approximate LCAO–MO using accurate atomic Hartree–Fock solutions is the presence of small contributions which can be shown to be equivalent to the ionic (or so-called charge transfer) contributions one usually discusses in more qualitative contexts. These terms are shown to be the most important, both in terms of their contribution to the wavefunction and to the energy, provided that the overlap is less than about 0.2.
Molecular Motions of Deuterated Isobutylenes in the Liquid State
Authors: Assink R. A. , Jonas J.
Published: February 2004

The temperature dependences of the deuteron spin–lattice relaxation times in 2-methyl-1, 1-propene-d2, and 2-methyl-d3-3,3,3-propene-d3 in the liquid state have been measured by the adiabatic fast passage method. Since the deuteron coupling constants were estimated from the known values for analogous systems, the experimental T1 data were analyzed in terms of the over-all reorientation of the molecules and the internal rotation of the methyl-d3 groups. The rotational diffusion constant for over-all reorientation was calculated to be D = 0.28 × 1012sec−1 at 0°C, and the upper bound for the internal rotation rate of the methyl-d3 groups was found to be 0.2 × 1012 sec−1 at 0° by assuming rotational diffusion and 0.6 × 1012 sec−1 at 0°C by considering random reorientation among three equivalent positions 120° apart.
Infrared Spectra of Matrix-Isolated Acetic Acid Monomers
Authors: Berney C. V. , Redington R. L. , Lin K. C.
Published: February 2004

Infrared spectra of monomeric acetic acids [CH3COOH(D) and CD3COOH(D)] isolated in Ar and N2 matrices near 4°K are reported. A large anharmonic potential-energy contribution is evident from the observed isotopic frequency shifts. It is proposed that this arises primarily from the double-minimum potential-energy curve for tunneling of the H atom between O atoms via the COH angle bending coordinate. In all cases, important coupling between the COH angle bending and C–O stretching coordinates is observed, with a strong addition of CD3 (umbrella) angle deformation in the case of CD3COOH. Exceptional matrix effects are observed for the coupled vibrations of the –OH molecules. A complete assignment is proposed for these acetic acids on the basis of the new data. As additional supporting evidence, the spectrum of matrix-isolated CH3COF and of acetic acid vapor at low pressure and long path length are reported.
Sedimentation and Diffusion of Wormlike Polymer Chains
Authors: Ullman Robert
Published: February 2004

Sedimentation and diffusion constants of wormlike polymer chains have been calculated using methods of Kirkwood and collaborators. The chain statistics are described according to Hermans and Ullman using the (Kratky–Porod) model. The results are obtained for several chain lengths at varying degrees of flexibility for several possible values of the segmental friction constant. Application of these calculations and an earlier calculation on intrinsic viscosity led to a Kratky–Porod persistence length of about 600 Å for double-stranded DNA.
Isotope Effects in the Photoionization Yields and the Absorption Cross Sections for Acetylene, Propyne, and Propene
Authors: Person James C. , Nicole Paul P.
Published: February 2004

The values of the absorption cross section Σ and the photoionization yield Γ (the probability that photon absorption produces ionization) have been experimentally determined for C2H2 and C2D2, C3H4 and C3D4, and C3H6 and C3D6 at various photon energies in the energy range of 11.25 to 11.78 eV, 10.23 to 11.78 eV, and 9.63 to 11.78 eV, respectively. In both propyne and propene there is an increase in ionization for the heavier isotopic molecules, which is evident both in Γ and in the photoionization cross section, Σi (Σi = ΓΣ). This increased ionization is discussed in terms of the Platzman competitive ionization mechanism, and the effects of methyl substitution on Σ and Σi are also discussed.
Particle Scattering Factor and Segment–Segment Distance Distribution of Macromolecules in Solution
Authors: Mijnlieff P. F. , Coumou D. J. , Meisner J.
Published: February 2004

The angular dependence of light scattering from solutions of three samples of poly-α-methylstyrene in various solvents has been measured over a wide range of scattering angles. For molecules with a radius of gyration larger than about 500 Å, differences (δP) are found between the actual angular dependence (P) and the one (Pid) predicted—with Debye's equation—for ideal Gaussian coils. By subjecting the measured data to a statistical analysis, a quantitative description of δP was attained. The quantities δP contain information about the function δW(r), which is the difference between W(r) and Wid(r); the function W(r) is the actual distribution function for the distances between the segments in a polymer coil, whereas Wid(r) is the one that follows from random-flight statistics, underlying the ideal Gaussian coil model. The information about δW(r) was obtained by applying a Fourier inversion to the measured δP curves. Although this information is necessarily limited, the typical features of δW(r) thus found correspond with those a realistic function W(r) must possess in order to account for the impossibility of chain intersections (excluded volume effect).
Optical Properties of Atoms and Diatomic Molecules Calculated by a Time-Dependent Coupled Hartree–Fock Method
Authors: Epstein Irving R.
Published: February 2004

A time-dependent coupled Hartree–Fock perturbation theory which employs basis sets more flexible than those of many previous methods is used to calculate the dynamic polarizabilities of He, Be, Ne, H2, LiH, Li2, and HF. From these values, transition frequencies, indices of refraction, and Verdet constants are computed. The calculations are carried out using several combinations of the dipole length, velocity, and acceleration operators. Approximations of the results by polynomials and by Padé approximants are discussed.
Relativistic Calculation of Anomalous Scattering Factors for X Rays
Authors: Cromer Don T. , Liberman David
Published: February 2004

Anomalous scattering factors Δf′ and Δf″ have been calculated relativistically for Cr, Fe, Cu, Mo, and Ag Kα radiations for the atoms Li through Cf. An interpolation scheme for other wavelengths is included in a separate report. Relativistic calculations of the photoelectric cross section have been made and the integral for the principal contribution to Δf′ has been evaluated numerically without approximation to the form of the cross section-vs-energy curve, as has been done in previous calculations. Many of the results are significantly different from previous calculations. Where experimental values exist, agreement for Δf″ is improved. For the rare gases, except for xenon, agreement between Δf′ and experiment is improved. Because of the more rigorous evaluation of Δf′ from cross-section information, it is presumed that the present Δf′ values are more accurate than previous calculated values. Calculated mass absorption coefficients for the elements are included as incidental information.
Skeletal Molecular Structure of closo-2,3-Dicarbahexaborane (6) from Microwave Spectral Studies
Authors: Beaudet Robert A. , Poynter Robert L.
Published: February 2004

The microwave spectra of 13 isotopic species of closo-2,3-dicarbahexaborane(6) have been studied. From these results, the cage structure of this molecule has been determined. The accuracy of the molecular structure has been evaluated. The bond lengths determined were: BondLengthB(1)–B(6)2.434 ± 0.005 ÅC(2)–B(4)2.297 ± 0.005 ÅB(1)–B(4)1.721 ± 0.015 ÅB(4)–B(5)1.752 ± 0.005 ÅB(4)–C(3)1.605 ± 0.005 ÅC(2)–C(3)1.540 ± 0.005 ÅB(1)–C(2)1.627 ± 0.015 Å The measured dipole moment was 1.50 ± 0.03 D.
Neutron-Diffraction Study of D3Co(CN)6
Authors: Güdel H. U. , Ludi A. , Fischer P. , Hälg W.
Published: February 2004

The crystal structure of D3Co(CN)6 has been determined by a neutron-diffraction analysis of a poly-crystalline sample at room temperature. The deuterated compound was prepared from H3Co(CN)6. It crystallizes according to space group D3d1-P3̄1m with a = 6.431(4) Å and c = 5.695(4) Å. The lattice constants do not show any isotope effect within the limits of error. Slightly trigonally distorted Co(CN)6 octahedra are three-dimensionally linked by N–D–N bonds. The Co–C, C–N, and N–N distances are 1.88, 1.15, and 2.60 Å, respectively. The latter distance is the shortest so far found in an N–D–N or N–H–N bond. These hydrogen bonds are either linear symmetric or statistically asymmetric (double minimum) centered on (12, 0, 12).
Vacuum Ultraviolet Photolysis of Fluoroethylenes. I. Vinyl Fluoride at 1470 Å
Authors: Kirk A. W. , Tschuikow-Roux E.
Published: February 2004

The photolysis of vinyl fluoride at 1470 Å and pressures in the range 1–150 torr, yields in addition to C2H2 and C2HF, 1,1-C2H2F2. The yield of 1,1-C2H2F2 with respect to that of C2H2, increases rapidly with decreasing pressure. It is proposed that F atoms are liberated as a result of the decomposition of certain excited C2H3F molecules. The F atoms in turn react with C2H3F to give a chemically activated difluoroethyl radical with a small excess energy with respect to a decomposition which yields almost exclusively 1,1-C2H2F2. The details of this preliminary proposal are discussed in relation to the measured relative yields of C2H2, C2HF, and 1,1-C2H2F2.
Electron Excitation Cross Sections for Light Atoms Using Slater Wavefunctions
Authors: Williamson W.
Published: February 2004

Electron excitation cross sections are given for light atoms using Slater radial wavefunctions and the Born and Bethe approximations. A simple result is found for the Bethe approximation, but in the case of the Born approximation only the analytic form of the scattering amplitude is given and integration over the momentum transfer must be done. The Na (32S → 32P) cross section is calculated as an example of the approximations.
Electron Attachment to SF6
Authors: Fehsenfeld F. C.
Published: February 2004

The attachment rate for electrons to SF6 has been measured between 293 and 523°K in a helium-buffered flowing afterglow over the pressure range of 0.1–1.5 torr. The attachment has a rate constant of 2.2 × 10−7 cm3/sec, independent of temperature and pressure. The primary reaction product over the measured range of temperatures is SF6−. However, the rate of production of SF5− increases rapidly with temperature.
Oxidation States and Site Symmetries of CaF2:U Crystals
Authors: McLaughlin R. , Abed U. , Conway John G. , Edelstein N. , Huffman E. H.
Published: February 2004

Brown, green, yellow, and red CaF2:U crystals were investigated by means of optical spectra and chemical analyses in order to determine the cause of the different colorations. Whereas earlier work claims the green coloration is due to U2+, we have shown it is due to U4+. Whereas earlier work claims the yellow coloration is due to U4+, we have shown it is due to U6+. The brown coloration appears to be due to U4+ with a different charge compensation that the green coloration.
Structure and Glass Transition Thermodynamics of Liquid Zinc Chloride from Far-Infrared, Raman, and Probe Ion Electronic and Vibrational Spectra
Authors: Angell C. A. , Wong J.
Published: February 2004

With a view to elucidating the structure of liquid ZnCl2 and deciding the extent of the analogy to SiO2 and BeF2 (liquid and vitreous) provided by ZnCl2 (liquid and vitreous), a variety of measurements on ZnCl2 have been performed. Density and expansivity measurements and probe ion electronic spectra are used to demonstrate the existence of an open tetrahedrally coordinated structure. Details supplied by x-ray patterns, Raman spectra, and particularly, by far-infrared spectral studies, are then used to establish a close similarity between the glass quasilattice and the network lattice of the α-ZnCl2 crystal polymorph. Finally a new technique, impurity ion (CN−) vibrational spectroscopy, is introduced to show that the defects characteristic of the glass structure involve small numbers of singly and doubly broken anion bridges, no evidence for intermediate states being found. On the basis of these observations a split-level cell model in which cells have discrete energies depending on the presence or absence of such Zn–Cl bonds, is constructed. The model proves capable of accounting for the volume data and for the characteristic changes in equilibrium thermodynamic properties which, it is known, must occur near (usually somewhat below) the glass transition temperature. The thermodynamic relations obtained for the simplest case are the same as those from the Macedo–Capps–Litovitz quasichemical two-state model. The liquid is best regarded as a weakened version of this network in which the progressive disruption of the anion bridges is associated with a greatly reduced structural relaxation time for the quasilattice.
Collision-Induced Dissociation of NO+ and O2+ at Low Kinetic Energies: Effects of Internal Ionic Excitation
Authors: Tiernan Thomas O. , Marcotte Ronald E.
Published: February 2004

The reactions resulting in dissociation of NO+ and O2+ upon impact with various atoms and molecules have been investigated in a double mass spectrometer of in-line geometry in the reactant ion kinetic energy range below 50 eV. Cross sections measured for these processes range from about 0.02 to 0.3 Å2. The dependence of the cross section upon kinetic energy is markedly affected by the energy of the ionizing electrons which produce the reactants. At the lowest electron energy, Ee = 13 eV, kinetic-energy thresholds are observed for the production of N+ from the NO+/NO reaction, and for O+ production from the O2+/Ar reaction, which are in excellent agreement with known adiabatic dissociation energies for the ground-state molecule ions. The production of O+ from the NO+/NO interaction is shown to involve a rearrangement process rather than simple collision-induced dissociation. At higher electron energies, the cross-section dependence plots exhibit structure which can be interpreted in terms of the participation of internally excited reactant ions. These results and related charge transfer experiments with NO+ indicate that the reactant ion beam formed by 20-eV electrons includes both vibrationally excited ground-state ions and NO+ ions in the δ3 electronic state. Isotopic experiments in which 15NO+ and N18O+, respectively, were impacted at low energies on normal NO demonstrate that the N+ and O+ products are derived in approximately equal amounts from the reactant ion and the reactant molecules. Products from the reaction of N18O+ with 15NO show that the major reaction process is simple charge transfer and that the extent of isotopic scrambling in this reaction is negligible. The implications of these observations for the mechanism of the dissociation reactions is discussed.
Intensity Perturbations and Breakdown of the Ordinary Selection Rules in the Infrared Spectrum of the Asymmetric Rotor
Authors: Braslawsky J.
Published: February 2004

The influence of first-order rotation–vibration interaction on line intensities in vibration–rotation fundamental bands of asymmetric-top molecules is treated here. The method of the preceding paper of Braslawsky and Ben-Aryeh is generalized. This generalization is attained by means of the group-theoretic method, using the symmetry properties of the zeroth-order rotational Hamiltonian. The influence of the interaction on ordinary lines is summarized in a simple formula for the intensities correction F factors. In addition, it is proved that the ordinary (zeroth-order) selection rules are violated due to the first-order centrifugal interaction. Such violation of selection rules did not obtain for diatomic or symmetric-top molecules.
Sequential Adsorption of Large Immobile Atoms on the Continuum and Regular Lattices
Authors: Verhagen A. M. W.
Published: February 2004

The nonoverlapping patterns of atoms which arise after sequential random adsorption of large immobile atoms on the remaining free sites of the continuum or regular lattices are for each adsorption density shown to be equally probable asymptotically. The adsorbed atoms therefore obey restricted Fermi–Dirac statistics in that only a subset of the set of all possible nonoverlapping patterns are equally probable. This characterization of the sequential adsorption process is used to derive the functional form of the equation relating the number of “excluded” sites to the adsorption density as the process proceeds. The functional form obtained contains a small number of constants, mathematically definite once the size of the atoms and the adsorption medium are specified. In the case of one dimension, the constants relate simply to the so-called filling density already known for special cases. In the case of the square and the triangular lattices in two dimensions, the constants in the functional form are estimated from simulation results for the function itself, published recently.
Raman Studies of Molecular Motion in Liquid and Solid HCl
Authors: Wang C. H. , Fleury P. A.
Published: February 2004

The Raman scattering technique was used to study molecular motion of HCl in its condensed phases. In the liquid and in cubic phases, the depolarization ratio, the spectral shape, and the spectral width of the H–Cl stretching mode were measured as functions of temperature. It was found that the modes of HCl molecular motion are not significantly changed as the liquid–solid phase transition was traversed. The depolarized linewidth measurements of the intramolecular stretching vibration also indicate that the energy barrier for HCl molecular reorientation is small, suggesting a nearly free tumbling of the HCl molecule in the cubic solid at temperature higher than 120°K. Below 120°K, satellite peaks develop, suggesting that evolution of the lower symmetry structure other than the cubic has already taken place at temperature higher than Tλ. The temperature study of the low-frequency spectrum in the 0–300-cm−1 region suggests that HCl polymeric fluctuations are responsible for the light scattering spectrum. Moreover, the libration time of HCl polymers (formed as a result of hydrogen bonding) is short compared with the mean reorientation time of a single molecule. The spectra of the lattice phonons as well as the internal stretching vibrations of HCl molecules below the first-order phase transition temperature (Tλ = 98.4°K) were also studied.
Quantum Mechanical Basis for the Cubes Models in Gas–Surface Scattering Theory, and an Experimental Test
Authors: Goodman Frank O.
Published: February 2004

It is suggested that there is a quantum mechanical basis for the classical flat surface (cubes) models of gas–surface scattering, the important characteristic of which is the assumption of conservation of tangential momentum of a scattering gas molecule. The theory is applied to the diffractive system He–LiF, and it is shown that existing experimental data confirm some predictions of the theory.
Electron Spin Resonance Investigation of the Conducting Dihydrides of Gadolinium and Gadolinium–Yttrium Alloys
Authors: Uhrich David L. , Schumacher David P.
Published: February 2004

Electron spin resonance linewidths and g shifts have been obtained in the temperature range 100–300°K for the gadolinium f electrons in the systems MHx (1.90 < x < 2.46; M ≡ Gd, Gd0.71Y0.29, or Gd0.50Y0.50). The systems behave differently depending on whether x > xcor x < xc; xc = 2.12, 2.07, 1.91 forM ≡ Gd, Gd0.71Y0.29, and Gd0.50Y0.50, respectively. For x > xc the g shifts (relative to g for Gd in insulators) were ≈ + 0.01, independent of x and of temperature. These shifts are interpreted as the result of the exchange interaction − JS·s between the gadolinium f electrons and the s conduction electrons, indicating that hydrogen enters the lattice as H−. The g values are slightly temperature dependent for x <x c. The linewidth is proportional to the temperature, characteristic of the Korringa formula, above x = xc. Below x = xc the linewidth becomes temperature independent and increases with decreasing hydrogen content. This temperature independent broadening is linked with vacant tetrahedral interstices, normally occupied by hydrogens in the face-centered cubic metal lattice.
High-Temperature Harmonic Free Energy of the fcc and hcp Lattices: Cell Cluster Method
Authors: Huckaby Dale A. , Salsburg Zevi W.
Published: February 2004

Previous applications to rigid sphere solids have indicated that the cell cluster expansion is more rapidly convergent in three dimensions than in two. In this paper this hypothesis is tested for the harmonic model by employing the cell cluster expansion to calculate the high-temperature limit of the Helmholtz free energy for both the hexagonal close-packed (hcp) and face-centered-cubic (fcc) lattices. The calculations were carried out both for a model involving first-nearest-neighbor interactions only and for a model involving both first- and second-neighbor interactions. The method could be taken to fifth order (involving all five particle clusters), and the results could be extrapolated to give four-decimal accuracy. Consistent with the known properties of these lattices the cell cluster method predicts that the fcc lattice has a slightly lower harmonic free energy at high temperatures.
Density Dependence of 129Xe Chemical Shifts in Mixtures of Xenon and Other Gases
Authors: Jameson A. Keith , Jameson Cynthia J. , Gutowsky H. S.
Published: February 2004

Unlike other chemical shifts in gaseous systems which have been found to have strictly linear dependence on density, we have found the 129Xe chemical shift in pure xenon gas to have a quadratic and cubic dependence in addition to the dominant linear dependence on density. This implies the importance of three or more body interactions in xenon. In mixtures of xenon with another gas (Ar, Kr, CO2, HCl, CH4, CH3F, CH2F2, CHF3, CF4), the dependence of the 129Xe chemical shift on the density of the other gas is found to be linear within experimental error, and varying from 2300–11 700 ppm/mol cc−1. These shifts are orders of magnitude greater than the reported H and F shifts in gases. Analysis of the results show that the density dependence cannot adequately be reproduced by the contributions, Σ1 = Σb − B〈ε2〉 − B〈F2〉, which had been adequate for H and F shifts. The general formulation for calculation of the A and B parameters, the coefficients of the linear and quadratic electric field terms in the theory of chemical shift in gases, is developed. An approximate calculation of B for atoms is given, and the repulsive and anisotropic contributions are estimated. The sensitivity of the chemical shift to the form of the intermolecular potential is suggested in the case of Xe and the fluoromethanes where the results are consistent with a noncentral field potential but not with a central field potential like the Lennard-Jones.
Equation of State for a Lennard-Jones Fluid
Authors: Toxvaerd So/ren , Praestgaard Eigel
Published: February 2004

The influence of higher-order terms in the Barker–Henderson approximation to the Zwanzig perturbation expansion is examined for a Lennard-Jones fluid. We find that the error introduced by ignoring these higher-order terms is negligible compared with the error due to the replacement of the reference system by a hard-sphere system in which the diameter of the spheres ensures coincidence to only first order between the free energy of the reference system and the free energy of the hard-sphere system. In calculating the contribution to the pressure due to the perturbation, one often uses the Percus–Yevick solution for the hard-sphere reference system. The error introduced by this approximation is found to be considerable.
Is Spin–Rotational Relaxation Effective in Solid NH4I?
Authors: Sharp A. R. , Pintar M. M.
Published: February 2004

Spin–lattice relaxation times for protons in NH4I and deuterons in ND4I are reported, and the dipolar and quadropolar relaxation rates are derived. In the cubic phase of NH4I, these rates are used to analyze the frequency-independent relaxation rate of proton spins, which increases approximately exponentially with increasing temperature, with a slope of 1.5 kcal/mole. The relaxation mechanism suggested is the spin–rotational interaction. There is qualitative agreement with both the rotational diffusion and transient rotation models, which were developed for liquids. However, the strength of the spin–rotational coupling predicted by these models is some one to two orders of magnitude too weak to quantitatively reproduce the observed relaxation rate in this solid.
93Nb NMR Study of the LiNbO3–LiTaO3 Solid-Solution System
Authors: Peterson G. E. , Carruthers J. R. , Carnevale A.
Published: February 2004

The 93Nb nuclear magnetic resonance has been studied in sintered powders of the solid-solution system LiNbO3–LiTaO3. The Nb nuclear quadrupole coupling constant decreases linearly with an increase in LiTaO3 concentration and this behavior agrees with the calculated electric field gradient. A variation of the 93Nb NMR linewidth with composition was observed which can be explained on the basis of a random distribution of electric field gradients across the solid-solution range. The linewidths follow closely the well-known expression for the entropy of a randomly distributed binary solid solution.
Effective Spin–Orbit Hamiltonian
Authors: Karayianis N.
Published: February 2004

Deviations from the Landé interval rule governing separations between the J multiplets of atomic terms are known to stem from term mixing by the spin–orbit interaction. A phenomenological interaction of the form HE = Σnλn(L·S)n with 1 ≤ n ≤ p can account for any such separations if p = 2S, one less than the multiplicity of the term. The general theory of effective operators is applied to the spin–orbit interaction for electron configurations of the form lN, and it is shown that, through pth order in the perturbation expansion of the spin–orbit Hamiltonian h = ηΣi1i·si, the above form for HE obtains where λn = ηΣm(η / F2)m−1λnm, n ≤ m ≤ p. Assuming hydrogenlike wavefunctions for f electrons, the λnm are shown to be constants depending only on the term being considered and the number of equivalent electrons N. Using η and F2 values given in the literature, the λn are calculated for the ground terms of the triply ionized rare earths. Values for the λn have been reported for Nd3+, Tb3+, Ho3+, and Er3+ in CaWO4 where the Stark-split energy levels of the ground terms were analyzed by treating the λn and the crystal-field parameters in HX = ΣkmBkm†Ckm as adjustable. The theoretical and experimental values for λ1, λ2 and λ3 agree to within 2.6%, 7.0%, and 23%, respectively. The calculation is then inverted and η and F2 values for the four ions are determined by requiring the theoretical λn parameters to equal those obtained empirically. The η and F2 values obtained in this manner differ from those reported by 2.3% and 5.1%, respectively. It is therefore feasible, through the use of the effective spin–orbit Hamiltonian HE, to determine η and F2 values for the ground configuration of an ion by considering the ground term alone.
Thermomagnetic Gas Torque in Apparatus with Varied Dimensions
Authors: Williamson Robert M. , Roy Thomas N.
Published: February 2004

An axial magnetic field causes a rarefied gas to exert a torque on a heated cylinder (r) suspended in a gas chamber (R). The torque is a peaked function of both H and of p and is related to the Senftleben effect in dilute gases. The direction of the torque is independent of pressure (0.001 < p < 4 torr) and of chamber dimensions. The shape θ(H) of torque peaks is the same for N2, CO, and CH4 and is independent of pressure and of chamber dimensions for R / r >̄ 1.3. The shape is identical to that of the transverse thermal conductivity and wider than that predicted by the volume effect theory of Levi and Beenakker (L–B). The O2 torque shape is pressure dependent and can be represented accurately as the sum of two component peaks related to the J = K and K ± 1 states of O2. The position H0 / p of the torque peak is almost independent of chamber dimensions for p > 0.05 torr. The size of the torque varies as (p lnR / r)−1 for p > 1.0 torr, in agreement with both volume and surface effect theories. The torque varies as p1.5 for p < 0.01 torr. The measured pθ0 for N2 above 1.0 torr agrees with the L–B volume effect theory for R / r = 18 but drops to 25% of the theory as R / r is decreased to 1.07 and (R − r) to 1.5 mm. Also for (R / r) = 1.5 mm, the entire HD torque curve and the O2 and NO torques for H > 10H0 reversed direction. These data support the validity of the second-order Chapman Enskog approximation used in the volume effect theory and suggest that a reverse surface creep flow becomes important for chambers with small wall spacings.
Three-Body Recombination and Dissociation of Nitrogen: A Comparison between Theory and Experiment
Authors: Shui Ven H. , Appleton John P. , Keck James C.
Published: February 2004

The modified phase-space theory of reaction rates is used to calculate the over-all recombination and dissociation rate coefficients of nitrogen in a heat bath of argon atoms. Substantial quantitative agreement is obtained between the theoretical predictions and the low-temperature (90–611°K) “discharge-flow-tube” measurements of the recombination rate coefficient and the high-temperature (8000–15 000°K) “shock-tube” measurements of the dissociation rate coefficient. The success of the theory in correlating the experimental measurements over such a wide temperature range clearly illustrates the importance of the weak attractive forces between the nitrogen and argon atoms for recombination at low temperatures, the marked reduction in the rates at high temperatures due to nonequilibrium distributions in the vibrational state populations of the molecules, and the major contribution to the over-all reaction rate coefficients due to reaction progress via the first electronically excited molecular state of nitrogen over the entire temperature range. The working relationships required for applying the modified phase-space theory to predict the dissociation and recombination rate coefficients of other diatomic molecules in the presence of weakly attracting collision partners, such as argon atoms, are summarized.
Profiles of Foreign-Gas-Broadened Atomic Lines and Satellite Bands
Authors: Kielkopf John F. , Davis John F. , Gwinn Joel A.
Published: February 2004

The intensity profiles of cesium principal series lines and the associated satellite bands produced by moderately high pressures of xenon are calculated on the basis of a Lennard-Jones potential and collision broadening. Experimental measurements of the contours show fairly good agreement, especially for high series members. Intensity ratios of the satellite to the parent line, calculated for binary interactions only, agree with measurements at several different pressures.
Fluorescence and Nonradiative Decay of Anthracene in Crystal and Solution
Authors: Sharf B. , Silbey R.
Published: February 2004

The marked decrease in fluorescent quantum yield of anthracene in going from crystal to solution is discussed theoretically. The relative importance of direct and indirect radiationless transitions from the excited singlet to the triplet manifold is calculated using a simple model.
Dissociative Attachment in CO and Formation of C−
Authors: Stamatovic A. , Schulz G. J.
Published: February 2004

The negative ions O− and C− produced by dissociative attachment of electrons in CO are measured using a quadrupole mass spectrometer. A trochoidal monochromator is used to give an electron energy distribution of about 0.07 eV. Within this resolution, the O− ion current exhibits a vertical onset at threshold and structure is observed at higher energies. The cross section of the production of C− shows an onset at 10.20 ± 0.04 eV, a peak at 10.4 eV, and a second peat at 10.9 eV. The cross section at the first C− peak is found to be smaller than the peak cross section for O− production by a factor of 3300 ± 20%. Using the previously determined value for the O− cross section (2 × 10−19 cm2) the peak cross section for C− formation is about 6 × 10−23 cm2.
Chemiluminescent Emission of CO Fourth Positive Bands in Nitrogen Atom/Oxygen Atom/Reactive Carbon Compound Systems. Relation to Chemi-ionization
Authors: Fontijn Arthur , Ellison Roy , Smith William H. , Hesser James E.
Published: February 2004

Emission of the CO Fourth Positive Bands (A 1π–X 1Σ+) is observed in the reactions of atomic N/O mixtures with C2F4, C2H4, C2H2, and C2N2 at about 300°K and pressures of 1–4 Torr. The N/O/C2F4 system, which has been investigated most extensively, produces CO A 1π–X 1Σ+ emission which, like the NO+ chemi-ion formation rate, follows [N]3[O] kinetics. The level of excitation extends to at least 10.3 eV above CO(X 1Σ+, υ = 0). It is shown that the formation of CO(A 1π) can be attributed to enhanced formation of excited N2 and NO molecules in atom-recombination reactions induced by a reaction intermediate (probably CN), followed by pooling of the energy of an N2* and an NO* molecule and energy transfer to the reaction product CO(X 1Σ+). The transfer reaction can take place before or after the pooling reaction. Formation of CO(A 1π) in the N/O/C2H4 system exhibits the same kinetic dependence on N and O atoms. For the N/O/C2H2 system, the observations indicate CO(A 1π) formation both via the above energy-pooling mechanism and via the mechanism operative in the O/C2H2 reaction [O+C2O→CO(A 1π)+CO(X 1Σ+)]. In the N/O/C reactions, the higher vibrational levels of CO(A 1π) are relatively more populated than in the O/C2H2 reaction.
Intrinsic Photoconductivity in Naphthalene Single Crystals
Authors: Braun C. L. , Dobbs G. M.
Published: February 2004

For naphthalene single crystals, we observe charge carrier generation which exhibits quadratic intensity dependence for photon energies from 3.8 to 5 eV. Our experiments appear to rule out involvement of triplet excitons, and we attribute ionization to the mutual annihilation of two singlet excitons. The room temperature rate constant for singlet–singlet annihilation is calculated to be γ ≤ 1.7 × 10−9cm3sec−1 and that for carrier generation is β = 2.3 × 10−14cm3sec−1. These values are compared with those for anthracene where the latter values are revised based on a recent estimate of the intrinsic anthracene singlet exciton lifetime. For photon energies above 5 eV, we observe increased carrier yield and decreased intensity dependence exponent and tentatively ascribe these results to onset of single-photon ionization at about 5 eV.
Dipole Moments and Hyperfine Structure of the Group IV/VI Diatomic Molecules
Authors: Hoeft J. , Lovas F. J. , Tiemann E. , Törring T.
Published: February 2004

Stark effect measurements on the pure rotational transitions of the Group IV/VI compounds are reported. The electric dipole moments of the molecules with the highest natural abundance were determined for the ground vibrational state. For SnS the variation of the dipole moment with vibrational state was also measured. The nuclear quadrupole hyperfine structure, which originates from the 33S and 73Ge nuclei, has been measured for nearly all of the molecules containing these nuclei. A systematic trend in the dipole moments of the Group IV/VI compounds is reported and compared with the alkali halides.
Extended Hartree–Fock Calculations for the Ground State and Hartree–Fock Calculations for the First Excited State of H2
Authors: Bowman Joseph D. , Hirschfelder Joseph O. , Wahl Arnold C.
Published: February 2004

The extended Hartree–Fock (double configuration self-consistent field–DCSCF) energies and orbitals for the ground state (1Σg+) of H2 and the Hartree–Fock energy and orbitals for the first excited state (3Σu+) of H2 are calculated for separations ranging from the united atom to 10a0. The Σg orbital obtained from the Σ1g+ is almost the same as that from the Σ3u+ for all separations. However, when R < 5a0, the Σu orbital from the Σ3u+ is significantly more diffuse than that from the Σ1g+. The correlation energy (EHF − E) is four times larger for the Σ1g+ than for the Σ3u+ at the equilibrium separation, 1.4a0. For separations greater than 8a0, the two correlation energies become equal.
Low-Energy Electron-Reflection Spectrometry for Thin Films of Aromatic and Aliphatic Molecules at 77°K
Authors: Lewis David , Merkel Paul B. , Hamill William H.
Published: February 2004

Low-energy electron-reflection (LEER) spectra have been measured for thin films of pyridine, mesitylene, toluene, aniline, 1-hexene, cyclohexene, 2-methylpentane, and cyclohexane at 77°K. Differential plots show clearly defined peaks which appear to correspond to near-resonant energy losses. For the aromatic molecules these peaks are matched by spectroscopically measured electronic energy levels, including both singlets and triplets. The aliphatic molecules show low-lying bands in the LEER spectra, indicating states which have not been proposed previously and do not correlate with spectroscopically measured energy levels. For 1-hexene the two lowest peaks occur at ∼ 3.1 and ∼ 4.6 eV; for cyclohexene at 3.0 and 4.7 eV; for 2-methylpentane at 3.9 and 4.9 eV; and for cyclohexane at 3.9 and 4.8 eV.
Magnetic Resonance Study of the V2O5–P2O5 Semiconducting Glass System
Authors: Landsberger F. R. , Bray P. J.
Published: February 2004

The 51V NMR of the diamagnetic V5+ and paramagnetic V4+ ions has been studied in the semiconducting V2O5–P2O5 glass system. From computer-simulation studies of the 51V NMR experimental spectra due to the V5+ sites, it has been determined that the spectra obtained at the higher spectrometer frequencies can be described by a skewed distribution of powder patterns arising from axially symmetric chemical shifts. The most probable value of the chemical shift distribution (Σ‖)mp of the chemical shift parameter Σ‖ decreases with increasing V2O5 content and (Σ⊥)mp is independent of glass composition. An estimate of the nuclear quadrupole coupling constant of the V5+ site has been made; it ranges from 1.0 to 1.5 MHz as the V2O5 content of the glass is increased. The NMR data are interpreted in terms of two types of VO5 units in the glass. One is a VO5 unit similar to that in crystalline V2O5 and the other is a VO5 unit with a PO4 unit substituted for the apex oxygen. From ESR intensity measurements, the ratio of the fractions of V4+ and V5+ ions (n4 / n5) present in the glass has been determined as a function of glass composition. There is some evidence to indicate that two V4+ sites are formed for each PO4 unit that is substituted for an apex oxygen of a VO5 unit. The implications of these data for the electrical conductivity of the glasses are discussed.
Inequalities for van der Waals Force Constants and Quantum Mechanical Sums
Authors: Kramer Howard L.
Published: February 2004

Quantum mechanical oscillator strength sums S(k) are used to find rigorous upper and lower bounds to van der Waals coefficients for two- and three-body interactions and to the sums themselves. It is shown that any two sums of any multipole order give a bound to any other sum and either a bound or an estimate for the like force constant for that multipole. An average energy function is defined in terms of the sums; a particular limiting case of this function gives an excellent approximation for force constants (rms deviation for 19 cases: 0.25%) and depends solely on the value and slope of S(k) at k = − 2. The results are tested on data for 18 dipole cases: H, He, Ne, Ar, Kr, Xe, Li, Na, K, Rb, Cs, Hg, H2, N2, CH4, He(21S), He(23S), He+; and 1 quadrupole case: H.
Electronic Absorption and Fluorescence Spectra of Anthracene, [D10]-Anthracene, and Acridine in n-Alkane Matrices at 4°K
Authors: Macnab Robert M. , Sauer Kenneth
Published: February 2004

A systematic examination has been made of the electronic absorption and fluorescence spectra of anthracene and [D10]-anthracene in n-hexane, n-heptane, and n-octane matrices at 4°K. Vibrational analyses have been made and the correspondence between most of the modes of the two molecules has been established. A number of errors in previously published anthracene data are pointed out. The existence, in the fluorescence spectrum of anthracene in n-heptane at 4°K, of a line which lies 391 cm−1 above the electronic origin has been shown to be part of the environment multiplet structure characteristic of these matrix spectra. It is clear that the line has no connection with the intense 390-cm−1 vibrational frequency since its position with respect to the main origin is the same in the spectrum of [D10]-anthracene, for which the vibrational frequency is 376 cm−1. A possible hypothesis that fluorescence was occurring from vibrationally excited molecules is therefore ruled out. The distinction between (a) “environment multiplets” for molecules in different crystal environments and (b) spectral components deriving from local lattice vibrations is stressed. The environment multiplet structure of the undeuterated and deuterated molecules are found to be virtually identical. The 4°K spectrum of acridine in n-heptane shows some weak sharp lines on broad background of absorption. The line shapes of the 77°K anthracene and 4°K acridine spectra are discussed in terms of the relative half-widths and intensities of components representing no-phonon and phonon transitions, respectively.
Magnetic Properties of TbIn3, TbPt3 and HoIn3
Authors: Nereson Norris , Arnold George
Published: February 2004

Neutron-diffraction measurements made on powder samples of TbIn3, TbPt3, and HoIn3 show that all of these compounds exhibit antiferromagnetism below Néel temperatures of 37 ± 1°K, 20.5 ± 0.5°K, and 11.5 ± 0.5°K, respectively. The magnetic reflections from TbPt3 can be indexed on a doubled chemical cell of cubic symmetry. A possible magnetic model consists of an arrangement where the Tb moments are aligned in opposite directions in adjacent (111) magnetic planes. The magnetic reflections from TbIn3 and HoIn3 can be indexed on a doubled chemical cell of tetragonal symmetry. An appropriate magnetic arrangement is one in which the rare-earth moments are aligned in opposite directions in adjacent (110) magnetic planes. For TbIn3 the moment direction makes an angle of 10 ± 5° with the unique or c axis of the magnetic cell; for HoIn3 the corresponding moment direction is 58 ± 5°. The saturation ordered moment on the rare-earth atom sites in all three compounds is approximately 10% less than the calculated or expected values. The magnetic structures are compared with predictions of the indirect exchange theory.
Initiation of Explosives by High-Energy Electrons
Authors: Phung P. V.
Published: February 2004

With very short and very intense pulses of fast electrons such as from a Febetron machine, many explosives can be initiated at doses clearly insufficient for uniform heating to initiation temperature. This provides an opportunity for further testing of the hot-spot theory on initiation of explosives. In this study, a stochastic model is presented to describe the nonuniform spatial distribution of energy packets arising from absorption of electrons or photons. Computer calculations using an appropriate heat conduction equation determine what average temperature (or dose) and corresponding hot-spot temperature are required for initiation. Critical doses calculated for PbN6, PETN, RDX, HMX, and nitroglycerine are 37.8, 21.4, 38.5, 37.3, and 21.6 cal/g, respectively. Experimental doses for dextrinated PbN6, BaN6, basic lead styphnate, and normal lead styphnate, are estimated from limited experimental data as 11, 19, 18, and 10 cal/g, respectively. Factors responsible for differences between calculated and observed values are indicated. The correct order of magnitude of the calculated results is considered as evidence supporting the thermal mechanism in initiation of explosives by high-energy radiations.
Apparent Heat of Formation of HDO in Mixtures of H2O and D2O
Authors: Duer Wayne C. , Bertrand Gary L.
Published: February 2004

The heats of solution of H2O and D2O in mixtures of H2O and D2O at 25°C have been measured. Results are interpreted in terms of the reaction to form HDO, assuming an ideal ternary solution. The possible contributions of nonideality to the standard heat of formation of HDO(l) are considered.
Three-Body Correlations in Simple Dense Fluids
Authors: Raveché Harold J. , Mountain Raymond D.
Published: February 2004

Correlations between triples of molecules in simple fluids at thermodynamic equilibrium are studied through their contribution to the isothermal density derivative of the pair probability density. Explicit computations are performed to indicate the role of the triplet function in accounting for the structure of the density derivative of the radial distribution function. The results imply that contributions from triplet correlations are in general quite appreciable. Various consequences of the results are disscussed, and the procedure is examined in general as a method for studying correlations between triples of molecules in simple fluids.
Fluorescence Lifetime of Benzene and Benzene-d6 Vapor Excited to Single Vibronic Levels
Authors: Selinger Benjamin K. , Ware William R.
Published: February 2004

The fluorescence decay curves and lifetimes of benzene and benzene-d6 vapor have been determined for excitation to each of several low-lying single vibronic levels of the B12u state. Measurements were made at pressures as low as 0.1 torr, where the benzene system is essentially collision free, and at higher pressures of both benzene and benzene plus an added thermalizing gas. The lifetime was found to vary with the vibrational level acting as the origin for emission. The variation of the radiative lifetime with vibronic level was consistent with the predictions of Herzberg–Teller Theory for vibronically induced transition. In addition, there appears to be a deuterium isotope effect on the nonradiative decay processes. The fluorescence lifetimes obtained permit a re-examination of some of the steady-state data in the literature pertaining to the variation of the quantum yield of fluorescence with pressure. Also, inefficient electronic quenching is found when isopentane is used as a thermalizing gas.