bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic Excitations of Open Shell Systems in the Grand Canonical and Canonical Time-Dependent Hartree-Fock Models. Applications on Hydrocarbon Radical Ions
Authors: Jo/rgensen Poul
Published: February 2004

Open shell extensions of the conventional monoexcited configuration interaction method and the time-dependent Hartree-Fock model are developed by using the grand canonical and the canonical ensemble averaging procedures. The electronic excitations are considered to be from a ground state which has fractional occupational numbers. The models are applied to conjugated hydrocarbon radical ions within the Pariser-Parr-Pople model.
Theory of Semiclassical Transition Probabilities for Inelastic and Reactive Collisions. V. Uniform Approximation in Multidimensional Systems
Authors: Marcus R. A.
Published: February 2004

An integral semiclassical expression for the S matrix of inelastic and reactive collisions was formulated earlier in this series. In the present paper a uniform approximation for the expression is derived for the case of multidimensional systems. The method is an extension of that employed in Part II for the case of one internal coordinate. The final result, Eq. (2), is highly symmetrical, thus making some of its properties immediately clear.
Convergence of the Hydrogenic Expansion in H+–H Scattering
Authors: Rapp Donald , Dinwiddie David
Published: February 2004

Calculations have been carried out for the processes H++H(nl)→ H(nl)+H+→ H++H(nl)using a two-centered expansion in travelling-wave hydrogen orbitals. New four-state calculations were done with 1s, 2s, 2p0 and 2p1 states included. After taking proper account of the asymptotic coupling between states, our cross sections are in close agreement with those of Cheshire, Gallaher, and Taylor. In addition, seven-state calculations were performed by including the 3s, 3p0, and 3p1 states. A comparison of four- and seven-state results shows that the convergence of the cross sections for the n=2 states appears to be fairly rapid.
Theory of Activated Radiationless Processes: Orientation Relaxation of Polar Molecules
Authors: Fong Francis K. , Diestler Dennis J.
Published: February 2004

A first-principles theoretical expression for the rate of a thermally activated process is derived. The treatment is specialized to the orientation relaxation of dipolar molecules, which is viewed as a radiationless transition between electronic states corresponding to the dipole in discrete orientations. The dipolar electronic states are calculated in the Born-Oppenheimer approximation and the associated adiabatic potential-energy surfaces are taken to be displaced, undistorted harmonic potentials. Transitions between the different electronic states arise through the nonadiabatic kinetic-energy operator associated with the librational degree of freedom of the reorienting dipole. The explicit expression for the transition rate is calculated to first order by evaluating the integral over the time-correlation function of the Kubo representation of the rate constant in the strong-coupling limit using the method of steepest descent. It is shown that the transition rate is actually a sum of five contributions arising from phonon scattering processes in which 0, ±1, ±2 quantum changes take place in the librational degree of freedom of the dipole. When kT is large compared with a quantum of librational energy, the expression for the transition rate simplifies to the functional form of the Arrhenius equation in which the pre-exponential factor and the activation energy are given in terms of specific molecular properties. The theory is applied to a quantitative treatment of the relaxation of dipolar impurities in ionic crystals. The dielectric behavior of polar organic molecules in liquids is also qualitatively examined, and the explicit dependence of the relaxation time on the molecular dimensions in a homologous series of compounds is obtained. Even though the theory is specialized to dipolar relaxation, the basic expressions are generally applicable to any thermally activated process involving the migration of ions or atoms, e.g., ionic conductivity, mobility, viscous flow, and diffusion.
Effects of Gamma Irradiation on Actinide Ions in Calcium Fluoride
Authors: Stacy J. J. , Edelstein N. , McLaughlin R. D.
Published: February 2004

Optical absorption and thermoluminescence measurements were used to study the effects of gamma irradiation on trivalent actinide ions in CaF2. Thermoluminescent glow curves for Np, Pu, Am, and Cm (and, for comparison, the lanthanides Er, Ho, and Tm) were measured between 100 and 300°K. These were found to be remarkably similar, with glow peaks occurring at nearly the same temperatures for each of the ions. High resolution measurements of the spectra of the thermoluminescence showed that the emission is similar to the fluorescence of the trivalent ions. Evidence is presented that the glow emission below 300°K originates from trivalent actinide ions in cubic sites. When the actinide-CaF2 crystals were irradiated at 300°K, the actinide ion was oxidized to the tetravalent state. A mechanism has been proposed to explain these observations.
Energy Distribution among Reaction Products. VII. H + F2
Authors: Polanyi J. C. , Sloan J. J.
Published: February 2004

The reaction H+F2→ HF+F has been studied by means of the ``arrested relaxation'' variant of the infrared chemiluminescence technique. Detailed rate constants k (V′, R′, T′), where V′, R′, and T′ symbolize the vibrational, rotational, and translational energies in the reaction products, are reported in the form of a contour plot. The total detailed rate constants into specified vibrational quantum states for H+F2 (summed over the rotational levels of each v′ level) are k (v′ = 1) = 0.12, k (v′ = 2) = 0.13, k (v′ = 3) = 0.25, k (v′ = 4) = 0.35, k (v′ = 5) = 0.78, [k (v′ = 6) = 1.00], k (v′ = 7) = 0.40, k (v′ = 8) = 0.26, k (v′ = 9) < 0.16 (relative to the highest rate constant k(v̂′)=1.00). In common with other members of the H+X2 family of reactions (X is a halogen) the H+F2 reaction exhibits a comparatively low fractional conversion of the total available energy into internal excitation of the new molecule. The mean fractions entering vibration and rotation are f̄V′=0.53 and f̄R′=0.03. This behavior appears to be characteristic of reactions involving substantial ``repulsive'' energy release and a light attacking atom. Evidence is presented to show that H+F2 occupies a special place in the H+X2 family in that the repulsive energy release (energy released as the products separate) is restricted to the period when the F atoms are at close range. An approximate relation log Ea∝ ρ2f[a⊥/(1−a⊥)], relating the activation energy Ea to the X–X bond extension at the termination of energy release, ρ2f, and the attractive energy release a⊥, is derived and is tested for the family of reactions H+X2. There is only a very small increase in product rotational excitation with decreasing vibrational excitation. It follows that the translational energy of the products is markedly greater for successively lower v′ states.
Three Atom Correlations in the Lennard-Jones Fluid
Authors: Raveché Harold J. , Mountain Raymond D. , Streett W. B.
Published: February 2004

The Monte Carlo method has been used to compute the triplet correlation function in a classical fluid with Lennard-Jones interactions. The computations were performed for particular configurations at five thermodynamic states of high density. The structure of the triplet function is discussed in the liquid and dense gas regions. Several closure approximations, which express the triplet function in terms of the pair correlation function, are compared to the Monte Carlo results.
Classical S-Matrix for Vibrational Excitation of H2 by Collision with He in Three Dimensions
Authors: Doll Jimmie D. , Miller William H.
Published: February 2004

Complex-valued classical trajectories have been computed by direct numerical integration of the equations of motion for three-dimensional collisions of He and H2, and from such trajectories classical S-matrix elements for transitions between specific rotational-vibrational states of H2 have been constructed. At the collision energies employed (∼1–2 eV) all vibrationally inelastic transitions are classically forbidden, thus the need for analytically continued, complex-valued trajectories. Comparison with the quantum mechanical calculations of Eastes and Secrest shows excellent agreement between the quantum and uniform semiclassical transition probabilities. Since, however, for three-dimensional collision systems one is seldom concerned with individual S-matrix elements, but rather sums over many of them, an important practical feature is developed which shows how one can combine the usual Monte Carlo classical treatment of some of the internal degrees of freedom with a semiclassical state-by-state description of others; i.e., one can ``quantize'' only those degrees of freedom that are highly quantumlike (e.g., vibration). This ``partial averaging'' approach also greatly simplifies the practical aspects of applying classical S-matrix theory to systems with several internal degrees of freedom.
Lifetimes of the A 2π and B 2Σ States of the CN Radical
Authors: Cook Thomas J. , Levy Donald H.
Published: February 2004

The lifetimes of the A 2Π(v=10) and B 2Σ(v=0) states of the CN radical have been measured from an analysis of the zero electric field limit linewidth of the level anticrossing spectrum. The measured value of τΣ=39.4± 9.3 nsec was found to be in reasonable agreement with the most recent phase shift measurement. The value of τπ=137± 45 nsec was in serious disagreement with previous direct decay measurements on v≤ 9 vibrational levels, but in essential agreement with previous microwave and intensity measurements on the perturbed v=10 level.
Effect of H2 Pressure on Pulsed H2+F2 Laser. Experiment and Theory
Authors: Suchard S. N. , Kerber R. L. , Emanuel G. , Whittier J. S.
Published: February 2004

Stimulated emission predictions and measurements for an H2+F2 laser are compared for H2 pressures from threshold to stoichiometric, a range of several orders of magnitude. Slowly flowing, helium-diluted, 50 torr mixtures are initiated photolytically. Two dilution ratios and two output couplers are considered, and good agreement is found for time-to-threshold and pulse duration vs H2 pressure. Spiking, relaxation oscillations, and possibly mode beating, features not modeled, are observable in some pulses; however, predicted intensity vs time generally agrees in pulse shape with laser output. Observed and predicted peak intensities nearly match for low H2 pressure, and the predicted increase of peak intensity with low H2 is followed fairly well. For H2 in the vicinity of one-tenth stoichiometric, the peak intensity data show an abrupt leveling off, while the calculations predict a continuing increase. This disagreement most probably cannot be attributed to uncertainties in the kinetic model. All rate modifications considered have proven incapable of producing a change sufficiently large or abrupt to explain this feature of the data. Experimental results are presented supporting the notion that parasitic oscillations cause this change in laser output.
Bond Order in LCAO Molecular Orbital Theory
Authors: Cohen Irwin
Published: February 2004

On the basis of LCAO theory, the following intrinsic definition of bond order is derived: Bij=pijSij+pijfijgij,where i and j index basis functions on different centers, pij is the corresponding charge-and-bond-order matrix element, Sij is the overlap integral, fij is a long range factor, and gij is an atomic hybridization and nonorthogonality factor. The term pijSij is the overlap population and the term pijfijgij is the associated net atomic population; the latter is defined by a reference homopolar bond constructed from normalized hybrids of compositions determined by the LCAO wavefunction. The bond order, Bij, reduces in the appropriate special cases to the Coulson, Mulliken, and Wiberg bond orders. However, Bij is not limited to these cases but is also valid for analysis of any LCAO wavefunction. It may also be combined with a Mulliken population analysis to resolve the total electron population into bonding terms and residual lone pair terms. Similarly, the valence electron population may be resolved into covalence, electrovalence, and free valence. Applications are given to HF, the Group I fluorides, and some organic molecules.
14N NQR Spectroscopy of Some Amino Acids and Nucleic Bases via Double Resonance in the Laboratory Frame
Authors: Blinc R. , Mali M. , Osredkar R. , Prelesnik A. , Seliger J. , Zupančič I. , Ehrenberg L.
Published: February 2004

The 14N nuclear quadrupole resonance spectra and spin-lattice relaxation times of the amino acids histidine, methionine, cystine, cysteine, and tyrosine as well as of the nucleic bases uracil, thymine, cytosine, and guanine have been determined by a 14N-proton double resonance technique in the laboratory frame. The experiments were performed on polycrystalline samples at 77°K or above this temperature. A theoretical estimate of the sensitivity of this method for a variety of experimental conditions is as well presented. It is shown that in contrast to usual double resonance techniques the method works even in the case of a short proton spin-lattice relaxation time if only the nitrogen relaxation time is long.
Self-Diffusion in Liquid Water to −31°C
Authors: Gillen Kenneth T. , Douglass D. C. , Hoch M. J. R.
Published: February 2004

The self-diffusion coefficient of water is reported to −31°C, where the activation energy reaches 11 kcal/mole compared with 4.5 kcal/mole at 25°C. The similarity of the temperature dependence of the fraction of broken hydrogen bonds, as inferred from Raman and infrared data, and the diffusion coefficient over the entire liquid range forms the basis of empirical support of the dominant role of hydrogen bonding in the fundamental diffusion mechanism.
Formation of HeH+ from Low-Energy Collisions of Metastable Helium and Molecular Hydrogen
Authors: Neynaber R. H. , Magnuson G. D. , Layton J. K.
Published: February 2004

A merging-beams technique has been used to study HeH+ formation resulting from the interaction of a composite beam of He(21S) and He (23S) with a beam of H2. The experiments were conducted over a range of interaction energy W from 0.05 to 10 eV. It was learned that the dominant process for producing HeH+ is the rearrangement ionization reaction He*+H2→HeH++H+e, where He* represents the metastable helium. The experiment consisted of measuring the lab-energy distributions of HeH+. Relative and absolute cross sections were obtained from these distributions. The distributions indicate that, in the center-of-mass system, most of the HeH+ is scattered in the direction of the incident He*. The energies associated with the HeH+ at the peaks of these distributions are very close to those expected for spectator stripping. Conservation of energy for the reaction can only be achieved by assuming that the electron carries away a substantial portion of the available energy. It is shown that the two-step model He*+H2→He+H2+†+e→HeH++H+e (where † indicates ground-state or vibrationally excited H2+) satisfactorily explains the measured lab-energy distributions near W=0.05 eV but does not agree completely with the results for W≥ 2 eV.
Infrared Study of Matrix-Isolated Lithium Isocyanide
Authors: Ismail Zakya K. , Hauge Robert H. , Margrave John L.
Published: February 2004

The infrared spectrum of lithium isocyanide trapped in solid matrices of neon, argon, and nitrogen was examined over the range 4000–33 cm−1. The effect of temperature cycling, which results in migration of trapped molecules to generate polymeric species, has been pursued. A careful study of the effect on the spectra of superheating the vapor species has been carried out. By these means it has been possible to identify the bands in the spectrum due to monomeric and to polymeric species. This paper is only concerned with the monomeric species. Isotope frequency shifts were measured for carbon-13, nitrogen-15, and lithium-6 enriched samples of lithium isocyanide. From the measured isotopic shifts it was possible to determine the structure of the monomer of the lithium compound as being linear LiNC. Accurate force constants were calculated using the measured frequency values of the three fundamental vibrational modes of the different isotopic molecules of LiNC.
Effect of Thickness on the Layer Structure of Grandjean Texture in a Sheared Cholesteric Liquid Crystal
Authors: Pochan J. M. , Marsh D. G.
Published: February 2004

Reflection spectra of the sheared (1.32 × 102sec−1) Grandjean texture of the cholesteric mesophase have been obtained as a function of sample thickness in the visible wavelength region. These studies substantiate a model of the sheared mesophase composed of layers consisting of normal Grandjean, perturbed Grandjean, and dynamic focal conic textures. Changes in sample thickness affect principally the perturbed Grandjean layers. The central dynamic focal conic layer is shown to be relatively thin (<7 μ).
NMR Relaxation Study of Liquid CCl3F. Reorientational and Angular Momentum Correlation Times and Rotational Diffusion
Authors: Gillen K. T. , Douglass D. C. , Malmberg M. S. , Maryott A. A.
Published: February 2004

Using pulsed NMR techniques, values of the self-diffusion constant Ds and the 19F spin-lattice and rotating frame relaxation times, T1F and T1ρF have been obtained for CCl3F over its entire liquid range. (∼150–450°K). The dependence of T1ρF on the rotating field strength ω1 has been used to derive temperature-dependent values of the 35Cl spin-lattice relaxation time T1Cl and the chlorine to fluorine spin-spin coupling constant JF19–Cl35(=11.9± 0.4 Hz, independent of temperature). Except at low temperatures where the intermolecular dipole-dipole relaxation mechanism is important, T1F is dominated by the spin-rotation interaction (T1F)sr. Using Ds data to separate the dipole-dipole contribution from T1srF allows us to estimate values of the angular momentum correlation time τJ over a 300° temperature range. Over the same temperature range, values of T1Cl give the correlation times for molecular reorientation τθ,2. Although possible anisotropy in molecular motion and in the spin-rotation interaction preclude rigorous quantitative comparisons with rotational diffusion theory, the results for τJ and τθ,2 are shown to be consistent with Gordon's extended J diffusion model. In particular, at high temperatures the molecular reorientation is no longer described by the small angular steps implied in classical theory: near the critical temperature τJ and τθ,2 become of comparable magnitude and correspond to angular steps approaching 1 rad.
Electron Spin Relaxation Studies of Manganese(II) Complexes in Acetonitrile
Authors: Lynds Lahmer , Crawford Jane E. , Lynden-Bell R. M. , Chan Sunney I.
Published: February 2004

The electron spin resonance spectra of [Mn(CH3CN)6]2+, [MnCl4]2−, and [MnBr4]2− in acetonitrile solutions have been examined as a function of manganese concentration, counterion concentration, halide concentration, temperature, and Larmor frequency. Particular emphasis was directed towards the elucidation of the dynamical processes which lead to broadening of the hyperfine components of the tetrabromomanganese(II) complex. The resonance widths of the [MnBr4]2− system were found to have the empirical formΔ H =a+b[Br−]+c[MnBr4]2−,which we have interpreted in terms of the following three contributions to the transverse relaxation rate: (a) an intrinsic part due to fluctuations in the solvent sheath around the ion, including ion pairing, (b) a part due to bromide ligand exchange, and (c) a part due to dipolar coupling between colliding sextet ions. Of these results, perhaps the most significant single finding is the lability of the tetrabromomanganese(II) complex towards ligand replacement in acetonitrile indicated by this work. The data suggest that the ligand exchange is bimolecular, involving one bromide and one [MnBr4]2−, and the bimolecular rate constant of ∼ 1× 109liter mole−1·sec−1 deduced over a wide temperature range suggests that the kinetics of the ligand exchange is almost diffusion controlled with essentially zero enthalpy of activation. The apparent entropy of activation was found to be −22 e.u., which is consistent with an association mechanism, possibly involving a five-coordinate intermediate presumably of C3 symmetry. A pronounced frequency dependence of the dipolar contribution to the relaxation rate has also been noted in the case of the [MnBr4]2− system, a result which we have taken to indicate that the spin-spin interaction between the sextet anions is modulated at a time scale which is no longer in the ``extreme narrowing'' region at K-band frequency. A density matrix treatment which accounts for these observations is presented.
Semiclassical Three-Dimensional Model for Vibrational Relaxation
Authors: So/rensen Gert Due Billing
Published: February 2004

In a semiclassical three-dimensional treatment of the atom-diatom scattering problem, we have included the intermultiplet transitions to infinite order, whereas simultaneous rotational and vibrational transitions are treated to first order. Thus we have found criteria for neglect of intermultiplet transitions. With this model we have investigated three systems, namely, the He–H2, the Ar–HCl, and the Ar–O2 systems. We found that transitions with change in both vibrational and rotational quantum number were dominant for the vibrational relaxation process in the Ar–HCl case, while they were important for the Ar–O2 system and negligible for the He–H2 system.
Statistical Mechanical Study of Hydrophobic Interaction. III. Generalization and Further Applications
Authors: Ben-Naim A.
Published: February 2004

The method of studying solute-solute interaction developed in previous papers [A. Ben-Naim, J. Chem. Phys. 54, 1387, 3696 (1971)] is extended to include more complex solute molecules. The modification in the general formalism is due to the possible existence of internal rotations. Some new applications based on available experimental data are demonstrated, and further possible uses are indicated.
Shock-Tube Study of the Acetylene-Oxygen Reaction. IV. Kinetic Study of CH, C2, and Continuum Chemiluminescence During the Induction Period
Authors: Matsuda Shimpei , Slagle Irene R. , Fife Dennis J. , Marquart John R. , Gutman David
Published: February 2004

The induction period of the C2H2+O2 reaction is accompanied by the exponential growth of chemiluminescent emission from several sources. The emission between 300 and 550 nm has been spectrally analyzed as well as dynamically monitored using an end-on shock-tube technique. Light emitted during the early portion of the induction period consists of emission from CH(A 2Δ), CH(B 2Σ−), and CH(C 2Σ+) superimposed on a weakly structured or unstructured emission (called the continuum) which has a maximum intensity near 350 nm. The exponential growth constants of these four emissions as well as their relative intensities were measured over a 1000°K temperature range (1100–2100°K), a tenfold range in [O2], and a twofold range in [C2H2]. All these emissions grow exponentially with the same growth constant as that of a major induction-period product CO, indicating that they originate from reactions first order in reaction intermediates. The relative intensities of pairs of these four emissions were found to be independent of mixture composition, and the relative intensities of the three CH emissions were also independent of temperature (the ratio of CH to continuum emission intensity has a 14 kcal ``activation energy''). The results suggest that all four emissions are produced by the same reactants. A mechanism for the production of the CH and continuum (attributed to electronically excited HCO) emissions observed during the C2H2+O2 induction period is proposed to account for the experimental findings. It consists of the reactionsC2H+O2→CH(A 2Δ, B 2Σ−, C 2Σ+)+CO2and C2H+O2→HCO(Ã, B̃, C̃) +CO.Reaction (1) has the same activation energy (25±10kcal/mole) for the production of all three CH electronic states, and this activation energy is 14 kcal/mole larger than that of Reaction (2). This and other mechanisms are discussed. During the latter portion of the induction period, chemiluminescent emission from C2(A 3Πg) was also observed. The exponential growth constants for this emission were indirectly compared with those of the induction-period product CO over a wide range of experimental conditions. In each experiment the growth constant for C2(A 3Πg) emission was twice that for CO production, indicating that C2(A 3Πg) is produced in a reaction second order in reaction intermediates. Several possible reactions responsible for this emission are discussed.
Dielectric Relaxation and Complex Formation in Dilute Solutions of Chloroform and Bromoform in Nonpolar Solvents
Authors: Boule P.
Published: February 2004

Measurements of dielectric permittivity and loss in the microwave region at 20°C have been made on dilute solutions of chloroform and bromoform in cyclohexane, benzene, mesitylene, and 1,4-dioxane, and on chloroform in 2,5-dimethylpyrazine. The shortest relaxation times are found in cyclohexane solution and the longest in mesitylene, dioxane, and dimethylpyrazine. For the solutions of chloroform in benzene and mesitylene use of literature values for constants of association allows calculation of the relaxation time of the complex. For the other mixtures a method is proposed for the determination of both the constant of association and the relaxation time of the complex. The relaxation times for the complexes of chloroform with mesitylene, dioxane, and dimethylpyrazine are comparable and can be attributed to rotational reorientation of the complex. A similar conclusion may be drawn for the complexes of bromoform with mesitylene and dioxane. In benzene the relaxation times of the complexes with chloroform and bromoform are too small to be attributed to rotational reorientation and presumably reflect the lifetime of the associated species instead. These conclusions are consistent with the enthalpies of mixing for the above solutions.
Mobilities of Uranium and Mercury Ions in Helium
Authors: Johnsen R. , Biondi Manfred A.
Published: February 2004

The mobilities of mass-identified U+ and Hg+ ions in helium have been determined in a drift tube-mass spectrometer apparatus. For uranium ions a reduced mobility value μ0=(16.0± 0.5) cm2/V·sec is obtained at 305°K and a standard gas density of 2.69× 1019cm−3. The mobility of mercury ions is μ0=(19.4± 0.5) cm2/V·sec at 292°K, in agreement with two previous determinations. The effect of fast ion injection in drift mobility measurements is discussed, and a new technique to circumvent these problems is described. The results are compared with existing theories of ion mobilities.
Orbital Description and Properties of the BH Molecule
Authors: Blint Richard J. , Goddard William A.
Published: February 2004

The ground state wavefunction of the BH system has been investigated as a function of internuclear distance. The electronic wavefunctions were calculated using the G1 and SOGI methods. These methods yield an orbital description of the wavefunction which is used to interpret the bonding and the changes in properties of the molecule. The effect of spatial symmetry restrictions was investigated, and it was found that allowing π character into the orbitals provides a significant improvement in the description of the BH molecular system. Also reported are the dipole moment, quadrupole moment, electric field gradient, and some other properties as a function of the internuclear distance. In addition, we compare the orbitals of BH and BH2 and find that the use of G1 atomic and molecular orbitals allows a qualitatively useful discussion of molecular geometry and symmetry.
Generalized Susceptibility Theory. IV. The Structure of Electronic Absorption Spectra of Linear Polymers
Authors: Ziv Alan R. , Rhodes William
Published: February 2004

The role of the monomer absorption spectrum in determining the absorption spectrum of a linear polymer is studied in terms of a linear response theory formulation of the complex, frequency-dependent polarizability of the polymer. It is pointed out that a Ketteler-Helmholtz dispersion (combination of positive and negative frequency Lorentzians) is a fundamental form in the theory and, accordingly, model absorption spectra for the monomer, consisting of sets of Ketteler-Helmholtz bands, are used. In each case the polymer spectrum is calculated for various intermolecular coupling strengths. Under the right conditions, all of the intensity from a set of monomer bands appears in a single collective mode of the polymer [cf. A. Herzenberg and A. Modinos, Proc. Phys. Soc. (London) 87, 597 (1966)]. The spectral requirements for such strong coupling and the spectral features of the collective mode are determined. The latter is observed to become a Ketteler-Helmholtz in the strong coupling limit. The contribution of monomer transitions outside the observed frequency range is also considered.
Dipole Moment Studies. VI. Dipole Moments of the Propylphosphines
Authors: Morse Joseph G. , Parry R. W.
Published: February 2004

The dipole moments of mono-n-propylphosphine and mono-i-propylphosphine, as measured in the gas phase with a heterodyne beat apparatus, are 1.09±0.06 and 1.22±0.03 D, respectively. The moments of mono-n-propylphosphine, di-n-propylphosphine, and tri-n-propylphosphine as measured in benzene solution and corrected for solvent effect are 1.17±0.05, 1.22±0.05, and 1.16±0.08 D, respectively. The moments for mono-i-propylphosphine, di-i-propylphosphine, and tri-i-propylphosphine as measured in benzene solution and corrected for solvent effects are 1.13±0.03, 1.32±0.12, and 1.32±0.04 D, respectively. Discrepancies between gas phase and solution values are considered, and values for each series are discussed in terms of a bond polarization model.
Dipole Moment Studies. VII. Dipole Moments of the Halofluorophosphines
Authors: Morse Joseph G. , Parry R. W.
Published: February 2004

Dipole moment measurements have been made on a series of halophosphines using gas phase heterodyne beat methods. Values obtained are: PF3, 1.00±0.03 D; PF2Cl, 0.93±0.01 D; PFCl2, 0.86±0.05 D; PF2Br, 0.83±0.05 D; and PF2I, 0.85±0.06 D. The value for PF3 is in reasonable agreement with the microwave value [R. G. Schulman et al., Phys. Rev. 78, 145 (1950)], of 1.025±0.009 D. Molecular moments for the mixed halophosphines are approximated reasonably well by the vector sum of P-X moments, but the value for PF2H cannot be estimated in this way. An explanation is offered in terms of a polarization model.
19F Chemical Shift Tensor in Group II Difluorides
Authors: Vaughan R. W. , Elleman D. D. , Rhim W-K. , Stacey L. M.
Published: February 2004

The multiple pulse nuclear magnetic resonance techniques have been used to measure the 19F chemical shift, or nuclear magnetic shielding, tensor in a series of Group II difluorides. For the cubic difluorides measured values relative to a C6F6 reference are: CaF2, −61 ppm; SrF2, −82; BaF2, −154 ppm; CdF2, +33 ppm; and HgF2, +32 ppm. For the two noncubic difluorides principal values of the chemical shift tensors relative to C6F6 are; for MgF2, +13, +28, and +43 ppm; and for ZnF2, +15, +38, and +59 ppm. The chemical shifts from the cubic difluorides were found to correlate well with electronegativities and a covalency parameter calculated from electron spin resonance superhyperfine interaction parameters. A theoretical calculation of the chemical shift tensor for MgF2 is presented and give good agreement with the experimental data, accounting for 90% of the chemical shift anisotropy.
Quantum Vibrational Transition Probabilities in Atom-diatomic Molecule Collisions. A Tractable Three-Dimensional Model
Authors: Wilson David J. , Locker David J.
Published: February 2004

The method of Cheung and Wilson [J. Chem. Phys. 51, 3448, 4733 (1969)] is applied to the computation of vibrational transition probabilities for collisions of atoms with spherically symmetric oscillators (``breathing spheres'') in three dimensions. Effects of the angular momenta associated with rotation of the sphere and with the relative motion of the atom and sphere are taken into account. Takayanagi's modified wave number method [Progr. Theoret. Phys. (Kyoto) 8, 497 (1952); J. Phys. Soc. (Japan) 14, 75 (1959)] is employed to obtain approximations in closed form to the integrals which are needed.
Molecular Structure of Gaseous d,l-1,2,3,4-Diepoxybutane from Electron Diffraction Data
Authors: Smith Z. , Kohl D. A.
Published: February 2004

The d,l isomer of 1,2,3,4-diepoxybutane was studied in the vapor phase by electron diffraction. The following values were found for the bond lengths and bond angles (standard deviations are given in parentheses): r(C–C)ring= 1.463(0.005) Å, r(C–C)= 1.521(0.009) Å, r(C–O)= 1.439(0.004) Å, and ∠CCC= 115.2°. The data were consistent with a single conformation corresponding to a nearly trans configuration for the carbon chain.
Triplet State of p-Chloroaniline
Authors: Marchetti Alfred P.
Published: February 2004

The lowest triplet state of p-chloroaniline has been studied using single crystals at 4.2°K. The origin shows two lines separated by 2.0 cm−1 which have been interpreted as the allowed B1u and B3u k≈ 0 factor group states from polarization and Zeeman data. Low-field Zeeman spectra indicate that the ordering of the magnetic substates (that is, the three spin levels of the triplet state) in the crystal is Tb>Ta>Tc, which implies that the ordering in the free molecule is TM(y) > TL(z) > TN(x). Mixed crystal stark measurements yield a value of 0.64 D for the change in molecular dipole moment on excitation to the 3A1 state.
Rotational Diffusion of Symmetric Top Molecules in Liquids
Authors: McClung R. E. D.
Published: February 2004

The extended M- and J-diffusion models previously proposed to describe the reorientation of linear and spherical molecules are used to treat liquids composed of symmetric top molecules. The reorientational correlation functions and their Fourier transforms are determined and shown to reduce, in the J-diffusion model in the limit of very short angular momentum correlation times τJ, to the results obtained by solution of the anisotropic diffusion equation. The applicability of the diffusion equation results, outside the domain of validity of the diffusion equation, are examined in detail. Magnetic spin relaxation due to motional modulation of spin-rotational interactions is also considered within the framework of the extended diffusion models, and the inertial motion of the molecules and the precessional motion of the angular momentum are shown to influence the contributions due to the modulation of the anisotropic part of the spin-rotational interactions.
Intermolecular Potentials from Crossed Beam Differential Elastic Scattering Measurements. VII. para-H2+para-H2
Authors: Farrar J. M. , Lee Y. T.
Published: February 2004

The differential elastic scattering cross section for para-H2+para-H2 has been measured at a collision energy of 2.71×10−14 erg. Oscillations due to nuclear symmetry were observed in the angular distribution. The data were fit to a spherically symmetric multiparameter potential form giving a well depth ε /k= 34.8°K, rm= 3.49 Å. This potential is closer to those obtained from the total elastic scattering cross section by Gengenbach and Toennies and from differential cross sections by Dondi, Valbusa, and Scoles than to those derived from bulk properties and early valence bond calculations. The position of the bound vibrational level has been computed and is found 2.0 cm−1 below the continuum.
Measurement of Total Inelastic Cross Sections for Electron Impact in N2 and CO2
Authors: Spence D. , Mauer J. L. , Schulz G. J.
Published: February 2004

We use the trapped-electron method to study the total inelastic cross sections for electron impact in N2 and CO2 in the energy region of the low-lying compound states. These compound states are centered energetically at about 2.5 and 3.8 eV above the ground state of N2 and CO2, respectively. The measurement gives the total cross section for the excitation of vibrational states which lie above about 1 eV. A reasonable extrapolation gives an indication of the total cross section for excitation of vibrational states at 2.5 eV in N2 and 3.8 eV in CO2. For N2, the value of 6.0×10−16 cm2 is in excellent agreement with the analysis of the swarm data by Engelhardt, Phelps, and Risk. For CO2 the total inelastic cross section in the region of 3.8 eV is 8.3×10−16 cm2, almost an order of magnitude larger than the previously published value.
Variational Solution of the Linearized Boltzmann Equation with Applications to Helium
Authors: Revercomb H. E. , Bruch L. W.
Published: February 2004

High accuracy variational solutions for the Navier-Stokes coefficients of thermal conductivity and viscosity are obtained from a linearized Boltzmann equation for a dilute one-component gas with spherically symmetric interactions. Results are presented for quantum-mechanical hard spheres and for three helium potential models at 1–20°K. The deviation of the one-particle distribution function from a local Maxwellian is also obtained from the variational solutions and is compared with an approximate treatment of the Hilbert-Enskog equation.
Thermal Conductivity in Vibrationally Excited Gases
Authors: Ahtye Warren F.
Published: February 2004

Comparisons between experimental and theoretical values of the thermal conductivity for a series of nonpolar polyatomic gases show large discrepancies at higher temperatures where the vibrational contribution becomes increasingly important. Examination of various theoretical approaches indicates that the discrepancies can be attributed to approximating the coefficient characterizing the diffusion of vibrational energy, Dvib, by the self-diffusion coefficient. The present theories cannot accurately predict Dvib, and its measurement is extremely difficult. Consequently, the approach taken in this paper is to obtain Dvib by using experimental values of the thermal conductivity. An attempt is made to correlate these values of Dvib with vibrational relaxation time, so that values of Dvib can be obtained for all nonpolar polyatomic gases.
Quantum Mechanical Derivation of the Correction to the Boltzmann Equation Due to the Duration of Collisions
Authors: van Santen Rutger A.
Published: February 2004

Expressions are studied for the rate of change of momentum distribution that result from using a recent formulation of a quantum mechanical derivation of the Boltzmann equation obtained by Snider and Sanctuary. The gas is supposed to consist of spherical and structureless particles. If the distribution function is restricted so as initially to be spatially uniform, it will remain spatially uniform and the rate of change of momentum distribution is as given by the Boltzmann collision term. In case the distribution function initially is spatially nonuniform, the nonuniformity persists in the Boltzmann equation, and in addition to drift terms, a term appears containing the duration of the collisions. This can be related to the Enskog modification of dense gases in the case of hard sphere particles. A new quantum mechanical expression of this correction is presented.
Semiclassical Approximation of Multidimensional Bound States
Authors: Pechukas Philip
Published: February 2004

We extend WKB theory of bound states to all states of nonseparable systems derived from separable states by transformations that do not destroy the basic nodal structure of wavefunctions. We discuss the quantum and classical mechanics of semiclassical approximation and analyze the relation to other recent work.
Theory of Natural Optical Activity in Crystalline NiSO4· 6H2O
Authors: Strickland R. , Richardson F. S.
Published: February 2004

The optical activity of crystalline NiSO4· 6H2O is exclusively a property of the solid state, arising from a dissymmetric crystal environment. The rotatory strengths and dipole strengths associated with electronic transitions within the Ni2+ 3d manifold of states are calculated to second order in perturbation theory. It is assumed that optical activity is induced in the ligand-field bands of the octahedral NiO6 clusters by the perturbing influence of the neighboring crystal environment which has C2 symmetry. A crystal-field representation is adopted so that the crystal perturber sites (the H atoms of the water ligands, and O and S atoms of the SO42− anions) are treated as point charges ``seen'' by the NiO6 chromophoric sites. The signs and relative magnitudes of the rotatory strengths calculated for the 3A2g→3T2g, 3A2g→3T1g(F), and 3A2g→3T1g(P) cubic transitions of Ni2+ are in substantial agreement with experiment. It is concluded that the CD spectrum of crystalline NiSO4· 6H2O in the near infrared, visible, and near ultraviolet regions can be understood in terms of a model on which the optical activity is assumed to arise from dissymmetric perturbations of individual Ni2+ sites rather than from chiral exciton motion over the helically arranged Ni(H2O)62+ clusters.
Quantitative Study of Luminescence in Aromatic and Heteroaromatic Molecules
Authors: Li R. , Lim E. C.
Published: February 2004

Quantum yields of fluorescence and phosphorescence were determined for about 20 organic compounds (and their deuterated analogs) using fluorescence yield of 9, 10-diphenylanthracene as a reference standard. The emission yields so obtained are combined with appropriate phosphorescence lifetime data to deduce conclusions related to radiative and radiationless transitions in these molecules. Evidence is presented which indicates that the dominant singlet→triplet intersystem crossing is S1→ T1 in benzene but S1→ Tj(j>1) in larger aromatic hydrocarbons. The results also indicate that deuteration of polycyclic hydrocarbons significantly increases the radiative lifetime of the lowest state, contrary to previously held views. Solvent and temperature dependence of fluorescence in nitrogen-heterocyclic compounds and phosphorescence characteristics of aromatic carbonyl compounds are discussed in terms of vibronic interactions between nearby ππ* and nπ* states.
Vibrational Relaxation of the Bending Mode of Shock-Heated CO2 by Laser-Absorption Measurements
Authors: Eckstrom D. J. , Bershader D.
Published: February 2004

The vibrational relaxation characteristics of shock-heated CO2 have been studied using a tuned CO2 laser absorption technique. Absorption-coefficient histories were obtained for a single rotational state in each of the 10°0 and 02°0 levels over the temperature range from 500 to 2000°K, and for 21 rotational states of the 10°0 level at 1000°K. These histories have been combined with translational-rotational temperature histories based on interferometer measurements to calculate vibrational relaxation times for the bending mode. The results verify the mutual equilibrium of the bending and symmetric-stretch modes due to Fermi resonance. The bending mode relaxation times are approximately 10% shorter than predicted from interferometer results using the ratio of specific heats. Furthermore, relaxation times based on measurements of different rotational states at 1000°K show a variation with quantum number J, indicating a possible rotational nonequilibrium during the vibration relaxation process.
Diagrammatical Method for Geminals. I. Theory
Authors: Paldus J.
Published: February 2004

A diagrammatical method, based on the second quantization formalism, time-independent Wick's theorem and Feynman-like diagrams, is developed for the calculation of arbitrary matrix elements of n-particle spin-independent operators between the states represented by antisymmetrized products of geminals of an arbitrary kind (i.e., strongly or weakly orthogonal, interacting, nonorthogonal or identical geminals). The method greatly facilitates any algebraic manipulation with geminal type functions, particularly when strong orthogonality is not assumed, and enables one to write down directly the algebraic expressions for the matrix elements of an arbitrary spin-independent operator between the single antisymmetrized products of arbitrary geminals, solely on the basis of simple graphs, which are easily constructed.
Electronic Structure and Dynamic Jahn-Teller Effect of Cobaltocene from EPR and Optical Studies
Authors: Ammeter J. H. , Swalen J. D.
Published: February 2004

In order to determine quantitative information about chemical bonding in metallocenes, we investigated the electronic structure of the cobaltocene molecule by EPR and magnetic susceptibility measurements at 4.2°K and by optical spectroscopy at 77°K. Diamagnetic ruthenocene, weakly paramagnetic nickelocene, and paramagnetic cobaltocene single crystals as well as polycrystalline ferrocene served as host systems. From the poorly resolved optical spectra of pure cobaltocene, approximate ligand field parameters were determined. The magnetic properties (g tensor, cobalt hfs tensor) of the lowest Kramers doublet are explained in terms of the relative magnitudes of (a) spin-orbit coupling, (b) static orthorhombic distortion, and (c) vibronic coupling (dynamic Jahn-Teller effect) in the orbitally degenerate 2E1g ground state. From the analysis of the EPR data of cobaltocene-doped ruthenocene, we conclude that covalency effects and vibronic interactions (``Ham effect'') are of comparable importance resulting in a drastic modification of the magnetic parameters compared to a free Co2+ ion in a static crystal field. In agreement with earlier qualitative and semiquantitative predictions, considerable covalency of the singly occupied e1g* orbital (42%± 5% ligand, 58%± 5% cobalt 3d character) was found. The strong change of the EPR parameters going from the ruthenocene to the ferrocene host lattice originates mainly in a strongly enhanced static orthorhombic splitting parameter in the tighter packed ferrocene environment. In cobaltocene single crystal, magnetic dipole-dipole interactions broaden the EPR lines beyond detection even at 2°K. Nickelocene, an S =1 case with a large positive zero field splitting, behaves as a pseudodiamagnet at liquid helium temperature; exchange interactions with the cobaltocene dopant cause significant modifications of the g values but leaves the cobalt hfs tensor almost unaffected.
Polymorphism and Molecular Dynamics in Beryllium Oxyacetate, Be4O(CH3COO)6
Authors: Albert Shmuel , Ripmeester John A.
Published: February 2004

The unusual polymorphism of beryllium oxyacetate, Be4O(CH3COO)6, was investigated by means of pulsed NMR between −196°C and the melting point (286.7°C). In the α modification of this substance, fast methyl group reorientation takes place, and the spin-spin and spin-lattice relaxation times proved to be insensitive to the presence of the reported second or higher order phase transitions. The onset of a relatively slow motion with an activation energy of 15.5± 0.8 kcal/mole was found above the first order phase transition at ∼148°C, and this motion is likely to be over-all molecular tumbling motion. The transition from a high symmetry cubic phase to a lower symmetry orthorhombic structure on raising the temperature is discussed. No self-diffusion is present up to the melting point. The high temperature γ phase of beryllium oxyacetate does not seem to be a plastic crystal as was proposed previously. In light of our results, the isotropic optical property by itself is of questionable usefulness as a criterion for plasticity of molecular crystals. Unusual behavior of the spin-lattice relaxation between −20°C and −196°C was determined, and the possibility that additional phase transitions are present has to be further studied.