bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluorescence of Eu3+-Activated Lanthanide Oxyhalides LnOX
Authors: Blasse G. , Bril A.
Published: January 2005

The fluorescence of Eu3+-activated lanthanide oxyhalides LnOX (Ln=Y, La, Gd, and X=Cl, Br) is reported. Although all host lattices under consideration are isomorphous, the spectral energy distribution of the Eu3+ emission depends strongly on the choice of X and to a lesser extent on that of Ln. This variation is accounted for qualitatively by assuming a strongly varying linear component of the crystal field. The excitation spectra of the Eu3+ fluorescence and the diffuse reflection spectra are also dependent on the choice of Ln and X. The quantum efficiency of GdOCl: Eu and YOCl: Eu is high (about 60%—70%).
Viscosity of Several Liquid Hydrocarbons as a Function of Temperature, Pressure, and Free Volume
Authors: Hogenboom David L. , Webb Wayne , Dixon Joseph A.
Published: January 2005

Viscosity measurements have been made on nine pure hydrocarbon liquids at six temperatures ranging from 15.56° to 135°C and at pressures as high as 4000 bars. The samples included rigid bicyclic compounds of relatively high symmetry and three n-alkanes, n-C12, n-C15, and n-C18. These data were analyzed using the Eyring significant-structure theory, the Cohen—Turnbull free-volume model, and the empirical Doolittle equations. All of these equations produced essentially identical fits to the data at atmospheric pressure. The hypothesis that the constants v0 and vs in these equations represent the specific volume of a ``solid'' or condensed phase was tested by comparing best-fit values of these constants with experimental values for the solid-phase specific volume.

The Cohen—Turnbull and Doolittle equations were modified for use at elevated pressures with the result that the values of v0 necessary to satisfy the equations at high pressures were shown to be analogous to the specific volumes of a glassy state at high pressures. Further, the change in v0 as a function of pressure was compared with the change in the experimental values of the solid-phase volume at the melting point as a function of pressure and a definite correlation between the two was established.
Spin—Orbit Effects and the Fine Structure in the 3Σg− Ground State of O2
Authors: Kayama Kunifusa , Baird James C.
Published: January 2005

The true microscopic Hamiltonian is used for the spin—orbit interaction in the description of the fine structure of the O2 3Σg− ground state. The inclusion of the true spin—orbit interaction Hamiltonian in place of the commonly used isotropic form AL·S causes a mixing of 1Σg+ into the ground state. The 1Σg+ character causes Hso to contribute to the same order to the effective spin dipole—dipole parameter λe as does the spin dipole—dipole Hamiltonian Hss and also results in slight changes in the experimental values of the spin—rotation coupling constant and the g value of the orbital angular momentum. The amount of 1Σg+ character is estimated to be 1.0×10−4 from the data of Tinkham and Strandberg, and a somewhat larger value 3.6×10−4 from the data of Bowers et al., these are to be compared with 1.3×10−4 obtained by a previous calculation. The amount of singlet character, obtained from a combination of experimental data and estimates of molecular parameters, is compatible with the experimental data obtained by Tinkham and Strandberg or Bowers et al. Our result is supported by agreement with a calculation of 〈Hss〉 and by comparing the values of this parameter for several 3Σ− molecules.
Molecular Collisions. V. Nearly Spherical Potentials
Authors: Curtiss C. F. , Hardisson A.
Published: January 2005

Expressions are developed for the coefficients in spherical harmonic expansions of the degeneracy-averaged cross sections for collisions between rigid diatomic molecules which interact through a soft nearly spherical potential. The development is based on a perturbation method which is essentially an extended distorted wave treatment. Explicit expressions for the moments of the cross sections to second order in the perturbation parameter are obtained in a form suitable for numerical evaluation and application to the kinetic-theory evaluation of the transport coefficients. The expressions are simplified through the use of an expression which is developed for a sum of products of 12—j symbols.
Isotope Effects on Franck—Condon Factors. VII. Vibrational Intensity Distribution in the H2 Lyman, H2 Werner, O2 Schumann—Runge, N2 First Positive, N2 Vegard—Kaplan, and LiH (A—X) Systems Based on RKR Potentials
Authors: Halmann M. , Laulicht I.
Published: January 2005

Franck—Condon factors, based on Rydberg—Klein—Rees (RKR) potential functions, were computed for the H2 and D2 Lyman and Werner, 16O2 and 18O2 Schumann—Runge, 14N2 and 14N15N first positive and Vegard—Kaplan, and 7LiH and 7LiD (A—X) band systems. While the vibrational intensity distribution within several band series differs considerably from that previously computed using Morse potentials, the isotope effect is very similar by both methods. Only for the hydrogen Lyman bands, even the isotope effect on Franck—Condon factors is markedly different by the two methods. The isotope ratio of Franck—Condon factors, q(7LiH)/q(7LiD), in a given band progression (e.g., v″=3), oscillates between very large (104) and very small (10−2) values at the minima between the locus of the Condon parabolas. The isotope effect on r centroids is negligible for transitions of oxygen and nitrogen molecules, but amounts to several percent for transitions of hydrogen and lithium hydride molecules. For the 16O2 Schumann—Runge absorption bands, the variation of the electronic transition moment Re with internuclear distance r is represented by Re=const(1–0.70r).
Theoretical Studies of Two-Photon Absorption Processes. I. Molecular Benzene
Authors: Honig Barry , Jortner Joshua , Szöke Abraham
Published: January 2005

The theory of two-photon transitions is extended to include vibronic-coupling effects. The theory of symmetry-forbidden two-photon transitions is applied for the theoretical study of excitations in benzene. The introduction of vibronic-coupling effects implies that the transition probability should be temperature dependent. A definite assignment of the electronic states of benzene can be obtained from theoretical predictions of the intensity ratios, the vibrational structure, and the polarization dependence for two-photon absorption cross sections in this system.
Magnetic-Dipole Character of the 3500-Å System of Formaldehyde
Authors: Lombardi John R. , Freeman D. E. , Klemperer William
Published: January 2005

The 3480-Å absorption band of formaldehyde, 0+2v6′←0, is photographed with linearly polarized radiation in an electrostatic field of 5 kV/cm. From the observed Stark splitting of the lines, this transition is shown to be magnetic dipole in character, in complete accord with the earlier conclusions of Callomon and Innes.
Open-Shell Orthogonal Molecular Orbital Theory
Authors: Harris Frank E.
Published: January 2005

Computationally convenient formulas are derived for the matrix elements of the identity and of one- and two-electron operators, for open-shell spin-projected wavefunctions built from orthogonal spatial orbitals. These formulas explicitly show the coefficients of the one- and two-electron integrals involved, and are not much more complicated than the closed-shell formulas in common use. Results are presented in tabular from for all types of configuration pairs possessing nonzero matrix elements, and examples of the use of the formalism are given. These results should facilitate the development of open-shell extended Hartree—Fock theory.
Hydroxyl-Radical Kinetics by Kinetic Spectroscopy. I. Reactions with H2, CO, and CH4 at 300°K
Authors: Greiner N. R.
Published: January 2005

Gas-phase reactions of OH radicals with H2, CO, and CH4 have been studied at total pressure of up to 200 mm. Hydroxyl radicals were generated rapidly in the presence of the reactant by flash photolysis of H2O—reactant mixtures in a quartz system, and the subsequent decrease in OH concentration was followed by quantitative kinetic spectroscopy (KS). Rate constants at 301°±1°K were found to be, in cubic centimeters per mole·second,  OH+H2→H2O+H,k6=4.0±0.2×109,OH+CO→CO2+H,k7=8.9±0.9×1010,  OH+CH4→H2O+CH3,k1=5.3±0.2×109,over 25-fold ranges of [OH] and > fivefold ranges of [H2], [CO], and [CH4]. These results confirm recently reported values obtained at pressures of ∼1 mm with a flow-system—ESR technique and extend the range of reactant concentration studied by factors of up to 125.
Kinetics of Positive-Charge and Electron Scavenging and the Kinetics of Charge Neutralization in the Radiolysis of Dielectric Liquids
Authors: Freeman Gordon R.
Published: January 2005

The kinetics of the scavenging of positive and negative species in spurs during the radiolysis of liquids is described. The motions of the positive ions, the solvated electrons, and the scavenger molecules are simultaneously taken into consideration. The kinetic equation contains two adjustable parameters, β and f, but in most instances f is taken as unity. The theory is tested, with moderate success, against previously published results of studies of positive-ion and electron scavenging in cyclohexane and n-hexane.

The recently observed complexity of the kinetics of ion-neutralization reactions in liquid cyclohexane, after a pulse of radiation, is explained in terms of the same model as that used in the scavenging kinetics.

It is concluded from the over-all success of the theory that solvated electrons form in suitable nonpolar molecular liquids, such as alkanes, during radiolysis.

The terms ``thermal electron'' and ``spur'' are not used by all authors to mean the same things, so they are discussed in the Appendixes in the hope that the meanings of these terms will become more standardized. The quasifree, trapped, and solvated states of electrons are distinguished in Appendix A. A definition of the term ``spur'' is offered in Appendix B.
Generalized Perturbation Method for a Basis of Orthogonalized Trial Functions
Authors: Ritchie A. B.
Published: January 2005

The generalized perturbation method employing nonorthogonal functions of Blinder, valid to second order in the energy and first order in the function, is extended to all orders. In addition it is shown that the set of n perturbed energies and functions obtained is identical to that obtained by Löwdin using partitioning techniques in the wave-operator formalism, provided an inverse operator is properly chosen.
Oxidation of Hydrogen Sulfide in Shock Waves. I. Absorption Studies of OH and SO2 in H2S–O2–Ar Mixtures
Authors: Bradley J. N. , Dobson D. C.
Published: January 2005

The oxidation of hydrogen sulfide in shock waves has been studied over the temperature range 1350°—2450°K by following the characteristic absorption of OH and SO2. Two different regions are apparent: below 1560°K the OH and SO2 appear together, after an induction period, while above 1700°K the SO2 appears before the OH. The oxidation at low temperatures is interpreted in terms of a branching-chain mechanism involving the reactions: SH+O2→SO+OH,OH+H2S→H2O+SH,SO+O2→SO2+O,O+H2S→OH+SH.At higher temperatures, participation of the reaction O+H2S→SO+H2provides an additional route for the formation of SO2 and hence leads to a reduced induction time for its appearance.
Formation of M Centers from ``Stored'' Photoelectrons
Authors: Hirai Masamitsu , Scott Allen B.
Published: January 2005

A process for producing M centers in additively colored KCl, by alternately bleaching F centers at − 120°C and warming up to about 0°C, is described. In undeformed crystals, M centers were formed only in samples in which an absorption at 600 mμ was observed as an intermediate. Plastic deformation enhanced the formation, in both KCl and KBr. M centers did not appear until temperatures well above the temperature of disappearance of F′ centers. A mechanism is proposed involving the migration of α centers to F centers; the F2+ centers thus formed are then neutralized by electrons released from the centers absorbing at 600 mμ.
Phase Transitions in Two-Dimensional Lattice Gases of Hard-Square Molecules
Authors: Bellemans A. , Nigam R. K.
Published: January 2005

The thermodynamic properties of lattice gases of hard-square molecules on a two-dimensional square lattice are investigated with special emphasis on the occurence of phase transitions. Three particular systems (called A, B, and C, respectively) are considered, corresponding to exclusion cores including (a) first neighboring sites, (b) first and second neighboring sites and (c) first, second, and third neighboring sites. Case A has already been extensively studied by several authors so that the original contribution of this paper essentially concerns Cases B and C. Three methods for computing the thermodynamic properties of these systems have been used: (a) the matrix method of Kramers and Wannier, (b) low- and high-density series, and (c) an approximate method due to Rushbrooke and Scoins. The situation from the point of view of phase transitions seems to be as follows: Case A presents a second-order continuous transition, Case B remains doubtful, although a first-order transition seems to be ruled out, Case C presents a first-order transition (of the fluid—solid type).
Double-Resonance Studies of the Conformations of the Free Radical in Irradiated Alpha-Aminoisobutyric Acid
Authors: Wells James W. , Box Harold C.
Published: January 2005

The technique of fast-passage double resonance (ENDOR) is applied to the study of the three conformations of the free radical found when warming single crystals of α-aminoisobutyric acid, x irradiated at 77°K. The ENDOR measurements, made at 4.2°K, allow the absorption signals for the protons of the two methyl groups to be distinguished at all crystal orientations. Both methyl groups in the first two conformations and one group in the third conformation are found to have a single representative ENDOR line, indicating the group's rotation. The hyperfine tensors for these groups are found to depart from the generally assumed axial symmetry. The nonrotating methyl group in the last conformation permits an accurate determination of the parameters in the coupling equation A=B0+B cos2θ. The experimental values are B0=3.2 Mc/sec and B=123.3 Mc/sec.
On the Quadrupole Moment of the Hydrogen Molecule
Authors: Karl G. , Poll J. D.
Published: January 2005

Various matrix elements of the quadrupole moment of H2 in the ground electronic state are calculated in the adiabatic approximation and the results are presented in Table II. These matrix elements are of the form , where v, J and v′, J′ denote the vibrational and rotational quantum numbers of the initial and final states, respectively; Q represents the quadrupole moment for fixed nuclei. The calculations are based on the adiabatic potential and electronic quadrupole moment computed by Koℏos and Wolniewicz. The results of the calculations are compared with experimental values obtained from the quadrupole absorption spectrum by Fink, Wiggins, and Rank. The influence of the rotation—vibration interaction on the matrix elements is discussed and an estimate of the contribution due to nonadiabatic effects is given for the case Δv=0.
Pulse-Radiolysis Studies of the Formation of Triplet Excited States in Cyclohexane Solutions of Naphthalene and Anthracene
Authors: Hunt John W. , Thomas J. K.
Published: January 2005

The radiolysis of solutions of napthalene and anthracene in cyclohexane and benzene has been studied using 3- to 11-nsec pulses of 3-MeV electrons. The triplet states of the solutes were observed via their characteristic absorption spectra, while the excited singlet states were observed via their fluorescence. The rate of formation of most of the triplet state was less than one nanosecond; a small portion, however, did grow in with a half-life of 100 nsec. The possible mechanisms of production of the triplet state are discussed, and it is concluded that they are formed either directly from subexcitation electrons or from ion recombination.
Ultraviolet Absorption Cross Sections for the Alkali Halide Vapors
Authors: Davidovits P. , Brodhead D. C.
Published: January 2005

The ultraviolet-absorption cross sections for the alkali halide vapors were measured in the region from 2000 to 4000 Å. Potential curves were constructed from these data and are compared to some previous work. From the cross sections the radiative lifetime of the excited molecular states is calculated to be on the order of 10−8 sec.
Ultrasonic Determination of Transport Properties of Monatomic Gases at High Temperatures
Authors: Carnevale E. H. , Lynnworth L. C. , Larson G. S.
Published: January 2005

An ultrasonic method has been used to determine experimentally the temperature and transport properties of monatomic gases. Both the sound velocity and the sound absorption were measured in helium up to 1300°K and argon up to 8000°K. The temperatures were calculated from the well-known theoretical temperature—sound velocity relation. The transport properties were calculated from the Stokes—Kirchoff expression for the sound absorption. The viscosity determinations agree with independent results of other investigators, thereby establishing the validity of this ultrasonic technique in determining transport properties of monatomic gases at high temperatures.
On the Infrared Spectra of HNCS and DNCS
Authors: Durig J. R. , Wertz D. W.
Published: January 2005

The infrared spectra of gaseous and solid HNCS and DNCS are reported. The infrared spectra of both molecules isolated in an argon matrix are also reported. The shift in frequency of the N–H and N—D stretching vibrations upon solidification is 440 and 293 cm−1, respectively, which suggests a very short and strong hydrogen bond. Two of the low-frequency bending modes are identified for the HNCS molecule and reassignments are given. Assignment of the six fundamental vibrations for the DNCS molecule are proposed and discussed. Gas-phase analysis of the N–H stretching vibration centered at 3537.9 cm−1 gives the A rotational constant of 43.35±0.5 cm−1 for the ground state and 36.67±0.5 cm−1 in the excited state. The mass defect is calculated to have a value at 0.184 amu·Å2.
Irreversible Thermodynamics of a Fluid System with Spin
Authors: Snider R. F. , Lewchuk K. S.
Published: January 2005

The linear irreversible processes occurring in a fluid system whose molecules possess internal angular momentum are completely classified by means of irreducible Cartesian tensors in terms of the symmetry properties of the local equilibrium state of the fluid. It is assumed that this local thermodynamic state is characterized by the specific internal angular momentum s as well as by the mass density ρ and the specific entropy S. Even with the resulting tensorial phenomenological coefficients, it is found, for example, that there can be no contribution to the heat flux from the velocity gradient.
Trifluoromethyl, and Other Radicals, in Irradiated Single Crystals of Trifluoroacetamide
Authors: Rogers Max T. , Kispert Lowell D.
Published: January 2005

ESR spectra characteristic of the trifluoromethyl radical are obtained when single crystals of trifluoroacetamide are irradiated and observed at 77°K. The isotropic hyperfine splitting found is the same as has been reported for the trifluoromethyl radical in solution. The anisotropic hyperfine splittings indicate that the radical is oriented in the lattice but can undergo rapid reorientation about the threefold axis at 77°K. The observed 19F and 13C hyperfine-splitting values provide evidence that the radical has essentially the same structure in the crystal as in solution with bond angles (∠FCF) near the tetrahedral value. The ·CF2CONH2 radical is also found and evidence for a new radical ·NH2CO reported.
Numerical Solution of Linear Integral Equations of the First Kind. Calculation of Molecular Weight Distributions from Sedimentation Equilibrium Data
Authors: Provencher S. W.
Published: January 2005

A new method is described for the numerical solution of g(x)= ∫ abK(x, t)f(t)dtfor f(t) when g(x) is experimental data of limited accuracy. Under these conditions, the well-known least-squares and algebraic methods often give poor solutions. The new method is an optimum combination of these two. It has the stability of the least-squares method to errors in g(x), but still does not assume a functional form for f(t). The problem of the determination of the molecular weight distributions (MWD) of ideal polydisperse solutes from sedimentation equilibrium data is discussed in detail. Results of the analyses of sets of computer-simulated experimental data containing pseudorandom errors show that the method can give good estimates of the MWD from data accurate to only two significant figures. This accuracy should be readily attainable experimentally. The method should be useful in the analysis of data from many other types of experiments, particularly scattering, where the presently used Fourier methods are often unsatisfactory because they require the extrapolation of data in order to evaluate the Fourier transforms.
Liquid—Vapor Phase Behavior and Liquid Phase Density in the System Neon—Argon at High Pressures
Authors: Streett W. B.
Published: January 2005

Liquid—vapor phase compositions for the system neon—argon are reported at five temperatures in the range 95.82°—129.92°K, and at pressures up to 9000 psia. The liquid—vapor critical curve has been determined for mixtures containing up to 60 mole % neon. The density of the liquid phase has been measured in the compressed-liquid region at temperatures of 101.94°, 110.78°, and 121.36°K, at pressures up to 8000 psia.
Nonreactive Scattering of K by HBr and DBr in Crossed Molecular Beams
Authors: Airey J. R. , Greene E. F. , Kodera K. , Reck G. P. , Ross J.
Published: January 2005

The nonreactive scattering of K by HBr and DBr has been measured in crossed molecular beams as a function of relative kinetic energy and for two configurations of the beams and the detector. The effect of averaging over residual energy distributions in the two beam and detector configurations is studied. It is shown that the density of the cross beam must be kept low because attenuations of 50% of the K beam by the cross beam affect the observed rainbow cross section appreciably. From the rainbow scattering and a kinematic restriction, the parameters for an assumed exp−6 potential (α=12) are found to be K+HBr: ε=0.565 kcal/mole, rm=4.2 Å; K+DBr: ε=0.564 kcal/mole, rm=3.9 Å. Chemical reaction affects the nonreactive scattering at large scattering angles. Probabilities of reaction, threshold energies, and total reaction cross sections are obtained from the nonreactive scattering with a simple model used previously. The total reaction cross sections are: K+HBr: 35.0±9.0 Å2, K+DBr: 26.0±8.0 Å2 in the energy range 2.6 to 5.8 kcal/mole.
Scattering of Potassium by a Series of Reactive and Nonreactive Compounds in Crossed Molecular Beams
Authors: Airey J. R. , Greene E. F. , Reck G. P. , Ross J.
Published: January 2005

The scattering of potassium by the nonreactive compounds C(CH3)4, C6H6, C6H12, (CH3)2C=C(CH3)2 and the reactive compounds CH3I, CCl4, SiCl4, SnCl4, SF6 has been studied in experiments with crossed molecular beams. In each case the nonreactive scattering of velocity-selected K was measured as a function of laboratory scattering angle at a number of initial relative kinetic energies. Except for K+SnCl4, rainbow scattering is observed. The differences which appear between the measured scattering in this region for the various nonreactive compounds show the influence of the molecular structure on the intermolecular potential. Potential parameters of an assumed spherically symmetric potential (exp−6, α=12) are calculated from the rainbow scattering. The scattering of K from the various reactive compounds differs markedly at larger angles and is an indication of a correspondingly large variation of reaction cross sections. Probabilities of reaction, threshold energies, threshold distances, and the energy dependence of the total reaction cross sections are obtained from an interpretation of the measurements.
Visible Continua in Xenon, Krypton, and Neon
Authors: Prince J. F. , Robertson W. W.
Published: January 2005

An investigation of the continua (2900–7000 Å) radiated from positive columns in xenon, krypton, and neon is described for discharges in the pressure range of 5 to 40 torr (neon, 20 to 120 torr) and current range of 0.7 to 5 mA, conditions chosen for normal glow, unconstricted discharges. Experimental results show definitely that these continua are not attributable to free—bound, free—free transitions and indicate that they are of molecular origin, the radiating states being populated by electron excitation of metastable molecules formed by three-body conversion of metastable atoms.
Plait Points in Two- and Three-Component Liquid Mixtures
Authors: Widom B.
Published: January 2005

It is pointed out that the plait-point (isothermal critical-mixing-point) phenomenon in a three-component liquid system of fixed pressure and temperature is identical in character to that in a two-component liquid system of fixed temperature but variable pressure. A lattice-gas model of a two-component liquid system is described. The intermolecular forces are all postulated to be infinitely strong repulsions, as between hard cores, but the model system nevertheless undergoes first-order phase transitions with an associated plait point. The behavior of the system with varying densities ρ1 and ρ2 of its two components, at fixed temperature, is shown to be exactly transcribable, with only a change of language, from the behavior of a reference one-component lattice gas of varying density and temperature, in which the interaction between neighboring molecules is attractive. The critical point of the reference one-component system is mapped into a plait point in the isothermal ρ1, ρ2 plane of the binary system. The two-phase coexistence curve (binodal curve) in the isothermal ρ1, ρ2 plane of the binary system is found to be of algebraic degree (1—α′)/β near the plait point, where α′ and 1/β are the index of the divergence of the constant-volume specific heat in the two-phase region of the reference system, and the algebraic degree of the reference system's temperature—density coexistence curve, respectively. Fluctuations in ρ1 and ρ2 in the one-phase region of the binary system are found to diverge as the —γ/(1—α) power of the distance from the plait point, where α and γ are the indices of the divergence of the constant-volume specific heat and of the compressibility, respectively, in the one-phase region of the reference system.
Emission Spectra of Chlorophyll-a in Polar and Nonpolar Solvents
Authors: Broyde S. B. , Brody S. S.
Published: January 2005

Emission spectra of chlorophyll-a, dissolved in ethanol and various nonpolar solvents, were measured at 300°, 77°, and 4°K, in both dilute and concentrated solutions. For this purpose, a new technique utilizing fiber optics was employed. In dilute ethanolic solutions, the observed fluorescence bands emanate from chlorophyll-a monomers, solvated at the central magnesium. At 4°K the fluorescence yield in this solvent is 0.85. In highly concentrated ethanolic solutions, a low-temperature band occurs at about 724 nm, which may be attributed to solvated dimers, formed by a π—π interaction between the chlorophyll monomers. In nonpolar solvents, a low-temperature luminescence band is observed at about 755 nm. This emission is attributed to π—π* phosphorescence from unsolvated monomers. Also in nonpolar solvents, highly concentrated solutions have a weak band in the neighborhood of 733 nm at 77°K, which originates from chlorophyll dimers formed by interaction between the magnesium of one monomer with the cyclopentanone carbonyl of another. The 733-nm band is either π—π* phosphorescence or fluorescence. In CCl4 and benzene the dimers are believed to be unsolvated. At room temperature unsolvated dimers fluoresce feebly at 713 nm. Cooling dilute solutions of chlorophyll a in benzene and 1,2-dichloroethane causes dimer formation, but this is less in CCl4 and absent in ethanol. A calculation of the upper limit for the rate constant for intersystem crossing to the triplet state, in ethanol, gives a value of 9.0×106/sec.
Molecular Charge Distributions and Chemical Binding
Authors: Bader R. F. W. , Henneker W. H. , Cade Paul E.
Published: January 2005

This paper presents an interpretation of the chemical binding found in the first-row homonuclear diatomic molecules. The interpretation is based upon the one-electron density distribution and the forces which it exerts on the nuclei. The general topographical features of the density distributions are discussed in relation to ``molecular size'' and the manner in which the total charge is partitioned between different spatial regions. The binding in these molecules is discussed in terms of the density difference distributions which picture the redistribution of charge which results from the formation of the molecule. It is proposed that the density difference distribution, or Δρ map, may be taken as the pictorial representation of the ``bond density.'' The forces exerted on the nuclei in the molecule are related to the changes in the charge distribution pictured in the ``bond density'' and a quantitative discussion of the manner in which electrostatic equilibrium is attained to give a stable molecule is given in terms of the forces. The concepts of bonding and antibonding are compared with the terms binding and antibinding, terms which are defined in terms of the forces exerted on the nuclei. In particular, both vertical and adiabatic ionization processes are viewed from the standpoint of the change in the electronic force contribution between the molecule and the molecule ion.

A definition of ionic and covalent binding based on the density difference distributions is presented. The Δρ map (or bond density) for covalent binding is shown to be characterized by a density increase located between the nuclei and shared equally by each. The Δρ map for ionic binding exhibits an increase in charge density which is localized on a single nucleus. A partitioning of the total electronic force in accordance with the Δρ maps demonstrates that in covalent binding, the nuclei are bound by the density which is shared between them, while in ionic binding the nuclei are bound by the density which is localized on a single nucleus.
Infrared Spectra of Solid and Matrix-Isolated (CH3)3N, (CD3)3N, and (SiH3)3N
Authors: Goldfarb Theodore D. , Khare Bishun N.
Published: January 2005

The argon-matrix infrared spectra of (CH3)3N, (CD3)3N, and (SiH3)3N are used to deduce a more complete assignment of the fundamental vibrational modes of these molecules than has previously been possible. Evidence is presented in support of a C3v rather than C3h point-group symmetry for trisilylamine. Barriers to internal rotation are calculated from the observed asymmetric torsional fundamentals of (CH3)3N in the solid and matrix phases and of (CD3)3N in the gaseous phase.
Infrared Studies of Dimethylsilylamine and Methyldisilylamine by the Matrix-Isolation Technique
Authors: Goldfarb Theodore D. , Khare Bishun N.
Published: January 2005

A partial assignment of the fundamental vibrational modes of (CH3)2SiH3N and CH3(SiH3)2N is proposed based on the infrared spectra of the matrix-isolated molecules. Calculated and observed vibration—rotation band envelopes are compared and are found to be indicative of a pyramidal skeleton for the monosilyl molecule and a planar skeleton for the disilyl molecule. Compelling evidence for the intermolecular association through p→d bonding in (CH3)2SiH3N is presented in the form of radical changes in the infrared absorption spectrum as the concentration of this molecule in an argon matrix is increased. Although the structure of the associated molecules can not be deduced from the complicated spectrum of the solid, several conceivable geometries are discussed.
Vibrational Spectra of Molten Salts. I. Infrared Spectra of Calcium Nitrate in Alkali-Metal Nitrate Solutions
Authors: Hester R. E. , Krishnan K.
Published: January 2005

Infrared spectra of thin films of Ca(NO3)2 dissolved in molten KNO3 and molten NaNO3 have been obtained over the spectral range 4000–200 cm−1. The changing appearance of the spectra as a function of calcium content indicates clearly an asymmetric perturbation of the nitrate ion by the calcium ion in these mixtures. The magnitude of the perturbation is, however, similar to that found in concentrated aqueous calcium nitrate solutions, consistent with the existence, in both types of systems, of contact ion pair complexes. A new band at 332 cm−1 in the spectra of melts containing Ca2+ characterizes the Ca–ONO2 stretching frequency. Addition of chloride ion in the form of KCl to KNO3/Ca(NO3)2 molten mixtures was found to restore the appearance of the spectrum towards that of pure KNO3, showing that Cl− ion displaced NO3− from the Ca2+–ONO2− complexes.
Hexagonal Diamond—A New Form of Carbon
Authors: Bundy F. P. , Kasper J. S.
Published: January 2005

A new crystalline form of carbon—hexagonal diamond—has been synthesized in the laboratory under conditions of static pressure exceeding about 130 kbar and temperature greater than about 1000°C. It is necessary to start with well-crystallized graphite in which the c axes of the crystallites are parallel to each other and to the direction of compression. There is electrical evidence that the transformation starts at room temperature but hexagonal diamond is not retrieved unless a setting temperature exceeding about 1000°C is applied. The electrical and crystal characteristics have been studied. The crystal structure is hexagonal with a=2.52 Å and c=4.12 Å. The theoretical density is 3.51+g/cm3, same as cubic diamond. It has also been prepared recently in another laboratory from crystalline graphite by a method involving intense shock compression and strong thermal quenching. More recently it has been discovered to be present to the extent of over 30% in the Canyon Diablo meteorite diamonds.
Positron Annihilation in the Normal Aliphatic Hydrocarbons
Authors: Gray Peter R. , Sturm G. P. , Cook C. F.
Published: January 2005

The lifetimes and intensities of triplet positronium have been measured in 15 normal aliphatic hydrocarbons. The lifetime of the triplet state in the deoxygenated samples decreases from 4.63±0.06 nsec for butane, C4H10, to 3.19±0.06 nsec for octadecane, C18H38. The intensities of the triplet positronium components are less variable; butane has the highest intensity, 38.4%±0.8%, and tetradecane, C14H30, the lowest, 29.6%±1.2%. The cross section for triplet-state quenching, 〈σv〉Av=(M/ρN0)(τ2 )−1, is shown to have a linear dependence on the length of the hydrocarbon chain. Partial cross sections for quenching by the component groups of the hydrocarbons, the methyl and methylene groups, have values of 0.895 and 0.965×10−14 cm3/sec, respectively. Back calculations of 〈σv〉Av using partial quenching cross sections lead to an average error of 0.56% in 〈σv〉Av for the 15 hydrocarbons. The largest back-calculated error is 1.92% for butane.
Production and Annealing of Br2 in the Radiolysis of Polycrystalline C2Br6: An In Situ Determination
Authors: Iyer R. M. , Willard J. E.
Published: January 2005

Spectrophotometric analysis of pressed tablets of polycrystalline C2Br6 has been used to study the growth and decay of Br2 in the matrix as a function of γ dose, and of time and temperature of annealing. G(Br2) is 4 or higher at doses of 1018 eV g−1 and below but falls rapidly in the range of 1018 to 1019 eV g−1, reaching values of a few tenths at doses above 1020 eV g−1, indicating radiation-catalyzed back reactions. On thermal annealing, part of the Br2 produced by irradiation is removed rapidly by reactions within the crystal at 76°, and faster at 100°, while part is relatively unaffected by the annealing. The rate of production of Br2 by irradiation is not affected by adsorbed Br2 incorporated in the crystals prior to pressing the tablets. Reaction intermediates which may be responsible for the observed phenomena are discussed. The exchange of Br2(82Br) with C2Br6 in the tablets and its loss by diffusion have been tested.
Kinetic Theory of Loaded Spheres. III. Transport Coefficients for the Dense Gas
Authors: Sandler Stanley I. , Dahler John S.
Published: January 2005

A kinetic equation is derived for a dense gas of loaded spheres. This is accomplished as a straightforward generalization of the Enskog theory for ordinary, rigid spheres. The normal solution of this kinetic equation is then constructed according to the method of Chapman and Enskog. From this solution accurate estimates are computed for the thermal conductivity and for the coefficients of shear, bulk, and vortex viscosity.
Kinetics of the Oxidation of Tungsten by CO2 at High Temperatures
Authors: Walsh Patrick N. , Quets Jean M. , Graff Robert A. , Ladd Irwin R.
Published: January 2005

The kinetics of oxidation of tungsten by CO2 was studied in a stagnation-flow reactor at CO2 partial pressures from 20 to 337 torr. The study covered the temperature range 2200°—3200°K and included measurements of the effect of CO on the reaction kinetics at CO/CO2 ratios up to 5:1. The rates observed below about 2650°K were essentially independent of the velocity of the impinging gas stream, indicating that the surface kinetics primarily determined the rates up to 2650°K at the lower gas-flow rates used and to higher temperatures at higher flow rates. Where surface kinetics were rate determining, the data could be explained simply by incorporating a temperature-dependent ``sticking coefficient'' for CO2 into the two-layer adatom model previously proposed for the W–O2 reaction. The effect of CO was small and was readily incorporated into the model. Velocity-dependent deviations from the rates predicted by the model were observed at temperatures above 2650°K. These were largely explained by simple mass-transfer theory. At low flow rates and high temperatures it appears that the reactions of desorbed oxygen became important.
Optical Absorption of Tetrahedral Co3+ and Co2+ in Garnets
Authors: Wood D. L. , Remeika J. P.
Published: January 2005

Optical spectra of single crystals of high-purity garnets YIG (Y3Fe5O12), YAG (Y3Al5O12), and YGG (Y3Ga5O12) containing cobalt ions have been investigated. With the pure materials the spectra for all three crystals are essentially the same, and the spectrum has been attributed to the rarely observed tetrahedral Co3+. Small amounts of tetrahedral Co2+ also in evidence in YAG and to a smaller extent in YGG can be eliminated by additions of Ca2+ with the cobalt. Addition of Si4+ with the cobalt produces a spectrum characteristic of tetrahedral Co2+ in YAG and YGG but not in YIG. In the latter Fe2+ is formed and the lines of tetrahedral Co2+ are missing. Evidence for lines due to octahedral and dodecahedral Co2+ and octahedral Co3+ is given together with the approximate crystal-field splitting parameter Dq for each. A qualitative interpretation of the fine structure in the spectra of the tetrahedral ions is discussed.
Singlet—Triplet Transitions in p-Dihalogenated Benzenes
Authors: Castro George , Hochstrasser Robin M.
Published: January 2005

The polarized absorption and also some emission spectra of single crystals of p-dichlorobenzene (DCB) and p-dibromobenzene (DBB) at 4.2°K are presented and vibrationally analyzed. The absorption to the lowest triplet states (T1) is sharp and displays a pure exciton origin in both crystals; at 27 890 cm−1 in DCB, and at 27 910 cm−1 in DBB. The assignment of these origins is confirmed by emission at 4.2°K. Modes belonging to species ag (all but one of the possible six are observed), b2g, b3g, b1u, b3u, and au are observed in the spectra, and the polarizations of the individual vibronic bands are presented. The bulk of the transition is polarized normal to the molecular plane. The identification of long-axis polarization in the 0, 0 band and normal polarization in a b2g vibronic level suggests an assignment 3B2u for the spatial character of the lowest triplet states.

The T1→S0 emission of DBB is very sharp and consists of an alternating intensity pattern in a long (∼20) sequence of the 278 b2g ground-state mode. From the analysis we conclude that the state T1 is slightly distorted into a trans chair form, and that the upper state contains a double minimum along this mode.

Crystal-field effects on the spectra T1←S0 are discussed in relation to lattice structure—which is prominent in the spectrum of DBB but not DCB—and in relation to the polarization ratios, and intermolecular spin—orbit coupling. The origin bands of DBB and DCB consist of triplets at about 0, 0+4.3, and 0+10.3 cm−1 and some possible reasons for this are discussed. The temperature dependence of the 0, 0 band of DBB is attributed to a subtle change in crystal structure with temperature.

For completeness the S1←S0 spectra were also examined, and a comparison is made of the vibrational frequencies in the states S0, S1, and T1.

The mechanisms of spin—orbit coupling in dihalobenzenes is examined in detail with reference to the symmetry and polarization assignments. Also a brief theoretical justification for the appearance of u-vibrational modes in the transition is given.
Vibrational Spectra of Molecules and Atoms on Surfaces
Authors: Smith William Hayden , Eckstrom Hartley C.
Published: January 2005

The vibrational spectrum of a monolayer film chemisorbed on a single-crystal metal substrate is discussed in the harmonic-oscillator approximation. Optical modes must be totally symmetric with respect to translation, and be of an appropriate symmetry species under the unit cell (or factor) group. The spectral implications of the presence of anharmonicity, quadrupolar-induced transitions, and non k=0 processes are discussed in considerable detail. This theory is applied to the case of CO and H2 or D2 adsorbed on nickel (100), (110), and (111) crystallographic surfaces.
Crystallization of Diamond and Graphite
Authors: Strong H. M. , Hanneman R. E.
Published: January 2005

The key features of the complete, stable nickel—carbon phase diagram have been established at pressures up to 54 kbar, and these features have been directly related to the nucleation and growth characteristics of diamonds in this system. At pressures in excess of 52.5 kbar, a liquid+graphite+diamond eutectic is formed at approximately 0.3 at. % nickel which results in an effective displacement of the diamond—graphite equilibrium line. This displacement is shown to be of considerable importance in the nucleation and growth of diamonds in this system. Under low and moderate driving forces, the rate-controlling mechanism in the growth on a speed diamond is the diffusion of carbon across the molten metal film surrounding the diamond. The observed diffusion coefficients at 54 kbar and 1660°<T<1800°K lie in the range: 2×10−5<D<4×10−5 cm2/sec.
Spectroscopic Evidence for the Formation of I3− in Silver Iodide at High Pressures
Authors: Moore Michael J. , Skelly David W.
Published: January 2005

The presence of an absorption maximum at 360 mμ indicates the formation of I3− in the fcc phase of AgI at approximately 4 kbar. The concentration of I3− is pressure dependent above 4 kbar and falls to zero somewhere below the transition to the new phase at approximately 100 kbar. A mechanism is proposed to explain the presence (in the fcc structure) and subsequent disappearance of I3−.
Low-pressure microreactor system for kinetic studies on high surface area catalysts in the pressure gap
Authors: Leppelt R. , Schumacher B. , Häring T. , Kinne M. , Behm R. J.
Published: January 2005

We describe a low-pressure plug-flow microreactor system for investigating reaction kinetics on high surface area catalysts over a wide pressure range, covering the so-called pressure gap. Well defined gas flows with rates spanning over six orders of magnitude, from 100Nml∕min down to 0.5Nμl∕min, are produced using a capillary system for gas flow control. This allows us to measure reaction rates at constant contact time, despite widely differing pressures. The performance of the reactor system is demonstrated by test measurements on the kinetics for CO oxidation over a Au∕TiO2 catalyst, covering a pressure range from 1000mbarto10−1mbar total pressure, equivalent to CO partial pressures of 10mbarto10−3mbar.
Performance of a reflection-type polarizer by use of muscovite mica crystal in the soft x-ray region of 1keV
Authors: Imazono Takashi , Ishino Masahiko , Koike Masato , Kimura Hiroaki , Hirono Toko , Sano Kazuo
Published: January 2005

To develop the polarizer functioning in the soft x-ray region of 1keV, the polarization performance of muscovite mica has been investigated theoretically with a simulation code based on dynamical theory. As the result of calculation, muscovite mica is found to be a promising candidate as a reflection-type polarizer with the reflectivity for s polarization of 0.03 at approximately 0.9keV at the angle of incidence of 45°. In order to verify the polarization performance of muscovite mica experimentally, a symmetric Bragg reflection measurement of muscovite mica(002) was carried out using a linearly polarized undulator radiation. As a result, the maximum reflectivity for s polarization and the extinction ratio of muscovite mica were approximately 0.018 and 200 at 878eV, respectively. This result indicates that muscovite mica works as a practical polarizer in the soft x-ray region.
XGef Mediates Early CPEB Phosphorylation duringXenopusOocyte Meiotic Maturation
Authors: Martínez Susana E. , Yuan Lei , Lacza Charlemagne , Ransom Heather , Mahon Gwendolyn M. , Whitehead Ian P. , Hake Laura E.
Published: January 2005

Polyadenylation-induced translation is an important regulatory mechanism during metazoan development. During Xenopus oocyte meiotic progression, polyadenylation-induced translation is regulated by CPEB, which is activated by phosphorylation. XGef, a guanine exchange factor, is a CPEB-interacting protein involved in the early steps of progesterone-stimulated oocyte maturation. We find that XGef influences early oocyte maturation by directly influencing CPEB function. XGef and CPEB interact during oogenesis and oocyte maturation and are present in a c-mos messenger ribonucleoprotein (mRNP). Both proteins also interact directly in vitro. XGef overexpression increases the level of CPEB phosphorylated early during oocyte maturation, and this directly correlates with increased Mos protein accumulation and acceleration of meiotic resumption. To exert this effect, XGef must retain guanine exchange activity and the interaction with CPEB. Overexpression of a guanine exchange deficient version of XGef, which interacts with CPEB, does not enhance early CPEB phosphorylation. Overexpression of a version of XGef that has significantly reduced interaction with CPEB, but retains guanine exchange activity, decreases early CPEB phosphorylation and delays oocyte maturation. Injection of XGef antibodies into oocytes blocks progesterone-induced oocyte maturation and early CPEB phosphorylation. These findings indicate that XGef is involved in early CPEB activation and implicate GTPase signaling in this process.