bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluorescence of Niobium-Activated Antimonates and an Empirical Criterion for the Occurrence of Luminescence
Authors: Blasse G.
Published: February 2004

The fluorescence of Nb5+-activated antimonates is reported. A relation has been found between the position of the maximum of the excitation (and absorption) band of the niobate octahedra and the quenching temperature of their fluorescence. This result can be explained using a simple configuration-coordinate diagram. The displacement of the equilibrium distance of the excited state relative to the ground state can be estimated from the set of radii derived by Waber and Cromer for the principal maxima in the radial wavefunctions of the elements. It is shown that the estimated displacement of the excited state should be 0.3 Å or less to have efficient fluorescence from this state.
Calculation of Anisotropic Excitation Cross Sections and the Polarization of Impact Radiation of He Atoms Induced by Electron and Proton Impacts
Authors: Lin Sheng Hsien
Published: February 2004

The excitation cross sections of helium atoms for the transitions 1s→2pm, 1s→3pm, 1s→3dm, and 1s→4dm induced by the proton impact (5–40 keV) and electron impact (50–200 eV) have been calculated. These excitation cross sections have been used in calculating the polarization of impact radiation for the radiative transitions D1→P1 and P1→S1. The theoretical values of excitation cross sections and polarization of impact radiation are compared with the experimental results.
Effects of Ion Pairing on the EPR Spectrum of the Naphthalene Anion in Solution
Authors: Dodson C. L. , Reddoch A. H.
Published: February 2004

Data are presented for the proton and cation hyperfine coupling constants and g values of the naphthalene anion as a function of solvent, cation, and temperature. The variation of the proton coupling with solvent and cation is comparatively small and somewhat irregular. The temperature coefficient of the proton coupling constants depends upon the cation. Cation coupling constants are reported for the five alkali ions, as well as the temperature dependence in some cases. The g value of ion pairs containing Rb or Cs differ considerably from that of the free anion, and is temperature dependent. These observations are discussed qualitatively in terms of the formation and structure of ion pairs, and of the effects of the electrostatic field and spin–orbit coupling of the cation.
Molecular-Orbital Studies of Hydrogen Bonds. An Ab Initio Calculation for Dimeric H2O
Authors: Morokuma K. , Pedersen L.
Published: February 2004

The hydrogen-bond energy and the most stable structure of the dimeric H2O system are calculated by the LCAO MO SCF method using a medium-sized Gaussian orbital basis set. The most stable structure, found by a limited variation of the interatomic coordinates, is a linear hydrogen bond (stabilization energy 12.6 kcal mole−1) with an H···O distance of 1.72 Å, and with the hydrogen-acceptor molecule almost freely rotating around its molecular axis. The stretching of the proton donor O–H bond is calculated to be 0.12 Å. A population anaysis near the energy minimum shows that the change in the population is distributed not only in the O···H–O fragment, but also delocalized into the neighboring O–H bonds. Hydrogen bonds of dimeric H2O other than the linear structure (cyclic and bifurcated) are also examined.
Electron Spin Resonance Studies of Free Radicals in γ-Irradiated Amino Acid Crystals: α-Amino Isobutyric Acid and l-Lysine·HCl·2H2O
Authors: Fujimoto M. , Seddon W. A. , Smith D. R.
Published: February 2004

Single crystals of α-amino isobutyric acid and l-lysine monohydrochloride dihydrate were γ irradiated at 77°K and electron spin resonance spectra were observed in the temperature range between 77° and 298°K. It was confirmed that the stable radicals, (CH3)2ĊCOOH and H2N(CH2)3CH2ĊHCOOH, formed from these two amino acids, respectively, are not direct products of irradiation but are formed thermally by an inter- (or intra-) molecular reaction involving intermediate radicals in the crystals. The intermediate radicals formed at 77°K are similar in these materials and are believed to be involved in the mechanism of amino-group abstraction. The radical H2N(CH2)3CH2ĊHCOOH in llysine·HCl·2H2O exhibits different ESR spectra at different temperatures corresponding to different conformations in the crystal lattice.
Distribution Function of Classical Fluids of Hard Spheres. I
Authors: Lee Yong-Teh , Ree Francis H. , Ree Taikyue
Published: February 2004

The Born–Green–Yvon (BGY) hierarchy is truncated by introducing a superposition approximation g1234≃g123g124g134g234 / [g12g13g14g23g24g34] to the quadruplet correlation function g1234. The resulting pair of two simultaneous integral equations including the triplet- and pair- correlation functions g123 and g12, which hereafter will be called the BGY2 equations, is reformulated to give mathematically simpler forms. The BGY2 theory is checked for its internal consistency by using the hard-sphere potential. The following results have been obtained: (a) The fifth virial coefficients B5 for the BGY2 theory is (0.1090 ± 0.0008) (B2)4 or (0.1112 ± 0.0005) (B2)4 depending on whether the virial theorem or the Ornstein–Zernike compressibility relation is used in the calculations of B5; the exact Monte Carlo value, B5 = 0.1103 (B2)4, lies between the two values. (b) The third coefficient g123 in the density series of g12 agrees exactly with the exact value given in the literature if r / σ ≥ 2 (σ is the sphere diameter), and it differs only slightly from the exact value if 0 ≤ r/σ ≤ 2, i.e., the deviation lies within 3%. (c) The data for the second coefficient g1232 of the density series of g123 show that the Kirkwood superposition approximation is better for a symmetric configuration than an asymmetric one when particles 1, 2, and 3 lie close to each other. (d) It is shown that the pressure calculated from the virial theorem using the BGY g(r) is exact for one-dimensional hard rods, and is independent of any approximation used to describe the g123. A proof that the BGY2 g(r) is exact for the hard-rod system is also given. These results indicate that the BGY2 theory compares favorably with any other theories for g(r) and that it satisfactorily describes low and moderately dense fluids.
Solvent Stark Effect and Spectral Shifts. II
Authors: Nicol Malcolm , Swain Julie , Shum Yin-Yan , Merin Robert , Chen Rosa H. H.
Published: February 2004

Solvent shifts of the energies of the lowest 1La bands of the electronic absorption spectra of anthracene, chrysene, phenanthrene, and tetracene and of the lowest 1Lb bands of the electronic absorption spectra of naphthalene, phenanthrene, chrysene, and picene are reported. The origins of these shifts are analyzed in terms of theories developed by Baur and Nicol and by Abe. Satisfactory fits of the experimental data are obtained for theoretical expressions derived from the theory of Baur and Nicol. The magnitudes of the parameters of this theory, which relate solvent shifts to the dielectric constant and refractive index of the solvent, are the same within experimental precision for the 1La transitions in all of the molecules studied. A similar regularity of the parameters also is observed for the 1Lb transitions. The theoretical expression for polar-solvent shifts derived by Abe is found to overestimate the contribution of the permanent dipoles of the solvent molecules.
Mössbauer Study of Soluble Prussian Blue, Insoluble Prussian Blue, and Turnbull's Blue
Authors: Ito A. , Suenaga M. , Ôno K.
Published: February 2004

The electronic structures of soluble Prussian blue, insoluble Prussian blue, and Turnbull's blue were investigated by the Mössbauer effect of 57Fe. The results below their Curie temperatures of 5.5° ± 0.5°K show that all of them are ferric ferrocyanide: one kind of irons is Fe3 +  with high spin and the other is Fe(II) with low spin. It is concluded that Prussian blue and Turnbull's blue have the same electronic structure. The results of a paramagnetic resonance absorption observed at room temperature also support the conclusions obtained by the Mössbauer study.
Structure of the H Function for a Hard-Sphere Gas
Authors: Harris S.
Published: February 2004

We consider, for a spatially uniform hard-sphere gas, the generalization of the connection between the H function and the thermodynamic entropy to that limit where gas imperfections are taken into account through the first virial correction term. Our choice of a generalized H function is based on the expansion of the canonical entropy density given by Nettleton and Green. We show that this choice allows us to give a physical significance to the condition that Boltzmann's H function be convex as a function of time.
Permeation, Diffusion, and Solution of Nitrogen in Tungsten and Molybdenum
Authors: Frauenfelder R.
Published: February 2004

Using ultrahigh-vacuum and mass-spectroscopic techniques, permeation, diffusion, and solution of nitrogen in W and Mo have been measured for pressures between 5 × 10−5 and 500 torr and for temperatures between 1100° and 2500°K. The following values for the permeation constants, P(torr·liter/cm·sec·torr1/2), diffusion constants, D (cm2/sec), and solubility constants, S(torr·liter/cm3·torr1/2), are obtained: P(W) = 1.6 × 10−4exp (− 52 000 / RT); P(Mo) = 6.1 × 10−3exp (− 47 500 / RT); D(W) = 5.4 exp (− 62 000 / RT); D(Mo) = 2.3 × 10−2exp (− 33 000 / RT); S(W) = 3.0 × 10−5exp (+ 10 000 / RT); S(Mo) = 2.6 × 10−1exp (− 14 500/RT). For nitrogen in Mo, the diffusion constant is in agreement with Zener's theory of interstitial atomic diffusion. For nitrogen in W, the frequency factor of the diffusion constant and the diffusion activation energy are abnormally high and not in agreement with Zener's theory.
Low-Temperature Viscosities of Argon, Krypton, and Xenon
Authors: Clarke A. G. , Smith E. B.
Published: February 2004

The viscosities of nitrogen, argon, krypton, and xenon have been measured by a capillary flow technique in the temperature range 114°–374°K relative to that of nitrogen at 273.15°K. The results for nitrogen and argon below 170°K are lower than those reported by previous workers, but at higher temperatures the results are consistent with other recent determinations.
Matrix-Isolation Study of the Infrared Spectrum of the Free Radical F2CN
Authors: Jacox Marilyn E. , Milligan Dolphus E.
Published: February 2004

In studies of the reaction of photolytically produced F atoms with FCN in Ar and N2 matrices at 14°K, the free radical F2CN is stabilized in sufficient concentration for observation of its electronic spectrum and of six infrared absorptions. A vibrational assignment has been made. The carbon–nitrogen bond is found to possess double-bond character. Products of further F-atom attack on F2CN, including CF2NF and CF3NF2, are observed in these experiments. Evidence is presented suggesting that the species FC=NF may also be stabilized.
Reduced-Density-Matrix Theory: The Energy Properties of the Be 2-Matrix
Authors: Barnett Gene P. , Platas Oscar R.
Published: February 2004

An energy analysis of the 2-matrix, the second-order reduced-density matrix, for the Weiss Be function is presented. The properties of the reduced Hamiltonian HR = (N − 1)−1 (H1 + H2) + H12 are first considered by discussing the simple example of the function 1s2 which can represent a two-electron atom or ion, or a natural geminal which is an eigenfunction of the 2-matrix, or an approximate eigenfunction of the reduced Hamiltonian which is referred to as a Bopp function. It is seen that the ordering of the natural geminal energies, εi, may be different than the ordering of the eigenvalues, λi, of the 2-matrix. The 2-matrix energy decomposition is defined as E = Σλiεi. This decomposition is discussed with reference to truncations of the natural geminal expansion of the Weiss Be function. The total Be energy from the truncated expansions is sometimes below the true energy; this is presumably due to a lack of antisymmetric behavior in the truncated expansions.
Observation of Metastable Ions in a Cycloidal Mass Spectrometer
Authors: Benz Hanspeter , Brown Harmon W.
Published: February 2004

The line shapes that result from metastable-ion transitions in a cycloidal mass spectrometer are discussed and some half-lives of metastable ions are determined by a new method.
Vibrational Analysis of Copper Chloride Dihydrate
Authors: Fifer R. A. , Schiffer J.
Published: February 2004

Room- and low-temperature infrared spectra of copper chloride dihydrate and its deutero modifications have been obtained, as well as polarized spectra of oriented polycrystalline films of the hydrate. Bands due to vibrations of H2O, D2O and HOD molecules in the hydrate, as well as their polarization, are considered in terms of the crystal structure and previous interpretations of the vibrational bands of the solid.
Charge Transfer of N2+, O2+, and NO+ to Sodium Atoms at Thermal Energies
Authors: Farragher A. L. , Peden J. A. , Fite W. L.
Published: February 2004

A steady-state flowing afterglow system has been utilized to measure the two-body rate coefficients for the charge-transfer reactions of N2+, O2+, and NO+ to sodium atoms. The reactions of N2+ and O2+ are rapid with rate coefficients of approximately 5.8 and 6.7 × 10−10 cm3/sec, respectively, whereas the NO+ reaction is found to be an order of magnitude slower.
Studies of Chemical-Shift Anisotropy in Liquid-Crystal Solvents. I. Experimental Results for the Methyl Halides
Authors: Ceasar Gerald P. , Yannoi Costantino S. , Dailey Benjamin P.
Published: February 2004

Analysis of the nuclear magnetic resonances of the methyl halides dissolved in nematic-liquid-crystal solutions has given measurements of the anisotropy of the proton chemical shift. The accurate determination of these anisotropies is made difficult by the effect of the liquid-crystal solvent on the chemical-shift measurements. The method was found to be unsuitable for the measurement of the methyl fluoride anisotropy, but the values for the other methyl halides have been determined to within ± 1.5 ppm. Methyl chloride and methyl bromide are found to have small proton chemical-shift anisotropies indistinguishable from zero, and methyl iodide appears to have a small positive anisotropy.
Coupled Equations and the Minimum Principle for Collisions of an Atom and a Diatomic Molecule, Including Rearrangements
Authors: Miller William H.
Published: February 2004

The collision of an atom with a diatomic molecule is treated by a coupled-channel expansion augmented by a finite number of square-integrable basis functions. The method is, in principle, capable of essentially exact results if the total energy is less than that necessary for the existence of three free atoms and should be practical for light systems at low energies. The coupled equations which must be solved to apply the minimum principle are obtained and the direct and exchange interaction matrices are given in reduced form. The case that two, or all three, of the atoms are identical simplifies the equations, and the cross-section formulas are given for arbitrary spin of the indentical nuclei.
Yield of Ozone in the Pulse Radiolysis of Gaseous Oxygen at Very High Dose Rate. Use of This System as a Dosimeter
Authors: Ghormley J. A. , Hochanadel C. J. , Boyle J. W.
Published: February 2004

A yield of ozone of 13.8 ± 0.7 molecules/100 eV was determined in the pulse radiolysis of oxygen at 1 atm by high-energy electrons at a dose rate of 1012 rads/sec. The absorbed dose was based on the energy absorbed in a thin aluminum calorimeter. While this yield is somewhat higher than a recently reported value of 10.2 ± 0.6 determined from the initial rate of formation in radiolysis by cobalt gamma rays at low dose rate, it is likely that there is little or no dose-rate dependence over the range from 102–1012 rads/sec. This system provides a simple and accurate dosimeter, and the in situ measurement of ozone offers the added advantage for system in which the electron flux is nonuniform, that the average absorbed dose in the optical path is measured.
Distribution Functions for Chain Molecules
Authors: Jernigan R. L. , Flory P. J.
Published: February 2004

Distribution functions W(r) of the end-to-end vectors r of chain molecules are calculated according to: (i) the exact expression of Rayleigh for a freely jointed chain, (ii) the Kuhn and Grün approximate function, (iii) a revised form of the KG function which amends a basic error in its traditional derivation, and (iv) the Gaussian function W∞(r). For chains of 4–20 freely jointed bonds, (iii) is a considerable improvement over (ii). Except at higher extensions, in the vicinity of r / nl≈1, the Gaussian expression (iv) offers the most satisfactory approximation to (i). Series expansion of W(r) / W∞(r) in Hermite polynomials in the even moments 〈r2〉0, 〈r4〉0, etc., readily yields results previously obtained by a more lengthy Fourier transformation method due to Nagai. For chains of short or moderate length the convergence of Nagai's series is slow, many higher moments being required for significant refinement of W(r) beyond its approximation by W∞(r). Analysis of the moment ratios 〈r4〉0 / 〈r2〉02 and 〈r6〉0 / 〈r2〉03 and their trends with chain length suggests that the Rayleigh (exact) distribution functions for freely jointed chains may afford very good approximations for real chains, provided, however, that the number of equivalent bonds for the freely jointed model is properly scaled in relation to the actual number of bonds in the real chain. About 20 bonds of a polymethylene chain are equivalent to one of the freely jointed model chain.
Electron Affinities of the Heavy Elements
Authors: Zollweg R. J.
Published: February 2004

Horizontal analysis has been employed to obtain quantitative estimates for the electron affinities of the elements of the three long periods of the Mendeleev periodic table. Values for the A subgroup were normalized to published experimental values for bromine and iodine and an estimate for astatine, while those for the B subgroup were based on experimental values for the alkali and noble metals. The results are consistent with most of the published experimental values and with those negative ions detected. The near agreement between the electron affinities of the alkali metals and those of the excited rare gases with similar outer-shell configurations suggests that most of the polarization contribution arises from the outer shell.
Argon–Methane Phase Diagram
Authors: Greer Sandra C. , Meyer Lothar , Barrett C. S.
Published: February 2004

The argon–methane solid phase diagram has been determined between 15° and 65°K by x-ray diffraction techniques. The diagram shows a miscibility gap with a critical point for unmixing at 35% methane and 63°K, whereas published liquidus and solidus lines indicate a eutectic point at 39% methane and 71°K. Calculation of the interaction energy of the mixture from the properties of the constituents using the simplest forms of solution theory yields values which deviate by an order of magnitude from those derived from the experimental data. The discrepancy is in part due to an appreciable contribution to the excess free energy from strain energy and otherwise due to inadequacies of the methods of calculating both interaction and strain energies and due to imprecision in the experimental parameters required for these calculations.
Nonadditive Third Virial Coefficient for Intermolecular Potentials with Hard-Sphere Cores
Authors: Fowler Ronald , Graben H. W.
Published: February 2004

An analytic evaluation of the triple-dipole dispersion energy contribution to the classical third virial coefficient is given for intermolecular pair potentials in which the interaction varies as a sum of inverse powers of the distance outside a hard-sphere core. Applications to a modified Lennard-Jones model, the Sutherland potential, and the weak-attraction Sutherland model are made. Comparisons with existing results are given.
Low-Temperature Hole Injection and Hole Trap Distribution in Anthracene
Authors: Schadt M. , Williams Digby F.
Published: February 2004

A solid and stable electrode has been developed, able to efficiently inject holes into anthracene crystals down to low temperatures. With such an electrode, dark-current measurements have been performed successfully, which show that the exponential distribution of hole traps does not extend to the valence band. In addition two shallow hole trap levels at energies E1 ≡ 0.53 and E2 ≈ 0.2 eV have been observed. This trap distribution has been found for a series of melt-grown anthracene crystals, prepared from different batches of carefully purified starting material.
Hulthén Approximations to 1s and 2p Orbitals of Atoms
Authors: Weare John H. , Weber Thomas A. , Parr Robert G.
Published: February 2004

It is shown that a substantial energy improvement over the Slater-orbital description is obtained by the variational use of 0s and 1p functions in the 1s and 1p shells of atoms, where 0s and 1p represent differences of two Slater 0s and Slater 1p functions, respectively. Reduction of the number of parameters by enforcing nuclear cusp conditions does not affect the variational energy significantly. The reduction yields one-parameter 0s and 1p orbitals giving a better energy than both one-parameter Slater functions and two-parameter variable-principal-quantum-number functions. It is also shown that in correlated functions of the form ψ = φ (r1) φ (r2) χ (r12), the orbital 0s when used as a one-term approximation to the best possible φ gives an excellent result for the energy of heliumlike systems and the correlation function 1 + c exp(− ηr12) gives a very good approximation to the best χ(r12). Reduction of the number of parameters by requiring both nuclear and correlation cusp conditions yields a two-parameter trial function which for helium has the energy − 2.90002 a.u., vs the Roothaan–Weiss best energy, − 2.90039 a.u., for a function of this form.
Electron-Impact Spectrum of Mercury Vapor
Authors: Skerbele Ausma , Ross Kevin J. , Lassettre Edwin N.
Published: February 2004

Electron-impact spectra of mercury vapor have been investigated at incident electron energies between 50 and 300 eV. An intensity anomaly (expected from previous studies of atomic spectra) has been observed in the 61S0 → m1P1 series due to a perturbation by the 6p′ 1P state. Relative cross sections and oscillator strengths have been calculated from the intensity data at 300 eV and zero scattering angle.
ESR of Spin-Labeled Nematogenlike Probes Dissolved in Nematic Liquid Crystals
Authors: Ferruti P. , Gill D. , Harpold M. A. , Klein M. P.
Published: February 2004

We show that qualitative information on molecular ordering in nematic mesophases is conveniently obtained from the ESR spectra of trace quantities of spin-labeled rodlike probe molecules dissolved in the nematogen. The synthesis of the probe [2,2,6,6-tetramethyl-4-(p-octyloxy)-benzoylamino-piperidine-1-oxyl] is described and ESR spectra of the probe taken in nine solvents at various temperatures are reported. Large contractions of the 14N hyperfine patterns, as well as positive g shifts, have been observed. This indicates that the alignment of the host molecules along the magnetic field direction constrains the motion of the probe molecules so that they too are aligned parallel to the field. The rotation of the radical group about the bond connecting it to the rest of the probe molecule is partly hindered. In general, the probe molecules described here are less ordered than the host molecules.
Analysis of the Ion–Molecule Reactions in Gaseous H2, D2, and HD by Ion Cyclotron Resonance Techniques
Authors: Bowers M. T. , Elleman D. D. , King J.
Published: February 2004

Using recently developed ion cyclotron resonance (ICR) techniques, the thermal-energy rate constants of the reactions H2++H2→H3++H, D2++D2→D3++D, H2D++D↗HD++HD↘HD2++H, have been measured. The values of k1 = 2.11, k2 = 1.60, k3a = 0.75, and k3b = 1.05 × 10−9cc molecule−1·sec−1 are in good agreement with the predictions of the Langevin theory and agree in general with other data where available. The energy dependencies of (R1)–(R3) were studied by a variety of double-resonance techniques. An analysis of the energy dependence implies that a dual reaction mechanism is operative in (R1)–(R3). At low energies (KE < 1 eV) both a complex formation and a stripping mechanism occur while at higher energies the stripping mechanism becomes dominant. The relative energy dependencies of (R3a) and (R3b) are interesting in that the ratio [H2D+]/[HD2+] considerably increases as the reactant-ion energy increases (at low energies). The implication is that as the stripping mechanism becomes more dominant the importance of the displacement of the center of mass from the center of the HD bond becomes significant. This variation of [H2D+]/[HD2+] with HD+ energy is used to calibrate the ion energy in the ICR cell by comparison with the tandem-mass-spectrometer data of Futrell and Abramson. Pulsed-ion-ejection studies of an 1:1 H2, D2 mixture indicate the reactions H2++D2↗D2++H2↘HD++HD do not take place at thermal energies for any value of the impact parameter. Also, using pulsed-ion-ejection methods, the relative thermal rate constants for Reactions (R5), H2D++D↗H2++D2↘HD2++H, k5a / k5b = 0.55, are considerably lower than the HD system, k3a / k3b = 0.73, indicating a proton is preferentially transferred in (R3).
Site Effects in Isotopic Mixed Crystals—Site Shift, Site Splitting, Orientational Effect, and Intermolecular Fermi Resonance in the Vibrational Spectrum of Benzene
Authors: Bernstein Elliot R.
Published: February 2004

A modification of the Davydov theory of energy levels in molecular crystals is applied to experimental observations on ground-state vibrations of various isotopic substituted benzene crystals. Three distinct and characteristic crystal-induced phenomena have been observed experimentally: (1) site group splitting—involving degenerate vibrations of the molecule that map into nondegenerate site modes; (2) orientational effects—concerning isotopic benzenes of symmetry D2h or lower, and their relative positioning in two or more distinct orientations at crystal sites; and (3) intermolecular Fermi resonance—near-resonance interaction between two adjacent-site molecules of different isotopic compositions. The first two phenomena give rise to 10 cm−1 ≲ splittings. A general mechanism is proposed to account for this, and the differences and similarities between these two effects are discussed. The importance of observed gas-to-crystal energy shifts is also discussed in the light of these experimental findings. In addition to intersite Fermi resonance, solid-enhanced intrasite Fermi resonance is reported and discussed qualitatively. Applying the Davydov theory for the case of no resonance interactions, one can obtain information concerning the shape of the molecule in the site field, the symmetry of the site field, and the general nature of the intermolecular interactions.
Zeeman Quadrupole Resonance in Powders
Authors: Creel R. B. , Segel S. L. , Anderson L. A.
Published: February 2004

The Zeeman patterns for spin-32 quadrupole interactions, for powder samples, have been determined for the special case of the perturbing rf magnetic field perpendicular to the small static magnetic field. A four-line Zeeman pattern has been observed experimentally for 35Cl in NaClO3 and HgCl2 using an induction spectrometer, and a theoretical calculation using a zero linewidth quadrupole resonance has provided an adequate explanation. The resonance shapes and positions are insensitive to the asymmetry parameter in the electric field gradient. The technique is useful for the determination of magnetic shift parameters, and a value of 0.5% has been obtained for the parallel component of the paramagnetic shift of 35Cl in K2ReCl6.
Nuclear Magnetic Resonance Powder Patterns in the Presence of Completely Asymmetric Quadrupole and Chemical Shift Effects: Application to Metavanadates
Authors: Baugher J. F. , Taylor P. C. , Oja T. , Bray P. J.
Published: February 2004

A procedure was developed for the determination of the locations of shoulders and singularities of polycrystalline nuclear magnetic resonance (NMR) powder patterns in the presence of both nuclear quadrupole and anisotropic magnetic shift interactions. This determination was performed for both the satellites and the central transition of the NMR spectra of nuclei with half-integral spin I. Typical theoretical behavior of the powder pattern for the central transition in the presence of both quadrupolar and magnetic shift interactions is shown in a series of examples. In addition, a computer program was devised which calculated the powder pattern for arbitrary quadrupolar and magnetic shift parameters, convoluted it with a function that simulates the effects of dipolar broadening, and took the derivative of the result. The quadrupolar and magnetic shift parameters in three metavanadate compounds (NH4VO3, KVO3, and NaVO3) are determined using these methods.
Vapor Pressure and Crystal Structure of Curium Metal
Authors: Smith P. K. , Hale W. H. , Thompson M. C.
Published: February 2004

The vapor pressure of liquid and solid curium metal between 1179° and 2068°K and between 10−10 atm and 0.1 mm was measured by the Knudsen effusion method to be log10P(atm) = (4.74 ± 0.37)–(18 060 ± 590) / T(°K); ΔH°1600°K and ΔS°1600°K for vaporization are 82.6 ± 2.7 kcal/g·atm and 21.7 ± 1.7 eu, respectively. A previously unreported fcc close-packed structure for curium metal was identified with a = 4.382 ± 0.004 Å, and a density of 19.26 g/cm3, corresponding to a +4 metallic valence. The entropy of condensed curium is indicated to be relatively high by the low heat of fusion and by the ΔH°298°K for vaporization calculated by second- and third-law methods. The heat of vaporization of curium may be explained from the opposing effects of a quadrivalent metal and a 5f66d27s2 to 5f76d7s2 transition on vaporization.
High-Order ESR Transitions of Eu2+ in CaF2, SrF2, and BaF2
Authors: Lee Sook , Menne T. J. , Paxhia E. C.
Published: February 2004

The X-band ESR spectra obtained from single crystals of CaF2, SrF2, and BaF2 containing dilute concentrations of Eu2+ ions show many weak absorption lines at magnetic field values lower than those of the allowed (ΔM = ±1) Eu2+ lines. These additional weak lines display a marked angular variation, but for a given crystal orientation, their relative field positions and ESR intensities are qualitatively similar for the three crystals. Calculations were made to determine the exact energy levels and probabilities of all high-order (ΔM ≠ ± 2) ESR transitions for various crystal orientations as a function of the magnetic field strength. These calculations were based on the diagonalization of the energy matrix for the S7/28 state in a cubic environment without use of the high-field approximation. The observed field positions and relative ESR intensities of the additional low-field lines were found to be qualitatively consistent with those calculated for the high-order ESR transitions corresponding to ΔM = ± 2, ± 3,···± 7 of Eu2+ ions in cubic sites of the crystals.
Resonance Theory of Termolecular Recombination Kinetics: H+H+M→H2M
Authors: Roberts Robert E. , Bernstein R. B. , Curtiss C. F.
Published: February 2004

A theory is formulated for atomic recombination reactions which is based upon the identification of the set of transition complexes, X2i, as specific quasibound states or orbiting resonances. The conventional “energy-transfer mechanism” is assumed, since it has been justified under many experimental situations. Calculations, based on a modified distorted-wave approximation, demonstrate that the main contribution to the rate is that arising from rotational (rather than vibrational) transitions downwards from the quasibound to the bound states. Computations were carried out for the reaction H+H+M→H2+M for M = He, H2, and Ar making use of detailed ab initio knowledge of the spectrum of quasibound states and their wave-functions. Good agreement was found between the experimental rate constant and that calculated by the present resonance theory. The theory predicts a maximum in the rate in the temperature range between 65° and 100°K, attributed mainly to one particular quasibound state: υ = 14, j = 5. This suggests a promising region for further experimental investigation which could provide a critical check of the present theory.
Melting Curves of Volatile Materials
Authors: Babb Stanley E.
Published: February 2004

The melting curves of 15 substances are reported over pressures up to 15 kbar.
Energy-Level Scheme for Nd3+ in CaWO4
Authors: Sengupta D. , Artman J. O.
Published: February 2004

The free ion parameters obtained from a least-squares fit to the six lowest free ion levels of Nd3+ in CaWO4, neglecting configuration mixing, are (in cm−1): E1 = 4859.7, E2 = 24.78, E3 = 482.0, and ζ = 873.8. The rms deviation of this fit was 21. Crystal-field parameters appropriate to S4 symmetry were fit to the structure of the I9/24 and I11/24 levels. Intermediate coupling wavefunctions obtained from the free ion fit were used. J−J mixing between the I9/24 and I11/24 states was considered. The crystal-field parameter values found are (in cm−1): B02 = 514.2, B04 = −1040.0, B06 = − 60.0, B44 = − 1071.6, B46 = − 980.0, B4′4 = 16.4, and B4′6 = 13.1. The rms deviation of this fit was 9.5. We calculate the ground-state g values to be g‖ = 1.633 and g⊥ = 2.621. The corresponding experimental values are 2.035 and 2.537. By changing the Bmn we can get a better g-value fit, namely, 2.025 and 2.593.
Theory of the Trapped Dielectron
Authors: Fueki Kenji
Published: February 2004

The theory of the trapped dielectron in polar media is developed on the basis of the continuous dielectric model. The theory leads to a remarkably simple criterion for the energetic stability of the dielectron. Application is made to the calculation of the electronic properties of the trapped dielectron in aqueous systems. The transition energies calculated with low static dielectric constants are in the right energy region as compared to the absorption spectrum observed at low temperature.
Screening Constants and Transition Energies
Authors: Veigele Wm. J. , Stevenson D. E. , Henry E. M.
Published: February 2004

Atomic screening constants usually are derived by minimizing ground-state energies or by comparing known system energies with those calculated with simple wavefunctions containing screening constants. One set was derived, however, by matching moments of simple wavefunctions containing screening constants to moments of Hartree–Fock wavefunctions. These screening constants represent the individual interactions of a given electron with each of the other electrons of the system. They are, therefore, particularly useful for calculating bound–bound and bound–free transition energies in ions. Neutral atom electron binding energies calculated using these screening constants disagreed with published best values. We adjusted the screening constants empirically to reproduce closely these best values of binding energies. Using the adjusted screening constants, we calculated K- and L-shell transition energies in neutral atoms and ions and obtained good agreement with accepted values.
Higher-Order Phase Transitions in Two-Stranded Macromolecules
Authors: Applequist Jon
Published: February 2004

A model for infinitely long two-stranded macromolecules, previously shown to exhibit phase transitions in association with the helix–coil equilibrium, is examined further. It is found that phase transitions of all orders from one to infinity are formally possible, depending on the value of a, which is defined by the fact that the probability of loop closure for a random flight of k steps is proportional to k−a. Defining h = a − 1 for the “matching case” and h = a − 2 for the “mismatching case,” a phase transition of order m (m = 2, 3,···,) is predicted if 1 / m ≤ h < 1 / (m − 1). The transition is of infinite order if h ≤ 0 and of first order if h > 1. From calculated transition curves it is found that the apparent order may be lower than the true order if the stacking parameter b is sufficiently small. The maximum slope of the transition curve is found to vary as b−1/(1−h) for b ≪ 1, h ≤ 12, in agreement with previous studies of certain special cases. An examination of mean sequence lengths shows that, for phase transitions of low order, the mean length of a helical sequence remains near a critical minimum value over a considerable range of the degree of bonding, while the mean loop size varies greatly over the same range. Experiments on the helix–coil equilibrium in two-stranded polyriboadenylic acid are reported, demonstrating the existence of a true equilibrium under certain conditions, and confirming the occurrence of an apparent second-order phase transition under these conditions. Attempts to fit the experimental data to the theory lead to the conclusion that a is in the range 2.3–2.5 if the mismatching model is correct but that a would be lower if some bias exists toward the matching condition.
Statistical Mechanics of Linear Molecules. IV. Nonspherical Polar Molecules
Authors: Sweet John R. , Steele William A.
Published: February 2004

A modified form of the Stockmayer potential for polar molecules is utilized to calculate: pair correlation functions in the limit of zero density, ordinary second virial coefficients, and dielectric second virial coefficients. The potential function is modified by allowing the dispersion–repulsion part of the energy to be that for a linear symmetric molecule whose potential energy is represented by Lennard-Jones functions for the two “atoms” in the molecule interacting with the two “atoms” in neighboring molecules. This is combined with an ideal dipole–dipole energy to give the over-all potential function. Changes in the statistical properties are calculated as a function of the molecular length, and results are presented which indicate that the ordinary second virial coefficients change by moderate amounts, but that the dielectric second virial coefficient is strikingly affected by the introduction of nonspherical molecular shapes.
Monte Carlo Simulation of Vacancies in Rare-Gas Crystals
Authors: Squire David R. , Hoover William G.
Published: February 2004

The thermodynamics of vacancy formation in rare-gas crystals is studied by using Monte Carlo simulation of a many-body system. In the calculations, we assume a pairwise-additive Lennard-Jones potential and measure the change in Helmholtz free energy associated with reversibly adding a particle to a crystal containing a single vacancy. The addition is carried out by varying a coupling parameter joining one particle to the others in the crystal. Results for both 32- and 108-particle systems show that in a macroscopic (rare-gas) crystal near the triple point (1) the fraction of lattice sites vacant is about 1/3000, and (2) relaxation of particles neighboring a vacant site is less than 1% of an equilibrium interparticle spacing. These calculations are in excellent agreement with the earlier work of Glyde.
Normal-Coordinate Treatment of Acetic Acid Monomer and Dimer
Authors: Fukushima Kunio , Zwolinski Bruno J.
Published: February 2004

A normal-coordinate treatment was carried out for acetic acid monomer, dimer, and their deutero analogs. All of the in-plane and out-of-plane vibrations were calculated using a modified Urey–Bradley force field. On the basis of the calculated frequencies and the potential-energy distributions, assignments of vibrational spectra were made for these substances.
Model for the Shock Hugoniot for Condensed Media
Authors: Salzman P. K. , Collings A. F. , Pings C. J.
Published: February 2004

A simple model is presented for the Hugoniot energy change in condensed media subjected to a strong shock. Approximating the structure of the liquid by a cell model and assuming an n − 6 pair potential, an expression is obtained for the configurational portion of the internal energy change. Changes in thermal energy and the effects of relaxation processes are ignored since the final state involved in the energy balance is selected to coincide with the state of molecules just after they have attained the particle velocity, but before significant translational relaxation has taken place. The Hugoniot energy change is then approximated by the change in configurational energy. The derived Hugoniot fits available data for five simple liquids within experimental error when a consistent and realistic value of n is chosen. A single value of 8.5 is found to produce good agreement between experimental and predicted results for argon at initial temperatures of 86° and 148°. Values of n for argon and nitrogen agree closely with those found from molecular beam scattering studies.
ESR Spectra of the Charge-Transfer Triplet States of Some Molecular Complexes
Authors: Hayashi Hisaharu , Iwata Suehiro , Nagakura Saburo
Published: February 2004

ESR spectra were measured with various solutions containing 1,2,4,5-tetracyanobenzene (TCNB) and benzene or methyl-substituted benzenes. We succeeded in finding the ESR absorption lines due to the Δm = ± 2 and Δm = ± 1 transitions of the lowest triplet excited states of TCNB complexes and also in measuring their lifetimes. From the analysis of the ESR spectral data, we determined the fine-structure constants D and E for the TCNB–toluene complex in ether–toluene mixed solvent, and D* for the TCNB complexes with benzene, toluene, xylenes, mesitylene, durene, pentamethylbenzene, and hexamethylbenzene. From the theoretical consideration on the D* value, an equation representing its dependence upon the charge-transfer degree (x) in the lowest triplet excited state of a molecular complex was derived. The combination of the equation with the observed D* values gave the x values for the TCNB complexes, for example 95%, 72%, and 42% for TCNB complexes with hexamethylbenzene, durene, and mesitylene, respectively. Thus, excited triplet states of great charge-transfer character were found to exist.
Nanosecond Pulse Radiolysis of Polystyrene and Poly(methylmethacrylate)
Authors: Ho S. K. , Siegel Seymour
Published: February 2004

Polystyrene and poly(methylmethacrylate) (PMMA) containing diphenyl and naphthalene as solutes have been studied by nanosecond pulse radiolysis. Solute negative ions were detected in PMMA but not in polystyrene. Solute triplet states were formed in times short compared with the lifetimes of the solute excited singlet states, whose presence was observed via their fluorescence. G (triplet) values of 0.2M samples of naphthalene and diphenyl in polystyrene were estimated to be 1.7 and 1.3, respectively. It is also shown that for high concentrations of solute less than 25% of the solute triplet states is derived from ion neutralization inside the spur. No evidence of intersystem crossing from solute excited singlet states was observed. Triplet yields in PMMA were smaller than those in polystyrene by a factor of 2 or more and were strongly affected by other transient species accumulated in previous irradiations. This latter quenching mechanism appears to involve the precursor, rather than the solute triplet states themselves. Possible modes of solute triplet-state formation are discussed, and it is concluded that triplet energy transfer from the solvent is the most probable mechanism.
Uniformly Valid Asymptotic Approximation for the Quantized Anharmonic Oscillator at High Energies
Authors: Handelsman Richard A. , Lew John S.
Published: February 2004

A uniformly valid asymptotic expansion is obtained for the solution of the one-dimensional time-independent Schrödinger equation which describes an anharmonic oscillator. As is well known this leads to a two-turning-point problem in asymptotic analysis. The expansion itself is determined for large values of an appropriately constructed parameter λ which can be specialized to 4J(E) / 2πℏ. It is shown to be valid for high energies which, in turn, implies that the turning points are well separated. The technique employed is an extension of that used previously by the authors for low energies, i.e., for nearby turning points. The energy levels are obtained implicitly to all orders in λ−1 by a relation which generalizes the traditional Bohr–Wilson–Sommerfeld quantum condition. For analytic potentials the leading terms in the asymptotic expansion of this relation are obtained in terms of contour integrals and are found to agree with the corresponding higher-order JWKB results of Dunham. Finally, the normalization of the eigen-solutions is discussed and a procedure for obtaining the asymptotic expansion of the normalization constant is indicated.
Energy-Level Scheme for Np4+ in PbMoO4
Authors: Sharma Kailash K. , Artman J. O.
Published: February 2004

Neptunium-doped (Na-charge-compensated) crystals of PbMoO4 were grown by the Czochralski method. The absorption spectrum between 5000 and 25 000 cm−1 was obtained on a Cary-14 spectrophotometer. The fluorescent spectrum between 4000 and 7000 cm−1 was obtained on a specially modified Beckman DK-2 spectrophotometer. The free-ion parameters obtained from a least-squares fit to the five lowest free-ion levels neglecting configuration mixing are E1 = 2970.0 cm−1, E2 = 14.01 cm−1, E3 = 312.1 cm−1, and ξ5f = 2.89.2 cm−1. The rms deviation for this fit was 17 cm−1. A first-order crystal-field perturbation calculation was carried out. A least-squares fit to the 10 lowest Stark levels yielded the following values for the crystal-field parameters: B02 = 2580.7 cm−1; B04 = − 3534.0 cm−1; B06 = − 1476.6 cm−1; B44 = − 2892.5 cm−1; B′44 = − 918.6 cm−1; B46 = − 663.9 cm−1, and B′46 = − 211.4 cm−1 with an rms deviation of 40 cm−1. A similar least-squares fit, including the effects of J–J mixing, could not be carried out due to rather large computation time requirements. However, it was found that for the lowest state [J = 9/2(I)], the mixing effects are less than 30 cm−1 whereas for the first excited state [J = 11/2(I)], J–J mixing can shift levels by as much as 80 cm−1. The results for Np4+ are compared with those for the “isoelectronic” ion Nd3+.
Diffusion in a Mixed Dense Fluid
Authors: McLaughlin E.
Published: February 2004

By using Thorne's extension of the Enskog theory for diffusion in a dense mixed hard-sphere fluid, equations are developed expressing the mutual diffusion coefficient as a function of composition. Use is made in the derivation of the expression for the contact distribution function obtained by Lebowitz from the theory of Percus and Yevick. For molecules of the same size, the equations reduce to those of Longuet-Higgins, Pople, and Valleau. Variation of the diffusion coefficient with density is discussed and the theory is applied to the diffusion of gases in liquids and to systems where either solute or solvent are dissimilar in size and mass. The Stokes–Einstein equation is also discussed.