bluuebunny/crossref_metadata_2025 huggingface dataset:
Deuterium quadrupole coupling in formic acid
Authors: Ruben D. J. , Kukolich S. G.
Published: February 2004

The 101 → 000 and 313 → 312 transitions of HCOOD were measured using a beam maser spectrometer with 4 kHz resolution. The measurements of three completely resolved components for each of these transitions allowed an unambiguous assignment of the spectrum. The quadrupole coupling strength in the O–D bond direction is eqzzQ = 272 ± 3 kHz with an asymmetry η = −0.075 ± 0.010. Analysis of previous data on DCOOH yields a deuterium quadrupole coupling strength along the D–C bond direction of eqzzQ = 166 kHz.
Thermodynamic properties for a simple model of solid carbon dioxide: Monte Carlo, cell model, and quasiharmonic calculations
Authors: Gibbons Trevor G. , Klein Michael L.
Published: February 2004

Monte Carlo, cell model, and quasiharmonic calculations are employed to investigate a molecular crystal model consisting of rigid linear molecules disposed in the cubic P a 3 structure and interacting with pair potentials between the ends. The errors involved in employing the cell model and quasiharmonic approximations are established. The thermodynamic properties for a model of solid CO2 are calculated as functions of temperature for a variety of 6—n potentials, including those obtained from previous lattice dynamical calculations in which fits were made to observed phonon frequencies. The results are compared with experimental measurements of the sublimation energy, heat capacity, bulk modulus, Grüneisen function, and thermal expansion. The best over-all fit was obtained with a 6–9 potential. However, a consistent inadequacy of this interaction model is that it exhibits too much thermal expansion for any reasonable fit to the other properties.
Exact quantum mechanical study of kinetic isotope effects in the collinear reaction Cl + H2 → HCl + H. The H2/D2 and the H2/T2 isotope effects
Authors: Persky Avigdor , Baer Michael
Published: February 2004

Exact quantum mechanical probabilities for the collinear reaction of chlorine atoms with H2, D2, and T2 in their ground and first vibrational excites states, were calculated for a semiempirical LEPS potential energy surface. These probabilities were used to calculate the kinetic isotope effects kCl+H2/kCl+D2 and kCl+H2/kCl+T2 which are compared with experimental data and with results of transition state theory calculations. The effect of the vibrational excitation of the hydrogen molecules on the rate of their reaction with chlorine atoms was also studied.
Proton spin-lattice relaxation studies of reorienting methyl groups in solids
Authors: Kumar Anil , Johnson Charles S.
Published: February 2004

Spin-lattice relaxation times (T1) have been measured at 30 MHz as a function of temperature for the methyl protons in a series of compounds, each of which contains a single methyl group. In all cases except one, the measurements were carried out above 77°K, where thermally activated motion predominates. The purpose was to determine activation energies for methyl rotation and to detect phase transitions. The following activation energies were found for polycrystalline samples from the slopes of plots of ln T1 vs T−1 on the low-temperature side of the T1 minimum: CH3PCl2 (2.25 kcal/mole), CH3PSCl2 (2.35), CH3SCN (2.4), CH3CD2I (3.2), and CH3CD2Br (3.6). For CH3COCl and CH3NCS, which apparently form glasses, only the high-temperature side of the T1 minimum could be observed. The apparent activation energies were 1.45 and 0.57 kcal/mole, respectively. Significantly nonexponential behavior was observed near and to the high-temperature side of the T1 minima. The relationship of these activation energies to internal rotation barriers obtained from ir, microwave, and cw NMR absorption studies is discussed.
Some theorems satisfied, and some theorems not satisfied by Born-Oppenheimer type wavefunctions
Authors: Epstein Saul T.
Published: February 2004

It is shown that within Born-Oppenheimer type approximations, hypervirial theorems and generalized Hellman-Feynman theorems are often not satisfied, the discrepancies between them and the theorems actually satisfied being of the order of the ``adiabatic correction.''
Green's function approach to the direct perturbation calculation of the excitation energies of closed shell fermion systems
Authors: Paldus J. , Čížek J.
Published: February 2004

General expressions to the third order of perturbation theory for a direct calculation of the excitation energies of closed shell atomic and molecular systems are presented. The Hartree-Fock ground state orbitals are used as single particle states, but the method may be easily generalized for an arbitrary one-electron basis. The derivation is based on the Green function and diagrammatic approaches. A ``double projection'' scheme is developed which enables not only a very effective reduction of the number of Feynman diagrams which must be explicitly considered but also yields very compact formulas for the excitation energies, which may be easily implemented on a computer. The method is illustrated and checked on a simple six-electronic model system for which the exact solutions are available.
Effect of high energy radiation on sodium methylarsonate and n-propylarsonic acid studied by electron spin resonance spectroscopy
Authors: Lyons A. R. , Symons M. C. R.
Published: February 2004

Exposure of Na2MeAsO3 and n-PrAsO(OH)2 to γ rays at 77°K gave radical species whose ESR spectra comprised sets of four lines assigned to hyperfine coupling to 75As nuclei. All centers had magnetic axial symmetry. Evidence is presented in favor of the identification of the trapped species as AsO32−, Na+ ⋯ AsO32−, MeAsO33−, H2CAsO32− (two equivalent protons also detected) in Na2MeAsO3, and the species AsO(OH)2, PrAsO(OH)2, and, less certainly, HAsPrO(OH)2 and PrAsO2− in n-PrAsO(OH)2. Structural aspects of the results are considered, and possible mechanisms for the formation of these radicals are proposed.
Crystal structure refinements with generalized scattering factors. I. 1,1′-Azobiscarbamide
Authors: Cromer Don T. , Larson Allen C.
Published: February 2004

1,1′-Azobiscarbamide, C2H4N4O2, is monoclinic, space group P21/c, with a = 3.593, b = 9.095, c = 7.635, β = 113°15′, and Z = 2. Data were collected with an automatic diffractometer using crystal monochromated MoK α radiation. The usual spherical free atom model with anisotropic thermal parameters was used in making the least-squares refinement. In addition, refinements were made with generalized scattering factors, and the scalar, dipole, and quadrupole one-center terms and scalar two-center terms were successively added. The aspherical distortions found are reasonable and produce a significant improvement between measured and calculated intensities. No significant difference in bond distances was observed.
Cross sections for production of O2− and C− by dissociative electron attachment in CO2: An observation of the Renner-Teller effect
Authors: Spence D. , Schulz G. J.
Published: February 2004

Using monoenergetic electron beams and mass analysis, we have measured the cross sections for production of O2− and C− by dissociative attachment in CO2. The cross section for O2− production has two peaks of the order of 10−24 cm2 at 11.3 ± 0.2 and 12.9 ± 0.2 eV incident electron energy. The double peaked nature of the cross section is interpreted as a manifestation of the Renner-Teller effect. The positions of these two peaks are in good agreement with INDO calculations. The cross section for C− production has three peaks at 16.0 ± 0.2, 17.0 ± 0.2, and 18.7 ± 0.1 eV, the largest of which, at 18.7 eV, has magnitude of approximately 2 × 10−21 cm2.
Nuclear corrections to molecular properties. IV. Theory for low-lying vibrational states of polyatomic molecules with application to the water molecule near the Hartree-Fock limit
Authors: Krohn B. J. , Ermler W. C. , Kern C. W.
Published: February 2004

Perturbation theory is used in conjunction with ab initio electronic structure calculations to obtain information about the effects of pure vibration on properties of polyatomic molecules. It is shown that any property P can be developed in the form ν=P′0+Σi=1NAi(νi+1/2)+Σi≤ jNBij(νi+1/2)(νj+1/2)+ ⋯, where P′0,{Aj}, and {Bij} are certain constants and (ν1, ν2, ⋯ νN) are the quantum numbers for N modes of vibration. Several isotopic variants of the water molecule with Σiνi ≤ 3 are used to illustrate the utility of the method at the Hartree-Fock level of approximation. The properties examined include the energy, the dipole moment, and the nuclear quadrupole coupling constants.
Infrared laser photolysis of Freon 12
Authors: Freeman M. P. , Travis D. N. , Goodman Myron F.
Published: February 2004

When the beam from a high power CO2 laser operating at 10.6 μ impinges on a laminar stream of CF2Cl2 at NTP, it inflames and undergoes high conversion to CF3Cl in an over-all nearly thermally neutral reaction. Experiments demonstrating the appreciable nonthermal component of the reaction, based on a null-hypothesis design, are reported. In a mixture of Freon 12/Ar/SF6, the partial pressure of CF2Cl2 is held constant; the total flow rate (residence time) is constant; the absorption coefficient is changed by varying the amount of SF6 in the (always optically thin) mixture; for each composition the laser power is changed until the average enthalpy increase of the Freon 12 component of the mixture would correspond to 24 kcal/mole (equilibrium temperature of [inverted lazy s] 1300°K); or the equivalent experiment is done by omitting SF6 and changing the intensity by vertical expansion or compression of the beam. The rate is found to be stimulated about two orders of magnitude by the radiation and depends on the fifth power of laser intensity.
Core-level binding-energy shifts in small molecules
Authors: Davis D. W. , Banna M. S. , Shirley D. A.
Published: February 2004

Binding-energy shifts of carbon and fluorine 1s levels in small (two to four carbon) fluorinated hydrocarbons are reported. Carbon, nitrogen, and oxygen 1s shifts for a number of other molecules are also reported. All samples were studied as gases: A total of 72 new shifts were measured. The results are analyzed using three electrostatic potential models. First the RPM theory, which includes final-state relaxation, was used. It correctly predicts the N(1s) shifts in methylamines, for which relaxation shifts are dominant, showing the importance of considering relaxation when comparing unlike molecules. The GPM theory involves only ground-state properties. It gives excellent results for carbon, oxygen, and fluorine shifts if only groups of similar molecules are compared. Finally, atomic charges are derived using the ACHARGE model. These charges agree very well with CNDO/2 charges. The ``charge alternation'' effect deduced in CNDO/2 calculations was observed— substitution of F for H on a carbon atom decreases the charge on the next carbon by about 0.025e in ethane, 0.045e in benzene, and 0.06e in ethylene, according to the ACHARGE analysis. Inductive charge transfer was found to be additive.
Coupling strength in the theory of radiationless transitions: f → f and d → f relaxation of rare-earth ions in YAIO3 and Y3AI5O12
Authors: Lauer Howard V. , Fong Francis K.
Published: February 2004

The dependences of the radiationless relaxation rate constant on the effective mediating phonon energy, the equilibrium displacement of the adiabatic surfaces, the energy gap, and the temperature are numerically examined in terms of a generalized theoretical rate expression derived from quantum statistical theory. Special emphasis is placed on the effect of the coupling strength on the behavior of the relaxation rate process. The validity of the generalized rate expression is illustrated by its application to the fit of the temperature dependences of several f → f and d → f radiationless relaxation rates observed for Er3+, Eu3+, Ho3+, and Pr3+ in YAIO3 and Y3Al5O12.
Reaction of atomic oxygen with carbon dioxide behind reflected shock waves
Authors: Baber S. Charles , Dean Anthony M.
Published: February 2004

The rate constant for O + CO2 → CO + O2 has been measured by monitoring CO2 infrared emission from dilute CO2/N2O/Ar mixtures behind reflected shock waves. Initial temperatures ranged from 3015°K to 4675°K and the total concentrations were 1.3–1.5 × 1018 molecule/cm3. Under these conditions, the CO2 concentration was observed to decay exponentially, and this decay is consistent with the mechanism: N2O + M → N2 + O + M, (1); N2O + O → NO + NO, (2); N2O + O → N2 + O2, (3); O + CO2 → CO + O2, (4); CO2 + M → CO + O + M, (5). Conditions are such that the oxygen atoms produced via (1)–(3) remain at a constant concentration (within 10%) during the time interval when the CO2 decay rate kapp is measured. Methods are described which estimate both the temperature and atomic oxygen concentration during this interval. These estimates then allow k4 to be calculated from kapp and known values for k5. The Arrhenius parameters (in units of cm3 molecule−1 · sec−1) are: log A = −11.1±0.2 and EA = 36.1±2.9 kcal/mole. This value is significantly lower than the previous direct measurement of k4 and is reasonably consistent with high temperature data on k−4.
Quantum efficiency of diffusion limited energy transfer in La1−x−yCexTbyPO4
Authors: Bourcet Jean-Claude , Fong Francis K.
Published: February 2004

In this paper, we present a detailed study of the concentration and temperature dependences of the quantum efficiency of energy transfer between Ce3+ and Tb3+ in La1−x−yCexTbyPO4. The determination of the transfer quantum efficiency in terms of the donor luminescence lifetimes is critically compared with that in terms of the donor luminescence intensities. It is shown that for the latter determination it is important to take into account any overlap in the donor and acceptor absorption spectra in the excitation wavelength region. From the exponential behavior, the concentration, and thermal dependences of the donor luminescence decay, it is shown that the diffusion of donor excitation plays an important role in the energy transfer process.
EPR of mixed valence complexes: Frequency and concentration dependent linewidths in hexaminechromium (III) chlorocuprates (I, II)
Authors: Scaringe R. , Kokoszka G.
Published: February 2004

Nine compounds in the mixed valence series of hexaminechromium (III) chlorocuprate (I,III), which may be formulated as [Cr(NH3)6] [CuCl4]1−x [CuCl5]x over a substantial range of x, have been investigated by a room temperature EPR study employing four microwave frequencies. The data indicate a variation of linewidth with concentration and frequency for each compound. The linewidth variation with concentration is largely associated with a concentration dependence of the internal dipolar and exchange fields while the frequency dependence is mainly associated with the ``10/3 effect.'' A simple model is employed to account for these variations and to evaluate the appropriate exchange fields. This is the first example of EPR study incorporating two different paramagnetic ions in which the concentration of one of the ions can be variated over a large range with little variation in other parameters.
Intramolecular kinetic isotope effect in the reaction of fluorine atoms with methane-d2 (CH2D2)
Authors: Persky Avigdor
Published: February 2004

The relative rates of the two competing isotopic reactions: F + CH2D2 → HF + CHD2 and F + CH2D2 → DF + CH2D, were measured in a fast flow system by direct mass spectrometric analysis of the reaction products in the temperature range of 159° to 298°K. The ratio of rates can be expressed as an Arrhenius type equation: kF+HCHD2/kF+DCH2D = (0.81 ± 0.03) exp[(275 ± 15)/R T]. The relation between the intramolecular isotope effect kF+HCHD2/kF+DCH2D and the intermolecular isotope effect kF+CH4/kF+CD4 is discussed with regard to secondary isotope effects in the fluorine atom + methane reaction.
Approach to the description of atoms using hyperspherical coordinates
Authors: Knirk Dwayne L.
Published: February 2004

We formulate and develop an expansion of N-electron atomic wavefunctions in hyperspherical coordinates. The expansion basis is a complete set of functions of 3N − 1 angle variables which describe the relative configuration of the electrons. The system is scaled in size by one extensive variable, the hyperradius r. All angular momentum and Pauli antisymmetry properties of the system are contained in the hyperspherical basis. A new, total angular momentum, antisymmetry-adapted basis is derived and used to construct an expansion of the total electronic wavefunction which has all of these properties term by term. These basis functions are called configurational normal modes and provide a new element of atomic structure to replace the electron orbital. Several methods of implementing calculations with this expansion are discussed.
Critical behavior of cross-transport coefficients
Authors: Papoular M.
Published: February 2004

We show that, near the consolute critical point of a binary mixture, thermodiffusion (kT), as well as baro- and electrodiffusion, should diverge like the isothermal osmotic compressibility. This is a stronger divergence than expected according to earlier predictions, and seems to be borne out by experiment. A few hydrodynamical consequences are discussed.
Application of matrix isolation spectroscopy to quantitative sputtering studies. I. Energies and oscillator strengths of the resonance transitions of gold atoms isolated in noble gas matrices
Authors: Gruen D. M. , Gaudioso S. L. , McBeth R. L. , Lerner J. L.
Published: February 2004

Matrix isolation spectroscopy has been applied to quantitative sputtering studies by developing techniques for the codeposition of backsputtered metal atoms with a large excess of noble gas atoms. The electronic spectra of Au atoms, produced by bombardment of an Au target with 50 keV Ar+ ions, were studied in Ar, Kr, and Xe matrices. Energy level shifts of the Au resonance doublet due to matrix perturbations have been interpreted in terms of changes in the spin-orbit coupling interaction of the Au 6p electron. The values in the various matrices were found to be ζXe = 2570 cm−1; ζKr = 2943 cm−1; ζAr = 3215 cm−1 (ζgas = 2543 cm−1). The [inverted lazy s] 650 cm−1 splitting of the 2P3/2 state in the various matrices was interpreted in terms of an axial distortion of a dodecahedral substitutional site. Simultaneous diagonalization of spin-orbit and ``crystal'' field interactions was employed to obtain ζ and A2 splitting parameters. Oscillator strengths for the resonance transitions were determined by developing an equation which relates the sputtering yield, integrated current, and a geometric factor based on a cosine law distribution of sputtered atoms to the integrated absorbance over an absorption band and the area sampled by the spectrometer beam. Oscillator strengths for the 2P1/2 and 2P3/2 transitions were found to be 0.132, 0.293 in Ar; 0.090, 0.200 in Kr; and 0.061, 0.145 in Xe. By comparison the gaseous atom values (``hook'' method) are 0.19, 0.41. It is pointed out that the matrix isolation method can determine Au atom concentrations equivalent to 0.1 monolayer and is therefore among the more sensitive and rapid techniques for measuring sputtering yields.
Phase-space theory of atomic dissociation and recombination reactions: H + H + H ⇄ H2 + H
Authors: Kung R. T. V. , Anderson J. B.
Published: February 2004

The modified phase-space theory of reaction rates was coupled with Monte Carlo trajectory calculations to yield rate constants and energy distributions of reactants and products for the three-atom dissociation and recombination reactions of hydrogen at 4000°K. Sample trajectories selected from those crossing a surface in phase space were calculated in forward and reverse directions to determine the classical rates of formation of bound and quasibound molecules from atoms. The probability of stabilization of quasibound molecules in successive collisions with hydrogen atoms was determined from additional trajectory calculations. The steady state correction factor for recombination, which accounts for dissociation of newly formed bound H2 molecules, was estimated from similar calculations. The computed rate constant for recombination is 3.1×1015 cm6 mole−2 sec−1. For equilibrium between free atoms and quasibound molecules established through tunneling, the over-all rate constant is estimated to be 75% higher. In either case the process of termolecular recombination to produce bound molecules makes a substantial contribution to the over-all rate.
Localized orbitals and short-range molecular interactions. II. Nonadditivity in He3 and He4
Authors: Musso G. F. , Magnasco V. , Giardina M. P.
Published: February 2004

Short-range interatomic interactions in singlet He3 and He4 are discussed in terms of a single determinant of doubly occupied nonorthogonal localized orbitals through the formalism described in Paper I [V. Magnasco and G. F. Musso, J. Chem. Phys. 60, 3744 (1974)]. Ab-initio calculations using a minimal set of Slater 1s orbitals are reported for several nuclear configurations. Nonadditive terms can be very well represented as resulting from penetration of pairs of nonorthogonal localized electron groups into the core provided by nuclei and ``unperturbed'' electrons of all other groups. Even in the small overlap region it is essential to include all the penetration effects, and this is easily done by the present formalism.
Mössbauer emission spectra of 119Sn after the EC decay of 119Sb in metals, oxides, and chalcogenides of antimony and tellurium
Authors: Ambe F. , Ambe S. , Shoji H. , Saito N.
Published: February 2004

Mössbauer emission spectra of 119Sn have been measured in metals, oxides, and chalcogenides of antimony and tellurium labeled with 119Sb or 119mTe. Sources of 119Sb and 119mTe metals gave an emission line in the Sn(0) region. In the emission spectra of 119Sb2O3 the species attributable to the Sn(II) state were dominant, while only the Sn(IV) species was found in the 119mTeO2 samples. The 119Sb2S3 and 119Sb2Se3 gave two emission lines attributable to the Sn(II) and Sn(IV) states. The 119Sb2Te3 showed only an emission line in the Sn(II) region. These observations were discussed in terms of the aftereffect of the EC (electron capture) decays and the properties of the matrices.
Compound-negative-ion-resonant states and threshold-electron-excitation spectra of monosubstituted benzene derivatives
Authors: Christophorou L. G. , McCorkle D. L. , Carter J. G.
Published: February 2004

Threshold-electron-excitation (TEE) spectra for fluorobenzene, benzaldehyde, and benzoic acid are presented, discussed, and compared with photoabsorption spectra. The TEE spectra clearly indicate excitation of optically forbidden states. Short-lived transient-negative ions have been found to form at low energies (< 2 eV) for nine monosubstituted benzene derivatives. These arise from the quasitrapping of slow electrons in the two lowest unoccupied π orbitals of the benzene-derivative molecules that, owing to the perturbation introduced by the substituent, are no longer degenerate as in benzene. The positions of these double compound-negative-ion resonant states are 1.27, 1.74; 0.61, 1.67; 0.55, 1.88; 0.4, 1.6; 1.30, 2.25; ?, 1.90; 0.66, 1.10; 0.71, 1.12; and 0.63, 1.33 eV for fluorobenzene, phenol, aniline, toluene, N-methylaniline, anisole, thiophenol, benzaldehyde, and benzoic acid, respectively. These are discussed in terms of the net π-charge transfer between the substituent and the benzene ring.
Magnetic resonance studies of the antiferroelectric phase transitions in the ammonium arsenates NH4H2AsO4 and ND4D2AsO4
Authors: Dalal N. S. , McDowell C. A. , Srinivasan R.
Published: February 2004

The antiferroelectric phase transitions in NH4H2AsO4 and ND4D2AsO4 have been studied through EPR and NMR. The paramagnetic center AsO44−, created by x irradiation of these compounds, was used as a microscopic probe in the EPR studies. Both 75As hyperfine structure and the superhyperfine structure from the hydrogen-bond protons showed changes on passing through the Curie point. Some features of the EPR spectra have been identified with the configurations postulated by Nagamiya in his model of the phase transition in these antiferroelectrics. In conjunction with earlier Raman scattering, Mössbauer effect, NMR, and dielectric studies of ADP, the present EPR studies point to a model of the phase transition where the NH4+ ions play a dominant role. At high temperatures, well above the Curie point (Tc), the NH4+ ions are thought to be executing fast reorientations such that they behave effectively like the spherical ions K+, Rb+, or Cs+. At Tc there is sudden change in the motion of the NH4+ ions, and each NH4+ ion is thought to make a permanent hydrogen bond with an oxygen of an AsO4 group, leading to a distortion of the group and hence a phase transition. Measurements of the proton nuclear spin-lattice relaxation times (T1) support this model. Comparison with earlier results on NH4H2PO4(ADP) suggests that the same mechanism is operative in the phase transition of ADP also. Several anomalies mentioned in earlier studies on KH2PO4-type compounds could also be rationalized on the basis of the present model. Implications of the present work to the Nagamiya model and further experiments to check the conclusions of the present work are also pointed out.
Nonexponential nuclear spin-lattice relaxation in polycrystalline dimethyl sulfone
Authors: Cutnell J. D. , Venable W.
Published: February 2004

Spin-lattice relaxation has been monitored via pulse techniques in polycrystalline dimethyl sulfone (DMS) and a 1:1 polycrystalline solution of DMS in the perdeutero analogue. The temperature range covered for DMS was between −90.1 and +75.8°C, while for the 1:1 solution it was between −111.4 and +72.8°C. The relaxation was found to be consistently nonexponential, with exponential relaxation being approached at the lowest temperature. The extent of the nonexponentiality is intermediate between that expected from the effects of cross correlations and that expected from a simple averaging of the exponential behavior of randomly oriented single crystals. A parameter δ=τnull/Tlav is introduced, which may be used to judge the extent to which cross correlations are increasing the observed nonexponentiality beyond that expected from a simple averaging of the exponential behavior of randomly oriented single crystals. Further, it is suggested, in situations where cross correlations do not completely determine the nonexponentiality, that the initial slopes of the decay plots be used to determine correlation times for methyl reorientation since these initial slopes are independent of cross correlation effects. Correlation times for methyl reorientation in DMS have been determined and yield an activation energy of 4.22±0.07 kcal/mole.
Connection between the macroscopic electric and mechanical susceptibilities
Authors: DiMarzio Edmund A. , Bishop Marvin
Published: February 2004

The following relationship is derived between the dielectric permittivity ε(ω) and the complex shear viscosity η(ω): [ε(ω)−ε∞]/(ε0−ε∞) = [1−i ωη(ω)K]−1. This equation is derived from both a generalization of Debye's treatment and from the Kubo formalism. The model consists of dipoles imbedded in rigid beads that are in turn imbedded in the viscous medium. A frequency dependent η(ω) implies nonexponential decay of the electric dipoles and results in skewed-arc behavior in the Cole-Cole plots except when η(ω) is represented by a single Maxwell element or by a single Viogt element. Consistency with the requirements of causality and reality is displayed. Experimental ε(ω) are predicted reasonably accurately by use of experimental η(ω) and the above equation.
C2H radical: 13C hyperfine interaction and optical spectrum
Authors: Graham W. R. M. , Dismuke K. I. , Weltner W.
Published: February 2004

C2H was prepared by photolysis of acetylene, 90% 13C-substituted acetylene, or iodoacetylene and trapped in solid argon at 4°K. A complete analysis of all hyperfine-interaction and g tensor components was made for the randomly oriented 2Σ molecules yielding the following parameters (in solid argon): g∥=g⊥=2.0025(5), A⊥(H)=41(1), A∥(H)=51(1), A⊥(13Cα)=863(1), A∥(13Cα) =980(1), A⊥(13Cβ)=139(1), A∥(13Cβ) =191(1) MHz. Approximate spin densities are compared with an extended basis INDO calculation of Morikawa and Kikuchi. Weak optical absorption spectra were observed at about 10 000 and 3300 Å. The former is assigned to an A2∏i←X2Σ transition with (0,0,0) at 10 080 Å (in solid argon) where the upper state C–C stretching frequency ν′3 =1560 cm−1. The assignment of the 3300 Å system (s) is less definite, but it may involve a B2A′←X2Σ, bent ← linear transition since the observed bands cannot definitely be assigned to an expected B2Σ←X2Σ transition.
X-ray energy analysis of Hg diffusion in Ag3Sn amalgams
Authors: Malhotra M. L. , Reynolds C. L. , Barker R. E.
Published: February 2004

Mercury diffusion in the various phases of dental amalgams is a matter of considerable importance with regard to mechanical strength and corrosive properties. At a sufficiently early stage of the diffusion of a diffusant into a diffusate it is appropriate to treat even very small diffusate particles as semi-infinite bodies. Using this idea, we discuss a convenient general scheme for determining the diffusion coefficient D from a profile of relative diffusant concentration C/Cs vs depth x from the surface where a constant concentration Cs exists. If the initial concentration is zero, the result of the method is that a plot of x vs erf−1 (1−C/Cs) should be a straight line through the origin and that the slope should be (4D t)1/2. X-ray energy dispersion spectrometry provides data well suited for analysis by the theoretical method. We have made analyses for Hg diffusion into Ag3Sn over a range of temperature so that D, D0, and ED from the relation D =D0e−ED/RT were obtained. The results are D0 =4.20 cm2/sec and ED =78±6 kJ/mol (18.6±1.5 kcal). At 37°C, D =3.2×10−13 cm2/sec.
On the theory of filamentary double injection and electroluminescence in molecular crystals
Authors: Hwang W. , Kao K. C.
Published: February 2004

A theoretical model based on the filamentary double injection in solids is presented, and expressions for the intensity of electroluminescence in molecular crystals as a function of applied electric field, current density, and temperature are derived. The computed results are in good agreement with presently available experimental results for undoped and doped anthracene crystals.
Absolute signs of hyperpolarizabilities in the liquid state
Authors: Levine B. F. , Bethea C. G.
Published: February 2004

We discuss several techniques for the determination of the absolute sign of molecular hyperpolarizabilities in the liquid state. One of these techniques is the comparison of the phase of the harmonic field produced by the liquid with that generated by the glass walls of the electroded cell. Another method is the phase comparison with a crystal quartz plate of known absolute orientation. Using these methods we determine that the hyperpolarizability of nitrobenzene is positive, i.e., the molecular nonlinearity and dipole moment are parallel. By mixing nitrobenzene with other liquids, we can readily determine their absolute signs, e.g., we have found that liquid CH3I is positive in agreement with measurements in the gas phase.
Crystal-field model study of the xenon hexafluoride molecule. I. Energy levels and molecular geometry
Authors: Wang Sylvester Y. , Lohr Lawrence L.
Published: February 2004

A crystal field approach is used to examine theoretically the energy levels and wavefunctions for a gaseous xenon hexafluoride molecule. The electronic configurations used are 5s2, 5s5p, and 5p2 of hexavalent xenon. Four electronic parameters are used to explore the variation in energy with the amount of distortion from 0h symmetry. These are the energy difference between the xenon 5s and 5p orbitals, the spin-orbit coupling constant for xenon 5p, and two Slater-Condon electron repulsion parameters. Pure bending vibrational modes of t1u, t2g, and t2u symmetries are examined. The mixing of xenon 5s and 5p orbitals by the t1u vibration is particularly important, leading to a possible pseudo-Jahn-Teller distortion from 0h symmetry. Calculations of the vibrational force constants and frequencies are made for the bending modes, and the results are compared with observed values for other hexafluorides and related complexes. Calculations made on the coupling between t1u and t2g modes indicate an enhanced stabilization of the C3v structure. These studies lend support to the interpretation of the observed anomalous properties of the xenon hexafluoride molecule in terms of nonrigid octahedral structure but not to interpretations requiring thermally populated spin-triplet excited states.
Crystal-field model study of the xenon hexafluoride molecule. II. Comparisons with other hexavalent xenon molecules
Authors: Wang Sylvester Y. , Lohr Lawrence L.
Published: February 2004

The two-electron crystal-field model previously used to describe in detail the energy levels of XeF6 is applied to other hexavalent xenon systems, namely the XeOF4, XeO2F2, and XeO3 molecules and the XeF82− ion. Comparisons of the calculated excited state energies of these three molecules at their observed geometries are made to those calculated for various geometries of XeF6. The strong low symmetry fields in XeOF4, XeO2F2, and XeO3 result in very high excitation energies, which may also be taken to represent greater energy stabilization for the lone electron pair relative to that for XeF6. The ground state energy of XeF82− is explored as a function of geometry, and within the two-space considered, the results match the structure observed in solid (NO)2XeF8.
Exact solution of the mean spherical model for strong electrolytes in polar solvents
Authors: Adelman S. A. , Deutch J. M.
Published: February 2004

The mean spherical model (MSM) for dense fluids is solved for an arbitrary mixture of equal radii charged hard spheres with permanent embedded dipole moments. The model provides a treatment of ionic solutions that includes the feature of a molecular solvent. Thus, it gives a basis for investigating deviations from the familiar continuum dielectric model in ionic solution theory. The arbitrary polar-ionic mixture is first reduced to an effective two component problem. One component is an effective charged species while the other is an effective polar species. This two component problem is solved in terms of three parameters closely related to the thermodynamic functions of the fluid. Nonlinear algebraic equations for these parameters are obtained. Although these equations appear to be analytically intractable for arbitrary ionic and dipolar strengths, explicit results are obtained for low ionic strength. In this limit, the ion-ion contribution to the Helmholtz free energy is given by the classical Debye-Hückel result. The dielectric constant in the Debye-Hückel formula is that of the MSM polar fluid mixture that results if the ionic components of the polar-ion solution are discharged. The self-energy of charging, however, differs from the classical result. The model also exhibits Debye shielding of the solven-solvent and solvent-solute interactions as well as the more familiar solute-solute shielding.
Microwave spectrum and dipole moment of 3,6-dihydro-2H-pyran
Authors: Wells John A. , Malloy Thomas B.
Published: February 2004

The microwave spectrum of 3,6-dihydro-2H-pyran has been studied in the R-band frequency range (26.5–40.0 GHz) with a Hewlett-Packard Model 8400 Stark-modulated microwave spectrometer. The spectrum is characterized by rather strong perpendicular transitions of a near oblate symmetric top. Transitions obeying a, b, and c dipole selection rules have been assigned and 24 low-J R -branch transitions have been used to derive the rigid rotor constants for the ground vibrational state: (in MHz) A = 5085.152±0.012; B = 4849.365±0.013; C = 2712.487±0.013. Stark displacement measurements were made for several transitions and the dipole moment components determined by least squares fitting the displacements; (in Debye) |μa|=0.892±0.006; |μb|=0.507±0.010; |μc|=0.771±0.002 yielding a total dipole moment μtot = 1.283±0.005. Model calculations to reproduce the rotational constants and the observed dipole moment components indicate that the data are consistent with a twisted ring skeleton with the oxygen atom on one side and carbon 2 on the other side of a plane formed by the other four carbon atoms. This oxygen atom and carbon 2 lie in a plane which passes halfway between carbons 3 and 6 and makes an angle τ=31.5° with the C3 − C4 = C5 − C6 plane. The valence angles within the ring are very near the values found in analogous open chain molecules.
Kinetic theory and rheology of bead—rod models for macromolecular solutions. II. Linear unsteady flow properties
Authors: Hassager Ole
Published: February 2004

The behavior of macromolecules in solutions undergoing slow unsteady flow is considered. A general procedure for obtaining the relaxation modulus of linear viscoelasticity of any bead—rod model without internal potentials is presented. The general form of the relaxation modulus is such as to predict dynamic moduli monotonically increasing with frequency; the storage modulus tends to a limiting value and the polymer contribution to the loss modulus increases beyond all bounds at large frequencies. The method is applied to the general rigid collection of beads with two principal moments of inertia equal. For this group of models only one relaxation time is found. A short table with values of the parameters of the storage and loss moduli is included for several models: the rigid dumbbell, rigid plane polygons, the rigid tridumbbell, and other structures. The method is also applied to the model of three beads connected by two rigid freely jointed rods. For this model two dominant relaxation times are found. It is shown that the introduction of a flexible joint in a rigid macromolecule decreases the loss modulus over the entire frequency range; the storage modulus on the other hand is decreased at low frequencies but increased at high frequencies.
Differential elastic scattering of potassium atoms by atomic iodine at thermal energies
Authors: Kaufmann Kenneth J. , Lawter J. Ronald , Kinsey James L.
Published: February 2004

The differential scattering of potassium atoms by atomic iodine has been measured in both the in-plane and out-of-plane mode. The energy range covered was 0.08–0.2 eV. The angular dependence was measured between 2° and 108° (center of mass). A semiclassical synthesis of the observed scattering from assumed potential curves gives insight into the nature of the covalent potentials for potassium iodide.
Potassium iodide chemiluminescence in diffusion flames and the KI upper-state potential
Authors: Kaufmann Kenneth J. , Kinsey James L. , Palmer Howard B. , Tewarson Archibald
Published: February 2004

Chemiluminescence spectra of potassium iodide have been measured in a dilute diffusion flame apparatus. A total of 54 fluctuation bands have been resolved. An analysis of these results as well as previous potassium iodide spectra is presented. By using molecular beam scattering data an extremely accurate excited state potential for potassium iodide is obtained.
Two-dimensional computer studies of crystal stability and fluid viscosity
Authors: Hoover W. G. , Ashurst W. T. , Olness R. J.
Published: February 2004

The mathematical analogy between the elastic stress due to particle displacements in Hooke's law solids and the viscous stress due to velocity gradients in incompressible fluids correlates two interesting phenomena. In a two-dimensional crystal the elastic restoring force opposing particle displacements approaches zero with increasing crystal size, leading to a logarithmically diverging rms displacement in the large-system limit. The vanishing of the solid-phase force is mathematically analogous to the lack of viscous damping for a particle moving slowly through a two-dimensional incompressible fluid. These two continuum results are compared with discrete-particle computer simulations of two-dimensional solids and fluids. The divergence predicted by macroscopic elasticity theory agrees quantitatively with computer results for two-dimensional harmonic crystals. These same results can also be correlated with White's experimental study of the viscous resistance to a cylinder (a falling wire) moving slowly through a viscous fluid. The agreement is good. Finally, we carried out a computer study of a two-dimensional fluid confined between two moving walls (plane Couette flow). Despite theoretical predictions that transport coefficients in two-dimensional systems diverge, no viscosity anomalies were observed under the conditions of the computer simulation.
Photoconductivity of chlorophyll-a induced by a tunable dye laser
Authors: Bromberg A. , Tang C. W. , Albrecht A. C.
Published: February 2004

Microcrystalline chlorophyll-a is seen to give a photoconductivity signal upon excitation by a pulsed dye laser. The shape of the signal pulse essentially matches that of the laser pulse both at room temperature and at 77°K. An upper limit of the photocurrent response time of about 1 nsec is accordingly estimated. The photocurrent signal can be easily saturated by the light pulse. The light exponent varies from nearly zero at high light intensities to unity at low light intensities. The action spectrum of photoconductivity is found to match the microcrystalline chlorophyll-a absorption spectrum. A lower limit of 0.15 cm2/V sec is estimated for the charge mobility. Electronic conduction in the microcrystal is implicated. The nature of photocharge generation is also briefly discussed in terms of either an intrinsic or an extrinsic process.
Atomic and molecular electronic spectra and properties from the electron propagator
Authors: Purvis George D. , Öhrn Yngve
Published: February 2004

Decoupling in terms of the superoperator formalism according to Pickup and Goscinski is employed to find tractable forms of the self-energy operator leading to direct calculation of the electron propagator or Green's function. This is applied to the calculation of photoelectron line spectra, ground state total energies, and electronic properties of atoms and molecules. Numerical results for the helium atom and the nitrogen molecule are reported.
High resolution differential cross sections and intermolecular potentials I: Li–Kr and Li–Xe
Authors: Auerbach D. J.
Published: February 2004

High resolution differential cross sections for the systems Li–Kr and Li–Xe have been measured with resolution sufficient to obtain the amplitudes as well as the positions of all features including the highest frequency oscillations in d σ/d Ω. Two different relative velocities per system were selected for measurement in the range from 1–3 × 105 cm/sec. The data were analyzed by fitting model potentials. Values obtained for the range and depth parameters of the potential are rm =4.78 Å, ε = 1.35 × 10−4 erg for Li–Kr, rm =4.80 Å, ε = 2.03 × 10−14 erg for Li–Xe. The data are of sufficient quality to yield additional information about the shape of the intermolecular potential.
Thermodynamics of small clusters of atoms: A molecular dynamics simulation
Authors: Kristensen W. Damgaard , Jensen E. J. , Cotterill R. M. J.
Published: February 2004

The thermodynamic properties of clusters containing 55, 135, and 429 atoms have been calculated using the molecular dynamics method. Structural and vibrational properties of the clusters were examined at different temperatures in both the solid and the liquid phase. The nature of the melting transition was investigated, and a number of properties, such as melting temperature, latent heat of melting, and premelting phenomena, were found to vary with cluster size. These properties were also found to depend on the structure of the solid phase. In this phase the configuration of lowest free energy was found to be icosahedral in the 55-atom system and face centered cubic for the two larger systems.
Multilayer theory of adsorption from a polymer solution
Authors: Roe Ryong-Joon
Published: February 2004

A general theory of multilayer adsorption from a polymer solution at an interface is presented. The free energy of the solution in the presence of a plane surface is evaluated in the quasicrystalline model. The configurational entropy of placing a linear chain molecule in the liquid medium of nonuniform concentration is derived by extension of the procedure used for deriving the Flory-Huggins entropy of mixing. The theory gives the adsorption isotherm and the concentration profile near the surface as a function of the chain length r, the Flory-Huggins parameter χ, the differential adsorption energy parameter χs, and the bulk concentration φ*2. The theory is also applicable to adsorption from a mixture of two polymers and from a partially muscible liquid mixture. Many of the results derived by previous theories of adsorption can be obtained, either exactly or approximately, as special cases of the present theory. The predictions of the theory are in good qualitative, and sometimes quantitative, agreement with experimental observations on surface tensions of polymer solutions and on adsorption from partially miscible oligomer solutions and from dilute polymer solutions.
Determination of the deuterium quadrupole coupling constant in phosphine-d3 from studies in a nematic solvent
Authors: Zumbulyadis Nikolaos , Dailey Benjamin P.
Published: February 2004

The phosphorus and deuterium Fourier transform NMR spectra of PD3 in a nematic solvent are reported. The experimentally determined value of the deuterium quadrupole coupling constant is 7.8±0.6 kHz in good agreement with the recent theoretical predictions of Rothenberg, Young, and Schaefer. The ordering parameter is unambiguously determined from the P–D splittings, thus enhancing the accuracy of the present results over previous measurements of this kind.
Phase separation of Lennard-Jones systems: A film in equilibrium with vapor
Authors: Liu K. S.
Published: February 2004

Classical ensembles interacting by Lennard-Jones (LJ) forces and exhibiting liquid vapor phase separations are investigated for reduced temperatures t = T/Tc from 0.515 to 0.955 by Monte Carlo methods. An external wall potential is introduced to stabilize the phase separation. The coexistence curve, surface tension, and surface energy are found to be in good agreement with experimental results. The film exhibits a layered structure at the lower temperatures. It is proposed that the method can be exploited to study inhomogeneous systems in general and phase coexistence in particular.
Electron-impact excitation of the 3S and 5S states of atomic oxygen
Authors: Stone E. J. , Zipf E. C.
Published: February 2004

Absolute cross sections for the excitation of the OI 3s(3S0) and 3s(5S0) states by electron impact on atomic oxygen have been measured for electron energies from threshold to 300 eV. The magnitude of the 3S0 cross section is 5.3×10−17 cm2±40% at its peak at 20 eV. This cross section differs in magnitude and shape from the result reported earlier. The 5S0 cross section has a maximum value of 2.5×10−17 cm2±50% at 15 eV. Both cross sections are larger than the values predicted by recent theoretical calculations. The experimental techniques used in the production of atomic oxygen and in the measurement of the OI density have been improved and are described in detail. Critical experimental problems concerning the loss of atoms at the system walls and the role of excited states of O2 in the discharged gas are also discussed.