bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrodynamic interaction and the dynamic intrinsic viscosity of a flexible polymer
Authors: Bixon Mordechai , Zwanzig Robert
Published: February 2003

We extend an earlier calculation by C. W. Pyun and M. Fixman [J. Chem. Phys. 44, 2107 (1966)], dealing with the effects of hydrodynamic interaction on the intrinsic viscosity of a flexible polymer in solution. We use their general formulation of the problem, but perform a more complete ’’infinite order’’ calculation of eigenvalues and eigenvectors. For the steady state viscosity constant, we obtain a new value Φ=2.76, which lies between their result 2.66 and the value 2.856 which is found when the preaveraged hydrodynamic interaction is used. Further, we obtain the spectrum of relaxation times, and the frequency dependent intrinsic viscosity. The effects of preaveraging are more striking here; for example, our calculated lowest relaxation time is about twice as large as that found in the preaveraged approximation.
Piezoelectric langbeinite-type K2Cd2(SO4)3 structure at four temperatures below and one above the 432°K ferroelastic–paraelastic transition
Authors: Abrahams S. C. , Lissalde F. , Bernstein J. L.
Published: February 2003

K2Cd2(SO4)3 is orthorhombic below 432°K, with space group P212121, and cubic above with space group P213. A structural study has been completed at 298, 351, 390.5, 417.5, and 443.5°K based, respectively, on 3383, 1377, 1803, 1724, and 1022 symmetry-independent structure factors. Measurement of integrated intensities was made using a CAD-4 diffractometer with Nb-filtered MoKα radiation. An air-flow microfurnace with temperature controller provided a thermal stability better than ±1°. The a, b, c lattice constants in the orthorhombic phase are 10.2084(8), 10.2813(8), 10.1684(8) Å at 298°K; 10.2239(8), 10.2897(8), 10.1821(8) Å at 351°K; 10.2386(8), 10.2951(8), 10.1981(8) Å at 390.5°K; and 10.2501(10), 10.2972(8),10.2132(9) Å at 417.5°K. In the cubic phase, a=10.2704(10) Å at 443.5°K. The model previously obtained at room temperature [J. Chem. Phys. 67, 2146(1977)] was refined by the method of least squares using the new sets of measurements. The final agreement factors are R=0.028 (298°K), 0.032(351°K), 0.026(390.5°K), 0.037(417.5°K), and 0.026(443.5°K). Normal probability plot analysis shows the furnace does not introduce new systematic error in the Fmeas. The amplitudes of thermal vibration for the oxygen atoms increase gradually between 298 and 417.5°K and sharply between 417.5 and 443.5°K. The metal atom vibrations have smaller increases. The metal ions undergo displacements averaging 0.16 Å at the transition, with displacements in the orthorhombic phase averaging 0.05 Å over a 120°K interval. The sulfur atom displacements are comparable, with simultaneous rotations of the SO2−4 tetrahedra. Two K–O contacts are broken and one is made in the K1+ coordination polyhedra at the phase transition. The Cd–O coordination polyhedra remain unchanged.
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o MC–SCF ground-state potential energy curves for LiH−, NaH−, and CsH−
Authors: Karo Arnold M. , Gardner Maureen A. , Hiskes John R.
Published: February 2003

We have evaluated the 1Σ+ ground states of LiH, NaH, and CsH and the 2Σ+ ground states of LiH−, NaH−, and CsH−, as well as the 2Σ+ ground state of CsH+, over a wide range of internuclear distances. Multiconfiguration-self-consistent-field wavefunctions were obtained with the optimized valence configuration approach to the description of chemical bonding. Four configurations for the neutral molecules and seven for the negative ions provided satisfactory descriptions. All of the negative-molecular-ion ground states are attractive and have lower potential energies than the neutral-parent-hydride molecules over the internuclear distances studied. Molecular electron affinities at the equilibrium internuclear separations were found to be 0.283 eV (LiH), 0.278 eV (NaH), and 0.357 eV (CsH).
Vibrational normal modes of SiO2. II. Cristobalite and tridymite
Authors: Etchepare J. , Merian M. , Kaplan P.
Published: February 2003

The valence force field previously obtained on α and β polymorphs of quartz is applied to the other principal crystalline forms of SiO2. A correlation is found between certain vibrations belonging to different symmetry species. The calculated frequencies of cristobalite fit the whole experimental spectra with as much accuracy as for α-quartz, proving that the forces acting on the atoms in the two polymorphs are quite the same. A detailed study of β-cristobalite showed a complete correlation between the α and β forms. Information about the phonon dispersion curves of this material is given and the existence of a soft mode near 230 cm−1 is proved. However, the calculations made on tridymite, based on the only structural form known, the β variety, do not explain satisfactorily enough the Raman spectra of the α form.
Theory of nonlinear transport in Burnett order
Authors: Wong C. K. , McLennan J. A. , Lindenfeld M. , Dufty J. W.
Published: February 2003

Constitutive equations for a fluid are worked out to the order corresponding to the Burnett approximation of kinetic theory, using the method of time-dependent correlation functions. These equations describe effects such as normal stresses which are characteristic of non-Newtonian fluids. Coefficients of the various terms are identified as integrals of correlation functions; those associated with linear second-order terms involve space and time moments of the same two-point correlation functions which occur in the first-order theory, while the coefficients of nonlinear terms depend in addition on three-point correlation functions.
Semiclassical exchange approximation for inelastic electron scattering
Authors: Truhlar Donald G. , Mullaney Nancy Ann
Published: February 2003

The semiclassical exchange approximation is extended to treat the inelastic exchange kernels which arise in the theoretical treatment of excitation of electronic states by electron impact. Cross sections for excitation of the 2s state of hydrogen are calculated using both the semiclassical exchange approximation and its high-energy limit (the high-energy exchange approximation). The results are compared to those obtained using the exact treatment of exchange and the agreement is generally very good. The method should be useful for the quantum mechanical treatment of intermediate-energy electron–molecule scattering for interpreting electron impact spectroscopic studies.
Theoretical study of the photodissociation of HOCl
Authors: Jaffe R. L. , Langhoff S. R.
Published: February 2003

A theoretical study of the uv photodissociation of HOCl is presented based on ab initio SCF-CI calculations. Results are reported for the low-lying singlet and triplet states of the HOCl molecule using both double-zeta and double-zeta-plus-polarization basis sets of contracted Cartesian Gaussian functions. The calculated equilibrium geometry for the ground electronic state [(1)1A′] is in excellent agreement with experiment. The vertical excitation energies to the two lowest singlet states are determined to be: (1)1A′→ (1)1A″ =4.50 eV and (1)1A′→ (2)1A′=5.66 eV. Electronic transition dipole matrix elements have been calculated for these two excitations. The (1)1A′→ (2)1A′ transition moment is more than a factor of 10 larger than that for the (1)1A′→ (1)1A″ transition. The photoabsorption spectrum for HOCl (vapor) between 200 and 400 nm has been computed based on the ab initio potential energies and transition moments. This spectrum exhibits a single peak, at 220 nm, with an extinction coefficient of 123 l mole−1 cm−1. The only previous experimental measurement is that of Fergusson, Slotin, and Style, who reported two peaks at 220 and 320 nm. Possible explanations for this discrepancy are discussed in light of recent unpublished experimental data. By examination of the topographies of the (1)1A″ and (2)1A′ potential energy surfaces, the photodissociation products are predicted to be Cl+OH. Since HOCl does not have significant photoabsorption cross section in the wavelength range 300–500 nm we conclude that it would be stable to photodissociation if formed in the stratosphere. The implications of HOCl as a reservoir for stratospheric chlorine are discussed.
Optical spectra of alkali metal anion and ’’electride’’ films
Authors: Dye James L. , Yemen Michael R. , DaGue Michael G. , Lehn Jean-Marie
Published: February 2003

The optical absorption spectra of thin films produced by evaporation of methylamine from solutions which contain cryptated alkali cations and either alkali metal anions or solvated electrons have been examined over the region 4 000–40 000 cm−1. Films prepared from solutions of Na− show a strong absorption band at 15 400 cm−1, a pronounced shoulder at 18 900 cm−1, and a small but distinct peak at 24 500 cm−1. Films prepared from solutions of K−, Rb−, and Cs− have maxima at 11 900, 11 600, and 10 500 cm−1, respectively, while those prepared from solutions which contain e−solv have maxima at 7400 cm−1 independent of whether the solution is made by contacting K or Rb metals. Films made by evaporating solutions which contain Na−, K−, Rb−, and Cs− are gold colored by reflectance and blue by transmittance and are presumed to be similar to the crystalline salt Na+C⋅Na−, in which C is 2,2,2-cryptand. The principal absorption maxima occur at wavelengths very close to those of M− in the corresponding metal solutions in amines and ethers. However, the films contain no solvent but rather cryptated cations and alkali metal anions. Films made by evaporating solutions which contain M+C and e−solv are dark blue in color and strongly paramagnetic. The location of the absorption maximum and its independence of metal suggest that these films are ’’electride’’ salts in which the charge on M+C is balanced by a localized electron.
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U (2s+1) ×ζr and spin statistical weights for tetrahedral XY4, trigonal bipyramidal XY5, and octahedral XY6
Authors: Galbraith Harold W.
Published: February 2003

Using Schur’s theorem we obtain explicit expressions for the spin degeneracies of the three molecular types; XY4 of Td symmetry, XY5 of D3h symmetry, and XY6 of Oh symmetry. Tables of spin statistical weights for all spins up to 11/2 are given.
Trajectory calculations on the H+O2→OH+O combustion reaction
Authors: Gauss Arthur
Published: February 2003

The endothermic H+O2→OH+O reaction is important in combustion processes. The system is characterized by a nonlinear intermediate, HO2. A modified LEPS surface is described which can successfully model this bent intermediate. Trajectory calculations using this surface indicate a considerable enhancement in the reaction probability due to vibrational excitation of the O2 molecule. The contribution to the total rate constant from the higher vibrational levels of O2 (v=4,5,6) appears to be significant at the high temperatures (1600 °K and above) encountered in combustion processes.
Infrared reflection and dipole derivatives of azide ion in KN3
Authors: Decius J. C. , Pastorek Chris
Published: February 2003

The infrared reflection spectrum of the 001 face of tetragonal KN3 has been studied between 100 and 2300 cm−1. Since an oblique incidence technique was employed, observations with TE polarized and TM polarized incident radiation allowed discrimination between crystal modes whose transition dipoles were parallel (A2u) or perpendicular (Eu) to the z axis. The reflection intensity was modeled by a classical harmonic oscillator formula for the dielectric constant, which associates three parameters with each mode. These parameters are νT, the transverse phonon frequency; S, the strength; and γ, the damping constant. From the value of S, taking into account the difference between external and internal effective fields, the values of the dipole derivatives ∂μ/∂q were determined for the several modes. These dipole derivatives were converted to internal coordinates, analyzed from the point of view of the effective charge–charge flux model, and compared with values found for the azide ion in NaN3.
Collisional quenching of fluorescence from S2(B 3Σ−u)
Authors: McGee T. Howard , Weston Ralph E.
Published: February 2003

Lifetimes of the B 3Σ−u state of S2 in the presence of various quenching gases have been determined from fluorescence decay measurements made with the single-photon time correlation technique. Fluorescence was excited with a pulsed light source of wavelength 292 nm, and observed at 370 nm. He, Ar, Xe, N2, S2, CF4, and C2F6 have been used as quenching gases. At high pressures (?100 torr), a single lifetime of ∼110 nsec is observed, whereas at lower pressures there is a second component with a lifetime of ∼5–30 nsec. From these lifetimes, bimolecular rate constants and thermally averaged cross sections for collisional quenching have been determined. The results are discussed in terms of a model in which two excited states of S2 are either coupled with each other or with the ground state.
Structures and electronic properties of copper clusters and bulk; comments on Mulliken–Walsh diagrams and on criticisms of the extended Hückel procedure
Authors: Anderson Alfred B.
Published: February 2003

Structures, energy levels, and binding energies of small copper clusters are determined using a two-step theory wherein (1) rigid atoms are superimposed giving rise to readily determined pairwise repulsion energies and (2) attractive energies due to electron delocalization are estimated with a molecular orbital theory derivable, with approximations, from the superimposed rigid atom Fock potentials. Optical spectra determined by Moskovits and Hulse for Ar matrix isolated Cu, Cu2, Cu3, Cu4, and Cu5 are interpreted in terms of molecular orbital energy levels. Matrix frequency shifts are seen to arise from antibonding interactions between Cu 4s and 4p orbitals with Ar matrix 3p orbitals. A fcc Cu13 cluster yields the major features of the Cu(100) surface photoemission spectrum due to Burkstrand et al. but not the lattice and force constants. The force constant is determined using a Poisson equation. It is suggested that small Cu microcrystals have structural and bond length disorder and may experience a phase transformation to fcc at a critical size. Less than bulk bond lengths in microcrystals are due to an absence of bulk coordination. It is suggested that the orbital energies, which are similar to extended Hückel ones, are obviously appropriate to Mulliken–Walsh diagrams and that past criticisms of extended Hückel orbital energy results have been premature.
The properties of ion clusters and their relationship to heteromolecular nucleation
Authors: Castleman A. W. , Holland Paul M. , Keesee Robert G.
Published: February 2003

Ion induced heteromolecular nucleation may be formulated in terms of either a kinetic or a steady-state thermodynamic model. In the case of the former, nucleation is expressed in terms of the rate constants of the individual association reactions leading to the formation of the ion cluster prenucleation embryos. The thermodynamic approach, on the other hand, leads implicitly to the concept of an energy barrier to nucleation. The two formulations are examined in detail and shown to be complementary. An assessment of the validity of the classical charged liquid drop expression, referred to as the Thomson equation, is made by comparing predicted and experimental values. Although the equation is shown to be useful for calculating the enthalpies of ligand attachment to ions at moderate and larger cluster sizes, in the case of entropies it is only moderately successful for hydration reactions and totally fails for ammonia clustering about ions. It is concluded that the Thomson equation is inadequate for treating the general heteromolecular phenomenon and that methods which are able to effectively take large numbers of configurations into account offer the most promise in describing the molecular properties of clusters. Experimental entropy data indicate that the structures of certain ion clusters are more ordered than accounted for by the classical charged liquid drop formulations. Further examination of these data in light of the Sakur–Tetrode equation indicates the existence of low lying excited internal vibrational modes in ion clusters. These considerations suggest that vibrational frequencies on the order of 1.7×1012 sec−1 are present in clusters of two or more ligands.
A photoionization study of carbon dioxide dimers in a supersonic molecular beam
Authors: Jones Gilbert G. , Taylor James W.
Published: February 2003

Photoionization of carbon dioxide clusters from a supersonic molecular beam using wavelength dispersed synchrotron vacuum uv radiation revealed that neutral cluster formation was dominated by growth in the molecular beam expansion jet. The photoionization efficiency curve of the carbon dioxide dimer at m/z 88 differed from the monomer curve at m/z 44 in that the onset was strongly curved and shifted 0.37 eV lower than the sharp m/z 44 onset at 13.77 eV. The few remaining autoionization features from the dimer ion were strongly broadened. Three broad autoionization features in the m/z 88 cruve were fitted to a Rydberg series with a convergence limit of 18.03 eV. This series had peaks and quantum defects close to that for the Henning diffuse series which converges to the B 2Σu+ electronic state of the CO2+ ion at 18.076 eV. The slight shift to lower energy of the m/z 88 series limit was tentatively related to the neutral dimer bond energy. The lower limit to the exothermicity of the reaction of CO2+ with CO2 to form (CO2)2+ was predicted to be 7.2 kcal/mole. Molecular beam conditions (80 torr, nozzle temp. =243 K) were found where the only detectable cluster was the dimer; other beam conditions (272 torr and 246 K) were identified in which the entire series of (CO2)+n, 1⩽n⩽20, ions up to a m/z of 880 were observed. These high clusters were found to fragment at photon wavelengths only 1 eV above the m/z 88 onset, producing ions at m/z 88. Higher cluster fragmentation accounted for over 87% of the total m/z 88 signal at 20 eV photon energy.
Analysis of the decay of molecular fluorescence in optically excited mercury vapor
Authors: Stock M. , Smith E. W. , Drullinger R. E. , Hessel M. M. , Pourcin J.
Published: February 2003

The decay of the 485 and 335 nm molecular fluorescence bands in optically excited mercury vapor is studied. A 10 nsec laser pulse at 256 nm is used to excite the vapor and the subsequent fluorescence intensity between 1 and 2000 μsec was recorded for each band. At late times following the laser pulse, both bands decay at the same exponential rate. This exponential decay coefficient was measured as a function of gas density from 1017 to 1019 cm−3 and as a function of temperature from 473 to 1048 °K. These decay data, as well as data on the relative intensities of the two bands, are analyzed in terms of a simple kinetic model for the mercury vapor system and various kinetic rates are determined. Some decay rates for the 6 3P atomic manifold are also obtained and compared with previous measurements.
Measurement of the rate coefficients for the bimolecular and termolecular ion–molecule reactions of He2+ with selected atomic and molecular species
Authors: Collins C. B. , Lee F. W.
Published: February 2003

This work reports the measurement of bimolecular and termolecular charge transfer reactions of He2+ into nonassociative product channels. In this study ion destruction frequencies have been experimentally determined from the selectively excited fluorescence of N2+ in high pressure afterglows of mixed gases excited by intense electron beam discharges. Data have been obtained as functions of helium pressure over the range from 400 to 1500 torr and as functions of the partial pressure of reactant from 50 to 200 mtorr. From these data pressure-dependent rate coefficients have been extracted and subsequently resolved into contributions from bimolecular and termolecular components for reactions of He2+ with Kr, H2, O2, NO, HBr, HCl, H2O, N2O, NO2, C3H8, NH3, and CCl2F2, selected because of their widely varying values of polarizability and dipole moment. The bimolecular components have been found to agree with the NOAA flowing afterglow results, where available, and to compare favorably with theoretical values obtained from Langevin, ADO, and locked dipole approximations. The sensitivity of the method has been sufficient to detect termolecular components as small as 2×10−30 cm6 sec−1 and values were found to range widely from 9×10−30 cm6 sec−1 for H2 to 140×10−30 cm6 sec−1 for HBr. A termolecular analog to the Langevin limit has been constructed which explains these values in terms of the rates at which third-body encounters change glancing collisions into inwardly spiraling trajectories. The data reported are in general agreement with this model and indicate that the reaction probabilities are near unity in both bimolecular and termolecular channels. The size of these termolecular rates suggests the general importance of three-body ion–molecule reactions in higher pressure plasmas such as those found in e-beam lasers.
Computer simulation of the nucleation and thermodynamics of microclusters
Authors: Rao M. , Berne B. J. , Kalos M. H.
Published: February 2003

Gas liquid equilibrium in finite systems is studied by computer simulation using molecular dynamics and Monte Carlo techniques. In overexpanded liquids, cavitation is observed. At some point in the expansion the system undergoes a transition from a liquid with cavities to a droplet in equilibrium with vapor. This transition is observed in the pV diagram of the system. A method is devised for counting the number of atoms in a cluster and thereby determining the cluster distribution function. The interface of a microcluster is compared to that of a planar sheet. The two are found to be very similar in density profile. In the course of this study homogeneous nucleation in a supersaturated gas is observed for the first time in a molecular dynamics study. Simple theories of nucleation in a finite system are considered. The free energy of formation of a droplet is found to have a maximum—a barrier to nucleation—and a minimum—stable equilibrium between a droplet and liquid. When gas imperfection is included, the barrier increases, and the stable cluster is destabilized.
Ionization of xenon atoms in selected high Rydberg states by collision with CH3I, C7F14, C6F6, and CH3Br
Authors: Hildebrandt G. F. , Kellert F. G. , Dunning F. B. , Smith K. A. , Stebbings R. F.
Published: February 2003

Absolute rate constants for the ionization of highly excited xenon atoms in collisions with CH3I, C7F14, C6F6, and CH3Br have been measured and the major charged reaction products have been identified for all but CH3Br. Theoretical models view such collisions as being dominated by the interaction between the target molecules and the ’’essentially free’’ Rydberg electrons. It is thus predicted that the negative ions produced during collisional ionization are identical to those resulting from free electron attachment, and that the rate constants for collisional ionization are the same as those for free electron attachment. The present data are reviewed in light of these predictions.
Electron mobilities in gaseous, critical, and liquid xenon: Density, electric field, and temperature effects: Quasilocalization
Authors: Huang Sam S.-S. , Freeman Gordon R.
Published: February 2003

The mobility μ of electrons in xenon varies with the density n : μαnx. For thermal electrons in the gas, x=−1.0 when n<4×1020 molecule/cm3, independent of temperature. This corresponds to pressures ≲20 atm at 300 K and is characteristic of simple scattering by a polarization field. At higher gas densities x becomes more negative, then passes through a minimum, and μ has a positive temperature coefficient at constant n. At these densities thermal electrons form quasilocalized states in, or suffer enhanced scattering by, microscopic regions of relatively high density. The localization effect maximizes in the critical fluid, at n=5×1021 molecule/cm3 and T=290 K, and appears to be negligible again at n?6.8×1021. In the liquid phase x?+9 up to n=1.2×1022 molecule/cm3, then changes to x?−8 at higher densities. The positive value of x is due to the formation of a conduction band, and the negative value to scattering by the repulsive cores of the closely spaced atoms. In the gas and the low density liquid (n<1.2×1022), μ at low electric field strengths E is independent of E; at higher E, μ increases with E then passes through a maximum and decreases. The maximum in μ reflects the Ramsauer–Townsend minimum in the scattering cross section σε of the atoms as a function of electron energy ε. Quasilocalization is inhibited by the application of moderate electric fields, and this enhances the observed dependence of μ on E at 4×1020<n<7×1021. The electron drift velocity vd tends to become independent of E at high E in the gas, liquid, and solid phases. The ’’plateau’’ drift velocity was 1.0 km/s at n?2×1021, 1.2 at 5×1021, 3.0 in the normal liquid at 1.4×1022, and 5.5 km/s in the solid at 1.6×1022 molecule/cm3 (Miller, Howe, and Spear). The velocity plateau in all phases is attributed to the steep rise in σε on the high energy side of the Ramsauer–Townsend minimum. The maxima in μ vs E curves for heavy ions are discussed.
Stochastic theory of phase transition in multiple steady state chemical system: Nucleation
Authors: Mou Chung Yuan
Published: February 2003

We study on a simple reaction–diffusion system the role of stochastic fluctuation in first order type transition between two stable steady states. We focus especially on the large fluctuations in a small spatial region that give rise to nucleation. The theory of nucleation developed is based on a birth-and-death type master equation. The result is different from the Langevin approach, and the physical basis of the difference is discussed.
Lattice parameters and thermal expansion of solid CD4
Authors: Baer D. R. , Fraass B. A. , Riehl D. H. , Simmons R. O.
Published: February 2003

Precise measurements of the lattice parameters of solid CD4 at saturated vapor pressure are reported for temperatures from 4.4 to 60 K. The x-ray results show two first order phase transitions near 26.9 and 22.0 K. For the two cubic phases, the lattice parameter a has been measured to an accuracy of 15 ppm, while for the low temperature tetragonal phase, the parameters c and a have been obtained to roughly 150 ppm. The thermal expansion for all three phases has been calculated from the data. Several interesting properties of the phase transitions are described.
Electron affinity of the HC2 radical
Authors: Vasudevan K. , Grein F.
Published: February 2003

Large scale ab initio self-consistent field and configuration–interaction calculations were carried out to evaluate the electron affinity of the HC2 radical. The adiabatic electron affinity is calculated to be 2.14, the vertical 2.09, and the vertical detachment energy 2.17 eV. Corrections for the zero-point vibrational energy amount to less than 0.01 eV. From these values, the heat of formation of HC2, and the C–H bond dissociation energy of C2H2 were estimated to be 121±3 and 119±3 kcal mole−1, respectively.
Further empirical testing of the suitability of a nonrandom integration method for classical trajectory calculations
Authors: Suzukawa Henry H. , Wolfsberg Max
Published: February 2003

A nonrandom method of approximating multidimensional integrals is compared to the traditional Monte Carlo method in the determination of average energy transfer values from classical trajectories. Two atom–diatomic molecule collision systems are studied. Estimates of error show that, for a given number of trajectories, the nonrandom method tends to be more accurate than the Monte Carlo method.
Pseudospectral dipole oscillator strength distributions and some related two body interaction coefficients for H, He, Li, N, O, H2, N2, O2, NO, N2O, H2O, NH3, and CH4
Authors: Margoliash D. J. , Meath W. J.
Published: February 2003

Pseudospectral dipole oscillator strength distributions (DOSDs) are presented for ground state Li, N, O, H2, N2, O2, NO, N2O, H2O, NH3, and CH4. The pseudo DOSDs are constructed by requiring the pseudodipole (excitation energy–oscillator strength) pairs to reproduce known values of the dipole oscillator strength sums S (k) evaluated from original DOSDs which are either completely continuous or contain a substantial continuum contribution. The pseudo DOSDs, which are a discrete representation of the original DOSDs, can be used to evaluate a variety of molecular properties in almost a trivial way, without sacrificing accuracy, by using discrete pseudospectral representations of the properties. The pseudostate DOSDs, together with ab initio pseudostates for the ground state H and He atoms, are used to evaluate orientation averaged dipole–dipole dispersion energy constants, C6(a,b), and the corresponding relativistic coefficients, W4(a,b), for all pairs of atoms and molecules taken form H, He, Li, N, O, H2, N2, O2, NO, N2O, H2O, NH3, and CH4. The results are used to assess the accuracy of semiempirical formulae for C6(a,a) and W4(a,a) and of combination rules for C6(a,b) and W4(a,b). The method of using pseudo DOSDs for the evaluation of molecular properties, relative to using the original DOSDs, becomes more important as the number of integrals over DOSDs in the definition of a property increases.
Spin–spin dipolar and exchange interactions in crystalline bisgalvinoxyl biradical
Authors: Mukai Kazuo , Sakamoto Jun
Published: February 2003

A static magnetic susceptibility study has been carried out on a stable crystalline bisgalvinoxyl biradical in the temperature range 55–330 °K. The study has established that a negative intramolecular spin–spin exchange interaction (J) exists between two unpaired electrons in the biradical, giving rise to a ground singlet state and a thermally accessible triplet state. The singlet–triplet separation (2‖J‖/k) derived from the observed temperature dependence of the magnetic susceptibility is 490±10 °K (341±7 cm−1). The existence of the triplet state of the bisgalvinoxyl has been further confirmed by the observation of the ESR zero-field splitting of its powder sample, by reducing the triplet entities at the low temperatures. The ESR spectrum at 77 °K can be clearly analyzed as the characteristic spectrum of a nonaxially symmetrical triplet, with the zero-field splitting parameters ‖D‖=76.0±0.3G, ‖E‖=14.5±0.3G, and gxx, gyy, and gzz, equal to 2.0042±0.0002, 2.0059±0.0002, and 2.0032±0.0002, respectively. When the temperature is increased, the zero-field components are broadened, then collapsed into a single, sharper line, because of temperature-dependent intermolecular exchange interactions. The D- and E-parameters were calculated for the assumed molecular structures, using McLachlan’s spin densities of galvinoxyl, which is a monoradical half of the bisgalvinoxyl.
The molecular Zeeman effect in vinyl formate using a conventional Stark modulated spectrometer and a Fourier transform spectrometer
Authors: Voss H. L. , Krajnovich D. , Hoke W. E. , Flygare W. H.
Published: February 2003

The rotational Zeeman effect in vinyl formate was observed. The rotational molecular g values are gaa=−0.1759±0.0010, gbb=−0.0342±0.0006, and gcc=−0.0059±0.0006. The magnetic susceptibility anisotropies in units of 10−6 erg G−2 mole−1 are 2χaa−χbb−χcc=9.5±1.3 and 2χbb−χaa−χcc=20.8±1.1. The molecular quadrupole moments in units of 10−26 esu cm2 are Qaa=−0.1±1.4, Qbb=+1.2±1.4, and Qcc=−1.1±2.2. The relative performance of Stark modulated and Fourier transform spectrometers is examined.
Crossed beam studies of O−+D2→OD−+D
Authors: Johnson S. G. , Kremer L. N. , Metral C. J. , Cross R. J.
Published: February 2003

Using the crossed-beam machine EVA we have measured the product angular and energy distributions of the reaction O−+D2→OD−+D in the relative energy range of 1.2–4.7 eV (5.7–23.1 eV LAB). Below 2.5 eV the product distribution is centered about the center of mass, indicating a long-lived complex. Above 2.5 eV the distribution slowly moves forward. Most of the available energy goes into internal energy of the products.
The potential energy curves of the X 1Σ+ and A 1Σ+ states of CsH
Authors: Hsieh Yin-Kang , Yang Sze-Cheng , Tam Andrew C. , Stwalley William C.
Published: February 2003

Potential energy curves for the X 1Σ+ and A 1Σ+ states of CsH are constructed using Rydberg–Klein–Rees (RKR) methods modified (a) to include the effect of nonzero rotational quantum number and (b) to simultaneously use data from more than one isotopic variant, respectively. In each case these potentials are considered to be the most complete and reliable available. Intensity calculations are also presented based on Franck–Condon factors obtained for these new potentials; comparison with experiment suggests a strong increase of A–X transition moment with increasing internuclear distance, as previously indicated for analogous transitions in LiH and NaH.
Study of variation in properties of ferric porphyrins with fifth ligands—magnetic hyperfine interactions
Authors: Mallick Mahendra K. , Chang Jane C. , Das T. P.
Published: February 2003

The electronic structures and magnetic hyperfine interactions in a series of related heme compounds have been studied. Four of these are high spin ferric heme compounds with different fifth ligands, namely, fluoride, chloride, bromide, and hydroxide, and the fifith is a low spin cyano–ferric–heme compound. The molecular orbitals needed in the calculation were obtained by the self-consistent charge extended Hückel method. For the four high spin ferric compounds the hyperfine constants of 57Fe and 14N nuclei were found to vary only 4% among themselves. In the chloride derivative, the only one for which 57Fe data are available, excellent agreement was found between our result and the 57Fe hyperfine field in hemin from Mössbauer measurements. For 14N nuclei in the fluoride, chloride, and bromide derivatives the near constancy of the predicted hyperfine constants is borne out experimentally, with the theoretical values being all about 50% of experiment. For the low-spin cyano compound we predict the 57Fe hyperfine field per spin to be 60% larger than the corresponding fields in high spin heme derivatives, while the hyperfine field at 14N per spin is substantially smaller than its counterpart in the high spin compounds. Our results for the hyperfine fields in the series of compounds studied and their agreement with available experimental data demonstrate that there is substantial delocalization of unpaired spin away from iron for these ferric heme compounds, especially the high spin ones. This feature is also supported by a similar calculated decrease in the iron 3s core energy splittings relative to that expected in free Fe+3 ion, which is verified by ESCA measurements in hemin chloride. The need for additional experimental data on the hyperfine fields, particularly for 57Fe and 14N in the cyano compound, is stressed.
Modeling of picosecond photobleaching in large molecules
Authors: Olson G. L. , Greve K. S. , Busch G. E.
Published: February 2003

A kinetic model which simulates ultrafast processes by numerical integration of rate equations based upon a homogeneously broadened two level system is compared with experimental results obtained by picosecond spectroscopy experiments. Application of the model to predict behavior of the saturable absorber DODCI using only measured molecular properties and no adjustable parameters illustrates the predictive ability of the present method in the nonlinear as well as linear regime of molecular response.
Collision-induced formation of positive and negative ions from diatomic and triatomic negative hydrogen ions
Authors: Schnitzer Rafael , Odom Robert W. , Anbar Michael
Published: February 2003

The collision-induced dissociation of diatomic (H2 −, HD−, D2 −) and triatomic (H3 −, HD2 −, D3 −) negative hydrogen ions to produce negative atomic ions and positive atomic and molecular ions upon collision with target gas has been investigated in the energy range 5–15 keV. The cross sections for collision-induced dissociation and double electron detachment have been measured by monitoring the growth of the fragment ions with target gas pressure. The total collisional attenuation cross sections of the parent negative molecular ions have also been determined.
Excitation of Hg(6 3P1) by low energy alkali ion impact: Optical polarization and cross sections for magnetic sublevels
Authors: Aquilanti Vincenzo , Casavecchia Piergiorgio , Grossi Gaia
Published: February 2003

Intensity and polarization of emitted light from collisions of ground state alkali ions and mercury atoms have been studied by a combined crossed-beam and optical technique. Both the intensity and polarization of the Hg(6 3P1→6 1S0)−253.7 nm line exhibit a pronounced structure as a function of ion energy in the investigated range (from apparent thresholds to 3 keV); in particular, the polarization fraction takes on in succession very large positive and negative values. From the emission cross sections for several transitions, after correction for cascade and anisotropy, absolute cross sections for the excitation process M++Hg(6 1S0) →M++Hg(6 3P1), where M+=Li+, Na+, and K+, have been obtained. Using emission and polarization data, cross sections for population of the magnetic sublevels, mj=0 and ‖mj‖=1, have also been obtained. These cross sections have apparent thresholds much larger than the endoergicities of the processes, but reach very high values (of the order of 10−16 cm2) in the investigated energy range; they also show a pronounced oscillatory structure. Although it appears that the primary excitation mechanism involves short-range repulsive forces, the final evolution of the collision system towards products is best depicted by reference to the adiabatic potential energy curves for the HgM+ molecules. In particular the prominent structure in the cross sections for the magnetic sublevels, which oscillate in antiphase, is discussed in terms of quantum mechanical phase interference between the quasimolecular terms dissociating to the 3P1 state of mercury.
Rotational partition function of the symmetric top
Authors: Kayser Richard , Kilpatrick John E.
Published: February 2003

The entire asymptotic high temperature expansion of the direct partition function for rotation of a rigid symmetric top is obtained by double application of the Euler–Maclaurin summation formula. The expansion is in the form of an exponential factor times the classical answer times a very simple descending power series in the temperature.
Effects of structural disorder on the optical properties of molecular crystals
Authors: Klafter Joseph , Jortner Joshua
Published: February 2003

In this paper we advance a model for absorption line shapes of Frenkel excitons in molecular crystals, considering the effects of static disorder and of weak exciton–phonon coupling. Our treatment is limited to single-particle excitations located at the bottom of the exciton band. The effects of disorder scattering at zero temperature are handled by applying the average t matrix, single-site approximation to a system characterized by a Gaussian distribution of site excitation energies. The simultaneous effects of disorder scattering and of phonon scattering were treated by the introduction of an effective exciton–phonon Hamiltonian, which was subsequently utilized within the framework of the single-site approximation for disorder scattering at finite temperatures. Model calculations of the optical absorption line shapes were performed for a quasi-one-dimensional system. The zero-temperature line shapes are asymmetrically broadened, the high energy edge being close to a Lorentzian, while the low energy edge decreases fast with decreasing energy. This asymmetry disappears at higher temperatures where phonon scattering effects dominate. We propose that the asymmetric low-temperature absorption band in 1, 4-dibromonaphthalene originates from the effects of structural disorder. The asymmetric optical line broadening experimental data for this quasi-one-dimensional system are well accounted for in terms of our theory.
Fluorescence excitation spectrum of s-tetrazine cooled in a supersonic free jet: Van der waals complexes and isotopic species
Authors: Smalley Richard E. , Wharton Lennard , Levy Donald H. , Chandler Dean W.
Published: February 2003

Van der Waals complexes between s-tetrazine and a number of light gases have been observed and characterized by laser spectroscopic studies of a free supersonic expansion of s-tetrazine in a helium carrier gas. The observed complexes are of the form X–s-tetrazine and X2–s-tetrazine where X=He, H2, and Ar. The spectra are consistent with the X species in both types of complexes being bound on or near the out-of-plane C2v axis on the top and/or bottom of the s-tetrazine ring. For the He and H2 complexes, analysis of the rotational structure indicates that the van der waals bond length is ∼3.3 Å. The observed red shift of the O–O band of the 1B3u←1A1g transition of all complexes relative to that of s-tetrazine indicates that a slighly stronger van der waals bond exists in the excited state. Observed vibrational progressions for these complexes indicates the stretching frequency of the van der waals bond in the excited state is 38.0, 95, and 110 cm−1 for the X=He, H2 and Ar complexes, respectively. The O–O band origins of the 12C214N4HD, 12C214N4D2, 12C13C14N2H2, and 12C214N315NH2 isotopic species relative to that of 12C214N2H2 are also reported.
Quantum rate theory for a symmetric double-well potential
Authors: Weiner J. H.
Published: February 2003

A quantum rate theory is presented for a symmetric double-well potential which is defined by a piecewise quadratic function. The theory is based on stationary states which are decomposed into- and left-moving states. The flux and transmission coefficients for the latter are found in terms of parabolic cylinder functions and are thermally averaged. Close analogies between the quantum and classical formulations are found when an appropriate phase space representation is used. The theory shows good agreement with experimental results for the diffusion of hydrogen and deuterium in niobium, but the agreement is poorer for the same process with the host metal vanadium and the theory does not predict the observed anomalous isotope effect for this process in palladium.
3ν3 vibrational spectra of SF6 and UF6
Authors: Fox Kenneth
Published: February 2003

The 3ν3 vibrational bands of SF6 and UF6 are discussed.(AIP)
Matrix isolation infrared and Raman spectra of binary and mixed zinc dihalides, isotopic effects, force constants, and thermodynamic properties
Authors: Givan A. , Loewenschuss A.
Published: February 2003

Raman and infrared matrix solution spectra for all ZnX2 and ZnXY (X,Y=Cl,Br,I) molecules are reported. These spectra complete the vibrational information on the binary dihalides and provide the first experimental evidence for the existence of the mixed zinc dihalides. All three fundamentals are assigned for all molecules. Isotopic effects due to the halogen atoms, the zinc atoms, or both were resolved in the stretching mode bands. From these, stretching, bending, and interaction force constants were evaluated, the criterion for their calculation being the reproduction of band frequencies within experimental error and the best fit of the frequency differences in the resolved isotopic fine structures. The possibility that not all molecules are linear is indicated. The effect of temperature and deviations from linearity on calculated entropies is shown.
X-ray diffraction study of choline chloride’s β form
Authors: Petrouleas V. , Lemmon R. M. , Christensen A.
Published: February 2003

The organic salt choline chloride exists in two crystalline polymorphs. One (the α form) is extraordinarily sensitive to ionizing radiation, the other (the β form) is not. The present report describes an x-ray diffraction study of the β form. The structure has been found to be highly disordered face centered cubic. A reasonable least-square refinement of the intensity data has been achieved in the centrosymmetric space group Fm3 or Fm3m by use of a molecular model with restrained bond lengths. The results show that in the β form the electronic density due to the choline cation is closely spaced around the N, so that hydrogen bonding to the chloride is unlikely. Comparison with infrared and NMR data indicates that the disordering is dynamic and can be ascribed to rotations of the choline ion around crystallographic symmetry axes. Possible connections of these results with the radiation stability of the β form are discussed.
The mechanism of the laser-induced infrared photolysis of ethylene
Authors: Hall John H. , Lesiecki Michael L. , Guillory William A.
Published: February 2003

The mechanisms for the production of C2(a 3Πu) generated from the laser-induced ir photolysis of 97.2% 12CH2 13CH2 ethylene has been determined as a function of initial sample pressure. The wavelength resolved laser-induced fluorescence spectra of 13,13C2, 12,13C2, and 12,12C2 indicate that at pressures greater than ∼0.2 torr, the C2 produced from the ir laser photolysis of C2H4, comes primarily from collisional processes such as C=C scission, followed by recombination of as yet undetermined single carbon fragments or a possible four center reaction between a pair of highly excited ethylene molecules. At pressures less than ∼0.2 torr, C2 is produced primarily by direct elimination of C2 from a single C2H4 molecule. Photolysis of a 0.30 torr sample of C2H4 (in natural abundance) resulted in an isotopic enrichment of ∼100% (or an enrichment factor of ∼2) of 12CH2 12CH2. The delay times between the CO2 photolysis laser pulse and the onset of observed C2 were studied as a function of pressure and extrapolated to the collisionless pressure region to give an estimated threshold energy fluence of 50±12 J/cm2 for direct elimination of C2. Rotational and vibrational temperatures were determined from the rovibronic distributions obtained as a function of sample pressure.
ESR of nitrogen atoms in multiple trapping sites—Nitrogen matrix
Authors: Delannoy D. D. , Tribollet B. , Valadier F. , Erbeia A.
Published: February 2003

ESR study of nitrogen atoms, produced by high frequency discharge through molecular nitrogen, and trapped at 7 and 14 K in a nitrogen matrix, has been carried out. At 7 K there is only one group A of three lines corresponding to hfs value Aa=4.25±0.1 G, while at 14 K there appears three groups A, B, and C of three lines corresponding to three hfs values Aa=4.25±0.1 G, Ab=4.85±0.1 G, Ac=4.5±0.1 G. All these spectra correspond to nitrogen atoms (4S3/2) trapped in different surroundings. Theoretical work of Adrian allowed us to determine them; a substitutional site for the A spectrum, an interstitial octahedral distorted site for spectrum B, and an interstitial tetrahedral distorted site for spectrum C.
The relative intensity of the ν (CO) and ν (MC) Raman bands of M(CO)6 species (M=Cr, Mo, W)
Authors: Kettle S. F. A. , Luknar N.
Published: February 2003

The relative intensities of the A1g and Eg ν (CO) and ν (MC) stretching modes of hexacarbonyl species are interpreted in terms of exact and approximate derived bond polarizability models. The two give results in good agreement for the 2000 cm−1 region but not for the lower frequency, ν (M–C), bands. For all cases transverse and the corresponding longitudinal derivatives are probably of opposite sign; some current ideas on polarizability derivatives are questioned in the light of this result.