bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystal lattice and polarization energy of tetrathiafulvalene
Authors: Sandman D. J. , Epstein A. J. , Chickos J. S. , Ketchum J. , Fu J. S. , Scheraga H. A.
Published: February 2003

The heats of sublimation of tetrathiafulvalene and thianthrene are reported. Minimization of the lattice energy, utilizing the known crystal structures and heats of sublimation of these compounds, served to determine the parameters of the Lennard-Jones potential for the nonbonded interaction between two divalent sulfur atoms. Using partial atomic charges, it is found by two independent calculations that the contribution of electrostatic interactions to the lattice energy is small, and that the lattice energy is determined primarily by nonbonded interactions. The attractive coefficient of the potential function provides an estimate of the polarizability of the sulfur atom, and the atomic polarizabilities are used to compute the static polarization energy of tetrathiafulvalene. The latter is in satisfactory agreement with experiment.
Solute structuring in aqueous iron(III) sulphate solutions. Evidence for the formation of iron(III) –sulphate complexes
Authors: Magini M.
Published: February 2003

Ferric sulphate solutions of high concentration have been investigated by an x-ray scattering technique. The experimental distribution curves show the main peaks at about 1.5, 2.0, 2.4, 2.8, 3.3, and 4.2 Å. The presence in solution of sulphate groups in tetrahedral configuration and of hydrated iron complexes in octahedral configuration is in agreement with the peaks found at 1.5, 2.0, and 2.4 Å. Water–water interactions from the solvent give the main contributions to the 2.8 Å peak. The 3.3 Å peak reveals the formation of inner iron–sulphate complexes of average form Fe(H2O)6−n(OSO3)+3−2nn, in which oxygens from sulphate groups substitute n water molecules of the hydrated Fe(H2O)3+6 ions. Least squares refinements of the i (s) curves are consistent with a structural unit in which about one sulphate tetrahedron shares a corner with one iron octahedron with an Fe–O–S angle of 135 deg. The same structural unit found for the iron–sulphate complexes in solution can be identified as the building unit of the crystalline structures of several iron–sulphate solid compounds. The large peak at about 4.2 Å in the distribution curves strongly suggests a hydrogen bonded structure outside the inner coordination sphere of Fe3+. The introduction into the model of (H2O)I– (H2O)II interactions between water molecules pertaining to two subsequent hydration shells of Fe3+ resulted in a marked better agreement between experimental and calculated i (s) curves. Some indications have been obtained supporting the presence of hydration water around the sulphate groups.
Unified statistical model for ’’complex’’ and ’’direct’’ reaction mechanisms: A test on the collinear H+H2 exchange reaction
Authors: Pollak Eli , Pechukas Philip
Published: February 2003

Miller’s unified statistical theory for bimolecular chemical reactions is tested on the collinear H+H2 exchange reaction, treated classically. The reaction probability calculated from unified statistical theory is more accurate than that calculated from ordinary transition state theory or from variational transition state theory; in particular, unified statistical theory predicts the highenergy falloff of the reaction probability, which transition state theory does not. A derivation of unified statistical theory is presented that emphasizes the dynamical and statistical assumptions that are the foundation of the theory. We show how these assumptions unambiguously define the ’’collision complex’’ in unified statistical theory, and we test these assumptions in detail on the H+H2 reaction. Finally, a lower bound on the reaction probability is derived; this bound complements the upper bound provided by transition state theory and is significantly more accurate, for the H+H2 reaction, than either transition state theory or unified statistical theory.
Multiphoton CO2 laser vibrational heating of the triplet state of biacetyl
Authors: Burak I. , Quelly T. J. , Steinfeld J. I.
Published: February 2003

Multiple infrared photon excitation of the metastable triplet state of biacetyl is carried out by irradiating the sample with an intense CO2 laser pulse following cutoff of the optical excitation at 4579 Å. Changes in the luminescence characteristics of the electronically excited molecules induced by the infrared pulse are studied in various visible spectral regions. In the region characteristic of the biacetyl phosphorescence, a fast decay of the emission is observed. A burst of delayed fluorescence, characterized by a very fast rise time, is observed in the short-wavelength emission region. Both the fluorescence and phosphorescence signals exhibit the same decay time, which is inversely proportional to the infrared fluence in the CO2 laser beam. The experimental results are explained in terms of mixed singlet–triplet vibronic wave functions characterizing the triplet vibrational manifold, with vibrational energy exceeding the 1Au–3Au separation. A calculation of the decay times as a function of the absorbed CO2 laser energy, which is based on a recent model for delayed fluorescence due to Jortner and Kommandeur, gives good agreement with the experimental data. The possibility of multiphoton induced emission from vibrationally excited ground-state molecules is also considered.
Emission spectrum of rare gas dimers in the vacuum uv region. I. Ar2
Authors: Tanaka Y. , Walker W. C. , Yoshino K.
Published: February 2003

Emission spectra of the argon dimer have been studied in the vuv region. Liquid nitrogen cooled argon was excited with a rf (Tesla coil) discharge. Four band systems, I, II, III, and x, were observed in the 1000–1500 Å region and classified as transitions to the common ground state X 1Σ+g (O+g) from the upper states A4s 3Σu+(1u), A4s 1Σ+u(Ou+), B5pσ  (O+u) and x, respectively. Except for band system I, they consisted entirely of diffuse bands.
Diffusion and thermal diffusion phenomena of neutral species in the high temperature range (2000–4000 K). I. Measurements of thermal diffusion factors in the high temperature range
Authors: Gouesbet G. , Valentin P.
Published: February 2003

In order to develop a new simple method using thermal diffusion phenomenon for measuring neutral temperatures and their gradients in a laminar plasma flow, we had to study diffusion and thermal diffusion phenomena of neutral species in the high temperature range. First, we measured thermal diffusion factors αT in a rf argon and helium plasma jet. The assumption of a binary mixture was made to derive the values of αT although the fluid also contained ions and electrons. That work is presented here. In Paper II, we report in the temperature range under interest theoretical calculations in the binary mixture hypothesis, namely, we shall calculate the Chapman–Cowling first and second approximations of the thermal diffusion factor αT, then the first and second approximations of αT in the Kihara scheme and finally we shall introduce a new scheme of approximations. We shall find that the experimental results fit very well with the theoretical calculations. The results given in Papers I and II can be valid only when the binary mixture approximation is true. Consequently, this assumption has to be checked. That work is related in Paper III where we determine the range in which the binary approximation is valid by examining the influence of electrons and ions upon the diffusion and thermal diffusion phenomena of the neutral species. The new method which is above announced for measuring neutral temperatures and their gradients in a laminar plasma flow will be explained in detail in another paper.
Dynamics of the helix-coil transition in biopolymers
Authors: Baumgärtner A. , Binder K.
Published: February 2003

The dynamics near and far from equilibrium of the linear Ising model of biopolymers is formulated in terms of the concentrations n+l(t) [n−l(t)] of sequences of l subsequent units in the helix [coil] state, investigating the limit of very long chains. The resulting set of kinetic equations can be truncated for large l in a controlled fashion with negligible error, in contrast to previous ’’closure’’ approximations whose accuracy was doubtful. The initial relaxation time τ* is obtained exactly by our method for relaxation near and far from equilibrium, in agreement with previous studies. Analytic approximation formulas are obtained for helix fractions ϑ close to zero or unity, complementing previous work valid for ϑ close to 1/2. For intermediate values of ϑ we solve our set of kinetic equations numerically to study the width of the relaxation spectrum. We find that the relaxation function becomes distinctly nonexponential for ϑ away from 1/2, 1, 0 and cooperativities σ=10−3–10−1. Applications of these results to temperature or pH jump, as well as NMR experiments, etc., are briefly discussed.
Isotope effects on order in nematic solutions. Deuterium quadrupole coupling constants for CDH2I
Authors: Wooten Jan B. , Savitsky G. B. , Jacobus John , Beyerlein A. L. , Emsley J. W.
Published: February 2003

In a previous publication [J. Chem. Phys. 66, 4226 (1977)] it was noted that dipolar coupling constants obtained from the NMR spectra of mixtures containing 13CH3I and 13CDH2I dissolved in nematic solvents revealed the presence of an isotope effect. In order to investigate this phenomenon quantitatively we have carried out vibrational averaging of the dipolar couplings, and derived the order parameters, Sαβ, for the two molecules. We also derived the quadrupole coupling constant, QD, for deuterium in 13CDH2I from the splitting observed in the deuterium spectrum and values of Sαβ obtained from vibrationally corrected dipolar couplings. The average value of QD was found to be 160.7±1.7 KHz if the asymmetry parameter, η, is assumed zero, and 153.9±1.6 KHz or 168.2±1.8 KHz for values of η=−0.04 and +0.04, respectively.
Pressure-induced phase transitions and pressure dependence of zone-center phonons of the K[Ag(CN)2] crystal investigated by Raman scattering
Authors: Wong P. T. T.
Published: February 2003

Pressure-induced phase transitions in K[Ag(CN)2] have been studied by Raman scattering. Two high-pressure polymorphs have been found in the pressure range below 18 kbar. The pressure induced frequency shift of the internal and external zone-center modes in the three phases has been measured. A dramatic change in the pressure dependence of the zone-center CN stretching mode among the three polymorphs has been observed and the mechanism of this change is discussed.
Studies in molecular dynamics. XVII. Phase diagrams for ’’step’’ potentials in two and three dimensions
Authors: Young David A. , Alder Berni J.
Published: February 2003

Molecular dynamics calculations in two and three dimensions for particles interacting with a repulsive step potential show local maxima and minima in the melting curve and solid–solid isostructural phase transitions. The phase diagrams constructed from the combination of high temperature perturbation theory, finite-temperature molecular dynamics, and exact zero-temperature calculations are compared with experimental cerium and cesium phase diagrams.
Equations of state for orthorhombic and triclinic lattices of polyethylene
Authors: Kobayashi Masamichi
Published: February 2003

P, V, T equations of state were theoretically constructed for the orthorhombic (o-PE) and triclinic modifications of polyethylene (t-PE). Helmholtz free energies A at arbitrary temperatures were calculated for the lattices with various unit cell dimensions. Contribution of the zero-point energy (for T=0 K) as well as of the phonon free energy (T〉 0 K) was evaluated directly from the density of states obtained by means of the normal coordinate treatment of the crystal lattices. The isotherms were obtained for various temperatures by numerical differentiation of A with respect to the unit cell dimensions. For the o-PE lattice the calculated volume thermal expansion at atmospheric pressure and the theoretical isotherms at room temperature agree well with the experimental results. The predicted anisotropy in linear thermal expansion as well as in linear compressibility, on the contrary, does not reproduce the experimental data. The calculated elastic moduli in the directions perpendicular to the c axis are in a good agreement with the values measured by x-ray diffraction method. The calculated Gibbs free energies of o-PE and t-PE were compared with each other for various temperatures and pressures, concluding that at atmospheric pressure o-PE is favored over t-PE in the temperature range higher than 130 K, and that the effect of hydrostatic pressure on the thermodynamic stability is not significant up to 10 kbar.
Determination of electronic energies from experimental electron densities
Authors: Bentley John
Published: February 2003

An electrostatic energy expression due to Politzer is used to calculate total electronic energies from experimentally determined charge density functions. When partitioned as suggested in this paper, Politzer’s expression is useful for indicating the effects of bond formation, and for diagnosing problems in the charge density function. Application to experimental charge density data for beryllium and diamond crystal is reported. Some binding energies are also calculated, but caution is recommended in interpreting these values.
Statistics of solubilizate distribution and its application to pyrene fluorescence in micellar systems. A concise kinetic model
Authors: Infelta Pierre P. , Grätzel Michael
Published: February 2003

A statistical model describing the distribution of solubilizate molecules in micellar systems is developed on the basis of their kinetic interaction with the host aggregates. The distribution function obtained corresponds to a Poisson law governing the occupancies of individual micelles by quest molecules. This model provides the basis for the development of a theory which treats the behavior of pyrene fluorescence emission in micellar systems. Solutions of this fluorophore in cetyltrioxyethylene sulfate micelles were examined experimentally both by steady-state and time-resolved fluorescence techniques. The results obtained are in excellent agreement with the predictions of the model. From this analysis the rate constant for the intramicellar encounter of two pyrene molecules was derived as ke=9×106 s−1. The aggregation number of CTOES was determined as 96 surfactant molecules/micelle.
The H3+ potential surface
Authors: Dykstra Clifford E. , Swope William C.
Published: February 2003

An accurate potential surface of 68 grid points for the H3+ system is given. The surface was calculated at the level of a full configuration expansion using a large basis set of 63 contracted Gaussian functions. This surface should be suitable for extremely accurate predictions of the vibration–rotation spectrum of H3+.
Laser induced fluorescence from fluorobenzene radical cations in the gas phase
Authors: Bondybey V. E. , Miller Terry A.
Published: February 2003

Laser induced fluorescence excitation spectra have been observed for a number of fluorobenzene radical cations including hexafluorobenzene, sym-trifluorobenzene and its deuterated analog, three isomers of tetrafluorobenzene, and pentafluorobenzene. In all cases vibrational structure is clearly resolved and the spectra assigned. It is shown that low frequency ring distortion modes and the C–F stretch dominate the spectra. There is an extremely strong correlation between vibrational frequencies in the neutral molecule and both electronic states of the ion involved in the transition. Convincing evidence is found for Jahn–Teller distortion of the ions with degenerate ground states.
Photoionization of argon at high resolution: Collisional processes leading to formation of Ar+2 and Ar+
Authors: Radler K. , Berkowitz J.
Published: February 2003

The photoionization mass spectrum of argon is presented at a photon resolution width of 0.02 Å from threshold to beyond the 2P1/2 continuum. A natural linewidth of ∼0.4 meV is estimated for the first sharp, s-like autoionization peak. Collisional processes leading to formation of Ar+ below threshold are attributed to a kind of Penning ionization of very high Rydberg states. The associative ionization process Ar*+Ar→Ar2+ +e− is also observed below threshold. In the competition between the two collisional processes, the formation of Ar+ is much more probable at high principle quantum number, but this cross section declines exponentially with energy deficit. The cross section for formation of Ar2+ surpasses that for Ar+ below n=13, but it declines at very high n values, asymptotically vanishing at the ionization threshold.
Rotational analysis of the 1B2u(ππ) ←1A1g, (61) band of benzene and helium–benzene van der Waals complexes in a supersonic jet
Authors: Beck S. M. , Liverman M. G. , Monts D. L. , Smalley R. E.
Published: February 2003

The fluorescence excitation spectrum of benzene at a rotational temperature of 0.3 K has been observed using both pulsed and continuously operated supersonic free jets. Individual lines of the 1B2u(ππ) ←1A1g, (610) band at 259 nm have been clearly resolved and analyzed. Rotational and Coriolis constants as derived from these resolved spectra are in agreement with those derived from previous band contour analyses of room temperature spectra. Rotational analyses of the corresponding spectral band for the He–benzene and He2–benzene van der Waals complexes are also reported. In both complexes the helium atoms are found to lie on the C6 symmetry axis approximately 3.5 Å above (and/or below) the plane of the benzene ring. Sufficient rotational cooling is achieved in these supersonic expansions so that ≳95% of all benzene molecules are relaxed to the lowest rotational state available (consistent with nonrelaxation of nuclear spins).
A numerical study of large amplitude motion on a chain of coupled nonlinear oscillators
Authors: Rolfe Timothy J. , Rice Stuart A. , Dancz John
Published: February 2003

We have studied the behavior of large amplitude compressive displacements on one-dimensional lattices of equal masses which interact with a variety of nearest neighbor potential energy functions. For the cases of cyclic and fixed end boundary conditions solitons are found to exist and to completely determine the dynamics. The shapes of the solitons on the several lattices are remarkably alike, and all are very close to the shape of a Toda lattice soliton. Because reflection at a free end creates a dilational displacement, a solitary wave does not survive on a lattice with free ends. Mass inhomogeneities in the lattice also scatter solitary waves and lead to their destruction, but the rate of the process depends on the defect to host mass ratio. When that mass ratio is 13/12 a solitary wave survives at least 500 collisions, and its energy is modulated cyclically. The rate of destruction increases as the defect to host mass ratio increases. The results of the calculations are discussed with respect to the ubiquity of solitary wave behavior, its relation with the shape of the potential energy curve, and the possible role of metastable solitary pulses in intramolecular energy transfer.
Normal coordinates calculation and valence force field for propyne, propargyl halides, butyne-2, halobutyne-2, and 1,4-dihalobutyne-2
Authors: Lichene Franco , Dellepiane Giovanna , Lorenzelli Vincenzo
Published: February 2003

Force constants and normal modes were evaluated using an overlay technique for CH3CCH, CH3CCD, CD3CCH, CD3CCD, CH2FCCH, CH2ClCCH, CH2BrCCH CH3CCCH3, CH3CCCH2Cl, CH3CCCH2Br, CH2FCCCH2F, CH2ClCCCH2Cl and CH2BrCCCH2Br. Particular attention has been given to diagonal and off-diagonal force constants related to linear bending coordinates. The force field reported in this paper gives such a satisfactory frequency fit that it should provide a good starting point for the explanation of the peculiar band widths of 1,4-dihalobutyne-2.
Theoretical excitation energies of the lowest quartet states of nitrogen dioxide
Authors: Jackels Charles F.
Published: February 2003

The quartet–doublet vertical transition energies of NO2 have been calculated by ab initio methods that include large-scale configuration interaction. A double-zeta quality basis set augmented with polarization functions is employed in this work. The 4B2←X 2 A1 and 4A2←X 2A1 vertical excitation energies are calculated directly and the corresponding vertical de-excitation energies are inferred from this and previous work.
Electronic three wave mixing spectra of transient species produced by UV laser photolysis of benzene
Authors: Gross Kenneth P. , Guthals Dennis M. , Nibler Joseph W.
Published: February 2003

Intense three wave mixing (3WM) spectra are reported for transient fragments produced by 266 nm laser photolysis of benzene and several substituted benzenes. Single pulse broadband 3WM spectra taken with an optical multichannel analyzer establish that the fragments are primary photoproducts obtained under collision-free condition. The spectra consist of many features at anti-Stokes frequency shifts of 900–3100 cm−1 from a 532 nm ω1 pump. 90° fluorescence studies of the photolysis zone show that C2 is produced in various electronic states and energetic consideration require that dissociation of C6H6 must involve two or more photons at 266 nm. 3WM spectra of C6D6 are identical to those of C6H6 in the anti-Stokes shift region near 3000 cm−1 and hence the transients do not contain CH bonds. 3WM spectra of C2H2 fragments are quite similar to those of benzene in the 3000 cm−1 region so that C2 is believed to be responsible for both 3WM and fluorescence spectra. The 3WM spectra cannot be interpreted in terms of simple CARS vibrational resonances of C2. Intensity considerations suggest that enhancement due to multiple resonance is likely, and various electronic–electronic and vibrational–electronic 3WM processes are discussed.
Intensity of light scattered from polymeric gels: Influence of the structure of the networks
Authors: Candau S. J. , Young C. Y. , Tanaka T. , Lemarechal P. , Bastide J.
Published: February 2003

The fluctuating part of the intensity of light scattered from polystyrene networks swollen to equilibrium in benzene has been measured by means of a technique using a spectrum analyzer or a rms meter. The scattered intensity is shown to decrease with the equilibrium concentration of the gel, according to a scaling law similar to that predicted from the theory of semidilute solutions. The fluctuating part of the intensity of light scattered from swollen networks containing controlled amounts of pendant chains has been shown to increase with decreasing cross-linking density. These results are in satisfactory agreement with the behavior of the elastic modulus determined from mechanical measurements.
Theoretical expression for Warman e
t a
l.’s parameter α in electron scavenging
Authors: Tachiya M.
Published: February 2003

We have derived a theoretical expression for Warman et al.’s parameter α in electron scavenging in the radiolysis of hydrocarbon solutions.
An exact treatment of the Kerr-effect relaxation in a strong unidirectional electric field
Authors: Morita Akio , Watanabe Hiroshi
Published: February 2003

The transient Kerr-effect relaxation is treated taking contributions from both permanent and induced dipoles into account with a view to obtaining exact expressions for the electric birefringence following the sudden application of a unidirectional electric field based on the rotational Smoluchowkski equation for a symmetrical body. As a special case (i) where the contribution from the induced dipole is ignored, the Laplace transform of the electric birefringence is obtained exactly in terms of a continued fraction. At the same time, the electric birefringence at a very high field is expressed in terms of a hypergeometric function as an explicit function of the time. Also, in the case (iii) where the contribution from the permanent dipole is neglected, we obtain an exact expression for the Laplace transform of the electric birefringence in terms of a continued fraction. In both cases (i) and (ii), effective relaxation times are expressed theoretically as an explicit function of the applied electric field. Further, the static birefringence for cases (i) and (ii) are calculated by means of continued fractions.
Electronic to vibrational energy transfer from I(5 2P1/2). III. H2, HD, and D2
Authors: Grimley A. J. , Houston P. L.
Published: February 2003

Deactivation of I* (*=2P1/2) by three isotropic forms of hydrogen has been monitored both by observing the decay of I* emission at 1.315 μm and by probing the E→V transfer from I* to hydrogen indirectly through observation of subsequent V→V transfer from hydrogen to CO2. I* is relaxed by H2, HD, and D2 at rates of (3.69±0.22) ×103 sec−1 Torr−1, (9.5±1.3) ×103 sec−1 Torr−1, and 45±14 sec−1 Torr−1, respectively. The minimum number of hydrogen vibrational quanta excited per deactivation of I* has been calculated from the amplitude ratio of CO2 (mn1)/I* fluorescence. These minima are 0.61±0.12 for HD, 0.51±0.10 for H2, and 0.10±0.03 for D2. The actual values may be higher due to uncertainties in corrections for radiative trapping and in the analysis of the kinetic scheme. The current results are compared to the predictions of recent quantum mechanical calculations.
Ground and n–π* excited state structures of the hydrogen bonded complexes pyrazine⋅H2O and pyrazine⋅ (H2O)2
Authors: Rossetti R. , Brus L. E.
Published: February 2003

Vibronically resolved 1Ag→1B3u(n–π*) excitation and 3B3u(n–π*) →1Ag emission spectra are reported for isolated pyrazine, and the complexes pyraxine⋅Kr, pyrazine⋅H2O, and pyrazine⋅ (H2O)2 in solid neon host at 4.2°K. Distinct zero phonon lines appear in the complex spectra, enabling a separation of vertical and adiabatic excitation with respect to the low frequency intermolecular normal modes. In the monohydrate and bihydrate complexes, the hydrogen bond is only slightly weaker in the n–π* excited states. An upper limit for excited state hydrogen bond lengthening is obtained. The monohydrate 3n–π* complex decays purely radiatively, but a competitive intersystem crossing appears to occur in the bihydrate. The hydrate spectra and lifetimes are unchanged for D2O. Zero phonon lines are just resolved in the spectra of pyrazine isolated in solid ice at 10°K. These data enable a distinction to be drawn between specific solvation due to hydrogen bonding and general solvation due to a polar water environment.
An accurate intermolecular potential for Ar–Kr
Authors: Aziz R.A. , Presley J. , Buck U. , Schleusener J.
Published: February 2003

A potential function for ArKr is derived which is able to reproduce the best data available for the differential and the integral cross section, the second virial coefficient and the diffusion and viscosity within the quoted experimental errors.
Near-resonant electronic energy transfer: Initial rotational state populations of NO(A 2Σ+, v′=0,1) produced by energy transfer from Zn(1P1)
Authors: Breckenridge W. H. , Blickensderfer R. P. , FitzPatrick John , Oba D.
Published: February 2003

The complete initial distribution of product vibrational and rotational state populations for the following energy transfer process has been determined for Boltzmann conditions at ∼550°K.: The initial population distribution of NO(A 2Σ+, v′=1) rotational levels is remarkably consistent with that predicted by the Cross–Gordon version of near-resonant dipole–dipole theory, except that a minor (∼7%) Boltzmann-like population must be included for satisfactory simulation of the sensitized NO γ (1,X) band spectra in the frequency regions dominated by transitions originating from lower rotational levels. The initial rotational level population distribution of NO (A 2Σ+, v′=0) is very similar to a Boltzmann population distribution at ∼1400°K. The rate of production of NO(A 2Σ+, v′=1) is ∼10 times that of NO(A 2Σ+, v′=0). Efficient ’’near-resonant’’ dipole–dipole interactions appear, therefore, to constitute the dominant mechanism for electronic energy transfer in this particular system. Production of NO(A 2Σ+, v′=0), and possibly the lower rotational levels of NO(A 2Σ+, v′=1), likely result from a minor competing mechanism. A Zn+NO− charge-transfer surface crossing is suggested as one possibility.
Intermolecular force field and normal vibrations of polyethylene crystals
Authors: Kobayashi Masamichi
Published: February 2003

A general expression of wave vector dependent dynamical matrices for calculating the harmonic normal frequencies of molecular crystals, including polymer crystals, has been presented, along with the method of factorization according to the irreducible representation of the space group. The first derivative of the atom–atom potential with respect to interatomic distance, which was ignored in the commonly used central force approximation, has been included in the intermolecular force field. The method has been applied to the case of the orthorhombic and triclinic modifications of polyethylene. It has been indicated that the effect of inclusion of the linear terms on the normal frequencies is remarkable for the librational lattice modes (it lowers the frequencies by as much as 20 cm−1), while it is not drastic for the translational lattice modes as well as for the correlation splittings of the internal modes. Agreement between the observed and calculated frequencies of the lattice modes is improved significantly by including the linear terms.
Effects of interatomic attraction on total cross sections in curve crossing collisions
Authors: Chang Albert M. , Pritchard David E.
Published: February 2003

We consider the influence of long range forces on the total cross section for a two-state curve crossing process. If the upper potential curve V22(R) is attractive and behaves as R−n for R beyond the crossing point, a centrifugal barrier will arise which affects the cross section. Using the Landau–Zener approximation, we derive cross sections accounting for this which can be expressed in terms of a simple dimensionless integral for both radial and rotational coupling. Results for suitably reduced cross sections are presented in a form which facilitates comparison with experiments for n=4, 5, and 6. Analytic expressions for the threshold behavior are also presented. We show that the low energy behavior of the cross section is very sensitive to the type of coupling, the value of n, and the potential at the crossing point.
Vibrational dephasing of heavy phosphine in the liquid and plastic crystalline states
Authors: Wilde Richard E. , Cohen Simon S.
Published: February 2003

Vibrational correlation functions obtained from the spontaneous Raman scattering of the P–D stretching vibration of liquid and solid PD3 have been modeled using a memory-functions approach. It has been shown that the modulation of the frequency is fast and most probably arises from the rotational motions of the molecules. Information about the molecular dynamics has been obtained out to 7 ps.
The B1Πu–X1Σg+ band system of the 7Li2 molecule
Authors: Hessel Merrill M. , Vidal C. R.
Published: February 2003

The absorption and laser induced fluorescence spectra of the B1Πu–X1Σg+ band system of the 7Li2 molecule has been photographed at high resolution (360 000) and high dispersion (0.4 Å/mm). Over 14 000 spectral lines have been assigned to this band system for a wide range of rotational quantum numbers (J=0−80) and vibrational quantum numbers (v′=0−13 and v″=0−18). Using a Dunham type analysis, we have obtained the molecular constants for both states and have fitted 8358 nonoverlapping lines with an rms error of 0.012 cm−1. The B1Πu lambda doubling constants, including the centrifugal distortion lambda doublet constant, have been determined. Quantum mechanical potential curves have been generated for this system using a novel variational procedure and Franck–Condon factors for different rotational quantum numbers have been determined over the range of vibrational levels observed. Excellent agreement has been achieved for the centrifugal distortion terms Dv, Hv, and LV which were obtained from the least squares fit of the assigned lines and those calculated from the quantum mechanical rotationless potential. Levels above the 2 2P+2 2S1/2 dissociation limit have been observed and a maximum for the barrier in the B 1Πu potential of 450±50 cm−1 above this atomic level was found. The dissociation energy, De′ of the X 1Σg state was found to be 8600±150 cm−1. A review of the literature on the Li2 molecule through 1977 is given.
Sublevel phosphorescence spectra of anthraquinone. I. Site selective sublevel phosphorescence spectra for octane and hexane solutions
Authors: Murao Toshiro , Azumi Tohru
Published: February 2003

The sublevel phosphorescence spectra originating from individual sites are observed. The sublevel phosphorescence spectra obtained have vibrational structures which are quite simple as compared with the complicated structures usually found in multisite and thermally averaged phosphorescence spectra. The vibrational analysis can be performed straightforwardly. The Tz spectrum has vibronic bands due to b1u vibrations only. These vibronic bands are mainly due to the vibronic coupling between the 3B1g(nπ*) lowest triplet state with a closely located 3Au(nπ*) state. The Tx spectrum has vibronic bands involving b3u vibrations only. The intensity mainly comes from a mechanism which involves vibronic coupling between 1B2g(σπ*) and 1B1u(ππ*). The Ty phosphorescence spectrum has vibronic bands involving b2u vibrations only. The intensity comes from a mechanism which involves vibronic coupling between 3B1g(nπ*) and 3B3u(σπ*). The rigorous selectivity in the appearance of vibronic bands in the sublevel phosphorescence spectra reveals that the molecule retains its D2h symmetry in the triplet state. This conclusion contradicts with the one given by Khalil and Goodman. Finally some comments are made concerning previously given vibrational assignments.
High-Rydberg atomic fragments from electron-impact dissociation of molecules
Authors: Schiavone James A. , Tarr Susan M. , Freund Robert S.
Published: February 2003

The production of high-Rydberg (HR) atomic fragments by electron-impact dissociation of 13 molecules has been compared to the results of previous work on the production of HR rare gas atoms. Measurements have been made of principal quantum number distributions, effective radiative lifetimes, and excitation cross sections including both the shapes as a function of electron-impact energy and the absolute magnitudes. Principal quantum number (n) distributions peak at lower n values than those for the rare gases and are consistent with the shorter times of flight of dissociation fragments. The HR atomic fragments appear to form in high angular momentum (l) states as a direct result of the dissociation process, whereas rare gas atoms form initially in low-l HR states and require subsequent electron collisions to reach high-l HR states. The energy dependence of the excitation cross sections, a slow rise from threshold with a peak near 100 eV, resembles that of other dissociative excitation processes and does not display the step function threshold characteristic of the rare gases. Magnitudes of the excitation cross sections are expressed in a form which separates the apparatus-dependent radiative decay factor from the initial excitation cross section. The result permits calculation of HR densities under a variety of electron-impact dominated conditions.
Unlinked cluster effects in molecular electronic structure. III. Potential curve for the CN− ion and the adiabatic electron affinity of CN
Authors: Taylor Peter R. , Bacskay G. B. , Hush N. S. , Hurley A. C.
Published: February 2003

A ground state potential curve (around equilibrium) is constructed for the CN− ion, using CI and coupled-pair methods. Significant changes arise in the values of the spectroscopic constants when the effects of unlinked cluster excitations are included; a set of predicted CN− spectroscopic constants is obtained using the uncertainties observed in a recent calculation on HCN. Evidence of the great similarity in the HCN, HNC, and CN− electronic structures is obtained by analyzing the CN− valence-shell correlation energy into pair contributions, thus giving a very simple picture of the correlation effects. Using the direct CI method extended to include approximate coupled pair techniques, the ground state energy for the CN radical is calculated, obtaining the adiabatic electron affinity of CN to form CN−; agreement with the very accurate experimental data is satisfactory.
Experiment and theory for CO2 laser-induced CF2HCl decomposition rate dependence on pressure and intensity
Authors: Stephenson John C. , King David S. , Goodman Myron F. , Stone James
Published: February 2003

A laser-excited fluorescence method has been used to determine the rates at which CF2HCl molecules dissociate into CF2+HCl fragments during CO2 laser pulses of uniform fluence and known intensity. A study has been made of the dependence of the rate on CO2 laser intensity and pressure of Ar buffer gas from the collision-free regime to atmospheric pressure. The effect of increasing Ar pressure is initially to increase the CF2HCl dissociation rate; above a moderate pressure (∼50 Torr), the rate is independent of Ar pressure up to atmospheric pressure. The data has been compared to a model, which adequately reproduces all the experimental data. The model treats the effect of collision between CF2HCl and the argon buffer gas in terms of rotational equilibration or ’’hole filling’’ in the discrete energy level region of CF2HCl. The discrete energy levels are interfaced to a quasicontinuum of vibrational–rotational states in a self-consistent manner which incorporates a background of nonpumpable CF2HCl states as a finite heat bath interacting with the pump mode. The model is used to calculate the rate of formation of product CF2 molecules as a function of argon pressure and CO2 laser intensity. The quasicontinuum for CF2HCl is predicted to begin about four quanta above the ground state. The absorption cross section in the quasicontinuum is shown to decrease from 10−18 to 10−20 cm2 at V=15. The energy distribution in CF2HCl is predicted to be decidedly nonthermal both below and beyond threshold.
A thermodynamic model for the monoclinic (ripple) phase of hydrated phospholipid bilayers
Authors: Doniach S.
Published: February 2003

A thermodynamic model for the formation of the ripple phase of hydrated phospholipid bilayers is proposed based on the idea of a spontaneous local curvature free energy. It is suggested that this arises from electrostatic coupling between water dipoles and the dipolar head groups of the lipid molecules. A bilayer profile for the ripple phase is calculated using the model and agrees qualitatively with x-ray structure measurements. The premelting transition is discussed in terms of a repulsive coupling between the spontaneous curvature free energy and the free energy of ordering of the head group array in the Lβ′ phase. This leads to a mechanism for the premelting transition based on the competition between head group ordering, which softens with increasing temperature, and the free energy of ripple formation, which is favored by an increase in the bilayer polarisability when the head groups become disordered.
The Green’s function method of sensitivity analysis in quantum dynamics
Authors: Hwang Jenn-Tai , Rabitz Herschel
Published: February 2003

The paper considers the sensitivity of quantum dynamics systems with respect to parameters or features in the underlying Hamiltonians. The analysis allows for the calculation of the gradient of the wave function with respect to any well defined system parameters in the Hamiltonian. Differential sensitivity equations are derived from the Schrödinger equation and their solution is expressed in terms of a Green’s function. For scattering problems the necessary asymptotic Green’s function can be exclusively expressed in terms of the available solution to the original Schrödinger equation. Also considered are problems from time-dependent quantum mechanics. In this latter situation sensitivity theory allows for the calculation of gradients of the time evolution operator with respect to system parameters. The scattering formalism is illustrated with a study of the anisotropy coefficient sensitivity in the collision of an atom and a ridgid rotor. The time-dependent theory was applied to a collision-free molecular beam translating through a region of space containing electric and magnetic fields. This case examined the sensitivity of the molecular transition probabilities to field parameters. These brief examples serve to illustrate the potential usefulness of sensitivity analysis in quantum dynamics.
Effect of n-butane impurity on electron mobility and electron–ion recombination rate constant in solid neopentane
Authors: Namba Hideki , Shinsaka Kyoji , Hatano Yoshihiko
Published: February 2003

The electron mobility and recombination rate were determined from measurements of the decay of the electron current induced by pulse x-ray irradiation under a DC electric field. (AIP)
Overlap integrals involving 4f orbitals
Authors: Urland W.
Published: February 2003

Overlap integrals are evaluated for rare earths with s, p+, and pPgr orbitals. (AIP)
An anisotropic Fermi interaction and isotropic dipolar interaction? A study of the 19F hyperfine interaction of Yb3+ in KMgF3
Authors: McGarvey Bruce R.
Published: February 2003

A theoretical analysis of the 19F hyperfine parameters of Yb3+ in KMgF3 reveals that the isotropic As and anisotropic Ap terms due to transferred spin can be accounted for by the same covalent model used earlier for Yb3+ in CaF2. The different ground states (Γ7 for CaF2, and Γ6 for KMgF3) account for the large numerical differences in As and Ap for the two systems. It is found that As is determined primarily by dipolar interactions with spin in 2p ortibals of F− and that Ap is determined mostly by the Fermi Contact interaction with spin in 2s states.
Nuclear magnetic relaxation and molecular reorientation of dicyanoacetylene in hydrocarbon solutions
Authors: Vold R. R. , Vold R. L. , Szeverenyi N. M.
Published: February 2003

14N linewidths and 13C spin lattice relaxation rates have been measured for dicyanoacetylene, neat and in several hydrocarbon solutions. The correlation between molecular reorientation rates and solvent viscosity is found to break down in the sense that the reorientation rates of dicyanoacetylene in aromatic solvents are significantly slower than in saturated hydrocarbon solvents. Without making reference to specific solute–solvent interactions this behavior may be rationalized by taking into account the isothermal compressibility of the various solvents.
Absolute rate constants for the reaction of atomic hydrogen with ketene from 298 to 500 K
Authors: Michael J. V. , Nava D. F. , Payne W. A. , Stief L. J.
Published: February 2003

Rate constants for the reaction of atomic hydrogen with ketene have been measured at room temperature by two techniques, flash photolysis–resonance fluorescence (FP–RF) and discharge flow–resonance fluorescence (DP–RF). The measured values are (6.19±1.68) ×10−14 and (7.3±1.3) ×10−14 cm3 molecule−1 s−1, respectively. In addition rate constants as a function of temperature have been measured over the temperature range 298–500 K by the FP–RF technique. The results are best represented by the Arrhenius expression k= (1.88±1.12) ×10−11 exp(−1725±190/T) cm3 molecule−1 s−1, where the indicated errors are at the two standard deviation level. These results are compared to two previous investigations both of which employed the discharge flow–mass spectrometric technique. Also they are compared to the analogous reaction, H+C2H4 (high pressure limit) since both reactions refer to addition across a carbon–carbon double bond. The reaction is considered theoretically from the activated complex point of view. Lastly, the suggestion that this reaction could contribute to the destruction of ketene in interstellar clouds is evaluated based on the present results.
Generalized hyperbolic coordinates for collinear reactive scattering
Authors: McNutt J. F. , Wyatt R. E.
Published: February 2003

An orthogonal coordinate system, designated generalized hyperbolic coordinates (GHC), is developed for the collinear exchange reaction AB+C→A+BC. The point transformation method is utilized and generalization is made to collisions with noninfinite central mass. Plots are presented of typical metric coefficients which compose the system Hamiltonian in GHC, and the Schrödinger equation for the collision (using a uniquely scaled wave function) is shown to lead to a set of close-coupled equations possessing a symmetric coupling matrix.
The effect of the central ligand on the surface photovoltage of phthalocyanine films
Authors: Dahlberg S. C. , Musser M. E.
Published: February 2003

The photovoltage at light intensity saturation, and the relaxation time associated with this photovoltage have been measured for a variety of phthalocyanine (Pc) thin films and the results are: H2Pc(10.60 mV, 25.3 msec), NiPc(10.99 mV, 43.2 msec), CuPc(5.24 mV, 47.2 msec), and ZnPc(0.01 mV, 18.4 msec). No observable photovoltage signal was obtained for either FePc or CoPc. The spectral dependence of the photovoltage signals for these phthalocyanine dyes is related to their optical absorption spectra. Efforts have been made in the literature to connect the semiconducting properties of these organic dyes to the differing luminescent, magnetic, and optical properties of the various phthalocyanines. This work indicates that the size of the photovoltage signal is unrelated to any of these parameters. The size of the photovoltage is, however, closely correlated with the alignment of the energies of the d orbitals on the central metal ligand with the valence band molecular orbital of the outer phthalocyanine ring, as determined from photoemission measurements.
Structural dynamics in phospholipid bilayers from deuterium spin–lattice relaxation time measurements
Authors: Brown Michael F. , Seelig Joachim , Häberlen Ulrich
Published: February 2003

The quadrupolar spin–lattice (T1) relaxation of deuterium labeled phospholipid bilayers has been investigated at a resonance frequency of 54.4 MHz. T1 measurements are reported for multilamellar dispersions, single bilayer vesicles, and chloroform/methanol solutions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), selectively deuterated at ten different positions in each of the fatty acyl chains and at the sn-3 carbon of the glycerol backbone. At all segment positions investigated, the T1 relaxation times of the multilamellar and vesicle samples of DPPC were found to be similar. The profiles of the spin–lattice relaxation rate (1/T1) as a function of the deuterated chain segment position resemble the previously determined order profiles [A. Seelig and J. Seelig, Biochem. 13, 4839 (1974)]. In particular, the relaxation rates are approximately constant over the first part of the fatty acyl chains (carbon segments C3–C9), then decreasing in the central region of the bilayer. In chloroform/methanol solution, by contrast, the relaxation rates decrease continuously from the glycerol backbone region to the chain terminal methyl groups. The contributions from molecular order and motion to the T1 relaxation rates have been evaluated and correlation time profiles derived as a function of chain position. The results suggest that the motions of the various methylene segments are correlated in the first part of the fatty acyl chains (C3–C9), occurring at frequencies up to 1/τc?1010 Hz. Beyond C9, the rate and amplitude of the chain segmental motions increase, approaching that of simple paraffinic liquids in the central region of the bilayer (1/τc?1011 Hz). The T1 relaxation rates of multilamellar dispersions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) deuterated at the 9, 10 double bond of the sn-2 chain were also determined and found to be significantly faster than those of the CD2 chain segments of DPPC bilayers. This is most likely due to the larger size and correspondingly slower motion of the chain segment containing the double bond. At segments close to the lipid–water interface the rate of motion is considerably less than in the hydrocarbon region of the bilayer.