bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural temperature dependence in α-lithium iodate: Neutron and x-ray study between 20 and 500 K
Authors: Svensson C. , Albertsson J. , Liminga R. , Kvick Å. , Abrahams S. C.
Published: February 2003

The crystal structure of hexagonal α-LiIO3 has been determined by neutron and x-ray diffraction at ten temperatures from 20 to 500 K. Four sets (1248 to 1622 reflections per set) of neutron data at λ=1.0467 Å were measured at the Brookhaven High Flux Beam Reactor at 20, 100, 200, and 295 K. The integrated intensities of a further 13 data sets (1706 to 2963 reflections per set) were measured using MoKα radiation on a CAD-4 x-ray diffractometer. The structure at each temperature was refined by least squares in space group P63. The final agreement factors fell between 0.023 and 0.050. The unit cell dimensions at 295 K are a=5.481 27(6), c=5.171 65(4) Å, with nonlinear thermal expansions. The principal change in structure with temperature is in the atomic positions of Li and O relative to I: The positions are given by O(z)T=−0.8435(3) Å+62(4)×10−9 T2 Å K−2 and Li(z)T=0.3808(10) Å+167 (18)×10−9 T2 Å K−2. The iodate ion has a constant I–O bond length of 1.8081(4) Å, but the O–I–O angle increases from 99.97(2)° at 20 K by 0.5° over the temperature range studied. The LiO6 octahedron has Li–O distances of 2.070 and 2.140(1) Å at 20 K, increasing to 2.106 and 2.151(9) Å at 460 K. The mean-square amplitude of thermal vibration varies nonlinearly with temperature for all atoms, with zero-point rms displacement of 0.040 Å for I, 0.067 Å for O, and 0.094 Å for Li. The Debye temperature is estimated as 350 K on the basis of the 295 K amplitudes of vibration.
Renormalization group treatment of polymer excluded volume by t’Hooft–Veltman-type dimensional regularization
Authors: Kholodenko A. L. , Freed Karl F.
Published: February 2003

The chain conformation space renormalization group method is transformed into a representation where the t’Hooft–Veltman method of dimensional regularization can directly be applied to problems involving polymer excluded volume. This t’Hooft–Veltman-type representation enables a comparison to be made with other direct renormalization methods for polymer excluded volume. In contrast to the latter, the current method and the chain conformation one from which it is derived are not restricted to the asymptotic limit of very long chains and do not require the cumbersome use of insertions to calculate the relevant exponents. Furthermore, the theory emerges directly in polymer language from the traditional excluded volume perturbation expansion which provides the correct weight factors for the diagrams. Special attention is paid to the general diagrammatic structure of the theory and to the renormalization prescription in order that this prescription follows from considerations on measurable quantities. The theory is illustrated by calculation of the mean square end-to-end distance and second virial coefficient to second order including the full crossover dependence on the renormalized strength of the excluded volume interaction and on the chain length. A subsequent paper provides the generalization of the theory to the treatment of excluded volume effects in polyelectrolytes.
Renormalization group treatment of excluded volume effects in a polyelectrolyte chain in the weak electrostatic coupling limit
Authors: Kholodenko A. L. , Freed Karl F.
Published: February 2003

We provide the first rigorous treatment of the electrostatic excluded volume for a polyelectrolyte chain which incorporates the effects of salt concentration. Our treatment involves an extension of the t’Hooft–Veltman method of dimensional regularization for polymer excluded volume, developed in the accompanying paper, to the case complicated by the presence of electrostatic interactions. The critical dimensionality for the polyelectrolyte chains with realistic interactions is shown to be four in sharp contrast to previous simplified analyses, which do not consider salt concentration effects explicitly and which lead to a critical dimensionality of six. Our results imply that expansions in ε=4−d (with d the dimensionality of space) can be applied, so the theory reduces to the limit of uncharged polymers with excluded volume when the electrostatic interactions become totally screened. Our renormalization group (RG) treatment indicates the absence of stable fixed points, so there is no simple scaling limit. The range of validity of the perturbation expansion is established on the basis of a RG analysis, and a physical meaning of the weak coupling limit is also determined. The predicted lack of universality for the polyelectrolyte chain is in accord with experimental information. Explicit renormalized expressions are derived for the mean squared end-to-end distance 〈R2〉 to lowest order in both excluded volume and electrostatic coupling constants. These expressions are combined with the solution of the RG equations to provide a generalized scaling representation for 〈R2〉 in terms of three scaling variables. A brief discussion of possible future biological and nonbiological applications is provided.
Effect of energetic C+6 irradiation on transport properties of trans-(CH)x and trans-(CD)x
Authors: Kaufer J. , Wells W. K. , Woerner T. , Lauchlan L. , Heeger D. J. , Etemad S. , Heeger A. J.
Published: February 2003

A study of the effects of irradiation by energetic carbon ions (56 MeV C6+) on the transport properties of polyacetylene is presented. The in situ monitoring of the resistivity ρ shows an initial nonlinear increase by a factor of 104 as a function of the integrated flux of particles Φ. For trans-(CH)x, the increase in ρ saturates at a plateau starting at Φ0≂5×1012 particles/cm2, whereas trans-(CD)x behaves differently. After a similar (but slower) initial increase, ρ of trans-(CD)x peaks at Φ0 and subsequently decreases for Φ≳Φ0. Parallel thermoelectric power data show an increase from ∼880 μV/K to ∼1100 μV/K for Φ≂Φ0 for both isotopes. Characterization studies of the irradiated film using infrared and Raman spectroscopy show no detectable change in backbone structure, and ESR measurements show no increase in the number of unpaired spins. Moreover, after irradiation the polymer can be doped to the highly conducting metallic regime. We therefore conclude that the conjugted (CH)x chains remain intact during irradiation; either the deposited energy is rapidly dissipated to avoid bond breaking or there is a high tendency to self-heal. The increase in ρ is primarily the result of a decrease in the number of carriers. The dramatic differences in the behavior of trans-(CH)x and trans-(CD)x suggest that hydrogen or deuterium is released during irradiation and that these radicals subsequently compensate the residual carriers in the polymer. Aside from this compensation effect, it is concluded that polyacetylene is relatively immune to damage by irradiation.
Vibronic coupling theory of infrared vibrational transitions
Authors: Nafie Laurence A. , Freedman Teresa B.
Published: February 2003

The theory of vibronic coupling is developed for infrared vibrational transitions. It is shown that the lowest order nonadiabatic Born–Oppenheimer correction terms contain an important adiabatic component which may be used to describe infrared transition intensity for imaginary Hermitian operators, such as the momentum and angular momentum operators. This previously unrecognized source of adiabatic infrared intensity forms a complement to the traditional Herzberg–Teller vibronic coupling expressions, which are active for the position operator, and resolves the paradox of vanishing electronic intensity for momentum operators in the Born–Oppenheimer approximation. Expressions for infrared absorption and vibrational circular dichroism are derived that utilize only ground electronic state wave functions; LCAO wave functions are used in these expressions to provide a more detailed description of these new momentum intensity contributions.
Ab initio infrared and Raman spectra
Authors: Fredkin Donald R. , Komornicki Andrew , White Steven R. , Wilson Kent R.
Published: February 2003

We discuss several ways in which molecular absorption and scattering spectra can be computed ab initio, from the fundamental constants of nature. These methods can be divided into two general categories. In the first, or sequential, type of approach, one first solves the electronic part of the Schrödinger equation in the Born–Oppenheimer approximation, mapping out the potential energy, dipole moment vector (for infrared absorption) and polarizability tensor (for Raman scattering) as functions of nuclear coordinates. Having completed the electronic part of the calculation, one then solves the nuclear part of the problem either classically or quantum mechanically. As an example of the sequential ab initio approach, the infrared and Raman rotational and vibrational-rotational spectral band contours for the water molecule are computed in the simplest rigid rotor, normal mode approximation. Quantum techniques are used to calculate the necessary potential energy, dipole moment, and polarizability information at the equilibrium geometry. A new quick, accurate, and easy to program classical technique involving no reference to Euler angles or special functions is developed to compute the infrared and Raman band contours for any rigid rotor, including asymmetric tops. A second, or simultaneous, type of ab initio approach is suggested for large systems, particularly those for which normal mode analysis is inappropriate, such as liquids, clusters, or floppy molecules. Then the curse of dimensionality prevents mapping out in advance the complete potential, dipole moment, and polarizability functions over the whole space of nuclear positions of all atoms, and a solution in which the electronic and nuclear parts of the Born–Oppenheimer approximation are simultaneously solved is needed. A quantum force classical trajectory (QFCT) molecular dynamic method, based on linear response theory, is described, in which the forces, dipole moment, and polarizability are computed quantum mechanically, using gradient techniques step by step along a classical trajectory whose path is determined by these quantum forces. We believe the QFCT method to be a more practical ab initio route to spectral band contours for large molecules, clusters, and solutions, and it can be equally applied to equilibrium and nonequilibrium systems. It is pointed out that a similar ab initio QFCT molecular dynamic approach could be used to compute other types of spectra, e.g., electronic absorption, as well as other parameters such as transport properties and thermodynamic functions and their quantum corrections. For parameters not depending on momenta, a parallel ab initio Monte Carlo approach would use electronic energies and other parameters of interest generated quantum mechanically, and ‘‘classical’’ trial moves of the nuclei.
Fluorescence lifetimes of single vibrational levels in HSO (Ã 2A′)
Authors: Kawasaki Masahiro , Kasatani Kazuo , Tanahashi Shigeo , Sato Hiroyasu , Fujimura Y.
Published: February 2003

Fluorescence lifetimes of single vibrational levels of the first excited state of HSO and DSO have been measured under effusive flow conditions following excitation by a pulsed dye laser. The lifetimes show a systematic decrease with v′3 (S–O stretch) from 74 μs (v′3=1) to 26 μs (v′3=8) for HSO. For DSO, the lifetimes are longer than the corresponding ones of HSO. These results are interpreted in terms of a second-order coupling model (Ã 2A′→X 2A′′→continuum) in which the final states are those of H+SO dissociation continuum. An analytical expression is given to compute the vibrational energy dependence of nonradiative rates assuming an energy dependent linewidth from the dissociation.
Electron attachment to the perfluoroalkanes n-CNF2N+2 (N=1–6) and i-C4F1 a)
Authors: Spyrou S. M. , Sauers I. , Christophorou L. G.
Published: February 2003

Parent and fragment negative ion formation from seven perfluoroalkanes [n-CNF2N+2 (N=1–6) and i-C4F10] under low-energy (0–10 eV) electron impact has been studied using a time-of-flight mass spectrometer. For CF4, C2E6, and C3F8 only fragment anions were observed, F− being the most abundant. For n-C4F10 a weak parent ion was observed, but again the predominant ion was F−. For i-C4F10, n-C5F12, and n-C6F14 the parent negative ions were the most abundant with relative cross sections peaking at 0.6 eV and autodetachment lifetimes from 10 to 100 μs depending on the molecular size and electron energy. In addition to the parent negative ion and the F− ion, fragment negative ions of the form CNF−2N+1, CNF−2N, CNF−2N−1 (with N=1–6) have been detected. The relative cross sections for all observed negative ions have been measured and corrected for the finite width of the electron pulse using an unfolding procedure. The positions of the dissociating negative ion states (NISs) shift to lower energies with increasing size of the molecule. Possible fragmentation mechanisms of the NISs leading to the production of the observed fragment negative ions have been suggested and discussed. From the appearance onsets of a number of fragment negative ions, various C–C and C–F bond dissociation energies, heats of formation, and electron affinities EA of certain fragments have been determined and are reported. It was found that the EA of perfluorocarbon radicals increases with increasing size of these radicals, and similar behavior is indicated also for the perfluoroalkane molecules themselves. The separation times of the dissociating fragments and the autodetachment lifetimes of the extremely short-lived (∼10−15 s) and dissociating NISs of CF4 and C2F6 were estimated. The intensity of the parent anion as a function of molecular size and geometry is discussed, and the relative intensities of the fragment negative ions are rationalized on the basis of the present work and relevant data from electron swarm studies.
Electronic Hamiltonian, wave functions, and energies, and derivative coupling between Born–Oppenheimer states in the vicinity of a conical intersection
Authors: Mead C. Alden
Published: February 2003

We derive the most general form of the first terms of the power-series expansion of the electronic Hamiltonian in the neighborhood of the conical intersection at the equilateral triangle configuration of the homonuclear triatomic system M3. Previous treatments of this problem had assumed that the derivative coupling between Born–Oppenheimer states could be transformed away by choosing a strictly diabatic basis of electronic states. It has recently been pointed out, however, that this is not possible, in general. Making full use of the symmetry of the problem, and also taking account of the molecular Aharonov–Bohm effect, we obtain explicitly the leading terms of the expansion for electronic energies and wave functions, and of the derivative coupling. In terms of the expansion parameter r, a measure of the distance from the equilateral triangle configuration, the derivative coupling can be transformed away through the first order, but in the second order, nonremovable terms appear which are expected to be important in some problems.
Atomic orbitals of the nonrelativistic hydrogen atom in a four-dimensional Riemann space through the path integral formalism
Authors: Grinberg Horacio , Marañon Julio , Vucetich Hector
Published: February 2003

The Kustaanheimo–Stiefel transformation together with the well-known expansion of the kernel of an isotropic harmonic oscillator is used to generate the atomic orbitals of the nonrelativistic hydrogen atom in a four-dimensional Riemann space through the path integral formalism. Group theoretical implications of the present problem are briefly discussed.
Some explicit results for finite wave vector double and triple molecular correlation functions
Authors: de Elvira A. Ruiz , Ortiz M. J.
Published: February 2003

Kawasaki’s mode-coupling theory is generalized to include evolution equations for the hydrodynamic fluxes. Using Martin, Siggia, and Rose’s method for stochastic nonlinear problems, we are able to determine explicit expressions for the wave vector and time dependence of double and triple molecular correlation functions for neutral fluids. Our results show a t−3/2 decay for triple shear-current correlation functions for the case of finite wave vectors.
Calculation of the phonon density of states and related thermodynamic properties for trigonal selenium
Authors: Hansen F. Y. , Alldredge G. P. , McMurry H. L.
Published: February 2003

The phonon density of states for trigonal selenium has been calculated on the basis of a short range force model giving good overall agreement with experimental room temperature phonon dispersion data. A qualitative comparison with an experimental determination of the phonon density of states shows similarities in the gross features, but the experimental data lacks many of the finer details shown by the theoretical results due to resolution effects. The lattice dynamical contribution to the heat capacity CV is calculated and is found to be in good agreement with experimental determinations of Cp after transforming Cp to CV.
The transition dipole moment of the ν2 band of the methyl radical
Authors: Yamada Chikashi , Hirota Eizi
Published: February 2003

By using a tunable diode laser, the decay of the absorption intensity with time, after the discharge generating the radical was switched off, was measured for a line [Q4(4)] of the CH3 ν2 fundamental band. When combined with a literature value of the CH3 recombination rate constant, the present result gave an estimate for the steady-state concentration of the CH3 radical. The steady-state absorbance of the Q4(4) line was also measured, and the two results were used to calculate the transition moment of the CH3 v2=1←0 band to be 0.280±0.049 D.
Experimental and theoretical analysis of the temperature dependence of rotational Raman linewidths of oxygen
Authors: Bérard M. , Lallemand P. , Cebe J. P. , Giraud M.
Published: February 2003

This paper is devoted to a detailed study of the self-broadening of rotational Raman lines in oxygen in order to obtain information concerning the anisotropic part of the intermolecular potential using the theory of Smith, Giraud, and Cooper. It first recalls the rotational structure of the energy spectrum of O2 due to its nonzero electronic spin, and shows that a pure Hund’s type b coupling scheme is not adequate. Then the experimental setup and data analysis procedure are discussed in detail to justify the accuracy of the results. In this part the presence of a weak background in addition to Lorentzian Raman lines is discussed but without giving a plausible physical origin. The next part extends the theory of Smith, Giraud, and Cooper to the case of a 3Σ state. The last part presents the experimental results together with theoretical results obtained with an intermolecular potential derived from that of Mingelgrin and Gordon. It is shown that a very good agreement between theory and experiment can be reached provided one takes a slightly steeper repulsive potential. Finally it is shown that the repulsive anisotropic potential is dominating in the whole temperature range that was studied.
Two-dimensional nuclear magnetic resonance in rotating solids: Time reversal effects in chemical shift-dipolar spectra
Authors: Munowitz M. G. , Griffin R. G.
Published: February 2003

Two magic-angle sample spinning experiments which separate chemical shift and dipolar interactions are described. Both experiments involve dipolar coupling periods employing partial or complete time reversed evolution. It is shown that partial reversal results in two unusual spectral effects. First, the intensity distributions of the dipolar spectra corresponding to individual chemical shift sidebands are symmetric. This is in contrast to the usual result described previously [M. G. Munowitz and R. G. Griffin, J. Chem. Phys.. 76, 2848 (1982)]. A second, and perhaps more unusual, result is that rotational sidebands spaced at half the spinning frequency are observed. In its simplest form, the time reversal effect can be utilized to eliminate phase twist in two-dimensional shift-dipolar spectra.
13C chemical shifts and spin-lattice relaxation in the nematic liquid crystal 4-cyano-4′-n-pentylbiphenyl
Authors: Lewis John S. , Tomchuk E. , Hutton H. M. , Bock E.
Published: February 2003

The 13C chemical shifts and spin-lattice relaxation times were obtained for all the resolved resonances in the isotropic and nematic phases of 5CB. NOE measurements were also obtained for the isotropic phase. A study of these data indicates that a flexibility gradient exists along the n-pentyl chain and that D⊥, the rotational diffusion constant for the entire molecule, can be estimated. A study of the chemical shifts of 5CB shows that a single average shielding tensor is inadequate for a proper description of the chemical shifts of this liquid crystal molecule. These results are compared with those previously obtained for MBBA and PAA.
The structure of BCl2: ESR study of an x-irradiated crystal of BCl3
Authors: Franzi R. , Geoffroy M. , Lucken E. A. C. , Leray N.
Published: February 2003

X irradiation of a single crystal of boron trichloride at 77 K yields at least two different radicals. A brief exposure to UV irradiation, however, leaves only one radical whose ESR spectrum shows hyperfine coupling from one boron (10B, 11B) and two equivalent chlorine (35Cl, 37Cl) nuclei and which is identified as BCl2. The g tensor, 11B magnetic hyperfine tensor, 35Cl magnetic and quadrupole hyperfine tensors have been determined and analyzed in terms of the radical’s electronic structure. Problems arising in relating the principal direction of the various tensors to the molecular geometry are discussed. BCl2 is nonlinear with a ClBCl angle of approximately 122° and with the unpaired electron mainly localized on the boron atom.
Beam maser investigation of inelastic scattering of NH3. III. Cross sections for rotational transitions induced by CO2, N2, and H2
Authors: Klaassen D. B. M. , ter Meulen J. J. , Dymanus A.
Published: February 2003

Cross sections for rotational transitions between various low-lying inversion doublets of NH3 in collisions with CO2, N2, and H2 are measured in a double-resonance beam maser setup. A modification of Anderson’s theory [D. B. M. Klaassen, J. J. ter Meulen, and A. Dymanus, J. Chem. Phys. 77, 4972 (1982)] yields values for the cross sections that are in good agreement with the experimental results for CO2 and N2. For the system NH3–H2, transition probabilities are evaluated in Anderson’s theory using ‘‘bent’’ trajectories. Induction and dispersion terms up to R−7 are considered in the long-range intermolecular potential. For the short-range repulsive part, two empirical potentials are proposed with parameters that are fitted to the experimental results. Integral cross sections for rotational transitions calculated with these potentials are also presented.
Rotation–translation coupling and the lattice dynamics of RbCN
Authors: Ehrhardt K.-D. , Press W. , Lefebvre J.
Published: February 2003

The dispersion of acoustic and optical phonons of rubidium cyanide has been measured at 300 K and near the phase transition from the disordered cubic high-temperature phase to the ordered monoclinic phase. At both temperatures the acoustic branches are affected by rotation–translation coupling in a way similar to the findings in KCN. The data are explained by a model which combines a shell model and rotation–translation coupling. The rotation–translation coupling is described on the basis of the theory of Michel and Naudts. Details of the model are discussed.
Application of time-resolved luminescence in the study of lipid aggregate symmetry. I. Theoretical discussion
Authors: Johansson Lennart B.-Å , Lindblom Göran
Published: February 2003

It is shown how the emission anisotropy of a time-resolved luminescence measurement depends on the symmetry of lipid aggregates containing solubilized lumiphores. A derivation of the relevant equations is performed. The potential applications of this method as a complementary technique for studies of aggregate shape at high amphiphile concentrations are discussed. In particular, the method is expected to be very useful in structural investigations of cubic liquid crystalline phases. The behavior of the emission anisotropy for a sample containing a mixture of two different aggregates is determined as a function of order parameter and changes in the fraction of the different aggregate symmetries.
Direct inelastic scattering Ar from Pt(111)
Authors: Hurst J. E. , Wharton Lennard , Janda K. C. , Auerbach D. J.
Published: February 2003

High resolution angularly resolved time of flight distributions are presented for a supersonic argon beam scattering from a clean well-characterized Pt(111) single crystal. A novel presentation of the resulting velocity and angular flux information in terms of iso-flux contour maps in Cartesian velocity space allows the scattering process to be decomposed into three mutually independent directions defined by the surface normal (z), parallel to the surface and in the scattering plane (y), and parallel to the surface but perpendicular to y and z(x). The iso-flux contour maps appear as nested ovals with principal axes oriented parallel to the above defined directions; axis length decreases in the order z, y, x. The corresponding variances in the x, y, and z velocities vary directly with the surface temperature. Three beam energy regimes are evident and are discussed in terms of the diminishing effect of the attractive well which occurs for increasing beam energies and the increasing effect of short range phenomena prevalent at high incident beam energies. Accomodation coefficients were defined and measured for the y and z directions and were 0.1 and 0.45, respectively, indicating the degree to which parallel and perpendicular momenta are not conserved in a single collision. Geometric scattering by instantaneous surface roughness is experimentally shown to be negligible for this system for beam energies <20 000 K indicating that the scattering is by the finite momentum of the surface.
Internal momentum equilibration and the configurational distribution function for dilute macromolecular solutions: Rigid dumbbells in steady flows
Authors: Olmsted R. D. , Snider R. F.
Published: February 2003

The Curtiss, Bird, and Hassager (CBH) theory of dilute macromolecular solutions [Adv. Chem. Phys. 35, 31 (1976)] uses an assumption of equilibration in momentum space as a principal simplifying approximation. Whereas, CBH assume each internal momentum of a polymer molecule to be normally distributed about a zero average value, the present work generalizes the treatment to allow nonzero configuration dependent average momenta. For rigid dumbbells, the usual rotational diffusion equation is replaced by three coupled nonlinear differential equations which simultaneously determine the average internal momenta and the configurational distribution function. Exact solutions of this set of nonlinear equations are obtained for irrotational and pure vortex flows.
Dynamical study of nonadiabatic unimolecular reactions: The conical intersection between the B̃ 2B2 and Ã 2A1 states of H2O+
Authors: Dehareng Dominique , Chapuisat Xavier , Lorquet Jean-Claude , Galloy Claudine , Raseev Gheorghe
Published: February 2003

The conical intersection connecting the B̃ 2A′ and Ã 2A′ states of the H2O+ ion is studied. The two potential energy surfaces are calculated ab initio by the SCF/CI method within the CS point group. The nonadiabatic coupling matrix elements 〈Ã‖∂/∂q‖B̃〉 are computed for several cross sections throughout the potential energy surfaces. A transformation to the diabatic representation is performed. The linear model is found to be a good approximation in the region close to the apex of the cone. The global functions t(s) and T(S) governing the nonadiabatic transition probability are calculated; their shapes are those predicted by the Landau–Zener model (in the Nikitin bidimensional version). A dynamical study is undertaken by means of classical trajectory calculations on the upper adiabatic potential energy surface. An averaged transition probability P̄tr is derived. Excitation of rotation or of the bending mode of H2O before photon impact has no influence on P̄tr. Excitation of the symmetrical or antisymmetrical valence modes of H2O lowers P̄tr. The shape of ln (1−P̄tr) as a function of time indicates the existence of two distinct regimes at short and intermediate time ranges, characterized by two different rate constants k1 and k2, respectively. The rate constants are of the order of 1014 s−1. k1 exhibits a maximum as a function of the absorbed energy Eabs, whereas k2 decreases as a function of Eabs.
Electron diffraction studies of supersonic jets. III. Clusters of n-butane
Authors: Heenan Richard K. , Bartell Lawrence S.
Published: February 2003

Expansions of n-butane from small nozzles were observed to generate large clusters, provided that enough helium or neon carrier gas accompanied the butane to remove the heat of condensation rapidly and to produce low temperatures. Although expansions of neat n-butane led to substantial conformational cooling under some circumstances, clusters were never seen without carrier gas. Both thin-plate and tapered glass nozzles yielded clusters. Diffraction patterns of the clusters are characteristic of a liquidlike structure perhaps as cold as 100 K. No evidence for inclusion of carrier gas atoms in the clusters was found. Cluster scattering signals are stronger and show less noise than those which could be expected from bulk liquid by x-ray or neutron diffraction. Their interference terms are dominated by nearest neighbor intermolecular C...H contacts. Information about the distribution of these contacts has not yet been deduced in computer simulations of liquid butane but should soon become available for comparison.
Vibrational predissociation of Ar ⋅ BCl3: A Monte Carlo-RRKM study
Authors: Adams John E.
Published: February 2003

Experiments have shown that absorption of a single photon by the high-frequency asymmetric stretching mode of boron trichloride will lead to dissociation of the Ar ⋅ BCl3 van der Waals molecule in 1–3 ps. Since it appears unlikely that a direct dynamical coupling of the excited mode to the weak van der Waals bond occurs, it is somewhat difficult to understand why the observed lifetime of the metastable complex is so short. One finds, however, that an RRKM calculation (performed within a Monte Carlo framework) correctly predicts that a short lifetime will be observed, suggesting that energy randomization amongst the low-frequency modes of oscillation may play an important role in the predissociation dynamics.
Photochemistry of acetylene at 1849 Å
Authors: Okabe Hideo
Published: February 2003

The photochemistry of acetylene at 1849 Å has been studied over the pressure range from 13 to 2660 N m−2 (0.1–20 Torr). The quantum yield of diacetylene formation has been measured as a function of C2H2 pressure and the partial pressures of He and N2. The quantum yield of H2 formation has also been obtained. It was concluded that the quantum yield for C2H2→hνC2H+H is 0.06 and that for C2H2→hνC2+H2 is 0.1. The remaining process is the formation of a metastable acetylene which reacts either with C2H2 to form C2H radicals or is deactivated by collisions with the walls or inert gases. The C2H radicals react 0.240±0.015 times as fast with C2H6 and 0.469±0.041 times as fast with C3H8 as with C2H2. The role of acetylene photolysis in the Titan atmosphere is briefly discussed.
Rotational energy transfer in collisions of internally excited molecules. Effect of initial conditions and potential energy surface
Authors: Thompson Donald L. , Blais Normand C. , Truhlar Donald G.
Published: February 2003

We report quasiclassical trajectory calculations for the effect of potential energy surface, initial vibrational state, and initial translational energy on cross sections for rotational energy transfer in nonreactive collisions. We also study whether the effect of initial vibrational excitation is different for systems that allow reaction from those that do not. The systems studied are Ar+HF, Ar+H2, and H(D)+H2 on a total of five potential energy surfaces. The most important trend is that if rotational-translational energy-transfer cross sections are small (<4a20) for the ground vibrational state, then they increase markedly upon vibrational excitation; but if they are large (20–110a20) for the ground vibrational state, then they change very little. We present tables of the state-to-state rotational energy cross sections to illustrate finer details of the trends.
Multimode Jahn–Teller and pseudo-Jahn–Teller effects in BF+3
Authors: Haller E. , Köppel H. , Cederbaum L. S. , von Niessen W. , Bieri G.
Published: February 2003

The ionic states of BF3 exhibit striking vibronic coupling effects. These effects are investigated using ab initio calculated ionization energies and vibronic coupling constants. The two Jahn–Teller active modes show interference effects in all bands of E symmetry. In the 2A1′ ionic state single quanta of a degenerate vibration are excited. They borrow their intensities from the adjacent 2E′ state by means of a pseudo-Jahn–Teller interaction. The symmetry of BF+3 in its nondegenerate ground state is predicted to be lowered due to two-mode pseudo-Jahn–Teller interaction with the second excited state. This interaction is surprisingly strong and leads to an interesting vibronic structure. The vibrational fine structure in all experimental spectra is reproduced satisfactorily. The earlier discrepancies in assigning the three lowest lying ionic states are explained.
Equivalence of ionization potential and magnitude of chemical potential in Hartree–Fock theory of atoms
Authors: Alonso J. A. , March N. H.
Published: February 2003

The result that the chemical potential is equal to the negative of the Koopmans ionization potential I in Hartree–Fock theory is obtained by invoking: (i) The asymptotic form of the electron density: ρHF (r)∼exp[−2(2I)1/2r]; and (ii) results for the large r behavior of the functional derivatives of kinetic energy T and exchange energy Ex: (δT/δρ)→−I, (δEx/δρ)→0.
A theoretical study of the lowest singlet and triplet surfaces of C2H2S
Authors: Siegbahn P. E. M. , Yoshimine M. , Pacansky J.
Published: February 2003

SCF-gradient and configuration-interaction (CI) methods have been used in a study of the lowest singlet and triplet states of various isomers on the C2H2S energy surface. The main conclusions drawn from the calculations are the following: the singlet isomer with the lowest energy is thioketene with mercaptoacetylene 17.4 kcal/mol higher in energy. The much debated antiaromatic isomer thiirene is found to be 37.5 kcal/mol above thioketene and is found to be considerably lower in energy than thioformylmethylene carbene (∼32 kcal/mol) or the 1,3-diradical (∼22 kcal/mol) formed by scission of a CS bond. Consequently, thiirene has some stability towards thermal rearrangement by this route.
The doubly excited autoionizing states of H2
Authors: Guberman Steven L.
Published: February 2003

Ab initio potential curves have been determined for 24 doubly excited autoionizing states of H2 having primarily the lowest 2Σ+u and 2Πu states of H+2 as the core orbital. Fourteen of these states have not appeared previously in the literature. The configuration interaction wave functions are constructed with a Feshbach projection operator formalism. The accuracy of the calculations is assessed by comparison with calculations in the same bases on the lower bound states of H2. The potential curves for the doubly excited states fall into three distinct groups with an energy ordering related to the Hartree energy of the most important configuration of each state. The relative energies of several doubly excited states and the corresponding singly excited bound states of H2 are compared. The grouping of doubly excited states is in good agreement with the electron impact results of Kollman. Both the lowest two states of Q1 1Σ+u symmetry and a Q2 1Σ+u state with thresholds of 25.5, 27.8, and 33.3 eV, respectively, are important in the interpretation of dissociative photoionization and electron impact experiments. A Q2 1Σ+g state with a threshold at 30.2 eV will contribute to the fast H atoms seen in several electron impact experiments.
Highly excited states of nitric oxide studied by two-color double resonance spectroscopy
Authors: Ebata Takayuki , Mikami Naohiko , Ito Mitsuo
Published: February 2003

Double resonance enhanced four-photon ionization spectra and fluorescence dip spectra of nitric oxide have been simultaneously measured using two tunable dye lasers. The spectra showed clear rovibronic structure of the 2Σ+ and 2Π states in the third-photon region (65 000∼70 000 cm−1). Laser power dependencies of the fluorescence dip and the ionization signals have been analyzed by kinetic rate equations and the absorption cross section of the transition between the second resonance state and the A 2Σ+ state has been obtained. Finally, by varying the delay time between the pulses from the first and second dye lasers, we have observed collision induced rotational relaxation in the A 2Σ+ state. The relaxation rate was found to be comparable to the electronic quenching rate.
Calculation of the frequencies and intensities in the infrared spectrum of the ethyl radical
Authors: Pacansky J. , Schrader B.
Published: February 2003

The infrared spectra of matrix isolated ethyl radicals have been simulated by a normal coordinate calculation, fitting the vibrational frequencies of all, partially and fully deuterated species CH3CH2⋅, CH3CD2⋅, CD3CH2⋅, and CD3CD2⋅, together with the calculation of the IR intensities by modified CNDO and INDO open shell procedures. A good reproduction of the observed spectra is in agreement with the theoretical molecular geometry and the special structural features shown by deviations of the force constants for the ethyl radical compared to that of related saturated and unsaturated hydrocarbons.
Photodissociation of HCHO in air: CO and H2 quantum yields at 220 and 300 K
Authors: Moortgat Geert K. , Seiler Wolfgang , Warneck Peter
Published: February 2003

New data are presented for the quantum yields of CO and H2 produced in the photodecomposition of formaldehyde. The data were obtained at the two temperatures 220 and 300 K, as a function of wavelength in the spectral range 253–353 nm, and as a function of pressure at the wavelengths 339.3 and 353.1 nm. The influence of temperature occurs mainly on the quenching constants describing the pressure dependence. The results are compared with previous data and improved quantum yield curves are presented for applications in atmospheric chemistry.
A test of the semiclassical energy conserving trajectory technique for low energy electron transfer reactions
Authors: DePristo Andrew E.
Published: February 2003

A semiclassical procedure, quantum internal states plus classical translational path, is detailed. Enforcing conservation of total energy leads to a coupling of the time-dependent Schrödinger equation and Hamilton’s equations through the use of an instantaneous expectation value of the interaction potential in the latter. A general computer program was written to solve the resulting set of coupled first order differential equations. We present two detailed tests of the numerical accuracy. Applications to electron transfer in the symmetric O+2+O2 reaction are presented using two interaction potentials. The first is identical to that used in previous semiclassical calculations. Comparison of state-to-state cross sections from the present and previous semiclassical calculations shows essentially no agreement, even though the dynamical equations, basis set, and interaction potential are identical in both studies. The second interaction potential is identical to that used in a previous exact quantal calculation. Comparison of state-to-state transition probabilites at 1 eV kinetic energy from the present semiclassical and previous quantal calculations shows good agreement for the resonant and near-resonant channels only. At 8 and 36 eV kinetic energies, the present semiclassical state-to-state cross sections are in excellent agreement with the quantal ones for all channels. These results demonstrate the quantitative accuracy of the semiclassical energy conserving trajectory approach to charge transfer even at low kinetic energies.
Diffusion controlled colloidal growth rates for nonspherical clustersa)
Authors: Deutch J. M. , Meakin Paul
Published: February 2003

The recent simulation results for particle cluster size R as a function of particle number N are combined with the classical theory of colloid growth by diffusion to arrive at a new rate law for the growth rate. In contrast to the classical prediction R2(t)∼t when N∼Rd, we find the simulation result N∼Rηd leads to Rα(t)∼t with α=2−(η−1)d.
A molecular dynamics simulation of the vitreous silica surface
Authors: Garofalini S. H.
Published: February 2003

The molecular dynamics (MD) computer simulation technique was used to simulate the vitreous silica (v-SiO2) surface. A modified Born–Mayer–Huggins potential function was used in the simulation; periodic boundary conditions in two dimensions (x and y) only were used to create a thin slab with free surfaces in the z directions. Radial distribution functions and bond angle distributions for interior regions and for surface regions (within several angstroms of the surface) were evaluated. In accordance with our understanding of the v-SiO2 surface, the MD simulation generated a surface in which the oxygen atoms rather than the silicon atoms predominated at the outer surface. Nonbridging oxygen (NBO) as well as bridging oxygen were found at the surface. The NBO–Si interatomic spacing was found to be about 0.08 Å less than the normal bridging O–Si spacing and is in accordance with calculations made from spectroscopic data of defects in v-SiO2. Also an additional peak at 110° was observed in the Si–O–Si bond angle distribution for bonds within several angstroms of the surface. These smaller bond angles indicate the presence of the expected strained siloxane bonds which can occur at the surface. The results of this work are significant in that they indicate the applicability of using the molecular dynamics technique to accurately simulate the v-SiO2 surface.
Semiclassical vibrational spectra for diatomic molecules: Application to HF, CO, and NO
Authors: Stine J. R. , Noid D. W.
Published: February 2003

Semiclassical dipole-moment matrix elements are calculated for HF, CO, and NO using realistic dipole-moment functions, and are in excellent agreement with those calculated from quantum mechanics. The first two cases have the Δn=1 vibrational transition exhibiting a maximum at excited vibrational states and then becoming nearly forbidden at higher vibrational states; in this region, the Δn=2 transition becomes more probable than the Δn=1 transition. This behavior is also expected for NO or for any heteronuclear diatomic molecule whose dipole moment has a maximum at an internuclear distance greater than re.
Equivalence between perturbatively calculated transition moments
Authors: Jo/rgensen Poul , Oddershede Jens
Published: February 2003

We have shown that equivalence between dipole length and dipole velocity transition moments is obtained in an order by order polarization propagator calculation of excitation energies and transition moments. Also, the Thomas–Reiche–Kuhn sum rule is shown to be fulfilled order by order. Numerical examples show good agreement between dipole length and velocity transition moments both in first and second order polarization propagator calculations using finite basis sets with approximately 50 Slater type orbitals for atoms and diatomic molecules.
Photoconductivity in crystals of charge-transfer complex anthracene–tetracyanobenzene
Authors: Samoc Marek , Williams Digby F.
Published: February 2003

Photoconduction in crystals of a CT complex anthracene–tetracyanobenzene has been studied employing a dc method and pulsed excitation from a dye laser. It has been found that the photogeneration in A–TCNB involves thermal dissociation of the lowest-lying CT state which is also responsible for the crystal fluorescence. The Onsager model of the dissociation of the geminate pair of charges has been found to be inappropriate for the quantitative description of the photogeneration efficiency. The generalized model of Hong and Noolandi is more physically justified, but a description in terms of a simple kinetical scheme is found to be sufficient to explain the general features of the photogeneration in anthracene–TCNB.
Pair correlations near a hard wall: An analytical theory
Authors: Giaquinta P. V. , Parrinello M.
Published: February 2003

A new integral equation for an average of the pair correlation function in the presence of a hard wall is derived. This equation is solved analytically for a fluid of hard spheres in the presence of an infinitely repulsive wall. The results compare favorably with Monte Carlo calculations.
A theory of the anomalous thermodynamic properties of liquid water
Authors: Dahl Larry W. , Andersen Hans C.
Published: February 2003

A theory of the anomalous thermodynamic properties of liquid water is presented. The theory is based on a simple model intermolecular potential that contains short-ranged repulsions, hydrogen-bonding attractions, and longer ranged interactions. Cluster theory approximations derived in the previous paper [J. Chem. Phys. 77,1962 (1982)] are used to calculate the thermodynamic energy and free energy for the model fluid as a function of temperature and density. The model fluid is found to have many of the anomalous properties of real water, such as a density maximum at low pressure, a compressibility minimum, and high heat capacity. The model is not a quantitatively accurate one for real water, nor does it have the anomalous properties observed for supercooled water. The reasons for the anomalies of the model fluid are discussed in order to shed some light on the physical basis for these anomalies in real water.
Concentration dependence of friction coefficients for polymer chains in solution
Authors: Freed Karl F.
Published: February 2003

The multiple scattering formalism is used to provide a theory of the concentration dependence of the cooperative and self-translational friction coefficients for polymer chains in solution. The specific case of Gaussian chains is analyzed, but extensions to good solutions can be made using previously described models. We stress the importance of dynamical contributions to the effective screened concentration dependent hydrodynamic interactions, which have been omitted in previous algebraically tractable static approximations. The theory is represented explicitly as an expansion in concentration and as an expansion in the effective medium representation. Explicit calculation of the leading concentration dependence is given in the following paper, fully including the dynamical effective hydrodynamic interactions.
Multiple scattering theory calculation of the concentration dependence of the tracer and cooperative friction coefficients for Gaussian polymer chains
Authors: Perico Angelo , Freed Karl F.
Published: February 2003

The cooperative and tracer translational friction coefficients of Gaussian polymer chains are evaluated through order c in concentration using the full dynamic bead-to-bead space time correlation functions within the framework of a recent reformulation involving a discrete bead-type model of the multiple scattering theory for the concentration dependence of the viscoelastic properties of polymer solutions. It is shown that, using preaveraging approximation of the Oseen tensor, the order c cooperative translational friction coefficient differs from the tracer friction coefficient due to a higher order contribution in the multiple scattering expansion, one varying as the inverse seventh power of the separations between chains. The leading term for the cooperative translational friction coefficient of order c, equal to the tracer one, is calculated to be 0.62, about 10%–25% smaller than recent experimental results obtained in sedimentation experiments. The difference may be due, in part, to the higher terms in the multiple scattering expansion which turn out to be very difficult to compute even in a very rough approximation.