bluuebunny/crossref_metadata_2025 huggingface dataset:
On the rotational angular momentum polarization in N+2–He. Classical trajectory and hard-ellipsoid model calculations
Authors: Follmeg Bernd , Werner Hans-Joachim , Rosmus Pavel
Published: July 2002

Classical trajectory and hard-ellipsoid methods are used to investigate collision-induced rotational alignment effects in N+2–He. Classical total, mf-resolved, and tensor cross sections for collision-induced rotational transitions are presented. Comparison of classical trajectory and quantum closed-coupled results show that total rotational inelastic cross sections are in good agreement, while mf-resolved and tensor cross sections agree only semiquantitatively. Velocity-averaged alignment parameters for N+2 ions drifting in a helium buffer gas are computed using a hard-ellipsoid model and a semiempirical two-dimensional velocity distribution. The alignment parameters are found to be smaller than the experimental values but lie in the range of the parameters obtained from a former quantum closed-coupled analysis.
Hilbert space multireference coupled-cluster methods. I. The single and double excitation model
Authors: Kucharski Stanislaw A. , Bartlett Rodney J.
Published: July 2002

Employing a Hilbert space ansatz the multireference, coupled-cluster single and double (MRCCSD) excitation equations are derived and presented in a transparent diagrammatic form for a complete active multireference space. All possible excitation levels are considered to be in the complete active space through hextuples, for which the MRCCSD equations are shown to be complete. The equations apply for open- and closed-shell cases.
π-electron calculations of polarizabilities in conjugated systems
Authors: Villesuzanne A. , Hoarau J. , Ducasse L. , Olmedo L. , Hourquebie P.
Published: July 2002

Finite-field technique has been applied to the calculation of π molecular polarizabilities within the Pariser–Parr–Pople Hamiltonian. This formalism allows to analyze large oligomers containing up to 400 atoms, and asymptotic behaviors may be attained in some cases. We have investigated the role of the polymer size, the backbone geometries, the presence of neutral and charged defects (solitons, polarons, bipolarons), the chemical nature of the monomeric unit on the polarizabilities. Generally, the polarizabilities are not deeply modified by a change of the geometrical characteristics, and may lead to huge values for sufficiently large chains. Besides, the charge of the system is the leading factor which determines the values of this property. The evolution of the polarizability with the number N of π atomic centers, αu depends largely on the charge, and on the defect. For the neutral systems, the polarizability per monomeric unit αu increases smoothly, and then exhibits an asymptotic behavior with N. For polymers with a defect, this variation is different: αu first increases with N, reaches a maximum αu max for Nmax, and finally tends to an asymptotic value. The values of αu max for Nmax depend on the type of defect (soliton vs polaron) and are rather sensitive to the dependence of the first-neighbor one-electron interaction with the interatomic distance. A large number of results on polyacetylene and polyheterocycles shows that there exists a simple law between the polarizability and the electronic gap, independently of the type of the monomeric unit. But this relation shows a deviation from a simple proportionality behavior, as soon as α reaches large values (as for example in polymers with defects).
Relaxation of the first B
u excited state in linear polyenes: From trans-butadiene to polyacetylene
Authors: Shuai Z. , Brédas J. L.
Published: July 2002

We present a multiphonon approach to analyze the lattice relaxation in the first Bu excited state of linear polyenes, from trans-butadiene to polyacetylene. Within a single-electron theory including bond-length bond-order relationship, we optimize the geometries of both the ground state and the first excited state and characterize the geometry of the excited state in terms of the formation of soliton–antisoliton pairs (bipolarons). The vibrational modes are simply taken as corresponding to soliton translation and amplitude vibrations (dimerization modes) within the harmonic approximation. Photoabsorption experiments carried out on a series of linear polyenes, β-carotene, polyenals/polyenones, and polyacetylene are discussed. The Huang–Rhys factor S and the relaxation energy are calculated for polyenes with various lengths. A major result is that we find that the S value first tends to decrease with increasing chain length but after some critical length the evolution completely modifies and S starts increasing. This feature is in good agreement with the experimental data and, in the case of polyacetylene, we are able to reproduce properly the experimental subgap absorption tail. The vibronic behavior for short chains is thus markedly different from that of long chains. We therefore caution that extrapolations based on observations on short polyenes might be invalid for the polymer.
Chord-length and free-path distribution functions for many-body systems
Authors: Lu Binglin , Torquato S.
Published: July 2002

We study fundamental morphological descriptors of disordered media (e.g., heterogeneous materials, liquids, and amorphous solids): the chord-length distribution function p(z) and the free-path distribution function p(z,a). For concreteness, we will speak in the language of heterogeneous materials composed of two different materials or ‘‘phases.’’ The probability density function p(z) describes the distribution of chord lengths in the sample and is of great interest in stereology. For example, the first moment of p(z) is the ‘‘mean intercept length’’ or ‘‘mean chord length.’’ The chord-length distribution function is of importance in transport phenomena and problems involving ‘‘discrete free paths’’ of point particles (e.g., Knudsen diffusion and radiative transport). The free-path distribution function p(z,a) takes into account the finite size of a simple particle of radius a undergoing discrete free-path motion in the heterogeneous material and we show that it is actually the chord-length distribution function for the system in which the ‘‘pore space’’ is the space available to a finite-sized particle of radius a. Thus it is shown that p(z)=p(z,0). We demonstrate that the functions p(z) and p(z,a) are related to another fundamentally important morphological descriptor of disordered media, namely, the so-called lineal-path function L(z) studied by us in previous work [Phys. Rev. A 45, 922 (1992)]. The lineal path function gives the probability of finding a line segment of length z wholly in one of the ‘‘phases’’ when randomly thrown into the sample. We derive exact series representations of the chord-length and free-path distribution functions for systems of spheres with a polydispersivity in size in arbitrary dimension D. For the special case of spatially uncorrelated spheres (i.e., fully penetrable spheres) we evaluate exactly the aforementioned functions, the mean chord length, and the mean free path. We also obtain corresponding analytical formulas for the case of mutually impenetrable (i.e., spatially correlated) polydispersed spheres.
Pure rotational spectrum and structure of the benzene–CO van der Waals complex
Authors: Brupbacher Th. , Bauder A.
Published: July 2002

Pure rotational transitions of four isotopic species of the benzene–CO van der Waals complex have been observed in the microwave region between 8–18 GHz. Double resonance experiments on the parent species have been used to confirm the assignment of the spectrum resembling that of a symmetric top. The structure of the complex, with CO above the plane of benzene, has been determined from the measured moments of inertia for the isotopic species. Only an upper limit for the barrier to internal rotation of CO may be derived in the absence of transitions in excited internal rotation states.
Photodissociation of methanol at 193.3 nm: Translational energy release spectra
Authors: Wen Y. , Segall J. , Dulligan M. , Wittig C.
Published: July 2002

Center-of-mass translational energy distributions of the dominant primary products resulting from 193.3 nm excitation of jet-cooled CH3OH, CH3OD, and CD3OH were obtained by using the high-n Rydberg time-of-flight (HRTOF) technique. The appearance threshold in the HRTOF spectrum yields a bond dissociation energy, D0(CH3O–H), of 105±1 kcal mol−1, in agreement with recent literature values. Translational energy release spectra from the three isotopomers exhibit progressions of 950±100 cm−1, which are attributed to excitation in the ν3 O–CH3 stretch of the methoxy product. The progressions peak at v=1, with population out to at least v=5. This differs from the results of a recent wave packet dynamics study on a calculated excited state potential energy surface [Marston et al., J. Chem. Phys. 98, 4718 (1993)], which predicted no O–CH3 stretch excitation in the methoxy fragment following photolysis of ground state methanol. The spatial anisotropy of the fragments (β∼−0.7) implies a dissociation time ≤1 ps. The impulsive model for rotational excitation is compared to the unresolved rotational contour of the vibrational peaks in the translational energy release spectra and is found to underestimate the extent of rotational excitation, though the model correctly predicts the increase in contour width observed for the O-deuterated species. The unresolved rotational contours are fit empirically. The inferred vibrational energy distributions are discussed in terms of a simple Franck–Condon model for the pseudotriatomic, Me–O–H. Implications of the vibrational and rotational photofragment distributions for the full 1 1A″ surface are discussed.
Measurement of the vibrational Zeeman effect for HCF3 using magnetic vibrational circular dichroism
Authors: Wang Baoliang , Keiderling Timothy A.
Published: July 2002

Magnetic vibrational circular dichroism (MVCD) for different vibrational modes of HCF3 have been measured. The vibrational Zeeman effect for the E-symmetry v4 and v5 modes have been determined for the first time from the solution and gas phase MVCD and absorption spectra. An analysis of the vibrational Zeeman effect for the other E-symmetry mode, v6, has been derived from the v3+v6 combination band MVCD and absorption spectra based on the vibronic coupling model. Comparisons of these results to predictions of different theoretical models have been used to analyze the sources of the observed Zeeman effect.
Study of the LiHg excimer: Blue–green bands
Authors: Gruber D. , Musso M. , Windholz L. , Gleichmann M. , Hess B. A. , Fuso F. , Allegrini M.
Published: July 2002

We report for the first time the production of the LiHg excimer by a photochemical reaction of excited Li2 molecules with Hg(6s2 1S0). Excitation energy was provided by a cw ultraviolet (UV)-argon ion laser operating in single-line and in multi-line mode or by pulsed XeCl excimer laser at 3080 Å. We observed bound-free emission of LiHg in the form of diffuse bands peaking at 4450 and at 4670 Å. The latter was structured by bound–bound emission lines. Relativistic ab initio calculations for the potential energy curves of LiHg and for the relevant dipole transition moments were performed. Using these results, an interpretation of the observed emission is presented.
Frustration-limited clusters in liquids
Authors: Kivelson Steven A. , Zhao Xiaolin , Kivelson Daniel , Fischer Thomas M. , Knobler Charles M.
Published: July 2002

We present a continuum theory of frustration-limited clusters in one-component glass-forming liquids that accounts, in part, for the recently reported [Fischer et al., J. Non-Cryst. Solids, 131–133, 134 (1991)], and quite unexpected, presence in simple glass-forming liquids of stable clusters at low temperatures (T) and the even less expected persistence for very long times of these clusters at higher T’s. The model is based on the idea that there is a local structure that is energetically preferred over simple crystalline packing, which is strained (frustrated) over large distances; although in a curved space the preferred packing could lead to ‘‘ideal’’ crystallization at temperatures that are usually above the actual freezing temperature, in ‘‘flat’’ space this transition is narrowly avoided. We are led to a new ansatz for the T dependence of the viscosity, which permits us to collapse data for many liquids onto a universal curve.
Many-body methods for excited state potential energy surfaces. II. Analytic second derivatives for excited state energies in the equation-of-motion coupled cluster method
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

Two distinct theoretical formalisms are developed for evaluating second derivatives of the energy analytically within the equation-of-motion coupled cluster approximation for excited electronic states (EOMEE-CC). In the first approach, both perturbations are treated equivalently. In the alternative formulation, the final operator expression is not symmetric with respect to interchange of the perturbations, and calculation of the second derivative requires that four systems of linear equations be solved for the first-order response of wave function parameters. However, only two systems need to be solved when the symmetric strategy is followed. While the symmetric approach superficially appears to be both more elegant and better suited for practical calculations, analysis shows that the former assertion is open to question and the latter only conditionally true. In particular, the asymmetric formulation is shown to be the preferred choice for all cases in which a large number of perturbations is involved. This is a rather important conclusion that holds not only for the EOMEE-CC method, but also for CC treatments of the electronic ground state and their finite-order many-body perturbation theory approximations.
Semiclassical calculations of tunneling splitting in malonaldehyde
Authors: Sewell Thomas D. , Guo Yin , Thompson Donald L.
Published: July 2002

We have devised a semiclassical procedure based on the Makri–Miller [J. Chem. Phys. 91, 4026 (1989)] model for calculating the eigenvalue splitting in many-atom systems and have used it to calculate the ground-state splitting in several isotopomers of malonaldehyde. A potential-energy surface that includes all twenty-one vibrational degrees of freedom was constructed based on the available theoretical and experimental information. The results for calculations in which all atoms are allowed full three-dimensional motion are in good agreement with the experimentally measured values. Restricting the molecular motion to a plane leads to an increase in the splitting due to a decrease in the average height and width of the barrier to tunneling when the molecule is not allowed to vibrate transverse to the molecular plane. Low energy mode-specific excitations were used to study the sensitivity of the splitting to the motions of heavy atoms. The results show that the heavy atom motions have significant influence on the tunneling. This study demonstrates that simple semiclassical methods can be used to treat proton tunneling in large systems.
Calculation of nuclear magnetic resonance shieldings and magnetic susceptibilities using multiconfiguration Hartree–Fock wave functions and local gauge origins
Authors: van Wüllen Christoph , Kutzelnigg Werner
Published: July 2002

A multiconfiguration generalization of the individual gauge for localized orbitals (IGLO) method for the calculation of magnetic properties such as susceptibilities and nuclear magnetic resonance chemical shifts is presented in detail. This includes a rederivation of the IGLO method by means of a nonlocal gauge transformation. The method is most successfully applied if strong (static) correlation effects have to be taken into account.
Ion–ion correlations in quenched disordered media
Authors: Bratko D. , Chakraborty A. K.
Published: July 2002

Monte Carlo simulation based on Widom’s test particle method is used to study the behavior of dilute ionic solutes in a quenched disordered medium with ionized obstacles. The structure of the medium is assumed to correspond to that of an ionic fluid equilibrated at certain prequenching temperature Tq and permittivity εq different from the corresponding values at the conditions of observation. The correlations among solute ions display qualitative differences from those observed in annealed systems. The ions of equal sign are attracted to domains of the disordered material characterized by charge opposite to that of the ions. The attraction of ions of the same sign to the same domains results in an apparent attractive contribution to the disorder-averaged interionic potential. At distances sufficiently exceeding the screening length pertaining to the ionized obstacles at the prequenching conditions, the disorder-induced term prevails over the direct Coulombic repulsion and a net attraction is observed. A similar mechanism leads to a long-ranged repulsion between oppositely charged ions. These findings are in agreement with earlier calculations of the disorder-averaged ion–ion potentials based on the asymptotic Debye–Hückel description of the disordered medium. The simulations are also used to estimate the effects of the medium on thermodynamic properties of the embedded ionic solute. The energies and activity coefficients are found to decrease with the product of the prequenching temperature and permittivity, a phenomenon explained in terms of the increase of the potential fluctuations in the disordered medium with growing εqTq. In spite of its global electroneutrality, the quenched medium displays a strong selectivity with respect to the valency of the solute, the tendency towards absorption increasing with the charge of the ions. The selectivity rapidly increases with increasing value of the characteristic product of the prequenching temperature and the permittivity εqTq.
On the origin of size inconsistency of the second-order state-specific effective Hamiltonian method
Authors: Heully J. L. , Malrieu J. P. , Zaitsevskii A.
Published: July 2002

The deviations of the second-order state-specific effective Hamiltonian method from the strict size consistency are analyzed. Provided that complete or separable model spaces are used, these deviations can be suppressed by a proper choice of nonuniform shifts of energy denominators.
Dynamics for the Cl+C2H6→HCl+C2H5 reaction examined through state-specific angular distributions
Authors: Kandel S. Alex , Rakitzis T. Peter , Lev-On Topaz , Zare Richard N.
Published: July 2002

Photolysis of Cl2 initiates the title reaction at a sharply defined collision energy of 0.24±0.03 eV. Nascent product rotational state distributions for HCl (v=0) are determined using resonance enhanced multiphoton ionization (REMPI), center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the C2H5 product is deduced from the dependence of the core-extracted signal on the photolysis polarization. The HCl product has little rotational excitation, but the scattering distribution is nearly isotropic. Although seemingly contradictory, both of these features can be accounted for by using the simple line-of-centers model presented to explain earlier results for the Cl+CH4 reaction. In contrast to the Cl+CH4 reaction, the data suggest that the Cl+C2H6 reaction proceeds through a loosely constrained transition-state geometry. The reactions of atomic chlorine with ethane, C2H6, and perdeuteroethane, C2D6, yield virtually identical results. These findings, along with the low energy deposited by the reaction into the ethyl product (200±120 cm−1), demonstrate that the alkyl fragment acts largely as a spectator in this hydrogen abstraction reaction.
Coupled-cluster calculations of the excitation energies of benzene and the azabenzenes
Authors: Del Bene Janet E. , Watts John D. , Bartlett Rodney J.
Published: July 2002

A series of equation-of-motion coupled-cluster (EOM-CC) calculations of the vertical excitation energies of benzene, pyridine, pyrazine, pyrimidine, pyridazine, symmetric triazine, and symmetric tetrazine have been performed. Single and double excitations have been included fully, and a noniterative approximation has been used to estimate triple excitation effects [the EOM-CCSD(T̃) method]. The basis set contains polarization functions and has reasonable diffuseness. Comparison is made with experimental data and second-order perturbation theory complete active space (CASPT2) theoretical data. The average EOM-CCSD(T̃) error for π→π* transitions is 0.11 eV and the error for n→π* transitions is 0.15 eV. Based on these small errors, several uncertain assignments for pyrazine and pyrimidine are substantiated.
Equilibrium yield for helium incorporation into buckminsterfullerene: Quantum-chemical evaluation
Authors: Patchkovskii S. , Thiel W.
Published: July 2002

The binding energy and equilibrium constant for the endohedral He@C60 compound have been determined from ab initio and density functional (DFT) calculations. Very large grids for the numerical integration are necessary to converge the DFT results to within 0.1 kcal/mol. Gradient-corrected DFT methods incorrectly predict He@C60 to be less stable than He+C60. At the highest ab initio level employed, i.e., second-order Mo/ller–Plesset perturbation theory (MP2) with extended basis sets and counterpoise corrections, He@C60 is bound by 2.0 kcal/mol. The equilibrium constant for He incorporation into C60 has been evaluated from Hartree–Fock and DFT interaction potentials adjusted to reproduce the MP2 binding energy. Computed equilibrium yields at 3000 atm and 900 K exceed 10%, compared with 0.1% observed in the experiment, which indicates that suitable catalysts could increase the observed yield significantly.
Quasielastic light scattering study of ternary polymer solutions of polystyrene and poly(methyl methacrylate) in dioxane
Authors: Sun Z. , Wang C. H.
Published: July 2002

The dynamic behavior of the mixture polystyrene (PS)–poly(methyl methacrylate) (PMMA)–dioxane in the semidilute concentration regime has been investigated by quasielastic light scattering. Two types of PS–PMMA–dioxane solution have been studied: One at a fixed composition but with different total polymer concentrations, and the other at a fixed total polymer concentration but with different compositions. For each solution, two distinct modes have been observed in the autocorrelation function of the scattered light; their relaxation times and relative amplitudes are studied as functions of composition and total polymer concentration. The present method differ from the previous one by not requiring one polymer to be isorefractive with solvent. By using the experimental results and with the help of theory, the self-diffusion coefficients of PS and PMMA and the Flory–Huggins interaction parameter χF have been deduced. The χF values obtained in this work are in good agreement with that obtained by other methods.
A Knudsen effusion mass spectrometric study of the molecule Si3N
Authors: Gingerich Karl A. , Viswanathan R. , Schmude, Jr. Richard W.
Published: July 2002

Condensed phase mixtures (Ge+Si+Si3N4) and (Si+Si3N4) were evaporated from boron nitride Knudsen cells and the partial pressures of Si3N(g),Si2NSi2(g), and Si(g) measured with a mass spectrometer at temperatures between 1770 and 2000 K. The thermal functions for Si3N(g) were calculated from theoretical molecular constants, and the enthalpy change for the dissociation reaction: Si3N(g)=Si2N(g)+Si(g) determined. With the revised atomization enthalpy of Si2N(g), ΔatomHmo=1011±12 kJ mol−1 at T=0 K and 1020±12 kJ mol−1 at T=298.15 K, the atomization enthalpy of Si3N(g) was derived as 1298±19 kJ mol−1 at T=0 and 1312±19 kJ mol−1 at T=298.15 K. These values in combination with the enthalpies of formation of Si(g) and N(g) yielded the enthalpies of formation ΔfHmo at T=298.15 K: 352±15 kJ mol−1 for Si2N(g), and 511±22 kJ mol−1 for Si3N(g). Experimental and theoretical bond dissociation energies have been compared and discussed, indicating a very strong bonding of nitrogen to Si3.
Local orientational correlations and short time anisotropic motion in molecular liquids: Computer simulations of liquid CO2
Authors: Fedchenia I. I. , Schröder J.
Published: July 2002

Molecular dynamic (MD) simulations of liquid CO2 at 250 MPa pressure and room temperature have been performed using a flexible model potential. A detailed analysis of the data reveal the full three-dimensional local structure of coordination shells that exhibit significant deviations from spherical symmetry with strong angular correlations among the molecules which form the inner coordination shell of the local liquid structure. Structures resembling T-shaped and offset-parallel CO2 dimers similar to those found in molecular beam and low temperature experiments have been identified, the T-shaped dimer having higher probability to be formed than the offset-parallel configuration. Local motion on short time scale is found to be different along three principal directions of a local coordinate frame.
Ab initio theoretical study of the methyl and phosphine torsion modes in ethylphosphine
Authors: Smeyers Yves G. , Meléndez F. J. , Senent M. Luisa
Published: July 2002

In this paper, the far infrared (FIR) methyl and phosphine torsional frequencies and intensities are determined theoretically in ethylphosphine from ab initio calculations. For this purpose, the potential energy function for the double rotation of the methyl and phosphine groups in the electronic ground state is determined in a standard calculation by using the MP2/RHF and a6-31G(3df,p) basis set, with full optimization of the geometry. The numerical results are fitted to a symmetry adapted analytical form and introduced together with the kinetic parameters into the Hamiltonian operator. The Schrödinger equation for these two motions is solved by developing the solutions on the basis of products of trigonometric functions. From the energy levels, the torsional functions and the dipole moment variations the FIR spectrum is synthesized. A new assignment is proposed for some trans-trans and the gauche-gauche transitions between the phosphine levels. Additional transitions between the methyl levels are also proposed. It is concluded that a two dimensional calculation is indispensable for reproducing the FIR torsional spectrum in the region from 200 to 140  cm−1.
Calculation of the reaction field due to off-center point multipoles
Authors: Kong Yong , Ponder Jay W.
Published: July 2002

Two general methods for calculating the reaction field generated by a set of off-center point multipoles in a spherical cavity are presented. The methods are a generalization of Kirkwood’s original theory for an arbitrary charge distribution. A polytensor formulation, similar to that previously developed for direct multipole interactions, serves to organize the computation and allows straightforward extension to higher derivatives of reaction field energy and gradients of the potential. The computation is reduced to calculation of the Cartesian derivatives of biaxial harmonics. Recursive and explicit formulas are given for the calculation. As an example, the incorporation of reaction field effects in computation of induced dipole moments is discussed. The second procedure, the central multipole method, scales linearly in calculation time with the size of the system. Methods to obtain derivatives analytically based on this method are also described. Our developments allow use of reaction field energy terms with atomic multipole-based empirical potential energy functions. Both methods show particular promise for use in simulation of heterogeneous systems, such as biopolymers, where the remainder of the cavity can be filled explicitly with solvent.
Brownian dynamics simulation of diffusion-limited polymerization of rodlike molecules: Anisotropic translation diffusion
Authors: Srinivasalu Gupta J. , Khakhar D. V.
Published: July 2002

Step-growth polymerization of rodlike molecules is qualitatively different from flexible molecules, because rotational and translational diffusion limitations result in a rate of reaction which is dependent on the molecular weights of the reacting oligomers. An understanding of the kinetics of polymerization is important for the manufacture of such polymers which have many applications. The theoretical basis and computation details of the pairwise Brownian dynamics method [Northrup et al., J. Chem. Phys. 80, 1517 (1984)] to determine the effective rate constant for reaction between rodlike molecules are presented. In this method the effective rate constant is obtained in terms of the first visit flux (for which an analytical expression is derived) and the reaction probability (which is obtained using Brownian dynamics simulations). A simple derivation for finite domain correction for the simulations is presented, which explicitly accounts for the spatial and orientational variations of the reaction probability. Computations are presented to validate assumptions of the technique. The computed rate constants decrease with decreasing rotational diffusivity to approach an asymptotic value, and decrease with decreasing values of the translational diffusivity perpendicular to the rod axis. The computations show that the effective rate constants obtained for the case of dilute solutions when ratio of the translational diffusivities perpendicular and parallel to the rod axis is equal to 12, is close to the case of isotropic translational diffusion.
Photodissociation of acrylonitrile at 193 nm: A photofragment translational spectroscopy study using synchrotron radiation for product photoionization
Authors: Blank David A. , Suits Arthur G. , Lee Yuan T. , North Simon W. , Hall Gregory E.
Published: July 2002

We have investigated the photodissociation of acrylonitrile (H2CCHCN) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used tunable vacuum ultraviolet synchrotron radiation for product photoionization. We have identified four primary dissociation channels including atomic and molecular hydrogen elimination, HCN elimination, and CN elimination. There is significant evidence that all of the dissociation channels occur on the ground electronic surface following internal conversion from the initially optically prepared state. The product translational energy distributions reflect near statistical simple bond rupture for the radical dissociation channels, while substantial recombination barriers mediate the translational energy release for the two molecular elimination channels. Photoionization onsets have provided additional insight into the chemical identities of the products and their internal energy content.
Influence of attractive forces on the solvent mediated potential of mean force between colloidal particles
Authors: Amokrane S.
Published: July 2002

The solvent induced potential of mean force between colloidal particles is computed by using the superposition approximation for the solvent–macrospheres triplet distribution function and the density functional theory for the solvent density profile at an isolated macrosphere. The influence of solute–solvent attractive forces is investigated for different diameter ratios and solvent bulk densities. For large colloidal particles, the mean force is shown to scale with the solvent–colloid hard core radius. At densities of liquid solvents, sufficient solvent–colloid attractions are found to strongly enhance the repulsive barriers in the potential of mean force and reduce the depth of the depletion well. At lower density, the mean force can become repulsive. Solvent–solvent attractions can suppress the oscillations in the potential and make it more attractive. These results are finally contrasted with those relative to purely repulsive interactions.
Calculation of electronic affinity and vertical detachment energy of the water dimer complex using the density functional theory
Authors: Bouteiller Y. , Desfrançois C. , Schermann J. P. , Latajka Z. , Silvi B.
Published: July 2002

Though the electron attachment on the water dimer has been observed, ab initio calculations performed at the Hartree-Fock and post Hartree-Fock levels do not succeed in predicting a bounded (H2O)2− anion. It is shown that the hybrid density functional approach (B3LYP) yields results in reasonable agreement with experiment, provided a systematic optimization of the basis sets in the variational sense. Neglecting the zero point energy (ZPE) correction, the adiabatic electron affinity (EA) is calculated to be 21.2 meV (30±2 experimentally) whereas the vertical detachment energy (VDE) is overestimated by about 10 meV (55.7 against 45). The ZPE correction has been estimated from the frequencies calculated at the harmonic level. It improves noticeably the EA and the VDE which corrected values are 35.6 and 41.9 meV, respectively. The sign and magnitude of the EA and VDE isotopic shifts between the hydrogenated and deuterated species are correctly predicted. The analysis of the electron density difference and of the spin density shows that the electron attachment occurs for one half between the two H2O molecules, the remaining being located outside in the dipole moment direction.
Density-functional study of hydration of sodium in water clusters
Authors: Ramaniah Lavanya M. , Bernasconi Marco , Parrinello Michele
Published: July 2002

The structures and hydration energies of small water clusters containing a sodium atom or ion are investigated within density-functional theory by using the Becke–Lee–Yang–Parr (BLYP) generalized gradient corrections to the local density approximation exchange and correlation energy, norm-conserving pseudopotentials, and a plane-wave expansion of Kohn–Sham orbitals. The hydration energies obtained for both neutral and ionized clusters, as well as the ionization potentials, are in good agreement with experiment and with available quantum-chemical calculations.
First spectroscopic investigation of the 4d transition metal monocarbide MoC
Authors: Brugh Dale J. , Ronningen Theodore J. , Morse Michael D.
Published: July 2002

The first optical spectroscopic investigation of MoC has revealed a complicated vibronic spectrum consisting of about 35 bands between 17 700 and 24 000 cm−1. Analysis has shown the ground state to be the Ω=0+ spinorbit component of a Σ3− state that derives from a 10σ211σ25π42δ2 configuration. The X 3Σ0+− rotational constant for Mo9812C was determined to be B0=0.553 640±0.000 055 cm−1, giving r0=1.687 719±0.000 084 Å. Consideration of spin-uncoupling effects in the X 3Σ− state requires that this value be revised to r0=1.6760 Å, which represents our best estimate of the true Mo–C bond length. Spectroscopic constants were also extracted for six other major isotopic modifications of MoC in this mass resolved experiment. All rotationally resolved transitions were found to originate from the ground state and terminate in electronic states with Ω=1. An attempt is made to classify the observed transitions into band systems, to rationalize the complexity of the spectrum, and to understand the bonding from a molecular orbital point of view.
Electron–phonon coupling in the one-dimensional crystals of negatively charged [18]annulene
Authors: Yoshizawa Kazunari , Kato Takashi , Yamabe Tokio
Published: July 2002

We discuss the Jahn–Teller effect and possible superconductivity in negatively charged [18]annulene, (CH)18, which is an interesting molecular system with a highly symmetric D6h geometry in the neutral state. Electron–phonon coupling constants in model one-dimensional crystals of (CH)18−, (CH)183−, (CD)18−, and (CD)183− are calculated and analyzed using the B3LYP density functional method of Becke and Lee, Yang, and Parr. The lowest two frequency modes (116 cm−1 and 405 cm−1) causing distortions of the C–C framework of [18]annulene and the highest frequency mode (3201 cm−1) of C–H stretching appear to give significant electron–phonon coupling constants. The computed coupling constants of the monoanion are larger than those of the trianion. Possible preconditions under which idealized one-dimensional crystals of (CH)18− and (CH)183− can exhibit superconductivity at finite temperature are considered.
Fluorescence quenching by reversible excitation transfer: Application of a hierarchy approach to a pseudo first-order model
Authors: Naumann Wolfgang
Published: July 2002

Fluorescence quenching by reversible excitation transfer is studied for a pseudo first-order model allowing only geminate-type back transfers from the acceptor to the donor. For the theoretical treatment a diffusion-kinetic hierarchy approach is applied derived on the base of reactive many-particle equations. It is demonstrated that the obtained nonlinear system of rate and pair equations is in the low concentration limit kinetically equivalent to a description by generalized rate equations (non-Markovian approach). By introducing an effective forward transfer coefficient, analogous to the definition of phenomenological rate coefficients for other type reversible reactions, compact and physically transparent formulas for the evolution of donor and acceptor concentrations during the quenching process are derived. The fluorescence quenching constant is explicitely given for a contact transfer mechanism. It is shown that for longer range transfers the calculation of the fluorescence constant calculation results in the solution of an integral equation. Some analytically solvable limiting cases are indicated and especially a Förster-type transfer is discussed.
Photodetachment studies of extended excited states in I−Xen clusters (n=1–54)
Authors: Becker Israela , Cheshnovsky Ori
Published: July 2002

We present a comprehensive experimental study of bound excited states in I−Xen clusters (n=1–54), using photoelectron spectroscopy and energy-dependent action spectroscopy. Starting at n=4, the electron detachment action spectra developed peaks lying in energy lower than the vertical binding energy. This behavior has been shown for both final spin states of the neutral iodine. It indicates the existence of bound electronic states extended over the xenon cluster. The peaks lying in energy below the J=1/2 detachment continuum of the iodine (n>4) were detected over the whole cluster size range of I−Xen using electron detachment action spectroscopy. For the bound states relating to the J=3/2 continuum, in the size range n=4–12, thermionic emission has marked the existence of bound states. For n>12, these states were stabilized and detected via two-photon excitations. The picture, unveiled from the above photoelectron and action spectra, is the gradual evolution of bound excited states on the xenon solvent cluster. The critical size for the binding of an excited electron is n=4. At the largest cluster interrogated, I−Xe54, the binding energy amounts to 170 meV. We compare these states to the ground electronic state of Xen− clusters, and discuss the differences between the experiments and model calculations.
Zero electron kinetic energy and threshold photodetachment spectroscopy of XenI− clusters (n=2–14): Binding, many-body effects, and structures
Authors: Lenzer Thomas , Furlanetto Michael R. , Pivonka Nicholas L. , Neumark Daniel M.
Published: July 2002

Xe n I − van der Waals clusters have been investigated by anion zero electron kinetic energy (ZEKE) and partially discriminated threshold photodetachment (PDTP) spectroscopy. The experiments yield size-dependent electron affinities (EAs) and electronic state splittings between the X, I, and II states accessed by photodetachment. Cluster minimum energy structures have been determined by extensive simulated annealing molecular dynamics calculations using Xe–I(−) pair potentials from anion ZEKE spectroscopy and various nonadditive terms. The EAs calculated without many-body effects overestimate the experimental EAs by up to 3000 cm−1. Repulsive many-body induction in the anion clusters is found to be the dominant nonadditive effect, though the attractive interaction between the iodide charge and the Xe2 exchange quadrupole is also important. Unique global minimum energy structures for the anion clusters arise from the influence of the many-body terms, yielding, e.g., arrangements with a closed shell of xenon atoms around the iodide anion for the clusters with n=12–14. The specific dependence of the EA curve on cluster size allows us to refine the absolute Xe–I bond lengths for the anion, X, I, and II state diatomic potentials to within ±0.05 Å.
Mass analyzed threshold ionization of phenol⋅CO: Intermolecular binding energies of a hydrogen-bonded complex
Authors: Haines Stephen R. , Dessent Caroline E. H. , Müller-Dethlefs Klaus
Published: July 2002

[ Phenol⋅CO ] + was studied using a combination of two-color resonant zero kinetic energy (ZEKE) spectroscopy and mass analyzed threshold ionization (MATI) spectroscopy to investigate the interaction of the CO ligand with a hydrogen-bonding cation. Vibrational progressions were observed in three intermolecular modes, the in-plane bend (42 cm−1), stretch (130 cm−1), and in-plane wag (160 cm−1), and are consistent with a planar hydrogen-bonded structure where the CO bonds through the carbon atom to the phenol OH group. Dissociation energies for the S0, S1, and D0 states were determined as 659±20, 849±20, and 2425±10 cm−1, respectively. The cationic and neutral dissociation energies of the phenol⋅CO complex are considerably stronger than those of phenol⋅N2, demonstrating the extent to which the larger quadrupole of CO affects the strength of binding.
Tunneling in jet-cooled 5-methyltropolone and 5-methyltropolone–OD. Coupling between internal rotation of methyl group and proton transfer
Authors: Nishi Kaori , Sekiya Hiroshi , Kawakami Hiroko , Mori Akira , Nishimura Yukio
Published: July 2002

The coupling of two large amplitude motions, the internal rotation of the methyl group and the intramolecular proton transfer, has been investigated for jet-cooled 5-methyltropolone, 5-methyltropolone–OD, and the 5-methyltropolone–(H2O)1 1:1 hydrogen-bonded complex by measuring the fluorescence excitation, dispersed fluorescence, and hole-burning spectra in the S1–S0 region. The vibronic bands in the excitation spectrum of 5-methyltropolone consist of four components originating from the transitions between the sublevels in the S1 and S0 states. The intensity of the bands, the frequencies, and the change in the stable conformation of the methyl group upon photoexcitation have been analyzed for 5-methyltropolone–(H2O)1 by calculating the one-dimensional periodic potential function, which provides the correlation between the internal rotational levels of 5-methyltropolone–(H2O)1 and the sublevels of 5-methyltropolone. It has been shown that the electronic transitions between the sublevels within the same symmetry are allowed in 5-methyltropolone. The tunneling splitting of the zero-point level in the S1 state is 2.2 cm−1 for 5-methyltropolone. The corresponding splitting for 5-methyltropolone–OD is less than 0.5 cm−1. A drastic decrease of the tunneling splitting for 5-methyltropolone as compared to that for tropolone (19.9 cm−1) is ascribed to a strong coupling between the two large amplitude motions in the S1 state. The existence of a similar coupling has been suggested in the S0 state of 5-methyltropolone. The excitation of the sublevel in the S1 state considerably promotes proton tunneling. This effect has been explained by the delocalization of the wave function of the internal rotation of the methyl group. The two-dimensional potential energy surface along the proton transfer coordinate and the rotational angle of the methyl group has been calculated to explain the effects of the coupling on proton tunneling.
The effect of intermolecular interactions on the electric properties of helium and argon. I. Ab initio calculation of the interaction induced polarizability and hyperpolarizability in He2 and Ar2
Authors: Hättig Christof , Larsen Helena , Olsen Jeppe , Jo/rgensen Poul , Koch Henrik , Fernández Berta , Rizzo Antonio
Published: July 2002

The frequency-dependent interaction induced polarizabilities and second hyperpolarizabilities are calculated for He2 at the coupled cluster singles and doubles and full configuration interaction levels and for Ar2 at the coupled cluster singles and doubles level. The frequency-dependence is approximated by a power series to second-order in the frequency arguments using Cauchy moments and hyperpolarizability dispersion coefficients. Using large correlation consistent basis sets, results close to the basis set limit are obtained. The computed curves for the interaction induced (hyper-) polarizabilities are tabulated for a range of internuclear distances. The data are employed in a companion paper to make for the first time a direct comparison between the experimentally determined pressure dependence of an ESHG hyperpolarizability and ab initio calculated hyperpolarizability second virial coefficients.
Noncommutative differential geometry and new models of gauge theory
Authors: Dubois-Violette Michel , Kerner Richard , Madore John
Published: July 2002

The noncommutative differential geometry of the algebra C∞(V)⊗Mn(C) of smooth Mn(C)-valued functions on a manifold V is investigated. For n≥2, the analog of Maxwell’s theory is constructed and interpreted as a field theory on V. It describes a U(n)–Yang–Mills field minimally coupled to a set of fields with values in the adjoint representation that interact among themselves through a quartic polynomial potential. The Euclidean action, which is positive, vanishes on exactly two distinct gauge orbits, which are interpreted as two vacua of the theory. In one of the corresponding vacuum sectors, the SU(n) part of the Yang–Mills field is massive. For the case n=2, analogies with the standard model of electroweak theory are pointed out. Finally, a brief description is provided of what happens if one starts from the analog of a general Yang–Mills theory instead of Maxwell’s theory, which is a particular case.
A thermodynamical approach to Eddington factors
Authors: Anile A. M. , Pennisi S. , Sammartino M.
Published: July 2002

Eddington factors are a common ingredient in many techniques for solving radiation hydrodynamics problems. Usually they are introduced in a phenomenological or ad hoc manner. In this paper a fundamental approach is devised for justifying Eddington factors on the basis of mathematical requirements arising from nonequilibrium thermodynamics.
On the generalization of the Boltzmann H-theorem for a spatially homogeneous Maxwell gas
Authors: Bobylev A. V. , Toscani G.
Published: July 2002

Sufficient conditions are given for a convex and isotropic functional that are monotonically decreasing in time along the solution of the spatially homogeneous Boltzmann equation for Maxwellian molecules. The conditions are valid for plane velocities and for axially symmetric solutions in any dimension. Explicit examples are given.
Comments on Good’s proposal for new rules of quantization
Authors: Navarro Miguel
Published: July 2002

In a recent paper [J. Math. Phys. 35, 3333 (1994)] Good postulated new rules of quantization, one of the major features of which is that the quantum evolution of the wave function is always given by ordinary differential equations. In this paper we analyze the proposal in some detail and discuss its viability and its relationship with the standard quantum theory. As a byproduct, a simple derivation of the ‘‘mass spectrum’’ for the Klein–Gordon field is presented, but it is also shown that there is a complete additional spectrum of negative ‘‘masses.’’ Finally, two major reasons are presented against the viability of this alternative proposal: (a) It does not lead to the correct energy spectrum for the hydrogen atom. (b) For field models, the standard quantum theory cannot be recovered from this alternative description.
Lie algebra of anomalously scaled fluctuations
Authors: Broidioi M. , Momont B. , Verbeure A.
Published: July 2002

For quantum lattice systems, it is proven that anomalously scaled fluctuations have a natural Lie algebra structure. The harmonic lattice in the ground state is given as an illustration of the general theorem.