bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical nonlinearities of glass doped with PbS nanocrystals
Authors: Tamulaitis G. , Gulbinas V. , Kodis G. , Dementjev A. , Valkunas L. , Motchalov I. , Raaben H.
Published: July 2002

Nonlinear absorption of glass doped with PbS nanocrystals is studied and application of this composite material as a saturable absorber for mode locking in lasers is discussed. By using time resolved absorption pump-probe investigation, bleaching due to filling of discrete states caused by quantum confinement in the nanocrystals as well as induced absorption are revealed and characterized. The origin of the induced absorption is discussed. Kinetics of the transient absorption is studied. Two components observed in the decay of the nonlinearities are observed and interpreted in terms of the carrier trapping.
Nonrelativistic Compton scattering in Furry’s picture. II. Bethe surface by means of the complex-coordinate method
Authors: Froelich Piotr , Flores-Riveros A. , Weyrich W.
Published: July 2002

Bethe surface as a means of characterizing the inelastic scattering of photons and electrons on atomic targets is discussed, and framed within the two-potential Furry’s picture of scattering theory. In particular, the cross section for inelastic photon scattering is considered, and its first distorted Born approximation is identified to be given in terms of Bethe surface along the path conserving energy and momentum transfer. The difficulties in obtaining accurate cross sections in situations where the energy transfer is close to the ionization threshold are indicated, and related to the so-called Compton defect. The method for calculating the inelastic photon scattering cross section, introduced in Part I of this work [J. Chem. Phys. 80, 5669 (1984)] is summarized. The scattering cross section, and the entire Bethe surface, is obtained by means of the L2 discretization of the continuum and implemented in terms of the complex-coordinate method, without explicit calculation of the final scattering waves. The method is tested for the case of photon scattering off the hydrogen atom. The results are encouraging, and may be relevant for applications of the complex-coordinate method to calculations of more general transition amplitudes. The method is predicted to be most useful in cases close to ionization threshold (e.g., appearance edges in Compton scattering, Compton defect). Although applied to the one-electron problem the procedure is readily applicable to many electron atoms.
Bonding of NH3 to Cu
Authors: Bauschlicher Charles W.
Published: July 2002

Chemisorption of NH3 into the threefold hollow of Cu(111) is considered using a cluster model, Cu18NH3, and the interaction is found to be repulsive. This result supports previous work for NH3/Cu(100) and NH3/Al(111), the combination of this work and previous work suggests that NH3 chemisorbs directly above a metal atom.
Elastic properties and equation of state of high pressure ice
Authors: Shaw George H.
Published: July 2002

Measurements of compressional and shear wave velocities at −25 °C at pressures up to about 0.7 GPa show that the elastic properties of the high pressure polymorphs of ice are significantly different from the values for the room pressure phase. Data were obtained from ice Ih, III, V, and VI. The bulk modulus decreases slightly when ice Ih transforms to ice III while the shear modulus increases by nearly 50%. Both moduli increase substantially going from ice III to ice V. The bulk modulus of ice VI at 0.625 GPa is more than 50% greater than ice Ih and the shear modulus is more than two and one half times as large as for ice Ih. Apart from the interest in these properties for a material as important as ice, the results may be used in interpreting the internal structure and orbital histories of the icy satellites of the outer planets.
Local density theory of ionic hyperpolarizability
Authors: Subbaswamy K. R. , Mahan G. D.
Published: July 2002

A self-consistent formulation for the computation of nonlinear susceptibilities within the local density approximation is presented. The formulation correctly takes account of wave function normalization. Numerical results for helium are presented as illustration.
On the connection between Fourier coefficient and Discretized Cartesian path integration
Authors: Coalson Rob D.
Published: July 2002

The relationship between so-called Discretized and Fourier coefficient formulations of Cartesian path integration is examined. In particular, an intimate connection between the two is established by rewriting the Discretized formulation in a manifestly Fourier-like way. This leads to improved understanding of both the limit behavior and the convergence properties of computational prescriptions based on the two formalisms. The performance of various prescriptions is compared with regard to calculation of on-diagonal statistical density matrix elements for a number of prototypical 1-d potentials. A consistent convergence order among these prescriptions is established.
Theory of initial yields of ions generated by electrons in binary mixtures. II
Authors: Inokuti Mitio , Eggarter Eugenia
Published: July 2002

Eggarter [J. Chem. Phys. 84, 6123 (1986)] showed a method of calculation of the yield of ionization in a mixture by electrons, and carried out a numerical solution for the Ar–H2 mixture. At sufficiently high electron energies, the yields NAr and NH2 of each component species closely follow the relation NAr/NH2 =K⋅CAr/CH2, where CAr and CH2 represent the concentration fractions, and K is a constant. We now present a theoretical interpretation of that relation, and show the microscopic meaning of the constant K; it is the ratio of the ionization cross sections of the two component species, evaluated for an electron of a certain kinetic energy, which is much greater than the ionization threshold energy. The interpretation rests on the fact that the Spencer–Fano electron degradation spectrum depends on electron energy only mildly and smoothly except near the initial source energy and near the first ionization threshold energy. The same relation should hold approximately for any combination of ordinary molecules, although for the Ar–H2 case the relation is obeyed especially close. We also discuss the connection of our treatment with earlier treatments based on certain assumptions about the radiation-energy partition among the mixture components.
Orbital stress and transition stress of diatomic molecules
Authors: Zhang Yongfeng , Hanson David M.
Published: July 2002

The orbital stress, which is the force of the electrons in a given molecular orbital opposing the repulsive stress (tension) between the nuclei, has been calculated for some diatomic molecules (N2, N+2, C2, O2, O+2, CO, CO+, NO, NO+, CF). The transition stress is defined as the difference between the orbital stress of an upper molecular orbital and that of a lower molecular orbital involved in an electronic transition. It is shown that the transition stress is a fundamental property of molecules. The bond lengths of various excited states are related to the transition stress and the bond length of the ground state. The rotational constants of excited states are linear functions of the transition stress, provided configuration interaction between the excited states is not very strong. The complexity of the vibrational structure in photoelectron spectra is related to the orbital stress. The theoretical results are in good agreement with experimental data. An analysis in terms of the orbital stress and the transition stress for polyatomic molecules may serve to predict the chemical relaxation paths that follow excitation of a core electron in a molecule and subsequent Auger decay.
Electronic states of the π-radical electron of a carbene chain end on a polydiacetylene chain: Comparison of the configuration model and the continuum model
Authors: Kollmar C. , Sixl H.
Published: July 2002

Electronic states of a single pz-radical electron moving on a polydiacetylene chain are described using a simplified version of the configuration model and a quantum mechanical continuum model. The two models are compared by a mathematical transition from the discrete sites of the configuration model to a continuous coordinate. It turns out that the energy difference between the butatriene and the acetylene structure per unit cell is related to the potential gradient of the continuum model. Moreover, the effective mass of the electron, which can be derived from the transfer integral of the configuration model, is about one-half of the real electron mass.
Fluorescence excitation and emission spectra of 1,5-dihydroxyanthraquinone-d2 in n-hexane at 10 K
Authors: Smulevich Giulietta , Foggi Paolo
Published: July 2002

The fluorescence excitation, between 430 and 505 nm, and emission, between 505 and 725 nm, spectra in n-hexane of 1,5-dihydroxyanthraquinone-d0 and -d2 at 10 K have been measured. Dual excitation and emission associated to excited state proton transfer were observed. Apart from the long wavelength emission, well resolved vibrational structures were obtained. A remarkable spectral shift (684 cm−1) of the origin of the high frequency transition was observed upon deuteration. The energy gaps between the transition origins both in excitation and emission as well as the isotopic shifts of the origins, were interpreted in terms of Lippincott–Schroeder asymmetric double minimum potential functions along the OH coordinate. An extra fluorescence occurs in the low frequency range, vanishing upon deuteration. It was explained as due to the ν(OH) stretching mode of the high frequency emission enhanced via vibronic coupling between the two ground states.
Nascent vibrational/rotational distribution produced by hydrogen atom recombination
Authors: Orel A. E.
Published: July 2002

The nascent vibrational/rotational distribution produced by the reaction H+H+H→H2(v, J)+H is calculated using resonance complex theory. These calculations are carried out using the accurate Siegbahn–Liu–Truhlar–Horowitz surface. Results are presented for the dependence of the total recombination rate coefficient on temperature and specific rate coefficients into individual vibrational/rotational states.
Investigation of finite size effects in a first order phase transition: High pressure Raman study of CdS microcrystallites
Authors: Variano B. F. , Schlotter N. E. , Hwang D. M. , Sandroff C. J.
Published: July 2002

We have used Raman scattering to explore finite size effects in the phase transition of 300 Å colloidal microcrystallites of CdS. By measuring the shifts in LO phonon frequencies as a function of pressure we conclude that these small microcrystallites undergo a well-defined first order phase transition similar to the one occurring in bulk CdS. However, the phase transformation in the colloids exhibits a smaller hysteresis, which we attribute to efficient annealing of defects in the high surface area colloidal material.
Polymer chain entanglements and brittle fracture
Authors: Mikos Antonios G. , Peppas Nikolaos A.
Published: July 2002

A stochastic model is presented to predict the molecular weight dependence of the polymer fracture energy and strength for polymers with molecular weights higher than the critical value corresponding to the onset of entanglements. A chain scission criterion is invoked for the polymer chain segments crossing the fracture plane and being entangled about it. The presence of dangling ends is shown to be responsible for the change of the fracture properties with molecular weight. The predictions of the model agree with experimental data of the fracture energy and strength of poly(methyl methacrylate) and polystyrene.
Critical appearance size of doubly charged Xe clusters revisited
Authors: Scheier P. , Walder G. , Stamatovic A. , Märk T.D.
Published: July 2002

In previous experiments on doubly charged Xe clusters, no Xe2+n cluster ions were observed for n less than 53 atoms and no Xe3+n for n less than 114 atoms. From recent computer simulations it was inferred that Xe2+51 ions have lifetimes ∼100 ps while Xe2+55 ions have lifetimes ∼10 μs in agreement with these previous experimental results. Using a high performance mass spectrometer we have repeated these experiments. In contrast to these earlier findings, we are here able to observe doubly charged Xe cluster ions with sizes down to n2=47 (at lifetimes of ∼400 μs). The critical appearance size for triply charged Xe clusters is n3=108. In accordance with a recently suggested sequential ionization mechanism for the production of stable doubly charged van der Waals cluster ions, the appearance energy of Xe2+n was found to lie well below the appearance energy of Xe2+ from Xe.
Neutron diffraction study of the intermolecular structure in liquid germanium and vanadium tetrachloride
Authors: van Tricht Jan B. , Sjoerdsma Jelle S.
Published: July 2002

Intermolecular scattering functions of liquid GeCl4 at 233 and 295 K and of liquid VCl4 at 295 K are determined by thermal neutron diffraction. Reference interaction site model (RISM) calculations with σCl =0.34 nm show good general correspondence with the experimental data. The RISM describes in much detail the difference between the two intermolecular scattering functions of GeCl4 at different thermodynamic states. The results for VCl4 are compared to experimental results by Gibson and Dore and to molecular dynamics calculations by Murad and Gubbins. The intermolecular scattering functions are tabulated.
Electronic spin resonance spectra of SiH3 radicals trapped in the low temperature matrix of nonmagnetic isotopes of xenon
Authors: Nakamura K. , Okamoto M. , Takayanagi T. , Kawachi T. , Shimokoshi K. , Sato S.
Published: July 2002

Xenon enriched with nonmagnetic isotopes, in which the content of magnetic isotopes is less than 0.4%, has been used as the low temperature matrix for the observation of electronic spin resonance (ESR) spectra of SiH3 radicals and H atoms. The SiH3 radicals were generated by the reaction of SiH4 with H atoms produced by the photolysis of HI. Well resolved proton hyperfine structures, which were smeared out into a broad singlet spectrum when natural xenon was used as the matrix, were observed. The ESR parameters were determined by numerical deconvolution and compared with those obtained for the radicals trapped in other matrices. Simple doublet spectrum of H atoms without superhyperfine structures was also observed after the photolysis of HI at 4.2 K.
Global potential-energy surfaces for H2Cl
Authors: Schwenke David W. , Tucker Susan C. , Steckler Rozeanne , Brown Franklin B. , Lynch Gillian C. , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

We present two new analytic potential-energy surfaces suitable for studying the competition between the abstraction reaction H+DCl→HD+Cl and the exchange reaction H+DCl→HCl+D. In the abstraction channel the surfaces are only slightly different from the Stern–Persky–Klein GSW surface, but the exchange barrier on both surfaces is raised by inclusion of a three-center term fitted to ab initio extended-basis-set multireference configuration interaction calculations with scaled external correlation. The two surfaces differ significantly only for the steepness of H–Cl–H bend potential. The exchange and abstraction saddle points are characterized by harmonic analysis for H2Cl, HDCl, and D2Cl, and we also compute vibrationally adiabatic barrier heights including anharmonicity. We also report thermal rate constants and activation energies for both reactions mentioned above.
Site selective spectroscopy and crystal field analysis of Eu3+ doped lanthanum–calcium–zirconium–silicon borate
Authors: Capobianco J. A. , Proulx P. P. , Raspa N. , Simkin D. J. , Krashkevich D.
Published: July 2002

The fluorescence of Eu3+ doped lanthanum–calcium–zirconium–silicon borate ceramic was studied at 77 and 300 K using laser excited site selective spectroscopy. The fluorescence spectrum excited at 514.532 nm reveals the presence of three distinct sites for Eu3+. The sites were assigned to Eu3+ substituting for lanthanum in a ninefold coordination site in LaBO3 and Eu3+ substituting for Ca2+ in six- and sevenfold coordination sites in Ca2B2O5. To assign the spectra to the definite sites a phenomenological crystal field analysis was conducted, using C2v symmetry. The crystal field for Eu3+ in Ca2B2O5 was found to be dominated by electrostatic effects. The large 7F1 level splitting and the value of the B20 parameter support the concept of strong ionic bonding between europium and oxygen in Ca2B2O5.
Comment on the interpretation of the spectral functions {a
j,k(z)} in dual Lanczos transformation theory
Authors: Wassam W. A. , Torres-Vega Go. , Balderas-López J. A.
Published: July 2002

We comment on the interpretation of the spectral functions {aj,k(z)} [J. Chem. Phys. 88, 1837 (1988)] appearing in dual Lanczos transformation theory. It is shown that these functions are not, in general, equivalent to the Laplace transforms of the matrix elements of the system propagator in the dual Lanczos representation. In fact, we show that these functions may be used to construct the Laplace transforms by performing a suitable contour integration. New and useful expressions for the Laplace transforms are obtained, which reveal that they differ from the spectral functions {aj,k(z)} by an analytic piece that is unimportant for time evolution computations but it is of critical importance in spectral density computations. The utility of our results is illustrated by constructing the appropriately corrected expressions for the Fourier–Laplace transforms of the elements of the correlation matrices associated with first and second moment coordinate and momentum fluctuations for the Brownian harmonic oscillator [J. Chem. Phys. 88, 1854 (1988)].
Derivative self-consistent field theory within the local space approximation. I. Polymatrix formulation
Authors: Kirtman Bernard , Dykstra Clifford E.
Published: July 2002

The SCF density matrix equations of the local space approximation are differentiated analytically. This is done through a polymatrix treatment which provides immediate generalization from ordinary Hartree–Fock to open-shell and MCSCF wave functions. Expressions are obtained in a form that is easily extended to arbitrary order.
Charge localization in negative ion dynamics: Effect on caging of Br−2 in Arn and (CO2)n clusters
Authors: Perera Lalith , Amar François G.
Published: July 2002

We present results of molecular dynamics calculations of the recombination dynamics of a model system intended to mimic the main features of Br−2 in clusters of argon and CO2. The calculation displays a number of novel features, most notably, a consistent treatment of the asymptotic localization of the ‘‘extra’’ negative charge on one of the bromine atoms at large rBr–Br. We simulate the photoexcitation of Br−2 to the 2Πg state and present a summary of results for the dissociation dynamics and caging behavior of minimum energy structures of Br−2Xn, 5<n<17 where X can be Ar or structureless CO2. The structural control of caging is most pronounced for these minimum energy configurations. We then present results of thermal averaging over an ensemble of trajectories for each cluster. These results are compared with recent experiments on Br−2(CO2)n.
Anisotropic reorientational relaxation of biphenyl: Transient grating optical Kerr effect measurements
Authors: Deeg F. W. , Stankus John J. , Greenfield S. R. , Newell Vincent J. , Fayer M. D.
Published: July 2002

Subpicosecond transient grating optical Kerr effect measurements have been used to evaluate the reorientation of biphenyl molecules in neat biphenyl and n-heptane solutions. Besides an ultrafast (100 fs time scale) component associated with librational damping/dephasing, two reorientational relaxation components are observed. The slow reorientation is due to rotation around the short axes of the molecule (tumbling motion), the fast reorientation is associated with internal rotation around the central C–C bond and/or rotation of the whole molecule around its long axis (spinning motion). Whereas the tumbling motion has been observed in earlier depolarized light scattering data, the time resolved Kerr data presented here are the first ones to reveal the dynamics of the fast reorientation component and the ultrafast librational dynamics. It is shown that the diffusive reorientational relaxation must be coupled to the ultrafast librational dynamics, and implications of this coupling are pointed out.
Similarity transference of molecular parameters. II. The bond distances, force constants and polar tensors of HC3N and HC5N
Authors: Herrera O. M. , Bruns R. E. , Neto B. B.
Published: July 2002

The similarity transference procedure is extended to the calculations of the geometries and vibrational frequencies of HC3N and HC5N. Similarity models are constructed using the experimental and STO-3G molecular orbital values of the bond distances and force constants of the HCN, C2H2, CH3CN, C4H2, CH3CCH, and C2N2 reference molecules. Simple linear regression calculations based on these similarity models and using STO-3G values of the bond distances and force constants of HC3N result in estimates of the experimental bond distances and frequencies which have root mean square errors about an order of magnitude smaller than the one for the molecular orbital values. Similarity models for the bond distances and vibrational frequencies and intensities of HC5N are also reported.
The reaction of methane with molecular oxygen: A semiquantitative estimate of the activation energy
Authors: Hamilton Tracy P. , Schaefer Henry F.
Published: July 2002

The geometry and vibrational frequencies at the transition state for the abstraction of hydrogen atom from CH4 by O2 have been determined by ab initio quantum mechanical techniques. Configuration interaction including all single and double excitations (a total of 164 377 configurations in C1 symmetry) was used in conjunction with a double zeta plus polarization basis set. The molecular structures and vibrational frequencies for the reactants and products are also given so that reaction rate constants may be obtained for the forward and reverse reactions. The ab initio activation energy agrees quite well with previous rough estimates based on kinetic modeling and by analogy with related reactions.
Quantum correction to solitary wave propagation in quasi-one-dimensional systems: Nonadiabatic theory of soliton damping
Authors: Ruckh R. , Sigmund E. , Eisele H.
Published: July 2002

The coupled electron–phonon dynamics of a one-dimensional system, which is described by a Su–Schrieffer–Heeger(SSH)-type Hamiltonian, is investigated by means of a time-dependent projection operator technique. The equations of motion for the vibrational and electronic expectation values are derived without using the adiabatic and continuum approximation. When solving these transport equations numerically a quantum correction to the usual soliton solutions which we calculate from the Heisenberg equations for the SSH Hamiltonian is obtained. It turns out that the soliton solutions become unstable when the extension of the soliton is of the order of a lattice constant.
Asymptotic low-temperature behavior of the classical path probability density for a Morse oscillator
Authors: Stanton John F.
Published: July 2002

A closed-form asymptotic low-temperature expression for the thermally averaged probability density of an ensemble of Morse oscillators is derived within the framework of a classical path approximation due to Miller. The resulting equation is of the correct quantum form [ρβ(x,x) =exp(−βε0)‖ψ0‖2], where ψ0 differs from the exact ground-state wave function by a simple multiplicative factor. Hence, the normalized probability distribution function converges to the exact result as T→0. The incomplete treatment of quantum effects afforded by the classical path approximation is manifested mostly in the Boltzmann factor, which converges to the pure harmonic result (ε0=ℏω/2).
Electronic spectroscopy and second harmonic generation in the benzimidazole crystal
Authors: Bree A. , Zwarich R.
Published: July 2002

The first excited electronic state of benzimidazole in the pure crystal at about 1.8 K has been studied using several techniques. Fluorescence is essentially b polarized and consists of a spectrum of doublets spaced 6 cm−1 apart. The doublets arise from two emitting states, one of which is the bottom of the lowest energy of the four exciton branches at 35 844 cm−1, and 6 cm−1 below it is some trap of unknown origin. The polarized reflection spectrum from a freshly cleaved bc face located the energies of two of the exciton branches; a c polarized origin band was observed at 35 844 cm−1 with a stronger b polarized band 34 cm−1 to the blue. The two-photon excitation spectrum was strong in (aa), (bb), and (cc) polarizations yielding an origin band at 35 844 cm−1, clearly marking this as the a1 Davydov component. There was a more intense line in the (cc) spectrum at 35 865 cm−1 arising from a resonance enhanced second harmonic generation (SHG) process in the solid; the frequency of the line varied as the crystal section was rotated about the b axis. Nonresonant SHG was observed in a bc section of benzimidazole at room temperature with ω equal to 15 533 cm−1. Making use of the fact that the energies of the exciton branches depend on the direction of the transition moment, an attempt was made to determine the position of the in-plane moment; the present data were insufficient to locate accurately the orientation of the moment in this way.
Addendum to the van der Waals constants among ground-state hydrogen atoms
Authors: Koga Toshikatsu
Published: July 2002

For the van der Waals interaction among N ground-state hydrogen atoms, the fundamental significance of the two- and three-body interactions is shown. Some accurate nonadditive three-body van der Waals interaction constants are reported based on the perturbation wave function obtained by a momentum-space method. Additive two-body interaction constants are also given with improved accuracy.
Molecular structure and nuclear quadrupole coupling tensors of 14N and 35Cl in 1-chloro-2,4-dinitrobenzene. Phase I
Authors: Sharma Surendra , Weiden Norbert , Weiss Alarich
Published: July 2002

The crystal structure of the orthorhombic, stable phase of 1-chloro-2,4-dinitrobenzene was studied at T=295 K by x-ray diffraction; space group D102h=Pccn; a=893.0(1) pm, b=1104.5(2) pm, c=1567.6(2) pm, Z=8, ρcalc=1.740 Mg m−3, all atoms in position 8(e). The Zeeman split single crystal 35Cl NQR gives e2ΦzzQh−1(35Cl)=74.287(4) MHz, η(35Cl)=0.1636(18) at T=295 K. At 110 K the values are: e2ΦzzQh−1(35Cl)=75.183(4) MHz, η(35Cl)=0.1572(20). Φzz(35Cl) is almost parallel to the bond C(1)–Cl(1) with a deviation of 2.8° from the bond direction. Φxx(35Cl) deviates from the normal to the ring plane by 9.9° and the deviation of Φyy(35Cl) from the ring plane amounts to 8.3°. By high field single crystal NMR the 14N nuclear quadrupole coupling tensors were determined. At T=295 K it is found: e2ΦzzQh−1 (14N(2))=0.9109(10) MHz, η(14N(2))=0.222(3); e2ΦzzQh−1 (14N(4))=1.0758(10) MHz, η(14N(4))=0.303(3). The (NO2)(2) group is twisted 41.9° against the ring plane whereas the group (NO2)(4) is much less twisted to the benzene ring plane (twist angle 12.3°). Both Φzz(14N(2)) and Φzz(14N(4)) are nearly parallel to the bond directions C(2)–N(2) and C(4)–N(4), respectively. In the group (NO2)(4), Φxx(14N) is almost in the plane O(4,1)–N(4)–O(4,2) and parallel to the direction O(4,1)⋅⋅⋅O(4,2). Accordingly, Φyy(14N(4)) is perpendicular to the plane (NO2)(4). Quite differently to the geometrical twist angle of 41.9°, Φxx(14N(2)) is only 22.2° twisted against the benzene ring plane thus forming an angle of 19.7° with the direction O(2,1)⋅⋅⋅O(2,2).
Infrared spectra of isotopic (HCl)3 clusters in solid neon
Authors: Andrews Lester , Bohn Robert B.
Published: July 2002

The codeposition of Ne/HCl mixtures at 5 K leads to the formation of a large yield of (HCl)3 which exhibits one sharp fundamental at 2801.5 cm−1 with chlorine isotopic splittings at 2800.2 and 2799.4 cm−1. Deuterium substitution gives rise to the three stretching fundamentals for (HCl)2(DCl) and (HCl)(DCl)2 with resolved chlorine isotopic splittings that identify these clusters. The neon matrix observations confirm the argon matrix assignments and characterization of (HCl)3 as a C3h cyclic species by the Perchard group and predict a strong gas phase band at 2816±3 cm−1.
Short-range and long-range interactions in Monte Carlo free energy calculations on polypeptide chains
Authors: Premilat Samuël
Published: July 2002

Monte Carlo simulations based on a partition of the conformational energy into short-range and long-range interactions are performed in order to estimate the free energy and entropy of short polypeptide chains. Values of these quantities are obtained by using an ‘‘umbrella-sampling’’method particularly adapted to the partition of energy. Reference systems are completely determined by considering short-range interactions alone. In this case, the chain model allows us to calculate the average energy, the free energy and entropy. Moreover, the configuration of minimum energy is determined by an algorithm proposed for chain models with only nearest neighbor interacting units. When long-range interactions are also considered, the relative effects of the two types of interactions on the chain configuration can be appreciated by estimates of entropy variations. Such variations are related to the folding of the chain which can be modulated by using different dielectric constants in order to take into account solvent effects. The different methods of calculation are applied on models of the polypeptide hormones enkephalin and β-casomorphin.
Helium wave functions from distinguishable electron model
Authors: Koga Toshikatsu , Matsuhashi Toshiyuki
Published: July 2002

The property of solutions to a hypothetical distinguishable electron model (DEM) is discussed, which separates a two-electron atomic equation into a set of two one-electron equations, one for the fast (inner-orbital) electron and the other for the slow (outer-orbital) electron. The former equation includes the slow electron coordinate parametrically and its energy provides an effective potential for the motion of the slow electron. The DEM solutions are not acceptable directly for lower electronic states, but its concept can be used to construct simple correlated trial wave functions for two-electron atoms. Some numerical results are shown for the ground and low-lying excited states of the helium atom.
Molecular dynamics simulation of (N2 )2 formation using the monotonic Lagrangian grid
Authors: Lambrakos S. G. , Boris J. P. , Guirguis R. H. , Page M. , Oran E. S.
Published: July 2002

In this report, we describe some general features of the monotonic Lagrangian grid (MLG) that are important in molecular dynamics simulations. The monotonic Lagrangian grid is a highly efficient and general algorithm for tracking particles and computing interactions in simulations of systems consisting of large numbers of particles. Further, the MLG algorithm is highly adaptable to vector or parallel processing. The mode of implementing the MLG algorithm depends on the nature of the particle simulation. We present results of simulations for calculating the dimer mole fraction as a function of temperature in a three-dimensional N2 gas system. The results are consistent with experimental and theoretical results and demonstrate the effectiveness of the MLG algorithm in simulating gas systems and monitoring dynamically rare processes.
Kinetic equations and fluctuations in electrochemical nucleation: Studies of many-body effects on diffusion-controlled particle growth on a substrate
Authors: Tokuyama Michio , Enomoto Yoshihisa
Published: July 2002

A new viewpoint on the kinetics of electrochemical nucleation of a new phase is presented by numerically solving the kinetic equations recently derived by one of the present authors (M.T.) to study the many-body effects on diffusion-controlled three-dimensional nucleus growth on a substrate. The static many-body (screening) effect is shown to cause the crossover of the growth exponent for the average nucleus radius from 1/2 to 1/6. Hence the electric current grows as t1/2 at short times and falls with t−1/2 at long times, according to the Cottrell equation. The dynamic many-body (correlation) effect is shown to give rise to a dispersion in the size of the nuclei and thus to cause appreciable corrections to the amplitudes for the radius and the current in the zero limit of volume fraction Q. The corrections go as Q1/2t1/3 and hence cause a large deviation of the average current from the Cottrell equation at long times. Nonthermal fluctuations around the mean motion generated by the correlation effect are also explicitly explored together with a comparison with thermal fluctuations existing at the beginning. A computer simulation is finally performed to confirm the validity of the kinetic equations. The theoretical results are shown to have excellent agreement with the simulation.
Theory and simulation of hard-chain mixtures: Equations of state, mixing properties, and density profiles near hard walls
Authors: Honnell Kevin G. , Hall Carol K.
Published: July 2002

A combination of theoretical modeling and computer simulation is used to study the equation of state of binary mixtures of hard chains, where each chain is modeled by a series of freely jointed, tangent, hard spheres. Three approximate equations of state are derived, based on our previous work on one-component fluids. These equations contain no adjustable parameters and relate properties of the chain mixture to properties of pure monomer and dimer fluids at the same total volume fraction. Their predictions are tested against Monte Carlo results for the pressure of mixtures of 8-mers and monomers and mixtures of 8-mers and 4-mers, obtained using a hard-wall technique. Very good agreement is obtained using an equation of state developed here, in which the compressibilty factor of the mixture is set equal to the molar average of the compressibility factors of the pure components at the same overall volume fraction, as well as from Wertheim’s second-order thermodynamic theory of polymerization (TPT2). Using the equations developed here, we also examine the mixing properties of hard-chain fluids. For mixing at constant pressure, the free energy and entropy of mixing range from ideal-solution behavior at low pressures to Flory–Huggins behavior at high pressures. For mixing at constant volume fraction, the free energy and entropy of mixing reduce directly to the Flory–Huggins result without recourse to the usual lattice approximations. Site-density profiles obtained from the simulations indicate that chains are depleted near the walls at low densities and are enhanced near the wall at high densities; monomers, by contrast, are enhanced near the walls at all densities.
The two-electron terms of the no-pair Hamiltonian
Authors: Samzow Reinhard , Hess Bernd A. , Jansen Georg
Published: July 2002

New programs have been developed which enable us to calculate the two-electron terms of the no-pair Hamiltonian. We present all-electron calculations for the atoms Ag and Au with nonrelativistic and relativistic Hamiltonians. Self-consistent field ground state energies and orbital energies are reported. The configuration-interaction method is used to determine excitation energies, ionization potentials, and electron affinities. By comparing the results for different operators the effect of the relativistic correction of the two-electron integrals is investigated.
Population transfer by stimulated Raman scattering with delayed pulses using spectrally broad light
Authors: Kuhn A. , Coulston G. W. , He G. Z. , Schiemann S. , Bergmann K. , Warren W. S.
Published: July 2002

The feasability of selective and complete population transfer between atomic or molecular levels by stimulated Raman scattering with delayed pulses involving spectrally broad light with characteristics typical for pulsed lasers is investigated. In extension of previous work, the effect on the transfer efficiency of phase fluctuations and of the detuning ΔR of the laser frequencies from the two-photon resonance is analyzed. The minimum pulse energy Pmin required to achieve a transfer efficiency of nearly unity is derived analytically, with some restrictions imposed on the type of phase fluctuations. Pmin increases approximately proportional to the square of the bandwidth of the laser radiation and to the square of ΔR. The conclusions are confirmed by the results of extensive numerical calculations. These studies also reveal a high sensitivity of the transfer efficiency to the autocorrelation of the fluctuating light.
Pulsed-ultraviolet-laser-induced chemiluminescence of NaCd and NaHg excimers
Authors: Musso M. , Windholz L. , Fuso F. , Allegrini M.
Published: July 2002

NaCd and NaHg excimers have been produced by pulsed uv laser irradiation of Na+Cd and Na+Hg vapor mixtures. Their characteristic bound-free emission in the blue region, previously observed in cw experiments, has been detected and its time behavior has been investigated.
Ligand-field splittings and core-level linewidths in I 4d photoelectron spectra of iodine molecules
Authors: Cutler J. N. , Bancroft G. M. , Tan K. H.
Published: July 2002

High-resolution I 4d photoelectron spectra (total instrumental resolution ∼0.06 eV) of seven simple iodine molecules, ICl, IBr, I2, HI, CH3I, CH2I2, and CF3I, have been recorded. Ligand-field splitting (as measured by the asymmetric C20 crystal-field term) splits the 4d3/2 and 4d5/2 lines into a doublet and triplet, respectively. In contrast to Si 2p spectra, the I 4d spectra generally show little evidence for vibrational splitting or broadening. The C20 values, like the binding energies E4d, generally increase as the ligand electronegativity increases. C20 also correlates well with the nuclear field gradient, eqn, as measured by Mössbauer spectroscopy or nuclear quadrupole resonance. For the first time we have been able to study the chemical dependence of the inherent lifetime width of a core hole. We show that the I 4d linewidths generally decrease as the electronegativity of the ligand increases, and as the valence I 5p electron density decreases. We also show that there are large relaxation effects on E4d, C20 and especially the linewidth.
Extending J walking to quantum systems: Applications to atomic clusters
Authors: Frantz D. D. , Freeman D. L. , Doll J. D.
Published: July 2002

The J-walking (or jump-walking) method is extended to quantum systems by incorporating it into the Fourier path integral Monte Carlo methodology. J walking can greatly reduce systematic errors due to quasiergodicity, or the incomplete sampling of configuration space in Monte Carlo simulations. As in the classical case, quantum J walking uses a jumping scheme to overcome configurational barriers. It couples the usual Metropolis sampling to a distribution generated at a higher temperature where the sampling is sufficiently ergodic. The J-walker distributions used in quantum J walking can be either quantum or classical, with classical distributions having the advantage of lower storage requirements, but the disadvantage of being slightly more computationally intensive and having a more limited useful temperature range. The basic techniques are illustrated first on a simple one-dimensional double well potential based on a quartic polynomial. The suitability of J walking for typical multidimensional quantum Monte Carlo systems is then shown by applying the method to a multiparticle cluster system consisting of rare gas atoms bound by pairwise Lennard-Jones potentials. Different degrees of quantum behavior are considered by examining both argon and neon clusters. Remarkable improvements in the convergence rate for the cluster energy and heat capacity, analogous to those found in classical systems, are found for temperatures near the cluster transition regions.
Scattering of neutral metal clusters: Long-range interactions and response properties
Authors: Kresin Vitaly V. , Scheidemann Adi
Published: July 2002

The absolute integral cross sections for low-energy collisions of neutral sodium clusters Nan (n=2–40) with atoms and molecules (Ar, N2, O2, and halogens) have been measured. The cross sections are found to be exceptionally large (up to thousands of square angstroms), showing the dominant role of long-range intermolecular interactions. Elastic scattering proceeding under the influence of the van der Waals force, and a reaction channel involving electron transfer can successfully describe the measurements. The strength of the van der Waals potential is defined by such cluster response properties as the electric polarizability and the frequency of the giant dipole resonance. The reactive electron-jump channel, in turn, is described by the ‘‘harpooning’’ mechanism which is sensitive to the cluster ionization potential. Employing parameters taken from spectroscopic studies of alkali clusters, we obtain good agreement with the observed cross sections. This provides a direct connection between beam scattering experiments and studies of cluster electromagnetic response properties.
Chaotic dynamics in small inert gas clusters: The influence of potential energy saddles
Authors: Hinde Robert J. , Berry R. Stephen
Published: July 2002

We have investigated the effect of potential energy saddles on the evolution of chaos in small (three- to seven-atom) inert gas clusters by comparing the local Kolmogorov entropy (K entropy) accumulated near a saddle with that accumulated in a potential well. We find that saddles which are relatively flat along the isomerization coordinate are most effective at reducing the local K entropy in the saddle region, indicating that these saddles tend to regularize the dynamics close to the saddle point. In three- and four-atom clusters, we also find that flat saddles decouple some or all of the cluster’s vibrational modes from one another; this leads to approximately quasiperiodic behavior in some degrees of freedom near the saddle regions of these clusters.
Rotational dependence of the predissociation linewidths of the Schumann–Runge bands of O2
Authors: Cheung A. S-C. , Mok D. K-W. , Jamieson M. J. , Finch M. , Yoshino K. , Dalgarno A. , Parkinson W. H.
Published: July 2002

The predissociation linewidths of vibrational levels v=0 –12 for 16O2, 16O18O, and 18O2 molecules in the B 3Σu− state with rotational quantum numbers N≤20 have been calculated taking into account the spin–orbit interactions of the B 3Σu− state with the 5Πu, 3Σu+, 3Πu, and 1Πu states, and the rotational coupling with the 3Πu state. The predissociation linewidths exhibit systematic variations with rotational quantum number for different vibrational levels. Good agreement between most of the calculated and experimental linewidths has been obtained for all three isotopic molecules, with the exception of the set of linewidths of 16O2 for v=0 and 2. The agreement can be improved by adjustment of the 1Πu potential and the strength of the spin–orbit interaction between the B 3Σu− and 1Πu states.
Time-resolved degenerate four-wave mixing studies of solid-state poly(p-phenylene) oligomers
Authors: Marcy Henry O. , Rosker Mark J. , Warren Leslie F. , Reinhardt Bruce A. , Sinclair Michael , Seager Carl H.
Published: July 2002

We have measured χ(3) for a series of solid-state samples of polyphenylene oligomers, (Ph)n, where n=4–8 is the number of phenyl ring units, using optical pulses of 140 fs duration and 650 nm wavelength. A degenerate four-wave mixing (DFWM) signal was only observable for n≥6, and thereafter the measured values of χ(3) increased monotonically with n. Measurements of the linear absorption are also reported. Although the DFWM measurements were performed at wavelengths far from resonance, the figure of merit χ(3)/α was essentially flat beyond n≥6. In contrast to previous studies of oligomers in solution, the principal nonlinear responses were instantaneous within the 140 fs temporal resolution.
Exchange interaction between two O2 molecules using the asymptotic method
Authors: Bussery B. , Umanskii S. Ya. , Aubert-Frécon M. , Bouty O.
Published: July 2002

The interaction between two O2(3Σ−g) molecules gives rise to three dimer states with spin quantum number S=0, 1, and 2. The splitting between the spin states is accurately represented by the Heisenberg Hamiltonian. Using the asymptotic method, an analytical expression for the exchange coupling parameter J is proposed. Such an expression, easily handable, furnishes a straightforward understanding of the behavior of J with the O2–O2 intermolecular separation, R. In the present paper, we have evaluated the exchange coupling parameter in a wide range of orientations of the monomers for various fixed values of R. Analytical expressions of J are given for various specific geometries in terms of a polynomial in (1/R) multiplied by an exponential part with an exponent of 3.6 Å−1. Results are compared with experimental data and previous ab initio computations. A rather good qualitative agreement is found with ab initio calculations in the selected range of orientations and distances.
Evaporation and condensation at a liquid surface. I. Argon
Authors: Yasuoka Kenji , Matsumoto Mitsuhiro , Kataoka Yosuke
Published: July 2002

Molecular dynamics computer simulations were carried out to investigate the dynamics of evaporation and condensation for argon at the temperature of 80 and 100 K. From the decrease of the survival probability of vapor molecules, the ratio of self reflection to collision is estimated to be 12%–15%, only weakly dependent on the temperature. This suggests that argon vapor molecules are in the condition of almost complete capture, and the condensation is considered to be a barrierless process. The total ratio of reflection which is evaluated with the flux correlation of condensation and evaporation is 20% at both temperature. The difference between these two ratios of reflection is ascribed to a phenomenon that vapor molecules colliding with the surface drive out other liquid molecules. This molecule exchange at the surface is as important as the self-reflection, and the conventional picture of condensation as a unimolecular chemical reaction is not appropriate.
Theoretical analysis of the emission spectra of the NaCd excimer
Authors: Angeli C. , Persico M. , Allegrini M. , De Filippo G. , Fuso F. , Gruber D. , Windholz L. , Musso M.
Published: July 2002

We present simulations of bound–bound and bound–free emissions of the NaCd* excimer, along with new experimental results concerning the red band system. The simulations, based on ab initio potential energy curves, confirm that the initial states are 2 2Σ+ for the red emission and 2 2Π for the blue emission of NaCd*. The two main peaks in the blue band system are due to spin–orbit splitting. The band shape obtained in the simulations is extremely variable, depending on the population distribution of the vibrational states of NaCd*. The comparison of simulated and measured spectra leads us to conclude that, in the experimental conditions adopted here and in previous work, the thermal equilibration of the excimer is far from complete.
A Born–Green–Yvon equation for flexible chain-molecule fluids. II. Applications to hard-sphere polymers
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

Using the method of Born, Green, and Yvon we have recently derived a coupled set of integral equations for the intermolecular site–site distribution functions which describe the microscopic equilibrium structure of a fluid of flexible chain molecules. Here, we carry out an average over this set of equations to arrive at a single equation for the average intermolecular distribution function. Intramolecular distribution functions appearing in this equation are approximated by results for isolated chain molecules. Numerical calculations have been performed over a range of fluid densities for chains composed of 8, 16, 20, and 50 tangent hard spheres. The resulting intermolecular distribution functions are in good to fair agreement with simulation data. This structural information is used to compute second virial coefficients and pressure equations of state via the virial route to thermodynamics. Although the theory consistently underestimates the pressure, it gives very accurate results for the second virial coefficient for all chain lengths studied.