bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulation of the vibrational properties of a disordered N2O crystal. II. The symmetric stretching mode
Authors: Cardini Gianni , Schettino Vincenzo
Published: July 2002

The analysis of a previous molecular dynamics study of crystalline N2O was extended to study the effect of dipolar disorder on the internal symmetric stretching mode ν1. Results are reported for infrared and Raman spectra. The effect of the simulation box size and of the degree of dipolar disorder was investigated. The shape of the Raman spectrum and its relation with the density of states are discussed. The importance of the topology of the distribution of the head-to-tail flipped molecules on the shape of the Raman spectrum is shown.
Lamb dip and infrared–radio frequency double resonance spectroscopy of 188OsO4
Authors: Ricci L. , Pavone F. S. , Prevedelli M. , Zink L. R. , Inguscio M. , Scappini F. , Sassi M. P.
Published: July 2002

The frequency of seven rovibrational transitions in the ν3 band, Q branch of 188OsO4 have been measured with high accuracy. The spectrometer used consists of two CO2 lasers [∼3 W on the 10R(0)], the first of which is stabilized on the 4.3 μm fluorescence Lamb dip of the CO2 and the second on the saturated absorption dip of the rovibrational transitions of 188OsO4. The OsO4 transition frequencies are obtained combining the known frequency of the first laser with the measured frequency of the beat note between the two laser beams. For the assignment of the transitions the recent Fourier transform analysis by Bobin et al. [J. Mol. Spectrosc. 122, 229 (1987)] was used. The access to the 10R(0) laser line has made it possible to investigate, for the first time, the Q branch of the ν3 band of OsO4 in a sub-Doppler regime. Furthermore, the tetrahedral fine structure in the ground and in the ν3 state has been analyzed using the infrared–radio frequency double resonance technique. The tetrahedral splitting constants, Dt for the ground state and Z3t for the ν3 state, have been calculated with higher accuracy than in previous works.
A molecular dynamics simulation of a water droplet by the implicit-Euler/Langevin scheme
Authors: Schlick Tamar , Figueroa Samuel , Mezei Mihaly
Published: July 2002

Results are presented from potential energy minimization of water clusters and from molecular dynamics and Monte Carlo simulations of a liquid water droplet model. A new method for molecular dynamics—the implicit-Euler/Langevin scheme—is used in combination with a truncated Newton minimizer for potential energy functions. Structural and thermodynamic properties are reported for the scheme (with time steps of 5 and 10 fs), compared to a standard explicit formulation (with Δt=1 fs), to a Monte Carlo simulation, and to available experimental data. Results demonstrate that the implicit scheme is computationally feasible for large-scale biomolecular simulations, and that the droplet model can reasonably reproduce general structural features of liquid water. Results also show that the desired behavior is obtained from the implicit formulation: stability over large time steps, and effective damping of the high-frequency vibrational modes. Thus, major ‘‘bulk’’ properties of the system of interest may be observed more rapidly.
Inter- and intramolecular interactions and rotational tunneling of methyl groups in tetramethyltin
Authors: Zhang Da , Prager Michael , Weiss Alarich
Published: July 2002

Rotational tunneling of methyl groups in partially deuterated tetramethyltin compounds, (CH3)xSn(CD3)4−x with x=1 and 3, and in various isotopic mixtures, [(CH3)4Sn]x[Sn(CD3)4]1−x, x=0.027, 0.20, and 0.50, and [CH3Sn(CD3)3]x[Sn(CD3)4]1−x, x=0.10, respectively, has been studied using the inelastic neutron-scattering (INS) technique. Compared with the INS spectrum of fully protonated tetramethyltin, (CH3)4Sn, the spectra obtained in the present study show remarkable shifts and broadenings or splittings of tunnel lines. The interpretation of the INS results is based on the assumption that all partially deuterated compounds and isotopically mixed systems have the same crystal structure as pure (CH3)4Sn. Combining the INS results with a detailed discussion of the crystal structure, all features can be explained in terms of both the intra- and intermolecular interactions of methyl groups. The overall decrease of tunnel splittings with deuteration is explained by the reduction of the lattice parameter and the increased octopole moment of the CD3 groups. The interaction between different molecules is mediated by a subset of methyl group pairs only.
Anisotropic solute–solvent interactions in liquid crystals: An analysis of dipolar and quadrupolar couplings of methane in nematic solvents
Authors: Lounila Juhani , Diehl Peter
Published: July 2002

The most complete sets of available dipolar and quadrupolar couplings obtained from the NMR spectra of methane and its deuterated analogs dissolved in two different liquid crystals are analyzed. The analyses are based on a generalized theory for the correlation between the vibration and rotation of partially oriented molecules, which retains the quadratic terms in the Taylor series describing the functional dependence of the orienting forces on the vibrational normal coordinates. These quadratic terms are essential for the cases where vibrational effects (e.g., isotope effects) on the molecular orientation are of importance. In both liquid crystals all the 17 couplings can be reproduced by three adjustable parameters which describe the forces acting on the CH or CD bonds. However, the resulting forces are unexpectedly strong, and physically more plausible solutions are obtained by allowing for an additional parameter Bdp+Bext. Bdp and Bext are contributions to quadrupolar couplings arising from direct perturbations of the molecular electron distribution and external electric field gradient due to the neighboring molecules, respectively. The values of Bdp+Bext corresponding to plausible solutions can be rationalized in terms of (1) orientation dependent direct perturbations of the order of 0.5% on the quadrupole coupling constant of the deuteron, or (2) an external field gradient produced by the quadrupole moments of the liquid crystal molecules surrounding a cylindrical cavity which encloses the methane molecule. Independent values of Bext, estimated previously by the quadrupolar couplings of the hydrogen molecule, are strikingly similar to the present values of Bdp+Bext. The results appear to be consistent with two different physical interpretations: the anisotropic forces experienced by the methane molecule may be dominated by (1) the van der Waals forces acting on its atoms, or (2) the interaction between its quadrupole moment and the external field gradient. The results also lend support to the assumption that the molecular distortions due to a liquid crystal environment are normally dominated by bond-bending deformations.
The electronic spectrum of NS2 : Low-lying quartet states
Authors: Yamaguchi Yukio , Alberts Ian L. , Xie Yaoming , Schaefer Henry F.
Published: July 2002

An investigation concerning nine electronically low-lying doublet states of the SNS molecule appeared recently. The present paper represents the conclusion of studies on the NS2 molecule, focusing on the low-lying SNS quartet states. Nine quartet states, including six bent structures (4A2, 4B2, two 4A1, and two 4B1 ) and three linear structures (4Πg and two 4Πu ), have been investigated at the self-consistent field (SCF), single and double excitation configuration interaction (CISD), and complete active space (CAS) SCF levels of theory with five basis sets, double zeta (DZ) through triple zeta plus double polarization (TZ+2P). Four of these quartet states lie within 1.8 eV (42.0 kcal mol−1 ) of the 2A1 ground state of NS2. Thus one or more of these quartet states (which lie much lower than for the valence isoelectronic NO2 molecule) should be observable. At the SCF level of theory the stability of the wave functions and the relative energies of the nine quartet states are discussed in conjunction with the molecular orbital (MO) Hessian.
Golden-rule treatment of hydrogen abstraction by photoexcited acridine guests in fluorene single crystals
Authors: Chantranupong Lek , Wildman Timothy A.
Published: July 2002

The kinetics of the photoreduction of acridine guest molecules in fluorene single crystals has been investigated theoretically. The abstraction has been modeled using the golden-rule approach and proceeds by vibrationally assisted tunneling. Parameter values have been taken from experimental data and from semiempirical force-field calculations. The calculated temperature and isotope effects are in good agreement with the experimental data for acridine and phenazine guests [B. Prass, J. P. Colpa, and D. Stehlik, J. Chem. Phys. 88, 191 (1988)]. While a fluorene lattice vibration is the likely promoting mode, NH wagging in the radical-pair product plays an essential role.
The precipitation of kinks on stepped Si(111) surfaces
Authors: Wei Jian , Wang Xue-sen , Bartelt Norman C. , Williams Ellen D. , Tung R. T.
Published: July 2002

High resolution low-energy electron diffraction, scanning tunneling microscopy (STM), and transmission electron microscopy (TEM) have been used to study the temperature dependence of a vicinal (stepped) Si(111) surface with a polar angle of 6° from (111) along an azimuth rotated about 10° away from the high symmetry [1̄1̄2] direction. At the (1×1) to (7×7) reconstructive transition, the kinks at the step edges precipitate into a phase with high step density and increased rotation from the [1̄1̄2] direction, leaving behind a [1̄1̄2] oriented phase with triple-layer height steps and (7×7) reconstruction. The inclination and azimuth of the orientation of the kinked phase change continuously with temperature. When the surface is cooled to room temperature, STM images confirm a coexistence between the [1̄1̄2] oriented step-tripling phase and the kinked phase. In the step-tripling phase, the steps are very straight (kinks are rare) and oriented in the [1̄1̄2] direction. There is a small fraction of single-layer height steps between the predominant triple-layer height steps. All the terraces in this phase are (7×7) reconstructed. In the kinked phase, steps with a high density of kinks are bunched together and rotated to a direction about 45° away from [1̄1̄2]. TEM images of the surface topography also confirm the phase coexistence on a macroscopic scale. This phase separation (azimuthal faceting) is reversible, and can be understood thermodynamically by analogy with phase separation in a two-component fluid.
Local structure of polyethylene melt studied by pulsed neutron total scattering
Authors: Misawa M. , Kanaya T. , Fukunaga T.
Published: July 2002

Structure factors S(Q) of deuterated polyethylene (PE) in the solid and liquid states have been measured at 20, 130, and 170 °C in a wide range of Q from 0.55 to 25 A−1 by means of pulsed neutron total scattering technique. Very local intrachain structure of the PE melt, such as C–D and C–C bond lengths and ∠CCC, ∠DCD, and ∠CCD bond angles, is determined from a high Q part of S(Q), and is compared with that of solid PE. Interchain structure of the melt is also discussed, assuming that the PE chains are built of small segments of five –CD2 – units like pearl necklaces and furthermore, that orientations between the segments are completely uncorrelated and chain connectivity is entirely ignored. The experimental S(Q) is reproduced reasonably well by this simple model with an effective diameter σ of the segments of 4.34 A and a packing fraction η of the segments of 0.28, which are consistent with the experimental number density of the PE melt. The different size of the segments like four or six –CD2 – units, however, gives poor results with the values of σ and η being inconsistent with the experimental number density.
Dynamic relaxation of rodlike micelles
Authors: Cates M. E. , Marques C. M. , Bouchaud J.-P.
Published: July 2002

We study theoretically the angular relaxation and translational diffusion of stiff rodlike micelles that can undergo reversible scission reactions, in both the dilute and the entangled regime. The very strong dependence of the angular diffusion constant on rod length leads to an anomalous superdiffusive behavior of the typical angular deflection of a subunit at short times. Despite this, angular correlation functions such as 〈u(t)⋅u(0)〉 (also the linear birefringence and entropic stress response) decay exponentially with a relaxation time that is a power law combination of the time scales for reversible scission and for angular rotation of a (hypothetical) unbreakable rod. This relaxation time corresponds to the waiting time for a subunit to find itself on a rod that is short enough to rotate through an angle of order π before recombining with another rod. The translational diffusion of rodlike micelles shows no anomaly in contrast to the angular case. However, the collective diffusion and self-diffusion constants differ by a factor of 2 at all concentration ranges for which rod micelles exist. The diffusion equation for collective motion is nonlinear (even below the overlap threshold) since the mobility of the rods depends on their local concentration. The dynamic structure factor S(q,t) is calculated to order q4, the quartic terms providing, in principle, a probe of the micellar scission time.
Towards organic polymers with small intrinsic band gaps. III. Geometric and electronic structures of the base and salt forms of polypyrrylenemethine
Authors: Toussaint J. M. , Brédas J. L.
Published: July 2002

We present semiempirical modified neglect of diatomic overlap (MNDO) geometry optimizations and valence effective Hamiltonian (VEH) pseudopotential band structure calculations on the base and salt (protonated) forms of polypyrrylenemethine. This compound is a conjugated polymer based on 2H-pyrrole rings which are bridged by methine-type carbons. In the base (neutral) form, the band gap is calculated to be 1.47 eV. The presence of imine-type nitrogens results in an electronic structure for polypyrrylenemethine which is significantly different from that of polypyrrole despite the presence of six π electrons per unit cell. After protonation of the imine nitrogens, we find that the band gap for the salt form decreases to 0.78 eV, i.e., a value half as big as that of the neutral form. This strong band gap reduction originates from the geometry modification imposed by the protonation process, which leads to a situation where the pyrrole rings alternate between aromatic-type and quinoid-type geometries. Comparison is made with, on the one hand, the oxidation of polybipyrrolemethenylene, and on the other, the protonation of emeraldine base.
Simultaneous forward–backward Raman scattering studies of D2 broadened by D2, He, and Ar
Authors: Rosasco G. J. , Bowers W. J. , Hurst W. S. , Looney J. P. , Smyth K. C. , May A. D.
Published: July 2002

Unavoidable beam crossings within a spherical-mirror, multipass stimulated Raman gain cell give rise simultaneously to forward- and backward-scattering Raman signals. In the Doppler-broadened/Dicke-narrowed regime of density, the lineshape is a function of the momentum transfer in the scattering process and thus the observed spectra will have more complex lineshapes than those seen with simple forward or backward scattering geometries. The analyses necessary to quantitatively account for such forward–backward spectra are summarized. These spectra enable unique experimental tests of the lineshape functions used for the description of the Raman Q-branch spectrum under conditions where Doppler contributions and Dicke narrowing are significant. Results for the D2:D2 and D2:He systems support the well-known Galatry, or soft collision, lineshape function. However, in the case of D2:Ar, our results suggest the need to employ the more general, complex soft collision function. In addition, these studies have provided data on linear-with-density line broadening coefficients (previously published) and line shifting coefficients (reported here) for these molecular systems.
Theory of rotational coherence spectroscopy as implemented by picosecond fluorescence depletion schemes
Authors: Hartland Gregory V. , Connell Leslie L. , Felker Peter M.
Published: July 2002

We present a perturbation theory analysis of four time-resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherence spectroscopy. The analysis shows that ground-state rotational constants determine the rotational coherence effects in fully resonant, time-resolved stimulated Raman-induced fluorescence depletion (TRSRFD), excited-state rotational constants determine such effects in time-resolved stimulated emission spectroscopy (TRSES), and both ground- and excited-state constants do so in time-resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance-enhanced shows that this method gives rise to qualitatively different rotational coherence effects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground-state rotational coherence spectroscopy. Expressions for the calculation of rotational coherence effects in TRFD, TRSRFD, and TRSES traces are also presented. Such expressions are used to show that the magnitudes of rotational coherence transients are similar in all three schemes. Finally, experimental results on molecular iodine are presented to show that, indeed, both ground- and excited-state rotational coherence effects are manifest in TRFD traces.
An intermolecular H–O potential for methyl rotations in solid nitromethane
Authors: Rice Betsy M. , Trevino S. F.
Published: July 2002

A reliable determination of the (H–O) intermolecular potential by which molecules of nitromethane interact with each other is presented. This effort is based upon a very complete body of experimental information which is available on the crystal structure and the rotational properties of the methyl group of the molecule in the solid state. The crystal structure is known in the temperature range of 4–233 K. The rotational properties of the methyl group of the molecule have been studied by inelastic neutron scattering with which the energy levels, including the ground state tunnel splitting, have been measured in both the protonated and deuterated samples. These studies provide a most comprehensive characterization of any such similar molecular crystal. The measured properties are directly related to the intermolecular potential but the inversion of the underlying intermolecular potential from the measured properties has not been straight forward. We describe the application of the principle of maximum entropy in the determination of an intermolecular H–O potential. The resulting potential is of a novel character. At 3.5 GPa x-ray diffraction studies indicate that the equilibrium orientation of the methyl group in crystalline nitromethane is rotated 45° from the position in the low-temperature ambient pressure form. Calculations of the potential energy as a function of methyl group orientation in crystalline nitromethane at a pressure of 3.5 GPa using the intermolecular potential described herein has reproduced this experimental observation.
A Hookean dumbbell model with Basset forces for dilute polymer solutions
Authors: Schieber Jay D.
Published: July 2002

An inertial Hookean dumbbell model with Basset forces in steady-state shear flow has been solved analytically to yield predictions for the dilute solution polymer contribution to viscosity and first normal stress coefficient. The generalization to the standard Hookean dumbbell model with only a Stokes law drag force is accomplished by utilizing the solution found by Landau and Lifshitz for the drag on a sphere undergoing arbitrary, time-dependent displacement assuming Stokes flow. The Basset forces depend upon the entire past history of the phase-space coordinates (an integral of a 1/(t)1/2 memory kernel times the acceleration of the beads in the dumbbell); thus the resulting equation of motion for the internal phase-space coordinates of the polymer are no longer Markovian. These equations may still be solved in any steady flow using harmonic analysis. It is found that neither the viscosity nor the first normal stress coefficient are shear rate dependent. However, both constant results are weakly rescaled by the presence of the Basset forces in a complicated way.
Observation of Δk=3n selection rule in collisionally-induced transitions in methyl flouride
Authors: Shin U. , Schwendeman R. H.
Published: July 2002

By varying the phase of the lock-in amplifier that processes the probe signal in an infrared–infrared double-resonance spectrometer, the four-level double resonances that result from collisionally-induced rotational energy transfer in 13CH3F are clearly distinguished from those that result from a vibrational energy swapping mechanism (V–V energy transfer). The spectrometer operates with a fixed frequency moderate power (∼1 W) pumping source and a tunable low power (∼20 μW) probe source. At one choice of phase the absorption that results from collisionally-induced rotational transitions are shown to have one sign in the spectrum and to obey the selection rule Δk=3n (n a positive or negative integer) whereas the peaks from all other collisionally-induced transitions (Δk≠3n) have the opposite sign. The sample spectrum is for J=22, which shows that the Δk =3n selection rule persists for many J states away from the originally-pumped level (J =5).
The role of the potential surface in transport and relaxation phenomena in the He–H2 system
Authors: Smith Maureen J. , Shi Shenghua , Rabitz Herschel , McCourt Frederick R. W.
Published: July 2002

This paper examines the role of the potential surface of the rigid rotor He–H2 system upon a variety of transport and relaxation cross sections, including some involving field effects. The technique of functional sensitivity analysis is used to explore these issues. Three different levels of cross sections were studied: microscopic, thermally averaged, and effective cross sections. The cross sections studied were found to be sensitive to differing components of the potential energy surface, with some cross sections being more sensitive to either the slope or the magnitude of the potential components. The degree of information loss in the progression from microscopic to bulk observables is highly dependent on the individual phenomenon.
Monte Carlo simulation and self-consistent field theory for a single chain on a diamond lattice
Authors: Yuan X.-F. , Masters A. J.
Published: July 2002

Monte Carlo simulations of self-avoiding walks with nearest-neighbor attractions have been performed on a diamond lattice for n≤210, where n is the number of steps. The data are compared with the scaling analysis of Daoud and Jannink and de Gennes, and, as found by previous simulation studies, a crossover exponent greater than the theoretical value is required for a good fit in the good solvent regime, though the theory works much better on the poor solvent side. For good solvent conditions it was found that the results do not accord too well with two-parameter theory, and the theoretical expression of Muthukumur and Nickel fit the data rather poorly. The Domb–Barrett interpolation formulas fit somewhat better. This lends weight to the view that lattice corrections to two-parameter theory are important. Finally the results are compared with a self-consistent field theory over the whole range of solvent conditions. Provided an n-dependent excluded volume parameter was used, fair agreement for many properties could be obtained for both good and poor solvents.
Fluctuation potentials for intrashell electron pairs in some He- and Be-like systems
Authors: Banyard K. E. , Sanders J.
Published: July 2002

Electron correlation is determined by the deviation of the Coulombic repulsion potential from the Hartree–Fock average potential. This deviation is termed a fluctuation potential. Investigated here, for the first time, are the Z-dependent characteristics and the range of such fluctuation potentials for a series of He- and Be-like ions. Each system is in its ground state. Results for the separate intrashell electron pairs are presented as potential profiles and scaled contour diagrams. For these K and L shells, the radial and angular components of electron correlation—and their changes in relative importance with increasing Z—can be seen to arise from specific features in the associated fluctuation potentials. The extentof each potential difference is assessed with respect to a defined ‘‘range of shell’’ distance. Previously, fluctuation potentials had been presumed to be short ranged. The present work indicates that this is not the case.
Experiments on an oscillatory system close to equilibrium
Authors: Hjelmfelt Allen , Ross John
Published: July 2002

A theoretical study of systems with multiple equilibrium states due to nonidealities predicts that such systems may show oscillations arbitrarily close to equilibrium and with arbitrarily small dissipation [X.-L. Chu and J. Ross, J. Chem. Phys. 93, 1613 (1990)]. A crystal of V2O3 shows bistability in electrical resistance for a certain range of temperature. When the crystal is placed in an electric current clamp and in a bath at a temperature lower than that of the bistability region oscillations occur due to the difference in heating on each branch of the hysteresis loop. The period of oscillations increases as the electric current in the clamp is decreased. Within a range of conditions the experiment supports the theory.
Cross sections of collision induced rotational transitions of polar molecules perturbed by isotropic projectiles
Authors: Kajita Masatoshi , Odashima Hitoshi , Shimizu Tadao
Published: July 2002

A semiempirical formula for the cross section of collision induced rotational relaxation is presented, which allows one to estimate fairly accurate values of cross sections for any pairs of polar molecules and isotropic perturbers. The cross section does not critically depend on the quantum states, but is described simply by the parameters of colliding atoms and molecules, such as van der Waals potential constants, dipole moment, polarizability, and ionization energy. Interpretation of the semiempirical formula on the basis of a proposed model for the collision is also given. The formula may be usefully employed to evaluate cross sections of collision pairs which are difficult to be obtained from experiment.
Dielectric constant of a hard-sphere fluid with induced dipoles and quadrupoles
Authors: Hinsen K. , Felderhof B. U.
Published: July 2002

We study the dielectric constant of a fluid of hard spheres with a polarizable point dipole and quadrupole at their center. The deviations from the Clausius–Mossotti formula are found at six volume fractions by computer simulation of a system with periodic boundary conditions. For small polarizability the deviation agrees well with the theoretical result up to a volume fraction of 30%, if calculated in superposition approximation for the triplet distribution function. We also determine the spectral density appearing in the spectral representation of the dielectric constant. The spectrum differs significantly from that for a system with only induced dipole interactions, especially at high volume fraction.
The photodissociation of dimethylnitrosamine studied by classical trajectories on a
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i
t
i
o potential energy surfaces
Authors: Persico Maurizio , Cacelli Ivo , Ferretti Alessandro
Published: July 2002

We have determined ab initio potential energy surfaces of the S0 and S1 states of dimethylnitrosamine, with particular care for the N–N bond dissociation pathway. The electronic correlation has been taken into account by a multireference perturbation method, CIPSI. Classical trajectories in the S1 surface have been run with statistically determined initial conditions. Computed lifetimes and orientation parameters for the recoil velocity and the NO fragment angular momentum are in agreement with experimental data. Large amplitude internal motions are of primary importance in determining such quantities.
Infrared laser spectroscopy of CNC+
Authors: Fehér Miklós , Salud Carlos , Maier John P.
Published: July 2002

The CNC+ ion has been detected in the gas phase by tunable diode–laser spectroscopy in a hollow cathode discharge. The (001)–(000), (011)–(010), (021)–(020), and the (031)–(030) transitions were observed and analyzed. The derived spectroscopic constants for the ground vibrational state are: ν0=1974.071 72(65) cm−1 and B0=0.460 609(39) cm−1.
Molecular integrals with Slater basis. III. Three-center nuclear attraction integrals
Authors: Rico J. Fernández , López R. , Ramírez G.
Published: July 2002

We discuss the calculation of the general three-center nuclear attraction integrals with Slater type orbitals using a one-center expansion method which exploits the algorithms previously developed by us for long-range integrals. From the analysis of the numerical stability, accuracy, and computational cost we conclude that the reported procedure is suitable for the calculation of these integrals with any desired precision. Moreover, we have found that most of these integrals can be obtained with high accuracy at a very low computational cost, but the procedure could be too expensive for a few of them.
Absence of increase in length scale upon approaching the glass transition in liquid glycerol
Authors: Wiltzius Pierre , van Saarloos Wim
Published: July 2002

Earlier experiments based on comparing viscometer measurements of the viscosity of glycerol with the viscosity inferred from dynamic light scattering of Brownian particles have suggested the existence of an increasing microscopic length scale upon lowering the temperature towards the glass transition. We have performed accurate experiments on glycerol using Brownian particles of different sizes. Our results do not support the earlier claims.
Fluorescence and absorption spectroscopy of the near-infrared vibronic transitions in matrix-isolated NpF6
Authors: Mulford R. N. , Dewey H. J. , Barefield J. E.
Published: July 2002

The 4Γ8u and 2Γ7u excited states of NpF6 have been characterized in a solid argon matrix using absorption, fluorescence, and fluorescence excitation spectroscopies. Analysis of the vibronic bands of these states gives excited state vibrational frequencies which are compared to those of the ground state. The 4Γ8u state fluoresces with a 4.6 ms lifetime and a quantum yield near unity.
Thermalization of photoelectrons in polar medium
Authors: Rips Ilya , Silbey R. J.
Published: July 2002

The process of thermalization of a hot subexcitation photoelectron in polar medium is studied. The master equation is used as a starting point to derive analytic expression for the thermalization distances distribution function. The resulting expression depends upon the mechanism of energy dissipation via the first two moments of the energy loss probability. Asymptotic decay of the distribution with the distance depends on the character of electron motion (exponential for the ballistic motion; Gaussian for the diffusion). A general scheme was developed for evaluation of the energy loss probability within the framework of the linear response theory. Explicit results are derived for electron thermalization in polar medium whose dissipative properties are characterized by the Debye macroscopic dielectric susceptibility function (the Fröhlich–Platzman model). Comparison of the estimates of thermalization distance in water with the experiment shows that the randomization of the direction of the electron motion is much faster than the thermalization process. The dependence of the most probable (average) thermalization distance on the initial kinetic energy, En, and on the dielectric relaxation time, τD, is found for the ballistic and the diffusive motion of the electron (∝E3/4nτ 1/2D for diffusive motion). An explicit relation is derived between the probability to escape geminate recombination and the excess kinetic energy as well as the polarity of the medium and its relaxation time. Results are used to interpret the recent experimental data on the yield and kinetics of geminate recombination in normal and heavy water.
Fully L2 methods for multichannel scattering problems. Partial widths
Authors: Martín F. , Riera A. , Sánchez I.
Published: July 2002

We analyze the use of L2 discretization methods to calculate, in the framework of the Feshbach theory, partial and total widths of resonances lying in multichannel continua. When interchannel coupling is strong, partial widths cannot be obtained without solving the scattering equations and for this purpose a fully L2 method is proposed, starting from the static exchange approximation as a zeroth order result. For weak interchannel coupling, we study the validity of Born-type approximations and we justify why neglecting this coupling may be appropriate to evaluate total widths even for cases where it is totally inadequate for the corresponding partial ones. The methods proposed are illustrated for moderately strong (He) and weak (O6+) coupling in atomic resonances.
A coupled-cluster study of the ground state of C+3
Authors: Watts John D. , Stanton John F. , Gauss Jürgen , Bartlett Rodney J.
Published: July 2002

We report calculations of the structure and harmonic frequencies of the 2B2 state of C+3 and the energy of this state relative to the linear geometry (2Σ+u) using self-consistent-field (SCF) and coupled-cluster (CC) methods, including the full coupled-cluster single, double, and triple excitation (CCSDT) model. The calculations on 2B2 C+3 are the most complete treatment to date and may be of assistance in further experimental detection of this species. The calculations of the 2Σ+u–2B2 energy difference support a bent structure. These calculations also show that, compared with full CCSDT and configuration interaction single, double, triple, and quadruple excitation (CISDTQ) results, CC methods which only approximately include effects of connected triple excitations seem for this example to give misleadingly small energy differences. A recent prediction by such approximate methods that C+3 may be quasilinear is therefore questionable. In the course of this work, certain practical difficulties in the SCF description of 2B2 C+3 were encountered, namely symmetry breaking of the restricted open-shell Hartree–Fock (ROHF) wave function and the existence of two distinct 2B2 unrestricted Hartree–Fock (UHF) solutions. We show that these can be alleviated by using our quasirestricted Hartree–Fock CC approach (QRHF-CC). Coupled-cluster single and double excitation (CCSD) calculations based on a QRHF reference function consisting of neutral molecule orbitals are able to provide all frequencies, unlike those based on a ROHF reference function. This work shows that QRHF-CC calculations offer a convenient single reference solution to certain problems involving symmetry breaking or other difficulties which traditionally have been solved by multireference methods. This quality of the QRHF-CC approach appears not to have been recognized previously.
An iterative method for the direct calculation of total electronic energy and correlated wave function through the use of the Colle–Salvetti correlation potential
Authors: Colle Renato , Salvetti Oriano
Published: July 2002

An iterative method is proposed for calculating the ground state energy and the corresponding correlated wave function of an effective all-electron Hamiltonian containing the Colle–Salvetti correlation potential. Through calculations on systems with highly polarizable cores and on excited states of organic molecules, it is shown that this approach allows one to properly correct the Hartree–Fock values of several important atomic and molecular properties.
Electron–ion coincidences studies of the fragmentation of thiophene and of tetrahydrothiophene upon core ionization
Authors: Hayes Robert G. , Eberhardt Wolfgang
Published: July 2002

We have measured the fragment ion spectra of thiophene and tetrahydrothiophene in coincidence with selected Auger final states following the ionization of a S 2p or C 1s core electron. Contrary to our earlier observations for diatomic and triatomic molecules, for thiophene and tetrahydrothiophene the fragmentation is quite unselective to the extent that many of the fragmentation channels are open immediately at the Auger threshold. Moreover, the kinetic energy released in the fragmentation is largely independent of the energy of the Auger final state.
Use of corrected centrifugal sudden approximations for the calculation of effective cross sections. II. The N2–He system
Authors: Thachuk Mark , McCourt Frederick R. W.
Published: July 2002

A series of centrifugal sudden (CS) and infinite-order sudden (IOS) approximations together with their corrected versions, respectively, the corrected centrifugal sudden (CCS) and corrected infinite-order sudden (CIOS) approximations, originally introduced by McLenithan and Secrest [J. Chem. Phys. 80, 2480 (1987)], have been compared with the close-coupled (CC) method for the N2–He interaction. This extends previous work using the H2–He system [J. Chem. Phys. 93, 3931 (1990)] to an interaction which is more anisotropic and more classical in nature. A set of eleven energy dependent cross sections, including both relaxation and production types, has been calculated using the LF- and LA-labeling schemes for the CS approximation, as well as the KI-, KF-, KA-, and KM-labeling schemes for the IOS approximation. The latter scheme is defined as KM=K=max(kj,kjI). Further, a number of temperature dependent cross sections formed from thermal averages of the above set have also been compared at 100 and 200 K. These comparisons have shown that the CS approximation produced accurate results for relaxation type cross sections regardless of the L-labeling scheme chosen, but inaccurate results for production type cross sections. Further, except for one particular cross section, the CCS approximation did not generally improve the accuracy of the CS results using either the LF- or LA-labeling schemes. The accuracy of the IOS results vary greatly between the cross sections with the most accurate values given by the KM-labeling scheme. The CIOS approximation generally increases the accuracy of the corresponding IOS results but does not completely eliminate the errors associated with them. For some cross sections, the CIOS results are more accurate than the corresponding CS or CCS results.
Fourier transform infrared spectroscopy of the ν3 band of cyclopropenylidene, C3H2
Authors: Hirahara Yasuhiro , Masuda Akimasa , Kawaguchi Kentarou
Published: July 2002

The gas-phase absorption spectrum of the ν3 band of cyclopropenylidene (cyclic C3H2 ) has been observed with a high-resolution Fourier transform infrared spectrometer. The ring molecule was generated by a microwave discharge in an allene and Ar mixture. The spectrum observed between 1250 and 1305 cm−1 was analyzed by using Watson’s A-reduced Hamiltonian to determine the band origin (ν0=1277.3711±0.0001 cm−1 ), rotational constants, and centrifugal distortion constants in the ν3 state.
Influence of two-photon absorption on third-order nonlinear optical processes as studied by degenerate four-wave mixing: The study of soluble didecyloxy substituted polyphenyls
Authors: Zhao Mingtang , Cui Yiping , Samoc Marek , Prasad Paras N. , Unroe Marilyn R. , Reinhardt Bruce A.
Published: July 2002

We have investigated the influence of two-photon absorption on the third-order nonlinear optical properties of model organic molecules using the technique of degenerate four-wave mixing (DFWM). A theoretical formulation developed here shows that the presence of two-photon absorption, which is related to the imaginary part of the third-order susceptibility χ(3), leads to an enhancement of the effective third-order nonlinearity and to the appearance of effects caused by the formation of two-photon generated excited states. The dynamic behavior of the nonlinearity is then governed by the properties of excited molecules. The nonlinear effects also involve contributions which depend on the fifth power of the electric field. We have performed a systematic study of third-order nonlinear optical properties of alkoxy (–C10H21OCH) substituted p-polyphenyl oligomers using the technique of time-resolved degenerate four-wave mixing with subpicosecond pulses at 602 nm. Experimentally determined values of the second-order hyperpolarizability γ for the oligomers increase smoothly from the monomer to the trimer, with a more rapid increase to the pentamer and to the heptamer. In addition, the hyperpolarizabilities for the pentamer and the heptamer appear to be complex. A smooth increase of the γ value is expected from an increase of the π conjugation from a shorter chain oligomer to a longer chain oligomer. The more rapid increase of the γ value in the pentamer, and especially in the heptamer, however, cannot be explained satisfactorily by only taking into account the π-conjugation length. Two-photon absorption for the pentamer and the heptamer at the measurement wavelength of 602 nm is suggested to be important as the observed dynamic behavior is satisfactorily explained by the predictions of the theoretical model presented here. It is shown that the effective γ value for a two-photon absorbing material is a function of optical intensity, pulse width, and sample length if one uses the conventional degenerate four-wave mixing description.
Photodissociation spectroscopy of benzene cluster ions: (C6H6)+2 and (C6H6)+3
Authors: Ohashi Kazuhiko , Nishi Nobuyuki
Published: July 2002

The photodissociation of size-selected benzene cluster ions, (C6H6)+m+hω→(C6H6)+n+ (m−n)C6H6, has been investigated in the 410–750 nm wavelength range using tunable dye laser radiation. The measurements were performed using a tandem mass spectrometer [a linear time-of-flight (TOF)/reflectron TOF] combined with multiphoton ionization (MPI) for ion preparation. Only C6H+6 was detected as a photofragment of (C6H6)+2 and (C6H6)+3, while both C6H+6 and (C6H6)+2 fragments were observed in the case of (C6H6)+4 photodissociation. Photodissociation spectra, i.e., photofragment yield spectra as a function of wavelength, of (C6H6)+m (m=2,3) were obtained. Two local excitation bands of (C6H6)+m were seen in this region and assigned to the C(A2u)←X(E1g) and the B(E2g)←X(E1g) transitions of a C6H+6 unit in the clusters. The origin of the B←X transition of (C6H6)+2 and (C6H6)+3 was redshifted relative to that of C6H+6 by about 1400 and 2400 cm−1, respectively, while the C←X bands of (C6H6)+2 and (C6H6)+3 were seen at the same wavelengths of 440 nm. Possible structures for the cluster ions are discussed based on the spectral shifts.
On the scattered-wave X alpha calculation of potential energy surfaces for transition metal impurities in ionic lattices. Mn2+ and Cu+ in NaF
Authors: Till Stephen J. , Parsons Ian W. , Huke Jeremy P.
Published: July 2002

Cu+ and Mn2+ impurities in NaF are studied within the perturbed cluster approximation. The MTXα method is used to compute potential energy curves for the a1g vibrational modes of CuF5−6 and MnF4−6 clusters embedded in the host. The clusters are stabilized by an electrostatic lattice potential evaluated analytically from Slater orbital (SO) approximations to the true lattice ion wave functions. This approach is superior to the common Watson Sphere model for the cluster environment, but gives unsatisfactory muffin-tin total energy curves. To remedy this, the implementation and evaluation of non-muffin-tin corrected total energies for embedded clusters is described and shown to give improved results. Further refinements to the model are discussed.
Contrasting the surface adsorption of comb and linear polymers
Authors: Balazs Anna C. , Siemasko Chris P.
Published: July 2002

Using Monte Carlo computer simulations, we contrast the surface adsorption of comb and linear polymers that contain the same number of monomers. Both homopolymers and copolymer chains are examined. The homopolymers are composed of units that are strongly attracted to the surface (‘‘stickers’’), while the copolymers contain both stickers and sites that do not have a high affinity for the surface. Comb polymers, which contain long ‘‘sticky’’ teeth, provide a higher or comparable surface coverage, thinner films, and greater polymer–surface adhesion and wettability than the equivalent linear chains. For homopolymer and copolymer combs that contain sticky teeth, the behavior of the long teeth dominate the properties of the chains at the interface. Thus differences between these two types of chains diminish as the tooth length is increased. Finally, combs that contain sticky backbones but nonsticky teeth provide a significant polymer density in the outer layers of the adsorbed film. Thus the width of the adlayer can be tailored by varying the location of the stickers in the comb polymers.
Methyl torsional fine structure in the high-resolution S→T(nπ*) excitation spectrum of acetophenone and its three methyl-deuterated isotopomers
Authors: Siebrand W. , Zgierski M. Z. , Amirav A. , Fuchs B. , Penner A. , Lim E. C.
Published: July 2002

The cold-beam excitation spectrum of the S0→T(nπ*) origin band of acetophenone shows two maxima separated by about 1.2 cm−1, interpreted as due to methyl tunneling. Consecutive deuteration of the methyl group leads to three maxima in acetophenone-d1, two maxima in acetophenone-d2 and a single broad maximum in acetophenone-d3, with maximum separations of about 7 cm−1 in -d1 and about 3.5 cm−1 in -d2. These separations are due to the occurrence of two rotamers for isotopomers without threefold permutation symmetry. The spectra are analyzed in terms of hindered rotor potentials. It is shown that S0 and T(nπ*) share the same torsional equilibrium configuration, relative to which the S1(nπ*) equilibrium configuration is rotated by 60°. However, both nπ* states have very shallow torsional potentials with barrier heights of 70 and 90 cm−1 for T and S, respectively, compared with a barrier of about 800 cm−1 in the ground state. Hence, relative to S0 the two potentials are quite similar despite the shift. The potential terms entering upon partial deuteration are quite similar for S0 and T(nπ*) and hence depend only weakly on the barrier height. They are ascribed to kinetic coupling of the asymmetric rotor of the CH2D or CHD2 group with other modes in the molecule.
Feshbach–Fano formalism in Hilbert space: Application to shape resonances in molecular photoionization
Authors: Winstead C. , Langhoff P. W.
Published: July 2002

An explicit Hilbert-space formulation of Feshbach–Fano theory is described which is particularly well suited for treating the problem of shape resonances in molecular photoionization. The prepared states of Fano and Cooper are employed to resolve the irreducibly infinite degeneracy of molecular electronic continua that arises from the noncentral nature of molecular (body-frame) potentials. Previously defined L2 Stieltjes states generated employing appropriate test functions are shown to converge to these prepared states, avoiding prior construction of the associated degenerate channel functions. A novel choice of zeroth-order state particularly suitable for applications of the L2 Feshbach–Fano method to molecular shape resonances is introduced and shown to avoid many of the difficulties associated with more conventional treatments employing resonance scattering theory. Specifically, the energy of this zeroth-order state is seen to be a weighted average over the correct photoionization cross section, the associated background cross section is shown to vanish for all energies, and calculations of the associated ρ(E) and q(E) parameters are avoided entirely. Computational applications in the static-exchange approximation to selected valence-shell channels in N2, O2, and N2O illustrate the attributes of the method and provide accurate cross sections in each case. The L2 Feshbach–Fano formalism is seen to provide improved results from precisely the same information—pseudostate energies and oscillator strengths—as is required in the conventional L2 Stieltjes development, with no significant increase in computational effort required.