bluuebunny/crossref_metadata_2025 huggingface dataset:
An exact coupled-channel study of the transition state spectroscopy of the collinear H+H2 reaction
Authors: Krause Jeffrey L. , Shapiro Moshe
Published: July 2002

We present an exact coupled-channels method for calculating free-bound transition dipole amplitudes in reactive chemical systems. The method employs a log-derivative propagator in hyperspherical coordinates and the artificial channel method to calculate the desired integrals implicitly. We apply this method to a study of the transition state spectroscopy of the collinear H+H2 reaction, analyzing in detail the effect of the reactive Feshbach resonances on the spectra. A statistical analysis of the spectral intensity fluctuations shows that at low collision energies the intensity distribution is poorly fit by the Porter–Thomas distribution. As the collision energy increases, the fit becomes very good, indicating strongly coupled degrees of freedom. These results indicate the importance of considering the preparation of the initial state when performing statistical analyses.
Statistical mechanics of a quenched van der Waals fluid
Authors: Singh Rajiv R. , Kovac Jeffrey
Published: July 2002

The equilibrium properties of a quenched one-dimensional fluid with a high density phase transition are calculated using the formalism of Mazo in which the free energy is first calculated as a function of a certain quenched parameter and the quenched free energy is evaluated by averaging over the probability that the constraint is realized in the quenching process. The model system studied here is one in which the particles interact through a pair potential with a shoulder in the repulsive wall. The quenched parameter is taken to be the width of the shoulder, corresponding to the introduction of disorder or free volume into the quenched phase. Assuming a Gaussian distribution of shoulder widths it is possible to calculate the quenched free energy exactly and compare the thermodynamic properties of the quenched fluid to those of the equilibrium fluid. The quenched disorder softens the first-order freezing transition into a second-order-like glass transition which qualitatively reproduces the quasiequilibrium properties of real glasses.
Reorientational motion of diphenylpolyenes in simple solvents as studied by depolarized light scattering and fluorescence anisotropy decay
Authors: Uphoff R. , Potthast R. , Dorfmüller Th.
Published: July 2002

The reorientational motion of diphenylpolyenes and related molecules in simple solvents is studied by depolarized light scattering and fluorescence anisotropy decay. The viscosity dependence of the reorientational correlation times obtained by the two techniques shows remarkable differences, although the Stokes–Einstein–Debye equation applies to all data. The light scattering results are consistent with hydrodynamic models and slip boundary conditions. The reorientation about an axis perpendicular to the long molecular axis is monitored. With the fluorescence anisotropy decay measurements of the diphenylpolyenes on the other hand, a much smaller viscosity dependence is found. The influence of photophysical processes in the excited state on the fluorescence depolarization is discussed and it is shown that the fluorescence anisotropy measurements are sensitive to reorientation about a perpendicular axis as well as about the long molecular axis.
Linear analysis of the polarization of macroions
Authors: Manning Gerald S.
Published: July 2002

The dipole moment induced in the ion atmosphere of a macroion by application of a steady electrical field is calculated when both the applied field and the field of the macroion are small. The macroion is modeled as a rigid array of point charges in general configuration. Linearized transport and equilibrium equations are then solved analytically by Fourier transform analysis. In the absence of relative motion between macroion and solvent the moment is isotropic and proportional to the number of charges on the macroion. It varies inversely as the concentration of supporting salt. For a symmetrical salt (same charge but for sign on counterion and co-ion), the dipole moment vanishes, as the salt ions of one sign prevent polarization of the ions of opposite sign. For a general salt the moment does not vanish. If the counterion charge is the larger of the two salt ions, the induced moment is positive. It is negative if the co-ion has the larger charge. There is a clear sense in which the polarization of co-ion charge may be said to control this complex behavior. The polarizability tensor can be decomposed into tensors αPl introduced by Fixman and Jagannathan [J. Chem. Phys. 75, 4048 (1981)] and computed numerically by them for a cylindrical macroion in a nonlinear context (small applied field but macroion field of general magnitude). Comparison with their tabulated results suggests that the properties of these tensors in the range of short cylinders and large Debye lengths reflect linear behavior. When the relative motion of macroion and solvent is included in the linear analysis, the induced dipole moment becomes anisotropic (and hence, in particular, does not vanish for a symmetrical salt). The convective effect is similar to what was found by Fixman and Jagannathan.
Anomalous line shapes in delayed optical–optical double resonance studies of N2
Authors: Li Leping , Chupka William A. , Pratt Stephen T.
Published: July 2002

Anomalous dips are observed in time-delayed, ionization-detected optical–optical double resonance (OODR) experiments on molecular nitrogen. These dips arise from the competition between radiative decay and photoionization of the upper level of the probe transition, and are therefore of considerable potential importance in the application of ionization-detected OODR to molecules.
Polarization fluctuations as a source of electron spin resonance linewidths and relaxation of spin probes in KH2PO4-type of polar solids
Authors: Dalal N. S. , Rakvin B.
Published: July 2002

While the CrO3−4 and SeO3−4 radicals have been used as spin probes in numerous electron spin resonance (ESR) studies of phase transitions in the KH2PO4-type of solids, discrepancies exist regarding the nature of molecular reorientations sensed by these probes, in particular, on whether the detected motion is thermally activated, with a well-defined activation energy, ΔE. Our new ESR data on these probes show that their linewidth, Γ, varies as Γ=Γ0+A exp(ΔE/kT)+B exp(−ΔE/kT). Here Γ0 is the temperature-independent, residual linewidth and A and B are parameters that depend on the ESR line splittings and the Zeeman field. ΔE has been estimated to be ≈0.22±0.02 eV (20±2 kJ/mol) for CrO3−4 in KD2AsO4, KD2PO4, NH4H2AsO4, ND4D2AsO4, and ND4D2PO4, and SeO3−4 in KD2PO4. The results suggest that the omission of the term B exp(−ΔE/kT) in all of the earlier analyses of the ESR linewidth data from these probes was the major cause of the reported non-Arrhenius behavior, especially in the high temperature (T≫Tc) regime. Measurements utilizing externally applied electric fields indicate that this new, Orbach-type, ESR line broadening process originates from thermally activated spin–flips via vibronic transitions, and ΔE provides a measure of the barrier height for the intrabond hydrogen transfer along the O–H⋅ ⋅ ⋅O bonds.
Quasi-one-dimensional dipolar ordering near the ferroelectric phase transition in t
r
i
s-sarcosine calcium chloride crystals
Authors: Fujimoto M.
Published: July 2002

A quasi-one-dimensional chain model of Ca(sarcosine)6 complexes is proposed for the ferroelectric phase transition in tris-sarcosine calcium chloride (TSCC) crystals. Although in the noncritical region the two states of distortion in the complex related by mirror reflection could be described by the Ising spin, when applied to the critical EPR anomaly the pseudospin was found to behave like a classical vector, giving rise to a modulated structure along the ferroelectric b axis. The modulation was sinusoidal near the transition temperature, but changed to a rectangular structure of a longer period. The pseudospin dynamics reflected in the EPR anomaly exhibited a quasi-one-dimensional character, which is consistent with the uniaxial properties of TSCC crystals. The modulated pseudospin structure is regarded as a condensate sustained by the lattice strain of a long wavelength, which is induced when soft phonons condense at the transition temperature. It occurs in phase with the phonons, and hence the observed EPR anomaly was analyzed in light of the theory of a one-dimensional anharmonic lattice. It is believed that such a structure is stabilized by a negative correlation energy among pseudospins. In this model a long-range dipolar order develops as the local electric field in the chain modifies the pseudospin modulation. The significance of uniaxial dipole–dipole interactions is discussed for the ferroelectric order in TSCC crystals.
Structure and dynamics of the fluorperovskite, RbCaF3
Authors: Nosé Shuichi , Klein Michael L.
Published: July 2002

A rigid-ion model has been used in conjunction with molecular dynamics calculations to investigate the structure and dynamics of RbCaF3. The effect of cooling the high-temperature cubic phase has been studied using the constant-pressure constant-temperature technique. Below about 150 K we find evidence of transitions to tetragonal and orthorhombic structures. The phase transition, which involves the cooperative rotation of CaF6 octahedra, is shown to be of the classic displacive type.
Numerical multiconfiguration self-consistent-field study of the hyperfine structure in the infrared spectrum of 3He4He+
Authors: Yu Nan , Adamowicz Ludwik
Published: July 2002

Numerical multiconfiguration self-consistent-field (MCSCF) procedure is employed to calculate the hyperfine interaction energy for the 3He4He+ cation at different internuclear separations. A conventional vibrational averaging of the energy allows the calculation of hyperfine splitting in the IR spectrum. This is done for several of the lowest vibrational states. We predict that the hyperfine splitting will get larger with the increasing vibrational excitation. Various different MCSCF wave functions are used in the study to verify the convergence of the hyperfine parameters and to determine the importance of the electronic correlation.
Relaxation in supercooled liquids: Similarities between projected and unprojected correlation functions
Authors: MacPhail Richard A. , Kivelson Daniel
Published: July 2002

We have examined and interrelated a number of different relaxation processes in supercooled liquids by use of molecular correlation functions, some of which are newly developed and have projected dynamics. Experimentally it is found that in supercooled liquids both the projected and unprojected correlation functions yield similar Cole–Cole plots with skewed arc behavior. The similarity between the projected and unprojected correlation functions is possible only for a certain range of the parameter μJr(0)[τm/τcd]β, where μ and Jr(ω) are the shear modulus and retardational compliance, respectively, τm is the Maxwell stress relaxation time, and τcd and β are the relaxation time and spectral width parameter, respectively, which describe a Cole–Davidson representation of the skewed arc behavior of Jr(ω). The fact that this experimental parameter is found to be of order 2, independent of temperature, has been known, but the fact that values close to this are required for the projected and unprojected correlation functions to be similar, which in turn is required for the Cole–Cole plots for dielectric correlation and retardational compliance to be similar (as observed) is a feature newly explored in this article.
Dynamics of nonspherical bodies: Relative diffusion of a sphere with respect to a fixed convex body
Authors: Evans Glenn T.
Published: July 2002

A Smoluchowski equation is derived that depicts the relative diffusion of a sphere with respect to a fixed nonspherical convex body. The transport equation is expressed in terms of the surface-to-surface separation s, measured along the surface normal k. In this coordinate system, the effects of particle shape anisotropy can be subsumed by an appropriate mapping of the radius of the convex excluded volume surface onto that of an equivalent spherical system. Steric effects in diffusion-controlled reaction rates and shape anisotropy corrections to the recollision kinetic theory of rotation are analyzed in light of the chosen coordinates and the explicit Smoluchowski equation.
Thermal electron attachment to carbon suboxide
Authors: Shimamori Hiroshi , Suzuki Etsuhito , Hatano Yoshihiko , Nonaka Hidehiko , Ohshima Yasuhiro , Kondow Tamotsu , Kuchitsu Kozo
Published: July 2002

Thermal electron attachment to C3O2 has been studied by the pulse radiolysis–microwave cavity technique. The time variation of electron concentration for mixtures C3O2 with Ar and N2 shows decays with two components and the profile changes significantly depending on both C3O2 and buffer-gas pressures. This can be interpreted by a fast electron attachment to C3O2 to form a compound negative ion in equilibrium with the free electron followed by a slow stabilization of the ion leading to a stable ion. Simulations for the observed decay profiles indicate the presence of two electron attachment–detachment equilibria, one associated with electron and C3O2 and the other including electron, C3O2, and the buffer-gas molecule. The slow stabilization occurs by two-step three-body processes involving intermediate dimer ion. This is supported by a ‘‘negative temperature dependence’’ observed for the stabilization rates between 270 and 325 K. Though the electron affinity of C3O2 is negative, that of (C3O2)2 is likely to be positive. The role of electron attachment has been discussed in relation to an anomalously large cross section for ionization of a high-Rydberg atom by C3O2 reported previously.
Optical refractive and reflective properties of resonantly absorbing media
Authors: Ahmed S. A. , Ali M. A. , Mitwally K. , El-Massry M.
Published: July 2002

Experiments on the reflection of polarized monochromatic laser light at the planar interface of resonantly absorbing medium and the air were made for a variety of situations, including Brewster’s angle in the spectral vicinity of the resonance wavelength. An ethanolic solution of Rhodamine B, an organic laser dye luminofor with a well defined resonance absorption spectrum, was chosen as the absorbing medium. The tunable polarized output of a cw dye laser was used to provide the collimated monochromatic beam. To compare the experimental results with theory, the Fresnel reflectivity equations combined with the Kramers–Kronig relations were used to predict the reflectance in terms of the known absorption coefficient. The experimental results generally confirm the theoretical predictions.
Pressure dependence of the polarized reflectance spectrum of the solid charge-transfer complex, perylene–TCNQ: Estimation of microscopic parameters
Authors: Ida Takashi , Yakushi Kyuya , Kuroda Haruo
Published: July 2002

The polarized reflectance spectrum of the perylene–TCNQ complex has been measured at high pressure up to 27 kbar using a diamond anvil cell incorporated into a microspectrophotometer. The degree of charge transfer (ρ), the transfer integral (t), and the site-energy difference (Δ) are estimated at each pressure from analysis of the charge-transfer band. The pressure dependences of t and Δ are found to be ∂ ln t/∂P=+2.3% kbar−1 and ∂Δ/∂P=−6.4 MeV kbar−1, respectively. It is shown that the observed pressure dependence of the stabilization energy of the charge-transfer excited state is mainly attributable to the change in the electrostatic energy accompanying the lattice contraction.
Vibrationally inelastic collision cross sections of He+SO2: Distorted wave approach
Authors: Novak M. M. , Balint-Kurti G. G. , Clary D. C.
Published: July 2002

Vibrational excitation and relaxation cross sections of He colliding with SO2 molecule are evaluated. The results obtained rely on several approximate methods. The infinite-order-sudden approximation provides the framework for treating the rotational motion, while the vibrational processes are described using both exact close-coupling and approximate distorted wave techniques. The latter scheme permits the computations to be extended to much higher collisional energies than so far reported in the literature. Vibrational excitation and relaxation cross sections are presented over a range of collision energies.
Highly efficient D/H scrambling and highly selective chlorination in γ-irradiated D- and H-octane mixture in freon 113: Trace back of transient processes by the observation of D/H scrambling with high resolution nuclear magnetic resonance
Authors: Okazaki Masaharu , Toriyama Kazumi , Nunome Keichi
Published: July 2002

A high resolution nuclear magnetic resonance (NMR) study on the γ-irradiated isotopic mixture of protiated and deuteriated octanes in freon 113 revealed that the protons and the deuterons are scrambled very efficiently (G>2000) at all the positions. While the solvent freon was substituted selectively with a proton and chlorination of protiated octane occurred selectively at the secondary position. These observations, coexistence of the random D/H scrambling and the selective substitution reactions, are the evidence that: (1) in the ‘‘hot’’ period rapid intermolecular radical migration occurs among the solute alkanes, the solvent freons, and the product chloroalkanes, (2) on the other hand, in the following ‘‘thermalized’’ period, chemical bonds within which radical site migration occurs are limited to those with smaller bonding energies, i.e., the secondary position of (protiated) alkane, the C–Cl bonds of freon, and the C–Cl bond of chloroalkanes.
Field dependence of the nuclear magnetic resonance static powder line shape: Studies of the mutual orientation of interaction tensors from the field dependence of the critical frequencies
Authors: Chu P. J. , Gerstein B. C.
Published: July 2002

A procedure for determining the interaction tensor orientations and the interaction parameters for mutually oriented electric field gradient (efg) or dipolar and shielding tensors has been developed based upon the magnetic field dependence of the critical frequencies from the polycrystalline nuclear magnetic response (NMR) spectrum. The central transition line shape of half-integer quadrupolar nuclei in the presence of mutually oriented shift anisotropy and second order quadrupolar interaction is discussed for the first time. Analytical expressions for the field dependent critical frequencies have been determined for special orientations when the shift principal Z axis lies on the XZ, YZ, or XY plane of the efg (or dipolar) tensor for the first order as well as the second order interactions. The plots resulting from the analytical expressions provide a convenient graphical approach in determining approximate tensorial orientations and interaction parameters through pattern recognition. For general orientations, a numerical procedure has been developed to determine these parameters by iteratively minimizing the squares of the differences of the calculated and the experimental critical frequencies. Higher order perturbation terms can be incorporated in the present treatment. The method is demonstrated by variable field static proton spectra of trichloro–acetic acid at three different fields (1.3, 2.3, 5.2 T). The near-orthogonal orientation between the dipolar and shielding tensors and the interaction parameters obtained from this approach are consistent with those obtained previously from single crystal studies.
New method for calculating atomic and molecular resonances by partitioning technique
Authors: Froelich P. , Höghede M. , Alexander S. A.
Published: July 2002

A new implementation of the complex coordinate method is presented. It is a combination of the stabilization method with the partitioning technique, supplemented with the idea of dilation-adapted basis sets. This method is designed to handle large scale calculations of many-electron resonant states, as to minimize the amount of complex arithmetics and in particular to avoid the diagonalization of the complex eigenvalue problem. Both a resonant eigenvalue and a corresponding eigenvector are obtained. An application to the lowest doubly excited autoionizing states of helium is presented.
Elastic properties of unpolymerized and polymerized cadmium octadecyl/fumarate and cadmium octadecyl/maleate Langmuir–Blodgett films determined by Brillouin scattering
Authors: Lee S. , Hillebrands B. , Stegeman G. I. , Laxhuber L. A. , Swalen J. D.
Published: July 2002

We have measured the elastic properties of saturated ODF, and of unpolymerized and polymerized regular ODF, ODF/ODM, and ω-ODF Langmuir–Blodgett films by means of Brillouin spectroscopy. Whereas the ODF/ODM films show a slight increase in the elastic constants, the regular ODF and ω-ODF films exhibit elastic constants which are significantly softer in the polymerized films due to the formation of microcracks on polymerization.
Continuous emission spectrum of ArAl+ excimer
Authors: Yamashita Takuro
Published: July 2002

A broad emission band spectrum was observed at 192.7 nm by passing dc discharge through low pressure argon gas with aluminum electrodes. The relative spectral intensity distribution of the band was estimated by calibrating the monochromator system by means of the molecular branching ratio method and was then compared with the theoretical result by Sato et al. The origin of the band was further investigated experimentally. All the results strongly suggest that the observed broad emission band originates from the ArAl+ excimer.
Polarized resonant Raman scattering of c
i
s polyacetylene
Authors: Lanzani G. , Luzzati S. , Tubino R. , Dellepiane G.
Published: July 2002

Polarized resonance Raman measurements of highly oriented films of cis polyacetylene have been carried out. The intensities of the three Raman fundamentals in the four different scattering configurations have been measured and corrected for the strong anisotropy of the optical properties. The experimental data are indicative of an almost perfect alignment of the cis polyenic chains along the stretching direction. Preliminary measurements of the depolarization ratios carried out on isotropic samples in the resonance region appear to be inconsistent with the view that the visible band is dominated by a single transition polarized along the chain axis.
Radiation pressure induced gratings in colloidal suspensions: Dynamics of formation and decay
Authors: Xia Keqing , Park Il Hyun , Johnson Charles S.
Published: July 2002

The growth and decay of radiation pressure induced (concentration) gratings (RPIGs) have been monitored by diffraction of a probe beam. In all cases the growth of diffracted intensity in the presence of radiation pressure followed a curve of the form I=I0 [1−exp(γt/τD )]2 where γ is a constant and τD is the characteristic time for diffusion. When the writing laser pulse was terminated, the decay of diffracted intensity was described by exp(−2t/τD ). The fitting parameter γ is equal to unity at low intensities and becomes linear in radiation intensity at high intensities. A simple equation relating γ to the ratio of the potential energy of a microsphere in the radiation field to its average kinetic energy has been derived. Diffusion coefficients can be determined from τD values even for turbid samples; however, errors were found when short path lengths (0.1 mm) were used. This is thought to result from particle-surface interactions under the influence of the scattering force.
Dynamics of helical worm-like chains. XI. Translational diffusion with fluctuating hydrodynamic interaction
Authors: Yamakawa Hiromi , Yoshizaki Takenao
Published: July 2002

The translational diffusion coefficient D of long enough polymer chains without excluded volume is studied on the basis of the discrete helical worm-like (HW) chain with partially fluctuating (orientation-dependent) hydrodynamic interaction (HI). In order to evaluate D(∞) in the long time limit, which corresponds to diffusion experiments usually done on the long time scales, the time-dependent part of D is explicitly treated by an application of the projection operator method. The result may be written as D(∞)=D(Z)(1−δ0−δ1), where D(Z) is the Zimm value with preaveraged HI, δ0 is the relative decrease at time t=0 due to constraints, and δ1 is the additional relative decrease at t=∞ due to coupling between the translational motion and the Rouse vector (dielectric) modes for the HW chain, especially the long wavelength ones. It is found that D(∞) is decreased as the local conformation of the chain becomes rather compact, and that D(∞)=D(Z) (i.e., δ0=δ1=0) in the stiff-chain limit. Thus the ratio ρ of the root-mean-square radius of gyration to the hydrodynamic radius defined from D is not a universal constant but depends on the chain conformation and stiffness. A comparison of theory with experiment is made with respect to ρ, and it is found that the theory may well explain experimental results. For comparison, also for the Gaussian chain (spring–bead model), which is an unconstrained chain, D(∞) is evaluated, and it is found that δ0=0 but δ1≠0.
Tunable vacuum ultraviolet laser spectroscopy of the D(O+u) ion-pair state of jet-cooled I2
Authors: Bartels M. , Donovan R. J. , Holmes A. J. , Langridge-Smith P. R. R. , MacDonald M. A. , Ridley T.
Published: July 2002

High-resolution vacuum ultraviolet fluorescence excitation spectra of the D(O+u) ion-pair state of I2 have been obtained using a pulsed supersonic jet. Tunable vacuum ultraviolet radiation in the region 184–200 nm was produced by Raman shifting (fifth and sixth anti-Stokes) the output from a tunable excimer pumped dye laser. Vibrational constants valid up to v′=201 in the D(O+u) state have been determined.
Soft repulsions in a lattice model for liquid crystalline order in self-assembling systems
Authors: Hentschke Reinhard , Herzfeld Judith
Published: July 2002

Herzfeld and co-workers have developed a lattice model describing the coupling between self-assembly and liquid crystalline order in aqueous solutions of amphiphilic molecules. Even for simple assembly into rod-like aggregates of constant cross section, they find unexpectedly complex phase behavior. Here we propose a direct extension of the earlier model to incorporate soft repulsions. The main effects are illustrated for labile rod-like aggregates. We show that the phase behavior of self-assembling systems is very sensitive to the steepness of repulsions between particles in certain parts of the phase diagram.
Librational lattice modulation in incommensurate phases of betaine calcium chloride dihydrate
Authors: Fujimoto M. , Kotake Y.
Published: July 2002

The librational lattice modulation in incommensurate phases of betaine calcium chloride dihydrate (BCCD) crystals was investigated by the EPR spectra of VO2+ impurities substituted for Ca2+ ions in the lattice. Below 164 K, the spectra exhibited anomalously broadened 51V-hyperfine lines, representing pinned condensates of librational order parameters. The angular variation of the anomalies indicates that the Ca2+-betaine complex librates between two states of twisted deformation. In contrast with the Mn2+ spectra from BCCD, the staircase transitions were not faithfully indicated by the VO2+ spectra, being consistent with the absence of dielectric dispersions in VO2+-doped BCCD. The asymmetry in the VO2+ complex is considered to be responsible for strongly pinned condensates, resulting in no further staircase to be evolved otherwise. Although the whole transition scheme in BCCD except at 164 K suffers a modification, the VO2+ spectrum has provided clear evidence for a collective libration of Ca complexes below 164 K.
Vibrational relaxation in pyridine
Authors: Cardamone M. J. , Rhodes W. , Eberlein T. H. , Cardamone D. M. , Stutzman B. A.
Published: July 2002

Vibrational relaxation in liquid pyridine was investigated by means of Brillouin scattering. Comparison of experimental results with theoretical predictions indicates a double relaxation with all but the two lowest vibrational modes having relaxed at lower than experimental frequencies. These results are in agreement with those of Takagi and Negishi [J. Acoust. Soc. Am. 65, 86 (1979)], rather than those of Jain and Whittenburg [J. Phys. Chem. 92, 2023 (1988)].
An experimental and theoretical study of the reactions Na+HCl and Na+DCl
Authors: Plane John M. C. , Rajasekhar B. , Bartolotti Libero
Published: July 2002

An experimental study is presented of the reactions Na+HCl/DCl from 590 to 820 K. Na atoms were produced in an excess of HCl/DCl and He bath gas, by the pulsed photolysis of NaCl vapor. The metal atom concentration was then monitored by time-resolved laser induced fluorescence of Na atoms at λ=589 nm. A fit of the data to the Arrhenius form yields (2σ uncertainty): k(Na+HCl)=(2.1±0.5)×10−9 exp[(−41.8±1.5 kJ mol−1)/RT] cm3  molecule−1 s−1; k(Na+DCl)=(2.2±1.0)×10−9 exp[(−45.5±3.8 kJ mol−1)/RT] cm3  molecule−1 s−1. The large activation energies are shown to be consistent with vibrational excitation of the hydrogen halides greatly enhancing the reaction cross-sections. A pair of ab initio potential surfaces for these reactions are then calculated at constant angle cuts through the lowest 2 A′ hypersurface, including a collinear surface and the surface containing the lowest saddle point which is found to occur at a bent configuration with θNaClH=54.7°. Both surfaces exhibit a late reaction barrier. The effect of reactant vibrational excitation is then demonstrated by using quasiclassical trajectories on the collinear surface.
Observation of Rh-subcarbonyls on stepped Rh surfaces during catalytic reactions
Authors: Kruse N. , Abend G. , Block J. H.
Published: July 2002

The interaction of CO, CO2, and CH3OH with stepped Rh field emitter surfaces has been studied by means of pulsed field desorption mass spectrometry (PFDMS) at 300 K and pressures at or below 1.3×10−4 Pa. A common feature in the PFD mass spectra of the different adsorbed layers is the occurrence of CO+ and Rh(CO)n+x ions (x=1–3, n=1,2). These species indicate that a surface reaction takes place between chemisorbed CO and Rh step sites to form adsorbed Rh-subcarbonyls. COad is formed by CO adsorption from the gas phase as well as CO2 and CH3OH decomposition on the surface. The relative and absolute abundances of the various species depend on the desorption field strength FD of the pulses. High index Rh(CO)x are observed in large quantities at low FD. Under these conditions only small amounts of chemisorbed CO are removed and the surface is kept close to the saturation coverage. At high FD, the surface layer is completely desorbed, so that the COad concentration which builds up during the field free reaction time, tR=1 ms, is insufficient for Rh-subcarbonyl formation. Coadsorbed oxygen and hydrogen formed during CO2 and CH3 OH decomposition, respectively, do not prevent Rh(CO)x production on the stepped Rh surface.
The transition from nonadiabatic to solvent controlled adiabatic electron transfer: Solvent dynamical effects in the inverted regime
Authors: Yang D. Y. , Cukier R. I.
Published: July 2002

We analyze the effect of dynamical solvent effects on the rate of a nonadiabatic electron transfer (ET) reaction. Starting from a Hamiltonian for a reaction coordinate for motion along the potential surfaces of donor and acceptor species, and a bath representing the solvent dynamical effects, we obtain a system of four coupled reduced equations of motion for the elements of the density matrix of the donor/acceptor system. In this derivation the dynamics along the reaction coordinate are reduced to a classical Fokker–Planck operator since we assume the temperature is high compared with bath frequencies. At temperatures where the nuclear motion describing the transition between the surfaces can be treated classically we show that the ET processes may be viewed as a consecutive reaction scheme with rate constant k=kNA kD/(kD+kNA), the steps are diffusion along the reaction coordinate with rate constant kD followed by crossing between the donor and acceptor surfaces at the point of intersection of the surfaces with rate constant kNA. kNA is given by an activated rate expression and is independent of solvent dynamics, e.g., a dielectric relaxation time. When the nuclear motion must be treated quantum mechanically, as is favored in the inverted regime (where the surfaces have slopes of the same sign at their crossing point), the transition between the surfaces is no longer confined to their crossing point. We obtain an approximate expression of the above form where kNA incorporates the width of the transition. Then kNAdepends on solvent dynamics. When the separation into diffusive and crossing motion is no longer appropriate, we use a basis set expansion method to directly solve the four coupled density matrix equations to obtain k. These results are compared with the approximate formula given above.
A theory for two-dimensional Fourier-transform ion cyclotron resonance mass spectrometry
Authors: Guan Shenheng , Jones Patrick R.
Published: July 2002

A theoretical model, based on the Lorentz equations for ion motion and the mass action law, is developed for two-dimensional Fourier-transform mass spectrometry known as 2D FT–ICR or 2D FTMS. The theory illustrates that the modulation of 2D FT–ICR ion signals in the additional time dimension comes from the modulation of the primary ion speed by the 2D excitation pulses. The modulation of the primary ion speed is found not to be sinusoidal and the modulation of the ion signals in 2D FT–ICR spectra is found to be complicated even in the simplest chemical system. The complex modulation creates higher harmonic components in the spectra. Based on the model, a data processing algorithm is proposed. The results show that the Fourier transformation should be performed stepwise in order to obtain complete information, and that the phase portion of the frequency domain generated by the second Fourier transformation should not be discarded since it contains useful information.
Electron paramagnetic resonance investigations of divalent cobalt in single crystals of perovskite (CaTiO3)
Authors: Boldú O. J. L. , Boatner L. A. , Abraham M. M.
Published: July 2002

The electron paramagnetic resonance (EPR) spectrum of Co2+ in CaTiO3 was observed at 9 GHz in the temperature range of 10–15 K. The results establish that Co2+ occupies four magnetically inequivalent sites in the orthorhombic CaTiO3 lattice. These sites become equivalent when the applied magnetic field is parallel to the so-called ‘‘pseudocubic’’ b axis of the crystal. The magnetic axes in the plane perpendicular to the pseudocubic axis make an angle of ±28.5°±0.5° with the a and c axes of the host lattice. The site symmetry is qualitatively similar to that of the aliovalent ions Er3+ and Yb3+ in CaTiO3.
The Baldet–Johnson (B 2Σ+–A 2Πi) system of 13C18O+
Authors: Reddy S. Paddi , Prasad C. V. V.
Published: July 2002

The rotational structure of the 1–0, 0–0, and 0–1 bands of the Baldet–Johnson (B 2Σ+–A 2Πi) system of the molecular ion 13C18O+, excited in the cathode glow of a hollow-cathode discharge tube, was recorded in the spectral region 3700–4225 Å and analyzed. An effective Hamiltonian proposed by Brown et al. [J. M. Brown, E. A. Colbourn, J. K. G. Watson, and F. D. Wayne, J. Mol. Spectrosc. 74, 294 (1979)] was used to obtain the molecular constants from the wave number data of these three bands. These molecular constants were first ‘‘merged’’ together and the resulting constants were then ‘‘merged’’ with those of the comet-tail (A 2Πi–X 2Σ+) system of 13C18O+ recently reported by us [C. V. V. Prasad and S. P. Reddy, J. Chem. Phys. 90, 3010 (1989)] to obtain a unique set of constants for the X 2Σ+, A 2Πi, and B 2Σ+ states. Some of the derived constants of 13C18O+ (in cm−1) are for X 2Σ+: Be=1.794 28(4), αe=0.016 61(3), ωe =2110.27(2), and ωexe =13.81(6); for A 2Πi : Be=1.441 92(4), αe=0.016 76(1), ωe =1488.99(2), ωexe =12.454(7), and ωeye =0.0279(8); for B 2Σ+: Be=1.632 62(4), αe=0.025 49(2), ωe=1657.56, and ωexe =27.42.
Extrapolation to the infinite length limit of polymer energies. I. Hückel approximation
Authors: Evangelisti Stefano
Published: July 2002

A method to extrapolate the energy of polymers (in the Hückel approximation) as a function of the number of monomers is presented. The method can be used to obtain the energy per monomer of the infinite chain, and test computations are presented. They show good results even in presence of a strongly oscillating behavior of the finite polymer energies.
Geometrical representation of the equations for solving quantum beat problems
Authors: Furuya Kenji , Gondo Yasuhiko
Published: July 2002

Equations for solving a quantum beat problem in a three-level system, which determine two complex variables of time dependence, are rewritten in terms of four real functions constructed from the two complex variables. The Minkowski space is reasonably introduced in order to represent the time evolution of the four real functions as the motion of a four-dimensional vector, though the equations are irrelevant to the special theory of relativity. It is found that the four-dimensional vector precesses around the zeroth axis on the cone which is constructed from all of the points whose norms are zero in the Minkowski space, and that the Euclidean norm of the vector decreases with the increase of time. Though the visualized motion of the vector is similar to those in the well-known magnetic resonance precession model, the picture obtained from the equations for quantum beats cannot be connected with such a phenomenon as photon echo.
Spectroscopy of the 1 2Πu state of Na+2
Authors: Bordas C. , Broyer M. , Vialle J. L.
Published: July 2002

We report here the first experimental observation of bound–bound transitions between the ground X 2Σ+g and excited 1 2Πu states of Na+2. The basis of our experiment is to study doubly excited Rydberg states of Na2 by preparing a well-defined nd 1Λg singly excited Rydberg state of Na2 and then, by exciting the Na+2 core with a tunable laser. In this paper, we show that the ionic transitions may be directly deduced from the doubly excited Rydberg states spectra [C. Bordas, J. L. Vialle, and M. Broyer (submitted)]. We demonstrate that this technique is one of the most powerful to study the excited states of diatomic ions which are not predissociated. A detailed spectroscopic analysis of the 1 2Πu state has been performed and the results are compared with the more recent ab initio and pseudo- (or model-) potential calculations.
Numerical experiments for effects of elastic strain energy on decomposition process of binary alloy in a two-dimensional system
Authors: Hatta N. , Kaneko T. , Ishii R.
Published: July 2002

This paper is concerned with phase decomposition of a supersaturated solid solution in a two-dimensional system. We have investigated the behavior of numerical solutions to the Cahn equation including an elastic strain energy term in such a way as to follow the time evolution of composition modulation by computer. It is found that in our numerical experiment the introduction of the elastic strain energy gives no anisotropic effect on the crystallographic direction of the composition modulation, but leads to the decrease in the decomposition rate and quantity at the equilibrium state. Again, it is demonstrated that for a high solute alloy system inside the spinodal region a large circular precipitate is formed at the center of the flat matrix through a very complicated process accompanied with the aggregation and annihilation of particles. The results so obtained are discussed from a numerical point of view.
Application of the molecular-dynamics technique for simulations of an oscillating thermochemical system
Authors: Gorecki J. , Kawczyński A. L.
Published: July 2002

The results of molecular-dynamics simulations of transient thermochemical oscillations in closed, nonadiabatic systems are presented. A system of 600 molecules has been studied. The simulations confirm qualitatively oscillatory behavior predicted by the phenomenological model. The quantitative agreement fails because far from equilibrium properties of the system appearing in simulations are missing in the phenomenological picture.
Optical transient bleaching of quantum-confined CdS clusters: The effects of surface-trapped electron–hole pairs
Authors: Wang Ying , Suna Andris , McHugh John , Hilinski Edwin F. , Lucas Patricia A. , Johnson Robert D.
Published: July 2002

We studied the optical transient bleaching of ∼40 Å, ammonia-passivated CdS clusters in a polymer with nanosecond and picosecond pump-probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30-ps pump laser pulse width, we tentatively attribute the bleaching to the exciton-exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron-hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron-hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron-hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron-hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron and hole are present, the lowest exciton absorption is red-shifted from the original exciton absorption, and this transition has a weak oscillator strength, which explains the observed efficient bleaching. The model also predicts that a trapped electron is more efficient than a trapped hole for bleaching the excitonic absorption of CdS clusters in the size regime considered here. This is confirmed by pulse radiolysis results. Finally, we discuss the possible effects of charged surface defects on the linear absorption spectra of semiconductor clusters.
Valence band photoemission spectra and molecular geometry of biphenyl in condensed and chemisorbed phases
Authors: Ramsey M. G. , Steinmüller D. , Netzer F. P.
Published: July 2002

In this paper, three distinct phases of biphenyl on the Pd (110) surface are characterized and studied by angle-resolved ultraviolet photoemission (ARUPS); a disordered condensed multilayer which desorbs at 218 K, a bilayer that desorbs at 240 K, and a strongly bound monolayer which breaks up at the surface for temperatures greater than 400 K. The multilayer ultraviolet photoemission (UP) spectrum is almost identical to that of the gas phase spectrum and it is inferred that the torsional angle between the phenyl rings in the condensed phase is unaltered from that in the gas phase. Changes observed in the π orbital emissions on the formation of the bilayer indicate a significant reduction in the torsional angle. Application of the selection rules for ARUPS to the strong angular effects observed in the emissions from the highest-lying π orbitals suggests that the molecules in this second layer have their molecular axes parallel to and molecular planes perpendicular to the surface, that is, edge-on to the molecules in the monolayer which are shown to be flat lying to the metal surface and bonded via the π orbitals.
Photodissociation of KrF by intense ultrashort pulses
Authors: Hill K. E. , New G. H. C. , Burnett K.
Published: July 2002

A theoretical model is presented for studying the photodissociation of KrF in the presence of an intense ultrashort optical pulse. The spectrum of the translational kinetic energies of the dissociated products is calculated and shown to be a clear probe for the coherent interaction.
Diffusion, sedimentation, and Taylor dispersion of a Brownian cluster subjected to a time-periodic external force: A micromodel of ac electrophoretic phenomena
Authors: Haber Shimon , Brenner Howard , Shapira Moshe
Published: July 2002

A generalized formulation of the Fokker–Planck equation is utilized to calculate the mean velocity and dispersivity of a flexible Brownian cluster of rigid particles which is acted upon by a time-periodic external force. It is shown that if the force consists of a nonzero mean part and a ‘‘fluctuating’’ (i.e., zero mean) part, their effects are decoupled. Similarly, if a Fourier expansion of the force is carried out, the effect of each term of the expansion can be treated independently of the others. A representative force term of the form Fn exp(iωnt) +F̂n exp(−iωnt) was selected to act upon a flexible dumbbell composed of two identical tethered spheres of radii a, with the inextensible tether acting as an ‘‘attractive’’ internal potential. The dispersion tensor is found to consist of a ‘‘parallel’’ contribution (directed along FnF̂n+F̂nFn) and a ‘‘hydrostatic’’ contribution. This dispersion tensor depends linearly upon the scalar (Fn⋅F̂n)a/24πμkT (μ=viscosity), approaches a constant asymptotic value for small nondimensional frequencies Ωn=12πμa3ωn/kT, and decreases asymptotically to zero for very large frequencies.
Vibrational and electronic excitation of hexatriacontane thin films by low energy electron impact
Authors: Vilar M. Rei , Schott M. , Pfluger P.
Published: July 2002

Thin polycrystalline films of hexatriacontane (HTC) were irradiated with low energy (E=0.5–15 eV) electrons, and off-specular backscattered electron spectra were measured. Below E∼7 eV, single and multiple vibrational excitations only are observed, which relax the electrons down to the bottom of the HTC conduction band. Due to the negative electron affinity of HTC, thermal electrons are emitted into vacuum. Structure in the backscattered electron current at kinetic energies about 1.5 and 4 eV are associated to conduction band density of states. Above E∼7 eV, the dominant losses correspond to electronic excitations, excitons, or above a threshold (energy of the electron inside the HTC film) at 9.2±0.1 eV, electron–hole pair generation. The latter process is very efficient and reaches a yield of the order of one ∼11 eV. Evidence for chemical reaction above E∼4 eV is observed.
Matrix polynomial extension of the sequence-generating function method for macromolecular binding
Authors: Woodbury Charles P.
Published: July 2002

A generalization of Lifson’s sequence-generating function method [J. Chem. Phys. 40, 3705 (1964)] for linear chain molecules is explored. The generalization replaces the various scalar series of statistical weights, used to generate sequence partition functions, by appropriate matrix series. These matrix SGFs permit a more compact treatment of complicated macromolecular binding models than can be achieved through the standard SGF method or by using combinatorial or regular transfer matrix methods. They also lead to appreciable gains in computational speed for such models. Applications of the matrix-SGF method are presented, including selective binding of ligands to helix–coil junctions in biopolymers and sequence-selective binding of large ligands on random copolymers.
On the correlation of inhomogeneously broadened site energies in different electronic states
Authors: Root Leslie J.
Published: July 2002

We investigate by molecular dynamics computer simulation the distribution of ground- and excited-state energies and the degree of correlation between them for a chomophore dissolved in an amorphous solid. The system studied is resorufin in amorphous glycerol. We find that there is considerable but not complete correlation between the absolute energy of the chromophore in the ground electronic state and its energy in the first excited electronic state. This is shown to be reasonable in view of the microscopic model of inhomogeneous broadening introduced by Laird and Skinner [J. Chem. Phys. 90, 3880 (1989)].