bluuebunny/crossref_metadata_2025 huggingface dataset:
Coulomb screening and exciton binding energies in conjugated polymers
Authors: Moore Eric , Gherman Benjamin , Yaron David
Published: July 2002

Hartree–Fock solutions of the Pariser–Parr–Pople and MNDO Hamiltonians are shown to give reasonable predictions for the ionization potentials and electron affinities of gas-phase polyenes. However, the energy predicted for formation of a free electron-hole pair on an isolated chain of polyacetylene is much larger than that seen in the solid state. The prediction is 6.2 eV if soliton formation is ignored and about 4.7 eV if soliton formation is included. The effects of interchain interactions on the exciton binding energy are then explored using a model system consisting of one solute and one solvent polyene, that are coplanar and separated by 4 Å. The lowering of the exciton binding energy is calculated by comparing the solvation energy of the exciton state to that of a single hole (a cationic solute polyene) and a single electron (an anionic solute polyene). It is argued that when the relative timescales of charge fluctuations on the solute and solvent chains are taken into account, it is difficult to rationalize the electron–electron screening implicit in the parametrization of a single-chain Hamiltonian to solid-state data. Instead, an electron–hole screening model is developed that includes the time scales of both the electron–hole motion and the solvent polarization. The predicted solvation energies, which are saturated with respect to solute and solvent chain length, are 0.07 eV for the exciton and 0.50 eV for a well separated electron–hole pair. Given this large, 0.43 eV reduction in the exciton binding energy due to interaction with a single chain, it seems likely that interchain interactions play a central role in establishing the solid-state exciton binding energy.
Density functional approach to the structure of uniform fluids
Authors: Patra Chandra N. , Ghosh Swapan K.
Published: July 2002

The structure of uniform fluids is studied here by using a density functional approach. The key property of interest is the radial distribution function which is essentially the density distribution in the field of a tagged fluid particle. The theory is applied to the neutral fluid mixture as well as ionic fluids. The only inputs used are the first and second order direct correlation functions. For the uniform fluid mixture, the different auto- and cross-distribution functions are found to show density oscillation depending upon the different diameters and the results are quite comparable to the results obtained through Fourier transform. The work is extended to a uniform electrolyte within the restricted primitive model using a correlation function corresponding to the mean spherical approximation. Once again the results are quite good and compare well with the results obtained from the usual Fourier transform technique.
Quantum interference in resonant Raman spectra of I2 in condensed media
Authors: Ovchinnikov M. , Apkarian V. A.
Published: July 2002

Both B(3Πu0) and B′′(1Π1u) surfaces contribute to resonance Raman scattering of iodine, and the interference between these two channels leads to modulation of the intensity profile of overtone progressions, as observed experimentally. The effect is simulated through mixed order semiclassical molecular dynamics.
Attractive potential effect on the self-diffusion coefficients of a solitary water molecule in organic solvents
Authors: Wakai Chihiro , Nakahara Masaru
Published: July 2002

H 1 -Fourier-transform pulsed field-gradient spin-echo NMR method is applied at 30 °C to measure the self-diffusion coefficients of water, acetonitrile, and benzene in dilute organic solutions of carbon tetrachloride, benzene, chloroform, dichloromethane, acetonitrile, acetone, and water. The dependence of the translational friction coefficients ζT of the apolar solute molecule, benzene, on solvent viscosity η is linear as predicted by the simple hydrodynamic theory. The η dependence of ζT of the polar solute molecule, acetonitrile, is linear, though the slope is different in apolar and polar solvents. The η dependence of ζT of the hydrogen-bonding solute molecule, water, is nonlinear and dependent on the strength of the solvent as a proton acceptor. The breakdown of the hydrodynamic theory is discussed in terms of the attractive part of the solute–solvent interaction. The role of the attractive interaction is illustrated through a linear relationship between the proton chemical shift of water and the residual friction coefficient, which is defined by removing the repulsive component from the overall translational friction coefficient.
The ground state (1A1) and the lowest triplet state (3B1) of the phenyl cation C6H5+ revisited
Authors: Hrušák Jan , Schröder Detlef , Iwata Suehiro
Published: July 2002

Ionization of the phenyl radical C6H5•(2A1) is examined by high level theoretical methods. A systematic comparison of MCSCF, B3LYP, and CCSD(T) results shows that the ground state of the phenyl cation C6H5+ is definitely the singlet (1A1) state and that the associated adiabatic ionization energy for the transition C6H5•(2A1)→C6H5+(1A1) is estimated to be 8.1±0.1 eV. The lowest triplet state C6H5+(3B1) is approximately 0.8 eV higher than the singlet ground state in energy. However, theoretical geometries of the phenyl radical and cations suggest that formation of the triplet cation is favored in photoionization experiments because the Franck–Condon factors are much larger for the transition C6H5•(2A1)→C6H5+(3B1) than for that of the singlet ground state C6H5+(1A1). By evaluating the Franck–Condon factors for both photoionization processes [i.e., (2A1)→C6H5+(1A1) and (2A1)→C6H5+(3B1)] of C6H5• and C6D5•, attempts were made to assign the reported photoelectron spectra.
Interaction of small boron cluster ions with HF
Authors: Sowa-Resat Marianne B. , Smolanoff Jason , Lapicki Adam , Anderson Scott L.
Published: July 2002

A combined experimental and theoretical study of the reactivity of small boron cluster cations with HF is reported on in this article. Cross sections for the reaction of mass-selected boron (Bn+, n=2–14) and boron monohydride (BnH+, n=4, 7, and 10) cluster ions with HF are reported for collision energies from 0.1 to 10 eV. The reaction proceeds via a chemically bound intermediate complex, and the major product channels involve elimination of BF or H from the BnHF+ intermediate. Both reactivity and product branching depend strongly on cluster size. Reactivity at low energies generally decreases with increasing size, with the appearance of significant bottlenecks to reaction. The effect of adding a hydrogen atom to the reactant clusters was examined for several cluster sizes. In contrast to the boron oxides, hydrogenation of elemental boron cluster ions has little effect on reactivity with HF, suggesting that H does not compete effectively for HF binding sites. The results provide a sensitive test for the accuracy of ab initio calculations on Bn+ and BnH+ and BnF+, and the calculations provide insight into the reaction mechanism and effects of cluster structure on reactivity.
Helium atom scattering and HREELS investigations of the H/Rh(311) adsorption system
Authors: Farías Daniel , Siebentritt Susanne , Apel Rainer , Pues Roland , Rieder Karl–Heinz
Published: July 2002

Hydrogen adsorption on Rh(311) at 110 K leads to the appearance of (1×1)H, (1×3)H, (1×2)H and (1×2)REC ordered layers. A previous He-atom scattering (HAS) investigation of the (1×1)H phase has revealed that its coverage is Θ=1 ML, whereby the H–atoms are adsorbed between the topmost bridge sites and the adjacent threefold hollow sites on the (111)-microfacets. In this paper we present a combined HAS and HREELS structural analysis of the higher coverage phases. The structures derived from the HAS data show that, starting with the (1×1)H phase, the (1×3)H is formed by the adsorption of H–atoms in the wells between every third close-packed row, whereas adsorption on the same sites but between every second row leads to the completion of the (1×2)H phase. Accordingly, the corresponding coverages are 4/3 ML and 3/2 ML. The HREELS results clearly demonstrate the existence of two different H-species in the (1×3)H phase and three in the (1×2)H phase, supporting the HAS results.
Comment on “On the relation between unimolecular reaction rates and overlapping resonances” [J. Chem. Phys. 101, 9672 (1994)]
Authors: Rotter I.
Published: July 2002

In a quantum mechanical many-body system at high level density different time scales are created by the trapping effect leading to a saturation of the average decay width of the narrow resonances. The trapping effect is a broadening of the distribution of the resonance widths, caused by the bifurcation of widths of neighboring resonances, which finally leads to a separation of the time scales. This saturation corresponds to that obtained by Peskin et al. for the decay rate. Thus, decay rate and average decay width behave in the same manner not only at low but also at high level density. The saturation should be proven experimentally by direct time measurements.
Rotational and spin-orbit effects in the dynamics of O(3Pj)+hydrocarbon reactions. II. Models for spin-orbit propensities
Authors: Sweeney Gillian M. , McKendrick Kenneth G.
Published: July 2002

The spin-orbit states of OH have been found in part I to be unequally populated in reactions of O(3Pj) with saturated hydrocarbons. We clarify the correlations in these systems within an adiabatic, Ω-conserving approximation, and show that the observed OH spin-orbit distributions are not well reproduced in this adiabatic limit. However, the inclusion of strong nonadiabatic coupling between surfaces of the same Ω and parity results in a much improved prediction. The necessary mixing is demonstrated to be compatible with a previous independent characterization of the entrance channel surfaces in O(3Pj) + CH4 collisions, and with the known propensities for inelastic, j-changing collisions of O(3Pj) with closed shell partners. The nonadiabatic model which agrees best with the observed OH spin-orbit ratios suggests that the O(3Pj) reactivity should vary only moderately with j.
Concentration dependence of static chain properties 2, off-lattice Monte Carlo simulations
Authors: Olaj Oskar Friedrich , Petrik Thomas , Zifferer Gerhard
Published: July 2002

Monte Carlo simulations of polymer chains in off-lattice systems under theta and athermal conditions have been performed in order to support results obtained for cubic lattice chains in assemblies of varying concentration. Global as well as local quantities are investigated in detail in order to point out the differences as well as the similarities of the results and the reliability of the two models, especially with respect to local packing effects. Furthermore, the results are discussed with reference to theoretical predictions of the scaling theory. Probably the most important finding is the fact that an isolated θ chain and a chain in bulk exhibit totally different microscopic structures (which is in full concordance with our previous lattice simulations), although the global dimensions of bulk polymers tend to converge to the values of an isolated θ chain.
Electro-deposition of polymer chains on an adsorbing wall: Density profiles and wall coverage
Authors: Foo Grace M. , Pandey R. B.
Published: July 2002

Growth of polymer density in an electro-deposition model of polymer chains on an impenetrable wall is studied on a two dimensional discrete lattice using a Monte Carlo simulation. Polymer-polymer repulsion and polymer-wall attraction for the adsorbing wall (along with the neutral and repulsive interactions) are considered in an external field. Effects of the field strength (B), temperature (T), and chain length (Lc) on the density profile of the polymer chains and wall coverage are investigated. The spatial density profile shows onset of oscillation near the wall at a characteristic field (Bc) which depends on chain length and temperature. In low field, adsorption-to-desorption transition at the wall appear on increasing the temperature (unlike neutral and repulsive walls). In high field regime, on the other hand, a non-monotonic dependence of coverage on temperature is observed with a maximum at a temperature (Tm) which increases on increasing B. The equilibrium value of the polymer density (pd) shows a power-law decay with the chain length, pd∼Lc−α, at the wall and in the bulk with corresponding values of the exponent αW and αB; these exponents differ substantially and depend on B, T, and Lc. The coverage decays monotonically with the chain length at a constant temperature and field.
The first adiabatic ionization potential of Ar2
Authors: Signorell R. , Wüest A. , Merkt F.
Published: July 2002

The high resolution zero-kinetic-energy (ZEKE) photoelectron spectrum of Ar2 has been recorded between 116500 and 128500 cm−1. The spectrum consists of a progression of 52 vibrational bands in the A Σ1/2u+2←XΣg+1 (X 0g+1 in Hund’s case (c) notation) photoelectronic transition. The absolute numbering of the vibrational progression in the A←X transition is achieved by measuring the isotope shifts of two vibrational bands of the Ar236 molecule. From the analysis of the vibrational progression the first adiabatic ionization potential of Ar2 has been determined to be 116593.5±6.0 cm−1 (14.4558±0.0007 eV) from which a dissociation energy D0 of 10601.2±6.0 cm−1 (1.3144±0.0007 eV) results for the A Σ1/2u+2 ground state of Ar2+. The potential curve of the ground ionic state in the vicinity of the potential minimum is adequately represented by a Morse potential with ωe+=307.0±0.4 cm−1 and ωexe+=2.05±0.05 cm−1. The position of higher members of the vibrational progression with v+>25 cannot be fitted accurately with a Morse potential.
Metal adsorption calorimetry and adhesion energies on clean single-crystal surfaces
Authors: Stuckless J. T. , Starr D. E. , Bald D. J. , Campbell Charles T.
Published: July 2002

The heats of adsorption of metals have been measured calorimetrically for the first time on clean, single-crystalline surfaces. A pulse of metal vapor from a chopped atomic beam adsorbs onto an ultrathin single crystal’s surface, causing a transient temperature rise. This heat input is detected by a pyroelectric polymer ribbon, which is gently touched to the back of the crystal during calorimetry. The sticking probability is measured by detecting the reflected fraction mass spectroscopically. The differential heat of adsorption is thus measured as a detailed function of coverage up through multilayer coverages. The integral heat of adsorption also provides the adhesion energy of the metal film, if the surface free energy of the clean metal surface is known. Adsorption and adhesion energies for metals (Pb or Cu) on clean Mo(100) and on well-defined surface oxides of Mo(100) and W(100) are reported.
Two-photon spectroscopy of xenon dimers in supersonic jets
Authors: Keto J. W. , Cai Hong , Kykta Martin , Lei Chen , Möller Thomas , Zimmerer Georg
Published: July 2002

We report experiments studying the fluorescence and two-photon excitation spectroscopy of xenon dimers and small clusters formed in supersonic jets. Under thermodynamic conditions for maximum dimer density, determined by two-photon resonant multiphoton ionized time-of-flight (TOF) spectroscopy, no fluorescence of free bound excimers correlating to Xe* 6p[1/2]0, 6p[3/2]2, or 6p[5/2]2 was observed; very weak excimer fluorescence was observed for the excimer correlated with Xe+Xe* 5d[1/2]1. Comparing the observed atomic fluorescence and measurements of the monomer–dimer ratio, we estimate predissociation lifetimes on the order of 10−3 of the expected radiative lifetime. TOF spectra are consistent with predissociation for all excimers except those correlating to 6p[5/2]2. At higher nozzle stagnation pressures, we observed fluorescence from vibrationally or electronically relaxed Xe2* excimers imbedded in helium clusters Hn, with 〈n〉 most probably 6 and 13. We report dispersed spectra, and using modeled reflection spectra, we assign the fluorescence spectra to 6p[1/2]00g+→6s[3/2]1B0u+, 6p[1/2]00g+→6s[3/2]11u, and 5d[1/2]10g+→6s[3/2]2A1u.
Ab initio quantum chemical calculation of electron transfer matrix elements for large molecules
Authors: Zhang Linda Yu , Friesner Richard A. , Murphy Robert B.
Published: July 2002

Using a diabatic state formalism and pseudospectral numerical methods, we have developed an efficient ab initio quantum chemical approach to the calculation of electron transfer matrix elements for large molecules. The theory is developed at the Hartree–Fock level and validated by comparison with results in the literature for small systems. As an example of the power of the method, we calculate the electronic coupling between two bacteriochlorophyll molecules in various intermolecular geometries. Only a single self-consistent field (SCF) calculation on each of the monomers is needed to generate coupling matrix elements for all of the molecular pairs. The largest calculations performed, utilizing 1778 basis functions, required ∼14 h on an IBM 390 workstation. This is considerably less cpu time than would be necessitated with a supermolecule adiabatic state calculation and a conventional electronic structure code.
From the sparse to the statistical limit of intramolecular vibrational redistribution in vibrational predissociation: ArCl2 as an example
Authors: Roncero Octavio , Caloto David , Janda Kenneth C. , Halberstadt Nadine
Published: July 2002

The dynamics of intramolecular vibrational relaxation (IVR) for ArCl2 are examined for a wide range of vibrational and rotational excitation. In order to describe the IVR more efficiently, and characterize it more quantitatively, we propose a refinement of the traditional Bixon-Jortner description in which the active states are prediagonalized to simplify the coupling scheme that must be considered. This allows for an explicit determination of the average density of states and average coupling strength for each initial excitation. We find that the IVR dynamics proceed from the sparse regime for v=11, for which the first open dissociation channel corresponds to the loss of two Cl2 quanta, to the intermediate-dense regime for v=25 which dissociates by the loss of 4 quanta. We find that over this range the increase in the density of states is less important than the increase in the coupling strength. For v = 18 we examine the effect of rotation in considerable detail. Initial states that couple via a manifold of 6000 channels can be considered since the calculation is performed on a parallel computer. The effect of increasing J, the total angular momentum excitation, is found to be less than that of increasing K, the degree of rotation about the van der Waals bond. This means that the main effect is not simply an increase of the available density of states due to Coriolis coupling. Understanding the details of IVR in a relatively simple system like ArCl2 should help us understand the dynamics of more complicated molecules. In particular, the case of ArI2 is discussed.
Modeling and structure of mercury-water interfaces
Authors: Shelley J. C. , Patey G. N. , Bérard D. R. , Torrie G. M.
Published: July 2002

The modeling and nature of the physisorption of water at the metal (Hg)-water interface is explored in this paper. We have evaluated potential models that fit into three general classes that are employed in the literature. These classes are distinguished by the manner in which the isotropic interactions between the metal and the water are modeled: namely, as non-attractive, weakly attractive, and strongly attractive. In the present studies the metal is described by a jellium model. In our model, in addition to the isotropic water potential there is an interaction between the jellium and the water molecules which depends on the orientation of the water molecule with respect to the metal surface. We find that hard potentials without isotropic attractive terms dewet. The density of water near hydrocarbon-like potentials remains close to the bulk value but the interaction isn’t strong enough to structure the water near the interface, nor are the adsorption energies sufficiently high. The strongly attractive potentials seem to be the most appropriate. For such models we have checked the sensitivity of the results to the parameters in our model including surface corrugation. We find that the structural results are insensitive to the parameters employed suggesting that they not only provide a good classical description of the Hg-water interface but may be readily adaptable to describe the physisorption of water on metals in general. Although we find the layering of water molecules and the pattern of hydrogen bonding near these surfaces, which have been described as ice-like in previous studies, the actual arrangement of the water molecules near these surfaces is distinctly different from the structure of ice.
Pressure dependence of the miscibility of poly(vinyl methyl ether) and polystyrene: Theoretical representation
Authors: An Lijia , Horst Roland , Wolf B. A.
Published: July 2002

The present calculations were performed on the basis of the Sanchez-Lacombe-Balasz lattice fluid theory. The two system specific parameters ε12* and δε* required for that purpose have been obtained from the spinodal temperatures measured (SANS) for mixtures of poly(vinyl methyl ether) (PVME) and deuterated polystyrenes (d-PS) by Schwahn and coworkers. The experimental data reported for atmospheric pressure and six representatives of the present system are well described theoretically, where ε12* does not depend on molar mass and δε* decreases only slightly as the chain length of d-PS is raised. The measured pressure influences on the spinodal conditions correspond to an approximately linear reduction of δε* with increasing P; this observation should reflect the volume changes associated with the formation of specific interactions. According to the present calculations the critical composition shifts markedly towards pure PVME as P is raised. Since experimental data are commonly expressed in terms of the Flory-Huggins theory, the current results were also translated into Flory-Huggins interaction parameters and evaluated with respect to the contributions of enthalpy and of entropy. The agreement between experimental information and that calculated from the Sanchez-Lacombe-Balasz lattice fluid theory is reasonable.
Intramolecular proton transfer direct dynamics in the glycolate anion: Isotope effects
Authors: Fernández-Ramos Antonio , Rodrı́guez-Otero Jesús , Rı́os Miguel A.
Published: July 2002

We studied the dynamics of intramolecular proton transfer in the glycolate anion, and of the analogous processes in the deuterated and tritiated derivatives, by means of variational transition state theory with various multidimensional semiclassical models of tunneling. Rate constants were obtained directly from MP2/6-31++G** ab initio results scaled so that the height of the barrier coincided with the value given by QCISD/6-31++G** calculations, 3.81 kcal/mol. In this kind of system, the effective barrier height can be significantly lower for the lighter isotope. Analysis of the kinetic isotope effects (kH/kD and kH/kT) showed them reduced by tunneling but favored by certain vibrational modes.
Fluids and fluid mixtures containing square-well diatomics: Equations of state and canonical molecular dynamics simulation
Authors: Gulati Harpreet S. , Hall Carol K.
Published: July 2002

We present new perturbation theory equations of state for square-well dimer fluids, square-well dimer mixtures, square-well dimer/monomer mixtures and square-well heteronuclear dumbbell fluids. Our first- and second-order perturbation terms are based on Barker and Henderson’s local compressibility approximation and Chang and Sandler’s perturbation theory, respectively. The perturbation approach requires knowledge of the radial distribution functions of the reference hard-dimer fluid and hard dimer/monomer mixture, which are obtained from molecular dynamics simulation. For mixtures we use one fluid mixing rules to approximate the average mixture structure and perturbation parameters. The predictions of the perturbation theory are compared to the compressibility factors obtained from discontinuous canonical molecular dynamics simulation, an adaptation of Anderson’s canonical ensemble molecular dynamics method to the case in which the potential is discontinuous.
Raman intensities of C=C stretching vibrational frequencies of polyenes: Nodal mode analysis
Authors: Lee Jin Yong , Lee Sang Joo , Kim Kwang S.
Published: July 2002

It is not clearly understood how and why the Raman intensity increases drastically with an increasing chain length of polyenes. We therefore investigated the vibrational intensities of the C=C stretching vibrational modes of four polyene systems of X–(HC=CH)n–Y, where X/Y=H/H, H/NH2, H/NO2, and NH2/NO2. The investigation was done using nodal mode analysis (based on the number of nodes formed by the alternations of stretches and contractions) combined with ab initio frequency calculations. The C=C stretching/contracting mode without node is found to have the strongest Raman intensities regardless of polyene systems because of the long-range cooperation effect by the concurrent stretch/contraction motion of all C=C bonds. The corresponding IR spectra have also the strongest intensities for the nonsymmetric polyene systems, whereas are inactive for the symmetric polyenes (by the exclusion rule). The intensities of the nonconcurrent C=C stretching/contracting modes (particularly for the Raman spectra) tend to decrease drastically (in proportion to [n/(m+1)]4) with increasing node number m, though weak (or zero) intensities appear somewhat alternately because the molecules have approximate (or exact) centrosymmetry.
Theoretical analysis of the vibronic structure of the zero-kinetic-energy photoelectron spectra from single vibronic levels of the S1-state manifold of naphthalene
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

The two-color zero-kinetic-energy (ZEKE) photoelectron spectra of naphthalene are simulated and analyzed by means of quantum-chemical calculations of molecular parameters followed by the modeling of vibronic intensities. Ab initio and semiempirical calculations are carried out to obtain molecular structures of the initial S1 and final D0 states of neutral and ionic naphthalene, respectively. The vibronic perturbations that couple these states to low-lying excited states are also evaluated and included in the model based on the perturbative expansion of vibronic states. It is shown that the simulated intensities reproduce the observed spectra in a very satisfactory fashion. The most prominent bands are identified and some reassignments of ground state frequencies of the cation are indicated on the basis of simulated vibronic intensities. This is, to date, the first completely theoretical simulation of ZEKE spectra of a medium-large molecule based on a model which goes beyond the usually assumed Franck-Condon selection rules.
A small optimal complete active space (CAS) for multiconfigurational spin tensor electron propagator method (MCSTEP) ionization potentials: Application to methane, acetylene, ethylene, and ethane
Authors: Heryadi Dodi , Jones Christopher T. , Yeager Danny L.
Published: July 2002

The multiconfigurational spin tensor electron propagator method (MCSTEP) is specifically designed to provide accurate determination of the low-lying principal vertical ionization potentials (IP’s) and electron affinities (EA’s) of atoms and molecules. In this paper we modify and extend our definition of an “optimal” small complete active space (CAS) for MCSTEP calculations. We then use MCSTEP to determine the vertical principal ionization potentials (IP’s) below 19 eV of the small organic molecules methane, acetylene, ethylene, and ethane. The calculations are done using the standard Dunning-core-valence pVDZ and pVTZ basis sets. The results are compared with experiments as well as with some other forefront theoretical methods. The MCSTEP IP’s are in very good to excellent agreement with experimental IP’s.
Orientational order and rotational dynamics of the amide ions in potassium amide. I. Neutron diffraction
Authors: Müller M. , Senker J. , Asmussen B. , Press W. , Jacobs H. , Kockelmann W. , Mayer H. M. , Ibberson R. M.
Published: July 2002

Potassium amide (KNH2/KND2) is an ionic molecular crystal. The amide ions (NH2−/ND2−)—isoelectronic with the water molecule OH2—display orientational disorder in the cubic and tetragonal high–temperature phases. In the monoclinic low–temperature phase they are orientationally ordered. We investigated the deuterated compound KND2 in all three phases using high resolution neutron powder diffraction and determined the time–averaged probability density function of the deuterons in detail. The structure of the intermediate tetragonal phase was solved, and deviations of the reorientations of the amide ions in the cubic phase from free rotational diffusion were quantified. From the structural properties of the different phases of KND2 and the structural changes at the phase transitions conclusions on the respective dynamics of the amide ions can be drawn.
Ultrafast study of interfacial electron transfer between 9-anthracene-carboxylate and TiO2 semiconductor particles
Authors: Martini Ignacio , Hodak José , Hartland Gregory V. , Kamat Prashant V.
Published: July 2002

The excited state dynamics of 9-anthracene-carboxylic acid adsorbed onto the surface of TiO2 semiconductor particles were examined with ca. 250 fs time resolution. A combination of transient absorption and time-resolved anisotropy measurements show that approximately 76% of the photo-excited dye molecules transfer an electron to the TiO2 particles. The time scale for the forward electron transfer reaction was determined to be ⩽1 ps. The 9-anthracene-carboxylate radical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electron transfer reaction time is not possible, due to the contribution to the transient absorption signal from adsorbed dye molecules that do not transfer electrons to TiO2. These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescence spectrum from the dye molecules bound to the TiO2 particles is very different to that of the free dye in solution. The free dye has a broad red-shifted spectrum, whereas, the adsorbed molecules have a structured spectrum that displays a small Stokes shift. The red shift in the free dye fluorescence spectrum is due to stabilization of the excited electronic state through torsional motion of the carboxylate group. This motion cannot occur when the dye is bound to the particle surface. Thus, the excited molecules emit from a nuclear configuration that is similar to the ground-state geometry, producing a structured fluorescence spectrum. The dual behavior of the adsorbed dye molecules (electron transfer versus nonradiative decay/fluorescence) is attributed to the existence of two different sites for adsorption at the surface of the TiO2 particles: electron transfer can occur from one site but not the other.
Hard-sphere Yukawa fluid near a planar slit
Authors: Yi Jong-Ho , Kim Soon-Chul
Published: July 2002

A density functional perturbative approximation, which is based on both the weighted-density approximation (WDA) of Tarazona and the density functional approximation of Rickayzen et al., has been employed to predict the density profiles of the hard-sphere attractive Yukawa fluid near a planar slit. The calculated density profiles show that the density functional perturbative approximation is a significant improvement upon those of the modified version of the Lovett–Mou–Buff–Wertheim (LMBW-1), which uses the exact contact value theorem, and compares very well with the computer simulation even for the low temperatures in the vicinity of the liquid–vapor fluid bulk transition.
The reaction rate constant in a system with localized trajectories in the transition region: Classical and quantum dynamics
Authors: Makarov Dmitrii E. , Metiu Horia
Published: July 2002

We examine here a model for a chemical reaction which has a large number of trajectories localized in the transition region to find out how they affect reaction dynamics and the rate constant. One issue is whether the incessant crossing and recrossing of the dividing surface by the trapped trajectories impairs our ability to define a rate constant. To study this we rewrote the correlation function formula for the rate constant in a form that allows us to follow the crossing and recrossing of the dividing surface in time. We find that due to a “dephasing” process the correlation function converges towards a well-defined rate constant, even though the recrossing of the dividing surface by the bound trajectories is still going on. It is well known that error in the transition state approximation is caused by recrossings. In view of this, it is surprising that the variational transition state theory applied to our model picks as the best dividing surface one that leads to an infinite number of recrossings. The transition state rate constant for another dividing surface, which has only a few recrossings, has large errors. We show that the variational transition state theory improves the value of the rate constant by diminishing the contribution from trajectories that recross an odd number of times, relative to those that undergo an even number of recrossings; it does not, and need not, diminish the total number of recrossings. Quantum calculations show that the bound trajectories correspond to high energy, long lived resonances. In spite of this change in the physical description, the evolution of the quantum correlation functions follows closely that of the classical ones.
Propagating fronts in chemical systems with coexisting multiple stationary states
Authors: Vives D. , Careta A. , Sagués F.
Published: July 2002

Propagating reaction-diffusion fronts in a two-variable chemical system, the so-called open unstirred cubic autocatalator, are analyzed. Both situations of stable/metastable and stable/unstable connections are considered. Numerical results for the front velocities and profiles are supplemented with analytical calculations either exact or obtained perturbatively. Special emphasis is devoted to the question of velocity selection. In this respect, abundant use is made of the marginal stability criteria, either in their linear or nonlinear formulations. Finally, some peculiar phenomenologies related to the appearance of double fronts are examined.
Ionization energies of benzo[a]pyrene and benzo[e]pyrene
Authors: Zakrzewski V. G. , Dolgounitcheva O. , Ortiz J. V.
Published: July 2002

Photoelectron spectra of benzo[a]pyrene and benzo[e]pyrene have been assigned on the basis of ab initio electron propagator theory. Electron correlation effects are included in the partial third order approximation and agreement with experimental peaks is very close. To each ionization energy, there corresponds a Feynman–Dyson amplitude that exhibits the change in electronic structure associated with the removal of an electron. Correlation corrections to the Koopmans description of the cationic states are large for many states, but the qualitative validity of this model remains valid. The lowest final states with σ holes occur around 11.0 eV.
The visible emission and absorption spectrum of C60
Authors: Sassara A. , Zerza G. , Chergui M. , Negri F. , Orlandi G.
Published: July 2002

The fluorescence spectrum of C60 in Ne and Ar matrices is revisited in the light of improved calculations of vibronically induced intensities for the transitions between S0 and the lowest three singlet excited states. The Jahn–Teller and Franck–Condon vibronic activities for the three transitions are also evaluated and combined with the computed Herzberg–Teller intensities to simulate the fluorescence spectra. The agreement between experimental and simulated spectra is very satisfactory and provides an assignment for almost every observed band. The excitation spectrum of C60 in Ne and Ar matrices in the 640–500 nm region is also presented. It is almost a mirror image to the fluorescence spectrum of C60 in Ar matrices and nearly all bands can be attributed by analogy with the fluorescence spectrum. However, it appears that some mode frequencies increase while others decrease as compared to the fluorescence spectrum. In the high-frequency region evidence for a number of bands whose intensity has no counterpart in the fluorescence spectrum shows that they correspond to ag(2) overtones of bands appearing at lower frequency. Finally a band is found which is attributed to Herzberg–Teller modes of a higher electronic state of T2u symmetry. Its electronic origin is estimated to lie at ∼16 700 cm−1.
Cross-polarization efficiency in INS systems using adiabatic RF sweeps
Authors: Hodgkinson P. , Pines A.
Published: July 2002

The theory describing nuclear magnetic resonance cross-polarization using adiabatic sweeps of the rf spin-lock fields through the Hartmann–Hahn matching condition is extended to small homonuclear coupled systems of the type INS. In particular, the connection is made between such experiments and the associated theoretical limits on polarization transfer—the “unitary bounds”—demonstrating that these techniques can achieve the maximum transfer of polarization from the I spins to the S spins, subject to the constraint of angular momentum conservation imposed by spin-locking. Factors such as permutation symmetry of the spins, imperfect adiabaticity of individual crossings and fast sample spinning are shown to have no fundamental impact on the validity of these results.
Multicomponent system with polydisperse species
Authors: Bilgen D. , Aykac M. , Gujrati P. D.
Published: July 2002

We consider a multicomponent system containing polydisperse species of polymers produced in equilibrium polymerization. We present a general theory of such a system by solving the model on a Bethe lattice. We show that the resulting free energy has certain universal features regardless of the number of components, their architecture, and dispersity.
Influence of exciton–exciton interaction on one-to-two exciton transitions in molecular aggregates with linear and circular geometries
Authors: Juzeliūnas Gediminas , Reineker Peter
Published: July 2002

One- to two-exciton transitions have been examined in molecular aggregates with linear and circular geometries at various strengths of the exciton–exciton interaction. For the interaction parameter a sufficiently different from its critical value acrit=1, the exciton–exciton interaction has been shown to have little influence on the transition dipole moments, as well as on the corresponding transition energies between the one-exciton states and the dissociated two-exciton states. The interaction between the excitons then may be represented in an effective manner by the replacement of the actual number N of molecules per aggregate by a nearby effective number Neff, the latter being a-dependent. Hence, inclusion of the exciton–exciton coupling does not affect substantially the previous analysis of one- to two-exciton transitions based on the model of noninteracting one-dimensional excitons. That is, effects such as the blue shift of the excited-state absorption and the enhancement of nonlinear susceptibilities are not sensitive to the exciton–exciton interaction. These findings are relevant, inter alia, to J-aggregates in which there is no evidence for the coupling parameter a to be in the critical region or beyond. On the other hand, for the critical value of the exciton–exciton interaction (a=acrit), the blue shift is either totally absent in the excited-state absorption, or extremely small compared with the ordinary case. The above is in full agreement with earlier calculation of the pump–probe spectrum showing a weak dependence on the exciton–exciton interaction for a<1, as well as a strong bleaching of the exciton band in the critical region.
The triatomic Eckart-frame kinetic energy operator in bond coordinates
Authors: Wei Hua , Carrington Tucker
Published: July 2002

We present an exact Eckart-embedded kinetic energy operator in bond coordinates for triatomic molecules. The kinetic energy operator is derived from the general equations of J. Chem. Phys. 107, 2813 (1997) for the Eckart frame and the method of Handy. The Coriolis terms of the Eckart-bond kinetic energy operator do not depend on the angle between the Eckart in-plane axis and the equilibrium bond vector. We compare the Eckart-bond kinetic energy operator to other kinetic energy operators and show that the Coriolis terms of the Eckart-bond operator are smallest. The Eckart-bond kinetic energy operator should be useful for calculating and analyzing ro-vibrational spectra.
Approximations based on the adiabatic treatment of rotation for resonances
Authors: Qi Jianxin , Bowman Joel M.
Published: July 2002

In the adiabatic treatment of overall rotational motion, the rotational energy is obtained by diagonalization of the inertia tensor at each nuclear configuration, and subsequent insertion of the rotation constants into the standard formalism for the energy for a symmetric or asymmetric top. We have tested this approximation previously for bound states and resonances in HCO, and found it to be quite accurate. This adiabatic approximation is justified here by deriving an approximation very similar to it (but less accurate) for a triatomic molecule. We then consider further approximations to the adiabatic rotation approximation. In one we assume that rotation constants for each resonance are independent of the angular momentum state J. This approximation requires a minimum of two calculations of resonance positions and widths for nonzero J in addition to the one for J=0. The second approximation we consider is standard first-order perturbarion theory. The adiabatic rotational energy is the perturbation relative to the J=0 Hamiltonian, and the complex L2 eigenfunctions of this Hamiltonian are the zero-order states. These two approximations are tested for HCO bound states and resonances, where those obtained from the full adiabatic rotation approximation are assumed to be the benchmark calculations.
Absorption cross section of NO2 by the reflection method from ab initio calculations involving the three low lying electronic states
Authors: Liévin J. , Delon A. , Jost R.
Published: July 2002

The potential energy surfaces of the three low lying electronic states of NO2, namely the X̃ 2A1, Ã 2B2, and B̃ 2B1 states, and the transition dipole moment surfaces between the ground state and both excited states have been calculated at two levels of ab initio theory; complete active space self-consistent field (CASSCF) and internally contracted multireference configuration interaction (CMRCI). Only 9 points of these surfaces, located around the equilibrium geometry of X̃ 2A1 and corresponding to C2v geometries, have been found sufficient for calculating the cross section, in the 10000–45000 cm−1 energy range, by means of a 2D version of the reflection method. The agreement between the experimental low resolution data and the ab initio absorption cross section is satisfactory, mainly at the CMRCI level, at which the energy at maximum amplitude, the width, the maximum amplitude and the effective transition dipole moment describing both involved electronic transitions are predicted within 4%, 6%, 20%, and 11%, respectively. The sources of errors coming from the reflection approximation and from the level of ab initio approximation are analyzed on the basis of test calculations. The relative importance of the different contributions to the total cross section (both electronic transitions, cold and hot bands) is also discussed. In addition, quantum calculations based on Franck–Condon factors have been performed in order to improve the description of the low energy part of the cross section and to interpret the observed radiative lifetimes.
Electron–ion coincidence measurements: The neutral dissociation cross section for CF4
Authors: Mi L. , Bonham R. A.
Published: July 2002

Pulsed electron beam time-of-flight measurements of the electron energy loss spectrum of CF4 at scattering angles of 28°, 45°, 71°, 112°, and 135° were made at impact energies of 22, 25, and 34 eV in coincidence with positive ions. The angular-dependent elastic, total dissociation, ionization, and neutral dissociation cross sections were measured. The data were integrated over electron energy loss for each scattering angle. These angular dependent data were then fitted with various analytic functions and integrated over angle to obtain individual total cross sections. All cross-section data were placed on an absolute scale by matching to total cross-section measurements reported in the literature. The angular-dependent elastic cross section and the integrated cross sections for elastic and total dissociation were found to be in good agreement with literature values. Values obtained for the neutral dissociation cross section agree with the difference between previous separate measurements of total dissociation by Winters and Inokuti and total ionization by Bruce and Bonham, but are inconsistent with those of the direct measurements of Sugai et al. The total ionization cross sections agree to within the experimental uncertainties with literature values.
A model of relaxation in supercooled polymer melts
Authors: Loring Roger F.
Published: July 2002

We present a dynamical mean-field model for molecular motions in a supercooled polymer melt. A macromolecule is represented by a harmonic chain undergoing Brownian motion whose bead mobilities fluctuate between zero and a finite value. These fluctuations mimic the dynamic obstacles formed by the chain segments surrounding a given segment, whose effects become more pronounced as T decreases. The rate of these mobility fluctuations is determined self-consistently by equating it to the asymptotic long-time relaxation rate of the shortest-wavelength Rouse mode. The resulting fluctuating rate vanishes as c, the equilibrium fraction of mobile beads, approaches a threshold value c*. As c→c*, relaxation times become arbitrarily large, permitting the modeling of fluids as T approaches Tg. Calculations of autocorrelation functions of Rouse mode coordinates and of segmental mean-squared displacements are presented and compared to results from recent simulations of melts at low temperatures. The deviations from the Rouse model observed in the simulations are features of this theory.
Sub-microhartree accuracy potential energy surface for H3+ including adiabatic and relativistic effects. I. Calculation of the potential points
Authors: Cencek Wojciech , Rychlewski Jacek , Jaquet Ralph , Kutzelnigg Werner
Published: July 2002

Sixty-nine points of the Born–Oppenheimer (BO) potential energy surface (PES) for the ground state of H3+ have been computed using explicitly correlated Gaussian wave functions with optimized nonlinear parameters. The calculated points have an absolute error of about 0.02 cm−1 (0.1 microhartree), i.e., they are by at least one order of magnitude more accurate than ever reported. Similarly accurate adiabatic and relativistic corrections have also been evaluated by means of the Born–Handy formula and by direct perturbation theory (DPT), respectively.