bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of a model isomerization reaction at the liquid/solid interface
Authors: Rose Daniel A. , Benjamin Ilan
Published: July 2002

The thermodynamics and dynamics of an isomerizing diatomic (modeled using a double-well potential) at the interface between a flat wall and a Lennard-Jones liquid are studied using molecular dynamics and are compared with those of the same reaction in the bulk liquid. The potential of mean force along the reaction coordinate is calculated in the bulk and at the surface and used to estimate the transition state theory rate constant. For the particular system studied here (weakly adsorbed isomerizing molecule), the liquid contribution to the activation free energy is much smaller at the interface, but the total activation energy is the same as in the bulk because of a contribution from the binding to the surface. Calculations of the transmission coefficients using molecular dynamics reactive flux correlation show that the surface and bulk liquid contributions are nearly equal even though the effective density of the liquid near the surface is a factor of three larger than the bulk density. Insight into these results is obtained by calculations of the dynamical friction as a function of orientation and bulk density.
Site-selective and Stark spectroscopy of the series [Os(2,2′-bipyridine)3−x(2,2′-bipyridine-d8)x]2+ (x=0–3) in single-crystal [Ru(2,2′-bipyridine)3](PF6)2. Localization of an intramolecular triplet metal-to-ligand charge transfer exciton by partial deuteration
Authors: Riesen Hans , Wallace Lynne , Krausz Elmars
Published: July 2002

Selective luminescence and excitation spectroscopy and Stark experiments of the title systems are reported. Dramatic differences of the x=1 and x=2 spectra, in comparison with the x=0 and x=3 spectra are observed. The lowest-excited state in the x=1 and x=2 spectra is a charge-transfer excitation localized on the protonated ligand(s). A superposition of ‘‘deuterated’’ and ‘‘protonated’’ transitions is observed for the x=1 and x=2 systems in the excitation spectra. The deuteration shift (≊32 cm−1) is much larger than the excitation exchange interaction β between the metal–ligand subunits. ‖β‖ is ≊2 cm−1, as directly determined from the observed splittings (5–7 cm−1) of I, the lowest-energy electronic origin. Stark shifts confirm the charge transfer character of the lowest-excited states. The splittings and relative intensities of the origins and their Stark effect can be quantitatively described by an exciton formalism.
The influence of neutral reagents on the effective recombination energy of the ArN+2 cluster ion in charge-transfer reactions at thermal energies
Authors: Tsuji Masaharu , Matsumura Ken-ichi , Aizawa Masato , Funatsu Tsuyoshi , Nishimura Yukio
Published: July 2002

In order to examine the influence of neutral reagents on the effective recombination energy of a simple cluster ion in charge-transfer (CT) reactions at thermal energy, CT reactions of ArN+2 with CH3Cl, CH2Cl2, CO2, and Kr are studied at near-thermal energy by using an ion-beam apparatus. Only CT channels leading to parent ion and/or fragment ions are found for all reagents. The reaction rate constants are (0.72–1.4)×10−9 cm3 s−1, which correspond to 55%–230% of the calculated values from Langevin or averaged dipole oriented theory. The effective recombination energy of ArN+2 in the CT reactions is found to depend on the ionization potentials of reagents. For CH3Cl and CH2Cl2 with lower ionization potentials of 11.27 and 11.35 eV than the effective recombination energy of ArN+2(∼13.5 eV), respectively, CT reactions occur without distortion of ArN+2. On the other hand, for CO2 and Kr with slightly higher ionization potentials of 13.78 and 14.00 eV than that, respectively, the bond length of (Ar–N2)+ slightly expands due to interactions with the reagent to increase the effective recombination energy of ArN+2 and to induce CT reaction.
Temperature dependencies of the reactions of CO−3(H2O)0,1 and O−3 with NO and NO2
Authors: Arnold Susan T. , Morris Robert A. , Viggiano A. A.
Published: July 2002

We have measured temperature dependencies of the rate constants for CO−3 and O−3 reacting with NO and NO2. In addition, the temperature dependence of the CO−3(H2O) reaction with NO was determined, and a 196 K rate constant was measured for the reaction of CO−3(H2O) with NO2. The reactions with NO all proceed by O− transfer to produce NO−2. The temperature dependencies of the rate constants for the reactions of CO−3 and O−3 with NO are represented as 1.5×10−7*T−1.64 and 4.4×10−7*T−2.15 cm3 s−1, respectively, and agree very well with previous measurements. The rate constant for the reaction of CO−3(H2O) with NO is 4.1×10−5*T−2.72 cm3 s−1. Previous measurements of the rate constants for CO−3, CO−3(H2O), and O−3 reacting with NO2 appear to be in error; our measured rate constants for the first two reactions are represented as 2.6×10−5*T−2.38 and 9.1×10−9*T−0.79 cm3 s−1, respectively. The rate constant for the reaction CO−3(H2O) with NO2 is 7.9×10−11 cm3 s−1 at 196 K. The reactions of CO−3 and CO−3(H2O) with NO2 proceed by O− transfer, producing NO−3. While the reaction of O−3 with NO2 proceeds mainly by charge transfer at room temperature, about half the reactivity at 200 K is due to charge transfer with the remainder arising from O− transfer. Atmospheric implications of the present results are discussed.
Theoretical study of Fermi resonance in the vibrational spectrum of HO2
Authors: Barclay V. J. , Hamilton I. P.
Published: July 2002

Fermi resonance complicates the vibrational spectrum of the hydroperoxyl radical, HO2. Using a recent three-dimensional potential energy surface, we calculate vibrational wave functions for H–16O–16O and H–18O–18O. We identify low-lying Fermi resonant pairs and determine the degree of coupling by constructing linear combinations of these Fermi resonant wave functions. The coefficients are systematically varied to find the linear combination which, through visual inspection, replicates the nodal patterns of ‘‘pure’’ vibrational wave functions. The energies of the pure vibrational levels and the Fermi resonance shifts are also calculated.
Valence shell photoionization dynamics calculations for oriented PF3 molecules
Authors: Powis Ivan
Published: July 2002

Photoelectron angular distributions (PADs) from the ionization of fixed-in-space, oriented PF3 molecules have been computed using the CMS-Xα method. These molecule frame distributions are richly structured and varied with a high harmonic content. Interference terms between odd- and even-partial waves create orientation in the PAD, and in some instances such oriented PADs may undergo a dramatic inversion, or reversal of direction, at a shape resonance. This phenomenon is attributable to the rapid rise in scattering phase of a single resonant partial wave component. A previously observed reversal in the experimental PF3 Ã band correlated photoelectron–photofragment ion recoil direction can be understood in these terms. Good agreement is found between the experimental and calculated molecule frame anisotropy for ionization of the 4e orbital over a range of electron kinetic energies. The assignment of the Ã band is thereby clarified, and the presence of a 4e→ka1(σ*) shape resonance at ∼3 eV can be confirmed without the necessity to scan through the resonance energy. Other shape resonances in the range 0–25 eV are identified by the calculations; all are investigated and the resonant behavior is scrutinized with the assistance of continuum eigenchannel plots. In this manner different trapping mechanisms can be visualized and the localization of the continuum scattering functions related to virtual atomic and molecular orbitals embedded in the ionization continuum.
Quantum effects in heat and mass transport processes
Authors: Eu Byung Chan
Published: July 2002

By using the evolution equations for heat and mass fluxes which have been derived from the kinetic equation for the Wigner distribution function of a quantum gas mixture, a quantum effect in mass and heat transport is discussed in this paper. We show that there are transport coefficients associated with mass and heat transport which are proportional to ℏ2 and the curvatures of the external force applied to the system. These quantum effect terms should become observable at low temperatures if the field gradients are comparable to the temperature gradient or the concentration gradients in magnitude. In the case of charge carriers, one of the field gradients can be related to the charge density if the external field is electric, and the quantum effect should be noticeable at low temperatures if the charge density gradient is comparable to the temperature gradient or the concentration gradients of neutral species in magnitude. The said quantum effect means that heat and mass can be driven by second-order gradients (e.g., the curvature) of the external field.
Theoretical study of geometrical and electronic structures of new π-conjugated thiophene copolymers
Authors: Hong Sung Y. , Kwon Si J. , Kim Shi C.
Published: July 2002

Theoretical work was performed to investigate the geometrical and electronic structures of four new hypothetical thiophene copolymers: poly(thienylene cyclopentadienylene) (PThPD), poly(thienylene silolylene) (PThS), poly(thienylene oxocyclopentadienylene) (PThOPD) and poly(thienylene thiocyclopentadienylene) (PThTPD). AM1 band calculations showed that the ground-state geometries of PThPD and PThS copolymers are of the aromatic forms whereas those of PThOPD and PThTPD are quinoid. Each intraring structure of the copolymers is predicted to be nearly identical to that of their corresponding parent homopolymers and the bond-length alternation (δr) of each copolymer is equal to the average of δr values for the two corresponding homopolymers. Modified extended Hückel band calculations produced that the band gaps (which correspond to the absorption peaks of π–π* band transition) of the copolymers in their ground states are 1.7 for PThPD, 1.55 for PThS, 1.9 for PThOPD, and 2.09 eV for PThTPD. These values, except for PThPD, are quite smaller than the band gaps calculated for the corresponding homopolymers in the ground states. Decomposition of the band gaps reveals that the gaps of the aromatic forms of PThPD and PThS are dominated by the δr contribution and the electronic effect of the bridging groups and that the gaps of the quinoid types of PThOPD and PThTPD correspond primarily to the δr contribution.
Theory of thermal effects in nuclear magnetic resonance spectra of metal hydrides undergoing quantum mechanical exchange
Authors: Szymański S.
Published: July 2002

Thermal effects in nuclear magnetic resonance spectra of transition metal hydrides exhibiting resolved quantum mechanical exchange splittings are consistently explained. Interactions of the relevant spatial degrees of freedom of the hydride protons with a quantum mechanical thermal bath are described in terms of Wangsness–Bloch–Redfield (WBR) theory. Upon elimination of the vibrational modes which relax too quickly to be observed on the NMR time scale, the WBR equation for the remaining, slowly relaxing modes (exchange modes) is shown to be equivalent to the Alexander–Binsch equation for classically exchanging nuclei, where the standard spin–spin coupling term is replaced by (or augmented with) quantum exchange term. Numerical calculations were performed for a one-dimensional model of the relevant spatial motions, where the vibrational relaxation effects were described in terms of two adjustable parameters only. The assumed motion includes correlated rotation of a pair of the hydride protons, where the interproton distance may vary with the rotation angle. These calculations confirm that the present approach affords a consistent theoretical reproduction of the effects observed experimentally, i.e., an increase of the effective splitting with increasing temperature, with a gradual emergence of stochastic exchange that ultimately leads to motionally narrowed NMR spectrum lacking any fine structure.
Gauge transformations of electron group functions
Authors: Zapol Boris P.
Published: July 2002

Within the scope of the electron group functions (EGF) theory, the concept of gauge transformations (GT) of EGFs is introduced as such transformations that leave the state of the entire system invariant. The variational equations for EGFs should contain additional terms representing the Pauli repulsion part of the pseudopotential and being consistent with the choice of EGFs (the requirement of gauge consistency). The GTs present a natural way of ab initio defining the generalized many-electron pseudopotentials produced by an internally correlated subsystem. Some specific, but rather general forms of GTs are proposed. One of the form is defined using properties of group functions with odd number of electrons. The GTs belonging to another class are defined using properties of antisymmetrically annulling (ASA) functions introduced in our earlier work and studied further in the present work. In particular, we introduce the ASA kernel basis set for a given group function and show that any function ASA the given group function can be expanded in terms of this set. The algebraic properties of GTs and of their sets are studied, both general ones and specific for the mentioned forms. In general case, the proposed GTs depend on a set of parameters which are functions rather than numbers, that can provide improved transferability of pseudopotentials. The linear transformations of one-electron functions of a determinant as well as the procedures of strong orthogonalization of a group function to a determinant (by Fock, Veselov, and Petrashen’, and by Szasz) are shown to be special cases of the GTs considered.
Photoemission and high resolution electron energy loss spectroscopy study of CO/K/Cu(110)
Authors: Christensen So/ren V. , Nerlov Jesper , Godowski P. Jan , Onsgaard Jens
Published: July 2002

Adsorption of CO on the Cu(110) surface modified with a low (near the work function minimum) potassium precoverage has been investigated by use of low energy electron diffraction, photoelectron spectroscopy based on synchrotron radiation, work function measurements (ΔΦ), and high resolution electron energy loss spectroscopy (HREELS). During potassium adsorption at 118 K the binding energy (BE) of the K 3p peak shifts −0.3 eV. For a thick potassium overlayer both a surface and a bulk K 3p peak, with a 0.9 eV difference in BE, are observed. The work function results at ΘK∼0.5 ML show after saturation with CO an increase of 1.11 eV. An electrostatic interaction energy between CO and K equal to 0.23 eV is determined. This energy indicates a moderate attractive interaction between CO and K. A CO overlayer on K/Cu(110) shows the 4σ satellite, the 4σ orbital, and the 5σ/1π joint feature. No observable change in the BE separation between the 4σ and the 5σ/1π band (3.1 eV) was found, thereby indicating a negligible CO–CO interaction. From the 4σ to the 5σ/1π intensity ratio (∼2) a weakening of the CO–metal binding with increasing CO coverage is deduced. In the HREEL spectrum, an intense feature at 1731 cm−1 besides the fundamental stretching frequency at 2030 cm−1 implies a substantial potassium induced weakening of the C–O bond.
Theoretical studies of CO interaction on Rh3 cluster
Authors: Majumdar D. , Balasubramanian K.
Published: July 2002

Geometries and energy separations of 27-low-lying electronic states of Rh3CO with different structures have been investigated. Atop, bridged, and threefold CO orientations with Rh3 were considered. Complete active space multiconfiguration self-consistent field method (CASMCSCF) followed by large scale multireference singles and doubles configuration interaction (MRSDCI) computations that included up to 2.1 million configurations were used. Three nearly degenerate electronic states were found for the threefold structure around the minimum energy region. At the highest level of theory, the A22 state prevailed to be the ground state. The assignment of minimum energy states for the atop and bridged structures are unambiguous. The B42 and B41 electronic states were found to be the most stable states for these two respective geometries. The nature of the metal–CO bond formation in different states is discussed, and the difference density maps of Rh3CO with respect to Rh3 and COs are presented. The dissociation energies and CO vibrational frequencies have been computed and the results are compared with the available experimental data.
On the “direct” calculation of thermal rate constants. II. The flux-flux autocorrelation function with absorbing potentials, with application to the O+HCl→OH+Cl reaction
Authors: Thompson Ward H. , Miller William H.
Published: July 2002

We present a method for obtaining the thermal rate constant directly (i.e., without first solving the state-to-state reactive scattering problem) from the time integral of the flux-flux autocorrelation function, Cff(t). The quantum mechanical trace involved in calculating Cff(t) is efficiently evaluated by taking advantage of the low rank of the Boltzmannized flux operator. The time propagation is carried out with a Hamiltonian which includes imaginary absorbing potentials in the reactant and product exit channels. These potentials eliminate reflection from the edge of the finite basis and ensure that Cff(t) goes to zero at long times. In addition, the basis can then be contracted to represent a smaller area around the interaction region. We present results of this method applied to the O+HCl reaction using the J-shifting and helicity conserving approximations to include nonzero total angular momentum. The calculated rate constants are compared to experimental and previous theoretical results. Finally, the effect of deuteration (the O+DCl reaction) on the rate constant is examined.
Phase diagrams of single-component fluids in disordered porous materials: Predictions from integral-equation theory
Authors: Kierlik E. , Rosinberg M. L. , Tarjus G. , Monson P. A.
Published: July 2002

We present the calculation of phase diagrams for fluids in disordered porous materials using theories based on the replica symmetric Ornstein–Zernike equations. We consider molecular models in which the porous medium is described by quenched disordered configurations of spheres and the fluid-fluid and matrix intermolecular potentials are the sum of a hard-sphere core and an attractive tail. Such models account for the combined effect of confinement, wetting, and disorder that are expected to be important to describe recent experimental observations. We use the replica method to derive the expressions relating the thermodynamic properties of the fluid inside the porous material to the pair distribution functions within the mean-spherical approximation and the optimized random-phase approximation (ORPA). We also consider higher-order corrections within the optimized cluster theory developed by Andersen and Chandler for bulk fluids. In most cases a vapor–liquid coexistence curve, similar to that observed for the bulk fluid, although displaced and somewhat narrowed, is obtained. The improved ORPA+B2/EXP approximation also predicts the appearance of a second fluid–fluid phase transition at a lower temperature.
Time correlation functions for mixed quantum-semiclassical systems
Authors: McWhirter J. Liam
Published: July 2002

A stationary phase analysis of the reduced propagator gives the semiclassical reduced propagator and the Pechukas equation. Taking the semiclassical limit, we derive a mixed quantum-semiclassical time correlation function using Pechukas’ stationary phase analysis. This derivation incorporates the initial configuration variations of Xiao and Coker and the initial value formalism of Heller and Miller. To simplify the correlation function further, we employ a zero back reaction approximation, while noting the nonuniqueness property of the solution to the Pechukas equation subject to initial conditions. The limitations of this approximation were discussed.
Doped ionic magnetic fluids: A resonance characterization
Authors: Da Silva G. J. , Morais P. C. , Tourinho F. A.
Published: July 2002

Electron paramagnetic resonance is used to investigate ionic cobalt ferrite-based magnetic fluids. Two magnetic fluid samples having particles with different average radius of 6.8 nm and 7.8 nm were doped with Cu2+ at different concentrations. Changes on the resonance line shape of the Cu2+-ion in the aqueous phase was analyzed taking into account the particle polydispersity, the contribution of the dipolar field due to the surrounding magnetic nanoparticles, and the electrostatic interaction between the surface charged magnetic nanoparticles and the Cu2+-probe. The particle polydispersity parameters we found from the resonance line shape analysis are in excellent agreement with the parameters obtained from electron microscopy. The Cu2+ spatial distribution function around the magnetic nanoparticles gives very reasonable values for both, the particle–particle distance and the ionic layer thickness at the particle surface.
Energy transfer investigations on pure Cr(III)-complex compounds using resonant emission measurements
Authors: Nierhauve Guido , Schmidtke Hans-Herbert
Published: July 2002

Time resolved emission spectra of Cr(III) pentammine complexes excited at resonance conditions are recorded yielding different lifetimes for zero-phonon and vibronic transitions from 2B1(C4v). The decay of zero-phonon transitions furthermore depends on the excitation frequency. The energy dependence is rationalized assuming a model which describes energy transfer rates between electronic levels of molecules on equivalent sites which compete with intramolecular rates for the decay into the ground states. Corresponding rate equations worked out for some special cases are shown to have analytical solutions. For a continuous set of excited levels, a differential equation is obtained which is known from diffusion or transport phenomena but contain the energy as variable. Simulation of the experiments leads to transfer rate parameters kd which are 1 (or 2) orders of magnitude larger than the decay rate k0.
Brownian dynamics simulation of diffusion-limited polymerization of rodlike molecules: Isotropic translational diffusion
Authors: Srinivasalu Gupta J. , Khakhar D. V.
Published: July 2002

Rigid rodlike polymers have considerable technological importance due to their excellent mechanical properties. The polymerization kinetics of such condensation polymers are qualitatively different from flexible polymers, and exhibit significant slowing at the later stages of the reaction. This is due to the slow rotational diffusion of the molecules to an appropriate configuration for reaction. In this work we have carried out Brownian dynamics (BD) simulations to obtain the effective rate constant for reaction between rodlike molecules in the presence of diffusional limitations. The theory of Northrup et al. [J. Chem. Phys. 80, 1517 (1984)] for pairwise BD simulation of reactions is extended to the case of rodlike molecules assuming isotropic translational diffusion. The computed results are compared to exact analytical predictions. Good agreement between computation and theory is obtained over a wide range of parameter values.
Spectroscopy and dynamics of rare gas-spherical top complexes. III. The infrared spectrum of the ν3 band of Ne–SiH4 ( j=1←1 and j=2←1 transitions)
Authors: Brookes Matthew D. , Hughes Daniel J. , Howard Brian J.
Published: July 2002

The infrared spectrum of the rare gas-spherical top complex Ne–SiH4 has been recorded in a supersonic jet in the region of the SiH4 ν3 triply degenerate stretching vibration at ∼2189 cm−1. The Ne–SiH4 spectrum is complex with no obvious regular band structure; a previous paper has reported the assignment and analysis of bands correlating with the SiH4 R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. This paper concludes the study of Ne–SiH4, with the assignment and analysis of bands correlating with the SiH4 Q(1) (K=0←0, K=1←0, K=0←1, K=1←1) and R(1) (K=0←0, K=1←0, K=1←1, K=2←1) transitions. This was facilitated by a twofold approach: The bands were identified by the use of a theoretical intermolecular potential, with potential parameters determined by reference to the two bands previously fitted; and the relative magnitudes of the Coriolis interactions between the K sublevels within the bands determined by explicit calculation of the Coriolis matrix elements. Having fitted the bands to appropriate energy-level expressions, an effective anisotropic potential for the internal rotation of the SiH4 molecule within the complex was fitted to the total of 15 observed band origins. With a fixed R approximation, the V3 anisotropic term was found to be ∼30 cm−1, approximately one-third that of Ar–SiH4, reflecting the freer rotation of the SiH4 molecule when complexed with the less polarizable Ne atom. However, the overall quality of the fit was an order of magnitude worse than that for Ar–SiH4, indicating increased importance of the radial dependence of the Ne–SiH4 potential compared to that of Ar–SiH4.
Polarization selectivity of nonresonant spectroscopies in isotropic media
Authors: Murry Robert L. , Fourkas John T.
Published: July 2002

We present an analysis of the contributions of the first- and second-derivative tensors of the many-body polarizability to third- and fifth-order nonresonant spectroscopies in isotropic media. Collision-induced effects are shown to have a notable influence on the second-derivative polarizability tensor (Π(2)) for intermolecular modes. As a result, polarization selectivity in nonresonant intermolecular spectroscopies can be achieved in fifth-order spectroscopies. Additionally, terms in fifth-order spectroscopy that arise from three interactions through Π(2) may not be negligible in many liquids. Our analysis shows that there exists no straightforward relationship between the observables in third- and fifth-order intermolecular spectroscopies. The predictions of this analysis are tested against the available experimental data for CS2.
Structure of Na(3 2P)–Arn clusters using semiempirical potentials
Authors: Tutein Alan B. , Mayne Howard R.
Published: July 2002

We have calculated the minimum energy geometries of clusters formed by electronically-excited sodium atoms in their (3 2P) state with argon atoms. The potential energy functions needed to describe interactions with an open-shell atom are not pairwise additive. Those used here were constructed using a first-order perturbation treatment. The semiempirical pair potentials used as input were derived from direct spectroscopic evidence, in the case of the A 2Π potential, and indirectly from matrix isolation data for the B 2Σ. We find the clusters to be planar for n=2–5. Additional argon atoms form an argon subcluster, avoiding the perpendicular to the Na*–Ar5 plane. We have investigated the sensitivity of the cluster geometry to variations in the potential parameters, and predict our qualitative conclusions will not be changed by further data on the B potential. We propose a naming convention to clarify discussion of these ring-containing clusters. We have also calculated geometric isomers for the Na*–Ar5 case. The significance of these findings for fluorescence experiments in cryogenic rare gas matrices and other condensed phase experiments is discussed.
On the numerical solutions of kinetic equations for diffusion-influenced bimolecular reactions
Authors: Kim Hyojoon , Shin Seokmin , Shin Kook Joe
Published: July 2002

Numerical methods for solving kinetic equations for diffusion-influenced bimolecular reactions are presented for three cases. Finite difference method is used to solve diffusion-reaction equations for the pair distribution function. The kinetic equation for the concentration is evolved by the Runge–Kutta method with adaptive time step. The boundary doubling method is introduced to study long time dynamics, where the truncation problem of the infinite boundary is crucial. The above methods are applied, in the first case, to the classical Smoluchowski approach to a binary reaction with random initial condition and the results are compared with ones in two dimension. In the second case, an isolated pair recombination dynamics with a delta function initial condition is investigated and the results are compared with analytic expression in three dimension with spherical symmetry. A more complicated system with the hierarchical Smoluchowski approach with the Kirkwood superposition approximation is also investigated in the third case. The efficiency and the accuracy of the numerical calculations are examined against the asymptotic analytical solutions and a Monte Carlo simulation in one dimension.
The anomalous adsorbate dynamics at surfaces in porous media studied by nuclear magnetic resonance methods. The orientational structure factor and Lévy walks
Authors: Zavada Tatiana , Kimmich Rainer
Published: July 2002

Diffusion of adsorbate molecules along surfaces of porous media was examined with respect to ordinary and Lévy walk diffusion mechanisms. The orientational structure factor formalism of the “reorientation mediated by translational displacements” (RMTD) mechanism originally derived for ordinary diffusion is generalized to Lévy walks. The two cases can be distinguished experimentally using field-cycling NMR relaxometry. The low-frequency spin-lattice relaxation dispersion is influenced by the dynamics on the surfaces as well as by the surface geometry. The experiments were carried out with polar and nonpolar liquids filled into porous glasses and fine particle agglomerates (ZnO, TiO2). The spin-lattice relaxation dispersion of polar and nonpolar adsorbate species shows dramatic differences, and reflects the limits of “strong” and “weak” adsorption, respectively. The low-frequency behavior is explained by RMTD along the surfaces. At temperatures below the freezing point of the confined liquids, one or two molecular diameter thick surface layers remain unfrozen. Molecular dynamics in the interfacial liquid in these nonfreezing surface layers (NFLs) were also studied. The propagators relevant for RMTD are shown to depend on whether the sample is frozen or not. In the NFL case, an ordinary Gaussian displacement distribution function applies, whereas a Lévy walk surface diffusion process with a Cauchy distribution tends to dominate in the strong-adsorption limit. On a much longer length scale beyond the so-called retention time when diffusion becomes normal, field gradient NMR diffusometry was applied. Confinement of the liquid adsorbate to the pore space or, in frozen samples, to the NFL reduces the diffusion coefficient mainly due to the geometrical restriction. In the case of NFLs, the reduction amounts to one order of magnitude relative to the bulk values.
Size-dependence of the electronic spectra of benzene⋅(N2)n clusters
Authors: Adams John E.
Published: July 2002

The evolution of the electronic spectral properties of benzene⋅(N2)n clusters to those of the corresponding bulk has been studied using a formalism that yields a direct correlation between cluster geometries and spectra. We show here that the calculated spectral shifts of the benzene chromophore as a function of cluster size are in good agreement with experimental values and that, as observed previously, the asymptotic limit of the shifts is considerably smaller in magnitude than the observed bulk values. An examination of the structures of these cryogenic clusters reveals a strong influence of quadrupolar interactions between dinitrogens, interactions that yield a degree of ordering not present in the ostensibly similar benzene⋅(Ar)n clusters. These observations suggest a resolution of the problem posed by the apparent lack of asymptotic convergence to bulk spectral shifts, that at the higher temperatures associated with the bulk measurements, higher local dinitrogen densities become possible as the thermal energy exceeds the quadrupolar orientational ordering energy. These higher “solvent” densities would be expected to produce the larger spectral shifts seen in experiments.
The near-infrared bands of NO2 observed by high-resolution Fourier-transform spectroscopy
Authors: Orphal J. , Dreher S. , Voigt S. , Burrows J. P. , Jost R. , Delon A.
Published: July 2002

Fourier-transform absorption spectra of NO2 at 298 K were recorded between 9000–15 000 cm−1 at Doppler-limited resolution (0.012 cm−1), in the region near the Ã 2B2–X̃ 2A1 conical intersection. The strong rovibronic bands are rather isolated and organized into polyads which can be assigned by the number of bending quanta of the Ã 2B2 state, corresponding to the transitions with the largest Franck–Condon factors. Around 9735 cm−1, where the origin of the Ã 2B2–X̃ 2A1 vibronic band system is located and up to about 11 200 cm−1, hot bands dominate the spectrum. The complexity of the spectrum increases with energy due to an increasing density of Ã 2B2 and X̃ 2A1 vibrational levels. The rotational structure is dense and irregular as already observed for higher bands.
Coherent nuclear motion in a condensed-phase environment: Wave-packet approach and pump–probe spectroscopy
Authors: Mühlbacher Lothar , Lucke Andreas , Egger Reinhold
Published: July 2002

A quantum-mechanical Gaussian wave-packet approach to the theoretical description of nuclear motions in a condensed-phase environment is developed. General expressions for the time-dependent reduced density matrix are given for a harmonic potential surface, and the exact quantum dynamics is found for a microscopic system-plus-bath model. Particular attention is devoted to the influence of initial correlations between system and bath for the outcome of a pump–probe experiment. We show that the standard factorized preparation, compared to a more realistic correlated preparation, leads to significantly different stimulated emission spectra at high temperatures. Recent experiments for the reaction center are analyzed using this formalism.
Doubly excited autoionizing states of H2 above the second ionization threshold: the Q2 resonance series
Authors: Sánchez I. , Martı́n F.
Published: July 2002

We report extensive calculations of energy positions, autoionization widths and branching ratios for the Q2 doubly excited states of H2 lying above the second ionization threshold 2Σu+. We present results for 60 doubly excited states of the 1,3Σg,u+, 1,3Πg,u, and 1,3Δg,u symmetries in the range of internuclear distances 0⩽R⩽6 a.u. The resonance parameters have been obtained using the Feshbach theory and an L2 representation of the nonresonant continuum with B-spline functions. Our results are compared with previous theoretical data. We have found significant discrepancies for states of the 1Σg+ and 1Σu+ symmetries.
Coherent stabilization of zero-electron-kinetic-energy states
Authors: Bellomo Paolo , Stroud C. R.
Published: July 2002

The accuracy of zero-electron-kinetic-energy (ZEKE) photoelectron spectroscopy rests on the ultralong lifetimes of the high-n, high-l Rydberg states that are responsible for the ZEKE signal. However, a few-photon process cannot excite electrons directly from the low-l ground state to the high-l ZEKE manifold. In this paper we show that using the dynamics of Rydberg Stark states in slowly time dependent external fields it is possible to control coherently the angular momentum of Rydberg electrons, and therefore also their lifetime. We derive explicitly two different schemes based on simple, short electric dc pulses, which populate precisely those high-l, long-lived Rydberg states that are necessary for accurate ZEKE experiments. The high-l states that we construct are also Stark eigenstates, therefore a moderate dc external field can eventually enforce cylindrical symmetry and lock the ZEKE electrons in the stable, long-lived high-l manifold.
The structure of the liquid-vapor interface of a gallium-tin binary alloy
Authors: Zhao Meishan , Rice Stuart A.
Published: July 2002

We report the results of self-consistent quantum Monte Carlo simulations of the structure of the liquid-vapor interface of the alloy Sn0.09Ga0.81. Our calculations are in very good agreement with the experimental results reported by Lei, Huang and Rice [J. Chem. Phys. 107, 4051 (1997)]. In particular, our calculations confirm the experimentally inferred existence of a partial second layer of Sn below the complete outermost layer of Sn in the stratified liquid-vapor interface of this alloy.
Quadratic response of molecules in a nonequilibrium and equilibrium solvation model: Generalizations to include both singlet and triplet perturbations
Authors: Poulsen Tina D. , Ogilby Peter R. , Mikkelsen Kurt V.
Published: July 2002

Quadratic response theory for equilibrium and nonequilibrium solvation has been extended to include both singlet and triplet perturbations. The approach is tested by investigating the effect of solvent on the phosphorescence lifetime of formaldehyde.
Electronic structure of polyglycine as a function of proton position
Authors: Howard I. A. , Springborg M.
Published: July 2002

Using a first-principles density-functional method, we have calculated the single-chain band structure for the α-helical protein polyglycine as a function of proton position along the N⋯H⋯O hydrogen bond. Calculations have been carried out using both the local-density approximation (LDA) and the Perdew–Wang generalized gradient approximation (GGA) for the exchange-correlation potential. We present results for the total energy, band structure, charge densities, and Mulliken charges as a function of proton position. The total energy gives an asymmetric double-well potential for simultaneous proton shift, with the global minimum (at a N–H distance of 0.95 Å) 2.02 eV (46.6 kcal/mol) below the top of the barrier with the GGA, and 1.83 eV (42.2 kcal/mol) below with the LDA.
Statistical thermodynamics of bond torsional modes
Authors: Chuang Yao-Yuan , Truhlar Donald G.
Published: July 2002

We present a formalism for treating torsions in complex molecules with nonsymmetry-related multiple minima along the internal rotation coordinate. We also present a comparison of rectilinear and curvilinear methods for estimating the moment of inertia of an internal rotor, and we find that they sometimes differ considerably. We point out that the usual method of estimating the free rotor limit of the moment of inertia may sometimes be inaccurate by an order of magnitude. We present an approach called the Cω-single-frequency approximation that provides a good compromise of accuracy and economy of information cost, in that it provides reasonably accurate partition functions for most systems over a wide range of temperature without requiring extra Hessians or information about saddle points.
Analysis of the intramolecular charge transfer in donor–acceptor α,α′-substituted oligothiophenes from their vibrational spectra
Authors: Hernández V. , Casado J. , Effenberger F. , López Navarrete J. T.
Published: July 2002

In this paper we report on the vibrational Fourier transform infrared (FT–IR) and Fourier transform Raman (FT–Raman) spectra of a few donor–acceptor–substituted bi- ter-, and quaterthiophenes, as solids. The relevant vibrational information has been interpreted with the help of spectroscopic data previously collected on a model series of α,α′-dimethyl end-capped oligothiophenes, in their neutral, radical cationic, and dicationic forms. We have also performed density functional theory (DFT) quantum-mechanical calculations to analyze the effects of the intramolecular charge transfer on the geometries and vibrational spectra of these systems.
Optical near-field excitation at the semiconductor band edge: Field distributions, anisotropic transitions and quadrupole enhancement
Authors: von der Heydt A. , Knorr A. , Hanewinkel B. , Koch S. W.
Published: July 2002

The optical near-field response of a three dimensional subwavelength aperture-semiconductor system is analyzed within a finite difference time domain scheme for Maxwell’s and excitonic material equations. The analysis includes the field modification due to the high refractive index environment and the excitonic response to a near-field distribution. The resonant optical response is illustrated for anisotropic dipole transitions in quantum wells and the enhancement of the quadrupole transition in materials with dipole forbidden interband transitions.
The improved Bardeen–Cooper–Schrieffer method in polymers
Authors: Piris Mario , Otto Peter
Published: July 2002

It is shown that in the framework of the improved Bardeen–Cooper–Schrieffer (IBCS) theory the correlation energy per unit cell of a periodic polymer can be computed. Furthermore, a new concept of effective correlated pairs is introduced and the coefficient ϰ modifies now the correlated term in the occupation number arised. The occupation numbers are expressed by means of quantities, which satisfy a trigonometric relation to warrant the nonidempontent condition. As a consequence a variational method can be introduced, which involves only an unconstrained minimization which may be performed using a conjugated gradient technique. Some results of calculations on nucleotide bases and the corresponding periodic polymers are presented.
Rydberg state decay in inhomogeneous electric fields
Authors: Softley T. P. , Rednall R. J.
Published: July 2002

An extension of the model of Merkt and Zare [J. Chem. Phys. 101, 3495 (1994)] is presented to describe the effects of static inhomogeneous electric fields, which arise experimentally from combinations of applied (or stray) homogeneous fields and the presence of charged particles, on Rydberg states of atoms and molecules. The effect of an arbitrary number of charged particles is included and the effects of nonzero quantum defects are investigated. A quantization axis rotation procedure is defined, allowing clear distinction between homogeneous and inhomogeneous field effects. Calculations are reported of the time-dependent decay of a coherent population of eigenstates for n=20, 33, and 50, involving diagonalization of the full n2×n2 matrix. Calculations are also carried out for n=100 by pre-diagonalization of the full homogeneous field perturbation followed by a restricted basis set diagonalization for the inhomogeneous part of the perturbation. The inclusion of nonzero quantum defects has a substantial impact on the ml mixing, confining significant mixing to a narrow range of radial and angular positions of the ion. An applied homogeneous field of order the Inglis-Teller field is required in combination with the field due to the ions. The dynamics are very different according to whether np or nf series carry the transition probability. For np-state population, the maximum stabilization is achieved at ion-Rydberg distances of around 5n2a0, with the ion almost perpendicular to the applied homogeneous field. For an initial nf population the ion perturbation may have a destabilizing effect at sufficiently small homogeneous field (⩽0.1FIT). Significant effects of laser polarization on the stability are reported. Calculations for a realistic pseudo-random distribution of ions and Rydbergs suggest that ml mixing by ions will never reach the complete mixing limit, but that at least an order of magnitude stabilization is achievable under a restricted range of conditions.
Site specific dissociation dynamics of propane at 157 nm excitation
Authors: Wu S. M. , Lin J. J. , Lee Y. T. , Yang X.
Published: July 2002

Photodissociation of propane at 157 nm excitation has been investigated using the photofragment translational spectroscopic (PTS) technique. Three dissociation channels have been experimentally observed: the atomic hydrogen (H) elimination, the molecular hydrogen (H2) elimination, and the methyl radical (CH3) elimination. Product translational energy distributions have been measured and relative branching ratios have also been estimated, which indicate that all these three processes are significant. The site effects on the H and H2 elimination have been quantified by investigating the photodissociation of the four propane isotopomers: CH3CD2CH3, CD3CH2CD3, CH2DCH2CH2D, and CD3CD2CD3. Experimental results indicate that the yield of the H elimination from the two terminal CH3 groups (terminal-H elimination) is larger than that from the internal CH2 group (internal-H elimination). H2 elimination from the internal carbon atom of propane (2,2-H2 elimination) is the dominant process while H2 elimination channels from the vicinal carbon atoms (1,2-H2 elimination) are less significant. Minor processes of H2 elimination from terminal carbon atoms (1,1-H2 and 1,3-H2 elimination) were also observed. The translational energy distributions also show that the dynamics of H and H2 elimination from different sites are significantly different. Relative branching ratios of H and H2 elimination processes from different sites have also been determined.
Structures, energies, vibrational spectra, and electronic properties of water monomer to decamer
Authors: Lee Han Myoung , Suh Seung Bum , Lee Jin Yong , Tarakeshwar P. , Kim Kwang S.
Published: July 2002

The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve as sensitive indicators of the status of proton donation (“d”) and acceptance (“a”) (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O–H frequencies (ν) for each cluster can be arranged in the following order: ν3da (single donor–single acceptor) ≅ν3daa (single donor–double acceptor) >ν3dda (double donor–single acceptor) >ν1dda>ν1da> (or ≅) ν1daa. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν3da and ν3daa). The intensities of ν1daa and ν1da are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν1dda modes are diminished by more than half. Most of the above data have been compared to the available experimental data. Keeping in view the recent experimental reports of the HOH bending modes, we have also analyzed these modes, which show the following trend: ν2dda>ν2daa≅ν2da. The present study therefore would be useful in the assignments of the experimental O–H stretching and HOH bending modes.
Comparative study of He3, Ne3, and Ar3 using hyperspherical coordinates
Authors: Blume D. , Greene Chris H. , Esry B. D.
Published: July 2002

We calculate the L=0 ground and excited state energies for the rare gas trimers He3, Ne3, and Ar3. An adiabatic representation is adopted to solve the nuclear Schrödinger equation, in which the Schrödinger equation in hyperangular coordinates is solved at a series of fixed hyper-radii using B splines. We compare results obtained in a strict adiabatic approximation with nonperturbative coupled-adiabatic-channel calculations. Structural properties such as the pair and angle distributions are monitored as functions of the hyper-radius. These structural studies pinpoint the locus of configurational changes that occur as the trimer fragments into a diatom plus an atom. Analysis of the angular channel functions and their associated radial components permits an approximate classification of the vibrational spectrum.