bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of vibration on the linear and nonlinear optical properties of HF and (HF)2
Authors: Bishop David M. , Pipin Janusz , Kirtman Bernard
Published: July 2002

Electronic and vibrational contributions to the linear and nonlinear optical properties of the HF dimer have been calculated for the first time. The vibrational components are very significant with mechanical anharmonicity effects, as determined by perturbation-theoretic formulas, playing a major role. We identify the important anharmonic potential constants and analyze the validity, as well as the possible extension, of our theoretical treatment. Parallel computations, for purposes of comparison, have also been performed for the monomer.
Optical properties of two-dimensional dye aggregate
Authors: Nabetani Akira , Tomioka Akihiro , Tamaru Hiroharu , Miyano Kenjiro
Published: July 2002

Absorption and emission spectra of a monolayer J aggregate of a cyanine dye have been studied in detail from room temperature to liquid helium temperature. Various sources of disorder that affects the degree of delocalization of the exciton have been investigated including the hitherto unnoticed effect of He gas. Simple numerical simulations were performed, which reproduce the basic features of the spectra fairly well. The spectral signatures indicate that we have a typical two-dimensional Frenkel exciton system characterized by large red shift and large cooperation number despite strong disorder, which is in contrast to the well-studied pseudoisocyanines (PICs) that have been analyzed as one-dimensional even in the Langmuir–Blodgett monolayers. The disorder stems partly from the exciton–phonon coupling but is dominantly of the static character at low temperatures.
Structural transformations of impurity centers in naphthalene crystals
Authors: Brovchenko Ivan V.
Published: July 2002

A new kind of structural transformation of impurity centers in molecular crystals was found. Light irradiation and annealing of naphthalene crystals, doped with some aniline derivatives, results in sharp changes of the optical spectra of impurity and local (X trap) states. Calculations of aniline sites in naphthalene crystals using the method of atom–atom potentials have been carried out. Sets of impurity sites, with and without hydrogen bond between impurity and host molecules, were obtained. Infrared-spectra investigations confirm results of the calculations. Possible mechanisms of the observed transformations are also discussed.
Bending of N-isopropylacrylamide gel under the influence of infrared light
Authors: Zhang Xiaomin , Li Yong , Hu Zhibing , Littler Chris L.
Published: July 2002

A CO2 infrared laser has been used to irradiate a straight cylindrical N-isopropylacrylamide gel. It is found that the infrared laser not only induces the volume phase transition in the gel, but also causes the gel to bend toward the laser beam. When the laser is blocked, the gel becomes straight again. The transition between the straight and the bending gel is fully reversible. The maximum bending strain of the gel is comparable to that obtained for poly(vinyl alcohol)–poly(sodium acrylate) copolymer gel under the influence of an electric field. The bending effect has been systematically studied as a function of CO2 laser power, time, and the sample cell temperature. The relaxation behavior for the gel restoring its original shape after blocking the infrared irradiation follows an exponential form. It is suggested that the bending effect is caused by a temperature gradient which produces an osmotic pressure difference between the front surface area of the gel and the remainder of the gel.
On the selective Λ-doublet relaxation in CH(X 2Π,v=0,N)
Authors: Heinrich P. , Stuhl F.
Published: July 2002

For large quanta of N, CH(X 2Π,v=0,N) radicals were observed to develop Λ-doublet population disequilibra in collisions with Ar. Both time and pressure dependences were measured. This imbalance is due to collisional depletion of the Π(A′) levels and the preferred population of the Π(A″) levels in ΔN−1 steps as described by Alexander and Dagdigian.
Microwave detected, microwave-optical double resonance of NH3, NH2D, NHD2, and ND3. II. Predissociation dynamics of the Ã state
Authors: Henck Steven A. , Mason Martin A. , Yan Wen-Bin , Lehmann Kevin K. , Coy Stephen L.
Published: July 2002

Using microwave detected, microwave-optical double resonance, we have measured the homogeneous linewidths of individual rovibrational transitions in the Ã state of NH3, NH2D, NHD2, and ND3. We have used this excited state spectroscopic data to characterize the height of the dissociation barrier and the mechanisms by which the molecule uses its excess vibrational and rotational energies to help overcome this barrier. To interpret the observed vibronic widths, a one dimensional, local mode potential has been developed along a N–H(D) bond. These calculations suggest the barrier height is roughly 2100 cm−1, approximately 1000 cm−1 below the ab initio prediction. The observed vibronic dependence of levels containing two or more quanta in ν2 is explained by a Fermi resonance between 2ν2 and the N–H(D) stretch. This interaction also explains the observed trends due to isotopic substitution. The rotational enhancement of the predissociation rates in the NH3 21 level is dominated by Coriolis coupling while for the same level in ND3, centrifugal effects dominate.
The thermal reaction rate of muonium with methane (and ethane) in the gas phase
Authors: Snooks Rodney , Arseneau Donald J. , Fleming Donald G. , Senba Masayoshi , Pan James J. , Shelley Mee , Baer Susan
Published: July 2002

Rates for the gas-phase thermal reaction Mu+CH4→MuH+CH3 (Mu=μ+e−), have been measured using the μSR (muon spin rotation) technique, over the temperature range 625–820 K. A good fit is obtained to the usual Arrhenius expression, k=A exp(−Ea/RT), giving an activation energy Ea=24.6±0.9 kcal/mol, ∼12 kcal/mol higher than that of the H-atom isotopic variant of this reaction, H+CH4→H2+CH3. This Ea difference is the largest yet seen at high temperatures between H and Mu in the gas phase, and seems much too high to be explained in terms of [zero-point-energy (ZPE)] differences in their respective transition states, indicating instead a dramatic difference in reaction dynamics. The possible sources of this difference include differing reactivities from vibrationally excited states and/or a more favorable tunneling path for the H+CH4 reaction due to its suspected much earlier (and thinner) reaction barrier. In contrast, the similar H-atom abstraction reactions with H2 and C2H6 gave Ea differences which matched expectations based on ZPE shifts, suggesting a qualitative difference in dynamics between these otherwise homologous reactions. It is suggested that reaction from vibrationally excited states may be relatively more important in the case of the Mu+CH4 reaction.
Rotational structure and dissociation of the Rydberg states of CO investigated by ion-dip spectroscopy
Authors: Komatsu Masaaki , Ebata Takayuki , Maeyama Toshihiko , Mikami Naohiko
Published: July 2002

In a series of spectroscopic work of the Rydberg states of CO, we present the rotational analysis of the v=0 and 1 levels of the singlet ns, np, nd and nf-Rydberg states (n=4–7). The spectra were measured by ion-dip spectroscopy with triple resonance excitation via the 3sσ:B 1Σ+ or the 3pσ:C 1Σ+ state. All the spectra were rotationally well resolved and the term value, quantum defect and the rotational constant were obtained for each state. Through the analysis of the rotational structure, the coupling between the Rydberg electron and the ion core has been investigated. For the np-Rydberg states, a switching from Hund’s case (b) to (d) was clearly observed with the increase of n. A significant perturbation was observed in the 6pπ 1Π and 7pπ 1Π states and it is suggested that these states are perturbed by the state with the same symmetry. For the nf-Rydberg states, the observed electronic energy was well analyzed by the long range force model and the precise ionization potential was obtained. The Rydberg↔valence and inter-Rydberg states interactions were also investigated. For the ns-Rydberg states, the interaction matrix element with the repulsive state was estimated from the measurement of linewidth of the rotational levels. The potential curve of the repulsive state to which ns-Rydberg states predissociate was also determined. Selective predissociation was found for the e-symmetry levels both in the v=0 and 1 levels of the nf-Rydberg state. A strong interaction between the v=0 levels of the 6d- and 7s-Rydberg states was observed.
Chemical structure effects on the equilibrium and under shear properties of thin films in confined geometries: A molecular dynamics simulation study
Authors: Padilla Paz
Published: July 2002

We have made a comparative study of confined thin fluid films, composed of either n-decane or 4-propyl-heptane. The films are studied in equilibrium and under shear using molecular dynamics (MD) simulations. The films composed of linear chains present density profiles of methylene subunits with higher degree of layering than those composed of branched molecules. There are no significant differences in the diffusion coefficients of the two molecules studied in bulk, or in confined geometries. The diffusion coefficients for the confined films are strongly dependent on the strength of the frictional forces exerted by the wall, rather than on the density of the films. They also indicate that the confined films remain in a fluidlike state in all the simulations. The bulk values of the diffusion coefficient of n-decane are in excellent agreement with the experimental data. When the confining walls move in opposite directions, the fluid films develop shear flow with a very weak shear rate. Superimposed to the shear flow, the films seem to perform an oscillatory motion, where they alternately flow following the direction of motion of either wall. The steady state values of the shear stress increase linearly with the pressure normal to the confining walls, as also found experimentally. The films composed of linear chains exhibit higher resistance to the displacement of the walls than those composed of branched chains. This is because the films composed of linear chains have higher density of methylene subunits in the region of the pore where the fluid molecules exert frictional forces on the walls.
Oscillator Hamiltonian representation of the linear response stochastic theory of outer-sphere electron transfer reactions
Authors: Basilevsky M. V. , Chudinov G. E.
Published: July 2002

We consider an interconnection between the oscillator (spin-boson) model of the continuum outer-sphere electron transfer (ET) reaction and its stochastic dynamical description in terms of generalized Langevin equation (GLE) for longitudinal polarization. The oscillator Hamiltonian derived from the GLE can be reduced to its conventional spin-boson form in the limit of the infinite frequency of the primary polarization mode, called solvaton coordinate. In the framework of the Golden Rule approximation the ET rate expressions of both (solvaton and spin-boson) theories are exactly the same. This is shown without invoking a condition of the infinite solvaton frequency. A hypothesis concerned with a physical nature of bath oscillators is suggested. These modes, embodying dielectric friction effects within the GLE description, are associated with transverse polarization waves of a continuum medium.
Short time dynamics of glass-forming liquids
Authors: Roland C. M. , Ngai K. L.
Published: July 2002

Calculations have been presented for the intermediate scattering function, dynamic structure factor, and dynamic susceptibility of a complex correlated system undergoing relaxation with independent vibrations. The vibrational contribution was approximated by a Debye spectrum, smoothed at high frequency, while the coupling model was used to describe the relaxation. This model asserts for nonpolymeric glass-forming liquids a crossover at a microscopic time from intermolecularly uncorrelated relaxation at a constant rate to intermolecularly coupled relaxation with a time-dependent, slowed-down rate. Although the model has previously been employed to successfully predict and otherwise account for a number of macroscopic relaxation phenomena, critical phenomena are not included in, and cannot be addressed by, the coupling model. Notwithstanding an absence of any change in transport mechanism for the supercooled liquid at a critical temperature, the coupling model data, when analyzed in the manner used for mode coupling theory, shows various features interpreted by MCT as critical dynamic singularities. These include an apparent fast ‘‘β’’ relaxation giving rise to a cusp in the temperature dependence of the Debye–Waller factor, a power-law divergence in the temperature dependence of the relaxation time for the α process, and critical exponents for the relaxation having a defined relationship to one another.

Additionally, other experimental features of the short-time dynamics, such as the anomalous Debye–Waller factor and the von Schweidler law, are also observed in results derived from the coupling model. Whatever similarities underlie the coupling model and MCT, a crucial difference is that only the latter predicts the existence of critical phenomena. Yet these and other distinct features are exhibited by the coupling model data. Evidently, any interpretation of short-time behavior in terms of MCT is ambiguous, if not necessarily incorrect. This indicates the importance of the many macroscopic-time relaxation properties found over the years in glass forming liquids (including polymers, small molecule van der Waal liquids, and inorganic materials), and the necessity that they be addressed by any theory, including MCT, purporting to offer a fundamental description of relaxation phenomena.
Vibronic activity in trans,trans-1,3,5,7 octatetraene: The S0→S1 spectrum
Authors: Buma Wybren Jan , Zerbetto Francesco
Published: July 2002

Various levels of configuration interaction are used to investigate the vibronic intensity pattern of the false origins of the 1 1Ag→2 1Ag transition of all-trans octatetraene. The vibronically induced mixing of the Ag with the Bu states is best simulated when polarization, i.e., d, functions are added to the basis set of atomic orbitals. Normal mode rotation upon electronic excitation plays an important role in the intensity distribution of the bu false origins. The progressions of the totally symmetric modes built on the bu bands is satisfactorily simulated at the CASSCF/6-31G* level. The calculations also show that the overtones of at least four out-of-plane modes should be present in the spectra whose assignment is tentatively proposed. Through comparison of the pure electronic intensities calculated for cis–trans octatetraene with the vibronically induced intensities of trans–trans octatetraene, it is found that the presence of a cis linkage induces a spectral perturbation similar to that of the most active bu mode.
Freezing of Yukawa systems near the one-component-plasma limit
Authors: Rosenfeld Yaakov
Published: July 2002

It is shown that the Yukawa freezing results as obtained recently by Farouki and Hamaguchi [J. Chem. Phys. 101, 9885 (1994)] using molecular dynamics simulations contradict the Monte Carlo results of Meijer and Frenkel [J. Chem. Phys. 94, 2269 (1991)], the molecular dynamics results of Stevens and Robbins [J. Chem. Phys. 98, 2319 (1993)], as well as several empirical freezing criteria. A systematic inquiry finally identified the cause for the Farouki and Hamaguchi incorrect result, and solved the discrepancy. The present analysis provides the corrected freezing line for Yukawa systems in the weak screening region, near the one-component-plasma limit.
‘‘Order–disorder’’ phenomena in a diffusion-reaction model of interacting dipoles on a surface
Authors: Garza-López Roberto A. , Byun Daniel , Orellana Kevin , Partikian Arthur , Siew David , Yu Anne , Kozak John J.
Published: July 2002

We study the reaction efficiency of a surficial process in which a diffusing, tumbling dipole A reacts (eventually and irreversibly) with a stationary target dipole B. In contrast to earlier studies of such irreversible diffusion-reaction events (A+B→C), we consider the situation where at each and every site of the space accessible to the diffusing coreactant A, there is also embedded a fixed dipole. To quantify the influence on the reaction efficiency of (angle-averaged, dipole–dipole) potential interactions between the tumbling dipole A and the ensemble of stationary dipoles, we design a lattice-statistical model to describe this problem and use both analytical methods and numerical techniques rooted in the theory of finite Markov processes to work out its consequences. Specifically, we define the reaction space to be an n×n=N square-planar lattice with the target dipole occupying the centrosymmetric site in that space and determine the mean number of steps required before the irreversible event, A+B→C, occurs. Our results reveal two qualitatively-distinct regimes of behavior for this diffusion-reaction process, a low temperature (or strong coupling) regime dominated by nearest-neighbor excursions only, and a high-temperature (or weak-coupling) regime dominated by non-nearest neighbor excursions of the tumbling dipole A, with the transition between these two regimes occurring over a relatively narrow range of interparticle couplings. This behavior has the character of an ‘‘order–disorder’’ transition and is interpreted here in terms of an ‘‘order parameter’’ W related to a generalized Onsager length. The behavior uncovered is studied as a function of system size and of the boundary conditions imposed.
Determination of Flory–Huggins interaction parameter and self-diffusion coefficients in ternary polymer solutions by quasielastic light scattering
Authors: Sun Z. , Wang C. H.
Published: July 2002

We present a new method to determine the Flory–Huggins polymer–polymer interaction parameter and self- (or tracer) diffusion coefficients in a ternary polymer solution made of two monodisperse polymers and a solvent by quasielastic light scattering (QELS). Previously proposed methods based on the random phase approximation (RPA) were restricted to special cases with two polymers having the same molecular weight and identical interaction parameters with the solvent. The present method (also using RPA) has removed these restrictions. It is applicable to an arbitrary ternary solution with polymers having different molecular weight and solvent interaction parameters. To illustrate, we have analyzed the published QELS data of the PDMS/PMMA/toluene ternary system to obtain self diffusion coefficients for each polymer and the Flory–Huggins polymer–polymer interaction parameter as a function of polymer composition in the system.
Transition state resonances by complex scaling: A three-dimensional study of ClHCl
Authors: Leforestier Claude , Yamashita Koichi , Moiseyev Nimrod
Published: July 2002

Four lowest-lying transition state resonance energies and lifetimes in the three-dimensional ClH+Cl←ClHCl→Cl+HCl reaction are reported in this paper. This is the first application of the complex coordinate method to a three-dimensional, triatomic molecule with a double dissociation continuum, which has been handled by means of the hyperspherical coordinates. Two numerical strategies have been considered in order to make the calculations feasible. The first one consists in minimizing the dimension of the Hamiltonian matrix by prediagonalization of the basis set. This has been achieved in terms of the successive adiabatic reduction method of Bac̆ić and Light [J. Chem. Phys. 85, 4594 (1986)], holding the hyperradius fixed. The second strategy is to compute a reduced set of eigenvalues corresponding to the resonances by using the complex Lanczos algorithm. The number of Lanczos recursions required to achieve convergence is dramatically reduced by applying the iterative scheme to the complex scaled resolvant operator (E0−H̄)−1, rather than to the complex scaled Hamiltonian. Inversion of a large complex symmetric matrix is avoided by using an LU decomposition of the complex scaled Hamiltonian matrix (E0−H̄). The proposed numerical approach appears to be very efficient and powerful for the study of large systems.
Electrochemical deposition of hydrogen on platinum single crystals studied by infrared-visible sum-frequency generation
Authors: Peremans A. , Tadjeddine A.
Published: July 2002

Under- and overpotential deposition of hydrogen on polycrystalline platinum, Pt(100), Pt(110), and Pt(111) surfaces in sulfuric acidic medium is monitored in situ by infrared-visible sum-frequency generation (SFG). In the underpotential range, the vibrational signature between 1800 and 2020 cm−1 reveals hydrogen bonding between the stable hydrogen adsorbed in terminal sites and the water molecules in the electrolyte. For Pt(100) and Pt(111), the observed double SFG resonance is most consistent with a bonding configuration where water dimers are each bounded to three hydrogen atoms adsorbed on the relaxed substrate. In the overpotential range, an additional hydrogen adspecies appears with a similar vibrational fingerprint on all the investigated platinum surfaces and is tentatively assigned to the intermediate of the hydrogen evolution reaction. Its SFG resonance frequency near 1770 cm−1 suggests a dihydride adsorption configuration. The detection at under- or overpotential of high frequency SFG resonances (≊2050 cm−1) correlates with the presence of corrugations on the disordered Pt and reconstructed Pt(110) surfaces.
Complex angular momentum analysis of resonance scattering in the Cl+HCl→ClH+Cl reaction
Authors: Sokolovski D. , Connor J. N. L. , Schatz George C.
Published: July 2002

Resonance effects in the differential cross sections of the Cl+HCl(v,j)→ClH(v′,j′)+Cl reaction are analyzed using Regge pole and complex angular momentum (CAM) techniques. This is the first detailed application of CAM theory to reactive molecular scattering. The rovibrational transitions studied are v=1, j=5→v′=0, j′=15, and v=1, j=5→v′=1, j′=5 at total energies E=0.66, 0.68, 0.70 eV. The CAM theory expresses the scattering amplitude as a background subamplitude plus a pole subamplitude. The uniform (and nonuniform) semiclassical evaluation of the background subamplitude is discussed. It is necessary to include explicitly the resonance Regge pole in the semiclassical theory because it has a small imaginary part. We derive a new generic semiclassical formula, involving the complementary error function for the resonance angular scattering. The position and residue of the resonance Regge pole at each E are extracted numerically from scattering matrix elements calculated by the centrifugal sudden hyperspherical (CSH) quantum scattering method. There is good agreement between the semiclassical CAM and CSH angular distributions. However, the latter involve summing a partial wave (PW) series with a large number of numerically significant terms—as a result the PW computations provide no physical insight. We also show that a simple semiclassical optical model becomes inaccurate when the rotational period of the ClHCl complex is comparable to the resonance lifetime. We derive a new ‘‘sticky’’ optical model which allows for rotation of the complex. All our calculations use the Bondi–Connor–Manz–Römelt semiempirical potential energy surface.
Transverse diffusion measurements and Monte Carlo simulation studies of Rb+ ions in Kr and Xe
Authors: Tan T. L. , Ong P. P.
Published: July 2002

The ratio of the transverse diffusion coefficient to the mobility (DT/K) for Rb+ ions drifting in Kr and Xe gases at about 303 K has been measured at electric field to neutral gas density ratio (E/N) values ranging from 5 to 500 Td. Calculations of the reduced mobility (K0) and the ratios of the longitudinal and transverse diffusion coefficients to mobility (DL/K and DT/K, respectively) for both Rb+–Kr and Rb+–Xe systems were made using a Monte Carlo simulation (MCS) technique and the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)]. Furthermore, DL/K and DT/K values were derived from K0 values obtained from the present MCS calculations and from experimental results reported in the literature, using the generalized Einstein relations which are based on the three-temperature theory. For the Rb+–Kr system, the experimental values of DT/K were found to be 2%–4% higher than those from MCS calculations for E/N values above 120 Td. For the Rb+–Xe system, the experimental DT/K values were similarly higher than the MCS values in the range 90–220 Td. Given that the standard error of the experimental measurements of DT/K is estimated to be better than 3% and that of the MCS calculations is better than 2.5%, the agreement between the present experimental DT/K data and those from MCS calculations can be considered to be fairly good. The interaction potential of Koutselos, Mason, and Viehland therefore appears to represent well the actual potential for both Rb+–Kr and Rb+–Xe systems.
Nonlinear optical and electroabsorption spectra of polydiacetylene crystals and films
Authors: Mukhopadhyay D. , Soos Z. G.
Published: July 2002

Vibronic structure of nonlinear optical (NLO) coefficients is developed within the Condon approximation, displaced harmonic oscillators, and crude adiabatic states. The displacements of backbone modes of conjugated polymers are taken from vibrational data on the ground and 1B excited state. NLO resonances are modeled by three excitations and transition moments taken from Pariser–Parr–Pople (PPP) theory and optimized to polydiacetylene (PDA) spectra in crystals and films, with blue-shifted 1B exciton. The joint analysis of third-harmonic-generation, two-photon absorption, and nondegenerate four-wave-mixing spectra of PDA crystals and films shows weak two-photon absorption to 2A below 1B, leading to overlapping resonances in the THG spectrum, strong two-photon absorption to an nA state some 35% above 1B, and weak Raman resonances in nondegenerate FWM spectra. The full π-π* spectrum contributes to Stark shifts and field-induced transitions, as shown by PPP results for PDA oligomers. The Stark shift dominates high-resolution electroabsorption (EA) spectra of PDA crystals below 10 K. The close correspondence between EA and the first-derivative I′(ω) of the linear absorption above the 1B exciton in PDA crystals provides an experimental separation of vibrational and electronic contributions that limits any even-parity state in this 0.5 eV interval. An oscillator-strength sum rule is applied to the convergence of PDA oligomers with increasing length, N, and the crystal oscillator strengths are obtained without adjustable parameters. The sum rule for the 1B exciton implies large transition moments to higher-energy Ag states, whose locations in recent models are contrasted to PPP results. Joint analysis of NLO and EA spectra clarifies when a few electronic excitations are sufficient, distinguishes between vibrational and electronic contributions, and supports similar π-electron interactions in conjugated molecules and polymers.
Molecular dynamics of biaxial nematic liquid crystals
Authors: Sarman Sten
Published: July 2002

We devise a constraint algorithm that makes the angular velocity of the director of a liquid crystal a constant of motion. When the angular velocity is set equal to zero, a director based coordinate system becomes an inertial frame. This is a great advantage because most thermodynamic properties and time correlation functions of a liquid crystal are best expressed relative to a director based coordinate system. One also prevents the director reorientation from interfering with the tails of the time correlation functions. When the angular velocity is forced to be zero the constraints do not do any work on the system. This makes it possible to prove that ensemble averages of phase functions and time correlation functions are unaffected by the director constraint torques. The constraint algorithm also facilitates generalization of nonequilibrium molecular dynamics algorithms to liquid crystal phases. In order to test the algorithm numerically we have simulated a biaxial nematic phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)]. The director constraint algorithm works very well. We have calculated the velocity autocorrelation functions and the self diffusion coefficients. In a biaxial nematic liquid crystal there are three independent components of the self-diffusion tensor. They have been found to be finite and different thus proving that we really simulate a liquid rather than a solid and that the symmetry is biaxial. Simulation of biaxial liquid crystals requires fairly large systems. We have therefore developed an algorithm that we run on a parallel computer instead of an ordinary work station.
A quantum Monte Carlo perturbational study of the He–He interaction
Authors: Huiszoon Cornelis , Caffarel Michel
Published: July 2002

Using a recently proposed quantum Monte Carlo method the exact first-, second-, and third-order Rayleigh–Schrödinger interaction energies of the He–He interaction have been calculated for internuclear distances in the range 1.5- to 7.0 bohr. Putting together these new data with the best ab initio results available, the relative importance of the different contributions appearing in the perturbational expansion of the He–He interaction energy is discussed. In particular, the results show that the third-order Rayleigh–Schrödinger term and the intra-atomic correlation contribution to the second-order component play a significant role. For intermediate and large distances (including the equilibrium distance), it is found that the perturbational expansion limited to the complete first- and second-order, plus the third-order Rayleigh–Schrödinger energy agrees with the best known values of the total interaction energy of the helium dimer.
Pressure and light effects on the phase transition of deuterated Cu(2,5-dimethyl-dicyanoquinonediimine)2
Authors: Gómez D. , Schmitt H. , von Schütz J. U. , Wachtel H. , Wolf H. C.
Published: July 2002

The deuterated dicyanoquinonediimine salt Cu(2-CD3,5-CD3-DCNQI)2, (d6), a highly conductive organic metal, exhibits a phase transition from the metallic to the insulating state at Tc≊60 K. In a wide temperature range we observe an anticoincidence of high conductivity and electron spin resonance (ESR). These experiments were performed simultaneously in our ESR apparatus equipped for application of He pressure up to 200 bar. Near Tc there is a coexistence of conducting (ESR-silent) and insulating (ESR-detectable) domains. This correlation of ESR and conductivity σ points to a percolation limited conductivity. Upper and lower limits of the size of the domains are given. Light shifts Tc to lower temperatures, pressure to higher ones. This could be due to shrinking of the lattice which induces the phase transition when critical structural parameters are approached. This is most probably explained by a simultaneous development of a charge density wave (CDW) and a spin pairing within the DCNQI stack together with the trimerization of the lattice. Systematic measurements by variation of temperature, pressure, and light led to phase diagrams of (d6) and to the understanding of the mechanism of the phase transitions.
Isomers of SO3: Infrared absorption of OSOO in solid argon
Authors: Jou Shen-Horng , Shen Min-yi , Yu Chin-hui , Lee Yuan-Pern
Published: July 2002

Sulfur trioxide (SO3) isolated in solid argon at 12 K was irradiated with light at 193 nm from an ArF excimer laser. Recombination of photofragments O and SO2 produces OSOO that absorbs at 1229.6, 1041.3, and 597.6 cm−1. The assignments are based on observed 34S- and 18O-isotopic shifts. Theoretical calculations using the B-P86 and the B3-LYP density functional methods were carried out for five isomers of SO3; energies and vibrational wave numbers were predicted for each one. Observed line positions, infrared intensities, and isotopic shifts fit well with those predicted for cis-OSOO at the B-P86 level. Further irradiation of the matrix sample with emission at 248 nm from a KrF laser bleached OSOO and enhanced absorption lines of SO2 and, to a lesser extent, of SO3. The mechanism of formation of OSOO in a matrix cage is discussed.
An efficient fast-multipole algorithm based on an expansion in the solid harmonics
Authors: Wang H. Y. , LeSar R.
Published: July 2002

We present an efficient variant of the fast-multipole method for calculating long-range interactions in three-dimensional Coulombic systems. Using a multipole expansion based on the solid harmonics instead of the more common spherical harmonics leads to a greater increase in computational efficiency than a recently-reported fast-Fourier transform method, with none of the overhead associated with that approach.
Normal mode and isomerization bending states in HCP: Periodic orbit assignment and spectroscopic signature
Authors: Farantos Stavros C. , Keller Hans-Martin , Schinke Reinhard , Yamashita Koichi , Morokuma Keiji
Published: July 2002

We present exact quantum mechanical bound-state calculations for HCP using an abinitio potential energy surface. The main result is the existence of two distinct families of bending states: one with normal-mode-type behavior and wave functions confined to small bending angles and the other one sampling the isomerization path all the way from H–CP to CP–H. Stable periodic orbits provide a clear-cut assignment. Possible relations to recent spectroscopic observations are discussed.
Optical Stark spectroscopy of molecular aggregates
Authors: Mukamel Shaul , Rott Pavel , Chernyak Vladimir
Published: July 2002

Effects of static disorder and interaction with phonons on the dynamics of Frenkel excitons in molecular aggregates are studied by calculating the absorption of a weak probe in the presence of a strong resonant and off-resonant pump field. To second order in the pump amplitude, the self-energy which determines the Stark shift and dynamical broadening of the probe absorption is expressed in terms of the single exciton Green function and the two-exciton scattering matrix. For stronger pump intensities the self-energy is calculated using higher-order optical response functions of the system.
A combined quantum chemical and transition state theory study of the C2H+2+CH4 reaction dynamics
Authors: Klippenstein Stephen J.
Published: July 2002

The stationary points of importance in the initial branching between complex formation and H transfer in the reaction of acetylene cation with methane are studied at the Gaussian-2 level of theory. Three separate C2H+2...CH4 ion-neutral complexes are found. One complex leads smoothly to the bridged form of C2H+3 plus CH3. A slightly exothermic saddlepoint also connects this ‘‘bridged’’ complex to a stable C3H+6 species. A ‘‘classical’’ complex leads to the more endothermic classical form of C2H+3. These classical and bridged ion-neutral complexes are nearly degenerate and are directly connected via a low energy saddlepoint which is substantially exothermic relative to reactants. The third, more weakly bound complex, is of a more electrostatic nature and appears to be of no importance in the subsequent branching between the C3H+6 and C2H+3+CH3 products. Transition state theory calculations employing the ab initio determined energetics and quadratic force fields provide estimated reactive cross sections which are in qualitative accord with recent experimental data. The quantitative discrepancies may be explained on the basis of the neglect of anharmonicites and/or errors in the estimated energetics. Consideration of the vibrational frequencies for the various stationary points provides a qualitative explanation for the observed variations in the reaction cross sections with initial vibrational excitations in the CC stretching and bending modes of C2H+2.
Spectroscopy of F2 in Ne matrices
Authors: Bressler C. , Lawrence W. G. , Schwentner N.
Published: July 2002

The excited states of free and matrix-isolated F2 were investigated in the windowless VUV region by absorption and excitation spectroscopy. In emission the f 3Πg→a 3Πu laser band of F2 at 7.72 eV with a radiative lifetime of 2 ns and a weaker and broader band at 7.47 eV attributed to emission of F2 aggregates are observed in an Ne matrix independent on excitation energy. The Franck–Condon envelope of the charge transfer state C 1Σ+u extends further to the blue by more than 1 eV in Ne (12 to 14 eV) compared to the gas phase due to a blueshift of the avoided crossing with the Rydberg 3pπu(1Σ+u) state. The H 1Πu Rydberg state remains unperturbed in the Ne matrix but is blueshifted by 1.24 eV and significantly broadened due to electron–phonon coupling. A previously unreported broad absorption is observed both in the gas phase and in the matrix around 15 eV and is assigned to a strongly perturbed charge transfer state with Σ symmetry corresponding to F+*(1S) and F−(1S) ions in the dissociation limit. This charge transfer state is strongly perturbed in the gas phase by the high density of npπu(1Σ+u) Rydberg states (n≥4) while in Ne matrix it is mixed mainly with the low-lying 4pπu(1Σ+u) state. The analogy of the resulting two separated groups of bands with irregular vibrational progressions to the Cl2 case is shown. Further npπu and npσu Rydberg progressions and the repulsive 3Σ+u valence state are treated. The utility of matrix-isolated F2 for a solid state laser is discussed.
First assignment of the rotational spectrum of a molecule containing two iodine nuclei: Spectroscopic constants and structure of CH2I2
Authors: Kisiel Z. , Pszczól/kowski L. , Caminati W. , Favero P. G.
Published: July 2002

The rotational spectrum of methylene iodide was assigned by combining information from broadband mm wave spectra of low-J transitions recorded with a supersonic-jet spectrometer and of high-J transitions measured for a room temperature sample. In the first step, the analysis of central frequencies of hyperfine multiplets for rotational transitions with J″ up to 189 resulted in constants of the rotational Hamiltonian HR, in which the fitted quartic centrifugal distortion constants were consistent with predictions made from ab initio calculations. The knowledge of HR allowed accurate prediction and assignment of the hyperfine structure due to the two iodine nuclei, which was made on the basis of well isolated splitting patterns observed in the jet and the effects of nuclear spin statistics visible therein. The final rotational constants are A=22 034.437(2), B=620.584(2), C=605.798(2) MHz and the hyperfine coupling constants are χaa=−1180.9(1), χbb−χcc=−892.42(5), |χab|=1358.9(5) MHz, and χzz=−2030.1(5), χxx=993.4(10), χyy=1036.7(1) MHz. The angle θza between the inertial a axis and the principal quadrupole axis z is 32.00(1)° and is consistent with ∠(CI⋅a)=33.0(2)° from the fitted structure. The structural parameters of the halogen in CH2I2 are r(CI)=2.134(2)Å and ∠(ICI)=114.0(3)° and compare well with extrapolation from the remaining methylene halides.
Taylor dispersion of particles in stratified media with random transition rates
Authors: Soto-Campos Gerardo , Mazo Robert M.
Published: July 2002

The dispersion of particles in a stratified medium with a flow normal to the stratification is studied for the case when the transition rates between the various strata are random variables. One- and two-dimensional stratified systems are considered. The effective diffusion coefficient in the flow direction is computed numerically. The numerical results are compared with the predictions of a mean-field theory known as the effective (or coherent) medium approximation.
On the calculation of absolute spectral densities
Authors: Smith Sean C. , Jeffrey Stephen J.
Published: July 2002

A new method of calculating the absolute spectral density of a Hamiltonian operator is derived and discussed. The spectral density is expressed as the solution of an integral equation in which the kernel is a renormalized one-sided energy correlation function of the full microcanonical density operator and a microcanonical density operator for a reference Hamiltonian. The integral operator associated with this equation transforms a known spectral density function for the reference Hamiltonian into the spectral density of the full Hamiltonian. The integral equation, by virtue of its formulation in energy space, is inherently one-dimensional and offers no storage difficulties, and the elements of its kernel may be computed by applying the Lanczos algorithm to randomly selected eigenfunctions of the reference Hamiltonian. This spectral density correlation method offers a number of advantages over variational methods. In particular, it has the potential for overcoming the hitherto largely insurmountable problem of tracing over a multidimensional Hilbert space in order to compute the spectral density of a nonseparable molecular Hamiltonian.
Green–Kubo relations for the viscosity of biaxial nematic liquid crystals
Authors: Sarman Sten
Published: July 2002

We derive Green–Kubo relations for the viscosities of a biaxial nematic liquid crystal. In this system there are seven shear viscosities, three twist viscosities, and three cross coupling coefficients between the antisymmetric strain rate and the symmetric traceless pressure tensor. According to the Onsager reciprocity relations these couplings are equal to the cross couplings between the symmetric traceless strain rate and the antisymmetric pressure. Our method is based on a comparison of the microscopic linear response generated by the SLLOD equations of motion for planar Couette flow (so named because of their close connection to the Doll’s tensor Hamiltonian) and the macroscopic linear phenomenological relations between the pressure tensor and the strain rate. In order to obtain simple Green–Kubo relations we employ an equilibrium ensemble where the angular velocities of the directors are identically zero. This is achieved by adding constraint torques to the equations for the molecular angular accelerations. One finds that all the viscosity coefficients can be expressed as linear combinations of time correlation function integrals (TCFIs). This is much simpler compared to the expressions in the conventional canonical ensemble, where the viscosities are complicated rational functions of the TCFIs. The reason for this is, that in the constrained angular velocity ensemble, the thermodynamic forces are given external parameters whereas the thermodynamic fluxes are ensemble averages of phase functions. This is not the case in the canonical ensemble. The simplest way of obtaining numerical estimates of viscosity coefficients of a particular molecular model system is to evaluate these fluctuation relations by equilibrium molecular dynamics simulations.
Raman rotational spectra of endohedral C60 fullerene complexes
Authors: Hernández-Rojas J. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

Low-temperature (T=50 K) Raman rotational spectra are calculated for the endohedral complexes Li+@C60, Na+@C60, CO@C60, LiF@C60, and LiH@C60. The systems Na+@C60 and CO@C60 produce very similar spectra dominated by pure rotational bands. The other systems present more complex and dissimilar spectra which suggest librational motions and hindered rotations. An assignment of these spectra is provided by analyzing the structure of the low energy rotational eigenvalues and eigenfunctions. Rotational energy quasiband and quasiresonance-band structures are found to be the origin of the features observed in the more complex spectra. Out of these analyses, a classification of the low-temperature rotational spectra of endohedral C60 complexes in two basic classes emerges.
Molecular rotations in vibronically coupled systems
Authors: Mayer M. , Cederbaum L. S.
Published: July 2002

A new general method to compute rovibronic energy levels and eigenstates in molecules with possibly strong vibronic coupling is presented. The method can be applied to arbitrary coupling mechanisms of electronic potential energy surfaces and is not limited to a special type of molecule. Explicit criteria for the applicability of the method are derived. To investigate different types of rovibronic interactions, parameter studies for linear and bilinear E×ε Jahn–Teller systems are performed. As an application to a real molecule we investigate the rotational structure of the Li3 electronic A state. A variety of possible nontrivial effects is found which cannot be explained by rotational–vibrational coupling on a single electronic potential energy surface. Attention is paid to construct explicit effective Hamiltonians describing the findings. The rovibronic motion on potential surfaces with equivalent minima is also discussed in this context.
Classical and quantal calculations for the Penning ionization system N2+He*(23S)→N+2+He+e−
Authors: Ishida Toshimasa , Horime Katsuyuki
Published: July 2002

Quasiclassical trajectory and quantal calculations are carried out for the Penning ionization system N2+He*(23S)→N+2(2Σg,2Πu,2Σu)+He+e− based on an ab initio resonance potential and energy widths which were obtained in previous work. Total and partial ionization cross sections are evaluated for the collision energy range of 0.1–1.0 eV. For the trajectory calculation, the collisional energy dependence of the cross section is in better agreement with a recent experiment on state resolved Penning ionization than calculations using the classical sudden approximation. The result in the high frequency rotation limit is significantly different from that for the sudden approximation, which is in contrast to the H2–He* system. The results for quantal calculations using the sudden and spherical-potential approximations confirm the reliability of classical treatments. The results obtained suggest that analyses with the widely used atomic-target models lose their validity for significantly anisotropic systems in which targets have large moments of inertia.
Microscopic description of nonadiabatic, nonequilibrium, and equilibrium solvations for solvated cluster reactions: (H2O)nCl−+CH3Cl→ClCH3+Cl−(H2O)n
Authors: Okuno Yoshishige
Published: July 2002

A microscopic theory was presented for each of the nonadiabatic- and equilibrium-solvation regimes in microsolvated cluster reactions to examine nonequilibrium-solvation effects, and applied to the SN2 reactions: (H2O)nCl−+CH3Cl→ClCH3+Cl−(H2O)n for n=0–4. To have pictures for nonadiabatic and equilibrium solvations, the potential-energy surface of the reacting system on the transition-state region was described with effective normal coordinates defined in each of these solvation limits. The solute dynamics in each of these solvation limits was considered to be determined by the effective frequencies characterizing the motions along the corresponding normal coordinates, and a rate-constant expression was approximately derived. Ab initio molecular-orbital calculations were carried out for the microsolvated SN2 reactions, and the ratio of nonadiabatic- to equilibrium-solvation rate constants was evaluated. It was found that the ratio provides a better approximate value of a transmission coefficient that corresponds to the ratio of the nonequilibrium- to equilibrium-solvation rate constants, for the larger values of number of microsolvated waters. It was supported that the nonadiabatic-solvation picture appropriately characterizes the dynamics on the transition-state region in such a reaction that the time scale of the reaction is very short compared to the motions of solvent reorganization. Furthermore, the finding that the transmission coefficients were quite small gave us a new understanding of the importance of the nonequilibrium-solvation effect. In addition, the activation free energy for the microsolvated reaction in the case of n=4 was found unexpectedly to give most of the activation free energy for the corresponding solution reactions.