bluuebunny/crossref_metadata_2025 huggingface dataset:
Internal conversion rates for single vibronic levels of S2 in azulene
Authors: Woudenberg Timothy M. , Kulkarni Sudhir K. , Kenny Jonathan E.
Published: July 2002

Simultaneous measurements of absorption and fluorescence in a pulsed planar supersonic jet are used to determine fluorescence quantum yields of nine single vibronic levels (SVL’s) in the S2 electronic manifold of azulene. The quantum yield at the spectroscopic origin is 0.042±0.004, somewhat larger than that obtained in earlier room-temperature measurements in solution. Most SVL’s have quantum yields within experimental error of this value, except for two SVL’s characterized by large-amplitude motion at carbons 1 and 3, whose quantum yields are about 0.06. Our quantum yield data are combined with recent lifetime measurements by other workers to obtain fluorescence and internal conversion decay rates for each SVL. The radiative rates vary by a factor of 2, as expected for the S2↔S0 transition, whose absorption strength is strongly enhanced by vibronic coupling between S2 and S4. The internal conversion rates increase with excess vibrational energy in a way that parallels the extent of intramolecular vibrational redistribution (IVR), as determined by other workers using SVL emission and time-and-energy-resolved emission. Thus, the internal conversion rates in this electronic state appear to exhibit behavior typical of large aromatic molecules.
On coil–stretch transitions in dilute polymer solutions
Authors: Wiest J. M. , Wedgewood L. E. , Bird R. Byron
Published: July 2002

In this paper we examine molecular stretching in the inception of uniaxial elongational flow of dilute polymer solutions. The polymer molecules are modeled as bead–spring chains with finitely extensible nonlinear elastic springs, and we use the Peterlin approximation. This work is distinguished from earlier work because we model the macromolecules with chains instead of dumbbells, and we examine the time dependence of three average quantities describing the chain conformation in unsteady flows: root-mean-square end-to-end distance, root-mean-square extensions of the individual links, and mean moment of inertia about the axis of elongation. We observe a gradual transition from the coiled equilibrium state of the chain to the stretched state after the inception of strong uniaxial elongational flow, and we describe the nature of this transition which takes place in roughly four stages: I equilibrium coil; II deformed coil; III spring stretched (‘‘locally unraveled’’); and IV unfolded chain. Inclusion of hydrodynamic interaction changes the macromolecular response quantitatively but not qualitatively.
Phase transformation kinetics in finite inhomogeneously nucleated systems
Authors: Weinberg Michael , Kapral Raymond
Published: July 2002

Phase transformation kinetics that occurs by a nucleation and growth process is investigated. A simple discrete space and time model is used for the dynamics and analytical results are obtained for the volume fraction of the material transformed for both finite systems and a special example of an inhomogeneously nucleated system. The theory is developed for two cases, initial nucleation, and continuous nucleation. The results are compared with simulations of the model.
Structure and growth of colloidal metal particles
Authors: Wales David J. , Kirkland Angus I. , Jefferson David A.
Published: July 2002

In this paper we discuss the results of electron microscopic studies of colloidal gold and silver metal particles in the context of a model for their structure and growth. The images obtained are compared with the results of computer simulations for various candidate structures and the actual morphologies present are thereby deduced. With reference to earlier work on inert gas atom clusters we have developed a scheme which models the growth of such particles and enables the structures present to be predicted and rationalized. In particular, we deduce that particles belonging to the icosahedral point group have Mackay-type structures, in agreement with the experimental results. Comparisons are also made between the morphologies adopted by gold, silver, and argon clusters, and the relationship between the bonding in these systems is discussed.
Polyelectrolyte bead model. I. Equilibrium
Authors: Fixman Marshall
Published: July 2002

A polyelectrolyte chain is modeled as a string of charged beads each of which is surrounded by a shell of salt solution. Within a shell the electrostatic potential is described by the nonlinear Poisson–Boltzmann equation. In the region outside the set of shells the equation is linearized and the potential behaves as if it originated in a set of effective charges immersed in a Debye–Hückel solvent. The effective charges are examined for rod-like chains in regard to dependence on bead number, on curvature of the rods, and the approach of other rods. Significant changes from straight-rod values occur only for configurations of very high energy. Ancillary calculations are reported for the second osmotic virial coefficient of cylindrical models of DNA fragments.
Shock temperature measurements of planetary ices: NH3, CH4, and ‘‘synthetic Uranus’’
Authors: Radousky H. B. , Mitchell A. C. , Nellis W. J.
Published: July 2002

Shock temperature measurements have been performed on several materials which have relevance to the modeling of the outer planets. These materials are methane, ammonia and a mixture of water, ammonia, and isopropanol known as synthetic Uranus. Temperatures have been measured in these materials over the pressure range 33–76 GPa for which there also exists measurements of equation of state and electrical conductivity. The temperatures are found to agree well with available calculations, with small discrepancies between data and theory ascribed to energy absorbing processes such as dissociation and molecular ionization.
Dielectric study of percolation in an oil–continuous microemulsion
Authors: Ponton A. , Bose T. K. , Delbos G.
Published: July 2002

Measurements of the electrical conductivity and the complex permittivity near a percolation threshold in the ternary microemulsion system composed of water, iso-octane, and AOT are reported. It is shown that the electrical conductivity, which implies charge transfer process, is well described by a dynamic percolation model. The frequency dependence of the dielectric constant and the behavior of the relaxation frequency are also found to be in close agreement with the scaling power laws of the dynamic percolation model.
A direct product decomposition approach for symmetry exploitation in many-body methods. I. Energy calculations
Authors: Stanton John F. , Gauss Jürgen , Watts John D. , Bartlett Rodney J.
Published: July 2002

An analysis of the matrix contractions involved in many-body perturbation theory and coupled-cluster calculations leads to a convenient strategy for exploiting point group symmetry, by which the number of floating point operations can be reduced by as much as a factor of h2, where h is the order of the molecular point group. Contrary to a statement in the literature, the significant reduction in computation time realized in coupled-cluster calculations which exploit symmetry is not due to nonlinearities in the equations. Rather, the savings of the fully vectorizable direct product decomposition (DPD) method outlined here is associated with individual (linear) contractions, and is therefore applicable to both linear and nonlinear coupled-cluster models, as well as many body perturbation theory. In addition to the large reduction in floating point operations made possible by exploiting symmetry, core memory requirements are also reduced by a factor of ≊h2. Implementation of the method for both open and closed shells is reported. Computer timings and hardware requirements are given for several representative chemical systems. Finally, the DPD method is applied to the calculation of the equilibrium geometry, totally symmetric harmonic force field and vertical ionization potentials of the cubane molecule at the coupled-cluster singles and doubles (CCSD) level.
Overtone line narrowing and intramolecular vibrational energy redistribution in substituted toluenes
Authors: Sowa Michael G. , Henry Bryan R.
Published: July 2002

The vibrational overtone spectra of some substituted toluenes are presented. The aryl CH stretching overtone regions have a simple local mode interpretation. Distinct aryl CH stretching overtone progressions can be identified for each structurally nonequivalent type of aryl oscillator. The methyl CH stretching overtone regions are more complex. Both conformationally nonequivalent methyl CH bonds and unresolved methyl rotational structure may contribute to the methyl spectral profiles. An additional factor involves a strong alkyl CH stretch-bend Fermi resonance. Two of the molecules studied, 2,3,5,6-tetrafluorotoluene and 2,3,4,5,6-pentafluorotoluene, stand out. The ΔvCH=3 methyl overtone linewidths of these molecules are markedly reduced compared to the other toluenes studied and reveal underlying structure. The narrowing is interpreted as arising from a detuning of the alkyl CH stretch-bend Fermi resonance due to the presence of multiple fluorine substituents.
Concerted hydrogen atom exchange between three HF molecules
Authors: Komornicki Andrew , Dixon David A. , Taylor Peter R.
Published: July 2002

We have investigated the termolecular reaction involving concerted hydrogen exchange between three HF molecules, with particular emphasis on the effects of correlation at the various stationary points along the reaction. Using an extended basis, we have located the geometries of the stable hydrogen-bonded trimer, which is of C3h symmetry, and the transition state for hydrogen exchange, which is of D3h symmetry. The energetics of the exchange reaction were then evaluated at the correlated level, using a large atomic natural orbital basis and correlating all valence electrons. Several correlation treatments were used, namely, configuration interaction with single and double excitations, coupled-pair functional, and coupled-cluster methods. We are thus able to measure the effect of accounting for size-extensivity. Zero-point corrections to the correlated level energetics were determined using analytic second derivative techniques at the self-consistent field (SCF) level. Our best calculations, which include the effects of connected triple excitations in the coupled-cluster procedure, indicate that the trimer is bound by 9±1 kcal/mol relative to three separated monomers, in excellent agreement with previous estimates. The barrier to concerted hydrogen exchange is 15 kcal/mol above the trimer, or only 4.7 kcal/mol above three separated monomers. Thus the barrier to hydrogen exchange between HF molecules via this termolecular process is very low.
Coherent laser control of bound-to-bound transitions of HCl and CO
Authors: Lu Shao-Ping , Park Seung Min , Xie Yongjin , Gordon Robert J.
Published: July 2002

We have used the method of Shapiro, Hepburn, and Brumer to control the ionization rates of HCl and CO. This method is based on the principle of quantum mechanical interference between two competing paths. In this case the two paths are one- and three-photon excitation of an intermediate Rydberg state. A vacuum ultraviolet (VUV) (110–115 nm) laser beam was produced by third-harmonic generation from an UV beam in Kr gas. The relative phases of the two beams were controlled by passing them through a chamber containing either Ar or H2. The laser beams were focused into a molecular beam, and the ionization products were measured in a time-of-flight mass spectrometer. As the pressure of the phase-tuning gas was varied, the ionization signal was found to oscillate with a period that is proportional to the difference of the indices of refraction of the tuning gas at the UV and VUV wavelengths. We have applied this technique to transitions involving different intermediate electronic states [the j 3Σ−(0+), H 1Σ+(0+), and m 3Π(1) states of HCl and the B 1Σ+ state of CO], different rotational lines of the same electronic transition, and different competing processes (ionization vs dissociation) following excitation of the intermediate state.
Fluorescence-detected wave packet interferometry. II. Role of rotations and determination of the susceptibility
Authors: Scherer N. F. , Matro A. , Ziegler L. D. , Du M. , Carlson R. J. , Cina J. A. , Fleming G. R.
Published: July 2002

The recently developed technique of time-resolved spectroscopy with phase-locked optical pulse pairs is further explored with additional experimental data and more detailed comparison to theory. This spectroscopic method is sensitive to the overall phase evolution of an optically prepared nuclear wave packet. The phase locking scheme, demonstrated for the B←X transition of gas phase molecular iodine, is extended through the use of in-quadrature locked pulses and by examination of the dispersed fluorescence signal. The excited state population following the interaction with both pulses is detected as the resultant two-field-dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from rovibrational wave packet dynamics of the molecule on the excited state electronic potential energy curve. Quantum interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two-pulse phase-locked interferograms are interpreted with first order time-dependent perturbation theory. Excellent agreement is found between the experimental interferograms and those calculated from literature values of the parameters governing the electronic, vibrational and rotational structure of I2. A relationship between the phase-locked interferograms and the time-dependent linear susceptibility is obtained. The in-phase and in-quadrature phase-locked interferograms together provide a complete record of the optical free induction decay. Thus by combining the in-phase and in-quadrature data, we obtain the contributions to both the absorptive and dispersive linear susceptibilities arising from transitions within the pulse spectrum.
A modified real particle method for the calculation of the chemical potentials of molecular systems
Authors: Kumar Sanat K.
Published: July 2002

We propose a new technique, which is a combination of the test and real particle methods proposed originally by Widom, for the calculation of the chemical potentials of molecular systems from computer simulation. The technique involves the creation of a hole in the fluid by removing a particle from the simulated system, and subsequently reinserting it as a test bead. The equations for the calculation of chemical potentials are derived, and it is shown that the results obtained by this method for Lennard-Jones fluids are accurate over larger ranges in temperature and density than other traditional methods. In spite of these advantages for simple systems, the proposed technique is expected to be of greatest value when dealing with complex systems containing ionic or polymeric molecules.
The coupled-cluster single, double, triple, and quadruple excitation method
Authors: Kucharski Stanislaw A. , Bartlett Rodney J.
Published: July 2002

A general implementation of the coupled-cluster (CC) single, double, triple, and quadruple excitation (CCSDTQ) method is presented and applied to several molecules, including BH, HF, H2O, and CO with DZP basis sets. Comparisons with full CI show average errors to be 14 μhartree at equilibrium and 26 μhartree at twice Re. CCSDTQ is exact for four electrons and is the first CC method correct through sixth order in perturbation theory.
Monte Carlo simulation of gas diffusion in regular and randomized pore systems
Authors: Burganos Vasilis N.
Published: July 2002

The rate of rarefied gas transport in clusters of hollow spheres and cylinders is determined using Monte Carlo simulation methods. The spheres and the cylinders, representing large pore bodies and narrow pore throats in actual porous materials, are either placed at the nodes and the branches, respectively, of a cubic network or arranged randomly in space allowed to overlap freely. The Knudsen diffusion coefficient is computed by simulating molecular trajectories and calculating statistical averages of the mean-square displacement of a sample of molecules per unit of travel time. The accuracy of approximating the transport resistance of regular structures of spheres and cylinders by the resistance of a commonly used network of sinusoidal conduits is checked in this work for the first time. It is found that networks of sinusoidal conduits approximate fairly well systems of spheres and cylinders in the low density region when Knudsen diffusivity estimates are pursued. However, at moderate and high densities, pore overlap becomes significant and the sinusoidal conduit geometry fails to represent efficiently the actual geometry during gas diffusion owing to the development of considerable end effects and strong interaction between remote overlapping pores. Regular arrays of spheres and cylinders yield diffusivities that compare well with those in structureless systems with randomized pore positions for the same number density of pores. Sphere-to-sphere, sphere-to-cylinder, and combined size correlations are introduced to regular structures using properly devised algorithms. Comparison of correlated-to-uncorrelated systems for continuous and discrete sphere and cylinder radius distributions reveals that sphere-to-sphere size correlation considerably facilitates the gas transport for any value of the pore system density. On the contrary, sphere-to-cylinder size correlation has a relatively small impact on the diffusivity results, which becomes non-negligible in low-density systems only and for discrete size distributions.
On the unimolecular fragmentation of C60+ fullerene ions: The comparison of measured and calculated breakdown patterns
Authors: Foltin M. , Lezius M. , Scheier P. , Märk T. D.
Published: July 2002

The stability of singly charged C60+ fullerene ions, produced by electron impact ionization of C60, has been studied as a function of the electron energy and the time elapsed from ionization in a Nier-type ion source/double-focusing, sector-field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) was observed for the dissociation process C60+→C58++C2. The ionization efficiency curves for C58+, C56+, and C54+ fragment ions have been recorded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time-resolved breakdown graph of the decaying C60+ fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice–Ramsperger–Kassel–Marcus (RRKM) expression, have been used to calculate the decay rates and the breakdown graph of the C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+−C2) of 7.1±0.4 eV. This value is somewhat larger than the dissociation energies predicted earlier from the kinetic energy release distributions, and possible reasons for this difference are discussed in this paper. Using a quasithermochemical cycle the dissociation energy (C58−C2) of the neutral C60 molecule has been estimated to be 7.64±0.4 eV.
Statistical thermodynamic theory of grafted polymeric layers
Authors: Carignano M. A. , Szleifer I.
Published: July 2002

We present a theoretical approach to study the conformational and thermodynamical properties of grafted polymer layers. The theory is based on the generalization of a mean-field approach previously used to treat chain packing in amphiphilic aggregates. The theory can be used for any chain model, for any mixture of grafted chains and for any quality and type of solvent. The theory is based on finding the probability distribution function (pdf) of chain conformations that is shown to depend on the local osmotic pressure. The equations needed to find the osmotic pressures (and thus the pdf) are a set of nonlinear equations with the chain conformations, surface coverage, and surface geometry as input parameters. The theory is applied to off-lattice and lattice chain models. For the former, excellent agreement is found with molecular dynamic simulations. For the latter, we look at the effect of chain branching on the spatial structure and thermodynamic properties of the grafted layer.
Two-dimensional femtosecond vibrational spectroscopy of liquids
Authors: Tanimura Yoshitaka , Mukamel Shaul
Published: July 2002

The nonlinear optical response of liquids subjected to a series of N femtosecond laser pulses is calculated using a multimode harmonic model for nuclear motions, with nonlinear coupling to the radiation field through the coordinate dependence of the electronic polarizability. Using electronically off-resonant optical fields, this multidimensional spectroscopy is shown to provide direct information regarding the homogeneous or the inhomogeneous nature of the spectral density obtained from optical birefringence measurements. Complementary information can be obtained using infrared pulses where the multiple time correlation functions of the nuclear dipole moment (rather than the electronic polarizability) are being probed.
Optimally controlled five-laser infrared multiphoton dissociation of HF
Authors: Kaluža Matjaž , Muckerman James T. , Gross Peter , Rabitz Herschel
Published: July 2002

Simulations of the quantum dynamics of the HF molecule immersed in a field of five overlapping, intense, linearly polarized, infrared laser pulses of subpicosecond duration are performed. The HF molecule, initially in its ground state, is modeled as a rotating oscillator interacting with a classical laser field via electric dipole interaction. Realistic potential and dipole functions are used. Optimal overlaps of the five laser pulses, as well as the optimal carrier frequencies of the laser pulses, are found which maximize the HF dissociation yield. A maximal yield of 45% in a single combined pulse is achieved using the best available potential and dipole moment functions. The optimal infrared multiphoton dissociation pathway for the HF molecule includes a series of the Δv=1 vibrational-rotational transitions followed by a series of Δv≥2 vibrational-rotational transitions. The latter is necessary as a consequence of the vanishing Δv=1 transition moment around v=12. In the Δv=1 regime, both P and R branch transitions are found to be important. The angular distribution of the dissociative flux is computed. Robustness of the results with respect to changes in the interatomic potentials, dipole functions and reduced mass, as well as to changes in laser pulse parameters (carrier frequencies, timings, phases, field amplitudes, and pulse durations) is investigated.
Spiral waves in a surface reaction: Model calculations
Authors: Bär M. , Gottschalk N. , Eiswirth M. , Ertl G.
Published: July 2002

A systematic study of spiral waves in a realistic reaction-diffusion model describing the isothermal CO oxidation on Pt(110) is carried out. Spirals exist under oscillatory, excitable, and bistable (doubly metastable) conditions. In the excitable region, two separate meandering transitions occur, both when the time scales become strongly different and when they become comparable. By the assumption of surface defects of the order of 10 μm, to which the spirals can be pinned, the continuous distribution of wavelengths observed experimentally can be explained. An external periodic perturbation generally causes a meandering motion of a free spiral, while a straight drift results, if the period of the perturbation divided by the rotation period is a natural number.
Molecular potential energy surfaces by interpolation
Authors: Ischtwan Josef , Collins Michael A.
Published: July 2002

A moving interpolation technique which provides an accurate representation of potential energy surfaces for polyatomic molecules is presented. The method uses the ab initio energy, energy gradient, and second derivatives calculated at dynamically important configurations. The interpolant of the energy and its derivatives converges to the exact value with increasing number of data. A procedure is given for finding the optimum configurations at which ab initio calculations are performed. The method is demonstrated by application to the six-dimensional surface of a diatomic plus diatomic reaction.
Control of quantum dynamics by a locally optimized laser field. Application to ring puckering isomerization
Authors: Sugawara M. , Fujimura Y.
Published: July 2002

We presented a theoretical method for controlling quantum dynamics by locally optimized nonstationary laser fields, within the semiclassical theory of the molecule–radiation field interaction. The external laser field is optimized based on the control theory of a linear time-invariant (LTI) system, so that both the summation of the population of the nontarget states and the total energies of the laser fields are minimized. The optimization procedure involves operation of the so-called feedback gain matrix to the time-dependent state vector. This procedure is carried out at every successive short stage, in which the time-dependent Schrödinger equation can be approximated to the equation of motion of the LTI system. As an example, the control theory was applied to laser-induced ring-puckering isomerization, the dynamics of which can be described as the wave packet in the one-dimensional double minimum potential under locally optimized laser fields. The result indicated that nearly 100% of the population can be transferred to the final product state by irradiation of the optimized laser fields. The optimized laser fields were analyzed to obtain information on the carrier frequencies or the frequency modulation by using the fast Fourier transform method. These results were then compared with the result of isomerization induced by nonoptimized laser fields.
Theory of nonadiabatic transition for general two-state curve crossing problems. I. Nonadiabatic tunneling case
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

Based on the achievements for the linear potential model, new accurate and compact formulas are established for general two-state nonadiabatic tunneling type curve crossing problems. These can cover practically the whole range of energy and coupling strength and can be directly applied not only to nonadiabatic tunneling itself, but also to the various problems such as inelastic scattering, elastic scattering with resonance, and perturbed bound state problem. All the basic potential parameters can be estimated directly from the adiabatic potentials and the nonunique diabatization procedure is not required. Complex contour integrals are not necessary to evaluate the nonadiabatic transition probability and thus the whole theory is very convenient for various applications. The previously proposed simple and compact formula, better than the famous Landau–Zener formula, is shown to be applicable also to general curved potentials. The explicit expressions are derived also for the nonadiabatic tunneling (transmission) probability. Now, the present theory can present a complete picture of the two-state curve crossing problems.
A nonequilibrium golden rule formula for electronic state populations in nonadiabatically coupled systems
Authors: Coalson Rob D. , Evans Deborah G. , Nitzan Abraham
Published: July 2002

A formula for computing approximate leakage of population from an initially prepared electronic state with a nonequilibrium nuclear distribution to a second nonadiabatically coupled electronic state is derived and applied. The formula is a nonequilibrium generalization of the familiar golden rule, which applies when the initial nuclear state is a rovibrational eigenstate of the potential energy surface associated with the initially populated electronic state. Here, more general initial nuclear states are considered. The resultant prescription, termed the nonequilibrium golden rule formula, can be evaluated via semiclassical procedures and hence applied to multidimensional, e.g., condensed phase systems. To illustrate its accuracy, application is made to a spin–boson model of ‘‘inner sphere’’ electron transfer. This model, introduced by Garg et al. [J. Chem. Phys. 83, 4491 (1985)] for the nonadiabatic transition out of a thermal distribution of states in the initial (donor) electronic level, is extended to include nonequilibrium, nonstationary initial nuclear states on the donor surface. The predictions of the nonequilibrium golden rule are found to agree well with numerically exact path integral results for a wide range of initial distortions of the initial nuclear wave packet from its equilibrium configuration.
Sign change of hyperpolarizabilities of solvated water
Authors: Mikkelsen Kurt V. , Luo Yi , Ågren Hans , Jo/rgensen Poul
Published: July 2002

The experimental work by Levine and Bethea [J. Chem. Phys. 65, 2429 (1976)] and by Ward and Miller [Phys. Rev. A 19, 826 (1979)] on the hyperpolarizability of solvated water and water in the gas phase, respectively, showed a very substantial effect of the solvent on the measured hyperpolarizability. The sign of the hyperpolarizability of solvated water changed compared to gas phase and the numerical value increased by a factor of 1.5. This article presents a theoretical investigation of the solvent effects of the hyperpolarizabilities and their frequency dispersions for liquid water using the continuum, semicontinuum, and supermolecular models. Calculations involving the semicontinuum and supermolecular models give the sign change of the hyperpolarizabilities, indicating that the hydrogen bonds and the static dipole interaction have substantial impact on the hyperpolarizability of liquid water. Hyperpolarizabilities calculated by the supermolecular approach are about one third of those calculated by the semi-continuum model. The continuum and semicontinuum models involve a recent implementation of a quantum mechanical reaction field response method.
Calculation of intermolecular interaction in aromatic molecular clusters from direction dependent atom-pair potentials
Authors: Rosenblum Gershon , Speiser Shammai
Published: July 2002

An expression for the interaction potential between two anisotropic molecules is derived. This expression is suitable for describing the van der Waals interaction between two chromophores within a bichromophoric molecular cluster. For the anthracene–naphthalene cluster the calculation predicts the existence of two isomers, in agreement with experimental observations. The model is also successfully applied to other clusters yielding better results than those obtained by alternative methods which do not take into account the anisotropy of molecular polarizability.
Vibrational relaxation in disordered 1,4-dihalobenzenes
Authors: Caldarone Barbara , Taiti Claudia , Bini Roberto , Schettino Vincenzo
Published: July 2002

The influence of structural disorder on vibrational relaxation in molecular crystals has been investigated by using high resolution Raman spectroscopy. The role of the disorder has been studied by analyzing the temperature dependence of lattice and internal modes linewidths in two isomorphic benzene derivatives: the 1,4-C6H4BrCl (p-BCB) and the 1,4-C6H4Br2 (p-DBB) crystals. The former shows a disorder arising from a random head-to-tail orientation of the molecular dipoles. It has been found that in p-BCB crystals the additional band broadening, ascribed to head-to-tail disorder, can be assumed to be temperature independent. It is shown that corresponding modes in the two crystals involve similar anharmonic decay processes.
Nanophase coexistence and sieving in binary mixtures confined between corrugated walls
Authors: Curry J. E. , Cushman John H.
Published: July 2002

The grand canonical Monte Carlo method is used to study a binary mixture of Lennard-Jones atoms confined to a corrugated slit micropore which is in thermodynamic equilibrium with its bulk phase counterpart. The micropore has atomically structured walls; one of the which possesses nanoscale structure in the form of rectilinear grooves (corrugation). The grooved surface divides the confined fluid film into two strip shaped regions, that inside and that outside the grooves. Transverse solidlike order in the film gives rise to shear stress. Transverse order coupled with packing restrictions give rise to a difference between the pore and bulk fluid mixture compositions. Solidlike order may appear within the grooves only, outside the grooves only, or in both regions simultaneously. As the relative alignment of the walls is shifted the pore fluid undergoes freeze–thaw cycles in one or both regions with associated changes in the shear stress and pore fluid composition. The degree of transverse order in the film is less than would be expected in a pure Lennard-Jones film and fluid-solid phase transitions are gradual as opposed to sudden as seen in pure Lennard-Jones films. The magnitude of the shear stress is greatest when a fluid–solid phase transition occurs in both regions of the pore.
Multiresonant four-wave mixing in diphenyloctatetraene doped bibenzyl crystals
Authors: Hawi Sharon R. , Wright John C.
Published: July 2002

A diphenyloctatetraene (DPOT) doped bibenzyl crystal is a model system for studying the spectroscopic properties of polyenes because the well-defined local environment of the DPOT eliminates most of the inhomogeneous broadening that obscures many transitions. The spectra of the three DPOT sites in this host are measured with site-selective laser spectroscopy to define the vibrational, electronic, and vibronic states of the S0, S1, and S2 electronic states. Multiply resonant four-wave mixing spectra were then obtained using a coherent anti-Stokes Raman spectroscopy method (CARS) for a series of different laser frequencies in order to define the CARS excitation profile (CEP). The CEP profile of the C=C and C–C stretch vibrations showed strong enhancement of the four-wave mixing from the S2 state but only the C=C stretch mode had observable enhancement from the S1 state. Contrary to previous studies in pentacene, azulene, and perylene, DPOT did not exhibit enhancement of vibronic transitions that would permit mode selective or site-selective nonlinear spectroscopy.
Phonons in graphitic tubules: A tight-binding molecular dynamics study
Authors: Yu Jin , Kalia Rajiv K. , Vashishta Priya
Published: July 2002

Using the tight binding molecular dynamics method (TBMD), the structural and dynamical properties of graphitic tubules are studied. The phonon dispersion and density of states of graphitic tubules with various helicities and diameters are calculated. Phonon modes in tubules are softened by the curvature when compared with graphite. Unique features of the graphitic tubule, with special emphasis on low-frequency modes, are discussed. The symmetry of phonon modes is analyzed, and infrared and Raman active modes are identified. Sound velocities in graphitic tubules are also calculated as functions of tubule helicity and diameter.
Diffusive torsional dynamics of malachite green molecules in solid matrices probed by fluorescence decay
Authors: Abedin Kazi Monowar , Ye Jing Yong , Inouye Hideyuki , Hattori Toshiaki , Sumi Hitoshi , Nakatsuka Hiroki
Published: July 2002

The torsional dynamics of phenyl rings of malachite green molecules in the excited state is studied in polymeric and monomeric glass matrices by measuring the fluorescence decay time as a function of temperature. It is shown that the phenyl rings rotate diffusively in solid polymers (polymethyl methacrylate and polyvinyl alcohol) quite rapidly even at low temperatures. To analyze the experimental results, we used the concept of microviscosity which controls the diffusive rotational motion of phenyl rings of malachite green molecules in solid matrices. By using the reaction-rate theory, we show that a horizontal excited-state potential surface rather than a downhill potential surface for the rotation of phenyl rings can more reasonably explain the rotational motion in polymers. If we assume that the potential is horizontal, the temperature dependence of the microviscosity can be described by Andrade equation with a definite activation energy which is known to be valid for many liquids over a wide range of temperatures. This implies that the microscopic dynamics of small molecular rotations in a solid polymer resembles the behavior in many liquids. By monitoring the fluorescence decay of malachite green molecules doped in ethanol monomeric glass during its phase transition, we show that the effects of phase transition are well represented in the fluorescence decay time. We then propose to use malachite green molecules as sensitive optical microprobes of local dynamics in various solid matrices and their phase transitions, etc.
Interactions between charged spherical macroions
Authors: Stevens Mark J. , Falk Michael L. , Robbins Mark O.
Published: July 2002

Monte Carlo (MC) simulations were used to study the screened interactions between charged spherical macroions surrounded by discrete counterions, and to test previous theories of screening. The simulations were performed in the primitive cell of the bcc lattice, and in the spherical Wigner–Seitz cell that is commonly used in approximate calculations. We found that the Wigner–Seitz approximation is valid even at high volume fractions φ and large macroion charges Z, because the macroion charge becomes strongly screened. Pressures calculated from Poisson–Boltzmann theory and local density functional theory deviate from MC values as φ and Z increase, but continue to provide upper and lower bounds for the MC results. While Debye–Hückel (DH) theory fails badly when the bare charge is used, MC pressures can be fit with an effective DH charge, ZDH, that is nearly independent of volume fraction. As Z diverges, ZDH saturates at zψmaxRm/λ, where z is the counterion charge, Rm is the macroion radius, λ is the Bjerrum length, and ψmax is a constant of order 10.
Fast assembly of the Coulomb matrix: A quantum chemical tree code
Authors: Challacombe Matt , Schwegler Eric , Almlöf Jan
Published: July 2002

Fast methods based on a representation of the electron charge density in a Hermite Gaussian basis are introduced for constructing the Coulomb matrix encountered in Hartree-Fock and density functional theories. Simplifications that arise from working in a Hermite Gaussian basis are discussed, translations of such functions are shown to yield rapidly convergent expansions valid in both the near- and far-field, and the corresponding truncation errors are derived in compact form. The relationship of such translations to hierarchical multipole methods is pointed out and a quantum chemical tree code related to the Barnes-Hut method is developed. Novel methods are introduced for the independent thresholding of ‘‘bra’’ and ‘‘ket’’ distributions as well as for screening out insignificant multipole interactions. Recurrence relations for computing the Cartesian multipole tensor are used to efficiently calculate far-field electrostatic interactions using high-order expansions. Application of the quantum chemical tree code to assembly of the Coulomb matrix for HF/3-21G calculations on sequences of polyglycine α-helices and water clusters demonstrate scalings as favorable as N1.6, where N is the number of basis functions. Comparisons with a commercial electronic structure program indicate that our method is highly competitive. Speed is obtained without sacrificing precision, truncation errors are controlled with a single parameter, and the method performs equally well with a contracted or uncontracted LCAO basis.
Quantum dynamics of ArI2 vibrational predissociation including low total angular momenta: The role of intramolecular vibrational energy redistribution
Authors: Roncero Octavio , Gray Stephen K.
Published: July 2002

Full-dimensional quantum dynamics calculations on ArI2(B,vi) vibrational predissociation with total angular momenta J=0, 1, and 2 are presented. Models based on a few interacting states are shown to reproduce important aspects of the dynamics, confirming that vibrational predissociation is mediated by a few-state intramolecular vibrational energy redistribution effect. As a consequence, vibrational predissociation rate constants exhibit large oscillations with vi, the initial I2 vibrational quantum number in the complex. The qualitative effect persists when alternative choices for the interaction potential parameters are considered. Similarly, despite the importance of Coriolis coupling, the effect persists as J is varied from 0 to 2. We also discuss how the effect may be difficult to observe in typical experiments that involve higher J values.
Sodium chloride vapor at very high temperatures: Linear polymers are important
Authors: Pitzer Kenneth S.
Published: July 2002

At temperatures above 1500 K, the measured vapor pressure of sodium chloride at saturation exceeds that calculated for the sum of the NaCl monomer and the Na2Cl2 ring (rhombic) dimer. The difference is primarily a dimer together with higher polymers of linear or chain structure. This dimer has much higher entropy and enthalpy than the ring dimer. Approximate values are reported for the thermodynamic properties and for structure calculations using a potential function that is based on the spectra of the monomer and was adopted previously for the ring dimer.
Product representation of potential energy surfaces
Authors: Jäckle A. , Meyer H.-D.
Published: July 2002

The recently proposed scheme for representing multidimensional potential energy surfaces as a linear combination of products of one-dimensional functions is extended. The extensions prove to be important if one proceeds to higher dimensions. An iteration procedure is introduced which can further improve the representation. The product representation of potential energy surfaces is especially well suited to be employed within the framework of the multiconfiguration time-dependent Hartree (MCTDH) approximation. The potential representation scheme cannot only be used to represent given analytical potential energy surfaces, but also to interpolate multidimensional surfaces on given, e.g. ab initio, product grid points. The product representation method is applied to the three-dimensional S1 electronic surface of NOCl and to a six-dimensional model Coulomb potential. To check the quality of the NOCl surface representation, the photoabsorption spectrum for an excitation from the S0 to the S1 surface is computed. Weight functions are shown to be easily implemented and, in the case of the NOCl surface, allow a substantial reduction of the number of required expansion coefficients. Exploiting the underlying symmetries of the potential under consideration can further reduce the computational effort, as is shown in the example of the Coulomb potential. Finally, the NOCl S1 potential surface defined on 616 ab initio points is interpolated, as an example for the product interpolation scheme.
Theoretical study of the CH4+F→CH3+FH reaction. I. Ab initio reaction path
Authors: Corchado J. C. , Espinosa-García J.
Published: July 2002

Using ab initio information, the reaction path for the CH4+F→CH3+FH reaction was traced and the coupling between the reaction coordinate and normal modes was analyzed along it. The FH product may be vibrationally excited due to the nonadiabatic flow of energy between the reaction coordinate and this bound mode, manifest in the large peak in the coupling term after the saddle point. It was concluded that the variational effects were due only to entropic effects. The rate constants were calculated for the temperature range 100–500 K using the variational transition state theory with different levels of calculation to calibrate the reaction path. Agreement was found with the experimental values when using the QCI/b3 shifted curve, avoiding the errors associated with the use of the single-point calculation.
Initial stages of multilayer growth and structural phase transitions of physisorbed benzene on Ru(001)
Authors: Jakob P. , Menzel D.
Published: July 2002

The initial stages of the multilayer growth of a model system for molecular solids, namely physisorbed benzene on Ru(001), have been studied in detail by infrared reflection absorption spectroscopy and thermal desorption spectroscopy. A variety of different phases have been discriminated spectroscopically and characterized in situ: the parallel oriented first physisorbed layer which is found to rearrange into a more crowded layer with a high tilt angle at slightly higher coverages; an amorphous layer which grows at low temperatures (T ≤55 K), and a crystalline layer to which the former converts at elevated temperatures. Clear evidence for structural disorder of the uppermost layer of the crystalline phase is found. The amorphous–crystalline phase transformation is irreversible and the required temperatures vary considerably with the layer thickness. This is attributed to two different processes: at high coverages (Θ ≥10 ML) crystallization is possible at low T without mass transport and requires only a reorientation and minor rearrangement of the benzene molecules. Low initial coverages (Θ=2.5–5 ML) require nucleation and diffusion of benzene molecules to form stable 3D crystallites with the former process acting as the kinetically limiting factor. Particular attention has been devoted to the unravelling of the nature of the metastable state observed in thermal desorption spectroscopy and its transformation into the more stable crystalline phase.
Polarized hyper-Rayleigh light scattering measurements of nonlinear optical chromophores
Authors: Kaatz Philip , Shelton David P.
Published: July 2002

Hyper-Rayleigh light scattering measurements at incident wavelengths of 1064 and 1319 nm are reported for several organic nonlinear optical chromophores in solution with approximately 5%–10% uncertainty in the relative first hyperpolarizabilities. The measured chromophores include representatives from C1, Cs, C2V, and D3 molecular point groups and include both neutral and ionic compounds. The measurements were made with 2–5 cm−1 spectral resolution and include polarization analysis of the incident and scattered light. Polarization ratios were measured with 2%–3% uncertainty for each molecule, and relative magnitudes of the hyperpolarizability components were deduced. Two-photon induced fluorescence from several chromophores was observed to overlap with the scattered second harmonic light spectrum. The use of a scanning monochromator, however, generally allows the separation of these two sources of photons. The measured first hyperpolarizabilities are consistent with previous electric field induced second harmonic generation measurements of the same compounds, provided that the standard reference value for the nonlinear susceptibility of quartz is taken to be d11=0.30±0.02 pm/V at 1064 nm.
Infrared laser spectroscopy of jet-cooled carbon clusters: The ν5 band of linear C9
Authors: Van Orden A. , Provencal R. A. , Keutsch F. N. , Saykally R. J.
Published: July 2002

The ν5 antisymmetric stretching vibration of 1Σ+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079–2081 cm−1 were assigned to this band. A combined least squares fit of these transitions with previously reported ν6 transitions yielded the following molecular constants for the ν5 band: ν0=2 079.673 58(17) cm−1, B″=0.014 321 4(10) cm−1, and B′=0.014 288 9(10) cm−1. The IR intensity of the ν5 band relative to ν6 was found to be 0.108±0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD).