bluuebunny/crossref_metadata_2025 huggingface dataset:
Cooperative radiative dynamics in molecular aggregates
Authors: Spano Francis C. , Kuklinski Jan R. , Mukamel Shaul
Published: July 2002

We theoretically investigate the radiative dynamics of molecular aggregates with physical dimensions much smaller than an optical wavelength. The fluorescence decay rate of a one-dimensional aggregate consisting of N electronically coupled two-level molecules interacting with acoustic and optical lattice phonons is calculated. The linear dependence (superradiance) of the radiative decay rate on the aggregate size N is shown to be quenched by exciton–phonon coupling. An increase of aggregate size N eventually leads to a convergent, size independent decay rate, which is N* times faster than the monomer decay rate. The coherence size N* is generally a function of the exciton–phonon coupling strength, the phonon bandwidth, and the aggregate temperature. For low frequency phonons, a scaling law is obtained and an empirical relation for the temperature dependence N*∼T−1/3 is derived.
Theory of solvation-induced reentrant coil–globule transition of an isolated polymer chain
Authors: Matsuyama Akihiko , Tanaka Fumihiko
Published: July 2002

This paper discusses the conformational phase change of an isolated polymer chain which is capable of forming physical bonds with solvent molecules. On the basis of the mean-field theory of Flory type, we derive a formula for the temperature dependence of the expansion factor of the chain. Our theory takes account of the extra mixing entropy change induced by the solvation. We predict that the physical-bond formation between polymer and solvent molecules causes a reentrant conformational change between coiled and globular state when temperature is varied. We also show that the polymer chain exhibits a sharp collapse near the Θ temperature as the temperature is increased to the lower critical solution temperature of the polymer solution. This behavior can be interpreted in terms of the breakup of the polymer–solvents complex. The result is compared with the observed coil–globule transition on poly (N-isopropylacrylalmide) in water.
A complete basis set model chemistry. II. Open-shell systems and the total energies of the first-row atoms
Authors: Petersson G. A. , Al-Laham Mohammad A.
Published: July 2002

The major source of error in most ab initio calculations of molecular energies is the truncation of the one-electron basis set. An open-shell complete basis set (CBS) model chemistry, based on the unrestricted Hartree–Fock (UHF) zero-order wave function, is defined to include corrections for basis set truncation errors. The total correlation energy for the first-row atoms is calculated using the unrestricted Mo/ller–Plesset perturbation theory, the quadratic configuration interaction (QCI) method, and the CBS extrapolation. The correlation energies of the atoms He, Li, Be, B, C, N, O, F, and Ne, calculated using atomic pair natural orbital (APNO) basis sets, vary from 85.1% to 95.5% of the experimental correlation energies. However, extrapolation using the asymptotic convergence of the pair natural orbital expansions retrieves from 99.3% to 100.6% of the experimental correlation energies for these atoms. The total extrapolated energies (ESCF+Ecorrelation) are then in agreement with experiment to within ±0.0012 hartree (root-mean-square deviation) and represent the most accurate total energy calculations yet reported for the first-row atoms.
Application of ion imaging to the atom–molecule exchange reaction: H+HI→H2+I
Authors: Buntine Mark A. , Baldwin David P. , Zare Richard N. , Chandler David W.
Published: July 2002

The bimolecular abstraction reaction H+HI→H2+I has been investigated in a neat molecular beam of HI using ion imaging to detect the H2 (v=1,J=11,13) products. Images obtained determine the laboratory-frame product velocity distribution and show evidence for reaction with fast and slow H atoms arising from the I (2P3/2) and I*(2P1/2) channels in the photolysis of HI, as well as formation of I and I* in the reaction product channel.
Glassy relaxation at surfaces: The correlation length of cooperative motion in the facilitated kinetic Ising model
Authors: Butler Scott , Harrowell Peter
Published: July 2002

A measure of cooperativity in glassy liquids is defined in terms of the kinetic correlation length of relaxation at a surface. This length and its temperature dependence is measured for the facilitated kinetic Ising model with both free and pinned surfaces. A simple power law is found to describe the relationship between the kinetic correlation length and the bulk relaxation time over four decades of data.
Molecular-dynamics computer simulation of an aqueous NaCl solution: Structure
Authors: Zhu Sheng-Bai , Robinson G. Wilse
Published: July 2002

A classical molecular-dynamics simulation of a 1.791 molal aqueous NaCl solution is performed using a flexible/polarizable five-site water model. Through an investigation of the ion-water pair-correlation functions and the relative orientation of the waters of hydration, we are able to study the solvation structure in this moderately concentrated salt solution. Under perturbations from the solvated ions, there appears a considerable reorganization of the water molecules. Some of the original intermolecular hydrogen bonding structure is broken down, as bonding with the neighboring cations and anions takes place. Also considered in this paper are the energetics of hydration, and the effect of ionic perturbations on properties such as the water intramolecular structure and vibrations, and the solution phase dielectric constant. An important conclusion from this work is that, in the NaCl solution studied here, perturbations on the water structure by the ions affect mainly intermolecular orientational properties. Although they may be large, these orientational effects are not sensitively detected by many experimental techniques, such as x-ray diffraction or vibrational spectroscopy.
Pseudospectral method for solving the time-dependent Schrödinger equation in spherical coordinates
Authors: Corey Gregory C. , Lemoine Didier
Published: July 2002

In this paper we describe a numerically efficient pseudospectral method for solving the time-dependent Schrödinger equation in spherical coordinates. In this method the translational kinetic energy operator is evaluated with a Fourier transform. The angular dependence of the wave function is expanded on a two-dimensional grid in coordinate space and the angular part of the Laplacian is evaluated by a Gauss–Legendre–Fourier transform between the coordinate and conjugate angular momentum representations. The potential energy operator is diagonal. Calculations performed for a model system representing H2 scattering from a static corrugated surface yield transition probabilities identical to those obtained with the close coupled wave packet (CCWP) method. The new algorithm will be more efficient than the CCWP method for problems in which a large number of rotational states are coupled.
Lattice Boltzmann computations for reaction-diffusion equations
Authors: Ponce Dawson S. , Chen S. , Doolen G. D.
Published: July 2002

A lattice Boltzmann model for reaction-diffusion systems is developed. The method provides an efficient computational scheme for simulating a variety of problems described by the reaction-diffusion equations. Diffusion phenomena, the decay to a limit cycle, and the formation of Turing patterns are studied. The results of lattice Boltzmann calculations are compared with the lattice gas method and with theoretical predictions, showing quantitative agreement. The model is extended to include velocity convection in chemically reacting fluid flows.
Positional time correlation function for one-dimensional systems with barrier crossing: Memory function corrections to the optimized Rouse–Zimm approximation
Authors: Perico Angelo , Pratolongo Roberto , Freed Karl F. , Pastor Richard W. , Szabo Attila
Published: July 2002

The one-variable Smoluchowski equation is used to study the influence of barrier crossing processes on the positional time correlation function. The memory function of this correlation function is evaluated for a 2–4 potential as a function of the barrier height using the Mori continued fraction expansion and an equivalent but more efficient matrix formulation. Higher orders in the expansions are required to obtain numerical convergence as the barrier height increases. An exact integral solution for the correlation time is derived and is compared with the approximations. A biexponential approximation, which describes the independent motion in a potential well and the transition between wells, is found to be very accurate for high barriers. Numerical simulations provide checks on the approximations to the correlation function for a barrier height of 2 kBT. The possibility of including the influence of more rapid barrier crossing processes into the many variable Smoluchowski description of long time polymer and protein dynamics is discussed.
Fit of the potential energy surface for the reaction Ne+H2+→NeH++H using three different functional forms
Authors: Pendergast Phil , Heck John M. , Hayes Edward F. , Jaquet Ralph
Published: July 2002

Three different functional forms are fit to a calculated coupled electron pair approach potential energy surface for the reaction Ne+H2+→NeH++H. Minimum energy pathways and stationary points of the various fits are discussed.
Inhomogeneous broadening of infrared and Raman spectral bands of amorphous and polycrystalline thin films
Authors: Ovchinnikov Mikhail A. , Wight Charles A.
Published: July 2002

Infrared and Raman spectroscopy are used to examine effects of solid structure on vibrational line shapes in thin solid films of CO2 and N2O prepared by vapor deposition. A new method is described for forming films consisting of weakly-interacting microcrystals by deposition of the solute in an argon matrix followed by complete evaporation of the argon. The infrared absorption bands of these films exhibit inhomogeneous spectral broadening in the range between the transverse and longitudinal frequencies of the bulk crystal. The origin of this broadening is shown to be the dependence of vibrational frequencies on size and shape of the microcrystals due to long-range dipole interactions. In contrast, the spectra of polycrystalline films formed by direct vapor deposition are the same as for single crystals because the long-range dipole interactions effectively couple vibrational motions across the grain boundaries. An analysis shows that inhomogenously broadened spectra are obtained whenever the separation between the individual crystal grains is comparable to or greater than their dimensions.
Near-threshold electron emission from impact of slow van der Waals clusters and fullerene ions on clean gold
Authors: Töglhofer K. , Aumayr F. , Kurz H. , Winter HP. , Scheier P. , Märk T. D.
Published: July 2002

Atomically clean polycrystalline gold has been bombarded by slow cluster ions (N2)+n (n≤120), Ne+n (n≤100), and Cq+60,70 (q=1–4) at kinetic energies from the apparent electron emission thresholds up to 6 kV times the cluster charge (qe). The resulting electron yields have been derived from the corresponding electron emission statistics. Grossly different electron yields were found for different cluster species of about equal masses and velocities, which is attributed to different partitions of the initial cluster kinetic energy among (a) inelastic binary collision processes involving cluster constituents and surface atoms, and (b) intramolecular vibrational excitation in the cluster ions (where possible). We found emission threshold velocities of ∼1×104 m/s, which is a lower velocity than where emission was observed in most other studies previously conducted. For (N2)+n cluster ions, clear evidence for a nonlinear behavior (i.e., increasing electron emission per cluster constituent with increasing cluster size) has been found.
Implications of rotation–inversion–permutation invariance for analytic molecular potential energy surfaces
Authors: Collins Michael A. , Parsons Drew F.
Published: July 2002

A molecular potential energy surface has the symmetry properties of invariance to rotation of the whole molecule, inversion of all atomic coordinates, and permutation of indistinguishable nuclei. While some of this invariance character can be easily incorporated in a local description of the surface, a formal application of these symmetry restrictions is useful in considering the form of the global surface which must account for large amplitude changes of the atomic coordinates. The form of a global molecular potential energy surface as a properly symmetrized analytic function of Cartesian coordinates is derived by extending Molien’s theorem of invariants for finite groups to cover the continuous rotation–inversion group. O(3), and the product of O(3) with the complete nuclear permutation group. The role of so-called redundant internal coordinates in molecular potential energy surfaces is clarified.
Nucleation in dipolar fluids: Stockmayer fluids
Authors: Talanquer V. , Oxtoby D. W.
Published: July 2002

We have used density functional theory to begin the study of nucleation in dipolar fluids. The simplest case of a system composed of Stockmayer molecules has been considered. Under a Weeks–Chandler–Andersen perturbation scheme we obtain the corresponding bulk phase diagram and interfacial properties of planar and spherical liquid–vapor interfaces. Molecules tend to align perpendicular to the interface on the liquid side of small droplets, but the overall effect of this on free energies is negligible. The resulting nonclassical effects on nucleation rates are, hence, close to those found in nonpolar Lennard-Jones fluids.
Free surfaces of polymer blends. II. Effects of molecular weight and applications to asymmetric polymer blends
Authors: Hariharan Arvind , Kumar Sanat K. , Russell Thomas P.
Published: July 2002

The composition of the free surface of a binary polymer blend has been investigated in this work as a function of molecular weights, energetic parameters, and composition. The approach involved the use of a compressible mean-field lattice model that was developed in the spirit of the Scheutjens and Fleer theory of polymer solutions [J. Chem. Phys. 98, 6516 (1993)]. For symmetric polymer blends it was found that the surface segregation was driven by the degree of incompatibility of the blend, with the segregation increasing monotonically with the quantity χ1,2 r. These results are in qualitative agreement with conclusions obtained from an incompressible model for polymer blends near a surface, and suggests that the inclusion of compressibility effects do not change the predicted surface segregation significantly in these cases. In contrast, the behavior of isotopic polymer blends of disparate molecular weights, which could not be reproduced in the case of incompressible models, can be captured by the compressible model [J. Chem. Phys. 98, 4163 (1993)]. This fact therefore stresses the importance of compressibility effects in the context of segregation to free surfaces, especially for blends of disparate molecular weight polymers, and suggests that such models have to be utilized to incorporate all of the physics in these situations.
Simulation of the adsorption of unsymmetric diblock copolymers at the interface between the two monomeric homopolymers
Authors: Wang Yongmei , Li Yong , Mattice Wayne L.
Published: July 2002

An unsymmetric diblock copolymer has been suggested to be more effective than a symmetric diblock copolymer for compatibilization of a blend of two homopolymers. It reduces the formation of the micelle which competes with the preferred adsorption of the diblock at the interface of the two homopolymer fields. The homopolymer fields used in the simulations are monomeric homopolymer fields. The composition of the chains is changed from symmetric to unsymmetric, and the equilibrium adsorption of the chains at the interface between the two monomeric homopolymer fields is studied as a function of the composition of the chains and the interaction energy. The new results obtained with unsymmetric diblock copolymers can be described by the same functional dependence that we have previously obtained [Y. Wang and W. L. Mattice, J. Chem. Phys. 98, 9881 (1993)] for the symmetric diblock copolymer, as long as the proper replacement of variable is made. In addition, the equilibrium of unsymmetric diblock copolymers in the two different monomeric homopolymer fields enables us to test the validity of Flory–Huggins theory of the thermodynamics of polymer mixing with the solvent. No major deviation from the Flory–Huggins theory has been observed.
Differential cross sections for fine structure transitions in O(3P2)+Ar collisions
Authors: Ma Z. , Liu K. , Harding L. B. , Komotos M. , Schatz G. C.
Published: July 2002

We present experimental and theoretical differential cross sections for the fine structure changing process Ar + O(3P2)→Ar + O(3Pjmj) with (j,mj)=(1,1), (1,0), and (0,0). The measured cross sections refer to a collision energy of 3.2 kcal/mol, and were obtained from Doppler line shapes associated with (2+1) resonance-enhanced multiphoton ionization of O(3Pjmj) after scattering in a crossed-beam apparatus. The theoretical results are based on 3Σ− and 3Π potential curves obtained from high quality ab initio calculations, and on quantum coupled-channel calculations. The calculated differential cross sections show strong Stuckelberg oscillations, with similar magnitudes and phases for all three final states. With slight adjustment (0.03 Å) of the hard wall on the 3Σ− curve, the calculated angular distributions match up well with the corresponding experimental results, much better than is found using previously derived empirical potentials. At the same time, the integral total cross sections obtained from the ab initio measurements are in satisfactory agreement with previous measurements.
An implementation of analytic second derivatives of the gradient-corrected density functional energy
Authors: Johnson Benny G. , Fisch Michael J.
Published: July 2002

We report an implementation of analytic second derivatives with respect to nuclear displacement of the Kohn–Sham energy for gradient-corrected functionals. The second derivative equations are given in a form well-suited for computer implementation, and the exchange-correlation contributions are discussed in detail. The algorithms presented have favorable asymptotic exchange-correlation cost scaling requirements relative to other aspects of the calculation. The results obtained show that analytic calculation of Kohn–Sham second derivatives is indeed a viable technique in practice.
Stationary phase surface hopping for nonadiabatic dynamics: Two-state systems
Authors: Webster Frank , Wang E. T. , Rossky P. J. , Friesner R. A.
Published: July 2002

Our technique for mixed quantum-classical simulations has the correct limiting behavior at short and long times. Two-state systems are used to investigate coherence effects and dephasing, and to compare with results from a simpler theory of nonadiabatic dynamics. Compared to fully quantal results, our method is shown to be accurate for a wide range of models over a wide range of energies. The two-state models do not explicitly include the bath dynamics which lead to dephasing, and the models do not include tunneling. These are above barrier scattering exercises for which the dynamics is fully coherent. We apply our theory including an arbitrary dephasing time scale, and quantitatively reproduce the correct results for long dephasing times and qualitatively reproduce the results for short dephasing times. We conclude that our method is applicable to relaxation and activated processes in condensed matter where the dephasing time is unknown. We propose that our method is appropriate for simulations of electronically and vibrationally nonadiabatic processes in solids and liquids.
Rheology of hydrodynamically interacting concentrated hard disks
Authors: Yuan X. F. , Ball R. C.
Published: July 2002

We have developed a novel Lagrangian–Eulerian numerical scheme to model the dynamics of dense hard-disk suspensions with many-body hydrodynamic interactions in an incompressible Newtonian solvent. A simple derivation of analytical solution of lubrication force between disks is given. Reasonable agreement of viscosity calculated by lattice–gas cellular automata and our method is obtained, despite the lack of Brownian forces in our work.
Laser-induced infrared fluorescence of O3 trapped in xenon matrices
Authors: Jasmin D. , Brosset P. , Dahoo R. , Raducu V. , Gauthier-Roy B. , Abouaf-Marguin L.
Published: July 2002

A strong IR fluorescence is observed following the excitation of O3 at the ν1+ν3 mode in xenon matrices. The emission frequency is characteristic of the 2ν3 to ν3 transition. The rise time and the decay time are both faster than a few nanoseconds. The excitation spectra are similar to the line profile of the ν1+ν3 absorption at 5 K. A spectral analysis of the fluorescence shows that no energy transfer occurs during the relaxation between molecules in the two trapping sites. The most perturbed molecules (high frequency component of the absorptions) seem to have a faster relaxation, exhibiting a stronger temperature dependence. The possible intramolecular vibrational cascades are discussed considering the literature data about vibrational relaxation of matrix isolated small polyatomic molecules in their ground electronic state.
The dynamics of one-dimensional excitons in liquids
Authors: van Burgel Mirjam , Wiersma Douwe A. , Duppen Koos
Published: July 2002

The properties of excitons in one-dimensional molecular aggregates, dissolved at room temperature in a liquid, were studied by means of femtosecond nonlinear optical experiments. Both the one-exciton band (i.e., Frenkel-excitons) and multiexciton bands contribute to the observed nonlinear optical response. The rapid motions in the liquid lead to ultrafast perturbations of the molecular energy levels. This localizes the excitons on limited sections of the chains of aggregated molecules. Ultrafast frequency-resolved pump–probe spectroscopy on the lowest two exciton bands was employed to determine the delocalization length of the optical excitations. The kinetics of the exciton populations was measured by ultrafast grating scattering experiments and time-resolved single photon counting. A model is described in which the multiexciton bands act as doorway states in the exciton–exciton annihilation process. These bands thereby determine the population decay of the Frenkel excitons at high excitation densities. Room temperature photon echo experiments show that stochastic perturbations of the exciton transition frequencies occur on two distinct time scales. In particular the slow components of the fluctuations are affected by motional narrowing, associated with the exciton delocalization length. It is therefore argued that the optical dephasing of excitons is directly related to the spatial extent of the excitation on the aggregate chain.
Flat surface study of the Eley–Rideal dynamics of recombinative desorption of hydrogen on a metal surface
Authors: Persson Mats , Jackson Bret
Published: July 2002

The dynamics of a direct reaction between a gas phase H atom and an adsorbed H atom, often referred to as an Eley–Rideal mechanism, is explored using a fully three-dimensional flat surface model for Cu(111). The model is based on a flat-surface approximation for a single electronically adiabatic potential energy surface (PES). This reduces the inherently six-dimensional reactive scattering problem (for a rigid surface) to a three-dimensional problem by introducing three constants of motion. The resulting scattering problem is treated quantum mechanically by solving the time-dependent Schrödinger equation, and also by quasiclassical trajectory calculations. We have considered four different model PESs which are all more or less compatible with available knowledge about the interactions between hydrogen atoms and molecules and Cu(111). We have studied the dependence of the reactive cross section and product translational energy and rovibrational state distributions on the kinetic energy and angle of the incident H atom, and the vibrational state of the adsorbed H atom.
Dissociation and pyrolysis of Si2H6 on Si surfaces: The influence of surface structure and adlayer composition
Authors: Xia L.-Q. , Jones M. E. , Maity N. , Engstrom J. R.
Published: July 2002

The reaction of disilane, Si2H6, with the Si(100) and Si(111) surfaces has been examined with supersonic molecular beam scattering techniques. The emphasis has been on elucidating the reaction mechanism operative under conditions leading to steady-state Si epitaxial growth. Two reaction mechanisms have been identified: (i) complete pyrolysis to form two adsorbed Si atoms and gas phase hydrogen; and (ii) a reaction forming one adsorbed Si atom, gas phase hydrogen, and silane, SiH4, as a gas phase product. The relative predominance of these two channels is sensitive to surface structure, adlayer composition, and incident kinetic energy. In particular, only complete pyrolysis is observed on the clean Si(100)-(2×1) and Si(111)-‘‘(1×1)’’ surfaces. The silane production channel, on the other hand, is observed on the Si(111)-(7×7) surface, and on the Si(100)-(2×1) surface in the presence of a finite coverage of either adsorbed hydrogen or phosphorus atoms. Examination of the reaction dynamics reveals that the probability of complete pyrolysis increases with increasing incident kinetic energy. Angular-resolved measurements of the scattered SiH4(g) product on the Si(111)-(7×7) surface suggest that silane is formed from the reaction of a chemisorbed intermediate. Comparison of the reaction probability of SiH4 and Si2H6 on the Si(111)-(7×7) surface as a function of incident kinetic energy suggests a similar decomposition mechanism for these two molecules, namely, Si–H bond activation. In this scenario, SiH4(g) is formed via unimolecular thermal decomposition of an adsorbed Si2H5(a) species.
Static yield stresses and shear moduli in electrorheological fluids
Authors: Clercx H. J. H. , Bossis G.
Published: July 2002

A previously introduced method to study many-body electrostatic interactions among spherical particles [H. J. H. Clercx and G. Bossis, Phys. Rev. E 48, 2721 (1993)] has been used to study yield stresses and shear moduli for simple cubic (SC), simple hexagonal (SH), and body-centered tetragonal (BCT) structures, with polarizable spheres on the lattice sites, to gain insight in the electrostatic response of these structures to externally applied stresses. The shear modulus G and the static yield stress τs have been calculated for several ratios of particle to fluid dielectric constant. It turned out that interchain interactions are very weak in SC and SH structures which is confirmed by the nearly linear φ dependence of G and τs (even no weak maximum appears in these curves). The results reported for the BCT structure are rather unexpected, because both G and τs suddenly decrease to zero at large volume fractions. We discuss the data for G and τs and compare some of these results with data obtained by employing the dipolar approximation and the Laplacian relaxation technique to calculate the electrostatics. This comparison shows that in general a multipolar approach is indispensable in obtaining correct values for G and τ; especially the dipolar approximation underestimates these quantities. As an example the strength of single vs double chains has been compared for both the dipolar approach and a multipolar calculation. In the dipolar approximation the double chain structure seems strongest. However, multipolar calculations show that the single chain structure is stronger than the double chain structure.
Coherent control of photodissociation in intense laser fields
Authors: Charron Eric , Giusti-Suzor Annick , Meis Frederick H.
Published: July 2002

The fragmentation dynamics of the hydrogen molecular ion H2+ and of its isotopic derivate HD+subjected to an intense pulsed laser radiation are studied using quantum wave packet propagations. It is shown that bichromatic optical excitations are subject to a high degree of control through the variation of the relative phase between the two fields. A phase-locked (ω,2ω) laser pulse is used to induce asymmetry in the angular distribution of the emitted fragments. In addition, an appreciable isotope separation in the fragmentation of HD+ is predicted. The critical role of quantum molecular interferences in such phase-controllable processes is demonstrated.
The structure of phenol(H2O) obtained by microwave spectroscopy
Authors: Gerhards Markus , Schmitt Michael , Kleinermanns Karl , Stahl Wolfgang
Published: July 2002

The microwave spectrum of phenol(H2O) has been recorded using a pulsed molecular beam Fourier transform microwave spectrometer. Twenty a-type transitions have been observed and assigned. All a-type transitions are doublets with splittings varying from 1.13 to 4.01 MHz. These splittings are interpreted to result from the internal rotation of the water molecule. The ground state of this torsional motion is split into two levels with different spin statistical weights. Both torsional levels are treated independently as asymmetric rotor states. The resulting rotational constants are A=4291.486 MHz, B=1092.1484 MHz, and C=873.7263 MHz for the lower torsional level (σ=0) and A=4281.748 MHz, B=1092.329 55 MHz, and C=873.906 81 MHz for the upper level (σ=1). A fit of the molecular structure is performed by optimizing the intermolecular OO distance and two angles, describing the H-bonding arrangement. The resulting trans-linear structure is in reasonable agreement with the ab initio calculations
Analysis of the vibrational, static and dynamic, second hyperpolarizability of five small molecules
Authors: Bishop David M. , Dalskov Erik K.
Published: July 2002

Various approximate methods for the calculation of vibrational second hyperpolarizabilities are assessed on the basis of results for CH4, NH3, H2O, HF, and CO2. Three specific variations of the perturbation-theoretic approach are analyzed. These are defined by the types of terms retained in expansions which involve electrical and mechanical anharmonicities. The lowest-order approximation being the double-harmonic one and the next higher one the relaxation method. For the dynamic hyperpolarizabilities, the replacement of the optical frequencies (ω) by ω→∞ (the infinite-frequency approximation) is also explored. On the evidence at hand it is concluded that the relaxation/infinite-frequency method is a satisfactory approximation for the nonlinear optical vibrational hyperpolarizabilities.
Efficient π electrons delocalization in α,α′-dimethyl end-capped oligothiophenes: A vibrational spectroscopic study
Authors: Hernández V. , Casado J. , Ramírez F. J. , Zotti G. , Hotta Shu , López Navarrete J. T.
Published: July 2002

α,α′-dimethyl substituted oligothiophenes with increasing chain length (from the dimer up to the hexamer) were recently synthesized by chemical methods. In this paper we have investigated the vibrational Fourier transform-IR and Fourier transform-Raman spectra of solid α,α′-dimethyl substituted oligothiophenes in the neutral state. The data are consistent with the existence of a chain-length dependent π electron delocalization: a large frequency dispersion with conjugation length is observed for some Raman and infrared active vibrational modes, particularly at the high-energy side of the aromatic C=C stretching region. Vibrational assignments are proposed for the main vibrational features in the whole spectral range. This vibrational spectroscopic analysis of the solid samples thus becomes a tool for deriving information on the structure of these neutral materials in solution and in the doped state.
Matrix isolation study of the interaction of excited neon atoms with O3: Infrared spectrum of O−3 and evidence for the stabilization of O2⋅⋅⋅O+4
Authors: Lugez Catherine L. , Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a Ne:O3 sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwave discharge, the most prominent infrared absorptions of the resulting solid are contributed by trans- and cyc-O+4 and by O−3. The failure to detect infrared absorptions of O+3 is consistent with the initial formation of that species in one or more dissociative excited states. The ν3 absorption of O−3 appears at 796.3 cm−1, close to its position in earlier argon-matrix experiments in which photoionization of an alkali metal atom provided the electron source and in which diffusion of the atomic cation would result in the stabilization of appreciable M+O−3. The identification of O−3 isolated in solid neon is supported by observations of O−3 generated from isotopically substituted Ne:O2:N2O samples, also codeposited with excited neon atoms. An upper bound of 810 cm−1 is estimated for the gas-phase band center of ν3 of O−3. Infrared absorptions which grow on mild warmup of the sample are tentatively assigned to an O2...O+4 complex.
Vibrational wave functions and energy levels of large anharmonic clusters: A vibrational SCF study of (Ar)13
Authors: Jung Joon O. , Gerber R. Benny
Published: July 2002

The vibrational ground state and the fundamental excited states of (Ar)13 were studied by vibrational self-consistent field (VSCF) calculations. These calculations treat the interaction between different modes through a mean potential approximation, and incorporate anharmonicity in full. The good accuracy of VSCF for such systems was demonstrated by test calculations for (Ar)3 and other clusters. The study of (Ar)13 focused on the properties of the wave functions and the excitation energies, on the role of the coupling between the modes and on the deviation from the harmonic approximation. It was found that SCF excitation energies for the fundamental transitions differ from the harmonic values by about 25% for the softest modes, and by about 10% for the stiffest modes. Coupling between the modes, treated by SCF, was found to be much more important than the intrinsic anharmonicity of the individual modes. For the ground state, the harmonic wave function compares well with VSCF, but for the fundamental excited states appreciable differences were found. The results for a potential field expanded to fourth-order polynomial in the normal mode displacements are found to be valid, almost indentical with those for a more elaborate sixth-order polynomial expansion. The fundamental excitation frequencies computed using the Aziz–Slaman Ar–Ar pair potential are very similar, with some quantitative deviations, to the values obtained with a Lennard-Jones potential. The differences are larger for certain specific modes, and very small for the others. These calculations demonstrate the computational power of VSCF as a tool for quantum-mechanical calculations for large clusters, at the level of specific wave functions.
Gas phase reactions of some positive ions with atomic and molecular hydrogen at 300 K
Authors: Scott Graham B. , Fairley David A. , Freeman Colin G. , McEwan Murray J. , Spanel Patrik , Smith David
Published: July 2002

The reactions of CO+, CO2+, SO2+, NO2+, CS2+, CN+, C2N2+, and C2H3+ with H atoms and H2 molecules have been studied in a selected ion flow tube operated at (300±5) K. The H atom reactions proceed variously by the processes of atom exchange and charge transfer (when allowed), none proceed at the Langevin rate, and the rates of several of them appear to be influenced by the spin states of the product species. Most of the H2 reactions proceed by H atom abstraction and at a large fraction of the Langevin rate.
Volume-phase transition of N-isopropylacrylamide gels induced by hydrostatic pressure
Authors: Kato Eiji
Published: July 2002

Pressure-induced volume phase transitions in neutral and ionized N-isopropylacrylamide-water (NIPA) gels were investigated. When the pressure is increased, the gels which shrink at atmospheric pressure undergo two volume phase transitions. One is the continuous phase transition (for neutral gels) or the discontinuous phase transition (for ionized gels) from the shrunken state to the swollen state at low pressures of several tens MPa. Another is the gradual shrinking transition occurred in the high pressure region above 100 MPa. The former transition shows that the derivative of the pressure with respect to the volume at constant temperature, (∂P/∂V)T, is positive. The results are explained by considering the pressure effects on the hydrophobic interaction. The phase transition pressure increases with increase of the temperature, and the critical pressure and temperature exist. The P–T coexistence curve resembling the evaporation curve of a liquid–gas system is obtained. These pressure-induced phase transitions are qualitatively explained by taking account of the pressure effects on the hydrophobic and hydrophilic interactions. It is assumed that the polymer solvent interaction parameter χ is a function of the pressure as well as the temperature, χ(T,P)=χ0(T)+χ1(T)P+χ2(T)P2. χ1 represents the pressure effects on the interaction between water molecules and the hydrophobic parts of NIPA, and it is responsible for the phase transition at low pressure. χ2 represents the pressure effects on the association between water molecules and the hydrophilic parts of NIPA, and it is responsible for the gradual shrinking transition at high pressure.
Quantum scattering studies of collisional energy transfer from highly excited molecules: Classical/quantum comparisons for collinear He+CS2
Authors: Schatz George C. , Lendvay György
Published: July 2002

We present the results of an accurate quantum scattering study of collisional energy transfer in the collinear He+CS2 system, considering energies up to 75 kcal/mol. These results are generated using a coupled channel calculation, with vibrational eigenfunctions obtained from a discrete variable representation method. Detailed comparisons with the results of classical trajectory calculations are performed so as to assess classical/quantum correspondence for energy transfer moments, and for the energy transfer probability distribution function. We find very good agreement of the energy averaged first moments over a wide range of molecular vibrational energies provided that the translational energy is not too low (translational temperatures significantly below 300 K). The second moments, as well as 〈ΔE〉up and 〈ΔE〉down show less quantitative agreement, especially at low temperatures. The energy transfer distribution functions show considerable mode-specific behavior, but the overall envelope is approximately exponential in |ΔE| except for a spike near |ΔE|=0. Only weak dependence of the energy averaged results on the strength of intramolecular coupling is noted.
Rotational state-resolved sticking coefficients for H2 on Pd(111): Testing dynamical steering in dissociative adsorption
Authors: Gostein Michael , Sitz Greg O.
Published: July 2002

We have measured the sticking coefficient of H2 on Pd(111) as a function of incident translational energy, over the range 31–94 meV, and initial H2 rotational state J, for J=0 through J=5, at a surface temperature of 423 K. These measurements test recent theoretical predictions that the H2/Pd sticking coefficient should decrease with increasing J, for low J, because more quickly rotating molecules are less easily “steered” to dissociation sites by interaction with the surface as they approach the metal [A. Gross, S. Wilke, and M. Scheffler, Phys. Rev. Lett. 75, 2718 (1995)]. We find that the sticking coefficient is sensitive to the rotational quantum number, first decreasing as J is raised from 0 to 3, then increasing again for J=4 and 5. The greatest sensitivity found occurs for an incident translational energy of 73 meV, for which the sticking coefficient decreases from 0.76±0.05 for J=0 to 0.54±0.13 for J=3. In addition, we have found that a fraction of the H2 incident in states J=0 and J=1 is excited to states J=2 and J=3, respectively, upon collision with the surface, with a probability that increases strongly with surface temperature and is roughly independent of incident translational energy. This rotational excitation does not result from dissociation followed by recombinative desorption, but occurs directly upon scattering from the surface. Furthermore, the majority of the excitation energy comes from the surface, and not from the incident translational energy of the molecule. The J-dependent sticking coefficients and the rotational excitation of the incident molecule by the surface are together strong evidence for a coupling between the surface and the rotational degree of freedom of the incident molecule, and support the model of dynamical steering in the adsorption process.
The temperature and pressure dependence of the reaction CH+H2⇔CH3⇔CH2+H
Authors: Fulle D. , Hippler H.
Published: July 2002

Thermal rate constants of the reaction of CH(2Π) with H2 have been measured at pressures between 1 and 160 bar and temperatures between 185 and 800 K. CH radicals have been generated using multiphoton laser flash photolysis of CHBr3 at 248 nm and detected by saturated laser-induced fluorescence near 430 nm. At low pressures the reaction leads to CH2(3B)and H, while at high pressures CH3 radicals are produced. S-shaped transition curves have been constructed to describe the pressure dependence of the rate constant. The high-pressure limiting rate constant for the recombination to CH3 has been evaluated to be k1,∞=2.0×10−10(T/300 K)0,15 cm3molecule−1 s−1. Using experimental low-pressure data from the literature, the rate constant for the second channel could be separated and has been analyzed in terms of SACM theory. A simple kinetic model has been applied to describe the overall rate constant k1 in an extended temperature and pressure range. Related rate constants for the reaction of CH2(3B) with H and the unimolecular two-channel dissociation of CH3 have also been analyzed.
Femtosecond photochemical ring opening of 1,3-cyclohexadiene studied by time-resolved intense-field ionization
Authors: Trushin S. A. , Fuß W. , Schikarski T. , Schmid W. E. , Kompa K. L.
Published: July 2002

We found that hydrogen ion formation due to multielectron dissociative ionization by an intense-laser field is much less efficient with 1,3-cyclohexadiene than with its isomer 1,3Z,5-hexatriene (Z-HT). Moreover by suppressing the ionization barrier an intense-laser field ejects electrons most efficiently from molecular states of low ionization potential. After pumping 1,3-cyclohexadiene at 267 nm to its 1B2 state we probe the system by intense-laser field ionization with delayed 800 nm pulses. Monitoring of the parent ion C6H8+, of the main fragment C6H7+ and of H+ allows us to follow the motion from the 1B2 surface to the dark 2A1 state and from there towards the 2A1/1A1 conical intersection to the ground-state surface of the product. The measured 1B2 and 2A1 lifetimes are 43±3 and 77±7 fs, respectively, and the primary photoproduct cZc-HT is produced within 200 fs.
An accurate potential energy curve for helium based on ab initio calculations
Authors: Janzen A. R. , Aziz R. A.
Published: July 2002

Korona, Williams, Bukowski, Jeziorski, and Szalewicz [J. Chem. Phys. 106, 1 (1997)] constructed a completely ab initio potential for He2 by fitting their calculations using infinite order symmetry adapted perturbation theory at intermediate range, existing Green’s function Monte Carlo calculations at short range and accurate dispersion coefficients at long range to a modified Tang–Toennies potential form. The potential with retardation added to the dipole-dipole dispersion is found to predict accurately a large set of microscopic and macroscopic experimental data. The potential with a significantly larger well depth than other recent potentials is judged to be the most accurate characterization of the helium interaction yet proposed.
Quantum corrections to the classical path equations: Multitrajectory and Hermite corrections
Authors: Billing Gert D.
Published: July 2002

The classical path theory can be derived from first principles by assuming a product type wave function and a Gaussian wave packet in those degrees of freedom for which the classical equations of motion is introduced. Here we give ways of correcting the theory so as to obtain the exact solution with enough correction terms.
Detection of cyclic carbon clusters. I. Isotopic study of the ν4(e′) mode of cyclic C6 in solid Ar
Authors: Wang S. L. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

The cyclic C6 cluster has been identified for the first time in Fourier transform infrared spectra of the products from the laser evaporation of graphite rods trapped in Ar at ∼10 K. Measurements on spectra produced using both 12C- and 13C-enriched rods combined with the results of new density functional theory calculations performed in the present work as well as previous calculations by Martin and Taylor, have resulted in the assignment of the most intense infrared active mode, ν4(e′)=1694.9 cm−1 of the cyclic C6 isomer with D3h symmetry. This assignment is based on excellent agreement of the frequency, 13C isotopic shifts, and relative intensities with the theoretical predictions.
Transfer of chloroform across the water–carbon tetrachloride liquid–liquid interface
Authors: Chang Tsun-Mei , Dang Liem X.
Published: July 2002

Using the constrained molecular dynamics technique, the mass transport of a chloroform molecule across the CC14/H2O interface is investigated. As expected, the transfer free energy is found to decrease monotonically from the aqueous phase into the nonpolar carbon tetrachloride liquid. The presence of the solute exerts essentially no perturbation to the interface, and to the peak positions of the solute–solvent atomic radial distribution functions. These observations suggest that the transport of the chloroform molecule involves a smooth change in the solvent composition of the solvation shells around the solute molecule.
Crossed beams study of C-H bond activation: Mo(5S2)+CH4→MoCH2+H2
Authors: Willis Peter A. , Stauffer Hans U. , Hinrichs Ryan Z. , Davis H. Floyd
Published: July 2002

The reaction of metastable Mo(5S2s1d5) with methane was studied using the method of crossed molecular beams. Angular and velocity distributions of the MoCH2 products were obtained at relative collision energies of 7.2 and 13.0 kcal/mol. The center-of-mass MoCH2 angular distributions exhibited forward–backward symmetry, consistent with a mechanism involving C–H bond insertion forming HMoCH3 followed by H2 elimination. The potential energy barrier for insertion of Mo(5S2s1d5) into CH4 is less than or equal to 7.2 kcal/mol.
Simulation of the athermal coarsening of composites structured by a uniaxial field
Authors: Martin James E. , Anderson Robert A. , Tigges Chris P.
Published: July 2002

We report the results of a computer simulation of the evolution of structure in a two component fluid consisting of a liquid phase and a dispersed colloidal phase subjected to a uniaxial field. Our primary objective is to understand the mechanism and kinetics of coarsening and the emergence of crystallinity. Using an efficient, linear-N simulation method we report studies of systems of N=10 000 particles over the concentration range of 10–50 vol %. We present a variety of methods of characterizing the structures that emerge, including the anisotropy of the conductivity, capacitance and dipolar interaction energy, the two-dimensional pair correlation function, principal moments of the gyration tensor, velocity correlation functions, microcrystallinity and coordination number, and the optical attenuation length. We conclude that athermal coarsening is effectively driven by the presence of defect structures and that as the concentration increases, the structures progressively lose the well-known “chain” anisotropy evinced at low concentration.