bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral quantization of high energy transition state resonances in the H+H2 reaction
Authors: Sadeghi R. , Skodje Rex T.
Published: July 2002

We have discovered 13 transition state resonances for the collinear H+H2 chemical reaction on the DMBE potential surface. The resonances were identified through a hypothetical transition state spectrum, I(ω), generated using a time-dependent wave packet method. The transition state resonances are manifested as strong peaks in spectrum. The peak positions and widths give the resonance energies and widths, respectively. Since the initial wave packet used to generate the spectrum can be chosen to maximize the overlap with the resonance states, the interference of the resonance peaks with the background continuum can be minimized. The resonance energies, lifetimes, and wave functions have been extracted for all 13 resonances. Unexpectedly, the lifetimes grow significantly longer at higher energy. The resonance wave functions form a single progression built up along the asymmetric stretch coordinate. The resonances appear to be in close correspondence with resonant periodic orbits trapped in the transition state region.
Photodissociation of HBr/LiF(001): A quantum mechanical model
Authors: Seideman Tamar
Published: July 2002

The photodissociation dynamics of HBr adsorbed on a LiF(001) surface is studied using time-independent quantum mechanics. The photodissociation line shape and the Br(2P1/2)/Br(2P3/2) yield ratio are computed and compared with the corresponding quantities for gas phase photodissociation. The angular distribution of the hydrogen photofragments following excitation of adsorbed HBr is computed and found to agree qualitatively with experimental data [Bourdon and co-workers, J. Chem. Phys. 95, 1361 (1991)]. The effect of polarization of the photon is illustrated and discussed. We find the field polarization to affect significantly the magnitude of the photodissociation signal but not the angular dependence of the photofragment distribution, in agreement with experiment and in accord with expectations for a strongly aligned adsorbed phase.
Silyl group internal rotation in S1 phenylsilane and phenylsilane cation: Experiments and ab initio calculations
Authors: Lu Kueih-Tzu , Weisshaar James C.
Published: July 2002

Resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure S1–S0 and cation–S1 spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP(phenylsilane)=73 680±5 cm−1, IP(phenylsilane ⋅Ar)=73 517±5 cm−1 and IP(phenylsilane ⋅Ar2)=73 359±5 cm−1. We assign many low lying torsion–vibration levels of the S1 (Ã 1A1) state and of X̃ 2B1 of phenylsilane+. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V6 are, in S1, B=2.7±0.2 cm−1 and V6=−44±4 cm−1; in the cation, B=2.7±0.2 cm−1 and V6=+19±3 cm−1. The sign of V6 and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a″1 and 3a″2 torsional levels. In S1 the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose absolute phase is known experimentally, the most stable conformer is staggered in the neutral states (S0 and S1 p-fluorotoluene, S1 toluene, S1 p-fluorotoluene) and eclipsed in the cationic states (ground state toluene+ and phenylsilane+). In phenylsilane+ we estimate several potential energy coupling matrix elements between torsional and vibrational states.

For small V6, the term PαPa in the rigid-frame model Hamiltonian strongly mixes the 6a′1 and 6a′2 torsional states, which mediates further torsion–vibrational coupling. In addition, the cation X̃ 2B1 vibrational structure is badly perturbed, apparently by strong vibronic coupling with the low-lying Ã 2A2 state. Accordingly, ab initio calculations find a substantial in-plane distortion of the equilibrium geometry of the X̃ 2B1 state, while the Ã 2A2 state is planar and symmetric. The calculations also correctly predict the lowest energy conformer for S0 states and for cation ground states. Finally, we adapt the natural resonance theory (NRT) of Glendening and Weinhold to suggest why sixfold barriers for methyl and silyl rotors are uniformly small, while some threefold barriers are quite large. The phase of the sixfold potential is apparently determined by a subtle competition between two types of rotor-ring potential terms: attractive donor–acceptor interactions and repulsive van der Waals interactions (steric effects).
Dynamics of helical wormlike chains. XII. Dynamic depolarized light scattering
Authors: Yoshizaki Takenao , Yamakawa Hiromi
Published: July 2002

Dynamic depolarized light scattering from dilute solutions of flexible chain polymers is studied on the basis of the discrete helical wormlike (HW) chain. A preliminary consideration leads to the conclusion that the spring-bead model with the Kuhn–Grün expression for the spring polarizability tensor is, in principle, inappropriate for the description of the depolarized scattering, so that the Ono–Okano theory is invalid for real chains. In the approximations that the orientational fluctuations of the subbodies composing the HW chain may be decoupled from their density fluctuation at very small magnitude of the scattering vector and that the former relax much faster than the latter, which are good enough under the condition of usual measurements, it is shown that the spectrum JΓ of the excess depolarized component of the scattered light may be written in terms of the same class of basic time-correlation functions with the ‘‘total angular momentum quantum number’’ L=2 and the number of ‘‘excited’’ subbodies n=1, as in the case of nuclear magnetic relaxation and fluorescence depolarization. Numerical evaluation of JΓ is carried out, taking atactic polystyrene (a-PS) and atactic poly(methyl methacrylate) as typical examples of flexible polymers. It is then found that for both polymers JΓ may be represented in terms of a small number of those eigenvalues λj2,k of the diffusion operator for the HW model with very small wave numbers k which belong to two branches specified by the superscript j, one corresponding to the low frequency modes and the other to the high frequency modes. For a-PS, this result is consistent with the experimental result obtained by Bauer, Brauman, and Pecora [Macromolecules 8, 443 (1975)] except that the present low frequency modes do not correspond to the Rouse–Zimm modes of the spring-bead model.
Selective ion solvation in mixed solvents: Vibrational spectroscopy of Cs+[(CH3)2CO]N(CH3OH)M cluster ions
Authors: Selegue Thomas J. , Cabarcos Orlando M. , Lisy James M.
Published: July 2002

Solvation of the Cs+ ion in a binary mixture of acetone and methanol is studied in mixed cluster ions of the form Cs+[(CH3)2CO]N(CH3OH)M. Vibrational predissociation spectra are recorded for mass-selected clusters with well-known solvent composition. The pure acetone cluster ions Cs+[(CH3)2CO]N do not absorb in the infrared within the range of our CO2 laser, leaving the CO stretch of methanol as the sole chromophore for excitation. Monte Carlo simulations of the heteroclusters are performed using pairwise additive intermolecular potentials. The infrared spectra indicate that a distinct change in cluster structure takes place in Cs+[(CH3)2CO]N(CH3OH)M cluster ions between a size of N=3, M=1 and N=4, M=1, as evidenced by a large shift in absorption frequency. No other significant change in cluster structure is noted through a cluster size of N=12, M=1. Spectra of clusters with M≳1 indicate that the methanol molecules occupy spectroscopically distinct sites. The simulations suggest that the distinct change in cluster structure is due to the formation of a hydrogen bond between an acetone and a methanol molecule before the first solvent shell is completely filled. A comparison of the spectra of the heteroclusters and those of Cs+(CH3OH)N and the (CH3)2CO–CH3OH dimer support this assertion. The sensitivity of Monte Carlo simulations to the choice of intermolecular potentials is discussed.
A new approach to the efficient basis set for accurate molecular calculations: Applications to diatomic molecules
Authors: Tao Fu-Ming
Published: July 2002

The method of the bond function basis set combined with the counterpoise procedure is studied in detail by the complete fourth-order Mo/ller–Plesset perturbation (MP4) theory, following from a recent communication report [J. Chem. Phys. 98, 2481 (1993)]. This method is applied to calculate molecular dissociation energies De as well as equilibrium bond distances re and harmonic frequencies ωe of a number of diatomic molecules (N2, O2, F2, Cl2, HF, HCl, and CO) and the results are compared with those from other methods, without either counterpoise procedure or bond functions or both. The usefulness of the method is shown by the results for all the molecules using a moderately polarized basis set (2p1d for H atom and 2d1f for heavy atoms) augmented with the universal bond functions 3s3p2d. The method has consistently recovered 98%–99% of the experimental values for De, compared to as low as 90% without bond functions. The effect of bond functions is less significant on the predictions of re and ωe, due primarily to the inadequacy of the MP4 theory, but our method is still shown to be favored over the other methods. The electric dipole moments of the polar molecules (HF, HCl, and CO) are also examined and it is found that the use of bond functions results in a significant improvement of the dipole values. Detailed discussions are given to explain the need for bond functions and the counterpoise procedure. The high linear independence with nucleus-centered basis functions is explained to be responsible for the efficiency of bond functions. The counterpoise procedure is logically justified from the conventional noncounterpoise procedure. Potential problems and limitations associated with the proposed method are also discussed.
Vector correlations in the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P)
Authors: Costen M. L. , Hancock G. , Orr-Ewing A. J. , Summerfield D.
Published: July 2002

The reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P) has been studied using translationally aligned oxygen atoms formed from the 355 nm polarized photodissociation of NO2. The nascent CO product was detected by laser-induced fluorescence (LIF) with sub-Doppler resolution in order to extract the pair correlations between the reagent and product relative velocities k and k′ and the product rotational angular momentum J′. Previous theories interpreting the Doppler profiles of photodissociation products in terms of vector correlations have been extended to the case of bimolecular reactions. The system studied was seen to yield a close to isotropic distribution of product velocities k′ about the k direction, and a rotational alignment of J′ with k close to zero. The CO molecule departs with its rotational angular momentum vector J′ aligned preferentially perpendicular to the product relative velocity k′, hence exhibiting a negative k′, J′ correlation. Further insight has been gained on these results by quasiclassical trajectory (QCT) calculations on a London–Eyring–Polanyi–Sato (LEPS) potential energy surface (PES).
Picosecond time-resolved multiplex coherent anti-Stokes Raman scattering spectroscopy by using a streak camera: Isomerization dynamics of all-trans and 9-cis retinal in the lowest excited triplet state
Authors: Tahara Tahei , Toleutaev Bulat N. , Hamaguchi Hiro-o
Published: July 2002

A picosecond time-resolved multiplex coherent anti-Stokes Raman scattering (CARS) measuring system using a streak camera was constructed. Picosecond uv laser pulses (50 ps) were used for the photoexcitation and nanosecond laser pulses (7 ns) were used as ω1 and ω2 for CARS probing. The multiplex CARS signals were spectrally analyzed by a spectrograph, and time-resolved and detected by a streak camera. This system enabled us two-dimensional (the time and frequency domain) detection of CARS signals with picosecond time resolution. The cis–trans photoisomerization dynamics of all-trans and 9-cis retinal was investigated. In the case of all-trans retinal, the CARS signals due to the ‘‘all-trans-like’’ T1 state appeared in accordance with the decay of the S1 fluorescence. In contrast, the appearance of the all-trans-like T1 signals was markedly delayed in the case of the 9-cis isomer. This indicates that there exists a T1 state having ‘‘9-cis-like’’ conformation in the picosecond time region. The lifetime of this 9-cis-like T1 state was obtained as 880±150 ps from the analysis of the rise of the all-trans-like T1 state. The mechanism of retinal photoisomerization is discussed on the basis of the obtained picosecond time-resolved data. It is concluded that the conformational change from the 9-cis to the all-trans form in the triplet manifold proceeds from the thermalized (not vibrationally excited) 9-cis-like T1 state.
Energetics of the sticking of Cl2 onto Ga-rich GaAs(100) c(8×2), As-rich GaAs(100) c(2×8), and stoichiometric GaAs(110)(1×1) surfaces
Authors: Flaum Harris C. , Sullivan Daniel J. D. , Kummel Andrew C.
Published: July 2002

Initial sticking probabilities are measured for monoenergetic molecular chlorine upon the Ga-rich GaAs(100) c(8×2), As-rich GaAs(100) c(2×8), and stoichiometric GaAs(110)(1×1) surfaces. The sticking probabilities are measured as a function of incident translational energy (0.038–0.66 eV), surface temperature (256–807 K), and angle of incidence (0°–37°). Our data indicate the presence of both precursor and direct activated chemisorption mechanisms on all three surfaces. The average barriers to direct chemisorption are quite similar on these three surfaces, suggesting both structural and elemental insensitivity for the direct component to chemisorption. In addition, there is evidence of direct barrierless chemisorption on the Ga-rich GaAs(100) c(8×2) and stoichiometric GaAs(110)(1×1) surfaces. In contrast, precursor mediated chemisorption is seen to differ for the Ga-rich GaAs(100) c(8×2) and stoichiometric GaAs(110)(1×1) surfaces with the precursor persisting to higher incident energies on the stoichiometric GaAs(110)(1×1) surface (∼0.5 eV) than on the Ga-rich GaAs(100) c(8×2) surface (∼0.35 eV). Total energy scaling is observed on both the Ga-rich GaAs(100) c(8×2) surface and the stoichiometric GaAs(110)(1×1) surface for both precursor mediated and direct activated chemisorption, indicating highly corrugated gas-surface interaction potentials.
L
2 calculations of resonances and final rotational distributions for HCO→H+CO
Authors: Wang Desheng , Bowman Joel M.
Published: July 2002

We present calculations for 40 resonances for nonrotating HCO, using an earlier fit to ab initio calculations of the HCO potential. The method used is a completely L2 approach, augmented by a localized negative imaginary potential which serves as an absorbing boundary in the asymptotic region. The real Hamiltonian is diagonalized in a large basis, using a truncation/recoupling approach, and a subset of the eigenvectors are then used to represent the complex Hamiltonian. This Hamiltonian is diagonalized yielding complex eigenvalues, some of which correspond to resonances. Final rotational distributions of the CO fragment are also calculated for two resonances by a straightforward scattering analysis of the complex L2 wave functions in the near asymptotic region. Comparisons of the new results are made with previous time-independent and time-dependent calculations, and with experiment, including two sets of very recent, preliminary experiments that have determined resonance widths.
Photodissociation of ketene: Vibrationally excited CH2(ã 1A1)
Authors: Garcia-Moreno I. , Lovejoy Edward R. , Moore C. Bradley
Published: July 2002

Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from 1460 to 2900 cm−1 above the threshold for singlet methylene CH2(ã 1A1) production. By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence (LIF) excitation spectrum is recorded and the 1CH2(0,1,0), (0,2,0), and (1,0,0) product state rotational distributions determined. As observed previously for the (0,0,0) state, the rotational state distributions of vibrationally excited methylene are in good agreement with phase space theory (PST) for excess energies less than 200 cm−1 above the appearance threshold of each vibrational state probed. For higher excess energies, 1CH2(0,1,0) and (0,2,0) rotational distributions like those for (0,0,0) are substantially colder than the statistical distribution given by PST. The quantum yields of vibrationally excited states are determined at several excess energies above the singlet threshold. These values are larger than predicted by PST and match values predicted by variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method.
Theory of natural circular dichroism in molecules oriented by photoselection
Authors: Che Diping , Goldbeck Robert A. , Kliger David S.
Published: July 2002

The polarization properties of light absorption by molecular chromophores partially aligned by photoselection are presented in complete form. Circular dichroism (CD) in a photoselectively oriented ensemble of absorbers arises from two types of magnetic dipole terms and, in systems of low symmetry, from electric quadrupole terms, the relative contributions of these depending on the polarization properties of the probe and actinic light beams. The anisotropic circular dichroism measured in such experiments may contain information about molecular moments that is not directly available from other spectroscopic techniques, although separation of the magnetic dipole and electric quadrupole contributions will be generally possible only in systems of C2 or D2 symmetry. Theoretical descriptions are presented of possible experimental configurations for measuring oriented CD by modulating circular polarization in the probe beam, exciting beam, or in dual beams. The possible effect of photoselection-orientation on the time evolution of circular dichroism signals measured in laser flash-photolysis experiments is discussed.
Photoinduced absorption of conjugated polymer/C60 solutions: Evidence of triplet-state photoexcitations and triplet-energy transfer in poly(3-alkylthiophene)
Authors: Janssen R. A. J. , Sariciftci N. S. , Heeger A. J.
Published: July 2002

We present spectral evidence of the efficient photogeneration of triplet-state excitations in poly(3-alkylthiophene), P3AT, solutions. The steady-state photoinduced absorption spectra display a PIA band centered at 1.50 eV which is attributed to a dipole-allowed triplet–triplet transition. Photoexcitation of P3AT solutions containing C60, results in an efficient energy transfer reaction and provides an estimate for the P3AT triplet-state energy of 1.57–1.72 eV.
Quantum simulation studies of metal–ammonia solutions
Authors: Deng Zhihong , Martyna Glenn J. , Klein Michael L.
Published: July 2002

Metal–ammonia solutions are examined from the insulating to the conducting regime using three different microscopic models. In model I, the ammonia molecules are treated via a classical point charge model and the cations as a positive neutralizing background. In model II, the ammonia solvent is made fully polarizable and the cations are again, treated as a positive background. Finally, in model III, the solvent is taken to be fully polarizable and the ions, here lithium, are explicitly included. At T=260 K, the following picture emerges of the electronic states as a function of metal/electron concentration: The dilute solution behaves like an electrolyte in which the electrons exist as polarons, on average spherical states localized in solvent cavities, far from the counterions. At 1 mole percent metal (MPM), the electrons spin pair, forming peanut-shaped species called bipolarons. At slightly higher concentration, 2 MPM, the electronic states oscillate (moderated by solvent fluctuations) between dimers of bipolarons and connected tubular states. At 9 MPM, a good liquid metal is formed in which the electron density forms tubular extended states. These observations are in good agreement with experiment. The quantitative differences between the three models are discussed. In general, the effects of solvent polarizability are found to be rather small. However, the explicit inclusion of the cations is found to slightly increase the metallic character of the solution at 9 MPM.
Vibrational and rotational dependence of NO B 2Π state quenching
Authors: Luque Jorge , Crosley David R.
Published: July 2002

Quenching studies of the NO B 2Π state with NO X 2Π and O2 X 3Σ−g colliders have been carried out for the vibrational levels v′=0–3 and 7. NO B 2Π was generated by laser excitation, and the temporal fluorescence decays were found to be single exponentials. The decays are predominantly due to quenching to dark states, and show little vibrational transfer or transfer into A 2Σ+. The quenching rates for NO do not show a change with v, and have an average cross section of 40 Å2. O2 is different, having a cross section increasing with v. For the vibrational levels v=0–3, it was possible to study the rotational dependence between J′=1.5 and 20.5 for NO as the collider. Over this range, a smooth decrease of about 10% in the quenching rate is observed.
Fine structure branching ratios and translational energies of O(3P
j) atoms produced from collision induced intersystem crossing of O(1D) atoms
Authors: Matsumi Yutaka , Inagaki Yousuke , Morley Gregory P. , Kawasaki Masahiro
Published: July 2002

Collision induced intersystem crossing of O(2p 1D) to O(2p 3Pj) j=0,1,2 was studied by vacuum-ultraviolet laser induced fluorescence. The nascent j-branching ratios of O(3Pj) produced from collision of O(1D) with Xe, Kr, N2, CO2, and CF3H at the collision energy of 15.6, 14.7, 11.4, 13.0, and 14.4 kcal/mol are [O(3P0)]/[O(3P1)]/[O(3P2)]=(0.28±0.04)/(0.58±0.05)/1, (0.33±0.05)/(0.45±0.07)/1, (0.23±0.05)/(0.35±0.05)/1, (0.14±0.05)/(0.41±0.05)/1, and (0.20±0.05)/(0.45±0.05)/1, respectively. The j-branching ratios are discussed in terms of potential curve crossing between singlet and triplet surfaces and nonadiabatic nature of the potential surfaces. Doppler profiles of the product O(3Pj) atoms were measured as a function of time delay between pump and probe laser pulses. By calculating the kinetic energy from the Doppler profiles, electronic energy transfer efficiencies to the internal energy of N2, CO, CO2, and CF3H are obtained to be (30±7), (31±7), (49±3), and (52±5)%, respectively.
Statistical mechanics of polymers with distance constraints
Authors: Gutin A. M. , Shakhnovich E. I.
Published: July 2002

The ensemble of polymer conformations satisfying some distance constraints is studied using the replica approach. Dependence of the number of such conformations on the number of constraints is obtained. Possible implications of these results for the physics of polymer gels and the problem of protein structure reconstruction from NMR are discussed.
Variable-angle-spinning nuclear magnetic resonance spectra of homonuclear spin pairs consisting of two crystallographically equivalent spins in the solid state
Authors: Wu Gang , Wasylishen Roderick E.
Published: July 2002

Variable-angle-spinning (VAS) nuclear magnetic resonance (NMR) spectra arising from a pair of crystallographically equivalent but magnetically nonequivalent spins have been investigated under conditions of rapid sample spinning. It is demonstrated that although the indirect spin–spin coupling, J, between two crystallographically equivalent spins is not observable in rapid magic-angle-spinning (MAS) NMR studies, it can be reintroduced by spinning the sample off the magic angle (54.74°). This J-recoupling phenomenon is more general than the previously reported J recoupling via homonuclear dipole–dipole interactions and can be observed for any J-coupled spin pair consisting of two crystallographically equivalent but magnetically nonequivalent nuclei. In addition, we demonstrate that VAS NMR spectra contain information concerning the relative orientation of two chemical shielding tensors which involve a pair of J-coupled nuclei. In contrast to dipolar-chemical shift NMR techniques which depend on the magnitude and orientation of the homonuclear dipolar interaction, the VAS method does not depend on the dipolar interaction; instead, the method makes use of second-order (AB) features present in high-resolution NMR spectra acquired by rapid sample spinning about an axis tilted slightly off the magic angle.
Collision-induced transitions from N2(A′ 5Σ+g) to N2(B 3Πg) via the gateway mechanism
Authors: Ottinger Ch. , Smirnova L. G. , Vilesov A. F.
Published: July 2002

A beam containing N2 in long-lived states was allowed to interact with target particles in a collision cell. Intramolecular transitions A→B and W→B are induced, such as were studied earlier by us by means of the subsequent B→A emission [R. Bachmann, X. Li, Ch. Ottinger, and A. F. Vilesov, J. Chem. Phys. 96, 5151 (1992)]. In the present work the product emission was observed under high resolution (1 Å FWHM). Most of the B→A bands show the typical quasithermal rotational contours. However, in the emissions from the B state vibrational level v=10 sharp superimposed features were observed. They were assigned to transitions from the rotation/fine structure/Λ sublevel 3Πe2(12). This particular level is perturbed by the A′ 5Σ+g state, serving as a so-called gateway to allow the otherwise spin-forbidden 5Σ+g→3Πg collision-induced transition. According to this mechanism, the collisions scramble only the levels within the A′ state, while the A′→B transition occurs spontaneously through S/O coupling. A similar, less well characterized perturbation was also found for the B,v=12, 3Πe0(16) level. Apart from the gateway mechanism, any direct collisional coupling between the A′ and B states appears to be insignificant. This result is contrary to one of the long-standing assumptions on the origin of the Lewis–Rayleigh nitrogen afterglow.
Flux redistribution during the photodissociation of ClNO in the T1 state
Authors: Vegiri Angeliki , Alexander Millard H.
Published: July 2002

The mechanism of the photofragmentation of ClNO in the first triplet state (T1) is investigated using our quantum flux method based on time-independent calculations using a new ab initio potential. Particular attention is focused on the development of the NO rotational distributions, as a function of the Cl–NO separation and as a function of excitation wavelength. The nodal structure of the ClNO bending wavefunction in the Franck–Condon region leaves indelible traces on the evolving photofragment flux, examined in coordinate space. The structure of the flux redistribution in terms of photofragment product states is less readily interpreted. Although the final product distributions are virtually adiabatic in the NO vibrational motion, considerable excitation of NO vibrational motion does occur during the photodissociation, which later disappears into rotation as the fragments separate. This internal energy flow can be seen clearly in an analysis of the flux redistribution among the adiabatic states of the internal motion, and is a consequence of strong vibration-rotation coupling in the Franck–Condon region. The picture obtained here of the mechanism of this complex process is complementary to that offered by earlier dynamical studies.
Impact of global interaction and symmetry on pattern selection and bifurcation
Authors: Middya Usuf , Luss Dan , Sheintuch Moshe
Published: July 2002

Global interaction refers to a nonlocal mode of information exchange (coupling) between the local states on a surface. Global interaction may produce a very rich class of spatiotemporal patterns. A system has an inversion symmetry if both φ(x,y,λ) and φ(−x,−y,−λ) are solutions. Here x and y are the two dynamic variables of the system and λ is a global control variable. The presence of inversion symmetry sharpens the distinction among the various motions and leads to bifurcation scenarios which have not been found in its absence. A heteroclinic connection between two inversion symmetric saddle foci leads to many shifts between back-and-forth and unidirectional pulse branches of solutions. The scenario by which the periodic orbits gain and lose stability via period-increasing or saddle-node bifurcations is similar to one predicted by Glendining for a system described by three ordinary differential equations having inversion symmetry. The dynamic features are robust and rather insensitive to the functional form of the kinetic expression.
Small angle neutron scattering from polymer blends in the dilute concentration limit
Authors: Briber Robert M. , Bauer Barry J. , Hammouda Boualem
Published: July 2002

Small angle neutron scattering has been used to study the radius of gyration and thermodynamics of dilute blends of deuterated polystyrene (PSD) in a strongly interacting matrix of poly(vinylmethylether) (PVME) and in a weakly interacting matrix of protonated polystyrene (PSH). The PSD chain is found to be slightly expanded in the PVME matrix over the PSH matrix with the radius of gyration for PSD being 123 Å in PVME vs 115 Å in PSH. The Flory interaction parameter, χ/v0, for the PSD/PVME blend is found to be in approximate agreement with an extrapolation of data from studies of χ/v0 for PSD/PVME at high concentration to the dilute concentrations studied in this work. The concentration dependence of χ/v0 is close to linear and shows no strong change in slope down to φ=0.02, the lowest concentration studied in this work. Analysis of the temperature dependence of χ/v0 and the second virial coefficient, A2 indicates that phase separation should occur for PSD/PVME in the range of 200 °C for a sample with φPSD≂5%.
Computer simulation of solid C60 using multiple time-step algorithms
Authors: Procacci Piero , Berne B. J.
Published: July 2002

The reversible reference system propagator algorithm (r-RESPA), based on a Trotter factorization of the classical propagator, is tested in a molecular dynamics simulation of solid C60. We show how, with an appropriate subdivision of the interaction potential and with a careful balancing of the integration parameters, one can adopt large time steps and impressively efficient r-RESPA integrators which yield the same dynamics obtained by means of the small time-step Verlet algorithm. The results presented here show that the use of r-RESPA integrators speeds up the simulation by a factor of between 20–40 with respect to the standard Verlet algorithms.
Dynamic scattering function for diblock copolymer chains in dilute solutions
Authors: Rey Antonio , Freire Juan J.
Published: July 2002

The quasielastic scattering function of a diblock copolymer chain in dilute solution has been evaluated according to the Rouse–Zimm theory (valid for Gaussian chains with preaveraged hydrodynamic interactions), and also through Brownian dynamics simulations which include net repulsions between units of different type (A or B) and fluctuating hydrodynamic interactions. Opposite contrast factors are defined for the A and B units (i.e., the solvent refractive index is assumed to be intermediate with respect to those of A and B). Both types of calculations agree in the main features: The scattering function can be expressed as a sum of exponential terms, whose relative contributions are completely different from the ones calculated from the Pecora analysis for homopolymer chains. For small or moderate values of the scattering variable, close to the peak for the total scattering intensity, a single exponential is obtained, but its exponent depends both on the translational diffusion coefficient, and on the chain first internal relaxation time.
First-principles molecular dynamics study of surface vibrations and vibrational mode coupling on the H/Si(111)1×1 surface
Authors: Gai Huadong , Voth Gregory A.
Published: July 2002

The vibrational dynamics of hydrogen on the H/Si(111)1×1 surface are studied using the first principles Car–Parrinello molecular dynamics method. The calculated frequencies of the surface vibrations are found to be in good agreement with experimental results. The combination band spectra indicate there are anharmonic couplings between the Si–H stretching mode and other lower frequency silicon surface phonon modes.
Response to ‘‘Comment on ‘The rheological behavior of concentrated colloidal dispersions’ ’’ [J. Chem. Phys. 101, 1757 (1994)]
Authors: Brady John F.
Published: July 2002

A reply to the comments of Cichocki and Felderhof concerning my paper on the rheological behavior of concentrated colloidal dispersions [J. Chem. Phys. 99, 567 (1993)] is given. Differences exist between my scaling predictions as maximum packing is approached for the reduced dynamic viscosity and the dilute analysis of Cichocki and Felderhof.
Anomalous pressure effects on the Raman spectra in hydrogen-bonded molecular chain systems
Authors: Moritomo Y. , Tokura Y. , Mochida T. , Sugawara T. , Oohashi T. , Kojima T. , Istubo A.
Published: July 2002

Effects of hydrostatic pressure on the Raman spectra have been investigated for four kinds of 1,3-diketone crystals with hydrogen-bonded molecular chains. In all the crystals we studied, intense CO stretching Raman mode shows pressure-induced softening reflecting compression of the hydrogen bonds. Furthermore, application of pressure broadens the both C=O and C–O stretching modes and intensifies several specific vibrational bands. We ascribed these spectral changes to formation or growth of the kink-type defects of the hydrogen-bonded sequence.
Many-body potentials of an open shell atom: Spectroscopy of spin–orbit transitions of iodine in crystalline Xe and Kr
Authors: Lawrence W. G. , Apkarian V. A.
Published: July 2002

Temperature dependent emission spectra of spin excited iodine in crystalline Xe and Kr are presented and analyzed in terms of nonadditive anisotropic pair interactions. In the octahedral trap site, the atomic 2P states split into E1/2 and G3/2 groups of the double valued representation. The fourfold degenerate G3/2 state is subject to strong Jahn–Teller instability and further splits by coupling to phonons into E1/2 and E3/2 Kramers pairs. Accordingly, the observed emission spectra are composed of two bands: 2E1/2→1E1/2 and 2E1/2→E3/2 transitions. Two pairs of bands are observed each in Xe and Kr. The long-lived pairs (at 15 K, τ=250 μs and 930 μs in Xe and Kr, respectively) are assigned to the isolated atom, while a short lived pair of bands (at 15 K, τ<1 μs in Xe, and τ=2.2 μs in Kr) are assigned to I atoms trapped as nearest neighbor to a localized charge, identified as (HRg)+. The isolated atom spectra are simulated by Monte Carlo methods which assume classical statistics in the heavy atom coordinates, and adiabatic following of the electronic coordinate. Angle dependent, gas phase pair interactions are used as a starting point. Minor modifications to the pair interactions, and a temperature dependent spin–orbit splitting constant, adequately reproduce the experimental spectra. Many-body contributions to the effective pair potentials can be estimated to change pair parameters by less than ∼3%.
Calculation of the dynamic structure factor in polymer melts
Authors: Chatterjee Avik P. , Loring Roger F.
Published: July 2002

In this work, we present calculations of the dynamic structure factor for monodisperse melts of linear polymers. Our calculations are based on a model in which a freely jointed chain encounters fluctuating obstacles whose relaxation is self-consistently determined from that of the chain conformation. The calculation of the structure factor is related to the solution of a one-dimensional random walk with dynamical disorder, which is treated within the dynamical effective medium approximation. This model has been applied previously to the calculation of the self-diffusion coefficient and the mean-squared displacement of a chain segment. We present calculations of the structure factor over a wide range of wave vectors, times, and entanglement molecular weights. Our predictions are compared with calculations from the theories of Ronca and of de Gennes, and with the experimental results for polyethylene-butylene-2 obtained by Richter et al. from neutron spin echo measurements. Our calculations show semiquantitative agreement with the experimentally determined structure factors for the time range accessible to these experiments.
Structure and optical anisotropies of critical polymer solutions in electric fields
Authors: Wirtz Denis , Werner Douglas E. , Fuller Gerald G.
Published: July 2002

We present simultaneous measurements of both the small angle light scattering (SALS) structure factor and the dichroism for near-critical polymer solutions in electric fields. The theory of flow birefringence and dichroism proposed by Onuki and Doi is extended to the electric field case. This theory relates the electric-field-induced form dichroism to the second moment of the structure factor. Excellent agreement is observed between the form dichroism calculated from measured scattering intensities and the dichroism measured by optical polarimetry. In addition, a possible mechanism of dipolar coupling between the large critical concentration fluctuations and the electric field is proposed. Similar simultaneous SALS and birefringence measurements confirm this model of dipolar interaction.
State-to-state dynamics of atom+polyatom reactions. III. The H+CDCl3→HD(v′,j′)+CCl3 reaction
Authors: Lanzisera Dominick V. , Valentini James J.
Published: July 2002

We report measurement of the HD product quantum state distributions for the H+CDCl3→HD(v′,j′)+CCl3 reaction at a collision energy of 1.6 eV using coherent anti-Stokes–Raman scattering (CARS). We determine relative HD(v′,j′) population distributions and absolute partial and total cross sections. The total cross section is 0.55±0.10 Å2. HD products are observed in v′=0, 1, and 2, with 7% of the available energy in HD product vibration. As with previous light+light–heavy→light–light+heavy reactions, the total energy in rotation, 15%, is also small. HD formed in v′=1 and 2 have greater fractions of rotational energy in relation to the energy available to them than HD in v′=0, with gr=〈Erot〉/(Etot−Evib)=0.15, 0.20, and 0.19 for v′=0, 1, and 2, respectively. Linear surprisal analysis shows that the excited vibrational levels have less bias against rotational excitation than the ground state level, respectively, if the CCl3 fragment is regarded as structureless. Comparison with previous state-to-state dynamics results of kinematically similar H+HX→H2+X (X=Cl, Br, I) and H+HR/DR→H2/HD+R (R=CD3, C2H5, C3H7) reactions indicate that the positive (v′,j′) correlation may be intrinsic to polyatomic hydrogen atom abstraction reactions.
Many-body trial wave functions for atomic systems and ground states of small noble gas clusters
Authors: Mushinski Andrei , Nightingale M.P.
Published: July 2002

Clusters of sizes ranging from two to five are studied by variational quantum Monte Carlo techniques. The clusters consist of Ar, Ne, and hypothetical lighter (‘‘1/2-Ne’’) atoms. A general form of trial function is developed for which the variational bias is considerably smaller than the statistical error of currently available diffusion Monte Carlo estimates. The trial functions are designed by a careful analysis of long- and short-range behavior as a function of inter-atomic distance; at intermediate distances, on the order of the average nearest neighbor distance, the trial functions are constructed to have considerable variational freedom. A systematic study of the relative importance of n-body contributions to the quality of the optimized trial wave function is made with 2≤n≤5. Algebraic invariants are employed to deal efficiently with the many-body interactions.
Hydrated proton clusters and solvent effects on the proton transfer barrier: A density functional study
Authors: Wei Dongqing , Salahub D. R.
Published: July 2002

The density functional calculations using the Perdew nonlocal corrections to exchange and correlation have been carried out for a sequence of hydrated proton clusters. The optimized structures were obtained up to H13O+6. It is found that H3O+ is indeed the central unit in all the lowest energy structures we found. Our results support the argument that the structure with a four-coordinate first solvation shell is very unlikely in small hydrated proton clusters. The density functional calculations with the Perdew nonlocal corrections to exchange and correlation give somewhat shorter hydrogen bond lengths, but slightly longer chemical bond lengths as compared with the post-Hartree–Fock calculations. The harmonic vibrational frequencies and IR intensities of various vibrational modes have been generated for all the structures optimized. Results for small clusters are compared with the high resolution experimental spectroscopy studies of Yeh et al. and Begemann et al. Results for larger clusters are used to interpret the low resolution spectra of Schwartz. Very good accord with experimental results is obtained. The solvent effects on proton transfer energy barriers in clusters have been studied by designing a few model systems. The barrier is found to be very sensitive to the solvent configurations. When the solvent water is replaced by the classical partial charge model, a significant change of the barrier is observed, indicating that a quantitative treatment will ultimately require a good pseudopotential to properly account for the quantum nature of the solvent. A combined density functional and molecular dynamics simulation was used to calculate the proton transfer energy and free energy barrier in aqueous solution. The barrier is found to be 3 kcal/mol higher than in gas phase. Very large solvent fluctuation is observed which may have a significant influence on the reaction rate.
Determination of the heats of formation of CCCN and HCCCN
Authors: Francisco Joseph S. , Richardson Steven L.
Published: July 2002

The electronic structure, vibrational spectra, and thermodynamic stability of CCCN and HCCCN have been determined using ab initio molecular orbital theory. The heat of formation for HCCCN was determined using Gaussian-2 theory (G2) and was found to be 91.5±2 kcal mol−1. There is good agreement between the G2 value and that determined using an isodesmic reaction (i.e., one in which both the number and types of bonds are conserved), which yields 90.7±2 kcal mol−1. The heat of formation for the CCCN radical is 178.2±2 kcal mol−1. These data are used to compute the bond dissociation energies in HCCCN, D0(C–C) and D0(C–H), which are important in assessing the chemistry in Titan’s atmosphere, as well as in other interstellar media.
An inversion procedure from rotational total cross sections
Authors: Belchior J. C. , Murrell J. N.
Published: July 2002

A hard shape potential has been used to interpret A+BC rotational inelastic cross sections as a function of collision energy. These hard shape surfaces have been shown to be close to the classical turning points of an appropriate soft potential, this providing a computationally simple route for deriving a soft potential. If the soft potential has exponential form then reducing the hard shape radii by an amount equal to the inverse of the exponent improves the results.
The determination of hyperpolarizabilities using density functional theory with nonlocal functionals
Authors: Lee Aaron M. , Colwell Susan M.
Published: July 2002

The theory for the coupled perturbed Kohn–Sham calculation of hyperpolarizabilities using nonlocal density functionals is presented. Results for calculations on formaldehyde, acetonitrile, and methyl fluoride using moderate size basis sets are reported. These results are compared with previous density functional calculations using the local density approximation, Hartree–Fock, and correlated methods, and with the experimental values.
Critical behavior of ionic micellar systems at different salt concentrations
Authors: Martín Ana , López Iñaki , Monroy Francisco , Casielles Arturo G. , Ortega Francisco , Rubio Ramón G.
Published: July 2002

The coexistence curves of the system dodecylammonium chloride+water+KCl have been measured at different salt concentrations. The results can be described with the usual Ising 3-D value for the critical exponent β=0.325. The analysis of the diameter indicates that the correct order parameter is defined in terms of an effective concentration calculated according to Eq. (4). Both the order parameter and the diameter of the coexistence curve point out that the range of validity of simple scaling decreases with the KCl concentration, i.e., as the system approaches a critical end point. The critical line, and an estimation of the Krafft temperatures have allowed us to estimate the position of the critical end point within the T-w-[KCl] space, w indicating the weight fraction of the surfactant. The pressure dependence of the critical temperature has been measured and found to be independent of the salt content for the present range of concentrations.
Microwave–millimeter-wave double resonance experiments on Ar–CO
Authors: Jäger Wolfgang , Gerry M. C. L.
Published: July 2002

A new double resonance technique has been developed to extend the accessible frequency range of a cavity pulsed microwave Fourier transform spectrometer beyond the microwave to the millimeter wave range. The technique has been used to measure indirectly pure rotational transitions of the van der Waals complex Ar–CO in the millimeter-wave range. These have led to an improved value for its A-rotational constant. The experiments have been done with the signal transition in the microwave range and the pump transition in the millimeter-wave range, in order to exploit the high sensitivity of the microwave spectrometer. All double resonance effects observed can be rationalized in terms of pump (millimeter-wave) radiation-induced population transfer.
Towards a new correction method for the basis set superposition error: Application to the ammonia dimer
Authors: Muguet Francis F. , Robinson G. Wilse
Published: July 2002

In this paper, we investigate a new method for correcting the basis set superposition error (BSSE) in ab initio quantum calculations of hydrogen-bonded (H-bonded) molecular complexes. In our scheme, the Hartree–Fock molecular orbitals (MOs) are first localized. The localized MOs (LMOs) are then separately attributed to each of the component or fragment molecules. Considering each LMO assigned to a specific fragment molecule, we set to a zero value the LMO coefficients relative to the AOs belonging to all the other partner molecules or molecular fragments. After purification of the ‘‘off-fragment’’ coefficients, the LMOs are then reorthonormalized. The resulting wave function constitutes a first level of approximation to a BSSE-corrected wave function. An iteration procedure is then implemented, comprising the following steps: HF MOs; localization; fragment attribution; off-fragment purification; reorthormalization; new Fock matrix; diagonalization of the Fock matrix; HF-type MOs; localization and so on. The converged wave function satisfies a self-consistent equation. The scheme can be extended to MCSCF wave functions. The MCSCF MOs are localized, then off-fragment LMOs components are eliminated. The resulting LMOs are reorthonormalized to generate a MO basis for a CI computation. This BSSE correction scheme is implemented within gamess. Applications to the ammonia dimer are described in this and the accompanying paper. We find that linear H-bonded geometries are artificially favored by the BSSE.