bluuebunny/crossref_metadata_2025 huggingface dataset:
Conrotatory and disrotatory pathways in the electrocyclic isomerization of cyclobutene to cis-butadiene: The spin-coupled viewpoint
Authors: Oliva J. M. , Gerratt J. , Karadakov P. B. , Cooper D. L.
Published: July 2002

Spin-coupled theory is used to study the changes which occur in the electronic wave function as a system moves along the intrinsic reaction coordinate (IRC) for the case of conrotatory and disrotatory motion in the ground-state electrocyclic interconversion of cyclobutene to cis-butadiene. It is shown that the reaction is already well under way in the conrotatory case at the transition point (considerable cis-butadiene character), but the system still retains an overwhelming reactant (cyclobutene) character for the disrotatory motion. In both the conrotatory and disrotatory cases, the sharp changes in the weights of the two spin couplings, associated with the onset of bond breaking or bond formation, are shown to occur at points along the IRC well after the transition point.
Phase coexistence in random copolymers
Authors: Dobrynin Andrey V.
Published: July 2002

We study the phase coexistence in random copolymer systems. In the framework of the one-loop approximation it is shown that the phase transition between liquidlike and lamellar phases in the symmetric random copolymers occurs through the two-phase region. We derive an analytic expression describing the volume fractions of the coexisting phases in this region. To illustrate the phase behavior of the random copolymers, the phase diagram of a polydisperse A–B multiblock copolymer melt is calculated.
Assessing the accuracy of the vibrational infinite order sudden approximation for He+I2(n=5)→He+I2(n′=4,6)
Authors: Fiedler M. J. , McDonald K. A. , Pfeffer G. A.
Published: July 2002

For vibrational excitation, the vibrational infinite order sudden approximation was qualitatively correct using an initial j of 0. The agreement was improved using an average over the initial j states of 0, 2, and 4. For vibrational relaxation, reasonable agreement with experiment was achieved only with the initial j average and n̄=n.
The local order in liquid water studied through restricted averages of the angular correlation function
Authors: Santis Alberto De , Rocca Dario
Published: July 2002

A structural study of TIP4P [W. L. Jorgensen et al., J. Chem. Phys. 79, 926 (1983)] water is presented. The method of structural analysis is based on the choice of restricted ranges of the pair angular configuration space. Such ranges, referred to as states or configurations Γ, are used to obtain restricted averages, gΓ(r), of the angular correlation function g(r,ω1,ω2). Eulerian angles are used to define molecular orientations. This allows one to analyze all the geometries of the configuration space and to pay due attention to the nonhydrogen bonded configurations. The local structures and their temperature evolution are studied using the restricted distribution functions of oxygen–oxygen, gOOΓ(r), and of oxygen–hydrogen gOHΓ(r) of the different configurations. As the temperature rises, the local population of nonhydrogen bonded configurations increases owing to the breakdown of the tetrahedral network. By comparing the gOOΓ(r) to the g(r) of simple fluids, scaled from liquid argon, the evidence of a residual tetrahedral network in liquid water at high temperature is obtained. Some restricted averages gOOΓ(r) reveal the existence of long-range direct correlation, masked by the orientational averages of the total distribution function gOO(r). The comparison between gOOΓ(r), evaluated at the liquid density and in the limit of zero density, shows that the direct correlation extends beyond four molecular diameters (about 12 Å for water) as in simple polar liquids. Finally, the dimer geometries contributing to form the first peak in gOHΓ(r) are identified. A practical method for determining the coordination numbers of hydrogen and nonhydrogen bonded molecules from the experimental data is suggested. Applied to TIP4P liquid water at various temperatures, the method yields coordination numbers in agreement with the random network model predictions.
Influence of chaos on the ionization induced fragmentation dynamics of van der Waals clusters
Authors: Garcia M. E. , Reichardt D. , Bennemann K. H.
Published: July 2002

Molecular dynamics simulations based on a self-consistent electronic model are performed to investigate the effect of ionization on the atomic motion of small van der Waals clusters. We find unimolecular dissociation (fragmentation) with time scales in the picosecond range. The dynamics during the relaxation process after ionization turns out to be extremely nonlinear, with fragmentation times which depend strongly on initial conditions. Our calculations show that the largest Liapunov exponent λ+ after ionization is much larger than λ0, the corresponding exponent before ionization. This indicates that the ionization process enhances the nonlinear character of the motion of small clusters. We also determined the distribution of fragmentation times as a function of the vibrational temperature of the clusters before ionization. Since the ionization process creates a state far away from thermodynamical equilibrium, a time-dependent fragmentation probability W(t) is obtained. Furthermore, W(t) reflects the ionization induced chaotic dynamics.
Photodissociation of CH≡CCH2X (X=Br and Cl) by translational spectroscopy
Authors: Lee Ya-Rong , Lin Shen-Maw
Published: July 2002

The photodissociation of CH≡CCH2X (X=Br and Cl) at 248 and 193 nm was studied using an angle-resolved beam apparatus. Both the translational energy distribution P(Et) and the anisotropy parameters β of the photoproducts were derived from the time-of-flight spectra. Excitation at 248 nm, CH≡CCH2Br underwent exclusively a direct C–Br bond fission with anisotropic recoiling fragments. By increasing the photon energy to 193 nm, three competing primary dissociation channels were observed: (1) Br+C3H3 (fast, 26%), (2) Br+C3H3 (slow, 44%), and (3) HBr+C3H2 (30%), with average translational energies of 34, 12, and 9 kcal/mol, respectively. From the measured β values, all reactions were shown to proceed extremely fast within a fraction of rotational period. On the other hand, excitation at 193 nm, CH≡CCH2Cl underwent a predominant C–Cl bond fission with a minor channel producing the HCl+C3H2 fragments. The results are discussed in terms of either a direct or an indirect dissociation process arising from the transition to the (π,π*) state.
Volume polarization in reaction field theory
Authors: Zhan Chang-Guo , Bentley John , Chipman Daniel M.
Published: July 2002

In continuum reaction field models of solvation, unconstrained quantum mechanical calculation of the solute electronic structure inevitably leads to penetration of some solute charge density outside the cavity and into the solvent dielectric region. This produces a rarely recognized or treated volume polarization that contributes in addition to the commonly considered surface polarization. In this work a new practical implementation is described for quantitatively evaluating both volume and surface polarization contributions to the solute-solvent interaction with an irregularly shaped cavity surface. For illustration, numerical results are presented on several representative small neutral, cation, and anion solutes. The volume polarization contributions to energies and dipole moments are found to be somewhat smaller than those from surface polarization, but not negligible. The results are also used to test several charge renormalization approaches that have been previously proposed in the literature. Compared to the exact volume polarization correction, these can sometimes lead to energy corrections of the wrong sign. A previously proposed method of simulating volume polarization through an additional surface polarization generally produces corrections of the right sign and of about the right magnitude.
The Ã 2Σ+ state of Ar⋅NO studied using resonance-enhanced multiphoton and zero-kinetic-energy pulsed-field ionization spectroscopy
Authors: Bush Andrew M. , Dyke John M. , Mack Paul , Smith David M. , Wright Timothy G.
Published: July 2002

Resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy pulsed-field ionization (ZEKE-PFI) techniques have been used to study the spectroscopy of the Ã 2Σ+ state of Ar⋅NO. Ã 2Σ+–X̃ 2Π REMPI spectra of Ar⋅NO have been interpreted using two different models that have allowed assignment of the observed rovibronic transitions to be made and an estimate of the strength of the Ã state intermolecular potential to be determined. Simulations of the spectra have indicated that the ground vibrationless level of the Ã state has a linear geometry while some higher vibrational levels have a skewed T-shaped structure. The analysis has also confirmed that the Ã state of Ar⋅NO is very weakly bound and is non-Rydberg in character. Reported for the first time are ZEKE-PFI spectra observed via the intermediate Ã 2Σ+ state. No structure was detected close to the ionization threshold, although peaks observed at higher excitation energy have been assigned to transitions to high-lying vibrational levels of the Ar⋅NO+ ion. ZEKE-PFI spectra recorded via the ground vibrational level of the intermediate Ã state, which has a linear structure, show progressions which correlate with transitions to highly excited van der Waals stretching and bending levels; however, spectra recorded via an intermediate level with a T-shaped geometry show progressions which correlate principally with the van der Waals stretching mode. These assignments have been interpreted in terms of the large changes in intermolecular bond length and bond angle upon ionization and fully support the interpretation of the REMPI spectra. The results of the REMPI and ZEKE-PFI spectra have also been used to investigate the character of the Ã state and the geometry that the complex adopts in each of the vibronic levels of this state has been rationalized in terms of the possible interactions involved.
Vibrational predissociation spectroscopy of the (H2O)6−⋅Arn, n⩾6, clusters
Authors: Ayotte Patrick , Bailey Christopher G. , Kim Jun , Johnson Mark A.
Published: July 2002

Solvation of (H2O)6− with several argon atoms suppresses the strong direct photodetachment background in the bare hexamer anion, allowing vibrational predissociation spectroscopy to be carried out in a background-free regime. In addition to the previously reported autodetaching resonances [C. G. Bailey, J. Kim, and M. A. Johnson, J. Phys. Chem. 100, 16782 (1996)] in the single donor hydrogen bonding region (∼3300 cm−1), the predissociation spectra reveal many weak bands scattered throughout the mid infrared (3200–3750 cm−1). Most of these new bands are evident in the bare hexamer spectrum after signal averaging, indicating that they are isolated using predissociation but not induced by solvation. The most intense bands display much stronger redshifts (>30 cm−1 by n=15) than the matrix shifts typically found for the neutral water clusters, indicating that these bands are unique to the negative ion.
Sub-microhartree accuracy potential energy surface for H3+ including adiabatic and relativistic effects. II. Rovibrational analysis for H3+ and D3+
Authors: Jaquet Ralph , Cencek Wojciech , Kutzelnigg Werner , Rychlewski Jacek
Published: July 2002

The 69 potential energy points of H3+ computed by Cencek et al. [J. Chem. Phys., 108, 2831 (1998), preceding paper] have been fitted to an analytical potential energy surface (PES). Rovibrational frequencies have been derived for the symmetric H3+ and D3+ isotopomers. A comparison with experiment shows residual discrepancies of a few tenths of cm−1 which can be ascribed mainly to nonadiabatic effects.
Absence of annealing effect in the vibrational density of states in a glassforming polymer
Authors: Zorn Reiner , Frick Bernhard
Published: July 2002

We describe experiments devised to detect a possible effect of annealing on the vibrational density of states (DOS) in a glassforming polymer with high accuracy from inelastic incoherent neutron scattering. Such an effect could be expected from the argument that annealing changes the behavior of many measured quantities, especially the specific heat cp(T). For the experiment polybutadiene was chosen because the effect on cp(T) as well as the optimal conditions of annealing are well known for this material. Contrary to our expectations an annealing effect could be detected neither in the inelastic spectra taken on IN6 nor in the elastic scans performed on IN16 at ILL, Grenoble. From this we conclude that the observed annealing effect on cp(T) does not originate from a difference in the DOS but rather from other sources, e.g. a change of configurational entropy. This result is in agreement with a theory of DiMarzio which gives a good prediction of the step Δcp at the glass transition in the case of polybutadiene and attributes only 8%–10% of it to vibrational terms.
Spurious cooperativity in alkylated succinic acids
Authors: Ben-Naim A.
Published: July 2002

The proton–proton correlation, as measured by the ratio between the second and the first dissociation constants of dibasic acid, is sometimes very large and far beyond what could be explained by electrostatic theories. We propose a novel interpretation of this phenomenon based on the idea of spurious cooperativity. The general theoretical framework underlying the onset of spurious cooperativity is developed first. The basic result is that whenever a binding (or dissociating) two-site (or more) system splits into a mixture of noninterconverting isomers the binding isotherm (or the titration curve) behaves as if it is more negatively cooperative compared with the genuine cooperativities of the individual isomer. The theory is applied to a specific system of α–α′ dialkyl succinic acid. It is known that the Meso form of these alkylated derivatives show a normal correlation of the same order of magnitude as in succinic acid. On the other hand, the Racemic form of these alkylated derivatives shows anomalous strong negative correlations when the alkyl groups become large (e.g., isopropyl and tert butyl). It is shown that the theory of spurious cooperativity can explain the different behavior of the Racemic and the Meso forms, as well as the onset of anomalous strong negative correlations when the alkyl groups become large.
Escape through a bottleneck undergoing non-Markovian fluctuations
Authors: Bicout D. J. , Szabo Attila
Published: July 2002

The Zwanzig model for the kinetics of escape through a fluctuating bottleneck is generalized to handle fluctuations that can be described by a non-Markovian Gaussian process. Our formulation is based on a multidimensional Fokker–Planck equation with a quadratic sink term. It is shown that the calculation of the time dependence of the survival probability can be reduced to a matrix eigenvalue problem. The elegant Wang–Wolynes expression, first derived using path integral techniques, for the effective rate constant that determines the asymptotic behavior of the survival probability, is recovered. When the fluctuations are described by Langevin dynamics (i.e., diffusion in position and velocity space) on a harmonic potential, analytical expressions for the survival probability and effective rate constant for all values of the friction, are derived. As a function of the friction constant, these exhibit “Kramers turnover” behavior.
193 nm photolysis of H2S in rare-gas matrices: Luminescence spectroscopy of the products
Authors: Khriachtchev Leonid , Pettersson Mika , Isoniemi Esa , Räsänen Markku
Published: July 2002

The 193 nm photolysis of hydrogen sulfide (H2S) in solid rare gases is studied at 7.5 K. In order to get the most reliable data of the photolysis process, Fourier transform (FT) infrared and time-resolved luminescence methods are used in the same experiment. The 193 nm photolysis of H2S in Ar and Kr matrices was found to be very similar to the gas phase. A kinetic scheme of H2S photolysis, which is consistent with all the experimental features, was constructed. The major channel is formation of (H+SH) pairs, which are stabilized in the matrix. Then SH radicals decompose to (S+H) pairs, providing the main source for S atoms. No experimental evidence of a cage-induced reaction H+SH→S+H2 was observed in our study, which can be connected with high probability for hydrogen-atom exit from the parent cage, and/or with high probability of the recombination reaction H+SH→H2S. The available spectroscopic information for S atoms and SH radicals in Ar and Kr matrices is further specified, and new spectroscopic data on the photolysis products in Ne and Xe matrices are reported. In particular, the luminescence data on SH radicals in solid rare-gas matrices (Ne, Ar, Kr, and Xe) were found to resemble the tendencies known for OH radicals. Also, the infrared absorptions of SH radicals in Ar and Kr matrices were identified to be at 2607 and 2594 cm−1, respectively, and a novel rare-gas molecule HXeSH with the Xe–H stretch at 1119 cm−1 was detected.
Static structure of polydisperse colloidal monolayers
Authors: Lado F.
Published: July 2002

A generalization of integral equation theory of simple liquids is used to study the structure and thermodynamics of a monolayer of spherical colloidal particles having a continuous distribution f(σ) of diameters σ. The quasi-two-dimensional fluid is modeled using both a repulsive Yukawa potential to represent charged hard spheres (with attendant charge polydispersity) and a Lennard-Jones potential to represent soft spheres with an effective attractive well. The numerical solution of the integral equations makes essential use of polynomials that are orthogonal with weight function f(σ), which is taken here to be a Schulz distribution.
Extended superstructure in Tl-rich liquid Cs–Tl alloys
Authors: van der Aart S. A. , Verkerk P. , Badyal Y. S. , van der Lugt W.
Published: July 2002

The structure factors of liquid caesium–thallium alloys with thallium concentrations of 50 to 85 at. % have been measured by means of neutron diffraction. They exhibit a prepeak which shifts to smaller wave vectors with increasing thallium content. This result is explained using a model in which neutral units CsmTln are dissolved in an excess of liquid thallium.
A fourth-order numerical integrator for stochastic Langevin equations
Authors: Hershkovitz Eli
Published: July 2002

Stochastic differential equations (SDE’s) can be numerically integrated using second-order accuracy methods. Higher order schemes are not in use because of the complexity of the algorithm and because of the difficulties in producing non-Gaussian noises. Yet for the case of the Langevin equation (LE) which is a subclass of SDE’s, high order integrators can be developed. A fast fourth-order integrator is presented here. The improved efficiency of the new integrator allows for solution of systems which could not be integrated accurately with the standard second-order methods.
Isomerization pathway of the aluminum monocarbonyl/isocarbonyl pair, AlCO/AlOC: Evidence of a cyclic minimum
Authors: Wesolowski Steven S. , Galbraith John M. , Schaefer Henry F.
Published: July 2002

The isomerization pathway between AlOC and AlCO has been explored at the self-consistent field, configuration interaction, and coupled-cluster levels of theory. Five stationary points on the Al+CO potential energy surface were located and show that the path of Al migration from the isocarbonyl to the monocarbonyl involves a very small barrier to a perhaps unexpected cyclic minimum structure followed by a second barrier to the AlCO isomer. A quantitative analysis of the relative stabilities of the isomers as well as the ZPVE-corrected isomerization barriers are presented and compared to the boron carbonyl analogs. At the coupled-cluster level with single, double, and perturbatively applied connected triple substitutions [CCSD(T)] using a TZ2P+f basis set, the cyclic minimum is 9.4 kcal/mol higher in energy than AlCO but is 11.4 kcal/mol more stable than AlOC. The barriers from AlOC to the cyclic isomer and to the dissociation products P2 Al and X 1Σ+ CO are only 3.5 and 1.0 kcal/mol, respectively, and leave the tentative experimental observation of AlOC in doubt. On the other hand, the cyclic structure lies in a substantial well with barriers of 19.4 and 14.9 kcal/mol to AlCO and AlOC, respectively. The barrier to Al+CO from the cyclic isomer is estimated to be near 2.5 kcal/mol. The C–O harmonic stretching frequency of the cyclic isomer at this level is predicted to be 1605 cm−1 and provides a guide for the possible experimental observation of this species.
Building protein lattice models using self-consistent mean field theory
Authors: Koehl Patrice , Delarue Marc
Published: July 2002

An optimization protocol for modeling protein structures on lattice is proposed which is based on self-consistent mean field (SCMF) theory. In this procedure, the protein residues are supposed to be independent, and their possible positions are given by a list of lattice sites. To do this, an effective larger system is considered, in which each residue i is supposed to occupy all possible sites j, each with a weight V(i,j) stored in the so-called lattice probability matrix V. The effective energy of the system is computed, and iteratively minimized with respect to the weights V, the lattice sites being fixed in space. The final self-consistent V matrix describes the conformational space available to the protein, based on the energy function implemented. This energy function contains two types of terms, namely simple geometric terms which ensure bond connectivity and prevent chain intersection, and energy terms specific to the problem of interest. The application of the above protocol to building a lattice model of a protein, given its three dimensional structure, is discussed and compared with other lattice fitting procedures.
Oscillating time correlation functions from dynamic light scattering of gold-labeled tracers
Authors: Schärtl Wolfgang , Roos Christopher , Gohr Kerstin
Published: July 2002

Nanoparticles containing small gold clusters as labels have been used to study tracer diffusion in concentrated colloidal dispersions by dynamic light scattering. Our samples consisted of a small amount of gold tracers in a concentrated dispersion of nonlabeled matrix particles of similar size. Unexpectedly, the intensity time correlation functions showed oscillations. The unusual signal has been attributed to convection which may be caused by light absorption of the tracer particles, creating a thermal gradient within the sample. However, the origin of the oscillations in our homodyne light scattering experiment is still not perfectly understood. Here, we present a qualitative discussion concerning the dependency of oscillations both on laser light power and the concentration of matrix particles.
Fragmentation, charge transfer and chemical reactions in C60+/C70+–SF6 collisions
Authors: Ehlich R. , Sprang H. , Westerburg M. , Campbell E. E. B.
Published: July 2002

Collisions of C60+ and C70+ with SF6 were investigated in a time of flight mass spectrometer and compared with the results of collisions with Ar and Xe. Reaction cross sections were determined as a function of the center of mass collision energy between 25 and 300 eV. The influence of thermal excitation on fragmentation was examined. The effects of additional vibrational degrees of freedom in the case of SF6 on the fragmentation cross sections are discussed. Charge transfer reactions producing C602+ or C702+ were observed only with SF6. Thermal electron emission from highly excited C60− was used for the formation of an intense neutral C60 beam with 1 keV kinetic energy which was used to investigate charge transfer in neutral C60+SF6 collisions. Reaction products CnF+, n=1–11, were found in SF6 collisions.
Comment on “On the direct complex scaling of matrix elements expressed in a discrete variable representation: Application to molecular resonances” [J. Chem. Phys. 104, 7008 (1996)]
Authors: Bludský Ota , Li Yan , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

The Cauchy–Goursat theorem is used to derive a surface integral correction term for the computation of complex scaled potential matrix elements in the sine-based DVR procedure originally proposed by Museth and Leforestier. Test results show that scatter in the complex energy trajectories is removed by including this term.
An effective method for the quantum mechanical description of photoionization with ultrashort intense laser pulses
Authors: Meier Christoph , Engel Volker , Manthe Uwe
Published: July 2002

We present a new method to calculate photoionization spectra which originate in the interaction of a molecule with ultrashort strong laser pulses of arbitrary time and frequency shapes. Concepts of the multiconfiguration time-dependent Hartree method, initially developed for the propagation of multidimensional wave packets are combined with the fast-Fourier-transform split-operator method. The presented method allows for a fast and stable nonperturbative description of multiphoton excitation and ionization processes in small molecules. As an example we calculate the kinetic energy distribution of photoelectrons from intense short pulse ionization of Na2.
Competition between radiation and photofragmentation in the Ã 2Σ+ state of the SH/D rare gas complexes
Authors: Applegate Brian E. , Yang Min-Chieh , Miller Terry A.
Published: July 2002

The natural lifetimes of a large number of the vibrational levels of the excited Ã 2Σ+ electronic state of the family of rare gas complexes, R⋅SH (R=Ne, Ar, and Kr) and their deuterides, are reported. It is well known that the natural lifetime of the Ã 2Σ+ state of isolated SH/D is markedly shortened by a photofragmentation process. Our results for the complexes show that the rare gas atom plays an important role in inhibiting this process. From a classical model of the molecular system we are able to explain the trends observed in our lifetime data. The data from the R⋅SD complexes where for some vibrational levels the deuterium atom appears to be trapped between the rare gas and sulfur atoms allows us to establish a radiative lifetime for these complexes and the SH/D monomer.
Low temperature CO oxidation triggered by the gas-phase D atom incident on Pt(111) covered with O2 and CO
Authors: Kim Jae-Young , Lee Jihwa
Published: July 2002

Exposing an O2-saturated Pt(111) surface at 85 K to a beam of D atom leads to desorption of O2 and D2O. A series of post D-exposure thermal desorption spectra shows that D2O is produced by consecutive D-addition reactions via adsorbed OD intermediate, i.e., O2(ad)→ lim DO(ad)+OD(ad)→ lim DD2O(ad)+D2O(g). When CO is coadsorbed with O2 on Pt(111) at 85 K, the incident D atom also induces prompt desorption of CO2 but not CO. We propose that CO is oxidized by the nascent hot O* and OD* formed in a highly exothermic initiation reaction D(g)+O2(ad)→DO2≠→O(ad)+OD(ad) with an energy release of ∼−4.6 eV before they become accommodated to the surface. Possible mechanisms for O2 desorption are also briefly discussed.
Variational resonance valence bond study on the ground state of C60 using the Heisenberg model
Authors: Flocke N. , Schmalz T. G. , Klein D. J.
Published: July 2002

A detailed variational resonance valence bond (RVB) study is performed for the S=0 ground state of the C60 molecule in the framework of the Heisenberg model. It is shown that the 12 500-dimensional Kekulé space can be divided into two subspaces of respective dimensions 5828 and 6672, of which the first one recovers 99.82% of the energy of the full Kekulé space. This 5828-dimensional subspace is derived from the main Kekulé function, which is formed from spin pairs on hexagon–hexagon bonds only, by simple rotations of the three spin pairs in disjoint sets of hexagonal rings of C60 in all possible ways. This indicates that the concept of the stability of the aromatic sextet still plays an important role even in this nonalternant system. Further, the inclusion of some longer range RVB functions like Dewar-type functions and functions involving Claus structures is investigated, and the effect on the ground-state energy as well as on the nearest neighbor correlation functions is examined.
Derivation of the phase factor and geometrical phase for an N-state degenerate system
Authors: Baer Michael
Published: July 2002

This work considers the case of degenerate vibronic coupling of N states all at one single point. For this case we first derived the extended single-state Born–Oppenheimer equation [M. Baer and R. Englman, Chem. Phys. Lett. 265, 105 (1997)] and then showed that such a system, like the two-state system, can be described in terms of a single phase factor and is characterized by a well-defined geometrical phase.
Anomalous enrichment of C2+ ions by laser ablation of graphite in Ar jet
Authors: Park Seung Min , Chae Heebyung , Wee Sangwook , Lee Inhyung
Published: July 2002

The formation mechanisms of C1+, C2+, and C3+ ions by laser ablation of graphite are investigated using time-of-flight (TOF) quadrupole mass spectroscopy. The laser ablation is performed in conjunction with a pulsed Ar expansion to elucidate the third-body effect on the formation of the small carbon cluster ions. Drastic enhancement of the C2+ ion signal is observed by an increase of the local pressure near the target, indicating that C2+ ions are efficiently formed by recombination of carbon atoms and subsequent ionization. The branching ratio of C1+, C2+, and C3+ ions and their mean translational energies are different from those of neutrals. Also, the TOF spectra for Cn+ ions show multiple peak structures, which implies that different mechanisms are involved in the formation of Cn+ ions.
Theory of time-resolved fluorescence under the interaction of energy transfer in a bichromophoric system: Effect of internal rotations of energy donor and acceptor
Authors: Tanaka Fumio
Published: July 2002

Theoretical expressions of time-resolved fluorescence were worked out in the system of intramolecular energy transfer where both donor and acceptor possess motional freedom of internal rotation. Förster-type mechanism was assumed for the intramolecular energy transfer. Effects of the internal rotations of the donor and acceptor on the fluorescence intensity and anisotropy decays were examined by numerical calculations for some model systems. It was suggested that anisotropy decay of the acceptor is very sensitive not only to molecular geometry of donor and acceptor, but also to the diffusion coefficient of the internal rotation of the donor.
Azimuthal ordering in Langmuir monolayers
Authors: Braun F. N. , Yokoyama H.
Published: July 2002

A simple mean-field model of azimuthal ordering in Langmuir monolayers induced by anisotropic dispersion forces is presented. The approach is removed from previous studies of this interaction in systems of grafted rods insofar as the tilt of amphiphiles is decoupled from azimuthal ordering and treated as an external variable. We examine the phase space of fixed tilt against temperature, finding that azimuthal ordering is second order for most tilts, except in the range 57°–66°, where it is first order. This is related to the existence of three different types of azimuthal order; polar, nematic, and polar–nematic.
Relationship of phase diagrams and surfaces of new phase nucleation rates
Authors: Anisimov M. P. , Hopke P. K. , Rasmussen D. H. , Shandakov S. D. , Pinaev V. A.
Published: July 2002

Experimental and theoretical investigations of vapor nucleation began about 100 years ago. Until the 1980s, experiments generally measured only critical supersaturation values. Since then, measurement procedures have substantially improved and nucleation rates can now be measured as a function of temperature, vapor activities, and pressure with high accuracy. Nucleation theory has made obvious progress, but the understanding of nucleation phenomenon is far from complete. New approaches to conceptualizing nucleation are necessary in order to identify possible new directions for further improvement of nucleation theory. One such approach is the analysis of the topology of nucleation rate surfaces. The creation of a nucleation rate surface is based on knowledge of phase equilibrium diagrams, limited experimental nucleation results, and a few plausible assumptions. In this article, the surfaces of the nucleation rates as a function of pressure or activity for single and binary systems for nucleation from metastable vapor, liquid, and crystalline states are constructed. By using surface topology analysis, some problems in nucleation theory are more clearly formulated and future directions for improvement can be found. Currently, it is not possible to create a universal nucleation theory based only on first principles. Partial theoretical success can be obtained only in the case of systems with well-known molecular interaction potentials. By scaling the experimental nucleation rate surfaces for portions of the phase diagrams that are identical to one another, a semiempirical nucleation rate surface for an unknown system can be created from its phase diagram. Scaling should give quantitative nucleation rates. Phase diagrams must be more fully incorporated into the interpretation of experimental nucleation results and nucleation theory development.
A molecular dynamics study of the Cr3+ hydration based on a fully flexible hydrated ion model
Authors: Martı́nez José M. , Pappalardo Rafael R. , Sánchez Marcos Enrique
Published: July 2002

A theoretical study of the Cr3+ hydration in aqueous solutions has been carried out by means of molecular dynamics (MD) simulations. Ion–water intermolecular interaction potentials are based on first principles using the idea of the previously developed hydrated ion–water interaction potential: The bare ion, Mn+, is replaced by its corresponding hydrate, [M(H2O)6]n+, and the water molecules interact with the hydrate by means of an ab initio [M(H2O)6]n+–H2O interaction potential. A new ab initio interaction potential has been developed to describe the Mn+–(H2O)first-shell interaction based on an examination of the hexahydrate potential-energy surface section that distorts the position of one of the cluster water molecules, the remaining five fixed at their equilibrium position. These two complementary interaction potentials, which describe ion–water interactions have been combined with the TIP4P model for water molecules. Structural and dynamical results derived from the analysis of 1 ns of simulation for a sample formed by [Cr(H2O)6]3+ and 512 H2O are presented. Rigidity effects of the cluster are examined by comparing the present results with those previously obtained with a model of rigid hexahydrate [J. Phys. Chem. B 102, 3272 (1998)]. A new definition of hydrated ion based on the rotational properties of its hydrate is supported.
Hydration of a B–DNA fragment in the method of atom–atom correlation functions with the reference interaction site model approximation
Authors: Tikhonov D. A. , Polozov R. V. , Timoshenko E. G. , Kuznetsov Yu. A. , Gorelov A. V. , Dawson K. A.
Published: July 2002

We propose an efficient numerical algorithm for solving integral equations of the theory of liquids in the reference interaction site model (RISM) approximation for infinitely dilute solution of macromolecules with a large number of atoms. The algorithm is based on applying the nonstationary iterative methods for solving systems of linear algebraic equations. We calculate the solvent–solute atom–atom correlation functions for a fragment of the B-DNA duplex d(GGGGG)⋅d(CCCCC) in infinitely dilute aqueous solution. The obtained results are compared with available experimental data and results from computer simulations.
Finding the low-energy forms of avian pancreatic polypeptide with the diffusion-process-controlled Monte Carlo method
Authors: Derreumaux Philippe
Published: July 2002

Ab initio folding of the avian pancreatic polypeptide using a diffusion-process-controlled Monte Carlo method is presented. This method differs from other Monte Carlo methods in that two successive conformations must be kinetically connected in a small period of time. The 36-residue polypeptide is represented using a hybrid level of structure description: the backbone is treated at an all-atom level, while the side chains are modeled as spheres. The conformations are evaluated on the basis of pairwise contact energies between the side chains, a main chain hydrogen bonding potential, and local bonded potentials. Starting from various extended conformations, the chain reaches the basin of lowest energy in ∼1000–3500 Monte Carlo steps and the predicted conformations deviate by ∼3.0 Å rms from the x-ray structure. The eight trajectories suggest a three-step mechanism: (1) early formation of the α helix in the region 14–33, (2) cooperative formation of long-range interactions, and (3) stabilization of the polyprolinelike conformation in the region 1–8 in the final steps of folding.
Ab initio theoretical study of dipole–bound anions of molecular complexes: Water tetramer anions
Authors: Smith Dayle M. A. , Smets Johan , Elkadi Yasser , Adamowicz Ludwik
Published: July 2002

We present results of ab initio calculations of the (H2O)4/(H2O)4− system. The main conclusions of this work are as follows: The calculated results predict that water tetramer anions are metastable systems in agreement with weak spectral manifestation of these systems in gas-phase experiments of Bowen and co-workers; the excess electrons in all four structural isomers of water tetramer anions found in the calculations are attached to the clusters by the virtue of dipole-electron interaction; all four (H2O)4− anions found in the calculations are almost isoenergetic but have different vertical electron detachment energies (VDEs) ranging from 22 to 279 meV; the most stable cyclic structure of (H2O)4 has a null dipole moment and does not form a dipole–bound state with an excess electron; the water tetramer anions observed experimentally probably are formed as a result of hydration of the water dimer anion, (H2O)2−, by a neutral water dimer or by hydration of the water trimer anion, (H2O)3−, by a single water molecule; based on calculations, we predict some specific IR vibrational features for the anions which can be used for identification of these species; vibrational analysis of all four anions found in the calculations indicate that they correspond to minima on the potential-energy surface.
Two-dimensional treatment of the methyl and aldehydic torsions in gas phase propanal
Authors: Vivier-Bunge A. , Uc V. H. , Smeyers Y. G.
Published: July 2002

The potential energy surface for the simultaneous methyl and asymmetric aldehydic torsions in propanal has been determined at the restricted Hartree–Fock/second-order Moller–Plesset level using the 6-311(3df,p) basis set. The fit of the energy values to a symmetry adapted functional form was carried out in two ways: using all the 56 energy values obtained in the optimization procedure, or dropping out the energy values which do not respect the C3 dynamical symmetry of the methyl group, i.e., using only 28 values. With these potentials, as well as with the kinetic parameters and the electric dipole moment variations, the far infrared frequencies and intensities for the methyl and aldehyde torsions were determined theoretically. It is found that the two-dimensional calculation for the cis conformer satisfactorily reproduces not only the methyl torsion and asymmetric aldehyde torsion spectrum, but furnishes also methyl torsionally excited progressions for the aldehyde torsion modes. Most of these transitions occur in the region of 220–100 cm−1 and are responsible for the complexity of the spectrum. From these theoretical results, a new assignment is proposed. The fit of the energy values which respect to the C3 dynamical symmetry seems to produce better results and also permits one to save computational time.
Dynamics of neutral cluster growth and cluster ion fragmentation for toluene/water, aniline/argon, and 4-fluorostyrene/argon clusters: Covariance mapping of the mass spectral data
Authors: Foltin M. , Stueber G. J. , Bernstein E. R.
Published: July 2002

Neutral cluster growth and ionic cluster fragmentation are studied for toluene/water (TWn), aniline/argon (AnArn), and 4-fluorostyrene/argon (FSArn). Clusters are created in a supersonic expansion and ionized by both one-color and two-color (near threshold) resonance enhanced laser ionization. Toluene/water clusters are known to fragment subsequent to ionization by loss of water molecules or by proton transfer and loss of a benzyl radical. This system is selected to test the applicability of covariance mapping techniques to investigate the fragmentation behavior of singly charged cluster ions. To explore sensitivity of the parent ion/fragment ion correlation coefficient to cluster fragmentation, correlation coefficients are measured as a function of ionization photon energy as thresholds for the various fragmentation processes are scanned. For TW3+ parent ions, correlation coefficients correctly reflect switching between the benzyl radical loss and water loss fragmentation channels as the photon energy is increased. For T2Wn+ cluster ions, fragmentation contributes only about 20% to the correlation coefficient—the other 80% contribution is due to neutral cluster growth. The growth-dominated correlation coefficients scale approximately with the square root of the product of the two ion signal intensities and linearly with the ionization laser intensity, and therefore are not good relative measures of correlations between ions and signals of different intensities. A normalized covariance (covariance/product of signal intensities) is introduced to eliminate this dependence. The laser intensity [∼(signal product)1/2] independent component of the normalized covariance arises from ion correlation due to neutral cluster growth and the laser intensity dependent component of the normalized covariance arises from ion correlation due to cluster ion fragmentation. These findings are applied to study the cluster growth dynamics of AnArn and FSArn clusters. Covariance mapping shows that the broad intensity maxima in the mass spectrum of FSArn clusters are not caused by fragmentation but can be attributed to neutral cluster growth. The observed neutral cluster distribution appears to be a superposition of three broad, overlapping, log-normal-like distributions peaking around cluster sizes n=4, 8, 20. The difference between the overall shapes of the AnArn and FSArn mass distributions appears to be due to faster dimer and cluster growth kinetics for the FSArn cluster system. The growth kinetics for the latter two cluster systems can be fully explained and modeled by a simple closed form algebraic kinetic equation that depends on three parameters: dimer growth rate, overall cluster growth rate, and a cluster growth cross section that scales with cluster size.
Rotating-frame level matching study of CH3 tunneling in solid propionic acid
Authors: Choi Changho , Pintar M. M.
Published: July 2002

A study in the proton spin-rotating frame of the Zeeman-tunneling level matching of strongly hindered palmetic acid rotators CH3 is reported. A set of consecutive spin-torsional matching resonances was employed to saturate the tunneling polarization. The observed tunneling saturation rate as a function of the rotating magnetic field strength gave a well-defined resonance peak when the Zeeman splitting was equal to one-half of the tunneling frequency. This peak defines the tunneling frequency accurately. At this resonance a bimodal process of polarization transfer occurs; a fast one driven by the intra-CH3 proton–proton dipolar interaction and a slow one driven by the inter-CH3 dipolar interaction.
The nature of the “vibrational modes” of the network-forming liquid ZnCl2
Authors: Ribeiro M. C. C. , Wilson M. , Madden P. A.
Published: July 2002

The atomic motions responsible for features seen in the spectra of network-forming liquids above the relaxational frequency domain are conventionally described as vibrational, with the nature of the vibrational modes described either as local vibrations of the tetrahedral units of the network or as extended, phonon-like vibrations, depending on the standpoint of the observer. The validity of these apparently conflicting pictures of the atomic motions is examined by an instantaneous normal mode (INM) analysis of the network-forming liquid ZnCl2. Projections of the INM eigenvectors onto the idealized modes provides a way of quantifying the relationship between the actual and idealized modes. It is shown that individual INMs do not remotely conform to the modes envisaged in the simple pictures. Nevertheless, at the more coarse-grained level of identifying contributions to the density of states, the simple pictures do provide a qualitative guide to the character of the features which are observed. These findings apply to the modes calculated with several different models of the interactions, and appear to be fairly general. A detailed examination of the nature of the INMs which appear in the frequency domain of the boson peak observed in ZnCl2 is carried out and it is shown that these modes have a mixed acoustic wave and local torsional character.
Nonequilibrium statistical thermodynamics: Nonequilibrium grand ensemble method for sheared dilute gases
Authors: Farhat Hikmat , Eu Byung Chan
Published: July 2002

The nonequilibrium grand ensemble method previously reported is rigorously implemented for a nonequilibrium dilute gas mixture sheared in plane Couette flow geometry and analytic results are presented for the nonequilibrium thermodynamic quantities of the sheared gas. The calortropy is shown to contain all the constitutive information of the system. The notions of temperature and pressure for the nonequilibrium gas are examined on the basis of the calortropy calculated from the nonequilibrium grand partition function. The shear rate dependence of the nonlinear shear and first normal stress coefficients is calculated numerically and also by means of an iterative method. The first iterative solutions are found to give a qualitatively correct behavior for all Peclet numbers.
Suspensions of adhesive colloidal particles in sedimentation equilibrium in a planar pore
Authors: Jamnik A.
Published: July 2002

The sedimentation equilibrium of adhesive spheres mimicking a system of interacting spherical colloidal particles in suspensions in planar pores is considered. The density profiles of the adhesive fluid in a gravitational field, and its distribution between the pores and the homogeneous phase are studied on the basis of the solution to the hypernetted chain/Ornstein–Zernike equation, obtained by using the analytic results for the direct correlation function of the bulk fluid. In a few cases, the Percus–Yevick closure is also used. In the hard sphere limit, both integral equation approaches are compared with the results of a grand canonical ensemble Monte Carlo simulation. This comparison shows, in particular in narrow pores, that the hypernetted chain approximation provides a better estimate for the structure of the hard sphere fluid in the pore, as well as for its partitioning between the bulk and the confined system. The calculated density profiles consist of an oscillatory part near the lower wall revealing layering, and a monotonically decreasing tail approaching the upper wall, their shapes being very sensitive to the strength of interparticle attraction, the strength of the gravitational field, and the degree of confinement. Increasing interparticle adhesive attraction together with gravity results in the particles occupying the region of lower altitudes in the gap and being partly squeezed out from the pore.