bluuebunny/crossref_metadata_2025 huggingface dataset:
Lennard-Jones bridge functions and triplet correlation functions
Authors: Attard Phil
Published: July 2002

Bridge functions and triplet potentials of mean force are calculated for the Lennard-Jones fluid using the binodal chain closure approximation. Inclusion of the former in the hypernetted-chain closure for the pair correlation functions leads to substantially improved values for the virial pressure. The triplet function also appears reliable, being in reasonable agreement with the solution to the spherically inhomogeneous Ornstein–Zernike equation.
Accurate calculation and assignment of highly excited vibrational levels of floppy triatomic molecules in a basis of adiabatic vibrational eigenstates
Authors: Bačić Z.
Published: July 2002

We show that the triatomic adiabatic vibrational eigenstates (AVES) provide a convenient basis for accurate discrete variable representation (DVR) calculation and automatic assignment of highly excited, large amplitude motion vibrational states of floppy triatomic molecules. The DVR–AVES states are eigenvectors of the diagonal (in the stretch states) blocks of the adiabatically rearranged triatomic DVR-ray eigenvector (DVR–REV) Hamiltonian [J. C. Light and Z. Bačić, J. Chem. Phys. 87, 4008 (1987)]. The transformation of the full triatomic vibrational Hamiltonian from the DVR–REV basis to the new DVR–AVES basis is simple, and does not involve calculation of any new matrix elements. No dynamical approximation is made in the energy level calculation by the DVR–AVES approach; its accuracy and efficiency are identical to those of the DVR–REV method. The DVR–AVES states, as the adiabatic approximation to the vibrational states of a triatomic molecule, are labeled by three vibrational quantum numbers. Consequently, accurate large amplitude motion vibrational levels obtained by diagonalizing the full vibrational Hamiltonian transformed to the DVR–AVES basis, can be assigned automatically by the code, with the three quantum numbers of the dominant DVR–AVES state associated with the largest (by modulus) eigenvector element in the DVR–AVES basis. The DVR–AVES approach is used to calculate accurate highly excited localized and delocalized vibrational levels of HCN/HNC and LiCN/LiNC. A significant fraction of localized states of both systems, below and above the isomerization barrier, is assigned automatically, without inspection of wave function plots or separate approximate calculations.
Spin–orbit coupling in the copper monoxide molecule
Authors: Hippe D. , Peyerimhoff S. D.
Published: July 2002

Diagonal as well as off-diagonal spin–orbit matrix elements were calculated for the ground state and a large number of excited states of CuO. They are based on previously calculated CI wave functions and energies. The spin–orbit splitting of the ground state X 2Π is analyzed in detail. The excitation energies for a large number of excited states are predicted, for Ω quantum numbers 1/2, 3/2, 5/2, and 7/2. They are compared with experimentally observed states. The results suggest a new designation of the ‘‘δ 2Σ− ’’ state as 4Π contaminated by spin–orbit effects. Also ‘‘k 4Σ− ’’ is most likely the Ω=1/2 component of 2 4Π.
Driven chemical kinetics: Optimalization of catalytic action of membrane proteins by rectangular alternating electric field
Authors: Fuliński Andrzej
Published: July 2002

The chemical kinetics driven by external force in the form of a train of alternating rectangular impulses is discussed. The model of the conformational transition of a membrane protein exposed to an ac electric field, proposed by R. D. Astumian and B. Robertson [J. Chem. Phys. 91, 4891 (1989)], is reconsidered. On the example of this model we show that the use of the driving field in the form of rectangular impulses has two distinct advantages over the usual sinusoidal driving. The first one is that the use of a rectangular driving field makes it possible to obtain the exact solution of the basic kinetic equation of the system. This in turn enables one to write down the simple and very good approximate solution for any form of the driving field, better than the harmonic expansion used by Astumian and Robertson. A more important advantage is the greater flexibility of the rectangular driving, which makes possible the better optimalization of the process of interest. Astumian and Robertson demonstrated that the movement of charge within the catalytic cycle provides a mechanism for the enzyme to absorb energy from an ac electric field and to use that energy to enhance the catalyzed process. In this paper we show that the use of the driving ac field in the form of alternating rectangular impulses of variable duration and amplitude (instead of the usual sinusoidal modulation) leads to further optimalization of the process. The efficiency of the energy transduction, for example, can be increased from about 25% for sinusoidal driving to about 37% for suitably chosen alternating rectangular pulses.
Photoexcitation spectra of transition region species in reactions of potassium with sodium halides
Authors: Barnes Michael D. , Brooks Philip R. , Curl R. F. , Harland Peter W. , Johnson Bruce R.
Published: July 2002

Emission at 589.0 nm from Na atoms is observed when beams of K and NaX (X=Cl, Br, I) are crossed inside the cavity of a cw dye laser tuned to wavelengths not resonant with either the isolated reagents or products. This emission is interpreted as the signature of light absorption by a system in the process of chemical reaction, and the variation of this emission intensity with probe wavelengths is an excitation spectrum of ‘‘transition region species’’ formed in these reactions. Each spectrum displays a ‘‘blue’’ feature where the emission intensity increases sharply as the probe wavelength approaches the Na D lines. A second ‘‘red’’ feature is also observed in all of the spectra. This feature appears as a well resolved maximum for the Br and I systems peaked at ∼610 nm, and is much broader for the Cl system with the maximum at ∼640 nm. Definitive assignment of these features is not yet possible, but several possible mechanisms are proposed which may give insight into the reaction dynamics.
The reaction of FO with HO2: An examination of the coupling of FOOx species with the HOx cycle
Authors: Francisco J. S.
Published: July 2002

The abstraction reaction FO with HO2 is examined for the coupling of fluorine oxide radicals and for its stability. (AIP)
A simple analysis of the classical hard-sphere one-component plasma. II. Density functional theory
Authors: Penfold Robert , Nordholm Sture
Published: July 2002

The generalized van der Waals free energy density functional formalism of inhomogeneous fluids has been applied to study the thermodynamics and structural properties of a system of identical charged hard spheres neutralized by a uniform penetrating background. Nonlocal entropic effects are included through a simple density independent coarse-graining kernel and the hard-sphere truncation of the Coulomb interaction in the ionic atmosphere is accounted for within a mean-field approximation. In the first instance a parametric charge density of known form is introduced yielding essentially analytic results. Second, the functional is optimized with full variational flexibility to produce a theory directly analogous with the Poisson–Boltzmann approach to Coulomb fluids. A linearized version corresponding to a Debye–Hückel approximation is also discussed. The results compare favorably with Monte Carlo simulation in the regime of low to intermediate bulk density. At high volume fractions (>0.1) the functional becomes ill conditioned as the excluded volume effect is exaggerated and the mean-field hole correction fails to account for the hard-sphere structure.
Coincidence counting for the study of hydrocarbon ion desorption in plasma desorption mass spectrometry
Authors: Park M. A. , Schweikert E. A. , da Silveira E. F.
Published: July 2002

Abstract-coincidence counting techniques have been used in conjunction with time-of-flight mass spectrometry (TOF-MS) to study the impact of MeV/amu ions on surfaces. Several samples were studied including Au metal and a series of polymers. The results are consistent with a Coulomb explosion model for the secondary ion desorption in plasma desorption mass spectrometry. Correlations were observed between the emission of H+ and the emission of certain hydrocarbon ions from the polymer samples. These correlations indicate a chemical relation in the formation of H+ and other hydrocarbon ions and imply that low mass hydrocarbon ions are formed close to the center of the impact site during the collision of primary ion with the sample. Also, we have shown that the coincidence data contains information about the structure of the samples and that with coincidence counting, an estimate of the yield of secondary ions can be obtained without knowledge of how many primary ions strike the surface.
Relativistic effective potential quantum Monte Carlo simulations for Ne
Authors: Lao Miaozhen , Christiansen P. A.
Published: July 2002

We have carried out benchmark relativistic effective potential quantum Monte Carlo calculations for Ne. The resulting energy from the fixed-node diffusion QMC simulations was −34.960(3) a.u., as compared to the Hartree–Fock value of −34.647 a.u., giving a valence correlation energy of 0.313(3) a.u. This is in excellent agreement with previous work, and we believe accounts for 98% to 100% of the total valence correlation energy. We detect no significant error due to the effective potential approximation, and in particular, the nodeless pseudoorbitals in the 2s shell. We observed no difficulties related to the use of the conventional effective potentials in fixed-node QMC simulations.
Solvent mobility in poly(methyl methacrylate)/toluene solutions by depolarized and polarized light scattering
Authors: Floudas G. , Fytas G. , Brown W.
Published: July 2002

Depolarized Rayleigh and polarized Rayleigh–Brillouin scattering are employed to examine the mobility of toluene in solutions of poly(methyl methacrylate) (PMMA) and for PMMA concentrations: 0≤cPMMA≤1. The depolarized Rayleigh measurements were performed in the temperature range 20–120 °C with several interferometer spacings, whereas the polarized Rayleigh–Brillouin measurements were made in the range from −20 to 140 °C. The reorientation times of toluene, obtained from a single Lorentzian fit to the experimental depolarized spectra, are in good agreement with earlier NMR and dielectric relaxation data. Two Lorentzians were necessary to fit the depolarized spectra of the PMMA/toluene solutions at all temperatures. The broader Lorentzian is due to fast toluene reorientation and the narrower Lorentzian contains contributions from both slowly relaxing polymer chains and slow solvent reorientation in the macromolecular environment. The reorientation time and the fraction of the ‘‘mobile’’ toluene molecules are obtained, respectively, from the width and the integrated intensity of the broader Lorentzian. We discuss the reorientational dynamics of the broader Lorentzian with respect to the normalized solvent relaxation time τ/τ0, where τ0 refers to the neat solvent. The principal conclusions of this work with respect to solvent dynamics are (i) the exponential concentration dependence of τ/τ0 at high temperatures and for polymer concentration up to 70% which is similar for the three polymer/solvent systems employed so far, (ii) the presence of two time scales for the solvent reorientation in these homogeneous polymer solutions, and (iii) these ‘‘fast’’ and ‘‘slow’’ relaxation processes resemble the toluene and PMMA bare dynamics, respectively, and their relative amplitudes depend on temperature. The experimental results are discussed in terms of recent models of orientational relaxation in dense systems. Furthermore, the polarized Rayleigh–Brillouin measurements on the PMMA/toluene system revealed the presence of significant rotational mobility of toluene acting as an initiator for the broad hypersonic attenuation observed at GHz frequencies.
Monte Carlo sampling for atomic and molecular clusters with fixed energy and angular momentum
Authors: Dumont Randall S.
Published: July 2002

This paper presents a Monte Carlo method for sampling atomic and molecular cluster states according to the fixed energy and angular momentum ensemble, i.e., the EJ ensemble. Features of the methodology include the avoidance of numerical problems inherent in a straightforward implementation of Monte Carlo to EJ-ensemble averaging. In addition, qualitative characteristics of atomic momentum distribution within a cluster are extracted from exact analytic formulas, and illustrated numerically for argon clusters.
Molecular dynamics of discotics in the isotropic phase
Authors: Maliniak Arnold
Published: July 2002

Molecular dynamics (MD) simulation and multifield carbon-13 nuclear spin relaxation data are reported and used in an investigation of dynamic processes taking place in the isotropic phase, at 95 °C, of benzene-hexa-n-heptanoate (BHA7). The relaxation data are analyzed by using a dynamic model based on separation between a fast internal and a slow overall motion. The overall reorientations defined by the motion of the rigid aromatic core, occurs on nanosecond time scale, whereas the internal motion in the flexible, aliphatic side chains is much faster, with the correlation times in the picosecond range. Increased mobility and decreased orientational order along the chains—characterized by order parameters—is predicted by the experiments and also confirmed in the simulation. The translational diffusion constants are calculated in the MD simulation and measured using FT-PGSE technique. The molecular structure obtained from the MD simulation indicates that the aliphatic chains are situated alternatively up and down relative to the aromatic core. The radial distribution functions show that the molecules in the liquid are associated in pairs, with a parallel orientation of the planes defined by the aromatic core.
Translational energy dependence of the reaction cross section: Reactions of Sr+CH3I, CD3I, and CH3Br
Authors: Xu Qi-Xun , Mackay R. Scott , Aoiz F. Javier , Bernstein Richard B.
Published: July 2002

The excitation functions for the crossed beam reactions of Sr+CH3I, CD3I, and CH3Br have been investigated over the collisional energy (ET) range 0.1 to ∼1.0 eV. A highly sensitive single photon ionization technique has been developed to detect the reaction products (SrI and SrBr). The integrated total reaction cross sections (σR) exhibit very different behavior in reactions with CH3I vs CH3Br. In the reaction of Sr+CH3I, σR decreases sharply with increasing ET to a minimum near 0.35±0.05 eV and then gradually increases with increasing ET. In the isotopically substituted reactant, CH3I, no difference in the excitation function is seen within the resolution of the experiment. However, for the reaction Sr+CH3Br, σR exhibits a threshold around 0.12±0.05 eV and gradually increases with increasing ET. The present data for the Sr reactions with CH3I and CH3Br are compared with analogous alkali metal reactions and with several theoretical models.
Trajectory modeling of organic chemiionization reactions
Authors: Yen Y. F. , Cross R. J.
Published: July 2002

We have used a Monte Carlo trajectory calculation to model the chemiionization reaction, HI+B→I−+HB+. B is, in our experiments, a polyatomic amine, but the model treats it as a single atom. The reaction occurs on two potential energy surfaces, one covalent dissociating to the reactants, and one ionic corresponding to the products. Several restrictions were included in the model to account for the fact that B is really a molecule. Classical trajectories were then run at a range of energies and for several variations of potential parameters. The model showed that the reactive cross section rises rapidly as a function of translational energy and that the dynamic threshold energy is 0.38 eV higher than the minimum energy required to form the products. Product angular distributions at low energies show a peak in the forward direction but give a large amount of scattering at and behind the center of mass, in qualitative agreement with the experiments. As the energy increases, the distribution shifts to smaller angles. The Coulomb interaction in the ionic surface introduces some novel features not seen in most reactions.
Ultraviolet photon-induced interaction of Cl2 with GaAs(110): Dissociation by means of charge transfer
Authors: Liberman V. , Haase G. , Osgood R. M.
Published: July 2002

We have studied the UV photon-induced interaction of molecular chlorine with a well-defined GaAs(110) surface under ultrahigh vacuum (UHV) conditions. Without illumination, chlorine was found to adsorb both molecularly and dissociatively at 85 K. Illumination of a molecular chlorine-covered surface at 85 K with 193, 248, and 351 nm radiation led to the desorption of Cl atoms and to the formation of AsCl3. We suggest that the surface reaction, that is responsible for the AsCl3 formation, is initiated by molecular chlorine dissociation. While direct absorption by an individual Cl2 molecule may be the cause for the chlorine dissociation at 351 nm, we propose an intermolecular charge-transfer absorption within the condensed Cl2 overlayer for the cases of 248 and 193 nm. Support for this mechanism comes from the dependence of time-of-flight distributions and AsCl3 coverages on the molecular chlorine surface coverage, on the laser wavelengths, and on the use of different substrates.
Thermal fluctuations of a three phase contact line. I. Two fluid phases and one solid phase
Authors: Clarke Andrew S.
Published: July 2002

We describe a phenomenological model for the thermal fluctuations of the line of contact between two fluid phases and one solid phase. The line of contact between the three phases is constrained to lie along the solid surface, which is assumed to be smooth, flat, and horizontal with respect to gravity. In order to model the fluctuations of the contact line we must account not only for the fluctuations of the contact line itself, but also the corresponding fluctuations of the fluid–fluid interface. We develop a model for the energy of a superposition of sinusoidal fluctuations of all possible wavelengths. Our model for the fluctuation energy includes the effects of line tension, surface tension, and gravity. This fluctuation energy is then used in a modified capillary wave model based on the usual capillary wave model for a two-phase fluid interface to calculate the mean square magnitude of thermal fluctuations. We find that if the line tension is zero the rms fluctuation amplitude of the contact line is roughly proportional to the corresponding amplitude for a two-phase fluid interface, where the factor of proportionality is the reciprocal of the contact angle. Positive line tension significantly reduces the rms fluctuation amplitude near a second order wetting transition. If the line tension is negative, the fluctuation energy becomes negative for short wavelength fluctuations and small θ, and our model breaks down. We discuss the implications of our results.
Dielectric relaxation in polar and viscoelastic fluids with internal rotation
Authors: Dávalos-Orozco L. A. , del Castillo L. F.
Published: July 2002

Using the formalism of extended irreversible thermodynamics we derive the generalized constitutive equations for the polarization vector and the symmetric and antisymmetric parts of the total stress tensors (the sum of the Maxwell and viscous stress tensors) of a viscoelastic polar fluid. The analysis of these equations shows several interesting features for a permanent dipolar system. The diffusive transport of polarization charge and the diffusion of the transverse polarization component are two elementary physical processes in terms of which complex cases of dielectric relaxation can be described, taking into account both translational and rotational motions of the dipolar particles in a viscoelastic fluid. The calculations for several cases of the complex dielectric susceptibility are presented. In these results, the coupling between polarization and hydrodynamics predicts several modes of dielectric energy dissipation. These various channels are due to the intimate coupling between different degrees of freedom, taken into account by the constitutive equations. The calculation of the complex shear and rotational viscosities complete the presentation.
Low energy helium diffraction studies of CH3Br overlayers physisorbed on C(0001), NaCl(001), and LiF(001) surfaces
Authors: Robinson Gary N. , Camillone Nicholas , Rowntree Paul A. , Liu Gang-yu , Wang Jia , Scoles Giacinto
Published: July 2002

Low energy helium diffraction has been used to determine the unit mesh parameters of overlayers of CH3Br physisorbed on C(0001), NaCl(001), and LiF(001) at ≊35 K. CH3Br forms a uniaxially commensurate overlayer on C(0001) with unit mesh parameters 4.26 Å×6.75 Å. On NaCl(001), CH3Br forms a high coverage and a low coverage phase. The high coverage phase is incommensurate and has unit mesh parameters 4.54 Å×6.73 Å, whereas the low coverage phase is commensurate with a ((2)1/2×3(2)1/2)R45° unit mesh. The structure of CH3Br/LiF(001) is essentially the same as that of the high coverage phase of CH3Br/NaCl(001) with unit mesh parameters 4.52 Å×6.71 Å. The unit mesh parameters (with the exception of low coverage CH3Br/NaCl ) are very similar to the lattice parameters of the a-b [or (001)] plane of bulk crystalline CH3Br at ≊153 K. By analogy with the bulk crystal, it is likely that there are two molecules per unit mesh and that the CH3Br dipoles are nearly perpendicular to the surface and antiferroelectrically ordered. Our results suggest that the unit mesh for the low coverage phase of CH3Br/NaCl contains four molecules and that the molecular axes are parallel to the substrate surface. The implications of these results for photodissociation studies of physisorbed CH3Br are briefly discussed.
257 nm photoinduced chemistry of methyl iodide adsorbed on MgO(100)
Authors: Trentelman K. A. , Fairbrother D. H. , Strupp P. G. , Stair P. C. , Weitz Eric
Published: July 2002

The 257 nm photochemistry of CD3I adsorbed on MgO(100) has been investigated. The photofragments ejected along the surface normal were probed using resonantly enhanced multiphoton ionization spectroscopy coupled with time-of-flight mass spectrometry. Methyl radicals and atomic iodine fragments in both the ground I(2P3/2)≡I and spin–orbit excited I(2P1/2)≡I* states were observed as direct photofragments produced at the surface. Laser-induced desorption of methyl iodide competes with dissociation at the surface. The total cross section for removal of adsorbed CD3I from the surface at 257 nm is comparable to the gas phase cross section. A significant portion of the atomic iodine fragments remain trapped at the surface with preferential trapping of I*. Additionally, nonresonant I+ and I+2 signals were detected and initially observed to increase as a function of the irradiation time and then decrease with further irradiation. This behavior is shown to be indicative of photoinduced production and photoinduced removal of a surface intermediate. A surface reaction mechanism and the existence of a common precursor resulting from the interaction of adsorbed atomic iodine with adsorbed methyl iodide or methyl iodide clusters is postulated.
Vibrationally enhanced inelastic atom–molecule cross sections at very low energies
Authors: Keller H. M. , Külz M. , Setzkorn R. , He G. Z. , Bergmann K. , Rubahn H.-G.
Published: July 2002

Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mean collision energy of the order of 1 meV in a supersonic seeded beam arrangement. The initial vibrational state v and rotational state j is varied in the range 4≤v≤35 and 5≤j≤25. The Na2 molecules are excited by Franck–Condon pumping near the transition region to free molecular flow and the flux of molecules which survive in the level (v,j) is monitored far downstream behind a small aperture. It is shown that elastic processes do not contribute to the observed signal. The magnitude of the cross section for the sum of all inelastic processes is as large as several hundred Å2. It is found that the vibrational motion of the molecule, which carries an energy of up to 500 meV, strongly influences the collision dynamics. The rate constant increases with v by about 35% within the range of vibrational levels studied here. Some state-to-state rate constants are also reported. The experimental results are in good agreement with data from quasiclassical trajectory calculations on a semiempirical bond-distance-dependent potential surface. The observations can be rationalized by realizing that the long range attraction increases with vibrational excitation since the polarizability of the molecule increases. Furthermore, the vibrational excitation in combination with the relatively long interaction time due to the low collision energy enhances the probability for a change of the initial quantum state of the molecule.
Kinetics of molecular association and relaxation in a pulsed supersonic expansion
Authors: Emilsson T. , Germann T. C. , Gutowsky H. S.
Published: July 2002

The formation of polar Ar/HCN dimers and trimers in supersonic jets has been studied by observing the intensity of rotational transitions in the expanded pulsed jet as a function of input gas composition at a backing pressure of 1 to 2 atm. A programmable gas flow system was employed for accurate control of the composition of the gas mixture supplied to the pulsed nozzle. The formation of Ar–HCN was found to be first order in [Ar] and in [HCN]; (HCN)2 to be second order in [HCN]; (HCN)3 to be second order in [HCN]; and both Ar2–HCN and (HCN)2–Ar to be first order in [Ar] and in [HCN]. The three trimers are formed in two consecutive bimolecular steps at steady state, with (HCN)2 as the intermediate for (HCN)3, and Ar–HCN as the intermediate for both Ar2–HCN and (HCN)2–Ar. Less extensive measurements were made on the HCN/CO2 system, in which formation of the T-shaped HCN–CO2 dimer was found to be first order in [HCN] and in [CO2]. The rare gases He, first-run Ne (70% Ne, 30% He), and Ar were used as carrier gases. The carrier gas plays a major role as a third body in the formation steps. The relative effectiveness of the carrier gases as a relaxant in forming the dimers and trimers is Ar>Ne (first run)≫He. The method and results are discussed.
A coupled-cluster study of the electron affinity of the oxygen atom
Authors: Strout Douglas L. , Scuseria Gustavo E.
Published: July 2002

Despite quantum chemists’ best efforts, a highly accurate ab initio prediction of the electron affinity of atomic oxygen has remained elusive. In this study the coupled cluster method including all single, double, and perturbative triple excitations [CCSD(T)] is employed in conjunction with very large uncontracted Gaussian basis sets. A systematic shell-by-shell optimization at this level of theory leads to an optimal 23s26p10d5f3g basis set. Second-order configuration interaction (SOCI) calculations are also carried out and the coupled cluster results are found to be in good agreement with them. Our best theoretical prediction (1.415 eV) is 0.047 eV smaller than the experimental result but still marks a substantial improvement over previous high-quality calculations. The potential sources of error in our predictions are discussed.
Theory for laser induced dichroism and its effect on absorption, fluorescence, and Raman spectra
Authors: Myslinski Piotr , Koningstein J. A. , Shen Yizhong
Published: July 2002

The theory is presented for a two beam pump and probe laser experiment where a strong pumping beam induces a saturated anisotropy in the distribution of ground and excited state molecules in solution. The probing laser beam is (i) tuned to another transition of the molecule, or (ii) used to induce nonresonant ground state Raman scattering. Formulas are given for the laser induced anisotropic absorption for the general case, where the directions of transition moments of the molecule for the absorption of pump and probe light form an angle anywhere between 0° and 90°. Also, a comparison of the results to the existing theory for polarized fluorescence is made for weak intensity of the excitation source. Expressions are formulated for the polarization properties of the intensities of Raman bands modified by the presence of pumping light. The creation of the spatial anisotropy by the pump results in a Raman photoselection rule which permits one to distinguish asymmetric vibrational modes belonging to different representations of the same point group.
Study of the electrical conductivity of four-component critical microemulsions
Authors: Rebbouh N. , Hamelin J. , Bose T. K. , Thoen J.
Published: July 2002

Experimental data for the electric conductivity are reported for two microemulsions of n-dodecane, water, n-pentanol, and sodium dodecylsulfate with lower critical solution points along a critical line ending at a critical end point (CEP). One microemulsion (C) had a lower critical solution point (LCSP) close to the CEP. For the other microemulsion (A), the LCSP was away from the CEP. For a third microemulsion (D′) with a composition close to that of C, measurements were carried out near an upper critical solution point (UCSP) about 6 °C below the LCSP of sample C. For all three cases, an anomalous behavior was observed near the critical point. The anomaly for sample A can be well described in terms of a power law with Ising exponent values. This is not possible for sample C (near the CEP), where the anomalous behavior of σ can be better described with an expression recently proposed by Rebbouh and Lalanne [J. Chem. Phys. 90, 1175 (1989)] on the basis of a microscopic model for the merging of dynamic clusters near the CEP. On the basis of our additional results for the microemulsion D′, we can find no evidence for the influence of critical fluctuations near the UCSP on the anomaly in σ at the LCSP of sample C.
Anomalous pressure effects on optical spectra of hydrogen-bonded molecular chain solids: Possible formation of kink solitons
Authors: Moritomo Y. , Tokura Y. , Oohashi T. , Kojima T. , Itsubo A.
Published: July 2002

Infrared (IR) and visible absorbance spectra were measured as a function of hydrostatic pressure for five kinds of organic crystals composed of hydrogen-bonded molecular chains. An unconventional vibrational band is observed in the IR spectra around 1800 cm−1 commonly in these compounds and its intensity rapidly increases with pressure. We have ascribed the observed band to a proton vibration localized around kink-type defects or solitons in the hydrogen-bonded molecular chains. In accordance with the pressure-induced growth of the IR band, a new electronic absorption band appears in the visible region below the ordinary molecular excitation, suggesting a change in the electronic structures within the molecules around the soliton.
On the excluded volume of neighbor-avoiding lattice walks
Authors: Barrett A. J. , Benesch B. C.
Published: July 2002

Like real polymer chains and unlike self-avoiding lattice walks, neighbor-avoiding lattice walks exhibit a reduction or mediation of the effective excluded-volume interaction, and thus merit study as a model of real polymers. For small excluded volume, we extend the perturbation series of Domb and Joyce to neighbor-avoiding walks. For large excluded volume we compute exactly the excluded volume per lattice site for short chains and estimate it by Monte Carlo for longer chains. We discuss the significance of chain stiffness for lattice models and suggest how the correct two-parameter function might be determined numerically.
Dynamics of ion–molecule recombination. V. A study of energy transfer pathways
Authors: Hase William L. , Darling Cynthia L. , Zhu Ling
Published: July 2002

The Li++H2O→Li+(H2O) association reaction is used as a model system to study translation to rotation (T→R) and translation to vibration (T→V) energy transfer pathways which lead to the formation of ion–molecule complexes. Classical trajectories are used to study the association probability vs H2O bend frequency, impact parameter, and rotational temperature. For low and high bend frequencies association only occurs by T→R energy transfer. For intermediate bend frequencies a T→V energy transfer pathway becomes available. There is a well-defined peak in the association probability vs bend frequency, which apparently arises from a resonance between the Li++H2O relative translational motion and the H2O bend. Energy transfer and association are affected by the orientation of the H2O dipole during the collision. There is considerable structure in an association probability contour map plotted vs impact parameter and rotational temperature. In contrast to ion–molecule capture which is most probable for an impact parameter of zero and a rotational temperature of 0 K, the formation of an ion–molecule complex by energy transfer is most probable for nonzero values of impact parameter and rotational temperature. Both short-range and long-range Li+(H2O) complexes are formed. The long-range complexes are important at large rotational temperatures and impact parameters. A discussion is given of the sensitivity of the trajectory results to the treatment of the zero point energy motion of H2O.
Comment on: Anisotropic molecular reorientation of liquid benzene
Authors: Werbelow Lawrence , Thevand Andre
Published: July 2002

In a recent publication [J. Chem. Phys. 94, 3361 (1991)], the motional anisotropy of benzene was determined from nuclear spin relaxation measurements. However, interpretation of the data was made in context of an expression which is valid only in the limit of isotropic reorientation. In this Comment, appropriate equations are given and additional clarification of the importance of relaxation-induced multispin order is presented.
1
3C high resolution nuclear magnetic resonance studies in polycrystalline tetracyanoquinodimethane
Authors: Nunes T. , Vainrub A. , Ribet M. , Rachdi F. , Bernier P. , Almeida M.
Published: July 2002

1 3C high resolution NMR (nuclear magnetic resonance) studies of solid neutral tetracyanoquinodimethane are performed using different polycrystalline samples: (i) natural isotopic abundance; (ii) 15N isotope enriched; (iii) 13C isotope selectively enriched on the CN groups; and (iv) 13C isotope selectively enriched on the first neighboring carbons of the CN groups. 14N quadrupole effects are observed on 13C spectra of the CN groups, and the quadrupole coupling constant for 14N is obtained: −3.84±0.12 MHz. The 13C isotropic chemical shifts are found: 112.6 ppm for CN groups; 90.8 ppm for the carbons directly bonded to these groups; 132.0 and 133.2 ppm for protonated carbons; and 151.7 ppm for the other carbons on the quinone ring. The principal components of the chemical shielding tensor, the shielding anisotropy, and the shielding asymmetry factor are also obtained for the 13C nuclei in the fragments C–(CN)2, by graphical analysis of CP/MAS spectra. The following values are obtained for 13C nuclei on CN groups: 211±11 ppm, −85±22 ppm (σ⊥ and σ∥); −296±33 ppm (Δσ), and 0 (η). For the other carbon nuclei the values obtained for σ11, σ22, σ33, Δσ, and η are: 157±2 ppm, 75±2 ppm, 39±3 ppm, −77±4 ppm and 1.6±0.1, respectively. The reliability of these results is confirmed by a computer fit of the line intensities.
Out of resonance infrared fluorescence of SF6, 12CF3I, 13CF3I in quasicontinuum states. I
Authors: Angelie C.
Published: July 2002

IR fluorescence of SF6 has been monitored in the spectral range 700–1300 cm−1 with a high sensitivity device, after IR multiphoton excitation at several vibrational energies up to 17 000 cm−1. Preliminary results have also been obtained for 12CF3I and 13CF3I. The sensitivity achieved allows us to focus on the out of resonance fluorescence which turns out to be directly proportional to the local intramolecular couplings represented by the parameter γ(E→E′)=πρ(E′)‖V(E→E′)‖2. The method is partly free from an inhomogeneous contribution and the result can be compared to the Lorentzian model of intramolecular redistribution. It is shown that the γ(E→E′) parameter exhibits a hierarchy of structures, the strongest couplings leading mainly to a perturbative redistribution, while the weakest couplings are responsible for the dissipative redistribution. The homogeneous dissipative part γd appears to be smaller than 1–1.5 cm−1.
The Mueller matrix for elastic scattering in static magnetic field
Authors: Shi Yaoming , McClain W. M.
Published: July 2002

We present a central multipole calculation of the effect of a static magnetic field on nonresonant elastic scattering by an isotropic solution of molecules that are small compared to wavelength. Results are presented as perturbations to the observable Mueller scattering matrix. It is particularly interesting that perturbations appear in the six retardation elements of the matrix because these are otherwise exactly zero. The perturbation formulas for the whole matrix yield a number of relations that are empirically testable without prior knowledge of central multipole tensors α̃, G̃, and Ã or their magnetic field derivatives.
Collective excitations in liquid methanol: A comparison of molecular, lattice-dynamics, and neutron-scattering results
Authors: Alonso J. , Bermejo F. J. , García-Hernández M. , Martínez J. L. , Howells W. S. , Criado A.
Published: July 2002

The collective dynamics of liquid methanol-d4 is studied by means of molecular-dynamics simulation. The model potential is validated by means of lattice energy calculations and shows a very good agreement with the experimentally obtained crystal structure. Center-of-mass density and momentum fluctuations are investigated in the (Q,ω) region which is also accessible to inelastic neutron-scattering (INS) techniques. A simple viscoelastic model previously used for the analysis of INS data is tested against the dynamic structure factor computed from the simulation. A direct comparison with the INS results themselves is also made and qualitative agreement is found. Also, a tentative assignment of the peaks appearing in the current–current correlations is made on the basis of lattice-dynamics calculations for the polycrystalline low-temperature α phase.
Classical trajectory simulations of photodissociation of CH3Br at surfaces
Authors: Watson Joseph M. , NoorBatcha I. , Lucchese Robert R.
Published: July 2002

We have simulated the photodissociation of CH3Br adsorbed at a variety of surfaces. We have considered photodissociation at a smooth LiF (001) substrate and at three rough LiF surfaces which were constructed by removing atoms from the smooth surface. We have also considered photodissociation from several surfaces which have the same structure as the β phase of solid CH3Br to simulate dissociation from high coverages of the adsorbate where CH3Br ice is formed. The simulations were performed using the stochastic classical trajectory method. The asymptotic photofragment kinetic energy and angular distributions were determined and compared with the experimental results of Harrison et al. [J. Chem Phys. 89, 1475 (1988)] and Tabares et al. [J. Chem. Phys. 86, 738 (1987)]. When CH3Br is oriented with CH3 toward a surface, the CH3 kinetic energy distributions are shifted to much lower energies due to energy loss from multiple collisions with Br and the surface; the angular distributions are also significantly broadened. Much of the energy loss in these collisions goes into the translational mode of the Br fragments, causing the Br kinetic energy distributions to have a high-energy tail. When the molecule is in this orientation in a restricted geometry, collisions from the CH3 fragment lead to more effective energy transfer causing the peak of the Br kinetic energy distributions to be shifted to much higher energies and the corresponding angular distributions to become narrower. The main features of the experimental results from photodissociation of CH3Br adsorbed on LiF can be qualitatively explained using the results of the classical trajectory simulations.
Vibrational potentials of the low frequency out-of-plane motion in the ground and excited singlet electronic states of 9,10-dihydrophenanthrene
Authors: Zgierski Marek Z. , Zerbetto Francesco , Shin Young-Dong , Lim Edward C.
Published: July 2002

The absorption and single vibronic level fluorescence spectra of jet cooled 9,10-dihydrophenanthrene possesses long progressions of a low-frequency vibration identified as the out-of-plane nontotally symmetric CH2–CH2 bridge deformation. The intensity distribution in the observed progressions indicate that the molecule is nonplanar in the ground and the first two excited singlet electronic states described as the plus and minus combinations of the Lb states of the two perturbed benzene rings. The barrier height between the two equivalent nonplanar configurations, with the planar configuration being the transition state, is 2640 cm−1 in S0 and 1430 cm−1 in S1. These values are much larger than those in 9,10-dihydroanthracene. The observed intensity distributions are reproduced well by model calculations with double minimum potentials whose parameters are in good agreement with those obtained by geometry optimization and force field and vibrational frequency calculations by the quantum chemical force field π-electron method. The analysis yields the following values for the angle between the average planes of the two benzene rings: 18.4° in S0 and 12.1° in S1. It is shown that the origin of the S0→S2 transition lies ca. 240 cm−1 above the origin of the S0→S1 transition and that its doublet structure results from a near resonance vibronic interaction between the S1 and S2 states via a b mode of the calculated frequency of 221 cm−1.
Stationary solutions of the master equation for single and multi-intermediate autocatalytic chemical systems
Authors: Zheng Qiang , Ross John , Hunt Katharine L. C. , Hunt Paul M.
Published: July 2002

In this work, we test a hypothesized form for the stationary solution Ps(X,Y) of the stochastic master equation for a reacting chemical system with two reactive intermediates X and Y, and multiple steady states. Thermodynamic analyses and the exact results for nonautocatalytic or equilibrating systems suggest an approximation of the form Pas(X,Y)=𝒩 exp(−φ/kT), where the function φ is a line integral of a differential ‘‘excess’’ work Fφ, which depends on species-specific affinities. The differential Fφ is inexact. In a preceding paper, we have given an analytic argument for the use of the deterministic kinetic trajectory, connecting (X,Y) to the steady state (Xs,Ys) as the path of integration for Fφ. Here, we show that use of the deterministic trajectories leads to a potential φdet which is continuous across the separatrix between the domains of attraction of the two stable steady states in the model studied.

We compare the approximate form of Ps(X,Y) thus generated with numerical solutions of the time-dependent master equation in the limit of attainment of a stationary distribution. Because the time required for convergence to the stationary distribution scales as eN with the particle number N in cases with two stable steady states, the numerical work is limited to systems with 𝒪(10–100) X and Y particles. System size affects the accuracy of the approximation. To isolate system-size effects, we compare numerical solutions and the corresponding approximations to Ps(X) for two single-intermediate master equations, since the approximation becomes exact in the limit of large particle number for such equations. Based on these comparisons, for the systems with two intermediates, the agreement between the approximation and the numerical solutions is reasonable. The agreement improves as the number of particles increases in those test cases where it has thus far been possible to vary the system size over an order of magnitude. The results obtained by integrating along deterministic trajectories are better than those from straight-line or line-segment paths. The numerical work on small, single-variable systems with two stable steady states leads to two new observations: (i) the relative heights of the steady state peaks in the exact stationary distribution may invert as the system size increases and (ii) an approximation used commonly for particle counting may give results inconsistent with the exact stationary distribution when the particle number is small, while an alternative approximation improves the agreement.
Production and fragmentation of molybdenum oxide ions
Authors: Cassady Carolyn J. , Weil David A. , McElvany Stephen W.
Published: July 2002

The production and fragmentation of molybdenum oxide cluster ions MoxO+/−y have been studied by Fourier transform mass spectrometry. The cluster ions were generated by direct laser vaporization (DLV) and fast-atom bombardment (FAB) of molybdenum trioxide and molybdenum dioxide. The ionization method has a dramatic effect on ion formation with DLV producing primarily MoxO+y with high oxygen-to-metal ratios, while FAB yields Mo+x and MoxO+y with low oxygen-to-metal ratios. Direct emission appears to be an important mechanism in laser vaporization of molybdenum oxides, while the FAB results are consistent with ion formation by recombination reactions. For molybdenum dioxide, a comparison is made between DLV with pulsed CO2 and Nd:YAG lasers. The spectra obtained are similar; however, differences in (MoO3)+n, n=1–3, production with the two lasers may be related to sample absorption at the CO2 wavelength of 10.6 μm. The low-energy collision-induced dissociation of MoxO+y is dominated by loss of (MoO3)n. In addition, (MoO3)−n are the major anionic species produced by FAB and DLV. The prominence of the MoO3 unit in both ion production and fragmentation suggests that it may be significant in MoxO+/−y structures.
Reaction dynamics of D+H2→DH+H: Effects of potential energy surface topography and usefulness of the constant centrifugal potential approximation
Authors: Takada Shoji , Ohsaki Akihiko , Nakamura Hiroki
Published: July 2002

Two findings are reported for the D+H2→DH+H reaction on the basis of the exact quantum mechanical calculation for J=0, where J is total angular momentum. First, with use of the Liu–Siegbahn–Truhlar–Horowitz (LSTH) surface and the Varandas surface, we demonstrate that a rather small difference in potential energy surface (PES) induces a surprisingly large effect on reaction dynamics. Two origins of the discrepancy are pointed out and analyzed: (1) Noncollinear conformation in the reaction zone contributes to the reaction significantly despite the fact that the minimum energy path and the saddle point are located in the collinear configuration. (2) A difference in the distant part of PES also causes a discrepancy in the reaction dynamics indirectly, although this effect is much smaller than (1). Secondly, we investigate the validity of the constant centrifugal potential approximation (CCPA) based on the accurate results for J=0. The use of CCPA to estimate total cross section and rate constant is again proved to have practical utility as in the cases of the sudden and adiabatic approximations.