bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear line narrowing of excited state resonances in perylene doped poly(methylmethacrylate) using multiply enhanced nonparametric spectroscopy
Authors: Hurst Gregory B. , Wright John C.
Published: July 2002

Line narrowing of perylene vibronic transitions in poly(methylmethacrylate) (PMMA) samples has been demonstrated using multiply enhanced nonparametric spectroscopy. Three lasers are used to generate a four wave mixing signal that is resonantly enhanced by three simultaneous perylene resonances involving a vibrational state, a vibrationless excited electronic state, and a vibronic state of the excited electronic state. There is a correlation between the position of the electronic and vibronic states within the inhomogeneously broadened absorption spectrum. The correlation can be used to achieve line narrowing by fixing one of the resonances to a subset of perylene sites within the inhomogeneous profile so the other resonance of those sites will be selectively enhanced. The position of the line narrowed vibronic peaks depends upon the electronic detuning from the center of the inhomogeneously broadened absorption band as different sites within the band are probed. Excited state coherent processes involving resonance with higher singlet states are shown to be unimportant from the detuning dependence on a simultaneous vibrational enhancement.
Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol
Authors: Levstein Patricia R. , van Willigen Hans
Published: July 2002

Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.
Dynamics of concentration fluctuations in multicomponent polymer systems. Effect of hydrodynamic interactions
Authors: Vilgis T. A. , Benmouna M.
Published: July 2002

This paper discusses the effect of hydrodynamic interactions in multicomponent polymer systems. We show that the mobility matrix now has off diagonal elements when compared to the case where hydrodynamics is neglected. The wave vector dependence comes from the Oseen tensor which is convoluted with the static structure-factor matrix. The inclusion of Rouse mobilities is also discussed.
Monte Carlo study of diffusion of an ideal ring polymer in a network of obstacles on a cubic and a square lattice
Authors: Reiter Johannes
Published: July 2002

On a cubic and a square lattice, the statics and dynamics of a cyclic random walk in a regular network of uncrossable obstacles is investigated with a Monte Carlo algorithm where the chain moves along its contour by defect diffusion. For chain lengths of N=16 to N=128, the root-mean-square radius of gyration is found to vary as about N0.33 for both lattices. The exponent is decreasing with increasing chain length. For the same range of chain lengths, the center-of-mass diffusion coefficient is found to vary as about N−2.1 and N−1.9, and the integrated autocorrelation time of the radius of gyration is found to vary as about N2.7 and N2.6 for cubic and square lattices, respectively. The cyclic chain diffuses by tube leakage, with a diffusion coefficient which is similar to that of a linear random walk chain in a network.
Entrainment effect on dissociation for a chemical oxygen–iodine laser
Authors: Quan Victor , Copeland Drew A.
Published: July 2002

The effect of mass entrainment on the performance of a chemical oxygen–iodine laser is investigated analytically. It is found that the iodine dissociation rate is governed by a source term and an amplification factor. The results indicate that gradual mixing of reactants can lead to superior performance than premixing.
Obtaining accurate pressure second virial coefficients for methane from an a
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o pair potential
Authors: Gay David H. , Dai Houfeng , Beck Donald R.
Published: July 2002

The pressure second virial coefficients, including fourth-order many-body effects, have been calculated for methane and are found to agree with the experiment on the average, to 2.8% over the temperature range 110–623 K using the basis set of Sadlej. This is a major improvement over the usual 30%–40% accuracy of ab initio potentials and also has been attained by us for H2O. Monte Carlo simulations have also been performed with the potential and a C–C radial distribution function and the internal energy is obtained. The latter (−0.0757 eV/molecule) is in good agreement with experiment (−0.0738 eV/molecule).
The rotational Brownian motion of a linear molecule and its application to the theory of Kerr effect relaxation
Authors: Kalmykov Y. P. , Quinn K. P.
Published: July 2002

Exact expressions in the low-field limit for the Laplace transform of the Kerr effect response (KER), including inertial effects, of an assembly of needlelike dipolar anisotropically polarizable molecules are obtained using the Fokker–Planck–Kramers equation for the probability density function in configuration-angular velocity space. The spatial part of the solution of this equation is expanded as a series of associated Legendre functions. The coefficients of Legendre functions of the first and second order may be solved separately, using an expansion in Laguerre polynomials. The coefficients of the Legendre polynomials of order 1 and 2 are evaluated to yield the dielectric and KER after-effect solutions. The dielectric response is in full agreement with the result of Sack. The investigation shows clearly that the KER cannot be obtained by any simple transformation of the dielectric response unlike rotation in two dimensions.
Hard sphere colloidal suspension of macroparticles in a multicomponent solvent
Authors: Heno Y. , Regnaut C.
Published: July 2002

Colloidal suspensions are represented as a mixture of macrospheres immersed in a multicomponent solvent of small spheres. The behavior of the macrospheres is analyzed on the basis of the Percus–Yevick theory when the ratios of the small to large sphere diameters go to zero at fixed packing fractions. Within the Baxter formalism the recent results of Biben and Hansen for the one component solvent are generalized. It is shown that the macrosphere suspension reduces to a one component system in which Baxter’s function goes exactly over to that of a one component adhesive sphere fluid. The expression for the adhesiveness is exactly expressed in terms of the packing fractions of the solute and the solvent components. Such a model permits the interpretation of the flocculation phenomenon induced by the addition of free polymers in a colloidal suspension.
17O nuclear magnetic resonance in aqueous solutions of Cu2+ : The combined effect of Jahn–Teller inversion and solvent exchange on relaxation rates
Authors: Powell D. Hugh , Helm Lothar , Merbach André E.
Published: July 2002

The dynamics of water molecules in Cu2+ aqueous solutions were investigated by 17O NMR at several temperatures and magnetic fields. A theory is developed for scalar relaxation in the presence of Jahn–Teller inversion of the aquaion and water exchange between the aquaion and the bulk, for a model where the inversion is much faster than the exchange and the scalar coupling is negligible for axially bound water molecules. It is shown that this model is consistent with the experimental results, while the standard expressions, ignoring the influence of the Jahn–Teller inversion, are clearly inconsistent with the observed longitudinal relaxation rates. The inversion process is characterized by τ 298i=(5.1±0.6)×10−12 s and Ei=3.5±1.5 kJ mol−1. The water exchange process is characterized by k298ex=(4.4±0.1)×109 s−1, ΔH‡=11.5±0.3 kJ mol−1 and ΔS‡=−21.8±0.9 J K−1 mol−1.
Comment on: Nonequilibrium molecular dynamics calculation of self-diffusion in a non-Newtonian fluid subject to a Couette strain field
Authors: Sarman Sten , Evans Denis J. , Cummings Peter T.
Published: July 2002

We have solved the convective diffusion equation for a non-Newtonian fluid subject to a large Couette strain field. In this case the self-diffusion tensor is nonsymmetric. We show that the macroscopic mean square displacements that can be derived from the solution of this equation are consistent with the microscopic mean square displacements of Cummings et al.1 obtained by statistical mechanical methods. We also show that the solution of the convective diffusion equation only involves the symmetric part of the diffusion tensor. This implies that it is impossible to evaluate the antisymmetric part of the diffusion tensor with concentration measurements.
Kinetic theory of Lennard-Jones fluids
Authors: Leegwater Jan A.
Published: July 2002

A kinetic theory that describes the time evolution of a fluid consisting of Lennard-Jones particles at all densities is proposed. The kinetic equation assumes binary collisions, but takes into account the finite time duration of a collision. Furthermore, it is an extension of a kinetic equation for the square well fluid as well as the hard sphere Enskog theory. In the low density limit, the Boltzmann theory is obtained. It is shown that the proposed theory obeys all the conservation laws. The exchange of potential and kinetic energies is studied and it is shown that at high density this is a fast process. The dominant mechanism for energy exchange is found to be collisions at the strongly repulsive part of the potential that are disturbed by third particles. The kinetic equation is also used to calculate the Green–Kubo integrands for shear viscosity and heat conductivity. The major structures found in molecular dynamics simulations are reproduced at intermediate densities quantitatively and at high density semiquantitatively. It is found that at high density, not only correlated collisions have to be taken into account, but that even the concept of collisions in the sense of sudden changes in the velocity is no longer useful.
On the interpretation of radial distribution functions determined from integral equations
Authors: Root Leslie J. , Lovett Ronald
Published: July 2002

Nonlinear integral equations are commonly used as a basis for determining radial distribution functions in equilibrium fluids. We show that a solution to such an equation does not provide sufficient information to fix a unique equilibrium state because no explicit external field is prescribed. It follows that many physically distinct interpretations may be associated with each numerically generated solution. This is illustrated by showing that a family of physically distinct external fields can be associated with the same integral equation. In this context the occurrence of multiple solutions and the occurrence of solutions with long-range oscillations—both formally impossible events in the context of a well-posed problem—can be given a reasonable interpretation, for external fields in which these solutions are in fact appropriate can be identified. This shows, however, that the physical interpretation of numerically generated radial distribution functions is nontrivial. The problems are illustrated explicitly with a simple model integral equation which does exhibit multiple solutions and long-range solutions even though these are absent in the system the integral equation is intended to model.
The effect of polymer geometry on polymer–surfactant association in solution
Authors: Balazs Anna C. , Gempe Michael C. , Lentvorski Andrew P.
Published: July 2002

We developed a Monte Carlo computer simulation to examine the effect of polymer architecture on polymer–polymer and polymer–surfactant association in solution. Initially, we compared the self-assembly of polymers containing four associating sites with the assembling behavior of polymers containing only two such ‘‘stickers,’’ one at each end of the chain. This small difference in the number of stickers lead to marked differences in the properties exhibited by the two types of chains. At short times, the two-sticker polymers form a percolating network, which fragments into small clusters at long times. The four-sticker chains do not form such a network, but form relatively large, stable clusters. Surfactant molecules were then introduced into the system and we compared the polymer–surfactant association for both polymer architectures. The surfactants effectively break up the polymer clusters for the four-sticker architecture, while they enhance the cluster size found in the two-polymer solution. The implications of this behavior on the solution viscosity are discussed.
Collective diffusion in colloidal crystals
Authors: Rundquist Paul A. , Kesavamoorthy R. , Jagannathan S. , Asher Sanford A.
Published: July 2002

Electrostatically stabilized colloidal crystals prepared from aqueous suspensions of dyed polystyrene spheres compress locally when illuminated by intense laser light of wavelengths absorbed by the dye. The compression, which derives from the temperature dependence of the interparticle repulsive interaction, results in a concentration gradient where particles diffuse into the illuminated regions. The concentration gradient relaxes when the pump light is removed. We experimentally constrain particle diffusion to one dimension by creating a spatially periodic intensity grating which results in a periodic concentration profile. We measure the relaxation time constant of this concentration grating by monitoring the time-dependent transmitted intensity of a low intensity probe beam when the diffraction conditions are almost satisfied for the probe wavelength. The collective diffusion coefficient is found from the relaxation time constant.
Vibrationally resolved inelastic scattering and charge transfer in H+ +C2H2 collisions
Authors: Aristov Natasha , Niedner-Schatteburg Gereon , Toennies J. Peter , Chiu Ying-Nan
Published: July 2002

Differential total cross sections and time-of-flight spectra for inelastic scattering and charge transfer of protons with acetylene have been measured in a crossed molecular beam apparatus at 30 eV collision energy. Both vibrational excitation of the C–C stretch and C–H stretch vibrations are resolved in the time-of-flight spectra. The angular distributions indicate the existence of a potential well in the entrance channel of 1.2 eV. The angle dependence of the time-of-flight spectra reveal that the stiff C–H stretch vibrations are preferentially excited in grazing collisions by long range electrostatic multipole interactions, whereas the C–C stretch vibrations although softer are only excited by a short range interaction. The vibrational normal modes observed to be excited can be explained in terms of a proposed potential hypersurface and the associated nonadiabatic couplings and vibronic symmetry correlations.
Indicator matrices for potential instabilities in open systems
Authors: Poland Douglas
Published: July 2002

In a previous work we showed that the kinetics of a mass-action mechanism in the neighborhood of a steady state arbitrarily far from equilibrium could be expressed in terms of a sum of matrices, one of which we could identify as the source of all potential instabilities. In the present work we examine various truncated versions of the full linearized kinetic equation, including the maximum truncation that contains only the single interesting matrix, for several classic models exhibiting instabilities far from equilibrium in order to see at what level of truncation the true nature of the instability emerges. We utilize dimensionless ratios of steady-state concentrations and currents, which can be displayed in appropriate graphs, as variables in order to minimize the number of kinetic parameters in the analysis. In general, we find that the interesting matrix by itself usually correctly signals potential instabilities. Since this single matrix is often much simpler than the total kinetic matrix, its use simplifies the task of locating interesting regions (i.e., instabilities) in parameter space.
Amphiphilic membranes in thin films of a complex fluid: Statics and dynamics in lattice models
Authors: Chowdhury Debashish , Stauffer Dietrich
Published: July 2002

We investigate the equilibrium structure as well as a specific dynamical aspect of thin films of complex fluids containing amphiphilic membranes. We explore the structures of ternary microemulsions, confined in a pore, resulting from various different types of interactions of the confining wall with the molecules. We compute the concentration profiles of the three components, namely, oil, water and amphiphiles, by carrying out extensive Monte Carlo simulations of the Widom model and its recently proposed generalizations on a simple-cubic lattice of size Lx×Ly×Lz under appropriate boundary conditions. We investigate both the wide-gap geometry (Lx=Ly≂Lz≫1) and narrow-gap geometry (Lx=Ly≫Lz). We observe novel concentration profiles in the narrow-gap geometry when Lz is of the order of a few molecular layers (i.e., in micropores). We also compute the lifetime of thermodynamically unstable bilayers of a specific initial conformation in the Widom model of microemulsions. For identical initial conformations of these model bilayers, we study the dependence of the lifetime on (a) the initial concentration of the amphiphilic molecules, (b) the temperature, (c) the curvature elasticity of the amphiphilic membrane, and (d) the thickness of water layers inside the two constituent monolayers. We also compare the models and results in our computer experiment with those in the laboratory experiment on the rupture of the Newton black films and common black films.
Solitons in polyacetylene: Magnetic hyperfine constants from a
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Authors: Kirtman Bernard , Hasan Muhammad , Chipman Daniel M.
Published: July 2002

The spin distribution of the neutral soliton in trans-polyacetylene is controversial, as evidenced by the markedly different interpretations given by the various active experimental groups to their ENDOR measurements of proton hyperfine coupling (hfc) constants. To help resolve the discrepancies, ab initio electronic structure calculations were carried out for proton and carbon-13 hfc on the series of model polyene radicals C5H7,C9H11,...,C21H23 and the most important characteristics of the spin distribution were extrapolated to the long chain limit. Here σ spin polarization was treated through single excitation configuration interaction, both with respect to Hartree–Fock and to multiconfiguration reference spaces that include the most significant effects of π electron correlation. Particular attention was paid to basis set requirements necessary for proper treatment of hfc. It turns out that the ab initio hfc does not agree with that obtained by any of the major experimental groups. In particular, the calculated width of the bell-shaped distribution for the central and other even proton sites is much narrower than all experimental reports, and the anisotropic coupling calculated for the central proton along the fibril axis is much larger. Several assumptions commonly made in analyzing the ENDOR spectra are shown not to be quantitatively justifiable, but their improvement is unlikely to resolve the large discrepancies we have found between theory and current experimental interpretations. Thus, it appears likely that the experimental spectra are significantly influenced by environmental effects and cannot be adequately modeled by an isolated long chain polyene radical as heretofore assumed.
On an explicit semiclassical analytic formula for model potential parameters of diatomic molecules
Authors: Fairén Victor , López Vicente
Published: July 2002

We provide numerical values of parameters of a closed form semiclassical formula which gives the power expansion coefficients of the internuclear Born–Oppenheimer potential of diatomic molecules in terms of the experimental Dunham coefficients. These power expansion coefficients are useful as a first-order approximation to the potential and as best initial guess for the obtainment of more refined quantum values.
Charge mobility in σ-bonded molecules: The infrared spectrum of polymethylene chains in the solid and liquid phases
Authors: Castiglioni C. , Gussoni M. , Zerbi G.
Published: July 2002

The experimental observation we wish to explain and which is common in most of the polymethylene systems with no polar substituents is that the absorption intensity in the infrared of the CH2 wagging motions is extremely weak for solid all-trans chains and becomes much stronger in the liquid phase in which molecules are conformationally distorted. Using current theories for the interpretation of the infrared intensities, it is shown that the observed phenomenon arises from the fact that for all transplanar chains, a sizeable ‘‘σ conjugation’’ along the CC chain takes place. It is shown that ‘‘charge fluxes’’ from second nearest neighbors play the main role in determining such intensity behavior.
Rovibrational distributions of CH(A 2Δ,B 2Σ−) produced in energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) atoms to CH3 radical
Authors: Tsuji Masaharu , Kouno Hiroyuki , Nishimura Yukio , Obase Hiroshi , Kasatani Kazuo
Published: July 2002

Energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) to CH3 radical have been studied by observing emission spectra from excited fragments in the flowing afterglow. CH3 radicals were generated by the F+CH4 reaction. The CH(A 2Δ–X 2Πr:v′=0−2) and CH(B 2Σ−–X 2Πr:v′=0) emission systems were observed in the Ar(3P2) reaction, while only CH(A–X:v′=0,1) emission system was found in the Kr(3P2) and Xe(3P2) reactions. The nascent rovibrational distributions of CH(A:v′=0–2) were N0:N1:N2 =100(T0 =3400±400 K):28±5(T1 =1700±400 K):4±1(T2 =700±300 K) in the Ar(3P2) reaction and 100(T0 =1000±250 K):<5(T1 <800 K):0 in the Kr(3P2) and Xe(3P2) reactions. The rotational distribution of CH(B:v′=0) in the Ar(3P2) reaction was reproduced by a single Boltzmann temperature of 2800±300 K. The average fractions of total available energies channeled into vibration and rotation of CH(A,B) were less than 15% for all cases, suggesting that most of the available energies was deposited as relative translational energy of products and/or rovibrational energy of H2. The observed rovibrational distributions of CH(A) were colder than those predicted from statistical theories including and excluding the conservation of total angular momentum. The best agreement between the observed and statistical distributions was obtained for the mechanism giving CH(A,B) in two-body dissociation steps by assuming that 78–92% of the total available energy is released as kinetic energy in the first step, Rg(3P2)+CH3→CH*3+Rg, then the rest remains in the precursor CH*3 state as an internal energy.
Photodynamics of extended molecular systems. II. Application to the photodissociation of CH3I from vibrationally excited initial states
Authors: Srivastava Deepak , Micha David A.
Published: July 2002

Photodissociation cross sections have been calculated using a collisional time-correlation function (TCF) approach to light–molecule interactions. The method is based on separating the total TCF into the molecular dipole TCF of the target and the electric-field TCF of the light source. A norm-conserving time-dependent self-consistent-field approximation is implemented for the molecular dipole TCF of the target, which factors into a primary-region TCF, a secondary-region TCF, and a time-dependent phase factor. We present an application to the photodissociation of CH3I from vibrationally excited initial states with up to three quanta in the C–I and CH3 umbrella modes. The dynamics of energy transfer between the primary and the secondary region and its effect on the line shape functions for each initial vibrational state are systematically studied. The evolution of the primary- and secondary-region amplitudes is considered first for the initially excited (1,0) and (0,1) states of CH3I and shows the contrast between the fast oscillations of the primary-region amplitude and the slow oscillations of the secondary-region amplitude. A detailed study of the photodissociation dynamics of CH3I from the other vibrationally excited initial states is presented next. We fixed the number of vibrational quanta in the secondary-region dynamics, and studied the effect of increasing the vibrational excitation energy in the primary-region dynamics. Some of the vibrational energy given to the primary-region dynamics is transferred to the secondary-region dynamics. The reverse situation, vibrational energy transfer from the secondary to the primary region, is also obtained by fixing the vibrational quantum states in the primary-region dynamics and varying the excitation energy in the secondary region.
Charge and spin transfer and optical properties in conducting porphyrin compounds
Authors: Liang X. L. , Flores S. , Ellis D. E. , Hoffman B. M. , Musselman R. L.
Published: July 2002

Anisotropic materials made from stacked macrocycles have potential applications as organic conductors and nonlinear optical materials. Using self-consistent-field local density theory, we have been exploring the ground and excited state electronic structures of metal substituted tetraazaporphyrins, phthalocyanines, and tetrabenzoporphyrins. The calculated and measured polarized optical absorption peaks are in satisfactory agreement. The theoretical dipole oscillator strengths provide absolute intensities for verification of transition assignments, which are helpful for identifying low-lying states and excitation mechanisms.
Rotational spectra and the phase diagram of CH4/Ar mixtures
Authors: Prager M. , Asmussen B. , Press W. , Blank H. , Carlile C. J.
Published: July 2002

The rotational dynamics, i.e., tunneling and almost free quantum rotation at low temperature and rotational diffusion at higher temperature, have been studied in methane–argon mixtures in the whole concentration range and at temperatures up to T=40 K. Changes of the rotational spectra allow parts of the phase diagram to be established in detail. At T=5 K, a transition from a perturbed partially ordered phase II to be a disordered glassy phase has been observed at an argon concentration of 10%. There is a small regime of coexistence of these two phases. The miscibility gap at larger argon concentrations is wider than determined by x-ray diffraction with methane being almost insoluble in argon.
Nonlinear Kerr effect relaxation including inertial effects in alternating fields
Authors: Déjardin Jean-Louis
Published: July 2002

The complex birefringence of a dielectric medium is found for the nonlinear stationary response arising from the application of a unidirectional field superposed on an alternating field. Effects arising from dipolar inertia are included. The starting point is the Fokker–Planck–Kramers equation which is solved by assuming that the molecules are polar and anisotropically polarizable and are compelled to rotate in two dimensional space (plane rotator). Kerr effect relaxation is characterized by the ensemble average 〈cos 2θ〉(t). Time evolution of this gives rise to harmonic components in ω and 2ω. They are given to second order in the driving field. Cole–Cole diagrams for each component are presented for various values of P, the ratio of the induced dipole moment to the permanent dipole moment, and τI/τ, the inertial parameter (τI and τ being the friction time and the Debye relaxation time, respectively). It is shown how these diagrams behave in the high-frequency region, and a method of extracting information about molecular parameters such as τI, τ, and P from them is proposed.
Orientational order in two-component plastic crystals 2,2-dichloropropane–carbon tetrachloride
Authors: Akimov M. N. , Bezrukov O. F. , Chikunov O. V. , Struts A. V.
Published: July 2002

1H nuclear magnetic resonance (NMR) spectra and birefringence of noncubic plastic single crystals (CH3)2CCl2–CCl4 have been measured. The investigation has demonstrated that in these crystals weak preferred orientations of molecules were observed. Concentration dependence of orientational ordering matrix of (CH3)2CCl2 molecules has been obtained from computer simulations of NMR line shape and birefringence data. It has been found that all ordering matrix elements become zero at mole fraction of 2,2-dichloropropane 0.5.
Potential surface symmetry and vibronic wave functions for methane cation
Authors: Reeves Melissa S. , Davidson Ernest R.
Published: July 2002

In attempting to understand the ground vibronic state of CH+4, a new formalism was developed for discussing molecular symmetry groups. Various diabatic basis sets and approximations for Jahn–Teller vibronic problems were examined, using the relatively simple E⊗e case. Finally, the symmetries and form of the ground vibronic states of CH+4 were produced.
Bound excited electronic states of anions studied by electron photodetachment spectroscopy
Authors: Brinkman Elizabeth A. , Günther Erhard , Brauman John I.
Published: July 2002

Excited electronic states of the radical anion of 7,7,8,8- tetracyanoquinodimethane (TCNQDN) which are stable with respect to electron loss have been observed by electron photodetachment spectroscopy. An onset for photodetachment is seen at 1.2 eV, considerably below the known binding energy (2.8 eV). We propose a sequential two-photon detachment process, supported by intensity dependence studies. TCNQDN is representative of a general class of anions which we believe will have such bound states.
Interaction of monovalent ions with the water liquid–vapor interface: A molecular dynamics study
Authors: Wilson Michael A. , Pohorille Andrew
Published: July 2002

Results of molecular dynamics studies on the ions Na+,F−, and Cl− near the water liquid–vapor interface are reported. The free energies required to move the ions to the interface are presented and shown to depend on the sign of the ionic charge, and not the size of the ion. F− and Cl− can approach to within 2 molecular layers of the interface without incurring a significant change in free energy, while it costs about 2.5 kcal/mole to move Na+ this same distance. The free energy differences between the cation and the anions arise from the interaction of the ions with the water molecules in the interfacial region. These water molecules are oriented with a slight preference for their molecular dipoles to point toward the liquid. Thus, the anions approaching the interface disrupt the water structure less than does the cation. The calculated free energy curves were compared with predictions of simple dielectric models. It was shown that these models do not provide a good description of ions at the water surface. The ions were found to retain their first solvation shells at the interface. The anions also retain part of their second solvation shells, while Na+ does not. As a result, a larger bulge in the water surface is observed above the anions than above Na+. The lateral mobilities of the ions increase at the interface, in qualitative agreement with predictions of hydrodynamic models.
Molecular dynamics simulation of a dense model bilayer of chain molecules with fixed head groups
Authors: Biswas A. , Schürmann B. L.
Published: July 2002

Molecular dynamics (MD) simulations have been performed on a model bilayer of 48 chain molecules with fixed head groups and periodic boundary conditions in the lateral direction. The effects of packing density and temperature on the extent of spatial and temporal correlations in the ensemble have been studied by analyzing the trajectories from the MD simulations. Quantities such as order parameters, number density contours, trans–gauche statistics and structure factors are evaluated as a function of distance from the plane containing the constrained head groups. The ensemble having the smallest area of 20.625 Å2/molecule can be characterized as having three dimensional order, even at high temperatures, with a few gauche defects. Decreasing the packing density of the chains results in the increase of the number of gauche defects, especially at the chain extremities along with a decrease in the extent of spatial and temporal correlations. Interestingly, shorter interchain spacings are evident in the middle of the layer which can be attributed to clusters of chains lying alongside less dense domains. At temperatures lower than 300 K, the ensemble having the largest area of 26.125 Å2/molecule reveals a rather complex interplay between chains wherein the lateral arrangement of the methylene units across the layer is transient and varies as a function of distance from the plane of the head groups. At temperatures higher than 300 K, thermal motion increases chain separation which effectively suppresses cluster formation.
Influence of additives on crystallization kinetics: Comparison between theory and measurements in aqueous solutions
Authors: Kresin M. , Körber Ch.
Published: July 2002

Differential scanning calorimetry (DSC) was applied to study the crystallization kinetics induced by homogeneous nucleation mechanisms. Aqueous glycerol solutions were emulsified to create a large number of droplets free of heterogeneous nucleation sites. The nucleation rates were calculated from scanning experiments according to an established method. The results, which are in good accordance with the classical nucleation theory, were used to determine the system-dependent nucleation parameters. The thus determined dependencies were then applied to calculate nucleation rates in the vicinity of the glass transition temperatures, neglecting the influence of the increasing viscosity on the nucleation rates. In the second part, a set of experiments was performed with microemulsions in the isothermal mode of the DSC by recording the release of latent heat after rapidly lowering the temperature into the undercooled range. An Avrami-type growth law was adapted to the thermograms, treating the case of spherical diffusion controlled growth from small dimensions implementing the estimates for the nucleation rates. Applying this theory to calculate time-temperature-transformation (TTT) curves, good agreement with the measurements was achieved. The theoretical description of the crystallization process has the advantage of not being restricted to microemulsions, which generally exhibit a different kinetic behavior due to growth limiting surfaces. TTT curves for bulk samples with any given crystalline fraction can thus be calculated. This puts the estimates for ‘‘critical’’ cooling rates required for vitrification on a more reliable basis.
Comment on: Time dependent quantum mechanics using picosecond time steps: Application to predissociation of HeI2
Authors: Sharafeddin Omar A. , Bowen H. Ferrel , Kouri Donald J. , Das Sanjukta , Tannor David J. , Hoffman David K.
Published: July 2002

The wave packe propagation study of the HeI2 system reported by Das and Tannor1 is commended upon. [AIP]
Polaron and size effects in optical line shapes of molecular aggregates
Authors: Lu Ning , Mukamel Shaul
Published: July 2002

Optical absorption and fluorescence line shapes of molecular aggregates are calculated using a variational method and the dynamical coherent potential approximation (DCPA) which account for strong exciton–phonon coupling. The formation and the quantum size effects of excitonic polarons are studied.
Molecular hydrodynamic theory of non-Markovian collective orientational relaxation in dense dipolar liquids
Authors: Vijayadamodar G. V. , Bagchi B.
Published: July 2002

A microscopic study of the non-Markovian (or memory) effects on the collective orientational relaxation in a dense dipolar liquid is carried out by using an extended hydrodynamic approach which provides a reliable description of the dynamical processes occuring at the molecular length scales. Detailed calculations of the wave-vector dependent orientational correlation functions are presented. The memory effects are found to play an important role; the non-Markovian results differ considerably from that of the Markovian theory. In particular, a slow long-time decay of the longitudinal orientational correlation function is observed for dense liquids which becomes weaker in the presence of a sizeable translational contribution to the collective orientational relaxation. This slow decay can be attributed to the intermolecular correlations at the molecular length scales. The longitudinal component of the orientational correlation function becomes oscillatory in the underdamped limit of momenta relaxations and the frequency dependence of the friction reduce the frictional resistance on the collective excitations (commonly known as dipolarons) to make them long lived. The theory predicts that these dipolarons can, therefore, be important in chemical relaxation processes, in contradiction to the claims of some earlier theoretical studies.
Laser induced thermal desorption: A time resolved study
Authors: Rosenzweig Z. , Asscher M.
Published: July 2002

Real time monitoring of laser induced thermal desorption of NH3 from Re(0001) at ns time scale was performed utilizing optical SHG. Coverage dependent desorption kinetic parameters determined under equilibrium conditions were found to correctly describe the 8 orders of magnitude faster desorption rates. The role of strong repulsive interactions is discussed.
Molecular open shell configuration interaction calculations using the Dirac–Coulomb Hamiltonian: The f 6-manifold of an embedded EuO9−6 cluster
Authors: Visser O. , Visscher L. , Aerts P. J. C. , Nieuwpoort W. C.
Published: July 2002

We present results of all-electron molecular relativistic (Hartree–Fock–Dirac) and nonrelativistic (Hartree–Fock) calculations followed by a complete open shell configuration interaction (COSCI) calculation on an EuO9−6 cluster in a Ba2GdNbO6 crystal. The results include the calculated energies of a number of states derived from the f6−manifold and 5D–7F luminescence transition wavelengths. The calculations were performed using the molecular Fock–Dirac (molfdir) program package developed in our laboratory. The theory and methods employed in this package are briefly described. The physical models used to analyze the Eu3+ impurity states range from a bare Eu3+ ion to an EuO9−6 cluster embedded in a Madelung potential representing the rest of the crystal. We show that it is necessary to use the embedded cluster model to get a reasonable description of the crystal field splittings of the states arising from the f6-manifold. Our results indicate that the calculated splittings are very sensitive to the orbitals used. It is therefore essential that relativistic orbitals be used from the outset.
Laser preparation and probing of initial and final orbital alignment in collision-induced energy transfer Ca(4s5p, 1P1) +He→Ca(4s5p, 3P2)+He
Authors: Smith Christopher J. , Driessen Jan P. J. , Eno Larry , Leone Stephen R.
Published: July 2002

In a crossed beam atomic energy transfer experiment, relative cross sections are measured between initial and final magnetic substates of atomic orbitals in a three vector correlation experiment. A pulsed laser beam prepares Ca(4s5p 1P1) in a single magnetic sublevel ‖j1mi〉 with respect to the laser polarization vector. Subsequent collision with He at a well-defined relative velocity yields Ca(4s5p 3P2). The near-resonant Ca(4s5p 3P2) is probed by a second polarized pulsed laser, revealing its magnetic sublevel ‖j2mf〉 distribution with respect to the probe laser polarization vector. The experiment is analyzed in the collision frame where the direction of the initial relative velocity vector serves as the quantization axis. In this frame, the initial and final Ca states are characterized by substates ‖j1m1〉 and ‖j2m2〉, respectively. Fourteen collision frame cross sections are needed to describe the energy transfer completely. Eight of these cross sections are real and positive (conventional type—describing population transfer) and six are complex (coherence type—containing phase information). By symmetry, only 15 unique parameters for the real and imaginary parts of the cross sections are required, nine of which are obtained here using linear polarizations and collinear laser beams.

Possible cases for circularly polarized light are also tabulated; measurement of these cases would provide several more parameters. For initial parallel preparation of the 1P1 p orbital with respect to the relative velocity (m1=0), the final 3P2 state is highly aligned in the m2=±1 and m2=±2 sublevels. Initial preparation of the p orbital perpendicular to the relative velocity vector (m1=±1) produces an aligned 3P2 state with greater population in the m2=0 and m2=±1 sublevels compared to the m2=±2 sublevels. Using the coherence information and symmetry, orientation cross sections are obtained into the m2=±1 sublevels (i.e., m preserving 1→1 and m-changing 1→−1) and are nearly identical. Preparation of the Ca p orbital perpendicular to the initial relative velocity enhances the state-specific collisional transfer of 1P1→3P2 by a factor of 2.2±0.2 over initial parallel preparation.
A theoretical study of the shake-up intensity of benzene
Authors: Sjögren B.
Published: July 2002

A semiempirical method, based on INDO (intermediate neglect of differential overlap), is employed to calculate the shake-up spectrum of the C1s peak of benzene. Intensities are computed within the framework of the equivalent-core approximation, including correlation in both final and initial states. The method yields intensity ratios which are better than previously obtained theoretical results and in close agreement with experiment. The factors contributing to the intensity of individual peaks are analyzed and an assignment is proposed for the main features in the experimental spectrum.
Acid-base chemistry in the gas phase. The c
i
s- and t
r
a
n
s-2-naphthol⋅NH3 complexes in their S and S1 states
Authors: Plusquellic D. F. , Tan X.-Q. , Pratt D. W.
Published: July 2002

A unique view of the nascent acid-base reaction between 2-naphthol and ammonia along the proton transfer coordinate is provided by analyses of the rotationally resolved S1←S0 electronic spectra of their hydrogen bonded complexes cis- and trans-2HNA in the gas phase. Both complexes, in both electronic states, have structures in which ammonia, acting as a base, forms an in-plane hydrogen bond with the hydroxy hydrogen atom of 2-naphthol. The ground state O–H⋅⋅⋅N heavy atom separations are R=2.77 Å in cis-2HNA and R=2.79 Å in trans-2HNA. Electronic excitation of the significantly more acidic S1 state of 2-naphthol produces large decreases in R in both complexes. S1 cis-2HNA has R=2.62 Å and S1 trans-2HNA has R=2.57 Å. Comparing these results to the Lippincott–Schroeder potential for the hydrogen bond shows that there is little change in the vibrationally averaged position of the hydroxy hydrogen atom. But decreasing R produces significant decreases in the barrier to proton transfer, in the distance from reactant to product along the reaction coordinate, and in the energy difference between them. We thus conclude that whether or not such transfer occurs is primarily dependent on the ability of the two heavy atoms to come into close proximity during the early stages of the reaction, a condition that is not satisfied in either cis- or trans-2HNA, in either electronic state. This view is supported by observed changes in the shapes of the potential surfaces along the NH3 torsional coordinate that occur on S1←S0 excitation of the two complexes.
Chemical and phase transformations of thiophene at high pressures
Authors: Pruzan Ph. , Chervin J. C. , Forgerit J. P.
Published: July 2002

Phase transformations of thiophene (C4H4S) and deuterated thiophene (C4D4S) were studied by visual observations and Fourier-transform infrared spectroscopy up to 30 GPa. At room temperature, thiophene freezes at 0.4 GPa and undergoes a solid-to-solid transition around 6 GPa and probably another one at 20 GPa. It is shown that an irreversible incomplete transition occurs at 16 GPa. The product of the reaction, a white solid, can be recovered at atmospheric pressure. The infrared analysis of the recovered product gives evidence of a polymer formed by coupling between rings derived from thiophene, but with saturated bonds. Physicochemical arguments suggest the formation of a fibrous compound.
Ornstein–Zernike derivative relations and thermodynamic functions
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

The consequences of the derivatives of the Ornstein–Zernike relation with respect to the density (ρ) and temperature (T) are examined. An approximate closure for the Ornstein–Zernike relation is used to evaluate the derivatives of the pair-correlation function to all orders without knowing explicitly the correlation functions higher in order than the pair-correlation function. The first- and second-order thermodynamic (ρ or T) derivatives of the pair-correlation function are calculated and compared with the experiments of Egelstaff et al. In addition, the thermodynamic functions involving these derivatives are evaluated to demonstrate the utility and accuracy of the method.
Competition between exchange and inelastic T→V, R in Cl+HCl collisions
Authors: Fulmer John P. , Aker Pamela M.
Published: July 2002

Quasiclassical trajectory (QCT) calculations have been used to simulate the dynamics of 0.62 eV Cl′+HCl (v=1, J=3) collisions. The calculations have been performed to test the feasibility of studying this reaction experimentally. The results show that essentially complete product state distributions for both reactive exchange and inelastic T→V, R collisions can be extracted in an experiment that uses stimulated Raman excitation to prepare vibrationally excited, isotopically selected reactants.