bluuebunny/crossref_metadata_2025 huggingface dataset:
Comment on ‘‘Fluid wetting on molecularly rough surfaces’’ [J. Chem. Phys. 103, 8201 (1995)]
Authors: Nijmeijer M. J. P. , Bruin C.
Published: July 2002

In molecular dynamics (MD) simulations of drying and wetting by a fluid on both molecular rough and smooth substrates, Tang and Harris calculate the surface tension of the substrate-fluid interface incorrectly. Their microscopic expresesion for the surface tension contains a contribution which, as has been shown previously, should not be there. We carried out MD simulations to estimate the magnitude of this contribution and find that it is non-negligible. However, even with this contribution taken into account, we do not manage to reproduce the results of Tang and Harris, that show suspect behavior in various situations.
Photoionization spectra and ionization thresholds of CH3SO, CH3SOH, and CH3SS(O)CH3
Authors: Hung Wen-Ching , Shen Min-yi , Lee Yuan-Pern , Wang Niann-Shiah , Cheng Bing-Ming
Published: July 2002

Photoionization efficiency (PIE) spectra of CH3SO, CH3SOH, and CH3SS(O)CH3 were measured in the spectral range 110–150 nm by means of a discharge flow system coupled to a photoionization mass spectrometer that employs a synchrotron radiation source. These species were generated by reacting O atoms with CH3SSCH3 in the flow tube; a microwave discharge was employed to produce O atoms from O2. Alternatively, C2H5SCH3 replaced CH3SSCH3 to form CH3SO, CH3SOH, and other products. Ionization thresholds of 8.99±0.02, 8.67±0.03, and 8.82±0.05 eV were determined for the first time for CH3SO, CH3SOH, and CH3SS(O)CH3, respectively. The former two values agree well with theoretical calculations with the G2 method. The latter value fits satisfactorily with those predicted for CH3SS(O)CH3 with the G2MP2 method; the G2 method was not employed in this case due to the numerous atoms in this molecule. Ionization thresholds of CH3S and CH3S2 were also determined to be 9.27±0.03 and 8.62±0.05 eV, respectively, in agreement with recent experimental reports.
r
structure of difluoromethanimine, CF2=NH, from microwave spectra of isotopic species
Authors: Groner P. , Durig J. R. , DesMarteau D. D.
Published: July 2002

The microwave spectra of the 13C-, 15N-, and 15N, D-substituted species of difluoromethanimine, CF2=NH, have been measured and assigned, and the rotational, centrifugal distortion, and quadrupole coupling constants have been determined. The ground state rotational constants of four isotopic molecules were sufficient to determine six independent parameters of the r0 structure. The difference between the C–F distances was indeterminate. The structure and the quartic centrifugal distortion constants are compared with the results from ab initio calculations with and without electron correlation. The r0 C=N double bond distance is 1.240(5) Å, halfway between the distance determined from single crystal x-ray diffraction and the ab initio distance (MP2/6-31G*). The molecular electric dipole moment forms an angle of about 8° with the direction of the N–H bond.
Dynamics of an adsorbed patch and a model for spreading of films of ultralow thicknesses
Authors: Neogi P.
Published: July 2002

The dynamics of ultrathin films of liquids with vanishing 3D volatility, on solid surfaces, has been modeled by assuming that the films can be taken to be adsorbed patches spreading under surface diffusion. When the phase behavior of the adsorbed films is taken into account, it is seen that a vapor patch will spread without limits and evolve into a Gaussian profile. For a liquid film which has low 2D volatility, a patch will appear to equilibrate and the surface ahead will remain dry. This is the ‘‘pancake,’’ a case where a wetting liquid stops spreading. The spreading kinetics is generally seen to be the usual square-root-of-time type, although exceptions are also seen. A simple case of non-Fickian diffusion has been included, but fails to give anticipated results at least in a simple model.
Saturation behavior in degenerate four-wave mixing with nonmonochromatic, non-Lorentzian fields
Authors: Tang Y. , Reid S. A.
Published: July 2002

We report experimental studies of the degenerate four-wave mixing (DFWM) response in weak field (no saturation) and strong field (saturation) regimes for the case of nonmonochromatic, non-Lorentzian fields. The saturation behavior of line-center intensities and linewidths of rovibrational transitions in the ν3/ν2+ν4+ν5 Fermi dyad region of jet-cooled C2H2 are in good agreement with nondegenerate two-level saturation models, suggesting that the on-resonance DFWM response is weakly dependent on laser line shape. We find that DFWM line shapes in the weak field regime are determined by the infrared power spectrum and cubic intensity dependence, as illustrated using broadband (0.35 cm−1) and narrowband (0.04 cm−1) infrared radiation. The saturation intensity (I0sat) is observed to decrease with decreasing bandwidth, in qualitative agreement with theoretical predictions.
The symmetry breaking problem in the triflouride anion: A multireference approach
Authors: Mota Fernando , Novoa Juan J.
Published: July 2002

The multireference MCSCF and MCSCF+MP2 methods were applied to the study of the optimum geometry of the F−3 anion, its vibrational frequencies and its dissociation energy into F2+F−. The system has a strong multireference character. The optimum geometry of the F−3 anion doing a MCSCF(3,4) or a MCSCF(9,16) computation presents a symmetry breaking from D∞h→C∞v. Using other active spaces, like the one constructed following the orbital doubling technique, the symmetry breaking problem is solved. Finally, the accurate computation of the dissociation energy requires the inclusion of the dynamical correlation: the MCSCF dissociation energy is about 1/3 of the final MCSCF+MP2 result. The best multireference results obtained here agree very well with the best monoreference results of Heard, Marsden, and Scuseria [G. L. Heard, C. J. Marsden, and G. Scuseria, J. Phys. Chem. 96, 4359 (1992)] obtained with a QCISD(T) and similar methods.
A general mixture theory. I. Mixtures of spherical molecules
Authors: Hamad Esam Z.
Published: July 2002

We present a new general theory for obtaining mixture properties from the pure species equations of state. The theory addresses the composition and the unlike interactions dependence of mixture equation of state. The density expansion of the mixture equation gives the exact composition dependence of all virial coefficients. The theory introduces multiple-index parameters that can be calculated from binary unlike interaction parameters. In this first part of the work, details are presented for the first and second levels of approximations for spherical molecules. The second order model is simple and very accurate. It predicts the compressibility factor of additive hard spheres within simulation uncertainty (equimolar with size ratio of three). For nonadditive hard spheres, comparison with compressibility factor simulation data over a wide range of density, composition, and nonadditivity parameter, gave an average error of 2%. For mixtures of Lennard-Jones molecules, the model predictions are better than the Weeks–Chandler–Anderson perturbation theory.
Theory and simulation of central force model potentials: Application to homonuclear diatomic molecules
Authors: Bresme Fernando , Abascal José L. F. , Lomba Enrique
Published: July 2002

Structure and thermodynamics of fluids made of particles that interact via a central force model potential are studied by means of Monte Carlo simulations and integral equation theories. The Hamiltonian has two terms, an intramolecular component represented by a harmonic oscillatorlike potential and an intermolecular interaction of the Lennard-Jones type. The potential does not fulfill the steric saturation condition so it leads to a polydisperse system. First, we investigate the association (clustering) and thermodynamic properties as a function of the potential parameters, such as the intramolecular potential depth, force constant, and bond length. It is shown that the atomic hypernetted chain (HNC) integral equation provides a correct description of the model as compared with simulation results. The calculation of the HNC pseudospinodal curve indicates that the stability boundaries between the vapor and liquid phases are strongly dependent on the bond length and suggests that there might be a direct gas–solid transition for certain elongations. On the other hand, we have assessed the ability of the model to describe the thermodynamics and structure of diatomic liquids such as N2 and halogens. To this end we have devised a procedure to model the intramolecular potential depth to reproduce the complete association limit (i.e., an average number of bonds per particle equal to one). This constraint is imposed on the Ornstein–Zernike integral equation in a straightforward numerical way. The structure of the resulting fluid is compared with results from molecular theories. An excellent agreement between the HNC results for the associating fluid and the reference interaction site model (RISM)-HNC computations for the atom–atom model of the same fluid is obtained. There is also a remarkable coincidence between the simulation results for the molecular and the associating liquids, despite the polydisperse character of the latter. The stability boundaries in the complete association limit as predicted by the HNC integral equation have been computed for different bond lengths corresponding to real molecular liquids. These boundaries appear close to the experimental liquid branch of the vapor–liquid coexistence line of the molecular systems under consideration.
Profile unbiased thermostat with dynamical streaming velocities
Authors: Bagchi Ken , Balasubramanian Sundaram , Mundy Christopher J. , Klein Michael L.
Published: July 2002

We have devised a reversible set of nonequilibrium molecular dynamics equations of motion to study viscous flow under an applied shear field utilizing a new profile unbiased thermostat (PUT). Based on a modification of the original equations presented by Evans and Morriss [Phys. Rev. Lett. 56, 2172 (1986)], we have extended the phase space to include dynamically evolving streaming velocities. The new method is advantageous because it allows control of the rate at which the streaming velocities evolve and thereby removes the effects of high frequency thermal fluctuations. Further, in tests on a Weeks–Chandler–Andersen fluid, we find that a shear-induced microstructure, the so-called ‘‘string phase,’’ was not destabilized by the new PUT scheme.
A Monte Carlo study of methanol clusters (CH3OH)N, N=5–256
Authors: Wright D. , El-Shall M. S.
Published: July 2002

The thermodynamic and structural properties of methanol clusters (CH3OH)N, N=5–15, 20, 30, 60, 128, 256 and the bulk liquid have been investigated using Monte Carlo simulation. Calculated properties as a function of size include electrostatic and dispersive contributions to the configurational energy, configurational heat capacities, fractal dimension, density profiles, order parameters characterizing dipole and bond vector orientation, and the Lindemann index. The cluster heat capacities as a function of N possess an interior maximum near N=128 and converge to the bulk value from above. Monocyclic, semiplanar structures are found to persist at liquidlike temperatures up to about N=12, followed by bi- and polycyclic structures for N=13–20, with the larger clusters gradually becoming more spherical. The average density of the larger clusters is fairly well represented by the bulk value. For clusters with 30 or more molecules, there is a net tendency for the molecular dipoles to lie flat on the cluster surface. The observed trends in heat capacities, density profiles, and dipole alignments parallel to the cluster surface are likely to be general features of clusters of polar molecules.
Condensed phase spectroscopy from mixed-order semiclassical molecular dynamics: Absorption, emission, and resonant Raman spectra of I2 isolated in solid Kr
Authors: Ovchinnikov M. , Apkarian V. A.
Published: July 2002

A method for spectral simulations in systems of very large dimensionality via semiclassical molecular dynamics is introduced and applied to the spectroscopy of iodine isolated in solid Kr, as a prototype of spectroscopy in condensed media in general. The method relies on constructing quantum correlation functions, C(t), using initial value propagators which correspond to the zeroth- and second-order approximations in stationary phase of the exact quantum propagator. The first is used for treating modes with high thermal occupation numbers, the lattice modes, while the second is used for treating the guest mode. The limits of validity of the bare propagators are tested vs exact treatments of gas phase I2, and shown to be quite broad. The mixed order simulations are then used to reproduce the structured A→X emission, the structureless B←X absorption, and the intensities in resonant Raman (RR) progressions of matrix isolated I2, connecting spectroscopic observables to molecular motions. Decompositions of the supersystem correlations into system and bath are used to provide perspectives about condensed phase spectroscopy. The system correlation can be regarded as the sampling function for the decaying bath correlation, which in turn is a summary of the many-body dynamics. The B←X absorption spectrum is determined by the coherent ballistic motion of the excited state density: Upon stretching, I2 pushes the cage atoms out of overlap in position density, and C(t) never recovers. Due to the compressive nature of the cage coordinate in the A→X transition, C(t) decays more gently, after being sampled three times. RR spectra, which are reproduced with adiabatic dynamics, sample the complete history of the many-body correlations, however, due to the breadth in space-time of scattering into high overtones, the sampling is coarse grained. The specific dynamics that control C(t) cannot be described as dissipative.
Segmental concentration profiles of end-tethered polymers with excluded-volume and surface interactions
Authors: Adamuţi-Trache Maria , McMullen William E. , Douglas Jack F.
Published: July 2002

The segmental concentration profile ρ of end-tethered flexible polymer chains is calculated for comparison with recent measurements on polymer layers having a low surface grafting density σ. Moments of ρ are also calculated as these quantities, rather than the full concentration profiles, are usually measured experimentally. Exact calculations of ρ are summarized for ideal flexible chains with an arbitrary short-range polymer–surface interaction. Though technical difficulties restrict our renormalization group (RG) calculations for swollen chains to limiting values of the polymer–surface interaction, our analysis indicates that the relatively simple, closed-form, Gaussian-chain expression for ρ provides a good approximation to the RG results if the average size of the Gaussian chain is replaced by its swollen-chain analog. This approximate concentration profile for chains with excluded volume and variable polymer–surface interaction should prove useful when interpreting experimental results for low-grafting-density polymer layers formed in good solvents. Recent measurements [Kent et al., J. Chem. Phys. 103, 2320 (1995)] of the second moment of ρ in the limit σ→0 agree well with our RG calculations, and the measured concentration profile ρ, whose determination by neutron reflection is somewhat uncertain, agrees qualitatively with our theoretical predictions.
Effect of temperature on the infrared and sum-frequency generation spectra of adsorbates
Authors: Liu W.-K. , Hayashi M. , Lin J.-C. , Chang H.-C. , Lin S. H. , Wang J.-K.
Published: July 2002

The dephasing and energy relaxation contributions to the line width in infrared (IR) and sum-frequency generation (SFG) spectra of adsorbates are derived from the generalized master equation approach. Expression for the line shift is also obtained. The anharmonic interaction between the adsorbate and the substrate is expanded in a polynomial in terms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to two-phonon processes, while two-phonon, three-phonon or four-phonon processes can contribute to energy relaxation, depending on the relative values of the adsorbate vibrational and the phonon frequencies. The temperature-dependence data of the IR absorption for C(111):H is found to be consistent with the theory, and the large line width for C(111):D can be accounted for by the efficient two-phonon energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H.
An effective potential for calculating free energies. I. General concepts and approximations
Authors: Verschelde H. , Schelstraete S. , Vandekerckhove J. , Verschelde J. L.
Published: July 2002

We present a new analytical method to calculate free energies of molecules based on a high temperature expansion of an effective potential which is a function of the mean position and fluctuation of the coordinates of the molecule. We first introduce an effective potential Veff(x̄,β) which is a convex function of the mean position x̄ and whose sole minimum gives the free energy. Then, we define a convex effective potential Veff(x̄,Δ,β) which after variation over the mean fluctuation Δ yields Veff(x̄,β). We expand Veff(x̄,Δ,β) in a high temperature series. The first two terms of the series yield an effective diffused potential method to calculate free energies. The diffusion times are calculated via a variational principle. Besides free energies, the method yields an analytical annealing method for energy minimization.
Relaxation in a confined and entangled polymer liquid
Authors: Loring Roger F.
Published: July 2002

We investigate the dynamics of a monodisperse polymer melt that is confined to the vicinity of a plane by a harmonic potential. Polymer molecules are represented by harmonic bead-spring chains with fluctuating bead mobility tensors. Mobility fluctuation rates are determined self-consistently from the chain dynamics. The calculation of the viscoelastic shear modulus is mapped onto the solution of a dynamically disordered random walk, in which a walker hops among lattice sites with fluctuating hopping rates. The relevant random walk problems are solved within the effective medium approximation, yielding the shear modulus and coefficient of shear viscosity as functions of chain length, entanglement molecular weight, and strength of the confining potential. Calculations illustrate the competition between entanglement effects, which retard chain dynamics, and the effect of the external field, which promotes relaxation.
Excited state kinetics of neutral transition metal atoms by stimulated emission pumping: V*(3d44s,a4D)+hydrocarbons
Authors: Wen Ye , Yethiraj Arun , Weisshaar James C.
Published: July 2002

Stimulated emission pumping (SEP) by two tunable, ns dye lasers selectively creates metastable V*(3d44s,a4DJ*) excited states near 8500 cm−1 in a fast flow reactor with 1.2 Torr He buffer gas at 300 K. Subsequent collisions with He equilibrate the spin-orbit levels J*=1/2–7/2 on a time scale of 1 μs but do not quench the 3d44s, a4D multiplet on a time scale of at least 50 μs. A third probe laser placed 5 mm downstream of the SEP lasers monitors the V* population at fixed reaction time t=40 μs. The exponential decay of V* vs calibrated hydrocarbon flow yields total removal rate constants that include both chemical reaction and electronic quenching of the V* excited state. Coupling of diffusion with reaction/quenching evidently does not cause the slight positive curvature observed in the kinetics plots. We demonstrate this by solving two simplified models of the reacting, diffusing, flowing gas for our experimental geometry, with SEP and probe laser beams transverse to the flow tube axis. Instead, the curvature is probably caused by the delayed, collisional cascade of a small population of higher excited states into the V* state. Ethylene removes the V* state on essentially every hard-spheres collision. The alkanes CH4, C2H6, and C3H8 remove V* on one in 75, one in 25, and one in 12 collisions. This is remarkably inefficient reaction/quenching, given that the low-spin 3d44s state is ideally configured for CH or CC bond insertion in alkanes. We interpret these new results and earlier work of Honma and co-workers on the sextet 3d44s excited state in terms of avoided intersections between repulsive and attractive diabatic potential surfaces (conserving electron spin and configuration).
Viscoelastic effects in early stage phase separation in polymeric systems
Authors: Onuki Akira , Taniguchi Takashi
Published: July 2002

We examine how viscoelasticity affects early stage spinodal decomposition in polymer solutions and blends when fluctuations of the stress and the composition are coupled in dynamics. The coupling is increased with increasing asymmetry between the two components. We introduce a long viscoelastic length ξve within which the stress relaxation governs the composition relaxation. It can be of the order of the tube length in the reptation theory in strongly asymmetric polymer blends. For shallow quenching phase separation proceeds on time scales slower than the stress relaxation time τ and the kinetic coefficient depends on the wave number q as q−2 for qξve>1. On the other hand, for deep quenching phase separation takes place as in gels on time scales faster than τ. We describe the crossover between these two cases by assuming a single stress relaxation time.
Semiclassical initial value representation for electronically nonadiabatic molecular dynamics
Authors: Sun Xiong , Miller William H.
Published: July 2002

The semiclassical initial value representation (SC-IVR), which has recently seen a great deal of interest for treating nuclear dynamics on a single potential energy surface, is generalized to be able to describe electronically nonadiabatic (i.e., multisurface) processes. The essential idea is a quantization of the classical electron-nuclear Hamiltonian of Meyer and Miller [J. Chem. Phys. 70, 3214 (1979)] within the SC-IVR methodology. Application of the approach to a series of test problems suggested by Tully shows it to provide a good description of electronically nonadiabatic dynamics for a variety of situations.
On coherent-state representations of quantum mechanics: Wave mechanics in phase space
Authors: Mo/ller Klaus B. , Jo/rgensen Thomas G. , Torres-Vega Gabino
Published: July 2002

In this article we argue that the state-vector phase-space representation recently proposed by Torres-Vega and co-workers [introduced in J. Chem. Phys. 98, 3103 (1993)] coincides with the totality of coherent-state representations for the Heisenberg-Weyl group. This fact leads to ambiguities when one wants to solve the stationary Schrödinger equation in phase space and we devise two schemes for the removal of these ambiguities. The physical interpretation of the phase-space wave functions is discussed and a procedure for computing expectation values as integrals over phase space is presented. Our formal points are illustrated by two examples.
Pure rotational spectra of the van der Waals complexes Ne–CO, Kr–CO, and Xe–CO
Authors: Walker Kaley A. , Ogata Teruhiko , Jäger Wolfgang , Gerry M. C. L. , Ozier Irving
Published: July 2002

The pure rotational spectra of the van der Waals dimers of Ne, Kr, and Xe with CO have been measured using a pulsed jet, cavity microwave Fourier transform spectrometer. All transitions measured were a-type R-branches, obeying selection rules ΔJ=+1, ΔKa=0, and ΔKc=+1. Spectra with Ka=0 were measured for 7 isotopomers of Ne–CO, 13 of Kr–CO, and 17 of Xe–CO. Transitions with Ka=1 were measured for Ne20–C12O16 and Kr-84C12O16. Rotational constants and centrifugal distortion constants have been determined for all species, as well as the O17 quadrupole coupling constants χaa for Kr-184C13O17 and Ne20C13O17. Effective structural parameters have been calculated from the rotational constants. Results derived from the O17 quadrupole coupling constants and centrifugal distortion constants indicate that Ne–CO is considerably more flexible than Ar–CO, Kr–CO, or Xe–CO. Failure to observe hyperfine structure due to the Ne21, Kr83, and Xe131 nuclei is discussed in terms of the weak rare gas–CO bonding. Comparisons have been made to the isoelectronic rare gas–N2 van der Waals complexes.
Ab initio calculation of potential energy surfaces for the three lowest triplet states (1 3A′′,1 3A,2 3A′′) of PH(X,A)–He
Authors: Kolczewski Ch. , Fink K. , Staemmler V. , Neitsch L.
Published: July 2002

Quantum chemical ab initio calculations at the complete active space SCF level and with inclusion of correlation effects have been performed for the potential energy surfaces of PH in its X 3Σ− ground state and its first excited triplet state, A 3Π, colliding with He atoms. The PH distance was fixed at its experimental value (of the A 3Π state), the PH–He distance and the HePH angle were varied. All three potential energy surfaces [1 3A′′ for PH(X)–He and 1 3A,2 3A′′ for the two components of PH(A)–He] are purely repulsive, except for very shallow van der Waals minima with well depths of about 15–40 cm−1. The interaction potentials decay approximately exponentially with increasing PH–He distance and show large angular anisotropies. Legendre expansions for the angular dependence of the potential surfaces converge slowly for V(1 3A′′) and the sum potential 1/2[V(2 3A′′)+V(1 3A)], but rapidly for the corresponding difference potential 1/2[V(2 3A′′)−V(1 3A)]. The present PH(A)–He potentials have been used in the companion paper by Neitsch et al. [J. Chem. Phys. 106, 7642 (1997)], for the calculation of thermal state-to-state rate constants for inelastic PH(A)–He collisions.
Ultralow interfacial tensions in water–n-alkane–surfactant systems
Authors: Sottmann T. , Strey R.
Published: July 2002

The interfacial tensions between water- and oil-rich phases in the presence of microemulsions have been measured for ternary systems of water, n-alkanes, and nonionic alkylpolyglycolether surfactants (CiEj). It is found that the minimum of the interfacial tension curve, which is observed for each system in conjunction with the well-known phenomenon of phase inversion, depends sensitively, but systematically, on the chemical nature of the oil and the surfactant. Specifically, the minimum value of the interfacial tension σ̄ab decreases by 1 order of magnitude on decreasing either the carbon number of the alkane k by 6, or the number of oxyethylene groups j by 3, or by increasing the number of carbon atoms in the surfactant tail i by 2. The numerical values of the interfacial tensions as a function of temperature are presented along with an empirical description previously suggested [R. Strey, Colloid and Polymer Sci. 272, 1005 (1994)]. From the analysis, in terms of bending energy one obtains estimates for the bending and saddle-splay constants. The similar shape of the interfacial tension curves permits a superposition of the data for all 19 systems in support of a scaling relation recently derived [S. Leitao, A. M. Somoza, M. M. Telo da Gama, T. Sottmann, and R. Strey, J. Chem. Phys. 105, 2875 (1996)]. Furthermore, we note a striking coincidence of the numerical values of critical amplitude ratio R=σ0ξ02=0.37 kT in near-critical systems and the product σ̄abξ̄2=0.44(±0.10) kT where ξ̄ is the maximum length scale in the bicontinuous microemulsions.
Instantaneous normal mode prediction for cation and anion diffusion in ionic melts
Authors: Ribeiro M. C. C. , Madden P. A.
Published: July 2002

Instantaneous normal mode (INM) analysis was undertaken for several ionic melts: NaCl at six distinct thermodynamic states, and for a particular state of liquid LiCl, LiF, KF, KI, NaI, ZnCl2, and CuCl. In this Communication, we show that, in most cases, the ratio between the diffusion constants for cations (Dca) and anions (Dan) is predicted from the average frequency of the real (“stable” 〈ωs〉) and imaginary (“unstable” 〈ωu〉) frequency modes of the projection of the total density of states on cations and anions, respectively. The proposed relationship, Dca/Dan=(mca−1〈ωu〉ca〈ωs〉ca−2)⋅(man−1〈ωu〉an〈ωs〉an−2)−1, where mi is the mass of a particular species, is suggested by Keyes’ INM theory for diffusion [J. Chem. Phys. 101, 5081 (1994)], with the further assumption that the parameters which are related to the topology of the multidimensional potential surface are equal for cations and anions. The above equation is shown to be valid for the simple melts NaCl, LiCl, LiF, KF, KI, and NaI, but to fail for the network forming melt ZnCl2 and for CuCl, which shows fast ionic diffusion characteristics.
Fourier-transform microwave spectrum, structure, harmonic force field, and hyperfine constants of sulfur chloride fluoride, ClSF
Authors: Preusser Jürgen , Gerry Michael C. L.
Published: July 2002

The rotational spectrum of sulfur choride fluoride, ClSF, has been observed for the first time in the frequency range 8–26 GHz by means of a pulsed molecular jet Fourier-transform microwave spectrometer. The unstable sample molecule has been prepared using a pulsed electrical discharge in jets containing a mixture of SF6 and SCl2 in Ne. Besides the parent species 35Cl32S19F, the isotopomers 37Cl32S19F and 35Cl34S19F could be observed in natural abundance. Rotational constants and quartic centrifugal distortion constants as well as nuclear quadrupole coupling constants due to 35Cl and 37Cl and spin–rotation constants due to 35Cl, 37Cl, and 19F are given. The data were used for the determination of r0, rΔP, rs structural parameters. Additionally, the new data were used for the refinement of the molecular harmonic force field. Results from those harmonic force field calculations were applied in the evaluation of the ground-state average structure, rz, and the estimation of the equilibrium structure, re. The rs structure is r(SF)=160.653(162) pm, r(SCl)=199.437(65) pm, and ∠(ClSF)=100.732(81)°. The diagonal elements of the 35Cl/37Cl quadrupole coupling tensors have been obtained, and are interpreted in terms of the bonding at 35Cl/37Cl. Negative 19F spin–rotation constants suggest a close analogy of the electronic structures of ClSF and SF2.
Multichannel quantum defect theory Stark effect calculation of autoionization lifetimes in high-n Rydberg states of Ar, N2 and H2
Authors: Softley T. P. , Hudson A. J. , Watson R.
Published: July 2002

Multichannel quantum defect theory simulations of excitation spectra to autoionizing high-n Rydberg states (n=13−150) are presented for Ar (spin-orbit autoionization), H2 (vibrational autoionization) and N2 (rotational autoionization), including the l-mixing due to homogeneous electric fields (Stark effect). The calculations, the first of their kind relevant to the ZEKE (zero-kinetic energy) photoelectron spectroscopy excitation range, are compared with previously published experimental results. Although in some cases the lifetimes derived from calculated linewidths are sufficiently long for the states to be observed by delayed pulsed-field ionization, they are generally found to be too short in the highest-n regions (n>80) to account for the very long lifetimes observed experimentally (τ>10 μs), pointing to the importance of alternative stabilization mechanisms. The effects of rotational channel couplings in H2 and N2 are investigated; these are very weak if both channels are above the Inglis-Teller limit, but show significant effects if only one channel is strongly l mixed. In H2 it is found that a window resonance is preserved in the presence of a strong field. In Ar, ortho-H2 and N2 fine-structure of the hydogenic manifolds is predicted, and the distribution of intensity and linewidth amongst the fine-structure components is investigated. The non-zero quantum defects cause a lifting of degeneracy in the manifolds between different ml components. It is proposed that this would cause a reduction in ml-mixing by inhomogeneous fields as the homogeneous field increases.
Lattice theory of polymer solutions with endgroup effects
Authors: Ryu Jong-Hoon , Gujrati P. D.
Published: July 2002

We present a unified lattice theory for a binary solution where endgroups are treated differently from middle groups. This is a simple example of a triblock and the present study provides a starting point for studying a general triblock system. We replace the original homogeneous lattice by a Bethe lattice of the same coordination number as the original lattice. The model is solved exactly on the Bethe lattice. The resulting solution goes beyond the random mixing approximation and provides us with an approximate theory of the model on the regular lattice. The contributions of endgroups on various thermodynamic properties of a binary solution are investigated in a quantitative way using the theory. In particular, our theory predicts that contributions to the energy are more important than to the entropy.
High resolution optothermal spectroscopy of pyridine in the S1 state
Authors: Becucci M. , Lakin N. M. , Pietraperzia G. , Salvi P. R. , Castellucci E. , Kerstel E. R. Th.
Published: July 2002

The optothermal technique has been utilized to obtain the first high resolution spectrum of pyridine in the region of the S1←S0 electronic transition. Rotational profiles for several vibronic bands (000,6a01,16b026a01,6a02,1201) were measured and found to be severely homogeneously broadened with linewidths of the order of 3–5 GHz, in agreement with previous lifetime measurements. Rotational constants of pyridine in the excited S1 vibronic levels were extracted by a band contour analysis. The values obtained are in good agreement with results from ab initio calculations, also presented here.
Determination of reaction geometries
Authors: Gericke Karl-Heinz , Kreher Christoph , Reinsch Ernst Albrecht
Published: July 2002

Using polarized light the reaction geometry of selected species can be controlled even in bulk experiments. One reactant A is generated in a photodissociation process and its spatial distribution is completely described by the anisotropy parameter β. The other molecular reactant B is excited in a specific rovibrational state. Its spatial distribution is given by the J- and branch-dependent alignment parameter A0(2). Equations have been developed that allow a relatively easy conversion of experimental results to the angle of attack, γ. The unnormalized probability of an attack of A on B under an angle γ is given by the simple expression P(γ)∝[1+15βA0(2)P2(cos γ)P2(cos δ)] where δ is the angle between the E⃗ vectors of the dissociating and the exciting laser beam. As an example, we have studied the reaction of A+HCN→HA+CN with A=H,Cl. The experimental results prove a preferred linear reaction geometry, i.e, an end-on attack of atom A on the terminating hydrogen atom of the HCN reactant. However, the cone of acceptance is higher for the Cl+HCN reaction than for the H+HCN one.
Anisotropic translational cooling: Velocity dependence of collisional alignment in a seeded supersonic expansion
Authors: Harich Steven , Wodtke Alec M.
Published: July 2002

Whether or not the collisional alignment resulting from seeded supersonic expansions may be utilized for chemical studies of aligned reactants will depend on our ability to understand this phenomenon. We have performed quantum-state-specific neutral time-of-flight measurements on molecular beams of dilute CO seeded in helium using polarized probe light, revealing the collisional alignment of individual rotational states of CO as a function of the terminal velocity. The derived peak velocities and translational temperatures depend on the probe-light polarization. CO molecules with their rotational angular momentum vectors, J⃗, aligned parallel to the flow velocity are more effectively accelerated by the light carrier gas and reach a lower terminal translational temperature than molecules with J⃗ aligned perpendicular to the flow velocity. The potential for using collisional alignment is discussed.
Brownian dynamics simulations of diffusion controlled reactions with finite reactivity
Authors: Srinivasalu Gupta J. , Khakhar D. V.
Published: July 2002

A new Brownian dynamics simulation technique is presented for the calculation of the effective rate constant for diffusion controlled reactions with a finite intrinsic reactivity. The technique is based on the calculation of the recollision probability of a molecule with a reactive site using a large number of Brownian trajectories, when the probability of reaction upon collision with the reactive site (φf) is less than unity. The technique is a modification of the earlier work of Northrup et al. [J. Chem. Phys. 80, 1517 (1984)], and is applied to the case of a uniformly reactive target sphere and a target sphere with axially symmetric reactive patches. A theoretical analysis is presented to relate φf to the intrinsic surface reaction rate constant (k). Computational results for the uniformly reactive sphere are in excellent agreement with theory, and those for the sphere with patches are in very good agreement with the results obtained using a different computational technique [Allison et al., J. Phys. Chem. 94, 7133 (1990)]. The proposed method requires the computation of the recollision probability to a high accuracy; however, this does not result in computational times greater than those of Allison et al. [J. Phys. Chem. 94, 7133 (1990)]. The new method has the advantage that the results of the Brownian dynamics simulation are independent of k and can subsequently be used to calculate the effective rate constant for any given value of k.
Thermodynamics and elastic moduli of fluids with steeply repulsive potentials
Authors: Heyes D. M.
Published: July 2002

Analytic expressions for the thermodynamic properties and elastic moduli of molecular fluids interacting with steeply repulsive potentials are derived using Rowlinson’s hard-sphere perturbation treatment which employs a softness parameter, λ specifying the deviation from the hard-sphere potential. Generic potentials of this form might be used to represent the interactions between near-hard-sphere stabilized colloids. Analytic expressions for the equivalent hard-sphere diameter of inverse power [ε(σ/r)n where ε sets the energy scale and σ the distance scale] exponential and logarithmic potential forms are derived using the Barker–Henderson formula. The internal energies in the hard-sphere limit are predicted essentially exactly by the perturbation approach when compared against molecular dynamics simulation data using the same potentials. The elastic moduli are similarly accurately predicted in the hard-sphere limit, as they are trivially related to the internal energy. The compressibility factors from the perturbation expansion do not compare as favorably with simulation data, and in this case the Carnahan–Starling equation of state prediction using the analytic effective hard-sphere diameter would appear to be a preferable route for this thermodynamic property. A more refined state point dependent definition for the effective hard-sphere diameter is probably required for this property.
Cytochrome-c in reverse micelles: Small angle x-ray scattering measurements, percolation process, and critical behavior: An interpretation with an association model
Authors: Cassin G. , Duda Y. , Holovko M. , Badiali J. P. , Pileni M. P.
Published: July 2002

It has been shown that solubilization of cytochrome-c in water in oil reverse micellar systems induces a change in the small angle x-ray scattering (SAXS) spectrum which suggests an increase in the attractive part of the intermicellar potential. In addition, from conductivity measurements a percolation threshold appears for a micellar concentration which is smaller than that observed without protein. Finally, for the liquid–liquid phase transition, a decrease in the critical temperature and concentration is observed. To explain these results, we introduce a pairing sticky hard sphere model. In this approach, the empty micelles are described by the sticky hard sphere model as is usually done. The presence of cytochrome-c is represented by an additional attractive potential characterized by an association parameter that leads to the pair formation of micelles. The association parameter was determined by fitting the experimental structure factors and from this parameter the decrease in the percolation threshold has been estimated. The change in the critical parameters induced by cytochrome-c can be interpreted in terms of formation of micelle-pairs and not by a van der Waals-type theory. These results are also supported by dynamic light scattering experiments.
Reaction dynamics of O(1D)+H2, D2, and HD: Direct evidence for the elusive abstraction pathway and the estimation of its branching
Authors: Hsu Yen-Tsung , Wang Jeng-Han , Liu Kopin
Published: July 2002

The dependencies of the integral cross sections for the title reactions on collision energies were determined from 0.6 kcal/mol to nearly 6 kcal/mol. The result provides, for the first time, direct and unambiguous experimental evidence for the existence of an abstraction pathway, in addition to the widely accepted inserted one. A reaction barrier of about 1.8 kcal/mol was found for this elusive abstraction channel. The branching of these two microscopic pathways was estimated. An intriguing H/D isotope effect was revealed, which called for further studies.
Kinetic effect of cluster-cluster processes on homogeneous nucleation rates in one- and two-component systems
Authors: Arstila Hanna
Published: July 2002

Cluster-cluster collisions and cluster dissociation into two smaller clusters have an effect on steady-state size distributions and nucleation rates. Clustering also affects nucleation rates by influencing the saturation vapour pressure, which is often ignored in nucleation studies. Both the kinetic and the vapour pressure effect of clustering on nucleation in one and two component systems are investigated. For water, the effect of cluster-cluster kinetics is in the order of 10%, and the effect via vapour pressure in the order of 100% in nucleation rate. The approximate method [Shugard et al., J. Chem. Phys. 75, 5298 (1974)] to take the kinetic effect of clustering into account works well for water. In acetic acid vapour the dimer concentrations are an order of magnitude higher than monomer concentrations. Even in this extreme case the approximate way to describe the kinetic effect of clustering gives reasonable estimates of the correct rates. In sulphuric acid-water mixture the kinetic effect is from one to four orders of magnitude in nucleation rate, depending on whether the clustering is accounted for in the saturation vapour pressure or not. The effect of clustering via vapour pressure is 5-10 orders of magnitude, depending on the kinetic model used. The approximate way to describe cluster kinetics works only in some of the cases.
Comparison of experiments and modern theories of crystal nucleation
Authors: Gránásy László , Iglói Ferenc
Published: July 2002

The temperature dependencies of the interfacial free energy γ(T) predicted by modern nucleation theories (density functional and phenomenological models) are confronted with temperature dependent data on crystal nucleation in hydrocarbons, molten metals, and oxide glasses. It is found that the semiempirical density functional approach and the phenomenological diffuse interface theory are fully consistent with the experiment. In contrast, the γ(T) functions from the density functional models developed primarily for densely packed crystal structures could not be brought into agreement with the nucleation data investigated. The possible origin of the success/failure of the theoretical approaches are discussed.
Collisional removal of NO (B 2Π, v=2 and 3) at 230 K
Authors: Hwang Eunsook S. , Lacoursière Jean , Copeland Richard A. , Slanger Tom G.
Published: July 2002

Removal rate coefficients for NO(B 2Π) in the v=2 and 3 levels are measured at 230 K for seven colliders: NO, N2O, CO2, O2, N2, Ar, and He. These measurements are the first below room temperature and are compared to earlier 295 K measurements. These NO(B 2Π) vibrational levels differ from each other in that the v=2 level is unperturbed, and the v=3 level is significantly perturbed by the v=12 level of the a 4Π state. Although there are large variations in removal rate coefficients between the two B 2Π vibrational levels, the effect of reducing the temperature on the removal rate coefficients is modest, the largest effects occurring with the least effective colliders, He and Ar.
Nonrandom mixing in polymer blends: Implications for phase behavior
Authors: Foreman K. W. , Freed Karl F. , Ngola Isaac M.
Published: July 2002

Nearest-neighbor pair distribution functions are computed from the semiflexible chain lattice cluster theory (LCT) for binary polymer blends and are compared with the predictions of simple random mixing theory. The LCT treats lattice model polymers with structured monomers and with variable chain flexibility by allowing the monomers to extend over several lattice sites and by introducing trans ↔ gauche bending energies. Comparisons with Monte Carlo simulations for polymer melts enable further tests for the accuracy and limitations of the LCT, while computations of nearest-neighbor pair distribution functions for a variety of binary polyolefin blends provide a link between the phase behavior of these blends and the microscopic local correlations induced by packing constraints and energetic interactions. Altering monomer structures leads to subtle changes in pair distribution functions but profound variations in phase behavior. The calculations of the nearest-neighbor pair distributions provide a simple microscopic explanation for the LCT predictions of the pressure dependence of blend phase diagrams. Variations of the nearest-neighbor pair distribution functions with chain stiffness, van der Waals interactions, temperature, polymerization indices, etc., are correlated with trends in phase behavior and other physical properties. The semiflexible chain LCT is used for further tests towards developing a computationally convenient thermodynamically equivalent linear semiflexible chain model to mimic the melt and blend properties of experimental (or theoretical) structured monomer chains.
Analytical one-particle approach to the π electronic structure of heterocyclic polymers
Authors: Onipko Alexander , Klymenko Yuriy , Malysheva Lyuba
Published: July 2002

The one-electron description of heterocyclic five-membered ring polymers is derived on the basis of the Su–Schrieffer–Heeger–Hückel type Hamiltonian which accounts for the electronic interaction of the heteroatom p-orbital lone pair with the π band structure of the carbon backbone. An explicit form of the fifth order equation, the solutions of which determine the dispersion relations for five π electron bands and closed expressions of molecular orbitals, is obtained. The main accent is put on the gross π electronic polymer structure. It is shown that there exists one-to-one correspondence between the structure considered as a function of basic system parameters [such as the electron on-site energies at carbon (C) and heteroatom (X), and resonance integrals associated with C–C and X–C bonds] and zeros of the Green function of polymer building blocks—monomers. This interrelation is expressed in the form of certain equations for the monomer Green function matrix elements, which predict the values of system parameters at which the π electron spectrum contains closed gaps (some bands join each other), in-gap states, and degenerate bands. Ten band-edge energies of five π bands of heterocyclic polymers are found as analytical functions of system parameters. Four of 10 band-edge energies are shown to be independent of heteroatom parameters due to the system symmetry. The heteroatom effects on the band edges are traced for polythiophene, polypyrrole, and polyfuran. Theoretical results are compared with available experimental data, and band gaps and bandwidths of π electrons in heterocyclic polymers are predicted.