bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanism and power dependence of the multiphoton ionization of bromobenzene
Authors: Koplitz Brent D. , McVey Jeffrey K.
Published: February 2003

The multiphoton ionization and fragmentation of bromobenzene is examined in the low and medium power regimes. Single-photon excitation to the 1B2 state is followed by a one-photon absorption that leads to the parent ion. Subsequent absorptions lead to the C6H+5 and C4H+x species. Mass-resolved studies of this resonance-enhanced two-photon process are in qualitative agreement with a kinetic model. The power dependencies of the parent, phenyl, and C4H+x ions are successfully predicted and simple relationships for the branching ratios are evident. Information on the relative cross sections for photon absorption by the ions is obtained. The analysis of the data suggests that the primary route is ionization to form the parent with subsequent formation of the fragments occurring via a ladder-switching mechanism.
Threshold behavior of endothermic reactions: C+(2P)+H2 → CH++H
Authors: Ervin Kent M. , Armentrout P. B.
Published: February 2003

The total reaction cross section for the title reaction has been measured using a new ion beam apparatus specifically designed for accurate examination of the translational energy dependence of ion–molecule reactions. Previous results show excellent agreement with the present data if corrections to the energy scale used by previous workers are ignored. Such corrections were motivated by the discrepancy observed between the endothermicity for the reaction and the apparent threshold. The reasons for this discrepency are analyzed and information concerning the true threshold behavior is obtained. It is found that a simple model for endothermic ion–molecule reactions successfully reproduces the general threshold behavior.
Observation of PbBi in neon matrices
Authors: Eberle B. , Sontag H. , Weber R.
Published: February 2003

The PbBi molecule which was observed recently, has been analysed in neon matrices by laser emission and excitation spectroscopy. The spectroscopic constants of the ground and a weakly bound excited state are given. The dissociation energy determined is in agreement with previous mass spectroscopic investigations.
A semiempirical crystal orbital investigation on the one-dimensional polyferrocenylene system
Authors: Böhm Michael C.
Published: February 2003

The band structure of polyferrocenylene has been studied in the crystal orbital formalism based on the tight-binding approximation; the computational framework is a semiempirical INDO model designed to reproduce the results of double-zeta ab initio calculations in organic molecules and transition metal compounds. It is shown that 4–6 neighbors are necessary to stabilize the results of the band structure calculations. The total energy of the polymer has been partitioned into elements of physical significance. Metal–metal and metal–ligand potentials have been separated into resonance, exchange, and classical electrostatic contributions. The highest filled band of polyferrocenylene is a ligand π band; Fe 3d bands are predicted at lower energies. The calculated bandwidths span a range between 0.4 to 2.6 eV. Avoided crossing regions in the reciprocal k space lead to a strong metal–ligand intermixing in some of the outer valence bands. It is shown that hole states in partially oxidized polyferrocenylene are probably unstable against the formation of a state with trapped valences. The close relation between this behavior and the soliton formalism is discussed. The various computational results are compared with experimental data.
High pressure properties of solid α-O2
Authors: Helmy A. , Kobashi K. , Etters R. D.
Published: February 2003

The static and dynamic properties of solid oxygen are calculated vs pressure at zero temperature using a pattern recognition optimization scheme and harmonic lattice dynamics method. The lattice parameters, phonon and libron dispersion curves, acoustic sound velocities, compressibility, root mean square translational and librational fluctuations from equilibrium, and the pressure dependence of the intramolecular stretching mode are calculated. It is shown that the attractive magnetic interaction strongly influences the behavior of the solid at all pressures. A soft mode induced phase transition from the monoclinic α structure to an orthorhombic structure is predicted at zero temperature near 6 kbar. No volume change is observed at the transition.
Extended Brownian dynamics. III. Three-dimensional diffusion
Authors: Lamm Gene
Published: February 2003

An improved version of the ‘‘extended Brownian dynamics’’ algorithm recently proposed [J. Chem. Phys. 78, 2713 (1983)] is given. This work represents the conclusion of an analysis of a Monte Carlo approach to diffusion. The algorithm suggested here and in the previous paper is a procedure for solving the full three-dimensional Smoluchowski diffusion equation, including general force and reaction terms and boundary conditions. The method is most suited for the investigation of diffusion-controlled systems with asymmetric geometries. In this paper a general overview of the Monte Carlo approach is presented. A slight modification to the one-dimensional algorithm introduced earlier is given and the extensions necessary to treat three-dimensional diffusion are discussed. The procedure is then applied to three-dimensional free diffusion and diffusion in a Coulomb force and the reaction yields for these processes are presented and compared with other numerical and analytic results. A summary of the Monte Carlo method is given and comments concerning the advantages and difficulties of the algorithm are made.
Equation of state of sodium
Authors: Fritz J. N. , Olinger Bart
Published: February 2003

The volume of sodium in the bcc structure was measured at 293 K to 9 GPa using a high pressure, x-ray diffraction technique. The compression of NaF was used as the pressure gauge. These data, the shock compression data of Rice and Bakanova et al., and the melting curve data of Luedemann and Kennedy, and Ivanov et al., are all used to establish a model for the equation of state of sodium.
Raman and infrared studies on t
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n
s-copoly(acetylene+acetylene-d2): Vibrational frequency dispersion and the origin of doping-induced absorptions
Authors: Takeuchi Hideo , Furukawa Yukio , Harada Issei , Shirakawa Hideki
Published: February 2003

Raman spectra of trans copolymers of acetylene and acetylene-d2 have been measured with various isotopic mixing ratios. From the spectral changes associated with varying isotopic ratio, some criteria have been derived concerning the slopes of vibrational frequency dispersion curves near the zone center and the frequency ranges of dispersion for both (C2H2)n and (C2D2)n. Comparison of the criteria with the calculated dispersion curves reported so far shows that the force field for the infinite trans polyene chains should be reexamined. Infrared spectra of iodine-doped copolymers have also been investigated. Upon isotopic mixing, the infrared bands characteristic of doped polyacetylene behave quite differently from the Raman bands of pristine polyacetylene. This finding indicates that the doping-induced infrared absorptions cannot be correlated simply with the Raman-active vibrations in the unperturbed polyene chain. The behavior of the infrared bands is qualitatively accounted for by assuming a charged domain consisting of several or more consecutive monomer units for which the force field is severely affected by the charge transfer reaction with a dopant.
Roughening transition in adsorbed xenon multilayers
Authors: Miranda R. , Albano E. V. , Daiser S. , Wandelt K. , Ertl G.
Published: February 2003

Experimental evidence for a roughening transition in adsorbed xenon multilayers on palladium is presented. The clearly layer-distinctive Xe 5p photoemission is used to measure adsorption isotherms as well as the relative equilibrium population of individual Xe layers. Below a transition temperature TR xenon is adsorbed layer by layer—each layer being completed before any substantial population of the next layer occurs—with sharp first order phase transition steps for the second and third layer. Above TR the adsorption proceeds in a continuous manner, with rounded steps in the isotherms and simultaneous population of different layers. Layer population and critical temperature analysis suggests that the transition temperature is indeed the roughening temperature of the Xe film.
Variational calculation of the dynamics of a two level system interacting with a bath
Authors: Silbey Robert , Harris Robert A.
Published: February 2003

A variational calculation of the dynamics of a two level system interacting with a bath is presented. The effective tunneling matrix element is renormalized by the interaction with the bath. For the case of a bath with ohmic dissipation (or infrared divergence), the variational calculation gives a vanishing tunneling at T=0 for critical values of the coupling and gives a highly unusual temperature dependence for the tunneling rate for large kT, all of which agree with recent path integral calculations. The present method also yields results for intermediate temperatures and coupling. This indicates that a judicious choice of the zeroth order Hamiltonian to include the major part of the coupling can lead to correct results even in low order perturbation theory.
A
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t
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o investigation of the 3Π states of NO+
Authors: Saxon Roberta P. , Tuan Debbie F.-t. , Helm H. , Liu B.
Published: February 2003

MCSCF+CI calculations on the five lowest 3Π states of NO+ strongly indicate transitions to the b 3Π state have been observed in the laser photofragment spectroscopy experiment by Cosby and Helm [J. Chem. Phys. 75, 3882 (1981)]. The 2 3Π state is shown to have a long range minimum bound with respect to separated atoms and a local minimum near R=1.2 Å which gives rise to a quasibound state 0.81 eV above the N+(3P)+O(3P) asymptote or 21.84 eV above ground state NO. This quasibound state is responsible for the prominent feature at 21.7 eV in the photoelectron spectrum of NO, which was previously assigned to a c 3Π state.
Atoms in molecules. I. A charge conservation rule for taking into account the ‘‘orthogonality hole’’
Authors: Colle Renato , Fortunelli Alessandro , Salvetti Oriano
Published: February 2003

An ab initio method for treating separately core and valence electrons in large systems by means of a generalized product function is presented here. The orthogonality constraints for each valence orbital with respect to the core orbitals are taken into account by means of an ‘‘orbital charge conservation’’ rule. Core functions and that part of each valence orbital, that is due to the orthogonality hole, are taken directly from atomic or molecular calculations.
Prediction of the valence shell ionization spectrum of hydrogen fluoride using orthogonalized direct diagonalization
Authors: Baker Jon
Published: February 2003

The orthogonalized direct diagonalization (ODD) method is used to predict the valence shell ionization spectrum of hydrogen fluoride within an EOM/propagator formalism. Three sets of calculations are presented equivalent to (i) a full 2p-h TDA, (ii) a full third order propagator, and (iii) a higher order calculation with an extended operator manifold (including h5 terms). The results show the viability of the direct diagonalization approach and the importance of the h5 manifold for the accurate prediction of features in the ionization spectrum. The advantages of ODD over the 2p-h TDA and related techniques are discussed.
Changes in reference frame with respect to barycentric velocity in isothermal diffusion: Duality between reference flows and potentials in mechanical equilibrium
Authors: Peusner Leonardo
Published: February 2003

Using a topological representation of mechanical equilibrium as applied to isothermal diffusion it is shown that the barycentric velocity, which serves as a reference in the definition of diffusional flows, can be replaced by an arbitrary velocity. This result is a stronger form of a theorem originally proved by Prigogine which requires that mechanical equilibrium and the Gibbs–Duhem relations be obeyed. In its present form, the result follows from the topological duality between forces and flows when mechanical equilibrium holds.
The effect of density on the inherent structure in liquids
Authors: Weber Thomas A. , Stillinger Frank H.
Published: February 2003

We have previously shown that diffusion and fluid flow within a liquid may be interpreted as transitions between mechanically stable arrangements of the molecules, i.e., local minima in the potential energy for the system as a whole. In addition, our previous work has shown that the equilibrium thermodynamic properties of the liquid and solid can be rigorously and usefully explained in terms of vibrational excitations within, and shifting equilibrium between, these stable molecular packings. Previously, we have determined transition rates between various packings as a function of temperature, system size, and extent of supercooling. In the present work, we study the effect of density on both the number and distribution of packing structures. Molecular dynamics simulations were carried out for systems of 32 and 108 argon-like particles at three densities. The primary obsrvations and conclusions are: (1) as the density decreases the total number of different amorphous packing structures increases; (2) the density at which the amorphous material has its minimal average energy is about 3% lower than the density at which the crystalline material has its minimal energy; (3) the activation energy to self-diffusion is found to increase with increasing density.
The electronic/positronic structure of positron/pseudohalide systems: [OH−; e+], [SH−; e+], [CN−; e+], and [N−3; e+]
Authors: Kao Chih-Mai , Cade Paul E.
Published: February 2003

A theoretical examination of the [OH−;e+], [SH−;e+], [CN−;e+], and [N−3;e+] at the approximate Hartree–Fock level is presented. Large Gaussian basis sets are used to represent both the electronic and positronic orbitals in conventional Hartree–Fock–Roothaan theory. Quality studies are given for [F−;e+] and [OH−;e+] and some consideration of dependence with molecular geometry is considered. The basic energy results are presented and a cycle argument is followed to suggest that [CN−;e+] and [N−3;e+] are stable to Ps detachment while [OH−;e+] and [SH−;e+] are not stable. The corresponding angular correlation curves N(θ) are given and compared. Finally, interpretative aspects are presented and discussed in terms of positron localization and contrast to proton behavior.
Relativistic effects on molecular hyperfine interactions: Application to ytterbium fluorides
Authors: Case David A. , Lopez Jesus P.
Published: February 2003

Dirac scattered-wave (DSW) calculations are presented for YbF3−6 and YbF5−8 clusters that model Yb+3 impurity sites in KMgF3 and CaF3 lattices, respectively. The calculations predict Γ6 and Γ7 ground states for these two cubic geometries and show good agreement with observed g tensors, spin-orbit splittings, and Yb hyperfine interactions, but overestimate the crystal field splittings and the amount of covalency between the metal and the fluorine 2p orbitals. Nevertheless, the fluorine hyperfine tensors are in qualitative agreement with experiment, and confirm the existence of a large p-orbital contribution to the isotropic part of the 19F hyperfine tensor for YbF3−6. The implications of these results for future relativistic calculations on lanthanide complexes are discussed.
Liquid state thermodynamics and the dynamic compression of solids
Authors: Jones Randall S. , Ashcroft N. W.
Published: February 2003

The thermodynamic properties of simple solids at high pressures may be calculated from liquid state thermodynamics by treating the solid for certain structural purposes as a supercooled liquid. In particular, the energy and pressure at arbitrary temperatures and densities can be obtained from expressions that are reasonably simple functionals of the pair potential. This permits the direct use of dynamic compression data as a test of a given pair potential without resorting to the usual methods that reduce the Hugoniot. Alternatively, the method can be used to perform a reduction of a shock Hugoniot by selecting from a general space of pair potentials the one that reproduces best both the known thermodynamic quantities and the Hugoniot curve. The resulting pair potential is then invoked to generate the zero temperature data, thereby also performing the required ‘‘reduction.’’ These methods are applied to a model of the inert gas solids.
Equilibrium pressures over the systems bismuth trisulfate-dibismuthmonoxydisulfate and dibismuthmonoxydisulfate-dibismuthdioxymonosulfate. Slow transformation between two crystalline forms of dibismuthmonoxydisulfate
Authors: Jones Wesley M.
Published: February 2003

Equilibrium total pressures over the named solid systems were obtained in a static system. Simultaneous establishment of equilibrium (4) by platinum black catalyst mixed with the solids allows the data to be represented in terms of the equilibria (1), (2), and (3). Bi2(SO4)3=β-Bi2O(SO4)2+SO3, (1) ΔH°=38.39,  ΔS°=43.9,  Tm=785, Bi2(SO4)3=α-Bi2O(SO4)2+SO3, (2) ΔH°=33.71,  ΔS°=38.1,  Tm=760, β-Bi2O(SO4)2=Bi2O2SO4+SO3, (3) ΔH°=41.93,  ΔS°=37.3,  Tm=950, SO3=SO2+1/2 O2. (4) The enthalpy (kcal mol−1) and entropy changes (cal K−1 mol−1) are assigned to the middle of the range of temperatures covered Tm (K). The unusually sluggish transformation between the low temperature α and a new high temperature β form of Bi2O(SO4)2 allowed equilibrium pressures for equilibrium (1) to be determined below the transformation temperature Tt, where the β form is metastable, as well as above Tt. Pressure data for equilibria (1) and (2) gave Tt=811±2 K. Annealing-x-ray diffraction methods gave Tt=808±3 K. The entropy change for the transformation is rather large (5.76±0.13 cal K−1 mol−1). The possible nature of the transformation is considered.
Diffusion controlled autocatalytic growth of revert periodic precipitation of cadmium sulphide in lyophillic colloid
Authors: Palaniandavar N. , Gnanam F. D. , Ramasamy P.
Published: February 2003

The experimental conditions for revert and direct periodic precipitation of cadmium sulphide in lyophillic colloid have been investigated. The theoretically derived Mathur’s spacing law, which suits both revert and direct types of precipitation has been experimentally verified. An attempt has been made to explain the formation of revert cadmium sulphide bands by considering preferential adsorption of sulphide ion, interpenetrating distribution of ions, and autocatalytic growth of cadmium sulphide particles within the electrical double layer. The flocculation is explained as being due to the specific adsorption of the hydrolyzed counter ion. The increase and decrease in the value of velocity constant is explained on the basis of the formation of autocomplex species.
Optical Zeeman spectroscopy of the lowest doublet excited states of t
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s(acetylacetonato)chromium(III)
Authors: Fields R. A. , Haindl E. , Winscom C. J. , Khan Z. H. , Plato M. , Möbius K.
Published: February 2003

The three principal phosphorescent bands of Cr3+ (1%)/tris(acetylacetonato)aluminium(III) single crystals at 1.8 K, have been examined at field strengths of 0–5.7 T in various orientations. The principal g values of the corresponding doublet excited states are determined to lie in the ranges ‖g∥ ‖:1.4–1.7, ‖g⊥ ‖:1.8–2.0, in marked contrast to those of the inorganic analogue, ruby.
Vibrationally resolved optical spectra of c
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s,t
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s-1,3,5,7-octatetraene
Authors: Kohler Bryan E. , Spiglanin Thomas A.
Published: February 2003

Vibrationally resolved one photon fluorescence excitation and fluorescence spectra have been measured for cis,trans-1,3,5,7-octatetraene in n-alkane matrices at 4.2 K. Except for absolute intensities and the details of vibronic development, these spectra are similar to those observed for the all-trans isomer. That is, there is a weak S0–S1 transition (0–0 at 28 645 cm−1 in n-hexane) approximately 3490 cm−1 below the strongly allowed S0–S2 transition at 32 134 cm−1. The vibrational development of the S0–S1 absorption and fluorescence is that of a symmetry allowed transition (this transition is symmetry forbidden in the all-trans isomer). Thus, the low intensity of the S0–S1 transition testifies to the importance of multiple excitations in S1. Vibrational modes in S1 are weakly coupled with S2 due to the small energy gap. This coupling is energy dependent. Finally, Franck–Condon factors for the S0–S2 transition have been estimated. These Franck–Condon factors together with the vibrational structure observed for the S0–S1 excitation and fluorescence spectra show that neither S1 nor S2 is significantly distorted with respect to the ground state. Nonetheless, cis–trans photoisomerization competes with fluorescence, even at 4.2 K.
Two angle dependent reactive infinite order sudden approximation
Authors: Jellinek J. , Kouri D. J.
Published: February 2003

The reactive infinite order sudden approximation is redeveloped in a manner in which the initial and final arrangement internal angles γλ amd γν enter as independent quantities. The analysis follows parallel to that due to Khare, Kouri, and Baer except that matching of the wave function from different arrangements is done in a manner such that no single γν angle is associated with a particular γλ angle. As a consequence, the matching surface parameter Bλν does not occur.
Intramolecular distributions. I. Theory
Authors: Kupka H. , Polansky O. E.
Published: February 2003

In this paper, a theory of intramolecular distributions for polyatomic molecules is presented. This approach has been found to be of great value in calculating radiative and nonradiative transitions. The intramolecular distributions (ID) are introduced by the aid of generating functions which incorporate typical molecular effects such as electronic origin and force constant shifts, and mode mixing. For some special cases, the main properties of multidimensional intramolecular distributions, which involve the effect of the density of final and initial states, are formulated in terms of an addition theorem; further use of these distributions in problems of molecular spectroscopy is discussed. Finally, the symmetry behavior of the ID’s is derived and its significance in establishing general relations and correlations between transitions in absorption and emission is discussed.
EPR investigation of the intermediate spin exchange regime
Authors: Bales Barney L. , Willett Dennis
Published: February 2003

The electron paramagnetic resonance spectra of di-tert butylnitroxide (DTBN) are studied as a function of the nitroxide concentration in three hydrocarbon solvents at constant temperature. In the concentration range studied, DTBN undergoes electron spin exchange which leads to broadening, shifting, and eventual collapse of the lines forming the 19-line proton hyperfine pattern. Particular emphasis is given to the spin exchange frequencies ωHE in the region in which they are comparable with the proton hyperfine coupling constants—the intermediate spin exchange regime. Both linewidths and line shapes are found to be in full accord with theory in the intermediate exchange regime which completes the demonstration that theory and experiment are in agreement, the slow and fast exchange regimes having been studied many times in the past. Modeling the spectral lines as Lorentzian–Gaussian convolutions allows a detailed exposition of the collapse of the hyperfine structure as ωHE increases and this collapse, too, is well described by theory. Bimolecular spin exchange rate constants are compared with the prediction of simple hydrodynamic theory.
The effect of thermal vibrations on extended x-ray absorption fine structure. I
Authors: Boland John J. , Baldeschwieler John D.
Published: February 2003

The form of the Debye–Waller factor in EXAFS is discussed in detail, and an expression is obtained for this factor in a general three-atom system of Cs symmetry. The normal modes which contribute to the Debye–Waller factors for each scattering path are dependent on the symmetry of the system. A series of model three-atom systems with C2v symmetry are studied and the Debye–Waller factors as a function of the bridging angle are discussed for each of these systems. The temperature dependence of the Debye-Waller factor for each scattering path is also studied. In a system of C2v symmetry, approximating the double and triple scattering Debye–Waller factors by the second shell single scattering factor is shown to be strictly valid only for a system close to linearity. The error due to this approximation is dependent upon the amplitude of the individual scattering paths and is shown to increase with temperature. When the single scattering contribution is unimportant, there is shown to exist a temperature where the above approximation is exact.
Anisotropic screening in polar fluids and the extent of its accommodation by molecular perturbation theories that use sphericalized reference states
Authors: Singh Shri , Ram Jokhan , Goldman Saul
Published: February 2003

We developed a function that is sensitive to many-body screening of the anisotropy in dense molecular fluids and applied it to dipolar fluids and to ST2 water. For these fluids, the function was found to be oscillatory with distance and to become damped, for dipolar fluids, with increasing strength of the dipole moment. We then compared this function to the screening implied by the Gray–Gubbins and RAM reference state potentials. For ST2 water these comparisons suggested that a composite potential made up of equal proportions of the RAM and the Gray–Gubbins potential might be better than either the RAM or the Gray–Gubbins potentials alone.
Magnetic susceptibility of Nd (BrO3)3⋅9H2O single crystal: Effects of the crystal field and calculation of the electrostatic crystal field parameters
Authors: Neogy D. , Chatterji A. , Purohit T.
Published: February 2003

Single crystals of neodymium bromate were grown and their principal magnetic susceptibility and anisotropy determined from 90 to 300K. The anisotropy, which is relatively small, decreases with lowering of temperature and suffers a reversal at ∼155 K. The magnetic moment peff shows a small linear variation with temperature. A least squares fit yields Curie constant C=1.666 and paramagnetic Curie temperature θ=−23.76 K for χ̄. The results were analyzed with one electron crystal field (CF) with the intermediate coupling reduced matrix elements. Inclusion of J mixing through the CF with all the 4I J manifolds leads to a considerable change in the Stark energies and the magnetic properties. A theoretical analysis of the observed optical absorption and magnetic data shows conclusively that the next nearest neighbor interactions which reduce the Nd3+ site symmetry to C3v do have a significant role to play. The effects of the CF on the thermal and hyperfine behavior of Nd3+ in bromate lattice have been worked out. An ab initio calculation of the CF parameters using the structural data on the crystal demonstrates the inherent weakness of the point charge treatment to account for the effective parameters obtained from experiments. Although it is felt that the factors primarily responsible for the discrepancy are the uncertainties in the structural data and in the exact location of the dipole and charge centers, this by no means discounts the importance of overlap and covalency.
The role of the anion electronegativity in semiconductor–electrolyte and semiconductor–metal junctions
Authors: Sculfort Jean-Lou , Gautron Jacques
Published: February 2003

The anion electronegativity of binary and ternary semiconductors plays an important part as well in electrolytic junctions as in metallic junctions. The position of the top of the valence band EV of a wide range of these compounds increases always monotonically with increasing of the anion electronegativity whoever the chosen scale. A compilation of the different scales is accomplished. A simple method of EV position evaluation in an indifferent electrolyte gives good results for a characterization a priori of the flat band potential for AB compounds. From the comparison between theoretical and experimental flat band potential, the existence of specific interactions associated, for example, with thin surface layers (oxide or adsorbed ions) is revealed.
Rayleigh–Brillouin scattering studies of the rotation-translation coupling and bulk viscosity relaxation of liquids composed of anisotropic molecules: p-Anisaldehyde and aniline
Authors: O’Steen B. L. , Wang C. H. , Fytas G.
Published: February 2003

The coupling of reorientational motion to longitudinal acoustic modes was investigated by studying the Rayleigh–Brillouin spectra of aniline and p-anisaldehyde over a wide range of scattering angles. Aniline and p-anisaldehyde were chosen for investigation since their depolarized spectra at the temperature of interest show a pronounced coupling between orientation and shear modes. The experimental results for attenuation and velocity of the longitudinal waves show that there is indeed relaxation in the shear viscosity. However, shear relaxation due to the coupling of the longitudinal acoustic mode to reorientation plays only a minor role in causing the dispersion of the hypersonic longitudinal waves. To account for the large dispersion, it is essential to consider the mechanism associated with the relaxation of the bulk viscosity through the T-V energy transfer.
Conductivity fluctuations in butyric acid/water and isobutyric acid/water mixtures
Authors: Kolb H.-A. , Woermann D.
Published: February 2003

We have measured the power spectrum SV( f) of noise voltage at current clamp of mixtures of (a) butyric acid/water, isobutyric acid/water, and KCl/water as functions of composition at constant temperature and (b) isobutyric acid/water of critical composition as function of temperature near the critical point in a frequency range of 0.1≤ f≤3.2 kHz. At temperatures away from the critical, all spectra can be described by power law SV( f)∝f −x with x≂1. The amplitude of SV( f) is proportional to the applied voltage V0. Approaching the critical temperature of the system isobutyric acid/water, the spectra can be characterized by the same power law but with an exponent x≂2. The amplitude of SV( f) is proportional to V20. A qualitative explanation of the properties of SV( f) of the system isobutyric acid/water of critical composition near Tc is given by taking into account that this system is reactive and that—according to Procaccia et al.—near the critical point the relaxation time τr is expected to increase according to a power law τr∝t−γ with t=(T−Tc)/Tc γ=1.25. This interpretation is in agreement with the experimental results of this study and that of Goldburg et al.
Study of the hydration of a Na+ ion tightly bound to H2PO−4. Importance of polarization
Authors: Perez P. , Prohofsky E. W.
Published: February 2003

We report some results of a molecular dynamics simulation of the interaction H2PO−4+Na++nH2O with n=9, 12, 14. We use a polarizable model for both H2O and H2PO−4. According to our calculation, a potential fitted to available ab initio studies produces a strong binding between Na+ and H2PO−4. It is such that hydration of the Na+ ion is not clearly observed. Regardless of this we can conclude that the effect of assuming the H2PO−4 polarizable is not significant.
Subnanosecond measurements of detonation fronts in solid high explosives
Authors: Sheffield S. A. , Bloomquist D. D. , Tarver C. M.
Published: February 2003

Detonation fronts in solid high explosives have been examined through measurements of particle velocity histories resulting from the interaction of a detonation wave with a thin metal foil backed by a water window. Using a high time resolution velocity-interferometer system, experiments were conducted on three explosives—a TATB (1,3,5-triamino-trinitrobenzene)-based explosive called PBX-9502, TNT (2,4,6-Trinitrotoluene), and CP (2-{5-cyanotetrazolato} pentaamminecobalt {III} perchlorate). In all cases, detonation-front rise times were found to be less than the 300 ps resolution of the interferometer system. The thermodynamic state in the front of the detonation wave was estimated to be near the unreacted state determined from an extrapolation of low-pressure unreacted Hugoniot data for both TNT and PBX-9502 explosives. Computer calculations based on an ignition and growth model of a Zeldovich–von Neumann–Doering (ZND) detonation wave show good agreement with the measurements. By using the unreacted Hugoniot and a JWL equation of state for the reaction products, we estimated the initial reaction rate in the high explosive after the detonation wave front interacted with the foil to be 40 μs−1 for CP, 60 μs−1 for TNT, and 80 μs−1 for PBX-9502. The shape of the profiles indicates the reaction rate decreases as reaction proceeds.
Computer simulation of end-linked elastomers. II. Bulk cured tetrafunctional networks
Authors: Leung Yu-Kwan , Eichinger B. E.
Published: February 2003

Sol–gel distributions at high conversions, simulated by computer with spatial constraints on the distribution of chain ends, are reported for end-linked tetrafunctional systems. The composition of the sol is found to be uniquely dependent upon the functionality f of the cross linker: for f=4, it mainly consists of linear monomers and oligomers containing one saturated cross linker. The molecular weight distribution of the sol exhibits a shoulder at the trimer due to the relative abundance of the ‘‘bow-tie’’ molecule. The populations of various types of dangling ends and loop defects in the gel are given and discussed. Our simulations reveal empirically that the single-loop probability varies with the −3/8 power of the molecular weight of the prepolymers. The power-law dependence of the loop formation on chain length of the prepolymers and the nonrandom nature of the cross-linking reaction is discussed.
Polymer chain statistics and universality: Crossover from random to self-avoiding walks
Authors: Family Fereydoon , Gould Harvey
Published: February 2003

We develop a position space renormalization group (PSRG) method to study the scaling properties of the Domb–Joyce model of an interacting random walk on a lattice. In this model each intersection of the walk with itself receives a weight (1−w). We study the crossover from unrestricted random walks (w=0) to self-avoiding walks (w=1) using a two-parameter PSRG approach, obtain the global flow diagram, and find the result consistent with universality, that the critical behavior for w>0 is described by the self-avoiding walk fixed point. Our PSRG method avoids the difficulty of enumerating the infinite number of spanning random walks in a finite cell. The results for the mean end-to-end length exponent ν in spatial dimension d=1, 2, and 3 are consistent with the exact result ν=1/2 for unrestricted random walks.
Photodetachment, photodissociation, and photochemistry of surface molecules of icy solids containing NH3 and pure H2O ices
Authors: Nishi Nobuyuki , Shinohara Hisanori , Okuyama Tohru
Published: February 2003

Photodetachment and photodissociation of surface molecules of pure and mixed ices containing NH3 and H2O at 90∼130 K have been investigated by using a mass spectrometer situated in a ultrahigh vacuum apparatus. NH3 molecules were excited to the lowest singlet state by an ArF laser at 193 nm (hν=6.4 eV). The detachment of NH3 molecules from pure NH3 solids was observed at a low laser fluence of 0.1 mJ/cm2. Two photons of 248 nm laser (2hν=10 eV) also detached H2O molecules from pure H2O ices. Mixed solids of NH3 and H2O released both molecules from the surface upon irradiation with 193 nm laser. The translational energies of the detached molecules were of the order of the intermolecular interactions of the constituent molecules (0.03∼0.2 eV) and the energy distributions were not changed by the increase of laser fluence. A detachment mechanism explaining all of the observations is proposed. The force acting on the detachment is ascribed to ‘‘electronic exchange repulsion between an excited molecule and a ground state molecule.’’ At a higher laser fluence (≳500 mJ/cm2), the detachment of ions was observed at m/e=17 and 18 with comparable intensities in a mixed solid of NH3 and H2O. Translational energy distributions of the ions peaked at ∼1 eV for both species. Dissociation of atomic hydrogen from the surface molecules was observed in the solid containing NH3. A fraction of the atomic hydrogen posseses translation energy (Et=17 kcal/mol=0.74 eV) which is similar to that observed in the photolysis of free NH3 molecles in a molecular beam. A very low energy component (Et=0.6 kcal/mol=0.026 eV) appeared and exhibited an intensity change depending upon the deposition and annealing conditions. Molecular hydrogen was also released with a translational temperature very close to the surface temperature. The photoproducts N2H4, N2H2, and NH2OH were detected in the mixed solid system of NH3 and H2O. Photochemical bimolecular condensation is proposed as a mechanism of the reaction forming a hydrogen molecule and a condensed molecule. The present study affords an experimental foundation of the photoejection model of physically adsorbed molecules on interstellar grains presented by Watson and Salpeter (Ref. 2).
Stabilization energies of weak hydrogen bonded molecular complexes. Comparison of simple models
Authors: Castiglioni C. , Gussoni M. , Zerbi G.
Published: February 2003

The stabilization energy ΔE of some weak molecular complexes is calculated via an electrostatic model based on the use of experimental atomic charges derived from infrared intensities. The results compare favorably with observed values. The comparison with the results of another model recently proposed by Flygare and based on the use of data completely independent from infrared intensities, is also discussed.
Action-angle variables for quantum mechanical coplanar scattering
Authors: Askar A. , Rabitz Herschel
Published: February 2003

A formulation of quantum mechanical scattering is presented with action-angle variables. The theory is confined to coplanar three atom scattering which reduces to the consideration of three coordinates taking into account conservation of energy and angular momentum. The resultant partial differential equation is treated within a finite element formulation with the spatial elements taken as parallelopipeds. It is argued that the choice of the three independent coordinates in a calculation is not critical and no channel matching is required, although it could be included. A simple illustration with rotationally inelastic scattering is considered for the H+H2 system. The results are in good agreement with earlier close coupling calculations.
The reaction of NH+⋅3 with H2S: Dependence on the translational and internal energy of NH+⋅3
Authors: Wagner-Redeker Winfried , Kemper Paul R. , Bowers Michael T. , Jennings Keith R.
Published: February 2003

The reactions of vibrationally and of kinetically excited NH+⋅3 ions with H2S were investigated using a tandem ion cyclotron resonance spectrometer. NH+⋅3 ions with internal energies ranging from 1–5 eV were generated by charge transfer reactions, whose energy partioning is known. The charge transfer reagents used included Ar+⋅, Kr+⋅, Xe+⋅, N2+⋅, CO+⋅2 , CO+⋅, and O+⋅2 . The ionic products formed in the reaction of excited NH+⋅3 with H2S were NH+4 , H2S+⋅ and H3S+. It is shown that the product distribution is sensitive to the NH+⋅3 internal energy. At low internal energies, NH+4 is the dominant product, while at 5 eV all three reaction products are of comparable intensity. Competition between formation of H2S+⋅ and H3S+ is effective only at NH+⋅3 internal energies above 2 eV. The total rate constant was found to be 6±2×10−10 cm3/s and appeared to be independent of the internal energy of NH+⋅3 . Kinetically excited NH+⋅3 ions were formed using ICR double resonance and the effect on product distribution and total rate constant in the reaction with H2S was studied. It is shown that kinetic energy exclusively drives the charge transfer channel. The total rate was independent of the NH+⋅3 kinetic energy in the energy range of the experiment.
Resonances by the complex coordinate method with Hermitian Hamiltonian. II. Error estimates
Authors: Moiseyev N. , Froelich P. , Watkins E.
Published: February 2003

Using the Hermitian representation of the complex coordinate method that recently has been developed (Ref. 1), error estimates for the approximate resonance position and width are calculated. We prove here that for a given estimate of the resonance position and width the exact solution is embedded in the surface of a crescent being part of an annular ring whose inner and outer radii can be obtained by variational calculations. An illustrative numerical example is presented.
Laser studies of methyl radical reactions with Cl2 and Br2: Absolute rate constants, product vibrational excitation, and hot radical reactions
Authors: Kovalenko Laurie J. , Leone Stephen R.
Published: February 2003

Reactions of methyl radicals with Cl2 and Br2 are studied by pulsed laser dissociation of CH3I followed by time-resolved detection of infrared vibrational fluorescence from the C–H stretch modes of the methyl halide product. This method provides a determination of the absolute rate constants for the methyl radical reactions. The rate constants for thermal reactions of CH3 with Cl2 and Br2 are 1.5(±0.1)×10−12 and 2.0(±0.4)×10−11 cm3 molecule−1 s−1, respectively. From the fluorescence intensity of the methyl halide product, an estimate can be made of the fraction of energy available upon reaction that goes into product vibration. For both reactions this is determined to be about 0.5. In addition, the reaction rates are observed to be considerably enhanced for hot methyl radicals produced in the dissociation of CH3I. The enhancement is shown to be due predominantly to the translational excitation, as opposed to the vibrational excitation, imparted in the photofragmentation.
Predissociation rates as a probe of intramolecular dynamics
Authors: Sundberg R. L. , Heller E. J.
Published: February 2003

We investigate the consequences of quasiperiodic and ergodic classical dynamics on predissociation rates from bound vibrational states into a continuum. We find a strong correlation between the quasiperiodic/ergodic motion and rate constants for certain locations of the predissociation surface. Other locations for the predissociative surface are insensitive to the difference between classical motion which is confined to small regions of phase space and motion which samples a large portion of its available phase space.
Quasiclassical trajectory study of the reaction O(3P)+H2→OH+H. The effects of the location of the potential energy barrier, vibrational excitation and isotopic substitution on the dynamics
Authors: Broida Michael , Persky Avigdor
Published: February 2003

Three dimensional quasiclassical trajectory calculations were carried out for the nearly thermoneutral reaction of oxygen atoms O(3P) with hydrogen molecules (H2,D2, and HD). Three LEPS potential energy surfaces, all having the same barrier height but with slightly different locations, were used for the calculations. Calculations for various initial vibrational states of H2 were performed for reaction temperatures of 300, 500, and 900 K. For the reactions O+D2 and O+HD calculations were carried out for the ground vibrational state for 500 and 900 K. The trajectory calculations showed that the rate constants are very sensitive to the position of the potential energy barrier. They increase or decrease considerably when the barrier is shifted slightly in the direction of the exit valley or entry valley, respectively. The rate of the reaction O+H2(υ)→OH+H was found to be enhanced significantly with vibrational excitation of the H2. At 300 K the rate constant for υ=1 was found to be larger than for υ=0 by three to four orders of magnitude, depending on the potential energy surface used. The vibrational energy of the reactants Ev was found to be converted very efficiently into vibrational energy of the products E′v. The results of the present trajectory calculations are compared with earlier results for another nearly thermoneutral reaction Cl+H2→HCl+H and with experimental results for the rate constants and kinetic isotope effects.
Resolution of the exchange anomaly in triplet exciton ion radical salts
Authors: Chesnut D. B. , Meinholtz Dore C.
Published: February 2003

The temperature dependence of the D and E zero-field splitting parameters in the triplet-exciton-containing ion radical salt (φ3AsCH+3)(TCNQ)−2 has been measured at low temperatures and used to correct observed splittings at higher temperatures where triplet–triplet exchange is present. Analysis of the exchange effects from the thermally corrected splittings leads to an exchange activation energy (0.110 eV) and frequency factor (1.4×1011 Hz) in good agreement with those obtained from previous width measurements.
Lack of M-preserving propensities in rotationally inelastic collisions of NO(X 2Π1/2)
Authors: Orlikowski Tadeusz , Alexander Millard H.
Published: February 2003

An expansion of the collision-frame scattering amplitude for JM→J′M′ transitions in irreducible tensor components facilitates the investigation of M-preserving propensities when quantization refers to the geometric or kinetic apse vectors. Specifically one can demonstrate that M preservation can be expected for processes where backward scattering dominates, but not for transitions which are forward peaked. For forward scattered processes involving open-shell diatomics, either even or odd changes in M (apse quantization) will be dominant, depending on the e/f symmetry label of the initial and final wave functions. These formal results will be illustrated by close-coupling calculations on collisions of NO(X 2Π1/2).
Solubility and thermodynamics of solution of xenon in liquid n-alkanes
Authors: Ben-Naim A. , Marcus Y.
Published: February 2003

Using data from Pollack and Himm on the solubilities of xenon in liquid n-alkanes, we have computed the standard free energies, entropies, and enthalpies of solution of xenon in these liquids. These quantities are devoid of extraneous terms in the densities of the alkanes that are inherent in the quantities presented by Pollack and Himm. A qualitative interpretation of the results is suggested and compared with that suggested by these authors.
A crossover integral equation for the structure of simple liquids
Authors: Foiles S. M. , Ashcroft N. W. , Reatto L.
Published: February 2003

We propose here a new choice of bridge function appearing in the modified hypernetted chain approximation for determining the structure and thermodynamic functions of simple classical fluids. It takes the form of a crossover between the bridge function for a hard sphere system, which is known to give a satisfactory short-range description of correlations in such systems, and the mean spherical model result which is more suitable for long-range behavior. We derive a thermodynamic consistency requirement which is entirely local in its implementation. The results of the method are compared with simulation studies of the Lennard-Jones fluid, the one-component plasma, and models of simple liquid metals.