bluuebunny/crossref_metadata_2025 huggingface dataset:
On the intermolecularity or intramolecularity of nitrosonium nitrate formation in thin films of nitrogen dioxide: A Fourier transform infrared study
Authors: Givan A. , Loewenschuss A.
Published: July 2002

The question of whether the formation of the NO+NO−3 (nitrosonium nitrate), the ionic form of nitrogen dioxide, via the decomposition of the ON–O–NO2, ‘‘D′’’ isomer, is an inter- or intramolecular process, is addressed via temperature and concentration dependent FTIR studies of mixed films of N2O4 with CCl4. The process of nitrosonium nitrate production, both during deposition of disordered solid films of nitrogen dioxide and during their subsequent warming, is concluded to be intermolecular but not order dependent. A mechanism is tentatively suggested.
Alignment of the E,F 1Σ+g, v′E=1 state of H2 by two-photon excitation
Authors: McCormack E. F. , Pratt S. T. , Dehmer J. L. , Dehmer P. M.
Published: July 2002

The alignment of the E,F 1Σ+g, vE=1 state of H2 produced by two-photon excitation from the X 1Σ+g, v″=0 ground state was investigated using two different techniques. First, in a single-color experiment, photoelectron angular distributions were measured for the two-photon resonant, three-photon ionization of H2 via the Q(0) and Q(1) transitions to the E,F 1Σ+g, v′E=1 state. The photoelectron angular distributions are consistent with an unaligned E,F 1Σ+g, vE=1, J′=1 state. Second, in a two-color experiment, the photoionization spectra of several vibrationally autoionizing (X 2Σ+g)np, v=1 Rydberg states excited from the v′E=1, J′=1 level of the E,F 1Σ+g state were measured as a function of the relative polarizations of the pump and probe beams. The polarization dependence of the relative intensities of the P(1) and Q(1) transitions was used to determine the alignment of the E,F 1Σ+g, vE=1, J′=1 state produced by the pump laser. Consistent with the angular distribution measurements, the polarization results indicate that the populations in the M′=0 and M′=±1 levels of the E,F 1Σ+g, v′E=1, J′=1 state are equal, within the experimental uncertainty of 15%.
Scaled-particle theory and the short distance behavior of continuum percolation
Authors: Given James A. , Stell George
Published: July 2002

In this paper, we use results from geometric probability theory to constrain the behavior of continuum percolation models. Specifically, we consider the random percolation of spheres, in which particles are distributed at random with density ρ̄, each pair being considered connected if its separation is less than a distance a. For this model we prove a zero-separation theorem, which gives the first three terms in a Taylor series expansion around zero separation of the two-point connectedness function. These expressions are then used in a closure for the Born–Green equations of percolation. The result is an approximate equation of state, or formula for the mean number of clusters 〈nc〉 as a function of density, which is quite accurate at moderate densities. Using the relation between continuum percolation and the continuum Potts model, we develop two different forms of scaled particle theory for continuum percolation. These theories are then combined with the zero-separation theorems to give several approximate equations of state for percolation, i.e., formulas for the mean number of clusters 〈nc〉 as a function of density ρ̄. Finally, we use an argument from geometric probability theory to provide a closure of the Born–Green hierarchy for percolation. The equation of state resulting from this procedure is exact to third order in a density expansion.
Search for a multiproperty empirical intermolecular potential for XeSF6
Authors: El-Sheikh S. M. , Tabisz G. C. , Pack R. T
Published: July 2002

An approximate anisotropic intermolecular potential for the Xe–SF6 interaction is developed by fitting the Morse–Morse–Morse–Spline–Van der Waals potential form to interaction virial coefficient, diffusion coefficient and collision-induced light scattering data.
Statistical mechanics of rubber elasticity
Authors: Altenberger Andrzej R. , Dahler John S.
Published: July 2002

A new, statistical mechanical theory is proposed for the elasticity of rubber-like materials. Deficiencies and contradictions of the traditional approach to the problem are pointed out. It is shown that anharmonic interactions are important to the successful modeling of the non-Gaussian behavior of elastomers.
The structure function of branched polymers in a good solvent: A lattice calculation
Authors: Lam P. M.
Published: July 2002

The fourth 〈r4〉 and the sixth 〈r6〉 moments of the average monomer–monomer separation of branched polymers is calculated in two and three dimensions in the lattice animal model. The universal ratios ζ≡〈r4〉/(R2g)2 and μ≡〈r6〉/(R2g)3 with R2g the average squared radius of gyration of the polymers are found to be ζ=0.550±0.003, μ=0.201±0.003 in two and ζ=0.600±0.005, μ=0.254±0.006 in three dimensions. They are significantly different from the Gaussian values calculated here to be ζ=0.9204 and μ=0.7889 in all dimensions. Using the values of ζ and μ the scaled structure function for small values of the momentum transfer can be calculated and compared to that of the Gaussian approximation.
Exact results for the adsorption of a dense fluid onto a triangular lattice of sticky sites
Authors: Huckaby Dale A. , Blum Lesser
Published: July 2002

A discussion of exact results for the interface between a dense fluid of molecules with a spherical hard core of diameter σ and a triangular lattice of sticky sites with spacing d on a wall with hard repulsive potential is given. When σ≤d and first neighbors attract, there is a first order transition when the stickiness parameter λ, singlet wall density ρ01(0), and pair correlation function g02(d) satisfy λρ01(0)[g02(d)]3=1. If d<σ< 7/8 d, and further neighbors do not interact, the model is equivalent to the hard hexagon model solved by Baxter. In this case there is a second order phase transition between an ordered 7/8 × 7/8 phase and a disordered one.
Quenching of chemical oscillations: General theory
Authors: Hynne F. , So/rensen P. Graae , Nielsen K.
Published: July 2002

We develop the theoretical basis for quenching analysis of small-amplitude chemical oscillations near a Hopf bifurcation in a concentration space of arbitrary dimension. (Quenching of a limit-cycle oscillation is carried out experimentally by instantaneously changing the concentrations of chemical species in a definite phase of the oscillation.) It is shown that quenching by addition of a species determines the reciprocal of the corresponding component of an eigenvector of the transposed Jacobi matrix. The set of all independent quenchings determines the tangent space of the stable manifold at the associated saddle focus even if only one species is monitored. Reconstruction of the oscillations from quenching data in an n-dimensional concentration space requires additional information if n>3. We show that if n−2 chemical species are monitored the oscillatory parts of the remaining two (unknown) concentrations can be exactly calculated from the result of n quenchings. In this case one additional quenching by dilution suffices to determine also the two unknown average concentrations. Reconstruction from m quenchings with m−2 species monitored may still be possible with m less than the dimension; it will be exact if the species tested by quenching are the only ones oscillating, and an accurate approximation if the remaining n−m species oscillate with sufficiently small amplitudes. Experimental quenching data can be used for quantitative tests of models of an oscillatory system, and we show how quenching data can be calculated from models.
Calculation of the solvent contribution to the potential of mean force between water molecules in fixed relative orientation in liquid water
Authors: Mezei Mihaly , Ben-Naim Arieh
Published: July 2002

The contribution of the solvent molecules to the potentials of mean force between two water molecules in selected fixed relative orientations was calculated as a function of the interoxygen distance using the probability ratio method with adaptive umbrella sampling in Monte Carlo computer simulations.
The librational ground state of monodeuteromethane adsorbed on the surface of graphite
Authors: Ball P. C. , Inaba A. , Morrison J. A. , Smalley M. V. , Thomas R. K.
Published: July 2002

The heat capacity of CH3D on graphite has been measured at low temperatures (0.3–7 K) and shown to have a Schottky anomaly resulting from the difference in energy between states with the single deuterium atom pointing away from the surface (D up) and states where the deuterium atom is part of the tripod of atoms pointing towards the surface (D down). The energy difference is 380±20 μeV with the D-down configuration being the more stable. Incoherent neutron scattering spectra of CH3D adsorbed on exfoliated graphite have been used to resolve the finer detail of the tunneling splittings in the system. The energy levels can be described quantitatively in terms of five parameters, the splitting between D-up and D-down configurations and two pairs of parameters, one pair related to the rotational potential about the unique C–H(D) bond normal to the surface and the other pair to rotation about one of the three equivalent C–H(D) bonds pointing towards the surface. Comparison of the results from CH3D and earlier results from CH4 show that both potentials contain two Fourier components, approximately 75% of a threefold and 25% of a sixfold component in phase with one another. The values originally obtained from the CH4 tunneling spectrum for the two barrier heights 202 and 172 cm−1 for rotation about the unique and nonunique axes, respectively, are shown to have an uncertainty of about 5% resulting from the approximations used for the shape of the potential in the interpretation of the earlier experiment.
Computer simulations of polyelectrolyte chains in salt solution
Authors: Christos George A. , Carnie Steven L.
Published: July 2002

We use Monte Carlo simulation methods to study the conformational properties of a partially ionized polyelectrolyte chain in 1:1 electrolyte solution at room temperature and the distribution of salt ions around the polyion chain. The present results are extensively compared with our previous work on polyelectrolyte chains interacting via a screened Coulomb potential with no ions. The two models give similar conformational properties for lower degrees of ionization of the polyion and/or higher salt concentrations. For higher degrees of ionization and/or lower salt concentrations, the counterions seem to position themselves so as to enhance their effect over the screening model counterpart, and the polyelectrolyte chains are generally much more coillike than in the screening model. Our conclusions partly agree with Valleau’s work on shorter chains. Although some aspects of counterion condensation are evident, our results do not quantitatively agree with the limiting form of Manning’s counterion condensation theory, since conformational properties of the chain continue to depend on the ionization beyond the critical linear charge density in his theory. Simulations for fully ionized chains were found to behave erratically and most of these results had to be discarded.
Competition between dissociation and exchange processes: Contrasting dynamical behaviors in collinear H+H2 and He+H+2 collisions
Authors: Dove J. E. , Mandy M. E. , Mohan V. , Sathyamurthy N.
Published: July 2002

Dissociative, exchange, and nonreactive collisions of the H+H2 and He+H+2 systems in collinear geometry are examined. The behavior of the two systems is found to differ qualitatively and quantitatively. For H+H2 (v=0), quasiclassical trajectory (QCT) calculations on the Siegbahn–Liu–Truhlar–Horowitz surface show that the dynamic threshold energy (Edyth) for dissociation is twice the energetic threshold (Eeth). For v=1, the elevation of Edyth is slightly less. There is vibrational enhancement of collision induced dissociation (CID) near threshold, but slight vibrational inhibition at higher energies. At energies above that required for dissociation, a second threshold to exchange is observed and the exchange process eventually takes over from dissociation. For He+H+2 (v=0,1), QCT calculations on the McLaughlin–Thompson surface yield Edyth∼Eeth for dissociation, but also show an antithreshold, with the exchange process becoming dominant at a higher energy. There is only vibrational enhancement of the dissociation process in the energy range investigated. The differences in the dynamical behavior of the two systems are analyzed in terms of reactivity band plots and individual trajectories. Examination of the dynamics for different mass combinations on the HeH+2 potential energy surface gives an insight into the kinematic factors governing CID in collinear geometries. We also consider the qualitative and quantitative differences between this collinear study and our earlier three dimensional work on the same systems.
Infrared diode laser spectroscopy of the ν3 fundamental of the CD3 radical
Authors: Fawzy Wafaa M. , Sears Trevor J. , Davies Paul B.
Published: July 2002

The infrared absorption spectrum of the ν3 fundamental band of the CD3 radical has been detected by diode laser absorption spectroscopy. The CD3 radical was produced by excimer laser photolysis of CD3I at 248 nm or (CD3)2CO at 193 nm. Molecular parameters of the v3=1 vibrational state were determined from a least-squares fit to 62 rotation–vibration transitions. In this fit, molecular parameters describing the ground state were constrained to those obtained from previous spectroscopic studies of the ν2 parallel IR band [J. M. Frye, T. J. Sears, and D. Leitner, J. Chem. Phys. 88, 5300 (1988)]. The molecular parameters determined in the present work are the band origin ν0=2381.088 60(84), B′=4.758 737(40), C′=2.373 297(34), (ζC)3=0.476 278(72), q3=0.003 76(59), D′N =0.000 187 9(5), DNK =−0.000 341 0(12), D′K =0.000 143 7(8), ηN =−0.000 005 5(36), η′K =0.000 060(35), and qN =0.000 063(17), all in cm−1 with one standard deviation in parentheses. The derived molecular parameters were compared with those for the CH3 radical v3=1 level determined previously [T. Amano, P. Bernath, C. Yamada, Y. Endo, and E. Hirota, J. Chem. Phys. 77, 5284 (1982)]. The molecular parameters of the v3=1 state of the CD3 and CH3 radicals follow the expected isotopic relationships. We have also found that the determined molecular parameters reasonably satisfy the approximate planarity relationships [J. K. G. Watson, J. Mol. Spectrsoc. 65, 123 (1977)] and the sign of the l-type doubling constant is consistent with a planar equilibrium structure.
Dynamic light scattering studies on charged rod-like fd-virus in dilute aqueous solution
Authors: Schulz Susanne F. , Maier Erich E. , Krause Rainer , Hagenbüchle Martin , Deggelmann Martin , Weber Reinhart
Published: July 2002

Time correlation functions of the scattered light intensity are studied in aqueous solutions of charged rod-like fd-virus (L=880 nm, d=6 nm) at various ionic strengths. The short time behavior of the correlation function is dominated by the static structure factor S(q) which is also independently determined from static light scattering experiments. Comparison of correlation functions of solutions with high ionic strength (screened Coulomb interaction) and those of solutions with liquid-like nearest neighbor order (strong Coulomb interaction) shows different single particle diffusion coefficients on medium time scales at high scattering vectors, where mainly single particle properties are observed by light scattering. The single particle diffusion coefficient decreases with increasing structure peak height of the solutions. At low scattering vectors an extra slow mode component of the correlation function is observed for solutions with Coulomb interaction.
Hyperfine structure of the MnH X 7Σ+ state: A large gas-to-matrix shift in the Fermi contact interaction
Authors: Varberg Thomas D. , Field Robert W. , Merer Anthony J.
Published: July 2002

Sub-Doppler spectra of the A 7Π–X 7Σ+ (0,0) band of gas phase MnH near 5680 Å were recorded by intermodulated fluorescence spectroscopy. The spectra reveal hyperfine splittings arising from both the 55Mn and 1H nuclear spins. Internal hyperfine perturbations have been observed between the different spin components of the ground state at low N″. From a preliminary analysis of several rotational lines originating from the isolated and unperturbed F1(J″=3) spin component of the X 7Σ+(N″=0) level, the 55Mn Fermi contact interaction in the ground state has been measured as bF=Aiso =276(1) MHz. This value is 11% smaller than the value obtained by Weltner et al. from an electron-nuclear double resonance (ENDOR) study of MnH in an argon matrix at 4 K. This unprecedented gas-to-matrix shift in the Fermi contact parameter is discussed.
A new mean-field theory for dilute polymer solutions: Phase diagram, conformational behavior and interfacial properties
Authors: Szleifer I.
Published: July 2002

A new mean-field theory for dilute polymer solutions is presented. The approach is based on looking at a polymer molecule in the mean-field of the solution. This has the advantage of allowing the conformational behavior of the chain molecules and the thermodynamic properties of the solution to be studied within the same theoretical framework. The phase diagram predicted by the theory has a critical temperature which is lower, and a width of its coexistence curve which is larger, than those obtained from the simpler mean-field (Flory) theory. The critical volume fraction is found to scale with the degree of polymerization n as φc∼n−0.40, in excellent agreement with experimental results. The theory allows one to study the size and shape of the polymer molecules as functions of the thermodynamic state of the solution. It is found that on the concentrated side of the coexistence curve the average shape of the molecules is an elongated ellipsoid, while in the dilute side the molecules are smaller in size and their shape becomes more sphere-like as the concentration is reduced. The theory is extended to study the properties of the interface separating two phases at coexistence. In particular, it is applied to study the variation of the sizes, shapes and orientations of the polymer molecules due to the inhomogeneous density in the interfacial region. It is found that as the interface is approached from the polymer-rich side the molecules tend to slightly decrease their size and their longest axes tend to orient parallel to the interface. At the edge of the polymer-poor side of the interface the molecules are very elongated and their long axes are oriented almost exclusively perpendicular to the interface. It is argued that the orientation profile in the interfacial region is a general property of nonspherical molecules.
Spin–orbit state selectivity in KrF*and XeF* formation from ion-recombination reactions of Kr+(2P3/2,1/2) and Xe+(2P3/2,1/2) with SF−6 in the flowing afterglow
Authors: Tsuji Masaharu , Furusawa Makoto , Nishimura Yukio
Published: July 2002

Appropriate filter gases have been used to select one of the spin–orbit levels of Kr+ and Xe+, 2P3/2 or 2P1/2 , in a flowing afterglow reactor. The separated reactions of Kr+ and Xe+ in the 2P3/2 and 2P1/2 levels with SF−6 were examined at 300 K by observation of KrF* and XeF* formation. The Kr+ and Xe+ ions in the 2P3/2 level give only the B and C states, while those in the 2P1/2 level give the B and D states. The B:C distributions from Kr+(2P3/2) and Xe+(2P3/2) were estimated to be 0.62:0.38 and 0.68:0.32, respectively, while the B:D distributions from Kr+(2P1/2) and Xe+(2P1/2) were measured as 0.05:0.95 and 0.06:0.94. The high spin–orbit state selectively can be generally explained by the conservation of the Rg+(2P3/2)+F− and Rg+(2P1/2)+F− characters.
The evolution of vibrational phase space during the collisional relaxation of highly excited collinear CS2
Authors: Bruehl Margaret , Schatz George C.
Published: July 2002

This paper presents a detailed theoretical analysis of the vibrational relaxation of highly excited CS2 (initially 32 640 cm−1) in collinear collisions with a thermal bath of He atoms. The relaxation is simulated by a classical molecular dynamics method in which CS2 undergoes successive collisions with thousands of He atoms. In most of our studies the CS2 coordinates and momenta at the end of one collision are used as input to the next collision, so it is possible to examine the detailed evolution of the CS2 vibrational phase space during the relaxation process. By restricting motion to being collinear, it is possible to characterize this evolution using surfaces of section and other methods. Comparisons of our collinear results with corresponding three-dimensional simulations indicates that the collinear restriction does not alter the relaxation process significantly.

Our phase space analysis indicates that individual relaxation sequences can evolve in a variety of different ways depending on the initial location in phase space and on the details of subsequent collisions. Much of the initial phase space is chaotic, and if a sequence starts in such a region then after usually less than 30 collisions, the CS2 has moved into a nonlinear resonance zone where the antisymmetric and symmetric stretch modes have frequency ratios of 5:2, 7:3, or 9:4. These nonlinear resonances do not greatly change the ensemble averaged energy transfer per collision 〈ΔE〉 compared to the chaotic regions, but they are collisionally stable relative to these regions. As a result, it takes an energetic collision to kick the molecule out of a nonlinear resonance. If kicked out, then usually within a few more collisions another nonlinear resonance (or perhaps the same) has been entered. As relaxation progresses molecules caught in nonlinear resonances eventually drop down to simple quasiperiodic regions where the frequency ratio is not constrained to be a ratio of integers. We do find a region of phase space that is quasiperiodic even at 32 640 cm−1, corresponding to a ‘‘hyperspherical mode’’ in which most of the vibrational energy is locked up in antisymmetric stretch motion. Molecules in this region of phase space relax much more slowly than in chaotic and resonant regions. In addition, molecules starting initially in a chaotic region can be kicked into this hyperspherical mode region, leading to an additional slowing of the relaxation as the molecule drops down the well. This additional slowing plays an important role in determining the dependence of 〈ΔE〉 on the molecular vibrational energy E.

In particular, we find that 〈ΔE〉 varies linearly with E if phase space undergoes forced randomization after each collision, but it shows a stronger than linear dependence when redistribution is not forced. This implies that deviations from linearity in the dependence of 〈ΔE〉 on E provide a measure of the division of phase space into regions that have very different relaxation characteristics.
Superdiffusion in structurally disordered systems in any spatial dimension
Authors: Dunlap David H. , Phillips Philip
Published: July 2002

We show that a model recently proposed by Dunlap, Kundu, and Phillips for the absence of localization in structurally disordered systems can be generalized. Specifically we show how the perfect matching condition on the diagonal and off-diagonal matrix elements can be relaxed. It is demonstrated within a tight-binding model that if the structural disorder is chosen from a bivalued distribution (that is, the nearest-neighbor potential has two minima) superdiffusion obtains even in one dimension for the majority of the site-diagonal and site-off-diagonal coupling constants characterizing real systems. In one dimension the mean-square displacement of an initially localized particle is found to grow in time as t3/2 regardless of the magnitude of the disorder. The conditions for observing this behavior experimentally are discussed.
Cavity field and reaction field in nonpolar fluids
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We discuss the cavity field and the reaction field appearing in the Onsager–Böttcher theory of the dielectric constant of a nonpolar fluid. The ratio of both fields to the Maxwell field is determined by computer simulation for a hard-sphere fluid at six volume fractions. We find that the reaction field is reasonably well predicted by the Onsager–Böttcher theory, provided the cavity radius is adjusted to reproduce the exact value at small polarizability. The difference of the cavity field from the Lorentz local field is poorly predicted by the Onsager–Böttcher theory.
Absorption spectra of alkali halide molecules in the vacuum ultraviolet
Authors: Kubodera S. , Wisoff P. J. , Sauerbrey R.
Published: July 2002

The ultraviolet (UV) and vacuum ultraviolet (VUV) absorption spectra of three alkali halide molecules CsF, CsCl, and RbF have been determined experimentally. The longest wavelength absorption features are attributed to bound–free or bound–weakly bound transitions where the upper state correlates to ground state alkali and halogen atoms. The absorption at shorter wavelengths is due to transitions from the bound ground state to excited states that correlate to excited alkali and neutral halogen atoms. The covalent repulsive curves of CsF were found to have a close resemblance to the curve corresponding to the first ionization potential of CsF. The peak absorption cross sections are similar for the CsF and RbF molecules. CsCl molecules have a three to four times larger absorption cross section for the analogous upper states. The absorption oscillator strengths have been determined from the measured absorption cross sections, and are compared to those of other alkali halide molecules that have UV absorptions.
A pulsed optical–optical double resonance study of the 1 1Πg state of 7Li2
Authors: Miller D. A. , Gold L. P. , Tripodi P. D. , Bernheim R. A.
Published: July 2002

The results of an optical–optical double resonance study of the 1 1Πg state of 7Li2 are presented. This completes the observation and characterization of all singlet states of Li2 correlating with the Li(2s)+Li(2p) dissociation limit. Data spanning the first 31 vibrational levels were used to obtain Dunham molecular constants and a Rydberg–Klein–Rees (RKR) potential corresponding to 96% of the estimated potential well. The 1 1Πg state, which lies at Te=21 998.25  cm−1, can be characterized by a limited set of Dunham coefficients with ωe=93.354 cm−1, ωexe =−1.874 cm−1, Be=0.291 89 cm−1, and a dissociation energy De=1422.5±0.3 cm−1. The results are compared with the predictions of recent theoretical treatments.
Effect of cluster scavenging on homogeneous nucleation
Authors: Shi G. , Seinfeld J. H.
Published: July 2002

A closed-form expression for the effect of cluster scavenging on the rate of homogeneous nucleation of a vapor in the presence of continuum regime particles is obtained by solving the kinetic equation of nucleation by the method of singular perturbation. The reduction in nucleation rate of a condensing species at a given supersaturation is shown to be dependent largely on the number concentration, the size of the sink particles, and the molecular number concentration of the background gas. The reduction in the rate of nucleation due to the cluster scavenging by transition regime particles is also discussed.
Interaction of reactive ions with Pt(100). II. Dissociative scattering of molecular ions near the threshold energy region
Authors: Akazawa Housei , Murata Yoshitada
Published: July 2002

Dissociative scattering of N+2, CO+, and CO+2 ions from Pt(100) has been studied at low energies. For dissociated N+ emergence, the threshold of incident kinetic energy was found to be 40 eV. The threshold of dissociated CO+ emergence in the CO+2 incidence was clearly observed at 25 eV. The threshold of dissociated C+ emergence in the CO+ incidence was observed at 70 eV. Correlation between the dissociation energy of a free molecule and the threshold of incident kinetic energy is clearly discernible. The angular distributions show that the dissociation product appears at larger scattering angles than that for the parent molecular ions scattered nondissociatively. These experimental results are consistent with the model that dissociation is due to translational-rovibrational energy transfer above the dissociation limit at the impulse collision with the surface.
High resolution neutron scattering study of rotational excitations in mixed (CH4)1−c(CD4)c systems
Authors: Prager M. , Press W.
Published: July 2002

Inelastic neutron scattering spectra of the mixed system (CH4)1−c(CD4)c, c=0.05, 0.15, 0.20, and 0.96, were measured with high energy resolution (δE>9μeV). For c=0.05 and 0.15 weakly disturbed tunnel transitions and almost free-rotor lines, both characteristic of phase II of methane, are observed. The line shifts and broadenings in this concentration range are almost quantitatively described by a statistical model, which assumes a linear decrease of the tunnel splitting with the number of ordered CD4 neighbors. The decrease of the tunnel splitting is compatible with an octopole moment of the CD4 molecules increased by about 20% compared to CH4 molecules at ordered sites. Changes of the lattice parameter are incorporated into the model. The model also accounts for a part of the linewidth. Additionally a splitting of T states occurs as a consequence of the symmetry reduction. The systems show strong relaxation effects. The shift of the free-rotor lines is described by a quadratic dependence on the potential strength. At c=0.2 the mixed compound undergoes a phase transition into a more ordered phase at Tc≊12.9 K. It is not certain that this phase is identical with the formerly established phase III, characteristic of pure CD4 and CH4 under pressure. A three-site model not very different from that used for CD4 III describes the spectra rather well, including that obtained for c=0.96. The spectra are incompatible with a proposed crystal structure P42/mbc [Maki et al., J. Chem. Phys. 70, 655 (1979)].
Multiplet-specific shape resonance and autoionization effects in (2+1) resonance enhanced multiphoton ionization of O2 via the d 1Πg state
Authors: Stephens J. A. , Braunstein M. , McKoy V.
Published: July 2002

In this paper we discuss the single-photon ionization dynamics of the d 1Πg Rydberg state of O2. Comparision is made with vibrationally resolved measurements of photoelectron spectra which employ (2+1) resonance enhanced multiphoton ionization (REMPI) through the d 1Πg state. A σu shape resonance near the ionization threshold leads to non-Franck–Condon vibrational branching ratios and a substantial dependence of photoelectron angular distributions on the vibrational state of the X 2Πg ion. Significant differences exist between our one-electron predictions and experiment. These are mainly attributed to electronic autoionization of repulsive 1∑−u, 1∑+u, and 1Δu states associated with the 1π3u1π3g configuration. A proposed singlet ‘‘K’’ 1Πu Rydberg state converging to the A 2Πu ion probably also contributes to autoionization in the d 1Πg state spectrum. We also show that autoionizing H and J 3Πu Rydberg states of O2 converging to the a 4Πu and A 2Πu ionic thresholds, respectively, may play a previously unsuspected role in the C 3Πg state one-color REMPI spectra. We discuss multiplet-specific (spin-dependent) effects via comparision of these results with recent experimental and theoretical studies of O2C 3Πg photoionization.
Heats of formation of SiHmFn calculated by a
b i
n
i
t
i
o molecular orbital methods
Authors: Ignacio Edgar W. , Schlegel H. Bernhard
Published: July 2002

Reliable heats of formation are known for SiHn and SiF4, but experimental data for some of the other perfluorosilanes and the mixed SiHmFn compounds are less reliable or unavailable. Because of the importance of these compounds in chemical vapor deposition (CVD) and etching silicon and because of the continued uncertainty in some of the heats of formation, we have extended our previous calculations [J. Phys. Chem. 88, 6254 (1988)] on the ΔH0f of these compounds. The present calculations have been carried out at a higher level of theory [Mo/ller–Plesset perturbation theory including single, double, triple, and quadruple (MP4SDTQ)] with larger basis sets [6-31G(2d,2p), 6-31++G(d,p), 6-31++G(2d,2p)] for the entire set of molecules. To establish accurate theoretical estimates of ΔH0f for SiHn and SiFn additional calculations were performed on SiX, SXY, SiH3, and SiH2XY (X,Y=H,F) at the MP4SDTQ level with even larger basis sets [6-31G(3d,3p), 6-31G(df,p), 6-311G(d,p), 6-311G(2df,2p), 6-311++G(2df,2p)]. Based on the experimental ΔH0f for SiHn and SiF4 and the calculated ΔHr for isodesmic reactions, SiF is found to be 9 kcal/mol more stable and SiF2 is found to be 12 kcal/mol more stable than the currently accepted values; the calculated ΔH0f of SiHn and SiF3 agree with the value advocated by Walsh. The theoretical estimated heats of formation (298 K, ideal gas) are: SiH, 89.9±2; SiF, −14.2±2; SiH2, 65.7±2, SiHF, −37.8±2; SiF2, −153.0±2; SiH3, 47.7±2; SiH2F, −44.9±2; SiHF2, −142.6±2; SiF3, −240.7±2; SiH3F, −85.8±2; SiH2F2, −186.3±2; SiHF3, −288.2±2 kcal/mol.
A generalized radiation boundary condition
Authors: Dayan Ido , Weiss George H.
Published: July 2002

The radiation boundary condition was originally proposed by Collins and Kimball as a means of avoiding the unphysical prediction of the Smoluchowski model for reaction rates that the calculated rate in three dimensions k(t) has the property k(0)=∞. A microscopic model that can be used to derive the boundary conditions uses the tacit assumption that an encounter between two molecules A and B gives rise to a reaction with a probability α<1. We consider a non-Markovian model in which the probability that exactly n encounters between A and B are required to produce a reaction is equal to θn. We show that when the expected number of such encounters is finite, one gets the usual radiation boundary condition. When the expected number is infinite, one finds a boundary condition that is nonlocal in time. The extension of our analysis to higher dimensions as well as to the Smoluchowski equation is readily generated.
Computer simulation study of the θ-point in three dimensions. II. Trails on a simple cubic lattice
Authors: Meirovitch Hagai , Lim H. A.
Published: July 2002

A self-attracting trail is a walk on a lattice which may intersect itself but two bonds are not allowed to overlap; an interaction energy ε (ε<0) is associated with each self-intersection. Using the scanning simulation method, we study the tricritical behavior at the collapse transition of self-attracting trails of N≤250 steps on a simple cubic lattice. In the preceding paper (paper I) tricritical self-avoiding walks (SAWs) on the same lattice have been investigated. The tricritical temperature of trails is −ε/kBTt=0.550±0.004 (one standard deviation). The results for the radius of gyration, G, and the end-to-end distance, R, lead to νt=0.515±0.003, which is larger than νt=1/2, the theoretical prediction for SAWs. The ratio G2/R2=0.1676±0.0001 is slightly larger than 1/6=0.1666 ... predicted by theory for SAWs; The results for the partition function at Kt lead to γt=1.040±0.005 (as compared to the theoretical prediction for SAWs γt=1) and to the growth parameter value μt=5.0023±0.0020. The crossover exponent, φt, is approximately 0.5 as expected for SAWs at tricriticality; this value is significantly smaller than that found for SAWs in paper I. The results of G, R, and Z at Kt are found to be inconsistent with logarithmic corrections to scaling. However, we do not think that the above differences between trails and SAWs are sufficient to suggest unequivocally that the two models belong to different universality classes.
A chemically induced dynamic electron polarization study on the acetone ketyl radical and radical pair in an alcohol solution
Authors: Tominaga Keisuke , Yamauchi Seigo , Hirota Noboru
Published: July 2002

A quantitative study of the chemically induced dynamic electron polarization (CIDEP) spectra of acetone and deuterated acetone ketyl radicals in i-propanol over a wide temperature range has been done to understand the properties of the radical pairs in solution, e.g., the interactions between the radicals, the diffusional and rotational motions of the radicals of the geminate radical pairs, and the microscopic solvent structure. The spin polarization of the separated radical is analyzed in terms of the stochastic-Liouville equation. It is shown that the observed polarization cannot be explained on the basis of the normal diffusion of the radicals in the pair. The modified diffusion models which presuppose slow diffusion of the radicals in a microscopic solvent structure can account for the observed polarization. The radical pair electron paramagnetic resonance (EPR) spectra observed at very low temperatures (<∼−70 °C) are explained by assuming that a considerable fraction of the radicals are held together in solvent structures whose lifetimes are a few microseconds. From the comparison with other alcohol and amine systems, we conclude that the observation of the radical pair EPR spectra is characteristic of alcohol solutions at very low temperatures.
Relativistic well-tempered Gaussian basis sets for helium through mercury. Breit interaction included
Authors: Okada Satosi , Shinada Masaki , Matsuoka Osamu
Published: July 2002

A systematic calculation of new relativistic Gaussian basis sets is reported. The new basis sets are similar to the previously reported ones [J. Chem. Phys. 91, 4193 (1989)], but, in the calculation, the Breit interaction has been explicitly included besides the Dirac–Coulomb Hamiltonian. They have been adopted for the calculation of the self-consistent field effect on the Breit interaction energies and are expected to be useful for the studies on higher-order effects such as the electron correlations and other quantum electrodynamical effects.
Radiative lifetimes of the
Ã 2
A
1 (
v
3=0–2) states of CH3S
Authors: Lee Yin-Yu , Chiang Su-Yu , Lee Yuan-Pern
Published: July 2002

The radiative lifetimes of the v‴3 = 0–2 levels of the CH3S (Ã 2A1) radical have been determined by means of the laser-induced fluorescence technique. CH3S was generated by either laser photolysis of CH3SSCH3 (both in cell and in supersonic expansion) or the reaction of F and CH3SH in a discharge-flow system. The radiative lifetimes for v‴3 = 0, 1, and 2 levels are 1105±25, 910±20, and 250±10 ns, respectively, greater than previously reported values. The rate coefficients for the v‴3 = 0, 1, and 2 levels for quenching by CH3SSCH3 have also been determined to be 7.8, 7.8, and 16.5×10−10 cm3 molecule−1 s−1, respectively.
Conformational instability of the lowest triplet state of the benzene nucleus. II. p-Xylene, the influence of substituents
Authors: Buma W. J. , van der Waals J. H. , van Hemert M. C.
Published: July 2002

A calculation of the potential-energy surface of the lowest triplet state of p-xylene as a function of the S8(ρ,φ) distortion coordinate of the benzene skeleton has been made to learn more about the influence of substituents on the vibronically induced distortion of benzene in its metastable triplet state.The results show the hexagonal conformation of the benzene nucleus in p-xylene to be unstable with respect to distortions along S8(ρ,φ). In contrast with benzene, for which a flat, virtually cylindrical trough was calculated, the lowest triplet state of p-xylene shows a preference for quinoidal conformations. The variation in energy with the position of the methyl groups in the quinoidal structure is insignificant within the accuracy of the calculation.
Continuous wave multi-quantum electron paramagnetic resonance spectroscopy
Authors: Sczaniecki P. B. , Hyde James S. , Froncisz W.
Published: July 2002

The equation of Anderson [Phys. Rev. 102, 151 (1956)] (ω2−ω1)2=(γH0−ω1)2+γ2H21, which describes resonance conditions if relaxation times are long and irradiation at two frequencies is applied to a spin system, has been studied experimentally in the context of continuous wave electron paramagnetic resonance (EPR) spectroscopy. Here ω2 and ω1 are the frequencies of two incident microwave levels, one of which is much stronger than the other and is of amplitude H1. γH0 is the resonant condition if just one frequency is applied. Magnetization at either ω1 or ω2 has been observed as a function of sweep of the static magnetic field, sweep of ω2 and also sweep of the amplitude H1. Observation of magnetization at frequency ω1 corresponding to the strong microwave field H1 replicates the rotary saturation experiment of Redfield [Phys. Rev. 98, 1787 (1955)]. Multi-quantum effects are studied with the two frequencies well separated and also when they lie within the width of a single homogeneous line. In addition, data are shown when both microwave amplitudes are similar and the Anderson equation is no longer correct. The thrust of the work is not only to study the spin physics, but also to develop a basis for our development of rotary resonance as an alternative to field modulation in EPR spectroscopy [J. Chem. Soc. Faraday Trans. 1 85, 3901 (1989)].
Dynamics of copolymer and homopolymer blends in strong solutions and bulk: The Edwards Hamiltonian approach
Authors: Borsali R. , Vilgis T. A.
Published: July 2002

We consider the dynamics of copolymers and homopolymers in strong solution and in the melt system. We start from the Edwards Hamiltonian and derive from that the dynamical equations for the density and concentration fluctuations. We show that this procedure is new, simple, and easy to handle, leading to the same results as the projection operator formalism, if applied to this dynamical problem. In order to illustrate this formalism we apply it to the case of a mixture of two homopolymers and the case of diblock copolymers in blend and in solutions.
Photochemistry of adsorbed molecules. VII. Ultraviolet photoejection and photodesorption of OCS on LiF(001)
Authors: Polanyi J. C. , Young P. A.
Published: July 2002

The UV photoejection and photodesorption of OCS on LiF(001) at 222 nm are reported. Translational energy and angular distributions were obtained by angle-resolved TOF mass spectrometry. Energetic photoejection (PEJ) product, characterized by a peak translational energy T′p∼0.3 eV, was detected for ≳0.5 ML coverages of OCS(ad). The PEJ angular distribution was sharply peaked around the surface normal, ∼cos 18θ. The initial absorption of energy was into the chromophore of an OCS molecule. Molecular photodesorption (PDES) product, characterized by low translational energy, was detected for coverages ≥10−2 ML. The translational energy distribution P(T′) was found to be a sensitive function of detection angle, adsorbate coverage and laser energy. Peak translational energies and FWHM’s were ∼0.05 and ∼0.20 eV, respectively. Angular distributions were also sensitive to coverage, narrowing from ∼cos θ to cos 11θ with increasing coverage. Photodesorption is thought to involve absorption of radiation by defect centers in the LiF(001), with conversion of the absorbed energy into phonons which, following propagation to the surface, induce desorption of OCS(ad).
Comment on: ‘‘Electron bombardment fragmentation of size selected NH3 clusters’’
Authors: Coolbaugh M. Todd , Peifer William R. , Garvey James F.
Published: July 2002

New experimental results are presented utilizing ammonia clusters, which demonstrate that the differences in ion intensity reported in Ref.1, as a function of cluster size, are in fact attributed to beam conditions.(AIP)
Fluorescence quenching of the phenanthrene excimer on Al2O3(0001): Coverage and distance dependence
Authors: Haynes D. R. , Helwig K. R. , Tro N. J. , George S. M.
Published: July 2002

The fluorescence from disordered phenanthrene adlayers on Al2O3(0001) was examined in ultrahigh vacuum at 20 K using laser-induced fluorescence techniques. The fluorescence spectra was consistent with emission from a phenanthrene excimer. The fluorescence lifetime of the phenanthrene excimer was studied versus phenanthrene coverage and versus distance from the Al2O3(0001) surface using xenon spacers. As a function of coverage on Al2O3(0001), the fluorescence lifetime decreased from τ=34±0.5 ns at phenanthrene coverages of Θ≥20 ML to τ=7±0.5 ns at Θ=1 ML. As a function of xenon spacer distance from the Al2O3(0001) surface, the fluorescence lifetime also decreased from τ=34±0.5 ns at distances of d>100 Å to τ=7±0.5 ns at d=7 Å. Fluorescence measurements versus phenanthrene coverage on xenon, butane, acetonitrile and methanol multilayers revealed that the phenanthrene excimer fluorescence lifetime was constant at τ≊35 ns on the molecular multilayer surfaces. These results indicated that the reduction of the fluorescence lifetime was particular to the Al2O3(0001) surface.

The fluorescence lifetimes versus phenanthrene coverage and xenon spacer distance on Al2O3(0001) were equivalent when the phenanthrene coverage was converted to total adlayer thickness. This correspondence suggested that the excited electronic energy in disordered phenanthrene adlayers transfers rapidly to phenanthrene excimers at the phenanthrene–vacuum interface. Subsequently, competition occurs between fluorescence quenching by the Al2O3(0001) surface and phenanthrene excimer fluorescence. In addition, fluorescence lifetime measurements vs phenanthrene coverage on CaF2 thin films displayed similar fluorescence quenching. Fluorescence lifetimes versus phenanthrene coverage on O2 molecular multilayers also revealed fluorescence quenching that was attributed to a charge-transfer mechanism. The observed fluorescence quenching on Al2O3(0001) indicates that the surface states of this ionic crystal may be accessible for electronic energy transfer even though Al2O3 is a known insulator. A Förster electronic energy transfer mechanism was used to analyze the observed fluorescence lifetimes vs phenanthrene coverage or xenon spacer distance on Al2O3(0001).
Photodissociation of RNCS and RSCN (R=H, CH3, C2H5) at 248 and 193 nm: CN product energy distributions
Authors: Northrup F. J. , Sears Trevor J.
Published: July 2002

Photodissociation of the molecules RNCS and RSCN (R=H, CH3, and C2H5) was investigated at 248 and 193 nm and the internal energy distributions in the CN resulting from the RS+CN fragmentation channel were probed by laser induced fluorescence. These CN distributions were identical for formation from the isomer pairs in agreement with an excited state isomerization postulated earlier. At 248 nm, all precursors lead to nearly thermal CN rotational distributions with rotational temperatures of approximately 1100 K in v=0 and 800 K in v=1. The vibrational distributions could not be characterized by the same temperatures. At 193 nm, contributions from two apparently different dissociation channels were observed. The CN rotational populations showed a narrow, strongly peaked distribution lying on a broad, approximately statistical distribution. For HNCS, this peak occurs near N=10 suggesting dissociation from an excited state with a linear NCS skeleton. For the alkyl precursors, the peak occurs at very high rotational quantum number (N=70) indicating an excited state with a bent NCS framework. Vibrational distributions with population out to v=6 were observed. In all cases, the apparent statistical part of the rotational distribution and the vibrational distributions can be described by a prior function derived assuming that the internal modes of the alkyl substituent are not involved in any energy redistribution during the dissociation.
Comment on: Symmetry considerations and the formation of isotopically heavy ozone and carbon dioxide
Authors: Bates D. R.
Published: July 2002

The mass−independent isotopic enrichment in the formation of ozone and carbon dioxide has been explained by symmetry considerations.(AIP)