bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of spacer layers on the orientation of a hemicyanine dye in Langmuir–Blodgett films
Authors: Stroeve P. , Saperstein D. D. , Rabolt J. F.
Published: July 2002

The differences in orientation and order are investigated, via polarized IR spectroscopy, between Langmuir–Blodgett (LB) films of only hemicyanine and films of hemicyanine interleaved with spacer layers of either arachidic acid or cadmium arachidate. The hemicyanine layers of LB films without spacer layers (Z-type configuration) lose their orientation with the surface of the substrate after only a few layers are deposited. The hemicyanine layers interleaved with spacer layers (Y-type configuration) maintain an approximately fixed orientation with respect to the surface. The spacer layers are fully deuterated which permits IR differentiation of the hydrocarbon tails of the dye from the tails of the fatty acid or salt so that the orientation of each species can be assessed. The IR results explain the second harmonic generation (SHG) response from these noncentrosymmetric films. In addition, the interleaved LB structures with cadmium arachidate have an increased temperature stability compared to LB films of purely hemicyanine dye.
Universal amplitude combinations in critical mixtures of triethylamine and water from refractive index, density, turbidity, and specific heat experiments
Authors: Zalczer G. , Beysens D.
Published: July 2002

We report new data on refractive index (n), density (ρ), and turbidity (τ) in a mixture of triethylamine and water at critical concentration. These data have been obtained simultaneously for the same sample in order to make the comparison of amplitudes the most reliable. The high sensitivity (a few ppm) of the refractive index and density measurements enables the leading critical amplitude to be obtained accurately in both cases. The ratio of these amplitudes is in good agreement with the ratio (dn/dρ) far from Tc or that deduced from a suitably corrected Lorentz–Lorenz formula. This justifies the use of refractive index measurements to determine thermodynamic divergences. From the turbidity measurements, values for the correlation length and osmotic compressibility amplitudes can be obtained, allowing the universal amplitude combinations R+ξ =0.29±0.02 and R+c =0.056±0.01 to be determined. Both values are in agreement with the 2-scale factor universality based theories. Negative corrections to scaling are found for the susceptibility.
Quantum mechanical study of the light-atom transfer reactions, O(3P)+XCl→OX+Cl (X=H,D). I. Reactions in the ground vibrational states
Authors: Park Seung C. , Nakamura Hiroki , Ohsaki Akihiko
Published: July 2002

We present a three-dimensional quantum mechanical study of the light-atom transfer reaction O(3P)+XCl(vi=0)→OX(vf=0)+Cl(X=H,D), where vα represents the vibrational state in the α channel. The adiabatic-bend approximation reformulated in terms of the hyperspherical coordinates is employed to calculate the cross sections and rate constants. The potential energy surface used here is the Persky–Broida’s LEPS-I. The results are compared with the available experimental data and quasiclassical trajectory calculations. A discrepancy is found between the present results and the quasiclassical trajectory results at low collision energies (low temperatures). This is a clear manifestation of the quantum mechanical tunneling effect. The present results of the rate constants and the kinetic isotope effect are generally in better agreement with experiment. The previously proposed constant centrifugal potential approximation (CCPA) is directly demonstrated to work well.
Four-wave mixing spectroscopy of NO E 2Σ+ state
Authors: Tsukiyama Koichi , Tsukakoshi Motowo , Kasuya Takahiro
Published: July 2002

The E 2Σ+ state of NO (nitric oxide) has been probed by four-wave mixing spectroscopy via the A 2Σ+ state. In the present scheme, the pump laser frequency ω1 was fixed on the A 2Σ+ (v′=0 and 1)←X 2Π3/2(v″=0) two-photon transition, while the probe laser frequency ω2 was remained variable to monitor the E 2Σ+←A 2Σ+ electronic system. The intensity of the resultant coherent VUV radiation (ωVUV=2ω1+ω2) was strongly wavelength dependent. The analysis of the spectrum revealed the following two respects: (1) The intensity of VUV radiation was enhanced by the resonance of ω2 to rotational levels of the E 2Σ+ state as well as of 2ω1 to the A 2Σ+ state. (2) The spectral structure corresponding to the E 2Σ+←A 2Σ+ system was governed by different rotational selection rules from ordinary single-photon transitions. These aspects were discussed in terms of the third order nonlinear process in isotropic media and of the two-photon line intensities for the A 2Σ+←X 2Π system.
Laser vaporization generation of PdCH3, 15PdCH3, and Pd13CH3 for electron spin resonance neon matrix study at 4 K
Authors: Knight Lon B. , Herlong John O. , Cobranchi S. T. , Kirk Thomas
Published: July 2002

The Pd12CH3, Pd13CH3, and 105Pd12CH3 radicals have been generated by reactive laser vaporization and isolated in neon matrices at 4 K for electron spin resonance (ESR) investigation. Apparently no previous monomethyl metal radical has been characterized by ESR despite the importance of such species as reactive intermediates. These results allow an experimental description of the electronic structure in the valence region to be obtained. A direct electronic structure comparison between PdH and PdCH3 is also presented. A significant amount of s/d hybridization on Pd is observed which agrees with earlier calculations on the bonding in Pd(CH3)2. The magnetic parameters (MHz) for PdCH3 in neon matrices are: g⊥=2.273(1); for 105Pd, A⊥=−946(2), A∥=−987(20); for H, ‖A‖=13(1); and for 13C, ‖A‖=10.2(4).
Many-body contribution to the van der Waals binding energy of nonpolar fluids
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the many-body contribution to the van der Waals binding energy of nonpolar fluids. We show that in electric dipole approximation the binding energy may be expressed exactly in terms of the renormalized atomic polarizability. The latter is related directly to the Onsager reaction field. We have evaluated the total van der Waals binding energy of a hard sphere fluid of Drude–Lorentz oscillators by computer simulation.
Proton and deuteron magnetic resonance study of the HD–He potential energy surface
Authors: Wagner Robert S. , Armstrong Robin L. , Bissonnette E. Carey , McCourt Frederick R. W.
Published: July 2002

The relaxation of hydrogen and deuterium nuclei in HD–He gas mixtures is studied both experimentally and theoretically in the temperature range 90–300 K. A rationalization is given for the temperature dependence of the proton and the deuteron relaxations in terms of the relative strengths of the proton and deuteron intramolecular couplings and the role played by those HD molecules in the ground rotational state. Using a recent ab initio potential, quantitative agreement is found between the temperature dependence of the spin–lattice relaxation time of the proton in HD, as calculated theoretically and determined experimentally. A similar comparison between the calculated and experimental temperature dependence of the spin–lattice relaxation time of the deuteron in HD gave only semiquantitative agreement. It is suggested that the difference in quantitative agreement may be attributed to the selectivity of the respective predominant relaxation mechanisms to slightly different aspects of the anisotropic components of the interaction potential.
Ammonia: Dynamical modeling of the absorption spectrum
Authors: Tang Sandra L. , Imre Dan G. , Tannor David
Published: July 2002

A quantum mechanical calculation involving a time-dependent formalism is used to study the Ã–X̃ transition in ammonia. The experimental spectrum exhibits a single progression in the bend despite evidence that there is a displacement in both the bond lengths and bond angles in going from the ground to first excited state. Two models for the excited state surface are presented which reproduce this single progression in the bend. The starting point for our study is an ab initio surface. The excited state surface is first adjusted to match the equilibrium geometries and rotational constants derived from experiment. The surface is then further refined to produce agreement with the experimental absorption and emission frequency and intensity patterns.
Ultrafast photodissociation of a metalloporphyrin in the condensed phase
Authors: Rodriguez Juan , Holten Dewey
Published: July 2002

The photodissociation of axial ligands from six-coordinate nickel(II) porphyrins in solution has been studied by transient absorption spectroscopy with subpicosecond resolution. Our observations indicate that the deligation process takes place during the first several picoseconds following optical excitation of the macrocycle as the initially observed 3(π,π*) state relaxes to a dissociative ligand field excited state. No evidence is found for geminate recombination of the two ejected ligands with the four-coordinate photoproduct. A small fraction of the complexes appear to loose only one ligand. The absorption bands of the initially observed excited state and of the deligated species are also found to undergo a spectral evolution within the first 10 ps similar to that uncovered recently in the photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents [J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. Several possibilities are considered for these time-dependent spectral shifts, including vibrational dynamics.
A composition density functional theory for mixtures based upon an infinitely polydisperse reference. I. Formalism and theory
Authors: Kofke David A. , Glandt Eduardo D.
Published: July 2002

A general, statistical mechanical theory which relates the properties of mixtures of different compositions is presented. It is developed within a semigrand canonical framework, and thus the mixtures are formally described by species chemical potential differences, rather than directly by composition. The introduction of a set of n-particle composition distribution functions leads to a composition-space superposition approximation (CSSA), which forms the only approximate part of the treatment. A functional expansion of the canonical partition function in terms of the composition density is used to develop systematic corrections to the CSSA. Infinitely polydisperse mixtures [D. A. Kofke and E. D. Glandt, J. Chem. Phys. 90, 439 (1989)] are shown to be the composition-space analogs of homogeneous pure fluids, and the scaling properties of these mixtures make them ideal as a reference in the theory. The required input is the density-invariant composition of the infinitely polydisperse reference. The validity of the method is demonstrated on hard-particle fluids using accurate equations of state from the literature. Although based on a polydisperse reference, the treatment is equally applicable to discrete, i.e., conventional mixtures. In its most stringent test—the prediction of pure-fluid properties—the theory based on an infinitely polydisperse reference displays quantitative agreement with known behavior.
Inversion of experimental data to generate state-to-state cross sections for rovibrationally inelastic scattering of CO2 by hot hydrogen atoms
Authors: Kreutz Thomas G. , Khan Farooq A. , Flynn George W.
Published: July 2002

State-to-state integral cross sections for rovibrationally inelastic collisions between CO2 and hot hydrogen atoms are obtained by inversion of rotationally resolved population measurements of CO2 (0001). The inversion/fitting procedure is based upon the infinite order sudden (IOS) scaling law and employs a spline interpolating model to reduce the amount of information to be extracted. The experimental data used in the analysis was obtained using time domain tunable diode laser absorption spectroscopy to measure rotationally resolved excitation in CO2 (0001) following the generation of translationally hot H atoms via H2S photolysis at 193 nm. The analysis utilizes data acquired at two temperatures, 292 and 223 K, in order to acquire additional information about the temperature independent scattering cross sections. Sensitivity analysis is employed to gauge the sensitivity of the extracted cross sections to uncertainties in the experimental data, and also to estimate the effect of additional measurements at different temperatures. The final extracted cross sections exhibit strong rotational inelasticity, with a propensity for scattering from 0000; J′ to both 0001; J=J′ and 0001; J=‖J′±41‖. The largest state-to-state collision cross section corresponds to the (0000; J′=0) to (0001; J=41) transition and has a magnitude of (2.0±0.4)×10−3 Å2.
Electron-impact excitation of the normal vibrational modes of NH3 in the intermediate region (12–50 eV)
Authors: Furlan M. , Hubin-Franskin M.-J. , Delwiche J. , Collin J. E.
Published: July 2002

Electron-impact excitation of the normal vibrational modes of NH3 in its ground electronic state has been studied by electron-energy-loss spectroscopy in the 12–50 eV impact energy range out of resonances. At 25 and 50 eV the differential cross sections (in arbitrary units) for the elastic scattering process and for the vibrationally inelastic v=0→1 transitions have been measured at various scattering angles ranging from 8° to 60°. The ratio of the vibrational differential cross sections to the elastic one, reported as a function of the impact energy, indicates that for energies above 10 eV, the vibrational excitation occurs mainly through a direct mechanism. At 50 eV the angular shape of the cross sections is strongly dependent on the specific normal mode excited. The ν2 differential cross section is forward peaked, whereas that of ν4 is isotropic. The differential cross section of ν1,3 composite has a minimum at 30°. At 25 eV the angular behavior of the differential cross sections is roughly similar for all the normal modes with a minimum at 30°, but the values (in arbitrary units) below and above 30° are quite different for each normal mode.
Coriolis-induced vibrational energy transfer in D2CO–D2CO collisions. II. A semiclassical sudden approximation
Authors: Parson Robert
Published: July 2002

Recent experimental studies of vibrational energy transfer in D2 CO–D2 CO collisions have demonstrated extraordinarily high cross sections for an intramolecular V–V process in which a vibrational quantum is transferred between the two coriolis-coupled modes ν4 and ν6 . In this paper a simple semiclassical theory of this process is presented. The theory combines a resonance Hamiltonian originally derived by Burleigh, Mayrhofer, and Sibert, and by Gray and Davis, with a classical-path treatment of translation. With the aid of a time-dependent sudden approximation, applied not to molecular rotation but to the 3.5 cm−1 energy gap between perturbed molecular eigenstates, opacity functions, and absolute cross sections can be calculated in closed analytic form. Agreement with experiment is surprisingly good.
Late stages of phase separation/gelation of isotropic solutions of rod-like polymers by video microscopy
Authors: Chowdhury Aslam H. , Russo Paul S.
Published: July 2002

The late stages of phase separation/gelation in concentrated solutions of poly(γ-benzyl-α, L-glutamate) in N,N-dimethylformamide containing 2% added water have been observed by video optical microscopy. Microscopic phase separation is directly evident on cooling homogeneous, isotropic solutions. Within the phase separating mixture, diffuse structural features are separated from one another by a characteristic distance. Fourier transforms of the real space images, equivalent to scattering patterns, show a radially symmetric ring, which collapses to lower wave number as gelation proceeds. The wave number associated with the maximum intensity, qm, obeys a scaling relationship consistent with the Lifshitz–Slyozov evaporation/condensation model, also consistent with the Binder–Stauffer cluster dynamics model: qm=t−1/3, where t is time. A more general scaling relationship proposed by Furukawa is obeyed very well if the dimensionality of the growth of the new phases is 3. The advantages of video microscopy for such studies and possible implications for the role of spinodal decomposition in rod-like polymer solutions are discussed.
Direct measurement of vibrational level populations in CO2 produced during CO oxidation on Pd
Authors: Coulston George W. , Haller Gary L.
Published: July 2002

The dynamics of carbon monoxide oxidation by O2 on a Pd foil were probed under nearly collision free conditions using high resolution infrared chemiluminescence. Auger electron spectroscopy was used to verify the absence of impurities on the Pd surface. The reactants were supplied to the surface through a free jet nozzle source while a Fourier transform infrared spectrometer (Bomem DA3002) operating at 0.012 cm−1 resolution was used to fully resolve the rotational structure of 14 vibrational transitions in the product CO2. It was found for reaction on a 600 K surface that the (1001, 2001, 3001) symmetric stretch levels were populated consistent with an apparent vibrational temperature of Tsymmetric=3150 K. However, the populations of the levels with no symmetric component (0001, 0111, 0201, 0221, 0311, 0331, 0002, 0401, 0421, 0441, 0003) were better characterized by a temperature Tnonsymmetric=1900 K, with individual mode temperatures for bending and antisymmetric stretch being equal to 1820 and 2130 K, respectively. Increasing the surface temperature by 75 K resulted in a disproportionate increase in each of the temperatures, i.e., Tsymmetric=3700 K, Tanitsymmetric=2530 K and Tbend=2050 K. It is likely that the observed changes in the vibrational populations are due to changes in the reactant coverages as the surface temperature is increased.
Rotational relaxation contributions to infrared pressure broadening in ozone
Authors: Flannery C. , Mizugai Y. , Steinfeld J. I. , Spencer M. N.
Published: July 2002

Rotational relaxation times in ozone–ozone collisions have been measured for selected rovibrational levels using the time-resolved infrared double-resonance technique. Rotationally inelastic processes account for at least 90% of the measured pressure-broadening rate. The rotational-state dependence of the relaxation cross sections is in good accord with the collision models used in current theories of pressure broadening. The V–V energy transfer rate between symmetric and asymmetric stretching modes of ozone has also been measured; the V–V cross section is approximately 15 Å2, about an order of magnitude smaller than that for rotational energy transfer.
Partial widths by asymptotic analysis of the complex scaled resonance wave function
Authors: Peskin U. , Moiseyev N. , Lefebvre R.
Published: July 2002

The complex scaled square-integrable resonance wave function describing the scattering of a particle at a distance r from a target with internal state energies and wave functions denoted εj and χj (x) is given by ∑jχj(x)φj(r), where the φj(r)’s are the channel functions. The partial widths Γj (i.e., the decay rates into the channels open for dissociation) are obtained by calculating ‖φj(r)(kj/m)1/2 exp[−ikjr exp(iθ)]‖2 as r→∞, where exp(iθ) is the complex scaling factor, m is the reduced mass of the two scattered entities, and kj=[2m(Eres −εj)]1/2. Eres is the complex resonance eigenvalues of the complex scaled Hamiltonian H(x,r exp(iθ)). The wave function is determined either from a propagation plus matching technique or using a basis of particle-in-a-box functions. The former procedure is applicable even in the limit of zero rotation angle. Illustrative examples are given for a two-channel model Hamiltonian studied previously by Noro and Taylor, and by Bačić and Simons, and for a Hamiltonian which describes the scattering of HD from a flat Ag surface.
Numerical solution of angular wave equations
Authors: Johnson Bruce R.
Published: July 2002

A new adaptation of the Numerov–Cooley integration method is made for equations with both first and second derivatives, as commonly encountered in quantum mechanical problems involving angles. The associated second-order eigenvalue correction formula is also generalized. The new methods are demonstrated for the Legendre and Jacobi equations, large-amplitude polyatomic bending, and rotation of an asymmetric top (the Lamé equation).
Comparison of analytic and numerical results for the mean cluster density in continuum percolation
Authors: Given James A. , Kim In Chan , Torquato S. , Stell George
Published: July 2002

Recently a number of techniques have been developed for bounding and approximating the important quantities in a description of continuum percolation models, such as 〈nc〉/ρ̄, the mean number of clusters per particle. These techniques include Kirkwood–Salsburg bounds, and approximations from cluster enumeration series of Mayer–Montroll type, and the scaled-particle theory of percolation. In this paper, we test all of these bounds and approximations numerically by conducting the first systematic simulations of 〈nc〉/ρ̄ for continuum percolation. The rigorous Kirkwood–Salsburg bounds are confirmed numerically in both two and three dimensions. Although this class of bounds seems not to converge rapidly for higher densities, averaging an upper bound with the corresponding lower bound gives an exceptionally good estimate at all densities. The scaled-particle theory of percolation is shown to give extremely good estimates for the density of clusters in both two and three dimensions at all densities below the percolation threshold. Also, partial sums of the virial series for 〈nc〉 are shown numerically to give extremely tight upper and lower bounds for this quantity. We argue that these partial sums may have similar bounding properties for a general class of percolation models.
The third continuum of the rare gases emitted by heavy ion beam induced plasmas
Authors: Griegel Thomas , Drotleff Heinz W. , Hammer J. Wolfgang , Petkau Knut
Published: July 2002

Rare gases have been excited by dc ion beams of He+, Ne+ and Ar+ in the energy range 0.5–3.9 MeV using the Stuttgart DYNAMITRON accelerator and the gas target system RHINOCEROS. The VUV emission of the first, second and third continua was observed in the wavelength range between 50 and 300 nm. Especially the third continuum of neon could be observed at 99 nm for the first time in this way. The relative intensities of the continua depend strongly on the mass of the exciting ion and on the presence of contaminations. The observations support strongly the assignment of the third continuum to the radiative decay of Rg2+2 ionic excimers.
Discrete-continuum hybrid model for dynamics with applications: Desorption of adsorbates and relaxation of lattice inclusions
Authors: Thacher Thomas , Rabitz Herschel A. , Askar Attila
Published: July 2002

The paper presents and extends an earlier model for the relaxation dynamics within solids and desorption of adsorbates on surfaces. The model retains the discrete nature of the adsorbates while adopting a continuum representation for the solid. Extensive one-dimensional model calculations are carried out along with sample calculations in the two-dimensional case. Specifically, in the one-dimensional case, the formulation is for a diatom with harmonic intramolecular bond and a Morse potential between the diatom and the remaining solid as well as an atomic inclusion in a two-dimensional bulk solid. The excitation mechanisms are initial disturbances that take the system away from its equilibrium configuration and acoustic pulses incident on the molecule. An extensive parametric study is carried out by varying the width, amplitude, and frequency content of the pulse along with the strength of the Morse bond. These calculations permit an understanding of the roles of the various parameters in determining the relaxation rate associated with energy transfer and desorption processes for the case of the diatom bound to the solid surface as well as an atom as a guest in the solid. The observed physical phenomena span the linear to the highly nonlinear regime.
Free-energy model for the inhomogeneous hard-sphere fluid: ‘‘Closure’’ relation between generating functionals for ‘‘direct’’ and ‘‘cavity’’ distribution functions
Authors: Rosenfeld Yaakov
Published: July 2002

A free-energy model for the inhomogeneous hard sphere fluid mixture was derived recently [Phys. Rev. Lett. 63, 980 (1989)] which is based on the fundamental geometric measures of the particles. It features convolution forms for the direct correlation functions as obtained by functional differentiation from the free energy. General features of the model are further explicated in this paper. In particular, fused-spheres overlap forms for the cavity distribution functions are obtained by recursive application of the model to the free energy of composite particles. It is shown how the scaled-particle theory provides an explicit ‘‘closure’’ relation between these two alternative description of the correlations.
Density distribution functions of confined Tonks–Takahashi fluids
Authors: Davis H. Ted
Published: July 2002

The density distribution functions of a confined one-dimensional fluid of particles obeying the Tonks–Takahashi nearest neighbor two-body potential are reduced to simple functions of the grand canonical ensemble partition function. The resulting formulas are analogous to those found by Robledo and Rowlinson for a hard-rod fluid. In the absence of an external field the partition functions can be evaluated by the method of Laplace transforms. The dependence of the pressure P on the separation L of the confining walls is investigated for three model potentials: (i) hard rod, (ii) square well, and (iii) triangle well. P is an oscillating function of L in all three cases. The oscillations arise from the ordering effect of the repulsive forces between particles. The attractive interactions of the triangle-well potential reinforces the ordering whereas those of the square-well potential diminishes the ordering. Results for semiconfined and homogeneous fluids are also presented.
Kinetics of liquid–liquid phase separation in a binary mixture with miscibility gap: Study of noncritical mixtures of 2,6-dimethyl pyridine/water near the lower critical point
Authors: Mayer W. , Woermann D.
Published: July 2002

Temperature jump experiments with a characteristic heating time of 0.5 ms are used to study the kinetics of liquid/liquid phase separation in noncritical mixtures of 2,6-dimethyl pyridine and water (‖x-xc‖>0.02; x, mole fraction of 2,6-dimethyl pyridine; xc, critical composition) as functions of composition and supersaturation. The response of the system to a temperature jump is monitored by measuring the intensity of light scattered by the sample at different scattering angles Θ (30°<Θ<90°) as a function of time (up to several seconds). Starting from a level of constant intensity reached with a time constant in the range of milliseconds after a temperature jump the scattered intensity increases with time. For small supersaturations consecutive maxima and minima of scattered intensity develop.

They reflect diffusion limited growth of droplets of the emerging second liquid phase (linear relationship between the square of the radius of growing droplets and time). The growth rate increases with increasing supersaturation. Practically no nucleation barrier is found for the onset of growth of droplets for small supersaturations and compositions (‖x−xc‖>0.03). The number density N of growing droplets is obtained from simultaneous measurements of scattered intensity and turbidity. For small supersaturations N is independent of time but changes with supersaturation in a characteristic way (classical nucleation model). There exists an upper limit of overheating δT* above which the number density N increases drastically. The ratio (δT*/ΔTc,p) is independent of the reduced temperature ε̃ of phase separation (ε̃=ΔTc,p/Tc; ΔTc,p=Tc−Tp; Tc, critical temperature, Tp, temperature of phase separation). This is expected for mixtures of noncritical composition. The theory proposed by Langer and Schwartz to treat nucleation and growth of droplets in metastable near critical fluids does not describe the phenomena observed in this study. The composition of the samples used (‖x−xc‖>0.02) appears to be too far away from the critical.
Kinetic energy released in positive ion pair formation from inner-shell excited oxygen molecule
Authors: Saito Norio , Suzuki Isao H.
Published: July 2002

Total kinetic energy released in dissociation from inner-shell excited states of O2 induced by soft x-ray irradiation has been measured using a time-of-flight mass spectrometer operated in a photoion–photoion coincidence mode. The distribution of the total kinetic energy released in the dissociation into O++O+ at a photon energy above the 1s ionization threshold shows three peaks at about 6, 11, and 15 eV and that at an energy of the 1s→2pπg transition has a broad maximum at 11 eV. The total kinetic energy distribution of O++O2+ shows a broad maximum at about 20 eV in all the photon energies. The dissociation pathways are discussed in detail using the kinetic energy distributions obtained.
Role of photoacoustics in optogalvanics
Authors: Kumar D. , McGlynn S. P.
Published: July 2002

Time-resolved laser optogalvanic (LOG) signals have been induced by pulsed laser excitation (lsj→2pk, Paschen notation) of a ∼30 MHz radio-frequency (rf) discharge in neon at ∼5 torr. Dramatic changes of the shape/polarity of certain parts of the LOG signals occur when the rf excitation frequency is scanned over the electrical resonance peak of the plasma and the associated driving/detecting circuits. These effects are attributed to ionization rate changes (i.e., laser-induced alterations of the plasma conductivity), with concomitant variations in the plasma resonance characteristics. In addition to ionization rate changes, it is shown that photoacoustic (PA) effects also play a significant role in the generation of the LOG signal. Those parts of the LOG signal that are invariant with respect to the rf frequency are attributed to a PA effect. The similarity of LOG signal shapes from both rf and dc discharges suggests that photoacoustics play a similar role in the LOG effect in dc discharges. Contrary to common belief, most reported LOG signal profiles, ones produced by excitation to levels that do not lie close to the ionization threshold, appear to be totally mediated by the PA effect.
H/D isotope effect in the predissociation of C2HD
Authors: Cool Terrill A. , Goodwin Peter M. , Otis Charles E.
Published: July 2002

Resonance-enhanced multiphoton ionization (2+1 REMPI) spectroscopy near 243 nm was used to detect H and D photofragments of the 193 nm photolysis of C2HD. Measurements were performed in a molecular beam, laser photolysis apparatus, equipped with a time-of-flight mass spectrometer. The measured H/D ratio of 2.85±0.3 may indicate nonadiabatic surface hopping occurs between 2 3 A′ and 1 3 A′ and/or 2 1 A′and 1 1 A′surfaces in exit channels leading to ground state C2H(X̃ 2 Σ+ )+H(2 S) fragments in the predissociation of C2H2.
Temperature dependence of vibrational relaxation in the very-low-energy collision regime: The ground electronic state of p-difluorobenzene prepared by stimulated emission pumping
Authors: Kable Scott H. , Knight Alan E. W.
Published: July 2002

Stimulated emission pumping is used to selectively populate the υ″3=1 vibrational level (εvib=1258 cm−1) in the S0 state of p-difluorobenzene cooled in a supersonic free-jet expansion of argon. The time-dependent population of the υ″3=1 level, as it is depleted by collisions with the argon carrier gas, is probed using single vibronic level fluorescence. By varying the point along the expansion axis (X/D) at which state preparation and population probing are carried out, the rate coefficient for vibrational relaxation of the 31 state is measured as a function of temperature in the range 1–12 K. The temperature dependence of the rate coefficient is compared with the temperature dependence of hard-sphere (HS) and Lennard-Jones (LJ) elastic encounter rates. Comparison with data for the same collision system (p-difluorobenzene–Ar) at room temperature in both the S1 and S0 electronic states suggests that the temperature dependence of the Lennard-Jones elastic rate provides a useful means of scaling the temperature dependence of polyatomic vibrational relaxation over a wide range of temperatures, i.e., from 300 K down to near absolute zero.
The effect of crystallography on the Hg 5d branching ratios
Authors: Varma Shikha , Kime Y. J. , LaGraffe D. , Dowben P. A. , Onellion M. , Erskine J. L.
Published: July 2002

The Hg 5d5/2 and 5d3/2 cross sections and branching ratios were measured in the photon energy range between 26 and 105 eV for Hg overlayers on Ag (100) and Cu (100). The branching ratios deviate from the nonrelativistic statistical value of 1.5, reaching values of 8.0. Data are presented that establish a direct relationship between the branching ratio and the long-range crystallographic structure of the Hg overlayers. This relationship is a consequence of the formation of a band structure from the shallow mercury 5d eigenstates.
Nuclear magnetic resonance and temporary elasticity in molten polybutadiene: Concentration, chain length, and microstructure dependences
Authors: Cohen-Addad J. P. , Labouriau A.
Published: July 2002

This work deals with effects of variations of chain length and polymer concentration observed upon the gel-like behavior of high molecular weight polybutadiene. The relaxation rate σ of the transverse magnetization of protons attached to polymer chains was found to be governed by the residual dipole–dipole interaction of nuclear spins, a pseudosolid mechanism already observed in several concentrated polymer solutions. The rate σ corresponding to a given microstructure was measured at room temperature; it was found to be a function of both the chain molecular weight M̄w and the polymer volume fraction φ, according to the formula σ−1(φ,M̄w)=φ−1[1+2400(1−0.85φ) (M̄w)−1/2]σ−1∞, which applies within the ranges of 0.5×105<M̄w<5.45×105 g mole−1 and 0.4<φ<1.0. Here σ−1∞ corresponds to infinite chain length. The microstructure of pure polybutadiene chains was varied by keeping a constant length (M̄w=2×105 g mole−1). Measurements were performed at several constant temperature intervals above the glass transition. The isofree volume relaxation time σ−1f was found to be a linear function of the inverse of the values of the modulus of temporary elasticity G0N, which are reported in the literature for similar microstructures. It is suggested to associate the network structure perceived from NMR to the transient network responsible for the temporary elasticity of polybutadiene.
High resolution Raman spectroscopy in the α and β crystalline phases of N2
Authors: Ouillon R. , Turc C. , Lemaistre J.-P. , Ranson P.
Published: July 2002

The line shape of the Eg libron at 32 cm−1 and of the two components (Ag and Tg) of the vibron at 2330 cm−1 of solid α-N2 at normal pressure were measured as a function of temperature between 5 and 35.6 K by means of high-resolution Raman spectroscopy with a limiting spectral resolution of 0.006 cm−1. A suitable deconvolution procedure provides the respective weights of Lorentzian and Gaussian contributions to the true phonon line shapes. The measured residual bandwidth of the Eg libron at low temperature is noticeably narrower than that previously quoted in the literature (0.16 cm−1 instead of 0.8 cm−1). At high temperature it is shown that the libron relaxation mechanisms are mainly governed by four-phonon processes. The temperature dependence of the vibron dephasing mechanisms are discussed in terms of contributions arising from depopulation, elastic and inelastic quartic processes as well as from disorder. The influence of the low energy optical phonons (ω≂32 cm−1) in the vibron relaxation mechanisms is emphasized. It is shown that between 25 K and the α–β transition temperature at Tc=35.6 K, the temperature dependent vibron linewidths are driven by inelastic processes. Finally some results on the frequency and the bandwidth of the ν1 mode in the β phase are reported.
Applications of modified weighted density functional theory: Freezing of simple liquids
Authors: Zeng X. C. , Oxtoby David W.
Published: July 2002

A new formulation of the weighted density approximation, the so-called modified weighted density approximation (MWDA), has been developed recently by Denton and Ashcroft [Phys. Rev. A39, 4701 (1989)]. In the present article, the MWDA theory is examined by applying it to the freezing of simple liquids in low dimensions such as one-dimensional hard rods and two-dimensional hard disks, and also applying it to three-dimensional adhesive hard spheres and classical one-component plasma systems. Comparisons with previous density functional investigations and computer simulations are also presented.
Influence of optimal cavity shapes on the size of polymer molecules in random media
Authors: Honeycutt J. D. , Thirumalai D.
Published: July 2002

The importance of noncompact (nonspherical) cavities in determining the size of polymer molecules in random media is studied by means of generalized Flory–Lifschitz arguments and computer simulations. The simulations are performed for a freely jointed chain in one and two dimensions using a novel Monte Carlo algorithm that effectively eliminates the effects of the finite size of the random medium. For the one-dimensional case, the simulation result for the exponent ν (=0.31±0.02), characterizing the scaling of the mean-square end-to-end distance of the chain R with the number of monomers, is in excellent agreement with the ν (=0.33) predicted by the previously developed Flory–Lifschitz theory based on the notion of compact cavities. A generalized version of the theory that accounts for noncompact (for d>1) ‘‘tube’’-like cavities with L(αR2 ) being the length of the tube, and D being the diameter in d-1 transverse directions, predicts that ν=1/(2d+4), or 1/6, depending on the nature of the tube for d>1. This result is consistent with simulation results for the Gaussian chain in two dimensions. The theory also predicts that when one end of the chain is anchored and self-avoidance is included ν=2/3, which suggests a certain similarity between this problem and that of the directed walk in a random environment.
Computer experiments on the internal dynamics of crystalline polyethylene: Mechanistic details of conformational disorder
Authors: Sumpter Bobby G. , Noid Donald W. , Wunderiich Bernhard
Published: July 2002

The atomistic details of the internal dynamics of a polyethylene-like crystal are studied using molecular dynamics. Crystals with up to 6100 chain atoms have been studied for up to 30 ps. A microscopic description of the atomic motion has been examined and a link to available experimental data on the macroscopic and microscopic motion is provided. The results show that the onset of a significant population of rotational isomers is strongly altered by the intermolecular forces. Typical rates for the formation of isomers are 1010 to 1012 s−1 at 350 K (depending on the size of the simulated crystal, which changes the overall nature of the intermolecular forces) and increase exponentially with temperature. The large number of created defects causes a continuous decrease in the end-to-end distance. Specific defects, however, have extremely limited lifetime (i.e., those suggested by molecular mechanics calculations). These results suggest that at the temperatures where annealing or deformation of metastable crystals is possible, only randomly generated defects cause the macroscopically observed changes. The defects should move under the free enthalpy gradient set up within the crystal toward a more stable location. The activation energy required for motion which ultimately results in mass transport or lamellar thickening can be shown to be temperature and chain-length dependent. The highly uncorrelated behavior of the creation and annealing of defects reveals the underlying chaotic nature of the ‘‘transition’’ from an ordered crystal to a conformationally disordered crystal (CONDIS crystal). In the simulated case, the transition to the conformationally disordered state occurs gradually, involving little or no cooperative motion. This continuous transition to the condis state was suggested earlier on the basis of experimental evidence and is expected to occur in many other polymers in addition to and at lower temperature than possible additional first-order transitions to the condis state. Thermodynamic and kinetic parameters of the simulations have been determined and compared to the available experimental data with good agreement.
Contributions of the polyion and counterions to the internal and free energies of polyelectrolyte solutions
Authors: Škerjanc Jože
Published: July 2002

From the solution of the Poisson–Boltzmann equation for the cell model of a polyelectrolyte solution, the contributions of the polyion and counterion to the electrostatic internal and free energies, and to the heat of dilution, are calculated. Computations are made not only for polyelectrolyte solutions containing one kind of counterions species, but also for mixtures of counterions differing in charge and size. The results are presented as functions of the polymer concentration, its degree of ionization, and valencies and mole fractions of the counterions.
The role of lateral interactions in determining the properties of a dipolar monolayer at a polarizable interface
Authors: Fawcett W. Ronald
Published: July 2002

A statistical-mechanical description of a monolayer of solvent dipoles at a polarizable interface is presented in which the dipoles can take up three orientations with respect to the electrical field, namely, in its direction, against it, or perpendicular to it. The approach is based upon the Kikuchi method in which dipole–dipole interactions within an elementary cluster containing, in the present case, three molecules are treated exactly whereas those outside of the cluster are treated in the mean-field approximation. The differential capacity and monolayer entropy are calculated with consideration of lateral interactions between dipoles in the parallel orientation, and without these interactions. It is shown that the lateral interactions play an important role in determining monolayer properties near the zero charge potential. It is concluded that a more realistic description of such a monolayer can be obtained if the number of molecules in a basic cluster is significantly increased.
The hydrogen atom channel in the photodissociation of ethylene
Authors: Satyapal Sunita , Johnston Grace W. , Bersohn Richard , Oref Izhack
Published: July 2002

The photodissociation of ethylene at 193 nm is studied by laser induced fluorescence of the hydrogen atom product. The quantum yield for producing deuterium atoms from C2D4 is 0.19±0.04. Their average translational energy is 7.3±1.3 and 15.1±1.4 kcal/mol at 193 and 157 nm, respectively. Considerable evidence, both experimental and theoretical, shows that dissociation takes place from a hot internally equilibrated ground state molecule. A remarkable isotope effect is found. The ratio of H atoms to D atoms released from CH2CD2 is 2.2±0.2, whereas that from t-CHDCHD is 1.2±0.1. An RRKM model explains these ratios. For CH2CD2 the vibrational frequencies of the transition state for releasing H atoms are quite different from those for the release of D atoms. In t-CHDCHD, the transition state frequencies are nearly the same; there is a cancellation between differing zero point energies and moments of inertia.
Collision energy effects in the Cs+ICH3→CsI+CH3 reaction
Authors: Bañares Luis , González Ureña Angel
Published: July 2002

Differential and total reaction cross sections for the Cs+ICH3→CsI+CH3 system have been measured as a function of the collision energy by using the crossed beam technique. The analysis of the center-of-mass angular and recoil velocity distribution of the products indicated: (a) a backward peak character corresponding to a direct rebound mechanism at low collision energy; (b) over the collision energy range from 0.15 up to 0.56 eV, the backward character shifts to near sideways scattering. A direct interaction with product repulsion (DIPR) model analysis of this collision energy evolution showed an increasing participation of bent transition state configurations, which might be in competition with the purely collinear one at lower collision energy; (c) that the average products’ translational energy Ē′T increases approximately linearly with increasing collision energy ĒT as follows: Ē′T/kJ mol−1 =0.62 ĒT/kJ mol−1+58.5. Complete laboratory differential reaction cross-section measurements were carried out at 21 different relative velocities, then integrated and normalized to yield the total flux and reaction cross section in relative units. Over the available range of ET, σR(ET) shows a minimum at ET≂0.23 eV. A comparison of the present excitation function shape with current theoretical treatments is also reported.
Temperature dependence of picosecond time resolved fluorescence depletion measurements
Authors: Smith P. G. , McDonald J. D.
Published: July 2002

This paper presents the results of a temperature dependence study of time resolved fluorescence depletion (TRFD) measurements of intramolecular vibrational energy redistribution in the molecule p-cyclohexylaniline. TRFD scans of five vibrational bands of p-cyclohexylaniline were taken at several molecular beam conditions corresponding to rotational temperatures in the range of 8–110 K. The results are attributed to two possible coupling mechanisms and are shown to be consistent with previous work. Although rotational effects are probably dominant, our data also indicate that excitation of low (40 cm−1) vibrations may contribute to enhanced relaxation.
Comment on: Deuterium nuclear fusion at room temperature: A pertinent inequality on barrier penetration
Authors: Mas Francesc , Paniagua Juan Carlos , Puy Jaume , Salvador José , Vilaseca Eudald
Published: July 2002

Gerald Rosen (Ref. 1) estimates the fusion rate for deuterium molecules trapped in a metallic lattice by applying WKB barrier penetration formula. This comment proves that Rosen’s formula is incorrect. (AIP)
The application of optical potentials for reactive scattering: A case study
Authors: Neuhauser Daniel , Baer Michael , Kouri Donald J.
Published: July 2002

Recently we introduced a new kind of optical potential [D. Neuhauser and M. Baer, J. Chem. Phys. 90, 4351 (1989)], which allows us to treat an exchange collision as if it were an inelastic one. In this article we present, for the first time, a systematic study with respect to the optical potential parameters, and we discuss how the actual parameters to be used are related to those determined a priori, employing analytical arguments. We also show that the resulting extended inelastic arrangement channel can be treated using the common propagation technique, although the potential is complex.
Direct observation of preferential bond fission by excitation of a vibrational fundamental: Photodissociation of HOD (0,0,1)
Authors: Bar I. , Cohen Y. , David D. , Rosenwaks S. , Valentini James J.
Published: July 2002

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H stretching vibrational energy is described. Stimulated Raman excitation and coherent anti-Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) vibrationally excited HOD. The OD and OH fragments are detected by laser induced fluorescence. The photodissociation of the HOD (0,0,1) molecules yields at least three times more OD than OH.