bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron microscopy and Rutherford backscattering study of nucleation and growth in nanosized W–Ti–O thin films
Authors: Ferroni M. , Guidi V. , Martinelli G. , Comini E. , Sberveglieri G. , Boscarino D. , Della Mea G.
Published: July 2002

Structural characterization of nanosized W–Ti–O rf magnetron-sputtered films was carried out by means of electron microscopy techniques and Rutherford backscattering spectroscopy. The evolution of the W–Ti–O films turned out to be quite complex as several nanophases were determined. Depending on annealing temperature and W/Ti abundance, stable nanophases of WO3, TiO, and TiO2 have been reliably achieved. The investigation highlighted the reason why the films remained nanostructured in spite of a relatively high annealing temperature. In fact, formation of a dispersed TiO nanophase in the W–Ti–O layers was recognized to inhibit grain growth and promote secondary recrystallization. This resulted in exaggerated growth of WO3 crystallites over the nanostructured layers.
Transverse compressive stress effects on the critical current of Bi-2223/Ag tapes reinforced with pure Ag and oxide-dispersion-strengthened Ag
Authors: Bray S. L. , Ekin J. W. , Clickner C. C. , Masur L. J.
Published: July 2002

We have measured the effect of transverse compressive stress on the critical current of two similar sets of Bi2Sr2Ca2Cu3Ox (Bi-2223) tapes in a direct comparison between tapes reinforced with pure Ag and oxide-dispersion-strengthened (ODS) Ag. The critical-current degradation at a transverse stress of 100 MPa was about 10% for the Bi-2223/pure Ag tapes and only about 3% for the ODS Ag tapes. In one case for each sample type the degradation was significantly greater, and there was an apparent correlation with porosity in the superconductor filaments. The degradation was the same for critical-current criteria from 5 μV/cm down to 0.1 μV/cm.
Calculation of the reactive cross section for alkali atoms reacting with bromine molecules
Authors: Goldfield Evelyn M. , Kosmas Agni M. , Gislason Eric A.
Published: July 2002

Reactive cross sections have been computed for the five alkali metals reacting with bromine molecules. The computations were carried out on the potential energy surfaces described in the companion paper. A new type of reaction mechanism, vibrational capture, is predicted to occur for these systems. The calculated cross sections agree well with the experimental data at low collision energies. At higher energies there are indications that not all reactions at small impact parameters are reactive.
Density-functional theory with self-interaction correction: Application to the lithium moleculea)
Authors: Pederson Mark R. , Heaton Richard A. , Lin Chun C.
Published: July 2002

The self-interaction corrected version of the local spin density (SIC-LSD) theory for electronic structure of many-electron systems has been applied to the ground state (1Σ+g) and three excited states (3Σ+u, 3Σ+g, and 3Πu) of the lithium molecule. Calculations are performed by using both the symmetry-restricted and symmetry-unrestricted molecular orbitals. To facilitate computation with high accuracy, a special curve-fitting technique is presented. With the Kohn–Sham exchange-only functional, our results are generally in good agreement with those of multiconfiguration self-consistent-field (MCSCF) calculations for the three excited states, but our calculated dissociation energy for the ground state is much lower than the MCSCF and experimental values. Inclusion of the correlation term in the density functional is found to bring the ground-state dissociation energy very close to the experimental value but has little effect on the dissociation energy of the three excited states. Based on an analysis from the standpoint of orbital charge removal energy in a manner similar to the Koopmans theorem in Hartree–Fock theory, the canonical orbitals for SIC-LSD theory are introduced and their physical significance discussed.
Effect of temperature on the dissociative electron attachment to CClF3 and C2F6 a)
Authors: Spyrou S. M. , Christophorou L. G.
Published: July 2002

The total electron attachment rate constant ka(〈ε〉) for CClF3 and C2F6 has been meausured using an electron swarm technique in the mean electron energy range 0.41 to 4.81 eV and over the range of temperature T from 300 to 750 K. At each value of T the total electron attachment cross section σa(ε) was determined from the measured ka(〈ε〉) using the swarm unfolding technique and was compared with the results of a mass spectrometric study. The σa(ε) for C2F6 shows a single peak (due to F− and CF3−) which shifts from 3.9 eV at 300 K to ∼3.3 eV at 750 K. (The onset shifts correspondingly from 2.3 to 1.5 eV.) For CClF3 the σa(ε) shows two peaks: at ∼1.5 eV (due to Cl−) and at ∼4.7 eV (due to Cl−, F−, CClF2−, and ClF−). The peak at ∼1.5 eV is especially sensitive to changes in T. The peak value of σa(ε) increased by a factor of ∼3, and the energy position of the peak and onset shifted to progressively lower energies when T increased from 300 to 700 K. The analysis of these results led us to conclude that the changes in ka(〈ε〉) and σa(ε) for the dissociative attachment processes of these molecules with increasing T result from the increase with T of the total internal (≂ vibrational) energy of the molecule.
Thermodynamic properties of water in the subcooled region. I
Authors: Leyendekkers J. V. , Hunter R. J.
Published: July 2002

An equation is derived for the free energy difference between water and ice in the subcooled (supercooled) region. The calculated vapor pressure difference between ice and water agrees well with experimental measurements. The free energy difference estimated for water in the form of emulsion droplets and bulk water, although small varies significantly with temperature in the subcooled region and is shown to be proportional to the excess free energy arising from the surface effects inherent to the emulsion systems. The accuracy of emulsion data at temperatures below −20 °C therefore could be much lower than previously assumed. In turn this would distort the anomaly functions and give incorrect critical temperature estimates.
Photoionization study of HgAr
Authors: Linn S. H. , Brom J. M. , Tzeng W.-B. , Ng C. Y.
Published: July 2002

Photoionization efficiency data for HgAr+ have been obtained in the region of 680–1240 Å. The ionization energy of HgAr was determined to be 10.217±0.012 eV. This value allows the calculation of the dissociation energy of HgAr+ to be 0.228±0.017 eV. The relative probabilities for the formation of HgAr+ via the reactions Ar* ⋅ Hg or Hg* ⋅ Ar→ HgAr+ +e− with Ar* and Hg* prepared in high Rydberg states in the energy range of 10.22–15.79 eV were estimated. Although the radii for the 3d and 5s Rydberg ortitals of Ar have similar values, the probabilities for the formation of HgAr+ from Hg ⋅ Ar* with Ar* in the 5s[3/2]01 and 5s′[1/2]01 Rydberg states are substantially greater than those when the Ar* excited atoms are in the 3d[1/2]01, 3s[3/2]01, and 3d′[3/2]01 Rydberg levels. The ratio for the cross sections for the formation of HgAr+ from Hg ⋅ Ar* with Ar* formed in the 3d[1/2]01 and 4d[1/2]01 states, as well as that with Ar* prepared in the 5d[1/2]01 and 6d[1/2]01 states, were found to be consistent with the predictions of the previous impact parameter calculations.
Kinetic Ising model for polymer dynamics. II. Generalized transition rates and the Williams–Watts nonexponential function
Authors: Budimir J. , Skinner J. L.
Published: July 2002

We continue our study [J. Chem. Phys. 79, 1955 (1983)] of the one- dimensional kinetic Ising model to investigate cooperative dynamics in amorphous polymers. In this paper we consider the most general single spin-flip model. The spin–spin correlation function is calculated approximately using continued fractions. We find that the Williams–Watts empirical function φ(t)=exp[−(t/τ)β] with 1/2≤β≤1 provides a good fit to this correlation function. The results are interpreted in terms of the dynamics of domain wall defects in which the two fundamental processes are defect migration and defect creation and annihilation. This suggests possible microscopic mechanisms for relaxation in solid amorphous polymers.
Quantum mechanical theory of isotope effect on thermally activated hydrogen migration on W(110)
Authors: Freed Karl F.
Published: July 2002

A quantum mechanical model is introduced to explain the exponential increase with isotopic mass of the preexponential factor for the thermally activated diffusion rate for atomic hydrogen and its isotopes on the W(110) surface as recently observed by Gomer and co-workers. The adsorbed hydrogen atom is taken to have self-trapped quantum levels at low energies, with mobile ones above the barrier to hopping. The kinetics of transitions among these levels is analyzed using a master equation, and the individual transition rates are evaluated from a model of the coupling between the adsorbed hydrogen atom and its environment. The preexponential factor is found to vary exponentially with terms having a −1/2 and −1 power of the hydrogen mass (the latter is multiplied by a more slowly varying function of mass), in excellent agreement with the experiments on hydrogen, deuterium, and tritium.
FTIR matrix isolation study of carbon-13 substituted SiC2
Authors: Shepherd Richard A. , Graham W. R. M.
Published: July 2002

The products from vaporizing silicon carbide at 2900 K and quenching in argon at 8 K have been studied using Fourier transform infrared spectroscopy. Under the assumption of various possible geometries, calculations based on two vibrations observed at 1741.1 and 824.4 cm−1 were made to predict frequencies for SiC2 singly and doubly substituted with carbon-13. Comparison with the spectra observed on isotopic substitution shows conclusively that these vibrations belong to SiC2, and that the molecule is cyclic (i.e., possesses C2v symmetry) in the ground state. This geometry is in agreement with recent results from a rotational analysis of the band system at 500 nm. Force constants derived in the present study suggest that the silicon atom is singly bonded to two doubly bonded carbon atoms.
Nonadiabatic semiclassical scattering. III. Time dependent surface hopping formalism
Authors: Herman Michael F.
Published: July 2002

A time dependent surface hopping formalism is developed for nonadiabatic scattering. Standard semiclasical propagation on single adiabatic surfaces constitutes the zeroth order approximation. Higher order terms include reflections and/or transitions between the adiabatic surfaces with zeroth order propagation occurring between the times when these nonclassical reflections and/or transitions occur. In one dimension this expansion formally satisfies the exact quantum mechanical Schrödinger equation if all terms are retained. In many dimensional problems, the analysis is restricted to the special case involving two adiabatic surfaces and terms which are identified as corrections to the single surface semiclassical propagation are ignored. The resulting nonadiabatic expansion is inherently semiclassical, as opposed to the formally exact one-dimensional case. This time dependent surface hopping formalism is utilized to derive a nonadiabatic generalization of the frozen Gaussian approximation.
Theory of spin-orbit coupling. Application to singlet–triplet interaction in the trimethylene biradical
Authors: Furlani Thomas R. , King Harry F.
Published: July 2002

Efficient methods are developed for the computation of spin-orbit coupling constants in polyatomic molecules using complete active space multiconfiguration self-consistent field wave functions. All electron–nuclear and electron–electron spin-orbit interactions in the Breit–Pauli Hamiltonian are retained without storing or transforming spin-orbit integrals. This technique is applied to the calculation of spin-orbit coupling constants between singlet and triplet electronic states. Allowing nonorthogonality of the singlet and triplet molecular orbitals in the active space improves the quality of the wave functions and presents no serious computational difficulties. To test the method, spin-orbit coupling constants are computed for the diatomic molecules NH, OH+, PH, and O2 and compared with similar calculations reported in the literature. Calculations are also carried out for the organic biradical trimethylene (ĊH2CH2ĊH2). The coupling constant is found to vary from 0 to 2.5 cm−1 depending upon geometry. It is very sensitive to rotation of the terminal methylenes but relatively insensitive to the CCC angle. These results contribute to our understanding of the role of the triplet state in biradical reactions.
Two-point matrix probability function for two-phase random media: Computer simulation results for impenetrable spheres
Authors: Haile J. M. , Massobrio C. , Torquato S.
Published: July 2002

Computer simulation results are reported for the two-point matrix probability function S2 in two-phase random, homogeneous systems composed of completely impenetrable spheres. The results compare favorably with the earlier analytic predictions of S2 made by Torquato and Stell.
Photoinduced nucleation of carbon disulfide
Authors: Kalisky Ofra , Heist Richard H.
Published: July 2002

The action of light on undersaturated and supersaturated vapors of carbon disulfide has been investigated using a batch photochemical reactor and a thermal diffusion cloud chamber, respectively. Photoinduced nucleation was observed in each case. In the batch reactor enough sulfur was produced to nucleate and grow a sulfur aerosol. A model for the photoinduced nucleation of supersaturated carbon disulfide is proposed based upon the photochemical production and subsequent nucleation of sulfur. The model predictions compare well with observed nucleation delay time and nucleation rate data. A variation of the model utilizing diradical polymerization instead of nucleation is used to explain photoinduced nucleation results in the literature involving dilute solutions of carbon disulfide in supersaturated ethanol vapor.
A study of phase transitions in Cd(ClO4)2⋅6H2O by adiabatic calorimetry and infrared spectroscopy
Authors: White Mary Anne , Falk Michael
Published: July 2002

A calorimetric study of Cd(ClO4)2 ⋅ 6H2O from 20 to 345 K shows three phase transformations at 124.8, 238.45, and 271.62 K. Infrared spectra indicate that the low-temperature structure (below 124.8 K) contains one type of [Cd(OH2)6]2+, of symmetry C3 or C3v, and two types of ClO−4, both strongly distorted from tetrahedral symmetry. There appears to be some disorder in this phase. The sharp transition at 124.8 K leads to major spectral changes, which indicate that hydrogen bonding is weakened, distortion of the perchlorate ions is diminished, and [Cd(OH2)6]2+ symmetry is increased to D3d. The more gradual phase transition at 238.45 K involves no observable infrared spectroscopic changes and may correspond to an increase in ClO−4 disorder. The very sharp transition at 271.62 K involves only very minor changes in the infrared spectra and may involve an increased disorder of the [Cd(OH2)6]2+ units.
Photofragmentation of mono- and dichloroethylenes: Translational energy measurements of recoiling Cl and HCl fragments
Authors: Umemoto Masayuki , Seki Kanekazu , Shinohara Hisanori , Nagashima Umpei , Nishi Nobuyuki , Kinoshita Minoru , Shimada Ryoichi
Published: July 2002

Translational energy (Et ) spectra of Cl and HCl fragments from vinylchloride, trans-dichloroethylene, cis-dichloroethylene, and 1,1′-dichloroethylene have been measured for the π*←π excitation at 193 nm. Et distribution and angular dependence of the Cl fragment indicate that the two-center dissociation occurs in a time faster than a rotation period and the recoiling organic radical (the counter fragment) is highly vibrationally excited. In dichloroethylenes, the presence of a second channel producing Cl atoms has been confirmed and attributed to the dissociation from the lower (n,σ*) state through a (π,σ*) state. The Et distribution of HCl fragments is nonstatistical and found to converge to null population at an energy less than half of the total available energy. This convergence point coincides with the value of the local available energy for the elimination reaction (activation energy —ΔH0reaction ). The yield of HCl molecules relative to Cl atoms is estimated to be ∼1.1 for vinylchloride. High efficiency of the HCl elimination is attributed to a rapid internal conversion from the (π,π*) state to the lowest (π,σ*) state from which the pathway to the transition state for HCl elimination is opened in ground electronic manifolds.
The dynamical basis set
Authors: Blanco Mario , Heller Eric J.
Published: July 2002

A new Cartesian basis set is defined that is suitable for the representation of molecular vibration-rotation bound states. The Cartesian basis functions are superpositions of semiclassical states generated through the use of classical trajectories that conform to the intrinsic dynamics of the molecule. Although semiclassical input is employed, the method becomes ab initio through the standard matrix diagonalization variational method. Special attention is given to classical-quantum correspondences for angular momentum. In particular, it is shown that the use of semiclassical information preferentially leads to angular momentum eigenstates with magnetic quantum number ‖M‖ equal to the total angular momentum J. The present method offers a reliable technique for representing highly excited vibrational-rotational states where perturbation techniques are no longer applicable.
Sensitivity of liquid-state inherent structure to details of intermolecular forces
Authors: Stillinger Frank H. , LaViolette Randall A.
Published: July 2002

Temperature dependence of short-range order has been systematically studied in two model atomic liquids, using molecular dynamics computer simulation linked to steepest-descent mapping of configurations onto potential minima. Owing primarily to differing attractive forces, the natural crystal structures differ for the two models (face-centered-cubic vs primitive hexagonal). Only one of these crystals is geometrically consistent with the number of particles (256) and boundary conditions (periodic, cubic cell) used for both. Nevertheless, both liquids were found to exhibit temperature dependence of short-range order arising solely from vibrational smearing of underlying temperature-independent inherent structure. The latter differs substantially for the two models, and in each case resembles a highly defective version of the corresponding crystal.
Effects of van der Waals interactions on molecular dipole moments: The role of field-induced fluctuation correlations
Authors: Hunt K. L. C. , Bohr J. E.
Published: July 2002

One contribution to the transient dipoles induced during molecular collisions comes from the van der Waals interactions between the fluctuating charge distributions of the colliding molecules. In this paper we show that the application of an external field to a molecular pair changes the van der Waals interaction energy in two ways. First, the field alters the response of each molecule to the nonuniform, fluctuating field of its neighbor. Second, the applied field induces new correlations between the fluctuating charge moments on each molecular center. Field-induced fluctuation correlations have not been included in earlier models for the van der Waals dipole. With the inclusion of these effects, the van der Waals dipole of a molecular pair is simply related to an imaginary-frequency integral of two or more terms, each involving the product of a linear response tensor for one molecule and a nonlinear response tensor for the neighboring molecule.
A coupled states distorted wave study of the O(3P)+H2 (D2, HD, DH) reaction
Authors: Schatz George C.
Published: July 2002

In this paper, the coupled states distorted wave (CSDW) method is used to study the quantum reactive collision dynamics of O(3P)+H2 → OH+H and its D2, HD, and DH counterparts. The potential surface used is the sum of a LEPS potential (due to Johnson and Winter) and a correction factor which raises the barrier for H+OH → HO+H exchange to a realistic value. Full basis set convergence of the CSDW transition probabilities is established at low energies where tunneling dominates the dynamics, which means that the calculated cross sections should be exact except for errors introduced by the CS approximation, and the latter are expected to be less than 30%. The results presented for all four isotopes include: reaction probabilities as a function of energy E and total angular momentum J, total and state to state integral cross sections (including an analysis of product state distributions), and thermal and state resolved rate constants. Comparison of the results with those of several previous dynamical calculations on the same or similar surfaces is made and the accuracy of the approximations made in those calculations is assessed. For example, the product rotational distributions predicted by vibrationally adiabatic distorted wave theory are found to be quite close to what we calculate, although the absolute magnitudes of the cross sections are quite different. Comparison with the results of quasiclassical trajectory calculations indicates good agreement of the reactive cross sections well above the classical threshold, but not of the rate constants (because of tunneling) or of isotope ratios. Wigner corrected conventional transition state theory is very inaccurate in predicting rate constants, but a method which uses collinear exact quantum (CEQ) transmission coefficients to correct transition state theory does quite well. Variational transition state theory estimates of the rate constants and isotope ratios are also quite good, with the CSDW results generally bracketed by results obtained using the least action ground state (LAG) and small curvature ground state (SCTSAG) tunneling approximations. Comparison with experimental rate constants and isotope ratios is studied, and we find that the CSDW results are just outside the experimental error bars in all cases. The present results

on the corrected Johnson and Winter surface are found to be slightly less accurate than the best estimates of rate constants and isotope ratios obtained using the modified POLCI surface of Walch and co-workers, suggesting that the latter surface is more accurate.
Multicomponent cluster ions. I. The proton solvated by CH3CN/H2O
Authors: Deakyne Carol A. , Meot-Ner (Mautner) Michael , Campbell Cynthia L. , Hughes Michael G. , Murphy Sean P.
Published: July 2002

Experimental and ab initio dissociation energies of the (H2O)n(CH3CN)mH+ ions are reported. The experimental energies range from 10–35 kcal/mol. The proton is best stabilized by placing the maximum number of acetonitrile molecules close to the protonated center in such a way that the formation of a network of strong hydrogen bonds is still possible. Other results from this work are: (1) Distinct solvent shells can be distinguished in these complex ions. (2) Mixtures of several isomeric structures are unlikely for n≤4. (3) When a water or an acetonitrile molecule clusters with (H2O)(CH3CN)H+, the proton is transferred from the acetonitrile to the water. (4) Although electrostatic interactions make the dominant contribution to the bonding in these systems, polarization and charge-transfer effects contribute also. (5) There is a cooperativity effect among the hydrogen bonds that leads to extensive changes in geometry and charge distribution as successive hydrogen bonds are formed. (6) The relative complexation energies along a series of reactions correlate with many properties of the electron donor and with several properties of the proton donor.
Infrared emission spectrum of the radical 28SiH: Observation and analysis of the rovibrational bands 1–0, 2–1, and 3–2 in the X 2Π ground state
Authors: Betrencourt M. , Boudjaadar D. , Chollet P. , Guelachvili G. , Morillon-Chapey M.
Published: July 2002

The emission spectrum of a plasma of silane flowing through a multipass reactor and excited by a radio frequency discharge has been recorded with a high information Fourier transform spectrometer. 192 rovibrational transitions of the 28SiH radical, between 1800 and 2150 cm−1, have been measured in the electronic ground state. These transitions are related to the fundamental band 1–0 and the two hot bands 2–1 and 3–2. A least squares procedure have led to the determination of a unique set of 25 accurate molecular constants for the vibrational levels v=0, 1, 2, 3 of the X 2Π state. The standard deviation on the calculated frequencies is 7×10−4 cm−1.
Analytic dipole moment geometric derivatives from nuclear electric shielding in molecules
Authors: Lazzeretti P. , Zanasi R.
Published: July 2002

We present ab initio calculations of dipole moment geometric derivatives for some first-row atom hydrides. Dipole moment derivatives, in terms of atomic polar tensors (APT), are equivalent to nuclear electric shieldings and were determined analytically, within the random phase approximation (RPA). Polarized basis sets were used, which give accurate results with small computer effort.
Theoretical description of the overtone spectra of acetaldehyde using the local mode approach
Authors: Findsen Leonore A. , Fang Howard L. , Swofford Robert L. , Birge Robert R.
Published: July 2002

The overtone spectra of the C–H bonds of the methyl group of acetaldehyde are analyzed using a coupled oscillator local mode theory. The higher energy absorption band of each overtone region is assigned to the in-plane C–H methyl bond while the lower energy absorption band is assigned to the two out-of-plane methyl C–H bonds. We conclude that the two out-of-plane methyl group C–H bonds are weaker and have longer bond lengths than the corresponding in-plane C–H bond. This observation is in agreement with predictions based on the trans effect theory. The calculations exemplify the problem of assigning overtone spectra based on relative intensities. In particular, the lower energy band of the methyl C–H group is calculated to be more intense than the higher energy band irrespective of assignment. This theoretical prediction is associated with the importance of oscillator coupling via the kinetic energy matrix element.
Lifetime measurements of selectively excited rovibronic levels of SO2
Authors: Kullmer R. , Demtröder W.
Published: July 2002

With a frequency doubled single mode cw dye laser, pulse modulated with a Pockels cell, lifetimes of selectively excited rovibronic levels of SO2 molecules in a collimated molecular beam have been measured. Under collision-free conditions single exponential decay curves were found for all observed levels. Their radiative lifetimes scatter considerably for the different levels. For the Clements band ‘‘E’’ a mean unperturbed lifetime of about 8 μs was found while for the band at λ=322.6 nm the average lifetime was 30 μs. Possible perturbations are discussed and experimental evidence is given which explains the causes of the greatly differing lifetimes.
Mean trajectory Gaussian wave packet approach to rotationally inelastic molecule–surface diffraction
Authors: Jackson Bret , Metiu Horia
Published: July 2002

The rotationally inelastic diffraction of H2 from a corrugated surface is investigated using a mean trajectory model. The center of mass motion is treated using Gaussian wave packets, which propagate on a rotationally averaged potential. This trajectory in turn drives the rotational transitions. The method is nonperturbative and allows for changes in mj, the rotational orientation, and agrees well with recent close coupling calculations. A connection is also made with the recent semiclassical trajectory work of DePristo. The effects of the attractive well depth on rotational excitation and diffraction are considered.
Composite pulse excitation in quadrupole echo spectroscopy
Authors: Siminovitch D. J. , Raleigh D. P. , Olejniczak E. T. , Griffin R. G.
Published: July 2002

An examination of several composite pulse quadrupole echo sequences designed for the broadband excitation of I=1 systems shows that these sequences reduce but do not eliminate finite pulse width effects. It is demonstrated both experimentally and theoretically that these sequences can also introduce additional distortions. The origin of these distortions and several aspects of the evaluation of broadband excitation sequences are discussed. It is shown that these distortions can be eliminated by employing higher order composite pulses. Finally the viability of composite pulse excitation for systems undergoing chemical exchange is discussed.
Rotational collisional narrowing in an infrared CO2 Q branch studied with a tunable-diode laser
Authors: Strow L. Larrabee , Gentry Bruce M.
Published: July 2002

Rotational collisional narrowing, or line mixing, has been observed in the infrared absorption spectrum of the Q branch of the (1110,0310)II – 0000 band of CO2 at 1932.47 cm−1 using a tunable diode laser. Spectra were recorded at pressures ranging from 100 to 745 Torr both inside the Q branch and below the band origin. The observed absorption coefficients below the band origin were up to 80% less than those calculated using an isolated Lorentz line approximation. A simple fitting law for the state-to-state rotationally inelastic rate constants is determined using the known line broadening coefficients. A calculation of line mixing using these rates agrees with the observed absorption coefficients to within several percent.
Bottlenecks for radiationless electronic and vibrational relaxation of N2 in solid Kr matrices
Authors: Kühle H. , Fröhling R. , Bahrdt J. , Schwentner N.
Published: July 2002

Infrared emission due to W 3Δu (v′=4) → A 3Σ+u (v″=1, 2) and B′ 3Σ−u (v′=6) → B 3Πg (v″=1, 2) transitions of N2 molecules isolated in Kr matrices has been observed. The bottlenecks for radiationless relaxation which allow for a competition by these rather slow radiative transitions are identified by selectively populating individual vibrational levels in the vibrational progression of the A 3Σ+u, B 3Πg, a 1Πg, and w 1Δu states.
A Gaussian wave packet method for studying time dependent quantum mechanics in a curve crossing system: Low energy motion, tunneling, and thermal dissipation
Authors: Sawada Shin-Ichi , Metiu Horia
Published: July 2002

We explore numerically the behavior of a method of describing the time dependent quantum mechanics of a curve crossing system. The two nuclear wave functions corresponding to the two electronic states are each described by a Gaussian wave packet. The packet describing the incident state mimics the initial wave function, and the other packet is created by the time dependent Schrödinger equation. They are both propagated by using a variational method. The packets interact and we do not assume that they have a small width. Exploratory calculations are made for curve crossing dynamics at low kinetic energy above the barrier of the lowest adiabatic state, for tunneling, for multiple crossings, and for a curve crossing system which is strongly coupled to a harmonic bath whose motion is described by a mean trajectory classical Langevin method.
Rotationally inelastic gas–surface scattering: NH3 from NH3-saturated W(100)
Authors: Kay Bruce D. , Raymond T. D.
Published: July 2002

Two-photon resonant three-photon laser ionization spectroscopy is employed to study rotationally inelastic NH3 molecular beam–surface scattering from NH3-saturated W(100). For all surface temperatures (Ts) examined (300 to 650 K), the scattered NH3 rotational distributions are Boltzmann-like. Complete rotational accommodation is observed for Ts≤ 380 K, while for Ts> 380 K, the NH3 rotational temperature is less than Ts. Angular and velocity analysis of the scattered NH3 for Ts= 300 K show cosine and Maxwellian (T=300 K) distributions, respectively. These findings indicate a trapping–desorption scattering mechanism with a surface residence time below 10 μs at Ts= 300 K. Scattering of a nuclear-spin-state (para) enriched NH3 beam prepared using electric deflection techniques indicates that nuclear spin equilibration (ortho–para) is not achieved during the gas–surface encounter.
On the entropy changes and fluctuations occurring near a tensile instability
Authors: Debenedetti Pablo G. , D’Antonio Michael C.
Published: July 2002

Several liquids exhibit an apparent loss of tensile strength (tensile instability) as their temperature is lowered. Assuming that such substances exhibit a true minimum in the PT projections of their spinodal curves, the thermodynamically consistent behavior that follows from this hypothesis displays a variety of unusual phenomena, of which the PVT aspects have been recently discussed. If, along the tensile instability isochore, the reciprocal compressibility vanishes linearly with respect to temperature (as is the case for a van der Waals fluid near the spinodal) an unusual metastable phase transition with discontinuous entropy and thermal expansion coefficient but continuous volume must occur if this isochore admits a metastable solution below the tensile instability temperature. The form of the specific heat divergence, as well as the equations of phase diagram loci of constant correlation length follow from the nature of the PVT surface in the vicinity of a tensile instability.
Reaction mechanism and kinetics of silane pyrolysis on a hydrogenated amorphous silicon surface
Authors: Robertson Robert , Gallagher Alan
Published: July 2002

Three regimes of pressure and temperature are identified in which silane pyrolysis has distinctly different initial kinetics: in two regimes the initial reactions are heterogeneous and in the third regime it is homogeneous. We report here a preliminary model for the heterogeneous reaction regime where the decomposition rate is nearly independent of pressure. In the model the silicon surface is saturated with hydrogen and hence is nonreactive. The rate limiting step for silane decomposition is the creation of reactive surface sites by release of hydrogen. These reactive sites are refilled by decomposition of SiH4 or reincorporation of H2. A new adsorbed state of SiH4 is proposed which is bound to the surface by a three-center bond. After making some simplifications to the full model the kinetics are solved for static- and flowing-gas hot wall reactor experiments. The implications of the proposed reactions for the other two pyrolysis regimes and for silane discharges are briefly discussed.
Studies of chemical reactivity in the condensed phase. II. Vibrational relaxation of iodine in liquid xenon following geminate recombination
Authors: Paige M. E. , Russell D. J. , Harris C. B.
Published: July 2002

The geminate recombination of iodine atoms in xenon solutions, has been studied by monitoring the transient absorptions at wavelengths between 350 and 1000 nm.(AIP) ;m
Rotational excitations and orientational disorder in solid mixtures of CH4 and Kr
Authors: Grondey S. , Prager M. , Press W. , Heidemann A.
Published: July 2002

The orientational disorder in a solid solution of methane and krypton (27% Kr) has been studied with inelastic neutron scattering. The spectra display a transition from diffusive high temperature rotations to rotational excitations of molecules with frozen-in orientational disorder at low temperatures. Of particular interest is a relatively sharp feature at an energy transfer of 0.78 meV which both broadens and shifts with increasing temperature. A simple model is based on a statistical distribution of local potentials due to the presence of Kr atoms and the state of excitation of the molecules. Finally, the isotope effect of the rotational excitations has been investigated.
Electronic fine structure transitions and rotational excitation in NO rare gas collisions
Authors: Joswig H. , Andresen P. , Schinke R.
Published: July 2002

The excitation of rotationally cold NO in the ( j=1/2,2Π1/2) state by collisions with different rare gases is studied for multiplet conserving and multiplet changing transitions. In the crossed beam experiment we use jet cooling to prepare the cold NO and LIF to measure the state distribution of the collisionally excited NO in the scattering center. The measured integral state to state cross sections are compared to theory. The calculations are performed in the CS approximation and based on recent advances in treating open shell molecules. For multiplet conserving transitions the agreement between experiment and theory is good. Although for the multiplet changing collisions the general structure is in qualitative agreement, the strength of the Ω=1/2→3/2 transitions is underestimated in the theory.
Rotationally inelastic collisions between a molecule in a 2Σ electronic state and a 2S atom: Sudden factorization, scaling, and symmetry relations
Authors: Corey Gregory C. , Alexander Millard H.
Published: July 2002

Sudden factorization and scaling relations are developed for the scattering of molecules in 2Σ electronic states by open-shell 2S atoms, with particular emphasis given to the influence of the open-shell character of the atomic perturber. The molecule-state-resolved cross sections are expressed as a sum of spin-independent and spin-correlated terms. Both terms can be expressed as a weighted sum of spin-free tensor opacities. In the sudden limit the tensor opacities satisfy factorization relations formally equivalent to those first derived for 1Σ–1S collision systems. However, in the collision of two open-shell species these dynamical factors cannot be identified with inelastic scattering off a single adiabatic potential energy surface. The breakdown in the scaling and symmetry relations, which are valid in the energy sudden limit, for 2Σ-state molecules scattering off closed-shell targets provides a qualitative measure of the strength of the electrostatic exchange forces which arise from the open-shell nature of both the molecule and the atomic target.
Photodissociation dynamics of cadmium and zinc dimethyl
Authors: Yu C. F. , Youngs F. , Tsukiyama K. , Bersohn R. , Preses J.
Published: July 2002

M(CH3)2 (M=Zn,Cd) vapor was photodissociated at 193 nm into CH3 and MCH3 (Ã 2E) radicals and also into CH3 radicals and M atoms. The kinetic energy distribution of the CH3 was measured by a molecular beam time of flight technique. The emission spectrum of the MCH3(Ã 2E) was recorded in the visible region (for the first time) and it was shown that the radicals were vibrationally cold. An excitation spectrum for visible emission was determined through the VUV region down to 125 nm. Two photon production of M(n 3P01 ) states was easily observable but shown to be negligible compared to one photon processes. The methyl radicals released in this dissociation are slower but vibrationally hotter than those dissociated from CH3I.
A theoretical description of the radiative decay processes (b 1Σ+, a 1Δ)→X 3Σ− in NF
Authors: Yarkony David R.
Published: July 2002

A recently developed methodology based on the Breit–Pauli approximation and the symbolic matrix element method is used to consider the spin-forbidden radiative decay processes, b 1Σ+→X 3Σ− and a 1Δ→X 3Σ− in NF. We report nonrelativistic potential energy curves for the b 1Σ+, a 1Δ, and X 3Σ− states and geometry dependent spin-forbidden transition moments, μ∥ ≡μ0(b 1Σ+0+, X 3Σ−0+), μ⊥ ≡−μ−1(b1Σ+0+, X3Σ−1), and μ1(a 1Δ2, X 3Σ−1) based on large scale MCSCF/CI wave functions (∼5–9×104 CSF’s). Mean values for μ∥, μ⊥, and μ1(a 1Δ2, X 3Σ−1) corresponding to v=0 harmonic oscillator functions are found to be 0.181×10−2, 0.467×10−3, and 0.278×10−3 a.u., respectively. The mean value of the ratio μ∥/ μ⊥ is found to be, 〈μ∥〉0,0/ 〈μ⊥〉0,0 ∼3.87. These results are in good agreement with the available experimental data.
Analytic dipole moment geometric derivatives from nuclear electric shielding in molecules. II. Application to two-heavy atom molecules
Authors: Zanasi Riccardo , Lazzeretti Paolo
Published: July 2002

Calculations of dipole moment geometric derivatives for C2H4, C2H6, CH2O, CH3F, C2H2, HCN, and CH3OH molecules are presented. Satisfactory results are obtained by using a previously introduced analytical method, based on the evaluation of nuclear electric shieldings combined with the use of ‘‘polarized’’ basis sets. Computer efforts are maintained reasonably low.
The intensity and self-broadening of overtone transitions in HCN
Authors: Smith Alice M. , Lehmann Kevin K. , Klemperer William
Published: July 2002

The intensity of the (0,0,5)–(0,0,0) transition at 15 552 cm−1 in HCN has been measured using a cw laser dye laser, and a White cell with an optical path length of about 0.5 km. From a photoacoustic spectrum of HCN, the relative band intensity of the (0,0,5)–(0,0,0) transition to the (0,1,5)–(0,1,0) hot band transition has been determined. The integrated band intensity of the (0,0,5)–(0,0,0) band is 2.91(7)×10−23 cm−1/(molecule cm−2) or 17.5(4) cm/mol, which implies a dipole transition moment of 6.70(8)×10−5 D. The (0,1,5)–(0,1,0) band is equal in intensity to the (0,0,5)–(0,0,0) band within experimental error. The self-broadening and pressure shifts of these bands have also been examined. In the (0,0,5)–(0,0,0) band, the self-broadening coefficients for both the P and R branch for J between 0 and 19 were measured. For the P branch, the self-broadening coefficient is 23.7 MHz/Torr for J=1, increases monotonically to a maximum of 48.9 MHz/Torr at J=9, and then decreases monotonically to 21.7 MHz/Torr at J=19. For the R branch, the coefficient is 24.1 MHz/Torr for J=0, rises to a maximum of 45.4 MHz/Torr at J=7 and then falls to 14.7 MHz/Torr at J=19. For both branches, the variation of the self-broadening coefficient with J follows the J thermal population factor, which suggests that the collisional broadening is dominated by rotational resonances. Even though the vibrational density of states in the upper state is much larger than the density of rotational states, the line broadening appears to be fully accounted for by rotational relaxation.
Theoretical studies on polyaniline
Authors: Boudreaux D. S. , Chance R. R. , Wolf J. F. , Shacklette L. W. , Brédas J. L. , Thémans B. , André J. M. , Silbey R.
Published: July 2002

Theoretical predictions for various neutral and charged forms of polyaniline are presented and discussed. The MNDO semiempirical method is used to predict geometries which serve as input for valence effective Hamiltonian (VEH) calculations of the electronic band structure. The VEH calculations provide predictions for the band gaps, ionization potentials, electron affinities, and redox potentials for the various forms of polyaniline. Where possible, comparison is made to experiment with generally favorable results. The discussion emphasizes the electrochemistry of polyaniline and the structural evolution of the polymer during electrochemical oxidation and reduction.
Numerical MSA solution for binary Yukawa mixtures
Authors: Arrieta E. , Jȩdrzejek C. , Marsh K. N.
Published: July 2002

An efficient numerical algorithm is given to find the Blum and Ho/ye mean spherical approximation (MSA) solution for binary mixtures of hard-core fluids with one-Yukawa interactions. The initial estimation of the variables is achieved by partial linearization (based on known, physical asymptotic behaviors) of the system of nonlinear equations which result from the Blum and Ho/ye method. The complete procedure is at least one order of magnitude faster than that recently outlined by Giunta et al. More importantly, it always seems to converge to the physical solution (if it exists). We delimit, for several specific mixtures, the density-temperature region where no real solution is possible. This corresponds, following Waisman’s interpretation, to thermodynamic conditions for which vapor–liquid or liquid–liquid separation occurs. The dependency of the MSA solutions on the Yukawa exponent z is studied in detail. For high values of z, adequate for generalized mean spherical approximation (GMSA) applications, we propose an accurate linear approximation, and we relate it to the solutions given by Giunta et al. For equal-sized, symmetric, equimolar binary mixtures, we show that Baxter’s factorized version of the Ornstein–Zernike equation, including the factor correlation functions, can be decoupled. We also find, for equal-sized mixtures, that one of the approximations recently proposed by Jedrzejek et al. using an effective potential method is in very good agreement with our exact (MSA) results. Finally, a theoretical analysis shows that if the Yukawa amplitudes satisfy K12=(K11K22)1/2, the coefficients Dij of the factor correlation functions outside the core are related as follows: D1i/K1i =D2i/K2i, for i=1,2.
The liquid–vapor interface of simple polar fluids. I. Integral equation and perturbation theories
Authors: Eggebrecht J. , Gubbins K. E. , Thompson S. M.
Published: July 2002

The structure and surface tension of the liquid–vapor interface of a polar fluid are investigated using integral equation and perturbation theories. A recently introduced integro-differential equation and a perturbation theory are employed to determine the extent to which purely multipolar anisotropies in the pair potential induce orientational ordering. Both of these theories agree that orientational ordering can be induced at the liquid–vapor interface by purely multipolar anisotropies in the intermolecular potential. The Yvon–Born–Green equation and the virial equation for the surface tension are written in terms of the spherical harmonic coefficients of the one and two particle distributions, and simple expressions are obtained for the density profile and the anisotropic contribution to the surface tension for molecular models of multipolar symmetry. The density profiles, density-orientation profiles, surface thickness, and surface tension are calculated for the Stockmayer fluid for a range of temperatures and dipole moments.
A spin–boson model for spectroscopy involving nonadiabatically coupled potential energy surfaces
Authors: Coalson Rob D.
Published: July 2002

A spin–boson model, analogous to models commonly used to study multidimensional tunneling, is formulated for the problem of computing optical spectra when nonadiabatically coupled Born–Oppenheimer surfaces are involved. The model is implemented numerically for a one-dimensional prototypical case. It is demonstrated that time kernels associated with low resolution electronic absorption spectra and Raman scattering cross sections can be obtained without excessive effort even in strong coupling regimes where perturbation expansions become inefficient. The outlook for extension to more complicated systems is discussed.
An experimental study of the in-plane distribution of atoms in the liquid–vapor interface of mercury
Authors: Thomas Britt N. , Barton Scott W. , Novak Frank , Rice Stuart A.
Published: July 2002

We report measurements of the in-plane (transverse) structure function, S∥(k), of the liquid–vapor interface of Hg at 298 K, obtained using the method of grazing incidence x-ray diffraction. Our results show that: (i) The transverse structure function of the liquid–vapor interface of Hg is very similar to the bulk liquid structure function, but there are subtle differences between the two functions. (ii) As k→0, S∥(k) appears to diverge as k−2, in agreement with theoretical predictions and computer simulation studies. (iii) The observable S∥(k) for different longitudinal (parallel to the normal) density distributions in the liquid–vapor interface are very nearly the same, the largest differences being for momentum transfers less than that at the first peak of S∥(k). For this reason the measurement of S∥(k) is not a good monitor of the longitudinal density distribution. (iv) The interpretation of the differences in S∥(k) in the small k region (but outside the region where the k−2 divergence overtakes other effects) requires consideration of the effect of the longer ranged part of the effective pair potential and assessment of the relative contributions of the ionic and electronic components of the liquid metal to the compressibility. A comparison of the shape of S∥(k) in the threshold region just before the first peak and that of the same region in a bulk liquid metal leads to the weak conclusion that our data are consistent with the Monte Carlo simulations which predict that the liquid–vapor interface of Hg is stratified for about three atomic layers.