bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of torsional spectra of molecules with two C3v internal rotors. IV. The torsional potential function for propane-d and -d3
Authors: Durig J. R. , Groner P. , Griffin M. G.
Published: February 2003

The Raman spectra of gaseous propane-d0 and -d3 have been recorded between 50 and 3500 cm−1. Considerable torsional data are reported and used to characterize the torsional potential function based on a semirigid model. The average ’’effective’’ V3 for the propanes is found to be 1139±10 cm−1. The sine–sine coupling term is similar in magnitude but opposite in sign to the cosine–cosine coupling term in propane-d0.
Configurations of macromolecular chains confined to strips or tubes
Authors: Wall Frederick T. , Chin John C. , Mandel Frederic
Published: February 2003

Theoretical and Monte Carlo studies have been carried out on self-avoiding random walks confined to relatively narrow strips or tubes. Such walks are used to simulate the behavior of macromolecular chains subject to corresponding external boundary constraints. It is proved that, if a chain is confined to a strip or tube of a width small compared to the contour length of the chain, the mean square end-to-end separation of the chain is asymptotically proportional to n2, where n is the number of links in the chain. In addition, Monte Carlo results are reported for strips of three and eight layers and for thin tubes with four and five connected channels. The results so obtained are consistent with the theory. It is conjectured that chains confined to a thin surface, not necessarily monomolecular, will behave as if the chains were limited to a plane, but with parameters different from those for the strictly two-dimensional problem.
Crystal nucleation in a three-dimensional Lennard-Jones system. II. Nucleation kinetics for 256 and 500 particles
Authors: Mandell M. J. , McTague J. P. , Rahman A.
Published: February 2003

We have, by molecular dynamics computer simulation, observed homogeneous, spontaneous crystal nucleation in the 256-particle Lennard-Jones fluid at ρ*=0.91, 0.92, and 0.93, and in the 500-particle fluid at ρ*=0.92, 0.93, and 0.94. For the 500-particle system a nuclear region may be identified, and the nucleation and growth processes are found to be temporally separate. In all these events the nucleated phase was a body-centered (nearly cubic) structure. We believe this to be a manifestation of the Ostwald step rule.
Equation of state of fluid n–H2 from P–V–T and sound velocity measurements to 20 kbar
Authors: Mills R. L. , Liebenberg D. H. , Bronson J. C. , Schmidt L. C.
Published: February 2003

The molar volume V and ultrasonic velocity u of fluid n–H2 were measured simultaneously in a piston–cylinder apparatus over the range 75<T<307 K and 2<P<20 kbar. An equation of state of the Benedict type was formulated to reproduce the approximately 2000 experimental data sets of V and u within an average deviation of 0.7%. Values of the thermal expansion, compressibility, and heat capacity were derived. The equation was extrapolated beyond its experimental limits to make comparisons with existing static measurements at low pressure and with dynamic experiments and calculations at ultrahigh pressure. This is the first comprehensive high pressure study of the properties of fluid hydrogen, a substance that should prove useful in energy related research.
A study of medium effects on the collisions of hexafluorobenzene with the free radical bisdiphenylenephenylallyl
Authors: Potenza J. A. , Schmitz H. , Müller-Warmuth W.
Published: February 2003

The collision mechanics of hexafluorobenzene with the free radical bisdiphenylenephenylallyl (BDPA) has been examined in a variety of hydrocarbon solvents. Frequency and temperature dependent dynamic nuclear polarization and spin–lattice relaxation measurements have been used to determine absolute scalar and dipolar coupling strengths. In all cases, translational diffusion accounts quantitatively for the dipolar coupling. The C6F6 scalar spectra were analyzed in terms of a modified diffusion model with two frequency components. Each system shows a short scalar time constant arising from fast, elastic collisions, and a longer time constant arising from stereospecific plane–plane collisions. Addition of benzene to the binary C6F6/BDPA system changes the intensity and the time scale of the BDPA/C6F6 interaction hardly at all. In contrast, acetone or ether increases the intensity but does not change the time constants of the interaction, while addition of hexane depletes the scalar spectrum in high frequency components. The data suggest a tendency for BDPA and C6F6 to partition themselves from aliphatic solvents. With hexane, the fraction of plane–plane collisions is also increased.
Kinetic isotope effects in the quenching of NH(b 1Σ+) and of ND(b 1Σ+) by some deuterated compounds
Authors: Zetzsch C. , Stuhl F.
Published: February 2003

In addition to previous measurements with nondeuterated compounds, absolute rate constants for the quenching of metastable NH(b 1Σ+) and of metastable ND(b 1Σ+) radicals by HD, D2, D2O, CD4, C2D2, and C2D4 were determined at 296 K. The metastable NH and ND radicals were generated by pulsed vacuum uv photolysis of NH3 and of ND3 and were monitored by their (b 1Σ+; v′=0→X 3Σ−; v″=0) emissions at 471 nm. Large isotope ratios of the rate constants were measured for the quenching by hydrogenated and deuterated compounds.
Infrared laser double resonance study of vibrational energy exchange between 12CH3F and 13CH3F
Authors: Preses Jack M. , Flynn George W.
Published: February 2003

Laser–laser infrared double resonance has been used to study energy transfer from the 12CH3F to the 13CH3F isotopic species. The 12CH3F ν3 mode was pumped by a Q-switched 9.6 μ CO2 laser operating on the P (20) line, while the 13CH3F ν3 absorption was monitored by a cw CO2 laser operating on the P (32) 9.6 μ transition. Equilibration of the ν3 modes in the two isotopes occurs in approximately six gas kinetic collisions. Evidence for a somewhat slower equilibration of the ν6 modes in the two isotopes has also been observed. These results give further support to the idea that ladder climbing collisions can lead to a rapid redistribution of energy into high lying states of polyatomic molecules.
Existence of the dielectric constant in rigid-dipole fluids: The functional-derivative approach
Authors: Ramshaw John D.
Published: February 2003

In a previous article [J. Chem. Phys. 57, 2684 (1972)] sufficient conditions were established for the existence of the dielectric constant ε in rigid-dipole fluids. One of these conditions was an unrealistic restriction on the angular dependence of the direct correlation function c (12) at short range. Here it is shown that this restriction can be removed without altering the previous conclusions. Consequently, ε rigorously exists if c (12) depends only upon relative positions and orientations of molecules 1 and 2, and becomes asymptotic to −φd(12)/kT at long range, where φd(12) is the dipole–dipole potential. The development is based upon the functional-derivative interpretation of c (12) in terms of the response of the single-molecule distribution function to a single-molecule external field. The consequences of formally separating c (12) and the total correlation function h (12) into short- and long-range parts are briefly explored. A relation between the angular moments of the short-range parts of c (12) and h (12) is derived in an appendix.
Configurations of macromolecules subject to intermolecular volume exclusions
Authors: Wall Frederick T. , Chin John C. , Mandel Frederic
Published: February 2003

The effects of intermolecular interactions on the mean square end-to-end separations of macromolecules were studied by statistical sampling of multiple nonintersecting random walks on a square planar lattice. The ’’slithering snake’’ procedure was used to obtain large numbers of interacting random configurations, representing chains of 10, 20, 34, 62, and 104 links at various concentrations. The mean square end-to-end separation was studied as a function both of overall concentration and of chain length. With increasing concentration, the chains become shorter and the relative shortening becomes more pronounced with increasing contour chain length.
Dynamics of CO2+−D2 collisions
Authors: Mahan Bruce H. , Schubart Peter J.
Published: February 2003

We have measured complete velocity vector distributions for several of the products of collisions between CO2+ and D2. The reactions CO2+(D2,D)DCO2+ and CO2+(D2,OD)DCO+ are the principal chemical processes in the range of relative energies from 1 to 15 eV. The former proceeds principally by a direct interaction mechanism, with some smaller contribution from the decay of a D2CO2+ persistent complex. The DCO+ appears to come exclusively from the decay of this persistent complex at low relative energies, and from a complicated direct interaction process at very high energies. Other energetically accessible products such as CO+, O+, D2O+, and OD+ prove to be of minor or no importance.
Quantum mechanical close coupling approach to molecular collisions: Averaged definite parity j
z approximation with Clebsch–Gordan weights
Authors: Shimoni Yair , Kouri Donald J.
Published: February 2003

The new averaged definite parity jz(ADPjz) approximation is considered in detail from the standpoint of the exact body frame CC cross section equations. These cross section equations are decomposed into contributions from the various possible orbital angular momenta and projections of the total angular momentum J onto the scattering vector. This decomposition then permits a clearer examination of the quantities actually approximated in jz-conserving approximations. The various contributions to the exact cross sections are regrouped into components having the same form as the jzCCS and DPjz approximate equations. It is found that the labeling of jzCCS amplitude densities by an orbital angular momentum quantum number enters in a very natural way and in fact that only labeling by l̄ is permitted for consistency. Further, it is shown that only the amplitude density is approximated and this does not require the body frame T-matrix to be diagonal in jz. After the approximate amplitude density is substituted into the exact partial cross section expression, the resulting equations for the partial cross sections σJ(j0→j) lead to an average over the effective orbital angular momenta with weights wJlλ(j ‖j0) =〈l0jλ‖Jλ〉2[(2l+1)/(2J+1)]. A similar analysis of the definite parity partial cross section leads to an average over the effective orbital angular momenta of the proper parity with weights wJplλ(j ‖j0) =〈l0jλ‖Jλ〉2[(2l+1)/(2J+1)][2/(1+δλ,0)]. This enables one to include effects of the various turning points associated with values of l satisfying ‖J−j‖⩽l ⩽J+j. The present method of including parity through the values of l averaged also can correctly describe situations where the odd parity partial cross section is larger than the even parity partial cross section. The result is an approximation which describes definite parity partial cross sections, integral cross sections, and differential cross sections with very high accuracy. Finally, the ADPjz is a general method for which other averaging weights are possible. The previously obtained constant weight version is compared to the present Clebsch–Gordan weight version by applying both to He+H2 collisions. Results for definite parity partial cross sections and integral cross sections are used as the basis of the comparison.
Identification of the partial wave parameter and simplification of the differential cross section in the j
zCCS approximation in molecular scattering
Authors: Parker Gregory A. , Pack Russell T
Published: February 2003

The identification of the partial wave parameter ? of the JzCCS approximation in molecular scattering theory is considered, and it is shown that if one chooses ?=l′ (the final orbital angular momentum) rather than ?=J (the total angular momentum) the full body-fixed expression for the scattering amplitude reduces directly to the simple formula used by McGuire and Kouri. Results are cited that show that ?=l′ is not only simple, but physically reasonable, and gives much better results than ?=J.
Classical and quantum centrifugal decoupling approximations for HCl–Ar
Authors: Tarr Susan M. , Rabitz Herschel , Fitz D. E. , Marcus R. A.
Published: February 2003

The centrifugal decoupling approximation previously employed in quantum mechanics is extended to classical mechanics. Both classical and quantum centrifugal decoupling (CCD and CD, respectively) calculations are performed for HCl–Ar. Total cross sections are obtained from the CCD and CD calculations. The CCD and CD total cross sections are shown to be in good agreement with the corresponding exact classical trajectory (EC) and close coupled (CC) cross sections, respectively. The conservation of the projection (Ω) of the rotational angular momentum along the body-fixed z-axis is studied as a possible explanation for the success of the CCD and CD approximations in predicting total cross sections. By examining the CC body-fixed S-matrix elements and studying the behavior of Ω during the course of an exact classical trajectory, it is found that Ω is conserved only under very limited conditions. Detailed examination of the calculations shows that the CD approximation may be good even when Ω varies rapidly.
Photoelectron and far-ultraviolet absorption spectra of nonaromatic azomethine compounds
Authors: Vocelle D. , Dargelos A. , Pottier R. , Sandorfy C.
Published: February 2003

The far-ultraviolet absorption and He I photoelectron spectra of seven compounds containing either the nonconjugated azomethine group with alkyl substituents only or the conjugated C=C–C=N or C=N–N=C unit have been measured. The n and π ionization potentials (IP) as well as the bands due to π*←n and π*←π transitions are readily identified. The Rydberg bands can be approximately located at observed shoulders of broad bands using the IP’s and the usual ranges of term values but the diffuse character of these spectra makes exact assignments difficult.
Classical trajectories for the H+H2 reaction on a spline-generated potential energy surface
Authors: Gray Stephen K. , Wright James S.
Published: February 2003

Using a functional form based on rotated Morse curves, cubic splines are used to fit the Shavitt–Stevens–Minn–Karplus (SSMK) potential surface for the collinear H+H2 reaction. The fit obtained by this method is shown by contour maps and difference maps to be excellent. The deviations which do occur are shown to result from the limitations of the method but also from an error in the SSMK function. Classical trajectories carried out on the spline-fitted surface show good agreement with the results obtained from the analytical SSMK surface, including a point-by-point matching of most of the individual trajectories. The reactivity bands and total reaction probability agree closely with the two methods. The spline-fitted potential function is more economical to use than the analytical function and is easily adjusted to give different surface features, which may be varied in an independent manner.
The determination of the potential function governing the low frequency bending mode of disiloxane
Authors: Durig J. R. , Flanagan M. J. , Kalasinsky V. F.
Published: February 2003

The Raman spectra (10–3500 cm−1) of gaseous and solid disiloxane and disiloxane-d6 have been recorded. The infrared spectra of the gas from 4000 to 30 cm−1 and of the solid from 4000 to 450 cm−1 have been investigated. An examination of the low frequency Raman spectrum of the gas under moderately high resolution conditions (1.0 cm−1) revealed a Q-branch series for each molecule attributable to the double jumps of the anharmonic, low-frequency skeletal bending mode. The observed Q-branches were assigned with the help of a potential function of the form V (cm−1) =1.07±0.02 q4−21.9±0.3 q2 for the ’light’ compound and V (cm−1) =0.973±0.015 q2−19.2±0.4 q4 for the deuterated compound, where q is one of the reduced polar coordinates q and φ. These functions lead to barriers to linearity of 112±5 and 95±5 cm−1, with the ground state energy levels at 42.6 and 37.8 cm−1, respectively. Transformation of the potential functions to dimensioned form by using the reduced masses for the bending vibration gives an average equilibrium skeletal angle of 149±2°. The vibrational data have been interpreted in terms of G†36 molecular symmetry for the gaseous state, consistent with the concept of the quasilinear molecule, while the spectra of the solid state indicate a bent skeleton. Resolvable fine structure on two perpendicular bands in the infrared indicate very nearly free internal rotation in disiloxane.
Note on the Thomas–Fermi and Thomas–Fermi–Dirac model calculation of atomic polarizabilities
Authors: Csavinszky P.
Published: February 2003

This paper discusses a recent calculation by Bruch and Lehnen of the static electric dipole polarizabilities of neutral inert gas atoms. These authors obtained the polarizabilities within the framework of statistical models by making use of an energy functional method. The method, as applied by Bruch and Lehnen, makes use of the exact solutions of either the Thomas–Fermi (TF) or of the Thomas–Fermi–Dirac (TFD) equations. This paper points out that a better agreement than the one obtained by Bruch and Lehnen between calculated and experimental values of the atomic polarizabilities can be obtained upon resorting to approximate (analytical) variational solutions of the TF and TFD equations. The improved agreement, in the case of the TF model, is attributed to the fact that the variational solution of the TF equation permits one to construct a radial electron density for a neutral atom that decreases with the distance from the nucleus exponentially. That this is an improvement is seen from the fact that in the exact TF model the radial electron density drops off with the distance from the nucleus as the inverse fourth power of this quantity. Essentially the same situation prevails in the case of the approximate (analytical) solution of the TFD equation, which makes use of the variational solution for a TF atom. In this case, the improved radial electron density manifests itself in a contraction of the atomic radius, relative to that which is obtained within the exact TFD model.
Studies on bond dissociation in CH4, NH3, and H2O by the multiconfiguration self-consistent-field method
Authors: Ishida Kazuhiro , Kondo Kunihiro , Yonezawa Teijiro
Published: February 2003

The lowest potential curve for the dissociation of an X–H bond of an XHm molecule is calculated for CH4, NH3, and H2O by the multiconfiguration self-consistent-field (MCSCF) method. The calculated well depth of the potential curve (in atomic units) obtained from the two configuration wavefunction constructed with MCSCF localized orbitals is 0.1707 for CH4 (0.1656 for NH3 and 0.1589 for H2O). The value is in good agreement with the experimental value of 0.1619 (0.1610 for NH3 and 0.1879 for H2O). This allows us to conclude that the potential curve of the dissociation process of any local bond in a polyatomic molecule may be satisfactorily reproduced by the use of the two configuration wavefunction constructed with MCSCF localized orbitals. This wavefunction gives very good asymptotic behavior at a dissociation limit.
Polymer dynamics in a binary critical mixture
Authors: Duzy Carolyn , Kapral Raymond
Published: February 2003

The dynamic structure factor of a dilute solution of flexible polymer molecules in a binary mixture near the critical point is studied using mode coupling techniques. The strong concentration fluctuations of the solvent near the criticcal point lead to modifications of the hydrodynamic interactions between the monomer units in a chain. A set of equations is derived which describes the decay of the coupled solvent and polymer modes. The critical fluctuations of the solvent lead to a change in the wave vector dependence of the dynamic structure factor and a change in the center of mass diffusion coefficient of the polymer.
Quasiclassical trajectory studies of the chlorine–hydrogen system. I. Cl+H2→HCl+H
Authors: Persky Avigdor
Published: February 2003

Detailed three-dimensional quasiclassical trajectory calculations were performed for the reaction Cl+H2(0,J) →HCl+H on a semiempirical LEPS potential energy surface. Calculations were carried out for initial vibrational state v=0, rotational states J=0–4, and collision energies E between threshold and 12.0 kcal/mole. From the trajectory calculations we obtained reaction probabilities Pr(v=0,J,E,b) as a function of impact parameter b and initial values of J and E; reaction cross sections Sr(v=0,J,E) as a function of initial J and E; detailed rate constants k0,J and total rate constants kt in the temperature range 250–600°K, and the partitioning of energy and angular distribution of the products for different initial conditions. Thermal rate constants were compared with results of other trajectory studies of this system, with results of transition state theory calculations, and with experimental results. The trajectory rate constants of the present study were found to be in good agreement with experimental results. The results for energy partitioning show that under thermal equilibrium of reactants the products are formed with nearly thermal vibrational and rotational energies. The angular distribution of the products is mainly in the backward direction.
Configuration interaction calculations on the planar 1(π,π*) state of ethylene
Authors: McMurchie Larry E. , Davidson Ernest R.
Published: February 2003

The 1(π,π*) state of ethylene has been thoroughly investigated at the configuration interaction level utilizing an extensive basis set and a configuration selection scheme which emphasizes a correct description of all correlation effects involving the π or π* electrons. The result, contrary to the Hartree–Fock prediction, but in agreement with the general experimental conclusion, is a mostly valence state as indicated by a value for 〈π*‖x2‖π*〉 of 8.8 a.u. using the π* natural orbital. A calculated excitation energy of 7.96 eV is in reasonable agreement with the 7.65 eV experimental absorption maximum. It is found that σ→σ* excitations relative to the ground state are essential to describe the left–right correlation of σ electrons with the π* electron. This in turn compensates for the ionic πA 2–πB 2 nature of the 1(π,π*) state, allowing the SCF state to become contracted at the CI level. The π* orbital in the second 1(π,π*) state appears to be well characterized at a CI level as a 3d orbital constrained to be orthogonal to the mostly valence π* orbital of the lower state. Calculations employing the same configuration selection scheme were done on the 3(π,π*), 1(π,3s), and positive ion states, the characterization of these states being consistent with other theoretical and experimental work.
Surface polaritons on molecular crystals: An experimental study of anthracene
Authors: Tomioka Kenichi , Sceats Mark G. , Rice Stuart A.
Published: February 2003

We report studies of surface polaritons on the (ab) face of crystalline anthracene by the method of attenuated total reflection. We find that: (a) At low temperature (less than 100 °K) a surface polariton propagating down the b axis on the (ab) face is a well defined excitation. The estimated dephasing time at 16 °K is 0.1 psec, and the surface polariton can move coherently along the surface for up to 2 μm. (b) At higher temperatures the surface polariton is not a well defined excitation by virtue of scattering by lattice phonons. (c) The single particle 391 cm−1 vibronic dipolar surface polariton is also a well defined mode at low temperature. (d) The two particle scattering resonance arising from the 391 cm−1 vibration is seen in both the region of nonradiative modes and the region of radiative modes. (e) Three types of nonradiative polariton mode are observed below the exciton origin in thin crystals. Each of these is characterized by a different form of dispersion relation.
On the quasiadiabatic description of the dynamics of electronically adiabatic chemical reactions
Authors: Mukamel Shaul , Ross John
Published: February 2003

We present a description and analysis of electronically adiabatic chemical reactions in terms of nonadiabatic basis sets. By using an orthogonal transformation we separate the Hamiltonian into reactive and nonreactive parts. Explicit expressions are given for the reactive part of the interaction and some conditions are discussed for expressing it in terms of Franck–Condon type nuclear overlap. Next we derive closed formal expressions for all the various cross sections (elastic, inelastic, and reactive) in a chemically reactive system. With the present approach we make a distinction between the Franck–Condon and first-order approximation in the reactive interaction. Closed expressions for all the cross sections in terms of finite-size matrices are derived by making use of the K matrix formalism. These expressions enable us to use the Franck–Condon approximation and at the same time retain reactive interactions to infinite order.
Low-lying electronic states of the rare gas oxides
Authors: Dunning Thom. H. , Hay P. Jeffrey
Published: February 2003

A b initio configuration interaction calculations have been carried out on the electronic states of RgO (Rg=Ne, Ar, Kr, and Xe) arising from the Rg(1S)+O(3P, 1D, and 1S) separated atom limits. All potential curves, with the exception of the 1 1Σ+ curve in KrO and XeO, are found to be repulsive (with at most van der Waals minima). The binding in the 1 1Σ+ state is due in part to the incorporation of Rg++O− character and increase in the series: NeO (unbound) to ArO (essentially flat) to KrO (De=0.25 eV) to XeO (De=0.70 eV). The admixture of ionic character into the 1 1Σ+ state also strongly enhances the 2 1Σ+–1 1Σ+ transition, increasing the collision induced emission rate from O(1S) and decreasing the lifetime of the 2 1Σ+ state. The calculations also provide information on the quenching of O(1D) by the rare gases.
Laser spectroscopic measurement of weakly attractive interatomic potentials: The Na+Ar interaction
Authors: Smalley R. E. , Auerbach D. A. , Fitch P. S. H. , Levy D. H. , Wharton L.
Published: February 2003

The van der Waals molecule NaAr has been prepared in a supersonic expansion of sodium vapor plus a few percent argon in a helium carrier gas. The fluorescence excitation spectrum of the X2Σ+→A2Π optical transition has been analyzed and the vibrational and rotational spectroscopic constants have been derived from this analysis. The ground state interatomic potential implied by these parameters is consistent with that measured or calculated by other workers. The observed Λ-doubling of the A2Π state suggests that the coupling between the A2Π and B2Σ states may be stronger than would be expected from previous calculations. The magnitude and sign of the dependence of the fine-structure splitting of the A2Π state on vibrational quantum number suggests that the van der Waals bond produces an observable perturbation of the core electrons of the sodium atom.
Nonlinear optical properties of some m-disubstituted benzene derivatives
Authors: Carenco A. , Jerphagnon J. , Perigaud A.
Published: February 2003

Optical linear and nonlinear properties of single crystals of m-aminophenol, m-nitroaniline, m-chloronitrobenzene, and m-bromonitrobenzene have been studied. The second order optical susceptibilities have been measured with a Nd–YAG laser using a generalization of the Maker fringes technique to the case of biaxial crystals; their relative signs have been determined. The calculated phase matching conditions have been experimentally confirmed. m-Chloronitrobenzene, m-bromonitrobenzene alloys have a high figure of merit and are noncritically phase matchable from 0.974 to 1.069 μm as a function of concentration. The validity of an additivity approach to account for crystal nonlinearities is verified. The vector portion of the nonlinear susceptibility is related to the crystalline spontaneous polarization.
NMR of 205Tl in Tl2O–SiO2, Tl2O–GeO2, and Tl2O–B2O3 glasses
Authors: Panek L. W. , Bray P. J.
Published: February 2003

The NMR of 205Tl was observed in the thallium silicate, thallium germanate, and thallium borate glass systems at six operating frequencies spanning the range 8–26.5 MHz. Two sets of glasses were used for each system spanning the glass-forming region, one using 205Tl in the natural abundance (70.5%) and the other using thallium enriched to 99.5% 205Tl. Large distributions in the values of the chemical shift tensor and a large Tl–Tl exchange interaction were observed in each of the glass systems. The results for the thallium silicate glasses indicate a pairing or clustering of the thallium atoms. A variety of complicated and distorted structural groupings appear to be present in an SiO2 matrix; the groupings change in size and structure with composition and link together at compositions having more than 20 molar percent Tl2O. Data for the germanate and borate glasses are similar to each other and indicate the following conditions. (1) The environment for the thallium atoms in low thallium content glasses is ionic. (2) The thallium environment gradually changes to a covalent environment with increasing thallium content, and there appears to be a variety of thallium-rich structural units in the region of 20–35 molar percent Tl2O. (3) The thallium atoms are paired or clustered in similar covalent structural units for high thallium contents. This situation produces a strong exchange interaction and a reduction in the size of the distribution of the values of the chemical shift tensor.
Random-walk model of isotope enrichment in cascades
Authors: Kiss I. , Vass Sz.
Published: February 2003

Steady opertional parameters, such as enrichment, flow rates, isotopic distributions in the product and in the waste, and their variation over the cascade are evaluated in terms of transit probabilities and average transit times of different gas molecules in their random walk from one stage to another of a cascade. The random-walk model is also used for describing transient properties, i.e., the output response to input perturbation and the relaxation time. Calculations in terms of the model are carried out for UF6 of natural abundance enriched in a diffusion cascade. With the appropriate numerical values the model can also be applied to centrifuge cascades.
Translational energy dependence of the Rb+CH3I reaction cross section: Evidence for a minimum near 0.9 eV
Authors: Pace S. A. , Pang H. F. , Bernstein R. B.
Published: February 2003

The present study is a detailed reinvestigation of the crossed-beam reaction Rb+CH3I→RbI+CH3 over the translational energy range 0.5–1.7 eV. The integrated reaction cross section σR decreases with energy reaching a minimum near 0.9 eV, then rises slightly as the energy is increased further. This confirms the tentative conclusion regarding the existence of a minimum in σR(E) advanced by Litvak, Ure?a, and Bernstein [J. Chem. Phys. 61, 4091 (1974)] on the basis of less extensive data. A comparison is made of the overall body of experimental results with four different theoretically predicted σR(E) dependencies from the literature.
Vacuum–ultraviolet photoelectron spectroscopy of hexatriacontane (n-C36H74) polycrystal: A model compound of polyethylene
Authors: Seki Kazuhiko , Hashimoto Shimpei , Sato Naoki , Harada Yoshiya , Ishii Kikujiro , Inokuchi Hiroo , Kanbe Jun-ichiro
Published: February 2003

Photoelectron spectra of hexatriacontane (n-C36H74) films evaporated in situ under vacuum were measured at 12 photon energies between 11.4 eV and 21.2 eV. The escape depth of the photoelectrons was found to be much less than 10 nm. Although five valence band features on the photoelectron spectra are expected in this region from the findings by the XPS method, only one or two valence band features were observed; four features from the conduction band were found to dominate the spectra. The short escape depth and the smearing out of the valence band structure are attributed to scattering, which includes the scattering by phonons, and the appearance of the conduction band structure is attributed to accumulation of the secondary electrons to the high density of states parts associated with the conduction bands. The threshold energy of photoemission was found to be 9.1±0.5 eV. The finding for the evaporated films of polyethylene, despite some degree of decomposition, gave essentially the same result. The results for the conduction bands were compared with the theoretical band calculation on an isolated polyethylene chain. The lack of reliable theoretical calculations of the conduction band of solid polyethylene is pointed out.
Cooperative instability phenomena in arrays of catalytic sites
Authors: Bimpong-Bota E. K. , Nitzan A. , Ortoleva P. , Ross John
Published: February 2003

Cooperative instability phenomena are shown to arise in arrays of localized catalytic sites immersed in a bulk system. Nonlinear reaction mechanisms may occur on the catalytic sites; the reactions in the bulk are stable. From the partial differential reaction–diffusion equations for the total system we derive ordinary integral equations and obtain from these, by linearization, stability conditions as a function of catalytic site density. The theory is applied to one-dimensional lattices of catalytic sites with several model reaction mechanisms. For a product activated enzyme mechanism occurring on each catalytic site we show that there exist cooperative effects among sites due to the intersite coupling via the bulk reactions and diffusion. For given constraints, the number of stationary states available (one or three) depends on the density of catalytic sites, such that as the density increases, first three, then one, then three stationary states exist. For the Prigogine–Lefever mechanism critical concentrations necessary to produce chemical oscillations are shown to depend on the catalytic site density; the functional dependence has a maximum due to cooperative interactions among the sites. We study next a linear array of two types of alternating catalytic sites on which occur reactions of mutual activation of two species (a generalization of a model of Shymko and Glass). Multiple stationary states are found, and again the number of such states available depends on the site density.
Interface dipolar plasmons in liquid multilayer structures
Authors: Banville M. , Caillé A. , Zuckermann M. J.
Published: February 2003

The dispersion relations of interface dipolar plasmons are obtained for mono- and multilayer structures of molecular dipoles embedded in a nonpolar liquid. A system composed of dry multilayers of fatty acid molecules in which the terminal CH3 have been halogenated is suggested as an ideal structure for the observation of such dipolar plasmons. The results suggest that interface dipolar plasmons can be distinguished experimentally from single particle librational modes, as compared to bulk dipolar modes in polar liquids where such distinction is not possible.
Critical point wetting
Authors: Cahn John W.
Published: February 2003

It is shown that in any two-phase mixture of fluids near their critical point, contact angles against any third phase become zero in that one of the critical phases completely wets the third phase and excludes contact with the other critical phase. A surface layer of the wetting phase continues to exist under a range of conditions when this phase is no longer stable as a bulk. At some temperature below the critical, this perfect wetting terminates in what is described as a first-order transition of the surface. This surface first-order transition may exhibit its own critical point. The theory is qualitatively in agreement with observations.
Kinetics of the reactions of ClO with O and with NO
Authors: Zahniser M. S. , Kaufman F.
Published: February 2003

The ClO+O(k2) and ClO+NO(k3) reaction rate constants were obtained by measurements of steady-state Cl-atom concentrations using resonance fluorescence at 134.7 nm in a discharge-flow system. ClO is made by the Cl+O3(k1) reaction and removed by either added O or NO whose concentrations are measured independently. Over the temperature range of 220 or 230–298 K, k2/k1= (1.55±0.17) exp[(246±30)/T] and k3/k1= (0.52±0.05) exp[(373±23/T]. Combined with k1= (2.17±0.18) ×10−11 exp[−(171±20)/T] cm3 sec−1 reported earlier, this yields k2= (3.38±0.50) ×10−11 exp[(75±40)/T] and k3= (1.13±0.14) ×10−11 exp[(200±30)/T] cm3 sec−1. The Cl+O3 reaction was reinvestigated and k1 found to be (2.12±0.11) ×10−11 exp[−(157±12)/T] cm3 s−1, in excellent agreement with the earlier value. The kinetic and stratospheric significance of these findings is discussed.
Study of the stationary state of nonisothermal ideal gas mixtures containing an inert and a dissociating–recombining constituent
Authors: Lengyel Sándor
Published: February 2003

The energy balance equation and component balance equations for continuous systems in stationary state are used to derive a certain function (the negative potential of the inner energy flux) of temperature, which includes the heat conductivity of the system, the partial pressures of the components, the diffusion coefficient, and thermal diffusion ratio for binary ideal gas mixtures, and the enthalpy and rate constant of the chemical reaction in the system. This function is shown to satisfy the Laplace equation, from the solution of which the temperature distribution in the system can be derived. From this, the following are calculated as functions of position: diffusion and thermal diffusion fluxes of the components, rate of dissociation (or recombination) of the reaction A2?2A, contribution of this reaction to the internal energy transport in the nonisothermal ideal gas mixture A1A2 and inert gas. A method is given for finding the steady state partial pressures in the nonisothermal system from equilibrium partial pressures of the isothermal system calculated for different temperatures. As an example, calculations are made for the system NO2–N2O4–N2 enclosed between two coaxial cylinders of different temperatures.
Discrete and continuous Franck Condon factors of the Mg2 A1Σ+u–X1Σ+g system and their J dependence
Authors: Scheingraber H. , Vidal C. R.
Published: February 2003

Selective and nonselective excitation of the Mg2 A1Σ+u–X1Σ+g system has been obtained using the three uv lines of an argon ion laser. The discrete laser induced progressions have been compared with calculations of the discrete Franck Condon factors. Excellent agreement has been obtained taking into account the large J dependence due to the different shape of the potential curves involved. The continuous Franck Condon factors have been employed for an accurate determination of the repulsive part of the rotationless potential of the ground state, which has been evaluated up to 3400 cm−1 above the dissociation limit. Direct observation of predissociation of the A1Σ+u state has been possible by measuring the 3p 3P–3d 3D absorption of the Mg atomic system.
The high temperature vaporization and thermodynamics of the titanium oxides. XII. The temperature coefficient of the vapor pressure of TiO
Authors: Wheatley Quentin D. , Sheldon Robert I. , Gilles Paul W.
Published: February 2003

Knudsen vapor pressure measurements over the temperature range 1921–1998 K are reported for stoichiometric titanium monoxide contained in a molybdenum crucible. The pressures are consistent with that obtained in the preceding paper at 1806 K with the same material in a tungsten crucible. The combined results are given by logPE(atm) = (−129.2±1.7) ×103/4.576T+(39.25±0.86)/4.576. The uncertainties should be increased by about a factor of 3 to recognize the uncertainties in the mass spectrometric ionization cross sections. This equation together with the Knudsen equation allows the calculation of the equilibrium rate of mass loss of incongruently vaporizing stoichiometric TiO over the range zero to about 8% mass loss.
Modulated perturbation theory for molecular interactions. II. Remarks on nonlinear parameter minimization in STO expansions
Authors: Battezzati M. , Magnasco V.
Published: February 2003

In connection with the modulated form of Murrell–Shaw–Musher–Amos (MS–MA) perturbation theory, some properties of nonlinear minimization of first and second-order energy functionals with respect to STO exponents are examined. A general formulation for the system of equations for the orbital exponents is deduced, which can be conveniently used in application to specific problems.
A proton and deuteron spin–lattice relaxation study of the partially deuterated nematogens PAA and MBBA
Authors: Visintainer J. J. , Dong Ronald Y. , Bock E. , Tomchuk E. , Dewey David B. , Kuo An-Li , Wade Chas. G.
Published: February 2003

Proton spin–lattice relaxation times (T1) have been measured in methyl deuterated para-azoxyanisole (PAA-d6) and in ring deuterated para-azoxyanisole (PAA-d8) samples as functions of frequency and temperature. When compared with recent order fluctuation theory, the agreement between theory and experiment is not as satisfactory for the intermolecular contribution to the spin–lattice relaxation as for the intramolecular contribution. Deuteron spin–lattice relaxation times (T1Q) have also been measured in a ring deuterated p-methoxybenzylidene p-n-butylaniline (MBBA-d8) sample. The above order fluctuation theory fails to predict the observed temperature dependence and the observed frequency independence of T1Q if the correlation time τc for local motions is taken to be one-third the dielectric relaxation time τD. However, a fair agreement is obtained if one uses τc= (1/100) τD, a value consistent with Ref. 6.
Structure of phase boundaries
Authors: Kikuchi Ryoichi
Published: February 2003

The boundary between plus and minus spin phases of the Ising model is studied for the bcc 〈110〉 boundary. The iterative calculation due to Weeks and Gilmer is interpreted as a modification of the Natural Iteration computation (with subsidiary conditions) for deriving a free energy minimum. A tetrahedron is used as the basic cluster in the cluster-variation scheme for this problem. Two formulations are presented: one uses the scalar-product formulation and the other the boundary sum method. Previous results of an order–disorder type phase transition within the boundary are confirmed. Results of calculations on the boundary excess free energy and the boundary profile are compared for the pair and the tetrahedron treatments.
Dynamical charge transfer through hydrogen bonds: Hartree–Fock calculations of vibrational properties of formic acid monomer and cyclic dimer
Authors: Bosi Paola , Zerbi Giuseppe , Clementi Enrico
Published: February 2003

In order to explain the observed changes in the vibrational spectra of formic acid from monomeric to cyclic dimeric form the SCF Hartree–Fock method has been applied to calculate minimum energy geometries, quadratic force constants, and net atomic charges. The set of ab initio quadratic force constants turns out to be acceptable and accounts for the observed vibrational changes. The ab initio interaction force constant between carbonyl stretching motions for the dimer accounts for what is considered to be an unusually large observed splitting of the C=O stretching frequencies in carboxylic acid dimers. Calculations suggest that such a large splitting originates from a dynamical charge transfer through the hydrogen bond. The possible existence of this phenomenon should in general be considered when dealing with the vibrational properties of hydrogen bonded systems.
Volumetric pretransition in cholesterics
Authors: Armitage David , Price Fraser P.
Published: February 2003

Volumetric studies of the phase transitions in the following cholesterol derivatives are reported: cholesteryl chloride, C. laurate, C. myristate, C. nonanoate, and C. oleate. Pretransition volumetric behavior in the smectic, cholesteric, and isotropic phases is interpreted in terms of a ’’critical’’ exponent. In the isotropic phase the data are consistent with the de Gennes–Landau theory. The transformation from the cholesteric focal conic texture to the ’’blue phase’’ has a transition enthalpy and volume of approximately 5% of the transition to the isotropic phase.
Effects of long range interactions on the conformational statistics of short polypeptide chains generated by a Monte Carlo method
Authors: Premilat S. , Maigret B.
Published: February 2003

Distributions of end-to-end distances and characteristic ratios are calculated for short poly-l-ala and poly-gly chains generated by a Monte Carlo method based on the procedure elaborated by Metropolis et al. Interactions between all the atoms of the chain are taken into account for the conformational energy calculations. The characteristic ratios obtained with the complete set of interaction terms are much smaller than those calculated according to the method of Flory. This collapse of the chain is interpreted as due to a bad solvent. Results in agreement with Flory’s are obtained by modifying the potential functions to simulate the behavior of the chain in a ϑ or even a good solvent. In this case an excluded volume effect appears for poly-gly chains, whereas poly-l-ala chains are too stiff to present this effect with less than 16 residues.
Classical theory of circular dichroism in infinite helical polymers
Authors: Levin Alan I. , Tinoco Ignacio
Published: February 2003

In this report we derive a new method for obtaining the optical activity of helical polymers. Helical symmetry and periodic boundary conditions are applied to classical polarizability theory. This allows us to express the infinite polymer result in closed form. Our result is all order in intermonomer interactions, and gives the polymer circular dichroism bandshape in terms of monomer transition bandshapes. We also explicitly treat a complex unit of symmetry containing many optical transitions.
Core and molecular orbital photoelectron spectra of tetraphosphorus and tetraphosphorus trisulfide vapors
Authors: Banna M. S. , Frost D. C. , McDowell C. A. , Wallbank B.
Published: February 2003

The 2s and 2p binding energy shifts in gaseous P4 and P4S3 have been measured relative to the corresponding levels in PF5 and SF6 using AlKα radiation. Theoretical Hartree–Fock and XαSW L-shell orbital energy values are compared with experimental binding energies. The following differential photoelectric cross section ratios were obtained: P2p/P2s, S2p/S2s, S2p/P2p, and S2s/P2s. The cross section computations of Nefedov et al. and Scofield are in fair agreement with our experimental results. In the valence electron region, both AlKα and ZrMζ excitation sources were used, enabling a clear differentiation between the 3s- and the 3p-derived molecular orbitals. The prediction by Head et al. of a 3.48 eV gap between s- and p-like molecular orbitals of P4S3 is supported by the experimental data.
The quenching of NO2 fluorescence by a magnetic field
Authors: Butler Shahla , Levy Donald H.
Published: February 2003

It has been observed that the fluorescence from NO2 excited in the visible region is partially quenched by an applied magnetic field. The pressure dependence of the fluorescence intensity versus magnetic field strength curve indicates that the magnetic quenching is due to an increase in the collisional quenching cross section caused by the magnetic field. The magnetic quenching is observed to be a strong and erratic function of the exciting wavelength and this is shown to be consistent with the admixture of a nonradiating state where the amount of admixture is everywhere dominated by local perturbations. Magnetically induced changes in the absorption coefficient are also observed, and these are attributed to the tuning of energy level differences into and out of resonance with the exciting light by the magnetic field.