bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical treatment of the spin–orbit coupling in the rare gas oxides NeO, ArO, KrO, and XeO
Authors: Langhoff Stephen R.
Published: February 2003

A b initio configuration-interaction calculations have been carried out on the six electronic states of RgO (Rg=Ne, Ar, Kr, and Xe) arising from the Rg(1S)+O(3P, 1D, 1S) separated atom limits similar to the calculations of Dunning and Hay [J. Chem. Phys. 66, 3767 (1977)]. The degree of charge transfer and, hence, the extent to which the interactions are attractive is observed to be sensitive to the method of configuration selection. The off-diagonal spin–orbit coupling matrix elements 〈3Π‖Hso‖1Σ+〉 and 〈3Σ−‖Hso‖1Σ+〉 were calculated for the lowest 3Π, 3Σ−, and 1Σ+ states of RgO as a function of internuclear distance using the configuration-interaction wave functions and the full microscopic spin–orbit Hamiltonian. All one- and two-electron integrals were included. These results are compared in detail with the spin–orbit matrix elements of Cohen, Wadt, and Hay [J. Chem. Phys. 71, 2955 (1979)] computed using equivalent wave functions and an effective spin–orbit operator composed only of one-electron, one-center terms. The good agreement between the results strongly supports the use of an effective spin–orbit operator of this type. The success of their model results from the fact that electron interaction effects can be adequately accounted for by a single adjustable parameter that can be determined from the atomic fine structure. Apparently for the RgO molecules, the numerical results are a more sensitive test of the wave functions (particularly to the extent of charge transfer) than to the exact evaluation of all terms in the full spin–orbit operator. In fact, for the heaviest system considered, XeO, the effective operator approach actually appears to be superior to the ab initio treatment.
Quantum theory of large amplitude vibrational motions in a one-dimensional Morse chain
Authors: Eu Byung Chan
Published: February 2003

By introducing appropriately generalized annihilation and creation operators for oscillators interacting by Morse potentials and generalized coherent states, we have derived the quantal equations of motion for displacements in a one-dimensional chain of Morse oscillators. The equations of motion derived have the same mathematical structures as their classical analogs which have been studied before. Since they are essentially identical in structure, it is possible to infer from the classical studies that the quantal system also would exhibit solitary waves and solitons. It is pointed out in the paper that the formalism used here for anharmonic systems should also be useful for other fields of study related to high amplitude vibrational motions of polyatomic molecules for which the usual normal mode analysis is not suitable.
Theory of selective excitation of multiple-quantum transitions
Authors: Warren W. S. , Weitekamp D. P. , Pines A.
Published: February 2003

The question of whether a molecule can be made to absorb and emit photons only in groups of n is treated. Pulse sequences are introduced which in effect selectively induce the absorption of only groups of n photons. This causes only n-quantum transitions even when many other transitions might be resonant. The technique involves repeated phase shifts of 2π/n in the radiation to build up the selected coherences and cancel all other coherences, and is applicable to a wide range of spectroscopic systems. Coherent averaging theory is extended to describe selective sequences and demonstrates that n-quantum selectivity is possible to arbitrarily high order in the average Hamiltonian expansion. High-order selectivity requires many phase shifts, however, and for this reason the residual nonselective effects of sequences which are selective to only a finite order are calculated. Selective sequences are applied to the multiple-quantum NMR of oriented molecules, where in combination with time reversal sequences they produce a much more efficient transfer of the population differences into selected coherences than is obtainable by normal wideband pumping. For example, the 10-quantum transition in a 10-spin system can be enhanced by more than four orders of magnitude. Experiments on selective excitaiton of the 4-quantum transitions in oriented benzene verify the expected enhancement.
Dynamics of ion channeling at low energies: Preliminary trajectory studies
Authors: Adams John E. , Doll Jimmie D.
Published: February 2003

In order to understand better recent experiments which have examined the channeling of deuterium ions in tungsten single crystals at low incident energies, we have computed classical trajectories using various sets of ion–atom interaction parameters. Both the penetration depth profiles and the backscattered ion energy spectra display strong dependences upon the particular input parameters but to a much lesser extent upon the level of accuracy to which the individual trajectories are calculated. On the basis of our results we are able to suggest experiments which may resolve present uncertainties about the appropriate description of the microscopic interaction forces.
The role of vibrational–rotational coupling in V–V and V–R,T energy transfer
Authors: Coltrin M. E. , Marcus R. A.
Published: February 2003

The effect of neglecting vibrational–rotational coupling in energy transfer calculations is studied for collisions of HF (v=1−7) with HF (v=0). An analog of a ’’classical path’’ method is considered in which rigid-rotor trajectories are used to determine a time-dependent forcing term on the vibrational motion of each molecule. The results are compared with our quasiclassical calculations in which no such approximation was used. At higher vibrational states the rigid-rotor forced-oscillator model is found to predict substantially smaller V–R,T rate constants than those found in the exact study.
Dynamics of the reaction of N2O+ with D2 and H2
Authors: Johgo Akira , Murata Yoshitada
Published: February 2003

The ion–molecule reaction of N2O+ with D2 and H2 was studied by using the beam-collision chamber method in the relative energy ranging from 2.1 to 6.3 eV. Three main product ions, N2OD+ (N2OH+), N2D+ (N2H+), and OD+ (OH+) were detected in N2O+–D2(H2) collisions. The collision energy dependence of the reaction cross sections and the velocity distributions of these ions were observed. The experimental results including isotope effects show that the reactions proceed by a direct and short-lived interaction and suggest that the reaction path depends on the geometry formed from an incident ion and a target molecule in the collision. A simple collision model, which provides a consistent explanation of the experimental results, is proposed.
A mechanism for the photochemical induced combustion of H2/F2 mixtures inhibited by O2
Authors: Taylor R. L. , Lewis P. F. , Cronin J.
Published: February 2003

Using the most recent mechanisms and rate constants for the chemistry of the H2/F2/O2 molecular system, we have performed detailed modeling studies of the stability data of Levy and Copland and have shown that these data are explainable with reasonable accuracy. This same chemistry model has also been used to explain the variation in performance of a H2/F2 pulsed chemical laser with added O2. The results show that although there is some gain in laser performance by operating at low O2 concentrations, the inverse linear dependence on O2 concentration rules out any dramatic increase in performance.
Target patterns in a realistic model of the Belousov–Zhabotinskii reaction
Authors: Tyson John J. , Fife Paul C.
Published: February 2003

Periodic expanding target patterns of chemical activity are observed in thin layers of solution containing bromate, malonic acid and ferroin in dilute sulfuric acid. Commonly these patterns appear as thin blue (oxidized) rings propagating out from a central point into red (reduced) bulk medium. Recently, the opposite pattern has been observed: red waves of reduction propagating through an oxidized bulk medium. We discuss both of these patterns under the assumption that there is a heterogeneity at the center of the pattern—most likely a dust particle or a scratch on the glass—which changes the kinetics locally from a stable excitable steady state to a stable periodic oscillatory state. The temporal oscillation at the origin triggers waves of chemical activity which propagate radially into the excitable medium. Our approach is to combine recent advances in the mathematical description of traveling wave front solutions of reaction–diffusion equations with a realistic model of the kinetics of the reaction medium. The model we use, the Oregonator, is based on known features of the reaction mechanism, gives an acceptable qualitative and semiquantitative account of the reaction dynamics, and yet is simple enough to yield to analytic techniques developed primarily for scalar reaction–diffusion equations. With this approach we can account in some detail for most of the important features of target patterns in the Belousov–Zhabotinskii reaction. In particular, the distinction between trigger waves and phase waves is clarified by our analysis, and the novel properties of reducing waves in an oxidized medium appear as natural consequences of our model.
Alkoxy radicals: γ proton hyperfine couplings
Authors: Box Harold C. , Budzinski Edwin E. , Potienko George
Published: February 2003

The relationship between γ proton hyperfine couplings and conformation in alkoxy radicals was investigated. It is shown that large γ proton couplings are present in alkoxy radicals in which significant spin polarization of the Cβ–Cγ bonding orbitals is expected. This principle finds application in analyzing the proton hyperfine couplings observed for the alkoxy radical in irradiated deoxycytidine monophosphate. The g tensor and hyperfine coupling tensors which characterize this radical were determined from ESR and ENDOR measurements.
Surface enhanced Raman scattering from cyanide adsorbed on copper
Authors: Laufer G. , Schaaf T. F. , Huneke J. T.
Published: February 2003

Surface enhanced Raman scattering was observed from cyanide adsorbed on copper. The cyanide was adsorbed electrochemically and the spectra were taken in air. The relevant parameters were optimized to produce the largest Raman signal. The adsorbed material is believed to consist of multilayer coverage of CuCN on copper metal. The Raman signal probably results from the first adsorbed layer with an enhancement on the order of 104.
Sedimentation of wormlike coils. II
Authors: Hearst John E. , Reese Dennis A.
Published: February 2003

An application of the theories of Hearst and Stockmayer for the sedimentation coefficient of wormlike coils of length shorter than 2.2 Kuhn statistical lengths, and Gray, Bloomfield and Hearst for longer wormlike coils with excluded volume to recent sedimentation data on homogeneous DNA samples is presented. The data is entirely consistent with the predictions of the theories. The molecular parameters obtained from the analysis of the data are a Kuhn statistical length at 0.2 ionic strength of 1150 Å; at 0.1 ionic strength of 1290 Å; and a chain backbone diameter of 20–30 Å.
Muonium spin-exchange in low pressure gases: Mu+O2 and Mu+NO
Authors: Fleming Donald G. , Mikula Randall J. , Garner David M.
Published: February 2003

The spin-exchange cross sections σSE for muonium collisions with O2 and NO have been measured in weak transverse magnetic fields in low pressure moderator at 300 K. Average values from runs at 1 and 2.5 atm are σSE(Mu+O2)?1.2)×10−16 cm2 and σSE(Mu+NO) ?1.2)×10−16 cm2. These numbers are in qualitative agreement with those determined earlier by Mobley et al. in ∼40 atm. moderator pressure using a longitudinal field technique. Comparison with recent measurements of the corresponding cross sections for H atom spin exchange at 300 K reveals no isotope effect, consistent with expectations based on a random-phase approximation.
Cluster dynamics: Further classical trajectory studies of A+A
n⇄A*n+1
Authors: Brady John W. , Doll Jimmie D. , Thompson Donald L.
Published: February 2003

Classical trajectory studies were performed on the dynamics of collisions of single atoms with small clusters of atoms An, leading to the formation of quasibound An+1* complexes, in an attempt to model the microscopic steps in homogenous nucleation processes. Model system of atoms representing argon interacting through pairwise Lennard-Jones potentials were used in a three-dimensional numerical simulation of collision events. Capture cross sections and decay rate coefficients were computed as a function of energy for the case in which n equals 5, and the results were combined with previous studies for n equal to 3 and 4 to make preliminary observations about the functional dependence upon cluster size of the forward and reverse rate parameters for this process. Studies were also performed in which opacity functions were calculated for the case where n equals 3 in order to test certain aspects of a simple model frequently used in nucleation studies to compute cluster formation rates.
Laser-production of NaH crystalline particles
Authors: Yabuzaki T. , Sato T. , Ogawa T.
Published: February 2003

Production of NaH crystalline particles of μm size is observed in sodium vapor mixed with ∼10 Torr H2. The particles are produced when Na2 is excited to the B 1Πu state by a cw Ar+ laser, and also when Na is excited by a cw dye laser tuned to the D1 or D2 line.
The interaction between excited and ground state rare gas atoms. II. Homonuclear excimers with spin–orbit coupling
Authors: Vallee O. , Tran Minh N. , Chapelle J.
Published: February 2003

A model of potential is proposed to describe the potential curves of the first excited states of rare gas homonuclear excimers [X(np6, 1So)−X(np5(n+1)s, 1,3P)]. The model is composed of three terms: the long range part of the interaction given by a partial summation of the multipole series, the short range term which is the exchange Fermi potential, and the core–core interaction deduced from the ion–atom asymptotic model of Duman and Smirnov. The spin–orbit coupling is added using the semiempirical method of Cohen and Schneider. Three adjustable parameters are introduced to obtain the best value of the O+u(3P1) and of the O+u(1P1) potential well. Comparison with ab initio potential curves is made, and also with experimental data for the Ne*2, Ar*2, Kr*2 and Xe*2 excimers.
Interaction energies for low-lying electronic states of NaH and NaH−: Scattering of H− by alkali atoms
Authors: Olson R. E. , Liu B.
Published: February 2003

Extensive ab initio configuration interaction calculations have been performed for the seven lowest states of NaH and the four lowest 2Σ states of NaH−. Representative potential well parameters Re and De for the X 1Σ, A 1Σ, and C 1Σ states of NaH are 3.558 ao, 1.922 eV; 5.992 ao, 1.239 eV; and 11.88 ao, 0.787 eV, respectively. From the NaH− calculations, the adiabatic molecular electron affinity of NaH is found to be 0.373 eV. The molecular calculations on the low-lying states of NaH− allow us to predict that the mechanism for low energy electron loss in the H−+Na→H°+... reaction is primarily due to interactions with the H°+Na− charge transfer state. Cross sections are presented fo the H−+Na, K, Rb, and Cs electron loss reactions.
Investigation of energy levels due to transition metal impurities in metal-free phthalocyanine
Authors: Loutfy Rafik O. , Cheng Y. C.
Published: February 2003

We have determined the energy levels due to transition metal impurities in metal-free phthalocyanine. These energy levels were computed using the Anderson model and appear as multiple charge states, analogous to the deep states in inorganic systems. Two types of localized states can be classified, a donor-type appearing in the lower half of the gap and an acceptor type appearing in the upper half of the gap. Only donors occurred when the d-band energy of the transition metal, ED, was small, for instance in the case Cd, Cr, Fe and Mn. Both donor and acceptor levels coexist at intermediate values of ED, e.g., in the case of Co and Zn. However, when the value of ED is large, then only acceptor levels appear in the gap, e.g., with Cu and Ni. The computed localized energy levels of transition metals in H2Pc agreed well with those estimated from experimental redox potential data of phthalocyanine metal complexes. The implication of the existence of these impurity levels in H2Pc’s is very important as they can have a very profound influence on the electrical and photoelectrical properties of phthalocyanines.
The structure of electrolytes at charged surfaces: Ion–dipole mixtures
Authors: Carnie Steven L. , Chan Derek Y. C.
Published: February 2003

The detailed structure of the double layer is investigated using a model fluid consisting of hard spheres with embedded point charges in a solvent of hard spheres with embedded point dipoles against a hard wall with smeared-out surface charge. Such a model treats solute and solvent particles on an equal basis, unlike the primitive model of electrolytes. The statistical mechanics is solved using the mean spherical approximation for all interactions. This limits the validity of any results to the regime of low ionic concentrations, where, in this approximation, the model fluid has the correct limiting behavior for bulk thermodynamic quantities. In this regime, simple analytic results for the surface properties are given, which are correct to order (κR). In particular, the surface potential has the classical Stern layer form, with solvent structuring responsible for the inner layer capacitance. This result is the first derivation, as opposed to postulation, of Stern layer behavior. In addition, the polarization density oscillates about the continuum theory result for 3–4 molecular diameters away from the surface. Such behavior shows the difficulty in defining a local dielectric constant close to the surface.
Crystal-field analysis of triply ionized lanthanides in Cs2NaLnCl6
Authors: Morrison C. A. , Leavitt R. P. , Wortman D. E.
Published: February 2003

Optical spectra of the elpasolite hexachloride compounds Cs2NaLnCl6, where Ln represents one of the lanthanide ions, are analyzed with a crystal-field Hamiltonian of Oh symmetry. Crystal-field parameters, Bnm, are found that minimize the rms deviation between calculated and experimental energy levels for Ln=Ce, Pr, Nd, Eu, and Tb. A set of ’’smoothed’’ Bnm is found for the lanthanide series that predicts the spectroscopic properties of the remaining eight lanthanide ions. Predicted energy levels and g values are compared with available experimental data for all the lanthanides except Pm and Gd, and magnetic dipole intensities are computed.
Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. II. Normal, branched, and cycloalkanes
Authors: Fang Howard L. , Swofford Robert L.
Published: February 2003

The C–H stretching overtones of n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane, n-dodecane, 2-methylbutane, 2-methylpentane, 3-methylpentane, 2,2,4-trimethylpentane, cyclopentane, cyclohexane, methylcyclohexane, cycloheptane, and cyclooctane in the liquid phase are reported as seen by conventional absorption (5600–15 800 cm−1) and thermal lensing (14 700–18 500 cm−1) spectroscopy. A local mode (LM) model based on uncoupled, anharmonic C–H oscillators is used to analyze the observed spectra. Relatively strong, well-resolved absorption peaks are observed and are assigned within the LM model as the ’’pure’’ vibrational overtones of nonequivalent C–H oscillators in the molecule. The relative intensities of the absorption bands assigned to primary and secondary C–H oscillators at a given level of excitation reflect the relative numbers of primary and secondary hydrogens in the molecule. Both LM anharmonicity constants and LM mechanical frequencies are obtained from the overtone analyses. A gradual increase in the bandwidths of the pure LM states with increasing vibrational quantum number is seen for the n-alkanes. The effect which the environment exerts on the C–H bond is demonstrated in the spectra of cyclopentane and cyclohexane, where the absorption of the axial and equatorial C–H bonds is resolved. Relatively less intense bands which appear in the spectra are interpreted within the LM model as combination bands. The partially resolved bands which appear as shoulders on the high-energy side of the ’’pure’’ LM overtones are interpreted as combinations in which the vibrational quanta are distributed over two LM modes. The more prominent combination bands are assigned as combinations of the C–H bending mode with the ’’pure’’ LM C–H stretching mode. Relatively large anharmonic coupling coefficients are obtained from a fit to the energies of this latter type of combination band. The magnitude of these coupling coefficients is thought to indicate the importance of the combination motion in the deactivation of the highly vibrationally excited molecule. Finally, the intensities of the combination bands relative to the pure LM overtones are seen to decrease with increasing vibrational quanta, in accord with the prediction of the LM model.
Laser enhanced addition reactions between hydrogen halides and unsaturated hydrocarbons: An information–theoretic approach
Authors: Zamir E. , Haas Y. , Levine R. D.
Published: February 2003

Detailed balance, and surprisal analysis and synthesis are used to provide guidelines for maximal utilization of laser excitation of the reagents in addition reactions. Specific examples are discussed.
Molecular beam photoionization study of carbon disulfide, carbon disulfide dimer and clusters
Authors: Ono Y. , Linn S. H. , Prest H. F. , Gress M. E. , Ng C. Y.
Published: February 2003

A new molecular beam photoionization mass spectrometer which is significantly superior to previous similar instruments in both wavelength resolution and ion intensity is described. Photoionization efficiency (PIE) data for CS2+ near the threshold (1210–1232 and 1075–1130 Å) were obtained with a resolution of 0.14 Å (FWHM). Rydberg series I, n=16–24, which converges to the spin–orbit excited state, 2Π1/2, of CS2+ was resolved on the first vibrational step in the PIE data. Observation of the autoionization resonance, series I (n=16), indicates that the value for the first ionization energy (I.E.) for CS2 obtained in previous absorption experiments (Ref. 23) is too high. The I.E.’s for the 2Π3/2 and 2Π1/2 states of CS2 were determined to be 10.0685±0.0020 and 10.1230±0.0020 eV, respectively. PIE data for (CS2)n+ with n ranging from 2 to 5 have been obtained in the wavelength region from 650 to 1350 Å with this apparatus. The observed I.E.’s for (CS2)2, (CS2)3, (CS2)4, and (CS2)5 are 9.36±0.02, 9.22±0.02, 9.10±0.02, and 9.04±0.02 eV, respectively. From the measured ionization energy, and the estimated dissociation energy (0.05 eV) of (CS2)2, the binding energy for CS2+⋅CS2 is deduced to be 17.5±1 kcal/mol. This is in agreement with the value obtained previously in a high pressure mass spectrometry experiment. The Rydberg series for CS2, which are manifested as autoionization lines in the PIE data for (CS2)2+ in the region 850–960 Å, are red shifted in energy by ∼0.075 eV for Rydberg series III and ∼0.14 eV for Rydberg series IV. Adapting the most recent assignment of Rydberg series III and IV by Larzilliere et al. symmetry arguments support a linear or near linear geometry for the carbon disulfide dimer.
Slow molecular motion detected in the NMR spectra of rotating solids
Authors: Suwelack D. , Rothwell W. P. , Waugh J. S.
Published: February 2003

We present an approach for detecting and analyzing slow molecular motions in solids based on changes in the zero order sideband of an inhomogeneously broadened spin system under magic angle spinning conditions. The effect is discussed theoretically from the standpoint of second order perturbation theory and the stochastic Liouville equation of Kubo, both for the case where the chemical shift is the main interaction. Experimental investigations of decamethylferrocene (resulting in Ea=3.3 kcal/mole) and hexamethyl benzene support the predicted trends. A five site jump model is used to interpret the results for decamethyl ferrocene, while the interpretation of the hexamethyl- benzene data is obfuscated by the presence of a chemical shift inequivalency presumably caused by a distortion of the aromatic ring above the phase transition (Tc=116 K).
Energy transfer in NH3–He collisions
Authors: Green Sheldon
Published: February 2003

The electron gas intermolecular potential for NH3–He developed previously is shown to be inadequate for a quantitative description of energy transfer in this system by comparing with available experimental data—pressure broadening, double resonance, and beam scattering. A new potential is constructed by combining the Hartree–Fock results of Davis, Boggs, and Mehrotra with semiempirical long-range induction and dispersion terms. The resulting potential is shown to be in harmony with beam scattering and pressure broadening measurements. It is found that the quadrupole-induced–dipole interaction, which has been invoked previously to account for pressure broadening in this system, is not, in fact, as effective as the short-range anisotropy. Although results are in general accord with double resonance data, a few unresolved discrepancies remain. It is suggested that these are due to improper consideration of the dependence of cross sections on degenerate magnetic quantum levels in the experimental analysis. Using this new intermolecular potential state-to-state rate constants are presented for temperatures to 300 K.
Kinetics of polymer gelation
Authors: Ziff R. M. , Stell G.
Published: February 2003

The kinetics of polymer growth and especially gelation are discussed for a system of reacting f-functional monomeric units. The gelation models of Flory and Stockmayer are examined, and their implications concerning sol–gel interaction after gelation has occurred are clarified. A new model, in which the gel does not cross link, is considered. A mathematical analysis shows that the onset of gelation is related to the formation of a shock-wave solution of the equation satisfied by the generation function.
Photoelectron energy distribution following UV laser ionization of gas phase benzene
Authors: Meek Jon T. , Jones Robert K. , Reilly James P.
Published: February 2003

Photoelectrons from KrF (249 nm) and ArF (193 nm) laser induced ionization of gas phase benzene have been observed. The ionization is shown to result from absorption of two photons only. The intensity dependence of the ArF results indicates that collisionless intramolecular relaxation following absorption of the first photon is detectable with this technique.(AIP)
The microwave spectrum of carbon dioxide–18O
Authors: Endo Yasuki , Yoshida Kazuhiko , Saito Shuji , Horota Eizi
Published: February 2003

Five rotational transitions of 16O12C180 were observed by using a source–modulation microwave spectrometer. The least-squares analysis of the measured transition frequencies gave the rotational constant that is three times more accurate than the most recent infrared value. The dipole moment of the molecule was estimated to be (7.0±1.5)×10−4 from a comparison of its spectral intensity with that of 18O13C34S. (AIP)
Calculation of rotational–vibrational preionization in H2 by multichannel quantum defect theory
Authors: Jungen Ch. , Dill Dan
Published: February 2003

Multichannel quantum defect theory is adapted to treat simultaneous rotational and vibrational preionization in H2. The strongly preionized spectrum between the N+=0 and N+=2 rotational thresholds of photoionization of H2X1Σg+(J″=0, v″=0) to produce H2+X2Σg+(N+, v+=0) is computed as example and good agreement is obtained with the photoionization data of Dehmer and Chupka.
Adsorption at the liquid–vapor interface of a binary liquid mixture
Authors: Beaglehole D.
Published: February 2003

It is shown that the coefficient of ellipticity measures the adsorption at the liquid–vapor interface of a binary liquid mixture. The cyclohexane–aniline system has been studied. Measurements as a continuous function of temperature are reported for concentrations about the critical point. A discontinuity in the adsorption is observed at the phase boundary, which becomes a logarithmic singularity at the critical point. Adsorption isotherms are presented.
Ionic association in model 2–2 electrolyte solutions
Authors: Rossky Peter J. , Dudowicz Jacek B. , Tembe B. L. , Friedman Harold L.
Published: February 2003

A detailed study has been made of the structural consequences of a charged soft sphere model for aqueous 2–2 electrolytes. The HNC approximation for this model gives structural features as well as thermodynamic functions that agree closely with the reported HNC results for a charged hard sphere model provided that R, the distance at which the pair potential u+−(r) is a minimum, is the same in both models. In both cases there is a strong enhancement of the like-charge correlation function g++(r) [equals g−−(r) in these models] in the range where r?2R. The trends in the HNC results are generally confirmed by Monte Carlo (MC) results for the same model, but the enhancement of g++ is greatly reduced compared to HNC. The MC results confirm earlier interpretation of the enhancement as due to larger than pairwise clusters even at concentrations below 0.005 M. Analysis of the discrepancy between the MC and HNC results leads to a revised form of the latter, BHNC, which incorporates a simple approximation for the sum of bridge diagrams and which gives g++(r) in much closer agreement with the Monte Carlo results.
Gas–gas equilibria with a theoretical equation-of-state. I. Symmetrical model
Authors: Pittion-Rossillon G.
Published: February 2003

The Mansoori et al. equation-of-state for hard spheres mixtures is shown to give immiscibility in binary mixtures at high pressure when combined with the attractive part of the van der Waals equation-of-state for mixtures. With a restriction of this model giving what we call ’’symmetrical model’’, qualitative agreement with previous work on the van der Waals equation-of-state is found. Semiquantitative agreement with some aspects of experimental work is obtained.
A self-consistent cell model for the melting transition in hard sphere systems
Authors: Cowley E. R. , Barker J. A.
Published: February 2003

The Bethe approximation is applied to the zero- and single-occupancy cell model for hard spheres. The resulting self-consistency equations are solved numerically, for both two- and three-dimensional systems. In both cases there are solutions of two kinds, one representing the solid phase and the other the fluid phase. Cluster corrections to the Bethe approximation are also evaluated. When a reasonable choice of cell size is made and cluster corrections are included, the predicted melting parameters agree well with the results of computer simulations. Thus the self-consistent cell model provides a unified theory of solids, fluids and melting.
Spectroscopy and relaxation of CS2+ in solid Ne
Authors: Bondybey V. E. , English J. H.
Published: February 2003

Absorption and laser induced fluorescence spectra of CS2+ in solid Ne are re-examined. Isotopic studies with 13CS2 confirm that most of the strong vibronic bands of the ? 2Πg→? 2Πu transition are due to an almost perfect mixture of ν1 and ν2 modes. Extension of the spectrum into the UV leads to identification of the overtone of the third vibrational mode, 2ν′3=3288 cm−1, whose frequency increases considerably in the upper state. The origin of the X 2Πg→? 2Σu+ transition is also observed. ? 2Σu+ state excitation results in efficient nonradiative transition into the ? 2Πu state. Study of spectral line shapes associated with the ? 2Πg→? 2Πu transition suggests inefficient energy transfer between ν3 and the lower frequency modes.
Synchrotron radiation study of the molecular xenon fluorescence around 2000 Å
Authors: Dutuit O. , Castex M. C. , Le Calvé J. , Lavollée M.
Published: February 2003

Time-gated fluorescence spectra have been obtained in xenon excited by synchrotron radiation around the first resonance line. Evidence of the emission from the left turning point of the Xe2 excited Ou+ (3P1) potential curve is obtained between 2000 and 2100 Å for different excitation wavelengths. From our results, we propose an experimental potential curve for the Ou+ state with an equilibrium distance Re of 3.25 Å and a well depth De of 5000 cm−1. Franck–Condon factors have been calculated with this curve for the bound–free transitions between different vibrational levels of the Ou+ excited state and the Og+ ground state. They are in excellent agreement with experiment.
Generalized Langevin theory for many-body problems in chemical dynamics: Reactions in liquids
Authors: Adelman S. A.
Published: February 2003

A general theoretical framework for classical trajectory simulations of chemical reactions in liquids is presented. This framework is a development of the molecular timescale generalized Langevin equation (MTGLE) theory [S. A. Adelman, Adv. Chem. Phys. 44, 143 (1980)] for condensed phase chemical reaction dynamics. This generalization permits one to treat solute configuration (r0) dependent generalized damping forces in a computationally straightforward manner. Thus, for example, dynamical effects of common caging of reagents are realistically accounted for in the present theory. The theory is based on the following method. The nonequilibrium solvent density induced by small displacements Δr0(t) of the solute (chemical system) from a configuration point r0 is computed by linear response theory. The time dependent reaction force that the solvent exerts on the solute is then computed from the nonequilibrium solvent density. This leads to a set of solute configuration dependent MTGLE parameters {ωep2(r0), ωp+12(r0)}, p=0,1,2,⋅⋅⋅, which account for dynamical effects of solvent–solute interaction. These parameters may be generated as a function of r0 by performing molecular dynamics simulations of the solvent for a number of solute configurations r0. A generalized equivalent harmonic chain representation for solute dynamics is then developed. The force constants of the equivalent chain are the MTGLE parameters. Techniques for approximating the non-Markovian chain equations by Markovian model chain equations are presented. These Markovian equations are directly applicable to simulation of liquid state reactions.
Electron detachment from Cl− in collisions with H2 and D2 (5≲Erel≲50 eV)
Authors: Cheung J. T. , Datz S.
Published: February 2003

Energy losses accompanying the detachment of an electron from Cl− in collision with H2 and D2 have been measured using a time-of-flight technique. Four different energy loss channels are identified and their excitation functions mapped: (1) ΔE?4 eV corresponding to ’’simple detachment’’ (E.A. Cl−=3.6 eV); (2) ΔE?8 eV corresponding to dissociative attachment; (3) ΔE ranging from 11 eV at Erel?15 eV to 14 eV at Erel?50 eV associated with detachment to the 2Σ+g state of H−2; (4) ’’spectator’’ backscattering from a single hydrogen atom.
Negative ion–uranium hexafluoride charge transfer reactions
Authors: Streit Gerald E. , Newton T. W.
Published: February 2003

The flowing afterglow technique has been used to study the process of charge transfer from selected negative ions (F−, Cl−, Br−, I−, SF6−) to UF6. The sole ionic product in all cases was observed to be UF6−. Data analysis was complicated by an unexpected coupling of chemical and diffusive ion loss processes when UF6− product ions were present. The rate coefficients for the charge transfer processes are (k in 10−9 cm3 molecule−1 s−1) F−, 1.3; Cl−, 1.1; Br−, 0.93; I−, 0.77; and SF6−, 0.69. The rate constants agree quite well with the classical Langevin predictions.
Temperature dependence of near-resonant vibration–vibration energy exchange in H35Cl+H37Cl and D35Cl+D37Cl collisions
Authors: Shin H. K. , Kim Y. H.
Published: February 2003

A collision model which accounts for the role of long-lived collisions in molecular interaction is used to calculate vibration–vibration energy exchange probabilities of H35Cl+H37Cl and D35Cl+D37Cl. The collision trajectory of local translation is used to describe the vibration–vibration energy exchange process as well as the transfer of vibrational energy mismatch. Results are in agreement with experiment over the temperature range of 200–600 °K. In particular, the observed temperature dependence of the probability for H35Cl+H37Cl seems to be well described by the model. The role of collisions with nonzero impact parameters, dependence of probabilities on the interaction range parameter, and isotope effects on deuteraction are also discussed.
Formation of the XeBr exciplex by Xe–Br2 (D′) collisions
Authors: Ehrlich D. J. , Osgood R. M.
Published: February 2003

Emission has been observed from the XeBr exciplex produced by collisional formation following sequential two-photon optical pumping of Br2 (D′). The vibrationally thermalized emission which results allows a study of the temperature dependence of the broadband XeBr III1/2→II1/2 fluorescence to be made. Calculated spectra based on ab initio potential curves confirm the band assignment and reproduce the observed temperature dependence. The collisional formation process has been determined to proceed with a rate constant of 1.7×10−10 s−1 cm3. An interpretation is given in terms of a harpooning mechanism.
Proton NMR in a hydrogen molybdenum bronze
Authors: Cirillo A. C. , Ryan L. , Gerstein B. C. , Fripiat J. J.
Published: February 2003

Solid state nuclear magnetic resonance (NMR) has been used to probe the nature of hydrogen in the hydrogen bronze H1.64MoO3. The line shape of the proton NMR at room temperature is found to be associated only with shielding anisotropy. Below 230 K, dipolar interactions become important in broadening the line. The response of the protons under a magic angle spinning experiment indicates that there is predominantly only one type of hydrogen in this sample. The shielding anisotropy indicates that at room temperature this species is not a hydroxyl proton. The longitudinal relaxation time of protons in this material exhibits a minimum between 190–320 K indicating that dipolar interactions modulated by motion are responsible for T1 relaxation. Comparison of longitudinal relaxation times of partially deuterated and nondeuterated samples indicates that there exists a contribution to T1 relaxation not dependent upon interproton distance. There does not appear to be a conduction electron contribution to the proton T1.
Solutions of the master equation for multiphoton dissociation
Authors: Troe J.
Published: February 2003

Iterative analytical solutions of a master equation for multiphoton dissociation are derived and compared with numerical results. Simple expressions for the incubation time and for the steady-state rate constant are obtained identifying the relevant molecular parameters.
Electron impact excitation of the neon isoelectronic sequence
Authors: Ganas P. S. , Green A. E. S.
Published: February 2003

An analytic atomic independent particle model potential adjusted to experimental energy levels is used to generate wave functions for the ground and excited states of the neon isoelectronic sequence from Z=10 to Z=20. Using these wave functions in conjunction with the Born approximation and the LS-coupling scheme, we calculate optical oscillator strengths for transitions from the 2p6(1S0) ground state. Generalized oscillator strengths and integrated cross sections for incident electron energies ranging from threshold to 5 keV are calculated for the 2p–3s resonance transition along the sequence. Regularities and systematic trends along the sequence are discussed.
Quantum chemistry by random walk: Higher accuracy
Authors: Anderson James B.
Published: February 2003

The random walk method of solving the Schrödinger equation is extended to allow the calculation of eigenvalues of atomic and molecular systems with higher accuracy. The combination of direct calculation of the difference δ between a true wave function ψ and a trial wave function ψo with importance sampling greatly reduces systematic and statistical error. The method is illustrated with calculations for ground-state hydrogen and helium atoms using trial wave functions from variational calculations. The energies obtained are 20 to 100 times more accurate than those of the corresponding variational calculations.
The spectra of carbonium ions, cyanine dyes, and protonated Schiff base polyenes
Authors: Schulten Klaus , Dinur Uri , Honig Barry
Published: February 2003

The origin of the strong bathochromic shift of the visual pigment supposedly lies in the carbonium ion character of the protonated Schiff base retinal. Following this conjecture, we have carried out a comparative study of the spectra of the carbonium ions, cyanine dyes, and protonated Schiff base polyenes. In our calculation, we have employed PPP (for the carbonium ions) and CNDO/S parametrized Hamiltonians. For a correct description of the electronic excitations in these linear π-systems, we have included all single, double, and in some cases also all triple and quadruple excited configurations in the CI expansion. Our study revealed a close resemblance of the spectral behavior of these three compounds. In particular, we found that higher excitations, accounting for the effect of electron correlation, induce a strong red shift of the main (S1←S0) absorption band. This behavior is due to the mainly covalent character of the S1 state which is poorly described by single excited configurations. As in the polyenes, there exists also a weakly absorbing state of double excited character which in contrast to the polyene case lies above the allowed state.
Tunneling frequencies of solids with two equivalent internal C3V rotors
Authors: Smith David
Published: February 2003

The tunneling frequencies of two-top molecules have been computed by the diagonalization of the Hamiltonian matrices. For three solids, (CH3)2O, (CH3)2S, and (CH3)2CH2, whose tunneling frequencies have been determined, it has been found that the ratio of V6/V3 ranges from −0.05 to −0.14. For given torsional frequencies, the F′ term in the Hamiltonian may have a significant effect on the tunneling frequencies, but the cosine–cosine and sine–sine coupling terms have little effect on the tunneling frequencies. The observed torsional and tunneling frequencies of (CH3)2CO and CH3C≡CCH3 are also discussed in terms of the two-top model.
Two-photon excited luminescence of thulium-activated lanthanum oxysulfide (La2O2S–Tm3+)
Authors: Bleijenberg K. C. , Kellendonk F. A. , Struck C. W.
Published: February 2003

Using the results from two-photon excitation experiments it is shown that a low lying FC shifted state is present in La2O2S–Tm3+. The thermal quenching of the 1I6 emissions can be explained in terms of thermally promoted crossovers to this FC shifted state.
Theory of single-phase solid-state polymerization reactions
Authors: Baughman R. H. , Chance R. R.
Published: February 2003

A theory of single-phase reactions is developed to explain the isotope, impurity, pressure, and substituent dependence of solid-state polymerization kinetics. This theory, which is applied to diacetylene reactions, incorporates the effect of changing structure during reaction on the rate of chain initiation and the length of propagating chains. The analysis provides a new method for determining chain-length parameters for insoluble and infusible polymers, which utilizes changing ambient measurements during solid-state polymerization. The conversion dependence of activation parameters is derived from elasticity theory for polymerization reactions which produce large dimensional changes in the reaction direction. However, the formulation of reaction kinetics is applicable to a much broader class of solid-state polymerization reactions.
The intermolecular potential and the crystalline field of solid CH4. II
Authors: Yasuda Hideo
Published: February 2003

The two-center expansion of an arbitrary function is reviewed from the standpoint of the application of it to the calculation of intermolecular potentials. The expansion of an exponential function is obtained in a form convenient for the present purpose. When the intermolecular potential is calculated by use of this expression together with that for an inverse power function on the assumption that the intermolecular potential between polyatomic molecules is expressed in terms of the summation of atom–atom potentials of the Buckingham (exp-6) type, it is derived in a form reflecting both molecular orientations and molecular symmetry. The method is applied to solid CH4. The result, containing the crystalline field, is compared with that obtained by using the Lennard-Jones (n-6) type potentials in place of the Buckingham ones.
Pulsed laser photolysis–long path laser absorption kinetics study of the reaction of methylperoxy radicals with NO2
Authors: Ravishankara A. R. , Eisele F. L. , Wine P. H.
Published: February 2003

The technique of pulsed laser photolysis–long path laser absorption is employed to study the kinetics of the reaction CH3O2+NO2+N2 k1→CH3O2NO2+N2 over the temperature range 253–353 K. CH3O2 is prepared by 193 nm (ArF) laser photolysis of azomethane in the presence of O2. The temporal profile of [CH3O2] is followed by measurement of the time resolved attenuation of a frequency doubled argon ion laser beam (257 nm). A path length of 860 cm is employed to obtain initial absorptions of (typically) 4%, corresponding to a CH3O2 concentration of 2×1013 cm−3. At all temperatures, k1 is found to increase monotonically but nonlinearly with increasing N2 pressure (76–722 Torr) indicating that Reaction (1) proceeds predominantly via an addition mechanism and that the reaction is in the falloff region between second and third order kinetics. The measured values of k1 are compared with results of previous investigations and their atmospheric implications discussed.
Quasiperiodic trajectories for a multidimensional anharmonic classical Hamiltonian excited above the unimolecular threshold
Authors: Wolf Ralph J. , Hase William L.
Published: February 2003

Classical trajectories are used to investigate the internal dynamics of the model molecule H–C–C. Two different anharmonic potential energy surfaces which exhibit intrinsic non-RRKM lifetimes are found to contain a large number of quasiperiodic trajectories at energies in excess of H–C–C→H+C=C dissociation threshold. Rotational excitation of H–C–C does not destroy the quasiperiodic motion. The quasiperiodic trajectories are displayed by projections into different coordinate planes and by surfaces of section. The relationship of these results to unimolecular rate theory is briefly discussed.
Vibrational energy distribution of the CH3 radical photodissociated from CH3I
Authors: Shapiro M. , Bersohn R.
Published: February 2003

The n→σ* transition in CH3I at 257 nm produces mainly excited I(2P1/2) atoms and the available energy is mainly released as translation. A recent experiment by Y. Lee and co-workers measured the distribution of the methyl radicals over the bending umbrella vibration, the only mode excited. This dissociation process is attractively simple because it can be expressed in terms of two linear coordinates rC–I, the distance between the carbon and iodine nuclei and rC–H3, the distance between the carbon nucleus and the plane of the three protons. The potential surface chosen for the excited surface dissociating to I(2P1/2) atoms was (atomic units) An essentially exact close coupling calculation using this potential energy together with an appropriate ground state potential fitted the absorption curve very well and the vibrational distribution approximately. Both the experimental and theoretical distributions for λ=266 nm peak at v=2. Calculations for other wavelengths show that the umbrella mode distribution will peak at higher (lower) v when a more (less) energetic photon is absorbed.
Ion fragmentation from noninterconverting electronic states
Authors: Meisels G. G. , Hsieh Tacheng , Gilman Jerome P.
Published: February 2003

The photoionization and fragmentation of CH3ONO+ were studied using threshold photoelectron-coincident photoion mass spectrometry, photoelectron spectroscopy, and triple sector mass spectrometry.(AIP)