bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of the reactive sudden and adiabatic BCRLM approximations for rotationally averaged cross sections
Authors: Walker Robert B. , Pollak Eli
Published: July 2002

The large differences between sudden and adiabatic approximate reactive cross sections are removed by rotationally averaging the bending corrected rotating linear model (BCRLM) cross sections and by shifting the sudden cross sections by the zero point bend energy at the transition state. For D + H2(n=0) we find that the BCRLM rotationally averaged rate constants are in excellent agreement with experiment. For D + H2(n=1), the BCRLM rates are a factor of 6–10 smaller than the most recent experimental values.
NO(A 2Σ+–X2Π) chemiluminescence from the reaction C(1D)+NO2 at 300 K
Authors: Sekiya Hiroshi , Tsuji Masaharu , Nishimura Yukio
Published: July 2002

The metastable-state atomic carbon C(1D,1S) as well as the ground-state atomic carbon, C(3P), were produced at 300 K from the He(2 3S)+CO dissociative excitation by using a flowing afterglow apparatus. An intense ultraviolet chemiluminescence corresponding to the NO(A 2∑+–X2Π) transition was observed when NO2 was added, which was attributed to the reaction C(1D)+NO2 → NO(A 2∑+)+CO. The specific production of NO(A 2∑+) from the C(1D) reactants is explained by adiabatic correlation arguments.
A charge flow model for the higher derivatives of the molecular dipole moment. I. The model
Authors: Overend John , Hylden Jeffrey L.
Published: July 2002

A charge flow model has been developed for the study of both mechanical and electrical anharmonicity in infrared intensities and average dipole moments. The approach used assumes that the molecular dipole moment is comprised of a number of point charges located on the atomic nuclei which move with the nuclei during molecular vibrations. The charge is expanded in a Taylor series as a function of the internal coordinates through quadratic terms. The charge flow parameters that appear in the expansion are found by least squares fitting to the observable matrix elements of the dipole moment operator. The application of this model to C2v, D∞h, C∞v, and Cs triatomics will be discussed and the number and type of allowed charge flow parameters will be elucidated. It will be shown that expressing charge flow parameters in terms of internal coordinates allows the prediction of the signs of the higher order matrix elements when the signs of the fundamentals are known and that in some situations the infrared intensities of combination bands can be used to find linear charge flow parameters which are otherwise unknown. However, the main advantage of this model is that the charge flow parameters generated are more chemically intuitive than the parameters of other models. Results obtained using this model are given in the following paper.
Accurate semiclassical determination of rigid asymmetric rotor eigenvalues
Authors: Pajunen Petri
Published: July 2002

Higher-order semiclassical phase integral methods are applied to the determination of rigid asymmetric rotor eigenlevels from the eigenvalues of Lamé’s equation. The method yields accurate eigenvalues for moderate J, J larger than ten, but is especially valuable for very large J because the accuracy of the method improves with increasing J with practically no increase in the computational cost per eigenlevel.
ESR and ENDOR studies of dl-proline⋅HCl single crystals
Authors: Nelson William H. , Carpenter P. Kevin
Published: July 2002

By means of ESR and ENDOR spectroscopy, single crystals of dl-proline⋅HCl, Cl−H2+N(CH2)3CαHCOOH, have been studied after x irradiation at 65 K and warming to 300 K. The ESR has been shown to be the result of two stable end products. One product, Cl−(CH2)3ĊαHCOOH, results from cleavage of the Cα–N bond, a reaction common to all proline-related molecules studied to date. The Cα–N cleavage reaction is analogous to the usual deamination reaction of other amino acids. The second product, not previously observed in proline-related molecules, has been identified as the result of abstraction of the hydrogen bound to Cα. This product, Cl−H2+N(CH2)3ĊαCOOH, is characterized by four  β-proton couplings. Evidently, the second product results in some manner from a primary cationic product of the proline molecule. Notable about the case of dl-proline⋅HCl is that the concentration of the second product is equal to, or greater than, that of the more common product.
Reactive adsorption of H2CO on Ni(110) at 95 K
Authors: Richter Lee J. , Ho W.
Published: July 2002

The reactions of H2CO on a clean Ni(110) surface have been studied by high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). At 95 K, H2CO adsorbs reactively, producing CO(a), CH3O(a), H(a), and small amounts of paraformaldehyde. Upon thermal treatment the CH3O decomposes to CO(a) and hydrogen while the paraformaldehyde reacts to form a surface intermediate which subsequently decomposes to CO2(g) and H2(g) at 350 K. Coadsorbed D2 is not incorporated into the CH3O(a) suggesting the methoxy is formed via a disproportionation reaction. Mixed paraformaldehyde and solid H2CO multilayers are formed at high exposures.
Experimental observation of the spin-orbit splitting in methoxy
Authors: Carrick P. G. , Brossard S. D. , Engelking P. C.
Published: July 2002

We report the direct observation of the spin-orbit splitting n the X̃ 2E state of methoxy radical, CH3O. The doubling of emission features in the Ã 2A1→X̃ 2E system by 61.5±1.0 cm−1 supports the analysis of the laser magnetic resonance data, but differs from theoretical prediction. The high frequency spin-orbit component of the origin band is relocated to 32 306±2 cm−1.
Phase response relationships as an analytical tool in investigating chemical oscillating reactions. I. A crucial test between Explodator and Oregonator-type models describing the Ag+-perturbed Belousov–Zhabotinsky reaction
Authors: Ruoff Peter
Published: July 2002

In this paper we show that the phase response technique is a useful tool for deciding whether certain proposed mechanisms for chemical oscillating systems are useful or not. We use the perturbation of the Belousov–Zhabotinsky (BZ) reaction by silver ions to test the predictions of the Explodator model proposed by Noszticzius, Farkas, and Schelly (NFS) compared with the original Oregonator model developed by Field and Noyes. The predictions from these two models compared with corresponding experiments lead to the conclusion that the Explodator (with the parameters presented by NFS) is unable to describe the essential features of silver ion perturbed BZ systems.
Light scattering theory from dispersions of nonspherical Rayleigh particles
Authors: Franses Elias I. , Caruthers James M. , Yarlagadda Sudhakar
Published: July 2002

We have determined what orientation and size information can be obtained for colloidal dispersions of Rayleigh particles from light scattering measurements. For noninteracting particles and single scattering, a maximum of 15 moments of the orientation/size distribution can be obtained by using a minimum of (i) three distinct orientations of linearly polarized incident light; (ii) three scattering angles θ in one scattering plane; and (iii) three different polarization orientations of the detector. The information obtainable from less complete experiments has also been determined. Specifically, only six combinations of the above fifteen moments can be determined if unpolarized incident light is used. For polydisperse systems with random orientation, two moments of the size distribution can be determined. For polydisperse particles of the same orientation, (i) the orientation and three moments can be determined for axisymmetric particles and (ii) the orientation and six moments can be determined for nonaxisymmetric particles. The results provide guidance for design of instrumentation and experiments.
Direct determination of single-crystal parameters for axial paramagnetic resonance centers in polycrystalline-amorphous substances
Authors: Lee Sook , Brown I. M. , Ames D. P.
Published: July 2002

The paramagnetic resonance powder spectrum of an electron or nuclear spin system involving an isotropic Zeeman and an axially symmetric, first-order fine-structure splitting has been numerically investigated for the purpose of finding a simple, direct, and accurate method for determining its single-crystal parameters in polycrystalline-amorphous samples. It is shown that from the positions and relative intensities of some selected first- and second-derivative peaks of the axial fine-structure powder spectrum, one can directly determine with high accuracy the associated coupling constant, single-crystal line shape, and single-crystal linewidth for the external magnetic field perpendicular as well as parallel to the axially symmetric axis. The results of an experimental application of this method to the case of a randomly oriented zinc chelate biradical system are presented. The new powder spectrum method is applicable to first-order axially symmetric ESR and NMR quadrupole and dipole-dipole pair centers with isotropic g values under rigid state or slow tumbling orientational motion in polycrystalline-amorphous substances.
Rotational energy transfer in HF–Li collisions
Authors: Raghavan K. , Upadhyay S. K. , Sathyamurthy N. , Ramaswamy R.
Published: July 2002

We report state-to-state integral inelastic cross sections for rotational energy transfer in rigid rotor HF–Li collisions, at a relative translational energy of 8.7 kcal mol−1. The results have been analyzed in terms of power gap law, information theoretic synthesis using energy and angular momentum constraints, and energy corrected sudden and energy corrected sudden-power law scaling relations.
Dynamics of collinear asymmetric light atom transfer reactions
Authors: Nakamura Hiroki , Ohsaki Akihiko
Published: July 2002

The Rosen–Zener formula in its sophisticated form is shown to be applicable to asymmetric collinear light atom transfer reactions with near degeneracy of asymptotic vibrational levels. The formula is applied to simple model systems and is demonstrated to work well over a wide range of energy for both systems with and without reaction barrier. It is also discussed that effects of surface topology and masses on reaction dynamics are effectively investigated by hyperspherical coordinates.
Resonant rotational energy transfer in HF
Authors: Vohralik P. F. , Miller R. E.
Published: July 2002

A molecular beam apparatus has been used to study resonant rotational energy transfer between HF molecules in the ground vibrational state. A low temperature beam formed from the supersonic expansion of a gas mixture containing 1% HF in helium is attenuated by HF molecules with a thermal energy distribution. State selection of the molecular beam is accomplished by using an F-center laser in conjunction with a bolometer detector. The attenuation measurements show effects which are attributed to resonant rotational energy transfer. The results have been analyzed using a kinetic model to estimate the relevant cross sections. The first order dipole allowed process HF(J=0)+HF(J=1) →HF(J=1)+HF(J=0) is found to have a cross section of ∼300 Å2. The results also suggest that the second order cross sections, having ΔJ=2, are about 40 Å2.
Rate constants for the reaction of hydroxyl radicals with acetaldehyde from 244–528 K
Authors: Michael J. V. , Keil D. G. , Klemm R. B.
Published: July 2002

The rate constant for the reaction of hydroxyl radical with acetaldehyde OH+CH3CHO → products has been measured from 244–528 K with the discharge flow-resonance fluorescence technique. The temperature dependence, expressed in Arrhenius form, is k1(T)=(5.52±0.80)×10−12 exp(610±103/RT) cm3 molecule−1 s−1, where R is in cal mol−1 and the errors are at the two standard deviation level. This result is compared to earlier flash photolysis-resonance fluorescence work. Mechanistic considerations are additionally discussed, and it is concluded that acetyl radicals are the most probable products of the reaction. Lastly, theoretical calculations are presented which delineate theoretical issues to be addressed in ab initio potential energy calculations.
Fluorescence and absorption in e beam excited neon
Authors: Walter W. , Schaller U. , Langhoff H.
Published: July 2002

The fluorescence of Ne*2 for pressures of up to 8 bar has been investigated. The spectra for the fast decaying Ne*2(1Σ0) and for the slowly decaying Ne*2(3Σ1) differ significantly. Ne*2(3Σ1) as well as the precursors Ne* in the 3P2, 3P1, 3P0, and 1P1 states are observed by their characteristic absorption. The kinetics are described readily by a rate constant model.
The vibrationally averaged, temperature-dependent structure of polyatomic molecules. III. N2O
Authors: Miller Bruce R. , Fink Manfred
Published: July 2002

The thermally averaged molecular structure of gas phase N2O has been measured at temperatures between 293 and 932 K. Excellent agreement with the predictions of force field calculations based on the variational wave function method and the moment method is found. The stretch–bend coupling constant in the force field is of particular importance in describing the temperature dependence of the vibrationally averaged structure parameters.
Infrared-microwave double resonance spectroscopy of the ν3 band of 11BF3 using a tunable diode laser
Authors: Yamamoto Satoshi , Kuchitsu Kozo , Nakanaga Taisuke , Takeo Harutoshi , Matsumura Chi , Takami Michio
Published: July 2002

Pure rotational transitions in the ν3 state (E′) of 11BF3 due to vibrationally induced dipole moment are observed by infrared-microwave double resonance using a tunable diode laser. The induced dipole moment is estimated to be 0.04+0.02−0.01 D from the power broadening of the double resonance signals. The rotational constant C0 is determined experimentally by use of the observed infrared and microwave transitions to be 5161.4(6) MHz.
Monte Carlo random walk study of recombination and desorption of hydrogen on Si(111)
Authors: NoorBatcha I. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

The recombination/desorption of H2 and the desorption of hydrogen atoms from a Si(111) surface have been investigated using Monte Carlo transition-state theory methods with a biased random walk. Rate coefficients, activation energies, preexponential factors, and angular desorption distributions have been computed for both reaction channels. The distribution of polarization angles for the H2 rotational angular momentum vector is also reported. The potential-energy surface is expressed as the sum of a lattice potential, a lattice–adatom interaction term, and an adatom–adatom interaction. Keating’s formulation as given by Weber is used for the lattice potential. A pairwise sum of 60 Morse potentials represents the adatom–lattice term. The adatom–adatom interaction is a Morse function multiplied by a hyperbolic switching function. The potential parameters are adjusted to fit the theoretical data for the Si(111)–H interaction potential and the measured adsorption energy of H2 on Si(111). The surface predicts a barrier of 0.61 eV for H2 adsorption and the existence of an H*2 precursor state in the recombination/desorption process. Thermal desorption of hydrogen atoms is predicted to be too slow to be an observable process. The computed activation energies are in good agreement with the experimental data. The calculated preexponential factor for H2 recombination/desorption is a factor of 103 smaller than the measured results. A detailed treatment of a reaction mechanism involving an H*2 precursor intermediate indicates that this difference is due to differences in the surface coverage present in the calculations and in the experiments. In general, the theoretical methods are shown to be well suited for the study of this type of rare-event process.
Comment on ‘‘Behavior of polymer end-to-end distance in two dimensions at the theta point’’
Authors: Dekeyser R. , Maritan A. , Stella A. L.
Published: July 2002

It is argued that a correct interpretation of the renormalization group results for the two-tolerant trail, as derived by Marqusee and Deutch, leads to the conclusion that this model belongs to the universality class of the self-avoiding walk, contrary to what these authors claim.
Natural population analysis
Authors: Reed Alan E. , Weinstock Robert B. , Weinhold Frank
Published: July 2002

A method of ‘‘natural population analysis’’ has been developed to calculate atomic charges and orbital populations of molecular wave functions in general atomic orbital basis sets. The natural analysis is an alternative to conventional Mulliken population analysis, and seems to exhibit improved numerical stability and to better describe the electron distribution in compounds of high ionic character, such as those containing metal atoms. We calculated ab initio SCF-MO wave functions for compounds of type CH3X and LiX (X=F, OH, NH2, CH3, BH2, BeH, Li, H) in a variety of basis sets to illustrate the generality of the method, and to compare the natural populations with results of Mulliken analysis, density integration, and empirical measures of ionic character. Natural populations are found to give a satisfactory description of these molecules, providing a unified treatment of covalent and extreme ionic limits at modest computational cost.
Absolute measurements of the stretching mode density of states in polycrystalline ice Ih
Authors: Andreani C. , Bosi P. , Sacchetti F. , Loong C. K.
Published: July 2002

An incoherent inelastic neutron scattering experiment on polycrystalline ice Ih at T=20 K is reported. The experiment was performed on the high-resolution chopper spectrometer at the Intense Pulsed Neutron Source (IPNS) of the Argonne National Laboratory. Scattering functions in the angular range from 4° to 20° were converted, after analytical corrections, into density of states of the system in the stretching frequency region. Analysis of these results leads to the following conclusions: (i) the number of modes in the stretching region of ice Ih is 8.1/cell which is that expected in the case of free molecules; (ii) the general shape of the density of states is in qualitative agreement with Raman scattering results, however the high energy side of the neutron data suggests that a non-Raman active band exists at about 3500 cm−1; (iii) the overall bandwidth observed in this experiment is in good agreement with what we estimated by a coherent inelastic neutron scattering study on heavy ice Ih.
Enskog theory and the Kirkwood instability
Authors: Cole R. G. , Keyes T.
Published: July 2002

A new test is proposed for approximate kinetic theories of dense fluids: they must reproduce known results concerning the ‘‘Kirkwood instability’’ (KI). Since analysis of the KI based upon hierarchy truncations has been shown to yield results which depend sensitively upon the choice of truncation, the proposed test is especially appropriate for kinetic theories based upon truncation. Since the KI arises in the theory of freezing, a kinetic theory used for the study of dynamics associated with freezing should also give a good treatment of the KI. It is shown that the Enskog approximation to kinetic theory passes the test. (AIP)
Equilibrium constants for clustering of neutral molecules about gaseous ions
Authors: Banic C. M. , Iribarne J. V.
Published: July 2002

A study of the clustering of neutral molecules to gas-phase ions was made using mass spectrometry. Equilibrium constants and the associated standard Gibbs free energy changes were determined for the attachment of H2O, SO2, and CO2 to a number of positive and negative ions. Mixed clusters were studied, as well as clusters containing only one type of ligand. A total of 49 constants were measured for ligand association and switching reactions; a further 21 new constants could be derived from thermodynamic cycles. Clustering of N2 about hydrated positive and negative ions was observed, but no equilibrium constants were determined. The observed trends in the cluster stabilities are explained by consideration of the properties of the ions and molecules. The ions used were produced by a high pressure source using aqueous solutions. This technique extends considerably the range of ions that can be studied.
Quantum theory of the full pressure dependence of collision induced intersystem crossing
Authors: Harrowell Peter R. , Freed Karl F.
Published: July 2002

The quantum theory of the pressure dependence of collision induced intersystem crossing is developed in the impact limit where collision times are short compared to time scales for molecular decay. A basic model is introduced which contains singlet–triplet intramolecular couplings and collisional processes that correspond in a pure spin basis to dephasing of singlet–triplet and triplet–triplet coherences and to triplet state population decay. Expressions are derived for the ratio of the emission yield at zero pressure divided by that at pressure p in the separate limits of low triplet level density and of short triplet–triplet coherence lifetimes. The first order corrections to the latter case are found to be negligible over a wide range of pressure and molecular parameters. The theory shows that pressure saturation of collision induced intersystem crossing emerges from a competition between dephasing and population decay processes induced by the collider. A strong sensitivity is exhibited to the minimum zeroth order singlet–triplet splitting.
Optical spectra of matrix-isolated palladium atoms
Authors: Schrittenlacher W. , Rotermund H. H. , Kolb D. M.
Published: July 2002

The optical properties of Pd atoms isolated in Ne, Ar, Kr, and Xe have been studied in the photon energy range of 2–10 eV by absorption, emission, and emission-yield spectroscopy. Strong matrix effects on the optical transitions are observed which lead to a breakdown of the selection rules and, hence, make a band assignment by AMCOR (atom to matrix correlation) impossible. The unusually large blue shift of the Pd 4d→5p transitions is rationalized by an interstitial site for the Pd atom in all four noble gas matrices. Such a site assignment is supported by the high mobility of the Pd atoms which becomes apparent in annealing experiments. For Pd in Ne, absorption bands due to 4d→4f transitions were found around 8 eV. Finally, a transient-absorption spectrum from a metastable state has been observed, which is in agreement with previously reported MCD measurements.
Hyperfine coupling constants of the CCN radical in the Ã 2Δ(000) state by microwave–optical double resonance spectroscopy
Authors: Suzuki Tetsuo , Saito Shuji , Hirota Eizi
Published: July 2002

Electric-dipole allowed transitions between hyperfine components and between rotational levels have been observed for CCN in the Ã 2Δ(000) state by a technique of microwave–optical double resonance. The observed transition frequencies have been analyzed to yield 14N hyperfine coupling constants: a=50.22(19), b=−5.9(12), c=7.8(20), and eQq=0.75(54), all in MHz with one standard error in parentheses. An attempt was made to observe Λ-doubling transitions and rotational transitions in the X̂ 2Π(000) state without success.
On the hydrodynamics of a solvent-saturated lipid bilayer. I. Model of repulsion
Authors: Bisch P. M. , Wendel H.
Published: July 2002

In two consecutive papers we study the stability of black lipid films against thermally induced hydrodynamic fluctuations. In the present part we develop a semiphenomenological theory of steric repulsion in these films. The model employs the statistical mechanical formulation of the van der Waals theory of simple liquid–vapor interfaces; it is based on the picture that upon diminishing the film thickness the adjacent interfacial layers start overlapping in the film center raising the degree of orientation of the hydrocarbon segments there. This causes a mutual repulsion of the two film surfaces. The general order parameter profile is found to depend on two parameters which can be determined from film compression measurements.
Optical detection and comparison of rare gas clathrate hydrate interfacial films with those of the simple n-alkane series
Authors: Giuliani J. F.
Published: July 2002

Stable rare gas clathrate hydrate films adsorbed on a glass surface have been successfully produced under saturated air–gas dynamic equilibrium flow conditions at room temperature and atmospheric pressure. With the exception of helium, these structures could be detected by transmittance changes employing the technique of total internal multiple reflection. It is shown experimentally that the measured transmittance variations between these hydrated gas films depend nearly linearly on their molar refractions as contrasted with the previously studied simple n-alkane series, which exhibited a highly nonlinear behavior as a function of molar refraction. The differences in the optical behavior contrasted between these two series of nonpolar gases were compared with their measured bulk solubility properties in water as a function of their size and shape within the clathrate structure.
A comparative study of the ground state internuclear potentials of alkali hydrides and estimation of dissociation energies with the use of the RPC (reduced potential curve) method
Authors: Jenč F. , Brandt B. A.
Published: July 2002

The RKR potential curves for the ground states of alkali hydrides, calculated in recent years by several authors on the basis of new measurements employing modern techniques, have been analyzed with the use of the RPC method. The ground state reduced potential curves of the alkali hydrides are found to coincide to a high degree of approximation. This group of molecules (together with the group of dialkali molecules reported elsewhere) are slightly anomalous compared to the ground states of other diatomic molecules. The RPC method is used to critically evaluate the dissociation energies of the alkali hydrides using the noncrossing rule of the RPC scheme. This evaluation is essentially in accord with other recent values of dissociation energy.
Single crystal vibrational spectroscopy: An alternative procedure for the application of the factor group method in different crystal settings
Authors: Durman Ray , Jayasooriya Upali A. , Kettle Sidney F. A.
Published: July 2002

A procedure is proposed which eliminates the need for correlation tables relating factor group irreducible representations in different settings of a crystal. The procedure is applicable to all space groups and, unlike the method currently employed, provides a unique correlation between the irreducible representations of the factor groups associated with different space groups.
The removal of DF (v=3 and 4) by H and D atoms
Authors: Bott J. F.
Published: July 2002

The deactivation rate coefficients for the removal of DF (v=3) and DF (v=4) by H and D atoms have been measured at 295 K by the technique of laser-induced fluorescence in a discharge flow-tube apparatus. The rate coefficients were found to increase dramatically with vibrational level v. Data obtained for the removal of DF (v=4) in the presence of H atoms have been used to infer a rate coefficient of (4.5±2)×1011 cm3/mol s for the reaction H+DF (v=4)→HD+F. This is in good agreement with a value calculated from the equilibrium constant and a previous measurement of the F+HD rate coefficient obtained by Berry.
The drainage of thin liquid films between solid surfaces
Authors: Chan D. Y. C. , Horn R. G.
Published: July 2002

We present measurements of the thickness as a function of time of liquid films as they are squeezed between molecularly smooth mica surfaces. Three Newtonian, nonpolar liquids have been studied: octamethylcyclotetrasiloxane, n-tetradecane, and n-hexadecane. The film thicknesses are determined with an accuracy of 0.2 nm as they drain from ∼1 μm to a few molecular layers. Results are in excellent agreement with the Reynolds theory of lubrication for film thicknesses above 50 nm. For thinner films the drainage is slower than the theoretical prediction, which can be accounted for by assuming that the liquid within about two molecular layers of each solid surface does not undergo shear. In very thin films the continuum Reynolds theory breaks down, as drainage occurs in a series of abrupt steps whose size matches the thickness of molecular layers in the liquid. The presence of trace amounts of water has a dramatic effect on the drainage of a nonpolar liquid between hydrophilic surfaces, causing film rupture which is not observed in the dry liquids.
The temperature dependence of the 19F resonance in isolated F2C=CFX molecules
Authors: Jameson Cynthia J. , Jameson A. Keith , Oppusunggu D.
Published: July 2002

The 19F nuclear magnetic shielding in a molecule of the type F2C=CFX in the zero-pressure limit is of particular interest since the shielding in three different electronic environments are averaged over the anharmonic motions described by a single force field. The temperature dependence of the nuclear shielding for the three 19F nuclei in the molecules F2C=CFX (X=H, Cl, Br, I) are reported here. There are systematic changes in the temperature coefficient of the nuclear shielding upon X substitution. For these molecules, the values of −(dσ0/dT)300 decrease in the order X=I, Br, Cl, F, H for the F nuclei which are gem and cis to X. On the other hand, for the F nucleus which is trans to X, the values of −(dσ0/dT)300 are very similar for X=I, Br, Cl, F, and all are greater than for X=H. Comparison of the coupling constants obtained in the gas phase with values in various solvents provide support for Barfield and Johnston’s theory of solvent effects on spin–spin coupling.
Thermochemistry of the molecule (ZnI2)2(g) and the vaporization of ZnI2(s)
Authors: Hilpert K. , Bencivenni L. , Saha B.
Published: July 2002

The vaporization of zinc(II) iodide was studied by high-temperature mass spectrometry with a Knudsen cell. The dimer (ZnI2)2(g) was identified for the first time. Enthalpies and entropies of dimerization, equilibrium partial pressures, as well as enthalpies and entropies of sublimation were evaluated for ZnI2(g) and (ZnI2)2(g). A second- and third-law evaluation was carried out using estimated molecular parameters for (ZnI2)2(g). Appearance potentials were determined for the various ions observed upon vaporizing ZnI2(s).
A study of the parallel molecule–surface reaction HCl+surface→H⋅⋅⋅surface+Cl: A comparison between quantum mechanical and quasiclassical trajectory results
Authors: Shima Yaakov , Baer Michael
Published: July 2002

This work is devoted to the study of quantum effects as encountered in reactions of gas–solid–surface interactions. The reaction that was chosen is HCl (vi=0,1,...,4) +surface→H⋅⋅⋅surface+Cl where the HCl molecule was assumed to approach the surface while being parallel to it (i.e., γ=π/2). Results due to quantum mechanical and quasi-classical trajectory treatments were compared and sometimes large differences were encountered indicating the importance of quantum effects. The model assumed for this treatment inherently includes corrugation and part of the study was devoted to the influence of corrugation on the results.
Analysis of electron spin stimulated echo modulation in randomly oriented solids: 15N modulation of bacteriochlorophyll a radical cation
Authors: Tang J. , Lin C. P. , Norris J. R.
Published: July 2002

An improved and more reliable method for the analysis of the electron spin stimulated echo modulation in randomly oriented solids is presented. In this method the uncertainty in the decay function of the spin echo modulation envelope is largely eliminated by dividing modulation traces of various τ in the three pulse spin echo measurements. A nonlinear least squares fit procedure is then used to determine the hyperfine couplings. We have applied this method to determine the isotropic and the anisotropic hyperfine coupling constants of the 15N nuclei in the bacteriochlorophyll a radical cation. In our model the hyperfine tensors of four nitrogen nuclei are assumed to be axially symmetric and parallel to each other. The results are consistent with the reported values obtained by the electron nuclear double resonance technique.
Stark and hyperfine properties of oxygen difluoride
Authors: DeLeon Robert L. , Prichard Diana , Muenter J. S.
Published: July 2002

Radio frequency transitions within six different rotational levels of oxygen difluoride have been observed in the presence of external electric fields. These data were analyzed in terms of the dipole moment, polarizability anisotropies, spin rotation, and spin–spin interactions: μ=0.308 18(3) D, αaa−αbb=1.389(8) Å3,αaa−αcc=1.883(10) Å3, Maa=59.60(7) kHz, Mbb=25.05(11) kHz, Mcc=51.39(8) kHz, and 〈1/R3〉=0.0941(5) Å−3.
Crystal field spectroscopy of the crystalline precipitates in NaCl:Co2+
Authors: de Andrés A. , Calleja J. M. , Pollini I. , Benedek G.
Published: July 2002

The formation of three precipitated phases of Co2+ in NaCl, observed in as grown samples and after quenching from different temperatures, has been put in evidence by means of low temperature optical absorption and Raman spectroscopy. As grown samples show the presence of CoCl2 precipitates; and in quenched samples a tetrahedrally coordinated phase (probably Na2CoCl4) and the octahedrally coordinated Suzuki phase CoNa6⧠Cl8 have been found. Crystal field assignments of the Co2+ ions in the different crystalline phases, together with the B and Dq values, are also given. Finally, a comparative discussion on the different behavior of Co2+ and Ni2+ in their respective precipitates is made.
Multiphoton dissociation of Ga(CH3)3
Authors: Mitchell S. A. , Hackett P. A. , Rayner D. M. , Humphries M. R.
Published: July 2002

UV and visible multiphoton dissociation (MPD) processes of Ga(CH3)3 have been investigated by time-resolved detection of Ga(4 2PJ ) (J=1/2, 3/2) and Ga(5 2S1/2) following pulsed laser photolysis in a low pressure gas cell, and by mass spectrometric detection of ionic photofragments in a molecular beam. The relative abundances of neutral and ionic fragments were monitored as a function of photolysis wavelength and flux. Measurements of the anisotropy of the velocity distribution and the average kinetic energy of nascent Ga(4 2PJ ) are also reported. Under conditions of moderate photolysis flux (≂108 W cm−2), MPD of Ga(CH3)3 in the wavelength range 420–520 nm produced mainly neutral fragments, including Ga(4 2PJ ) and a metastable molecular fragment which is identified as GaCH3. Photolysis at 222 nm produced GaCH3 to the exclusion of Ga(4 2PJ ), or primarily ionic fragments at relatively high flux. At 308 nm, ionic Ga(CH3)+n species were produced in relatively high yield even in the limit of low photolysis flux. The mechanisms of MPD and MPI of Ga(CH3)3 are discussed with reference to simple rate equations models.
The infrared spectra of solid solutions. I. The infrared spectrum of a dilute solution of NO−3 in KBr at 4 K
Authors: Schutte C. J. H.
Published: July 2002

The infrared FT spectra of a dilute solution of NO−3 in KBr were examined between 4 and 300 K. The resulting spectrum at 4 K shows distinct features enabling the site symmetry to be determined as D3. The weak side peaks occurring near the ν3 mode at ∼1385 cm−1 at 4 K are explained as due to the lowering of the ion symmetry by site effects; rotational motions of the ion are ruled out by the observed spectra. In addition, a band which occurs at −107 cm−1 away from the ν3 peak is assigned to a difference mode of ν3 with the main KBr absorption at ∼113 cm−1; the gap modes are also identified.
Electron spin echo envelope modulation (ESEEM) caused by 14N and 2D hyperfine interactions in the triplet state of acridine at room temperature
Authors: Seidel H. , Mehring M. , Stehlik D.
Published: July 2002

Electron spin echo envelope modulation (ESEEM) with two- or three-pulse echoes has been observed in the triplet state of acridine in fluorene host crystals. Fourier transformation of the modulation pattern yield the quadrupole and hyperfine spectra of 14N and 2D nuclei. The hyperfine tensor elements for 14N as well as for m, α, and β deuterons are determined, rendering a refined spin density map of the acridine triplet state.
Morphology of dynamic electron transfer characteristic of chemical reaction dynamics
Authors: Tachibana Akitomo , Asai Yoshihiro , Kohno Masaru , Hori Kenzi , Yamabe Tokio
Published: July 2002

The morphology of the dynamic electron transfer characteristic of chemical reaction dynamics has been studied by introducing the new criterion of an isomorphism of the manifold of electron orbitals at the different points of the reaction coordinate. This criterion is a natural development of the famous Amos–Hall corresponding orbital. Natural orbitals are obtained from the dynamic Fock equation, then it is possible to describe the electronic process of a chemical reaction by the way that the occupation number of an electron orbital changes within the electron orbital manifold which is constructed to have isomorphic equivalency along the reaction coordinate. The electronic process accompanying the chemical reaction CH3+HF → CH4+F has been studied as a demonstration of the present theory. The electron orbital which is transformed by the new criterion is strongly localized, though the transformation operator itself does not contain any explicit localization terms, such as self-energy integrals. The transition of the occupation number inherent to the spatially clamped orbital is clearly demonstrated, and gives a new interpretation of the electronic process of the reaction mechanism. This has also been analyzed in the light of the dynamic electron transfer and dynamic spin transfer characteristic of a chemical reaction.
Influence of substitutional impurities on soliton dynamics in t
r
a
n
s-polyacetylene
Authors: Förner W. , Seel M. , Ladik J.
Published: July 2002

For different chains of trans-polyacetylene with various substituents the equations of motion of the coupled electron–phonon system are integrated within the Su–Schrieffer–Heeger model. NH+, N, and O+ as isoelectronic substitutions for a CH group as well as the effect of an NH and a CO group are investigated. The calculations for the time evolution of an end-generated kink show that neutral solitons can pass a nitrogen atom and an oxygen ion, but not an NH+, NH, or CO group. The negatively charged soliton is not able to pass any of the investigated substitutions. The CO unit, which is of special interest in the light of recent experimental results for acetylene–CO–copolymers with similar properties as trans-polyacetylene, represents a trap for both neutral and negatively charged kinks and a repulsive barrier for a positively charged kink. The limitations of the soliton model are discussed.
Infrared photodissociation of the CH3CN:HI hydrogen bonded 1:1 complex trapped in a nitrogen matrix
Authors: Schriver Louise , Schriver André , Perchard Jean Pierre
Published: July 2002

Infrared spectra are reported of a weakly hydrogen-bonded 1:1 complex of acetonitrile with hydrogen (deuterium) iodide in nitrogen matrices. Two forms of the complex have been found. The hydrogen bonded form (C), characterized by HI stretching frequency at 2142 cm−1, is dissociated by infrared irradiation and also at low temperature (below 16 K) in the dark. The process may be reversed by warming above 16 K. The nonhydrogen bonded form (U) which results from the photodissociation of the hydrogen bonded one has its HI stretching frequency very close to that of monomer HI in N2 at 2237 cm−1. The data are better interpreted as occurring from a reversible conversion C⇄U, in which the stable form C is preferentially photodissociated because of larger absorption coefficients but is thermally regenerated. The activation energy for the U→C conversion is found of the order of 1.5 kJ mol−1. This interpretation however implies the existence of an unusually efficient dark process, which suggests that the C→U photo-conversion occurs in several steps requiring low energy photons and involving correlated reorientation of both HI and N2 molecules.
Curvilinear coordinate formulation for vibration–rotation–large amplitude internal motion interactions. I. The general theory
Authors: Guan Yuhua , Quade C. Richard
Published: July 2002

A theory for vibration–rotation–large amplitude internal motion interactions is developed using curvilinear coordinates for the vibrational degrees of freedom. An essential feature to the theory is our coordination of two transformations developed previously for the separation of vibration from rotation and vibration from the LAM, in zeroth order. Series expansion in the vibrational coordinates is used to obtain the full vibration–rotation–LAM Hamiltonian. A Van Vleck perturbation approach is used to obtain the effective rotation–LAM Hamiltonian, HeffτR, for the molecule in the nth vibrational state. Reduction of the effective Hamiltonian has been made to (1) the zero angular momentum state of the molecule, (2) the zeroth order rotation–LAM Hamiltonian, and (3) the usual vibration–rotation Hamiltonian when the LAM takes on a small amplitude.
Vibrational spectra and phase transitions of crystalline hexachloroethane
Authors: Woost B. , Bougeard D.
Published: July 2002

The vibrational spectra of hexachloroethane (HCE) were investigated between 10 and 1000 cm−1 in the temperature range from 40 to 360 K. The three phases are characterized. The spectra are interpreted with the help of a normal coordinate analysis and a calculation of the crystal dynamics. A mechanism of the phase transition is proposed.
Electron spin echo study of nitroxide solutions adsorbed on silica gels
Authors: Romanelli Maurizio , Martini Giacomo , Kevan Larry
Published: July 2002

The two pulse electron spin echo (ESE) technique was applied to study the behavior and properties of aqueous solutions of two nitroxide radicals, negatively and positively charged, when adsorbed on different pore size silica gels. The study is carried out at two different concentrations, and the experimental patterns are interpreted via computer simulation. From evaluation of the ESE phase memory, time matrix nuclei were found to play a relevant role in determining the relaxation of the electron spin system. Analysis of the nuclear modulation of the electron spin echo revealed contributions both from water and radical protons. Conclusions are drawn about the radical concentration in the silica gel pores. The larger pore silica gels can reach greater radical concentrations. This confirms a result obtained from previous ESR studies at higher temperatures.
Excitation of helium by positron: A distorted wave polarized orbital approach
Authors: Srivastava Rajesh , Kumar Mukesh , Tripathi A. N.
Published: July 2002

The differential and total cross sections for the 1 1S–2 1S excitation of helium by positron in the energy range 22–200 eV are calculated using a distorted wave polarized orbital approach. The results are compared with other available theoretical and experimental results.
Electron energy distribution functions in He–CO vibrationally excited post discharges
Authors: Gorse C. , Paniccia F. , Ricard A. , Capitelli M.
Published: July 2002

Electron energy distribution functions (edf) and related parameters have been calculated in the post-discharge regime of vibrationally excited He–CO mixtures by solving the Boltzmann equation including superelastic vibrational collisions. The calculations point out to the importance of small concentration of CO molecules in affecting both edf ’s and related quantities in the post-discharge reigme. In particular, the mixture closely follows the behavior of pure CO for molar fractions of CO (XCO) approximately equal to 20%. Helium affects edf ’s by increasing elastic losses in the post discharge, for XCO<20%, leading to average electron temperatures, Te, lower than the corresponding vibrational temperatures, ϑ1. The results obtained by using Treanor’s and Boltzmann distributions emphasize the role of excited vibrational levels (v>2) in affecting edf and the average electron energy.
Nonadditivity effects in generalized Langevin dynamics simulation of interacting particles
Authors: Guárdia E. , Giró A. , Padró J. A.
Published: July 2002

An analysis of the generalized Langevin dynamics simulation method is carried out. Simple Lennard-Jones liquid mixtures with identical solute and solvent particles have been considered. The mean force potential and the memory function are obtained from a suitable molecular dynamics simulation at infinite dilution. Radial distribution functions, velocity autocorrelation functions, and self-diffusion coefficients of the solute at different concentrations are calculated. The comparison with the results obtained from molecular dynamics simulations of the full systems allows an analysis of the reliability of generalized Langevin dynamics as a function of the solute concentration.
Thermodynamic investigation of gaseous yttrium germanides and silicides
Authors: Kingcade Joseph E. , Gingerich Karl A.
Published: July 2002

The equilibria involving the gaseous species above the Y–Ge and Y–Si alloy systems have been experimentally investigated by the high temperature mass spectrometer technique. The gaseous molecules YGe, YSi, and YSi2 were identified and their partial pressures were measured as a function of temperature. The experimentally determined reaction enthalpies were combined with appropriate literature data to obtain the following atomization energies, ΔH○a,0 and standard heats of formation, ΔH○f,298 , in kJ mol−1: YGe, 275.4 ± 11.4 and 522.4 ± 11.4; YSi, 253.8 ± 17.3 and 625.3 ± 17.3; YSi2, 800 ± 21 and 528 ± 21.
One possible way to promote the bond-selective chemical reactions by frequency-modulated lasers
Authors: Bo-Min Xie , Jia-Qiang Ding , Xie Wei
Published: July 2002

It is proposed in this paper that we can use frequency-modulated (FM) lasers to realize bond-selective chemical reactions or to raise the efficiency of molecular isotope separation. Examples are given for HF molecule and the C–H bond in some hydrocarbons.
Hylleraas–Pekeris treatment of D-dimensional two-electron atoms
Authors: Loeser J. G. , Herschbach D. R.
Published: July 2002

The algorithm of Pekeris for S states of two-electron atoms is generalized to spaces of arbitrary dimensionality. Numerical calculations are reported for the ground state (1 1S) and first two excited states (2 3S and 2 1S) for a wide range of dimensions, 1<D<∞, and nuclear charge, 1≤Z≤6. The accuracy is typically better than one part in 108. The energy eigenvalues may be continued to arbitrary real values of the parameter δ=1/D. Real atoms, with D=3, connect smoothly with simple, exactly known limits at D→1 and D→∞. Analysis of the data permits several further terms in the 1/D expansion for the ground state energy to be determined, up to order D−12. This indicates that the expansion does not converge for D=3 but terms of third to sixth order do conform approximately to a geometric series form, as previously postulated in order to carry out dimensional interpolation. The excited state data exemplify near continuum motion at D→1 and quasivibrational asymmetric and symmetric stretching modes of electron motion as D→∞.
O2 and C2H4 molecular fluorescence enhanced by aerosol surface
Authors: Manzanares E. R. , Suto Masako , Lee L. C.
Published: July 2002

The fluorescence spectra produced from ArF laser (193 nm) excitation of O2 and C2H4 either in the gas phase or adsorbed on aerosol surfaces were observed. The aerosols were produced from the reaction of O3+C2H4 or O3+C2H4+SO2 in O2 in a high pressure flow tube. The fluorescence spectra of the adsorbed molecules were identified from their gas phase spectra. It was observed that the aerosol surface greatly enhanced the molecular fluorescence cross section.
Mapping a hard wall potential into a soft wall potential for helium diffraction from surfaces
Authors: Aten Ralph M. , Engel Thomas
Published: July 2002

A method is presented to map a corrugated Morse potential, whose well depth and softness parameter are given, into a hard wall potential. This demonstrates that there is a one-to-one correspondence between hard wall and corrugated Morse potentials. The inverse mapping from a best fit hard wall corrugation function to a Morse potential is sufficiently accurate that the best fit Morse potential can be determined in a gradient search with minimal computational effort.