bluuebunny/crossref_metadata_2025 huggingface dataset:
The ground state inversion potential method: Application to the calculation of photoionization cross sections
Authors: Hilton P. R. , Nordholm S. , Hush N. S.
Published: February 2003

We present a simple one-electron central potential method which allows atomic photoionization cross sections to be calculated to an accuracy comparable to that of the best Hartree–Fock calculations available. The method relies upon the frozen core assumption and the fact that a good approximation to the Hartree–Fock potential seen by the photoelectron can be obtained by inversion of the corresponding ground state orbital as tabulated in the literature. The potential is assumed to be local and independent of final state energy in distinction to the true Hartree–Fock potential which is generally nonlocal due to exchange. Subshell photoionization cross sections for photon energies between 0 and 1500 eV have been calculated for helium, neon, argon, and krypton and compared with the Hartree–Fock results of Kennedy and Manson and available experimental values. The agreement is good except for near-threshold cross sections for deep lying orbitals. The method is compared with the Herman–Skillman approximation. The simplicity and computational efficiency of the present method suggest that it will be of use in practical calculations, particularly for heavy atoms. It offers a promising alternative to the widely used Hartree–Slater and Herman–Skillman calculations.
Magnetic interactions in halobis(morpholyldithiocarbamato) iron(III). I. Solvation effects in iodo and bromo homologues
Authors: Grow J. M. , Hopkins T. E. , Robbins G. L. , Silverthorn W. E. , Wickman H. H.
Published: February 2003

Mössbauer hyperfine structure and related magnetic properties of solvent adducts or allotropes of Fe(S2CNC4H80)2X, with X=Br− and I− are reported. Cooperative magnetic transitions in the helium temperature range were observed in most of these complexes. Unit cell data for several adducts and the crystal structure of a dichloromethane adduct of the iodo homologue are reported. Mössbauer spectral features near magnetic critical points were found to depend on crystallization techniques and materials properties. A summary is given of materials in which cooperative transitions have been observed in this and related studies. Systematics emerging from the available data on intermolecular magnetic superexchange in iron bis(dithiocarbamate) complexes are noted.
Polarization effects in the measurement of luminescence yields
Authors: Poole J. A. , Findeisen A.
Published: February 2003

It is shown how the formula ordinarily used to compute the quantum yield of luminescence relative to that of a ’’standard’’ substance should be changed in order to take account of polarization effects in the excitation monochrometer and emission spectrometer, as well as nonzero emission anistropy of the emitting systems. A simplification results in a ’’correction factor,’’ and the magnitude of this factor, calculated for systems with varying degrees of emission anistropy, is seen to be outside of experimental error for highly viscous systems.
The molecular motion in the solid phases of t-butyl chloride by cold neutron scattering
Authors: Månsson T. , Larsson K. E.
Published: February 2003

Cold neutron scattering experiments have been performed on tertiary butyl chloride at the temperature 173 °K in phase III, 205 °K in phase II, and 235 °K in phase I. The crystal is plastic in phases I and II and ordinary crystalline in phase III. The experimental data are interpreted by fitting of computer calculated cross sections, based on various physical models. The main aim was to study the rotational motion. By use of the data from the phase III a reasonalbe Debye form for the translational spectrum is obtained and used also in phases II and I. Similarly the rather well separated torsional spectrum of the methyl groups is simply approximated. The rotational spectra dominate the elastic and near elastic scattering. Models ranging from free rotation to undamped libration, including isotropic Langevin rotational diffusion as an intermediate case when the rotational relaxation times are short, are tested. It is found, that no extreme model corresponding either to almost free rotation, or to almost undamped libration, or to uniaxial rotational diffusion can describe the data. On the other hand, either isotropic Langevin rotational diffusion or a stochastic two-step rotational diffusion model involving consecutive free rotations and librations fit the data well. It is found that the time a molecule needs for moving from one potential well to another is about 30% of the time it stays in a well. It is also found that both times are short, of order 0.2<τ<2.10−12 sec. It is therefore concluded that the Frenkel model type is not applicable to the rotational motion of t-butyl chloride in plastic phases I and II.
Collision induced dissociation of OH− by inert gas atoms
Authors: Herbst E. , Mulholland K. A. , Champion R.L. , Doverspike L.D.
Published: February 2003

The dissociation of the negative ion OH− by collisions with inert gas atoms has been studied with an ion beam, gas target scattering apparatus at relative collision energies under 100 eV. The only observed fragment ion, O− is preferentially scattered at angles significantly different from 0° and velocities smaller than that of the primary OH− beam. The detailed dynamics of the fragmentation process are discussed in terms of a simple two-step model and also analyzed by a one-surface classical trajectory study.
On the existence of a nonmonotonic nuclear density profile at the jellium–vacuum interface
Authors: Allen J. W. , Rice Stuart A.
Published: February 2003

By means of a generalization of the method of Lang and Kohn, in which we allow the positive jellium to relax, we show that in the transition region between jellium and vacuum self-consistent electron and positive charge distributions can, for large rs, have a nonmonotonic variation. For values of rs less than 4.3 we find monotonic charge distributions, while for values of rs greater than 4.3 we find nonmonotonic distributions. This trend is in accord with the expected decrease in screening with increasing rs of the electron-positive fluid interaction. The widths of the positive charge distributions exhibit a nonmonotone variation with rs and are quite small, lying between one and two atomic diameters in thickness. This width compares well with that case of a dielectric liquid at its triple point where, taking argon as an example, the width is of the order of two atomic diameters. The monotone distributions, however, deviate markedly from the hyperbolic tangentlike form characteristic of liquid argon. Although the jellium model is strictly valid only at the absolute zero of temperature, we suggest the possiblity that for some liquid metals thermal motion might not be so large as to wipe out the structure in the positive ion distibution at the surface. We also emphasize the importance of a stable bulk phase to surface calculations.
Initial slips and transient effects in relaxation phenomena
Authors: Sanctuary B. C. , Snider R. F.
Published: February 2003

For a set of observables, it is shown how the solution of a master equation can approximate the solution of the generalized master quation at long times, but with an aproximate initial slip. The slip phenomena is discussed theoretically and then made more concrete by the exact and approximate solution of a simple but realistic system, namely the initial time dependence for the relaxation of a nuclear spin.
Potential-energy calculations for molecular reorientations between unequal wells in solid t
r
a
n
s,t
r
a
n
s-muconodinitrile
Authors: Polak Micha
Published: February 2003

The possible occurrence of large-amplitude molecular reorientations in solid trans,trans-muconodinitrile (TMD), C6H4N2, is investigated by calculating the dependence of the van der Waals potential energy on molecular orientation in the crystal. The ’’6-exp’’ potential function is used with atom–atom interaction parameters from different sources. In spite of some diversity among the various resultant potential-energy profiles, due mainly to uncertainties in the parameters involving nitrogen, there are some important features common to all the profiles: (1) relatively low energy barriers for hindered rotation of the TMD molecule around its long axis; (2) existence of two subminima associated with the highly anistropic motion at molecular orientations of 120°–130° and 210°–230° from the low-energy equilibrium minimum. The magnitudes of the energy barriers involved indicate that molecular reorientations can occur at different rates between the unequal potential wells in TMD. The calculations show that at room temperature not more than half a percent of misoriented TMD molecules occupy the metastable states. These predicted dilute orientational defects, which did not affect the x-ray diffraction analysis made earlier, were detected and studied by NMR relaxation measurements which we reported in a recent paper.
Collisional energy dependence of Penning ionization and dissociative fluorescence of CS2 by metastable argon
Authors: Foreman P. B. , Parr T. P. , Martin R. M.
Published: February 2003

Using the crossed molecular beam method with time-of-flight velocity selection the collisional energy dependence was determined for total Penning ionization of CS2 by Ar(3P2,0) over the energy range 0.015–1.72 eV, and for dissociative fluorescence producing CS(A1Π) over the energy range 0.026−0.428 eV. The cross sections for the two competing processes were found to have nearly the same functional velocity dependence. Both functions decreased as v−n up to 0.08 eV, with n values of 1.02 for ionization and 0.88 for formation of CS(A1Π). Above 0.08 eV the velocity dependence becomes more gradual with n=0.60 for both processes up to 0.40 eV, and at higher collision energies the ionization velocity dependence again becomes steeper, corresponding to a larger n value than at low energies. The similarity of the cross section energy dependence functions indicates that the major dependence at low energy is for formation of a common ArCS2* precursor from which either autoionization or predissociation to form CS(A1Π) occurs, and the low energy n values suggest the presence of a chemical well in the Ar*–CS2 potential. Initial experiments using an electronically pulsed metastable argon beam gave cross section energy dependence functions which were dependent on the argon source pressure and temperature, and these effects were attributed to Ar*–Ar electronic energy exchange collisions within the time-of-flight beam. This was confirmed by changing to mechanical pulsing, which removed the dc Ar beam and eliminated the source pressure and temperature effects.
The origin of optical dephasing times and line shape functions for electronic transitions to localized and delocalized states in solids
Authors: Harris C. B.
Published: February 2003

A theory for optical dephasing times and the homogeneous component of the absorption line shape function to localized states and to the k=o levels of Frenkel exciton states is presented. The theory is based upon exchange between two or more optical transitions which are separated by a small frequency difference. Exchange averaging of the optical transition is brought about via phonon–exciton scattering which couples the optical transitions. This process causes both a temperature dependent line broadening and a temperature dependent frequency shift in the optical spectrum when quadratic phonon–exciton coupling terms are present. The optical dephasing time in such cases result from energy differences of the phonon branch in the ground and excited states and consequently it need not necessarily reflect the complete loss of coherence of the exciton wavepacket at k=o. A simple criterion is derived that allows one to determine whether the phonon–exciton interaction causes an optical transition line shape function to reflect random frequency modulation by the exchange of low frequency phonon or vibrational quanta. The theory allows the phonon–exciton scattering times for individual modes to be determined from an analysis of the frequency shift and line broadening relationships in favorable cases. This information coupled with knowledge of the exciton dispersion allows one to semiquantitatively determine the effect individual phonon and vibrational modes have on the exciton coherence time at a particular temperature.
Variation of electronic polarizabilities and sizes of ions in crystals under hydrostatic pressure
Authors: Shanker Jai , Agrawal S. C. , Sharma O. P.
Published: February 2003

A model, based on the energy level analysis performed by Ruffa and empirical relation between polarizability and ionic radius, has been developed to study the variation of electronic polarizabilities and sizes of ions in crystals under hydrostatic pressure. The model has been applied to investigate the photoelastic behavior of alkali halides and MgO. The strain derivatives of electronic polarizabilities and dielectric constants thus evaluated present a good agreement with experimental data and other theoretical investigations.
Nonlinear pressure dependence of fluorescence quenching, intermolecular intersystem crossing
Authors: Naaman R. , Yakhot V. , Fischer G.
Published: February 2003

A more general theory of collision induced intersystem crossing is described which contains the treatment of Thayer and Yardley as a special case. The fluorescence quenching depicts a strong nonlinear pressure dependence. It is shown that intermolecular intersystem crossing (intermolecular energy transfer) can provide a large and significant contribution to the overall rate of S–T crossing and can provide an explanation of the large self-quenching. A measure of success is achieved when the theory is applied to the pressure dependence of the fluorescence quenching of quinoxaline and benzophenone. The discrepancy between the experimental and theoretical results is attributed to the neglect of the ground state continuum. A similar problem is encountered in the interpretation of the fluorescence decay of intermediate case molecules when a restricted two state model is considered.
Time and temperature dependence of aftereffects in [57Co(phen)3] (ClO4)2⋅2H2O from time-differential Mössbauer emission spectroscopy
Authors: Grimm R. , Gütlich P. , Kankeleit E. , Link R.
Published: February 2003

Using the time-differential Mössbauer coincidence technique we have measured emission spectra of [57Co(phen)3] (ClO4)2⋅2H2O (phen=1, 10-phenanthroline) in the temperature range 10–80 K versus K4[Fe(CN)6]⋅3H2O as absorber. The lifetimes of the unstable 57Fe(II) high-spin states (5T2), which are formed as a consequence of the EC process of 57Co in the source matrix, have been derived. Including the results from earlier time-integral Mössbauer emission studies on the same system we find lifetimes ranging from 23 ns at 223 K to 393 ns at 10 K. The temperature dependence of the lifetime τ (5T2) is discussed assuming two possible decay mechanisms: (a) the tunneling of an electron to recombine with a ligand radical formed by autoradiolysis, and (b) the radiationless transition of an electronically excited spin state.
Study of molecular dynamics in a homologous liquid crystal series by nuclear spin thermodynamics
Authors: Thompson R. T. , Kydon D. W. , Pintar M. M.
Published: February 2003

The homologous series of benzoic acid liquid crystals has been studied in the nematic phase by proton spin lattice relaxation at high fields and in the rotating frame. A very slow cooperative mode has been found to contribute to the relaxation of the dipolar energy and the Zeeman energy in the rotating frame at fields H1≲2 G.
Electronic and rotational energy relaxation in molecular helium: Observation of reactive collisions by isotope-selective laser excitation
Authors: Delpech J.-F. , Gauthier J.-C. , Devos F.
Published: February 2003

We have shown that reactive collisions play a substantial role in the rotational energy relaxation of He2(e3Πg) molecules: about 16% of the total energy transfer from a given rotational level occurs through the exchange of helium nuclei. These observations may be relevant to problems of astrophysical interest, as well as to proposed schemes of laser isotope separation in more complex molecules where similar processes may lead to fast excitation exchange with atoms or molecules of undesirable isotopes. Radiative and collisional relaxation rates for the He2(e3Πg) molecule are derived from the analysis of a large set of experimental data; rotational redistribution is well described by the simple Polanyi–Woodall exponential gap law. Some new results concerning the dynamics of the d3 Σ+u, f 3Σ+u, and f 3Πu electronic levels are also reported.
Macromolecular self-diffusion and momentum autocorrelation functions in dilute solutions
Authors: Jacobs Glenn , Harris S.
Published: February 2003

We use an improved version of the solution to the one particle generalized Smoluchowski equation found earlier to calculate the self-diffusion coefficient D and the long time behavior of the momentum autocorrelation function φ (t) for a dilute system of uncharged, spherical macromolecules in solution. For D we find the infinite dilution result which is consistent with other theories based on Oseen’s model which we also use; our calculation does eliminate some other assumptions which are used in previous calculations. The asymptotic decay of φ (t) is found to be algebraic, as t−5/2; the −5/2 exponent is probably due to the use of Oseen’s model, and for a higher order hydrodynamic theory which gives a nonvanishing o (c) contribution to D we expect that φ (t) will decay as t−3/2.
Differential scattering of spin ± (1/2) polarized neutrons by chiral systems. II. Condon oscillator model
Authors: Cox J. Neal , Richardson Frederick S.
Published: February 2003

The general theory of polarized neutron scattering by chiral (nonmagnetic) systems (see Ref. 1) is applied to a specific model system. The model scattering system is taken to be a ’’spinless’’ nucleus vibrating in an anisotropic, anharmonic potential of the general form V=kxx2+kyy2+kzz2+Axy z. This potential is dissymmetric and the vibrational motions of the scattering nucleus are chiral. Double differential cross sections for the scattering of spin +(1/2) polarized neutrons versus spin −(1/2) polarized neutrons are calculated and compared under various conditions of neutron energy and (nuclear) oscillator parameters.
Statistics of one-dimensional cluster motion
Authors: Wrigley John D. , Reed David A. , Ehrlich Gert
Published: February 2003

The statistics of clusters, made up of metal atoms in adjacent one-dimensional diffusion channels, are developed quantitatively. Kolmogorov’s equation is used to find the mean square displacement for clusters capable of existing in energetically different configurations at the same position of the center of mass; this is done under steady state conditions, for which the probability of finding a specified configuration does not vary in time. Two systems are examined: (1) dimers capable of existing in an infinite number of states, a situation realized if dissociation is allowed, and (2) trimers diffusing on planes, such as W(211), on which nine distinct jump processes may contribute to the cluster motion. In dimer diffusion it is demonstrated that dissociation may be important even if the fraction dissociated is minor. For trimers, previous attempts to approximate the motion through the use of average transition rates are compared with the exact solutions and found wanting. Important statistical quantities beyond the mean square displacement are presented for simple dimers capable of existing in only two states. The generating function is derived, together with the higher moments of the displacement. Probability density functions for the number of jumps in an interval t, for the waiting time up to a specified jump, as well as for the displacements are all presented. These differ significantly from the density functions for an ordinary random walk. However, an averaging technique allows simple approximations for the behavior of dimers.
Single spin-configuration singlet states
Authors: Jaffé H. H. , Kondo N. , Singerman J. , Bouscasse L.
Published: February 2003

It is shown that no single spin-configuration singly excited singlet state can exist which is at the same time self-consistent and orthogonal to the closed shell ground state, unless the orthogonality is assured by differing symmetry of the two states.
The vibrational energy levels of the A 1Σ+u state of Li2
Authors: Konowalow Daniel D. , Olson Mark L.
Published: February 2003

The Kusch–Hessel assignment of Dunham constants Yi,O for the A 1Σ+u state of Li2 are corroborated by the theoretical vibrational energy eigenvalues we base on our accurate multiconfiguration self-consistent field (MCSCF) calculations. The MCSCF potential curve suggests that dissociation occurs at v′=118 at an energy De=9299 cm−1; a semiempirical scaling of this result gives our best estimate of D′e=9400 cm−1.
Rate constants for the reaction of OH radicals with dimethyl ether and vinyl methyl ether over the temperature range 299–427 °K
Authors: Perry R. A. , Atkinson R. , Pitts J. N.
Published: February 2003

Absolute rate constants for the reaction of OH radicals with dimethyl ether and vinyl methyl ether have been determined over the temperature range 299–427 °K using a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained were k (CH3OCH3) =1.29×10−11 e−(770±300)/RT cm3 molecule−1⋅sec−1, and k (CH2=CHOCH3) =6.10 ×10−12 e(1015±300)/RT cm3 molecule−1⋅sec−1, with rate constants at room temperature of k (CH3OCH3) = (3.50±0.35) ×10−12 cm3 molecule−1⋅sec−1 and k (CH2=CHOCH3) = (3.35±0.34) ×10−11 cm3 molecule−1⋅sec−1.
The vibrational energy levels and dissociation energy of O+2 (X 2Πg)
Authors: Samson James A. R. , Gardner J. L.
Published: February 2003

The absolute vibrational energy levels of the X 2Πg state of O+2 have been determined by the method of photoelectron spectroscopy in the range v=0 to 25. The precision of the method is comparable to that of conventional high resolution optical spectroscopy. The dissociation energy obtained by a linear extrapolation of the vibrational separations is found to be D0=6.67±0.18 eV.
Study of Ni(II) coordination in aqueous solution by EXAFS analysis
Authors: Sandstrom D. R. , Dodgen H. W. , Lytle F. W.
Published: February 2003

Extended x-ray absorption fine structure (EXAFS) spectroscopy has been applied to the study of water coordination of the Ni(II) ion in aqueous solution. High resolution x-ray spectra were measured for Ni(NO3) ⋅6H2O salt and its 0.1M aqueous solution using a synchrotron radiation source. Analysis of the spectra by Fourier transform reveals first sphere coordination in agreement with x-ray diffraction results. In addition, a second coordination sphere is detected and a tentative distance from the Ni(II) ion to the second sphere water molecules of 3.2±0.1 Å is measured.
Optical potential for molecular rearrangement collisions: Computational study of collinear CH3I+K→KI+CH3
Authors: Stone James , Kouri Donald J. , Lin Jeong-long
Published: February 2003

The imaginary part of the optical potential Uα for molecular collisions involving rearrangements developed by Lin and Kouri is calculated for a collinear reaction K+ICH3→KI+CH3. The potential surface employed is that of Blais and Bunker as modified by Raff and Karplus. Calculations have been made for several total energies ranging from 0.1 up to 0.3 eV. The total absorption is related to (φα+[Uα−Uα]φα+), where φα+ satisfies a Lippmann–Schwinger equation in which Uα acts as a potential. The present calculations approximate (Uα−Uα) φα+ by a Born approximation so that we compute ImUαχ, with χ representing the initial unperturbed state in channel α. Then ImUαχ is a local quantity and we examine its spatial variation to gain a picture of the region where chemical reaction occurs. The result is a proposed mechanism for the reaction of K+ICH3 on the particular model surface used. The resulting picture is that (1) vibrationally excited ICH3 reacts more readily, (2) the reaction occurs most readily when the K–I distance is highly compressed and concommitantly energy disposal in the reaction favors vibrationally excited KI product molecules, and (3) the reaction cross section is an increasing function of the total energy of reactants over the energy range studied. An alternative method for analyzing the reaction is to examine complete (Born) matrix elements of ImUα. It can be shown that this is a negative semidefinite matrix and we compute the eigenvalues and eigenvectors of this matrix at several total energies. We arrange the eigenvalues in increasing order and define them to be the ’’reactivity’’ of the corresponding eigenstates. Analysis of the associated eigenvectors shows that each is dominated by a single ICH3 vibrational state so that we gain information on the role of vibrational excitation of the reactants in the collision. It is found again that the v=0 vibrator state has the lowest reactivity, v=1 is next, and v=2 is highest. These conclusions are consistent with the results of earlier studies on this K+ICH3 surface using, e.g., classical trajectories. Because of the computational simplicity of the present approach, it is felt that the calculation of such optical potentials and their associated eigenvalues (reactivities) and eigenvectors can provide significant insight into reactive scattering relatively inexpensively. The present approach to reactive scattering also leads in a natural way to Franck–Condon type factors which govern the probability of reaction. These enter because the matrix elements of ImUα are of the form 〈χ‖Vβδ (E-Kβ) Vα‖χ〉 and these can be expressed in terms of Franck–Condon factors.
Excess electrons in condensed media. A model for migration in dilute molecular solutions
Authors: McHale Jeanne , Simons Jack
Published: February 2003

A theoretical framework is developed for analyzing the migration of excess electrons in one-component molecular solids and liquids. The mean-square distance traveled by an electron as a function of time 〈r2〉t is shown to depend both upon the diffusion of the solvent molecules and the rate of hopping and tunneling of the electron from one molecule to another. By introducing several physically motivated approximations, the quantities appearing in the final expression for 〈r2〉t are expressed in terms of solvent structure information (distribution function) and the electron–solvent interaction potential. Planned extensions of this initial work are also discussed.
The scattering of He atoms from a hard corrugated surface model using the GR method. I
Authors: García N.
Published: February 2003

Extensive computations on the scattering of atoms from a hard corrugated surface model (HCS) under the Rayleigh hypothesis by using the GR numerical method are presented. The computational applicability, the limits of convergence, as well as the sensitivity of the method are studied as a function of the scattering and corrugation parameters. We also calculate a set of curves, called topographic curves, of constant diffraction probabilities versus the incident angle and the corrugation parameters for a square unit cell. The effect of the strength and number of the Fourier coefficients of the corrugation function are analyzed obtaining rainbow disappearance when these coefficients increase in strength and number. A tentative analysis of the crystallography of the LiF(001) surface by using monoenergetic thermal helium beams is given in terms of the hard sphere and ’’contact’’ hard sphere model proposed here. The agreement with the experimental data on He/LiF(001) is excellent. We also discuss an empirical method for determining Debye–Waller factors by fitting the calculated peaks to the observed ones. We calculate the mean square displacement of the atoms vibrating at the surface for different directions on the He/LiF (001) system. These displacements and consequently the Debye–Waller factors are different than if the incident particles were electrons. Finally, several proposals for future experimental work are made.
High resolution 2H NMR study of single crystals of ammonium hydrogen oxalate hemihydrate
Authors: van Willigen H. , Haberkorn R. A. , Griffin R. G.
Published: February 2003

Proton decoupled 2H NMR spectra of ammonium hydrogen oxalate hemihydrate (AHOX, 5% deuterated) single crystals at room temperature have been employed to determine the electric field gradient tensors at the NH3D+, –OD (carboxyl), and HDO sites. In addition, the high resolution afforded by the narrow spectral lines (of the order of 102 Hz) and high magnetic field employed (6.8 T) made it possible to determine the chemical shift tensors of the HDO and –OD groups. The spectra show the presence of two inequivalent NH3D+ sites. At room temperature both are reorienting stepwise with rates in excess of 2×105 Hz. The motionally averaged quadrupole coupling constants for the two sites are (e2qQ/h) =5.1 kHz, η=0.53 and (e2qQ/h) =8.0 kHz, η=0.34. The quadrupole coupling and chemical shift tensor elements of the carboxyl deuterons are found to be (e2qQ/h) =129 kHz, η=0.30 and σxx=−18, σyy=−14, and σzz=6 ppm (relative to external D2O), respectively. The orientation of the tensor components can be related to the O–D bond direction, whereas the magnitude of the quadrupole coupling constant gives information on the O–D⋅⋅⋅O hydrogen bond structure. The HDO spectra show the effect of 180° ’’flips’’ of the molecule around the bisector of the bond angle. Values of the quadrupole and chemical shift tensor components are (e2qQ/h) =122 kHz, η=0.91 and −12, −1, and 13 ppm, respectively.
Rydberg states in condensed phases: Evidence for small ’’bubble’’ formation around NO 3sσ (A 2Σ+) in solid rare gases
Authors: Goodman Julie , Brus L. E.
Published: February 2003

The lowest Rydberg state of NO has been observed to fluoresce with unity quantum yield in solid Ar, Kr, and Xe hosts. The purely radiative lifetimes lengthen in the sequence vacuum, Ar, Kr, Xe, thus directly demonstrating an increasing delocalization of the Rydberg wavefunction in media with increasing dielectric coefficient. A simple, semiquantitative theory of the relative stability of bubble and Wannier Rydberg states favors bubble (∼10 Å diameter) formation around low n Rydberg states in the lighter rare gas solids. Asymmetric phonon contours in absorption and fluoresence, as well as the experimental extent of delocalization compared with Wannier model predictions, both support bubble formation around NO 3sσ (A 2Σ). Sequential two photon excitation spectra from 3sσ to higher Rydberg states near 7.5 eV are broad in all three hosts. The Rydberg 3sσ spectra and photophysics are compared with those of the nearly isoenergetic B 2Π and a 4Π states in both 14NO and 15NO. Rydberg–valence perturbations in the solid phase, as well as the difference between vertical and adiabatic solid phase ionization, are also discussed.
The solute orientational order in a nematic binary solution by NMR
Authors: Hillenbrand David F. , Yu Hyuk
Published: February 2003

The orientational order of a solute (monofluorobenzene) dispersed in a nematogenic solvent (p,p′-dihexyloxyazoxybenzene) is examined by NMR as functions of temperature and composition of the binary solution over the nematic phase region. The order is found to be solely characterized by the reduced temperature whereby a correspondence rule between T and mole fraction is established, and the coexistence curve of the nematic and isotropic phases is shown to serve as the minimum equiorder profile.
Temperature dependence of the collisional quenching of electronically excited bismuth atoms: 6p3 (2D3/2) and (2D5/2)
Authors: Trainor Daniel W.
Published: February 2003

Temperature dependent rate constants are reported for the collisional deactivation of the 6p3 (2D03/2) and 6p3 (2D05/2) states of atomic bismuth by Ar, Xe, N2, H2, D2, CO, O2, CO2, and SF6 at 300, 450, and 550 °K. These results are discussed within the context of correlation diagrams constructed using (J,Ω) coupling approximations. Where appropriate, these data are reduced to provide Arrhenius parameters and likely relaxation mechanisms are discussed.
Measurement of the rate constant for NO–O chemiluminescence using a calibrated piston source of light
Authors: Woolsey G. A. , Lee P. H. , Slafer W. D.
Published: February 2003

A calibrated piston source of light, which provides a uniform disc of luminosity, has been used to study NO–O chemiluminesence produced in a cylindrical volume. The source was moved along the axis of the cylinder, with the luminous disc simulating cross-sectional layers of chemiluminescent gas. Direct comparison was thus made between the light collected from the calibrated source and from the volume of chemiluminescent gas, without the need for geometrical analysis of light paths. This comparison allowed the absolute photon output of the chemiluminescence to be measured as a function of wavelength. From these measurements the rate constant for the reaction NO+O→NO2+hν has been determined over the wavelength range between 0.54 and 1.27 μm.
Photoelectron spectroscopy of carbonyls. Biological molecules
Authors: Dougherty D. , McGlynn S. P.
Published: February 2003

Photoelectron spectra of cylosine and guanine have been obtained with HeI excitation. The correlation between electronic structure and biological function of these biologically significant molecules is discussed in detail. (AIP)
Density gradient measurements of Cl2 dissociation in shock waves
Authors: Santoro Robert J. , Diebold Gerald J. , Goldsmith George J.
Published: February 2003

The dissociation of Cl2 in the presence of Ar and He has been studied behind incident shock waves over the temperature range 1700–2800 K. Dissociation rate constants were determined from laser schlieren measurements where a quadrant photodiode difference amplifier system was used to record the density gradient behind the shock front. Mixtures containing 20% Cl2–80% Ar, 10% Cl2–90% Ar, 5% Cl2–95% Ar, and 20% Cl2–80% He were studied to establish the rate constant for each collision partner. The over-all rate constant for each mixture was found to correspond closely to the Arrhenius rate constant expression. The relative efficiencies of Ar, He, and Cl2 as collision partners were found by assuming that the over-all rate constant for the dissociation reaction is given by the linear or additive mixture expression involving the individual collision partner rate constants. A Marquardt’s computer fitting procedure minimizing the percent error for the fit was used to obtain the rate constants. The results for the measured rate constants are given by kArd=2.65×1013 exp(−47 400/RT) cm3 mole−1 sec−1, kCl2d=1.83×1014 exp(−47 400RT) cm3 mole−1 sec−1, kHed?kArd. On a per collision basis relative to Ar, Cl2 was found to be 6.9 times more efficient as a collision partner, while He had a relative efficiency of 0.58.
Thermal accommodation coefficients by high speed vibration of solid samples. IV. Determination of translational and internal energy accommodation coefficients
Authors: Rosenblatt Gerd M. , Lemons Richard S. , Draper Clifton W.
Published: February 2003

A method has been developed to measure both translational and internal energy accommodation in gas–surface collisions from the surface temperature rise which occurs upon ultrasonic oscillation of a solid sample in a rarefied test gas. Thin, flat samples are vibrated normal to their exposed faces at rms velocities of ∼3×104 mm sec−1. About 35% more gas–surface collisions occur when the sample moves into the gas than when it recedes, so that, relative to the surface, incident gas molecules have more translational energy than do surface-equilibrated molecules. Gas–surface energy transfer causes the surface temperature to rise. A steady-state surface temperature is attained due to a balance between the translational energy transferred to the surface by the gas with thermal emission, conduction, and energy transfer involving gas internal modes. Incident gas molecules are cooler internally than the heated surface because gas internal energies are independent of the surface motion. Analysis of the energy balance enables a determination of both translational γt and internal γi energy accommodation coefficients from measured steady-state temperature rises as a function of gas pressure. For the diatomics N2 and O2, γi=γrot. Values of γrot for N2 on Ni, Cu, and Pt are, respectively: 0.09, 0.04, and 0.05. The values for O2 on the same surfaces are: 0.06, 0.05, and 0.08. For the polyatomics—CO2, C2H2, and C6H6−on Cu, the values of γi are 0.45, 0.54, and 0.07. The implications of the results and the experimental method are discussed.
The dielectric constant and related equilibrium properties of molecular fluids: Interaction site cluster theory analysis
Authors: Chandler David
Published: February 2003

The interaction site formalism for molecular fluids is used to develop a microscopic theory for the dielectric constant ε. A rigorous correlation function expression for ε is obtained from an elementary application of linear response theory. The formula is applicable to mixtures and fluids composed of polarizable and deformable molecules. The derivation shows that the correlation functions which determine ε are short ranged so that ε can be expected to be independent of sample shape or any other macroscopic boundary conditions. The theories for ε developed by Nienhuis and Deutch and by Ho/ye and Stell are shown to be special cases of the formalism given herein. A cluster diagramatic analysis is presented to examine the origin and significance of the large distance behavior of the correlations between neutral polar molecules. The diagramatic analysis is also used to show that the large distance form for the potential of mean force between two microscopic ions at infinite dilution in a dielectric liquid solvent is zAzA′/εr where zA and zA′ are the charges of the two ions. This asymptotic form is independent of the short-ranged interactions between the ions and solvent molecules.
Accurate a
b i
n
i
t
i
o potential curve for the ground state of O2
Authors: Guberman Steven L.
Published: February 2003

An ab initio approach is described for obtaining a quantitative description of the shapes of molecular potential curves. Emphasis is placed upon correlating all the configurations that lead to proper molecular dissociation. For the ground state of O2 calculations with several different wavefunctions are used to assess the importance of various configurations. The calculated energies of the first ten vibrational levels above the bottom of the well are within 3 cm−1 of the experimental values, while the first 22 levels are within 16 cm−1 of the experimental values. The calculated dissociation energy is 4.99 eV, which is 95.7% of the experimental value.
Jahn–Teller degeneracies of Thorson and Moffitt
Authors: Judd B. R.
Published: February 2003

In computer studies of the Jahn–Teller system Γ8⊗τ2g, Thorson and Moffitt showed that periodic coalescences of the energy levels occur as the interaction between the electronic part Γ8 and the vibrational mode τ2g is increased. These near degeneracies have been analyzed by drawing a correspondence between the Γ8⊗τ2g system and a displaced one-dimensional oscillator. It is shown that the degeneracies correspond to the zeros of the Bessel functions J1(z). In terms of the interaction parameter D and the oscillator number nτ of Thorson and Moffitt, z is 4D1/2(nτ+3/2)1/2 in the limit of large nτ. Second-order effects have been cast in an analytic form and do not affect the existence of the degeneracies in that limit. Third-order effects have also been studied. The theory is applied to the Jahn–Teller system E⊗ε.
Applications of perturbation theory to atom–surface diffraction
Authors: Wolfe Karen L. , Malik David J. , Weare John H.
Published: February 2003

A model for atom–surface diffraction is treated by two perturbative schemes: the distorted wave Born series and the reaction matrix method. The first order reaction matrix calculation is identical to the approximate close coupling equations developed by CCGM for a potential with no bound states. Higher order perturbation terms are calculated numerically for a purely repulsive interaction potential. We find good agreement using both perturbative methods with Wolken’s exact close coupling calculations. The convergence of the distorted wave Born series is found to be best for small incident mass and low incoming energy. A similar perturbative treatment of the Rayleigh equation is applied to the atom-surface diffraction problem with a hard corrugated surface potential. Results are compared to exact and semiclassical results. Improvements are found in the low energy region where errors are less than 0.1% in fourth order perturbation theory for specular diffraction and less than 1% in third order perturbation theory for first order diffraction events.
A simple model of rigid rod adsorption
Authors: Boehm R. E. , Martire D. E.
Published: February 2003

A simple lattice model is developed to describe the adsorption at a planar surface of rigid rods restricted to three mutually orthogonal orientations—two parallel and one normal to the surface. Surface exclusion effects by the adsorbed rods are automatically included in the present lattice model treatment. General expressions for the surface pressure versus coverage and adsorption isotherms are obtained over the entire range of surface coverage. When nearest neighbor attractive interactions prevail between the rods, phase separation results at temperatures below a critical temperature. The coexisting equilibrium phases are characterized by different surface coverages and order parameters. The critical temperature, coverage, and order parameter depend on and are determined for various choices of the length-to-breadth ratio of the rods and the strength of the surface–rod interaction. A brief comparison is made of the surface pressure isotherms obtained from the present model with those observed for monolayers formed by lyotropic molecules.
Singlet molecular oxygen annihilation luminescence in polymers
Authors: Kenner Rex D. , Khan Ahsan U.
Published: February 2003

In this investigation, we characterize three types of oxygen stimulated emission from organic chromophores embedded in polymer matrices: (i) singlet oxygen annihilation fluorescence from the S1 state of the chromophore; (ii) specific sensitization of excimer fluorescence in the stimulated emission; and (iii) oxciplex emission. The oxciplex emissions are the first reported examples of emissions originating from transitions within an organic molecule–singlet oxygen complex. To summarize the spectral analysis, the major emitting transition in singlet oxygen annihilation luminescence is the most radiative transition of either the organic chromophore or the oxygen–organic chromophore complex, which is of equal or of lower energy than [E02(1Δ)+ET1]. A kinetic analysis shows that the behavior of the annihilation fluorescence resembles triplet–triplet annihilation fluorescence. In this case, however, the dependence of the kinetics on the organic triplet population varies with the ability of singlet oxygen to participate in sequential collisional energy transfer to adjacent ground state oxygen molecules, minimizing the necessity for physical diffusion of singlet oxygen through the matrix. The kinetic behavior of the annihilation fluorescence can be understood in terms of the diffusion of an energy packet, an oxciton, composed of the electronic excitation energy of O2(1Δ) through the polymer matrix, hopping from one oxygen molecule to the next.
On the theory of translational energy distributions of product molecules of molecular beam reactions involving transient complexes. II
Authors: Worry Gary , Marcus R. A.
Published: February 2003

A statistically adiabatic model for chemical reactions involving a tight or loose transition state in the exit channel was used in Part I to obtain an integral equation for the individual reaction probabilities, i.e., for the magnitude of the S matrix elements. In the present paper this integral equation is explicitly solved for the general case of product orbital (l) and rotational (j) angular momenta constrained only by energy and angular momentum conservation. The reaction probabilities are shown to be related to a contour integral of a product of canonical partition functions. The theory includes an effect of the evolution of the bending vibrations of the transition state into free rotations of the product molecules. The distribution of final translational energy for the general (l,j) case is then obtained by averaging the reaction probabilities over various quantum states of the product molecules. The results are compared with the special cases in the literature for which (i) the transition state in the exit channel is loose (’’phase space theory’’), (ii) this case but with l≫j, and (iii) tight transition state theory with l≫j (Part I). The results are also compared with experimental data obtained from the molecular beam reaction F+(CH3)2C=CH2 →F(CH3)2ĊCH2*→CH3+FCH3C=CH2. The data and the theoretical results are now in better agreement. In the treatment described here and in Part I a loose transition state in the entrance channel was assumed. Expressions for the energy distribution are also given for the case when the entrance channel transition state is tight. Finally, a statistically adiabatic S matrix, which is useful for reactions proceeding through long-lived collision complexes having tight transition states, is described, and its possible application to angular distributions and angular momentum polarization experiments is discussed.
Weyl’s theory applied to predissociation by rotation. III. Jeffreys’ approximation
Authors: Hehenberger Michael
Published: February 2003

It is shown that a method due to Jeffreys can be used at the outermost classical turning point of a barrier penetration problem to construct a good approximation to the asymptotically correct ’’Weyl solution.’’ Compared to the previously introduced Weyl–Titchmarsh–Riccati technique, this Weyl–Titchmarsh–Jeffreys method allows a more efficient calculation of the m function, particularly for sharp resonances. Both alternatives are numerically compared to LeRoy and Bernstein’s Airy function boundary condition method for some predissociating states of HgH.
Coulomb holes and correlation coefficients for electronic shells: The Be-like ions
Authors: Banyard K. E. , Mashat M. M.
Published: February 2003

An expression has been obtained for the Coulomb hole associated with any pair of occupied HF spin orbitals for a many-electron system. The required partitioning of the known correlated second-order density matrix was achieved here, up to and including the pair-correlation effects, by using the many-particle theory proposed by Sinanoğlu. Ground-state wavefunctions were then analyzed for the Be-like ions when 3⩽Z⩽8 which, besides validating the partitioning technique, provided insight into the ’’radial’’ and ’’angular’’ components of intrashell correlation effects. The correlation coefficients τϑ, τr and τ1/r, taken from classical statistics, were also determined and their variation was examined with respect to the atomic number Z. These techniques provided a clear-cut means for comparing and contrasting, in a quantitative manner, the role of correlation within individual atomic shells. Moreover, the procedure is applicable to the comparative assessment of different correlated wavefunctions for the same state of a given system.