bluuebunny/crossref_metadata_2025 huggingface dataset:
Cross section for single and double ionization of carbon monoxide by electron impact from threshold up to 180 eV
Authors: Hille E. , Märk T. D.
Published: February 2003

Electron impact ionization of carbon monoxide has been studied as a function of electron energy up to 180 eV with a double focusing mass spectrometer Varian MAT CH5. Absolute partial ionization cross section functions have been obtained for the processes CO +e→CO++2e and CO+e→CO+++3e by normalization against the Ar+ cross section with help of the method of molecular effusive flow. In the case of CO++ the results suggest, in accordance with previous authors, that this ion exists in a metastable state with a mean half-life in the range of 6–15 μs. Using nth root extrapolation, the following minimum ionization potentials have been derived from the low energy cross section function: CO+(X 2Σ+) = (14.07±0.05) eV and CO++= (41.8±0.5) eV. All experimental results presented are compared with previous determinations where available.
Determination of potential energy surfaces for Ar–HCl and Kr–HCl from rotational linebroadening data
Authors: Kircz Joost G. , van der Peyl Gerald J. Q. , van der Elsken Jan , Frenkel Daan
Published: February 2003

Using recent rotational linebroadening data at different temperatures of HCl–Ar and HCl–Kr mixtures, an attempt is made to determine anisotropic intermolecular potential surfaces for both systems. The parameters characterizing the anisotropic part of the potential were determined from numerical fitting of computed linebroadening cross sections using the model of Smith, Giraud, and Cooper. The potential for HCl–Ar obtained that way is found to be rather different from potential surfaces that have been put forward in the literature. A comparison between the different potentials is made. In the HCl–Kr case no other ’’experimental’’ potential surfaces are available for comparison.
Classical and semiclassical theory of symmetric exchange collisions: An inversion procedure for low-energy He* (2 1S, 2 3S)+He differential scattering
Authors: Jordan R. M. , Siska P. E.
Published: February 2003

A straightforward derivation based on an intuitive classical optical model description of collisions of the A*+A type leads to a simple semiclassical theory of direct/exchange scattering. The theory is shown to give an accurate description of low-energy He*–He collisions by comparison with exact quantum mechanical calculations, and is used as the basis for an analytical inversion procedure which yields both of the potential curves from a single angular distribution measurement. The inversion method is applied to our crossed beam measurements of He* (2 1S, 2 3S)+He differential scattering at 0.94 kcal/mole nominal collision energy to derive the relevant potential curves at large internuclear distance. The success of the method depends strongly on the accurate inclusion of apparatus resolution effects in the analysis.
Hydrogen bonding in systems containing imidazoline nitroxide radicals
Authors: Gerardi Gary J. , Wagner Burkhard E. , Potenza Joseph A.
Published: February 2003

Hydrogen bonding between imidazoline nitroxide free radicals and hydrogen donors of varying strength [acetone, chloroform, trifluoroacetic acid (TFA)] has been examined using magnetic resonance techniques. Low-field dynamic nuclear polarization and spin–lattice relaxation measurements have been used to determine absolute scalar and dipolar coupling strengths. Dipolar coupling was found to increase in the order acetone <CHCl3<TFA, while only TFA systems showed substantial scalar coupling. For acetone, observed dipolar relaxation rates are consistent with translational modulation and little, if any, complexation. With CHCl3, both translational and rotational modulation are required; the data suggest that CHCl3 forms weak hydrogen bonds with the radical chromophore with little spin density transferred to the bridge proton. In contrast, the protons of TFA show strong scalar coupling with the various radicals which results in positive proton NMR enhancements or strong hyperfine coupling (AH?5 G). The 1H scalar relaxation rates are consistent with strong hydrogen bond formation at the imine site and complex lifetimes of ∼10−9–10−8 sec. The 19F nuclei of TFA show much weaker, radical-independent scalar coupling and little rotational dipolar modulation consistent with, at best, weak, nonstereospecific complex formation.
Photolytic production of S(1S) from OCS by two-quantum and vibrationally dependent mechanisms
Authors: Kligler D. J. , Pummer H. , Bischel W. K. , Rhodes C. K.
Published: February 2003

Studies of photolytic production of S(1S) atoms by two-photon dissociation of OCS at 193 nm are reported. The excited sulfur atoms are detected by observation of the collisionally-induced XeS(2 1Σ+ →1 1Σ+) emission near the S(1S→1D) atomic line at 773 nm. The measured OCS two-quantum absorption cross section is in accord with theoretical estimates based on previous established spectroscopic data. In addition, it is found that excitation of the OCS (ν2) bending vibration by CO2 laser irradiation causes the S(1S) yield to be considerably enhanced. The experimental data obtained indicate the presence of vibrational mode specificity in the mechanism of photolytic enhancement. Vibrationally dependent quenching arising from OCS† +S(1S) collisions is also observed. The mechanisms of vibrational enhancement are discussed in relation to their general applicability for photolytic production of electronically excited species.
Analytic fits to several atom–diatom a
b i
n
i
t
i
o potential hypersurfaces
Authors: Ewing Thomas F. , Detrich John , Conn Robert W.
Published: February 2003

Some recent available ab initio atom–diatom potential hypersurfaces are investigated. It is found that the behavior of these interaction potentials with respect to the atom–molecule distance and molecular bond distance can be rather accurately described by a simple functional form involving two exponential terms. The description appears to apply equally well to SCF or Gordon–Kim electron gas results. Fit parameters at various angles are presented for the systems Ar–HF, He–HF, He–CO, Li+–CO, Li+–N2, and Li+–H2. These potential fits are particularly well adapted to use as input to treatments of vibrationally inelastic collision processes.
Photoabsorption in formaldehyde: Intensities and assignments in the discrete and continuous spectral intervals
Authors: Langhoff P. W. , Orel A. E. , Rescigno T. N. , McKoy B. V.
Published: February 2003

Theoretical investigations of total and partial-channel photoabsorption cross sections in molecular formaldehyde are reported employing the Stieltjes–Tchebycheff (S–T) technique and separated-channel static-exchange (IVO) calculations. Vertical one-electron dipole spectra for the 2b2(n), 1b1(π), 5a1(σ), 1b2, and 4a1 canonical molecular orbitals are obtained using Hartree–Fock frozen-core functions and large basis sets of compact and diffuse normalizable Gaussians to describe the photoexcited and ejected electrons. The calculated discrete excitation spectra provide reliable zeroth-order approximations to both valence and Rydberg transitions, and, in particular, the 2b2(n) →nsa1, npa1, npb2, and nda2 IVO spectra are in excellent accord with recent experimental assignments and available intensity measurements. Convergent (S–T) photoionization cross sections in the static-exchange (IVO) approximation are obtained for the 15 individual partial channels associated with ionization of the five occupied molecular orbitals considered. Resonance features in many of the individual-channel photoionization cross sections are attributed to contributions from valencelike a1σ* (CO), a1σ* (CH), and b2σ* (CH)/πy* (CO) molecular orbitals that appear in the photoionization continua, rather than in the corresponding one-electron discrete spectral intervals. The vertical electronic cross sections for 1A1→1B1, 1B2, and 1A1 excitations are in generally good accord with previously reported CI (S–T) predictions of continuum orbital assignments and intensities, although some discrepancies due to basis-set differences are present in the 1B1 and 1B2 components, and larger discrepancies apparently due to channel coupling are present in the 1A1→1A1 cross section. Partial-channel vertical electronic cross sections for the production of the five lowest parent-ion electronic states are found to be in general agreement with the results of very recent synchrotron-radiation photoelectron branching-ratio measurements in the 20 to 30 eV excitation energy interval. Most important in this connection is the tentative verification of the predicted orderings in intensities of the partial- channel cross sections, providing support for the presence of a strong ka1σ* (CO) resonance in the (5a1−1)2A1 channel. Finally, the total vertical electronic cross sections for absorption and ionization are in general accord with photoabsorption measurements, photoionization–mass–spectrometric studies, and the previously reported CI (S–T) calculations. Although further refined calculations including vibrational degrees of freedom and autoionization line shapes are required for a more precise quantitative comparison between theory and experiment, the present study should provide a reliable zeroth-order account of discrete and continuum electronic dipole excitations in molecular formaldehyde.
Mössbauer, magnetic susceptibility, and EPR studies of intermediate spin iron (III) dithiooxalato halides
Authors: Niarchos D. , Kostikas A. , Simopoulos A. , Coucouvanis D. , Piltingsrud D. , Coffman R. E.
Published: February 2003

Mössbauer, magnetic susceptibility, and EPR measurements have been performed in the series of intermediate spin [Fe(S2C2O2)2X]2−, (X=Cl, Br, I) complex anions. All members of the series are paramagnetic down to 1.4 °K. Mössbauer and EPR results establish the axial character of the electronic spin Hamiltonian for the iodide derivative and increasing rhombicity for Br and Cl. Axial zero field splitting parameters D are positive and near 5 °K. The quadrupole splittings are large in the range 3.4–3.6 mm/sec and typical for the S=3/2 state of Fe(III). Isomer shifts are near 0.30 mm/sec indicating substantial d electron delocalization in the π system of the ligand. The magnetic hyperfine constant decreases from the Cl to the I derivative indicating increasing covalency of the metal–halide bond. Pronounced relaxation effects are present in the Mössbauer spectra varying with the halogen ligand. Spectra in applied fields up to 60 kG have seen fitted with a relaxation theory based on a stochastic model.
Theoretical determination of molecular structure and conformation. I. The role of basis set and correlation effects in calculations on hydrogen peroxide
Authors: Cremer D.
Published: February 2003

Equilibrium structure and barriers to internal rotation of hydrogen peroxide have been accurately determined with the Hartree–Fock method and Rayleigh–Schrödinger perturbation theory using a (9s5p1d/4s1p)  [4s3p1d/2s1p] contracted and (11s6p2d/6s2p) uncontracted basis set. Extensive rescaling of the contracted basis accompanied by complete geometry optimization leads to barrier values of 0.7 (trans) and 8 (cis) kcal/mole. Results obtained with the uncontracted basis indicate an improvement of the barriers to 1.1 and 7.4 kcal/mole comparable to the refined experimental values of Ewig and Harris. Inclusion of correlation does not change the barriers significantly. The latter, however, is necessary to obtain correct equilibrium parameters. The computed bond lengths [R (OO) =1.451 Å, R (OH) =0.967 Å] and angles [α (OOH) =99.3° and ϑ (HOOH) =119.3°] are in good agreement with experiment while near HF values lead to a false structure [R (OO) =1.390 Å, R (OH) =0.943 Å, α (OOH) =102.9°, ϑ (HOOH) =111.2°]. The importance of optimum scaled polarization functions in the perturbation approach is demonstrated.
Theory of Raman scattering by molecules adsorbed on electrode surfaces
Authors: King Frederick W. , Van Duyne Richard P. , Schatz George C.
Published: February 2003

In this work, we provide a simple classical model to explain the enormous intensity enhancement observed for Raman scattering from molecules adsorbed on electrode surfaces. It is proposed that the origin of the intensity enhancement arises from very large changes in the polarizability derivative with respect to a normal coordinate, by virtue of the image field at the admolecule. A qualitative discussion of the role of adsorbed counter ions is presented. We tentatively propose that the dependence of the intensity enhancement on counter ion concentration may be understood in terms of nearest neighbor dipole–dipole stabilization of surface clusters of counter ions with the adsorbate molecule. We also discuss some limitations of the classical model, and propose some further experiments that may lead to clarification of the ideas presented in this work.
Semiclassical approximations for low-energy inelastic scattering
Authors: Cross R. J.
Published: February 2003

Several semiclassical methods are developed to treat low-energy vibrationally and rotationally inelastic scattering. The quantal close coupling (CC) equations are approximated by treating the orbital angular momentum in a classical limit. This removes l from the coupled equations and introduces a dependence in the potential on the classical orbital angle. Unlike the coupled states (CS) approximation, l is not frozen. Next, the rotational motion is treated in the sudden approximation. For vibrationally inelastic scattering, this yields an approximation which involves the solution of N coupled differential equations for N vibrational states. For the pure rotational case, the theory reduces to a sudden approximation superior to the current alternatives. Unlike the traditional approach, it is not a perturbation theory based on an elastic trajectory. Unlike the infinite order sudden approximation (IOS), it converges properly at large impact parameters. Indeed, the IOS is obtained as a special case by freezing the orbital motion. A fast numerical procedure is derived for evaluating the matrices occurring in the sudden approximation.
Calculations of deuterium quadrupole coupling constants employing semiempirical molecular orbital theory
Authors: Barfield Michael , Gottlieb Hans P. W. , Doddrell David M.
Published: February 2003

Deuterium quadrupole coupling constants (DQCC) for deuterium nuclei in a variety of bonding situations have been obtained by means of semiempirical molecular orbital wavefunctions in the INDO approximation. All integrals of the operator entering the electronic contributions have been included with no approximations in their evaluation. Analytical expressions are tabulated for the relevant two-center integrals; three-center integrals were obtained by numerical integration techniques. The factors responsible for the very strong dependence of deuterium QCC on internuclear distance, and the relative constancy of the parameters for a given bonding situation are discussed with regard to the types of terms which enter the integral expressions and the molecular electronic distributions. Some interesting and possibly useful predictions are also included in the tabulated results; the DQCC for the bridging deuterium in diborane is predicted to have a value of −110 kHz, which may be the first negative value for a DQCC; the effects of hydrogen bonding can be significant, the result of lengthening of the D–X bond on hydrogen bond formation.
Resonance Raman spectra of octaethylporphyrinato-Ni(II) and m
e
s
o-deuterated and 15N substituted derivatives. I. Observation and assignments of nonfundamental Raman lines
Authors: Kitagawa T. , Abe M. , Ogoshi H.
Published: February 2003

High resolution resonance Raman spectra of octaethylporphyrinato-Ni(II) [Ni (OEP)], its meso-deuterated and 15N substituted derivatives were measured. With excitation near the O–1 transition, about 90 nonfundamental modes and previously unrecognized fundamental modes were observed for each derivative. The analysis of the nonfundamental Raman lines allowed the determination of all the fundamental frequencies in the A1g, B1g, A2g, and B2g species of D4h symmetry. Experimental classification of depolarized Raman lines to the B1g and B2g species was worked out. Combinations of B1g and B2g modes give rise to anomalously-polarized Raman lines while combinations of B1g modes and of B2g modes give rise to polarized lines. The Raman lines of the B1g species of Ni(OEP) with D4h symmetry are correlated with those in the Ag species of H2(OEP) with D2h symmetry while the B2g species of D4h is correlated with the B1g species of D2h. The experimental frequencies of fundamental modes thus determined satisfied the product rule for the isotopic substitution.
The energy levels of the CH3 asymmetric bending vibrations of toluene
Authors: Kanesaka Isao , Kawai Kiyoyasu
Published: February 2003

The energy levels of the CH3 asymmetric bending vibrations of toluene were calculated by the use of a finite element method for the two dimensional vibrational Schrödinger equation on the basis of a C2v symmetry of the molecule. The totally symmetric CH3 torsion was only considered and some boundary conditions were used for the CH3 asymmetric bending vibrations. The splittings of the energy levels were examined in relation to the interaction potential V2 or V6′ with the CH3 asymmetric bending and the CH3 torsional coordinates. The ir spectrum was observed in the region of those vibrations at room temperature and the Raman experiments at about −140 °C. The observed bands were assigned by comparing them with the calculated levels.
Characterization of the unimolecular behavior of recoil hot reaction products in inert bath gases. Application to c-C4H7T
Authors: Ferro L. J. , Spicer Leonard D.
Published: February 2003

A model for the kinetically controlled, nuclear recoil, chemical activation process is further developed to characterize the generation of excited cyclobutane and its subsequent unimolecular behavior. This approach specifically accounts for the overall effect of mixed bath gases in order to utilize previously reported pressure dependent data for cyclobutane in He, Ne, Xe, N2, and CF4. By incorporating appropriate relative energy transfer efficiencies from the activated molecule to the bath gases, a consistent interpretation for all of the experimental data is obtained. This model also provides information on the primary chemical activation process. The results indicate that ∼46% of the recoiling tritium energy is deposited into internal energy of the excited product cyclobutane-t during the T for H replacement reaction and that the energy distribution of activated molecules is relatively independent of the bath gas present in these mixed bath gas systems.
Radiation damage studies by x-ray photoelectron spectroscopy. II. Electron irradiated Li2CrO4 and Li2WO4
Authors: Sasaki T. , Williams R. S. , Wong J. S. , Shirley D. A.
Published: February 2003

Single crystals of Li2CrO4 and Li2WO4 were irradiated in situ with 0.3–1.6 keV electrons in an UHV x-ray photoelectron spectrometer. Chemical shifts of core-level peaks in XPS spectra indicate formation of Cr(III) on the irradiated Li2CrO4 surface and of W (IV) and W (V) on the irradiated Li2WO4 surface. Accompanying formation of the reduced species, new peaks attributable to photoelectrons from non-bonding orbitals appear at binding energies just below the respective Fermi levels. A lower limit of the G value for Cr(III) formation is estimated to be about 1.3×10−2 from the differential energy loss of the incident electrons. Comparison of this G value with previous data determined by ESR suggests that solid surfaces are far more sensitive to radiation damage than the bulk. The irradiation of Li2WO4 causes a shift in the entire XPS spectrum to lower binding energy, initially ∼2 eV, which decreases over a 20 h period until the original peak positions are recovered. This is interpreted in terms of surface charging caused by electron irradiation.
HCO production, vibrational relaxation, chemical kinetics, and spectroscopy following laser photolysis of formaldehyde
Authors: Reilly J. P. , Clark J. H. , Moore C. Bradley , Pimentel George C.
Published: February 2003

Formaldehyde vapor was photolyzed with a tunable pulsed uv laser. Flash kinetic absorption spectra of the HCO produced were recorded by intracavity dye laser spectroscopy with a time resolution of 1 μs. The energy threshold for radical production was confirmed to be at 86±1 kcal/mole. Photolysis at 294.1 nm produced HCO in its ground vibronic state (∼2/3) and with one quantum of vibrational excitation in either the bending (∼1/3) or CO stretching (10−1–10−2) vibrations. Observation of the CO stretching hot band absorptions allowed that frequency to be determined as 1868.4±1 cm−1. Quantitative, state-resolved measurements of concentration vs time were made in pure H2CO and in mixtures with O2, NO, or Ar. The vibrational relaxation rate for the bending vibration of HCO in collisions with H2CO was (4.3±1) ×10−12 cm3 molecule−1 s−1. Reaction rates for HCO+NO→HNO+CO and HCO+O2→HO2+CO were measured as (1.4±0.2) ×10−11 and (4.0±0.8) ×10−12 cm3 molecule−1 s−1, respectively. Approximate rates were determined for the radical–radical reactions H+HCO→H2+CO and 2HCO→H2CO+CO as 10−9.26±0.3 and 10−10.2±0.5, respectively.
Collisional relaxation of CO2 (X 1Σ+g) (00°1) and CO(X 1Σ+)(v=1) by some perfluoroalkyl bromides and iodides
Authors: Macke Edward T. , Hellfeld Heinrich , McFarlane Ross A. , Davis Christopher C.
Published: February 2003

The rate constants for the collisional relaxation of vibrationally excited CO2 (00°1) and CO (v=1) molecules by some perfluoroalkyl bromide and iodide molecules have been determined by the laser induced fluorescence technique. The measured rates were kCO2–CF3Br=1.47±0.05×105 s−1⋅torr−1, kCO2–C2F5Br=1.67±0.05 s−1⋅torr−1, kCO2–CF3I=2.69±0.08×105 s−1⋅torr−1, kCO2–C2F5I=0.99±0.04×105 s−1⋅torr−1, kCO2–n–C3F7I=1.16±0.07×105 s−1⋅torr−1, and kCO–CF3Br=819±39 s−1⋅torr−1. The relevance of these rates to E→V transfer lasers is discussed. A new and very liable determination of the self-relaxation rate constant of pure CO2 has also been determined: this rate is kCO2–CO2=305±6 s−1⋅torr−1.
Near-resonant electronic energy transfer from NF (a 1Δ) to Bi
Authors: Capelle Gene A. , Sutton David G. , Steinfeld Jeffrey I.
Published: February 2003

The rate constant for the near-resonant electronic energy transfer process NF(a 1Δ)+Bi(4S3/2) → NF(X 3Σ) +Bi(2D3/2)+16 cm−1 has been measured to be 1×10−9 cm3 molecule−1 sec−1. The effective cross section (∼200 Å2) is one of the largest that has been measured for such a process, and cannot be accounted for in terms of a long-range interaction between charge distributions.
Electronic structure of the noble gas dimer ions. I. Potential energy curves and spectroscopic constants
Authors: Michels H. H. , Hobbs R. H. , Wright L. A.
Published: February 2003

A systematic study of the electronic structure and chemical binding in the dimer ion sequence, Ne2+, Ar2+, Kr+2, and Xe2+, has been carried out using density functional methods. For comparison, ab initio configuration-interaction calculations were also performed for the Ar2+ ion. These studies include detailed calculations of the pertinent potential energy curves and an analysis of the calculated spectroscopic properties of the bound states of these ions. A regular progression is found in the spectroscopic properties for the ground A 2Σ+1/2u state which leads to some remarkably simple conclusions concerning the nature of the binding and the size of these dimer ions. For the heavier systems, Kr2+ and Xe+2, spin–orbit coupling becomes important, resulting in a strong mixture of the Λ–S coupled Σ and Π states. This mixing affects the strength of the binding in the ground state. A comparison with other ab initio studies and an analysis of the asymptotic behavior at large internuclear separations is given. These dimer ion species illustrate the classic Hartree–Fock symmetry dilemma arising from improper dissociation character. The nature of this problem for ionized homopolar species is discussed.
Classical trajectory model for electronically nonadiabatic collision phenomena. A classical analog for electronic degrees of freedom
Authors: Miller W. H. , McCurdy C. W.
Published: February 2003

A theoretical model of electronically nonadiabatic collision processes is proposed which describes all degrees of freedom, electronic and heavy particle (translation, rotation, and vibration), by classical mechanics. The advantage of such an approach is that since all degrees of freedom are treated on the same dynamical footing (i.e., via classical mechanics), all dynamical effects are described correctly (even resonance effects in electronic to vibrational/rotational energy transfer). Calculations can be carried out within the framework of the standard quasiclassical trajectory method. The key element in making this approach successful is being able to construct a classical model for the electronic degrees of freedom, and several approaches for doing this are described.
Molecular beam focusing of ICl in rotational states with positive induced electric dipole moments
Authors: Lübbert A. , Rotzoll G. , Günther F.
Published: February 2003

Molecular beams of ICl in selected rotational states with positive effective electric dipole moments have been produced using a new electrostatic alternate gradient focuser (AGF). It has been possible to select the states ‖J,M〉=‖0,0〉, ‖1,±1〉, and ‖2,±2〉. The experimental arrangement consists of a seeded supersonic nozzle beam source for ICl, the AGF selector based on four-wire fields, and a quadrupole mass filter detector with electron bombardement ionizer. The velocity selector, usually used in state selection experiments, could be eliminated because of the narrow velocity distribution (Δv/v?0.05) of the ICl beam. On account of the strong rotational cooling during the nozzle beam expansion, the relative populations of lower J states are increased substantially. An additional advantage of the seeded beam technique is the possibility of changing the velocity of the state-selected molecules by choosing a suitable noble gas mixture. The flux density of the state-selected ‖1,±1〉 beam exceeds that of the direct unfocused beam; its absolute value is estimated to be greater than 3×1012 molecules cm−2 s−1.
Determinations of relaxation rate constants on the 22 GHz rotational transition of H2O by coherent transient spectroscopy
Authors: Kasuga Takashi , Kuze Hiroaki , Shimizu Tadao
Published: February 2003

The relaxation time constants, T2’s of the 616←523 rotational line of H2O, self-broadened by H2O and foreign-gas broadened by He, H2, N2, and CO, are determined by the frequency switched microwave transient method. The relaxation constants of the M=5 and M=4 components in the presence of Stark fields are also examined. A novel method to stabilize the modulated microwave frequency is successfully developed.
SIMS study of the mechanism of cluster formation during ion bombardment of alkali halides
Authors: Honda Fumihiro , Lancaster Gerald M. , Fukuda Yasuo , Rabalais J. Wayne
Published: February 2003

The secondary ion mass spectra (SIMS) of the salts NaX (X=F,Cl,Br,I) and CsX (X=F,I,ClO4) are reported. A UHV spectrometer system designed for SIMS studies is described in detail. Large secondary ion clusters of high mass, e.g., Na14F13+(569 amu) and Cs4I3+(913 amu), whose minimum lifetimes are of the order of tenths of milliseconds, have been detected. The long series of detected cluster ions of the type MnXn−1+ indicates that there is a high degree of stability associated with these clusters. The intensities I of the different series of observed clusters of the type MnXn−m+ (m=−2,−1,0,1,2) vary as a function of cluster mass M according to I=aMb, where b takes values from −1.98 to −9.02 for various series. The kinetic energy distributions of the clusters have a maximum in the region 1–2 eV which, in general, shifts to lower energy and sharpens with increasing cluster size. A clustering mechanism which is qualitatively consistent with all of the experimental observations is proposed. This mechanism involves formation of a bound cluster from a collection of secondary particles moving through the selvedge region of the surface as a result of a single primary ion collision. The formation of a tightly bound cluster after the particles are moving through the selvedge region accounts for the molecular rearrangement observed during sputtering.
Energy and angular differential cross sections for electron transfer in the systems K+Br2 and K+Cl2
Authors: Kimura M. , Lacmann K.
Published: February 2003

Doubly differential cross sections, in energy and angle, were measured in a crossed beam for the electron transfer reaction K+X2→K++X−2 (or X+X−) at relative energies of 4.4 to 29.8 eV for X=Br and 9.6 to 36.6 eV for X=Cl. The translational energy-loss spectra show maxima which we attribute to X2− ions formed in the ground state and an electronically excited state. The excited X−2 ions are scattered through wider angles than the ground state ions. Also, the cross section for excited X−2 production is strongly dependent on beam velocity and is only significant at the lower end of the energy ranges examined. Both effects are interpreted in terms of bond stretching of the halogen molecular ion during the collision.
Vibrational relaxation of NH3(ν2)
Authors: Hovis Floyd E. , Moore C. Bradley
Published: February 2003

The method of laser-excited vibrational fluorescence is used to measure the vibrational relaxation rates of NH3(ν2) in collisions with NH3, He, Ar, N2, and O2. The rates are found to be 3.8×10−11, 2.8×10−13, 1.8×10−13, 3.6×10−13, and 4.2×10−13 cm3 molecule−1 sec−1, respectively. Exciting to different rotational levels gives the same result within 10%.
Mobilities and interaction potentials for Rb+–Ar, Rb+–Kr, and Rb+–Xe
Authors: Gatland I. R. , Lamm D. R. , Thackston M. G. , Pope W. M. , Eisele F. L. , Ellis H. W. , McDaniel E. W.
Published: February 2003

The mobilities of Rb+ ions in Kr and Xe gases have been measured at 300°K in a drift tube mass spectrometer over a wide range of the ionic energy parameter E/N (the ratio of the electric field intensity to the neutral gas number density). The Viehland–Mason kinetic theory of ionic mobility (valid for arbitrary E/N) is used to derive theoretical mobilities from the electron gas-Drude model potentials of Gordon and Waldman for Rb+–Ar (mobilities measured previously), Rb+–Kr, and Rb+–Xe. The results are compared with the experimental data. Further, by using an iterative technique, new potentials are developed which are determined directly by the experimental data, and these ’’experimental’’ potentials are tabulated and compared with the Gordon–Waldman potentials.
Excited states and photochemistry of saturated molecules. IX. Vertical excited states of silane
Authors: Gordon Mark S.
Published: February 2003

The vertical excited states of silane are investigated using ten different basis sets to systematically compare the effects of (a) Rydberg functions, (b) diffuse d functions, (c) valence d functions, and (d) a split valence shell. Wavefunctions are generated using all singly excited configurations, excluding inner shells. The low-lying singlet states are dominated by excitations into the diffuse orbitals, while low-lying triplets have significant valence character. Valence d functions have a minor effect on excitation energies if an adequate valence sp basis set is used. The main effect of Rydberg d functions is to introduce additional Rydberg states below the first ionization potential and above the s and p Rydberg levels.
Kinetic theory derivation of a pair configuration space diffusion equation
Authors: Cukier Robert I. , Mehaffey J. R. , Kapral Raymond
Published: February 2003

A repeated ring kinetic theory is derived to describe the thermal fluctuations of two test particles in a fluid. The kinetic equation incorporates the interaction of the two particles via the potential of the mean force, their uncorrelated Enskog propagation, and multiply correlated collisions among the test particles and the bath particles. By projecting on the configuration space of the test particles, an equation of the form of a diffusion equation is obtained, but with a microscopically defined diffusion tensor. The configuration and time dependent diffusion tensor is given by a time correlation function and can be evaluated for a given molecular model.
Raman spectroscopy of structural phase transitions in Ag26I18W4O16
Authors: Habbal F. , Zvirgzds J. A. , Scott J. F.
Published: February 2003

Raman spectra of single crystals of silver iodide tetratungstate (Ag26I18W4O16) have been obtained as a function of temperature from 77 to 350 K. These data indicate the presence of structural phase transitions near 199 and 285 K. Dielectric and conductivity measurements perpendicular to the unique twofold axis confirm the presence of these two transition temperatures. Plots of log (σT) versus 1/T reveal three distinct regions of linearity with activation energy 2.4 eV below 199 K; 0.07 eV between 199 and 285 K; and 0.42 eV above 285 K.
On the relative importance of spin–spin and spin–rotation relaxation in gas phase NMR
Authors: Kouri Donald J.
Published: February 2003

The ratio R of the gas phase spin–spin and spin–rotation relaxation times TSS1 and TSR1 is examined in detail using the factorization of infinite order sudden (IOS) approximation generalized cross sections given recently by Goldflam, Green, and Kouri. General results are obtained in the extreme narrowing limit for both homonuclear and heteronuclear systems. The results are quite simple and, in the low temperature limit, depend primarily upon the characteristics of the relaxing nucleus rather than the intermolecular interaction. Calculations illustrating the theory are presented for He+o-H2.
Kinetics of order–disorder transformations in alloys. II
Authors: Gschwend Klaus , Sato Hiroshi , Kikuchi Ryoichi
Published: February 2003

As a continuation of previously published work, isothermal ordering and disordering processes (of B2 type structure) in bcc binary alloys from initial states both near and far from equilibrium are investigated theoretically. The vacancy mechanism for atomic migration is assumed, interactions among nearest neighbors are taken into account, and the pair approximation of the path probability method of irreversible, cooperative phenomena is utilized. This leads to a set of nonlinear rate equations derived in a systematic, statistical fashion which describes the dynamical behavior of the system. The rate equations can be linearized near equilibrium and are shown to obey in this form the formalism of irreversible thermodynamics of Onsager. The driving forces are identified as the partial derivatives of the free energy function of the system with respect to state variables. Several relaxation times are obtained which characterize near equilibrium relaxation, and their meanings are discussed. Nonlinear coupling of relaxation modes occurs in relaxation processes from states far from equilibrium. Especially, a distinct three step relaxation of internal energy and a two step relaxation of the long range order are predicted when the alloy is quenched from states having zero or small long range order. The behavior is strikingly similar to the three step relaxation process of the internal energy observed in Ni3Fe near the order–disorder transition point. These relaxation effects are related to the geometry of the free energy surface.
Valence bond perturbation theory for electron correlation
Authors: Kirtman Bernard , Cole Samuel J.
Published: February 2003

A second-order valence bond perturbation treatment (VB–PT) of electron correlation which utilizes the self-consistent valence bond wavefunction as an initial approximation is described. In trial calculations on H2 and LiH, at the equilibrium intranuclear distance, the accuracy is comparable to third-order Hartree–Fock perturbation theory (HF–PT). However, the computation time for VB–PT increases only as NK4 (N is the number of electrons; K the number of basis functions) whereas HF–PT, and other treatments of similar accuracy, are N2K4 procedures. Other advantages of VB–PT include a correct qualitative description of bond dissociation in zeroth-order; an exact breakdown of the perturbation energy into a sum over pairs; potentially weak mixed pair correlations; and the possibility of atoms-in-molecules methods to do away with the basis set problem.
Collision induced dissociation in collinear H+H2: Quantum mechanical probabilities using the time-dependent wavepacket approach
Authors: Kulander K. C.
Published: February 2003

A time-dependent wavepacket approach is used to calculate quantum mechanical probabilities for reaction, dissociation, and inelastic transitions for the collinear H+H2 system. The interaction used is a realistic LEPS-type surface. Results are reported for collision energies between 3 and 12 eV and for three different initial vibrational states of the H2. The probability of reaction is found to be very small at these energies. We find vibrational enhancement of dissociation and the inelastic transition probabilities from a vibrationally excited diatom have an oscillatory structure. We solve the time-dependent Schrödinger equation using a newly developed predictor–corrector method.
A new assignment of the uv spectrum of thiophene. A
b i
n
i
t
i
o configuration interaction energies and the single crystal uv spectrum
Authors: Bendazzoli G. L. , Bertinelli F. , Palmieri P. , Brillante A. , Taliani C.
Published: February 2003

The uv spectra of single crystals of thiophene have been recorded in the spectral region 38 000–50 000 cm−1 for temperatures down to 12 K. In the crystal spectrum the ? and ? absorption systems observed in the vapor phase have different polarization ratios and the ? system is not observed. Wavefunctions and energies have been computed for the lowest excited states by a relatively large orbital expansion and configuration mixing. The two intense electronic transitions are assigned to π–π* states of A1 and B2 symmetry and the intermediate absorption in the vapor spectrum is attributed to a Rydberg state.
Luminescence from Al2xGa2(1−x)O3:Cr3+
Authors: Morgan G. P. , Glynn T. J. , Imbusch G. F. , Remeika J. P.
Published: February 2003

Detailed luminescence and lifetime data are presented for the mixed crystal system Al2xGa2(1−x)O3:Cr3+, where x=0, 0.02, 0.20, 0.35, 0.98, 1.0. The experimental data, obtained under broadband excitation at 77 K and 4 K, indicate that our pressure-grown microcrystalline samples all have the corundum (α-Al2O3) structure. The main features of the luminescence are explained in terms of the shifts, due to strain, of the chromium ion energy levels. Some experimental evidence for energy transfer among the optical centers is also presented.
Self-diffusion of small ions in polyelectrolyte solutions
Authors: Yoshida Nobuo
Published: February 2003

A method of calculating the self-diffusion coefficients of small ions in polyelectrolyte solutions is derived on the basis of the cell model with cylindrical symmetry. The method is applied to a salt-free polyelectrolyte solution and a limiting law for the self-diffusion coefficient of the counterion is formulated in a rigorous manner. The calculation leading to this result shows that at infinite dilution there is a condensation of counterions about the polyion chain. The limiting law is compared with Manning’s theory and with experimental data in the literature.
Escape probability of a random walker on a lattice doped with absorbers
Authors: Watanabe Hiroshi
Published: February 2003

We present a method for calculating the escape probability of a particle performing a random walk on a lattice doped with one or more absorbers. This method utilizes generating functions. We apply this method to several cases of practical interest, including the escape probability from a pair of absorbers. Comparison is made between the present discrete approach and the continuous one based on the Smoluchowski equation.
Study of the structure of molecular complexes. XVI. Doubly charged cations interacting with water
Authors: Corongiu G. , Clementi E.
Published: February 2003

The interaction energy between a molecule of water and Be2+ has been computed near the Hartree–Fock limit and in the CI approximation. Crossing of 1A1 states (representing either Be2++H2O or Be++H2O+) show that doubly charged cations can behave differently, when interacting with water, than singly charged cations.
Energy level cluster analysis for E (ν2) vibration rotation spectrum of spherical top molecules
Authors: Harter William G. , Galbraith Harold W. , Patterson Chris W.
Published: February 2003

The theory of level clusters, originally developed for ground state (A1) and weakly coupled infrared active (F2, ν3) excited states, is extended to include the Raman-active (E) states of vibration–rotation. Simple approximate formulas are given for the eigenvalue spectrum of the dominant second order vibration–rotation Hamiltonian and the results are compared with exact computer reductions.
Centrifugal and Coriolis effects on level cluster patterns for T (ν3) rovibrational bands in spherical top molecules
Authors: Harter William G. , Patterson Chris W. , Galbraith Harold W.
Published: February 2003

Fundamental T (ν3) type high-J rovibrational fine structure is derived for a range of values of the Coriolis and [2×2]4 centrifugal constants. The theory of level clusters is developed further. Correlations are made between cluster states corresponding to well separated P+, Q0, and R− branches, and the opposite case in which Π± and Σ0 labels are appropriate.
Analysis of torsional spectra of molecules with two internal C3v rotors. X.a) Low frequency vibrational spectra, methyl torsional potential functions, and internal rotation of ethylmethylether (-d and -d3)
Authors: Durig J. R. , Compton D. A. C.
Published: February 2003

The vibrational spectra of gaseous ethylmethylether C2H5OCH3 and C2H5OCD3 have been investigated at high resolution below 500 cm−1. Considerable torsional data are reported and assigned on the basis of an equilibrium between the s-trans and the high energy gauche conformers. Asymmetric potential functions for the CH2–O torsions have been calculated from observed transitions, leading to values for the enthalpy difference between the gauche and s-trans conformers of 1.36 and 1.28 kcal/mole, respectively, for the -d0 and -d3 compounds. Analysis of the transitions above 130 cm−1 indicates that the two methyl rotors have strongly coupled torsional vibrations in both compounds, and that previous assignments are insufficient for explaining the data. Transitions due to the methyl torsions of the molecules in the first excited states of the asymmetric CH2–O torsions are reported; also, some weak bands have been assigned tentatively as methyl torsions of gauche-ethylmethylether. Torsional potential functions and barrier heights to internal rotation of the methyl tops have been calculated for both compounds; for the ground state of ethylmethylether the C–CH3 and O–CH3 barriers are calculated to be 3.08 and 2.61 kcal/mole, respectively.
Collisionless decay, vibrational relaxation, and intermediate case quenching of S1 formaldehyde
Authors: Weisshaar James C. , Baronavski A. P. , Cabello A. , Moore C. Bradley
Published: February 2003

The decay of fluorescence from the 40 and 41 levels of the S1(Ã 1A2) state of H2CO and D2CO has been monitored as a function of pressure after selective, pulsed laser excitation. For D2CO, single exponential decays modified by 40↔41 energy transfer were observed over the entire pressure range 4×10−5–4 Torr. The zero pressure lifetimes τ0(40) =7.8±0.7 μs and τ0(41) =6.0±0.4 μs are probably the radiative lifetimes. The rate of 41→40 energy transfer in D2CO was found to be (9.6±0.4) ×10−10 cm3 molecule−1 s−1, about three times the gas kinetic rate. For H2CO at pressures above 0.1 Torr, fluorescence decays were also single exponentials modified by 40 ↔41 energy transfer. However, in the range 2×10−4–0.1 Torr, the decays of the individual 40 and 41 vibronic levels were typically biexponential. The zero pressure decay occurs on a timescale at least 20 times faster than the radiative lifetime of ∼5 μs. The Stern-Volmer plots of τ−1 vs pressure give quenching rates between 2.2×10−9 and 6.5×10−9 cm3 molecule−1 s−1 for both fast and slow components below ∼20 mTorr. The relative amplitude of the fast component decreases rapidly with pressure and approaches zero at 0.1 Torr. The slow component plots are dramatically curved and give quenching rates of only about 2.2×10−11 cm3 molecule−1 s−1 above 1 Torr. The low pressure quenching rates and zero pressure lifetimes for H2CO depend significantly on the K′ rotational quantum number within 40. The biexponential decays for H2CO may result from variations in lifetime among the J′ states excited by the laser. The large quenching rate constants and the curvature of the Stern–Volmer plots can be qualitatively understood in terms of recent mixed-state models of collision-induced radiationless decay.
Melting in a two-dimensional Lennard-Jones system
Authors: Toxvaerd So/ren
Published: February 2003

From molecular dynamics calculations on a two-dimensional Lennard-Jones system we find a first order solid–fluid transition with an estimated triple point temperature in good agreement with the triple point temperature of adsorbed monolayers of xenon. The melting obeys Ross’s melting criteria.
Piezo- and elasto-optic properties of liquids under high pressure. I. Refractive index vs pressure and strain
Authors: Vedam K. , Limsuwan Pichet
Published: February 2003

The variation of the refractive index of liquid with pressure to 14 kbar has been determined by an optical interferometric method. The liquids studied are carbon tetrachloride, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, benzene, chlorobenzene, methyl alcohol, and water at room temperature. Since most of these liquids freeze at room temperature even before 14 kbar is reached, the measurements were carried out up to the freezing point or 14 kbar, whichever pressure is lower. In every case the refractive index increases with pressure with pronounced nonlinearity, particularly at high pressure. This nonlinearity is reduced considerably when the same data on Δn are considered as a function of the volume strain indicating that the nonlinear piezo-optic behavior of liquids is just a manifestation of nonlinear P–V relationship. Further, it is found that the slight nonlinearity noticed in the Δn−ΔV/Vo relationship could be still further reduced when the variation in refractive index is treated as functions of the Lagrangian and Eulerian strains. In particular, the Eulerian strain ε is found to yield a much larger range of strain over which a simple linear relationship between Δn and ε is obtained irrespective of the nature of the liquid or the equation of state used to compute the strain.
Monte Carlo calculations of the number of ways to pack nonoverlapping rods on a square lattice
Authors: McCrackin Frank L.
Published: February 2003

The number of configurations of nonoverlapping rods on a square lattice is computed for various packing fractions and orientations of the rods. From the number of configurations, the entropies of the configurations are computed and compared with the results of approximate formulas of DiMarzio that are much used in statistical–mechanical theories of liquid crystals. For rods of three lattice sites, our calculations and Dimarzio’s formulas agree to within 0.5% for packing fractions less than 0.5. Some calculations for rods of ten lattice sites also showed good agreement.