bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of magnetic anisotropy distribution in longitudinal thin film media
Authors: Hee Ching Hian , Wang Jian Ping , Gong Hao , Low Teck Seng
Published: July 2002

The effects of anisotropy axes distribution on the properties of longitudinal recording media are simulated using the micromagnetic modeling. The angle between the anisotropy axis and the preferred direction is assumed to follow the Gaussian distribution. Hysteresis loops are simulated with the field applied in the preferred direction and perpendicular to it. Orientation ratios (OR) are calculated for different anisotropy distribution. OR (coercivity) increased from 1.4 to 79.2 when standard deviation of anisotropy distribution σ decreased from 0.7 to 0.05. The effect of the anisotropy distribution on the recording performance is evaluated, too. The results show that narrowing the anisotropy distribution will increase the OR and decrease the transition noise power and transition length. It was found that a critical point occurred at σ=0.15, below which the OR can be maintained at a sufficiently high value and yet the integrated noise power can be kept low.
Hydrogenation of defects in edge-defined film-fed grown aluminum-enhanced plasma enhanced chemical vapor deposited silicon nitride multicrystalline silicon
Authors: Jeong Ji-Weon , Rosenblum Mark D. , Kalejs Juris P. , Rohatgi Ajeet
Published: July 2002

Gettering of impurities and hydrogen passivation of defects in edge-defined film-fed grown (EFG) multicrystalline silicon were studied by low-cost manufacturable technologies such as emitter diffusion by a spin-on phosphorus dopant source, back surface field formation by screen-printed aluminum, and a post-deposition anneal of plasma enhanced chemical vapor deposited (PECVD) silicon nitride antireflection coating. These processes were carried out in a high-throughput lamp-heated conveyor belt furnace. PECVD silicon nitride-induced hydrogenation of defects in EFG silicon was studied in conjunction with screen-printed aluminum back surface field formation to investigate the synergistic effect of aluminum gettering and silicon nitride hydrogenation of bulk defects. It was found that post-deposition anneal of PECVD silicon nitride at temperatures ranging from 450 to 850 °C, without the coformation of aluminum back surface field on the back, does not provide appreciable passivation or hydrogenation of bulk defects in EFG material. However, simultaneous anneal of PECVD silicon nitride and formation of aluminum back surface field at 850 °C significantly enhanced the hydrogenation ability of the PECVD silicon nitride film. PECVD silicon nitride deposition and a subsequent anneal, after the aluminum back surface field formation, was found to be less effective in passivating bulk defects. It is proposed that aluminum-enhanced hydrogenation from a PECVD silicon nitride film is the result of vacancy generation at the aluminum-silicon interface due to the alloying process. The affinity of hydrogen to react with vacancies provides a chemical potential gradient that increases the flux of atomic hydrogen from the silicon nitride film into the bulk silicon. In addition, vacancies can dissociate hydrogen molecules, increasing the atomic hydrogen content of the bulk silicon. This enhances defect passivation and improves the minority carrier lifetime.
Nanoindentation investigation of Ti/TiN multilayers films
Authors: Daia M. Ben , Aubert P. , Labdi S. , Sant C. , Sadi F. A. , Houdy Ph. , Bozet J. L.
Published: July 2002

The hardness of Ti/TiN nanolaminated films is investigated in this study. Monolithic Ti and TiN films and Ti/TiN multilayers were deposited on silicon substrates by radio-frequency sputtering. The period thickness of multilayers was decreased from 20 to 2.5 nm. Grazing x-ray reflectometry showed that the modulation of composition of Ti/TiN multilayers exists for all the period thickness considered. From nanoindentation measurements, we determined the hardness and Young’s modulus of multilayers. Hardness increased with decreasing period thickness to go beyond the rule-of-mixture value for samples with period thickness of Λ⩽5 nm. The maximum hardness, 1.6 times higher than the value obtained by the rule of mixture, is obtained for Λ=2.5 nm. Our results are compared to a dislocation-based model previously introduced by Lehoczky.
Nonlinear alternating current susceptibility studies of high-Tc rings: Influence of the structuring method and determination of the flux creep exponent
Authors: Streubel S. , Mrowka F. , Wurlitzer M. , Esquinazi P. , Zimmer K.
Published: July 2002

We have studied the influence of the patterning procedure on the critical current density of high-Tc YBa2Cu3O7−δ thin rings using the nonlinear ac susceptibility method. At no applied dc magnetic field have we found that laser ablation strongly degrades the critical current density whereas ion beam etching has only a weak influence on it. From the measurements at different frequencies and at dc magnetic fields we analyzed the influence of flux creep and obtained the field dependence of the flux creep exponent. Our data reconfirm the recently observed scaling relation for the nonlinear susceptibility response of type-II superconductors.
Effect of multiple current spikes on the enhancement of ion charge states of vacuum arc plasmas
Authors: Yushkov George Yu. , Oks Efim M. , Anders André , Brown Ian G.
Published: July 2002

Ion charge state distributions of vacuum arc ion sources are correlated to the arc operating voltage. Recent research has shown that an enhancement of ion charges via an increase of the arc voltage can be achieved utilizing the transient processes that accompany an arc current spike. The idea investigated is to further enhance the ion charge states by multiple current pulses. It is shown that although the ion charge states are enhanced compared to quasi-dc operation, the application of a sequence of pulses does not lead to the desired additional increase in charge states. This can be attributed to the additional plasma production that is caused by higher arc currents: The additional power supplied to the plasma is distributed over a larger number of plasma particles. One can expect that in the limiting case of many current spikes, the ion charges state distribution approaches the one known for arc plasmas at higher discharge current.
Direct measurements of strain in a shock-loaded, lead filled glass
Authors: Millett J. C. F. , Bourne N. K. , Rosenberg Z.
Published: July 2002

Commercial strain gauges have been used to measure the plate impact behavior of type D, extra dense flint, a high density, lead filled glass. Impact stresses were in the range 2.0–4.6 GPa, just above the materials quoted Hugoniot elastic limit of 4.3±0.2 GPa. This glass (and similar materials) has been shown to display a reduction in shear strength across a slowly moving front (the failure wave) behind the main shock wave. At stresses below which this glass displays this feature, measured strains agree well with values calculated from the elastic relations. Above this stress, the failure wave leads to an increase in compressive strain, which has been correlated with measurements using other techniques. In this case, we show that the calculated values of strain agree only with the measured values behind the failure wave.
The low energy ion assisted control of interfacial structure: Ion incident energy effects
Authors: Zhou X. W. , Wadley H. N. G.
Published: July 2002

The properties of multilayered materials are often dependent upon their interfacial structure. For low temperature deposition processes where the structure is kinetically controlled, the interfacial roughness and the extent of interlayer mixing are primarily controlled by the adatom energy used in the deposition. Inert gas ion assistance during the growth process also enables manipulation of the interfacial roughness and intermixing. To explore inert gas ion assistance, a molecular dynamics approach has been used to investigate the role of ion energy and ion species upon the flattening of various surfaces formed during the growth of the Ni/Cu/Ni multilayers. The results indicated that ion energies in the 1–4 eV range could flatten the “rough” copper islands on either copper or nickel crystals. To flatten the rough nickel islands on copper or nickel crystals, higher ion energies in the 9–15 eV range would have to be used. Significant mixing between nickel island atoms and the underlying copper crystal atoms started to occur as the ion energy was increased to around 6–9 eV. However, little mixing was observed between the copper island atoms and the underlying nickel crystal atoms in the same ion energy range. At a given ion energy, the heavier (xenon) ions were found to produce more surface flattening and mixing than the lighter (argon) ions.
Modulated optical reflectance characterization of high temperature superconducting thin film microwave devices
Authors: Almond D. P. , Nokrach P. , Stokes E. W. R. , Porch A. , Foulds S. A. L. , Wellhöfer F. , Powell J. R. , Abell J. S.
Published: July 2002

The modulated optical reflectance (MOR) technique is shown to provide a room temperature, noncontact, nondestructive and high spatial resolution means of assessing high temperature superconducting (HTS) thin film quality. Room temperature MOR characterizations of a number of 8 GHz planar HTS resonators indicating a range of property variations and local degradations in HTS film performance are shown to be consistent with results obtained at low temperatures by the electron beam induced voltage contrast technique. The microwave performances of some of the resonators are found to exhibit nonlinear characteristics that can be explained by HTS defects revealed by the MOR technique.
Electron paramagnetic resonance of platinum impurities in KTiOPO4 crystals
Authors: Garces N. Y. , Stevens K. T. , Halliburton L. E.
Published: July 2002

Single crystals of KTiOPO4 (KTP) often contain trace amounts of isolated platinum impurities. When present in sufficient concentration, these ions increase the KTP crystal’s susceptibility to form gray tracks during frequency doubling of high-power laser beams. Electron paramagnetic resonance (EPR) has been used to characterize three platinum centers in a crystal of KTiOPO4. Initially, the platinum ions are present in nonparamagnetic forms. Exposure to ionizing radiation (i.e., x rays or above-band-gap laser beams) at room temperature converts them into paramagnetic centers. Once formed, these centers are stable for weeks at room temperature; however, their EPR spectra can only be observed at temperatures near or below 30 K. An angular study provided principal values and principal directions for the g matrices and the platinum hyperfine matrices. The Pt(A) center has principal g values of 1.9397, 2.4463, and 2.5900 and is assigned to a Pt3+ ion substituting for a Ti4+ ion. In contrast, the Pt(B) center has principal g values of 1.6019, 1.9754, and 2.7788 and the Pt(C) center has values of 1.4656, 1.8597, and 2.9576. We suggest that these latter two centers are Pt+ ions substituting for K+ ions.
Ultrathin boron-doped microcrystalline silicon as a novel constant band gap buffer inserted at the
p-a
-SiC:H/
i-a
-Si:H interface of amorphous silicon solar cells
Authors: Lee Chang Hyun , Jeon Jin Wan , Lim Koeng Su
Published: July 2002

The properties of thin p-type microcrystalline silicon (p-μc-Si:H) films prepared onto boron-doped amorphous silicon carbide (p-a-SiC:H) have been investigated. At the initial growth regime (<100 Å) of the p-μc-Si:H onto p-a-SiC:H, Si nanocrystallites were proved to be formed in amorphous matrix. The thin p-μc-Si:H was introduced as a novel constant band gap buffer at the p/i interface of amorphous silicon solar cells. The open circuit voltage and the blue response of the cell were improved significantly by inserting the p-μc-Si:H at the p/i interface as a buffer when compared with those of the bufferless cell. Our numerical modeling on the constant band gap buffer elucidates clearly that the buffering effects of the thin p-μc-Si:H originate from the shrinkage of highly defective region with a short lifetime in the vicinity of the p/i interface.
Photoluminescence of perovskite lanthanum aluminate single crystals
Authors: Kawabe Y. , Yamanaka A. , Hanamura E. , Kimura T. , Takiguchi Y. , Kan H. , Tokura Y.
Published: July 2002

Photoluminescence was observed from pure perovskite lanthanum aluminate (LaAlO3) single crystals grown with a floating-zone method in reduced atmosphere. Emission was composed of a fast ultraviolet (UV) component and a broad visible emission with a lifetime of 13 μs. From the lifetime measurement, absorption spectra, excitation spectrum, and comparison between samples as-grown and annealed in oxygen, we assigned the UV emission to band-to-band emission or emission from polaron pairs. The visible component was assigned to emission from the excited state formed at oxygen defect sites. These emissions demonstrate the possibility of light-emitting devices made of perovskite oxides.
Collisional ionization of Na by HBr in weak to strong electric fields
Authors: Safinya K. A. , Gallagher T. F. , Sandner W. , Gounand F.
Published: July 2002

We report the effect of static electric fields on the collisional ionization of highly excited sodium atoms by HBr. The binding energy dependence of the collisional ionization cross section is measured at zero field and in static electric fields up to that point at which the atom field ionizes. The applied electric field lowers the ionization threshold of the atom from its zero field value. Therefore an atom near the ionization threshold in an electric field is of smaller size than a free field atom with the same binding energy. Thus measuring the binding energy dependence of the cross section at different values of the electric field allows us to study the effects of the physical size of the atom on the cross section. The effect of the electric field was to lower the measured ionization cross section. However, the binding energy dependence of the cross section remains unchanged at the level of our measurement accuracy. The measured cross sections are larger for larger atoms, exhibit a drop with increasing binding energy characteristic of rotational to electronic excitation transfer, and are of order 10−12–10−11 cm2. A simple calculation based on dipole (J→ J−1) excitation transfer from the molecule to the atom predicts, with good agreement, the binding energy dependence of the cross section. The electric field dependence of the data however, is not shown in the theory.
Resolvent operator method for solving rheological equations of state
Authors: Armstrong Robert , Jhon Myung S. , Dahler John S.
Published: July 2002

Fano’s tetradic representation of Liouville–Neumann operators is used as the basis of a systematic procedure for solving an important class of rheological equations of state. This procedure is complementary to the techniques which Bird and his co-workers have used in their studies of complicated rheological models. To illustrate how our method works it is applied to a rheological model of the Maxwell–Oldroyd type. Explicit formulas are derived for the stress tensors associated with a number of time-independent, homogeneous flows.
Intramolecular vibrational relaxation of a polyatomic in the solid state. I. DCN in Xe
Authors: Abbate Alison D. , Moore C. Bradley
Published: July 2002

Vibrational relaxation of DCN isolated in Xe matrices has been studied by laser-excited vibrational fluorescence. The C–D stretch ν3 was excited and fluorescence was observed from overtones and combinations of the ν1 and ν2 modes. The decay rates of the vibrational levels range over several orders of magnitude from 3×108 s−1 for ν3 to 2.1×105 s−1 for ν2. A kinetic model for the relaxation was solved to give a complete set of rate constants for the vibrational transitions. The rates, together with the relative fluorescence intensities, show that there is a dominant relaxation channel. The relative importance of competing channels depends on changes in energy and in vibrational quantum number of the transfers. Within the ν2 manifold, the rate constants for Δv2=1 transitions follow kν,ν−1=vnk1,0 with n<1.3. The rate constants increase only modestly with temperature indicating that the excess vibrational energy is transferred to guest modes such as rotation or translation rather than to bulk phonons.
Solvation studies in argon matrices: Details of the stepwise formation of Sn⋅Li+NO3− ion pair solvates where S is H2O, THF, DMF, and glyme
Authors: Ritzhaupt Gary , Consani Keith , Devlin J. Paul
Published: July 2002

If an ion pair such as Li+NO−3 forms a solvate in a liquid solvent S of formula Snmax⋅Li+NO−3 it is possible to prepare each of the nmax-different solvates in low temperature matrices by using a range of solvent–argon matrix compositions. Each successive S molecule coordinated to the lithium reduces the cation distortion of the nitrate ion through a combination of cation charge neutralization and coordination-shell steric forces which act to increase the cation–anion bond length. As a result the splitting of the ν3(e) nitrate degeneracy (Δν3) of 262 cm−1 collapses in a surprisingly uniform stepwise manner as n increases for the matrix-isolated solvates. In this study Δν3 has been measured for the nmax different Li+NO−3 solvates for each of the solvents glyme, THF, DMF, and H2O which appear to display nmax values of 3, 4, 4, and 5, respectively (glyme is bidentate so nmax=3 but CNmax=6). Thus, part of the infrared spectrum of each solvate has been observed and assigned, the nmax solvation stages enumerated and the magnitude of the effect of each solvent molecule on the strength of the cation–anion interaction (δΔν3) noted. A comparison of the matrix data with liquid solution ν3(e) measurements indicates that, in general, CNmax (matrix)≳CNmax (solution) and that solution equilibria involving different solvates can be identified and characterized. The new H2O matrix data are remarkable both in the clarity with which the bands for five different hydrates were observed and in the very close correspondence of the observed δΔν3 values, for n=1, 2, and 3, with the values previously reported from an overlap-population analysis for the N–O bonds as deduced from an ab initio SCF calculation. The matrix data for the flexible ‘‘bidentate’’ ligand (glyme) show that there is at least a 50% chance of the ligand isomerizing to the bidentate cis structure during coordination, while the data for the liquid glyme solution suggest that two of the three ligands assume a bidentate form.
Experimental study of H+O2 reaction dynamics at collision energies of 2.6, 1.9, and 1.0 eV
Authors: Kleinermanns K. , Linnebach E.
Published: July 2002

The nascent rotational and fine structure state distributions of OH produced in the reaction H+O2→OH (N,v, f)+O were probed by fast atom-laser induced fluorescence experiments. Translationally hot H atoms were formed by photolysis of HJ and HBr at 248 and 193 nm leading to H+O2 c.m. collision energies E of 2.6, 1.9, and 0.9 eV. The rotational state distributions are compared with trajectory calculations using the ab initio potential energy surface of Melius and Blint. The OH λ-doublet distributions show preference for the π+ component which increases with increasing E suggesting less importance of out of plane rotation of the HO2 complex at high E. The alignment of OH relative to the flight direction of the H atoms is measured by polarizing analysis and photolysis laser beams. The large polarization effects directly demonstrate that the OH angular momentum vectors are preferentially parallel to the electrical vector of the dissociation laser and perpendicular to the flight direction of the H atoms.
Reactions of methyl and ethyl radicals with uranium hexafluoride
Authors: Lyman John L. , Laguna Glenn
Published: July 2002

We have measured the rates of reaction of both methyl and ethyl radicals with uranium hexafluoride (UF6) in the gas phase. The method we used was to photolyze samples of UF6 in the presence of either methane or ethane. The radicals produced by reaction of fluorine atoms with these species then react with either themselves or with UF6. We inferred the rate constants from ratios of the reaction products and the published rate constants for radical recombination. The diagnostic technique was gas chromatography. The resulting rate constants for reaction with UF6 were (1.6±0.8)×10−14 cm3 molecule−1 s−1 for methyl radicals and (4±2)×10−11 cm3 molecule−1 s−1 for ethyl radicals.
Local structure and dynamics in concentrated aqueous solutions and glasses of univalent ions
Authors: Guillaume François , Perrot Michel , Rothschild Walter G.
Published: July 2002

We have analyzed the VV- and VH-Raman band profiles of the N–O symmetric stretching fundamental of the nitrate anion of 1 mol/ℓ  NaNO3 (or LiNO3) in Li+Cl−-enriched aqueous solutions for the liquid phase between 263 and 353 K and for the vitreous state at 77 K. From the observed isothermal correlation time of the orientational mobility of the C3 axis of NO−3 in Li+-rich solution and from the band asymmetry of the corresponding isotropic profile at increasing temperature, we propose an endothermic local-structure-reforming process that favors incorporation of a few Li+ ions into the solvation sphere of the anions. The vitreous state of 1 mol/ℓ  LiNO3 or NaNO3 in the LiCl-rich systems, which shows considerable local order about the nitrate anion, is conceptualized as a percolation cluster of solvated cation–anion interaction that breaks up into cluster distributions for the liquid phase.
Quantitation of homogeneous and inhomogeneous broadening mechanisms in t
r
a
n
s-stilbene using absolute resonance Raman intensities
Authors: Myers Anne B. , Trulson Mark O. , Mathies Richard A.
Published: July 2002

The sensitivity of resonance Raman cross sections to the excited state homogeneous width is exploited to separate the homogeneous and inhomogeneous contributions to the optical absorption linewidth in trans-stilbene. Absolute Raman cross sections have been measured using excitation at 356, 299, 282, and 266 nm, and calculations have been performed to model the intensities of the eleven strongly Raman-active modes as well as the absorption spectrum. A simple analysis neglecting explicit temperature effects requires a homogeneous Lorentzian linewidth (Γ) of 310 cm−1 and an inhomogeneous Gaussian distribution of zero–zero energies with a standard deviation (θ) of 500 cm−1. Consideration of thermal excitation in the lowest-frequency single-bond torsional mode significantly improves the fit to the experimental absorption by broadening the spectrum asymmetrically to higher energies, while allowing Γ and θ to be reduced to 120 and 450 cm−1, respectively. The spectral broadening generated by thermal excitation of ground-state torsional modes thus contributes significantly to the homogeneous linewidth in trans-stilbene. The relatively large effective homogeneous width at room temperature is consistent with the low Raman intensity in the ∼200 cm−1 C=C–φ bend compared with the prominence of this vibration in resolved low-temperature electronic spectra.
Cluster expansion of the wave function. Valence and Rydberg excitations and ionizations of pyrrole, furan, and cyclopentadiene
Authors: Nakatsuji H. , Kitao O. , Yonezawa T.
Published: July 2002

The symmetry-adapted cluster (SAC) expansion and the SAC-CI theory have been applied to the calculations of the valence and Rydberg excitations and ionizations of the five-membered ring compounds, pyrrole, furan, and cyclopentadiene. For almost all cases, the experimental values were reproduced to within 0.3 eV for the excitation energy, and to within 0.5–0.7 eV for the ionization potential. We have given several new assignments for the excitation spectra of pyrrole and furan, and the first ab initio assignments for cyclopentadiene. However, there were remarkable disagreements of about 1.2 eV for the singlet π → π* excitations of 2 1A1 state for furan, and of 1 1B2 state for cyclopentadiene. This is the same tendency as those due to the MRSD-CI method. For ionization potential, our assignments of the peaks are essentially the same as those due to the Green’s function method by von Niessen et al., but the present SAC-CI method reproduces the experimental data better than this method.
Quasiclassical trajectory studies of the chlorine–hydrogen system. V. The reactions of Cl atoms with T2, HT, and DT
Authors: Persky Avigdor , Broida Michael
Published: July 2002

Three dimensional quasiclassical trajectory calculations were carried our for the reactions Cl+T2, Cl+HT, and Cl+DT, using two LEPS potential energy surfaces which were previously used in trajectory studies of the reactions Cl+H2, Cl+D2, and Cl+HD. Reaction cross sections, rate constants, and kinetic isotope effects were obtained from the trajectory calculations. The intermolecular kinetic isotope effects kCl+H2/kCl+T2, kCl+H2/(kCl+HT+kCl+TH) and kCl+H2/(kCl+DT+kCl+TD) calculated for both surfaces were found to be in good agreement with experimental results. The trajectory results indicate a very significant increase of the intramolecular isotope effect kCl+HT/kCl+TH with the initial rotational state J, and only a small increase in the case of the intramolecular isotope effect kCl+DT/kCl+TD. The results of the present study are discussed in relation to earlier trajectory results for the reactions Cl+H2, Cl+D2, and Cl+HD.
Theoretical investigations of small multiply charged cations. I. SiH2+
Authors: Koch Wolfram , Frenking Gernot , Chang Christian C.
Published: July 2002

In excellent agreement with experiments, high levels ab initio MO calculations of the multireference CI (MRD-CI) and complete active space SCF (CASSCF) type show a deep potential well for the X 2Σ+ ground state of the diatomic SiH2+ dication with a barrier of 1.06 eV (CASSCF) and 1.07 eV (MRD-CI) for the charge separation reaction. This result, which is in distinct contrast to the findings for the valence isoelectronic CH2+ dication, is explained by the charge polarization character of the SIH2+ ground state. Also the first excited A 2Π state was found by MRD-CI to be weakly bound.
Trajectory calculations of ion-quadrupolar molecule collision rate constants
Authors: Bhowmik Pradip K. , Su Timothy
Published: July 2002

The trajectory method is used to calculate collision rate constants for ion-quadrupolar molecule interactions for molecules belonging to the D∞h point group. The calculations utilize a framework similar to the trajectory technique developed previously for ion-polar molecule interactions. The trajectory method gives a good upper bound to reaction rate constants. It predicts a larger effect of the quadrupole moment on collision rate constants than the AQO (average quadrupole orientation)theory. It is shown that the moment of inertia of the neutral is an important factor in determinng the collision rate constant.
Thermal effects on the optical spectra of Al2O3:Ti3+
Authors: Powell Richard C. , Venikouas George E. , Xi Lin , Tyminski Jacek K. , Kokta Milan R.
Published: July 2002

The results of an investigation of thermal effects on the fluorescence spectra of Ti3+ in sapphire are presented. As-grown samples synthesized by the Czochralski method were found to exhibit a fluorescence band characteristic of Ti4+. After thermal annealing this band was greatly reduced and the Ti3+ fluorescence was seen. The fluorescence lifetime was found to be constant between 10 and 300 K and then to decrease significantly as temperature was raised to 500 K. This decrease is shown to be associated with thermal activation to the higher Jahn–Teller component of the excited state. The temperature dependences of the intensities, widths and lifetimes of the two zero-phonon lines were monitored between 10 and 80 K. The lifetimes remained constant as the widths broadened and intensities decreased with temperature.The intensity change is associated with the increased rate of vibronic emission while the line broadening is attributed to the phonon processes coupling spin-orbit components of the ground state and to Raman scattering of phonons.
Electron affinity of fluorine: A quantum Monte Carlo study
Authors: Barnett R. N. , Reynolds P. J. , Lester W. A.
Published: July 2002

The total nonrelativistic energies of the fluorine atom and its negative ion are calculated using the fixed-node quantum Monte Carlo (QMC) method. Over 90% of the correlation energy is obtained for both the neutral and the anion. Subtracting these energies yields an electron affinity of 3.45±0.11 eV, in excellent agreement with the recommended experimental value of 3.40 eV. The observed dependence of our Monte Carlo energies on the time step is discussed within the short-time QMC formalism. As in previous QMC studies in this series, only a single determinant, constructed with a small (double-zeta) basis set, multiplied by simple functions of electron–electron and electron–nuclear separation, is required as an importance function.
A systematic study of basis set effects in a
b i
n
i
t
i
o SCF calculations: Well-tempered Gaussian-type function basis set calculations on transition metal halides
Authors: Huzinaga Sigeru , Klobukowski Mariusz , Barandiarán Zoila , Seijo Luis
Published: July 2002

Geometric structure and one-electron properties were calculated from the SCF wave functions of ScF3, YF3, CuF, AgF, CuCl, and AgCl. The large well-tempered Gaussian basis sets were used in the SCF calculations. The results thought to be approaching the near-Hartree–Fock quality were compared with results of previous calculations in which smaller basis sets were used. The comparison indicates that most of the information on these transition metal compounds, obtainable from near-Hartree–Fock calculations, may be reliably predicted using the smaller basis sets.
Decoupled isotopomer vibrational frequencies in cubic ice: A simple unified view of the Fermi diads of decoupled H2O, HOD, and D2O
Authors: Devlin J. Paul , Wooldridge Paul J. , Ritzhaupt Gary
Published: July 2002

The existing infrared spectroscopic data for the isotopomers of water [H2O, D2O, (HOD)2, and HOD] decoupled in cubic ice at 90 K are reviewed and combined with new results to complete the infrared data for the internal vibrational modes. An assignment of the observed absorption bands, including the perturbed Fermi diads for νs in resonance with 2ν2, that largely follows established views is offered. This assignment is shown to be internally self-consistent by the analysis of the Fermi diads within a single framework based on the simplest representation of the effects of Fermi resonance and using the Burneau–Corset value for the Fermi-interaction parameter appropriate to cubic ice (60 cm−1 for H2O). It is shown that the inclusion of the decoupled-HOD diads significantly lowers the estimated value of νs for H2O and, consequently, allows the downshift of this mode to the observed frequency value (3225 cm−1), through resonance with 2ν2 (where ν2=1735 cm−1), to be closely modeled using the relatively small Fermi-interaction parameter of Burneau and Corset.
Theoretical study of the electric dipole moment function of the ClO molecule
Authors: Pettersson Lars G. M. , Langhoff Stephen R. , Chong Delano P.
Published: July 2002

The potential energy function and electric dipole moment function (EDMF) are computed for ClO X 2∏ using several different techniques to include electron correlation. The EDMF is used to compute Einstein coefficients, vibrational lifetimes, and dipole moments in higher vibrational levels. Remaining questions concerning the position of the maximum of the EDMF may be resolved through experimental measurement of dipole moments of higher vibrational levels. The band strength of the 1–0 fundamental transition is computed to be 12±2 cm−2 atm−1 in good agreement with three experimental values, but larger than a recent value of 5 cm−2 atm−1 determined from infrared heterodyne spectroscopy. The theoretical methods used include SCF, CASSCF, multireference singles plus doubles configuration interaction (MRCI) and contracted CI, coupled pair functional (CPF), and a modified version of the CPF method. The results obtained using the different methods are critically compared.
The reactions of mass selected aluminum cluster ions, Al+n (n=4–25), with oxygen
Authors: Jarrold Martin F. , Bower J. Eric
Published: July 2002

The reactions between aluminum cluster ions Al+n (n=4–25) with oxygen have been studied using a low energy ion beam apparatus at a center of mass collision energy of 1 eV. Some extraordinary reactions were observed. Oxygen very specifically cleaves the aluminum clusters, Al+n, resulting in Al+n−m cluster ions where m=4, 5 and for the larger clusters 10. Al+ is also an important product ion for clusters with n≤13. No oxygen containing product ions were observed. We suggest that addition of oxygen to the cluster raises the ionization potential and makes AlxO+ and AlxO+2 products energetically unfavorable, thus neutral oxide species are formed.
Optical emission and kinetic reactions of a four-atomic rare gas halide exciplex: Ar3F
Authors: Sauerbrey R. , Zhu Y. , Tittel F. K. , Wilson W. L.
Published: July 2002

A new broadband emission from electron beam excited high pressure Ar/F2 and Ar/NF3 mixtures at (435±50) nm is reported. This gas phase spectrum as well as a previously unassigned broadband emission at (455±55) nm observed in optically excited liquid Ar/F2 mixtures is assigned to the four-atomic rare gas halide exciplex Ar3F. Ar3F is produced by three-body collisions from Ar2F, but is easily destroyed by two-body collisions due to its small binding energy.
The argon melting curve to very high pressures
Authors: Zha C-S. , Boehler R. , Young D. A. , Ross M.
Published: July 2002

The melting curve of Ar has been measured to 717 K and 60 kbar using a new interferometric technique in a diamond anvil cell. Theoretical calculations are in excellent agreement with the measurements
Measurements of the conductivity and relaxation times for the micellar nematic phase of the system ammonium perfluorononanoate/H2O
Authors: Photinos P. J. , Saupe Alfred
Published: July 2002

Measurements of the electric conductivity for two mixtures of ammonium perfluorononanoate and H2O are presented. The molar ratios for the two mixtures are 0.0159 (sample S2) and 0.0112 (sample S3). The mixtures show a nematic (NL) range about 8 °C wide, between a lamellar smectic and an isotropic micellar solution. The conduction is predominantly due to the NH4+ ions, and it is estimated that about 50% of the NH+4 ions have free mobility. At the same reduced temperature in the NL phase, the lower concentration mixture is more anisotropic, indicating larger micelles for the lower concentration. The reorientation of the director of the NL phase in a magnetic field is also measured. Near the NL to isotropic transition the relaxation times at 10 kG are 30 s for S2 and 80 s for S3. The divergence behavior of the relaxation times at the NL to lamellar transition is different for the two samples. Also, at corresponding temperatures, the relaxation time for the lower concentration is longer than the high concentration mixture. It is also observed that in the lamellar phase, increasing the temperature may lead to undulation of the layers.
Photodissociation of molecules at structured metallic surfaces
Authors: Leung P. T. , George Thomas F.
Published: July 2002

Direct photodissociation of molecules at structured metallic surfaces is considered, where the concepts of optimal and critical distances are introduced into the description of this phenomenon. Numerical results for the distortion of the line shape and the enhancement ratio are obtained for a shallow sinusoidal grating.
Dynamics of the reaction O(1D2)+H2→OH(X 2Π,v″=2,3)+H: Full characterization of product energetics
Authors: Cleveland Cheryl B. , Jursich Gregory M. , Trolier Michael , Wiesenfeld John R.
Published: July 2002

The product OH(X 2Π) resulting from the subject reaction has been detected in v″=2 and v″=3 with full resolution of N″, f″, λ″ sublevels using LIF spectroscopy in the off-diagonal Δv=−2 sequence bands in the region 385–409 nm. As was noted previously for OH in the v″=0 and 1 states, strongly inverted rotational distributions were found; in the present case, efficient OH production was observed up to the available exoergicity of the reaction. Production of the π+ component was again seen to be significantly more probable than that of the π−. The ratio of summed populations in the two observed vibrational levels P(v″=3)/P(v″=2)=0.39±0.02 is smaller than that observed by other methods; experimental uncertainties in all methods used to date are discussed. The current observations are consistent with a mechanism in which O(1D2) inserts into the H–H bond to form a highly excited H–O–H complex which then dissociates. No evidence was obtained for a parallel process in which an H atom is directly abstracted by the excited oxygen atom.
Electronic structure of emeraldine and related molecules
Authors: Duke C. B. , Paton A. , Conwell E. M. , Salaneck W. R. , Lundström I.
Published: July 2002

The CNDO/S3 molecular orbital model is applied to describe the electronic structure of aniline, (C6H5)NH2, emeraldine, (C6H5)N(C6H4)N(C6H4)NH(C6H4)NH2, its hydrogenated and dehydrogenated forms, and its singly and doubly protonated forms. The cation eigenvalue spectrum, i.e., density of valence states (DOVS), is found to be sensitive to molecular conformation which, in turn, can be inferred from comparison of the calculated DOVS with suitable measured photoemission spectra. For fixed molecular conformation, the major effect of hydrogenation of emeraldine is the occupation of its lowest empty molecular orbital with accompanying electronic and atomic relaxation. Dehydrogenation leads to the emptying of its highest filled molecular orbital. Protonation leads to rearrangements of the valence electron charge among the filled orbitals but not to deviations from a closed shell system. Protonation-induced atomic relaxation can create major changes in the character of the highest filled or lowest empty orbitals. Changes in molecular conformation (e.g., upon hydrogenation or protonation) can yield large alterations in both the DOVS and the character of the radical cation eigenstates. Comparison of calculated DOVS with photoemission spectra from thin films of polyemeraldine does not suffice to determine the molecular conformation, however, because of the low resolution of the measured spectra.
Four-photon selection rules and line strengths in symmetric and spherical-top molecules and time-reversal invariance
Authors: Chiu Ying-Nan
Published: July 2002

Irreducible spherical tensor method is used to derive the angular momentum selection rules and rotational line strengths in the three- and four-photon transition of random symmetric-top and spherical-top molecules. Generally different as well as specifically identical photons are considered, with attention to the boson character of the latter case. It is shown from symmetry that for three identical photons of the same linear polarization, only 13 out of 27 transition operators exist for the triple product of three transition dipole ‘‘vectors.’’ Similarly only 28 components exist out of the 81 possible for the four-photon case. Warning is given against the use of nonexistent transition operators to connect spectroscopic states. The explicit expression for the different tensors of three- and four-photon operators are worked out and tabulated. The point-group selection rules for molecules of D6h, D3h, Oh, and Td symmetries are also deduced and tabulated for these three- and four-photon operators that exist. For four-photon transitions involving degenerate initial and final states, the ‘‘hidden’’ permutational selection rule imposed by time reversal invariance is also derived. It was found that for a four-photon fourth-order electric dipole transition, no hidden selection rule exists, because the permutational factor (±i)8N is even, regardless of the number (N) of electrons. However, for a special four-photon, third-order process the permutational factor was found to be (±i)6N and dictates a selection rule of even permutation of degenerate state product for even number and odd for odd number of electrons.
Collisional quenching and energy transfer in NS B 2Π
Authors: Jeffries Jay B. , Crosley David R.
Published: July 2002

Total collisional removal rate constants kd for the B 2Π excited electronic state of the NS free radical are measured for several collider molecules. For the lowest vibrational level (v′=0), kd is the electronic quenching rate constant; and for the vibrationally excited v′=1 and v′=6 levels, kd is the sum of those for electronic quenching and vibrational relaxation. Nitrogen sulfide free radicals are produced in a discharge flow reactor and the B 2Π state is prepared and monitored by laser-induced fluorescence. Measurements are made for nine different collision partners: He, N2, O2, SF6, N2O, H2, CH4, CO2, and NH3. Except for NH3, the thermally averaged quenching cross sections at room temperature are less than 10 Å2; this is a small value for electronic quenching of open shell diatomic radicals. For v′=6, kd is smaller than for v′=1 for N2, SF6, N2O, and CO2, but larger for H2, O2, and He. The vibrational relaxation pathway from v′=1 to v′=0 constitutes about 25% of the total v′=1 collisional removal for O2, N2O, and SF6.
Three-dimensional analytical quantum mechanical theory for triatomic photodissociation: Role of angle dependent dissociative surfaces on rotational and angular distributions in the rotational infinite order sudden limit
Authors: Grinberg Horacio , Freed Karl F. , Williams Carl J.
Published: July 2002

An analytical quantum mechanical model is developed for calculating fragment energy distributions in photodissociation of linear triatomic molecules when the repulsive potential energy surface is anisotropic. The translational continuum function is taken to be given by the infinite order sudden approximation, but an equivalent adiabatic interpretation leads to a natural choice for the diatomic angular momentum j and for the retention of asymptotic rotational energy differences. Normal coordinates are used for the initial bound state before dissociation, while scattering coordinates are applied for wave functions on the dissociative surface. These natural choices lead to three-dimensional nonseparable bound–continuum transition amplitudes. The translational continuum wave function is further approximated using Airy functions, and additional approximations are introduced based on the presence of small amplitude vibrations in the initial bound state. The three-dimensional transition amplitudes are then analytically reduced to sums of one-dimensional quadratures. The theory has been applied to the photodissociation of several rovibronic states of N2O+(Ã 2∑+) (predissociation) and ICN(C̃ 1A′) (direct photodissociation), and the rotational distributions for J=0 are in good agreement with three-dimensional close-coupled calculations except when the potentials become highly anisotropic. Our photodissociation infinite order sudden approximation is tested against various versions of the rotational infinite order sudden approximation for N2O+ and are found to be in good agreement with previous results. The present theory readily permits calculations for J>0 and may be used for the calculation of rotational distributions for excited rotational and/or vibrational states. In the limit of isotropic potentials the remaining integrals are evaluated to provide analytical approximations for the transition amplitudes.
Lack of translational energy activation of the dissociative chemisorption of CO on Ni(111)
Authors: Lee M. B. , Beckerle J. D. , Tang S. L. , Ceyer S. T.
Published: July 2002

The activation of the dissociative chemisorption of CO on Ni(111) by translational and vibrational energy is probed. Molecular beam techniques produce CO molecules with high kinetic energies and with some vibrational excitation. Thermal desorption and high resolution electron energy loss spectroscopy detect the product of the chemisorption event. The maximum translational and vibrational energies attainable in these experiments, 45 and 18 kcal/mol, respectively, are observed not to activate the dissociative chemisorption of CO. These experiments are sensitive to dissociation probabilities as small as 2×10−6 and 9×10−4 at the maximum values of translational and vibrational energy, respectively. It is concluded that translational energies greater than 45 kcal/mol do not contribute to the CO dissociation rate at high pressures. Rather, the potential energy surface of the CO–Ni(111) interaction likely requires vibrational excitation greater than the amount that can be achieved in this experiment for activation of the C 3/4 O bond.
Reactive scattering from double minimum potentials: Li+ catalyzed elimination reactions of alkyl halides
Authors: Creasy W. R. , Farrar J. M.
Published: July 2002

We present a study of the dehydrohalogenation reactions of Li+ with isopropyl chloride (IPC), n-propyl chloride (NPC), and isopropyl bromide (IPB) over the collision energy range from 0.6 to 1.7 eV. Full differential cross sections at collision energies up to 1.2 eV show increasing asymmetry in the angular distributions as a function of initial collision energy and suggest that the reactions take place on a time scale of 0.2 ps. The observation of Li+ nonreactively ejected from the initial encounter complex formed by approaching reactants is consistent with a double minimum reaction coordinate and a comparison of the nonreactive flux with the flux for elimination products allows an estimate of the heights of intermediate isomerization barriers separating the wells on the potential surface. The barrier heights are in the order IPC<IPB<NPC, consistent with a transition state at the isomerization barrier in which a significant charge has developed on the carbon attached to the halogen. The kinetic energy distributions for Li+(HX) and Li+(C3H6) products and nonreactively scattered Li+ are in reasonable agreement with phase space theory calculations which include all product vibrational modes. The energy dependence of the Li+(HX)/Li+(C3H6) branching ratio can be accounted for quantitatively by statistical calculations which include the increasing importance of dissociation of the products with increasing collision energy. The discrepancy between the statistical recoil energy distributions and apparent complex lifetimes several orders of magnitude shorter than the predictions of RRKM theory can be understood in terms of incomplete transfer of energy from the incident Li+ to the internal degrees of freedom of the initial encounter complex, resulting in a density of complex states much lower than statistical.
Density functional theory of freezing: Analysis of crystal density
Authors: Laird Brian B. , McCoy John D. , Haymet A. D. J.
Published: July 2002

The density functional theory of freezing is used to study the liquid to crystal phase transition in the hardsphere and Lennard-Jones systems. An important step in the calculation is the parametrization of the solid phase average single particle density ρ(r). In this work two popular parametrizations are compared. The first method is a general Fourier decomposition of the periodic solid density in which the amplitude of each (non-symmetry-related) Fourier component is treated as an independent parameter. The second parametrization, which is more restrictive but easier to implement, approximates the solid density as a sum of Gaussian peaks centered at the sites of a periodic lattice. The two methods give essentially identical results for the phase diagrams for the two systems studied, but the crystal density predicted by the Fourier method exhibits significant anisotropies which are excluded from the Gaussian representation by construction.