bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of three-dimensional grain boundary topography in a YBa2Cu3O7−d thin film bicrystal grown on a SrTiO3 substrate
Authors: Ayache J. , Thorel A. , Lesueur J. , Dahmen U.
Published: July 2002

The topography and crystallography of YBa2Cu3O7−d (YBCO) bicrystal films grown epitaxially on oriented SrTiO3 (STO) bicrystals have been characterized by scanning and transmission electron microscopies (SEM and TEM) and atomic force microscopy (AFM). The YBCO films were formed by laser ablation on melt-grown Σ13 STO bicrystals with a misorientation of 24° around the 〈001〉 tilt axis. In agreement with previous reports, TEM analysis revealed that the grain boundary in the film did not always follow the planar substrate grain boundary faithfully, but undulated about the average boundary plane. High resolution electron microscopy observations of the apparently complex undulating boundary structures could be explained as a result of an overlap between different orientation variants of the orthorhombic YBCO film. Cross correlation between SEM, AFM, and TEM imaging gave a clear evidence that an island growth mechanism is responsible for the observed grain boundary structure and morphology for which a schematic model is presented. It is seen that meandering of the YBCO grain boundary (GB) is necessarily coupled to a wide range of inclination of the GB plane in the z direction. The implications of this interfacial structure for the behavior of GB based Josephson junctions are discussed and compared to models proposed in the literature. It is also seen that inclination of the GB may be responsible for the poor correlation usually found in the literature between calculations and experimental curves of current density Jc versus the GB angle since the most elaborate models proposed up to now take into account only pure tilt GB plane facets, that is to say facets in the zone of the tilt axis. Moreover, such a GB structure may affect the interpretation of recent phase sensitive experiments done on bicrystal or tricrystal high Tc superconductors to determine the symmetry of the order parameter.
A methodology for automated quantitative microstructural analysis of transmission electron micrographs
Authors: Carpenter D. T. , Rickman J. M. , Barmak K.
Published: July 2002

It is generally recognized that proper quantification of microstructural behavior is necessary for the optimization of materials properties. In the specific case of polycrystalline thin films, transmission electron microscopy (TEM) is required for microstructural interrogation due to the small (nm–μm) inherent length scales in these systems. While a meaningful study requires large grain populations, typical data sets are relatively small due to the need for human interpretation of the contrast in TEM micrographs. To overcome this limitation, a general methodology has been developed to fully automate the grain boundary identification procedure by using a combination of both conventional and newly developed image processing algorithms to extract and combine information from multiple, optimized TEM micrographs. This technique has been validated by systematically analyzing microstructures of thin Al films, as obtained from TEM micrographs, and comparing these results with those obtained by a conventional, manual approach. Indeed, a statistical analysis shows that the agreement between these two methods is quite good. We further show that based upon a large population (8185 grains) one can estimate the number of grains required to draw meaningful conclusions about this microstructure. While the emphasis of this work is on TEM image processing, the techniques developed here are expected to be sufficiently general and flexible so as to be applicable to other (e.g., focused-ion beam) microscopies.
Series resistance and its effect on the maximum output power of 1.5 μm strained-layer multiple-quantum-well ridge waveguide InGaAsP lasers
Authors: Elenkrig B. B. , Smetona S. , Simmons J. G. , Takasaki B. , Evans J. D. , Makino T.
Published: July 2002

The series resistance of InGaAsP/InP multiple quantum well ridge waveguide laser diodes is investigated experimentally over a wide temperature range for both Fabry–Perot and distributed feedback type lasers. From the temperature dependence of the series resistance is found that it is defined mostly by the resistance of heterobarriers, although the semiconductor bulk resistance is also found to be a substantial part of ridge waveguide laser series resistance. The effect of the laser series resistance on other performance characteristics also investigated. The theoretically predicted strong correlation between the series resistance and laser maximum operating power is confirmed experimentally. The ways of reducing of series resistance are discussed.
Oscillatory exchange bias effect in FeNi/Cu/FeMn and FeNi/Cr/FeMn trilayer systems
Authors: Mewes T. , Roos B. F. P. , Demokritov S. O. , Hillebrands B.
Published: July 2002

The first experimental observation of a spacer-thickness dependent oscillatory exchange bias effect in ferromagnet(FM)/spacer/antiferromagnet trilayers is reported. The period of the oscillatory exchange bias field is found to be half of the period of the oscillatory interlayer coupling in the corresponding FM/spacer/FM systems with the same spacer, indicating that the observed effect is caused by an analogous coupling mechanism, being, however, sensitive to the absolute value of the coupling strength and not on its sign.
Optical absorption in PbSe spherical quantum dots embedded in glass matrix
Authors: Andreev A. D. , Kolobkova E. V. , Lipovskii A. A.
Published: July 2002

A phosphate glass embedded with PbSe semiconductor quantum dots (QDs) has been synthesized and investigated. Optical absorption spectra of PbSe spherical QDs have been studied both theoretically and experimentally. Theoretical analysis of the energy spectra and optical transitions has been carried out in the framework of four-band k⋅p model taking full account of anisotropy effects. It is demonstrated that the strong anisotropy of the energy bands of bulk PbSe gives rise to additional optical transitions in the quantum dots. These anisotropy-induced optical transitions have been clearly observed in the measured absorption spectra of the PbSe quantum dots. The calculated energy positions of the optical transitions are in good agreement with experiment. Five optical transitions were distinguished and interpreted in the absorption spectra measured at T=12 K. For the set of samples with a narrow size distribution of PbSe QDs (diameters in range of 40–110 Å), the energies of three or four optical transitions have been determined from the measured absorption spectra at T=300 K. The calculated dependencies of the energies of the first five optical transitions with QD size are in good agreement with those obtained from the experimental data.
On the temperature dependence of the electrical and optical properties of Cu2GeSe3
Authors: Marcano G. , Bracho D. B. , Rincón C. , Pérez G. Sánchez , Nieves L.
Published: July 2002

The Hall effect and electrical resistivity measurements on p-type Cu2GeSe3 crystals were measured in the temperature range from 80 to 300 K. The temperature variation of the hole concentration p from about 200 to 300 K is explained as due to the thermal activation of a shallow acceptor level with an ionization energy of around 50 meV. At low temperatures the impurity band conduction dominates the electrical transport processes. From the analysis of the p vs T data, the density-of-states hole effective mass is estimated to be of the same magnitude as the free electron mass. The temperature variation of the hole mobility in the valence band is analyzed by taking into account the scattering of charge carriers by ionized impurities and acoustic phonons. In the impurity band, the mobility is explained as due to thermally activated hopping transport. The optical absorption coefficient spectrum shows the presence of three absorption narrow bands below the fundamental gap. From the analysis of their temperature dependence, these bands are attributed as due to free–to–bound transitions related to intrinsic defect acceptor states. Activation energies of these states are estimated to be around 0.12, 0.24, and 0.30 eV. Tentative assignment of the nature and origin of these defect states were also made.
Monte Carlo simulation of electron thermalization in gases. V. Subexcitation electrons in rare gases
Authors: Koura Katsuhisa
Published: July 2002

The thermalization of subexcitation electrons in rare gases is studied by the Monte Carlo simulation (MCS). The electron velocity distribution is found to deviate significantly from the local Maxwellian distribution (MD) even for the initial Maxwellian distribution. Comparisons with available experimental results of the electron energy degradation as well as the electron thermalization time are made. A considerable discrepancy between the theoretical results obtained by the MCS and the approximate theory based on the MD assumption and the experimental results is revealed especially for rare gases with the Ramsauer minimum. The measured electron energy degradation is likely to be much less sensitive to the Ramsauer minimum than the theoretical one.
Fractal behavior in classical collisional energy transfer
Authors: Noid D. W. , Gray Stephen K. , Rice Stuart A.
Published: July 2002

A plot of final vibrational action vs initial vibrational phase for a two degree of freedom model for He + I2 collisions is examined and shown to exhibit a chattering region. The chattering region is shown to exhibit a very intricate structure, including an infinite number of regions of regularity which we term icicles. Structure is evident on all scales of examination and a fractal dimension close to 2 is obtained for some parts of the chattering region. The survival probability associated with the complexes shows an initial fast decay, due to the icicles, but can be roughly characterized over a longer and wider time range by a more slowly decaying exponential.
Exciton and vibronic effects in the spectroscopy of bianthracene in supersonic beams
Authors: Khundkar Lutfur R. , Zewail Ahmed H.
Published: July 2002

Excitation and dispersed fluorescence spectra of 9,9′-bianthracene in a supersonic expansion are reported. The spectra are anthracene-like, indicating that the rings are weakly coupled. Exciton effects are considered in the interpretation of the spectra. The torsional potential in S1 is modeled as a double-well (Gaussian perturbation on a one-dimensional harmonic oscillator) with barriers to perpendicularity and planarity of ∼30 and ∼1100 cm−1, respectively. The S0 torsional potential shows negative anharmonicity which is modeled as a quartic perturbation. Anthracenic modes in S1 and S0 are also assigned. Finally, measurements of S1 fluorescence lifetimes up to ∼6000 cm−1 excess energy in the excited state show no evidence of charge transfer.
Nonadditivity of a
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o pair potentials for molecular dynamics of multivalent transition metal ions in water
Authors: Curtiss Larry A. , Halley J. Woods , Hautman Joseph , Rahman Aneesur
Published: July 2002

A b initio molecular orbital pair potentials for the interaction of Fe2+ and Fe3+ ions with H2O are reported. Molecular dynamics calculations of the static structure of the solvation shell of Fe2+ and Fe3+ in water using the ab initio pair potentials gives physically incorrect results, i.e., the coordination numbers are eight instead of six as observed experimentally. This problem has also been encountered by other workers for divalent transition metal ions in water. By computing three-body energies from the interaction of two water molecules with the cations, we show that the origin of the problem is most likely in the assumption of the additivity of the pair potentials, i.e., neglect of many-body forces. Empirical potentials are reported which take approximate account of the three-body forces and give coordination numbers of six for both Fe2+ and Fe3+ in water.
A classical trajectory study of the high CH and CD overtones in benzene and perdeuterobenzene
Authors: Clarke David L. , Collins Michael A.
Published: July 2002

The intermolecular vibrational energy redistribution in high overtones of CH(D) stretching modes in benzene and perdeuterobenzene has been studied using classical trajectories. Using the exact kinetic energy and the best available potential energy surface for planar benzene, the rapid decay of these local modes is calculated to be in reasonable quantitative agreement with experimental lifetimes. The long time dynamics of benzene is found to be characterized by periodic energy transfer between CH stretching local modes via a 2:1 Fermi resonance with normal ring modes. Preferential energy transfer between para CH local modes is observed.
Density dependence of attractive forces for hydrogen stretching vibrations of molecules in compressed liquids
Authors: Zakin M. R. , Herschbach D. R.
Published: July 2002

An empirical analysis of pressure-induced shifts for quasidiatomic C–H or O–H stretching vibrations in solution shows that the portion attributable to attractive forces exhibits an approximately quadratic dependence on the solvent density, with exponents in the range sA=2.1±0.3 for ten systems. This is much stronger than the linear density dependence (sA=1) postulated for attractive forces by Schweizer and Chandler. Theoretical estimates of the attractive part of the frequency shifts are derived by averaging attractive R−n potentials (n≥6) using the hard-sphere radial distribution function. This yields a simple, explicit formula with an exponential dependence on density. We find attractive exponents of sA∼2.0±0.1 for the experimental density regime, whereas sA→1 for the zero-density limit. The sA values at high density are insensitive to the potential exponent n and comparable to the repulsive exponents sR∼3.5 obtained from the hard-sphere model. The density dependence for attractive as well as repulsive contributions thus is governed chiefly by packing effects and is only mildly affected by the form of the intermolecular potential. This situation permits attractive potential parameters to be evaluated by inverting experimental data on pressure-induced vibrational frequency shifts.
A variational localized representation calculation of the vibrational levels of the water molecule up to 27 000 cm−1
Authors: Bačić Z. , Watt D. , Light J. C.
Published: July 2002

We have calculated variationally highly excited vibrational (J=0) levels of the water molecule up to ∼27 000 cm−1 (relative to the minimum of the potential surface), for a global Sorbie–Murrell-type potential surface. The calculation has been performed in Radau coordinates, using the recently developed DVR-DGB variational approach [Z. Bačić and J. C. Light, J. Chem. Phys. 85, 4594 (1986); 86, 3065 (1987)]. 110 symmetric and 77 antisymmetric vibrational levels have been determined accurately, requiring diagonalization of relatively small Hamiltonian matrices of dimension ∼600. Many of the calculated levels correspond to large amplitude bending vibrations. Nearest neighbor level spacing statistics for the calculated levels above 18 000–20 000 cm−1 conform closely to a Wigner distribution, suggesting classically chaotic behavior in this energy range. Convergence rates of these variational calculations for H2O are comparable to those seen earlier for LiCN/LiNC and HCN/HNC. The DVR-based vibrationally adiabatic approach introduced by Light and Bačić [J. Chem. Phys. 87, 4008 (1987)] has also been tested here. Perturbative inclusion of the nonadiabatic corrections has allowed reliable identification of vibrational (J=0) levels of H2O up to 18 000–20 000 cm−1. With this model potential energy surface, reasonable agreement (∼1%) is obtained with experimentally known vibrational states to ∼20 000 cm−1.
The vibrational spectra (100–1600 cm−1) and scaled a
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o STO-3G and 3-21G harmonic force fields for norbornane, norbornene, and norbornadiene
Authors: Shaw R. A. , Castro C. , Dutler R. , Rauk A. , Wieser H.
Published: July 2002

The vibrational spectra in the region of 100–1600 cm−1 of norbornane and norbornadiene are reinvestigated, and those of norbornene reported for the first time. On the basis of 3-21G ab initio force fields, evaluated for each molecule and scaled using overlay refinements of 12 scaling factors, the spectra are assigned and the observed transition frequencies reproduced with an overall average error of 6.2 cm−1. STO-3G force constants, modified to correct for deficiencies in the stretch/bend interaction force constants but otherwise scaled analogously to 3-21G, reproduce the final set of assignments to within 10.1 cm−1 on average. The STO-3G basis is shown to be suitable as a less costly alternative to split-valence basis sets, particularly for describing the vibrational dynamics of low frequency ring bending and torsional modes.
Theory of geminate recombination on a lattice. IV. Results using large Coulomb radii on the simple cubic and square lattices
Authors: Rackovsky S. , Scher H.
Published: July 2002

We have expanded the molecular theory of geminate recombination of charge-carrier pairs to the consideration of arbitrary Coulomb cutoff lengths. The results on the simple cubic and square lattices exhibit behavior typical of that observed experimentally. This behavior is controlled by a set of molecular parameters as well as the initial distribution of the electron about the hole. It is shown also that the temperature dependence in the infinite-sink limit is qualitatively Onsager-like, but does not quantitatively match the predictions of the Onsager formulation, reflecting previously noted deficiencies in the continuum approximation. It is also shown that the two parameters φ0 and r0 of the Onsager model cannot, in general, be regarded as giving the actual, physical initial quantum efficiency and initial separation of the charge carriers, respectively. In the course of these calculations, methods are developed for calculating two- and three-dimensional lattice Green’s functions in the presence of an external electric field. Analytical and numerical methods are established, as well as recursion relations useful in calculation. A relationship is exhibited and utilized between two- and three-dimensional field-dependent Green’s functions.
High temperature thermodynamic studies of some gaseous thorium fluorides
Authors: Lau K. H. , Brittain R. D. , Hildenbrand D. L.
Published: July 2002

The gaseous species ThF, ThF2, ThF3, and ThF4 were generated in a heated effusion beam source, and were identified and characterized by mass spectrometry. An oxygen impurity in the sample also led to the characterization of the gaseous oxyfluoride ThOF. Reaction equilibria involving these species were studied over large temperature ranges, leading to the derivation of both enthalpy and entropy data for these species. The vapor pressure and vaporization thermodynamics of ThF4(s) were also determined. Derived bond dissociation energies in ThF4 oscillate about a mean value of 670±25 kJ mol−1, with a distinct pattern similar to other metal tetrafluorides. Absolute entropies calculated from the equilibrium data indicate moderate electronic level contributions for the odd-electron molecules ThF and ThF3, but not for the even-electron species ThF2 and ThF4. Furthermore, the experimental entropy of ThF4 is compatible with a regular tetrahedral configuration, rather than the distorted configuration of lower symmetry found earlier for UF4 and UCl4. Thermochemical properties of the Th–F species are presented and the comparison with the corresponding uranium fluorides is discussed.
Structure and dynamics of carbon dioxide clusters: A molecular dynamics study
Authors: Cardini Gianni , Schettino Vincenzo , Klein Michael L.
Published: July 2002

Molecular dynamics calculations have been used to explore the structure and dynamics of clusters of carbon dioxide, ranging in size up to 164 molecules. The most detailed calculations have been carried out around T=90 K with a view to interpreting the results of infrared data on clusters produced in a seeded argon beam. Under appropriate conditions, in addition to liquid-like clusters, we have been able to produce clusters with the following structures: (i) an ordered solid—analogous to the bulk crystal, (ii) a bulk–solid core with liquid-like outer layers, (iii) an amorphous solid, and (iv) an amorphous solid core with liquid-like outer layers. We have calculated the dispersion of the infrared active intramolecular Q3 mode for these clusters using the transition-dipole transition-dipole interaction that is usually invoked to explain the infrared spectrum of the bulk crystal. The resulting spectrum for the Q3 mode of each type of cluster is quite distinct in appearance. This observation suggests that molecular dynamics calculations could be a valuable diagnostic tool in the study of clusters. The dynamical behavior of these clusters has also been investigated, and we find evidence for the existence of large-amplitude, low-frequency symmetric breathing modes with frequencies in the range 2–4 cm−1. The overall shape of the experimental Fourier transform infrared spectra of clusters produced in a seeded argon jet is very similar in appearance to the calculated spectrum for a cluster of 116 molecules with an ordered solid core and liquid-like outer layers.
Phase separation dynamics of polycarbonate/polymethyl methacrylate blends. I. Temperature jumps into an immiscibility loop
Authors: Lim Dong-Soo , Kyu Thein
Published: July 2002

Time-resolved light scattering was employed to elucidate the dynamics of phase separation in mixtures of polycarbonate (PC) and polymethyl methacrylate (PMMA). Blends of PC and PMMA of moderately low molecular weight reveal the coexistence of an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST), forming an immiscibility loop. Temperature jump (T jump) experiments were conducted on the 40/60 PC/PMMA composition from a single phase (150 °C) to a two-phase immiscibility loop region (180 and 190 °C). Phase separation takes place through spinodal decomposition (SD). The early stage of SD is explicable in terms of the linearized Cahn–Hilliard theory. The late stage of SD follows the power law relationship with the exponents of −1/3 and 1 as predicted by the cluster theory of Binder and Stauffer. The universal curve with reduced variables Qm and τ was established. The late stage data were analyzed in accordance with the recent scaling law of Furukawa. The temporal phase decomposition appears to be self-similar. The shape of the structure function can be scaled with slopes of 2 and −6 for qqm regions, respectively.
Evidence for anisotropic vibration of diatomic adsorbates—NO and CO chemisorbed on stepped Pt(112)
Authors: Szabò A. , Henderson M. A. , Yates J. T.
Published: July 2002

Both NO and CO preferentially chemisorb on step sites of the Pt(112) crystal orienting the intermolecular bond in the downstairs direction. Using the digital electron stimulated desorption-ion angular distribution method (ESDIAD), an elliptical angular distribution of the desorbing O+ ions was detected from the NO/Pt(112) system, with the longer axis of the ellipse normal to the step-edge direction. On the other hand, the O+ ESDIAD pattern from the CO/Pt(112) system shows an approximately cylindrical symmetric shape. Heating of the crystal leads to broadening of the ion desorption patterns in both cases without change in the patterns’ elliptical or circular cross-sectional geometry. These results are interpreted as being due to ion desorption from NO molecules bonded to two Pt atoms on the step edge and vibrating with a longer amplitude in the direction perpendicular to the step. In the case of CO, bonded to a single Pt atom, the amplitude of vibration is approximately the same in directions parallel and perpendicular to the step edge. Thus, in certain cases ESDIAD patterns may be used to determine the hybridization state of adsorbates.
Infrared spectroscopy of CO on NaCl(100). II. Vibrational dephasing and band shapes
Authors: Noda Chifuru , Richardson Hugh H. , Ewing George E.
Published: July 2002

Temperature dependence of the fundamental vibrational transition of CO on NaCl(100) single crystal surfaces has been measured between 4 and 55 K using Fourier-transform infrared spectroscopy. The absorption profiles at various temperatures are very nearly symmetric. At 4 K, the transition is centered at 2154.93 cm−1, and the full-width at half-maximum (FWHM) is 0.17 cm−1; at 55 K, the transition shifts to 2155.14 cm−1, and the FWHM broadens to 0.39 cm−1. The temperature-dependent frequency shift and broadening of the fundamental transition are interpreted by a vibrational phase relaxation process, in which the coupling between the stretching mode of CO and a low frequency CO-surface mode causes the dephasing of the excited state vibrational motion. The Boltzmann factor associated with the spectroscopic temperature dependence allows us to determine that the surface mode has a frequency of 40 cm−1 . We associate this mode with the frustrated translational motion of CO over the NaCl(100) surface. The observed temperature-dependent absorption band undoubtedly is a consequence of collective motions among the CO stretching vibrations within the monolayer. However, the current model, that assumes this collective motion is coupled by the CO transition dipoles, cannot explain the data. Further theoretical work will be required to understand the infrared spectroscopy of CO on NaCl(100). Below 10 K, the CO fundamental absorption becomes temperature independent. However, the limiting bandwidth and bandshape depend on the NaCl(100) crystal surface preparation. For one set of crystals, the 4 K bandshape was Lorentzian with a FWHM of 0.17 cm−1 . For another set, the 4 K bandshape was Gaussian with a FWHM of 0.11 cm−1. We believe these bandwidths, narrower than those reported previously for any adsorbate system, are limited by heterogeneities of the monolayer and/or substrate. The 0.11 cm−1 bandwidth provides a lower limit of 45 ps for the lifetime of vibrationally excited CO on NaCl(100).
Kinetic oscillations in the NO + CO reaction on Pt(100): Experiments and mathematical modeling
Authors: Fink Th. , Dath J.-P. , Imbihl R. , Ertl G.
Published: July 2002

The reaction of NO and CO on Pt(100) exhibits two branches of steady state production of N2 and CO2 and the occurrence of kinetic oscillations. This system was studied under steady flow conditions in the 10−6 mbar total pressure range using low-energy electron diffraction-(LEED), work function measurement, and mass spectrometry for determination of the reaction rate. These studies revealed that kinetic oscillations can only be initiated from one of the two stable reaction branches. Two separate existence regions were detected in which the oscillations are always damped. Oscillations can be very reproducibly excited by slight decreases in temperature. The 1×1■hex phase transition of the surface structure was observed to take place only in one of the two regions of reaction rate oscillations. Its influence seems to be of minor relevance to the mechanism of oscillations as oscillations in one region occur on the surface that maintains a 1×1 structure. The experiments were modeled by a set of coupled differential equations based on knowledge about the elementary reaction steps. The model calculations reproduced the steady states of the reaction as well as the occurrence of kinetic oscillations in different ranges in excellent agreement with experimental observation. In the model, the phase transition also has no relevance for the oscillation mechanism. The occurrence of oscillations can be rationalized in terms of a periodic sequence of autocatalytic ‘‘surface explosions’’ and the restoration of an adsorbate-covered surface. The damping, experimentally observed, is attributed to insufficient spatial coupling between different regions of the surface.
High-resolution infrared spectroscopy of 2-fluoroethanol in a molecular beam
Authors: Brummel Christopher L. , Mork Steven W. , Philips Laura A.
Published: July 2002

The high-resolution infrared spectrum of 2-fluoroethanol (2FE) in a molecular beam was obtained in the region of 2990–2977 cm−1. This spectral region contains the asymmetric CH stretch of the fluorinated carbon. Excitation of the CH stretch has previously been observed to photochemically isomerize 2FE from the Gg′ to the Tt conformation. The high-resolution spectrum of this transition provides the information necessary to quantitatively evaluate the amount of vibrational mode-coupling between the asymmetric CH stretch and the torsional reactive coordinate. Minimal amounts of vibrational mode coupling were observed in the spectrum which is consistent with the slow photoisomerization rate. The correlation of the amount of mode coupling and the isomerization rate supports the conclusion that mode-selective vibrational coupling plays an important role in the photochemical dynamics. It is further suggested that the strong intramolecular attractive interactions limit the magnitude of the vibrational mode coupling.
Infrared laser absorption spectroscopy of the SiH+3 cation
Authors: Smith D. M. , Martineau P. M. , Davies P. B.
Published: July 2002

Many vibration–rotation components of the ν2 fundamental of the SiH+3 cation have been measured using diode laser velocity modulation spectroscopy. By comparing the intensities of these components with their nuclear statistical weights, the ion is shown to be planar (D3h). The position of the ν2 band center is 838.0674(7) cm−1 and the ground state rotational constant is B0=5.2153(1) cm−1, r0=0.1462 nm. These results are compared with ab initio calculations.
The interaction of CO with Ni(111): Rainbows and rotational trapping
Authors: Hines Melissa A. , Zare Richard N.
Published: July 2002

Angularly resolved rotational state distributions of CO scattered and desorbed from a clean, single-crystal Ni(111) surface were measured using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered from the surface displayed highly non-Boltzmann rotational distributions that varied with incident translational energy and detection angle, but not with surface temperature. A rotational rainbow was seen in the scattering distribution and interpreted as arising from the interaction of the weakly attractive O end of the CO molecule with the Ni(111) surface. Up to total rotational-to-translational energy conversion was seen at incident translational energies of 0.26–0.45 eV. This energetic cutoff was the result of rotational trapping and was caused by the strongly attractive interaction of the C end of the molecule with the surface. The rotational state distributions of molecules desorbed from the Ni(111) surface were well fit by Boltzmann distributions each with a temperature which is 0.82±0.08 of the surface temperature.
Study of the viscoelastic properties of the metallorganic palladium mesogens Azpac and Azpac2 by photons self-beating spectroscopy
Authors: Versace C. , Formoso V. , Lucchetta D. , Pucci D. , Ferrero C. , Ghedini M. , Bartolino R.
Published: July 2002

We report on the first measurements of light scattering studies of the viscoelastic properties of two new metallorganic nematic liquid crystals and of their parent ligand 4–4′–bis(hexyloxy)–azoxybenzene. When the magnetic properties of the liquid crystal are not known, light scattering is an efficient technique to get qualitative and quantitative information on the elastic constants and viscosities. The experimental results are discussed in the light of the molecular structure of the compounds.
Subpicosecond pump–probe measurements of the electronic relaxation rates of the S1 states of azulene and related compounds in polar and nonpolar solvents
Authors: Wagner Brian D. , Szymanski Marian , Steer Ronald P.
Published: July 2002

The lifetimes of the S1 states of azulene, azulene-d8, 4,6,8-trimethylazulene, and guaiazulene (1,4-dimethyl-7-isopropylazulene) have been measured in three nonviscous solvents of different polarity and structure using a two-photon, two-color, pump–probe method with subpicosecond time resolution. A significant solvent effect is measured. The rate constants for S1■S0 internal conversion in all four compounds in all three solvents exhibit one common S1–S0 energy gap law correlation, indicating that variations in the electronic relaxation rates are governed exclusively by changes in the Franck–Condon factors for the transition. No effect is observed when the exciting wavelength is changed, indicating that vibrational relaxation is occurring on a time scale which is faster than that of electronic relaxation in these systems. No significant deuterium isotope effect is measured in azulene, indicating that high frequency C–H(D) stretching vibrations do not act as significant accepting modes in the radiationless transition.
Electron localization in metal–molten salt solutions: An optical study with in situ variation of composition
Authors: Nattland D. , Rauch Th. , Freyland W.
Published: July 2002

Optical absorption spectra are reported for liquid KxKCl1−x (10−5≤x≤5×10−2) and NaxNaBr1−x (10−3≤x≤3×10−2) solutions at 800 °C in the energy range 0.5≤hν≤5 eV. The composition has been varied in situ by Coulometric titration of the metal. With this method of simultaneous measurement of optical spectra and metal activity relative changes in the spectra are detected with high precision. In this way it has been possible, for the first time, to separate distinct components in the optical spectra of metal solutions. A contribution which is quantitatively described by the Drude model is attributed to weakly localized electrons in conduction band tail states with low mobility. The determined electronic mobilities are of the order of 0.1 cm2 V−1 s−1. The concentration dependence of the corresponding electronic conductivity quantitatively agrees with independent measurements of the dc-electronic conductivity. The main component in the absorption spectra is due to excitations of localized electrons which are different in character in the KxKCl1−x and NaxNaBr1−x melts. This is concluded from the distinct concentration dependence and is discussed in the light of quantum molecular dynamics calculations and further results of the electronic properties.
Diffusion of atoms and dimers on metal surfaces
Authors: Ruckenstein Eli , Tsekov Roumen
Published: July 2002

Experiments from literature have shown that the diffusion coefficients of dimers of rhenium atoms on tungsten are larger than those of the single rhenium atoms for the cross-channel configuration of the dimer and smaller for the in-channel configuration. In the present paper, expressions are derived for the diffusion coefficient which explain the above observations. They are based on Langevin equations of the motion of a single atom and dimer on a metal surface, in which the relation derived previously by the authors for the friction coefficient is employed. It is also demonstrated that the diffusion coefficient of the dimers exhibits a periodical dependence upon the ratio between the dimer length and the metal lattice parameter, phenomenon which is called resonant diffusion.
A supersonic molecular beam spectroscopic study of platinum monocarbide, PtC
Authors: Steimle T. C. , Jung K. Y. , Li B.-Z.
Published: July 2002

The gaseous products generated in the supersonic coexpansion of laser ablated platinum vapor with methane or acetylene were probed by visible laser induced fluorescence (LIF) spectroscopy. Both platinum monocarbide, PtC, and an unidentified Pt-containing polyatomic molecule were detected. The intense (0,0)A′ 1Π→X 1Σ+ (T00=13 196.13 cm−1) and (0,0)A 1Π→X 1Σ+ (T00=18 510.71 cm−1) band systems of PtC were recorded at a resolution of ∼0.001 cm−1. The magnetic hyperfine splitting exhibited in the spectral features of the 195PtC isotopomer was analyzed and indicates that the A′ 1Π and A 1Π states arise primarily from a...σ1π1 and a...δ3π1 configurations, respectively.
Characterization of molecular shape resonances using different decouplings of the dilated electron propagator with application to 2Π CO− and 2B2g C2H4− shape resonances
Authors: Medikeri Milan N. , Mishra Manoj K.
Published: July 2002

The zeroth (Σ0), second order (Σ2), quasiparticle second order (Σ2q), diagonal two-particle one-hole Tamm Dancoff approximation (Σ2ph-TDA) and the quasiparticle diagonal 2ph-TDA (Σq2ph-TDA) decouplings have been applied to investigate the 2Π CO− and 2B2g C2H4− shape resonances. An examination of the resonant roots and the corresponding Feynman Dyson amplitudes (FDAs) reveals that the most economic and effective description is offered by the second order decoupling. The more demanding diagonal two-particle one-hole Tamm Dancoff approximation (2ph-TDA) is shown to be less effective and the quasiparticle decouplings are shown to be no better than the zeroth order (bivariational self-consistent field) approximation in the description of molecular shape resonances. The correlation and relaxation effects incorporated by the Σ2 and Σ2ph-TDA decouplings are shown to assist resonance formation by lowering the antibonding nature of the lowest unoccupied molecular orbitals (LUMOs) on the real line and by turning these into anionic diffuse orbitals suitable for metastable electron attachment for the optimal value of the complex scaling parameter. The use of complex resonance energies calculated here to construct a nonempirical optical potential for the investigation of vibrational dynamics of these resonances is suggested.
Electron spin resonance studies of the methane radical cations (12,13CH+4, 12,13CDH+3, 12CD2H+2, 12CD3H+, 12CD+4) in solid neon matrices between 2.5 and 11 K: Analysis of tunneling
Authors: Knight Lon B. , King Gina M. , Petty Jeffrey T. , Matsushita Michio , Momose Takamasa , Shida Tadamasa
Published: July 2002

The radical cation of methane isolated in neon matrices exhibits highly unusual electron spin resonance (ESR) spectral features between 2.5 and 11 K. The anomaly has been clarified by invoking large amplitude tunneling motions of the hydrogens among several symmetrically equivalent Jahn–Teller distorted structures. The effect of the tunneling motions upon the ESR spectrum was investigated by an analysis scheme based upon permutation–inversion group theory. All the deuterium substituted cations, i.e., CDH+3, CD2H+2, CD3H+, and CD+4 were also studied. The hyperfine coupling constant of 13C was obtained from the study of 13CDH+3 and 13CH+4. Several independent generation methods were employed during the course of these methane cation studies, including photoionization, electron bombardment, x-irradiation, and a pulsed laser surface ionization technique.
Pulse length control of Na+2 photodissociation by intense femtosecond lasers
Authors: Machholm Mette , Suzor-Weiner Annick
Published: July 2002

Fragmentation dynamics of the molecular ion Na+2 irradiated by an intense femtosecond pulse laser is studied using quantum wave packet propagations. It is demonstrated that the pulse duration (20–250 fs) can be used as a control parameter for both the total dissociation probability and the branching ratio between different dissociation channels. This pulse length effect is important when the duration of the pulse is shorter than the vibrational period of the molecular ion in the ground state. The effects of laser intensity (1011–3×1012 W/cm2), wavelength (680–780 nm) and initial vibrational level on the dissociation dynamics are also studied.
Accuracy of approximate kinetic energy functionals in the model of Kohn–Sham equations with constrained electron density: The FH⋅⋅⋅NCH complex as a test case
Authors: Wesolowski Tomasz Adam , Chermette Henry , Weber Jacques
Published: July 2002

Ground-state properties of a linear hydrogen-bonded FH...NCH complex are studied by means of the ‘‘freeze-and-thaw’’ cycle of Kohn–Sham Equations with constrained electron density (KSCED) [T. A. Wesolowski and J. Weber, Chem. Phys. Lett. 248, 71, (1996)]. For several geometries of the complex, the electron density and the total energy are compared to the ones obtained by means of the standard Kohn–Sham calculations. The comparisons are made to assess the accuracy of several gradient dependent approximate kinetic energy functionals applied in the KSCED equations. It was found that the closest results to the Kohn–Sham ones were obtained with the functional whose analytical form was proposed by Perdew and Wang for exchange energy [J. P. Perdew and Y. Wang in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] and parametrized by Lembarki and Chermette for kinetic energy [A. Lembarki and H. Chermette, Phys. Rev. A 50, 5328 (1994)]. Around the interaction energy minimum as well as for larger intermolecular distances, the ‘‘freeze-and-thaw’’ cycle of KSCED leads to very similar potential energy surface as the standard supermolecule Kohn–Sham calculations.
Jet spectroscopy, structure, anomalous fluorescence, and molecular quantum beats of silylidene (H2C=Si), the simplest unsaturated silylene
Authors: Harper Warren W. , Waddell Kevin W. , Clouthier Dennis J.
Published: July 2002

The jet-cooled B̃ 1B2–X̃ 1A1 spectrum of silylidene, the simplest unsaturated silylene, has been observed for the first time. H2C=Si and D2C=Si have been produced by an electric discharge through tetramethylsilane and tetramethylsilane-d12 vapor diluted in argon at the exit of a supersonic expansion. Rotational analysis of the 000 bands yielded the following substitution structures: rs″(CSi)=1.706(5) Å, rs″(CH)=1.099(3) Å, θs″(HCH)=114.4(2)°, rs′(CSi)=1.815(5) Å, rs′(CH)=1.073(4) Å, and θs′(HCH)=133.7(1)°. The electronic transition consists primarily of strong electronically allowed perpendicular bands, but a weaker system of vibronically induced parallel bands has also been assigned. Transitions involving Δv=2 changes in the ν6 (b2) mode show up prominently in the spectrum, due to a very large change in the vibrational frequency on excitation. Silylidene has very interesting excited state decay dynamics. Anomalous S2−S0 fluorescence is observed due to the very large S2−S1 energy gap. Rotational level specific intensity anomalies are found in the laser induced fluorescence spectra. Collision-free fluorescence decay curves exhibit superimposed quantum beats for almost all the accessible rotational levels in the 000 bands of H2CSi and D2CSi. Density of states arguments lead to the conclusion that most of the beat patterns are due to coupling with high vibrational levels of the ground state, although two examples of hyperfine splittings associated with singlet–triplet interactions have also been found.
H
2
S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies
Authors: Rodriguez J. A. , Chaturvedi S. , Kuhn M. , van Ek J. , Diebold U. , Robbert P. S. , Geisler H. , Ventrice C. A.
Published: July 2002

The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O3<Cr3O4<Cr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for H2S on the metal, and a low dissociation probability for the molecule on the oxides. In the case of Cr3O4 and Cr2O3, there is a correlation between the size of the band gap in the oxide and its reactivity toward H2S. The uptake of sulfur by the oxides significantly increases when they are “promoted” with gold. The Au/Cr2O3 surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate H2S than polycrystalline Au or pure Cr2O3. The results of ab initio SCF calculations for the adsorption of H2S on AuCr4O6 and AuCr10O15 clusters show a shift of electrons from the gold toward the oxide unit that enhances the strength of the Au(6s)↔H2S(5a1,2b1) bonding interactions and facilitates the decomposition of the molecule.
Mathematical modeling of the NO+H2/Pt(100) reaction: “Surface explosion,” kinetic oscillations, and chaos
Authors: Makeev A. G. , Nieuwenhuys B. E.
Published: July 2002

A mathematical model, consisting of six ordinary differential equations and taking into account the lateral interactions in the adlayer, has been developed for simulating the NO+H2/Pt(100)-(1×1) reaction. This model provides a good theoretical description of temperature programmed desorption and temperature programmed reaction (TPR) experiments, including the occurrence of a “surface explosion” in TPR studies. In addition, the model is capable of reproducing many kinds of nonlinear behavior observed in the experiments such as kinetic oscillations and the transition to chaos through period-doubling bifurcations followed by a period-3 or period-5 limit cycle. The simulation results suggest that the (1×1)⇔hex surface phase transition of Pt(100) is not essential to describe the kinetic oscillations and chaos in the NO+H2/Pt(100) system. The most important step in the oscillation mechanism is the autocatalytic increase in the number of vacant sites for NO dissociation.
Infrared photodissociation spectra of CH3+–Arn complexes (n=1–8)
Authors: Olkhov Rouslan V. , Nizkorodov Sergey A. , Dopfer Otto
Published: July 2002

Infrared photodissociation spectra of the ionic complexes CH3+–Arn (n=1–8) have been recorded in the vicinity of the ν3 asymmetric stretching vibration of the CH3+ monomer. The CH3+–Ar dimer has also been investigated in the spectral range of the first CH stretching overtones, resulting in the characterization of its 2ν1, ν1+ν3, and 2ν3 vibrational states at the level of rotational resolution. The spectrum of CH3+–Ar is consistent with a pyramidal C3v minimum structure of the complex predicted by ab initio calculations at the MP2 level, whereby the Ar atom is attached to the empty 2pz orbital of the CH3+ moiety. The rotationally resolved ν3 spectrum of the CH3+–Ar2 trimer indicates that the two Ar atoms are located on opposite sides of the CH3+ moiety on the C3 axis, with significantly differing intermolecular C–Ar bond lengths. The splittings observed in the trimer spectrum are attributed to a tunneling motion between two equivalent C3v minimum configurations via a symmetric D3h transition state. The spectra of larger clusters (n⩾3) lack rotational resolution, however the positions and profiles of the ν3 band suggest that the additional Ar atoms are weakly attached to CH3+–Ar2 trimer, which acts as the effective nucleation center for the cluster growth. The stretching fundamentals of the CH3+ ion core in the CH3+–Arn clusters are intermediate between those of the methyl radical and the methyl cation, implying a substantial charge transfer from the rare gas atoms to the unoccupied 2pz orbital of CH3+.