bluuebunny/crossref_metadata_2025 huggingface dataset:
Giant magnetoresistance and superparamagnetic grains in Co–Ag granular films
Authors: Honda S. , Nawate M. , Tanaka M. , Okada T.
Published: July 2002

Co–Ag granular films having various Co grain sizes are prepared by rf sputtering under various sputtering conditions. The Co grain sizes are derived from the magnetization curves by dividing them into ferromagnetic and superparamagnetic components. As the Co content decreases, the radii of the superparamagnetic Co grains, rg, decrease and the distances between the Co grains, tg, increase. The magnetoresistance is proportional to the volume density of superparamagnetic grains or related to krg3 exp(−tg/l) with k=3.8×102 μΩ/cm2 and l=20 Å, implying that the giant magnetoresistance is caused by the spin-dependent scattering in the inner part of the superparamagnetic Co grains as well as at the grain surfaces. Furthermore, it is found that the extraordinary Hall effect arises mainly from the side jump mechanism, and that significant perpendicular magnetic anisotropy appears in the Co composition range between 40 and 70 at %.
Enhanced crystallization of amorphous Si containing hydrogen without oxygen during ion-beam irradiation at 310 °C and during furnace annealing below 450 °C
Authors: Nakata Jyoji
Published: July 2002

The epitaxial crystallization rates of amorphous Si layers on crystalline Si substrates containing a considerable number of hydrogen atoms are markedly increased in the absence of oxygen atoms. This enhanced crystallization occurs both during 1-MeV Xe-ion-beam irradiation at 310 °C and during furnace annealing in vacuum at temperatures below 450 °C. Implantation-amorphized crystal Si layers epitaxially grown on the (100)-crystalline Si substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD) are epitaxially crystallized by furnace annealing in vacuum at temperatures below 450 °C. Implantation-amorphized bulk-crystal Si substrates, however, are not entirely crystallized by the same low-temperature annealing. Nanometer-scale microcrystallites, remaining at the near-surface region in the amorphous Si layer after 80-keV phosphorus implantation into the UHV-CVD epitaxial Si layer, grow three-dimensional during 1-MeV Xe-ion-beam irradiation at 310 °C, but not during furnace annealing at 600 °C in dry Ar ambient. This three-dimensional crystal growth does not occur in implantation-amorphized bulk-crystal Si substrates, even during 1-MeV Xe-ion-beam irradiation. Amorphous Si layers directly deposited by low-pressure CVD (LP-CVD) are crystallized epitaxially by 1-MeV Xe-ion-beam irradiation at 310 °C. The crystallization rate of the LP-CVD sample heated in the deposition furnace after evacuation is twice as high as that of the sample heated before evacuation. All these results are explained consistently by the presence of a considerable amount of hydrogen in polyhydride states in the amorphous Si layers in the absence of oxygen. It is suggested that the mechanism of low-temperature crystallization of amorphous Si containing hydrogen atoms in polyhydride states is closely related, in terms of vacant spaces and dangling-bonds, to the mechanism of ion-beam-induced epitaxial crystallization.
Spin valves exchange biased by Co/Ru/Co synthetic antiferromagnets
Authors: Leal J. L. , Kryder M. H.
Published: July 2002

Spin valves exchange biased by a Co/Ru/Co synthetic antiferromagnet were fabricated and characterized. Room temperature measurements show giant-magnetoresistance ratios above 5% and that fields in excess of 5000 Oe are needed to saturate the synthetic antiferromagnet. The saturation field of the synthetic antiferromagnet is higher than 3000 Oe at 250 °C, which yields the possibility of observing the spin valve effect at very high temperatures. The transfer curve of the patterned structure shows a much weaker magnetostatic coupling than that observed in standard spin valves of identical dimensions. This is a consequence of the particular magnetic configuration of the synthetic antiferromagnet, which promotes an almost vanishing magnetostatic field.
Current–voltage characteristics of long wavelength quantum-well laser diodes
Authors: Lau Pey-Kee , Makino Toshihiko
Published: July 2002

This article presents an analysis of the derivatives of the current–voltage characteristics of long wavelength quantum-well laser diodes. It is found that the quantum size effect and the Auger recombination process significantly affect both the electrical derivatives and the diode ideality factor.
A proposed TlGaAs/AlGaAs pseudomorphic heterostructure field effect transistor
Authors: Svensson Stefan P. , Crowne Frank J.
Published: July 2002

A new heterostructure field effect transistor is proposed consisting of a pseudomorphic TlGaAs channel combined with an AlGaAs charge supply layer, the whole structure grown on a GaAs substrate. The properties of TlGaAs from previously published calculations and some simple scaling rules for the effective mass, mobility, and conduction band separation are predicted. The expected charge density in a realistic channel is calculated self-consistently and the expected critical thickness is discussed. The results suggest that a marked improvement in device performance can be expected over similar structures containing InGaAs.
Monte Carlo simulation of the laser-induced plasma plume expansion under vacuum: Comparison with experiments
Authors: Garrelie F. , Aubreton J. , Catherinot A.
Published: July 2002

The laser induced plasma plume expansion in vacuum is studied by a Monte Carlo simulation. An original method, which allows the simulation with no size restrictions on laser spot width or ablated depth, is presented. The global shape of the plume created above a copper target is followed in time by using a three-dimensional algorithm. Particles evaporation from the sample surface during the laser pulse duration is done by taking into account a radial distribution of laser energy and the influence of vapor pressure on surface temperature. The simulation indicates that, when much more than few monolayers are ablated, the laser energy absorption by the evaporated particles has dominant effects on the plume shape during the expansion process. An approximation of these effects has been done by considering that a fraction of the recombination of ionic and excited species leads to a delayed kinetic energy transfer in the plume. It was found that this contribution has a significant effect on the angular and kinetic energy distributions of the evaporated particles. Results of Monte Carlo simulations are compared with experimental results obtained by spectroscopic time of flight measurements and fast photography of the luminous component of the plume, a particular good agreement is obtained for kinetic energy distributions of particles.
The dynamic compressive strength of aluminas
Authors: Murray N. H. , Bourne N. K. , Rosenberg Z.
Published: July 2002

The plate impact loading response of three aluminas with varying glass content (purity) and porosity has been investigated using the 50 mm gas-gun facility at the University of Cambridge. The elastic precursor amplitude has been shown to be sensitive to both shock propagation distance and impact stress. The precursor decay effect is greatest in the low purity aluminas. The effect of the material microstructure on the compressive strength at high rates of strain has been studied. Porosity was shown to play the dominant role although some grain-size effects were also noted.
Electrical and optical properties of light-emitting p–i–n diodes on diamond
Authors: Melnikov A. A. , Denisenko A. V. , Zaitsev A. M. , Shulenkov A. , Varichenko V. S. , Filipp A. R. , Dravin V. A. , Kanda H. , Fahrner W. R.
Published: July 2002

Electrical and optical properties of light-emitting p–i–n diodes made on natural and synthetic single-crystal diamonds were studied at temperatures up to 500 °C. The diodes were fabricated by boron (p-type doping) and lithium or phosphorous (n-type doping) ion implantation in the form of two adjacent p- and n-type areas separated by an undoped i gap of a width varying from 2 to 200 μm. The current–voltage characteristics of the diodes showed typical rectifying behavior. Onset of the forward current is accompanied by electroluminescence (EL) and occurred at a certain voltage depending on the width of the i base and the type of the diamond substrate. At the onset voltage a current-controlled negative resistance regime was observed. Comparison of the electrical characteristics of the diodes with the optical properties of the corresponding diamond substrates revealed that in diamonds with nitrogen content over 5×1017 cm−3 the nitrogen defects are the major recombination centers affecting the carrier lifetime. EL of the A band, the nickel related 484 nm center, and the nitrogen related H3 and 575 nm centers, were excited in the diodes. The H3 and 575 nm centers retained considerable emitting intensity at temperatures as high as 500 °C. EL of the 484 nm center was possible to observe up to a temperature of 400 °C. EL of the A band was quenched rapidly with temperature and it disappears at above 200 °C.
Depletion layer thickness and Schottky type carrier injection at the interface between Pt electrodes and (Ba, Sr)TiO3 thin films
Authors: Hwang Cheol Seong , Lee Byoung Taek , Kang Chang Seok , Lee Ki Hoon , Cho Hag-Ju , Hideki Horii , Kim Wan Don , Lee Sang In , Lee Moon Yong
Published: July 2002

The electrical conduction properties of rf sputter-deposited (Ba, Sr)TiO3 (BST) films on Pt and IrO2 electrodes and metalorganic chemical vapor deposited (MOCVD) BST films on a Pt electrode were investigated and a new energy band model that satisfactorily explains the observed leakage current characteristics and film thickness dependent dielectric properties is proposed. The BST and Pt junction constituted a blocking contact with interface potential barrier heights of 1.6–1.7 eV and 1.2 eV for the sputtered and MOCVD films, respectively. Schottky emission behavior was observed at measurement temperatures higher than 120 °C and tunneling related conduction behavior appeared below that temperature for a film thickness of 40 nm. A partial depletion model with a very thin (about 1 nm) layer devoid of space charge at the interface with the Pt electrode is proposed to explain the V1/2 dependent variation of ln(Jo) as well as the decreasing dielectric constant with decreasing film thickness.
Quantitative analysis of room temperature photoluminescence of c-Si wafers excited by short laser pulses
Authors: Timoshenko V. Yu. , Petrenko A. B. , Stolyarov M. N. , Dittrich Th. , Fuessel W. , Rappich J.
Published: July 2002

Effect of surface nonradiative recombination on kinetics and total yield of the interband photoluminescence (PL) of c-Si wafers excited at room temperature by short laser pulses is studied. Numerical simulations show that a correlation of the PL quenching with the surface defect density takes place even at the high excitation level in spite of Auger recombination in the bulk. The quantum yield of PL reaches some percent for Si wafers with low bulk and surface defect concentrations. The calculations are confirmed by the experimental correlation between the PL quenching with the density of interface states (Dit) at the Si/SiO2 interface which has been obtained by conventional capacitance–voltage measurements. The express characterization of the Dit of Si surfaces by the pulsed PL can be carried out for the defect density in the range from 108 to 1014 cm−2 at room temperature.
Amorphous state and pulsed laser deposition of YBa2Cu3O7−δ thin films
Authors: Okunev V. D. , Samoilenko Z. A. , Svistunov V. M. , Abal’oshev A. , Dynowska E. , Gierłowski P. , Klimov A. , Lewandowski S. J.
Published: July 2002

Pulsed laser deposition of high Tc compounds onto unheated substrates, resulting in amorphous thin films, preserves to a great extent the composition of matter ejected from the target. This composition is of primary interest, both for understanding the dynamics of laser–target interaction and for practical (optimization) reasons. We have investigated the structure of amorphous and crystalline YBaCuO films obtained both in on-axis and off-axis deposition geometries, and correlated the results with optical and transport properties of these films. X-ray scattering reveals in amorphous films the existence of: (i) amorphous continuum of spatially disordered atoms, (ii) small (10–40 Å) amorphous clusters which can be considered as mesoscopic order fluctuations in the amorphous continuum, and (iii) slightly larger (50–250 Å) crystalline clusters exhibiting quasi-two dimensional (00l) or (11l) long range order. Crystalline films are predominantly (00l) oriented. Optical spectra of both crystalline and amorphous films show regions of enhanced attenuation caused by free charge carriers. Spectra of amorphous samples containing small crystalline clusters exhibit features which we relate to the electron localization caused by quantum size effects. Transport measurements are in good agreement with the structural and optical results. Conductivity of the on-axis films is 3–4 orders of magnitude higher than that of the off-axis films. The (nonlinear) conductivity of amorphous films increases with temperature and remains constant below 200 K. We suggest that besides the usual variable range hopping conduction mechanism, classical tunneling of charge carriers at constant energy between metallic (crystalline) clusters is present. Interestingly, the amorphous off-axis films exhibit a periodic repetition of the elements of atomic order or disorder along the direction of the plasma plume and undergo a structural transition of (11l)→(00l) type.
Electroluminescence of organic light emitting diodes with a thick hole transport layer composed of a triphenylamine based polymer doped with an antimonium compound
Authors: Yamamori Asuka , Adachi Chihaya , Koyama Toshiki , Taniguchi Yoshio
Published: July 2002

We investigated the electroluminescence (EL) performance of organic light emitting diodes having a thick doped hole transport layer [(DHTL):650 nm–1.5 μm]. The basic cell structure is an anode/DHTL/hole transport layer [(HTL):50–60 nm]/emitter layer [(EML):50–60 nm]/cathode. We examined various combinations of host polymers and guest molecules as a component of DHTL in this device structure. During the course of the materials’ search, we found that the best combination of a hole transport polycarbonate polymer (PC–TPD–DEG) and a tris (4-bromophenyl) aminium hexachroloantimonate (TBAHA) as a dopant enabled us to form a uniform thick DHTL (typically 650 nm–1.5 μm thick), which resulted in excellent EL performance. The thick DHTL not only showed considerable reduction in cell resistance compared with a conventional anode/DHTL (without doping)/HTL/EML/cathode device with the same thicknesses of the organic layers, but also greatly contributed to the enhancement of the device stability, particularly to pinhole problems that can occur with conventional 100-nm-thick thin devices. Furthermore, the interposed HTL between DHTL and EML was confirmed to function not only as a HTL but also as electron and exciton blocking layers. Without the HTL, the EL quantum efficiency (ΦEL) was low, because of the serious exciton energy transfer and/or electron migration from EML to DHTL where the PC–TPD–DEG:TBAHA complex layer had absorption at around 485 nm based on a charge transfer complex between them. We could increase it by interposing a thin transparent N,N′-diphenyl-N,N′bis(3-methyl phenyl)-1,1′-biphenyl-4,4′-diamine or 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (α-NPD) layer between DHTL and EML, while keeping the driving voltage low. With the DHTL (650 nm, 10 wt % of TBAHA) showed a luminance of 4004 cd/m2 at 10.0 V and 220 mA/cm2, of which the performance was comparable with that of typical thin film devices. Furthermore, we could expand the DHTL thickness up to 1.5 μm. An indium tin oxide (ITO)/DHTL (10 wt %)(1.5 μm)/α-NPD (60 nm)/Alq (60 nm)/MgAg device showed a luminance of 2600 cd/m2 at 18.0 V and 210 mA/cm2 with enhanced duration stability. In addition, the duration properties of the devices were also examined in the device structure of an ITO/DHTL (650 nm)/α-NPD (60 nm)/Alq(doped with rubrene 4.2 wt %) (60 nm)/MgAg. The half decay of the initial luminance successively exceeded over 1000 h under a constant current density of 10 mA/cm2.
Growth of GaN layers onto misoriented (0001) sapphire by metalorganic chemical vapor deposition
Authors: Pécz B. , di Forte-Poisson M. A. , Huet F. , Radnóczi G. , Tóth L. , Papaioannou V. , Stoemenos J.
Published: July 2002

The growth process of GaN layers grown by metalorganic chemical vapor deposition on sapphire is characterized by transmission electron microscopy and atomic force microscopy. The nitridation of the sapphire substrate and GaN buffer layers as well as film structure and the nature of defects are studied. Nitridation causes the formation of a 4 nm thick AlN layer on sapphire. GaN buffer layers grown at 510 °C are found to be hexagonal single crystals in their as-grown state with a mosaic structure. Annealing of the buffer layers leads to substantial smoothening of their surfaces due to the coalescence of the grains. GaN layers themselves are single crystalline, hexagonal, and epitaxial to the substrate. Layers grown on exactly oriented (0001) type substrate as well as on miscut substrate are compared. Smooth surfaces have been achieved on exactly oriented and on miscut substrates as well, but the range of the deposition parameters is wider when miscut substrates are used.
Raman scattering characterization of Si(100) implanted with mega-electron-volt Sb
Authors: Dey Soma , Roy Chaitali , Pradhan Asima , Varma Shikha
Published: July 2002

Dose dependent structural modifications in Si(100) due to 1.5 MeV implantation of Sb have been characterized using Raman spectroscopy and Rutherford backscattering spectrometry/channeling (RBS/C) techniques. With increasing fluence, an intensity reduction of the first order Raman peak, characteristic of crystalline Si, is observed. The amorphicity in Si lattice appears at a dose of 1×1013 ions/cm2 and it increases with each dose. For a dose of 5×1014 ions/cm2 the Raman spectrum resembles that of amorphous Si. RBS/C studies also support a fully amorphized lattice at this dose though for smaller doses it suggests lower disorder. For the fluences of 1×1013 and 1×1014 ions/cm2 a coexistence of undamaged crystalline Si regions and amorphous zones is observed. Consequently, phonon confinement is observed. Lattice recovery achieved by subsequent annealing has also been investigated using Raman spectroscopy. By annealing at 600 °C, sample crystallinity is fully recovered in all the cases up to the fluence of 5×1014 ions/cm2. For higher doses small amorphicity still remains. Depth dependent measurements of the shifts in the Raman peaks demonstrate a gradient in stress which is of compressive nature near the surface region but is tensile in deeper layers. Maximum stress in the lattice appears for a dose of 1×1012 ions/cm2 which gets relaxed by the incorporation of amorphous zones at higher fluences.
Nonprobe radio-frequency plasma diagnostics method based on the power balance in an asymmetric capacitively coupled discharge
Authors: Mateev Emil , Zhelyazkov Ivan
Published: July 2002

An analytical semiquantitative theoretical model founded on the self-bias voltage dependence on the absorbed radio-frequency (rf) power of a strongly asymmetric capacitively coupled rf discharge has been developed. The model is applicable under gas-discharge conditions typical of technological etching devices and suggests an experimental method for determining basic plasma parameters, more specifically the ion flux onto the powered electrode, without using any additional probes.
Spatial variation of ferroelectric properties in Pb(Zr0.3, Ti0.7)O3 thin films studied by atomic force microscopy
Authors: Christman James A. , Kim Seung-Hyun , Maiwa Hiroshi , Maria Jon-Paul , Rodriguez Brian J. , Kingon Angus I. , Nemanich Robert J.
Published: July 2002

Imaging of the phase and magnitude of the piezoelectric strain in Pb(Zr0.3, Ti0.7)O3 (PZT) capacitors is performed with an atomic force microscope. The imaging reveals a significant spatial dependence of the ferroelectric properties of both fatigued and unfatigued PZT films. We propose that the variation is related to the domain structure of the PZT. Through the measurement of local piezoelectric hysteresis loops and imaging of the piezoelectric strain, areas are observed in fatigued PZT that exhibit hysteresis loops shifted along the polarization axis. In some regions of fatigued samples, the hysteresis loops are shifted such that both remanent points of the hysteresis curve have the same polarization direction. These results have important implications for the scalability of nonvolatile ferroelectric random access memory to higher device densities.
Analytical solutions for strain in pyramidal quantum dots
Authors: Pearson G. S. , Faux D. A.
Published: July 2002

Analytical solutions for strain in buried pyramidal quantum dots (QDs) of arbitrary truncation, assuming isotropy of the elastic constants, are presented for the first time. The expressions allow the strain to be evaluated extremely rapidly compared to atomistic, finite-element, or other numerical techniques. As a demonstration of the new results, calculations are performed for InAs QDs buried within a GaAs matrix for a range of truncations for the cases of an isolated QD, an isolated QD with a graded composition profile, and a QD array. Results show that strain magnitudes and trends are highly dependent upon geometry and composition. For example, the biaxial strain at the apex of a pyramid is shown to be negative, but becomes positive as the truncation increases.
Dynamics of intramolecular vibrational-energy redistribution (IVR). I. Coherence effects
Authors: Felker Peter M. , Zewail Ahmed H.
Published: July 2002

In this series of papers, theoretical and experimental results concerning the dynamical manifestations of intramolecular vibrational-energy redistribution (IVR) in temporally resolved fluorescence are presented. In this paper (I) we present a general treatment of IVR and coherence effects in multilevel vibrational systems. Specifically, the concern is with the derivation of the characteristics of the beat-modulated fluorescence decays which arise from vibrational coupling among N levels within a molecule. Relations connecting quantum beat frequencies, phases, and modulation depths to coupling parameters are presented. Likely sources of deviation of experimental results from theoretical predictions are considered. And, finally, the direct link between IVR and time-resolved fluorescence experiments is discussed with emphasis placed on the physical interpretation of vibrational quantum beats and the nature of IVR as a function of vibrational energy in a molecule.
A critical comparison of molecular reorientation in the ground and excited electronic states: Cresyl violet in methanol
Authors: Blanchard G. J. , Wirth M. J.
Published: July 2002

Picosecond techniques were used to study the rotational diffusion of the ground and first excited electronic states of cresyl violet in methanol. The anisotropy decay was studied at five different wavelengths, three in the ground state and two in the excited state. It is known that the decay of the anisotropy is dependent on both the transition polarization and rotational diffusion constants. Experimentally, a different decay of the anisotropy is observed at each wavelength. The data are consistent with changes in the transition polarization but with the same diffusion constant in all cases. It is concluded that cresyl violet in methanol reorients identically in its ground and excited electronic states.
Analysis of the experimental orientation dependence of the reaction CH3I+Rb→RbI+CH3
Authors: Bernstein Richard B.
Published: July 2002

Experiments by Parker et al. (1981), deconvoluted for the precession effect by Stolte et al. (1982), yielded the dependence of reaction probability upon the ‘‘angle of attack’’ γ for the CH3I+ Rb reaction. Inversion of these results is carried out via the kinetic theory model of reactive collisions of rigid nonspherical molecules of Evans et al. (1985), yielding the angle dependence of the activation barrier as well as an estimate of the colliding-pair anisotropy (asphericity) parameter.
Classical dynamics of a coupled double-well oscillator in condensed media. III. The constraint of detailed balance and its effects on chemical reaction process
Authors: Fonseca Teresa , Grigolini Paolo , Pareo Diego
Published: July 2002

The reacting system is assumed to interact both with an ideal thermal source at the temperature T1 and a (fast) nonreactive oscillator coupled to an ideal thermal source at temperature T2. Special attention is devoted to the case T1=T2 which is characterized by equilibrium distributions of canonical kind. We use the same adiabatic elimination procedure (AEP) as applied in our earlier works [J. Chem. Phys. 79, 3320 (1983); 80, 1826 (1984)] with a projection operator that allows the canonical property to be recovered in the reduced equation of motion. It is shown that in the underdamped regime the chemical reaction rate increases with increasing strength of the coupling between reactive and nonreactive mode. In the high-friction regime the chemical reaction rate decreases with increasing reactive–nonreactive mode coupling. Arguments are also provided for a more complete picture covering the whole friction region. It is also shown that the projection operator used in the two preceding papers implies that T2≫T1.
Spectroscopic properties and conductivity of thin films of partially reduced metallo-phthalocyanines
Authors: Le Moigne J. , Even R.
Published: July 2002

Thin films of reduced phthalocyanine have been prepared by co-condensation, under high vacuum, of cobalt or iron phthalocyanine (Pc) with potassium. The absorption spectra of the reduced Pc have been determined and found to be strongly dependent upon the reduction state of the film, especially in the visible region. Four reduction states corresponding to the mono-, di-, tri-, and tetraanion of CoPc were observed and assignment of the electronic structure was made on the basis of similarity with the spectra of phthalocyanine in solution. The conductivity of each film was measured as a function of its state of reduction. The film conductivity was found to be isotropic. Large increases of conductivity from 10−10 to 1–2 Ω−1 cm−1 occurred by steps which correlated well with the state of reduction of the Pc. The temperature dependence of the conductivity was determined between 80 and 330 K. Based on the transport results, a Mott–Davis type model with hopping between localized states is proposed for the conduction mechanism.
Dispersive triplet excitation transport in organic glasses
Authors: Richert R. , Bässler H.
Published: July 2002

Time dependent transport of triplet excitations in amorphous benzophenone, anthraquinone, and phenanthrene has been investigated in the temperature range 4.2 K≤T≤100 K by monitoring diffusion-controlled decay of the inhomogeneously broadened 0–0 phosphorescence band of bulk states. The results are in excellent agreement with both the analytic theory of Grünewald et al. and computer simulation of nonequilibrium transport. They indicate the applicability of a stochastic transport model based on asymmetric hopping rates with unrestricted energy relaxation. The time dependence of the ensemble averaged hopping frequency has been determined within three decades of time. It can be approximated by a ν(t)∼(t/t0)α−1 law with time dependent dispersion parameter α.
Classical models for electronic degrees of freedom: The second-quantized many-electron Hamiltonian
Authors: Miller William H. , White Kim A.
Published: July 2002

A classical Hamiltonian is constructed which corresponds to the general second-quantized Hamiltonian operator for a many-electron system. This classical electronic Hamiltonian properly accounts for the fermion character of electrons (i.e., the anticommutivity of the electron creation and annihilation operators). By having a classical description of the electronic degrees of freedom one is able to incorporate electronically nonadiabatic effects into a classical trajectory simulation of nuclear motion in a dynamically consistent way. Several applications are described which illustrate the nature of the model. It can be viewed, for example, as a generalization of an earlier classical valence bond model (based on a Heisenberg–Van Vleck spin Hamiltonian), the generalization being to include the effects of ionic as well as covalent configurations in the description. The present model, therefore, is able to describe charge transfer aspects of the electronic degrees of freedom.
High pressure Raman study of solid CS2
Authors: Bolduan F. , Hochheimer H. D. , Jodl H. J.
Published: July 2002

Solid molecular CS2 is investigated in a diamond anvil cell in the temperature and pressure range of 6–300 K and 0–20 GPa, respectively, by means of Raman spectroscopy. The pressure dependence of external and internal vibrational frequencies is determined, and used to draw conclusions about the p–T phase diagram. A very strong influence of sample preparation conditions on the crystal quality, especially at low temperatures, is noticed. It turned out that the librational Raman spectrum provides a sensitive test with respect to the crystalline or amorphous structure of the solids under investigation. In addition the origin of the ν2 fundamental and phonon sidebands, some-times observed in spectra of amorphous CS2, will be discussed. Finally the influence of temperature and pressure is shown in the so-called ‘‘black’’ CS2, a polymerized modification of CS2, which is formed at pressures above 10 GPa.
Benchmark full configuration-interaction calculations on HF and NH2
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Taylor Peter R. , Handy Nicholas C. , Knowles Peter J.
Published: July 2002

Full configuration-interaction (FCI) calculations are performed at selected geometries for the 1∑+ state of HF and the 2B1 and 2A1 states of NH2 using both DZ and DZP Gaussian basis sets. Higher excitations become more important when the bonds are stretched and the SCF reference becomes a poorer zeroth-order description of the wave function. The CASSCF-MRCI procedure gives excellent agreement with the FCI potentials, especially when corrected with a multireference analog of the Davidson correction.
Theory of activated rate processes: Exact solution of the Kramers problem
Authors: Mel’nikov V. I. , Meshkov S. V.
Published: July 2002

The Kramers theory for the escape rate of a Brownian particle from a potential well is extended to the full damping range. It is shown that the most adequate description of the underdamped Brownian motion in a deep potential well is provided by a Green function of the Fokker–Planck equation in the energy-position variables. The problem of lifetime of a particle in a single potential well is reduced to an integral equation in energy variable, with the Green function being the kernel of this equation. The straightforward solution by the Wiener–Hopf method yields an explicit expression for the lifetime, which describes the crossover from the extremely underdamped regime to that of a moderate damping. With the use of Kramer’s result for moderate-to-large damping an expression for the lifetime is presented, which holds at arbitrary damping. The problem of the rate of transitions between the two minima of a double-well potential is reduced to a system of two integral equations, which is also solved by the Wiener–Hopf method. An explicit expression for the relaxation time of nonequilibrium populations of the two minima is given.
Multiphoton spectroscopy of X–NO (X=Kr, Xe, CH4) van der Waals molecules
Authors: Miller John C.
Published: July 2002

The vibronic spectroscopy of the van der Waals species KrNO, XeNO, and CH4NO has been investigated via mass-resolved multiphoton ionization (MPI) spectroscopy. Spectra are presented for the two-photon excited electronic transition of the various complexes associated with the C 2Π (v=0) state of nitric oxide. Dissociation pathways are analyzed for XeNO, which appears at the Xe+ mass following MPI. Trends in bonding and spectroscopic constants are discussed.
A full CI treatment of the 1A1,1B1, and 3B1 states of SiH2
Authors: Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

Full CI calculations are presented for the 1A1,3B1, and 1B1 states of SiH2 at their respective equilibrium geometries and at geometries with the SiH bonds stretched. These results are compared with those obtained from single reference and multireference CI calculations. The computed Te values agree well with the full CI results provided that the effects of higher than double excitations are accounted for either by the Davidson correction or by a multireference approach. When the SiH bonds are stretched the single reference methods are not sufficiently flexible, and only CASSCF/MRCI achieves chemical accuracy (i.e., agrees with the full CI to 1 kcal/mol). Overall, the accuracy of the various approximate methods is very similar to that found for H2O, NH2, and CH2.
On the contributions of van der Waals interactions to vibrational level mixing. Torsion–vibration coupling in p-fluorotoluene
Authors: Moss David B. , Parmenter Charles S. , Ewing George E.
Published: July 2002

As an example demonstrating the importance of van der Waals interactions to vibrational level mixing, a model is developed for the intramolecular coupling of internal methyl torsion to aromatic ring vibrations in p-fluorotoluene (pFT). The coupling is a consequence of van der Waals interactions between the methyl rotor and the ring. Fermi resonance matrix elements are generated in a manner analogous to previous calculations for collision-induced vibrational energy transfer and van der Waals molecule predissociation. Using these matrix elements, the extent of vibrational mixing is calculated for the level 6a1 in S1 pFT. The calculated result is in agreement with spectroscopic observations. Arguments for the generality of van der Waals interactions as mediators of intramolecular vibrational level mixing are presented.
Effects of capillary and van der Waals dispersion forces on the equilibrium profile of a wetting liquid: Theory and experiment
Authors: Truong Jack G. , Wayner Peter C.
Published: July 2002

The equilibrium profile of a wetting liquid near the transition region, where both capillary and van der Waals dispersion forces are equally important, was determined using interferometry and ellipsometry. Measurements were made in the thickness range 0.02≤δ≤1 μm for both hexane and octane wetting a single crystal silicon surface. Theoretical profiles were also obtained numerically from a model based on the balance of hydrostatic force, capillarity, and the retarded van der Waals dispersion force. We used the Dzyaloshinskii–Lifshitz–Pitaevskii theory to calculate the van der Waals interactions between the alkanes and silicon. Fairly good agreements between the theoretical and experimental film profiles were obtained. The measured equilibrium film thicknesses for hexane and for octane on silicon were ∼250 and ∼195 Å, respectively. Both experimental and theoretical results indicate that hexane and octane wet and tend to form a zero contact angle with silicon.
The decomposition of methanol on Ru(001) studied using laser induced thermal desorption
Authors: Deckert A. A. , Brand J. L. , Mak C. H. , Koehler B. G. , George S. M.
Published: July 2002

The decomposition reaction of methanol on Ru(001) was studied using laser induced thermal desorption (LITD). The LITD studies, combined with temperature programmed desorption and Auger electron spectroscopy measurements, allowed absolute product yields for the three competing surface pathways to be determined over the entire range of chemisorbed methanol coverages at a heating rate of β=2.6 K/s. At the lowest methanol coverages of θ≤0.07θs, where θs is the surface coverage of a saturated chemisorbed layer, all the methanol reacted between 220–280 K. This methanol decomposition reaction yielded desorption-limited H2 and CO as reaction products. At higher coverages, molecular desorption and the second methanol decomposition reaction involving C–O bond breakage became increasingly important. At θ=θs, 50% of the initial methanol coverage desorbed, 24% produced H2 and CO and 26% left C on the surface. Isothermal LITD kinetic measurements were carried out at low methanol coverages of θ≤0.07θs at various temperatures from 180 to 220 K. The initial decomposition rates obtained from the isothermal LITD studies displayed first order kinetics. The decomposition kinetics at later times could not be fit by first order kinetics and suggested a self-poisoned reaction. Subsequent LITD studies revealed that CO inhibited the decomposition reaction. The product CO inhibition was modeled by first order kinetics with a CO-coverage dependent activation barrier. The observed first order reaction kinetics at low methanol coverage could be expressed by the pre-exponential ν=106 s−1 and the coverage-dependent activation barrier E=7 kcal/mol+αθCO/θCO,s, where α=20 kcal/mol and θCO/θCO,s is the dimensionless CO coverage normalized to the CO saturation coverage θCO,s. Isotopic LITD studies revealed that the decomposition kinetics of CH3OH, CD3OH, and CH3OD were identical. This equivalence suggested that the hindered rotation of the surface methoxy species is the reaction coordinate for the rate-limiting step in the decomposition reaction.
The microwave spectrum of CO in the a 3Π state. I. The J=0–1 transitions in CO, 13CO, and C18O
Authors: Saykally Richard J. , Dixon Thomas A. , Anderson Thomas G. , Szanto Peter G. , Woods R. Claude
Published: July 2002

The J= 0–1 transitions of a 3Π carbon monoxide have been measured in several isotopic forms and vibrational states: CO (v=0–4), 13CO (v=0–3), and C18O (v=0–4). These new data were combined with the many lambda doublet transition frequencies previously available from the molecular beam electric resonance experiments of Klemperer and co-workers and fit to a model Hamiltonian essentially the same as that used for previously published fits of combined optical–lambda doubling data. Many different fits were made including single state–single isotope, multistate–single isotope, single state–multi-isotope, and multistate–multi-isotope ones. Among the important conclusions are (1) that the explicit perturbation due to the v′=0 level of the a′ 3∑+ state must be included for adequate fits of the v=0–3 data (and of course the more strongly perturbed v=4 data), (2) that the spin–orbit coupling constant A can be determined well from just the microwave–rf data if several isotopic forms are fitted together, and (3) that much additional data is needed for a completely satisfactory characterization of the a 3Π state.
Determination of static and dynamic interactions between monodisperse, charged silica spheres in an optically matching, organic solvent
Authors: Philipse A. P. , Vrij A.
Published: July 2002

A light scattering study is presented on static and dynamic interactions between monodisperse, charged silica spheres suspended in an optically matching, salt free mixture of ethanol and toluene up to volume fractions of 10%. The static structure factor S(K), well described by calculations in the RMSA approximation, is combined with the wave vector (K) dependent (short-time) diffusion coefficient De(K) to give the function H(K) which represents the hydrodynamic interactions. From H(K), obtained for the first time for charged particles, we conclude that the long-range electrostatic repulsion between the spheres has a pronounced influence on the hydrodynamics of large-scale, collective particle motions, whereas small-scale single-particle diffusion is relatively unaffected.
Proton spin–lattice relaxation in nematic microdroplets
Authors: Vilfan M. , Rutar V. , Žumer S. , Lahajnar G. , Blinc R. , Doane J. W. , Golemme A.
Published: July 2002

The frequency and temperature dependences of the proton spin–lattice relaxation rate have been studied in nematic droplets embedded in a solid polymer matrix. The comparison of these data to those for bulk nematic and pure polymer samples shows that cross-relaxation between liquid crystal and polymer protons dominates the liquid crystal relaxation in the MHz frequency region, while at low frequencies the translationally induced molecular rotation becomes important. The cross-relaxation rate is found to be ≈103 s−1 giving an order of magnitude estimate 10−4–10−3 s for the time for which a liquid crystal molecule is bonded to the surface.
A unitary group formulation of the complete active space configuration interaction method. I. General formalism
Authors: Gould M. D. , Chandler G. S.
Published: July 2002

A group theoretical formulation of the complete active space CI (CASCI) method is presented based on the subgroup chain U(n=n1+n2)⊇U(n1)×U(n2), where n1 is the number of active orbitals and n2 the number of external orbitals. The internal-external factorization of the U(n) generator matrices and the isolation of all singly, doubly (and higher) excited states fall out naturally in our approach. Moreover it is shown that the matrix elements of arbitrary products of U(n) generators may be obtained from simple products of the single generator matrices, to any order of excitation. The method is ideally suited to modern vector array processors and affords sufficient flexibility to allow core excitations and partitioning of the active space. Further simplifications occur in the all singles and doubles case, which will be discussed in the second (and final) paper of the series.
The collocation method for calculating vibrational bound states of molecular systems—with application to Ar–HCl
Authors: Peet Andrew C. , Yang Weitao
Published: July 2002

The ability of the collocation method to calculate vibrational bound states of molecules is investigated. The technique is simpler to implement than conventional variational methods; no integration over the basis functions is involved. We apply the method to the weakly bound complex Ar–HCl, a real multidimensional system of considerable physical interest, and find the procedure to be of equivalent accuracy to the corresponding variational approach at all times. This confirms the conclusions of our previous studies on one-dimensional test problems [W. Yang and A. C. Peet, Chem. Phys. Lett. (in press)]. Both low lying and highly excited states are examined and the conclusions hold even for levels very close to the dissociation limit. A test of the wave functions obtained also finds these to be of good accuracy and very similar to the ones given by the variational procedure.
Exponential power series expansion for the quantum time evolution operator
Authors: Makri Nancy , Miller William H.
Published: July 2002

The coordinate matrix element of the time evolution operator, exp[−iĤt/ℏ], is determined by expanding (its exponent) in a power series in t. Recursion relations are obtained for the expansion coefficients which can be analytically evaluated for any number of degrees of freedom. Numerical application to the tunneling matrix element in a double well potential and to the reactive flux correlation function for a barrier potential show this approach to be a dramatic improvement over the standard short time approximation for the propagator. Its use in a Feynman path integral means that fewer ‘‘time slices’’ in the matrix product exp[(−i/ℏ)ΔtĤ]N, Δt=t/N, will be required. The first few terms in the present expansion constitute a fully quantum version of the short time propagator recently obtained by us using semiclassical methods [Chem. Phys. Lett. 151, 1 (1988)].
van der Waals interaction at a material wedge
Authors: Lambin Ph. , Lucas A. A. , Derycke I. , Vigneron J.-P. , Derouane E. G.
Published: July 2002

The problem of evaluating the van der Waals attraction energy of an atom or microparticle adsorbed near the edge of a straight material wedge of arbitrary opening angle is considered. In order to disentangle more clearly the effect of the wedge geometry from the material parameters, a simple model is used in which the particle is represented by a harmonic oscillator and the substrate by a perfect conductor. By a straightforward extension of the method of images of classical electrostatics, the dipolar coupling of the particle to the substrate is constructed from which new oscillator frequencies can be determined as a function of its position relative to the edge and of the wedge angle. The van der Waals energy is given by the shift in zero-point energy of the oscillator coupled to itself via its images. For a physisorbed atom crossing the edge of a wedge material, the detailed shape of the van der Waals energy barrier (respectively, well) offered by a convex (respectively, concave) wedge is displayed and compared with approximate results deduced from a pairwise summation of Lennard-Jones interactions. The interest of these results for the dynamics and kinetics of sorption by microporous solids is briefly discussed.
A theoretical study of the dissociation of H2/Cu
Authors: Hand M. R. , Holloway S.
Published: July 2002

We present calculations for the dissociative adsorption of hydrogen molecules on a Cu surface as a function of initial translational energy and vibrational quantum state. Classical, semiclassical, and fully quantum calculations are performed and the results compared. The potential energy surface was based upon a total energy calculation for H2 on a small Cu cluster and has been previously employed in dynamical simulations. Our results show that for low primary beam energies, dissociation occurs primarily via tunneling through the activation barrier in the vibrational coordinate. Populating the initial vibrational states is shown to enhance reactivity, but not simply by a total energy shift. By changing the hydrogen isotope it is shown that tunneling effects can persist up to quite high molecular masses. This occurs because the activation barrier lies in the vibrational coordinate, where the reduced mass of the molecule determines the dynamics.