bluuebunny/crossref_metadata_2025 huggingface dataset:
Negative ion photoelectron spectroscopy of the group 5 metal trimer monoxides V3O, Nb3O, and Ta3O
Authors: Green Susan M. E. , Alex Simson , Fleischer Nancy L. , Millam Evan L. , Marcy Timothy P. , Leopold Doreen G.
Published: July 2002

The vibrationally resolved 488 nm negative ion photoelectron spectra of V3O, Nb3O, and Ta3O provide measurements of their electron affinities, vibrational frequencies, and low-lying electronic state energies. The electron affinities increase smoothly down the triad with values (eV) of 1.218±0.008 for V3O, 1.393±0.006 for Nb3O, and 1.583±0.010 for Ta3O. The metal–oxygen symmetric stretching fundamental frequencies are 750±20, 710±15, and 710±15 cm−1, respectively, for the neutral clusters and 770±20 cm−1 for V3O−. Lower frequency symmetric modes are also active, with frequencies (±15 cm−1) of 415 and 340 cm−1 for V3O, 320 cm−1 for Nb3O, 225 cm−1 for Ta3O, and of 355, 300, and 215 cm−1 (±20 cm−1), respectively, for their anions. Weaker transitions also showing vibrational structure are observed to excited states at 3350±30 cm−1 for V3O, 4300±100 and 6190±120 cm−1 for Nb3O, and 3180±40 and 3870±50 cm−1 for Ta3O. The spectra indicate that the extra electrons in the anions occupy essentially nonbonding orbitals, and that the neutral and anionic clusters have planar structures with doubly bridging oxygen atoms. Results are discussed in light of previous studies of Nb3O and the Group 5 M3 and MO molecules, and tentative assignments for the observed M3O and M3O− states are proposed.
Intrinsic point defects in oxidized 3C epitaxial layers on Si substrates
Authors: Macfarlane Patricia J. , Zvanut M. E. , Janowski Gregg M.
Published: July 2002

We have used electron paramagnetic resonance to study two intrinsic defects in oxidized epitaxial layers of 3C SiC, a potential substitute for Si in high speed, high power electronics. One center can be described by an isotropic g value of 2.0044. The defect is distinguished by a strong temperature dependence and is located throughout the 3C epitaxial layer. Although the detailed structure has not been determined, the g value is consistent with theoretical predictions for a Si dangling bond surrounded by carbon atoms. The second Si-related center, induced by post oxidation dry heat treatment, is characterized by an axial g tensor with g∥=2.0023 and g⊥=2.0082. The spectroscopic and chemical properties of the latter defect are consistent with those of the Si Pb center, a Si dangling bond located at a Si/insulator interface. Experiments show that in our 3C samples, the Pb signal is probably located on the Si side of a SiO2/Si interface between the SiC epilayer and the Si substrate.
Separation of spin–orbit coupled metastable states of Kr+ and Xe+ by ion mobility
Authors: Bluhm Brian K. , North Simon W. , Russell David H.
Published: July 2002

The spin–orbit coupled ground states of Kr+⋅ and Xe+⋅ (RG P1/22 and RG P3/22, RG=rare gas) are separated on the basis of ion mobility. Ion mobility separation of Kr+⋅ and Xe+⋅ is based on different interaction potentials of the P1/22 and P3/22 ionic states with helium. The ion mobility instrument used for these studies is a novel instrument based on a Fourier-transform ion cyclotron resonance mass spectrometer. The assignment of the spin–orbit coupled states of Xe+⋅ is consistent with results from charge exchange ion–molecule reactions of Xe+⋅ with N2O.
The role of rotation in the calculated ultraviolet photodissociation spectrum of ozone
Authors: Alacid Mercedes , Leforestier Claude
Published: July 2002

We present exact quantum calculations of the photodissociation of ozone in the Hartley band. These calculations rely on an hyperspherical description of the system, including rotation. A pseudospectral approach has been used for an efficient implementation of this scheme. The autocorrelation function has been directly computed by means of a Lanczos scheme, augmented by a complex absorbing potential. Using a single excited potential energy surface (D 1B2), calculations up to J=17 are reported. It is shown that in these conditions rotation has only marginal effects over the first 500 fs. The origin of the observed experimental temperature dependence is discussed in this context.
Cycloaddition reaction of furan with Si(100)-2×1
Authors: Qiao M. H. , Tao F. , Cao Y. , Li Z. H. , Dai W. L. , Deng J. F. , Xu G. Q.
Published: July 2002

The adsorption configuration of furan on Si(100)-2×1 at 125 K has been investigated using x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), high resolution electron energy loss spectroscopy (HREELS), and semiempirical molecular orbital (MO) calculation. A chemisorbed furan species is identified, which does not desorb until 300 K. Our results clearly demonstrate the covalent attachment of furan onto Si(100), possibly through a [4+2] cycloaddition reaction. Based on the frontier molecular orbital (FMO) theory and work function measurements, an electron donation mechanism from furan to Si(100)-2×1 is proposed to be involved in the cycloaddition reaction.
Evidence for small interstitial clusters as the origin of photoluminescence W band in ion-implanted silicon
Authors: Giri P. K. , Coffa S. , Rimini E.
Published: July 2002

We have investigated the origin of the photoluminescence (PL) W band in ion-implanted Si by studying the temperature evolution and depth profile of the related defects. Evolution of the PL spectra induced by postimplant annealing is correlated to a transition of small interstitial clusters to extended {311} defects in self-ion-implanted Si. Growth of W band intensity after step-by-step removal of the damaged layer rules out the involvement of vacancy-related defects in the formation of the W center and establishes that migrated and clustered interstitials give rise to an intense W band. The annealing behavior and the thermally activated growth of the W center suggest the involvement of small interstitial clusters, larger than di-interstitial. In accordance with recent results based on simulational studies, we argue that the W center consists of tri-interstitial clusters of silicon.
Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy
Authors: Seccombe D. P. , Tuckett R. P. , Fisher B. O.
Published: July 2002

Using tunable vacuum–ultraviolet radiation from a synchrotron, the decay pathways of the valence electronic states of CF2X2+ (X=Cl, H, Br) in the range 10–25 eV have been determined by threshold photoelectron–photoion coincidence spectroscopy. The ions are separated by a linear time-of-flight mass spectrometer. Coincidence spectra are recorded continuously as a function of energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are recorded with improved time resolution, allowing the mean total translational kinetic energy, 〈KE〉t, into some dissociation channels to be determined. By comparing the 〈KE〉t values for single-bond fragmentations with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. The excited states of all three parent cations show some evidence for isolated-state behavior. With CF2Cl2+ and CF2H2+, this is apparent from the form of the ion yields in the range 11–15 eV, whereas interpretation of the yields for CF2Br2+ is hampered by an absence of thermochemical data. New upper limits at 298 K for the enthalpies of formation of CF2H+ (593±3 kJ mol−1) and CF2Br+ (570±9 kJ mol−1) are obtained. At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond. With CF2Cl2 and CF2Br2, the appearance energies of the fragment ions are close to the thermochemical energy for production of that ion with neutral atoms, suggesting that these ions form by bond-fission processes only. With CF2H2, the one ion unambiguously assigned, CFH+, can only form at certain energies with molecular neutral fragments (i.e., CFH++HF), involving simultaneous bond-breaking and bond-making processes. The 〈KE〉t values for cleavage of a single C–F or C–X bond suggest a relationship between the part of the molecule where ionization occurs and the bond that breaks; impulsive values of 〈KE〉t are more likely to be obtained when the breaking bond lies close to the part of the molecule from which ionization occurs, statistical values when ionization occurs further away from the breaking bond. Furthermore, for all CF2X2+ cations there is a trend from impulsive to statistical behavior as the photon energy is increased.
Frequency-domain fluorescence based fiber optic fire alarm system
Authors: Sun T. , Zhang Z. Y. , Grattan K. T. V.
Published: July 2002

Erbium and thulium doped fibers have been investigated in this work, for the first time, to determine their potential in fire or high temperature alarm applications based on the use of fluorescence decay in the frequency domain. Two configurations of the sensing system have been tested and compared, with particular reference to their minimum spatial resolution and location discrimination characteristics. The initial results show that temperature excursions of 125 °C within 10 cm along a length of a 1.2 m sensing loop can be determined within a period of some tens of seconds, which is adequate for the sensor purpose, and work is continuing to increase the system’s sensitivity.
Stress evolution during Fe(001) epitaxy on GaAs(001)
Authors: Wedler G. , Wassermann B. , Nötzel R. , Koch R.
Published: July 2002

We report on in situ stress measurements of Fe/GaAs(001), which enlighten the dynamics of the interface formation of this important magnetic metal/semiconductor system. At deposition temperatures of 300 and 450 K, the stress evolution during growth is very similar. In Fe films thicker than 6–7 nm, the stress is compressive owing to the misfit between the lattices of Fe and GaAs. Thinner films surprisingly are dominated by a tensile stress contribution due to considerable As (and Ga) interdiffusion even at 300 K.
Influence of preformed shock wave on the development of picosecond laser ablation plasma
Authors: Mao Samuel S. , Mao Xianglei , Greif Ralph , Russo Richard E.
Published: July 2002

Using picosecond time-resolved shadowgrams, we measured the development of an expanding air plasma during double picosecond pulse laser ablation of solids. Prior to the formation of the air plasma, a shock wave was established by the first laser pulse interaction with the solid target. The second picosecond laser pulse interacted with the shock wave and induced the air plasma at the interface between the shock wave front and the ambient air. After the end of the second picosecond ablation laser pulse, the temporal expansion of the air plasma was found to follow the relation r∼t2/5. Results from this air plasma suggest that the shock wave induced by the first laser pulse only absorbs approximately 5% of the energy of the second picosecond laser pulse. Nevertheless, the expansion of the air plasma towards the target results in an enlarged heat-affected zone at the target surface.
Unique broad-spectrum neutron sensing instrument
Authors: Weaver J. A. , Joyce M. J. , Peyton A. J. , Roskell J. , Armishaw M. J.
Published: July 2002

This article reviews the design and operation of a transportable neutron spectrometer. Four of these instruments were produced and have been widely used for applications such as the assessment and classification of radiation hazards at field sites in the United Kingdom and Europe. An overview of the instrument hardware is presented, together with a description of the six sensor channels, their associated processing electronics and the host data logger computer. Spectral unfolding is also summarized and a sample spectra, which was measured from outside a fuel flask, is given.
Dielectric relaxation in Bi2O3–ZnO–Nb2O5 cubic pyrochlore
Authors: Nino Juan C. , Lanagan Michael T. , Randall Clive A.
Published: July 2002

Frequency dispersion associated with dielectric relaxation phenomena in polycrystalline cubic pyrochlore with normal composition (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 is analyzed. Measurements at cryogenic temperatures and high frequencies reveal a broad distribution of relaxation times. The associated dielectric loss data can be modeled with a function convoluting the Vogel–Fulcher law and a Gaussian distribution. This function, dependent only on measuring frequency and temperature, describes exceptionally well the phenomena observed over a frequency range covering seven decades and over 300 K.
Thermally assisted decay of pinning in polycrystalline exchange biased systems (invited)
Authors: Carey M. J. , Smith N. , Gurney B. A. , Childress J. R. , Lin T.
Published: July 2002

The exchange field decays when spin valves are subjected to a field that rotates the pinned layer towards the reverse direction. The decay results from a competition between the torque on the interfacial AF spins from the pinned layer, which lowers the barrier for thermal switching, and the KuV product of the AF grains, which provide stability. Typical values of the decay rates at 125 °C vary between 5–35%/decade, depending upon the AF. A comparison of the thermal decay-rates for many AF’s important for spin valve heads shows that IrMn (blocking temperature, Tb∼250 °C) is the most stable, followed by NiMn (350 °C), PtMn (325 °C) and NiO (200 °C). An Arrhenius-type model is presented which fits the data well. This model is used to estimate the anisotropy constants of NiMn and IrMn to be 5×105 and 2×106 erg/cm3, respectively. Thermally activated reversal of the AF results in Tb often being less than TN, and a peak in the pinned layer coercivity observed near Tb.
Growth of pellet imperfections and laser imprint in direct drive inertial confinement fusion targets
Authors: Schmitt Andrew J. , Velikovich A. L. , Gardner J. H. , Pawley C. , Obenschain S. P. , Aglitskiy Y. , Chan Y.
Published: July 2002

Simple hydrodynamic models for describing the Richtmyer–Meshkov (RM) growth and the Rayleigh–Taylor (RT) instability are tested by simulation. The RM sharp boundary model predictions are compared with numerical simulations of targets with surface perturbations or stationary intensity perturbations. Agreement is found in the overall trends, but the specific behavior can be significantly different. RM growth of imprint from optically smoothed lasers is also simulated and quantified. The results are used to calculate surface perturbations, growth factors, and laser imprint efficiencies. These in turn are used with standard RT growth formulas to predict perturbation growth in multimode simulations of compression and acceleration of planar and spherical targets. The largest differences between prediction and theory occur during ramp-up of the laser intensity, where RT formulas predict more growth than seen in the simulations.
Spin polarized Auger electron spectroscopy of Fe and Ni
Authors: Anilturk O. S. , Koymen A. R.
Published: July 2002

Surface sensitive experiments, in which the spin-polarized electrons are involved, play an important role for magnetic characterization, since the spin-polarized electrons are fingerprints for the local magnetization. Scanning electron microscope with polarization analysis (SEMPA) is one of the most powerful tools to investigate the surface magnetic domain structure of magnetic materials. On the other hand, at energies high enough to generate a two-hole final state arising from Auger transitions, it is possible to observe the spin polarization of the Auger electrons. These electrons reveal element-specific local magnetic information, particularly valuable for surface magnetic studies with composite systems. By using the uniqueness of the UTA-SEMPA tool, one can obtain the magnetic domain picture and also perform spin-polarized Auger electron spectroscopy studies by probing a single domain at the surface. In this study, precisely knowing the probed domain, spin polarization of electrons from super Coster–Kronig MMM Auger emissions on Fe and Ni samples have been investigated. The polarization enhancement above the 3p(M23) threshold is observed on both samples.
Multiplet structures of tetrahedrally coordinated Cr4+ and Cr5+ in Y3Al5O12
Authors: Ishii Takugo , Ogasawara Kazuyoshi , Adachi Hirohiko , Tanaka Isao
Published: July 2002

Multiplet structures of tetrahedrally coordinated Cr4+ and Cr5+ in Y3Al5O12 (yttrium aluminum garnet) were calculated with use of the ab initio electronic-structure calculation method. The calculated absorption spectrum of Cr4+ showed that two bands at the near-infrared and visible spectral regions originated from the same T13 parent multiplet term. The calculated levels originating from Cr5+ in the near-infrared region overlapped with the levels originating from Cr4+. Both the lowest-spin-allowed transitions of Cr4+ and Cr5+ were revealed to have the same polarization dependence. The result indicated that confusion on the assignment of the peaks could be ignored if the Cr5+ state really exists.
Solvent nuclear quantum effects in electron transfer reactions. II. Molecular dynamics study on methanol solution
Authors: Ando Koji
Published: July 2002

Solvent nuclear quantum effects in outer-sphere electron transfer (ET) reactions in methanol solution are examined via a molecular dynamics simulation analysis. The energy gap law of the quantum mechanical ET rate constant is decomposed into contributions from solvent intramolecular vibrations and other low-frequency intermolecular (collective) modes. It is shown that the high-frequency stretching and bending vibrations from the hydroxyl part of the solvent methanol exhibit marked quantum effects on the ET rate despite of their fractional contributions to the reorganization energy (computed to be <4%). A scaling property of the quantum energy gap law is proposed, which would be useful to coordinate data from variety of donor–acceptor systems where the solvent spectral density may have similar profile but the other parameters such as the reaction distance and the reorganization energy may vary. The results are compared with our previous study on aqueous ETs [K. Ando, J. Chem. Phys. 106, 116 (1997)].
The (n00), n=3, 4, and 6, local mode states of H3SiD: Fourier transform infrared and laser photoacoustic spectra and ab initio calculations of spectroscopic parameters
Authors: Bürger H. , Lecoutre M. , Huet T. R. , Breidung J. , Thiel W. , Hänninen V. , Halonen L.
Published: July 2002

The rotational structure of the local mode Si–H stretching vibrational bands (n00 A1/E), n=3, 4, and 6, of H328SiD have been studied by high-resolution Fourier transform infrared and by photoacoustic laser spectroscopy. The recorded bands have been rotationally analyzed with a Hamiltonian model which makes use of simple arithmetic relations between some of the rovibrational parameters. While the (300 A1/E) states were found to be unperturbed, severe perturbations by unknown dark states affect the (400 A1/E) and (600 A1/E) states for J values exceeding 8. Ab initio calculations have been performed to form the quadratic and the cubic potential energy surfaces which have been used to calculate spectroscopic parameters for the Si–H stretching fundamentals. These results, together with the local mode relations, have been successfully used to model the vibrational dependence of effective rovibrational parameters in the excited local mode states.
Spectroscopy and MRCI calculations on CrF and CrCl
Authors: Koivisto R. , Launila O. , Schimmelpfennig B. , Simard B. , Wahlgren U.
Published: July 2002

The B 6Π–X 6Σ+ band system of the CrCl radical has been recorded in thermal emission with FTS techniques in the region 6900–11 700 cm−1, using a resolution of 0.025 cm−1. An analysis of the (0,0) band of this system has been carried out. A rotational constant of B0=0.1615 cm−1 and a spin–orbit parameter of A0=51 cm−1 have been estimated for the B 6Π state. Improved values for the γ and λ parameters of the X 6Σ+ ground state have been obtained. MRCI calculations have been performed on CrF and CrCl and comparisons have been made with both experiments and DFT calculations [Bencheikh et al., J. Chem. Phys. 106, 6231 (1997)]. The calculations on CrF have provided valuable aids in the interpretations of the fluorescence excitation and dispersed fluorescence spectra of a state observed at 31 700 cm−1. This state has now been assigned as D 6Π. A number of transitions in the dispersed fluorescence spectra are believed to involve the hitherto unknown C 6Δ state. Spin–orbit-configuration-interaction (SO-CI) calculations have been carried out on both CrF and CrCl. It is shown that the second-order spin–orbit parameters λe of the X 6Σ+ ground states are due mainly to interactions with the B 6Π states.
An operator valued extension of the super Korteweg–de Vries equations
Authors: Andrea S. , Restuccia A. , Sotomayor A.
Published: July 2002

An extension of the super Korteweg–de Vries (KdV) integrable system in terms of operator valued functions is obtained. Following the ideas of Gardner, a general algebraic approach for finding the infinitely many conserved quantities of integrable systems is presented. The approach is applied to the above described system and infinitely many conserved quantities are constructed. In a particular case they reduce to the corresponding conserved quantities of super KdV.
Ribonucleic acid folder: The earliest moves of a good structure seeker
Authors: Fernández Ariel , Appignanesi Gustavo
Published: July 2002

We identify the earliest folding events of a classical soft-mode ribonucleic acid chain under renaturation conditions. Due to hard-mode elimination, when this problem is cast in terms of intrinsic (dihedral torsional) coordinates, the weight of the differential volume, resulting merely from the transformation from Cartesian onto intrinsic coordinates, is not constant. This inherent geometry resulting from the embedding of the infinite Cartesian space onto the compact torsional manifold has physical consequences, especially in the earliest folding times, when the intramolecular potential has not yet had a chance to outweigh it. Thus, the early folding events are induced by a coupling between the inherent geometry and the hydrodynamic drag, and they predate the occurrence of nonbonded intrachain interactions. Thus, by contrasting the early geometrically determined probability distribution in torsional conformation space with the energetically favorable regions, we elucidate how the interplay between hydrodynamic control and potential energy surface during the early stages of folding (10 ps–1 μs) determines the expediency of the process. In this way we are able to decide in simple cases whether geometry and Watson–Crick complementarities represent reinforcing or conflicting propensities, defining a good seeker or a bad folder.
Microstructure of laterally overgrown GaN layers
Authors: Liliental-Weber Z. , Cherns David
Published: July 2002

Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed light on the optical properties of such samples.
Spectroscopic properties of nitrogen doped hydrogenated amorphous carbon films grown by radio frequency plasma-enhanced chemical vapor deposition
Authors: Hayashi Y. , Yu G. , Rahman M. M. , Krishna K. M. , Soga T. , Jimbo T. , Umeno M.
Published: July 2002

Nitrogen doped hydrogenated amorphous carbon thin films have been deposited by rf plasma-enhanced chemical vapor deposition using CH4 as the source of carbon and with different nitrogen flow rates (N2/CH4 gas ratios between 0 and 3), at 300 K. The dependence modifications of the optical and the structural properties on nitrogen incorporation were investigated using different spectroscopic techniques, such as, Raman spectroscopy, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, ultraviolet-visible (UV-VIS) spectroscopy, electron spin resonance (ESR), photoluminescence (PL) and spectroscopic ellipsometry (SE). Raman spectroscopy and IR absorption reveal an increase in sp2-bonded carbon or a change in sp2 domain size with increasing nitrogen flow rate. It is found that the configuration of nitrogen atoms incorporated into an amorphous carbon network gradually changes from nitrogen atoms surrounded by three (σ bonded) to two (π bonded) neighboring carbons with increasing nitrogen flow rate. Tauc optical gap is reduced from 2.6 to 2.0 eV, and the ESR spin density and the peak-to-peak linewidth increase sharply with increasing nitrogen flow rate. Excellent agreement has been found between the measured SE data and modeled spectra, in which an empirical dielectric function of amorphous materials and a linear void distribution along the thickness have been assumed. The influence of nitrogen on the electronic density of states is explained based on the optical properties measured by UV-VIS and PL including nitrogen lone pair band.
Grating-tuned external-cavity quantum-cascade semiconductor lasers
Authors: Luo G. P. , Peng C. , Le H. Q. , Pei S. S. , Hwang W.-Y. , Ishaug B. , Um J. , Baillargeon James N. , Lin C.-H.
Published: July 2002

Grating-coupled external-cavity quantum-cascade lasers were studied for temperatures from 80 to 230 K. At 80 K, a tuning range of ∼65–88 nm are obtained for 4.5 and 5.1 μm laser amplifiers, respectively. The tuning ranges for both narrowed substantially with increasing temperature, to ∼23 nm at 203 K. The threshold varied slowly versus wavelength, while the efficiency appeared to be close to optimum toward wavelengths shorter than the free running wavelength.
Strong photoluminescence emission from polycrystalline GaN layers grown on W, Mo, Ta, and Nb metal substrates
Authors: Yamada K. , Asahi H. , Tampo H. , Imanishi Y. , Ohnishi K. , Asami K.
Published: July 2002

Polycrystalline GaN layers were grown on W, Mo, Ta, and Nb metal substrates by gas-source molecular-beam epitaxy using an ion-removal, electron-cyclotron-resonance radical cell. X-ray diffraction rocking curves showed preferential GaN(0002) or GaN(10–11) orientations. The grain sizes ranged from 100 to 800 nm. Strong photoluminescence (PL) emission without yellow luminescence was observed from these polycrystalline GaN layers. At 77 K, PL peaks at 3.46 and 3.26 eV were observed, and their temperature dependence fit a simple relation based on the number of phonons. The higher-energy peak probably was due to the free excitonic transition in hexagonal GaN.
A lifetime distribution study on the incoherent excitation transfer in systems with static disorder: Multiparticle approximations
Authors: Mollay B.
Published: July 2002

The topic of this work is to discuss the nonexponential relaxation behavior of electronic excitations caused by static disorder in terms of the lifetime distribution χ(τ) associated with the direct electronic transfer process. Special emphasis is given to the correlation of χ(τ) with the underlying spatial distribution functions. A multiparticle approximation is proposed which can be applied to randomly distributed donor–acceptor systems interacting in Δ-dimensional spaces via some arbitrary coupling mechanism. The method yields the distribution of lifetimes in terms of a hierarchy of integral representations of orders m=1,2,3,… corresponding to the interaction of a donor with m surrounding acceptors. For the multipolar case, the first approximation orders are elaborated and discussed in terms of the moments of the next neighbors distance distribution functions.
Vibrational and collisional energy effects in the reaction of ammonia ions with methylamine
Authors: Flad Jonathan E. , Everest Michael A. , Poutsma John C. , Zare Richard N.
Published: July 2002

We have investigated the reactions of vibrationally state-selected ammonia ions with d3-methylamine over the center-of-mass collisional energy range of 0.5 to 10.0 eV and for ammonia ion vibrational states ranging from ν2=1–9. Under these conditions, five major products appear: NH4+, NH3D+, CD2NH2+, CD3NH2+, and CD3NH3+. The cross section for each product is a decreasing function of collision energy and also a decreasing function of energy in the ν2 mode of the ammonia ion, except for CD2NH2+ that shows about a twofold enhancement with increasing internal energy, most notably at low-collision energies. Examination of the velocity scattering profiles shows that the mechanism for formation of each major product does not involve complex formation in this energy range. Branching ratios for each product are measured, and a comparison is presented for CD2NH2+ and CD3NH2+ arising from reactions with ammonia ions prepared in two nearly isoenergetic states. One state has no quanta in the symmetric stretch and five quanta in the umbrella bending mode (1025) and the other has one quantum in the symmetric stretch and two quanta in the umbrella bending mode (1122). Comparison indicates that this reaction is vibrationally mode selective, although the extent of mode selectivity is small.
Low lying vibrational excitation energies from equilibrium path integral simulations
Authors: Ramı́rez Rafael , López-Ciudad Telesforo
Published: July 2002

The centroid density is a function defined for quantum systems in thermodynamic equilibrium that is readily obtained by path integral simulations. The physical information provided by the centroid density is a static response of the system under isothermal conditions, namely, the change in the expectation value of the position operator of the quantum particles upon application of constant external forces. An interesting application of this function is the study of vibrational properties of atomic nuclei in molecules and solids. In particular, the analysis of the tensor defined by the second cumulants of the centroid density (i.e., the static isothermal susceptibility tensor) leads to the definition of the linear response vibrational modes, which are characterized by a response of the quantum system parallel to the applied force. The eigenvalues of the susceptibility tensor provide the linear response of the system. This response function is the basis for the formulation of two approximations to evaluate vibrational frequencies. The capability of these approximations is tested in the study of the tunneling frequency of a particle in a double-well potential, the anharmonic shift in the vibrational frequencies of molecules (H2 and HOCl), and the anharmonic shifts of phonon frequencies in diamond.
Quasigraded Lie algebras on hyperelliptic curves and classical integrable systems
Authors: Skrypnyk T.
Published: July 2002

A new family of infinite-dimensional Lie algebras on hyperelliptic curves is constructed. We show them to be quasigraded and explicitly find their central extensions. We also show, that constructed algebras in the case of zero central charge possess infinite number of invariant functions. Besides, they admit a decomposition into the direct sum of two subalgebras. These two facts together enables one to use them to construct new integrable systems. In such a way we find new integrable Hamiltonian systems, which are direct higher rank generalizations of the integrable systems of Steklov–Liapunov, associated with the e(3) algebra and Steklov–Veselov associated with the so(4) algebra. Besides we give hyperelliptic Lax representation for the generalized Euler tops.
Prediction of amorphous phase stability in the metal–silicon systems
Authors: Liu Y. Q. , Shao G. , Homewood K. P.
Published: July 2002

The stability of the amorphous phase with respect to the liquid phase in metal–silicon systems is modeled thermodynamically as a second-order phase transformation. The glass transition temperature of amorphous silicon is estimated according to the experimentally determined heat of crystallization and the Third Law of Thermodynamics. The feasibility of the model has been demonstrated using the Pd–Si, Co–Si, and Au–Si systems as examples. The predicted glass transition temperatures and heat of formation of the amorphous phase are consistent with available experimental data. The predicted amorphization stabilization at low temperatures in the Co–Si systems agrees with experimental observations.
Isotropy vs anisotropy in small-scale turbulence
Authors: Biferale Luca , Vergassola Massimo
Published: July 2002

The decay of large-scale anisotropies in small-scale turbulent flow is investigated. By introducing two different kinds of estimators we discuss the relation between the presence of a hierarchy for the isotropic and the anisotropic scaling exponents and the persistence of anisotropies. By direct measurements on a channel flow numerical simulation we show that the presence of a hierarchy for the isotropic and the anisotropic scaling exponent is not in contradiction with the persistence of anisotropies at small scales.
Optical characterization of 4H-SiC by far ultraviolet spectroscopic ellipsometry
Authors: Lim Seung-Gu , Jackson Thomas N. , Mitchel W. C. , Bertke R. , Freeouf J. L.
Published: July 2002

We have developed a far UV spectroscopic ellipsometer system working up to 9 eV, and applied it to the characterization of three 4H-SiC samples with different surface conditions [i.e., as-received and chemical mechanical processing (CMP) processed 4H-SiC bulk substrates and a 4H-SiC epi sample]. Pseudodielectric functions ε1 and ε2 clearly demonstrate the excellent surface sensitivity of the far UV ellipsometry system as it distinguishes the improvements provided by CMP process. Simulation results of ellipsometer data indicate the existence of a damaged subsurface layer in the as-received 4H-SiC bulk substrate. The investigation of sample surfaces using atomic force microscopy confirms the results of ellipsometry measurements.
Geometric quantization on the super-disk
Authors: Turgut Osman Teoman
Published: July 2002

In this article we discuss the geometric quantization on a certain type of infinite dimensional super-disk. Such systems are quite natural when we analyze coupled bosons and fermions. The large-N limit of a system like that corresponds to a certain super-homogeneous space. First, we define an example of a super-homogeneous manifold: a super-disk. We show that it has a natural symplectic form, and it can be used to introduce classical dynamics once a Hamiltonian is chosen. Existence of moment maps provide a Poisson realization of the underlying symmetry super-group. These are the natural operators to quantize via methods of geometric quantization, and we show that this can be done.
The reaction field of a water molecule in liquid water: Comparison of different quantum/classical models
Authors: Chalmet Stéphanie , Ruiz-López Manuel F.
Published: July 2002

The reaction field of a water molecule in liquid water has been computed with the help of continuum, discrete-continuum, and discrete models, using density functional theory calculations. In the continuum model, the liquid is simply described by a polarizable dielectric medium. The solute is placed in a cavity defined by a scaled van der Waals surface. Two different sets of van der Waals radii have been used for the atomic spheres. The discrete-continuum model consists of a quantum molecule surrounded by four classical molecules, the resulting aggregate being embedded in a dielectric continuum. Finally, in the discrete model, a molecular dynamics simulation is carried out for a quantum molecule in a box containing 215 classical molecules with periodic boundary conditions. The reaction field and the induced dipole moment in the standard continuum model are substantially underestimated. However, the use of optimized van der Waals radii for the atomic spheres produces a notable improvement. The discrete-continuum and discrete models lead to close results that are in good agreement with experimental data and previous theoretical estimations. For instance, the induced dipole moment (0.80 and 0.82 D for discrete-continuum and discrete models, respectively) compares well with the experimental estimate (0.75 D) and with Car–Parrinello simulations (1.08 D). The reaction field potential is analyzed in terms of multipole moment contributions. The role of the first shell and bulk solvent are also examined.
Influence of low-temperature buffer layers on the optical waveguiding properties of AlxGa1−xN alloys
Authors: Dogheche E. , Ruterana P. , Omnes F.
Published: July 2002

The influence of buffer layers on the optical waveguide properties of nitride related semiconductors films grown on sapphire by metalorganic vapor phase epitaxy has been investigated. Using the prism coupling technique, we measured the ordinary (nTE) and extraordinary (nTM) refractive indices of AlxGa1−xN/AlN and AlxGa1−xN/GaN heterostructures, where AlN, GaN are low temperature buffer layers. We investigated the relationship between the refractive indices of AlxGa1−xN alloys and the Al content (x) at a wavelength of 632.8 nm: nTE=2.3240–0.33x. This investigation demonstrates optical waveguiding in gallium nitride materials. It is shown that the indices of thick AlxGa1−xN films are independent of the buffer layer (AlN, GaN). However, the optical propagation losses are highly sensitive to the crystalline quality of the layers, particularly to the density of dislocations in the film. The best optical result of 1.2 dB/cm is obtained when the GaN buffer layer was employed, in comparison to 1.8 dB/cm with the AlN buffer layer. We interpret these results in relation with the transmission electron microscopy analysis.
Miniband transport in vertical superlattice field-effect transistors
Authors: Deutschmann R. A. , Wegscheider W. , Rother M. , Bichler M. , Abstreiter G.
Published: July 2002

We study the nonequilibrium transport of two-dimensional electrons through a periodic potential. Our samples are fabricated using the cleaved-edge overgrowth technique to provide a vertical field-effect transistor with an undoped GaAs/AlGaAs superlattice channel orthogonal to the current flow. We find a pronounced negative differential resistance, the magnitude of which increases with increasing modulation strength. The data are qualitatively consistent with the Esaki–Tsu transport model in minibands, which we calculate for the given samples. We emphasize the significance of the two-dimensionality of the electron system and the gate to inhibit domain formation. Weak features in the source-drain current are attributed to Bloch-phonon resonances.
A two-component variant of the Douglas–Kroll relativistic linear combination of Gaussian-type orbitals density-functional method: Spin–orbit effects in atoms and diatomics
Authors: Mayer Markus , Krüger Sven , Rösch Notker
Published: July 2002

The scalar relativistic variant of the linear combination of Gaussian-type orbitals—fitting functions—density-functional (R-LCGTO-FF-DF) method is extended to a two-component scheme which permits a self-consistent treatment of the spin–orbit interaction. The method is based on the Douglas–Kroll transformation of the four-component Dirac–Kohn–Sham equation. The present implementation in the program PARAGAUSS neglects spin–orbit effects in the electron–electron interaction. This approximation is shown to be satisfactory as long as bonding is restricted to s, p, and d orbitals. The method is applied to the diatomics Au2, Bi2, Pb2, PbO, and TlH using both a local density (LDA) and a gradient-corrected approximation (GGA) of the exchange-correlation functional. At the LDA level, bond lengths and vibrational frequencies are reproduced with high accuracy. For the determination of binding energies the open-shell reference atoms Au, Tl, Pb, Bi have been treated by a jj coupling approach based on a self-consistent noncollinear spin density-functional scheme and with an intermediate coupling procedure. The atomic state energies obtained with the jj coupling scheme agree well with experiment, but they are somewhat too high due to the incomplete inclusion of static correlation. Binding energies of diatomics at the GGA level are considerably improved due to the inclusion of spin–orbit interaction. The jj derived values are somewhat overestimated (by about 10%) compared to experiment, and they compare slightly worse with experiment than results based on the intermediate coupling approximation.
Effect of neodymium (Nd) doping on the dielectric and ferroelectric characteristics of sol-gel derived lead zirconate titanate (53/47) thin films
Authors: Majumder S. B. , Roy B. , Katiyar R. S. , Krupanidhi S. B.
Published: July 2002

The results of the studies on the effect of rare earth Nd doping on the phase formation behavior and electrical properties of sol-gel derived Pb1.05(Zr0.53Ti0.47)O3 (PZT) thin films are presented. The perovskite phase is obtained up to 5 at. % doping and beyond that pyrochlore phase was found to coexist with the perovskite phase in all the films. The transition temperature of undoped lead zirconate titanate (PZT) film was found to be reduced with Nd doping. The Nd doped films also exhibited typical relaxor-type behavior and a diffuse phase transition, similar to that observed in relaxor materials. The introduction of Nd into the PZT lattice probably introduces disorder in the B site of ABO3 lattice, which causes the observed dielectric relaxation. Efforts were made to isolate the irreversible component contributions in low field dielectric and high field polarization switching behavior.
Coalescence of C20 fullerenes
Authors: Ehlich Rudolf , Landenberger Peter , Prinzbach Horst
Published: July 2002

Coalescence of C20(H)n fullerenes with n=0–4 was studied by laser desorption and ionization in a time-of-flight mass spectrometer (337 nm N2 excimer laser). The fullerenes were produced by elimination of bromine from extensively brominated dodecahedranes (mixture, mean composition [C20H1Br13]≡Y, and mixtures with somewhat higher hydrogen content [C20H2Br12]≡X) accessible by photobromination of the C20H20 cage hydrocarbon. For Y probes at 80 mJ/cm2 laser fluence, coalescence of the generated C20 fullerenes was identified as the dominant process providing a series of (C20)k+ oligomers (k=1–13); there is evidence for the formation of C60+ from C20. At lower fluences (30 mJ/cm2) in the mass spectra measured near the desorption threshold or in the low ion velocity regime, the bromine substituents were not (totally) eliminated, the original degree of bromination has even been raised. High laser fluences (1500 mJ/cm2) primarily effect the bromine content, generating C20HnBrm ions with m ranging from 0 to 18, fullerene coalescence is suppressed. For X probes the only slightly higher hydrogen content inhibits C20 coalescence; a C36H4+ ion is interpreted as fullerene.
The escaped charge problem in solvation continuum models
Authors: Cancès Eric , Mennucci Benedetta
Published: July 2002

In the present article, we provide an analytical proof of the accuracy shown by integral equation formalism (IEF) continuum solvation method in dealing with the solute electronic charge that quantum mechanically penetrates outside the cavity embedding it. In particular we show that the approximation of the solute–solvent interaction energy obtained by this method is always an upper bound of the exact electrostatic interaction energy and that a lower bound of the exact energy can also be defined. A numerical example for which both approximated bounds and exact energies can be computed is presented.
Free surface Stokes flow over topography
Authors: Mazouchi Ali , Homsy G. M.
Published: July 2002

We study two-dimensional viscous flow over topographical features under the action of an external body force. The Stokes equations are written as a set of harmonic and biharmonic equations for vorticity and stream function. A direct biharmonic boundary integral equation method is used to transform these equations to a pair of integral equations on the boundary of the domain. These equations are solved with a preassumed free surface profile to obtain values of the flow variables on the boundary. The location of the free boundary is then updated by considering the normal stress condition and using an iteration technique. We have studied the flow over steps and trenches of different depths and for a wide range of capillary numbers, Ca. Our computation shows that for small Ca, the free surface develops a ridge before the entrance to a step down and a depression region right before a step up. The magnitude and location of these features depend on the capillary number and the step depth. For large capillary numbers the free surface nearly follows the topography and the ridge and depression are found to be exponentially small in the capillary number. On the other hand, our results agree well with lubrication theory for small capillary numbers on the order of 10−2 or less, even for steep features.
Ultrafast exciton dynamics of J-aggregates in room temperature solution studied by third-order nonlinear optical spectroscopy and numerical simulation based on exciton theory
Authors: Ohta Kaoru , Yang Mino , Fleming Graham R.
Published: July 2002

We report a study of the exciton dynamics in 1,1′-diethyl-3,3′-bis(sulforpropyl)-5,5′,6,6′ -tetrachlorobenzimidacarbocyanine (BIC) J-aggregates in water solution at room temperature by third-order nonlinear optical spectroscopy and numerical simulations based on exciton theory. The temporal profiles of the transient grating signals depend strongly on the excitation intensity as a result of exciton–exciton annihilation. On the other hand, the peak shift measurement gives information on the fluctuations of the transition frequency of the system. The peak shift decays with time constants of 26 and 128 fs. There is no finite peak shift on a longer time scale. The electronic state of J-aggregates is described by a Frenkel exciton Hamiltonian, and the exciton population relaxation processes is described by Redfield equations. Based on the numerical simulations, the peak shift data can only be explained even qualitatively when both exchange narrowing and exciton relaxation process are included in the model. The 128-fs component is assigned to a “hopping” time between exciton units. We confirmed that while the static disorder within an exction state that is partially delocalized due to static disorder is exchange-narrowed, the exchange narrowing of the dynamical disorder is not complete but appears as lifetime broadening, which competes with the exchange narrowing of the fluctuations. The effect of the exciton relaxation on the absorption spectrum is discussed.
Dimer-formation in the bis(arene)chromium fulleride Cr(C7H8)2 C60
Authors: Hönnerscheid Andreas , van Wüllen Leo , Jansen Martin , Rahmer Jürgen , Mehring Michael
Published: July 2002

Bis(toluene)chromium fulleride Cr(C7H8)2 C60 has been synthesized as a microcrystalline powder from C60 and Cr(C7H8)2 in toluene. One electron is transferred from the chromium complex to the fullerene resulting in a magnetic moment of about 2.5±0.2 μB at room temperature, which corresponds to two electron spins per formula unit. At 250 K a reversible phase transition from a primitive cubic high temperature paramagnetic phase to a triclinic low temperature dimer phase occurs. The high temperature phase (Pm3̄m, a=9.986 Å) is constituted of dynamically disordered fulleride anions and bis(toluene)chromium (I) cations in a CsCl-type arrangement. The low temperature modification is triclinic (P1̄, a=13.541 Å, b=13.864 Å, c=14.122 Å, α=120.59, β=91.78, γ=116.88) and carries one unpaired electron per formula unit (μ=1.72±0.01 μB), which according to ESR measurements is localized at the chromium site (giso=1.9870). The phase transition has been monitored by x-ray diffraction, ESR and solid state NMR spectroscopy as well as by SQUID measurements. All experimental results can consistently be described within the scenario of dimer formation of the C60 monoanions during the transformation.