bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray induced photoemission of a localized electron and its application to site-selective x-ray absorption fine structure measurement
Authors: Ishii Masashi , Yoshino Yoko , Takarabe Ken-ichi , Shimomura Osamu
Published: July 2002

For local structure analyses of carrier trap centers in semiconductors, site-selective x-ray absorption fine structure (XAFS) measurement, the “capacitance XAFS” method, is proposed. The concept of capacitance XAFS measurement is based on the fact that the amount of x-ray absorption of trap centers, not the bulk, may be evaluated from the capacitance change due to x-ray induced photoemission of a localized electron. In order to verify this model, characteristics of x-ray induced photoemission from defects are investigated by capacitance–voltage measurement. The temperature dependence of the photocarrier concentration in a semiconductor corresponds to that of the capacitance XAFS signal amplitude. On the other hand, no influence of the thermal excitation of defects on the capacitance XAFS signal amplitude is observed. These results indicate that the capacitance XAFS signal originates from localized electron emission via the inner-shell excitation of defect atoms, resulting in site selectivity to the trap centers.
Conductivity and Hall effect characterization of highly resistive molecular-beam epitaxial GaN layers
Authors: Kordoš P. , Morvic M. , Betko J. , Van Hove J. M. , Wowchak A. M. , Chow P. P.
Published: July 2002

Highly resistive molecular beam epitaxial GaN layers are characterized by temperature dependent conductivity and Hall effect measurements. Seven n-type GaN samples with room temperature layer resistivity ranging between 8 and 4.2×106 Ω cm are used in this study. The experimental data are analyzed by considering various transport models such as band and hopping conduction, scattering on charged dislocations and grain boundaries controlled transport. The same defect level of 0.23 eV, attributed to nitrogen vacancy, is found for layers with ρ300⩽3.7×103 Ω cm. The Hall mobility for two lower resistivity layers is influenced mainly by phonon scattering (μH∼Tx, x=−1.4). However, higher resistivity layers show positive mobility power, x=0.5–0.9, which can be explained by dominating scattering on charged dislocations. Properties of layers with the highest resistivity (1×105 and 4.2×106 Ω cm) and extremely low Hall mobility (6 and <0.1 cm2 V−1 s−1) are consistent with grain boundary controlled transport. The barrier height between grains of 0.11 eV and an average grain size of 200 nm are found. Neither nearest-neighbor or variable range single phonon hopping nor multiphonon hopping can be clearly attributed to the conduction of the layers investigated.
Interpolated ab initio quantum scattering for the reaction of OH with HCl
Authors: Yu Hua-Gen , Nyman Gunnar
Published: July 2002

We present an interpolated ab initio quantum scattering (AIQS) approach to studying chemical reactions. The dynamics calculations were performed by solving the time-independent Schrödinger equation. The potential energy surface used was interpolated from a set of grid energy points by a generalized discrete variable representation method. The guided spectral transform technique developed by the authors and the single-sector hyperspherical projection method for boundary conditions were extended to the rotating bond approximation model. The AIQS approach has been applied to the OH+HCl→Cl+H2O reaction, where the grid energy points were calculated by a dual level ab initio theory. The results obtained show that there exist a van der Waals complex in the entrance channel and an early saddle point with nonplanar geometry. Calculated thermal rate constants are in good agreement with the experimental results. They are nearly temperature-independent at low temperatures (T<300 K) while a sharp increase with temperature is observed at higher temperatures. In addition, it was found that the long range interaction potential plays an important role in the dynamics.
A configurational-bias approach for the simulation of inner sections of linear and cyclic molecules
Authors: Chen Zhong , Escobedo Fernando A.
Published: July 2002

A novel Monte Carlo technique, the rebridging configurational-bias (RCB) method, has been developed to rearrange inner sections of chain molecules having strong intramolecular interactions along the backbone. The ability of sampling inner chain segments is important for the simulation of systems with low concentration of chain ends, such as polymers and molecules with cyclic structures. In the RCB method, inner segments are removed and then regrown site-by-site in a configurational-bias fashion. However, a short preliminary simulation is carried out to calculate weighting functions based on histograms of the separation distance between pairs of sites along the chain; these functions are used to bias the trial positions of growing inner sites so as to promote efficient chain closure. A look-ahead search scheme is employed for the sampling of the last two sites to increase the overall acceptance rate of the reconstruction process. The validity and performance of the new algorithm were tested by studying linear alkane systems of various chain lengths. Fast conformational equilibration was observed, from the level of local bond orientations to the level of the chain end-to-end vector orientations. Cyclic alkanes containing from 8 to 100 carbon atoms were also studied by using the RCB method. Our results for the conformational properties of cyclooctane are generally consistent with previous molecular dynamics (MD) results and with experimental data. The vapor–liquid coexistence curve of cyclooctane was also mapped out by using the RCB method in conjunction with a histogram reweighting technique for the analysis of isothermal–isobaric simulations.
Active control of laminar-turbulent transition using instantaneous vorticity signals at the wall
Authors: Gmelin Christoph , Rist Ulrich
Published: July 2002

Many approaches with the objective to actively delay the laminar-turbulent transition in boundary layers are currently under investigation. These approaches, which are mostly based on the superposition of anti-phase disturbances, fail in cases where high (nonlinear) disturbance amplitudes occur. One possible solution to overcome this problem is the direct feedback of instantaneous flow signals from the wall. In our case the spanwise vorticity (ωz) on the wall is sensed, multiplied by a certain factor A and prescribed as a new boundary condition at the wall with some time delay Δt. This procedure (called ωz-control ) yields a robust algorithm which is less influenced by nonlinearities than other processes based on the linear superposition of disturbances (waves). The method was developed and evaluated using both linear stability theory and a three-dimensional spatial DNS code solving the complete Navier–Stokes equations.
Isotope effect on the translational and rotational motion in liquid water and ammonia
Authors: Hardy Edme H. , Zygar Astrid , Zeidler Manfred D. , Holz Manfred , Sacher Frank D.
Published: July 2002

The dynamic isotope effects (IE) on the translational and rotational motion in liquid ammonia and water are reassessed by NMR measurements. For H2O/D2O the translational and rotational IE are clearly distinct. At 298 K, 23% and 30% are obtained, respectively. Both effects as well as the slopes of the temperature dependencies increase with decreasing temperature. For NH3/ND3 a rotational IE of 37% was observed at 298 K. A small increase to 40% at 222 K could be ascertained. The translational IE is about 15% at room temperature and exhibits a stronger temperature dependence. It is suggested that the observed deviations of the IE’s from the square root of mass and square root of moments of inertia laws are caused by translation-rotation coupling as well as quantum effects. The experimental data obtained in the present paper are also of importance for the correct interpretation of all kinds of experiments on water and ammonia, where isotopic substitutions are involved.
Observation of exciplex emission from the mixed dimer of naphthalene and 2-methoxynaphthalene: A laser-induced fluorescence study in supersonic jet
Authors: Das Aloke , Mahato K. K. , Chakraborty Tapas
Published: July 2002

Exciplex formation in the van der Waals mixed dimer of naphthalene and 2-methoxynaphthalene has been investigated by using laser-induced fluorescence spectroscopy in supersonic jet. In contrast to the barrierless excimer formation in the S1 state of naphthalene homodimer, the exciplex formation in the mixed dimer occurs with a vibrational energy barrier of 466 cm−1. The energy gap between the S1 states of two molecules in the mixed dimer is 1000 cm−1. Therefore, the contribution of the exciton resonance interaction to the stability of the exciplex state is considered to be insignificant. Charge transfer interaction and the mixing of the charge transfer state with the locally excited state due to orbital overlap between two molecules at suitable geometry are proposed to be the decisive factors in formation of the exciplex.
Ab initio calculations of adiabatic and diabatic potential energy surfaces of Cl(2P)⋯HCl(1Σ+) van der Waals complex
Authors: Kłos Jacek A. , Chałasiński Grzegorz , Szczęśniak M. M. , Werner Hans-Joachim
Published: July 2002

Adiabatic and diabatic potential energy surfaces for the Cl(2P) atom interacting with the HCl molecule are calculated at the restricted coupled cluster singles, doubles, and noniterative triples [RCCSD(T)] level of theory and with the extended augmented correlation-consistent polarized valence-triple-zeta basis set supplemented with bond functions. An approximate counterpoise correction is applied to evaluate interaction energy of three adiabatic states: 1 2A′, 2 2A′, and the 1 2A″. Next, the adiabats are transformed to four diabats. The mixing angle of the adiabatic–diabatic transformation is determined from the transition matrix elements of the angular momentum operator L̂y calculated using the adiabatic multireference configuration interaction wave functions. At the RCCSD(T) level of theory the global minimum of the 1 2A′ surface occurs for the T-shaped geometry at θ=90° and R=3.0 Å with the well depth De=586 cm−1. There is also a local minimum at the collinear geometry Cl⋯H–Cl. The global minimum of 2 2A′ occurs for the collinear arrangement H–Cl⋯Cl at R=3.75 Å and with the well depth De=126 cm−1. The 1 2A″ state exhibits two collinear minima, and the global one is for the Cl⋯H–Cl arrangement at R=4.0 Å with the well depth De=429 cm−1. State crossings were also detected: one crossing between the 2Σ+ and 2Π states near R=3.0 Å for the Cl⋯H–Cl form, and two others for the H–Cl⋯Cl form, around R=3.4 Å and R=6.0 Å.
A Langevin equation for turbulent velocity increments
Authors: Marcq Philippe , Naert Antoine
Published: July 2002

Recently, Friedrich and Peinke demonstrated empirically that a Fokker–Planck equation describes the scale dependence of probability distribution functions of longitudinal velocity increments vr in fully developed turbulent flows. Thanks to the analysis of an experimental velocity signal, the stochastic process vr is further investigated by examining the related Langevin equation. This process is found to be Markovian in scale because the turbulent velocity field is correlated over distances much larger than the correlation length ρ of its spatial derivative. A Gaussian approximation for the random force yields evolution equations for the structure functions 〈vrn〉. Analytic solutions are obtained, in agreement with experimental data for even-order moments when the scale r is larger than a few times ρ. The third-order moment 〈vr3〉 is found linear in r, as predicted by Kolmogorov’s four-fifths law.
On the energy dependence of the hyperfine interaction in excited states of NO2
Authors: Xin Ju , Reid Scott A. , Santoro Fabrizio , Petrongolo Carlo
Published: July 2002

We report on the hyperfine structure of N=1, J=3/2 levels of 62 NO2 vibronic states in the 16 850–21 500 cm−1 region, as determined via quantum beat spectroscopy. The hyperfine structure of these levels of mixed Ã 2B2/X̃ 2A1 electronic character is dominated by the X̃ 2A1 Fermi-contact interaction, and a decrease in the hyperfine splittings with increasing energy is revealed when our results are compared with previous studies in the 11 200–13 700 cm−1 region. This comparison also reveals the loss of a correlation between band intensity and Fermi-contact constant. A detailed comparison of our results with theoretical predictions for the 16 600–18 700 cm−1 region is presented. We find that vibrational averaging of the X̃ 2A1 Fermi-contact interaction is reflected in the hyperfine interaction of bands in this region, and should also be a factor in producing the small hyperfine splittings observed at energies near dissociation threshold.
Crystal-polarity dependence of Ti/Al contacts to freestanding n-GaN substrate
Authors: Kwak Joon Seop , Lee K. Y. , Han J. Y. , Cho J. , Chae S. , Nam O. H. , Park Y.
Published: July 2002

The effect of crystal polarity on the electrical properties of Ti/Al contacts to n-GaN substrate has been investigated. The Ti/Al contacts prepared on Ga-face n-GaN substrate became ohmic with a contact resistivity of 2×10−5 Ω cm2 after annealing at temperatures higher than 600 °C for 30 s. On the contrary, the contacts on N-face n-GaN substrate exhibited nonlinear current–voltage curve and high Schottky barrier heights over 1 eV were measured at the same annealing conditions. These results could be explained by opposite piezoelectric-field at GaN/AlN heterostructure resulted from different polarity of the GaN substrate.
Energy transfer assisted frequency upconversion in Ho3+ doped fluoroindate glass
Authors: Rakov N. , Maciel G. S. , de Araújo Cid B. , Messaddeq Y.
Published: July 2002

Experimental results are reported which show a strong evidence of energy transfer between Ho3+ ions in a fluoroindate glass excited by a pulsed laser operating at 640 nm. We identified the origin of the blue and green upconverted fluorescence observed as being due to a Ho3+–Ho3+ pair interaction process. The dynamics of the fluorescence revealed the pathways involved in the energy transfer assisted upconversion process.
Variational inequalities in the flows of yield stress fluids including inertia: Theory and applications
Authors: Huilgol R. R.
Published: July 2002

Using the constitutive relations for a yield stress fluid in which the viscosity is shear rate dependent while the yield stress is a constant, a systematic procedure to include the local and convected acceleration components into a variational inequality applicable to each and every flow of such a yield stress fluid is developed. As examples of motions for which variational inequalities can be derived quite easily, several flows including some with free surfaces are exhibited. As an application of the variational inequality, a new nondimensional number, called the halting number, which determines whether a fluid in a pipe flow will come to rest in a finite amount of time is found, and is shown to be related to the minimum pressure drop per unit length necessary for a pipe flow to exist. As additional applications of the variational inequality, the rate at which an unsteady pipe flow approaches the steady state, and an upper bound for the steady flow rate in a pipe are investigated, both for Bingham fluids and more general viscoplastic fluids, with the halting number playing a part here as well.
Extension of quantized Hamilton dynamics to higher orders
Authors: Pahl E. , Prezhdo O. V.
Published: July 2002

The quantized Hamilton dynamics (QHD) method, which was introduced and developed in J. Chem. Phys. 113, 6557 (2000) to the second order, is extended to the third and fourth orders. The QHD formalism represents an extension of classical mechanics and allows for the derivation of a hierarchy of equations of motion which converge with the quantum-mechanical limit. Here, the second, third, and fourth order QHD approximations are applied to two model problems: the decay of a particle in a metastable cubic potential and the intermode energy exchange observed in the Henon–Heiles system. The QHD results exhibit good convergence with the quantum data with increasing order yet preserve the computational efficiency of classical calculations. The second order QHD approximation already does an excellent job in maintaining the zero-point energy in the Henon–Heiles system and describing moderate tunneling events in the metastable potential. Extensions to higher orders substantially improve the QHD results for deep tunneling and are capable of describing the finer details of energy exchange.
Josephson cross-sectional model experiment
Authors: Ketchen M. B. , Herrell D. J. , Anderson C. J.
Published: July 2002

This paper describes the electrical design and evaluation of the Josephson cross-sectional model (CSM) experiment. The experiment served as a test vehicle to verify the operation at liquid-helium temperatures of Josephson circuits integrated in a package environment suitable for high-performance digital applications. The CSM consisted of four circuit chips assembled on two cards in a three-dimensional card-on-board package. The chips (package) were fabricated in a 2.5-μm (5-μm) minimum linewidth Pb-alloy technology. A hierarchy of solder and pluggable connectors was used to attach the parts together and to provide electrical interconnections between parts. A data path which simulated a jump control sequence and a cache access in each machine cycle was successfully operated with cycle times down to 3.7 ns. The CSM incorporated the key components of the logic, power, and package of a prototype Josephson signal processor and demonstrated the feasibility of making such a processor with a sub-4-ns cycle time.
Study of antiferromagnetic structures by polarized neutrons: Application to PrAg
Authors: Givord D. , Morin P. , Schmitt D.
Published: July 2002

Polarized neutron scattering with polarization analysis on single crystals may allow one to remove some ambiguities in the determination of a magnetic structure, especially for high symmetry systems. The direction of the Fourier components of the magnetic moments and the relative population of different associated magnetic domains are, in particular, obtained. Application is made on a cubic antiferromagnetic rare-earth compound, PrAg. Among the different possibilities, it is proposed that this compound orders in a double-Q (biaxis) magnetic structure with local moments lying along twofold directions.
Scattering matrix approach to thermal wave propagation in layered structures
Authors: Iravani M. Vaez , Wickramasinghe H. K.
Published: July 2002

In this paper we describe a new technique, based on Fourier optics, to explain the propagation, as well as loss, of three-dimensional thermal waves in isotropic, homogeneous materials. Using this, the dependence of temperature distribution at any arbitrary infinite parallel plane on the aperture distribution is derived. In addition, the temperature distribution at the aperture plane, due to a given perpendicular source is formulated, by applying the boundary conditions in the spatial frequency domain. A scattering matrix theory is developed to analyze the propagation of thermal waves in multilayered structures. This directly relates the heat source characteristics to the temperature distribution at any level. The contrast mechanism in the subsurface mode of operation is explained, and the dependence of the response of a typical system on depth is explored. In addition, the theoretical amplitude and phase images of a cylindrical void are presented; the results are in good agreement with the published experimental subsurface images of voids. Finally, transform techniques in both temporal and spatial frequency domains are employed to analyze the pulse response of layered structures. It is found that the decay rate of the surface temperature is strongly influenced by the presence of inclusions within a sample.
Experiments on shock-wave focusing in an elliptical cavity
Authors: Gustafsson Göran
Published: July 2002

Experimental work on reflection and focusing of weak cylindrical shock waves in a liquid-filled elliptical cavity is presented. The shocks are generated at one focus of the cavity by electrical discharges, and converge at the other focus after reflection in the cavity wall. High-speed photographs of the resulting wave system, which appears to be considerably more complex than the corresponding one in air, are shown and discussed. The results are of interest in the design of a transient water-jet generator, which utilizes the energy in the converging shock wave to produce a fast liquid jet.
Hall measurements as a function of temperature on monocrystalline SiC thin films
Authors: Tachibana T. , Kong H. S. , Wang Y. C. , Davis R. F.
Published: July 2002

Hall measurements were conducted at temperatures up to 1000 K on unintentionally doped n-type β(3C)- and α(6H)-SiC thin films epitaxially grown on both on-axis and vicinal Si (100) and α(6H)-SiC (0001) by chemical vapor deposition. The carrier concentration versus temperature data were analyzed using a compensation model. The β-SiC films grown on Si were highly compensated (NA/ND=0.73–0.98). The compensation ratio was not as large in the SiC films grown on α-SiC (NA/ND=0.36, for β-SiC on α-SiC, and 0.02, for α-SiC on α-SiC). The donor ionization energy for β-SiC on Si was calculated to be 14–21 meV. Analogous values for β- and α-SiC films on α-SiC were 33 and 84 meV, respectively. These values are smaller than those for N determined from photoluminescence studies.
Schlieren measurements of the hydrodynamics of excimer laser ablation of polymers in atmospheric pressure gas
Authors: Ventzek Peter L. G. , Gilgenbach Ronald M. , Sell Jeffrey A. , Heffelfinger David M.
Published: July 2002

Pulsed schlieren photography and fast helium-neon laser deflection are used to study the hydrodynamics of laser ablation of polyethyleneterephthalate and polymethylmethacrylate by pulsed KrF (248 nm) radiation in atmospheric air, Ar and N2. Schlieren measurements show the evolution of shock waves, sound waves, and reduced-density, hot gas plumes. A transition from sound to shock at the ablation threshold for both polymers is observed. The shock velocity of PET tends to approach agreement with blast wave theory at fluences higher than 1 J/cm2. Plumes in air are consistently larger than those produced in Ar and N2 (at fluences below 5 J/cm2) suggesting that combustion may occur. Laser deflection measurements for PET at 150 mJ/cm2 indicate a plume density of 0.6 kg/m3 (50% atmospheric density).
Electric-field dependence of the hole drift mobility in molecularly doped polymers: Importance of the disorder of hopping sites
Authors: Kanemitsu Yoshihiko , Funada Hiroshi , Masumoto Yasuaki
Published: July 2002

The electric-field dependence of the drift mobility of holes in a polymer matrix doped with various different molecules was studied by means of time-of-flight (TOF) photoconductivity measurements. It was first found that the degree of the positive field dependence of the drift mobility increases with decreasing magnitude of the drift mobility at zero field. The tail of the Gaussian TOF current signal becomes broad in molecularly doped polymers having a large disorder of hopping site energies. It was pointed out that the field dependence of the mobility and the energy barrier for charge transport are largely related to energetic disorder of hopping sites.
Silver diffusion and pattern formation on polycrystalline tin oxide films
Authors: Zhang Jianping , Colbow Konrad
Published: July 2002

We report on some quantitative observations of silver diffusion and pattern growth on thin tin oxide film under different operating conditions. The patterns obtained from our experimental method have both fractal and dense packed structures. The fractal dimension of self-similar cluster in two dimensions is found to be 1.68±0.05 which is in good agreement with the theoretical value of 1.70 predicted by diffusion limited aggregation cluster growth. In addition, the scanning electron microscope picture and x-ray diffraction measurements give insight into this new type of aggregate.
Luminescence processes in Tm3+- and Er3+-ion-activated, Yb3+-ion-sensitized infrared upconversion devices
Authors: Mita Yoh , Hirama Katsumi , Ando Nobuaki , Yamamoto Hajime , Shionoya Shigeo
Published: July 2002

Frequency upconversion characteristics in Er3+- or Tm3+-ion-activated and Yb3+-ion- sensitized luminescent materials have been investigated. The principal goal was to obtain high-brightness, blue and green light sources under 980 nm emitting laser diode light excitation. Processes leading to the efficient upconversion have been investigated both by experimental and analytical methods. It has been shown that upconversion efficiencies are principally determined with Yb3+ ion excited state lifetime, which is highly influenced by device optical confinement as well as material characteristics. The effect of the optical confinement has been analyzed quantitatively on the basis of a rate equation model. Criteria for realizing efficient infrared upconversion devices have been presented. A strong tendency toward temperature quenching has been observed especially in blue emitting materials. This temperature dependence has been shown to be principally due to increase of nonradiative decay in the Yb3+ ion excited state.
Growth of CuO films by pulsed laser deposition in conjunction with a pulsed oxidation source
Authors: Willmott P. R. , Timm R. , Felder P. , Huber J. R.
Published: July 2002

Copper (II) oxide films of high chemical and crystal purity have been grown on MgO(100) substrates using pulsed laser ablation (248 nm) of copper in conjunction with a pulsed oxidizing gas source (N2O). A novel experimental geometry is presented, in which the gas pulse and ablation plume cross close to their respective origins, where the particle and energy densities are highest. The properties of the films have been analyzed by x-ray diffraction and scanning electron microscopy. Growth rates are typically 0.025–0.035 Å/laser pulse within the substrate temperature region of 50 to 600 °C. Results in the region of 350 °C are presented. Analysis by time-of-flight mass spectroscopy has identified the predominant gas-phase species during ablation and transportation to the growing surface.
Bloch states of a Bloch wall
Authors: Braun Hans-Benjamin , Loss Daniel
Published: July 2002

Bloch walls in mesoscopic ferromagnets can tunnel between periodically arranged pinning sites leading to a Bloch band. The quantum spin phase gives rise to a spin parity dependent shift in the dispersion. Static external magnetic fields induce magnetization oscillations and provide a magnetic analogue of the Josephson effect.
Magneto-optical studies of GaN and GaN/AlxGa1−xN: Donor Zeeman spectroscopy and two dimensional electron gas cyclotron resonance
Authors: Wang Y. J. , Kaplan R. , Ng H. K. , Doverspike K. , Gaskill D. K. , Ikedo T. , Akasaki I. , Amono H.
Published: July 2002

Magneto-optical experiments have been performed on free and bound electrons in GaN films and GaN/AlGaN heterojunctions. Cyclotron resonance of two dimensional electron gas in GaN/ AlxGa1−xN heterojunctions has yielded m* = 0.23 m0, x-dependent scattering times consistent with values from transport measurements, and an apparent unexplained level crossing at 70 cm−1. Infrared absorption of doped GaN films has shown that the binding energy of Si donors, 29.0 meV, is much smaller than that of residual donors, in agreement with transport measurements, thus suggesting donor spectroscopy as a useful technique for defect/impurity qualitative analysis. Zeeman effect of the donor spectra has been used to determine the GaN low frequency dielectric constant, ε0 = 10.4.
Field characterization of a D-shaped optical fiber using scanning near-field optical microscopy
Authors: Huntington S. T. , Nugent K. A. , Roberts A. , Mulvaney P. , Lo K. M.
Published: July 2002

Scanning near-field optical microscopy is used to measure the mode profile and evanescent field of a Ge-doped D-shaped optical fiber. The structure of the fiber is determined by differential etching followed by an investigation of the resultant topography with an atomic force microscope. This information is then used to theoretically model the expected behavior of the fiber and it is shown that the theoretical results are in excellent agreement with the experimentally observed field.
Red-green-blue emission of parahexaphenyl devices with color-converting media
Authors: Niko A. , Tasch S. , Meghdadi F. , Brandstätter C. , Leising G.
Published: July 2002

The realization of efficient red, green, blue (RGB) light emission can be achieved by covering a blue light emitter with color-converting dye/matrix layers. We present theoretical calculations and experimental data of an electroluminescence device based on parahexaphenyl, an organic semiconducting material, which emits bright blue light when used as an active layer in a thin film device. The waveguiding effects in the various layers making up the RGB device are investigated in order to maximize the brightness, and optimize the angular light intensity distribution of the external emitted light. Quantum efficiencies (QE) for green color conversion of 48%, and red conversion of 13% from blue emission were determined by calculating over all emission angles and confirmed by experimental QE measurements. The efficiency can be drastically improved when the device layers are directly coupled to each other by refractive index matching, where color-conversion efficiencies of 90% for blue to green and 80% for blue to red can be achieved.
Density of states in hexagonal SiC polytypes
Authors: Persson C. , Lindefelt U.
Published: July 2002

Densities of states for the hexagonal polytypes 2H, 4H, and 6H of SiC have been derived for energies within a few hundred meV around the band gaps. The nonparabolic energy dispersions of the valence bands have been taken into consideration, as well as the camel’s back structure of the lowest conduction band in 6H–SiC. The nonparabolicities of the energy bands are shown to strongly influence the densities of states in these polytypes.
Study on the effect of ion beam bombardment during deposition on preferred orientation in rutile-type titanium dioxide films
Authors: Zhang Feng , Zheng Zhihong , Chen Yu , Liu Duo , Liu Xianghuai
Published: July 2002

Rutile-type titanium dioxide films, which exhibits tetragonal structure, were synthesized by ion beam assisted deposition with ion beam incidence inclined 45° to the substrate. The influence of ion bombardment during deposition on the texture of films was studied. A change of preferred orientation from (110) to (200) with increase of ion to deposited atom arrival ratio was observed in case Ne+ and Xe+ ion bombardment were used during film growth, respectively. It is found that in rutile type TiO2 films (110) plane exhibits the lowest surface free energy and crystallites with (100) orientation have the widest channel to ions when ion beam injects with direction 45° to the film. The preferred orientation of titanium oxide films from (110) to (200) is accounted for in terms of channeling of ions and surface free energy. When arrival ratio of ion to atom is low, crystallites with (110) orientation which has lowest surface free energy grow preferentially. As the increase of arrival ratio, the damage induced by ion bombardment compresses the growth of crystallites with shallow ion channeling. Then (200) preferred orientation is observed because of its deep ion channeling. Compared to the cases Ne+ is used, lower arrival ratio is required to change the orientation from (110) to (200) for Xe+ bombardment, because heavier ion leads to stronger radiation damage.
Thickness dependence of the magnetic percolation threshold in as-deposited and annealed Fe–SiO2 granular thin films
Authors: Zhou J. N. , Butera A. , Jiang H. , Barnard J. A.
Published: July 2002

The magnetic properties and microstructure of as-deposited and annealed Fe–SiO2 granular thin films were studied. As-deposited films have a maximum in coercivity at an Fe volume fraction (Fe vol %) ∼62% independent of film thickness. Iron grains in as-deposited films are well defined, nearly equiaxial and ∼5 nm in diameter. From 66 to 90 Fe vol %, some as-deposited films showed an unusual well defined in-plane uniaxial anisotropy. The magnetic percolation threshold, xp, as indicated by the maximum in the Hc vs Fe vol % curve, changed after the films were annealed. The percolation threshold (xp) of films annealed at 420 °C for 30 min shifted to ∼47 Fe vol % except for the 5 nm films, whose xp remained unchanged. After annealing at 510 °C for 3 h, a strong thickness dependence of the percolation threshold was revealed in films thinner than 40 nm, with values ranging from 78 Fe vol % to less than 44 Fe vol %. The shifts of xp in both 420 and 510 °C annealed films can be explained by the effects of reduced dimensions in very thin films.
Ultrasonic waves in multilayered superconducting plates
Authors: Pan Ernian , Datta Subhendu K.
Published: July 2002

Guided waves and the transient response of multilayered superconducting tapes have been studied in this article. These tapes are usually composed of layers of a superconducting material (like YBa2Cu3O7−δ, or YBCO, for simplicity) alternating between layers of a metallic material (like nickel or silver). The tapes are thin, the thickness being in the range of 100–200 μm. The superconducting layer is orthotropic with a thickness of 5–10 μm. In applications, the tapes are long and have a finite width. In this article, we focus our attention on the dispersion of two-dimensional guided waves and the transient response of homogeneous and three-layered tapes assuming that the thickness of the superconducting layer is much smaller than the metal layer. Three tape geometries are considered: a homogeneous nickel layer, a three-layered YBCO/nickel/YBCO and a three-layered nickel/YBCO/nickel. It is found that although the total thickness of the YBCO layer is very small, the dispersion and transient response in the last two cases are quite different from each other and from the homogeneous nickel plate. These differences can be used for the ultrasonic characterization of the in-plane material properties of the superconducting layers.
Optical limiting and thermal lensing studies in C60
Authors: Harilal S. S. , Bindhu C. V. , Nampoori V. P. N. , Vallabhan C. P. G.
Published: July 2002

Optical limiting and thermo-optic properties of C60 in toluene are studied using 532 nm, 9 ns pulses from a frequency-doubled Nd:YAG laser. Optical limiting studies in these fullerene molecules lead to the conclusion that reverse saturable absorption is the major mechanism for limiting properties in these molecules. Thermal lensing measurements are also performed in fullerene solutions. The quadratic dependence of thermal lens signal on incident energy confirms that enhanced optical absorption by the sample via excited triplet state absorption may play a leading role in the limiting property.
Dipole moment function of carbonyl sulfide from analysis of precise dipole moments and infrared intensities
Authors: Tanaka Keiichi , Tanaka Takehiko , Suzuki Isao
Published: July 2002

Precise (±3×10−5 D) dipole moments of OCS in 11 vibrational states (0000, 1000, 2000, 0110, 0200, 0220, 0310, 0400, 1200, 1420, 2200) of the normal species and two states (0000 and 0200) of both OC34S and O13CS derived from our recent LMDR (infrared laser–microwave double resonance with intense electric field) measurements were combined with the previous MBER (molecular beam electric resonance) results for the 0000 and 0110 states of the normal, 34S, 13C, and 18O species and the transition moments for various vibrational bands obtained from infrared intensity measurements, to determine the dipole moment function. Curvilinear coordinates were used to describe the vibrational displacements. The dipole moment vector was represented in terms of the components parallel and perpendicular to the C–O bond. Thus the dipole moments of various isotopic species may be analyzed simultaneously. The dipole moments and the transition moments were calculated as the diagonal and nondiagonal matrix elements of dipole moment operator between the vibrational wave functions which were obtained by direct diagonalization method using an accurate anharmonic force field. The signs of the transition moments were inferred from the vibrational and isotopic dependence of dipole moment in the course of the analysis. The dipole moment function thus determined well reproduced the observed values of the average dipole moments and the transition moments. The vibrational changes and the slight isotopic dependence of the dipole moment were adequately accounted for.
Phonon sudden theory of Debye–Waller attenuation. Temperature dependence of rotational energy transfer in molecule/surface scattering
Authors: Schinke R. , Gerber R. B.
Published: July 2002

A model for the description of thermal attenuation in atom, molecule/surface scattering is presented. It is based on the energy sudden approximation for all degrees of freedom, i.e., phonons, diffraction, and rotation, and leads to a generalized Debye–Waller factor that depends on the rotational transition and is valid for arbitrary interaction potentials. The traditional Debye–Waller factor is recovered for a hard potential. Assuming a Debye frequency spectrum for the phonons we present two model calculations for molecule/surface scattering. In the first case we assume a pairwise interaction between the atoms of the molecule and the surface atoms and observe a temperature dependence of the rotational transition probabilities, which is due to both the rotational energy transfer and the rotational dependence of the Debye–Waller factor. In the second case we model NO/Ag(111) scattering and conclude that a variation of the surface temperature has only a slight influence on the final rotational state distribution which is in accordance with the experimental findings of Auerbach et al. The mean rotational energy transfer shows a slight linear increase with the temperature as recently observed by Kubiak et al.
Pairwise additive semi a
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i
o potential for the elastic scattering of He atoms from the LiF(001) crystal surface
Authors: Celli Vittorio , Eichenauer Dieter , Kaufhold Achim , Toennies J. Peter
Published: July 2002

The interaction potential for the elastic diffractive scattering of low-energy He atoms from the highly corrugated LiF(001) crystal surface is derived from semi ab initio pair potentials in the framework of the recently developed Tang–Toennies potential model [J. Chem. Phys. 80, 3726 (1984)]. In addition to the sum of all He atom-crystal ion two-body potentials the induced dipole potential caused by the electric field of the ion lattice is taken into account, leaving only one free parameter, the C6 dispersion constant of the He–F− interaction. By simple adjustment of this parameter, it is possible to fit all of the experimental bound states of the atom-surface potential well, to within experimental error. Diffraction probabilities calculated by the close coupling method with this potential are shown to be in good agreement with the available experimental results. Two different empirical potential models based on the Morse potential are also investigated, but do not provide as good a description of the bound states and diffraction intensities.
Product state distributions for inelastic and reactive H+D2 collisions as functions of collision energy
Authors: Blais Normand C. , Truhlar Donald G.
Published: July 2002

We have calculated state-to-state reaction cross sections for two new energies and state-to-state vibrationally inelastic cross sections for four energies for H+D2 collisions on the most accurate available potential energy surface. The dynamics calculations are based on the quasiclassical trajectory quadratic smooth sampling method. We present a detailed analysis of the theoretical inelastic and reaction cross sections and compare the trends in the reactive scattering results to those calculated from Gerrity and Valentini’s experiments and extrapolations thereof. The agreement between theory and experiment for the reactive scattering at 0.98–1.3 eV relative translational energy is stupendously good.
Detection of SiH(X 2Π) fundamental and hot band transitions by diode laser absorption spectroscopy
Authors: Davies P. B. , Isaacs N. A. , Johnson S. A. , Russell D. K.
Published: July 2002

Infrared absorption spectra of the SiH free radical have been detected in a silane plasma discharge. In addition to fundamental band transitions some hot band lines have been identified from predictions derived from optical and FTIR data. Low Q-branch transitions in both 2Π1/2 and 2Π3/2 states of the fundamental have been accurately measured and incorporated in a new set of predictions based on the present work and earlier FTIR data.
Multipolar polarizabilities and Rydberg states
Authors: Krishnagopal S. , Narasimhan S. , Patil S. H.
Published: July 2002

We have calculated multipolar polarizabilities for several atoms and ions, using some semiclassical relations and incorporating the correct asymptotic behavior in the electronic densities. We have also obtained simple expressions for penetration energies and nonadiabatic corrections which simplify the analysis of Rydberg states.
Neon–methane and argon–methane isotropic interaction potentials from total differential cross sections
Authors: O’Loughlin Michael J. , Reid Brian P. , Sparks Randal K.
Published: July 2002

Total differential cross sections have been measured at three collision energies each for Ne–CH4 and Ar–CH4 using the crossed molecular beams technique. The differential cross sections were used along with literature viscosity and second virial coefficient data to determine reliable isotropic interaction potentials. The Ne–CH4 potential has a well depth of 66±4 K and an intermolecular separation of 3.68±0.02 Å at the minimum while the Ar–CH4 potential has a well depth of 170±8 K and an intermolecular separation of 3.85±0.04 Å at the minimum.
Bond selective photochemistry in CH2BrI through electronic excitation at 210 nm
Authors: Butler L. J. , Hintsa E. J. , Lee Y. T.
Published: July 2002

To explore the possibility of bond selective photochemistry in an excited electronic state, we have studied the photolysis of CH2BrI in a molecular beam at 210 nm. Following the direct local excitation of a repulsive transition on the C–Br bond at 210 nm, the fragments were detected by time-of-flight mass spectrometry. The dominant channel was found to be C–Br fission (60%) releasing an average of 15 kcal/mol into translation with the remainder reacting to form CH2+IBr and CH2+I+Br. There was no evidence for the primary fission of the C–I bond, making this the first clear example of the selective cleavage of a stronger bond in a molecule over the weakest one.
The Zeeman effect as an aid to electronic assignment: The NiH A 2Δ5/2 state
Authors: Gray J. A. , Field R. W.
Published: July 2002

Sub-Doppler Zeeman spectra of the B 2Δ5/2–X 2Δ5/2(1–0) Q(2.5) line at 17 462.95 cm−1, the A 2Δ5/2–X 2Δ5/2 (1–0) Q(2.5) line at 17 021.54 cm−1, and the 2Φ7/2–X 2Δ5/2(0–0) R(2.5) line at 17 132.48 cm−1 were recorded at magnetic fields up to 900 G. The lowest-j levels of the B 2Δ5/2, X 2Δ5/2, and 2Φ7/2 states are found to behave as nearly pure Hund’s case (a)basis states with g values (Λ+2Σ) near g=3,3, and 4, respectively, but the A 2Δ5/2 is found to have a g value very different from that required for a 2Δ5/2 state. Present experiments are unable to distinguish between the 4Φ5/2 (g=2) and 4Π5/2 (g=4) reassignment possibilities for the ab initio inexplicable A 2Δ5/2 state.
Pore size distributions in random assemblies of equal spheres
Authors: Gotoh Keishi , Nakagawa Masafumi , Furuuchi Masami , Yoshigi Akira
Published: July 2002

The pore size distributions in random assemblies of equal spheres are obtained from the computer experiments and the theoretical study is made of the results within the framework of the Percus–Yevick approximation.
Wavelength dependent photoelectron spectra of rotationally resolved autoionizing levels of nitric oxide
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Photoelectron spectra obtained following two photon excitation to rotational levels of the 9dσπ, v=2 state, which can autoionize into the NO+ v+=0 and 1 continua, are presented. The photoelectron spectra obtained along the laser polarization axis show a distinct wavelength dependence across the rotationally resolved autoionizing resonances; the angular distributions vary dramatically, becoming more isotropic on resonance. The branching ratio into the NO+ v+=1 state increases significantly on the resonance, in accord with the Δv=−1 propensity rule for vibrational autoionization.
Spin-forbidden radiative decay of the a 4Πu state of O+2
Authors: Bustamente S. W. , Okumura M. , Gerlich D. , Kwok H. S. , Carlson L. R. , Lee Y. T.
Published: July 2002

The spin-forbidden radiative decay of a 4Πu O+2 has been measured in a radio frequency octopole ion trap. Photodissociation is used to probe the a 2Πu population as a function of trapping time. We have found that the a 4Πu state exhibits a multiple exponential decay, ranging from a few milliseconds to hundreds of milliseconds. The state dependence of the decay is seen in the photodissociation spectrum (b 4Σ−g←a 4Πu), which changes dramatically from 0.1 to 100 ms. The major changes in the spectrum are simulated by assuming that the F2 and F3 spin components of the a 4Πu state decay faster than the F1 and F4 components. We can account for this dependence on spin sublevel by assuming that the primary mechanism for radiative decay arises from spin-orbit coupling of the a 4Πu and A 2Πu states. Our results suggest that the a 4Πu radiative lifetime of 0.22 s measured by O’Keefe and McDonald reflects the decay of only the longest living a 4Πu sublevels.
Microwave spectra and structure for SO2⋅⋅⋅H2S, SO2⋅⋅⋅HDS, and SO2⋅⋅⋅D2S complexes
Authors: Bumgarner R. E. , Pauley D. J. , Kukolich S. G.
Published: July 2002

Microwave spectra for the SO2⋅⋅⋅H2S, SO2⋅⋅⋅HDS, and SO2⋅⋅⋅D2S complexes were measured using a pulsed beam, Fourier transform microwave spectrometer. Both a-dipole and c-dipole transitions were obtained. A total of 24 transitions were obtained for SO2⋅⋅⋅H2S, yielding A=8447.3(2), B=1762.004(7), C=1538.483(7) MHz, ΔJ=5.04(2) , ΔJK=65.46(9) , ΔK=−323(240) , δJ=0.63(1) and δK=38(3) kHz. For SO2⋅⋅⋅HDS, nine transitions yielded A=8229.7(6), B=1737.99(1), C=1519.69(2) MHz, ΔJ=4.4(4) and ΔJK=60(2) kHz, and for SO2⋅⋅⋅D2S, 11 transitions yielded A=8017.6(6), B=1715.24(2), C=1501.24(2) MHz, and ΔJ=3.8(4) , ΔJK=51(2) kHz. For the H2S data only, there are four possible structures for the complex which fit the data. When the deuterium isotope data are included, only two possible structures fit the data. There is only one structure which allows two O⋅⋅⋅H hydrogen bonds, and this is the structure we favor. This analysis basically gives a ‘‘stacked’’ structure with two O⋅⋅⋅H hydrogen bonds and a near Van der Waals radius contact between the two sulfur atoms.
Electron transfer and protein dynamics
Authors: Knapp E. W. , Fischer S. F.
Published: July 2002

The electron transfer process from cytochrome c to the special pair is revisited. In the present model the transfer rate in photosynthetic reaction centers of Chromatium vinosum is modulated by fluctuations of the dielectric constant of the protein environment between the donor and acceptor site. The fluctuations enter the rate expression as the inverse of a Lamb–Mössbauer factor. The strong temperature dependence of the transfer rate can thus be related to protein specific dynamics. Experimental evidence about such dynamics is provided by recently recorded Mössbauer spectra of iron containing proteins [Parak et al., J. Mol. Biol. 161, 177 (1982)].