bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and structural and magnetic characterization of the FePd ordered alloy on CdZnTe II–VI semiconductor
Authors: Bourgognon C. , Tatarenko S. , Cibert J. , Gilles B. , Marty A. , Samson Y.
Published: July 2002

We have studied growth conditions and structural and magnetic properties of FePd ferromagnetic layers deposited on (001) CdZnTe semiconductors. By using Au and Pd as buffer layers between the II–VI compound and the FePd, a two dimensional growth of the alloy is obtained. The alloy is well chemically ordered with the L10 tetragonal structure and is perpendicularly magnetized with a perpendicular magnetic anisotropy constant of about 6.6.106 erg cm−3. Magnetic force microscopy images showed the perpendicular magnetic configuration with up and down domains whose size is about 70 nm. The results compare favorably with FePd layers elaborated in the same run on an MgO substrate.
Photoluminescence in erbium-doped Pb(Mg1/3Nb2/3)O3–PbTiO3 thin films
Authors: Zheng Jianjun , Lu Yalin , Chen Xuesheng , Cronin-Golomb Mark , Zhao Jing
Published: July 2002

Erbium-doped Pb(Mg1/3Nb2/3)O3–PbTiO3(PMN–PT) thin films were grown on (10 1̄2) sapphire substrate with a purpose of developing an active optical material combining both the large electro-optic (EO) effect and lasing or amplification functions. Strong characteristic Er3+ intra-4f shell emission around 1.54 μm is observed at room temperature and a compositional quenching occurred at where the Er content is larger than 0.5 mol %. The introduction of Er in PMN–PT decreased the EO effect, but the EO coefficient still maintain a reasonable value, which is about 0.4×10−16 (m/V)2 at Er content of 0.5 mol %. These results indicate that Er3+-doped PMN–PT can be a potential candidate for integrated optic active devices.
Nanocomposite CoPt:C films for extremely high-density recording
Authors: Yu M. , Liu Y. , Moser A. , Weller D. , Sellmyer D. J.
Published: July 2002

Nanocomposite CoPt:C films were investigated as potential media for extremely high-density recording. An annealing temperature of over 600 °C is necessary to form nanocomposite CoPt:C films consisting of C matrix and fct CoPt nanocrystallites with grain sizes of 8–20 nm and coercivities of 3–12 kOe. Coercivity and grain size increase with increasing annealing temperature and decreasing C concentration and they are insensitive to film thickness. The average activation volumes are about 0.9×10−18 cm3. The properties of these nanocomposite CoPt:C films can be tailored to satisfy the thermal stability, coercivity, and media noise requirements for extremely high-density recording.
Many-body effects on excitons properties in GaN/AlGaN quantum wells
Authors: Traetta Giampiero , Cingolani Roberto , Di Carlo Aldo , Della Sala Fabio , Lugli Paolo
Published: July 2002

The many-body effects on excitons properties in GaN/AlGaN quantum wells are theoretically investigated by using a Green’s function model and the electron and hole wave functions calculated either in the envelope function approximation or in the frame of a self-consistent tight-binding model. We show that the built-in field induced by the piezoelectric and spontaneous polarization charge causes a reduction of the exciton binding energy and of the absorption coefficient well below the values expected for the quantum well with flat band. At high carrier concentrations, the many-body screening prevails over the screening of the built-in electric field, causing complete exciton bleaching at typical densities of the order of 1012 cm−2.
Structures of nitridated layers on sapphire studied by x-ray reflectivity and diffraction
Authors: Kim Ki-Sung , Kim Seon-Hyo , Lee Dong-Ryul
Published: July 2002

A nitridated layer of c-plane sapphire was studied by using synchrotron x-ray reflectivity and diffraction measurements. The nitridation of the sapphire surface produced an epitaxial AlN layer with a low mosaicity (1 arcmin) normal to the surface. The nitridation temperature determined the limiting thickness for the nitridated layer. Excessive nitridation beyond the limiting thickness caused strain relaxation by a reduction of lateral domain size and an increase in lateral mosaicity. Therefore, the nitridated layer with compressive strain may provide better nucleation sites for the subsequent GaN overlayer growth than the strain-relaxed one.
High characteristic temperature of near-1.3-μm InGaAs/GaAs quantum-dot lasers at room temperature
Authors: Mukai Kohki , Nakata Yoshiaki , Otsubo Koji , Sugawara Mitsuru , Yokoyama Naoki , Ishikawa Hiroshi
Published: July 2002

This letter reports on the high characteristic temperature of InGaAs/GaAs quantum-dot lasers at room temperature. Self-assembled quantum dots were grown using low-growth-rate molecularbeam epitaxy, and continuous-wave lasing occurred at the dot ground level of 1.26 μm at 25 °C. The characteristic temperature of the threshold currents was 120 K, and ground-level lasing was observed up to 100 °C. Comparing the lasing performances and the spontaneous emission spectra with those of 1.3 μm emission dots, we found that the large volume density, deep potential, and high quantum efficiency were key points for improving the temperature characteristics.
In situ transmission electron microscopy study of electric-field-induced microcracking in single crystal Pb(Mg1/3Nb2/3)O3–PbTiO3
Authors: Xu Z. , Tan X. , Han P. , Shang J. K.
Published: July 2002

In this letter, we report in situ transmission electron microscopy (TEM) study of effect of a cyclic electric field on microcracking in a single crystal piezoelectric 0.66Pb(Mg1/3Nb2/3)O3–0.34PbTiO3. A TEM heating stage was modified to permit the in situ application of an electric field on the TEM sample surface. Microcrack initiation from a fine pore under an applied cyclic electric field was directly observed in the piezoelectric single crystal. Experimental procedures for in situ TEM studies were described.
Multi-soliton complexes
Authors: Akhmediev Nail , Ankiewicz Adrian
Published: July 2002

In this paper we introduce the concept of multi-soliton complexes (MSC). A particular example of a MSC is an incoherent soliton in a multimode fiber or in a photorefractive crystal, but there are many examples in other areas of physics. We discuss a variety of profiles of MSCs, their unusual collisional properties, the possibility of a MSC on a background and some other interesting properties of MSCs. Some of their features are also shared by single solitons, but there are many differences between the properties of the two types.
Hydride abstraction by NO+ from ethanol: Effects of collision energy and ion rotational state
Authors: Green Richard J. , Qian Jun , Kim Ho-Tae , Anderson Scott L.
Published: July 2002

The effects of NO+ rotational state and collision energy on the reaction NO++C2H5OH→HNO+C2H4OH+ were studied in a guided-ion-beam instrument over the collision energy range from 50 meV to 3.7 eV. Integral cross sections for the reaction are presented. NO+ is prepared in specific rotational levels (N+=0,1 and N+=10) by means of mass-analyzed threshold ionization. Ab initio calculations were used to probe stationary points on the potential energy surface. The reaction is sharply inhibited by collision energy, suggesting a bottleneck for reaction. If rotational energy had a similar effect, ∼50% inhibition from N+=10 excitation would be observed at low collision energy. Instead, rotation is found to have no effect within experimental error. A precursor complex mechanism is proposed to explain the results.
Collisional quenching of CO B 1Σ+(v′=0) probed by two-photon laser-induced fluorescence using a picosecond laser
Authors: Di Teodoro F. , Rehm J. E. , Farrow R. L. , Paul P. H.
Published: July 2002

We report measurements of room-temperature, species-specific quenching cross sections of CO B 1Σ+ (v′=0) in collisions with He, Ne, H2, N2, Ar, CO, Kr, CH4, O2, Xe, CO2, C3H8, and H2O. The measured quenching cross sections (in Å2) were 0.25±0.02, 0.54±0.04, 11.0±0.4, 24.6±0.5, 27.7±0.5, 37±2, 42±2, 81±4, 85±5, 99±6, 133±5, 144±7, and 170±8, respectively. Two-photon excitation of the CO molecules via the Hopfield–Birge system (X 1Σ+→→B 1Σ+) was performed using the frequency-tripled 690 nm emission of a custom-built picosecond dye laser. Blue-to-green fluorescence in the Angström bands (B 1Σ+→A 1Π) was detected using a microchannel-plate photomultiplier tube and recorded with a digital storage oscilloscope. The quenching cross sections were directly obtained by time resolving the temporal decay of the fluorescence signal and observing its variation as a function of the quencher pressure. The effect of radiative trapping on the observed fluorescence was also quantitatively modeled.
Electrophoresis simulated with the cage model for reptation
Authors: van Heukelum A. , Beljaars H. R. W.
Published: July 2002

The cage model for polymer reptation is extended to simulate gel electrophoresis. With increasing electric field strength E, the drift velocity v of a long polymer with length L shows three different regimes: (a) the linear regime where v∼E/L; (b) the quadratic regime where v∼E2, independent of the length of the polymer; and (c) a regime where the velocity decreases exponentionally with E. The transition between regimes (a) and (b) occurs for field strengths E∼L−1. The transition between regimes (b) and (c) occurs for some value Eh, for which L−1≪Eh≪1. The behavior in the first two regimes is in agreement with earlier reports on simulations of the Duke–Rubinstein model, and with experimental work on DNA polymers in agarose gel. The third regime is not reported for the Duke–Rubinstein model, probably because in this model, stored length cannot compile into hernias.
Computer simulation of the thermodynamic properties of high-temperature chemically-reacting plasmas
Authors: Lı́sal Martin , Smith William R. , Nezbeda Ivo
Published: July 2002

The Reaction Ensemble Monte Carlo (REMC) computer simulation method [W. R. Smith and B. Tříska, J. Chem. Phys. 100, 3019 (1994)] is employed to predict the thermodynamic behavior of chemically reacting plasmas using a molecular-level model based on the underlying atomic and ionic interactions. Unlike previous plasma simulation studies, which were restricted to fairly simple systems of fixed composition, the REMC approach is able to take into account the effects of the ionization reactions. In the context of the specified molecular model, the computer simulation approach gives an essentially exact description of the system thermodynamics. We develop and apply the REMC method for the test case of a helium plasma. We calculate plasma compositions, molar enthalpies, molar volumes, molar heat capacities, and coefficients of cubic expansion over a range of temperatures up to 100 000 K and pressures up to 400 MPa. We elucidate the contributions of the Coulombic forces, ionization-potential lowering, and short-ranged interactions to the thermodynamic properties. We compare the results with those obtained using macroscopic-level thermodynamic approximations, including the ideal-gas (IG) and the Debye–Hückel (DH) approaches. For the helium plasma, the short-ranged forces are found to be relatively unimportant, but we expect these to be important for molecular systems. The DH theory is always more accurate than the IG approximation. The DH theory yields compositions that slightly underpredict the overall degree of ionization. For the molar heat capacity and the coefficient of cubic expansion, the DH theory is accurate at lower pressures, but at 400 MPa yields results that are up to 40% in error for the molar heat capacity.
Separation length in high-enthalpy shock/boundary-layer interaction
Authors: Davis Jean-Paul , Sturtevant Bradford
Published: July 2002

Experiments were performed in the T5 Hypervelocity Shock Tunnel to investigate nonequilibrium real-gas effects on separation length using a double-wedge geometry and nitrogen test gas. Local external flow conditions were estimated by computing the inviscid nonequilibrium flow field. A new scaling parameter was developed to approximately account for wall temperature effects on separation length for a laminar nonreacting boundary layer and arbitrary viscosity law. A classification was introduced to divide mechanisms for real-gas effects into those acting internal and external to viscous regions of the flow. Internal mechanisms were further subdivided into those arising upstream and downstream of separation. Analysis based on the ideal dissociating gas model and a scaling law for separation length of a nonreacting boundary layer showed that external mechanisms due to dissociation may decrease separation length at low incidence but depend on the free-stream dissociation at high incidence. A limited numerical study of reacting boundary layers showed that internal mechanisms due to recombination occurring in the boundary layer upstream of separation cause a slight decrease in separation length relative to a nonreacting boundary layer with the same external conditions. Correlations were obtained of experimentally measured separation length using local external flow parameters computed for reacting flow, which scales out external mechanisms but not internal mechanisms. These showed the importance of the new scaling parameter in high-enthalpy flows, a linear relationship between separation length and reattachment pressure ratio, and a Reynolds-number effect for transitional interactions. A significant increase in scaled separation length was observed in the experimental data at high enthalpy. The increase was attributed to an internal mechanism arising from recombination in the free-shear layer downstream of separation, perhaps altering its velocity profile. This real-gas effect depends on the combined presence of free-stream dissociation and a cold wall.
Compound droplet in extensional and paraboloidal flows
Authors: Palaniappan D. , Daripa Prabir
Published: July 2002

Exact analytical solutions are found for the steady state creeping flow in and around a vapor–liquid compound droplet, consisting of two orthogonally intersecting spheres of arbitrary radii (a and b), submerged in axisymmetric extensional and paraboloidal flows of fluid with viscosity μ(1). The solutions are presented in singularity form with the images located at three points: the two centers of the spheres and their common inverse point. The important results of physical interest such as drag force and stresslet coefficient are derived and discussed. These flow properties are characterized by two parameters, namely the dimensionless viscosity parameter: Λ=μ(2)/(μ(1)+μ(2)), and the dimensionless parameter: β=b/a, where μ(2) is the viscosity of the liquid in the sphere (part of the compound droplet) with radius b. We find that for some extensional flows, there exists a critical value of β=βc for each choice of Λ in the interval 0⩽Λ⩽1 such that the drag force is negative, zero or positive depending on whether β<βc, β=βc, or β>βc respectively. For other extensional flows, the drag force is always positive. The realization of these various extensional flows by simply changing the choice of the origin in our description of the undisturbed flow field is also discussed. In extensional flows where the drag force is always positive, we notice that this drag force De for vapor–liquid compound droplet is maximum when β≈1 (i.e., two spheres have almost the same radii). Moreover, we find the drag force De is a monotonic function of Λ, i.e., the drag force for vapor–liquid compound droplet lies between vapor–vapor and vapor-rigid assembly limits. We also find that the maximum value of the drag in paraboloidal flow depends on the viscosity ratio Λ and significantly on the liquid volume in the dispersed phase.
Structural decomposition of the chemical shielding tensor: Contributions to the asymmetry, anisotropy, and orientation
Authors: Herzfeld Judith , Olbris Donald J. , Furman Efim , Benderskiy Vadim
Published: July 2002

The nine elements of chemical shielding tensors contain important information about local structure, but the extraction of that information is difficult. Here we explore a semiempirical method that has the potential for providing relatively accessible structural correlations. The approach entails approximating the field-induced electron current density as entirely perpendicular to the applied field. This has two interesting consequences. (1) The resulting shielding tensor is perfectly symmetric. Thus, asymmetry in a shielding tensor is an indication of current density that is not orthogonal to the applied field. (2) The orientation dependence of the chemical shielding at a point of interest is related explicitly to the isotropic average of the chemical shielding at every point in the surrounding region. This suggests a relatively simple relationship between the orientation dependence of the chemical shielding and the molecular structure. Good correlation with experimental tensors is obtained with just one or two adjustable parameters in several series of compounds, including silicates, phosphates, hydrogen bonds, carboxyls, and amides. As expected, the results indicate that for a given center, the contribution to the shielding anisotropy that is associated with each bonded neighbor increases as the number of electrons at either the center or the neighbors increases.
Friction dynamics for curved solid surfaces with long-range elasticity
Authors: Persson B. N. J.
Published: July 2002

In this work I introduce a model that takes into account the effect of long range elasticity and apply it to study the boundary lubrication for curved solid surfaces. In particular, I investigate the sliding dynamics when the block and the substrate are separated by a molecular thin lubrication film. The role of elasticity and the origin of stick–slip motion is discussed.
Experimental and theoretical study of methyl-p-aminobenzoate/ammonia complexes. I. MAB(NH3)1
Authors: Fernández J. A. , Longarte A. , Unamuno I. , Castaño F.
Published: July 2002

Methyl-p-aminobenzoate(NH3)1 complex, henceforth MAB(NH3)1, prepared in a pulsed supersonic expansion, has been examined by laser mass-selective spectroscopies and density functional theory calculations, aiming to ascertain its isomer number, structures, identification, ionization energies, and vibrational assignments. Resonance enhanced multiphoton ionization and hole burning spectra of the species in supersonic beams show two 000 transitions redshifted by −715 and −709 cm−1 from that of bare MAB band origin and are plausibly associated with two different isomers, whereas ab initio calculations indicate the likely existence of five stable isomer structures. Identification of the experimental isomer spectra with the calculated structures is reported and, in particular, several isomer vibrational bands are identified by contrast with the calculated modes. Properties and features of the MAB(NH3)1 are compared with those of the MAB/water complexes.
Rotational coherence spectroscopy of para-cyclohexylaniline by stimulated Raman-induced fluorescence depletion and stimulated emission pumping
Authors: Weichert Andreas , Riehn Christoph , Brutschy Bernhard
Published: July 2002

A high-resolution rotational coherence spectroscopy (RCS) investigation of para-cyclohexylaniline (pCHA) was performed using the methods of time-resolved stimulated Raman-induced fluorescence depletion (TRSRFD) and time-resolved stimulated emission pumping (TRSEP). TRSRFD and TRSEP are sensitive to ground state or excited state rotational constants, respectively and allow the deconvolution of the time-resolved fluorescence depletion (TRFD) spectrum to which both ground state and excited state rotational constants contribute. Moreover, from a detailed analysis of the presented experimental data it is deduced, that photoionization and internal vibrational relaxation (IVR)—not included in the picture of TRSEP, TRSRFD and TRFD—also contribute to the RCS spectra. The obtained rotational constants are in very good agreement with our previous high-resolution TRFD investigation of pCHA, allowing additionally for the unambiguous assignment of the excited state J-type transients. From a linear regression analysis of 25 transient positions (B+C)′=719.74(53) MHz was calculated, confirming set (II) of our former investigation [C. Riehn, A. Weichert, U. Lommatzsch, M. Zimmermann, and B. Brutschy, J. Chem. Phys. 112, 3650 (2000)].
The design of laboratory experiments to produce collisionless shocks of cosmic relevance
Authors: Drake R. P.
Published: July 2002

Naturally occurring shocks transport energy and accelerate particles throughout the cosmos. The problem of producing collisionless shocks in the laboratory that are of relevance to such cosmic shocks is considered. Such an experiment must meet a number of constraints, several of which can be expressed by algebraic scaling relations. The relations for magnetization, plasma beta, Alfvén Mach number, temperature, magnetic field, and collisionality are described here. Taken together, the limits imposed by these constraints upon possible experiments are specified. The growth of magnetohydrodynamic (MHD) turbulence and the degree of particle acceleration are examined, demonstrating that it is feasible to contemplate studies of such phenomena in the laboratory. Finally, some discussion of how an experiment might meet the other qualitative constraints, and of how a laser might be used to drive the shock, is also included.
On the reliability of mean-field methods in polymer statistical mechanics
Authors: Tsonchev Stefan , Coalson Rob D. , Chern Shyh-Shi , Duncan Anthony
Published: July 2002

The reliability of the mean-field approach to polymer statistical mechanics is investigated by comparing results from a recently developed lattice mean-field theory (LMFT) method to statistically exact results from two independent numerical Monte Carlo simulations for the problems of a polymer chain moving in a spherical cavity and a polymer chain partitioning between two confining spheres of different radii. It is shown that in some cases the agreement between the LMFT and the simulation results is excellent, while in others, such as the case of strongly fluctuating monomer repulsion fields, the LMFT results agree with the simulations only qualitatively. Various approximations of the LMFT method are systematically estimated, and the quantitative discrepancy between the two sets of results is explained with the diminished accuracy of the saddle-point approximation, implicit in the mean-field method, in the case of strongly fluctuating fields.
Path integral Monte Carlo study on the structure and absorption spectra of alkali atoms (Li, Na, K) attached to superfluid helium clusters
Authors: Nakayama Akira , Yamashita Koichi
Published: July 2002

Path integral Monte Carlo calculations have been performed to investigate the microscopic structure and thermodynamic properties of the Ak⋅HeN (Ak=Li, Na, K,N⩽300) clusters at T=0.5 K. Absorption spectra which correspond to the P2←S2 transitions of alkali atoms are also calculated within a pairwise additive model, which employs diatomic Ak–He potential energy curves. The size dependences of the cluster structure and absorption spectra that show the influence of the helium cluster environment are examined in detail. It is found that alkali atoms are trapped in a dimple on the helium cluster’s surface and that, from the asymptotic behavior, the Ak⋅He300 cluster, at least semiquantitatively, mimics the local structure of experimentally produced large helium clusters in the vicinity of alkali atoms. We have successfully reproduced the overall shapes of the spectra and explained their features from a static and structural point of view. The positions, relative intensities, and line widths of the absorption maxima are calculated to be in moderate agreement with experiments [F. Stienkemeier, J. Higgins, C. Callegari, S. I. Kanorsky, W. E. Ernst, and G. Scoles, Z. Phys. D 38, 253 (1996)].
Molecular dynamics simulations of gas–liquid nucleation of Lennard-Jones fluid
Authors: Laasonen Kari , Wonczak Stephan , Strey Reinhard , Laaksonen Ari
Published: July 2002

Grand canonical molecular dynamics simulations have been performed to determine the sizes of critical nuclei in gas–liquid nucleation. The studied system consists of Lennard-Jones (LJ) argon atoms with a potential cutoff of 4.9σ at a reduced temperature of 0.694 and at vapor supersaturations between 3.5 and 6.2. To facilitate comparison with nucleation theories, we have also determined the equilibrium vapor pressure of LJ argon as a function of temperature. The results are compared with other simulation studies, and with predictions of the classical (CNT) and density functional (DFT) nucleation theories. We find that the semiempirical version of the DFT is in excellent agreement with the simulation results, and that the CNT underestimates the number of LJ atoms in the nuclei only very slightly.
Potential sources of error in electron beam induced current simulation
Authors: Ong Vincent K. S. , Phua Poh Chin
Published: July 2002

Computer simulation has been used extensively to verify physical phenomena in semiconductor devices. However, if care is not properly taken, serious errors may be introduced into the results. The objective of this article is to characterize and identify these errors in the simulation of the electron beam induced current effect using the Medici two-dimensional computer simulation software. This article characterizes the effect of grid generation, the Neumann boundary, the generation source lateral dimension, the depth, and carrier injection levels on the accuracy of the results.
Measuring system for the rapid determination of the concentration of coated micrometer-sized paramagnetic particles suspended in aqueous buffer solutions
Authors: Hawkins Peter , Luxton Richard , Macfarlane James
Published: July 2002

Coated paramagnetic particles (PMPs) with diameters typically 1–10 μm are widely used in biomedical investigations, including immunoassays and molecular biology. In this article we describe an investigation into a measuring system to determine rapidly the concentration of PMPs in suspension in a buffer solution in a pot. The measuring system is based on a coil in a parallel resonant circuit with a capacitor. The sample pot is placed in the coil, causing the inductance of the coil to increase and the resonant frequency to decrease. The decrease in resonant frequency is directly related to the concentration of PMPs in the pot. Using a circuit based on a Colpitts oscillator it is possible to determine the concentration of PMPs in the pot with an error of about 10% over the range 0–9 mg g−1. Much more accurate measurements (error <1%) are possible using a circuit design based on a phase-locked loop.
Lifetime of small polarons in strontium-barium-niobate single crystals doped with cerium or chromium
Authors: Woike Th. , Berben D. , Imlau M. , Buse K. , Pankrath R. , Krätzig E.
Published: July 2002

Small polarons are excited by pulsed illumination (λ=532 nm) of strontium–barium–niobate single crystals doped with 0.66 mol % cerium or with 0.025 mol % chromium. The dark decay of the polarons is observed by monitoring the light-induced absorption at λ=632.8 nm and at λ=785 nm. The relaxation fulfills a stretched-exponential behavior over at least five decades. Using the Arrhenius law the temperature dependence yields the activation energy EA=(0.58±0.02) eV and a frequency factor of Z=(5±3)×1013 s−1. Delayed double-pulse technique (λ=532 nm pulse followed by a λ=1064 nm pulse) reveals a distance dependent recombination rate of the small polarons, i.e., the lifetime of a created polaron is a function of the distance to the next available deep electron trap. In SBN:Cr the R branch can be excited by pulses of λ=1064 nm with energy transfer via an unknown X center. Excitation and recombination show a single-exponential temporal evolution without any significant temperature dependence.
Rotational and vibrational wave packet motion during the infrared multiphoton excitation of HF
Authors: Hervé Stephen , Quéré Frédéric Le , Marquardt Roberto
Published: July 2002

The time dependent quantum dynamics of molecular rotations and vibrations during coherent infrared multiphoton excitation are investigated by calculation of the time evolution of the wave packet in the rotational and vibrational configuration space. Results are presented for the rovibrational motion of HF using recent potential energy and dipole moment functions developed elsewhere by ab initio calculations. Several initial conditions are studied, from the single rovibrational ground state to a superposition of states with different J quantum numbers corresponding to highly oriented molecules. A careful examination of the rovibrational wave packet motion allows for a simple interpretation of the rotational motion and the effects of different initial conditions on the intramolecular kinetics. It is found that, depending on the degree of orientation, the HF molecule undergoes very fast deorientation, faster than its “classical” rotational period, thus perturbing the generation of a semiclassical vibrational motion during the excitation process.
Rotational and diffractive inelastic scattering of a diatom on a corrugated surface: A multiconfiguration time-dependent Hartree study on N2/LiF(001)
Authors: Heitz M.-C. , Meyer H.-D.
Published: July 2002

Theoretical investigations of molecule–surface scattering are performed using the multiconfiguration time-dependent Hartree method. Rotational and diffractive inelastic scattering of a rigid diatomic molecule from a corrugated static surface is investigated. The numerical simulations concern a five-dimensional N2/LiF(001) model system for collision energies ranging from 60 to 300 meV. A correction scheme of the energy distribution of the initial wave packet allows the simulation to be started close to the surface, in a region where the interaction potential is not negligible. The analysis of the propagated wave packet is performed using a combined flux operator/complex absorbing potential approach to extract partially summed transition probabilities and average energy transfers to selected internal modes. The scattering mechanism is investigated in detail. The surface corrugation is seen to damp the quantum interferences in the rotational excitation process and to induce rotational excitation to the low excited rotational states. The diffraction process and the impact of the initial rotational state of the diatom, in particular its initial orientation with respect to the surface, are discussed.
Electroluminescence from semitransparent Au film/SiO2/(amorphous-Si/SiO2) superlattice/p-Si structure
Authors: Heng C. L. , Chen Y. , Ma Z. C. , Zong W. H. , Qin G. G.
Published: July 2002

SiO 2 layer/four-period amorphous-Si/SiO2 superlattices (ASSOSLs) with amorphous-Si layers having 12 thicknesses in a range of 1.0–3.2 nm were deposited on p-Si substrates using the two-target alternation magnetron sputtering technique. Electroluminescence (EL) from semitransparent Au film/SiO2/ASSOSL/p-Si diodes and from a control diode without any amorphous-Si layer in the ASSOSL has been observed when the applied forward bias exceeded about 5 V; under reverse biases, however, no EL was observed. Every EL spectrum of the Au/SiO2/ASSOSL/p-Si diodes along with the control one could be decomposed into two Gaussian bands with peak energies of 1.82 and 2.22 eV, and full widths at half maximums of 0.40 and 0.65 eV, respectively; and their intensities and the current swung synchronously with increasing Si layer thickness with a period length being consistent with half a De Broglie wavelength of the carriers. The experimental results indicated that the EL originates mainly from the radiative recombination of electron-hole pairs via two types of luminescence centers with luminescence energies of 1.82 and 2.22 eV in the SiO2 layers, rather than within the nanometer Si quantum wells in the ASSOSLs.
Energy transfer, scattering and dissociation in ion atom collisions: CO2+/Ar
Authors: Tosh Ronald E. , Shukla Anil K. , Futrell Jean H.
Published: July 2002

Collision-induced dissociation (CID) and nondissociative scattering of CO2+ ions following collision with a supersonic molecular beam of argon has been studied at low collision energies by crossed-beam tandem mass spectrometry. The center-of-mass (c.m.) velocity contour diagram at 23.8 eV collision energy showed that the scattering of CO2+ ions have two energetically distinct components: elastic collisions at smaller angles in which momentum exchange apparently involves Ar/O repulsive interactions and inelastic collisions at larger angles in which internally excited CO2+ ions recoil from the two-body CO2+/Ar c.m. The most probable energy transfer in the inelastic process is 4.8±0.5 eV, just below the lowest dissociation threshold. The CID processes at the same collision energy leading to fragment ions, CO+ and O+, show similar characteristics. CID occurs via both spectator knock-out and two-body collisions that result into two distinct scattering patterns. The energy transfers for the two pathways for O+ fragment ions are 4.7±0.5 eV for knock-out collisions and 7.6±0.5 eV for the two-body inelastic recoil collision mechanism. It is suggested that CID for O+ via the latter process must involve an electronic state higher than the C state and proceed via curve crossing. Energy transfers for CO+ fragment ions via the two corresponding processes are 5.7±0.5 eV and 7.6±0.5 eV, respectively, clearly suggesting similar mechanisms for energy transfer and dissociation for this CID process also. It is suggested that the bent geometry of the CO2+ ions may be an important factor in promoting two distinct mechanisms.
Computer simulation of a Langmuir trough experiment carried out on a nanoparticulate array
Authors: Fenwick N. I. D. , Bresme F. , Quirke N.
Published: July 2002

We have used molecular dynamics simulation and thermodynamic arguments to analyze the pressure-area curve resulting from the compression of a monolayer of nanoparticulates at a liquid–vapor interface. The pressure–area isotherm produced by molecular dynamics simulation shows characteristics common to experimental isotherms obtained using the Langmuir-trough technique. The surface pressure exhibits a “knee,” which signals the onset of surface collapse. Visual inspection of the computer simulated monolayers, along with theoretical results from a free energy model of the Langmuir-trough experiment, suggests that the monolayer of particulates buckles under compression. This result is in agreement with recent experimental work on colloids at liquid–liquid interfaces.
Chemical sputtering of ta-C: Implications for the deposition of carbon nitride
Authors: Morrison N. A. , Rodil S. E. , Robertson J. , Milne W. I.
Published: July 2002

The majority of attempts to synthesize the theoretically predicted superhard phase β−C3N4 have been driven towards the use of techniques which maximize both the carbon sp3 levels and the amount of nitrogen incorporated within the film. However, as yet no attempt has been made to understand the mechanism behind the resultant chemical sputter process and its obvious effect upon film growth. In this work, however, the chemical sputtering process has been investigated through the use of an as-deposited tetrahedrally bonded amorphous carbon film with a high density nitrogen plasma produced using an rf-based electron cyclotron wave resonance source. The results obtained suggested the presence of two distinct ion energy dependent regimes. The first, below 100 eV, involves the chemical sputtering of carbon from the surface, whereas the second at ion energies in excess of 100 eV exhibits a drop in sputter rate associated with the subplantation of nitrogen within the carbon matrix. Furthermore, as the sample temperature is increased there is a concomitant decrease in sputter rate suggesting that the rate is controlled by the adsorption and desorption of additional precursor species rather than the thermal desorption of CN. A simple empirical model has been developed in order to elucidate some of the primary reactions involved in the sputter process. Through the incorporation of various previously determined experimental parameters including electron temperature, ion current density, and nitrogen partial pressure the results indicated that molecular nitrogen physisorbed at the ta-C surface was the dominant precursor involved in the chemical sputter process. However, as the physisorption enthalpy of molecular nitrogen is low this suggests that activation of this molecular species takes place only through ion impact at the surface. The obtained results therefore provide important information for the modeling and growth of high density carbon nitride.
Thickness dependence of density of gap states in diamond films studied using space-charge-limited current
Authors: Gan Bo , Ahn J. , Rusli  , Zhang Qing , Yoon S. F. , Ligatchev V. A. , Yu J. , Chew K. , Huang Q.-F.
Published: July 2002

Structural and electrical characteristics of chemical vapor deposited (CVD) diamond films have been studied as a function of film thickness. The samples comprise a set of codeposited, nominally undoped diamond films with average grain size on the growth surface increasing linearly with the film thickness. Raman scattering analysis reveals a decrease of nondiamond phase and intragrain defects with increasing film thickness. Temperature dependent dc conductivity results indicate that, as the film thickness increases, the Fermi level moves towards the valence band. There is a corresponding decrease in the density of states at the Fermi level, as deduced from the space-charge-limited current in the bulk of the samples. The spatial variation in the density of states through the material closely reflects the changes observed in the structural and electrical properties of the films. Such characteristic has the implication on the application of CVD diamond in the area of electronics.
First-principles structural stability study of nonstoichiometry-related planar defects in SrTiO3 and BaTiO3
Authors: Suzuki Toshimasa , Fujimoto Masayuki
Published: July 2002

First principles (ab initio) pseudopotential total energy calculations have been made to theoretically interpret the structural stability of various planar defects appearing in the perovskite oxide compounds SrTiO3 and BaTiO3. The defects are a consequence of nonstoichiometric chemical compositions occurring during conventional high-temperature sintering or low-temperature thin-film deposition. The calculations, based on the density functional theory using wave functions expanded by a plane wave basis set, were applied to newly found defect structures in the perovskite materials and have been successfully used to interpret these structures. The results showed the applicability of first principles-based calculations to the theoretical interpretation of the stability of perovskite lattices containing defect structures, comparable to the use of the conventional empirical shell model.
Excellent electric properties of free-standing InAs membranes
Authors: Yamaguchi Hiroshi , Dreyfus Remi , Hirayama Yoshiro , Miyashita Sen
Published: July 2002

We fabricated semiconducting free-standing-beam and Hall-bar structures with a high slenderness ratio, a minimum thickness of 50 nm, and a typical length of several tens of microns using InAs membranes processed from InAs/GaAs heterostructures. These structures showed clear electric conductivity without any intentional doping. We obtained the carrier concentration and mobility by means of standard Hall measurements, thus confirming that both parameters were much larger than those of as-grown heterostructure samples. These results indicate that this material system is promising for micro/nanoelectromechanical system applications.
Scaling the near-wall axial turbulent stress in the zero pressure gradient boundary layer
Authors: Metzger M. M. , Klewicki J. C. , Bradshaw K. L. , Sadr R.
Published: July 2002

Based upon high resolution LDA measurements over a range of momentum deficit thickness Reynolds numbers (Rθ=U∞θ/ν) from 1430 to 31 000, DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] propose a new mixed scaling for the near-wall region profile of the axial turbulent stress, u2¯. The present results support the validity of this scaling over an extended Reynolds number range 1000⩽Rθ⩽5×106.
Photodissociating methyl vinyl ether to calibrate O+ethylene product branching and to test propensity rules for product channel electronic accessibility
Authors: Morton M. L. , Szpunar D. E. , Butler L. J.
Published: July 2002

These experiments investigate the photodissociation of methyl vinyl ether at 193 nm in a crossed laser-molecular beam apparatus. We observe two C–O bond fission channels, a minor channel producing CH3+CH2CHO (X̃ 2A″) and the major channel yielding CH3+CH2CHO (Ã 2A′). Some of the neutral Ã state vinoxy product undergoes secondary dissociation to produce ketene+H. These experiments on the photodissociation of methyl vinyl ether, which produce nascent vinoxy and methyl radicals cleanly in a one-to-one ratio, serve two purposes. First, using the measured photofragment velocities and product branching we calibrate the relative sensitivity of mass spectrometric detection to the methyl and vinoxy polyatomic radical products at the m/e=15 daughter ion, taking into account the loss of neutral vinoxy to ketene+H formation. This relative mass spectrometric sensitivity calibration factor is determined to be 0.116±0.022. Knowledge of this factor allows us to extract the product branching ratio between the two major competing primary product channels from the O(3P)+ethylene reaction, a branching ratio that has been much disputed in the literature. Our results give a CH3+HCO/H+CH2CHO product branching ratio of R=0.61±.11, corresponding to 38% (±5%) branching to the CH3+HCO channel. Second, we use the result that the channel producing Ã-state vinoxy dominates over the formation of ground-state vinoxy to test propensity rules being developed to help predict what product channels may be suppressed by electronically nonadiabatic effects in chemical reactions. These propensity rules discriminate between channels that are “electronically facile” and “electronically difficult/prohibitive.” We find that a qualitative analysis of the changes in electronic configuration along the reaction coordinates for the photodissociation of methyl vinyl ether correctly predicts the dominant channel to be the production of excited state (Ã) vinoxy, the electronically facile channel.
Optical characterization of a laser dye in a solid state host
Authors: Sardar Dhiraj K. , Yow Raylon M. , Mayo Michael L.
Published: July 2002

The optical properties of C18H16N2O2B2F4H2O, an organic laser dye embedded in solid plastic host, have been characterized for a number of laser wavelengths in the visible region. The index of refraction of the dye in plastic host is measured by the conventional method of minimum deviation at these wavelengths. The inverse adding doubling method based on the diffusion approximation and radiative transport theory have been employed to determine the absorption, scattering, and scattering anisotropy coefficients of the dye in plastic host from the measurements of total diffuse transmittance, total diffuse reflectance, and collimated transmittance using an integrating sphere. The total attenuation coefficients obtained by this method have been compared with those determined by the collimated transmission and from the total attenuation measurement.
Calculation of ion mobilities from electrospray ionization high-field asymmetric waveform ion mobility spectrometry mass spectrometry
Authors: Guevremont Roger , Barnett David A. , Purves Randy W. , Viehland Larry A.
Published: July 2002

Careful analysis is made of the accuracies possible when gaseous ion mobilities are determined from data obtained by combining electrospray ionization (ESI), high-field asymmetric waveform ion mobility spectrometry (FAIMS) at atmospheric pressure and room temperature, and mass spectrometry (MS). New results are reported for the mobilities of positive and negative amino acid ions in air at elevated values of E/N, the ratio of the electric field strength to the gas number density.
Two channel vapor nucleation in the vicinity of the triple point
Authors: Anisimova Lyubov , Hopke Philip K. , Terry Jason
Published: July 2002

Considering the topology of semiempirical nucleation rate surfaces originate from lines describing the appropriate phase equilibria, there will be two nucleation rate surfaces that exist for the different physical states of the critical embryo phases that are formed near the triple point. Each rate surface is independently related to a nucleation channel and is described by individual equations for nucleation rates. Because of the differences in the sticking probability of vapor molecules when they collide with a physical surface (in this case, with the surface of the nucleation embryos) in the different phases, the growth rates of the different phase clusters will be different. As a result of this difference, one expects to find different sized particles for two cluster phases in the vicinity of the triple point for first order phase transitions. In the present study, particle size distributions were measured near glycerin triple point. A clearly bimodal size distribution was observed. This result suggests that there are two independent nucleation channels that exist near the triple point. This experimental system has the ability to discern particles produced through the two separate nucleation channels so that the nucleation rates can be measured for each channel.
High-density InP self-assembled quantum dots embedded in In0.5Al0.5P grown by metalorganic chemical vapor deposition
Authors: Ryou J. H. , Dupuis R. D. , Mathes D. T. , Hull R. , Reddy C. V. , Narayanamurti V.
Published: July 2002

We describe the characteristics of high-density InP self-assembled quantum dots embedded in In0.5Al0.5P cladding layers grown at 650 °C on GaAs (100) substrates by metalorganic chemical vapor deposition. Quantum dots grown with different deposition times are characterized by atomic force microscopy, photoluminescence, and transmission electron microscopy. For certain growth conditions, we observe the formation of a high density of quantum dots on the order of 1010 cm−2. The quantum dot average height increases from ∼5 to ∼25 nm with deposition time, while the quantum dot density changes insignificantly. Photoluminescence (4 K) shows a gradual shift of emission spectral peak from 2.06 eV (for 7.5 ML) to 1.82 eV (for 22.5 ML), corresponding to changes in the dominant quantum dot size. Also, incoherent quantum dot formation is not observed for up to 15 ML growth.
Theoretical calculation for the multiplet structure of the tetrahedrally coordinated Cr4+ in Y3Al5O12
Authors: Ishii Takugo , Ogasawara Kazuyoshi , Adachi Hirohiko , Tanaka Isao
Published: July 2002

The multiplet structure of the tetrahedrally coordinated Cr4+ in Y3Al5O12 [yttrium aluminum garnet (YAG)] was calculated by the ab initio electronic-structure calculation method. The authors examined the dependence of the multiplet structures on the size of cluster models, with the use of (CrO4)4− (without point charges), (CrO4)4− (with point charges), (CrY6Al4O44)54−, and (CrY18Al42O148)112− models. The covalency of the impurity-level molecular orbitals was analyzed, and it was revealed that the wave functions of the atoms outside the CrO4 tetrahedron should not be neglected. The validity of the assignment of the peaks in the absorption spectrum written in the literature was evaluated by the calculated magnitude of the spin-orbit splittings and the oscillator strengths of the transitions, which were obtained by the fully relativistic many-electron calculation. The effect of the codopant on the transition probability was also estimated by (CrCaY5Al4O44)55− models. It was indicated that the nearest Cr–Ca pairs would reduce the symmetry, and could produce some satellite peaks. The mixing of the wave functions of the triplet states was numerically analyzed, and the results were correlated with the transition probability. The traditional nephelauxetic parameter was estimated, and the value 0.51 was obtained. It was confirmed from first principles that the magnitude of the nephelauxetic parameter in some literature was appropriate.