bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermolecular vibrations of 1-naphthol⋅NH3 and d3-1-naphthol⋅ND3 in the S and S1 states
Authors: Henseler Debora , Tanner Christian , Frey Hans-Martin , Leutwyler Samuel
Published: July 2002

Hydrogen-bonded complexes of the photoacid 1-naphthol with NH3 and ND3 were investigated by resonant two-photon ionization, spectral hole burning, and fluorescence spectroscopies. Although the intermolecular vibrations are weak in both absorption and emission, with typical Franck–Condon factors <2% relative to the electronic origin, all six intermolecular modes were identified, namely the hydrogen bond stretch σ, the ammonia torsion τ, two in-plane wags β1 and β2, and two out-of-plane rocking motions ρ1 and ρ2. Several ammonia torsional excitations were observed, with spacings in good agreement with the S0- and S1 state effective torsional barriers derived by Humphrey and Pratt [J. Chem. Phys. 104, 8332 (1996)]. The β1, β2, and ρ2 vibrational excitations exhibit large (2–8 cm−1) torsional splittings, which indicate strong anharmonic coupling with the ammonia internal rotation. The observed Franck–Condon factors of the intermolecular stretching vibration imply a contraction of the O–H⋅⋅⋅N hydrogen bond by ≈0.01 Å upon S1←S0 excitation.
Flux-bias stabilization scheme for a radio-frequency amplifier based on a superconducting quantum interference device
Authors: Mück Michael , Clarke John
Published: July 2002

A magnetic flux stabilization scheme involving a low-frequency flux-locked loop has been developed to regulate the gain of a radio-frequency amplifier based on a superconducting quantum inteference device (SQUID). The flux-locked loop largely eliminates drifts in the gain arising from drifts in the flux bias. Tests on a microstrip SQUID amplifier operating at 777 MHz inside a superconducting shield show that the peak-to-peak drift in the gain is no more than 0.3 dB/day.
Nonlinear response functions for the three-dimensional spectroscopies
Authors: Cho Minhaeng
Published: July 2002

Theoretical descriptions of the three-dimensional spectroscopies are presented in this paper by calculating the associated nonlinear response function. Previously, the harmonic approximation with the Wick’s theorem was used to obtain theoretical expression of the three-dimensional vibrational response function, by treating the mechanical and electrical anharmonicities perturbatively. However, the bath-induced memory effect and anharmonicity-induced frequency shift were not correctly taken into account by the previous theory. By incorporating the system–bath interaction properly, the general nonlinear response functions for the three-dimensional vibrational, vibrational-electronic, or electronic spectroscopies are obtained and discussed in detail. By using the resultant nonlinear response function, two-color vibrational photon echo, three-dimensional sum- and difference-frequency generation spectroscopies, and two-color infrared pump–probe spectroscopy are theoretically proposed.
Special stability advantages of position-Verlet over velocity-Verlet in multiple-time step integration
Authors: Batcho Paul F. , Schlick Tamar
Published: July 2002

We present an analysis for a simple two-component harmonic oscillator that compares the use of position-Verlet to velocity-Verlet for multiple-time step integration. The numerical stability analysis based on the impulse-Verlet splitting shows that position-Verlet has enhanced stability, in terms of the largest allowable time step, for cases where an ample separation of time scales exists. Numerical investigations confirm the advantages of the position-Verlet scheme when used for the fastest time scales of the system. Applications to a biomolecule, a solvated protein, for both Newtonian and Langevin dynamics echo these trends over large outer time-step regimes.
Time domain analysis of a rational harmonic mode locked ring fiber laser
Authors: Zhu G. , Chen H. , Dutta N.
Published: July 2002

In this article, we present a theory of the pulse train generated by a rational harmonic mode locked ring fiber laser. The pulse width is calculated as a function of the rational harmonic order and the optical transfer function of the modulator. The theoretical work is based on a time domain analysis, which predicts that the pulse width decreases when the rational harmonic order goes up. The pulse width as a function of the modulation amplitude and bias level of the modulator was measured, and the experimental results agree with the theory.
Room temperature electroluminescence from a c-Si p-i-n structure
Authors: Dittrich Th. , Timoshenko V. Yu. , Rappich J. , Tsybeskov L.
Published: July 2002

The time dependent electroluminescence (EL) of c-Si (at 1.1 eV) is investigated at room temperature for a p-i-n structure under excitation with forward biased current pulses. The EL intensity increases by square law at shorter times (<3 μs) and reaches a steady state value at longer times (>10 μs). The parabolic dependence of the EL intensity on the current density at the shorter times points to the bimolecular recombination mechanism. The EL response time has been decreased to less than 200 ns for the given p-i-n structure by application of a reverse bias potential. The maximal EL quantum efficiency is of the order of 0.01% for the investigated p-i-n structure and possible ways to increase this value are discussed.
Ellipsometric study of InAs wetting layer in InAs/GaAs quantum dots at the threshold of quantum dot formation
Authors: Lee Hosun , Kim S. M. , Park Y. J. , Kim E. K.
Published: July 2002

Using room temperature spectroscopic ellipsometry, we measured the pseudo-dielectric function of self-assembled InAs/GaAs quantum dots at the onset of quantum-dot nucleation in the spectral range from 0.8 to 6 eV. The nominal In coverage varied from 1 to 2.6 ML. In addition to the quantum-dot-related feature at 1.1 eV, we observed two high energy transitions near 1.34 and 1.38 eV which arose from the InAs wetting layer. These two high energy features merged in 1-ML-thick wetting layer. We fitted the dielectric function of InAs wetting layer in the visible range performing multilayer analysis, and found strong excitonic enhancement and blue shift of the E1 peak.
Aerosol generation system for development and calibration of laser-induced breakdown spectroscopy instrumentation
Authors: Hahn D. W. , Carranza J. E. , Arsenault G. R. , Johnsen H. A. , Hencken K. R.
Published: July 2002

An aerosol generation system is described that enables the production of precise mass flow streams of well-characterized, submicron-sized aerosol particles. A pneumatic-type nebulizer is used to nebulize aqueous solutions of dissolved metals, which subsequently dry in a gaseous co-flow, producing a gaseous stream of dispersed, fine solid particles with a known mass concentration. Gravimetric calibration of nebulizer demonstrates the precise nature of the device for aerosol generation. Representative iron-based (spherical) and titanium-based (nonspherical) particles are analyzed using transmission electron microscopy and x-ray diffraction. Both aqueous metals form metallic oxides, namely, FeO and TiO, and the measured particle sizes are in the range 10–100 nm. The measured particle size and composition are used to calculate the size distribution of droplets produced by the nebulizer, which yield a mean droplet diameter of 524 nm. The nebulizer droplet distribution is well described by a log–normal distribution. Using the aerosol generator as a calibration source of aerosol particles for laser-induced breakdown spectroscopy, linear calibration curves were produced for titanium over mass concentrations ranging from 0 to 4425 μg/m3.
Volume and surface photoemission from a niobium target by polarized ultraviolet radiation
Authors: Donateo A. , Martina L. , Nassisi V.
Published: July 2002

This work reports on the experimental results of a Nb photocathode stimulated by two different lasers, of wavelengths 308 and 222 nm. A fast current detector, Rogowski coil, recorded the high output currents, while the beam angular distributions were measured by an array of small Faraday cups placed along the propagation axis of the beam. With p-polarized light, we found quantum efficiencies (QEs) of 1.6×10−5 and 1.0×10−6 for λ=222 nm and λ=308 nm, respectively. The efficiency of the photocathode was measured to be higher for s-polarized laser beams than for the p-polarized ones. They resulted in QEs of 5.3×10−5 and 2.1×10−6 for λ=222 nm and λ=308 nm, respectively. From the electron-beam angular divergence values, we determined the upper limit emittance. The lowest upper limit normalized emittance value found was 7 (π mm mrad), achieved by the s-polarized radiation of 308 nm. In both cases the s-polarized light generated electron beams of lower emittance.
On the structure and thermodynamics of heat flow in fluids: A molecular dynamics study
Authors: Baranyai András
Published: July 2002

We explore an approach to determine the entropy of a liquid in nonequilibrium steady state. Soft-sphere particles interacting with an inverse twelfth-power potential are simulated under the impact of a steady temperature gradient using a realistic algorithm. The thermodynamic relationship between the internal energy and the entropy is integrated in terms of the temperature for identical temperature gradients. Exploiting the scaling property of the interaction, the actual calculations are carried out at different densities starting from dilute systems. The energy of the middle layer is compared to that of an equilibrium system with identical temperature and density. The very small nonequilibrium increase of the energy is connected to the field-induced change in the local structure.
Inertial control of the mirror suspensions of the VIRGO interferometer for gravitational wave detection
Authors: Losurdo G. , Calamai G. , Cuoco E. , Fabbroni L. , Guidi G. , Mazzoni M. , Stanga R. , Vetrano F. , Holloway L. , Passuello D. , Ballardin G. , Braccini S. , Bradaschia C. , Cavalieri R. , Cecchi R. , Cella G. , Dattilo V. , Di Virgilio A. , Fidecaro F. , Frasconi F. , Gennai A. , Giazotto A. , Ferrante I. , La Penna P. , Lelli F. , Lomtadze T. , Marin A. , Mancini S. , Paoletti F. , Pasqualetti A. , Passaquieti R. , Poggiani R. , Taddei R. , Vicerè A. , Zhang Z.
Published: July 2002

In order to achieve full detection sensitivity at low frequencies, the mirrors of interferometric gravitational wave detectors must be isolated from seismic noise. The VIRGO vibration isolator, called the superattenuator, is fully effective at frequencies above 4 Hz. But the residual motion of the mirror at the mechanical resonant frequencies of the system is too large for the interferometer locking system and must be damped. A multidimensional feedback system, using inertial sensors and digital processing, has been designed for this purpose. An experimental procedure for determining the feedback control of the system has been defined. In this article a full description of the system is given and experimental results are presented.
Neutral C60 effusive source for atomic collisions with fullerene
Authors: Lobo R. F. M. , Silva N. T.
Published: July 2002

A neutral C60 effusive source was assembled in a crossed molecular-beam apparatus in order to perform atom–molecule electron transfer collisions and its calibration by time-of-flight mass spectrometry. This source consists of a stainless-steel oven where the C60 sublimation takes place. Typical operation temperatures are in the range of 723 up to 888 K. Through this setup the study of the ion pair formation process K+C60→K++C60− (where K is a potassium atom) can be performed in a wide range of collision energies (10 up to 500 eV). The results show evidence of negative ion formation (being the ratio of intensities between the parent ion and fragment ions) clearly dependent on the collision energy used.
Raman mapping of epitaxial lateral overgrown GaN: Stress at the coalescence boundary
Authors: Kuball M. , Benyoucef M. , Beaumont B. , Gibart P.
Published: July 2002

Using micro-Raman scattering spectroscopy we have investigated stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy using a two-step growth method. The presence of an increased compressive stress at the coalescence boundary of two adjacent wings of ELO GaN was identified. From changes in the E2 (high) phonon frequency we estimate the magnitude of the stress concentration at the coalescence boundary to be on the order of ≈0.07 GPa with respect to the ELO GaN wing. Mechanisms for the stress concentration at the coalescence boundary were studied. Differences in stress and crystalline quality between wing and window regions of ELO GaN were also investigated.
Dissipation in helical turbulence
Authors: Ditlevsen P. D. , Giuliani P.
Published: July 2002

In helical turbulence a linear cascade of helicity accompanying the energy cascade has been suggested. Since energy and helicity have different dimensionality we suggest the existence of a characteristic inner scale, ξ=kH−1, for helicity dissipation in a regime of hydrodynamic fully developed turbulence and estimate it on dimensional grounds. This scale is always larger than the Kolmogorov scale, η=kE−1, and their ratio η/ξ vanishes in the high Reynolds number limit, so the flow will always be helicity free in the small scales.
Spectroscopy of a ferroelectric plasma cathode
Authors: Dunaevsky A. , Chirko K. , Krasik Ya. E. , Felsteiner J. , Bershtam V.
Published: July 2002

Results of spectroscopic investigations of the plasma formed on the surface of a ferroelectric cathode upon the application of a driving pulse are presented. The ferroelectric plasma cathode was made of a solid solution of Sr, Ba, Ti, Nb, Pb, and O. Its front side was covered by Cu grounded strip electrodes. A driving pulse with an amplitude ≲18 kV and pulse duration of ∼400 ns was applied to the rear Cu disk electrode. A Jobin-Yvon 750M spectrometer was used for visible light dispersion. Spectral line profiles were obtained by a fast framing camera. It was shown that light is emitted from the excited ions and neutral atoms of Cu, Pb, Sr, Ba, Ti, and H within the first 50 ns after the beginning of the driving pulse. By analyzing the Doppler broadening of the observed spectral line profiles it was found that the ion and neutral atom temperature is ⩽0.8 eV. Analysis of the Stark broadening of the Hα and Hβ spectral lines showed the absence of a high (>1 kV/cm) electric field which could be developed at the surface of the ferroelectric due to the appearance of noncompensated surface polarization charges. The same Stark analysis also showed that the plasma density does not exceed 1013 cm−3. By comparing the relative intensities of the Hα and Hβ spectral lines obtained with the results of collision radiative modeling, the plasma electron temperature was found to be ∼3 eV.
Enhancement of magnetic surface anisotropy of Pd/Co/Pd trilayers by the addition of Sm
Authors: Okamoto Satoshi , Nishiyama Katsuya , Kitakami Osamu , Shimada Yutaka
Published: July 2002

We have studied the effect of Sm on the magnetic surface anisotropy (MSA) of Pd/Co100−xSmx/Pd (111) trilayers, where Sm content x was varied from 0 to 11. All samples show perpendicular anisotropy due to the strong MSA at the interfaces. The MSA significantly increases with x and attains a maximum value of 0.60 erg/cm2 at x≃8.3, which is 36% larger than that of pure Co (x=0). The volume term of the magnetic anisotropy shows a similar behavior as the MSA. The appreciable increase in MSA is considered to be due to the enhancement of orbital moment of Co by the addition of Sm.
Pulse bias sputtering of copper onto insulating surfaces
Authors: Barnat E. V. , Lu T.-M. , Little J.
Published: July 2002

The effects of charge accumulation during bias sputtering of copper onto an insulating material are studied. To eliminate the effects of charge accumulation, we periodically apply a pulse bias to the electrode. Microstructure and electrical properties of the sputtered copper films grown on silicon dioxide are measured as a function of the pulse bias frequency. By comparing the observed properties of these films grown under a pulse bias to those grown under a dc bias (with a conducting path to the electrode), the effectiveness of the pulse bias in controlling the ion energy distribution is demonstrated.
Structural, electrical, and optical properties of low-doped 4H–SiC chemical vapor deposited epitaxial layers
Authors: Kalinina E. , Kholujanov G. , Zubrilov A. , Solov’ev V. , Davydov D. , Tregubova A. , Sheglov M. , Kovarskii A. , Yagovkina M. , Violina G. , Pensl G. , Hallén A. , Konstantinov A. , Karlsson S. , Rendakova S. , Dmitriev V.
Published: July 2002

Results of complex studies of structural, optical, and electrical characteristics of 4H–SiC n-type low-doped epitaxial layers grown by the chemical vapor deposition (CVD) method are presented. Characteristics of CVD layers grown on commercial wafers with and without a thin (<0.1 μm) buffer layer grown by liquid phase epitaxy (LPE) between the commercial wafer and CVD epitaxial layer were compared. It has been shown that the LPE filling process caused a significant improvement in the structural quality of CVD layers. Cathodoluminescence (CL) data and electrical characteristics of Schottky barriers (SB) revealed that the LPE layer also made the concentration profiles of the uncompensated donors and recombination centers for holes as well as the hole diffusion lengths more uniform over the CVD layer. According to CL and secondary ion mass-spectroscopy measurements the presence of aluminum as an impurity was detected in initial commercial wafers, as well as having LPE and CVD epitaxial layers. Forward current–voltage characteristics of SBs formed on CVD layers with and without a LPE buffer layer followed an exponential relationship with an ideality factor of 1.04–1.06 over six to seven orders of magnitude in current density. A noticeable increase of the breakdown voltage was also observed in samples with a LPE layer.
Four-wave mixing spectroscopy of metastable centers in semiconductors
Authors: Koziarska-Glinka B. , Suchocki A.
Published: July 2002

An application of the continuous wave four-wave mixing technique for studying the nature of metastable centers in semiconductors is presented. The results of measurements of the four-wave mixing scattering efficiency dependence on the intensity of the write beams for CdF2 crystals doped with In or Ga are presented and theoretically explained. It is also shown that the obtained results can be used for identification of the sign of the Hubbard correlation energy of metastable centers in semiconductors.
Spectroscopic and laser properties of Nd3+ in LaSc3(BO3)4 host
Authors: Sardar Dhiraj K. , Castano Francisco , French Joey A. , Gruber John B. , Reynolds Thomas A. , Alekel Theodore , Keszler Douglas A. , Clark Benjamin L.
Published: July 2002

Spectroscopic and laser properties have been characterized for Nd3+ in LaSc3(BO3)4. The Judd–Ofelt analysis has been applied to the measured room temperature absorption spectrum to determine the radiative decay rates and branching ratios of Nd3+ transitions from the F3/24 metastable state to the IJ4 lower-lying manifolds. The parameters Ω2, Ω4, and Ω6 are larger than those reported for Nd3+ in other laser host crystals. The value of Ω4/Ω6 is approximately 3.0 times larger than that of Nd3+ in yttrium–aluminum–garnet (YAG) and about 1.4 times larger than that of Nd3+ in the β phase of LaSc3(BO3) reported recently. The measured room temperature fluorescence lifetime of the F3/24→I11/24 transition is 150 μs, while the Judd–Ofelt analysis predicts a radiative lifetime for the F3/24 state to be 249 μs, resulting in the fluorescence quantum efficiency of 60%. The emission cross sections of the F3/24→I11/24 and F3/24→I13/24 intermanifold transitions have been also determined at room temperature. Finally, these results are compared with those of the standard laser material Nd3+:YAG.
Measurement and analysis of leaded glass polycapillary optic performance for hard x rays
Authors: Suparmi  , Cari  , Wang Lei , Wang Hui , Gibson W. M. , MacDonald C. A.
Published: July 2002

The properties of borosilicate glass polycapillary x-ray optics have been extensively studied. Small-area scatter rejection borosilicate glass polycapillary optics have been demonstrated with good results. Many medical imaging and industrial radiographic applications for x rays would require large-area optics with good scatter rejection. Since shorter optics are easier to manufacture, optics with a shorter length would provide a faster route to bringing the benefit of polycapillary x-ray optics to these applications. Leaded glass would allow the optic to be much shorter and still give good contrast enhancement, because of the superior absorption of lead glass. In order to investigate the feasibility of using leaded glass polycapillary x-ray optics for these applications, measurements and simulations have been performed on the behavior of leaded glass polycapillary fibers in the 9–80 keV energy range. The transmission efficiencies of these fibers of different types and lengths were measured as a function of source location and x-ray energy. The measurements were analyzed using a geometrical optics simulation program, which included roughness, waviness, bending effects, and a leaded glass filter layer. Despite low transmission at low energies, leaded glass polycapillary x-ray optics with a length of 30–60 mm seem promising for many high-energy (>20 keV) x-ray applications. The longer fibers have transmission efficiency of up to 50% in the 35–40 keV, and very low scatter transmission of less than 0.06% up to 80 keV.
Resonant excitation of Er3+ by the energy transfer from Si nanocrystals
Authors: Watanabe Kei , Fujii Minoru , Hayashi Shinji
Published: July 2002

Photoluminescence (PL) properties of SiO2 films containing Si nanocrystals (nc-Si) and Er were studied. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separations between the discrete electronic states of Er3+. PL from exciton recombination in nc-Si and the intra-4f shell transition of Er3+ were observed simultaneously. At low temperatures, periodic features were observed in the PL spectrum of nc-Si. The period agreed well with the optical phonon energy of Si. The appearance of the phonon structure implies that nc-Si which satisfy the energy conservation rule during the energy transfer process can resonantly excite Er3+. For the PL from Er3+, a delay was observed after the pulsed excitation of nc-Si hosts. The rise time of the PL showed strong size dependence. The effects of the quantum confinement of excitons in nc-Si on the energy transfer process are discussed.
The effect of carrier gas pressure and wall heating on the operation of the thermal diffusion cloud chamber
Authors: Ferguson Frank T. , Heist Richard H. , Nuth Joseph A.
Published: July 2002

Experimental observations indicate that the nucleation behavior within the thermal diffusion cloud chamber (TDCC) changes with increasing carrier gas pressure and applied sidewall heating, even though such an effect is not predicted by typical nucleation theories and it is not seen in typical expansion-based nucleation studies. In this work we present a model of the chamber which shows that both of these effects are likely due to buoyancy-induced convection within the TDCC. As the chamber pressure is increased, the calculated critical supersaturation within the chamber decreases. Results from a simple model of the chamber wall heating are also presented. Previously, it was argued that unheated chamber walls result in a significant, radial concentration gradient which lowers the vapor concentration and condensation flux within the chamber center. In contrast, we show that this reduction is due primarily to a convective flow induced by the sidewall concentration gradient. The model has been applied to recent experimental data for n-pentanol. Results indicate that, with respect to buoyancy-induced convection, the typical 1D model should be regarded as an upper limit to the maximum attainable supersaturation within the chamber.
Phosphorous implantation in silicon through thin SiO2 layers: Oxide damage and postoxidation thermal treatments
Authors: Vedda A. , Martini M. , Spinolo G. , Crivelli B. , Cazzaniga F. , Ghidini G. , Vitali M. E.
Published: July 2002

A significant increase of HF etching rate and mean surface roughness (monitored by atomic force microscopy) was observed after P ion implantation on thin thermal SiO2 films (150 Å). The dependence upon the ion fluence (in the range 3×1012–5×1013 ions/cm2) and energy (in the range 270–500 keV) was analyzed, together with the recovery effect of a postimplantation annealing in N2 atmosphere. Moreover, the impact of P implants on oxides grown by different sequences, considering postoxidation annealing in N2O or N2 atmospheres, was also studied. The effect of ion irradiation was investigated by thermally stimulated luminescence (TSL) above room temperature in order to obtain information on point defects present in the layers. The results showed that postoxidation annealing treatments in N2 atmosphere carried out not only after, but also before ion implantation, were particularly useful in order to lower the concentration of TSL active defects. This can be interpreted as a role of N2 annealing in favoring a structural rearrangement of the SiO2 layers.
Femtosecond photoelectron imaging on pyrazine: Spectroscopy of 3s and 3p Rydberg states
Authors: Song Jae Kyu , Tsubouchi Masaaki , Suzuki Toshinori
Published: July 2002

Two-color and one-color resonance enhanced multiphoton ionization photoelectron spectroscopies (REMPI–PES) have been applied to Rydberg states (n=3) of jet-cooled pyrazine. The 3s and 3p members of Rydberg series converging to the ground state (n−1) of the cation and the 3s member of a Rydberg series converging to an excited state of the cation (π−1) were observed. The photoelectron angular distributions (PADs) measured via the 3s(n−1) state drastically differed for the two-color and one-color REMPI experiments. This behavior is ascribed to different molecular axis alignments created by the two-photon excitation schemes. The PADs were also used to discriminate between the different Rydberg series.
Coexisting photoluminescence of Si and Ge nanocrystals in Ge/Si thin film
Authors: Zhu Y. , Yuan C. L. , Quek S. L. , Chan S. S. , Ong P. P. , Li Q. T.
Published: July 2002

Thin films of composite germanium/silicon (Ge/Si) were prepared by pulsed laser ablation alternately on Ge and Si materials on a rotary target, followed by vacuum deposition of the ablated materials on an ultraclean glass substrate. X-ray diffraction and atomic force microscopy phase analysis confirmed that the film structure consisted of a mixture of Si and Ge nanoparticles which could exist in two possible phases. Most of the particles are of less than 30 nm in diameter even after the sample was annealed at 500 °C for 6 h. With different excitation light of wavelengths 280 and 380 nm the composite film yielded independent photoluminescence emissions corresponding, respectively, to the Si and Ge nanoparticles which did not interfere with each other. These results demonstrate that there was very little interaction between the Si and Ge emissions arising from their coexisting mixture in the thin film, even after high temperature annealing of the film in the atmosphere. It opens up the possibility for application of the Si/Ge composite film in multiple function optoelectronic devices.
Transient photoconductivity properties of tungsten oxide thin films prepared by spray pyrolysis
Authors: Hao Jianhua , Studenikin S. A. , Cocivera Michael
Published: July 2002

Tungsten oxide (WO3) thin films were deposited by spray pyrolysis of an ammonium tungsten oxide solution. The effect of postannealing on the structural, transport and optical properties of the films has been studied. Under steady-state illumination, slow photoconductivity growth and relaxation transients were observed at room temperature. The contributions of carrier concentration and mobility to the photoconductivity were determined from photo-Hall and photoconductivity data. The transient photoconductivity was found to be mainly due to photoinduced excess electrons over a wide time range from 0.0 to 104 s. This slow relaxation was characterized by two exponential decays indicating two discrete subband gap levels were involved. The fast photoconductivity relaxation over the time range from 10−8 to 10−1 s was more complicated and probably involved a distribution of subband gap states.
Control of the noncentrosymmetry of thick polymer films by nonresonant all-optical poling
Authors: Si Jinhai , Qiu Jianrong , Hirao Kazuyuki
Published: July 2002

All-optical control of the noncentrosymmetry of azodye-doped polymer films was demonstrated by nonresonant dual-frequency coherent excitation. Three kinds of noncentrosymmetries of the polymer films were optically tailored using appropriate combinations of the writing beam polarizations, linearly polarized, and circularly polarized light. The tensor analysis of the photoinduced noncentrosymmetry was performed by monitoring second harmonic generation (SHG) of the samples. The quadratic dependence of SHG on film thickness was observed, indicating that a χ(2) grating satisfying the phase-matching condition for SHG was optically induced in the polymer films and that this nonresonant all-optical poling can be used to tailor the symmetry of bulk samples.
Effects of P doping on photoluminescence of Si1−xGex alloy nanocrystals embedded in SiO2 matrices: Improvement and degradation of luminescence efficiency
Authors: Toshikiyo Kimiaki , Tokunaga Masakazu , Takeoka Shinji , Fujii Minoru , Hayashi Shinji , Moriwaki Kazuyuki
Published: July 2002

The effects of P doping on photoluminescence (PL) properties of Si1−xGex alloy nanocrystals (nc-Si1−xGex) in SiO2 thin films were studied. P doping drastically decreases the electron spin resonance (ESR) signals that are assigned to the Si and Ge dangling bonds at the interfaces between nc-Si1−xGex and SiO2 matrices (Si and Ge Pb centers). With increasing P concentration, the signal from the Ge Pb centers are first quenched, and then the signal from the Si Pb centers start to be quenched. The quenching of the ESR signals is accompanied by a drastic enhancement of the PL intensity. The PL intensity has a maximum at a certain P concentration, which depends on the Si:Ge ratio. By further increasing the P concentration, the PL intensity becomes weaker. In this P concentration range, optical absorption emerges due to the intravalley transition of free electrons generated by the P doping. The observation of the free-electron absorption provides direct evidence that carriers in nanometer-sized Si1−xGex alloy crystals can be controlled by impurity doping.
Inclusion of mean-field spin–orbit effects based on all-electron two-component spinors: Pilot calculations on atomic and molecular properties
Authors: Iliaš Miroslav , Kellö Vladimı́r , Visscher Lucas , Schimmelpfennig Bernd
Published: July 2002

An implementation of a two-component all-electron (jj/ωω) treatment of both scalar and spin–orbit relativistic effects in the MOLFDIR program suite is presented. Relativity is accounted for by Douglas–Kroll transformed one-electron operators: scalar (spin-free) and so called mean-field spin–orbit terms. The interelectronic interaction is represented by the nonrelativistic Coulomb operator. High-level correlated calculations of properties of several systems (FO, ClO, Cl, O2+, O2−, Tl, and TlH) where spin–orbit effects play a dominant role are presented and compared with other data. Agreement with Dirac–Coulomb(–Gaunt) reference values is in general very good.
Influence of nucleation nature on Ca mineral/substrate structural synergy and implications for biomineralization in microgravity
Authors: Liu Xiang Yang
Published: July 2002

This paper presents a study of the effect of heterogeneous/homogeneous nucleation and microgravity on the structural synergy between Ca minerals and substrates. The structure of human bone is determined to a large extent by the turnover of the bone, which includes nucleation of biominerals adjacent to collagen molecules. Heterogeneous-like nucleation will promote a strong structural correlation between biominerals and substrates. This may lead to a compact bone structure. On the other hand, homogeneous-like nucleation will create a poor correlation between biominerals and substrates. This may lead to a porous bone structure. Microgravity will suppress convection, and consequently promote homogeneous-like nucleation during biomineralization. This could be responsible for the formation of the porous structure of bone in space.
Niobate-based microwave dielectrics suitable for third generation mobile phone base stations
Authors: Hughes H. , Iddles D. M. , Reaney I. M.
Published: July 2002

High unloaded quality factor (Qu), zero temperature coefficient of resonant frequency (τf) and high relative permittivity (εr) microwave dielectric ceramics have been fabricated based on BaZn1/3Nb2/3O3. Properties have been optimized for the composition, 0.9Ba([Zn0.60Co0.40]1/3Nb2/3)O3–0.1Ba(Ga0.5Ta0.5)O3 for which Qu=32 000 @ 3.05 GHz, εr=35, and τf=0. The new compounds are disordered according to x-ray diffraction (XRD) and may be indexed using a simple perovskite unit cell, a=4.09 Å. Small peaks (e.g., d≈3.01 Å, relative intensity, 4.5) attributed to a barium niobate second phase are also present in XRD patterns. These ceramics are suitable in terms of cost and performance for base stations supporting third generation architecture.
Energy levels of HCN+ and DCN+ in the vibronically coupled X 2Π and A 2Σ+ states
Authors: Tarroni Riccardo , Mitrushenkov Alexander , Palmieri Paolo , Carter Stuart
Published: July 2002

The X 2Π and A 2Σ+ electronic states of HCN+ have been studied using a previously developed method [Carter et al., Mol. Phys. 98, 1967 (2000)] suitable for triatomic molecules showing three-state (Renner-Teller+vibronic) interactions. Ab initio three-dimensional diabatic potential energy surfaces for the Π(1 2A′2,1 2 A″) and Σ+2(2 2A′) states have been computed at the multireference configuration interaction level of theory, using extended Gaussian basis sets. Additional computations were done to determine the barrier to isomerization over the three surfaces and the spin–orbit constant for the Π2 state. Energies, spin–orbit splittings, and rotational constants have then been calculated for all rovibronic levels of Σ and Π symmetry up to 5800 cm−1 for HCN+ and 4800 cm−1 for DCN+. Assignments based on plots of vibrational wave functions are also provided. These computations have been finally used to revise previous interpretations of photoelectron spectra.
Molecular dynamics simulation of liquid N2O4⇌2NO2 by orientation-sensitive pairwise potential. I. Chemical equilibrium
Authors: Katō Toshiko , Hayashi Soichi , Machida Katsunosuke
Published: July 2002

This paper, the first of a series of papers, examines equilibrium properties of N2O4⇌NO2 in liquid state by classical molecular dynamics simulations of liquid NO2. An ab initio MO calculation has been carried out to elucidate NO2–NO2 potential, and an orientation-sensitive pairwise potential (OSPP), which can reproduce highly anisotropic character of covalent bonding between N–N, has been formulated. The OSPP potential is parameterized by the well depth De and by two anisotropy factors: Aθ (0⩽Aθ⩽1) the anisotropy factor for the rocking angle between NN bond and ONO direction, and Aτ (0⩽Aτ⩽1) for torsional angle of the two NO2 about NN bond. The reactive liquid N2O4 is modeled as liquid NO2 which interacts with the OSPP potential between N–N atoms and Lennard-Jones potentials between N–O and O–O atoms. Equilibrium properties were found to be very sensitive to the well depth De and anisotropy factors of OSPP. The population of more than the NO2 dimer (3-mer, 4-mer,…) is considerable when anisotropy factors of the NN bond are small. On the other hand, the equilibrium liquid N2O4⇌2NO2 is formed, that is, most NO2 form monomer or dimer and the population of more than 3-mer is very small when Aθ+Aτ⩾0.4–0.5. In simulated liquid NO2/N2O4, concentration of N2O4 is found to increase as De increases, Aθ increases, and Aτ decreases. The equilibrium constant for the dissociation reaction has been derived by computing the potential of mean force as a function of the N–N distance rc (the reaction coordinate). The OSPP potential for De=0.12×10−18 J, Aθ=0.5 and Aτ=0.1 is found to reproduce the observed liquid phase equilibrium properties fairly well.
Quantitative sensing of nanoscale colloids using a microchip Coulter counter
Authors: Saleh O. A. , Sohn L. L.
Published: July 2002

We have fabricated a microchip Coulter counter on a quartz substrate, and have used it to detect individual nanoscale colloidal particles with a sensitivity proportional to each particle's size. We demonstrate the ability of this device to sense colloids as small as 87 nm diameter, and to distinguish between colloids whose diameters differ by less than 10%. Further reductions in the pore size, easily done with current nanofabrication techniques, make our device applicable to measuring biological macromolecules, such as DNA and proteins.
Suppression of interlayer coupling and enhancement of magnetoresistance in spin valves with oxide layer
Authors: Li Kebin , Wu Yihong , Qiu Jinjun , Han Guchang , Guo Zaibing , Xie Hong , Chong Towchong
Published: July 2002

The interlayer coupling field (Hint) between the free and pinned ferromagnetic (FM) layers in an exchange-biased spin valve deposited by magnetron sputtering is usually dominated by the Néel “orange peel” coupling. The orange peel coupling is suppressed dramatically when an oxide layer (OXL) is inserted into the free/pinned FM layers due to smoother surface roughness at the interface. Hint decreases with increasing oxygen exposure dose of the free/pinned FM layers to pure O2 gas. And it is found that Ruderman–Kittel–Kasuya–Yoshida exchange coupling dominates in OXL spin valves due to smoother surface roughness and enhanced specular electron scattering at the interface between OXL and pinned/free FM layers, which also results in enhancement of magnetoresistance effect.
Spectroscopy on rare gas–doped silver clusters in helium droplets
Authors: Diederich Thomas , Tiggesbäumker Josef , Meiwes-Broer Karl-Heinz
Published: July 2002

The optical spectrum of Ag8 in a helium droplet, first measured by Federmann et al. [Eur. Phys. J. D 9, 11 (1999)], is studied over a broad wavelength range (237–450 nm) by resonant two photon ionization. A strong resonance is found in accordance to recent ab initio calculations. Doping the droplet additionally with rare gas atoms leads to a shift of the mean resonance position, which depends on the type and the number of attached atoms. In contrast to the red-shift obtained for argon, krypton, and xenon, for neon a net shift of the resonance to shorter wavelengths is observed. The dosage-dependence of the displacements will qualitatively be explained.
RECRIS-Romanian ECR ion source: Performances and experimental developments
Authors: Dobrescu S. , Schachter L.
Published: July 2002

The 14 GHz electron cyclotron resonance (ECR) ion source—RECRIS, developed at the National Institute for Physics and Nuclear Engineering (IFIN-HH) in Bucharest, Romania is presented. The source is conceived as a facility for atomic physics and material studies. The main constructive characteristics and performances of RECRIS are presented. The construction has some original characteristics: modular construction allowing simple dismounting and modification of the inner parts, high extraction voltage, up to 50 kV, and the possibility to use some techniques to enhance the performances, such as a biased disk, mixing gas, and others. The source is provided with a 90° analyzing magnet and with experimental devices and are presented as well. The main experimental device connected to the ECR ion source is a highly sensitive and wide mass range time-of-flight mass spectrometer for recoil ions.
Study of the extracted beam and the radial magnetic field of electron cyclotron resonance ion source at HIMAC
Authors: Kitagawa A. , Muramatsu M. , Sasaki M. , Yamada S. , Biri S. , Jincho K. , Okada T. , Sakuma T. , Takasugi W. , Yamamoto M.
Published: July 2002

Experimental results on an 18 GHz electron cyclotron resonance (ECR) ion source (NIRS-HEC) and a 10 GHz ECR ion source (NIRS-ECR) at National Institute of Radiological Sciences (NIRS) show that an extracted beam intensity strongly depends on the radial magnetic field distribution generated by a permanent sextupole magnet. In order to understand these results, we simulated beam extraction under strong influences of space charge. In the simulation, a current intensity at a different position of an extraction slit is assumed to be roughly proportional to a corresponding transverse area of the ECR zone under the assumption that ions are extracted along a longitudinal magnetic flux line. The calculations show that an optimum value of the sextupole magnetic field strength may exist for a given extraction configuration and beam intensity. Based on the simulation, beam intensity measurements have been performed with six sextupole magnets having different magnetic field strengths for two ion sources, and the experimental results are consistent with the calculations.
Photodissociation dynamics of phosgene: New observations by applying a three-dimensional imaging technique
Authors: Einfeld Tina , Chichinin Alexei , Maul Christof , Gericke Karl-Heinz
Published: July 2002

The photodissociation dynamics of COCl2 has been studied by monitoring ground Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) fragments by applying a novel technique where the three-dimensional momentum vector of a single reaction product is directly determined. The photodissociation at 235 nm produces exclusively three fragments: COCl2+hν→CO+2Cl. The kinetic energy distributions of Cl and Cl* are bimodal and exhibit a different behavior for the different spin–orbit states. Our attention was turned to the dependence of the anisotropy parameter β on the fragment velocity which was observed for the first time. For both spin–orbit states the anisotropy parameter differs clearly for slow and fast chlorine atoms, where a pronounced change from the value ∼0.7 to zero at about 20 kJ/mol is observed. Slow chlorine atoms are released isotropically and predominantly in the ground state Cl whereas fast chlorine atoms have an anisotropy parameter close to the theoretically limiting value and are distributed between ground and excited state Cl. These observations can be explained by a sequential decay where the first Cl fragment is released in a fast process characterized by the nonvanishing positive β parameter and a lifetime of ⩽210 fs, whereas the second Cl fragment is released after a period which is long on a rotational time scale. A significant contribution of a symmetric mechanism can be excluded.
Ab initio studies of the reactions of Cu(2S, 2D, and 2P) with SiH4 and GeH4
Authors: Luna-Garcı́a H. , Ramı́rez-Solı́s A. , Castillo S.
Published: July 2002

The interactions of Cu(2S:3d104s1, 2D:3d94s2, and 2P:3d104p1) with SiH4 and GeH4 were studied by means of Hartree–Fock–SCF and multiconfigurational-SCF followed by variational and multireference second order Möller–Plesset perturbational configuration interaction (CIPSI) calculations, using relativistic effective core potentials. The Cu atom in its P2(3d104p1) state is inserted in the Si–H and Ge–H bonds. In both interactions their corresponding 5 2A′ potential energy surfaces are initially attractive and become repulsive only after having encountered the avoided crossing region with the initially repulsive 4 2A′ surface adiabatically linked with the Cu(2D:3d94s2)–SiH4(GeH4) fragments. The three A′2 curves derived from the interaction of the Cu(2D:3d94s2) atom with silicon (or germane) molecule are initially repulsive. Each one of them shows two avoided crossings and its lowest lying 2 2A′ curve goes sharply down until it meets the X 2A′ curve adiabatically linked with the Cu(2S:3d104s1)+SiH4(GeH4) asymptotes. The 2 2A′ curve becomes repulsive after the avoided crossing with the X 2A′ curve. The lowest-lying X 2A′ potential leads to the HCuZH3 X 2A1 (Z=Si, Ge) intermediate molecule. This intermediate molecule, diabatically correlated with the Cu(2P:3d104p1)+ZH4 fragments which lie 5.8 and 1.6 kcal/mol, respectively, above the ground state reactants, have been carefully characterized as well as the dissociation channels leading to the CuH+ZH3 and H+CuZH3 products. These products are reached from the HCuZH3 intermediates without activation barriers. This work suggests that the simultaneous photoexcitation of the Cu atom in presence of silane and germane molecules in the gas phase could be used to produce better quality a-SiGe:H thin films.
Extraction of highly charged ions from the electron beam ion trap at LBNL for applications in surface analysis and materials science
Authors: Schenkel T. , Persaud A. , Kraemer A. , McDonald J. W. , Holder J. P. , Hamza A. V. , Schneider D. H.
Published: July 2002

We describe results from highly charged ion extraction experiments at the Electron Beam Ion Trap (EBIT) facility which is now operated at Lawrence Berkeley National Laboratory after transfer from Lawrence Livermore National Laboratory. Requirements on ion source performance for the application of highly charged ions (e.g., Xe44+) in surface analysis and materials science are discussed.
Completion of the ATLAS ECR-I ion source upgrade project
Authors: Moehs D. P. , Vondrasek R. , Scott R. H. , Pardo R. C. , Montgomery J. M.
Published: July 2002

A new 10 GHz electron cyclotron resonance ion source (ECRIS) has been constructed and commissioned for the ATLAS accelerator. This new source replaces the original ATLAS ECRIS that has been in operations since 1987. The goal of this upgrade project was to significantly improve the source performance while maintaining maximum operational flexibility for solid material feeds. The new source design includes a large magnetic-field gradient, aluminum plasma chamber, and bias disk following modern ECRIS design concepts. Eight solenoid coils from the original source along with a new iron yoke form the magnetic mirror. Hall Probe measurements showed the axial B field to be within 1% of the POISSON design model calculated at 400 A per coil. The injection and extraction mirror ratios are approximately 4.4 and 2.9, respectively, with a minimum field of 3.0 kG. A new aluminum plasma chamber houses the NdFeB hexapole magnets, which are encased in austenitic stainless steel to allow for direct water cooling. An open hexapole configuration provides six radial access ports, 1.7 cm×4.1 cm, to the plasma chamber for solid material feeds and vacuum pumping at an estimated rate of 20 l/s per radial port. Measurements of the hexapole field near the plasma chamber wall, 4 cm in radius, were within 13% of the designed B field of 9.3 and 5.7 kG along the poles and pole gaps, respectively. The first plasma in the new source was obtained on October 10, 2000. Already it has exceeded the best O6+16 beam current obtained from the original ECR-I by a factor of roughly 2.3, achieving 140 e μA with a biased disk. The source is back in regular operation and ATLAS experiment runs have been performed with He, O, Ar, Kr, Ni, and Zr.
Low-noise variable-temperature preamplifier for piezoelectric tuning fork force sensors
Authors: Patil N. G. , Levy J.
Published: July 2002

The development of a low-noise variable-temperature preamplifier for self-sensing piezoelectric tuning fork force sensors is presented. The preamplifier utilizes a pair of commercially available GaAs field effect transistors to achieve high impedance and low noise over a wide range of temperatures. Using a standard 32 kHz quartz tuning fork, the base noise level achieved is 20 dB below the thermal noise resonance at room temperature and at 4.2 K. The circuit diagram, biasing points, and noise specifications are presented, and the application for variable temperature scanning probe microscopy is discussed.