bluuebunny/crossref_metadata_2025 huggingface dataset:
A laser-induced fluorescence measurement technique for obtaining neutral hydrogen densities in plasmas
Authors: Yao X. Z. , Yang T. F. , Chang-Díaz F. R.
Published: July 2002

The resonance fluorescence of neutral hydrogen illuminated by Hα radiation has been used as a technique for the spatially and temporally resolved density measurements of neutral hydrogen in high temperature plasmas, such as in the tokamak and magnetic mirror plasma fusion devices. The fluorescence signal, usually very weak and buried in the background of stray laser light and Hα emission, is very difficult to extract and its measurements are inaccurate. This paper discusses the improvement of the signal extraction using two optical path laser-induced fluorescence (LIF) methods. One optical path carries the fluorescence signal and the background (the stray laser light and Hα emission), whereas the other path carries only the background signal. By combining these two signals, a clean fluorescence signal can be isolated by subtracting out the background using a differential amplifier. The measurement is obtained instantaneously from these two signals which are taken simultaneously in one pulse rather than being extracted from two separate spectra taken in two sequential pulses (double pulses). This method, therefore, makes a significant improvement on the double pulse technique in terms of the accuracy of the measurement and the time resolution. Using this LIF technique the measurement of the neutral density profile in the exhaust of a tandem mirror plasma propulsion device is obtained and presented.
Analytical study of a linear variable differential transformer transducer
Authors: Cabrera Mario L. , Saca José M.
Published: July 2002

A theory to explain the functioning of a linear variable differential transformer (LVDT) transducer has been developed in this work. By using the magnetic vector potential A in cylindrical coordinates for a thin circular conductor carrying a current I, the expressions for the self- and mutual inductances involved in the theory were obtained. Kirchhoff laws for alternating currents were used to obtain an expression for the transfer function or gain ‖vo/vi‖ of the transducer. Then, the variation of gain with displacement θ at a fixed frequency, as well as the dependence of gain with frequency at a fixed displacement θ are examined. Finally, it is concluded that the results here found correspond to a self-compensated LVDT.
Self-calibration of the same silicon photodiode in the visible and soft x-ray ranges
Authors: Krumrey M. , Tegeler E. , Goebel R. , Köhler R.
Published: July 2002

For the first time, the spectral responsivity of the same silicon n on p photodiode has been determined with self-calibration techniques in the visible and soft x-ray regions. Together with results from other calibrations the spectral responsivity is presented for the whole photon energy range from 1.2 eV to 3 keV.
1.5 μm wavelength compressively strained GaInAs/AlGaInAs multiquantum-well lasers grown by molecular-beam epitaxy with high differential gain and low threshold current density
Authors: Shimizu H. , Fukushima T. , Nishikata K. , Hirayama Y. , Irikawa M.
Published: July 2002

1.5 μm GaInAs/AlGaInAs multiquantum-well (MQW) lasers with 1% compressively strained quantum wells were grown by molecular-beam epitaxy. The effective differential gain (g0) determined from the squared relaxation oscillation frequency versus output power relations is a high value of 9.3×10−16 cm2 in long-wavelength lasers. On the other hand, the effective transparent carrier density (n0) of strained-layer MQW lasers determined from the measurement of the spontaneous carrier lifetime was found to be very high, which is different from the theory of strain effects. However, by taking the carrier transport effect into account, it was shown that (1) for the strained MQW lasers the intrinsic transparent carrier density is lower than that of the lattice-matched MQW lasers, and (2) the intrinsic value of g0 is estimated to be 28–56×10−16 cm2, which is close to the theoretically predicted value. By improving the laser structure to have better carrier transport, much higher effective differential gain and lower effective transparent carrier density can be expected.
A new instrument for the simultaneous determination of pH and redox potential
Authors: Nicolini Claudio , Sartore Marco , Zunino Marco , Adami Manuela
Published: July 2002

A significant upgrading of a recently introduced biosensor system, called potentiometric alternating biosensor is presented. The transducer consists of a light-addressable silicon chip, which provides regions properly modified and functionalized to yield sensitivity to either pH or redox potentials. The computer controlled system drives an array of light sources, allowing two-dimensional signal acquisition and processing. The system can provide multiparameter information related to the local modifications made on the sensing surface of the transducer. This work presents a detailed description of the measuring principles and the spatial resolution obtainable with the system. In addition, the complete circuit design is presented, both for signal conditioning, and for the bidimensional matrix addressing. A typical experiment for selective measurements is presented.
Consideration of several factors in the method for mapping x-ray rocking curves
Authors: Gao Dachao , Wilkins Stephen W. , Stevenson Andrew W. , Pogany Andrew P.
Published: July 2002

The resolution and dispersion arising from the diffraction geometry used in the method developed by Gao et al. for mapping x-ray rocking curves are discussed in detail. The effects of the resolution of the one-dimensional detector on the measurement of rocking curves and on the measurement of rocking curve contour maps have also been investigated. The dispersive effect (i.e., Δλ/λ) in the diffraction geometry is reduced by an order of magnitude and is negligible compared with the intrinsic angular width of the crystal for a wide range of sample crystal Bragg angles (say the range varies from 10° to 60°, using Cu Kα1 radiation), provided that the resolution of the detector is better than 0.2 mm. This means that the dispersive effect of nearly all crystal samples will be negligible if rocking curves are measured using the present method and Cu Kα1 radiation. The results also apparently show that the full width at half maximum of the rocking curves increases and the resolution of contour maps decreases when the resolution of the detector is poorer than 0.2 mm.
High-pressure strand burner system for propellant flame studies using absorption spectroscopy
Authors: Mallery C. , Kim E. , Thynell S. T.
Published: July 2002

The design and operation of a high-pressure strand burner capable of withstanding pressures up to 6.9 MPa are described. The associated components for use with the chamber, which is employed to study the combustion behavior of solid propellants burning at a constant pressure, are specified. By using this design, it is possible to perform absorption measurements through the flame zone with a line-of-sight that is free from the effects of recirculating product gases. Absorption measurements using Fourier-transform infrared spectroscopy are performed during the combustion at a spatial resolution of approximately 1 mm. The obtained spectra are of sufficient signal-to-noise ratio to allow a determination of species concentration and temperature profiles. Such data are of utmost importance for the elucidation of the complex chemical kinetics of solid propellant ignition and combustion.
An interferometric method of laser beam analysis
Authors: Liesenhoff A. , Rühl F.
Published: July 2002

A new interferometric method is described which allows the separate analysis of the individual transverse modes of multimode laser beams. The determination of the phase and the amplitude distribution of the modes by means of a Mach–Zehnder-type interferometer is possible with a homodyne or heterodyne measurement method. The required tunable single-frequency reference wave which serves as an optical local oscillator is generated from the multimode beam itself by means of acousto-optical frequency shifting and mode selecting devices which are positioned in the reference branch of the interferometer. In homodyne operation the individual modes are characterized by specific stationary interference patterns obtained by the synchronization of the reference wave frequency with the respective mode frequency. The use of conventional phase-shifting methods yields the phase distribution of the individual modes. The corresponding mode order can be determined from the phase jumps by π occurring in the fringe pattern. Results of the investigation of a slow-flow cw-CO2 laser with stable resonator configuration are presented, proving the feasibility of on-line diagnosis provided by the system.
Three-dimensional wavefront imaging by near-field scanning optical microscopy
Authors: Levy J. , Cohen A. , Awschalom D. D.
Published: July 2002

We have designed a near-field scanning optical microscope head compatible with an existing commercial scanning probe system. Near-field images of coherent scattering from a 500 nm fused silica grating are collected in transmission mode as a function of tip-sample separation, wavelength, and incident polarization. The images are compared with numerical simulations of scattering using both scalar wave and vector dipole approximations. The simulations suggest that the procedure can be inverted, allowing one to measure the dielectric constant ε(x,y,z) of an embedded three-dimensional object using information obtained in the near field.
Application of commercial antennas to very long baseline interferometry radio astronomy
Authors: Costa M. E. , Blair D. G. , Buckingham M. J. , Sinclair M. W. , Ferris R. H. , Jauncey D. L. , Reynolds J. E. , Tzioumis A. K. , Preston R. A.
Published: July 2002

Groundstation antennas located in Western Australia have been used as elements in the Southern Hemisphere very long baseline interferometry (VLBI) experiment. These commercial antennas of 27.5 and 15 m diameter provide a very economical improvement to the imaging capability of this array and the practice of using such antennas can be extended to other sites in the Southern Hemisphere in preparation for the forthcoming space-VLBI experiments.
Kurchatov Synchrotron Radiation Source status report
Authors: Belyaev S. T. , Stankevitch V. G.
Published: July 2002

A brief description of the Kurchatov Synchrotron Radiation Source is given. Commissioning of the injection complex is presented. Installation of the big storage ring is almost finished and high-vacuum conditions are fulfilled in half of the ring. Active proposals for investigations are described.
Summary of the satellite workshop on x-ray microbeam techniques and applications
Authors: Cargill G. S. , Bilderback D. H.
Published: July 2002

Papers at this workshop described tapered capillary optics, Bragg–Fresnel optics, and phase zone plate optics for producing micron and submicron beams of hard x rays. The usefulness of these optical systems was illustrated in diffraction, fluorescence, and microscopy applications.
Polarization measurement of SR from a helical undulator using a quarter-wave plate for a wavelength of 12.8 nm
Authors: Kimura H. , Miyahara T. , Goto Y. , Mayama K. , Yanagihara M. , Yamamoto M.
Published: July 2002

At a wavelength of 12.8 nm, polarization states of nominally circularly polarized light emitted from a helical undulator and monochromatized by a grating monochromator were measured (KEK-PF BL-28A). With a transmission-type multilayer quarter-wave plate and a multilayer mirror polarization analyzer mounted on a beamline ellipsometer, all polarization parameters of the circularly polarized SR at various conditions were determined. The best degree of circular polarization after the grating monochromator was found to be as high as 0.95.
Versatile double-crystal fixed exit monochromator for x-ray synchrotron radiation
Authors: Ramanathan Mohan , Montano Pedro A.
Published: July 2002

We describe a new monochromator that was designed and constructed for applications in x-ray scattering and extended x-ray-absorption fine structure experiments. The monochromator consists of two separate crystals of the same type. The monochromator covers a wide angular range from 58° to 5.1° which is equivalent to an energy range of 2.3–22 keV using Si 111 crystals. A constant offset of 1 in. is maintained between the incident white radiation and the exiting monochromatic radiation. The monochromator employs a simple mechanism to achieve this result. The monochromator is housed inside a stainless steel vacuum chamber and is currently operating in a high vacuum environment. The first monochromator crystal is water cooled in order to avoid temperature drifts due to the high heat load synchrotron radiation. This article presents the design details, characteristics, and data collected with this monochromator.
Absolute calibration of TFTR helium proportional countersa)
Authors: Strachan J. D. , Barnes Cris W. , Diesso M. , Jassby D. , Johnson L. , Loughlin M. , McCauley S. , Munsat T. , Roquemore A. L.
Published: July 2002

The TFTR helium proportional counters are located in the central five (5) channels of the TFTR multichannel neutron collimator. These detectors were absolutely calibrated using a 14 MeV neutron generator positioned at the horizontal midplane of the TFTR vacuum vessel. The neutron generator position was scanned in centimeter steps to determine the collimator aperture width to 14 MeV neutrons and the absolute sensitivity of each channel. Neutron profiles were measured for TFTR plasmas with time resolution between 5 and 50 ms depending upon count rates. The He detectors were used to measure the burnup of 1 MeV tritons in deuterium plasmas, the transport of tritium in trace tritium experiments, and the residual tritium levels in plasmas following 50:50 DT experiments.
Spectroscopic diagnostics of tritium recycling in TFTR
Authors: Skinner C. H. , Stotler D. P. , Adler H. , Ramsey A. T.
Published: July 2002

Spectroscopic measurements of tritium Balmer-alpha (Tα) emission from a fusion plasma are presented. A Fabry–Perot interferometer is used to measure the Hα, Dα, Tα spectrum in the current D–T experimental campaign on TFTR and the contributions of H, D, and T are separated by spectral analysis. The Tα line was measurable at concentrations Tα /(Hα+Dα+Tα) down to 2%.
Plasma potential measurements on text-upgrade with a 2 MeV heavy ion beam probe (abstract)a)
Authors: Ouroua A. , Heard J. W. , Crowley T. P. , Schoch P. M. , Connor K. A. , White R. R. , Schwelberger J. , McLaren P. E. , Demers D. , Russell W.
Published: July 2002

The plasma potential is measured in TEXT-upgrade tokamak by injection and detection of high energy heavy ions (thallium and cesium with a single charge) using a 2 MeV accelerator and a parallel plate energy analyzer. The change in beam energy, as it crosses the plasma, gives the local plasma potential at the measurement volume. Recent results of high energy beam operations are presented.
An ion gun and detector system for ion drift surface studies on toroidal devices
Authors: Sasser G. E. , Knowlton S. F. , Gandy R. F. , Lin H. , Thomas E. E. , Owens M. A.
Published: July 2002

An ion gun and detector system have been designed and tested to study energetic ion trajectories and single-particle confinement in the vacuum field and low-density plasma of the Compact Auburn Torsatron (CAT). Thermionic Li+ and Mg+ sources are used. The gun positioning is capable of two degrees of freedom, allowing selection of the pitch angle and initial flux surface. The injection energy of the ions is variable from 5 to 100 eV, allowing investigation of the orbits of particles with large Larmor radii. The ion currents of typical magnitude I=10 nA are detected with a movable paddle probe making use of a boxcar-averaging technique. The detected ion trajectories are compared with the predicted trajectories from a guiding center code that makes use of a detailed model of the CAT magnetic configuration. The ion gun and detector system will be used to study the effect of magnetic islands on energetic ion trajectories.
Investigation of ion sawtooth activity by Rutherford scattering
Authors: Tammen H. F. , Moorman S. A. H. , Oyevaar T. , Schüller F. C. , Donné A. J. H.
Published: July 2002

The Rutherford scattering diagnostic at TEXTOR has been used to investigate the time evolution of the ion velocity distribution during sawtooth activity. Coherent averaging techniques have been employed to obtain better statistics. The time evolutions of central ion and electron density were found to be strongly correlated in four out of five cases. In one case, where saturation of the sawteeth occurred, a discrepancy between the two has been found, which could be attributed to an influx of impurities towards the end of the sawtooth. Changes of about 20% in the central toroidal rotation of the bulk ions have been found during sawtooth crashes of neutral beam injected discharges, whereas no changes were found in the ohmic case. No statistically reliable statements can be made about changes in the ion temperature.
New type of radial collimator for strain measurements by neutron diffraction
Authors: Kocsis M. , Farago B. , Ceretti M.
Published: July 2002

We present a new type of radial collimator made of boron fibers. Coupled with a position sensitive detector system it is optimal for neutron diffraction studies of internal strain measurements. We show the results of the calibration and first test measurements on such a system. The obtained characteristics are excellent and allow a substantial (30) gain factor in data acquisition time.
A magnetic spectrometer for intense pulsed proton beams
Authors: Noonan W. A. , Young F. C. , Donovan E. P.
Published: July 2002

A magnetic spectrometer has been constructed and used to measure energy distributions of intense proton beams in the range ∼0.3–2.5 MeV from ion diodes. Rutherford scattering foils are used to attenuate the beam before magnetic analysis. CR-39 track detectors provide time-integrated records, while an array of p-i-n diode detectors provides time-resolved data at discrete energies. The spectrometer was calibrated in the range between 0.3 and 2.0 MeV with monoenergetic protons from a Van de Graaff accelerator. The overall precision of this calibration is better than ±10 keV. Spectral measurements of intense proton beams transported in low-pressure gases are presented.
Escaping charged fusion product spectrometer on Alcator C-Mod
Authors: Lo Daniel H. , Boivin Réjean L. , Petrasso Richard D.
Published: July 2002

A spectrometer for studying escaping charged fusion products has been designed, built, and installed on the Alcator C-Mod tokamak. The ultimate goal of deploying this diagnostic is to evaluate the effectiveness of 3He minority heating through the broadened spectrum of unconfined 14.7 MeV protons from D-3He reactions. Other physics issues that will be addressed with this diagnostic include: first-orbit losses, central ion temperature, 3He burnup, etc. The spectrometer is located 25.8 cm below the midplane and at least 5 cm behind the shadow of the rf limiter. The diagnostic is equipped with a 300 μm thick ion-implanted-silicon detector that is bakeable up to 200 °C. Eighteen apertures of various dimensions and viewing angles, with aluminum foils of various thicknesses, allow for a choice of different charged fusion products and a control of the signal level. In-vessel calibration is provided by two weak radioactive sources of α particles.
Edge parameters from an energy analyzer and particle transport on TEXT-U
Authors: Crockett D. B. , Phillips P. E. , Craven W. A. , Wootton A. J. , Wan A. S. , Yang T. F.
Published: July 2002

The energy distributions of ions and electrons in the scrape-off layer of TEXT are measured with a bidirectional retarding field analyzer (RFA). The probe provides simultaneous measurements parallel and antiparallel to the plasma current. Large asymmetries in this direction in the flux and temperature that were found with the RFA appear partly due to differences in the connection lengths Lc. The measurements from the analyzer are compared with the results of a simple edge model to infer the edge particle diffusion coefficient. While the measurements are consistent with the model for longer connection lengths (∼10 m in TEXT-U), the agreement deteriorates for shorter Lc.
Vertical and horizontal transfer function measurements on a magnetic gas spring
Authors: Braccini S. , Bradaschia C. , Del Fabbro R. , Di Virgilio A. , Flaminio R. , Galeotti S. , Giazotto A. , Kautzky H. , Montelatici V. , Morganti M. , Passuello D. , Zhang Z. , Di Fiore L. , Calloni E. , Holloway L. E.
Published: July 2002

A magnetic gas spring (MGS) has been developed to be the filter element of the super attenuator (SA), the device designed to support the optical components of the gravitational interferometric antenna VIRGO. Measurements have been performed on a MGS in order to study the resonance structure of the attenuation spectra in both the vertical and horizontal directions. Results guarantee an attenuation of at least 40 dB almost everywhere from 10 Hz to 1 kHz. By using experimental data the calculated performances of the SA turn out to be quite adequate for the VIRGO project requirements.
High pressure electrophoresis in narrow bore glass tubes: One- and two-dimensional separations of protein subunits
Authors: Erijman Leonardo , Clegg Robert M.
Published: July 2002

A mini-gel tube electrophoresis apparatus that is easily constructed and simple to operate has been developed. The system can be accommodated in standard commercially available high pressure tubing, and has been tested at up to 200 MPa. The narrow diameter of the glass tubes allows rapid and efficient dissipation of heat. Adequate buffer capacity is maintained in the low volume anode reservoir by increasing the concentration of the buffer. Analytical separations can be achieved in short times with high resolution. After the electrophoresis has been carried out at elevated pressure, the gel can easily be extruded from the tube and loaded onto a standard slab gel for a second-dimensional run at atmospheric pressure. We illustrate the application of this apparatus with the high pressure gel electrophoresis separation and subsequent identification of the constituent subunits of E. coli RNA polymerase.
Novel capillary pulse viscometer for non-Newtonian fluids
Authors: Kokal Sunil L. , Habibi Brian , Maini Brij B.
Published: July 2002

This article describes the design and principle of operation of an unsteady-state capillary pulse viscometer for measuring the rheology of non-Newtonian fluids at elevated pressures and elevated temperatures. The relevant equations for the flow of a power-law fluid in a capillary tube under a transient pressure gradient that decays at a rate proportional to the fluid flux in the tube are derived. The viscometer does not require a pump to generate a steady flow and is relatively simple and cheap to manufacture. Only a small amount of the test fluid is required and it is comparatively faster than other steady-state atmospheric pressure viscometers. Furthermore, the viscosity of the test fluid is measured over a range of shear rates with each run. The pressure decay across the capillary tube is recorded, and the apparent viscosity is calculated as a function of shear rate from the relevant flow equations. The viscometer has been used to measure the viscosity of polymer solutions over a wide range of concentrations [250–2500 parts per million by weight (wppm)] and the results compared with data using a steady-state commercial viscometer. The accuracy of the instrument is comparable to or better than that of other types of viscometers.
Excitation of ruby fluorescence at multimegabar pressures
Authors: Chen Nancy H. , Silvera Isaac F.
Published: July 2002

We demonstrate the excitation of strong ruby fluorescence at multimegabar pressures by red incident radiation. We have studied the ruby R1 and N2 lines to 251 GPa and measured the R1 line intensity as a function of pump wavelength. This new method of exciting ruby fluorescence enables easy pressure determination at pressures beyond the 250 GPa limit previously predicted by Ruoff.
A chamber for laboratory studies of atmospheric aerosols and clouds
Authors: Narus M. L. , Schoenfelder N. C. , Na Y. , Chavasse L. A. , Disselkamp R. S.
Published: July 2002

A stainless-steel chamber has been constructed and interfaced to a Fourier transform infrared spectrometer for the purpose of studying laboratory simulated atmospheric aerosols and clouds. The chamber is cylindrical in design and is comprised of a double-walled inner assembly that resides within an outer vacuum jacket. The volume of the aerosol sample region is 28 L. By circulating refrigerated methanol between the double walls of the inner assembly, constant temperature control of the sample region can be maintained between 187 and 300 K. A study of temperature uniformity within the chamber at 291, 240, and 187 K revealed a standard deviation in temperature of 1.6 K as determined from measurements made using five copper–constantan thermocouples. Good agreement is obtained between thermocouple measured temperatures and rotational temperatures computed from infrared absorption spectra of methane gas. The chamber described here has been used to examine heterogeneous chemistry of solid powder samples. A technique of generating an aerosol sample by rapidly dispersing a solid powder in a gas is presented. The half-life of a γ-alumina aerosol sample was measured to be 25 min.
Instrument for depositing Langmuir–Blodgett films composed of alternating monolayers using a protective layer of water
Authors: Troitsky V. I. , Sartore M. , Berzina T. S. , Nardelli D. , Nicolini C.
Published: July 2002

A novel instrument has been developed for depositing Langmuir–Blodgett (LB) films composed of alternating molecular monolayers of amphiphilic molecules and adsorbed layers of soluble compounds. The main feature of the deposition method is the possibility to prevent the transfer of the monolayer onto the substrate when dipping up. This can be achieved by keeping the external boundary of the deposited monolayer in contact with the aqueous solution by means of a protecting hydrophilic plate which covers the substrate with the film while still immersed in water. After that, the plate is fixed to the substrate parallel to its surface and both can be transferred to other compartments of the instrument for adsorption of soluble compounds onto the active hydrophilic surface or for deposition of monolayers of other types. Principal design elements, electronics, and software of the LB instrument based on this principle are described. Results on the deposition of LB films composed of alternating monolayers of amphiphilic donor molecules of hexadecylbis(ethylenedithio)tetrathiafulvalene and adsorbed layers of the soluble protein cytochrome P 450scc are reported as an example of the device functioning.
The principle of operation of the avalanche transistor-based Marx bank circuit: A new perspective
Authors: Chatterjee Amitabh , Mallik Kanad , Oak S. M.
Published: July 2002

The principle of operation of the transistor-based Marx bank circuit has been examined. It was experimentally observed that stage-wise increase of reverse voltage does not occur. This cannot be explained by the principle of operation understood so far. A new explanation, consistent with the experimental observations and associating current-mode second breakdown of transistors, is proposed. A few experimental observations made by earlier workers have also been justified in light of the new current-controlled mechanism.
Excitonic recombination in GaN grown by molecular beam epitaxy
Authors: Smith M. , Chen G. D. , Li J. Z. , Lin J. Y. , Jiang H. X. , Salvador A. , Kim W. K. , Aktas O. , Botchkarev A. , Morkoç H.
Published: July 2002

Time-resolved photoluminescence has been employed to probe the free-excitonic transitions and their dynamic processes in GaN grown by molecular beam epitaxy (MBE). The exciton photoluminescence spectral line shape, quantum yield, and recombination lifetimes have been measured at different excitation intensities and temperatures, from which the binding energy of an exciton, the energy band gap, and the free-exciton radiative recombination lifetimes of GaN grown by MBE have been obtained. Our results have demonstrated the superior crystalline quality as well as ultrahigh purity of the investigated sample, implying a new major breakthrough in MBE growth technologies for GaN.
Decay rate measurements in a negative ion rich plasma
Authors: Jimbo Kouichi , Iima Masashi
Published: July 2002

By artificially terminating the hydrogen arc discharge in the sheet plasma experiments, the decay rates of the negative and the positive saturation currents of a Langmuir probe could be measured. This technique made it possible to observe, under a deep potential well geometry, two single decay curves of saturation currents, which decayed at the same rate reciprocally with time in the negative ion dominant cold region. This result illustrates that the negative saturation current, which consisted of chiefly electrons, was still proportional to negative ion density.
A novel divided cell for quantitative Raman and resonance Raman spectroscopy
Authors: Mitambo Musilli M. , Zhang Shuliang L. , Loppnow Glen R.
Published: July 2002

A novel four nuclear magnetic resonance (NMR)-tube holder is described for quantitative Raman and resonance Raman spectroscopy. This cell has advantages over other divided cell designs in producing high precision quantitation and eliminating off-axis errors. Raman spectra were obtained for both absorbing and nonabsorbing solutions, and the results compared to those obtained using a single tube holder. The relative Raman intensities in a 1:1 mixture of benzene and chloroform taken in the single holder were within 6% of those of the pure solvents taken with the four NMR tube holder. The resonance Raman scattering cross section of chromate was determined to be within 6% of the value obtained from a single-tube holder. All of these errors in accuracy and precision are within the errors normally associated with the quantitative measurement of Raman cross sections. These results show that the four-tube holder can be used for precise and accurate measurement of both Raman and resonance Raman scattering.
High quality hydrogenated amorphous germanium prepared by the hot wire technique
Authors: Zanzig L. , Beyer W. , Wagner H.
Published: July 2002

Hydrogenated amorphous germanium films were deposited by thermal dissociation of undiluted germane. The optical and electronic properties of the material were investigated using infrared spectroscopy (IR), photothermal deflection spectroscopy, spectroscopic ellipsometry, and conductivity measurements. The results show that homogeneous material with steep band tails (50 meV) and without any sign of postoxidation can be obtained. A strong correlation between the defect density and the IR Ge–H stretching modes is observed.
Compact and freely movable single atom detector
Authors: Westphal P. , Koch S. , Horn A. , Schmand J. , Andrä H. J.
Published: July 2002

A compact fluorescence detector has been developed which allows to count slow, single Na atoms. By using glass fibers for the exciting and the fluorescence light we obtained free movability of the detector. The detection efficiency has been analyzed and optimized by an extensive investigation of all processes concerning the laser-induced fluorescence, especially the random walk of the atom, saturation effects, and optical pumping. Single atoms are identified by registration of photon bursts which stand out from the Poisson distributed background. An absolute calibration of the detection efficiency has been achieved by a time-resolved fluorescence measurement in combination with a corresponding Monte–Carlo simulation. Atomic fluxes well below 1 atom/s can be determined with an uncertainty of less than ±10%. Assuming a continuous flux of 1 atom/s with 25 m/s typically about 50% of these atoms are counted. The detector is an excellent tool for differential scattering experiments with slow atomic beams.
Convertible electrodynamic levitator trap to quasielectrostatic levitator for microparticle nucleation studies
Authors: Arnold S. , Goddard N. L. , Wotherspoon N.
Published: July 2002

This article describes an apparatus for obtaining nucleation data from a levitated solution microdroplet, automatically. A particularly novel feature is that it uses an electrodynamic levitator trap (ELT) which converts to a quasielectrostatic levitator (QEL), at any time during an experiment. The conversion is accomplished by using asymmetrically applied potentials on the ELT structure. With this modification one can trap a particle in the ELT mode and then convert to the QEL mode for automatic operation. By eliminating the need for the alternating gradient forces which are intrinsic to the ELT, the system in its QEL mode is shielded from unwanted noise and parametric instabilities associated with the ELT’s alternating potential. To test the system theoretically, we calculate the effect which molecular collisions have on the positional variance in a spherical void QEL. Following this, we describe the components of our servosystem, and demonstrate the robustness of our design by following the nucleation of a solution droplet as the ambient relative humidity is reduced by evacuation.
Instrumentation for a retarding potential analyzer experiment. Part I. Ion parameters
Authors: Rajput S. S. , Garg S. C.
Published: July 2002

This article presents the performance of an instrument designed to measure the current collected by an ion trap to get the plasma current–voltage (Ii–V), and dIi/dV–V characteristics for derivation of ion density, ion composition and their energy distribution. The instrument is capable of measuring the ion density in the range of 5×102–5×106 cm−3, ion temperatures from 500 to 5000 K, and ion density fluctuations of the order of 0.02% or more with full scale accuracy of 0.2%.
Obtaining absolute spatial flux measurements with a time-resolved pinhole camera
Authors: Baker K. L. , Porter J. L. , Ruggles L. E. , Fehl D. L. , Chandler G. A. , Vargas M. , Mix L. P. , Simpson W. W. , Deeney Chris , Chrien R. E. , Idzorek G. C.
Published: July 2002

A technique is described to determine the spatial x-ray flux emitted from a hohlraum wall and subsequently transmitted through a diagnostic hole. This technique uses x-ray diodes, bolometers, and a time-resolved pinhole camera to determine the spatial flux of x rays emitted through a hohlraum’s diagnostic hole. The primary motivation for this analysis was the relatively long duration, nearly 100 ns, of the x-ray drive present in z-pinch driven hohlraums. This radiation causes plasma to ablate from the hohlraum walls surrounding the diagnostic hole and results in a partial obscuration that reduces the effective area over which diagnostics view the radiation. The effective change in area leads to an underestimation of the wall temperature when nonimaging diagnostics such as x-ray diodes and bolometers are used to determine power and later to infer a wall temperature. An analysis similar to the one described below is then necessary to understand the radiation environment present in x-ray driven hohlraums when these diagnostics are used and hole closure is important.
Contactless angle measurement
Authors: Ponikvar Dušan
Published: July 2002

The apparatus described uses polarized light to trace the rotation angle of an object. The result of the measurement does not depend on the distance between the object and the base surface within a wide range, and the resolution is better than 0.25° with the full range of 180°. There are no electrical contacts to the rotating object.
A novel analog current-conveyor multiplier circuit
Authors: Kumar Umesh
Published: July 2002

A four-quadrant analog current multiplier circuit is proposed by using Gilbert’s current Cell (Ref. Reference 1) and the modified current conveyor (Ref. Reference 2).
Preparation of platinum/iridium scanning probe microscopy tips
Authors: So/rensen A. H. , Hvid U. , Mortensen M. W. , Mo/rch K. A.
Published: July 2002

We report on the development of an etching setup for use in the preparation of platinum/iridium tips for atomic force microscopy and scanning electrostatic force microscopy as well as scanning tunneling microscopy. The etching process is based on a two step electrochemical procedure. The first step is a coarse alternating current (ac) etching in which the etching is continuous and stops automatically when the lower part of the wire drops off, while the second step is a fine etching made by a number of ac pulses, each of a certain duration and separated by a certain interval of time. When the tip material being etched is platinum/iridium (10%) the influence of the stop phase of the ac current terminating each pulse in the second etching is found to be negligible, while in the case of second etching of tungsten wires it is important to break the pulse in a certain phase to avoid formation of a thick oxide layer. In order to explain the relatively high etching rates observed for the otherwise noble metal platinum we suggest that besides anodic corrosion of the platinum by the electrolyte containing chloride ions, a different etching mechanism causes a substantial increase of the etching rate. This mechanism is based on the formation of oxygen and hydrogen at the platinum/iridium electrode when the potential is above the dissociation potential of water (∼1.23 V) and storage of these products interstitially in the outer layers of the platinum wire. This leads to “microexplosions” that detach fragments of platinum from the wire surface and hereby give rise to “etching” of the wire. In the second etching blunt tips become sharp while tips which are already sharp apparently stay sharp. Therefore, the second etching scheme with pulses separated by pauses is found to be a very important factor for the production of sharp tips. After being etched the tips are ready for use in scanning tunneling microscopes, or they may be bent to form integrated tip/cantilever systems in ordinary commercial atomic force microscopes, being applicable as tapping mode tips and as electrostatic force microscopy tips.
Diameter control of an extremely thin cylindrical microprobe by electrochemical etching
Authors: Lim Y. M. , Kim S. H.
Published: July 2002

Electrochemical etching is shown to produce slender cylindrical tungsten probes used as microelectrodes for micromachining or in electrochemical studies. A mathematical model is derived for diameter control of the microprobes and its validation is investigated through experiments.
Radiative recombination in type-II GaSb/GaAs quantum dots
Authors: Hatami F. , Ledentsov N. N. , Grundmann M. , Böhrer J. , Heinrichsdorff F. , Beer M. , Bimberg D. , Ruvimov S. S. , Werner P. , Gösele U. , Heydenreich J. , Richter U. , Ivanov S. V. , Meltser B. Ya. , Kop’ev P. S. , Alferov Zh. I.
Published: July 2002

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self-organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan-view transmission electron microscopy studies reveal well developed rectangular-shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.
High-field direct-current conduction in epitaxial ZnS films
Authors: Bringuier E. , Briot O.
Published: July 2002

The current-voltage characteristic of a semiinsulating, monocrystalline ZnS film grown by metalorganic vapor-phase epitaxy is reported. High-field conduction is found to occur in the same field range (1.0–1.5 MV/cm) as in highly defected, electroluminescent material. It is concluded that the conduction mechanism underlying the operation of ZnS-based electroluminescent devices is bulk controlled.
Broadband pump wavelength tuning of a low threshold N-(4-nitrophenyl)-L prolinol near infrared optical parametric oscillator
Authors: Khodja S. , Josse D. , Samuel I. D. W. , Zyss J.
Published: July 2002

We report the pump wavelength tuning of a low threshold N-(4-nitrophenyl)-L-prolinol based nanosecond pulsed optical parametric oscillator in the near infrared. A broad tuning range from 1 to 1.5 μm corresponds to a limited pump wavelength excursion from 583 to 590 nm and a record low oscillation threshold of the order of 0.5 MW cm−2.
Spectroscopic and photorefractive properties of molybdenum-doped barium titanate
Authors: Schwartz Robert N. , Wechsler Barry A. , West Linda
Published: July 2002

Molybdenum-doped barium titanate crystals have been grown and characterized. Electron paramagnetic resonance spectroscopy has confirmed the presence of Mo5+ centers in reduced crystals that correlate with a strong optical absorption band at ∼600 nm. Self-pumped phase-conjugation and electro-optic beam-coupling measurements indicate that the reduced crystals are photorefractive in the visible, as well as the near-infrared wavelength region.
Intensity scan and two photon absorption and nonlinear refraction of C60 in toluene
Authors: Taheri Bahman , Liu Huimin , Jassemnejad B. , Appling D. , Powell Richard C. , Song J. J.
Published: July 2002

A single beam technique is introduced to determine the magnitude and sign of third order nonlinear optical coefficients of materials using intensity dependence of the complex refractive index. It is applicable to a large range of materials including those with a low damage threshold such as polymers and absorbing glasses. It is used to determine the two photon absorption and nonlinear refraction coefficients of C60 in toluene. Concentration dependence of the nonlinear optical coefficients results in second hyperpolarizabilities of 10−31 (esu) for C60.
Roughening kinetics of chemical vapor deposited copper films on Si(100)
Authors: Vázquez L. , Albella J. M. , Salvarezza R. C. , Arvia A. J. , Levy R. A. , Perese D.
Published: July 2002

The roughening kinetics of copper films synthesized by low pressure chemical vapor deposition (LPCVD) on Si(100) substrates was investigated by scanning tunneling microscopy (STM). By applying the dynamic scaling theory to the STM images, a steady growth roughness exponent α=0.81±0.05 and a dynamic growth roughness exponent β=0.62±0.09 were determined. The value of α is consistent with growth model predictions incorporating surface diffusion. The value of β, while higher than expected from these models, can be related to LPCVD processing conditions favoring growth instabilities.
Mechanically induced structural relaxation in an amorphous metallic Fe80B20 alloy
Authors: Fan G. J. , Quan M. X. , Hu Z. Q.
Published: July 2002

A melt-spun metallic Fe80B20 glass was subjected to high-energy ball milling. Microstructural changes of the glassy sample during milling were characterized by means of x-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, and Curie temperature measurements. It was found that the metallic glass may relax towards a low energetic configuration by mechanical milling, leading to a reduction of the heat release associated with crystallization of the amorphous phase and an increase of the average hyperfine field as well as of the Curie temperature. These results can be attributed to the occurrence of a strong short-range order in the amorphous state. Our experimental observations suggest that mechanical milling may induce structural relaxation in the amorphous Fe80B20 alloy.
Role of codopants in the luminescent output from Si:Er
Authors: Anderson Frederick G.
Published: July 2002

A crystal-field model for the electronic structure of the Er3+ ion in silicon is presented. The parameters within the model are adjusted in order that the Stark energy splittings within the f shell reproduce the observed spectral features in a Si:Er sample that is codoped with nitrogen. The dominant crystal field possesses tetrahedral symmetry. Several additional spectral features are attributed to local mode vibrational sidebands. From this analysis, I conclude that the role of the codopants is to isolate the Er ion so that the relaxation energy does not go into the production of lattice phonons.
Photo-oxidation of electroluminescent polymers studied by core-level photoabsorption spectroscopy
Authors: Sutherland D. G. J. , Carlisle J. A. , Elliker P. , Fox G. , Hagler T. W. , Jimenez I. , Lee H. W. , Pakbaz K. , Terminello L. J. , Williams S. C. , Himpsel F. J. , Shuh D. K. , Tong W. M. , Jia J. J. , Callcott T. A. , Ederer D. L.
Published: July 2002

The C 1s and O 1s core-level photoabsorption spectra of poly[2-methoxy,5-(2′-ethyl- hexoxy)-1,4-phenylene vinylene] (MEH-PPV) before and after exposure O2 and broadband visible light were recorded to determine the degradation pathway for this polymer. The change in the O 1s spectra as a function of exposure demonstrates that the O adds to the polymer chain to form a carbonyl group. Exposure to only O2 or only light causes no change in the C 1s or O 1s spectra. In the C 1s spectra, the change in the dependence on the photon angle of incidence after exposure demonstrates that O attacks the polymer at the double bond in the vinyl group thereby altering the extended conjugation of the polymer.
Rapid thermal annealing of arsenic-implanted Si0.6Ge0.4 alloys: Temperature effects
Authors: Tishkov V. S. , Gaiduk P. I. , Shiryaev S. Yu. , Nylandsted Larsen A.
Published: July 2002

Structural transformations produced in strain-relaxed, low-dislocation-density Si0.6Ge0.4 films by As+ implantation and rapid thermal annealing were studied by transmission electron microscopy and x-ray microanalysis. The type of residual defects was found to be strongly dependent on annealing temperature. Only perfect dislocation loops were observed in implanted layers after annealing at 800 °C. Annealing at higher temperature (900 °C) results in complete removal of irradiation damage accompanied by the formation of GeAs precipitates of monoclinic phase and spherical shape. The results show that the behavior of As in Si–Ge alloys during thermal processing is remarkably different from that in Si. In particular, precipitation of As atoms in Si0.6Ge0.4 was found at an As concentration of 9×1020 cm−3 which is at least one order of magnitude lower than the critical As concentration for As precipitation in Si.
Identification of compositionally invariant deep levels introduced by plastic strain in epitaxial Ge0.30Si0.70 and deformed bulk Si
Authors: Grillot P. N. , Ringel S. A.
Published: July 2002

Deep hole traps introduced by dislocations and other strain relaxation-induced defects in relaxed, epitaxial Ge0.30Si0.70 layers grown on Si are quantitatively compared to hole traps introduced by plastic deformation of bulk Si. Deep level transient spectroscopy (DLTS) spectra on these samples are shown to be virtually identical in every detail, with the key difference being a translation in temperature that results from the change in valence band energy with GexSi1−x composition. A one-to-one correlation is demonstrated for each hole trap detected in p-type Ge0.30Si0.70 to a given level in p-type plastically deformed (PD) bulk Si, with the measured differences in DLTS activation energy between each correlated level being approximately equal to the difference in valence band energy (∼0.12 eV) between relaxed Ge0.30Si0.70 and bulk Si. These hole traps are explained to be characteristic of defects induced by plastic strain within the GexSi1−x material system for x=0–0.30, independent of the composition and deformation method, implying invariant defect configurations. This comparison also reveals a hole trap at H(0.60) in Ge0.30Si0.70 not previously reported in PD Si, which we tentatively attribute to hole capture at the dislocation kink sites.