bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular Phylogenetics of Bromeliaceae: Evidence from trnL(UAA) Intron Sequences of the Chloroplast Genome
Authors: Horres R. , Zizka G. , Kahl G. , Weising K.
Published: July 2002

Abstract: Phylogenetic relationships within Bromeliaceae were assessed by comparative sequencing of chloroplast trnL(UAA) intronic DNA. Sixty‐seven taxa were analyzed, encompassing 62 species, two subspecies and two varieties from 32 bromeliad genera. One representative of Rapateaceae and two of Velloziaceae were used as outgroups. Introns were amplified by the polymerase chain reaction and conserved flanking primers. Amplification products were sequenced directly. Of 700 base pairs total trnL (UAA) intron size, 551 bp could be unambiguously aligned. Sequence divergence among ingroup taxa ranged from zero to 4.6 %. Phylogenetic reconstruction by parsimony analysis of 73 informative base substitutions yielded 1092 most parsimonious trees. The topology of the resulting strict consensus tree was only partially consistent with current classification schemes. Brocchinia and Ayensua grouped close together and formed a well‐supported basal clade, suggesting a sister group relationship of both to the remainder of the family. The remaining ingroup species were divided into an unresolved polytomy of three lineages, comprising (1) all members of subfamily Tillandsioideae, (2) the three investigated Hechtia species, and (3) all Bromelioideae and the remaining Pitcairnioideae (except Brocchinia, Ayensua and Hechtia). The latter clade was split into another polytomy of five sublineages, one of which comprised all Bromelioideae. Whereas monophyly of Tillandsioideae and Bromelioideae was supported by trnL intron data, Pitcairnioideae as traditionally circumscribed are clearly polyphyletic. There is evidence that the genera Tillandsia, Vriesea, Guzmania, Abromeitiella, Deuterocohnia, Dyckia, Pitcairnia and Aechmea in the present circumscription are not monophyletic.
The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing
Authors: Dyckmans J. , Flessa H. , Polle Andrea , Beese F.
Published: July 2002

Abstract: A continuous dual 13CO2 and 15NH415NO3 labelling experiment was undertaken to determine the effects of ambient (350μmol mol‐1) or elevated (700μmol mol‐1) atmospheric CO2 concentrations on C and N uptake and allocation within 3‐year‐old beech (Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO2 but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO2]. Under ambient [CO2] 37 %, and under elevated [CO2] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf‐growth and the partitioning of nitrogen was not altered under elevated [CO2]. The C/N ratio was unaffected by elevated [CO2] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO2].
Stem Elongation and Cell Wall Proteins in Flowering Plants
Authors: Kutschera U.
Published: July 2002

Abstract: The growth of stems (hypocotyls, epicotyls) and stem‐like organs (coleoptiles) in developing seedlings is largely due to the elongation of cells in the sub‐apical region of the corresponding organ. According to the organismal concept of plant development, the thick outer epidermal wall, which can be traced back to the peripheral cell wall of the zygote, creates a sturdy organ sheath that determines the rate of stem elongation. The cells of the inner tissues are the products of secondary partitioning of one large protoplast; these turgid, thin‐walled cells provide the driving force for organ growth. The structural differences between these types of cell walls are described (outer walls: thick, sturdy, helicoidal cellulose architecture; inner walls: thin, extensible, transversely‐oriented cellulose microfibrils). On the basis of these facts, current models of cell wall loosening (and wall stiffening) are discussed with special reference to the expansin, enzymatic polymer remodelling and osmiophilic particle hypothesis. It is concluded that the exact biochemical mechanism(s) responsible for the coordinated yielding of the growth‐controlling peripheral organ wall(s) have not yet been identified.
Quality improvement of metalorganic chemical vapor deposition grown GaP on Si by AsH3 preflow
Authors: Kohama Y. , Uchida K. , Soga T. , Jimbo T. , Umeno M.
Published: July 2002

GaP epilayers are grown on Si substrates after AsH3 preflow. Electron beam induced current observation and double-crystal x-ray diffraction show that the AsH3 preflow drastically improves crystalline quality of GaP epilayers. The full width at half-maximum of the (400) reflection obtained from 4.8 μm GaP is as small as 115 arcseconds. Secondary ion mass spectroscopy shows that As atoms accumulate at the GaP/Si interfaces, playing an important role in preventing Si outdiffusion into the GaP epilayers.
The optical gain lever: A novel gain mechanism in the direct modulation of quantum well semiconductor lasers
Authors: Vahala Kerry J. , Newkirk Michael A. , Chen T. R.
Published: July 2002

A new gain mechanism active in certain quantum well laser diode structures is demonstrated and explained theoretically. It enhances the modulation amplitude produced by either optical or electrical modulation of quantum well structures. In the devices tested, power gains of 6 dB were measured from low frequency to frequencies of several gigahertz. Higher gains may be possible in optimized structures.
Nonlinear optical properties of highly oriented polydiacetylene evaporated films
Authors: Kanetake T. , Ishikawa K. , Hasegawa T. , Koda T. , Takeda K. , Hasegawa M. , Kubodera K. , Kobayashi H.
Published: July 2002

Third-order harmonic generation has been measured on highly oriented polydiacetylene (PDA) films prepared by the vacuum deposition method. Experimental values of the third-order electric susceptibility χ(3) measured for pumping laser polarized parallel to the polymer chain show a sharp three-photon resonance to the 1Bu exciton in PDA. Potential usefulness of these highly oriented PDA evaporated films in nonlinear optical applications is suggested.
Storage and time reversal of femtosecond light signals via persistent spectral hole burning holography
Authors: Rebane A. , Aaviksoo J. , Kuhl J.
Published: July 2002

Time and space domain holography by persistent spectral hole burning in photochemically active media is shown to permit storage, recall, and conjugation of temporal profiles of light signals as short as 100 fs. This limit for the temporal resolution is set by the finite spectral width of the impurity absorption bands of presently available recording materials.
Grating-assisted InGaAsP/InP vertical codirectional coupler filter
Authors: Alferness R. C. , Koch T. L. , Buhl L. L. , Storz F. , Heismann F. , Martyak M. J. R.
Published: July 2002

We report a novel wavelength selective coupler based upon grating-assisted forward coupling between two, nonidentical, vertically stacked InGaAsP/InP waveguides. Using a 14.4 μm period to provide phase matching between the asynchronous waveguides, we achieve a filter bandwidth (full width at half maximum) of 65 Å and coupling efficiency of ∼65% with a device length of only 1.0 mm.
Characterization of single-crystal sapphire fibers for optical power delivery systems
Authors: Jundt D. H. , Fejer M. M. , Byer R. L.
Published: July 2002

Sapphire single-crystal fibers with diameters of 110 μm and lengths of over 2 m have been grown by the laser-heated pedestal growth method. The fibers are free of imperfections such as voids and bubbles. The minimum loss of 0.5 dB/m was measured in the near infrared at 1064 nm. Absorption loss at 2936 nm was 0.88 dB/m with a damage threshold higher than 1.2 kJ/cm2 for 110-μs-long pulses making tissue ablation feasible with fibers several meters in length.
Effect of low-level boron doping and its implication to the nature of gap states in hydrogenated amorphous silicon
Authors: Yang Liyou , Catalano A. , Arya R. R. , Balberg I.
Published: July 2002

Large simultaneous changes in ambipolar diffusion length (Ld) and photoconductivity (σph) were observed with boron doping below 1 ppm. The results can be explained satisfactorily by postulating that electrons and holes interchange their roles as majority or minority carriers at ∼0.4 ppm. The μτ products for both carriers are determined as a function of doping. The light intensity dependences of Ld and σph present new evidence for the existence of the hole trapping centers in a-Si:H and show that doping enhances the sensitizing effect due to these centers.
Determination of activation energies for diamond growth by an advanced hot filament chemical vapor deposition method
Authors: Kondoh Eiichi , Ohta Tomohiro , Mitomo Tohru , Ohtsuka Kennich
Published: July 2002

The activation energies for diamond growth were determined by an advanced hot filament chemical vapor deposition (AHF-CVD) method, which accurately controls the substrate temperature independently against other CVD parameters. The activation energies, as determined from an Arrhenius plot, were 22–24 kcal/mol in the range of 740–930 °C. These values are the lowest level reported in the literature. Reported growth mechanisms were evaluated in view of the obtained activation energies.
Investigation of KTiOPO4 as an electro-optic amplitude modulator
Authors: Wang X. D. , Basséras P. , Miller R. J. Dwayne , Vanherzeele Herman
Published: July 2002

We report on an experimental study of a hydrothermally grown KTiOPO4 (KTP) Pockels cell, utilizing the effective electro-optic coefficient, rc1=28 pm/V. Compensation of the static birefringence of KTP, when it is used as amplitude modulator, has been achieved by temperature tuning. Our study revealed that a KTP Pockels cell does not suffer from piezoelectric-induced parasitics (acoustic ringing). Stable high-repetition rate pulse slicing has been obtained at 30 kHz. Intracavity use of a KTP Pockels cell also has been successfully demonstrated in a Q-switched Nd:YLF laser system.
Monolithically Peltier-cooled vertical-cavity surface-emitting lasers
Authors: Berger Paul R. , Dutta Niloy K. , Choquette Kent D. , Hasnain Ghulam , Chand Naresh
Published: July 2002

We report the first tunable monolithically integrated thermoelectric controlled GaAs/AlGaAs vertical-cavity surface-emitting laser diode. The thermoelectric element is the n+-GaAs substrate based on the Peltier effect. A variation of active region temperature of ±7.5 °C has been achieved using ±100 mA of thermoelectric cooler current. The observed wavelength tuning associated with this temperature shift is ±6 Å. The device is useful for applications that require a high degree of frequency stability or small frequency tuning. Some examples of potential applications are in high data rate lightwave transmission, self-electro-optic device switches, and spectroscopy.
In situ diamond growth rate measurement using emission interferometry
Authors: Snail Keith A. , Marks Cheinan M.
Published: July 2002

The growth rate of diamond synthesized in an oxygen-acetylene flame has been measured in situ with an infrared pyrometer. During the growth, the sample’s emitted radiation intensity is modulated in time by interference phenomena in the growing diamond film, causing a periodic variation of the pyrometer’s apparent temperature reading. Diamond film growth rates determined from the period of these oscillations agree well with rates derived from the film thickness. This technique is used to determine the variation of diamond growth rate with substrate temperature over the range of 444–1200 °C. An Arrhenius analysis of the data shows two distinct diamond growth regimes.
Evidence for quantum confinement in porous silicon from soft x-ray absorption
Authors: van Buuren T. , Gao Y. , Tiedje T. , Dahn J. R. , Way B. M.
Published: July 2002

Measurements of x-ray absorption in the vicinity of the silicon L edge in porous silicon show a blueshift and a broadening of the conduction band edge, consistent with a distribution of quantum confinement energies. The absorption spectrum for porous silicon can be fit by a broadened and energy-shifted version of the crystalline silicon absorption spectrum. The average quantum shift and broadening used in the fit to the absorption spectrum are in reasonable agreement with the corresponding parameters derived from the photoluminescence spectrum.
Temperature dependence of photoconductivity in buckminsterfullerene films
Authors: Mort J. , Machonkin M. , Ziolo R. , Huffman D. R. , Ferguson M. I.
Published: July 2002

We report the temperature dependence from 330 to 230 K of the photoconductivity measured in sublimed films of buckminsterfullerene, specifically, C60/70, up to 3 μm thick. In contrast to the temperature dependence of the dark conductivity, where reversible structure, associated with the onset of a phase transition to orientational ordering below 250 K, is observed, the photoconductivity shows an essentially monotonic decrease to lower temperatures. This provides additional evidence that the thermal generation rate increases as the C60/70 becomes orientationally ordered. It also suggests that the increased order below the transition temperature does not lead to significantly increased carrier mobilities.
Growth kinetics of (100), (110), and (111) homoepitaxial diamond films
Authors: Chu C. J. , Hauge R. H. , Margrave J. L. , D’Evelyn M. P.
Published: July 2002

We present the first investigation of the growth kinetics of diamond films grown homoepitaxially by hot-filament chemical vapor deposition on (100), (110), and (111) natural diamond substrates. Growth rates on the various faces exhibited different functional dependencies on CH4 flow rate, being linear on (100), slightly sublinear on (110), and sigmoidal on (111). The temperature dependence of the growth rate was also crystal-face-dependent, yielding effective activation energies of 8±3, 18±2, and 12±4 kcal/mol for [100], [110], and [111] growth, respectively, at substrate temperatures between 735 and 970 °C. The apparent activation energies were considerably larger at substrate temperatures between 675 and 735 °C. We propose that these crystal face and temperature effects are due principally to differences and changes in the nanometer-scale morphology and surface hydrogen coverage with methane concentration and surface temperature.
Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition
Authors: Narayan J. , Tiwari P. , Chen X. , Singh J. , Chowdhury R. , Zheleva T.
Published: July 2002

We report epitaxial growth of TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using x-ray diffraction, Rutherford backscattering, four-point-probe ac resistivity, high resolution transmission electron microscopy and scanning electron microscopy techniques and epitaxial relationship was found to be 〈100〉 TiN ∥ 〈100〉 Si. TiN films showed 10%–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Four-point-probe measurements show characteristic metallic behavior of these films as a function of temperature with a typical resistivity of about 15 μΩ cm at room temperature. Implications of low-resistivity epitaxial TiN in silicon device fabrication are discussed.
Photoluminescence of porous Si, oxidized then deoxidized chemically
Authors: Nakajima A. , Itakura T. , Watanabe S. , Nakayama N.
Published: July 2002

We examined the change in photoluminescence spectra of porous Si when it is oxidized then deoxidized chemically. After both steps, photoluminescence shifted to higher frequencies and increased in intensity. These shifts to higher frequencies indicate the photoluminescence is a result of the quantum size effect. Moreover, the increase in photoluminescence intensity after oxidation suggests that termination by hydrogen on the porous Si surface does not always play a key role in the photoluminescence mechanism.
Novel oxidation process of hydrogenated amorphous silicon utilizing nitrous oxide plasma
Authors: Masuda Atsushi , Morimoto Akiharu , Kumeda Minoru , Shimizu Tatsuo , Yonezawa Yasuto , Minamikawa Toshiharu
Published: July 2002

A novel oxidation process in hydrogenated amorphous silicon (a-Si:H) using nitrous oxide (N2O) plasma was studied in detail for the first time. The N2O-plasma oxidized a-Si:H has an excellent interface whose interfacial defect density is largely reduced compared with the O2-plasma oxidized a-Si:H. It was elucidated that this oxide layer has almost stoichiometric composition and contains a small amount of N piling up at the interface between the oxide layer and a-Si:H layer. It also turned out that this process has less ion damage than the O2-plasma oxidation process. The reason for the reduction of the interfacial defect density is attributed to the presence of N at the interface and/or less ion damage in this process.
Layer-by-layer growth of epitaxial SnO2 on sapphire by reactive sputter deposition
Authors: Cavicchi R. E. , Semancik S. , Antonik M. D. , Lad R. J.
Published: July 2002

Epitaxial films of stoichiometric tin oxide were grown on sapphire (11̄02) substrates by reactive sputter deposition. X-ray diffraction showed the films to have a single (101) orientation. Lateral registry of film growth with respect to the substrate lattice was demonstrated by low energy electron diffraction. Atomic force microscopy was used to examine surface morphology and roughness. The films are extremely flat, having a rms roughness of 3 Å over a 4×4 μm2 area. Atomic steps, observed on the sapphire substrate and attributable to a 0.24° miscut, were also observed on the surface of a 400 Å film. The results indicate that the film grew via a layer-by-layer growth mechanism which was controlled by diffusion of the adatoms to the step edges.
Emission spectroscopy during direct-current-biased, microwave-plasma chemical vapor deposition of diamond
Authors: Shigesato Yuzo , Boekenhauer Rachel E. , Sheldon Brian W.
Published: July 2002

Optical emission spectroscopy was used to investigate dc biasing during diamond film synthesis in a microwave plasma. These measurements show that biasing produces significant changes near the substrate (i.e., close to the sheath region). Increasing the negative bias voltage (Vb) from 0 to −180 V in a CH4/H2/Ar (4/496/30 sccm) mixture increases the intensities of the hydrogen Balmer α and β lines. The relative concentrations of neutral atomic hydrogen were estimated by using an Ar(750.4 nm) emission line as an actinometer. At 38 Torr, increasing Vb from 0 to −150 V increased the concentration of atomic hydrogen by more than 20%. In addition, increasing Vb also increased the electron temperature near the substrate. These effects are likely to play an important role in the enhanced diamond nucleation that has been observed after negative-biased pretreatment.
Electrical conductivity of combustion flame synthesized diamond
Authors: Ravi K. V. , Koch C. A. , Olson D. S. , Choong P. , Vandersande J. W. , Zoltan L. D.
Published: July 2002

The electrical conductivity, from room temperature to 1000 °C, of combustion flame synthesized diamond films and free-standing diamond slabs are demonstrated to be up to two orders of magnitude lower than that of type IIa natural diamond crystals. The low conductivity, indicative of high purity, has been achieved at diamond growths rates of 5–10 μm/h, considerably higher than that achievable with other diamond synthesis techniques. These high growth rates have been achieved over areas of 5 cm×5 cm and both thin (10 μm) films on silicon substrates and thick (∼80 μm), free-standing diamond slabs exhibit similar electrical behavior. The high purity of this diamond is attributed to the presence of oxidizing species in the flame ambient which are more effective than hydrogen in removing any nondiamond forms of carbon and other impurities from the growing diamond film.
Electromagnetic radiation shielding by intrinsically conducting polymers
Authors: Joo J. , Epstein A. J.
Published: July 2002

The shielding efficiency of various intrinsically conducting polymers (ICPs) as a function of their intrinsic properties (conductivity and dielectric constant), thickness, and temperature is determined. Two types of shielding, reflection and absorption, by ICPs are discussed. The high shielding efficiencies of highly conducting doped polyaniline, polypyrrole, and polyacetylene are reported and compared to that of copper. The easy tuning of intrinsic properties by chemical processing suggests the wide applications of ICPs, especially polyaniline for shielding.
InGaAs/GaAs multiple strained-layer structure grown on a lattice-matched InGaAs substrate wafer
Authors: Fritz I. J. , Klem J. F. , Schirber J. E. , Olsen J. A. , Bonner W. A.
Published: July 2002

Growth of In0.15Ga0.85As/GaAs multilayers are reported on lattice matched In0.04Ga0.96As buffer layers using two different starting substrates: (1) a bulk, lattice-matched, Czochralski-grown In0.04Ga0.96As wafer, and (2) a bulk (unmatched) GaAs wafer. The structures, grown by molecular-beam epitaxy, consist of a 200-nm-thick undoped buffer plus a 28-1/2 period modulation-doped multilayer having 10-nm-thick In0.15Ga0.85As quantum wells and 25-nm-thick GaAs barriers doped over their 10-nm central region. The 4 K Hall mobility of the multilayer grown on the InGaAs substrate is 57% larger than the structure grown on GaAs, despite a low dislocation density observed in transmission electron micrographs of the latter. These results suggest that bulk ternary substrates can provide enhanced performance for future electronic and optoelectronic devices.
All-optical picosecond switching of a quantum well etalon using spin-polarization relaxation
Authors: Nishikawa Yuji , Tackeuchi Atsushi , Nakamura Satoshi , Muto Shunichi , Yokoyama Naoki
Published: July 2002

All-optical picosecond gate operation is demonstrated using the spin relaxation in a quantum well etalon. Electron spin-polarization photoexcited by circularly polarized light in GaAs quantum wells decays in picoseconds. With the use of the spin relaxation in a quantum well etalon, all-optical full switching is achieved with a decay time of as fast as 7 ps, by adopting a simple optical differential method, which uses only a quarter-wave plate and a polarizer in addition to the conventional all-optical switching setup.
Phase transformation of cobalt induced by ball milling
Authors: Huang J. Y. , Wu Y. K. , Ye H. Q.
Published: July 2002

We have found that phase transformation can occur in cobalt when subjected to ball milling. The two modifications of cobalt, i.e., face-center-cubic (fcc) and hexagonal close-packed (hcp) phases, which usually coexist at room temperature and are often difficult to be separated from each other, can now be easily separated by using the simple ball milling technique. The phase formation of cobalt was found to depend on the mill intensity. Under different mill intensity or different milling time, the phase transformations follow the routes of hcp+fcc→hcp, hcp+fcc→hcp→fcc+hcp, and hcp+fcc→hcp→fcc+hcp→fcc, respectively. Our results indicate that the phase formation of cobalt induced by ball milling was determined by the accumulation of structure defects. Different mill intensity may adjust the rate and level of the accumulation of defects.
Fabrication of two-dimensional arrays of nanometer-size clusters with the atomic force microscope
Authors: Schaefer D. M. , Reifenberger R. , Patil A. , Andres R. P.
Published: July 2002

An atomic force microscope tip is used as a vector positioner to manipulate nanometer-size preformed Au clusters deposited on atomically smooth substrates. Using this technique, two-dimensional cluster nanostructures can be assembled at room temperature.
Fast-closing beam valve for use in ultrahigh vacuum
Authors: Engel Thomas , Braid Donald N.
Published: July 2002

A commercially available straight-through valve has been modified for use in ultrahigh vacuum. Since it is actuated by a solenoid coil, opening and closing can easily be triggered from outside the vacuum system. The time elapsed between the interruption of the solenoid current and the closing of the valve is 3×10−2 s.
Pulsed laser desorption method for volatilizing thermally labile molecules for supersonic jet spectroscopy
Authors: Li Liang , Lubman David M.
Published: July 2002

A technique using pulsed laser desorption is described as a means of volatilizing nonvolatile and thermally labile molecules into the gas phase for entrainment in supersonic jet expansions. By carefully controlling the laser desorption power and by using a glycerol matrix to provide a thin even film of sample, repetitive desorption can be achieved over an extended period for performing jet spectroscopy. Using resonant two-photon ionization spectroscopy while monitoring only the molecular ion in a time-of-flight mass spectrometer, cold sharp spectra are observed for molecules which are difficult to volatilize by conventional heating methods.
Holmium laser temperature-jump apparatus for kinetic studies of muscle contraction
Authors: Smith J. J. , McCray J. A. , Hibberd M. G. , Goldman Y. E.
Published: July 2002

A solid-state holmium3+ –YLF laser-based temperature-jump apparatus has been designed, constructed, and used to produce sudden 2–10 °C temperature increases in aqueous media. This temperature jump technique can be used to perturb biological macromolecular assemblies, such as single skeletal muscle fibers, without their disruption and sufficiently rapidly to elicit biologically relevant kinetic information.
Results of a new ‘‘OCTOPUS’’ ECR ion source at 6.4 GHz
Authors: Dupont C. , Jongen Y. , Arakawa K. , Yokota W. , Satoh T. , Tachikawa T.
Published: July 2002

The first OCTOPUS electron cyclstron resonance (ECR) multicharged heavy ion source was built in 1985 at the Centre de Recherches du Cyclotron of the University of Louvain (Belgium). This first source used an ECR frequency of 14.3 GHz in the injector stage and 8.5 GHz in the main confinement stage. A new OCTOPUS source has now been built for a new cyclotron to be installed at the Japan Atomic Energy Research Institute (JAERI). The design of this new OCTOPUS source is identical to the first OCTOPUS source, but uses an ECR frequency of 6.4 GHz in the main confinement stage. The experimental results are described, and a comparison is made between the two sources. However, the available data does not allow any clear conclusion to be drawn on frequency scaling.
Scanning probe tips formed by focused ion beams
Authors: Vasile M. J. , Grigg D. A. , Griffith J. E. , Fitzgerald E. A. , Russell P. E.
Published: July 2002

Probe tips for scanning tunneling microscopy have been sharpened using focused ion beam milling. Reproducible tips were formed on polycrystalline W and Pt-Ir shanks, but this technique is not limited to these materials. The tips were found to have cone angles of 12±3° and radii of curvature as sharp as 4 nm. Focused ion beam machining allows precise control of the final shape of the tips which is important in metrology measurements of various nanostructure devices.
Wölter x-ray microscope characterization measurements on Nova
Authors: Remington B. A. , Glendinning S. G. , Wallace R. J. , Rothman S. , Morales R.
Published: July 2002

We have made in situ measurements of the spatial resolution function for two sectors of the 22× magnification Wölter x-ray microscope. Our experimental technique using backlit grid shots on the Nova laser and our method of analysis are described.
Multipass optical detection in reflected shock waves: Application to OH radicals
Authors: Su M.-C. , Kumaran S. S. , Lim K. P. , Michael J. V.
Published: July 2002

An UV multipass optical absorption method to increase the sensitivity for radical species detection has been developed for high temperature chemical kinetics experiments in a shock tube. The specific illustration is for OH radicals in the reflected shock wave regime. With a resonance lamp source, 12 optical passes were found to give a sufficient signal-to-noise ratio for a large range of [OH]. Two different calibration procedures using the reaction systems H2/O2 and C2H5I/NO2 were used, and a curve of growth was determined. The measured absorbance (ABS), was found to be dependent on both temperature and [OH]. The results can be expressed in a modified Beer’s law form as,(ABS)=9.49×10−12T−0.5281[OH]0.8736.Using this curve of growth, the absorbance data from the above kinetics experiments were converted to concentration profiles. These were fully modeled with previously established mechanisms, giving excellent fits. The multipass method is compared to earlier systems that used both resonance lamp and laser absorption sources, and the increase in sensitivity is found to be substantial due primarily to the increased path length. This increased sensitivity inhibits the effects of any possible secondary chemistry thereby allowing chemically isolated experiments on OH + molecules to be performed at high temperatures.
Measurement of thermal diffusivity by optical excitation and infrared detection of a transient thermal grating
Authors: Graebner J. E.
Published: July 2002

A contactless technique is described for measuring the thermal diffusivity in a direction parallel to the surface of a thin sample by pulsed-laser excitation and thermal-radiation detection of a thermal grating on the surface. The transient thermal grating (TTG) with periodicity Λ in the range 20–1000 μm is created with a novel interferometer which does not require a long-coherence-length laser. Fast thermometry is achieved with an infrared detector focused on a spot ∼30 μm in diameter to examine in detail the shape and the time dependence of the TTG. The performance of the system is demonstrated on a thin Ti foil with Λ ranging over an order of magnitude. Data are also presented for an 11 μm thick diamond film with a factor-of-20 higher diffusivity than for Ti. The results indicate that the system is well suited to measuring local diffusivities in regions as small as ∼100 μm in thin, free-standing films.
The ISOL–MAFIOS Source (invited)
Authors: Geller R. , Tamburella C. , Belmont J. L.
Published: July 2002

ISOL–MAFIOS is a high-efficiency Min B ECRIS charge state breeder for short-lived ISOL beams in the PIAFE project. The ISOL species issuing from a bombarded target are locally ionized (1+) in a given ion source (ISOLDE type). They are then accelerated, magnetically selected, and transported in a long beam line, before being adequately introduced into ISOL–MAFIOS operating in continuous mode. In this system, an ECR plasma (created with a support gas) replaces the usual solid catcher. The ISOL–MAFIOS system captures the (1+) ions and transforms them into (q+) ions. If we call, overall efficiency of ISOL–MAFIOS the ratio of the beam intensities: I(q+) after the magnetic selector/I(1+) before entering in the source, we find for example for Rubidium IRb9+/IRb1+≊120 enA/1000 enA and a ratio at least two times larger for IKr9+/IKr1+. Moreover, an even more important result of ISOL–MAFIOS is that the release time+ionization time is of the order of 10−2 s instead of many seconds for a solid-state catcher. Thus in the PIAFE project isotopes whose half life times are ≳10−2 s become utilizable for acceleration in the two-cyclotron SARA. Note that in the PIAFE project the (1+) ISOL ions are produced by neutron bombardment of a U target inside the high flux reactor ILL and not by the bombardment of a target by a primary ion beam.
High-resolution digital quadrature detection
Authors: Villa Marco , Tian Feng , Cofrancesco Pacifico , Halámek Josef , Kasal Miroslav
Published: July 2002

We illustrate the principles of digital quadrature detection and call attention to its various benefits (ghost-free spectra and high immunity to low-frequency interference) and its intrinsic capability of generating data sets with different aliasing behaviors. A function describing the filtering efficiency is introduced, and the digital filters of our detector are compared with their analog counterparts of conventional nuclear magnetic resonance spectrometers. With an appropriate analog-to-digital converter (ADC), our digital detector has a dynamic range which is essentially limited by the analog noise, and increases when the spectral bandwidth is reduced. These nearly ideal performances are achieved through dithering, which randomizes the quantization error and oversampling, which reduces the quantization noise in the band of interest. We introduce a ‘‘figure of merit’’ for AD converters which estimates the noise performances of ADCs, and allows to compare products which achieve different compromises between speed and accuracy. The distortions due to the nonlinearities of the ADCs are analyzed through simulations. We find that the majority of the spurious signals (i.e., the errors other than noise) occur outside the band of interest, and are disposed through digital filtering. An unexpected result of the simulation is that, in some circumstances (e.g., large-scale narrowband dithering), an increase in the number of bits of the ADC may actually reduce the distortion-free dynamic range. In Sec. VIII we analyze practical problems like the role of the aperture jitter and the selection of the sampling frequency.
A fast scan submillimeter spectroscopic technique
Authors: Petkie Douglas T. , Goyette Thomas M. , Bettens Ryan P. A. , Belov S. P. , Albert Sieghard , Helminger Paul , De Lucia Frank C.
Published: July 2002

A new fast scan submillimeter spectroscopic technique (FASSST) has been developed which uses a voltage tunable backward wave oscillator (BWO) as a primary source of radiation, but which uses fast scan (∼105 Doppler limited resolution elements/s) and optical calibration methods rather than the more traditional phase or frequency lock techniques. Among its attributes are (1) absolute frequency calibration to ∼1/10 of a Doppler limited gaseous absorption linewidth (<0.1 MHz, 0.000 003 cm−1), (2) high sensitivity, and (3) the ability to measure many thousands of lines/s. Key elements which make this system possible include the excellent short term spectral purity of the broadly (∼100 GHz) tunable BWO; a very low noise, rapidly scannable high voltage power supply; fast data acquisition; and software capable of automated calibration and spectral line measurement. In addition to the unique spectroscopic power of the FASSST system, its implementation is simple enough that it has the prospect of impacting a wide range of scientific problems.
Measurement of critical micelle concentration of nonionic surfactant solutions using impedance spectroscopy technique
Authors: Chang Hong-Chi , Hwang Bing-Joe , Lin Yi-Yung , Chen Li-Jen , Lin Shi-Yow
Published: July 2002

Critical micelle concentrations (CMCs) of nonionic surfactant solutions are determined by a conductance technique. A four-terminal cell and an impedance spectrometer are employed. An equivalent circuit of the sample cell is determined and the solution resistance is obtained from the impedance plane plot. The CMC of the nonionic surfactant solution is then determined from the slope break in a conductivity concentration (κ-C) diagram. Some nonionic surfactants, including both pure and industrial surfactants, are used to examine this technique. The CMCs obtained are in good agreement with that from the surface tension method, but the time required is much less than that for the surface tension method. The conductivities of nonionic surfactant aqueous solutions are only several times that of pure water, whereas they are two to three orders of magnitude lower than that of ionic surfactant solutions. The technique of impedance spectroscopy is found to be an effective tool for measuring the CMC of nonionic aqueous surfactant solutions.
Review of black surfaces for space-borne infrared systems
Authors: Persky M. J.
Published: July 2002

Low reflectivity (“black”) surface treatments for space-borne infrared systems are reviewed. The uses of black surfaces in general, as well as for specific space-borne applications are discussed. Compositions of a wide variety of surface treatments with examples of experimental data to characterize performances are provided. Specific treatments included are: Ames 24E paint; AZKO 463 (Sikkens, Cat-A-Lac) paint; Ball IR black paint; Chemglaze (Aeroglaze) Z306 and Z302 paints; Eccosorb 268E paint; Parsons Black paint; black anodize; black Hardlub; black Hardcoat; Martin Black; InfraBlack; Enhanced Martin Black; Ebonal C; Teflon; ion beam textured; appliqués; black chrome; black etched beryllium on beryllium; plasma sprayed boron on beryllium; plasma sprayed beryllium on beryllium; boron carbide on POCO graphite; and Kapton. Data presented for some but not all of the surfaces include: spectrally integrated, 5–25 μm hemispherical-directional reflectance; spectral reflectance at wavelengths between 2 and 500 μm for a variety of incident angles from 5° to 80°; and bidirectional reflectance at a number of wavelengths between 5 and 300 μm for a variety of incident angles from 0° to 80°. The instrumentation employed to obtain these data is briefly described. Long term stability of optical performance, as well as manufacturing reproducibility is demonstrated for several of the surfaces. Outgassing and atomic oxygen interaction information is also included. Methodology for calorimetric measurement of hemispherical emittance as an alternative to optical measurements is given.
Axisymmetric drop shape analysis as penetration Langmuir balance
Authors: Cabrerizo-Vı́lchez M. A. , Wege H. A. , Holgado-Terriza J. A. , Neumann A. W.
Published: July 2002

A new Langmuir-type pendant-drop penetration film balance has been developed combining a Langmuir-type pendant-drop film balance with a new rapid-subphase-exchange technique. In addition to the determination of surface pressure—molecular area isotherms of insoluble monolayers deposited on the surface of a pendant drop, it allows the study of reactions with some surfactant added to the subphase. The monolayer is spread on the surface of a drop suspended from a capillary, which is the outer one of an arrangement of two coaxial capillaries connected to the different branches of a microinjector. Once the film is brought to the desired state of compression by varying the drop volume with the microinjector, the subphase liquid in the drop can be exchanged quantitatively by means of the coaxial capillaries. This exchange is complete for a through-flow of at least three times the drop volume, and the monolayers endure it at all tested film pressures. The determination of surface tension as a function of surface area is performed using axisymmetric drop shape analysis (ADSA). The complete set-up, i.e., the image capturing and microinjector system is fully computer controlled by a user-friendly and fully Windows integrated program, including the ADSA surface tension calculus algorithm. As a penetration film balance, pendant-drop methodologies offer a wide range of advantages such as a more stringent control of the environmental conditions and therefore, more uniform temperature, pressure and concentration along the interface, small amounts of material needed, and a 20 times greater interface/volume ratio than in conventional Langmuir toughs.
Microfabrication of a mechanically controllable break junction in silicon
Authors: Zhou C. , Muller C. J. , Deshpande M. R. , Sleight J. W. , Reed M. A.
Published: July 2002

We present a detailed description of the fabrication and operation at room temperature of a novel type of tunnel displacement transducer. Instead of a feedback system it relies on a large reduction factor assuring an inherently stable device. Stability measurements in the tunnel regime infer an electrode stability within 3 pm in a 1 kHz bandwidth. In the contact regime the conductance takes on a discrete number of values when the constriction is reduced atom by atom. This reflects the conduction through discrete channels.
Selection criteria for microphones used in pulsed nonresonant gas-phase photoacoustics
Authors: Calasso I. G. , Sigrist M. W.
Published: July 2002

Theoretical and experimental investigations aimed at optimizing the detection performance in nonresonant pulsed photoacoustic (PA) studies in the gas phase by selecting microphones with appropriate characteristics are presented. An analytical function is introduced that allows the simulation of the measured signal-to-noise-ratio (SNR) for given microphone specifications. The crucial parameters for maximum SNR are the microphone responsivity, its bandwidth, and noise characteristics. Good agreement between calculated and measured PA signal shapes and their fast Fourier transform spectra is obtained. Comparisons of experimental data recorded in a gas mixture of 100 ppm ethylene buffered in synthetic air with various condenser and electret microphones with different characteristics confirm our theoretical predictions. The peak amplitude of the recorded microphone signal increases with decreasing laser beam radius rb. However, for standard microphones with a bandwidth of ≈20 kHz, there is no reason to use rb<0.5 mm because the spectral composition of the signal shape for frequencies <20 kHz remains unchanged. A large microphone bandwidth of a few 100 kHz is advantageous for resolving the temporal evolution of the PA signal but it can be disadvantageous for trace gas monitoring due to an enhanced noise level. Our simulation permits the selection of the most appropriate microphone for maximum SNR under given experimental conditions. Finally, an analysis on the pressure dependence of the responsivity of a selected microphone is presented that is of interest when performing trace gas measurements at reduced pressure to enhance the detection selectivity.
Calibration of a multiple microchannel plate detectors system by α-induced secondary electrons
Authors: Villette J. , Barat M. , Roncin P.
Published: July 2002

α particles emitted from an Am241 radioactive source at energies of 5.4 MeV generate bursts of about ten electrons when passing through an aluminized Mylar foil. Besides the typical surface barrier electrons, the energy spectra of the secondary electrons clearly reveal two additional peaks. One at 66 eV is ascribed to aluminum LVV Auger electrons and another at 10.5 eV is attributed to the decay of aluminum volume plasmons. The well-resolved angular and energy distributions of these secondary electrons are used to calibrate the relative detection efficiencies of a large set of individual detectors of a complex multicoincidence system.
Experimental apparatus for radiometric emissivity measurements of metals
Authors: Furukawa T. , Iuchi T.
Published: July 2002

An apparatus of the radiometric emissivity measurement of a metal has been developed. The apparatus possesses some features that it can keep a metallic sample under specific conditions such as vacuum, oxidizing, and deoxidizing atmosphere. Using the apparatus, spectral (from visible to 10 μm infrared wavelengths), directional (normal and 80°), and polarized emissivities (p and s polarized) of metals can be measured and can be utilized to investigate the behavior of the emissivity of the specimen. In this article, the concrete design of the apparatus is described and some examples of measurements of emissivities under some specific conditions are introduced. The peculiar behaviors of the emissivity of a metal in the early stage when the oxide film is grown on its surface are discussed.
Crystallization of layered metal-dichalcogenides films on amorphous substrates
Authors: Galun E. , Cohen H. , Margulis L. , Vilan A. , Tsirlina T. , Hodes G. , Tenne R. , Hershfinkel M. , Jaegermann W. , Ellmer K.
Published: July 2002

Anisotropic materials with layered structure, like MoS2 and WSe2, play an important role in a number of technologies. Some of these applications (lubrication, photovoltaics) require polycrystalline films oriented with their c axis perpendicular to the substrate surface (type-II texture), which is the thermodynamically favorable texture. However, films with the substrate ∥c (type-I texture) are usually obtained. We report that an ultrathin (<10 nm) metal-chalcogenide interlayer eutectics, like Ni3Se2, SnSe2, or InSe disentangle the growth mode of the film from the underlying amorphous substrate, and hence, WSe2 films with a perfect type-II texture and crystallites at least a few mm2 large are obtained at temperatures as low as 700 °C (van der Waals rheotaxy–vdWR). The mechanism for this growth mode is proposed.
Dominance of tunneling current and band filling in InGaN/AlGaN double heterostructure blue light-emitting diodes
Authors: Casey H. C. , Muth J. , Krishnankutty S. , Zavada J. M.
Published: July 2002

Measurement of the room temperature forward bias current-voltage behavior of InGaN/AlGaN double heterostructure blue light-emitting diodes demonstrates a significant departure from the usual Is exp(qV/ nkT) behavior where n is the ideality factor which varies between 1 and 2. The observed current-voltage behavior at room temperature may be represented as I=2.7×10−11 exp(5.7V) which suggests a tunneling mechanism. Measurement of the electroluminescence for currents from 0.5 to 100 mA demonstrates that the emission peak shifts to higher energy while increasing in intensity. The shifting peak spectra is due to band filling, a process which results from the injection of holes via tunneling into an empty acceptor impurity band and vacant valence band tails. At currents near 100 mA, a non-shifting band-to-band emission approaches the intensity of the shifting peak spectra. The active layer of these diodes is codoped with both the donor Si and the acceptor Zn.
High-resolution x-ray diffraction of self-organized InGaAs/GaAs quantum dot structures
Authors: Krost A. , Heinrichsdorff F. , Bimberg D. , Darhuber A. , Bauer G.
Published: July 2002

The structural properties of highly strained buried InxGa1−xAs layers on GaAs substrates are investigated by high-resolution x-ray diffraction. Such layers of a few monolayers in thickness serve for the formation of self-organized quantum dots by the Stranski–Krastanow growth mode. Exceeding a critical layer thickness the growth mode changes from two-dimensional Frank–van der Merwe to the three-dimensional Stranski–Krastanow mode resulting in the formation of coherently strained InxGa1−xAs islands. X-ray spectra of such structures below the growth mode transition can be perfectly simulated using dynamical theory allowing for determination of layer thickness with submonolayer sensitivity and composition within 5%. Dot formation manifests itself in a decrease of the effective In content of the wetting layer.
Luminescence efficiency of cerium doped insulators: The role of electron transfer processes
Authors: Raukas M. , Basun S. A. , Schaik W. van , Yen W. M. , Happek U.
Published: July 2002

Insulating host materials doped with trivalent cerium show quantum efficiencies of the Ce3+ emission ranging from zero to unity. Comparing optical and photoelectrical properties of a very efficient scintillator material (Lu2(SiO4)O:Ce) to those of cerium doped oxides with quenched emission, the radical differences for these materials are demonstrated to originate from the location of the cerium energy levels with respect to the host conduction band. Photoionization and subsequent nonradiative relaxation processes responsible for the luminescence quenching are discussed in a donor–acceptor model for the impurity ion and a rule for luminescence efficiency is derived, applicable to a variety of phosphor and scintillator materials.