1,034 research outputs found
Avalanches in a Bose-Einstein condensate
Collisional avalanches are identified to be responsible for an 8-fold
increase of the initial loss rate of a large 87-Rb condensate. We show that the
collisional opacity of an ultra-cold gas exhibits a critical value. When
exceeded, losses due to inelastic collisions are substantially enhanced. Under
these circumstances, reaching the hydrodynamic regime in conventional BEC
experiments is highly questionable.Comment: 4 pages, 2 figures, 1 tabl
Etching of Si through a thick condensed XeF2 layer
Etching of silicon by XeF2 is studied in a multiple-beam setup. Below 150 K XeF2 condenses and forms a layer on the silicon, which blocks the etching. Upon ion bombardment, this layer is removed and etching will resume. As a function of the layer thickness, the various removal mechanisms of the layer are studied. For a thick condensed layer it is found that 1 keV Ar+ ions sputter the condensed layer with a yield of 160 XeF2 molecules per ion for 1 keV Ar+ ions and 280 for 2 keV ions. For thinner layers (below 9 nm for 1 keV ions), this sputter rate by ions decreases significantly. Here, the removal is mainly due to consumption of XeF2 by etching at the bottom of the layer. This consumption rate reaches a maximum for a layer thickness of about 5 nm. In the steady-state situation, the layer thickness is further decreased, resulting in a smaller consumption and etch rate. Here, sputtering is the most important removal mechanism for the deposited XeF2 layer. From this, it is concluded that a pulsed ion beam should be used in cryogenic etching to obtain the highest etch rate
Anharmonic mixing in a magnetic trap
We have experimentally observed re-equilibration of a magnetically trapped
cloud of metastable neon atoms after it was put in a non-equilibrium state.
Using numerical simulations we show that anharmonic mixing, equilibration due
to the collisionless dynamics of atoms in a magnetic trap, is the dominant
process in this equilibration. We determine the dependence of its time on trap
parameters and atom temperature. Furthermore we observe in the simulations a
resonant energy exchange between the radial and axial trap dimensions at a
ratio of trap frequencies \omega_r / \omega_z = 3/2. This resonance is
explained by a simple oscillator model.Comment: 9 pages, 6 figure
An intense, slow and cold beam of metastable Ne(3s) ^3P_2 atoms
We employ laser cooling to intensify and cool an atomic beam of metastable
Ne(3s) atoms. Using several collimators, a slower and a compressor we achieve a
^{20}Ne^* flux of 6 10^{10} atoms/s in an 0.7 mm diameter beam traveling at 100
m/s, and having longitudinal and transverse temperatures of 25mK and 300microK,
respectively. This constitutes the highest flux in a concentrated beam achieved
to date with metastable rare gas atoms. We characterize the action of the
various cooling stages in terms of their influence on the flux, diameter and
divergence of the atomic beam. The brightness and brilliance achieved are 2.1
10^{21} s^{-1} m^{-2} sr^{-1} and 5.0 10^{22} s^{-1} m^{-2} sr^{-1},
respectively, comparable to the highest values reported for alkali-metal beams.
Bright beams of the ^{21}Ne and ^{22}Ne isotopes have also been created.Comment: 18 pages, 9 figures, RevTe
Thiobacillus
File replaced (incorrect version) on 18/7/22 by KT (LDS) (1.4)Thi.o.ba.cil'lus. Gr. neut. n. theĂźon, sulfur, brimstone; L. masc. n. bacillus, a short rod, a short wand; N.L. masc. n. Thiobacillus, sulfur rodlet. Proteobacteria / Betaproteobacteria / Nitrosomonadales / Thiobacillaceae / Thiobacillus Cells are short rods. Cytochrome c oxidaseâpositive and catalaseâpositive when grown on thiosulfate. Gramâstainânegative. Endospores, exospores, and cysts are not produced. Metabolically obligate chemolithoautotrophs, supported by reduced sulfur species and elementary sulfur, and some methylated sulfur compounds. Genes encoding Form IAc, Form IAq, and Form II dâribulose 1,5âbisphosphate carboxylase/oxygenases (RuBisCO) are present in the genomes. Carboxysomes are produced in some species and are repressed at high CO2 partial pressures. Volutin (polyphosphate) granules formed in most species. Produce tetrathionate as a detectable intermediate of thiosulfate oxidation. Obligately respiratory, with molecular oxygen and nitrate the only known terminal electron acceptors, with the latter only used in some species. Mesophilic, growing optimally at 25â32°C, and one psychrophilic species capable of growth down to â2°C. The major respiratory quinone is ubiquinoneâ8 (UQâ8). Dominant fatty acids are C16:0, C16:1, C15:0, and C17:1. DNA G + C content (mol%): 61.5â66.0. Type species: Thiobacillus thioparus Beijerinck 1904b, 153AL
Si/XeF2 etching: Temperature dependence
The temperature dependence of the Si(100)/XeF2 etch reaction is studied quantitatively in a molecular beam setup. At a sample temperature of 150 K the reaction probability reaches unity initially, after which the XeF2 condenses on the surface and blocks the etching process. For increasing temperatures the XeF2 reaction probability initially decreases from 100% at 150 K down to 20% around 400 K, but for temperatures above 600 K it increases again up to 45% at 900 K. In a simple reaction scheme the high etch rate at low temperatures is explained by a XeF2-precursor, with an activation energy for desorption of 32±4 meV. Furthermore the increased etch rate at high temperatures is explained by the desorption of SiF2 with an activation energy of 260±30 meV. The steady-state fluorine content of the SiFx reaction layer, measured using thermal desorption spectroscopy, reaches a maximum of 5.5 monolayers at 300 K. For increasing temperatures it decreases to a submonolayer coverage above 700 K. The temperature dependence of the formation of the reaction layer is described well by including the XeF2-precursor in a previously developed adsorption model. © 1996 American Vacuum Societ
Time-of-flight analysis of neutrals from a plasma as a diagnostic for the ion velocity distribution
In a plasma energy is transferred from ions to neutrals by elastic collisions and charge exchange. By measuring the velocity distribution of the fast neutrals with a time-of-flight spectrometer, one gets very detailed information on the ion velocity distributio
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