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

Silicon etch rate enhancement by traces of metal

We report the effect of nickel and tungsten contamination on the etch behavior of silicon. This is studied in a molecular beam setup, where silicon is etched by XeF2 and Ar+ ions. The etch process is directly monitored by the SiF4 reaction products which leave the surface. The effect of contamination appears very pronounced after the ion beam is switched off: it leads to a temporary enhancement of the spontaneous etch rate on a time scale of 500 s. With traces of contamination on the order of 0.01 ML, the etch rate may be enhanced by a factor of 2 for W and somewhat less for Ni. It is concluded that the contamination moves into the silicon by diffusion to vacancies created by the Ar+ ions. For 1 keV Ar+ ions the contamination moves to a depth of 25 Å, comparable to the penetration depth of the ions. After etching a 170 Å thick layer, the catalytic effect of contamination is reduced to less than 5%. A simple model, which describes the measured effect of contamination very well, indicates that only 3% of the contamination is removed when a monolayer of silicon is etched away. Besides this catalytic effect there are indications that contamination can also lower the etch rate under certain conditions, because of the formation of silicides. From the measurements no conclusions could be drawn about the underlying mechanism of etch rate enhancement. © 1999 American Vacuum Society

Quantification of depth of anesthesia by nonlinear time series analysis of brain electrical activity

We investigate several quantifiers of the electroencephalogram (EEG) signal with respect to their ability to indicate depth of anesthesia. For 17 patients anesthetized with Sevoflurane, three established measures (two spectral and one based on the bispectrum), as well as a phase space based nonlinear correlation index were computed from consecutive EEG epochs. In absence of an independent way to determine anesthesia depth, the standard was derived from measured blood plasma concentrations of the anesthetic via a pharmacokinetic/pharmacodynamic model for the estimated effective brain concentration of Sevoflurane. In most patients, the highest correlation is observed for the nonlinear correlation index D*. In contrast to spectral measures, D* is found to decrease monotonically with increasing (estimated) depth of anesthesia, even when a "burst-suppression" pattern occurs in the EEG. The findings show the potential for applications of concepts derived from the theory of nonlinear dynamics, even if little can be assumed about the process under investigation.Comment: 7 pages, 5 figure

Anaesthesia and PET of the Brain

Although drugs have been used to administer general anaesthesia for more than a century and a half, relatively little was known until recently about the molecular and cellular effects of the anaesthetic agents and the neurobiology of anaesthesia. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies have played a valuable role in improving this knowledge. PET studies using 11C-flumazenil binding have been used to demonstrate that the molecular action of some, but not all, of the current anaesthetic agents is mediated via the GABAA receptor. Using different tracers labelled with 18F, 11C and 15O, PET studies have shown the patterns of changes in cerebral metabolism and blood flow associated with different intravenous and volatile anaesthetic agents. Within classes of volatile agents, there are minor variations in patterns. More profound differences are found between classes of agents. Interestingly, all agents cause alterations in the blood flow and metabolism of the thalamus, providing strong support for the hypothesis that the anaesthetic agents interfere with consciousness by interfering with thalamocortical communication.</p

ICAR: endoscopic skull‐base surgery

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Reaction layer dynamics in ion-assisted Si/XeF2 etching: temperature dependence

We study the dynamics of the reaction layer during Ar+ ion-assisted Si etching by XeF2 in the temperature range T = 150–800 K. Depending on temperature, the etch rate can be enhanced a factor of 8 by ion bombardment. The dynamics are studied with ion-pulse measurements on a time scale of 1–100 s in a molecular beam setup. A reaction layer with a submonolayer fluorine coverage and dangling bonds is found to be formed on the Si(100) surface during ion bombardment. The dangling bond concentration increases with ion flux and is independent of temperature in the range 150–600 K. Chemisorption on these dangling bonds results in a higher reaction probability of XeF2. The temperature dependence of the reaction probability of XeF2 is fully determined by the temperature dependence of the XeF2 precursor state. A simple model gives a very good description of the reaction probability as a function of both temperature and ion flux. The model description of the behavior of the precursor concentration as a function of ion flux and temperature is confirmed by ion pulse measurements on a time scale of 1 s. Further, it is concluded that the mechanisms for enhanced SiF4 formation during ion bombardment are the same over the temperature range studied

Reaction layer dynamics in ion-assisted Si/XeF2 etching: Ion flux dependence

The etch rate of Si by XeF2 can be enhanced by more than a factor of 8 by ion bombardment. This enhancement is studied in a multiple-beam setup by looking at the response of reaction product signals upon ion pulses on time scales of 1–100 s in a multiple-beam setup. On a time scale of 100 s, it is found that ion bombardment causes fluorine depletion of the reaction layer and changes the structure of the reaction layer. This lower fluorine content results in a lower contribution of the spontaneous SiF4 production during ion bombardment. For the enhanced SiF4 production two processes are found from pulse measurements on the time scale of 1–10 s. First, ion bombardment creates weakly bound surface species, e.g., SiF2, that can react in the reaction layer to SiF4. Second, XeF2 reacts with these species with a higher reaction probability, thus enhancing the SiF4 production. The relative importance of both mechanisms is determined. Further, the limiting steps during spontaneous and ion-assisted etching are discussed, revealing that the creation of dangling bonds is the reason for the higher sticking probability of XeF2 during ion-assisted etching. © 1999 American Vacuum Society