Torr

Abstract: Early in the next century, several space missions are planned with the goal of landing craft on asteroids, comets, the Moon, and Mars. To increase the scientific return of these missions, new methods are needed to provide (1) significantly more analyses per mission lifetime, and (2) expanded analytical capabilities. One method that has the potential to meet both of these needs for the elemental analysis of geological samples is laser-induced breakdown spectroscopy (LIBS). These capabilities are possible because the laser plasma provides rapid analysis and the laser pulse can be focused on a remotely located sample to perform a stand-off measurement. Stand-off is defined as a distance up to 20 m between the target and laser. Here we present the results of a characterization of LIBS for the stand-off analysis of soils at reduced air pressures and in a simulated Martian atmosphere (5-7 torr pressure of CO2) showing the feasibility of LIBS for space exploration. For example, it is demonstrated that an analytically useful laser plasma can be generated at distances up to 19 m by using only 35 mJ/pulse from a compact laser. Some characteristics of the laser plasma at reduced pressure were also investigated. Temporally and spectrally resolved imaging showed significant changes in the plasma as the pressure was reduced and also showed that the analyte signals and mass ablated from a target were strongly dependent on pressure. As the pressure decreased from 590 torr to the 40-100 torr range, the signals increased by a factor of about 3-4, and as the pressure was further reduced the signals decreased. This behavior can be explained by pressure-dependent changes in the mass of material vaporized and the frequency of collisions between species in the plasma. Changes in the temperature and the electron density of the plasmas with pressure were also examined and detection limits for selected elements were determined. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Laser spark; Soil analysis; Space exploration.

Abstract: The oxidation of , , and has been studied at temperatures of 1300°–2000°K, and at oxygen partial pressures of 2–20 Torr. Below 1970°K, at oxygen pressures around 10 Torr, measurements of the rate of total oxygen consumption vs. time show an initial period of high linear rate, an intermediate region where the rate declines sharply with time, and a final plateau region of virtually zero oxidation rate. For each of the silicides, the total oxygen consumed prior to the attainment of the plateau decreases with temperature, and under given experimental conditions is largest for , less by a factor of about six for , and less by another factor of six for . The oxidized samples display a smooth glassy outer oxide layer that bridges across the tops of the cracks in the alloys and an inner oxidized zone, consisting of several irregular phases. Electron micro‐probe analysis and x‐ray photomicrographs of the oxidized specimens show that the outer protective oxide is pure , within the limits of detectability, i.e., the outer oxide contains less than 0.1% Mo. The inner oxide layer contains , molybdenum rich phases, including the terminal solid solution phase, and the original alloy phase. Evidence is presented that suggests that the oxide layers grow by inward diffusion of oxygen to the oxide/alloy interface where preferential oxidation of silicon occurs. The excellent oxidation resistance of the molybdenum silicides derives from the formation of this continuous layer of pure silica. Above 1970°K, at oxygen pressures above 10 Torr, an increased oxidation rate is observed due to formation of at the alloy/oxide interface, which causes rupture of the protective oxide. Below 1970°K, at oxygen pressures near 2 Torr, the rate of oxidation of is linear at all times since the oxygen partial pressure is insufficient to maintain a protective layer on the alloy surface.

Abstract: Reducing the diameter of the cathode hole in a plane anode - hollow cathode geometry to m has allowed us to generate direct current discharges in argon at atmospheric pressure. Up to pressure times cathode hole diameter (pD) values of approximately 5 Torr cm, and at sub-mA currents, glow discharges (predischarges) are observed with a shape which is determined by the vacuum electric field. In the same pD range, but at higher currents of up to approximately 4 mA, the discharges are of the hollow cathode discharge type. At pD values exceeding 5 Torr cm the predischarges turn into surface discharges along the mica spacer between the electrodes. At currents > 4 mA filamentary, pulsed discharges are observed. Qualitative information on the electron energy distribution in the microdischarges has been obtained by studying the VUV emission from ionized argon atoms and the argon excimer radiation at 130 nm. The results of the spectral measurements indicate the presence of a relatively large concentration of electrons with energies > 15 eV over the entire pressure range. The fact that the current - voltage characteristic of the microdischarges has a positive slope over much of the current range where excimer radiation is emitted indicates the possibility of forming arrays of these discharges and using them in flat panel excimer lamps.

Abstract: Various probe diagnostic methods have been applied to rf plasmas with non‐Maxwellian electron energy distribution functions (EEDF) and the results of these diagnostic methods have been compared. Plasma density and electron temperature were obtained using standard procedures from the electron retardation region (classic Langmuir method), the ion saturation region, and the electron saturation region of the measured probe I/V characteristic. Measurements were made in a 13.56‐MHz capacitive argon rf discharge at two gas pressures: p=0.03 Torr, where stochastic electron heating is dominant, and p=0.3 Torr, where collisional electron heating dominates. Thus, the measured EEDF at each gas pressure manifests a distinct departure from thermodynamic equilibrium being bi‐Maxwellian at 0.03 Torr and Druyvesteyn‐like at 0.3 Torr. Considerable differences in electron density and temperature were obtained from the different parts of the probe characteristic and these values differ dramatically in many cases from those f...