Knudsen oven

A Knudsen oven and quartz crystal thickness monitor have been attached to a HPLP system, enabling accurate, reproducible dosing of submonolayer quantities of gold under the ultraclean conditions of the UHV chamber. Results indicate that atoms of gold inhibit the catalytic activity of the platinum in heptane isomerization. However, annealing to form a 2-D alloy (and the consequent separation of surface platinum into small ensembles), actually led to an increase in isomerization activity over the dean platinum value. 

More ambitious attempts at measuring the heterogeneity of the atmospheric aerosol have been undertaken as well. Single-particle analysis by mass spectrometry was demonstrated by Sinha and co-workers (31, 32). In this technique, an aerosol sample is introduced into a vacuum chamber in the form of a particle beam. The particles are injected into a Knudsen cell oven, where they undergo many collisions with the cell wall and are ultimately vaporized and ionized. The ions are then mass-analyzed with a quad-rupole or sector mass spectrometer. So that individual particles can be analyzed, the flux of particles into the Knudsen cell is limited so that coincidence errors are minimized. Ion pulses from individual particles allow the determination of the amount of the species being analyzed in the particular particle. The sensitivity of the technique is limited. For sodium, the detection... 

Molecular weight from vapor pressures Molecular weight from P-V-T measurements Double oven effusion with mass spectrometer Knudsen effusion with mass spectrometer Knudsen effusion with mass spectrometer Velocity distribution analysis Velocity distribution analysis... 

A critical point in the work presented in this communication was the generation of a steady stream of silicon atoms which have to be condensed together with the substrate molecule and an excess of matrix material onto the cooled window. In our early experiments, silicon was vaporized from a tantalum Knudsen cell (Figure 1.2, Type A) or a boron nitride crucible which was surrounded by an aluminum oxide tube. The oven was resistively heated to temperatures of 1490-1550 °C by means of a tungsten wire wound around the alumina tube (Figure 1.2, Type B). In later runs, a rod of dimensions 0.7-2-22... 

In this method the sample is vaporized in a micro-oven placed in the ion source or out of a Knudsen-cell. Many metals can be analyzed qualitatively and quantitatively by this technique as metal organic compounds (see Refs. ). The metal chelates have lower volatilities than the metals and in many cases the mass spectra reveal higher sensitivities for these compounds compared with the analysis using direct evaporation of the metal. The latter technique of direct metal analysis by EIMS is only applied if the ionization energies of the metals are too high for thermal ionization mass spectrometry . 

ESA leads to an obstruction, for example, the ovens, Knudsen cells, etc. needed for deposition of thin films prevent a direct view of the substrate. This is one reason why RHEED is in rather widespread use in thin-film deposition systems—the electron gun and screen are remote and grazing incidence does not interfere with film deposition. Therefore, in many cases RHEED is a real in situ technique. By comparison, the conventional front view LEED system blocks essentially the full space and the rear view system blocks half of the space available. In this respect MEIS and RBS are also remote systems that do not take much space around the substrate or target. In some systems transport is installed as a solution (it is the only solution when adding STM to the tool box). 

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