Windowless system

Lithium fluoride is transparent down to about 105 nm, below which windowless systems must be used with differential pumping. 

Below the LiF transmission limit (1050 A.), differential pumping in windowless systems has been employed.7 This technique has produced an intensity of 3 X 10u quanta/sec. at 584 A. 

A very interesting technique has been used by Walker and Back124 in which the photolyses of methane, ethylene, and ethane have been carried out in a windowless system at 584 A., the helium resonance line. Since this is a study which has crossed the line into the realm of radiation chemistry, no discussion of the work will be presented. It does, however, represent a radiation chemical study using monochromatic ionizing radiation and it deserves, therefore, the attention of interested researchers. 

Figure 4. Windowless system for resonance spectrometry of F Pj atoms. B, buffer chamber C, collimated hole structures, two of which were mounted on specially fabricated glass discs, D K, inlet to lamp L M, M , differential manometers R, section of flow tube S, spectrometer slit sealed via an O-ring seal to the silica apparatus V, vacuum spectrometer.

Information exists about the use of measuring cells made entirely of diamond or graphite with or without embedded diamond windows. Diamond cells were used, for instance, by Toth and Gilpatrick [333] in the investigation of the Nb(IV) spectrum in a LiF - BeF2 molten system at 550°C. Windowless graphite cells for the IR spectroscopy of melts were developed by Veneraky, Khlebnikov and Deshko [334]. Diamond, and in some cases windowless sapphire or graphite micro-cells, were also applied for Raman spectroscopy measurements of molten fluorides. 

A photoionization system usually consists of a windowless, differentially pumped rare gas resonance lamp coupled with the ionization chamber of the mass spectrometer. Argon, krypton, or other inert gases are used in the lamp. Energies of 11.6 and 11.8 (Ar I) eV are produced by argon, and 10.0 and 10.6 (Kr I) eV for krypton. The pressure inside the ion source is usually about 10 Torr. 

McCarroU and Thomson have described the use of a solid-state windowless device for the direct monitoring of surfaces under UHV conditions. The detection system had excellent vacuum properties, a high detection efficiency for [14-C] 3 particles, and good background characteristics. However, since the detector was light sensitive, the apparatus had to be operated in total darkness. 

As a general rule, the electron energies and intensities measured in ESCA are both relatively low because of the various factors discussed above. The low electron-energies dictate the use of windowless detectors and the low intensities dictate the use of pulse-counting techniques most of the available ESCA instruments employ both. The low counting-rates also make automated data-acquisition and analysis attractive thus, many commercial instruments offer on-line computers as part of the entire ESCA system. 

Recent developments, such as the windowless EDX detector, have allowed the light element range to be extended down to C. Automated WDX spectrometers with computer control of the operating parameters are now available, such as the Microspec WDX-2A system. This considerably simplifies WDX analysis. The ideal system, however, requires both an EDX and WDX system mounted on the microscope simultaneously. This would permit the rapid determination of the elements present with the EDX system, and a detailed analysis of these elements using the WDX spectrometer. Combined EDX/WDX systems have already been developed where components of the hardware are shared, such as a computer to perform corrections on the measured data for atomic number, absorption, and fluorescence effects. These corrections are necessary when performing quantitative analysis. 

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