Ultrahigh vacuum, ionization

Hot-Cathode Ionization Gauges. For pressures below approximately lO " Pa, it is not possible, except under carehiUy controlled conditions, to detect the minute forces that result from the coUision of gas molecules with a soHd wall. The operation of the ion gauge is based on ionisa tion of gas molecules as a result of coUisions with electrons. These ions are then subsequendy collected by an ion collector. Ionisa tion gauges, used almost exclusively for pressure measurement in high, very high, ultrahigh, and extreme ultrahigh vacuums, measure molecular density or particle dux, not pressure itself. 

In addition to the conventional ionization gauge, whose electrode structure resembles that of a common triode, there are various ionization vacuum gauge systems (Bayard-Alpert system, Bayard-Alpert system with modulator, extractor system) which more or less suppress the two effects, depending on the design, and are therefore used for measurement in the high and ultrahigh vacuum range. Today the Bayard-Alpert system is usually the standard system. 

Figure 10 Thermal positronium-laser beam interaction region. Positronium is formed by a bunch of positrons that is stopped by a clean A1 surface in ultrahigh vacuum. Positronium atoms thermally desorbed from the surface are ionized by the laser and the e fragments are collected by a single particle detector. The laser pulse is narrowed in frequency by the Fabry-Perot interferometer.

Almost all pressure gauges customary in vacuum work can be incorporated in an ultrahigh vacuum system, provided they have been adapted to withstand the rigors of baking. Although they may lack the versatility and simplicity of the ionization gauge they are important for specialized measurements. Commercially available types are therefore briefly listed. 

We describe below the rotating-beam-source-photofragmentation apparatus [70] of the Wilson design used in our laboratory (see Fig. 1). The apparatus can be divided into three main components an excimer excitation laser, a photodissociation chamber in which a rotatable supersonic molecular beam intersects the laser beam, and a linearly movable, ultrahigh vacuum-electron ionization mass spectrometer detector. 

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