Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

186 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSFig. 1. Circuit diagram for electron and ion bombardment. Position A, filament only; B, filamentand grid; C, electron bombardment (sample positive); D, ion bombardment (sample negative).2.1.2 High vacuum techniqueConventionally, the term “high vacuum” is applied to pressures aroundtorr, and the term “ultra high vacuum” to pressures around lo-’’ torr. In fact thereis no great difference between the techniques required for producing “high vacuum”and “ultra high vacuum”, but the requirements become more stringent at thelower pressures. Prior to 1948, the lower limit of pressure observed in the conventionalion gauge was about low8 torr. However, it was found that this reading wascaused by a residual photoelectric current and was not a true ion current proportionalto the pressure7’. A eonsideration of this problem led Bayard and Al~ert~~to develop the ‘inverted’ ionisation gauge which is capable of measuring pressuresdown to lo-” torr. From this point, the development of ultra high vacuumtechnique was very rapid. The appreciation of these experimental techniques is, inmany instances, a necessary adjunct to the kinetic study of adsorption and catalysis.However, the subject matter is so extensive that it cannot be adequately reviewedin this chapter, and it is therefore necessary to provide reference to the manyexcellent publications on this subje~t~~-~~**~.For most studies of adsorption kinetics it is necessary to employ vacuum techniques.The question of whether high vacuum or ultra high vacuum conditions arerequired depends on the gas, the surface being studied and the time available to