The Use of the Field Emission Electron Microscope in Absorption Studies of W on W and Ba on W

W. MULLER, in 1936, described a tube in which the field emission electrons from a very sharp tungsten point were made to impinge on a fluorescent screen and there portray a magnified image of the variation in emission density from different regions on the point. He showed that magnifications approaching a million fold could be obtained. In subsequent papershe showed how such a tube can yield direct and striking information on the surface structure and on the effects of adsorbed films. Jenkins,3 in 1943, summarized the progress to that date and showed that fields of the order of 1()7 volts/cm produced pronounced changes in the surface configuration. More recently F. Ashworth4 has reviewed the field emission from clean metallic surfaces. In Fig. 2, (a) and (b) are two examples of photographs of the screen when field emission electrons are drawn from a single crystal of tungsten. The bright and dark regions are caused by variations in the intensity of electron emission from different regions of the tungsten surface. From such photographs it is possible to deduce how the electron work function varies for different crystallographic planes, how adsorbed atoms change this work function, and how the surface deviates from a smooth hemisphere when the tungsten is subjected to a range of temperatures and fields. It is quite apparent that this new and powerful tool will reveal, on an 907 908 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1 9 5 1 almost atomic scale, the nature of adsorption phenomena which are basic to thermionic, photoelectric and secondary electron emission, to catalysis and also to biological processes.