10. Optical method for investigating alkali antimonide photocathodes

Abstract

Some physical aspects of S-type photocathodes have been studied for many years in different laboratories. Among these investigations, optical methods are of great importance. As a matter of fact, an optical measurement can be used for investigating alkali antimonide photocathode deposition inside an actual tube. For instance, optical reflectance and/or transmittance of the layer measure during processing can be performed. It is shown that the optical reflectance gives information about several physical parameters of the photocathode under growth: o (i) the photocathode thickness can be continuously measured and, therefore the growth speed of the layer controlled; (ii) simultaneously, some composition information may be obtained. The mixture of two phases such as SbNa 3 and SbNa 2 K can be analysed as well as the deposition of pure Sb or SbNa 3 layers on the top of an SbNa 2 K layer. Moreover, chemical displacement between two alkalis inside the compound itself can be observed in this way. Whatever the kind of alkali antimonide photocathodes considered, it will be shown that the optical reflectance and/or transmittance are very worthwhile tools to control the growth and its reproducibility, as complementary measurements to photoemissivity generally used to monitor the growth of S-type photocathodes.