Quantitative XPS analysis of the chemical composition of S1(Ag-O-Cs) photocathodes and its relation to the photosensitivity

The chemical composition of a large number of transmission mode S1 photocathodes, prepared by a standard process in situ in an ESCA spectrometer has been examined using XPS. In particular the stoichiometry of the caesium oxide layer has been determined at the different stages of cathode formation. The method involves the measurement of XPS peak areas and the use of Schofield's values for the sub-shell photoionisation cross-sections. The silver content of the cathode oxide occurs. Thus the XPS peak area measurements and not the oxygen 1s peak position must be used to determine the stoichiometry of the caesium oxide. The cathodes with the highest photosensitivity and greatest stability were found to have a caesium-to-oxygen ratio very close to 2:1. The results are therefore not consistent with suggestions that the caesium suboxide Cs II O 3 is an important constituent of the S1 photocathode. In support of the XPS results the authors measured the photoresponse at different wavelengths from 555 nm to 1.1 mu m and photoelectron energy distribution curves from which the work-functions of the cathodes are obtained.