Interaction of Cesium-Potassium Antimonide Photocathode Materials with Oxygen: an X-Ray Photoelectron Spectroscopy Study

The interaction of oxygen with photocathode materials based on alkali antimonide at room temperature has been studied by means of X-ray photoelectron spectroscopy (XPS). In the first step, the antimonide is oxidized, forming alkali suboxide, (Cs 11 O 3 ) and elemental Sb (Sb 0 ) is segregated. Further oxidation (>20 L) causes the formation of an alkali oxide overlayer (i.e., Cs 2 O) and the oxidation of the elemental Sb to form Sb 5+ species. The kinetics of oxidation has been found to be much faster than the logarithmic rate due to strong diffusion of alkali ions toward the surface. The in-depth distribution of alkali oxides for exposures larger than 20 L is the opposite of that which produces a good photoemissive surface (Cs 11 O 3 /semiconductor). However, slight oxidation (<20 L) improves the spectral response of the photocathode. A Cs suboxide overlayer (one-cluster size) has been proposed as the optimum activation surface.