Optical properties of an EMI K2CsSb bialkali photocathode

A measurement of the angular and polarisation dependence of the reflectance of an EMI 9124B K2CsSb bialkali photocathode, with the photomultiplier immersed in water, has allowed an unambiguous determination of the K2CsSb complex refractive index (2.7 ± 0.1 + i(1.5 ± 0.1) at 442 nm wavelength) and thickness (23 ± 2 nm). The wavelength dependence of the refractive index has been obtained from a re-analysis of “three intensities” data from Timan [H. Timan, Optical characteristics and constants of high efficiency photoemitters, Revue Technique Thomson-CSF 8 (1976) 49]. A detailed study of K2CsSb optical properties (absorptance, reflectance and transmittance) has been performed using the above results as reference points.