Optical Properties of Cesium Telluride

Cesium telluride is the photoemissive material used in many laser-driven RF-guns like in TTF-FEL. The material has been already used since few years but a detailed study of its optical properties is still missing. In this paper, we present the measurements of the cesium telluride reflectivity versus wavelength and incidence angle. These measurements are used to calculate the real and complex part of the refractive index and to have a better understanding of the energy band structure of the cesium telluride. The knowledge of these properties is relevant in view of the measurement of the thermal emittance of this material that is in progress in our lab using a Time of Flight spectrometer. The thermal emittance is the key parameter for choosing the photocathode for the future high brightness machines.