Multiwavelengths optical diagnostic during Cs2Te photocathodes deposition

The production of Cs2Te photoemissive films used as laser driven electron sources in the high brightness photoinjectors at FLASH and PITZ, is a well established activity at INFN Milano since the '90s. Our total production is of more than 100 photocathodes, with an average QE of 8.6 % (λ = 254 nm) for fresh films and an operative lifetime that nowadays is of some months at FLASH. In the last two years, we have improved the standard diagnostic used during the cathode growth to better understand the photoemissive film properties. This activity is motivated by the need to improve the photocathode performance, mainly the energy distribution of the photoemitted electrons that influences the thermal emittance. The multiwavelengths diagnostic, i.e. the online measurements of the photocurrent and reflectivity from the film during its growth in the 239 nm - 436 nm range, has been applied on several cathodes and the potentiality of this technique are discussed in this paper.