XPS INVESTIGATIONS ON Cs2Te PHOTOCATHODES OF FLASH AND PITZ

Cesium telluride (Cs2Te) photocathodes are used as sources for electron beams because of their high quantum efficiency (QE) and their ability to release high peak cur-rent electron bunches in a high gradient RF-gun. A rapid unexpected decrease of the initial QE, from 10 % to values below 0.5 % in only a few weeks of operation, was ob-served end 2006/beginning 2007. In 2007 a peak of Fluo-rine possibly originating from Teflon was identified in XPS measurements. After identification and removal of this spe-cific contaminant, the lifetime of the cathodes increased to several months. In addition in 2008 we have investigated the response of fully functional photocathodes to extensive usage, bad vacuum conditions, and oxidation by means of XPS measurements. The experiments- carried out at the ISISS and the PM3 beam lines at the synchrotron facility BESSY II of the Helmholtz Zentrum Berlin HZB- com-pare the chemical composition and electronic structure of freshly prepared, contaminated, used, and oxidised Cs 2Te cathodes