EFFECTS OF THE FLUENCE OF IMPLANTED POTASSIUM-IONS ON THE PHOTOELECTRIC SENSITIVITY OF TUNGSTEN PHOTOCATHODES

Measurements of single-photon photoelectric emission sensitivity of po lycrystalline tungsten surface implanted by potassium ions, with incid ent fluences 1, 2, 3, 4, and 5 x 10(17) K cm(-2), are reported and com pared to the data for pure tungsten under the same experimental condit ions. The photoelectric performance of these photocathodes has been me asured at 213 nm for different incidence angles of p-polarized and 16 ps pulses produced by harmonic generation from a Nd:YAG mode-locked la ser. Significant improvements of the photoelectric efficiency of tungs ten implanted with alkali ions have been demonstrated, as well as a go od stability of performance during more than a full month irradiation. The enhancement depends both on the effective density of potassium at oms and on the microstructure of the implanted layer. At last, the enh ancements of the photoelectric yield are principally related to a shif t of the work function (WF) at the surface of implanted tungsten, obse rvations supported by a recent model of which the main features are re called. (C) 1998 Published by Elsevier Science B.V.