Characterisation of surface processes on metals under pulsed picosecond laser irradiation by photoelectric work function measurements

Picosecond pulsed laser irradiation of several metallic photocathodes (W, Ta, Al and Au) was carried out along with work function measurements during the irradiation process, work function measurements were performed before irradiation and after different irradiation periods. Metals used in this study behaved differently according to their af®nity towards oxygen. Most of these metal surfaces were covered with a native oxide layer, except gold. Photolythic or thermal processes could occur depending on the laser wavelength, in the former case, the native oxide layer was removed pulse by pulse while in the latter, surface oxidation was enhanced. For the tungsten and tantalum samples, the kinetics of oxide removal occurred in distinct stages relative to the native oxide multilayer structure. In the case of the Au sample, no native oxide layer formed on its surface (noble metal) and the laser irradiation induced a surface smoothing only. A linear increase of the photoelectric signal was observed during laser irradiation of the aluminium sample probably due to the partial surface oxide removal and the subsurface oxide incorporation. The presence of various solid phases of the oxide layer could be determined from pulsed laser irradiation measurements. # 2001 Elsevier Science B.V. All rights reserved