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. T a, Al and Au) was carried out along with work function measurements during the irradiation process, work function measurements were performed before i rradiation and after different irradiation periods. Metals used in this stu dy behaved differently according to their affinity towards oxygen. Most of these metal surfaces were covered with a native oxide layer, except gold. P hotolythic or thermal processes could occur depending on the laser waveleng th, in the former case, the native oxide layer was removed pulse by pulse w hile in the latter, surface oxidation was enhanced. For the tungsten and ta ntalum samples, the kinetics of oxide removal occurred in distinct stages r elative to the native oxide multilayer structure, in the cast: of the Au sa mple, no native oxide layer formed on its surface (noble metal) and the las er irradiation induced a surface smoothing only. A linear increase of the p hotoelectric 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 lay er could be determined from pulsed laser irradiation measurements. (C) 2001 Elsevier Science B.V. All rights reserved.