DEPENDENCE OF PHOTOEMISSION EFFICIENCY ON THE PULSED-LASER CLEANING OF TUNGSTEN PHOTOCATHODES .2. THEORY

The efficiency of picosecond laser cleaning of pure tungsten photocato des with a Nd3+:YAG mode-locked laser with 5(th) harmonic generator, i n high vacuum (a few 10(-8) Torr), at wavelength 213 Mm, has been prev iously reported in an experimental study (Part 1). The way of probing surface decontamination consisted of following the increase of the pho toelectric signal versus time. Parameters as the laser fluence and pre ssure of residual gas appeared to be the most important for cleaning p rocess. A model is built which takes these parameters into account, in order to describe pulsed laser action. This model is based on three p henomena occuring consecutively at the photocathode surface: (i) the i nitial contamination, (ii) the decontamination of the surface produced by each laser pulse, and (iii) the recontamination due to the residua l gas in the vacuum cell. Parameters concerning the laser cleaning of photocathodes are involved: the number of contaminent monolayers and t he work function, respectively before and after the cleaning, The appl ication of the model to experimental data reported in Part 1 allows to estimate orders of magnitude of these quantities.