ULTRAFAST PHOTOELECTRIC-EMISSION FROM NEW SOLID MATERIALS

We show that alkali ion implantation is a new valuable tool to improve significantly the photoelectric yield of pure metals. The enhancement by a factor greater than or equal to 10 of the single-photon photoele ctric sensitivity was measured in potassium ion implanted-polycrystall ine-tungsten surface irradiated by subpicosecond 248 nm laser pulses w ith a peak intensity of a few GW/cm(2). An investigation of physical a nd chemical properties supposed to be at the origin of this enhancemen t is reported. We demonstrate that the improvement of the tungsten pho toelectric sensitivity is related to an effective lowering of the work function at the surface, while the nonlinear photoelectric performanc es of these photocathodes indicate a change in electron-electron and e lectron-phonon relaxation times due to alkali ions implantation.