We report the first measurements of two-photon photoelectric emission sensitivity of tungsten at 496 nm, using a p-polarized laser beam of 450 fs pulse duration under incidence angle theta =83 degrees . The beam was produced by an XeCl excimer pumped dye laser, with energy up to 45 mu J. As for gold, nonlinear growth of photoelectric efficiency versus incident laser intensity is observed in the range 12-18 GW cm-2. Inversion of single- and two-photon sensitivities is also clearly demonstrated for laser peak intensity higher than 15 GW cm-2. Such effects are typical of subpicosecond metal behaviour and improve significantly the photoelectric performances of metallic photocathodes.

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The sensitivity of photoelectric emission of polycrystalline tungsten, produced by 248 nm laser pulses with 450 fs duration, has been measured. A nonlinear increase of photoemission efficiency, as a function of laser peak intensity, was observed, which confirms earlier observations with gold. This nonlinear behaviour is a direct consequence of non-equilibrium between electron gas and lattice temperatures produced at the surface by the sub-picosecond laser pulses. The nonlinear multiphoton photoemission order k was measured and shown to depend only on the laser intensity absorbed by the tungsten surface.