FEMTOSECOND NONLINEAR SINGLE-PHOTON PHOTOELECTRIC-EMISSION FROM TUNGSTEN AT 248 NM

The sensitivity of photoelectric emission of polycrystalline tungsten, produced by 248 nm laser pulses with 450 fs duration, has been measur ed. A nonlinear increase of photoemission efficiency, as a function of laser peak intensity, was observed, which confirms earlier observatio ns 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 multipho ton photoemission order k was measured and shown to depend only on the laser intensity absorbed by the tungsten surface.