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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Time-resolved single-photon photoelectric emission measurements from polycrystalline copper, using an autocorrelation system of 450 fs duration at 248 nm laser pulses, are reported. This experimental technique makes possible the direct measurement of the electron energy relaxation time at the surface as well as the evaluation of the electron - phonon scattering time in metallic samples. A nonlinear increase of the electron photocurrent versus the laser intensity is observed. This nonlinearity is attributed to the non-thermal equilibrium between the electron gas and the lattice produced by the subpicosecond laser pulses and is related to the dynamics of energy relaxation between the electrons and the lattice. The influence of the non-thermal electrons on the photoemission and their thermalization time is discussed. Our experimental results may be explained by a model based on the Fowler - Dubridge equation for photocurrent and on thermal-balance equations between electron and lattice subsystems.