NONLINEARITY AND INVERSION OF FEMTOSECOND SINGLE-PHOTON AND 2-PHOTON PHOTOELECTRIC-EMISSION SENSITIVITIES FROM GOLD

We report measurements of the two-photon photoemission sensitivity of gold at 496 nm, for three incidence angles of a p-polarized laser beam, with a 450-fs pulse duration, produced by a KrF pumped dye laser. The nonlinear growing of the photoelectric efficiency versus the laser peak intensity, in the range of 5-9 GW/cm2, is demonstrated to be consecutive to the thermal nonequilibrium between the electrons and the lattice at the surface of the metal. We also observe with a solid that the single-photon photoelectric sensitivity, measured by us previously, can be lower than the one for the two-photon process, when the incident laser.intensity is high enough. The crossing point of respective sensitivities for 248- and 496-nm laser radiations is about 5.5x10(-6) A/W and corresponds to a laser intensity of about 6.5 GW/cm2. This effect, which can be only observed with ultrashort laser pulses, represents a significant step towards the realization of very high brightness electron beams.