ELECTRON RELAXATION PHENOMENA ON A COPPER SURFACE VIA NONLINEAR ULTRASHORT SINGLE-PHOTON PHOTOELECTRIC-EMISSION

Time-resolved single-photon photoelectric emission measurements from p olycrystalline copper, using an autocorrelation system of 450 fs durat ion at 248 nm laser pulses, are reported. This experimental technique makes possible the direct measurement of the electron energy relaxatio n time at the surface as well as the evaluation of the electron-phonon scattering time in metallic samples. A nonlinear increase of the elec tron photocurrent versus the laser intensity is observed. This nonline arity is attributed to the non-thermal equilibrium between the electro n gas and the lattice produced by the subpicosecond laser pulses and i s related to the dynamics of energy relaxation between the electrons a nd the lattice. The influence of the non-thermal electrons on the phot oemission and their thermalization time is discussed. Our experimental results may be explained by a model based on the Fowler-Dubridge equa tion for photocurrent and on thermal-balance equations between electro n and lattice subsystems.