Experimental evidence of above-threshold photoemission in solids
  Nonlinear photoemission from a silver single crystal is investigated by femtosecond laser pulses in a perturbative regime. A clear observation of above-threshold photoemission in solids is reported for the first time. The ratio between the three-photon above-threshold and the two-photon Fermi edges is found to be 10^{-4}. This value constitutes the only available benchmark for theories aimed at understanding the mechanism responsible for above-threshold photoemission in solids. 
Francesco Banfi, Claudio Giannetti, Gabriele Ferrini, Gianluca   Galimberti, Stefania Pagliara, Daniele Fausti, and Fulvio P...
cond-mat.mtrl-sci