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.

Violation of the electric-dipole selection rules in indirect multiphoton excitation of image-potential states on Ag(100).

Photoemission from image potential states on Ag(100) is investigated using angle resolved multiphoton photoemission induced by 150 fs laser pulses. For the first time we demonstrate that image potential states populated by indirect transitions can be observed with light polarized parallel to the plane of incidence and light polarized normal to the plane of incidence. The latter is a process normally forbidden by the dipole transition selection rules. These findings are related to the creation of a hot-electron population whose properties largely remains to be understood.