Origin of the high-energy electron emission from metals under laser irradiation

We interpret recent experimental results concerning high-energy-electron production when an aluminum surface is irradiated by laser light in the visible range of 35-ps pulse duration in a moderate (${10}^6$ to 2×${10}^8$ W ${\mathrm{cm}}^{\mathrm{-}2}$) intensity range. We show that two-photon absorption governs the process of photoemission. High electron energies measured in such experiments are shown to be an effect due to the transport of a cloud of electrons. Its physical origin is the work of the Coulomb field on the center-of-mass trajectory of the electron cloud, depending on the external fields needed to cancel the space-charge effects.