Observation of high energy photoelectrons from solids at moderate laser intensity

We investigate the photoemission for a set of wide band-gap crystals irradiated by femtosecond Ti-Sapphire laser pulses at intensities varying from 0.5 to 6 TW/cm(2) (below the optical breakdown threshold). The measured total electron yield increases linearly with the laser intensity in this intensity range. An intense and wide plateau of high energy electrons appears in the photoelectron spectra at excitation intensities larger than 1 TW/cm(2). The exponential cut-off of this plateau reaches 40 eV at maximal applied intensities. In order to explain such a behavior, we propose a mechanism where the heating is due to a sequence of direct interbranch one- and multi-photon transitions in the conduction band.