Skobeltsyn Institute of Nuclear Physics
Lomonosov Moscow State University

CsI photoelectron spectra induced by 40 fs laser pulses with intensity of 5×10^11 - 3×10^12 W/cm2 are
presented. High-energy region of photoelectron spectra is characterized by the plateau, the width of
which increases with excitation intensity. This plateau has exponential cut-off; e.g., at 3×10^12
W/cm2 the plateau is extended up to 27 eV, and 90%% of emitted electrons have energy above 3.5
eV. This feature could hardly be explained only in terms of either free-electron models, or models
of electron heating in crystals due to electron-photon-phonon scattering. An alternative model of
electron heating is proposed. The main feature of the proposed model is the account for direct inter-
branch electron transitions in conduction band of insulator under intense laser radiation.