Electron heating in the crystal conduction band under high-power femtosecond laser pulses has been studied. It is demonstrated that the electron heating mechanism in the insulator conduction band based on multiple electron—photon— phonon scatterings may become ineffective in exciting some insulators (in particular, Csl) by ultrashort laser pulses with intensity higher than $10^{11}$ W/cm$^2$. To describe electron heating by these pulses to an energy of several tens of electron-volts, a mechanism that accounts for direct interbranch transitions in the conduction band is proposed.
$^1$Centre Lasers Intenses et Applications, CELIA, Universite de Bordeaux I et CNRS 33405 Talence, France
$^2$Department of Optics and Spectroscopy, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
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