We measure the kinetic energy of photoelectrons emitted from an α-quartz surface irradiated by short (100 fs) and intense laser pulses. In the high intensity regime (above 100 GW/cm²), we observe a high-energy tail in the photoemission spectra at 620 and 560 nm. This effect disappears for the doubled frequencies. These observations give direct evidence of free-electron multiple photon absorption in the conduction band, in keeping with recent Monte Carlo simulation results.
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Daguzan et al. (1994) studied this question.
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