MULTIPHOTON PHOTOEMISSION FROM LASER-IRRADIATED ALPHA-SIO2

Multiphoton photoemission of laser irradiated alpha-SiO2 is studied as a function of different laser parameters. The laser is a frequency-do ubled or -tripled Nd-YAG laser, operated in the picosecond pulse durat ion regime. The sample is a single crystal, cut perpendicularly to the C-axis direction, kept under ultra-high vacuum (UHV) conditions. The intensity dependence of the photocurrent reveals that emission at the second harmonic frequency is throughout the whole accessible intensity range governed by defect states, whereas at the third harmonic freque ncy we observe a transition from a two- to three-photon dynamics, whic h could reveal the onset of the valence band emission. However, the mo st probable explanation is still in both cases a two-photon, defect-dr iven, process, eventually followed by electron heating in the conducti on band through electron-photon-phonon collisions, or by photon absorp tion by electrons trapped in the bottom of the conduction band. The de pendence of the photocurrent from the laser polarization direction is shown to differ depending on whether all the emitted electrons are col lected or only the ones emitted along the C-axis. In this last case, a selection rule seems to apply to the emission process that yields inf ormation about the symmetry of the emitting state.