Multiphoton Processes at the (111) Surface of BaF₂Crystals

The subject of this contribution is multiphoton interaction of laser light in the blue and green spectral ranges with cleaved and polished (111) surfaces of BaF2 in ultrahigh vacuum and air. We claim that photoemission and laser-induced non-thermal desorption of positive ions are triggered by nonlinear absorption of some order between two and five. Photoemission and ion desorption yields are strongly wavelength dependent and display conspicuous resonance structures, with striking similarities between electron and ion yield spectra. Cluster calculations are performed in an attempt to understand the origin of these resonances. They predict the existence of occupied surface states near the middle of the bandgap together with a high density of resonances in the upper half of the bandgap. Measurements of the surface second harmonic generation in air showed a similar wavelength dependence as the electron yield in vacuum. The transition from pure second harmonic generation to plasma discharge was studied and evidence is presented that the discharge is ignited by resonant multiphoton absorption in adsorbed gas molecules.