LASER-STIMULATED DESORPTION AND DAMAGE AT POLISHED CAF2 SURFACES IRRADIATED WITH 532-NM LASER-LIGHT

Laser-stimulated emission of Ca neutrals from polished CaF2 (111)-surfaces was studied with a quadrupole mass filter system under UHV conditions. For intensity dependent measurements performed at various spots of the crystal surface a threshold pulse energy could be determined where a sharp decrease in transmitted light intensity occurred. Only for a few measurements this threshold coincided with the ablation threshold (type 1 spots). In most cases, however, the transmission breakdown threshold was found to be distinctly lower than the ablation threshold (type 2 spots). For type 2 spots a weak emission of delayed calcium atoms was observed at the transmission breakdown threshold. Irradiated spots have been examined by optical and scanning electron microscopy. Damage phenomena observed included coloration, cracking along crystal axes, surface erosion and surface melting depending on incident laser intensity. Pronounced differences between type 1 and type 2 spots with respect to laser light entrance and exit surface damage morphology were found.