LASER-STIMULATED DESORPTION FROM CAF2 CRYSTALS

Optical transmission and laser-induced desorption (LID) of neutral par ticles and positive ions from optical-grade CaF2 has been studied, usi ng nanosecond laser pulses of 532 nm. Two different emission character istics were found: one for which the breakdown of transmission coincid es with the onset of ablation; and another for which the transmission breakdown occurs at about 20-30% lower fluences compared to the ablati on threshold. In the latter case the transmission breakdown is accompa nied by delayed (tenths of seconds) particle emission. We propose that the particle emission below the ablation threshold is defect-assisted LID, in the sense that light absorption by preexisting defects leads to local heating and thermal expansion, causing microcracks from which particles are ejected.