LASER-STIMULATED DESORPTION FROM CAF2 CRYSTALS

Optical transmission and laser-induced desorption (LID) of neutral particles and positive ions from optical-grade CaF2 has been studied, using nanosecond laser pulses of 532 nm. Two different emission characteristics were found: one for which the breakdown of transmission coincides with the onset of ablation; and another for which the transmission breakdown occurs at about 20-30% lower fluences compared to the ablation threshold. In the latter case the transmission breakdown is accompanied 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.