Mechanisms of ablation-rate decrease in multiple-pulse laser ablation

We present two sets of experimental results on the ablation-rate decrease w ith increase of the number of consecutive laser pulses hitting the same spo t on the target surface. We have studied laser ablation of a carbon target with nanosecond pulses in two different interaction regimes: one with a XeC l laser (lambda = 308 mn) and the other with a Nd:YAG laser (lambda = 1064 nm), in both cases at the intensity similar to 5 x 10(8) W/cm(2) Two differ ent mechanisms were found to be responsible for the ablation-rate decrease; they are directly related to the two different laser-matter interaction re gimes. The UV-laser interaction is in the regime of transparent vapour (sur face absorption). The increase of the neutral vapour density in the crater produced by the preceding laser pulses is the main reason for the decrease of ablation rate. With the IR laser each single laser pulse interacts with a partially ionised plume. With increase of the number of pulses hitting th e same spot on the target surface, the laser-matter interaction regime grad ually changes from the near-surface absorption to the volume absorption, re sulting in the decrease in absorption in the target and thus in the decreas e in the ablation rate. The change in the evaporation rate was considered f or both vacuum and reactive-gas environments.