MODEL FOR THE UV LASER ABLATION OF POLYMERS IN THE LOW-FLUENCE LIMIT

The mass density spectrum and its dependence on the fluence is calculated for the first shot at a very low fluence (far below the single shot ablation threshold). At a given mass of fragment the spectrum is a linear combination of power functions of the fluence. For repeated shots a random degradation of the upper monolayers develops which can be identified as a percolation phenomenon. Close to the percolation threshold (that is, close to the critical number of shots) the correlation length of "cracks" becomes macroscopic. This critical phenomenon leads to a catastrophe-like behaviour at a sufficient number of shots. This picture is in accordance with the so-called incubation effect observed in several experiments.