TIME EVOLUTION OF LASER-INDUCED POLYMER ABLATION STUDIED BY ATTENUATION OF A PROBE HENE LASER-BEAM

The temporal evolution of ablation at different fluences was studied i n the polymers polyethyleneterephthalate (PET) and polyimide using a K rF excimer laser at 248 nm and a TEA-CO2 laser at 9.2 mum. The novel m ethod applied measures the attenuation of a probe HeNe laser beam in r eal time by the ablation products ejected from the polymer. This techn ique is at least one order of magnitude faster than similar methods ba sed on acoustic and thermal deflection of a probe beam. The delay in t he onset of attenuation with respect to the start of the pump laser pu lse is found to vary from several hundred nanoseconds to about 10 ns w hen the laser energy deposited per unit volume increases from a few to more than 100 kJ/cm3. Only at high values of the absorption coefficie nt and fluence the measurements indicate fast particle emission within several nanoseconds. The strong fluence dependence of the delay time is consistent with a thermally activated ejection of attenuating parti cles from the polymer.