SCHLIEREN MEASUREMENTS OF THE HYDRODYNAMICS OF EXCIMER LASER ABLATION OF POLYMERS IN ATMOSPHERIC-PRESSURE GAS

Pulsed schlieren photography and fast helium-neon laser deflection are used to study the hydrodynamics of laser ablation of polyethyleneterephthalate and polymethylmethacrylate by pulsed KrF (248 nm) radiation in atmospheric air, Ar and N2. Schlieren measurements show the evolution of shock waves, sound waves, and reduced-density, hot gas plumes. A transition from sound to shock at the ablation threshold for both polymers is observed. The shock velocity of PET tends to approach agreement with blast wave theory at fluences higher than 1 J/cm2. Plumes in air are consistently larger than those produced in Ar and N2 (at fluences below 5 J/cm2) suggesting that combustion may occur. Laser deflection measurements for PET at 150 mJ/cm2 indicate a plume density of 0.6 kg/m3 (50% atmospheric density).