TIME-RESOLVED STUDY OF ARF-EXCIMER-LASER ABLATION PROCESSES OF POLYMETHYLMETHACRYLATE

The processes of the ArF-excimer-laser ablation of PolyMethylMethAcrylate (PMMA) has been investigated. We studied the surface reflection of the polymer sample during UV photoablation. It was observed that a dark spot appeared in the reflected probe nitrogen laser beam (the reflection decreased below 4%) with about 5 ns delay after the onset of the ablating excimer pulse, while the transmitted probe laser intensity was increased to about 104%. A fast stroboscopic photographic arrangement was built for investigation of the ejection. A protuberance developed on the irradiated surface at about 100 ns delay. This expanded with about 600 m/s average velocity until 4 mu s, then contracted slowly, and finally at 16-18 mu s delay the ejection started. We also examined the process of the UV photoablation of PMMA in the vacuum chamber. In this case, the protuberance also appeared, but it left the surface nearly in one block at 3 mu s delay. Using a double-flash exposure photographic arrangement based on a dye laser, the propagation velocity of the shock waves developed in the air above the surface of the PMMA sample as a function of time was measured, and good agreement with shock wave theory was found. The pressure in the shock front can be as high as 1939 bar.