ABLATION OF POLYMETHYL METHACRYLATE FILMS BY PULSED (NS) ULTRAVIOLET AND INFRARED (9.17-MU-M) LASERS - A COMPARATIVE-STUDY BY ULTRAFAST IMAGING

Ablation of the surface of a polymethyl methacrylate film by single pulses of 193 or 248 nm laser radiation (approximately 20 ns) or 9.17 mum laser radiation (approximately 170 ns) was studied by photographing the emergence of the plume by a pulse (< 1 ns; 596 nm) of visible laser light. The dynamics of the blast wave was similar in the ultraviolet (UV) and in the infrared (IR) but the composition of the product mixture was entirely different. Essentially the only product that IR laser pulses gave was a volatile refractive vapor which is presumed to be the monomer, methyl methacrylate (MMA). UV (248 nm) laser pulses gave gaseous products with very little MMA and a large fraction of solid polymer as has been documented in the literature. Examination of the solid material in the plume at high magnification showed that it was structured in the early phase (< 3 mus) of its emergence from the surface but at longer times melted and formed liquid drops in the region above the surface of the polymer. UV (193 nm) laser pulses yielded gaseous products which included MMA. The solid material in the plume did not show any sign of melting. Gas production at the surface as well as in the first few micrometers below the surface appears to be an important factor in the dynamics of the plume in UV ablation. The relevance of photochemical and photothermal mechanisms in laser ablation is discussed in the light of the chemistry of the products seen in this study.