Incubation: Subthreshold ablation of poly-(methyl methacrylate) and the nature of the decomposition pathways

A study of the decomposition of poly-(methyl methacrylate) (PMMA) by pulsed UV laser ablation in the incubation regime is presented here. We propose that incubation proceeds via the photoinduced formation of defects centers; while ablation following incubation, at subthreshold fluences, is a thermally driven phenomena. The light scattering data, in the incubation regime, is consistent with molecular weight reduction via backbone cleavage. The defect centers, C=C at chain ends following backbone cleavage, increase the UV absorption coefficient thereby lowering the ablation threshold. Similarities between the mass spectra of ablative PMMA following incubation and pyrolysis suggest that ablation following incubation is thermally driven. The implications of these results in an extensive and contradictory body of available data are discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)05018-0].