Product Formation in the Laser Irradiation of Doped Poly(methyl methacrylate) at 248 nm:  Implications for Chemical Effects in UV Ablation

This paper examines the modifications that are induced to photolabile haloaromatic compounds (bromo- and iodonaphthalene, iodophenanthrene, and bromoanthracene) dispersed within poly(methyl methacrylate) (PMMA) films upon irradiation with intense nanosecond laser pulses at 248 nm. To this end, laser-induced fluorescence is employed for the characterization and quantification of the emitting aryl products that remain in the substrate following irradiation. Product formation is examined as a function of laser fluence over 50−2500 mJ cm^-2. Employed dopant concentrations are ≤2 wt %, corresponding to absorption coefficients ≤1000 cm^-1. At low fluences, ArH-type products are formed via one-photon photolysis of the dopants. However, close to the swelling onset, product formation increases sharply. In parallel, at least in the NapX-doped systems, formation of biaryl species (Nap₂ and perylene) is observed. It is argued that these changes do not result from differences in the excitation/fragmentation of the dopants. They must, instead, reflect differences in the reactivity of the photoproduced radicals. In particular, the increase in ArH product can be ascribed to the enhanced reactivity of the radicals due to the high attained temperatures and heat diffusion to the sublayers. The formation of biaryl species strongly indicates a very low polymer viscosity (≈10³ Pa s), consistent with transient polymer melting. In fact, biaryl formation provides a direct probe for assessing melting upon laser irradiation. In all, the observations are well accounted for by a thermal model of the laser interaction with PMMA and illustrate the factors that may control reactivity in the UV ablation of polymer substrates.