Incubation and photoablation of poly(methyl methacrylate) at 248 nm. New insight into the reaction mechanism using photofragment translational spectroscopy

The decomposition of poly(methyl methacrylate) (PMMA) under the action of 248 nm, 16 ns laser pulses at 250 mJ/cm2 was studied in vacuum. A sensitive electron impact ionizer/quadrupole mass spectrometer system was used to measure mass spectra and time-of-flight (TOF) distributions of the neutral photoproducts. The incubation and photoablation reactions were unambiguously resolved in the TOF distributions. The incubation reaction produces slow-moving species with mass peaks at m/e = 15, 28, 29, 30, 31, 32, 59, and 60 which can be assigned to methanol, CO, and methyl formate. These products are consistent with the incubation mechanism proposed by Stuke and co-workers, namely, single-photon absorption at the carbonyl side-chain chromophore followed by side-chain scission. The incubation reaction produces a partially unsaturated and more strongly UV-absorbing polymer backbone. The ablation of this degraded material at 250 mJ/cm2 produces mainly species with molecular weights < 150 and translational energies around 0.3 eV. The relationship of these results to an extensive and contradictory body of data in the literature is discussed.