UV laser ablation of halonaphthalene doped PMMA: chemical modifications above versus below the ablation threshold

Laser-induced fluorescence is employed to monitor the photochemical modifications that bromo- and iodon- aphthalene incorporated in PMMA films undergo following ablation at 248 nm. Following irradiation at low fluences with nanosecond pulses, photolysis of the dopants is found to result in the formation of naphthalene-like photoproducts, whereas above the threshold additional products are observed. Based on concentration studies, these are tentatively ascribed to species containing two naphthyl groups. In close correspondence to the formation of the new species, the photolysis yields for both dopants are found to increase sharply above the threshold. Finally, the nature and extent of these photochemical effects is indicated to depend sensitively on the laser pulse width. The mechanistic implications of these results are discussed.