Analysis of neutral fragments from ultraviolet laser irradiation of a photolabile triazeno polymer

Abstract

A photolabile triazeno polymer was irradiated with pulsed excimer laser light at 248 nm and 30 ns pulse width. The ablation fragments were analyzed using time-of-flight (TOF) mass spectrometry. At fluences below 1.3 ${\mathrm{J/cm}}^2,$ only neutral products were found. At these fluences, ${\mathrm{N}}_2$ is by far the most intense neutral signal along with measurable phenyl radical (mass 76) production. The ${\mathrm{N}}_2$ TOF shows a fast shoulder corresponding to kinetic energies of about 1.1 eV and a long slow tail persisting for hundreds of microseconds. The tail is attributed to delayed emission of reaction products from the polymer. The kinetic energy of the fast peak is attributed to direct ejection of products from surface sites undergoing exothermic decomposition. A weaker signal due to the phenyl radical is also observed. The observed fluence dependence of the two major products is highly nonlinear and is shown to fit an Arrhenius equation. We discuss the implications of these measurements regarding photochemical versus photothermal processes.