Dissociation of Adsorbates via Electronic Energy Transfer from Aromatic Thin Films

Photofragment translational spectroscopy has been used to characterize the energetics and the cross sections for photodissociation of CH₃I and CF₃I adsorbed on thin films of a variety of aromatic molecules, initiated by near-UV light. Thin films (nominally 10 monolayers) of benzene, five substituted benzenes and two naphthalenes have been employed to study systematic changes in the photochemical activity. Illumination of these systems with 248nm light is found to result in a dissociation process for the CH₃I and CF₃I mediated by initial absorption in the aromatic thin film, followed by electronic energy transfer (EET) to the dissociating species. The effective cross sections for dissociation are found to be substantially increased via this mechanism (from 1.8x--20x), amounts differing depending on the aromatic molecule thin film used, and is connected to the aromatic photabsorption profile and the particular excited states being accessed. Distinctive translational energy distributions for the CH₃ and CF₃ photofragments are found to vary systematically for the different aromatic molecule thin film used, and are related to the energy of the lowest electronic excited singlet state of the aromatic molecule. The CH₃ and CF₃ photofragment kinetic energy distributions found for the aromatic thin films suggest that the dissociation occurs via EET to the ³Q₁ excited state of CH₃I and CF₃I.