Dissociation of Adsorbates via Electronic Energy Transfer from Aromatic Thin Films

Photofragment translational spectroscopy has been used to characterize the photodissociation of CH$_3$I and CF$_3$I 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 CH$_3$I and CF$_3$I 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, by amounts that differ depending on the aromatic molecule thin film used, and is connected to the aromatic photabsorption profile. Distinctive translational energy distributions for the CH$_3$ and CF$_3$ photofragments are found to vary systematically for the different aromatic molecule thin film used, and are related to the aromatic molecule excited states. The CH$_3$ and CF$_3$ photofragment kinetic energy distributions found for the aromatic thin films suggest that the dissociation occurs via EET to the $^3Q_1$ excited state of CH$_3$I and CF$_3$I.