Adsorbate dissociation due to heteromolecular electronic energy transfer from fluorobenzene thin films.

Study of the near-UV photodissociation dynamics for monolayer (ML) quantities of CHI on thin films of a series of fluorobenzenes and benzene (1-25 ML) grown on a Cu(100) substrate finds that in addition to gas-phase-like neutral photodissociation, CHI dissociation can be enhanced electronic energy transfer to the CHI following photoabsorption in several of the thin films studied. Distinct CH photofragment kinetic energy distributions are found for CHI photodissociation on CHF, 1,4-CHF and CH thin films, and distinguished from neutral photodissociation pathways using polarized incident light. The effective photodissociation cross section for CHI on these thin films is increased as compared to that for the higher F-count fluorobenzene thin films due to the additional photodissociation pathway available. Quenching by the metal substrate of the photoexcitation this new pathway suggests a significantly longer timescale for excitation than that of neutral CHI photodissociation. The observations support a mechanism in which neutral photoexcitation in the thin film ( an exciton) is transported to the interface with CHI, and transfers the electronic excitation to the CHI which then dissociates. The unimodal CH photofragment distribution and observed kinetic energies on the fluorobenzene thin films suggest that the dissociation occurs the Q excited state of CHI.