Adsorbate Dissociation Due to Heteromolecular Electronic Energy Transfer from Fluorobenzene Thin Films

Study of the near-UV photodissociation dynamics for monolayer (ML) quantities of CH₃I on thin films of a series of fluorobenzenes and benzene (1--25ML) grown on a Cu(100) substrate finds that in addition to gas-phase-like neutral photodissociation, CH₃I dissociation can be enhanced via photoabsorption in several of the thin films studied. Distinct CH₃ photofragment kinetic energy distributions are found for CH₃I photodissociation on C₆H₅F, 1,4-C₆H₄F₂ and C₆H₆ thin films, and distinguished from neutral photodissociation pathways using polarized incident light. The effective photodissociation cross section for CH₃I 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 via this new pathway suggests a significantly longer timescale for excitation than that of neutral CH₃I photodissociation. The observations support a mechanism in which neutral photoexcitation in the thin film (i.e. an exciton) is transported to the interface with CH₃I, and transferring the electronic excitation to the CH₃I which then dissociates. The unimodal CH₃ photofragment distribution and observed kinetic energies on the fluorobenzene thin films suggest that the dissociation occurs via the ³Q₁ excited state of CH₃I.