  Study of the near-UV photodissociation dynamics for monolayer (ML) quantities
of CH$_3$I 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, CH$_3$I dissociation can be enhanced via photoabsorption in
several of the thin films studied. Distinct CH$_3$ photofragment kinetic energy
distributions are found for CH$_3$I photodissociation on C$_6$H$_5$F,
1,4-C$_6$H$_4$F$_2$ and C$_6$H$_6$ thin films, and distinguished from neutral
photodissociation pathways using polarized incident light. The effective
photodissociation cross section for CH$_3$I 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 CH$_3$I photodissociation.
The observations support a mechanism in which neutral photoexcitation in the
thin film (i.e. an exciton) is transported to the interface with CH$_3$I, and
transferring the electronic excitation to the CH$_3$I which then dissociates.
The unimodal CH$_3$ photofragment distribution and observed kinetic energies on
the fluorobenzene thin films suggest that the dissociation occurs via the
$^3Q_1$ excited state of CH$_3$I.
