  The mechanisms for photodissociation of methyl halides (CH$_3$X, X= Cl, Br,
I) have been studied for these molecules when adsorbed on thin films of
C$_6$H$_6$ or C$_6$F$_6$ on copper single crystals, using time-of-flight
spectroscopy with 248nm and 193nm light. For CH$_3$Cl and CH$_3$Br monolayers
adsorbed on C$_6$H$_6$, two photodissociation pathways can be identified:
neutral photodissociation similar to the gas-phase, and a dissociative electron
attachment (DEA) pathway due to photoelectrons from the metal. The same methyl
halides adsorbed on a C$_6$F$_6$ thin film display only neutral
photodissociation, with the DEA pathway entirely absent due to intermolecular
quenching via a LUMO-derived electronic band in the C$_6$F$_6$ thin film. For
CH$_3$I adsorbed on a C$_6$F$_6$ thin film, illumination with 248nm light
results in CH$_3$ photofragments departing due to neutral photodissociation via
the A-band absorption. When CH$_3$I monolayers on C$_6$H$_6$ thin films are
illuminated at the same wavelength, additional new photodissociation pathways
are observed that are due to absorption in the molecular film with energy
transfer leading to dissociation of the CH$_3$I molecules adsorbed on top. The
proposed mechanism for this photodissociation is via a charge-transfer complex
for the C$_6$H$_6$ layer and adsorbed CH$_3$I.
