Contrasting Mechanisms for Photodissociation of Methyl Halides Adsorbed on Thin Films of C$_6$H$_6$ and C$_6$F$_6$

Abstract

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.