Contrasting mechanisms for photodissociation of methyl halides adsorbed on thin films of C6H6 and C6F6

Abstract

The mechanisms for photodissociation of methyl halides (CH₃X, X = Cl, Br, I) have been studied for these molecules when adsorbed on thin films of C₆H₆ or C₆F₆ on copper single crystals, using time-of-flight spectroscopy with 248 nm and 193 nm light. For CH₃Cl and CH₃Br monolayers adsorbed on C₆H₆, 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 C₆F₆ thin film display only neutral photodissociation, with the DEA pathway entirely absent due to intermolecular quenching via a LUMO-derived electronic band in the C₆F₆ thin film. For CH₃I adsorbed on a C₆F₆ thin film, illumination with 248 nm light results in CH₃ photofragments departing due to neutral photodissociation via the A-band absorption. When CH₃I monolayers on C₆H₆ 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 CH₃I molecules adsorbed on top. The proposed mechanism for this photodissociation is via a charge-transfer complex for the C₆H₆ layer and adsorbed CH₃I.