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

TL;DR

This study compares photodissociation mechanisms of methyl halides on C6H6 and C6F6 thin films, revealing different pathways and energy transfer processes influenced by the film's electronic properties.

Contribution

It identifies distinct photodissociation pathways on different aromatic films and proposes a charge-transfer mechanism for energy transfer in C6H6.

Findings

C6H6 allows both neutral and DEA pathways for CH3Cl and CH3Br.

C6F6 suppresses DEA pathway due to electronic quenching.

Additional dissociation pathways occur for CH3I on C6H6 via energy transfer.

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.