Mechanisms for the Near-UV Photodissociation of CH$_3$I on D$_2$O/Cu(110)

TL;DR

This study investigates the mechanisms of near-UV photodissociation of CH$_3$I on D$_2$O/Cu(110), revealing dominant neutral dissociation pathways, the role of excited states, and electron attachment effects at different coverages.

Contribution

It provides detailed insights into the dissociation pathways and the influence of water coverage on CH$_3$I photodissociation on Cu(110).

Findings

D$_2$O layers influence dissociation pathways.

Dissociation mainly via the $^3Q_0$ excited state.

Electron attachment contributes at submonolayer coverage.

Abstract

The system of CH$_3$I adsorbed on submonolayer, monolayer and multilayer thin films of D$_2$O on Cu(110) has been studied by measuring the time-of-flight (TOF) distributions of the desorbing CH$_3$ fragments after photodissociation using linearly polarized $\lambda$=248nm light. For multilayer D$_2$O films (1-120ML), the photodissociation is dominated by neutral photodissociation via the "A-band" absorption of CH$_3$I. The polarization and angle dependent variation in the observed TOF spectra of the CH$_3$ photofragments find that dissociation is largely via the $^3Q_0$ excited state, but that also a contribution via the $^1Q_1$ excitation can be identified. The photodissociation results also indicate that the CH$_3$I adsorbed on D$_2$O forms close-packed islands at submonolayer coverages, with a mixture of C-I bond axis orientations. For submonolayer quantities of D$_2$O we have observed a contribution to CH$_3$I photodissociation via dissociative electron attachment (DEA) by photoelectrons. The observed DEA is consistent with delocalized photoelectrons from the substrate causing the observed dissociation -- we do not find evidence for a DEA mechanism via the temporary solvation of photoelectrons in localized states of the D$_2$O ice.