Excitation and quenching mechanisms in the near-UV photodissociation of CH3Br and CH3Cl adsorbed on D2O or CH3OH on Cu(110)

Abstract

Photochemical processes for CH₃X (X = Cl, Br, I) adsorbed on top of thin films of D₂O or CH₃OH on a Cu(110) substrate is studied by time-of-flight mass spectrometry for a range of UV wavelengths (351–193 nm). Photodissociation via dissociative electron attachment by photoelectrons and by neutral photodissociation is identified and quantified based on the observed dynamics of the desorbing CH₃ fragments. Photoelectron-driven dissociation of CH₃X is found to be a maximum for monolayer quantities of the D₂O or CH₃OH on Cu(110), but with differing kinetic energy release on the two substrates. The dynamics of CH₃Br and CH₃Cl photodissociation qualitatively differ on CH₃OH/Cu(110) as compared to D₂O/Cu(110), which is ascribed to differing molecular structures for these systems. Evidence is presented for an efficient inter-molecular quenching mechanism for neutral photoexcitation of CH₃Cl and CH₃Br on the CH₃OH/Cu(110) substrate.