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.
Highlights
There has been much recent work reported on UV photon- and electron-driven chemical processes in heterogeneous molecular thin films, for water ices
We have studied a range of halomethanes (CH3X, X = Cl, Br, I) adsorbed on thin films of D2O or CH3OH on Cu(110) substrate
The diminution of the photoelectron-driven signal does show that the average distance from the substrate to the CH3Br target that is detected in TOF increases with the D2O coverage
Summary
There has been much recent work reported on UV photon- and electron-driven chemical processes in heterogeneous molecular thin films, for water ices. We recently published a study of CH3I/D2O/Cu(110) photodissociation at l = 248 nm in which the CH3 photofragment translational energies were analyzed to highlight the varying contributions from neutral photodissociation and photoelectron dissociative electron attachment mechanisms.[28] A study of CH3I on thick D2O layers[29] analyzed the ground- and excited state I atoms emitted subsequent to l = 260 nm and 290 nm photodissociation. These studies showed evidence for a proportion of the I-atoms having larger than gas-phase translational energies which was ascribed to ‘‘chattering’’ during dissociation from ‘methyl-down’ oriented molecules on the ice surface as well as fast CH3 photofragments leaving the surface from ‘methyl-up’ oriented molecules
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