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.

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

“…Fig. 3a) diminished to undetectable levels, which is similar to the observations for the same methyl halide on D 2 O and CH 3 OH films in which the CT-DEA yield also diminished for thicker films 13 . We would expect similar behaviour for CH 3 I if CT-DEA from substrate photoelectrons was the operative mechanism for either or both of the A and B features observed on C 6 H 6 films.…”

“…The fast peak is consistent with that expected for neutral photodissociation, and the slower peak consistent with photoelectron mediated CT-DEA. These observed features are broadly consistent with surface photodissociation of CH 3 Cl seen in other heterogeneous systems such as CH 3 Cl/D 2 O/Cu(110) 13 . The magnitude of these features correlate with the CH 3 Cl coverage for a fixed amount of C 6 H 6 .…”

“…2) finds that CH 3 Br/C 6 H 6 has a CH 3 photofragment TOF spectrum with both a neutral photodissociation (peak at ∼41µs, 1.9eV) and photoelectron CT-DEA (peak at 62µs, 0.8eV) features. These peak assignments and energies are similar to those seen previously for CH 3 Br/D 2 O/Cu(110) 13 . For CH 3 Br adsorbed on a 3ML C 6 F 6 thin film, the TOF spectra display only the neutral photodissociation peak.…”

“…That quenching of neutral photodissociation is observed for the 1ML film indicates that either the LUMO-derived C 6 F 6 state shifted upward in energy is sufficient to couple with the neutral excitation or that the methyl halide excited state can couple with the Cu substrate states to facilitate quenching through the C 6 F 6 monolayer. It is noteworthy that the observations here for methyl halides on C 6 F 6 thin films are the obverse of findings for photodissociation of CH 3 Br and CH 3 Cl on CH 3 OH thin films 13 , where CT-DEA was enhanced but neutral photodissociation at λ=193nm was quenched, likely due to resonant hole-filling by the CH 3 OH layer. This highlights the importance of the molecular film electronic structure and electron transfer in how the outcomes for excitations of co-adsorbed molecules can be altered 62 .…”

“…We have studied a range of methyl halides (CH 3 X, X= Cl, Br, I) adsorbed on thin films of C 6 H 6 or C 6 F 6 on Cu single crystal substrates. The stimulated dissociation properties of these methyl halides have been studied in some detail in both the gas-phase [5][6][7][8] as well as condensed on surfaces [9][10][11][12][13] . Depending on the context, these molecules can display low-energy photoelectron driven Dissociative Electron Attachment (DEA) or neutral photodissociation processes when in the adsorbed state, with outcomes dependent on the details of the particular molecule, intermolecular interactions and electronic structure of the environment.…”