Photochemistry of methyl bromide on the α-Cr 2O 3(0001) surface

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The photochemical properties of the Cr-terminated α-Cr 2O 3(0001) surface were explored using methyl bromide (CH 3Br) as a probe molecule. CH 3Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr 2O 3(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH 3Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH 3Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr 3+ sites and near complete removal of CH 3Br TPD states above 150 K. The photochemistry of chemisorbed CH 3Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH 3Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10 − 22 cm 2. Photodissociation of the CH 3–Br bond was evidenced by both CH 3 detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr 2O 3, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH 3Br. It is unclear to what extent band-to-band versus direct CH 3Br photolysis play in CH 3–Br bond dissociation initiated by more energetic photons.