PNNL

Abstract

The photochemical properties of the Cr-terminated a-Cr2O3(0001) surface were explored using methyl bromide (CH3Br) as a probe molecule. CH3Br adsorbed and desorbed molecularly from the Cr-terminated a-Cr2O3(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH3Br 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 CH3Br 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 Cr3+ sites and near complete removal of CH3Br TPD states above 150 K. The photochemistry of chemisorbed CH3Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH3Br 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 cm2. Parent PSD was indicative of molecular photodesorption, but CH3 was also detected in PSD and Br atoms were left on the surface, both reflective of photo-induced CH3-Br bond dissociation. 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 a-Cr2O3, occurring at photon energies