Visible light induced photodesorption of NO from the α-Cr2O3(0001) surface

Nitric oxide chemistry and photochemistry on the Cr-terminated surface of α-Cr2O3(0001) were examined using temperature programmed desorption (TPD), sticking coefficient measurements and photodesorption. NO exposed to α-Cr2O3(0001) at 100 K binds at surface Cr cation sites forming a strongly bound surface species that thermally desorbs at 320–340 K, depending on coverage. No thermal decomposition was detected in TPD in agreement with previous results in the literature. Sticking probability measurements at 100 K indicated near unity sticking for NO up to coverages of ~ 1.3 ML, with additional adsorption with higher exposures at decreased sticking probability. These results suggest that some Cr cation sites on the α-Cr2O3(0001) surface were capable of binding more than one NO molecule, although it is unclear whether this was as separate NO molecules or as dimers. Photodesorption of adsorbed NO was examined for surface coverages below the 1 ML point. Both visible and UV light were shown to photodesorb NO without detectable NO photodecomposition. Visible light photodesorption of NO occurred with a greater cross section than estimated using UV light. The visible light photodesorption event was not associated with bandgap excitation in α-Cr2O3(0001), but instead was linked to excitation of a surface Cr3 +–NO− charge transfer complex. These results illustrate that localized photoabsorption events at surface sites with unique optical properties (relative to the bulk) can result in unexpected surface photochemistry.