Observation and destruction of an elusive adsorbate with STM: O₂/TiO₂(110).

When a slightly defective rutile TiO₂(110) surface is exposed to O₂at elevated temperatures, the molecule dissociates at defects, filling O vacancies (V(O)) and creating O adatoms (O(ad)) on Ti(5c) rows. The adsorption of molecular O₂ at low temperatures has remained controversial. Low-temperature scanning tunneling microscopy of O₂, dosed on TiO₂(110) at a sample temperature of ≈100 K and imaged at 17 K, shows a molecular precursor at V(O) as a faint change in contrast. The adsorbed O₂ easily dissociates during the STM measurements, and the formation of O(ad)'s at both sides of the original V(O) is observed.