Abstract: Provided is an optical element for a lithographic apparatus. The optical element includes a capping layer that includes oxygen vacancies therein. The oxygen vacancies prevent attack of the capping layer by preventing hydrogen and other species from penetrating the capping layer and underlying layers. The capping layer provides a low hydrogen recombination rate enabling hydrogen to clean the surface of the optical element. The capping layer may include an alloyed metal, a mixed metal oxide or a doped metal oxide and it may be a ruthenium capping layer that includes one or more dopants therein.