Effects of ion irradiation damage on the initial interactions of oxygen with polycrystalline gadolinium

The effects of surface and near-surface defects, induced by Ar+ ion irradiation, on the interactions of oxygen with polycrystalline gadolinium surfaces were investigated over the temperature range 140-300 K, utilizing Auger electron spectroscopy, direct recoil spectrometry, work function change measurements and density functional theory calculations. This enabled the distinction between the topmost surface and subsurface processes involved in this reaction. It turns out that the formation of bulk gadolinium oxide (at the near-surface region) involves in general three sequential steps: (i) surface random chemisorption, (ii) clustering of the chemisorbed atoms and the formation of a type of “surface oxide”, (iii) dissolution of surface oxygen atoms into the sub-surface and nucleation of the “bulk” oxide. However, the time duration of these steps becomes shorter (i.e. faster kinetics) as the corresponding rate controlling parameters (i.e. surface reactivity and temperature) become more favorable. Consequently, all the above steps are discernable only for the less favorable kinetic parameters (annealed surfaces at low temperatures). For the more active surfaces (sputtered or annealed at higher temperatures) the very initial random surface chemisorption step is practically absent and a two-step process occurs, with an initial stage associated with direct clustering and formation of the “surface oxide”, followed by the subsurface penetration and bulk oxide formation. It was also found that the irradiation induced damage does not affect significantly the topmost surface accumulation of oxygen. However, it accelerates appreciably the penetration of oxygen into the near-surface region and the development of the bulk oxide.