Valence transition 2+ $\to$ 3+ induced in ytterbium nanofilms by CO and O$_2$ molecules chemisorbed on their surface

Transformations of the surface and bulk of nanoscale ytterbium films during the surface interactions of these films with different ligand molecules have been studied. It has been shown that a combination of two factors, i.e., the existence of a lone electron pair in CO and O$_2$ molecules and the unoccupied 5d level lying near the Fermi level in metallic divalent ytterbium, causes the formation of a stable chemisorption state of the molecule-nanofilm surface layer in which donor-acceptor bonds between gas molecules and surface metal ions are formed. As a result, ytterbium on the surface and in the bulk of the nanofilm is oxidized to a non-autonomous trivalent electronic state. The depth to which this transition propagates in the nanofilm has been determined; its anomalously large value (from 9 to 22 layers according to different estimates) has been explained. It has also been shown that the ligand molecule layer on the ytterbium surface is filled in two stages. The two-stage mechanism of this process is reflected by the nonmonotonic behavior of concentration dependences of the work function of CO–Yb and O$_2$–Yb structures.