X-ray photoelectron spectroscopy of ytterbium nanofilms with oxygen adsorbed on their surface

X-ray photoelectron spectra of nanoscale-thickness ytterbium films along with adsorbed oxygen layers have been studied for the first time. It is found that the spectra include features from both divalent and trivalent ytterbium. The intensity ratio of these features raises upon increasing the film thickness (the size effect). It is shown that the above behavior is due to the formation of two final states related to divalent and trivalent ytterbium at the photoionization of 4$f$-level by photons with the energy of 1253.6 eV, while only the trivalent state is realized by using the photons with the energy of 142 eV.