Mechanism of the Yb$^{2+}$ $\to$ Yb$^{3+}$ valence transition in ytterbium nanofilms upon chemisorption of CO and O$_2$ molecules on their surface

The dependences of the work function of ytterbium nanofilms with a thickness ranging from 1 to 32 monolayers on the amount of CO or O$_2$ molecules chemisorbed on their surface have been investigated experimentally. It has been found that these dependences have a pronounced nonmonotonic character. The mechanism of the Yb$^{2+}$ $\to$ Yb$^{3+}$ valence transition, which occurs upon the chemisorption of CO or O$_2$ molecules on the surface of ytterbium nanofilms, has been developed using the results of this study together with the previously obtained data.