Surface core-level shifts and surface valence change in mixed-valent YbAl₂

High-resolution photoemission studies show that about two surface layers of mixed-valent YbAl2 are divalent. These layers exhibit large surface core-level shifts of Δs1=0.92 eV for the topmost and Δs2=0.35 eV for an underlying surface layer, allowing a separation of the divalent part of the spectrum into bulk and surface contributions. In this way, a mean valence of 2.4±0.1 can be derived for bulk YbAl2. The observed Δs1/Δs2 ratio is consistent with a recent theoretical prediction. © 1982.