What determines the probing depth of electron yield XAS?

Comparative Ni K-edge (8332.8 eV) total electron-yield (TEY) and conversion electron-yield (CEY) XAS spectra were collected from Ni wafers covered by NiO of varying thickness. In contrast to previous predictions, the surface sensitivity of TEY detection was found to be actually higher than for CEY detection. Examination of signal amplification factors upon switching from TEY to CEY detection indicates that the role of the surface insensitive inelastic KLL Auger electrons has been underestimated in the past. These energetic electrons constitute a major component of the emitted flux which is selectively amplified in CEY mode. Analysis of the TEY attentuation characteristics can be carried out solely in terms of Auger electron penetration ranges.