Stoichiometry-related Auger lineshapes in titanium oxides: Influence of valence-band profile and of Coster-Kronig processes

The ability to determine the nature and the occurrence of defects is a central need of ceramic surface chemistry. In titanium oxides, the $\mathrm{Ti}-\mathrm{LMV}$ Auger decays line shape is very sensitive to the titanium degree of oxidation, and has long been empirically used as a qualitative probe of the stoichiometry. In the present work, resonant Auger and resonant valence-band measurements at the $\mathrm{Ti}-L_{2,3}$ edges in ${\mathrm{TiO}}_2,$ ${\mathrm{TiO}}_{2-x}$ and metallic titanium provide a clear evidence that the evolutions of the $\mathrm{Ti}-\mathrm{LMV}$ Auger line shape are due to drastic changes in the valence-band profile and in the probability of $L_2{L}_{3}V$ Coster-Kronig decay processes when a fraction of titanium ions is reduced. Both are shown to be consequences of the filling of $\mathrm{Ti}-3d$ states near the Fermi level on defects sites. These conclusions are confirmed by Auger-photoelectron coincidence experiments.