  Oxygen, the third most abundant element in the universe, plays a key role in
the chemistry of condensed matter and biological systems. Here, we report
evidence for a hitherto unexplored Auger transition in oxides, where a valence
band electron fills a vacancy in the 2s state of oxygen, transferring
sufficient energy to allow electron emission. We used a beam of positrons with
kinetic energies of $\sim$1 eV to create O 2s holes via matter-antimatter
annihilation. This made possible the elimination of the large secondary
electron background that has precluded definitive measurements of the
low-energy electrons emitted through this process. Our experiments indicate
that low-energy electron emission following the Auger decay of O 2s holes from
adsorbed oxygen and oxide surfaces are very efficient. Specifically, our
results indicate that the low energy electron emission following the Auger
decay of O 2s hole is nearly as efficient as electron emission following the
relaxation of O 1s holes in TiO$_2$. This has important implications for the
understanding of Auger-stimulated ion desorption, Coulombic decay, photodynamic
cancer therapies, and may yield important insights into the radiation-induced
reactive sites for corrosion and catalysis.
