AbstractIn non-resonant Auger electron spectroscopies, multi core-ionized states lead to numerous energetically close-lying electronic transitions in Auger spectra, this hampering the assignment and interpretation of the experimental results. Here we reveal a new method to overcome this intrinsic limitation of non-resonant inner-shell spectroscopies. In a proof-of-principle experiment performed for the O₂ molecule, most of the Auger final states are dissociative, and we measure in coincidence the kinetic energy of the photoelectron and the kinetic energy release of the (O⁺, O⁺) ion pairs produced after the Auger decay of the O 1s⁻¹ core-ionized states. The Auger final states are assigned using energy conservation. We fully separate the contributions from the ⁴Σ⁻ and ²Σ⁻ intermediate ionic states and conclusively demonstrate that the Auger decay probability can dramatically depend on the different O₂ 1s⁻¹ intermediate multiplet states. In addition, a metastable Auger final state also exists, with lifetime longer than 3.8 μs, and clear changes are observed in both branching ratio and spectral profile of the O 1s photoelectron spectrum when they are recorded in coincidence with either ${{\bf{O}}}_{{\bf{2}}}^{{\boldsymbol{+}}{\boldsymbol{+}}}$ O 2 + + or with other ionic species. These changes are attributed to the population of the metastable ${{\boldsymbol{B}}}^{{\boldsymbol{^{\prime} }}3}{{\boldsymbol{\Sigma }}}_{{\boldsymbol{u}}}^{-}({\boldsymbol{\nu }}{\boldsymbol{^{\prime\prime} }}{\boldsymbol{=}}0)$ B ′ 3 Σ u − ( ν ″ = 0 ) Auger final state via different intermediate states.