Three-Electron Interatomic Coulombic Decay from the Inner-Valence Double-Vacancy States in NeAr

We have unambiguously identified interatomic Coulombic decay in NeAr from the inner-valence double-vacancy state Ne-Ar²⁺(3s^-2) to outer-valence triple-vacancy states Ne⁺(2p^-1)-Ar²⁺(3p^-2) by momentum-resolved electron-ion multicoincidence. This is the first observation of interatomic Coulombic decay where three electrons ($3e$) participate. The results suggest that this $3e$ interatomic Coulombic decay is significantly faster than other competing processes like fluorescence decay and charge transfer via curve crossing.