| Summary: | The dispute about whether the 1 s core-hole is localized on one atom or delocalized over both in a homonuclear diatomic molecule has continued for decades, which has been extensively studied by the photoelectron and electron–ion coincidence spectroscopies. For N _2 , if the 1 s core-hole is delocalized, the K-shell excitation into the 1 π _g orbital should split into two components, i.e., the dipole-allowed transition from the ungerade 1 σ _u state and the dipole-forbidden transition from the gerade 1 σ _g state. However, only the dipole-allowed transition has been observed up to now. Here, we report the inner-shell electron energy loss spectra of N _2 at different scattering angles with an incident electron energy of 1500 eV and an energy resolution of 65 meV. The vibrational structures of both the dipole-allowed ${(1s{\sigma }_{u})}^{-1}{(1{\pi }_{g})}^{1}$ and dipole-forbidden ${(1s{\sigma }_{g})}^{-1}{(1{\pi }_{g})}^{1}$ states of N _2 have been identified at different momentum transfers. The splitting between the ${(1{\sigma }_{g})}^{-1}{(1{\pi }_{g})}^{1}$ and ${(1{\sigma }_{u})}^{-1}{(1{\pi }_{g})}^{1}$ states with the reverse symmetry is determined to be 67 ± 7 meV. Moreover, the momentum transfer dependence behavior of the transition intensity ratio agrees with the theoretical predictions, as increasing to a maximum and then decreasing. The experimental observations clearly show that the inner most electrons can be described by 1 σ _g and 1 σ _u , which indicates that the inner-shell 1 s core-hole of N _2 is delocalized over two N atoms based on the excitation process.
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