Probing the delocalized core-hole via inner-shell excitation in N2

The dispute about whether the 1s 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 N2, if the 1s core-hole is delocalized, the K-shell excitation into the 1π g orbital should split into two components, i.e., the dipole-allowed transition from the ungerade 1σ u state and the dipole-forbidden transition from the gerade 1σ 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 N2 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 ( 1 s σ u ) − 1 ( 1 π g ) 1 and dipole-forbidden ( 1 s σ g ) − 1 ( 1 π g ) 1 states of N2 have been identified at different momentum transfers. The splitting between the ( 1 σ g ) − 1 ( 1 π g ) 1 and ( 1 σ