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

The dispute about whether the is 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 is core-hole is delocalized, the K-shell excitation into the 1 pi(g) orbital should split into two components, i.e., the dipole-allowed transition from the ungerade 1 sigma(u) state and the dipole-forbidden transition from the gerade 1 sigma(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 sigma(g) and 1 sigma(u,) which indicates that the inner-shell Is core-hole of N2 is delocalized over two N atoms based on the excitation process.