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

Abstract

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 1πg orbital should split into two components, i.e., the dipole-allowed transition from the ungerade 1σu state and the dipole-forbidden transition from the gerade 1σg state. However, only the dipole-allowed transition has been verified up to now. Herein, 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 (1sσu)-1(1πg)1 and dipole-forbidden (1sσg)-1(1πg)1 states of N2 have been observed at different momentum transfers. The splitting between the (1sσu)-1(1πg)1 and (1sσg)-1(1πg)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 the decreasing. The experimental results clearly shows that the inner most electrons can be described by 1σg and 1σu, which indicates that the inner-shell 1s core-hole of N2 is delocalized over both two N atoms based on the excitation processes.

Time-Dependent Resonant Inelastic X-ray Scattering of Pyrazine at the Nitrogen K-Edge: A Quantum Dynamics Approach

“…Within the dipole approximation, RIXS follows electronic symmetry selection rules that offer valuable insights into the symmetry of occupied and unoccupied orbitals in molecules. − For instance, by strictly adhering to the dipole selection rules during both the absorption and emission steps, the parity of the system must remain unchanged throughout the RIXS process. However, molecules with equivalent atoms always possess multicenter core orbitals that are delocalized over these atoms, resulting in nearly degenerate core-excited states, commonly referred to as the core-hole localization problem. − These states couple vibronically through a non-totally symmetric normal mode, thus corresponding to a symmetry-allowed conical intersection and resulting in a final localization of the core holes. ,,, This dynamical distortion of symmetry can enable transitions that are otherwise electronically symmetry-forbidden ,, and significantly impact the intensity of their RIXS signal …”

“…However, molecules with equivalent atoms always possess multicenter core orbitals that are delocalized over these atoms, resulting in nearly degenerate core-excited states, commonly referred to as the core-hole localization problem. 41 − 46 These states couple vibronically through a non-totally symmetric normal mode, thus corresponding to a symmetry-allowed conical intersection 47 and resulting in a final localization of the core holes. 18 , 37 , 48 , 49 This dynamical distortion of symmetry can enable transitions that are otherwise electronically symmetry-forbidden 38 , 39 , 50 and significantly impact the intensity of their RIXS signal.…”