Theoretical Calculations of the Resonant Double Core-Excitation and Autoionization Decay of N₂

Here we present theoretical calculations supporting the experimental observation of a molecular resonant double-core excitation process in N₂. This process was driven by a single few femtosecond soft X-ray Free Electron Laser (XFEL) pulse sequentially that excites two core level electrons to the same unoccupied molecular orbital. Multiconfigurational electronic structure calculations of X-ray absorption near-edge structure show a photon energy tuned to the 1s_σ → 1π^*_g resonance requires a minimum bandwidth around 2 eV to drive the double core-excitation and that the second core-excitation energy is lowered in energy with respect to the ground state value. High-level Auger-decay calculations of the single and double core-excited states, explicitly considering the wavefunction of the ejected electron, validate the signatures of the formation and decay of neutral two-site double core hole states are observed in the experiment. The excitation process should be general, enabling the production of exotic neutral multi-core hole states with high site- and orbital-selectivity, providing a resonant X-ray pump/probe scheme and being of general interest for XFEL measurements performed in resonant conditions.