Resonant Double-Core Excitation in N₂

We report on the experimental observation of a molecular resonant double-core excitation process in N₂ performed at the SQS endstation of the European XFEL, where a single few-femtosecond soft X-ray pulse sequentially excites two core-level electrons to the same unoccupied molecular orbital. The production of these neutral two-site K⁻¹K⁻¹V² double core hole states in the molecule are accessible due to few-eV bandwidth of the pulses. Scanning the central photon energy in the vicinity of the 1sσ→ 1π^*_g resonance in the ground state molecule and recording the electron de-excitation spectrum, the unambiguous signature of the decay of the doubly-excited molecules is observed through the appearance of new emission lines, including the so-called double-participator decay where both excited electrons take part in the de-excitation process. A full discussion of the measured spectra as well as a comparison with calculations is presented.