Double-core-hole resonance spectroscopy at the iron L-edge of iron pentacarbonyl

Transition-metal complexes are central for catalysis, material sciences and biochemistry, owing to their diverse non-radiative decay pathways driven by femtosecond-scale metal-ligand charge transfer. Resonant double-core-hole (DCH) ion-yield spectroscopy enables direct observation of ultrafast electronic and nuclear dynamics of such complexes with a direct coupling between strongly localized, transient charges and the chemically active valence electrons. Yet, previous DCH investigations were confined to atoms and small molecules at the K -edge. Here, we extend resonant DCH ion-yield spectroscopy to the L -edge of the transition-metal carbonyl Fe(CO) ₅ in the gas phase to investigate fragmentation dynamics and sequential DCH ionization of the Fe center. Using intense femtosecond X-ray pulses, we observe signatures of multiply charged Fe cations, up to Fe ¹⁵⁺ . Comparison with atomic-level ionization-pathway modeling indicates a photon-energy range between 740eV and 780eV in which cations originating from core-excited DCH states dominate. Competing DCH channels, (i) resonant excitation following ionization or (ii) resonant-resonant excitation, govern the ionization and subsequent relaxation cascade. These channels enable charge build-up far beyond the single-core-hole (SCH) limit and induce fluence-dependent depletion of lower-charge fragments. Our results establish L -edge DCH resonance spectroscopy as a powerful approach for probing ultrafast charge redistribution and fragmentation in transition-metal complexes.