Femtosecond core-level spectroscopy reveals signatures of transient excited states in the photodissociation of Fe(CO)5

Excitation of iron pentacarbonyl (IP), a prototypical photocatalyst, at 266 nm causes sequential loss of two CO ligands in the gas phase, creating catalytically active, unsaturated iron carbonyls. Thus far, the electronic states involved in the dissociation have eluded experimental observation, hindering a comprehensive understanding of IP photochemistry. Using femtosecond extreme ultraviolet transient absorption spectroscopy near the Fe M2,3-edge, we present the first spectroscopic characterization of valence electronic dynamics during IP photodissociation. Informed by electronic structure calculations, we uncover the spectroscopic signatures of the intertwined structural and electronic evolution among the manifold of metal-centered excited states during first CO loss from IP on a 100-fs timescale. Furthermore, spectroscopic signals associated with the formation of Fe(CO)4 on its lowest singlet excited state and in structures fluctuating between C2v and C3v geometries, and its subsequent picosecond dissociation to Fe(CO)3 in the Cs geometry, are corroborated using quantum chemistry calculations.