Insights from femtosecond core-level spectroscopy and theory*

After 266 nm excitation, gas-phase Fe(CO)₅, a prototypical photocatalyst, undergoes sequential CO dissociation to form Fe(CO)₃. The initial photoexcitation is ascribed to metal-to-ligand charge transfer states, which rapidly relax to dissociative metal-centered states. However, spectroscopic detection of the intricate excited state pathways responsible for sequential dissociation have long been elusive. Using femtosecond extreme ultraviolet transient absorption spectroscopy near the Fe M_2,3-edge, we have achieved the first spectroscopic characterization of the electronic dynamics during Fe(CO)₅ photodissociation. Using non-orthogonal configuration interaction singles calculations, which can treat core-to-valence transitions of two-electron open-shell states, we uncover the spectroscopic signatures of the intertwined structural and electronic evolution among the metal-centered excited states during the first CO loss from Fe(CO)₅ on a 100-fs time scale. Furthermore, the evolution of spectroscopic signals associated with the formation of Fe(CO)₄ in its closed-shell singlet surface in C_2v and C_3v geometries, and its subsequent picosecond dissociation into Fe(CO)₃ in its C_s geometry, are corroborated with electron-affinity time-dependent density functional theory.