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Stanford HAICohereColumbiaDepartment of PhysicsDeutsches Elektronen-Synchrotron DESYEdinburghEuropean XFELFlatironUC DavisMar 18, 2026arXiv:2603.18339This paper investigates the photochemical dynamics of acetophenone, a model aromatic carbonyl compound, to elucidate the Norrish type I reaction mechanism. Using time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy at the oxygen K-edge, combined with ab initio multiple spawning (AIMS) simulations, the researchers tracked the population transfer between different electronic states. They found that the reaction proceeds from an initially excited $^1\pi\pi^*$ state to a $^1n\pi^*$ state (0.13 ps), followed by intersystem crossing to a long-lived $^3n\pi^*$ state (3.17 ps), which is likely responsible for the Norrish type I chemistry.
Ultrafast X-ray spectroscopy reveals the hidden choreography of electronic state transitions that drive Norrish Type-I reactions, pinpointing the long-lived $^3n\pi^*$ state as the key player.
Norrish type I reactions selectively cleave carbon-carbon bonds directly adjacent to carbonyl groups. Despite their broad use in combination with aromatic carbonyls for additive manufacturing and dental UV curing applications, the nature of the photochemically active state and its population mechanism remain insufficiently understood. Detailed mechanistic insight requires mapping of the photoexcited population flow involving internal conversion and intersystem crossing. We present a time-domain study of gas phase acetophenone as a prototypical aromatic carbonyl combining soft X-ray time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy at the oxygen K-edge with ab initio multiple spawning (AIMS) simulations. Exploiting the specific sensitivity of TR-NEXAFS spectroscopy to states with $n\pi^*$ character, we observe population transfer from the initially excited $^1\pi\pi^*$ state to the $^1n\pi^*$ state with a time constant of $(0.13 \pm 0.02)$ ps after an initial induction period of $(0.12 \pm 0.02)$ ps without population transfer, in quantitative agreement with the AIMS simulations. The population in the $^1n\pi^*$ state subsequently decays via intersystem crossing, likely mediated by a $^3\pi\pi^*$ state, within $(3.17 \pm 0.66)$ ps to a long-lived $^3n\pi^*$ state, which is presumed to be active towards Norrish type I chemistry.
