Norrish type I reactions cleave carbon-carbon bonds adjacent to carbonyl groups — the basis of UV curing, additive manufacturing, and photoinitiated polymerization. The reaction has been used industrially for decades. The mechanism was assumed but never directly observed at the electronic level: which states does the molecule pass through, and on what timescale?
This paper tracks acetophenone's Norrish pathway using time-resolved soft X-ray absorption spectroscopy. The initial photoexcitation transfers to an intermediate electronic state in approximately 0.13 picoseconds, then converts to a triplet state over roughly 3 picoseconds. The triplet state is the one that initiates bond cleavage.
The 130-femtosecond intermediate had been invisible to previous methods. It's fast enough that optical spectroscopy couldn't isolate it from the initial excitation. X-ray absorption sees it because core-level transitions are element-specific and state-specific — the carbon and oxygen environments change as the electronic state changes. The mechanism that drives industrial UV curing passes through a state that lasts for 130 millionths of a nanosecond, then waits twenty times longer in the triplet before breaking the bond.