Few-femtosecond photoelectron spectroscopy reveals fundamental photodynamics of the carbon-carbon double bond in ethylene.

Conjugated π-systems, often composed of carbon-carbon double bonds as key structural units, are among the most important organic chromophores and serve as prototypical systems for studying ππ* excitation. Their ultrafast excited-state dynamics govern the outcomes of fundamental photochemical processes, including vision, photoisomerization, photosynthesis, and solar energy conversion. Here, we investigate the ultrafast dynamics of the simplest isolated C=C chromophore, ethylene, and its per-deuterated isotopologue using broadly tunable sub-4 femtosecond pulses in the 150-200 nm range combined with time-resolved photoelectron spectroscopy. Supported by advanced quantum dynamics calculations, our results reveal the important role of the 1B3g(σπ*) state, which is strongly coupled through torsional motion to the optically populated 1B1u(ππ*) state. These findings establish a revised framework for understanding and controlling photochemical reaction in conjugated molecular systems.