Time-resolved Coulomb explosion imaging of a photochemical ring opening reaction.

Photoinduced ring-opening of cyclic organic molecules is fundamental to many photochemical processes, from vitamin D biosynthesis to molecular optical switching. Despite advances in several ultrafast techniques, direct and unambiguous imaging of the ultrafast nuclear motion is still a major challenge. As a consequence, reaction mechanisms remain controversial even for extensively studied prototypical systems. Here, we show that time-resolved Coulomb explosion imaging, together with molecular dynamics simulation, can be used to identify and map the ring-opening reaction of gas-phase furan following ultraviolet photoexcitation, a reaction for which widely contradicting predictions and observations have been reported. By directly imaging the transient carbon-backbone structure, we reveal the presence of a strong ring-opening pathway that occurs on average at approximately 70 fs. With the development of higher-repetition-rate lasers, we anticipate that our approach will enable mapping of a broad range of ultrafast photochemical reactions.