The initial photochemical event that allows animals to observe light is a cis to trans isomerization within the molecular chromophore retinal, which forms part of the photoreceptor rhodopsin. The process is known to occur very quickly (200 femtoseconds) and is thought to take place through a dynamical phenomenon known as a conical intersection, but probing such reaction dynamics has proved troublesome in the past because of the short timescales involved. Now, a team led by Marco Garavelli and Giulio Cerullo from the Università di Bologna and Politecnico di Milano, respectively, have been able to provide1 compelling evidence for the involvement of a conical intersection in retinal isomerization.

The Born–Oppenheimer approximation is used to help describe reactions that occur in the ground electronic state. It allows reaction progress to be conceptualized by plotting a 'potential energy surface' as a function of atomic position and, as the reaction develops, the atoms smoothly 'traverse' the surface. Conical intersections are where two of these surfaces meet and are thought to provide a pathway, rather like a funnel, for relaxation from the photoexcited state to the ground state.

Garavelli, Cerullo and co-workers used ultrafast pump-probe spectroscopy, supported by simulations, to follow in time the energy gap between the photoexcited and ground states involved as the isomerization took place. They observed a pattern consistent with the presence of a conical intersection: during the first ∼80 fs the energy gap decreases as the potential energy surfaces approach one another; the reaction then passes through the conical intersection as the surfaces meet, and once the photoproduct forms, the probed energy gap increases.