formation of a compound due to photo-isomerisation
Stilbene has been extensively studied with pump-probe absorption spectroscopy as well as time resolved fluorescence.

In the ground state (S0) there is a large activation energy to go from trans  to cis and vice versa so that both species are stable at room temperature. This is not the case in the first excited state (S1)and the trans converts to the cis in a few tens of picoseconds and it is even faster for the cis excited state to convert to the trans.
The potential energy scheme is shown below (Not to scale). On the left is the trans species with a large barrier to for the cis (on the right). This has a higher energy than the trans but also has a large barrier to form the trans.
Detailed analysis suggest that the reaction occurs by a lengthening of the double bond before twisting. Exciting the trans puts it into the first singlet excited state (S1) where there is a small barrier on the pathway to cis, but as it happens there is an intersection with the ground state (by 3ps label &amp; twisted species) and from here either the ground state trans or ground state cis can be formed. If the cis ground state is excited then there is effectively no barrier to form the trans ground state and the excited cis molecule lasts for only a few ps.

For a review or organic photochemistry calculations see Bernardi et al. Chemical Society Reviews, p321, 1996, in particular fig 1. A lot of the original experimental work was done by the Hochstrasser group, see Annu. Rev. Phys. Chem. 2006. 57:37–64 for many references.