Ground state recovery and molecular structure upon ultrafast transition through conical intersections in cyclic dienes

Signatures of molecular structures immediately after the ultrafast crossing through a conical intersection (CI) were found in 1,2,3,4,5-pentamethyl-cyclopentadiene (PMCPD) and 1,3-cyclohexadiene (CHD). The molecules were excited at 267 nm, and the structures immediately after reaching their ground electronic states were observed using a structure-sensitive photoionization/photoelectron technique. PMCPD is found to revert to its original structure, but CHD is immediately launched toward the ring-opened structure upon crossing through the CI, proving that the ring opening reaction mechanism proceeds completely within an ultrafast time scale. The resulting hexatriene is observed to have a poorly defined fluxional structure.