Photochemical processes: usual and less usual pathways

 The study of photoinduced dynamics in isolated molecules by time-resolved techniques has recently received a new impulse by the possibility of performing pump-probe experiments where the probe is high-resolution valence photoelectron spectroscopy, i.e. a technique which is sensitive to both electronic and geometrical structural changes. This new possibility has in particular been made available at the free-electron laser (FEL) FERMI, Elettra, Italy, which being a seeded source provides intense and highly monochromatized light pulses with negligible photon energy jitter, at variance with the majority of existing FELs

Two examples will be discussed. The first concerns the dynamics of photoexcitation-deexcitation of acetylacetone. By pumping with an optical laser and probing with valence photoelectron spectroscopy, the photochemical process is described in detail as consisting of a photoexcitation to a bright singlet state, followed by a conical intersection with a singlet dark state, and then another conical intersection with a triplet state [1].

The second example concerns a photochemical isomerization reaction, and namely the ring-opening reaction of 1,3 cyclohexadiene to hexatriene, which is possibly the most studied one with single-molecule advanced ultrafast techniques. The usual description of the reaction pathway in three steps (photoexcitation to the lowest-lying bright state followed by a conical intersection with the first dark state and then either another conical intersection with the open-chain isomer ground state or a return to the cyclic isomer ground state) is oversimplified. The overall picture is much more consistent if diabatic rather than adiabatic states are analyzed. In particular, we demonstrate that an initially high-lying double excited state is the gateway to the reaction [2].