Photodissociation pathways of acetone upon excitation into the 3s Rydberg state: Adiabatic versus diabatic mechanism

Photolysis of acetone upon excitation to the 3s Rydberg excited state has been studied by means of high level ab initio methods. The calculations have been performed on multi-configurational self consistent field (MCSCF) level with a subsequent addition of dynamical correlation both by perturbation theory (CASPT2) and via a configuration interaction expansion up to double excitations (MR-CISD). In addition to the major photoreaction that is well known Norrish type α-cleavage, the formation of hydrogen and acetonyl (1-methyloxy or 2-oxopropyl) radical has been discussed. The major question addressed is whether the dynamical processes occur adiabatically on the S₂ surface or diabatically, with non-adiabatic transitions taking place in the course of the reactions.