The 3s Rydberg state as a doorway state in the ultrafast dynamics of 1,1-difluoroethylene

The deactivation dynamics of 1,1-difluoroethylene after light excitation is studied within the surface hopping formalism in the presence of 3s and 3p Rydberg states using multi-state second order perturbation theory (MS-CASPT2). Due to the proximity of the Rydberg -3s state with the * state, the states are mixed favoring ultrafast exchange of population via a conical intersection that closely resembles the equilibrium structure. After excitation, it is found that the -3s state acts as a doorway state, trapping the population and delaying internal conversion to the * state, from which deactivation to the closed-shell ground state takes place. Besides the conical intersection between the -3s and * states, five additional conical intersections between the * state and the ground state are found, indicating that after the system is excited, it stretches the C?C bond before it twists and pyramidalizes at any of the carbon atoms, in the spirit of a hula-twist mechanism.