Coherent oscillation and ultrafast internal conversion of tetrafluoroethene after excitation at 197 nm

Abstract

C2F4 was excited by using a 150 fs pulse in its longest-wavelength band to the Rydberg (3 s) state and then probed by photoionization techniques at 810 nm. The molecule relaxes in two consecutive steps (time constants 29 and 118 fs), probably via the ππ* state, which is lowered in energy by stretching and twisting the C=C bond. A coherent oscillation (350 fs) was found, which we assign to an overtone of the twist vibration (47.6 cm-1) in this state. We also conclude that dissociation to singlet and some triplet CF2 only takes place in the hot ground state of C2F4, from where also the C 2F4 triplet state is populated. The potentials and their conical intersections are discussed with respect to relaxation and dissociation, including also some new considerations of thermal processes.