Ultrafast two-step process in the non-adiabatic relaxation of the CH2molecule

CASSCF and MR-CI calculations have been used for the optimization of conical intersections and stationary points and for surface hopping dynamics on the excited-state energy surfaces of CH2 . Scans of the potential energy surfaces in the stretching and torsional coordinates were performed with inclusion of the 3s and 3p Rydberg states. The non-adiabatic relaxation of the system follows a two-step mode. First, the photoexcited system decays from the S2 to the S1 state along the CN stretching coordinate through the planar minimum on the crossing seam (MXS) in just 10 fs. After that, the system needs 30 fs to populate the ground state. Although the S1/S0 MXS occurs at a 90°-twisted structure, the system reaches the seam far from this geometry. Different from analogous systems, the evolution on the S1 state does not follow the torsional coordinate, but the stretching and bi-piramidalization modes.