Ultrafast two-step process in the non-adiabatic relaxation of the CH₂ molecule

Abstract

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.

Keywords:

• MRCI photodynamics
• Ultrafast chemistry
• Surface hopping

Acknowledgements

The authors thank Dr J. Pittner and Professor Bonačić-Koutecký for the surface hopping dynamics program and helpful discussions. The authors acknowledge support by the Austrian Science Fund within the framework of the Special Research Program F16 and Project P14817-N03. Mario Barbatti thanks for financial support from the Brazilian funding agency CNPq. The calculations were performed in part on the Schrödinger II cluster of the University of Vienna.