Ultrafast non-radiative decay via conical intersections of molecular potential-energy surfaces: C 2H 4+

Abstract

Conical intersections of molecular potential-energy surfaces are shown to lead to ultrashort (decay time ⪡ 1 ps) non-radiative decay of electronically excited molecules. In this work we calculate the ultrashort decay time for the Ā-X̄ conical intersection in the ethylene radical cation with the aid of ab initio derived parameters and obtain a value of 0.7 × 10 −14 s. We perform the calculations by exact (within the model hamiltonian adopted) numerical integration of the time-dependent Schrödinger equation. The results are compared to those obtained within a Green's function approach. They show a pronounced mutual influence of the vibrational levels of the upper potential-energy surface on the non-radiative decay time. By analyzing the time evolution of the molecular wavefunction on a microsecond time scale we give arguments for a strong quenching of the radiative rate. This rationalizes the absence of detectable emission in C 2H 4 − and other radical cations. The importance of conical intersections and their impact on molecular dynamics in open-shell systems is pointed out.