Conical intersections and strong nonadiabatic coupling effects in singlet-excited acetylene: An ab initio quantum dynamical study

The conically intersecting potential energy surfaces of the S₁ and S₂ excited states of acetylene and the resulting strong nonadiabatic couplings are investigated theoretically. The adiabatic potential energy surfaces are obtained from high-level MRCI calculations. They are diabatized using the concept of regularized diabatic states and then used as a basis for the subsequent wave-packet dynamical treatment of the nuclear motion. All three angular degrees of freedom are included in the present study, while the bond lengths are kept frozen. The importance of the nonadiabatic interactions for the fine structure of the VUV spectrum of acetylene in the 6.5-8 eV excitation energy range is established. The electronic populations display an S₂ → S₁ internal conversion process on the order of 50 fs, which is, however, incomplete owing to the relatively small S₂-S₁ energy gap and the present reduced-dimensionality treatment. Snapshots of the wave-packet as well as angular probability densities are analyzed and reveal, for the first time, an incipient excited-state cis-trans isomerization in this system.