Ab initio MO studies on the photodissociation of C 2H 2 from the S 1 ( 1A u) state. Non-adiabatic effects and S-T interaction

Abstract

Ab initio MO calculations at the EOM-CCSD and CASSCF level have been carried out for several low lying electronic states of C 2H 2. Many minima on the seam of crossing and transition states are located. For photodissociation from the S 1 state, a non-adiabatic path involving a S 1 → S 2 transition followed by a S 2 → S 0 crossing which produces mainly C 2H(X 2Σ +) is preferred at low energy to the S 1 adiabatic path producing mainly C 2H(A 2II), in agreement with experimental propensity. A minimum on the seam of crossing between S 1 and T 3 is found. T 3 may be a strong perturbing state in the ZAC spectrum of S 1.