Ab initio MO studies on the photodissociation of C2H2 from the S1 (1Au) state. Non-adiabatic effects and S-T interaction

Ab initio MO calculations at the EOM-CCSD and CASSCF level have been carried out for several low lying electronic states of C2H2. Many minima on the seam of crossing and transition states are located. For photodissociation from the S1 state, a non-adiabatic path involving a S1 → S2 transition followed by a S2 → S0 crossing which produces mainly C2H(X2Σ+) is preferred at low energy to the S1 adiabatic path producing mainly C2H(A2II), in agreement with experimental propensity. A minimum on the seam of crossing between S1 and T3 is found. T3 may be a strong perturbing state in the ZAC spectrum of S1.