Ab initio MO studies on the photodissociation of C2H2 from the S-1((1)A(u)) 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 slates of C2H2. Many minima on the seam of crossing and transition slates 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 C2H(X(2) Sigma(+)) is preferred at low energy to the S-1 adiabatic path producing mainly C2H(A(2) Pi)), 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.