Ab initio MO studies on the photodissociation of C2H2 from the S1(1Au) state. II. Mechanism involving triplet states

Ab initio calculations at the EOM-CCSD and CASSCF-CASPT2 level have been carried out for low lying triplet states of C2H2. Minima on the seam of crossing and dissociation transition states have been found. At a photon energy slightly above the H + C2H(X2Σ) asymptote, we propose the following pathway for photodissociation from the S1 state; C2H2 first hops to T3 upon symmetric HCC bending and torsion, then adiabatically travels onto T2 along asymmetric HCC bending and finally hops to T1 to dissociate to H + C2H(X2Σ). The proposed mechanism and the reverse barrier height on T1 are consistent with recent experimental observations.