Ab initio MO studies on the photodissociation of C 2H 2 from the S 1( 1A u) state. II. Mechanism involving triplet states

Abstract

Ab initio calculations at the EOM-CCSD and CASSCF-CASPT2 level have been carried out for low lying triplet states of C 2H 2. Minima on the seam of crossing and dissociation transition states have been found. At a photon energy slightly above the H + C 2H(X 2Σ) asymptote, we propose the following pathway for photodissociation from the S 1 state; C 2H 2 first hops to T 3 upon symmetric HCC bending and torsion, then adiabatically travels onto T 2 along asymmetric HCC bending and finally hops to T 1 to dissociate to H + C 2H(X 2Σ). The proposed mechanism and the reverse barrier height on T 1 are consistent with recent experimental observations.