On the S1→S0 internal conversion in the photodissociation of HNCO: the role of the NC stretch as a promoting mode

A possible pathway for the S1/S0 internal conversion in the photodissociation of HNCO in the first absorption band is discussed on the basis of electronic structure calculations for the two relevant potential energy surfaces. The ab initio calculations are performed at two levels of theory: multi-reference configuration interaction (MRCI) and complete-active space second-order perturbation theory (CASPT2). The NCO bending angle and the NC stretch coordinate are varied. It is concluded that the combined motion of both modes is most efficient in promoting the transition from S1 to S0, which is in qualitative accordance with the recent experimental findings.