On the S-1 -> S-0 internal conversion in the photodissociation of HNCO: the role of the NC stretch as a promoting mode

A possible pathway for the S-1/S-0 internal conversion in the photodissocia tion of HNCO in the first absorption band is discussed on the basis of elec tronic structure calculations for the two relevant potential energy surface s. 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 stret ch coordinate are varied. It is concluded that the combined motion of both modes is most efficient in promoting the transition from S-1 to S-0, which is in qualitative accordance with the recent experimental findings. (C) 200 0 Elsevier Science B.V. All rights reserved.