On the S1 → S0 internal conversion in the photodissociation of HNCO

We discuss a possible pathway for the S-0/S-1 internal conversion in the photodissociation of HNCO in the first absorption band. For this purpose, two-dimensional potential energy surfaces for the lowest two singlet states are calculated using the multi-reference configuration-interaction ab initio method; the NCO bending angle and the out-of-plane torsional angle are varied. According to our calculations, the transition from the S-1 to the S-0 state is likely to occur near a conical intersection at small NCO angles, which can be reached by large-amplitude NCO bending motion. (C) 1999 Elsevier Science B.V. All rights reserved.