Photoinitiated decomposition of HNCO near the H plus NCO threshold: Centrifugal barriers and channel competition

The decomposition of jet-cooled HNCO is investigated near the H + NCO chann el threshold [D-0(H+NCO) = 38 370 cm(-1)]. Dissociation to H + NCO at energ ies 17-411 cm(-1) above D-0 (H + NCO) proceeds on the ground potential ener gy surface (S-0), apparently without a barrier. The rotational state distri butions of the NCO(X(2)Pi(3/2),00(1)0) fragment are well described by phase space theory (PST), provided that dynamical constraints are included. Thes e constraints are associated with long range (4-7 Angstrom) centrifugal bar riers, which are significant even near threshold because of the small reduc ed mass of H + NCO, and result in a fraction of energy deposited in fragmen t rotation much smaller than predicted by unconstrained PST. The influence of orientation averaging on the attractive, long-range part of the potentia l is discussed, and it is argued that angular averaging with respect to the center of mass of the rotating polyatomic fragment results in a shift in t he effective potential origin, accompanied by an attenuation of the magnitu de of the potential compared to its value for fixed H-N distance. Following initial S-1((1)A ") S-0 ((1)A') excitation and internal conversion to S-0, HNCO(S-0) decays both via unimolecular decomposition of H + NCO and inters ystem crossing to the dissociative first triplet state, T-1 [yielding NH(X( 3)Sigma(-))+ CO products]. The competition between the two processes is int errogated by monitoring changes in the relative yields of NCO and NH(X(3)Si gma(-)) as a function of excitation energy. It is concluded that near D-0(H + NCO), the S-0 --> T-1 intersystem crossing rate is several-fold faster t han the H + NCO unimolecular decomposition rate. (C) 1999 American Institut e of Physics. [S0021-9606(99)00722-9].