The decomposition of jet-cooled HNCO is investigated near the H+NCO channel threshold [D0(H+NCO)=38 370 cm-1]. Dissociation to H+NCO at energies 17-411 cm-1 above D0(H+NCO) proceeds on the ground potential energy surface (S0), apparently without a barrier. The rotational state distributions of the NCO(X2Π3/2,0010) fragment are well described by phase space theory (PST), provided that dynamical constraints are included. These constraints are associated with long range (4-7 Å) centrifugal barriers, which are significant even near threshold because of the small reduced mass of H+NCO, and result in a fraction of energy deposited in fragment rotation much smaller than predicted by unconstrained PST. The influence of orientation averaging on the attractive, long-range part of the potential 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 the effective potential origin, accompanied by an attenuation of the magnitude of the potential compared to its value for fixed H-N distance. Following initial S1(1A″)←S0(1A′) excitation and internal conversion to S0, HNCO(S0) decays both via unimolecular decomposition of H+NCO and intersystem crossing to the dissociative first triplet state, T1 [yielding NH(X3∑-)+ CO products]. The competition between the two processes is interrogated by monitoring changes in the relative yields of NCO and NH(X3∑-) as a function of excitation energy. It is concluded that near D0(H+NCO), the S0→T1 intersystem crossing rate is several-fold faster than the H+NCO unimolecular decomposition rate.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry