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

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(X²Π_3/2,00¹0) 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.