Correlated distributions in the photodissociation of HNCO to NH(X3 Σ−, a1Δ) + CO(X1 Σ+) near the barrier on S1

The photodissociation of HNCO is examined at 230.1 nm, near the threshold to NH(a1 Δ) + CO(X1 Σ+). CO products are generated from NH(X3 Σ−) + CO(X1 Σ+) and NH(a1Δ) + CO(X1Σ+). Although the two contributions overlap spectroscopically, they are resolved in velocity space when using photofragment ion imaging. For both channels the CO rotational distribution is cold and non-statistical. A high resolution variant of imaging is employed to obtain NH(a1Δ) distributions correlating with specific JCO levels. Combining these measurements with NH(a1Δ) photofragment yield spectra and rotational distributions, we establish that decomposition to NH(a1Δ) + CO(X1 Σ) evolves over a small barrier on S1, estimated at 400–600 cm−1.