Vibrational state controlled bond cleavage in the photodissociation of isocyanic acid (HNCO)

We report the bond selected photodissociation of isocyanic acid (HNCO). This molecule dissociates from its first excited singlet state, breaking either the N–H bond to form H+NCO (X 2Π) or the C–N bond to form NH (a 1Δ)+CO (1∑+). The threshold for production of NH lies about 3900 cm−1 above that of NCO, and we detect both of these channels by laser induced fluorescence on either the NH or the NCO fragment. Dissociating the molecule out of a vibrationally excited state on its ground electronic surface containing four quanta of N–H stretch (4ν1) enhances the efficiency of the NCO channel over the NH channel by a factor of at least 20. We reach this conclusion by comparing the results of such a vibrationally mediated photodissociation experiment to those from a conventional single photon dissociation at the same total energy (about 1000 cm−1 above the threshold for the NH channel). Our estimate of the branching ratio in the one photon dissociation at this energy is roughly ΦNCO/ΦNH≊20, and it grows to ΦNCO/ΦNH≥400 in the vibrationally mediated photodissociation.