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

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Abstract
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.