We report the bond selected photodissociation of HNCO from a vibrationally excited state containing three quanta of N–H stretch (3ν1) and demonstrate that initial vibrational state preparation strongly influences the photochemical branching in HNCO, producing either NCO (X  2Π) from cleavage of the N–H bond or NH (a 1Δ) from cleavage of the C–N bond. Initial excitation of the N–H dissociation coordinate by excitation of the N–H stretching overtone enhances the probability for breaking the N–H bond in the electronically excited state. Compared to isoenergetic photolysis of the ground vibrational state, photodissociation of the 3ν1 state alters the NCO quantum yield by roughly a factor of 4 at the largest photolysis energy used in this work, changing the channel that breaks the N–H bond from the minor to the major dissociation pathway. In addition, the experiment measures the quantum yields for production of NCO (ΦNCO) in the one-photon dissociation at three different photolysis wavelengths and provides a correction for the influence of photodissociation from vibrationally and rotationally excited states.

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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.

Initial vibrational excitation of a state containing three quanta of N–H stretch (3ν1) decreases the fractional photolysis yield of NH (a 1Δ) relative to NH (X 3Σ−) by a factor of approximately two compared to the isoenergetic photodissociation of a 300 K thermal sample of HNCO. At a total energy of 43 480 cm−1, NH (a 1Δ) accounts for 24% of the total NH yield in the direct photolysis but only 10% in the photodissociation of 3ν1. At 44 440 cm−1, the NH (a 1Δ) yields are 65% and 32% in the single photon and two-step photodissociations, respectively. The variation in branching ratio may arise from dynamical behavior that is closely related to the preferential production of NCO in the photolysis of vibrationally excited HNCO. The initial vibrational excitation has no influence on the rotational and vibrational distributions of NH (X 3Σ−), but it significantly increases the amount of energy in rotation of NH (a 1Δ). These results, along with several recent experimental and theoretical studies, suggest the participation of at least three different potential energy surfaces in the photodissociation of isocyanic acid.