Vibrationally mediated photodissociation of isocyanic acid (HNCO): Preferential N-H bond fission by excitation of the reaction coordinate

We report the bond selected photodissociation of HNCO from a vibrationally excited state containing three quanta of N-H stretch (3v(1)) and demonstrate that initial vibrational state preparation strongly influences the photochemical branching in HNCO, producing either NCO (X (2) Pi) from cleavage of the N-H bond or NH (a (1) Delta) 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 3v(1) 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 (Phi(NCO)) 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. (C) 1996 American Institute of Physics.