Nonadiabatic effects in the photodissociation of vibrationally excited HNCO: The branching between singlet $(a 1Δ)$ and triplet $(X 3Σ−)$ NH

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.