Potential-energy surface control of the NH product state distribution in the decomposition reaction HN3(X 1A′)→ NH(a 1Δ)+ N2(X 1Σ+g)

A model is presented to explain quantitatively the Λ doublet propensities observed by King, Stephenson, Foy and Casassa in the infrared multiphoton and vibrational overtone decomposition of HN3 to yield NH(a 1Δ) fragments. This Franck–Condon-type model involves the projection of the electronic vibrational wavefunction of the HN⋯NN system at the transition state onto the electronic-rotational wavefunction of the free NH rotor. Ab initio calculations are used to estimate the angular extent of the torsional degrees of freedom which subsequently become converted to rotations of the NH fragment. This model deals explicitly with the two-electron character of the 1Δ state. The experimentally observed ratio of Δ(A′) to Δ(A″)Λ doublet populations vs. the rotational quantum number J of the NH fragment, as well as the observed average rotational excitation of the NH products, are well predicted. In contrast, the observed v, J correlation is not as well predicted. The model presented here represents an advance over those previously developed to explain Λ doublet propensities in that it is based on properties of the specific system under consideration.