Energetics and spin selectivity in the infrared multiphoton dissociation HN3(X̃1A’)→N2(X1Σg+)+NH(X3Σ−,a1Δ)

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The dissociation of HN3(X̃1A’) into ground state N2(X1Σ+g) and the NH radical in its ground (X3Σ−) or first excited (a1Δ) electron state, studied experimentally by Stephenson, King, Cassassa and Foy at NBS using infrared multiphoton and overtone pumping, involves a crossing between the lowest energy singlet and triplet HN3 potential energy surfaces. Ab initio CASSCF and MCSCF calculations have been used to locate the transition states for both channels. The symmetry of the ground state HN3 wavefunctions as well as of the relevant spin‐orbit Hamiltonian with respect to reflection of the spatial and spin components of all the electrons in the plane of the molecule implies that NH products can be formed only in states in which the wavefunction (electronic+rotational) is also symmetric with respect to this operation. This implies that for NH in the X3Σ− electronic state only the F1 and F3 multiplets will be populated in the dissociation process, as seen experimentally.