Predissociation of the c 1Π state of NH (ND): The role of dipolar spin–spin coupling

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Abstract
The lifetimes of the NH(c1 Π, v’=0,1) and ND(c 1Π, v’=0) levels are considered. Both radiative c 1Π→(a 1Δ, b 1Σ+) decay and radiationless decay are investigated. It is shown that a direct coupling of the c1Π state to the dissociative 15Σ− state, of magnitude approximately 1 cm−1, explains the known predissociation of the NH (c 1Π, v’=1) level. In this case the theory predicts that in the v’=0 manifold of NH even the low J’ rotational levels will also be predissociated. Predissociation of the v’=0 level in NH was not previously anticipated. The origin of the c 1Π∼1 5Σ− perturbation is considered. It is shown that this coupling originates principally from the dipolar spin–spin interaction, with a smaller contribution from second order spin–orbit coupling. Because the predissociation results from quantum mechanical tunneling, it is of reduced importance in ND.