The monoenergetic vibrational predissociation of expansion cooled NCNO: Nascent CN(V,R) distributions at excess energies 0–5000 cm−1

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Abstract
We report detailed vibration, rotation distributions for nascent CN(X 2∑+), following the one‐photon photodissociation of expansion cooled NCNO via π*←n excitation throughout the region 450–585 nm. At the observed threshold for dissociation (585.3 nm), >90% of the CN product is in v″=0, N″=0, with the remainder in N″=1, corresponding to 〈Erot〉 −1. CN(X 2∑+, v″=0) rotational distributions are obtained at many photolysis wavelengths and rotational levels are observed up to, but never above, the limit imposed by energy conservation: [B″vN″(N″+1)]<E p−D0(v″), where D0(v″) is the dissociation energy to produce CN(X 2∑+,v″) and Ep is the photon energy. CN(X 2∑+,v″=1) and CN(X 2∑+,v″=2) thresholds are observed at photolysis wavelengths which correspond exactly to Ep−D0(v″=1) and Ep−D0(v″=2). These observations can only be reconciled with a vibrational predissociation mechanism and spectroscopic observations suggest that this occurs following internal conversion to the ground state surface. With Ep−D0(v″) less than ∼2000 cm−1, the phase space theory of unimolecular reactions (PST) predicts the CN rotational distributions with high accuracy. However, when product vibrations are accessible, PST cannot be used, since it does not take proper account of the parent being vibrationally excited but rotationally cold. When explicitly taking this into account, we are able to reconcile the present experimental findings with a statistical model and we believe that the behavior observed for NCNO has a sound physical basis and is quite general.

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