Photofragment orientation as a probe of near-threshold non-adiabatic phenomena in the photodissociation of ICN

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Abstract
Cyanogen iodide (ICN) is photodissociated at 249 nm. The CN(X 2Σ+, v = 2) fragments are probed by sub-Doppler laser-induced fluorescence (LIF) using circularly polarized light, which allows resolution of the F1(J = N + ½) and F2(J = N - ½) spin-rotation doublets. These components are shown to be oriented, i.e. to possess a non-statistical non-centrosymmetric MJ quantum-state distribution. The sign and magnitude of the orientation vary between components and are shown to have some correlation with the energetic threshold for production of the I(2P1/2) spin-orbit state. These observations are briefly discussed in the context of breakdown of the Born-Oppenheimer approximation in this system.