Potential energy surfaces and nonadiabatic transitions in the asymptotic regions of ICN photodissociation to study the interference effects in the F and F spin-rotation levels of the CN products.

One of the most spectacular yet unsolved problems for the ICN -band photodissociation is the non-statistical spin-rotation F = N + 1/2 and F = N - 1/2 populations for each rotation level N of the CN fragment. The F /F population difference function f(N) exhibits strong N and λ dependences with an oscillatory behavior. Such details were found to critically depend on the number of open-channel product states, namely, whether both I ( P ) and I ( P ) are energetically available or not as the dissociation partner. First, in the asymptotic region, the exchange and dipole-quadrupole inter-fragment interactions were studied in detail. Then, as the diabatic basis, we took the appropriate symmetry adapted products of the electronic and rotational wavefunctions for the F and F levels at the dissociation limits. We found that the adiabatic Hamiltonian exhibits Rosen-Zener-Demkov type nonadiabatic transitions reflecting the switch between the exchange interaction and the small but finite spin-rotation interaction within CN at the asymptotic region. This non-crossing type nonadiabatic transition occurs with the probability 1/2, that is, at the diabatic limit through a sudden switch of the quantization axis for CN spin S from the dissociation axis to the CN rotation axis N. We have derived semiclassical formulae for f(N) and the orientation parameters with a two-state model including the 3A' and 4A' electronic states, and with a four-state model including the 3A' through 6A' electronic states. These two kinds of interfering models explain general features of the F and F level populations observed by Zare's group and Hall's group, respectively. © 2018 Wiley Periodicals, Inc.