One of the most spectacular yet unsolved problems for the ICÑ A-band photodissociation is the non-statistical spin-rotation F 1 = N + 1/2 and F 2 = N − 1/2 populations for each rotation level N of the CN fragment. The F 1 /F 2 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 ( 2 P 3/2 ) and I ( 2 P 1/2 ) 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 1 and F 2 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 0 and 4A 0 electronic states, and with a four-state model including the 3A 0 through 6A 0 electronic states. These two kinds of interfering models explain general features of the F 1 and F 2 level populations observed by Zare's group and Hall's group, respectively.It is known that the ground state of the I atom has j = 3/2, thus yields the non-zero quadrupole moment due to the non-spherical charge distribution, while the spin-orbit excited state has j = 1/2, WWW.C-CHEM.ORG FULL PAPERWiley Online Library J. Comput. Chem. 2019, 40, 482-499 485 the space-fixed (SF) Z 0 axis is taken as the propagation direction for circularly polarized light (Fig. 2). Then, we focus on the orientation of the SF total angular momentum of photofragment CN. This orientation is defined asHere, J Z 0 M SF J is the space-fixed Z 0 component of the total angular momentum CN, and ρ J ð Þ M SF J M SF J is the relative population observed in the level M SF J and is normalized as P M SF J ρ J ð Þ M SF J M SF J

ICN is one of the few simple triatomic molecules whose photodissociation mechanisms have been thoroughly investigated. Since it has a linear structure in the electronic ground state, the dissociation follows a photoexcitation at a linear or slightly bent structure. It is generally believed that the Ã-band consists of the dominant excitation to 3Π0+ (4A′) with the transition dipole moment (TDM) parallel to the molecular axis (z), a slightly weaker transition to 1Π1 (5A′, 4A″), and a much weaker transition to 3Π1 (3A′, 2A″), both of the latter two having perpendicular TDMs. In the present work, we have theoretically studied the geometry dependence of these TDMs and found a pronounced θ (bending angle) dependence in the parallel (z) component of the TDMs to 1Π1 (5A′) and 3Π1 (3A′), both of which should be zero at a linear geometry by symmetry and thus have been previously ignored. We estimated that the z component TDM to 1Π1 (5A′) has a contribution of 15–20% to the total absorption cross-section at 249 nm at room temperature. Interestingly, the TDM to 3Π0+ (4A′) does not exhibit such θ dependency and thus has only the z component. We compare the TDMs of ICN and CH3I molecules having similar excited states. The fact that all the TDMs to 3A′, 4A′, and 5A′ have nonnegligible z components implies the importance of the coherent excitation contributions to various observables of CN fragment, such as the anisotropy parameter, the orientation parameter, and the rotational level distribution as well as the rotational fine structure level distribution.