The atom-atom model for ion-pair formation
explains chemi-ionization in collisions *via* crossing of the diabatic potential energy surfaces of the system, or in other words, *via* a transition between two states of the system. A transition between two states can take place when the states are electronically coupled
.

The atom-atom model for ion-pair formation takes into account the Landau-Zener transition probability between electronically coupled states of the same species . The model can be further refined by also taking into account rotational coupling .

The differential cross section of chemi-ionization in sodium iodine collisions can be calculated *via* the potential curves of the system and the deflection function
. For the semiclassical treatment of the cross section, the different contributions to the differential cross section are to be approximated. Here the stationary phase approximation, the uniform approximation and JWKB phase shifts are used
.