For a quantitative explanation of the measured differential cross sections for the chemi-ionization process in collisions between *Na* and *I* , we calculate the theoretical cross sections using a semi-classical atom-atom model for ion-pair formation in molecular collisions .

In the first specific calculation of the differential cross section, we only take into account the Landau-Zener coupling between the potential curves of the system when we determine the theoretical differential cross section. Comparison between the experimental and the theoretical curves shows some persisting discrepancies . Therefore we take into account rotational coupling as well, which explains the discrepancies .