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 order to calculate the differential cross section , the deflection function of the collision process has to be determined as a first step, which in its turn depends on the potential of the system. The potential parameters and the deflection function are determined by fitting the theoretical cross sections on the experimental ones in the Quantitative interpretation A08-m5bi .

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.