The differential cross sections
for chemi-ionization in M + X_{2} M^{+} + X_{2}^{-}
collisions, in particular in K + Br_{2} K^{+} + Br_{2}^{-},
are explained qualitatively
with a simple classical atom-atom model for ion-pair formation in molecular collisions
* via* the general shape of the deflection function. A quantum mechanical explanation is suggested for the small angles in which the classical interpretation is not valid
.

In order to interpret the results more quantitatively , the potential parameters and the classical deflection function of the system are determined via fitting the results calculated according to the classical model with the experimental results . Based on the potential parameters and the deflection function thus determined, the differential cross section of the collision is calculated and compared to the measurements However, quantitative agreement is poor.