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## Abstract=A08-m1d

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The purpose of this study is to test a semiclassical calculation method and the suitability of the atom-atom model for ion pair formation to explain the chemi-ionization that forms the first step in a harpoon reaction .

Relative differential cross sections have been measured for the atom-atom collision process at kinetic energies from 13 to 85 eV:

We have used a molecular beam set-up , including a charge exchange sodium source and a hybrid between a collision chamber and a secondary beam for the iodine. This set-up allows for the experimental resolution of the Stueckelberg oscillations and the rainbow structure. The Stueckelberg oscillations are due to the interference of scattering from different potentials inside the pseudo-crossing of the covalent and ionic potential curves. Interference due to scattering from the ionic interatomic potential causes a rainbow structure that has been resolved completely.

The measurements allow estimations of the covalent potential parameters and the pseudocrossing parameter H12 . Based on the potential, semiclassical differential cross sections have been calculated via the deflection curve :

In this calculation, we have used the lowest-order stationary-phase approximation, JWKB phase shifts and the Landau-Zener transition probability .

When the known ionic potential and the determined covalent one are substituted, there is a very good agreement between the calculated and measured differential cross sections due to collisions with small and intermediate impact parameters . For large impact parameters a rather serious disagreement arises between the relative intensities as well as the oscillation wavelengths of the corresponding cross section . The intensity discrepancy has been removed taking into account the phenomenon of rotational coupling .

We conclude that the atom-atom model for ion-pair formation via potential curve crosssing by means of Landau-Zener coupling and rotational coupling, indeed explains the experimental results for atom-atom collisions if the semiclassical calculation method is used . Therefore, in future research the case of atom-molecule collisions can be addressed .