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Abstract=A08-m1d

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The purpose of this study [To the module Central problem (link type: to contents/is detailed in/is focused on in/is clarified in; target: A08-m2b] is to test a semiclassical calculation method and the suitability of the atom-atom model for ion pair formation [This model is clarified in a mesoscopic Theoretical methods module (link type: 'IS CLARIFIED IN/is detailed in/is focused on in/Project/To contents'; target: MESO-m3c-mod] to explain the chemi-ionization that forms the first step in a harpoon reaction [To the module Situation (link type: to contents/is detailed in/is focused on in/is clarified in; target: A08-m2a].

Relative differential cross sections have been measured for the atom-atom collision process $\Na+\I\to\Na^++\I^-$ at kinetic energies from 13 to 85 eV:
[The experimental results are given in a module Treated results (link type: TO CONTENT/is detailed in/is focused on in/is clarified in; target A08-m4bi1)]
We have used a molecular beam set-up [To the module Experimental methods (link type: to contents/is detailed in/is focused on in/is clarified in; target: A08-m3a], 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 [The parameters are estimated in a Quantitative interpretation module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m5bi)]. Based on the potential, semiclassical differential cross sections have been calculated via the deflection curve [The calculation is given in a Quantitative interpretation module (link type: TO CONTENT/is detailed in/is focused on in/is clarified in; target A08-m5bii1)]:
[To a detailed and focused account of the theoretical treated results (link type: TO CONTENT/is detailed in/is focused on in/is clarified in; target A08-m4bii2)]
In this calculation, we have used the lowest-order stationary-phase approximation, JWKB phase shifts [Details about these approximations are given in a Theoretical methods module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m3cii)] and the Landau-Zener transition probability [This transition probability is given in a Theoretical methods module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m3ci1)].

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 [The qualitative discussion of the results takes place in the Qualitative interpretation module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m5a)]. For large impact parameters a rather serious disagreement arises between the relative intensities as well as the oscillation wavelengths of the corresponding cross section [This disagreement is discussed in a Quantitative interpretation module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m5bii1)]. The intensity discrepancy has been removed taking into account the phenomenon of rotational coupling [Rotational coupling is given in a Theoretical methods module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m3ci2)][The impact of rotational coupling is discussed in a Quantitative interpretation module (link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m5bii2)].

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 [This conclusion is presented in the module Findings [link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m6a)]. Therefore, in future research the case of atom-molecule collisions can be addressed [This suggestion is made in the module Leads for further research [link type: TO CONTENTS/is detailed in/is focused on in/is clarified in; target A08-m6b)].