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According to the atom-atom model for ion-pair formation in molecular collisions, chemi-ionization in two-body collisions takes place via pseudo-crossing of the potential energy surfaces of the interacting atoms
shows the lowest ionic and covalent
potential curves of sodium iodide. The general shape of the curves is explained in a mesoscopic module, and the precise shape is determined in another module
The species of the ionic electronic state, indicating the symmetry and multiplicity
properties, is given by . By building up the molecule from
the two separate neutral particles Na(2S1/2)
LS-coupling gives rise to eight molecular states. One of them has the
same species as the ionic state, which can according to the Neumann-Wigner rule give rise to transitions. We assume that we can ignore the exited covalent state, only taking into account the lowest states of the system, thus reducing the case to a two-state problem.
Then, for our collision process, one of the eight collisions has the probability Pb for a diabatic transition at a single passage of the pseudo-crossing at Rc, given by the Landau-Zener formula .
Summarising, the following approximations have been used: