According to the atomatom model for ionpair formation in molecular collisions, chemiionization in twobody collisions takes place via pseudocrossing of the potential energy surfaces of the interacting atoms
Fig. A08m3ci1F1 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(^{2}S_{1/2})
and
I(^{2}P_{3/2}), the
LScoupling gives rise to eight molecular states. One of them has the
same species as the ionic state, which can according to the NeumannWigner 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 twostate problem.

For the lowest states the important parameter H_{12} has been estimated experimentally by Moutinho et al.
from total cross section measurements for chemiionization of
Na + I, giving a value H_{12}=0.05 eV. Two different types of
theoretical calculations by Herschbach and Grice
result in the values of 0.06 and 0.09 eV. These values of H_{12} all give, in
our energy range from 10 up to 100 eV, a transition probability P_{b} of
the order of 1/2 for the pseudocrossing of the ionic and lowest
covalent state.
An estimation of the internuclear crossing distance
of the ionic and excited covalent state leads to a value of the
relevant H_{12}
much smaller than for the former pseudocrossing. Then the diabatic transition probability for the outer pseudocrossing
hardly differs from unity, so the excited covalent state is an unimportant outgoing channel.
The first excited covalent state with species ^{1}MS^{+} does have an avoided crossing with the ionic state at large internuclear distance,
but this covalent state is not an incoming channel in the collisions
considered, because all the thermal dissociated iodine atoms are in
the ^{2}P_{3/2} state.
Therefore we shall ignore the excited covalent state in our considerations. 


Then, for our collision process, one of the
eight collisions has the probability P_{b} for a diabatic transition at a single passage of the pseudocrossing at R_{c}, given by the
LandauZener formula
.
Summarising, the following approximations have been used:
 1.
 The LandauZener transitionprobability formula.
 2.
 The use of the LandauZener formula to collisions where the
distance of closest approach R_{0} and the distance of pseudocrossing R_{c} are not well separated
 3.
 The use of the diabatic potentials
in the classical deflection function in spite of small deformation of
the curves at the pseudocrossing.
 4.
 The use of a transition point in spite of a transition region
around the pseudocrossing predicted by the LandauZener theory.
 5.
 The neglect of rotational coupling so far