What happens is that the electron jump not only can occur via the `normal' LZ transition, involving the
covalent state, but also via rotational coupling, involving another covalent state, the
state (which is also a low lying state, agreeing with the previous assumption that only low lying states are considered).
The atom-atom model for ion-pair formation in molecular collisions takes into account the Landau-Zener coupling, but not the rotational coupling. The introduction of rotational coupling actually introduces a third state, so that this case then is a three-state problem. However, this three state problem is dealt with as if the cross section is a sum of two two-state problems.
In this module, however, we do not go into detail, as this issue is not explored very thoroughly in the original article either. The necessary background knowledge and computer facilities were not available at that time.
An important theoretical article published a year later by Levine and Bernstein showed that the experimental results could be reproduced by simply applying a radial coupling and an adiabatic representation, such that rotational coupling is not required to explain the results.
This is the module that readers can consult when they want to know how to take the rotational coupling into account in the explanation of chemi-ionisation This module does not argue that rotational coupling is indeed necessary in the explanation, because that part of the Interpretation, where the measurements are compared with the calculations.
The first part of the original section 8.Rotational coupling is recast in this Theoretical methods module. The rest of the section is recast in the Quantitative interpretation module A08-m5bii2.
This module appears late in the essay-type sequential path: after the Interpretation module A08-m5bii1 and before A08-m5bii2, where the rotational coupling is taken into account (see the Map of contents). The same sequence was apparent in the original version of the article: the results were first discussed using the Landau-Zener coupling, before a section was devoted to the idea of rotational coupling.