The experimental differential cross sections for chemi-ionization in M + X2 -> M+ + X2- are analysed using a simple classical atom-atom model for ion-pair formation in molecular collisions . The transition to the ionic state takes place via crossing of the neutral and ionic ground states. The electron transition probability is calculated applying the Landau-Zener approximation; trajectories are calculated using the impact parameter approximation . We assume an isotropic intermolecular potential and neglect the internal states of the X2 molecule.
Actually the interatomic M-X-X potential should be described by hypersurfaces to include for instance the vibrational states of the and X2- molecules. However, we cannot resolve the vibrational structure from the differential cross section, so these effects only have an averaging effect on the measurements.
Unfortunately the effect of vibrational excitation is rather large for the collisions of interest.
From the semi-empirical Br2 and Br2- interatomic-potential curves of
it can be seen that even at a very restricted low vibrational-state distribution of Br2, the exothermic Br2 -> Br2-
transition can cause a high vibrational excitation or even
dissociation of the Br2- molecule, giving a wide range of possible values of the electron affinity
=I(M) - A(X2) ,
where I(\rm M) represents the ionization potential of the alkali atom and A (X2) the electron affinity of the halogen molecule, this means that in a chemi-ionization process is very dependent on the vibrational state of the halogen molecule before and after the collision.
Indeed, the largest possible bromine-electron affinity A(Br2), as measured by Baede and Los , 2.8 eV, is very different from our value of 1.2 eV for the vertical electron affinity the Quantitative interpretation.
Therefore this simplification is not really valid.
Therefore this simplification is not really valid either.