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Theoretical methods = A05-m3c

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 [Details on the model in a mesoscopic module (link type: 'IS DETAILED IN/wider range/project'; target: MESO3c-mod)]. 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 [Details on the calculation methods in a mesoscopic module (link type: IS DETAILED IN/wider range/project; target: MESO-m3c-defl)]. We assume an isotropic intermolecular potential and neglect the internal states of the X2 molecule.

Applicability of the model to the measured collison:

If we apply this model to the system of K-Br2, we assume that the following simplifications are valid

Simplification 1: no internal states Br2

We simplify the potential surfaces by neglecting the vibrational-state distribution and describe the collision as a two-body problem.

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 Person [The role of this vibrational excitation is argued and explained in R2  (link type: 'IS ARGUED IN/IS EXPLAINED IN/input from/external'; target:  Rf(A05)2-m*] 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 A(Br2). As
  =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 [Standpoint supported by disagreement with earlier measurements (link type: IS ARGUED IN/disagrees with/project; target:  A03-m4b)], 2.8 eV, is very different from our value of 1.2 eV for the vertical electron affinity [Input of this value from the Quantitative interpretation to support standpoint (link type: 'INPUT FROM/is argued in'; target:  interpretation A05-m5bii] the Quantitative interpretation.

Therefore this simplification is not really valid.

Simplification 2: isotropic intermolecular potential

Another simplification is made by neglecting anisotropy effects. The anisotropic potentials will also have an averaging effect on the measurements. [Calculations supporting the standpoint that the simplification is unwarranted (link type: IS ARGUED IN target:  R(A05)3)] has determined by classical trajectory calculations the chemi-ionization differential cross section of K + Br2 taking into account a nonspherical bromine molecule. His results for collinear and perpendicular collisions are quite different.

Therefore this simplification is not really valid either.