## Qualitative interpretation=A05-m5aii

In the measured differential cross section of the chemi-ioniation reaction M + X2 M+ + X- a small maximum is observed in the small-angle region, seen in all cross-section curves, that cannot be explained by a classical model stating the general shape of the deflection curve as sketched in figure A05-m5aii-F2 and the equation for the differential cross section
 (E1)

Quantum mechanically this phenomenon can be explained by constructive interference of some contributions, for instance the contributions of the two net repulsive branches of the b- curve. Because of the nearly equal slopes and about equal collision parameters of these branches at small scattering angles, the wavelength of the interference is rather long; a rough estimate has shown that the wavelength has a value of a few degrees at the energies used, which is of the same order of magnitude as the widths of the measured maxima.

Quantum mechanically the simple addition of contributions to the differential cross section according to Eq. (E1), i.e. must be replaced by a formula adding the scattering amplitudes. Especially in the case of an interference structure with a long wavelength the differential cross sections, calculated classically and quantum mechanically could be very different. The composition of the steep repulsive'' branches of the deflection curve (see Fig. A05-m5aii-F2)1 it is expected to give such a long-wavelength structure on the differential cross section. Now we shall give a rough estimation of this wavelength. Following the Ford and Wheeler semiclassical treatment of the scattering amplitude and using the notation of Bernstein the amplitudes of the contributions for repulsive scattering to the differential cross section is given by

where the indices 1 and 2 indicate the covalent'' and ionic'' contributions. The combined amplitude of these two contributions can be written by
From Fig. A05-m5aii-F2 it can be seen that the branches for net repulsive scattering have about equal values of b and about equal slopes for a certain value of . Considering the relations

we conclude
and
from which we obtain directly

In an elementary way the intensity can be found as follows:

and, according to ,

Now we compare two scattering angles and . Then the difference in the argument of the cosine is given by:

Fig. A05-m5aii-F2 shows that in the interesting region the curves are about straight lines so we simplify the last equation into

For two not very different scattering angles and we can roughly estimate a value of by . Then

Using the relation b= L/k (where  Å-1 for K + Br2 at 10.35 eV) it can be seen graphically that at small positive scattering angles the interference of these two contributions has a wavelength of the order of a few degrees at colliding energies of about 10 eV.

Although we have only considered a two-contribution interference instead of four contributions, this interference can explain the observed bump in the differential cross section at small angle.