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
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.
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.