In the measured differential cross section of the chemi-ioniation reaction
M + X_{2} 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.

.

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.