Using a simple classical atom-atom model for ion-pair formation in molecular collisions , we interpret the experimental differential cross section for chemi-ionization in alkali halide systems, by comparing the experimental cross sections to theoretical cross sections calculated with the model.

The differential cross sections for chemi-ionization of
K + BR_{2}
at colliding energies of 10.35 and 6.9 eV have been calculated
,
in a procedure in which the differential cross section is determined * via* the potential curves of the system and the classical deflection function
,
by fitting the calculated cross section with the experimental one
.

The theoretical cross section is expressed by

(E1) |

(E2) |

where E

The resulting classically calculated chemi-ionization differential
cross section for K + BR_{2}
are shown in Fig.A05-m5bii-F1.

The general shape of this calculated differential cross section agrees with the measured cross section, and therefore the simple classical atom-atom model gives a qualitative interpretation of the measurements.

The qualitative agreement between the calculated curves with the measured curves is good but there is only a poor quantitative agreement.

Of course, a bad agreement for the ``ionic''
part of the differential cross section is expected because of the
very different results for the rainbow structure as calculated
classically and quantum mechanically. Due to the choice of
and *H*_{12}, Fig. A05-m5bii-F1 shows the agreement of the inelasticity shifts and curve
ratio; at the same time the sensitivities of the determination of the
parameters
and are shown.

For the the estimated value of *H*_{12}, the value of
* P _{b}*(1-

Now let us make a comparison between the differential cross section
of K + BR_{2} and the measured one of Li + BR_{2}.
A few estimates can be made easily. For Li + BR_{2}
the minimum in the cross-section curve for b = R_{c}
scattering occurs at as compared to
135 eV . degree for K + BR_{2}.
Because
the inelasticity of the
Li + BR_{2}
collision will be larger. Indeed the endothermicity must be
1.1 eV larger due to the differences of ionization potential:
I(Li) = 5.4 eV
and
I(K) = 4.3 eV. The classical rainbow at
indicates a larger well depth of the ionic potential curve of Li-Br_{2}.

The relative differential cross sections of
K + BR_{2} and K + I_{2}
are nearly completely identical so a good similarity of the molecular
constants can be expected. Duchart et al.
have measured the K +I_{2} differential cross section for elastic scattering at a kinetic collision energy of l00 eV. The distances between the maxima of their
resolved rainbow are equal to the supernumerary spacing that we
should predict for K + BR_{2} ionization scattering at 100 eV.

Thus the potential parameters we determined are rather reliable.