[Legenda]
[Contents of the thesis]
[Comments on this module]
[Legenda]
[Contents of the thesis]
[Comments on this module]

According to the atom-atom model for ion-pair formation in molecular collisions, chemi-ionization in two-body collisions takes place via pseudo-crossing of the potential energy surfaces of the interacting atoms

Potential curves

Fig. A08-m3ci1-F1 shows the lowest ionic and covalent potential curves of sodium iodide. The general shape of the curves is explained in a mesoscopic module, and the precise shape is determined in another module . The species of the ionic electronic state, indicating the symmetry and multiplicity properties, is given by . By building up the molecule from the two separate neutral particles Na(²S_1/2) and I(²P_3/2), the LS-coupling gives rise to eight molecular states. One of them has the same species as the ionic state, which can according to the Neumann-Wigner rule give rise to transitions. We assume that we can ignore the exited covalent state, only taking into account the lowest states of the system, thus reducing the case to a two-state problem.

For the lowest states the important parameter H12 has been estimated experimentally by Moutinho et al. from total cross section measurements for chemi-ionization of Na + I, giving a value H12=0.05 eV. Two different types of theoretical calculations by Herschbach and Grice result in the values of 0.06 and 0.09 eV. These values of H12 all give, in our energy range from 10 up to 100 eV, a transition probability Pb of the order of 1/2 for the pseudo-crossing of the ionic and lowest covalent state. An estimation of the internuclear crossing distance of the ionic and excited covalent state leads to a value of the relevant H12 much smaller than for the former pseudo-crossing. Then the diabatic transition probability for the outer pseudo-crossing hardly differs from unity, so the excited covalent state is an unimportant outgoing channel. The first excited covalent state with species 1MS+ does have an avoided crossing with the ionic state at large internuclear distance, but this covalent state is not an incoming channel in the collisions considered, because all the thermal dissociated iodine atoms are in the 2P3/2 state. Therefore we shall ignore the excited covalent state in our considerations.

Then, for our collision process, one of the eight collisions has the probability P_b for a diabatic transition at a single passage of the pseudo-crossing at R_c, given by the Landau-Zener formula .

Summarising, the following approximations have been used:

1.

The Landau-Zener transition-probability formula.

2.

The use of the Landau-Zener formula to collisions where the distance of closest approach R₀ and the distance of pseudo-crossing R_c are not well separated

3.

The use of the diabatic potentials in the classical deflection function in spite of small deformation of the curves at the pseudo-crossing.

4.

The use of a transition point in spite of a transition region around the pseudo-crossing predicted by the Landau-Zener theory.

5.

The neglect of rotational coupling so far