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
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(2S1/2)
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 Pb for a diabatic transition at a single passage of the pseudo-crossing at Rc, given by the Landau-Zener formula .
Summarising, the following approximations have been used: