Nonadiabatic ionic–covalent transitions. Exponential–linear model for the process H++H−→H(2s,2p)+H(1s)

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Abstract
Ion–ion recombination and asymmetrical charge transfer are two of a series of physical phenomena which are commonly explained through transitions taking place in the region where the energies of covalent and ionic states cross. To calculate the transition probability in this region, the Landau–Zener model is currently used, assuming that (i) ionic and covalent states are orthogonal in the crossing region; (ii) their radial coupling is negligible; (iii) the transition region is well localized. For the particular case of the states involved in the process H++H−→H(2s,2p)+H(1s), we show that none of these assumptions is fulfilled. As a conclusion, ab initio, asymptotic, and semiempirical methods which employ Landau–Zener theory to treat ion–covalent transitions should be tested with regards to the validity of the basic assumptions (i), (ii), and (iii); if necessary, an exponential–linear model should replace Landau–Zener theory.