Effects of ion excitation on charge transfer reactions of the Mars, Venus, and Earth ionospheres

Abstract

The absolute reaction cross sections and reaction rate coefficients as a function of photoionisation energy for 25 ion–molecule reactions (charge transfer reactions except for one) have been measured between the most abundant species present as ions or neutral in the Mars, Venus and Earth ionospheres: O 2, N 2, NO, CO, Ar and CO 2. This study shows the strong influence of electronic as well as vibrational internal energy on most ion–molecule reactions. In particular endothermic charge transfer reactions are driven by electronic excitation of O 2 + and NO + ions in their a 4 Π u and a 3 Σ + metastable states, respectively. Moreover, it is shown that lifetimes of these metastable states are sufficient to survive the mean free path in the lowest part of ionospheres and therefore express their enhanced reactivity. The reactions of O 2 + with NO as well as the reactions of CO 2 + with NO, O 2, CO and to a less extent N 2 are driven by vibrational excitation. N 2 + and CO + reactions vary much less with photon energy than the other ones, except for the case of reactions with Ar. The effects of the molecular ion internal energy content on their reactivity must be included in the ionospheric models for most of the reactions investigated in the present work. It is also the case for the effect of collision energy on the CO ++M reactions as we expect that a significant proportion of these CO + could be produced with translational energy by dissociation of doubly charged CO 2 2+, in particular in the Mars ionosphere. Recommended effective rate constant values are given as a function of VUV photon energy.