Penning ionization of simple molecules and their possible role in planetary atmospheres

Ionic species are formed in the atmosphere of Earth and of other planets of the Solar System in various ways, the importance of which depends on specific conditions of the considered environment. They are extremely important for the transmission of radio and satellite signals, and in the upper atmosphere of planets they govern the chemistry of ionospheres. Molecular ions have also been detected in comet tails. In this paper recent experimental results concerning production of simple ionic species of atmospheric interest are presented. The discussed data concerns the formation of free ions in collisional ionization of H2O, H2S and NH3 induced by highly excited species (Penning ionization). The detailed laboratory characterization of such relevant elementary processes leading to the formation of charged atomic and molecular ions could be of importance to understand the atmospheric chemistry of terrestrial and extraterrestrial environments. In fact, the effect of Penning ionization has not been considered in the modeling of terrestrial and extraterrestrial objects so far, even though metastable helium is known to be formed by radiative recombination of He+ ions with electrons. Because helium is the second most abundant element of the Universe, Penning ionization of molecules by He*(23S1) is plausibly an active route of ionization in relatively dense environments exposed to cosmic rays.