Production of ions at high energy and its role in extraterrestrial environments

Ions in space are formed in various ways, the relevance of which depends on the specific conditions of the considered extraterrestrial environment. The interaction of neutral molecules with cosmic rays, UV photons, X-rays and other phenomena such as shock waves are all important processes for their production. Once formed, ions are able to drive some chemistry that leads to molecular ions and, via dissociative recombination, molecules of increasing complexity. Therefore, ion-chemistry plays a pivotal role in the chemistry of the interstellar medium and upper planetary atmospheres (ionospheres). Molecular ions have also been detected in comet tails. In this paper, we briefly review experimental results obtained in our laboratory and concerning the formation of free ions in two different types of processes: double photoionization and collisional ionization induced by highly excited species (Penning ionization). A detailed characterization of these processes in laboratory experiments is of crucial importance to understand the chemistry of extraterrestrial environments. The double photoionization occurs when high energy photons are available and, as dications are generally metastable and rapidly decay into two fragment ions, contributes to the global budget of ion formation. The effects of Penning ionization have never been considered in modeling 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.