Ionisation and dissociation of cometary gaseous organic molecules by solar wind particles I: Formic Acid

In order to simulate the effects of energetic charged particles present in the solar wind colliding with the cometary gaseous formic acid molecule (HCOOH), laboratory experiments have been performed. The absolute ionisation and dissociation cross sections for this molecule interacting with solar wind particles were measured employing fast electrons in the energy range of 0.5 to 2 keV and energetic protons with energies varying from 0.128 to 2 MeV. Despite the fact that both projectiles lead to a very similar fragmentation pattern, differences in the relative intensities of the fragments were observed. Formic acid survives about 4-5 times more to the proton beam than to the energetic electron collision.The minimum momentum transfer in the electron impact case was estimated to be 3-38% larger than the minimum momentum transfer observed with the equivelocity protons. The UV photodissociation rates and half-lives for HCOOH are roughly closer to the values obtained with energetic electrons. It is consequently important to take electron impact data into account when developing chemical models to simulate the interplanetary conditions.