Ionization Cross Section Measurement for Molecules in Mercury's Exosphere

Electron impact ionization is widely used for neutral mass spectrometry; its energy and species dependent efficiency must be empirically characterized for instrument calibration. Ionization cross sections for a variety of elements and molecules have been measured, notably for simple molecules, atmospheric compounds, and hydrocarbons. Past investigations have usually focused on species that are gaseous in standard laboratory conditions which are easiest to handle and most commonly encountered by ground-based mass spectrometers. Space-based applications (e.g. studying planetary atmospheres or cometary outgassing) involve a much wider variety of species, including refractory molecules in gas phase, that have never been measured. A new facility has been built to determine the cross sections of these species using an evaporative source to generate a molecular beam of refractory compounds. In addition to the absolute cross sections for ion production of the original species, this experiment also examines the fragmentation products and their dissociation energies. Ionization energies are probed from threshold to 1 keV. We are specifically interested in molecules that may contribute to the exosphere of Mercury that could explain the anomalous hot distributions of magnesium and calcium that have been seen by Messenger. By targeting possible progenitor molecules of the observed atomic species (i.e. CaO, MgO, and other oxides/hydroxides) we aim to prepare for the next mission to Mercury by BepiColombo. Preliminary results will be presented for these new compounds as well as baseline cross sections of well-studied species for validation.