An apparatus for studying momentum-resolved electron-impact dissociative and non-dissociative ionisation

A spectrometer for recoil ion momentum measurements has been built for studying electron impact ionisation and dissociation of molecules. The apparatus is described in detail, highlighting its capabilities, as well as differences in design from the ones already in use elsewhere. Momentum spectra of ions resulting from 1300 eV electron impact on CO₂ are presented. We observe a broad momentum distribution for the dissociative ionisation reaction leading to the formation of C⁺, and two momentum groups in the CO⁺ and O⁺ channel. By recording multiple ions arising from the same dissociative ionisation event, we also demonstrate the formation of fragment pairs O⁺:CO⁺, C⁺:O⁺, and O⁺:O⁺.