Electron stimulated desorption from solid CO₂ and a hydrogen containing CO₂ matrix: A comparative study

We have studied electron stimulated desorption of positive ions from ∼15 monolayer (ML) film of pure solid CO₂ adsorbed on polycrystalline Pt surface at ∼40 K using incident electrons in the energy range 1-31 eV. The positive ions O⁺, C⁺, CO⁺, CO₂⁺, and O₂⁺ were observed and their threshold energies are discussed. Hydrogenation of the CO₂ film was found to reduce the O₂⁺ production but the threshold remained the same as for the pure CO₂ matrix. After hydrogenation of the CO₂ matrix additional ions - H⁺, H₃O⁺, HCO⁺, and H₂CO⁺ - are observed. The formation of formaldehyde cations (with a threshold energy of 17 ± 1.5 eV) in the hydrogenated CO₂ matrix has been observed for the first time and is a product of electron stimulated ion-molecule reactions, possibly via a process analogue to the so-called photosynthesis. These results could be relevant to a better understanding of the atmosphere of Mars.