We have studied electron stimulated desorption of positive ions from similar to 15 monolayer (ML) film of pure solid CO2 adsorbed on polycrystalline Pt surface at similar to 40 K using incident electrons in the energy range 1-31 eV. The positive ions O+, C+, CO+, CO2+, and O-2(+) were observed and their threshold energies are discussed. Hydrogenation of the CO2 film was found to reduce the O-2(+) 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.