A new experimental set-up aimed at analyzing surfaces by the MIES technique is described. It was successfully tested on CO₂-exposed polycrystalline Mg. Spectra exhibited structures related to the CO₃^2- electronic orbitals. Mg oxidation leads to the occurrence of a peak at 6 eV binding energy and attributed to the O2p_z orbital. The present study allowed us to report the initial stages of growth of MgO. The first oxygen atoms were randomly chemisorbed, and then oxide islands were formed. Using He*(2¹S) atoms, a structure at 1 eV binding energy was clearly observed at room temperature: we attributed it to electron confinement in under-stoichiometric MgO. This peak disappeared at 420 K: the non-stoichiometric oxygen atoms in excess oxidised Mg and were directly incorporated into oxide islands.