“…In order to discuss the observed thresholds, firstly in pure solid CO 2 , we list the threshold energies of the same ions but in the gas phase ͑see Table II͒ obtained by electron impact ionization, as reported by different groups. 13,[18][19][20][21][22][23][24] The first three thresholds of O ϩ ions in the gas phase are at ϳ19 and 20, 18-21 ϳ22.5, 21,23 and 24.5 eV, 21 respectively. In the solid phase ͑Table I͒ the observed thresholds of the O ϩ ions are at 21.5Ϯ0.5 eV and 25Ϯ0.5 eV.…”