Determination of the Abundance of Excited O+ Ions in Beams Produced by Electron Impact on O2, CO2, N2O, NO2, and H2O

The fraction of excited ions present in O⁺ ion beams produced by impact of 60‐eV electrons on various oxygenated molecules has been determined approximately 23 μsec after formation. The technique employed involves the measurement of apparent rate coefficients for charge‐transfer reactions of O⁺ to N₂ and CO at very low kinetic energies $\text{(∼0.3\hspace{0.17em}}\mathrm{eV})$ in a tandem mass spectrometer. Under these conditions, charge transfer of O⁺(⁴S) is endothermic and the formation of N₂⁺ and CO⁺ products is due solely to excited O⁺ ions in the reactant beam. By normalizing the rate coefficients for the reactions of O⁺ from the several sources to that for an O⁺ beam for which the excited ion abundance is known, the fractional abundance of O^+* in each of the respective ion beams is calculated. These fractions for O⁺ beams from the indicated sources were found to be: CO₂, 0.04; N₂O, 0.43; NO₂, 0.34; H₂O, 0.90. The fractional abundance data are utilized for the study of the reactions of both ground‐state and excited‐state O⁺with Ar and Kr. Ground‐state O⁺(⁴S) was found to be completely unreactive with Ar even at ion kinetic energies up to 170 eV. The cross section for charge transfer from excited O⁺ ions to Ar is quite small at very low kinetic energies but increases sharply at energies above $E_{1\mathrm{ab}}\text{∼20\hspace{0.17em}}\mathrm{eV}$ . Both O⁺(⁴S) and O⁺(²D) undergo charge transfer to Kr over the energy range investigated and a maximum is observed in the cross section for the O⁺(⁴S) reaction. The results are qualitatively consistent with the adiabatic hypothesis for nonresonant charge transfer.