Electron Affinities from Endothermic Negative-Ion Charge-Transfer Reactions. II. O2

Electron‐transfer reactions of negative ions were studied utilizing an in‐line tandem mass spectrometer. Cross sections were measured as a function of translational energy for the reactions of D⁻, O⁻, S⁻, SH⁻, Cl⁻, Br⁻, and I⁻ with O₂. All of these reactions exhibit thresholds, their cross sections rising sharply with increasing energy above onset. The low energy tails of the electron‐transfer cross sections which are observed are shown to be caused by Doppler broadening of the effective center‐of‐mass energy, owing to the thermal motion of the target molecules. Center‐of‐mass energy distributions were calculated for several of the reactions (O⁻, S⁻, Br⁻, and I⁻) using an approximate one‐dimensional treatment of the collisions. These energy distributions were convoluted with various trial threshold functions, and the calculated curves were fitted to the experimental cross sections to obtain the true energy thresholds. Within the limits of error of this method, thresholds derived for the various reactions yield a consistent lower limit for the electron affinity of O₂ of $\text{0.45 ± 0.1\hspace{0.17em}}\mathrm{eV}$ .