Cross Sections and Electron Affinity for ${\mathrm{O}}^{-}$ Ions From ${\mathrm{O}}_2$, CO, and C${\mathrm{O}}_2$ by Electron Impact

Cross sections for the formation of ${\mathrm{O}}^{-}$ from ${\mathrm{O}}_2$, CO, and C${\mathrm{O}}_2$ have been measured using an electron beam experiment in which an attempt is made to eliminate kinetic energy and angular discrimination by collecting most of the positive and negative ions produced in the tube. The cross sections are independent of electron current, pressure, and magnetic field. The values of these cross sections at their peak are 1.3×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at 6.7 eV in ${\mathrm{O}}_2$, 1.6×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ at 10.1 eV in CO, and 4.5×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ at 8.2 eV in C${\mathrm{O}}_2$. ${\mathrm{O}}^{-}$ from C${\mathrm{O}}_2$ exhibits a second smaller peak at 4.2 eV with a cross section of 1.5×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$. Kinetic-energy measurements on the ${\mathrm{O}}^{-}$ ion from ${\mathrm{O}}_2$ and C${\mathrm{O}}_2$ are not compatible with the value for the electron affinity (1.465 eV) obtained from photodetachment experiments but yield an electron affinity of 2.0±0.1 eV and so suggest the existence of excited states of ${\mathrm{O}}^{-}$. The experimental evidence from triatomic molecules ${\mathrm{N}}_2$O, N${\mathrm{O}}_2$, and S${\mathrm{O}}_2$ is reviewed and is consistent with the higher value for the electron affinity for oxygen.