Measuring electron affinities with the photodetachment microscope

Photodetachment microscopy, which was originally proposed as a new method for direct measurements of ionization or detachment energies, has been applied for a determination of the electron affinity of oxygen. The electron affinities of ${}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O}$ could be measured separately from a natural sample. The experimental values (obtained as wave numbers, to be multiplied by $\mathrm{hc}$ to give energy units) are 11 784.682 (20) and 11 784.612 (29) ${\mathrm{cm}}^{\mathrm{-}1}$, respectively. The measured ${}^2{P}_{1/2}{-}^2{P}_{3/2}$ fine structure of ${}^{16}{\mathrm{O}}^{\mathrm{-}}$ is 177.085 (27) ${\mathrm{cm}}^{\mathrm{-}1}$. The observed discrepancy of the electron affinity of oxygen with the value admitted so far, 11 784.648(6) ${\mathrm{cm}}^{\mathrm{-}1}$, is yet unexplained.