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 ¹⁶O and ¹⁸O could be measured separately from a natural sample. The experimental values (obtained as wave numbers, to be multiplied by hc to give energy units) are 11 784.682 (20) and 11 784.612 (29) cm^-1, respectively. The measured ²P1/2-²P3/2 fine structure of ¹⁶O^- is 177.085 (27) cm^-1. The observed discrepancy of the electron affinity of oxygen with the value admitted so far, 11 784.648(6) cm^-1, is yet unexplained.