Sunlight photodetachment rate of ground state O₂⁻

The rate of photodetachment of O2− has been determined in white light and the rate adjusted to yield a value for the lifetime of O2− in sunlight. The technique used was to measure the rate of production of free electrons from ions in a buffer gas of pure oxygen. The buffer gas pressure was high enough to permit any vibrational or electronic excitation to be relaxed by ∼106 collisions with the 300°K oxygen molecules. Identification of the ions was by determination of their mobility. The sunlight photodetachment rate, as determined by the measurement, is 0.3 ± 0.1/sec. The only previous measurements were more controlled in most respects, but they were made on ions with an unknown amount of excitation. Having ions that are relaxed makes the data more relevant for use in applications to the mesosphere. There is the risk that O2− reacts with oxygen to make O4−. If this is true in the present measurement, it is likely to be true in applications also.