Emission of negative ions of oxygen during the activation of oxide-coated cathodes

Oxide-coated cathodes have been used as ion sources in a simple mass spectrometer and have been found to emit atomic negative ions of oxygen during their activation at high temperatures (1150-1275° K). A retarding potential at the collector was used to separate the ions emitted by the cathode from the ions formed in the residual gas. Most of the oxygen ions from the cathode arrived at the collector with more energy than they could have obtained from the potential difference across the electron gun, and their number increased as the cathode became more active. The evidence suggests that the ions were formed by dissociation of the oxide coating, then diffused to the surface and were removed by positive ion bombardment. Such a process would leave oxygen vacancies in the oxide which would act as electron donors and increase its electron emission.