Negative Ion Formation by the Thermal Surface Ionization of Oxygen-Bearing Gases (O2, CO2, CO, NO, and NO2)

The formation of the oxygen negative ion O^- from simple molecules such as O₂, CO₂, CO, NO, and NO₂ is of fundamental importance in environmental, atmospheric, and biological processes. This study investigates the mechanisms of O^- ion generation from these gases by analyzing the dependence of O^- ion current intensity on filament temperature. Optimum temperatures for O^- formation were identified for each gas, ranging from 1548 to 1721 °C. A comparison with the calculated thermal decomposition temperatures of the respective compounds indicates that distinct ion formation pathways are involved. For NO₂, the process likely involves a two-step dissociation mechanism, with molecular oxygen (O₂) formed in the first step, subsequently dissociating into O^- and O atoms. In contrast, for CO, O^- formation predominantly occurs through electron capture followed by molecular dissociation. These findings underscore the complex nature of negative surface ionization, which includes contributions from the capture of emitted from the cathode electrons by molecules.