Laser-induced photodetachment of negative oxygen ions in the spatial afterglow of an atmospheric pressure plasma jet

Negative ions are an important constituent of the spatial afterglow of atmospheric pressure plasmas, where the fundamental plasma-substrate interactions take place that are vital for applications such as biomedicine, material synthesis, and ambient air treatment. In this work, we use laser-induced photodetachment to liberate electrons from negative ions in the afterglow region of an atmospheric pressure plasma jet interacting with an argon-oxygen mixture, and microwave cavity resonance spectroscopy to detect the photodetached electrons. This diagnostic technique allows for the determination of the electron density and the effective collision frequency before, during and after the laser pulse was shot through the measurement volume with nanosecond time resolution. From a laser saturation study, it is concluded that O- is the dominant negative ion in the afterglow. Moreover, the decay of the photodetached electron density is found to be dominantly driven by the (re)formation of O- by dissociative attachment of electrons with O2. As a consequence, we identified the species and process responsible for the formation of negative ions in the spatial afterglow in our experiment.