Electron release in the afterglow of a pulsed inductively-coupled radiofrequency oxygen plasma

Abstract

A pulsed inductively-coupled radiofrequency plasma in oxygen is investigated by means of time-resolved microwave interferometry in a wide pressure range from 0.5 to 200 Pa. In the afterglow a peak of the electron density is observed. The effect is maximum for pressures around 50 Pa. The time-resolved measurements of the electron density are interpreted in the framework of a fluid model. This model points out the significance of negative ions. The overall electron density is comparatively small. Attachment and detachment processes nearly balance during the power-on phase. But when the power is switched off, the electron temperature drops very quickly. This means that the production of new negative ions is inhibited so that the negative ions are destroyed by collisions. These reactions quickly set free electrons in the afterglow and are the reason for the observed peak in the electron density after switching off the power.