Plasma chemistry and surface processes of negative ions

TLDR

Negative‑ion chemistry in plasmas is complex, yet most studies focus solely on volume production via dissociative electron attachment. This review examines formation processes of negative ions in low‑pressure laboratory plasmas, emphasizing the need to investigate sheath chemistry for a complete understanding of electronegative plasmas. Both volume reactions—excitation and chemical conversion of parent gases that enhance attachment rates—and sheath collisions involving high‑energy ions with neutrals or surfaces must be incorporated into plasma models. Experiments consistently show higher negative‑ion densities than predicted by ground‑state attachment rates, indicating the presence of additional, more efficient production channels.

Abstract

This work reviews formation processes of negative ions in low-pressure laboratory plasmas. There are many topics of discussion in the chemistry of negative ions. In most studies only volume production by dissociative electron attachment is considered. However, a typical problem is that experiments reveal higher negative ion densities than one would expect based on attachment rates to ground-state molecules. Apparently, there exist other, more efficient ion production channels. Excitation and chemical conversion of the parent gas under plasma conditions can significantly increase the effective attachment rates. This is due to extremely high attachment cross sections of rovibrationally and electronically excited molecules, as well as radicals and large polymeric species. Alternatively, negative ions can be efficiently generated in the plasma sheath, due to interactions of high-energy positive ions with neutrals or with the surface. Sheath chemistry can have a large impact on the bulk plasma, so it has to be studied in more detail to obtain a complete understanding of electronegative plasmas. Both chemical reactions in the volume and sheath collisions must be included in plasma models.