Plasma chemistry and surface processes of negative ions

This work reviews formation processes of negative ions in low-pressure labo ratory plasmas. There are many topics of discussion in the chemistry of neg ative ions. In most studies only volume production by dissociative electron attachment is considered. However, a typical problem is that experiments r eveal higher negative ion densities than one would expect based on attachme nt rates to ground-state molecules. Apparently, there exist other, more eff icient ion production channels. Excitation and chemical conversion of the p arent gas under plasma conditions can significantly increase the effective attachment rates. This is due to extremely high attachment cross sections o f rovibrationally and electronically excited molecules, as well as radicals and large polymeric species. Alternatively, negative ions can be efficient ly generated in the plasma sheath, due to interactions of high-energy posit ive ions with neutrals or with the surface. Sheath chemistry can have a lar ge impact on the bulk plasma, so it has to be studied in more detail to obt ain a complete understanding of electronegative plasmas. Both chemical reac tions in the volume and sheath collisions must be included in plasma models .