Argon metastable state densities in inductively coupled plasma in mixtures of Ar and O₂

We have measured the densities of 1s₅ and 1s₃ argon metastables as a function of the abundance of molecular oxygen in an inductively coupled plasma (ICP) in mixtures of Ar and O₂. Laser absorption spectroscopy was used to determine the densities of the metastables. It was found that even small abundances of oxygen lead to large increases in metastable density, mostly due to the reduction in the electron number density, since electron-induced quenching determines the metastable density. At abundances higher than 7% to 15% for powers between 50 and 150 W, quenching by oxygen molecules begins to dominate and the metastable density drops again.