The quenching effect of oxygen addition on an argon capacitively coupled plasma: experimental and computational study of the argon metastable atom kinetics

The effect of oxygen admixture in an argon capacitively coupled radio-frequency plasma is investigated experimentally and computationally in a symmetrical discharge cell, at pressures ⩽10 Pa. In the experiments, tunable diode laser absorption spectroscopy is applied to monitor the densities of the Ar 1s5 ( →772.376nm 2p6) and 1s3 ( →772.421nm 2p2) metastable atoms in the plasma as a function of the oxygen content in the working gas. In order to enhance computational efficiency, the modelling is divided into two steps. First, the electron energy probability function (EEPF) is obtained from a particle-in-cell/Monte Carlo collision simulation of the Ar/O2 plasma without considering the dynamics of the excited levels of Ar atoms. As the second step, this EEPF is fed into a code that solves the balance equations of Ar atoms in numerous excited levels. These equations comprise the effects of diffusion, direct and stepwise excitation processes, stepwise and pooling ionization, as well as radiative transfer between the various Ar atomic levels, and the quenching of the excited Ar atoms by O2 molecules. Using this approach is justified by the fact that the EEPF is insensitive to the excited level dynamics at low pressures, as shown in previous studies. The measurements and simulations are found to yield consistent results, indicating the correctness of the literature values of the quenching coefficient of Ar 1s5 and 1s3 by oxygen molecules.