Understanding Plasma Optical Emission Diagnostics using Ar Excitation Cross Sections

Electron-atom collisions are the primary excitation mechanism of low-temperature plasmas. Nevertheless, the emissions of most lines are the result of many excitation channels: direct excitation from the ground state, excitation into higher levels that radiatively cascade into the level of interest, and excitation from long-lived metastable atoms in the plasma. Incorporating these processes into a radiative model, we have used Ar emissions to determine the electron energy distribution functions of inductively coupled plasmas[1]. Emissions from the Ar($3p_5$) and Ar($3p_9$) levels, at 419.8~nm and 420.1~nm respectively, are found to exhibit a particularly interesting intensity ratio that is very sensitive to plasma conditions. The 420.1/419.8 line ratio varies from $\sim 1$ at high electron temperatures ($T_e$), to $\sim 3$ at low $T_e$. These variations can be explained in terms of the nearly equal ground state optical emission cross sections, and the vastly different cross sections out of the metastable levels[2]. We explore the atomic physics that drives this particular line ratio and how it can be used to non-invasively measure the electron temperature of argon containing plasmas. \\[4pt] [1] Plasma Sources Sci. Technol. \textbf{19}, 065001 (2010);\\[0pt] [2] Phys. Rev. A \textbf{75}, 052707 (2007).