Spatial characterization of pressure-based plasma regimes in a radiofrequency glow discharge by using optical emission spectroscopy

A new experimental set-up has been developed in order to perform spatially resolved measurements of the optical emission in a radiofrequency glow discharge (rf-GD). The emitted radiation is not only detected through an end-on window (as usually done in commercial GD-OES instruments), but the emission along the plasma plume is also measured axially-resolved by side-on measurements. The GD source is a modified Grimm-type source and follows a previous configuration designed for analytical applications in glow discharge coupled to mass spectrometry (GD-MS) instruments. Depending on the argon flow rate and pressure, and for a pure copper sample, two different plasma regimes have been observed and characterized by analyzing the spatial distribution of different excited species present in the plasma. Keeping a constant Ar flow rate, pressures higher than a certain value lead to enhanced Cu (analyte) emission (green regime) while at lower pressures a violet plasma is observed. In the violet regime, the spatial extension of Cu and Ar excited species along the plasma axis is found to be broader.