Comparative study on the excitation mechanism of chromium emission lines in the argon radio-frequency inductively-coupled plasma, nitrogen microwave induced plasma, and argon or nitrogen glow discharge plasmas

Boltzmann plots of both atomic and ionic chromium emission lines are investigated to compare the excitation mechanisms in four different plasmas: an argon inductively-coupled plasma (Ar-1CP), a nitrogen high-power microwave induced plasma (N₂-MIP), an argon glow discharge plasma (Ar-GDP), and a nitrogen glow discharge plasma (N₂-GDP). The plots of the atomic lines and the ionic lines give both linear relationships as well as similar excitation temperatures in the case of the Ar-ICP, the N₂-MIP, and the N₂-GDP. It implies that a thermodynamic process such as electron collision would control their excitations. However, only in the case of the ionic-line plot in the Ar-GDP, a departure from linear relationship is observed and the estimated excitation temperature is rather higher than that with the atomic lines, meaning that a specific excitation mechanism exists in the Ar-GDP. A possible explanation for these results is that a charge-transfer collision between chromium atom and argon ion plays a dominant role in exciting highly-lying energy levels of chromium ion, especially in the Ar-GDP.