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.
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