Emission characteristics of okamoto-cavity microwave-induced plasma using nitrogen-argon mixed plasma gas

An Okamoto-cavity microwave-induced plasma (MIP) with nitrogen-argon mixed gas was investigated to employ an alternative excitation source instead of the conventional nitrogen plasma. The emission intensities of atomic emission lines of copper and iron having small excitation energy were clearly elevated by adding argon gas to the nitrogen plasma, giving the maximum intensity at the argon mixing ratio of 40%. On the other hand, the excitation temperature, which was estimated from Boltzmann plots using Fe I emission lines ranging from 358 to 380 nm, was hardly changed when the amount of argon increased up to 50%. Also, the ionic-to-atomic intensity ratios of calcium, magnesium, and yttrium were not changed so much by mixing the argon gas. These results concerning the excitation temperatue as well as the ionic-to-atomic ratio imply that the excitation characteristics of the N₂-Ar MIP are similar to those of the N₂ MIP. However, it was observed that the emitting zone of the N₂-Ar MIP could be expanded compared to the N₂ MIP. Energetic argon species, which are produced through collisions with electrons and nitrogen species, can expand the plasma region due to their smaller cross-section compared to nitrogen molecule species, and then can take part in excitation collisions of analyte atoms at the outer zone of the plasma, leading the increased emission intensities of the analyte atoms.