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Argon metastable and resonant densities in a low-pressure Ar–N2 inductively coupled plasma

Argon metastable (3P2 and 3P0) and resonant (3P1 and 1P1) atom densities have been measured in a low-pressure (1–20 mTorr) inductively coupled plasma generated in argon–nitrogen gas mixtures with nitrogen contents ranging from 0 to 90% using optical absorption spectroscopy techniques. A significant decrease of the metastable density is found for increasing nitrogen contents, more pronounced as the pressure is increased. As for the resonant atoms, their behaviour differs from that of metastable atoms, which results from the growing influence of radiation trapping with pressure. A simplified model accounting for several Ar metastable population/depopulation mechanisms (electron excitation/ionization, diffusion and excitation transfer from Ar metastable to nitrogen molecules) has been used to interpret these results. It shows that excitation transfer reactions from Ar metastable atoms to nitrogen become the main depopulation process, especially for large nitrogen contents and pressures above 10 mTorr.