Determination of metastable level densities in a low-pressure inductively coupled argon plasma by the line-ratio method of optical emission spectroscopy

The line-ratio method of optical emission spectroscopy (OES) is used for the diagnosis of plasma parameters. In this work, electrostatic probe-assisted OES is employed to measure metastable level densities from spectral lines and electron energy distribution functions (EEDFs) in a low-pressure inductively coupled argon plasma. Emission spectroscopy is based on plasma modelling through a simple collisional-radiative model. The line intensities of Ar(3p ⁵4p → 3p⁵4s) are modified due to the plasma reabsorption at relatively high pressures where the plasma becomes optically thick. To consider this effect, a pressure dependence factor α_ij(P) is first derived from both the measured intensity and pressure-dependent cross-section for electron excitation. It is found that the obtained metastable densities range from 1.3 × 10⁹ to 1.2 × 10¹⁰ cm^-3 and their ratios are nearly constant by a factor of about 3-5 in the investigated pressure range (3-50 mTorr). The effect of non-Maxwellian EEDF on the metastable densities is also discussed. The results measured by the line-ratio method are consistent with that of the OES-branching fraction method taking into account the photon escape factor to treat the radiation trapping.