Modified state enhanced actinometry for measuring atomic oxygen density in a micro-scaled atmospheric pressure plasma jet

Abstract

Plasma-produced oxygen atoms (O) are powerful oxidative radicals that play important roles in plasma-induced chemical modification of material surfaces. Measuring O density in plasmas is important to enhance our understanding and control. Two-photon absorption laser-induced fluorescence (TALIF) is widely used for measuring O density, but is experimentally complex and often incompatible with in-situ process control. Actinometry, based on analysing spectral lines from optical emission spectroscopy, requires only simple hardware, but relies on more complex analysis of experimental data. Advanced actinometry methods such as state enhanced actinometry (SEA) have recently been developed and are consistent with TALIF measurements. In this work, a modified version of SEA is investigated. Including cascading effects in the SEA analysis model and reducing the effect of metastable states on the SEA emission line by considering the 728.1 nm He(31S→21P) line, instead of the 706.5 nm He(33S→23P) line, are considered. Importantly, Bayesian inference is used for matching experimental and theoretical data from the non-linear equations of each model. O density and mean electron energy are determined in a micro-scaled atmospheric pressure plasma jet (µAPPJ), operated with 1 slm of He and admixtures of 0.5% O2 and 0.1% Ar. It is found that the Bayesian process minimises the associated error to less than 1%. Both including cascading emission and considering the alternative He line result in significant changes to the derived O density, in the range of 20%-60% overall. However, because of the lack of accurate data from alternative methods, e.g. TALIF has an accuracy of 67% approximately, and the uncertainty of SEA itself (≈20%), due to uncertainties in the parameters used in the models, it is not possible to determine which model is the most appropriate for SEA and whether our modifications lead to improvements. The work highlights the currently achievable accuracy of advanced actinometry methods such as SEA.