Argon 420.1–419.8 nm emission line ratio for measuring plasma effective electron temperatures

We explore the feasibility of using the ratio of two argon emission line intensities at 420.1 and 419.8 nm to measure the effective electron temperature in argon-containing plasmas. Experimental measurements in numerous plasma sources reveal the ratio varies from a value of approximately 1 at high electron temperatures, to a value near 4 at low electron temperatures. This variation is understood in terms of the magnitudes of the electron excitation cross sections into the upper energy levels of the two transitions. At high electron temperatures, the upper levels of the two emission lines, the J = 3 3p(9) level for the 420.1 nm line and the J = 0 3p(5) level for the 419.8 nm line, are both primarily populated by excitation from the ground state and have similar optical emission cross sections. At low electron temperatures, excitation is dominated by excitation from the metastable levels which have very different cross sections into the two levels. Temperatures obtained with this line pair ratio in an inductively coupled plasma are found to be consistent with values obtained from a Langmuir probe as well as an analysis of the entire set of 2p(x) -> 1s(y) emission lines (665-1150 nm) under a wide variety of plasma conditions.