Measurements of argon 4<i>p</i> state population densities in a capacitively coupled discharge

Abstract Optical emission spectroscopy is performed in a capacitively coupled 13.56 MHz radio-frequency argon plasma discharge with a 2 cm gap between parallel plate electrodes. Cycle-averaged measurements are made at the center of the discharge volume at pressures of 0.1 Torr to 10 Torr and peak-to-peak voltages <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mtext>⩽</mml:mtext> </mml:mrow> </mml:math> 1 kV. Spectra are calibrated for absolute spectral radiance, and populations of excited electronic states are inferred from the atomic transition lines. Measurements of all ten states comprising the Ar(4 p ) grouping are reported individually, and the population distributions analyzed as functions of both pressure and voltage. It is found that the population densities increase with applied voltage. The state distribution is highly non-equilibrium and the relative populations are insensitive to voltage or pressure at pressures <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mtext>⩽</mml:mtext> </mml:mrow> </mml:math> 1 Torr. At higher pressures, the relative population distribution becomes a function of both pressure and voltage. Uncertainty quantification is performed using a Bayesian approach, making the reported population densities useful for plasma modeling purposes and validation of numerical simulations.