Spectroscopic Studies in Non-equilibrium Plasmas

Abstract A near-room-temperature capacitively-coupled plasma (CCP) glow discharge cell is used for fundamental studies of low and intermediate pressure Ar plasmas. The cell consists of two cylindrical plate electrodes powered by a 13.56 MHz radio-frequency source, forming a diffuse low-pressure plasma between the electrodes inside a cylindrical glass envelope 10 cm in diameter. The cell is operated at pressures from 0.1 to 10 Torr and electrode voltages up to $$\sim $$ ∼ 2 kV depending on pressure, producing plasmas in which the free electron temperature ranges up to several electron-volts while the gas remains close to ambient temperature ( $$\sim $$ ∼ 300K). The emission lines from Ar in the visible and near infra-red (NIR) correspond primarily to transitions from 4 p states to the metastable and resonant 4 s Ar states. Calibrated Emission Spectroscopy using Absolute Radiance (CESAR) measurements on these transitions are used to determine population densities of Ar 4 p states. Tunable laser absorption spectroscopy is used to measure the metastable and resonant Ar 4 s state populations. These densities are presented in combination with electron density and energy distribution function measurements performed with a Langmuir probe. These measurements enable us to assess the degree of non-equilibrium in the plasma as a function of pressure and electrode voltage. We use Monte Carlo techniques to quantify the uncertainty of all the measurements. The experimental results are used to enhance our understanding of non-equilibrium plasmas and to validate kinetic models of excitation and ionization processes in plasmas.