Role of Nitrogen in Modifying Argon Metastable Dynamics in Microwave-Driven Ar-N₂Plasmas

A microwave-driven plasma operating at 2.45 GHz is investigated by means of OES, LAS, LIF, probe diagnostics, and the plasma simulation package Zapdos [1]. The working gas is an Ar/N₂ mixture with a N₂ partial pressure range from 0 % to 25 % of total gas pressure (mTorr range) and delivered power density (W/cm3 range). LIF measurements probe the (2P^3/2₀ ) 4s² [3/2]₀, J=2 metastable level of Ar using a tunable Nd:YAG laser pumped OPO at 696.73nm, exciting it to the (2P^1/2₀ ) 4p² [1/2], J = 1 level. Fluorescence is observed at 727.49 nm and captured with an ICCD camera along with a bandpass filter. LIF signal is calibrated by measuring the absolute species density with LAS via absorption. A tunable diode laser was used to measure the line integrated Ar (1s₅) density. By scanning the laser wavelength over a specific absorption line of the metastable state, such as the 727.49 nm (1s₅ → 2p₂) transition, the population density of Ar(1s₅) can be measured. The absorbed light intensity is measured by a photodetector and is used to calculate the absolute population density using the Beer–Lambert law. An overall reduction in 1s₅ density with increasing N₂ admixture is observed. By calibrating the fluorescence intensity with the absolute density, the spatial distribution of the metastable species can be determined. The natural lifetime of fluorescence for pure Ar and the impact of introducing nitrogen is also examined. To explain the metastable decrease, relative concentrations of molecular N₂, ionized molecular N₂⁺ and atomic N as well as bright Ar emission lines are obtained through optical emission actinometry as a function of N₂ partial pressure. A decline in the intensity of Ar emission lines, particularly those arising from transitions involving the 4p levels, indicates a change in the population of excited Ar states. The simultaneous increase in nitrogen species suggests that Penning ionization processes are enhanced, contributing to the observed reduction in Ar metastable density. [1] PSST 32 (4), 044006