He(2³ S ₁) and He₂(a ³Σ_u ⁺) metastables densities measured in an RF-driven helium plasma using broadband absorption spectroscopy

Both atomic (He(2³ S)) and molecular (He₂(a ³Σ_u ⁺)) metastable species of helium are an important source of ionization in low electron density atmospheric pressure plasmas due to the efficient Penning ionization with impurities. In this contribution, we report for the first time the measured absolute densities of these metastable species in a capacitively coupled parallel plate atmospheric pressure helium glow discharge excited by radio frequency using broadband absorption spectroscopy. The spatial distribution profiles of these metastable species correlate well with RF-driven sheath structure operating in an α-mode regime. The peak density of He(2³ S) atoms at the sheath edge (∼6 × 10¹⁸ m^-3) is about 2.5 times larger than the peak density of He₂(a ³Σ_u ⁺) molecules at the same location. The study of the time-evolution of the densities when pulsing the discharge, along with a zero-dimensional chemical kinetics model, allow the determination of effective lifetimes of these metastable species in the afterglow and the characterization of the plasma studied in terms of the level of impurities present in the system.