Photoionization of air species as impurities in atmospheric pressure helium plasma

We revisit the problem of photoionization of small admixtures of nitrogen and oxygen molecules in atmospheric pressure helium plasma originally formulated in the pioneering work of Naidis (2010 J. Phys. D: Appl. Phys. 43 402001). The radiation trapping of resonance emission lines in atomic helium is quantified, and it is demonstrated that photoionization occurs due to radiative decay of the electronic A state of helium molecules. The collisions and atomic precursors that populate the excited A state of the helium molecule are clearly identified. The Einstein probabilities for the transition from bound and quasi-bound rovibrational levels of the A state to the continuum of the ground X state are provided. A kinetic scheme for the production of the fast component of ultraviolet emissions in atmospheric pressure helium plasma is proposed. The photoionization of molecular oxygen and molecular nitrogen as impurities in 99.9% and 99.99% purity helium is studied.