The importance of cascade emission and metastable excitation in modeling strong atomic oxygen lines in laboratory plasmas

Abstract

A collisional-radiative model for optically accessible atomic oxygen lines in laboratory plasmas has been developed to identify dominant mechanisms and assess the validity and utility for plasma diagnostics. A rate sensitivity analysis of the common 777 nm/844 nm line ratio was carried out and used to determine the importance of often ignored mechanisms including cascade emission and metastable excitation. Commonly used basic models for emission of these lines is compared to the model developed here and the mechanisms responsible for disagreement are identified. At low pressures metastable excitation from the state becomes important and at high pressures collisional relaxation of to is also non-negligible. At all pressures cascade emission from higher lying states was found to be crucial in accurately estimating the 777 nm/844 nm line ratio. Effective cascade excitation cross sections were calculated and used to include a cascade rate in the basic analytic models, which improves overall agreement with the collisional radiative model, particularly in the medium pressure range. Finally, the 777 nm/844 nm line ratio is calculated and presented for a variety of common laboratory plasma parameters.