In argon plasma, the collisional radiative (CR) model has been developed by Duck-Hee Kwon and Kil-Byoung Chai. In this website, Ar CR-model is based on the paper [1] and the population kinetics can be described as follows.

[1] D.-H. Kwon and K.-B. Chai, "Optical emission spectroscopy and collisional-radiative modelingl for low temperature Ar plasmas", J. Quant. Spectrosc. Radiat. Transfer 227, p136-144 (2019)