Rydberg state, metastable, and electron dynamics in the low-pressure argon afterglow

In this work a time-dependent collisional-radiative model for recombining plasmas is developed. It tracks the collisional and radiative capture of electrons into highly-excited (Rydberg) states and their consecutive deexcitation through collisions and radiation to the ground or the metastable state. The model allows the calculation of the net recombination rate and the electron energy gain by recombination. It is coupled to the volume-averaged balance equations for the electron density and temperature. The numerical solution of these equations includes a model for the diffusion cooling of the electrons (Celik et al 2012 Phys. Rev. E 85 046407) and a simplified model for the gas cooling. Using as only input the experimentally determined initial values of the electron density and temperature, gas temperature and metastable density, the temporal evolution of all parameters in the afterglow is calculated and compared with measurements. The results reproduce very well the measured quantities (electron density, light emission and metastable density) without the need to invoke adjustable parameters. This gives confidence in the validity of the model that allows it to be used not only to deepen the understanding of afterglow plasmas but also to tailor their properties as required for applications. The analysis of the model results further shows that gas heating and cooling must be explicitly taken into account to reproduce experimental observations. The electron heating by recombination is another process that is important for the good agreement. Both of these effects were largely ignored in previous works on afterglows.