Collisional-radiative models with molecules

Collisional-radiative models were originally developed to model the competi tion between collisional excitation and radiative decay under circumstances in which neither is dominant. However, in the context of fusion plasma phy sics, collisional-radiative models are now often used to separate plasma co mponents into 'slow' and 'fast' species, so that only the collisional-radia tively determined time development of the slow species need be considered e xplicitly. This paper extends this idea to the description of plasmas which contain molecules. In order to do so it makes a rather thorough (and some might say, boring) analysis of the requirements of a valid collisional-radi ative model, and derives a new set of validity criteria, which are not rela ted to the equilibrium time-scales. These criteria can also be used to gene rate all possible collisional-radiative models for a given set of plasma ra te processes. Details of how this is done and an analysis of the situation that applies when molecular hydrogen is present in the plasma are presented .