Inferring gas temperature from N2 emission via rotational distribution of the N2 B3πg and C3πu states

Optical emission spectroscopy (OES) is frequently used to measure the rotational temperature of a plasma. The technique is relatively simple to implement and non-intrusive. However, in the visible and ultraviolet spectral regions, it relies on emission from excited vibronic states whose rotational and vibrational temperatures might not be equilibrated with the bulk temperature of the plasma. Oftentimes, rapid equilibration between the rotational and translational modes occurs so that these temperatures are equilibrated. However, many cases exist where these modes are not equilibrated. In this work, we have developed a simple model that can be used as an experimental tool when using OES. An updated criterion for ensuring rotational-translational equilibrium is proposed. In cases where nonequilibrium is expected, the model may be used to provide an estimate of the bulk gas temperature. An illustrative example is given where we use this model to analyze previous measurements made in a nonequilibrium discharge.