Determination of the neutral gas temperature of nitrogen-containing low-pressure plasmas using a two-temperature model

The neutral gas temperature, which is an important process parameter in low-pressure plasmas, is often determined by evaluating the shape of rotational spectra of appropriate molecules. However, the observed population of the rotational levels is determined by the excitation mechanisms and the rotational level population of the electronic states from which the upper electronic state of the chosen molecule band is populated. We used the first negative system of the nitrogen molecule ion for temperature determination in different low-pressure discharges. We will show that the spectra can be fitted best assuming that they are composed of two contributions representing two populations of the nitrogen molecule ion with distinctly different rotational temperatures. Thus, at least two excitation channels of the upper electronic state have to be considered which, in our opinion, are the electron impact on the ground state neutral nitrogen molecule connected with ionization and excitation (rotational level distribution remains essentially unchanged and reflects the gas temperature) and the impact of nitrogen molecules in high vibrational states on the ground-state nitrogen molecule ion (connected with rotational excitation, rotational temperature much higher than the gas temperature). Considering this, more reliable results are obtained compared to the conventional method where a single Boltzmann distribution of the rotational levels is assumed. Here, often, too high temperatures (up to several 100 K) are determined.