On the validity of neutral gas temperature by emission spectroscopy in micro-discharges close to atmospheric pressure

The neutral gas temperature (T-g) of a single micro-hollow cathode discharge (MHCD) elaborated on silicon wafer is investigated. The MHCD is continuously powered in DC yielding a plasma in He, Ar and N-2 gases close to atmospheric pressure. Values of T-g are determined by means of optical emission spectroscopy inside and in the vicinity of a single cavity of 100 mu m diameter. In this work, the use of the resonant broadening to infer T-g is thoroughly reconsidered taking into account the reevaluation of an empirical coefficient as well as including the weaker contribution of the Van der Waals interaction in the line profile analysis. T-g is found to be around 360 K in He while in Ar it ranges from 460 to 860 K depending on the current. A temperature gradient is observed between the cavity and the surrounding gas, which is correlated with the design of the MHCD reactor. A second approach involves the study of the relative rotational population distributions of several N-2(C-B) vibrational bands to infer the rotational temperature (T-rot). While the latter is commonly assumed to approximated T-g, its validity will be discussed based on experimental results. Discrepancies between T-rot from different vibrational bands as well as with values found with the resonant broadening are investigated using different operating regimes of the MHCD. The validity of both approaches will be discussed supported by a comprehensive evaluation of the T-g and T-rot uncertainty estimations. The analysis of resonant atomic line profile appears to be a reliable and accurate method to measure T-g in absolute room temperature atmospheric pressure plasma sources used in industrial processes as well as in environmental and biomedical applications.