On the validity of neutral gas temperature by N2 rovibrational spectroscopy in low-pressure inductively coupled plasmas

Measurement of the rotational temperature of the second positive system of N2 was used as a diagnostic of the gas temperature in low-pressure inductively coupled Ar, Kr and N2 plasmas. The rotational temperatures determined from the rovibrational bands (ν', ν'') = (0, 0), (1, 0), (0, 2) and (4, 2) of the N2 C 3Πu → B 3Πg system differ by about 300 K depending on the operating gas pressure in the 0.4-20 mTorr range. Important discrepancies exist between the temperatures found from each of the rovibrational bands of N2. This shows that the method has important intrinsic uncertainty that may be due either to errors in the transition probabilities of N2 C 3Πu → B 3Πg or to inefficient thermal coupling between translational and rotational temperatures. In the case of argon, the population of the emitting C 3Πu states by energy transfer from Ar 3P0,2 metastable atoms is also considered as a possible factor influencing the rotational structure of some rovibrational bands. Based on these measurements, it is shown that, in the range of experimental conditions studied herein, the uncertainty of the method should be carefully accounted before considering one of the rotational temperatures of the N2 second positive system equal to the gas temperature.