On the determination of the vibrational temperature by optical emission spectroscopy

Over the years, until the present days, a persistent mistake has been found in the literature: the use of the ‘vibrational temperature’ of an emitting electronic state as somewhat representative of the vibrational temperature of the gas in the discharge. Such a temperature is determined by fitting the spectra measured by optical emission spectroscopy. Besides the misuse of the word temperature, the results of such fittings are ambiguously named ‘vibrational temperature’ and sometimes used to argue about the vibrational non-equilibrium and its variation with discharge conditions. What has this temperature to do with the vibrational excitation of the molecules’ ground state, i.e. of the large majority of gas components? It is well established that the connection between the vibrational population of the excited and the ground state exists through the excitation process, the collisional quenching, and the vibrational relaxation in the manifold of the excited state. Nevertheless, this is very often ignored in the literature. In this note, we discuss this subject with the example of the ‘vibrational temperature’ of the N-2(C, v) manifold, showing how much all the mentioned parameters can drive to incorrect deductions from an anyway conceptually wrong measurement.