A theoretical study of the interpretation of emission-absorption intensity ratio temperature measurements in the absence of thermal equilibrium

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Emission-absorption intensity ration temperature measurements of diatomic molecules under thermal non-equilibrium conditions are investigated theoretically. It is shown that the rotational population distribution must be taken into account when a small part of a vibration-rotation band is viewed in experiments for measuring a vibrational temperature. An emission-absorption intensity ratio of unity does not always mean equality of effective brightness temperature of background source and a vibrational temperature. Such an assumption could lead to errors of several per cent in experimentally deduced vibrational temperatures, particularly in situations where the ratio of rotational and vibrational temperatures is less than unity. The rotational temperature dependence is explained in terms of transitions taking place in vibration-rotation bands and this dependence decreases when a larger part of a band is viewed. Emission-absorption measurements are also investigated for cases where the vibrational population distribution deviates from Boltzmann, for polyatomic and for molecular electronic spectra. Finally it is shown that, in principle, both rotational and vibrational temperatures may be determined from two simultaneous emission-absorption measurements.