Oxygen Rotational Temperature Determination Using Empirical Analyses of C 3 Π(v′ = 2) ← X 3 Σ(v″ = 0) Transitions

The spectra of molecular oxygen through C³Π(v′ = 2) ← X³Σ(v″ = 0) transitions have been obtained by coherent microwave Rayleigh scattering (radar) from resonance-enhanced multiphoton ionization (REMPI). Measurements of rotational temperatures of molecular oxygen have been demonstrated based on the empirical analyses of the O₂ spectra without the requirement of highly resolved rotational features. Three methods, including (1) linewidth fitting, (2) linear fitting, and (3) area fitting have been investigated for temperature measurements within pure oxygen, ambient air, and H₂–air and CH₄–air flame environments. The first two methods were applied in a moderately low temperature environment with measurement uncertainties less than 11% and 26%, respectively. The area fitting method covered a wider temperature range, from room temperature (∼300 K) to flame temperature (∼1700 K), with minimal dependence on the fine structures of the O₂ spectra. Less elaborate than Boltzmann plot analyses of ultrafine rotational lines from congested upper rotational energy levels in O₂(C³Π(v′ = 2)), these empirical analyses are predictably sensitive to the thermal distribution of molecular oxygen and have been successfully demonstrated as simple and quick methods for remote gas-phase temperature measurement.