Measurement of oxygen temperature fluctuations by laser-induced fluorescence

This paper describes a noninvasive optical method for measuring temperature and density fluctuations in turbulent airflow in a wind tunnel. In this approach we use an ArF excimer laser tuned to one of the Schumann-Runge absorption lines of O₂ near 193 nm. The absorbed photons produce transitions between the lowest vibrational level of the ground state $\left(X^3{\displaystyle \sum {}_g^{-}}\right)$ and the fourth vibrational level (v′ = 4) of the excited state. The rotational sublevels of the excited state rapidly assume a thermal population distribution. Some of the excited molecules decay radiatively back to various levels of the X state, producing the fluorescence band spectrum shown in Fig. 1. The intensities and wavelengths in the figure were computed from published spectroscopic constants. Generally the fluorescence extends from ~220 nm to the visible, with most of the energy in the 220-250-nm range.