Abstract

The application of ArF laser-induced fluorescence (LIF) for temperature measurement of O₂ has been previously demonstrated in hot¹ and cold² low-speed air flows. In those measurements, one or more rotational lines within the Schumann-Runge $\left(B^3{\displaystyle {\sum }_{\mathrm{u}}^{-}-{\mathrm{X}}^3{\displaystyle {\sum }_g-}}\right)$ band are excited. Owing to the strong predissociation of the excited state, the fluorescence is proportional to the population of the initial absorbing state, which, in turn, is sensitive to temperature and density of O₂. When the density is known^1,2, a single line excitation may suffice for temperature measurement. Otherwise, excitation of two rotational lines is required and the temperature is determined from the fluorescence intensity ratio. Although the population of the rotational lines reaches thermal equilibrium within a few molecular collisions, the long lifetime of the vibrational levels in the $X^3{\displaystyle {\sum }_g-}$ state prevents their rapid thermalization. Therefore, the measurement accuracy behind sharp temperature gradients is limited by fluorescence from unrelaxed vibrational states. Accordingly, measurements in high speed or reacting flows requires isolated excitation of rotational lines from the same vibronic manifold. We determined the conditions for such isolated excitations by ArF, KrF, and Raman-shifted ArF lasers for O₂ temperatures up to 1900 K. We also determined the temperature measurement uncertainty for realistic flow conditions.

© 1991 Optical Society of America