Two-photon excitation of nitric oxide fluorescence as a temperature indicator in unsteady gasdynamic processes.

A laser-induced fluorescence technique, especially suitable for measuring fluctuating temperatures in cold turbulent flows containing very low concentrations of nitric oxide, is described and analyzed. Temperatures below 300 K may be resolved with SNRs of >50:1, using commercially available high-peak-power tunable dye lasers. The method relies on the two-photon excitation of selected ro-vibronic transitions in the NO(A(2)Sigma(+), upsilon' = 0 ? X(2)II, upsilon'' = 0) gamma band. The analysis includes the effects of fluorescence quenching and shows the technique to be effective at all densities below ambient. SNR estimates are based on a preliminary measurement of the two-photon absorptivity for a selected rotational transition in the NO gamma(0,0) band.