Velocimetry by femtosecond laser electronic excitation tagging (FLEET) of air and nitrogen

Supersonic air speeds are measured using femtosecond laser tagging of molecular nitrogen in unseeded air and in pure nitrogen by monitoring fluorescent emission with a time-delayed, fast-gated camera. Single and multiple lines are written in air by focusing a 120 fs, 1.2 mJ, 810 nm laser and relying on nonlinear nitrogen excitation and dissociation, and subsequent recombination resulting in fluorescent emission in the narrow focal region. The emission occurs in the spectral window from 560 to 660 nm corresponding to the first positive nitrogen emission (or nitrogen afterglow). The fluorescence mechanism is outlined and the accuracy of the technique is assessed through measurements in a small supersonic air jet.