Doppler-free laser-induced fluorescence of oxygen atoms in an atmospheric-pressure flame.

Doppler-free laser-induced fluorescence of oxygen atoms has been measured in the burnt gases of an atmospheric-pressure flame, where the Doppler and collisional widths are similar. The 3p(3)P level was excited using counterpropagating beams at 226 nm. Upper-state fine-structure splitting could be partially resolved, and a collisionally broadened Lorentzian width of 0.42 cm(-1) was measured. This corresponds to a broadening cross section of 10(-13) cm(2), which is larger than that expected for collisional quenching alone.