Doppler-free two-photon-excited fluorescence spectroscopy of OH in flames

We have used Doppler-free two-photon-excited fluorescence spectroscopy to fully resolve the pressure-broadened line shape of OH A2Σ+ ← X2Π transitions in low-pressure flames. This method uses two counterpropagating laser beams to excite the transition. Since the Doppler shifts for the two beams exactly cancel, the resulting signal consists of a sharp Doppler-free peak centered on a Doppler-broadened and much weaker background. Because measurements were made in the postflame gases of low-pressure stoichiometric hydrogen–oxygen flames, water was the dominant collision partner. A narrowband pulse-amplified cw ring dye laser system with a FWHM bandwidth of 45 MHz at 620 nm made it possible to fully resolve the OH linewidth at pressures down to 20 Torr. All the observed transitions consisted of doublets due to hyperfine splitting of the OH A2Σ+ state. Values of the broadening and shift of OH transition line shapes due to collisions with water measured using this technique are reported.