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

Abstract
High-resolution, Doppler-free fluorescence excitation spectra of OH A2Σ+X2Π(0, 0)-band transitions were measured in low-pressure hydrogen-oxygen flames. A pulse-amplified cw dye-laser system with a 40-MHz bandwidth was used in a counterpropagating-beam geometry to excite the OH transitions by two-photon absorption. Hyperfine splittings of 15 rovibronic transitions were determined, and water-vapor pressure-broadening coefficients were measured for four of the transitions by varying the total pressure of the flame. Direct observation of the collision-broadened line shape made it unnecessary to resort to any deconvolution procedures. To our knowledge, this is the first application of a Doppler-free spectroscopic technique in a combustion environment.