Two-photon-excited fluorescence measurements of OH concentration in a hydrogen–oxygen flame

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Abstract
Two-photon-excited fluorescence (TPF) has been used to determine the OH concentration profile in an atmospheric-pressure hydrogen–oxygen flame. Individual rovibronic transitions within the A2Σ+−X2Π (0,0) system of OH were identified by comparing calculated wavelengths to emission spectra obtained by laser excitation from 604 to 638 nm. Relative OH concentrations were determined by individually exciting the O1(5), O2(5), and P2(5) transitions and monitoring the resulting fluorescence with a filtered photomultiplier and a gated integrator system. The expected quadratic dependence of the fluorescence signal on laser power was verified indicating the probable lack of photochemical effects. The OH profile determined by the TPF method was found to be in good agreement with that determined by absorption spectroscopy. The estimated OH detection limit using TPF is ∼1014 molecules/cm3 at atmospheric pressure.