The potential for two-dimensional visualization of combustion species by using two-photon laser-induced fluorescence (LIF) has been investigated. The technique was applied for two-dimensional (2D) imaging of carbon monoxide, ammonia, oxygen, and hydrogen atoms in flames. Approaches for compensating the signal intensity for the quadratic laser intensity dependence in two-photon imaging are discussed. For the case of CO and H atom visualization, a potential problem is the interference from nonresonantly excited C2, whose emission spectrally and spatially coincides with the fluorescence from CO. Different strategies for elimination of the C2 emission were investigated. It was found out that the emissions from CO and C2 can be separated in time. For the case of the oxygen atoms, it was observed that the relation between the intensities of the fluorescence signals at 845 and 777 nm changes with the equivalence ratio of the investigated flame. An attempt to estimate the 2D detection limit for these species in flames is also made.
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