Multiphoton photochemical and collisional effects during oxygen-atom flame detection

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Abstract
A Nd:YAG-pumped dye-laser system was used to two-photon excite oxygen atoms at 225.6 nm in an atmospheric-pressure CH4–N2O–N2 flame. Subsequent emission at 844.7 nm from the directly populated state as well as a stronger emission at 777.5 nm that was due to the O(3p 3P → 3p 5P) collisional-energy transfer process was monitored. Two-photon-resonant oxygen-atom and hydrogen-atom (656.3-nm) emissions were also observed in the absence of a flame. Closer examination revealed that the tightly focused probe beam was producing these atoms by promoting multiphoton photolysis of the oxidizer as well as of the fuel molecules. Thus this type of laser-diagnostic probe is potentially quite intrusive, depending on the combustion region that is probed as well as on the laser energies used.