We report detection of stimulated emission from the atomic-oxygen 3 (3)P-3 (3)S transition at 845 nm after two-photon excitation of the 2 (3)P-3 (3)P transition using 226-nm laser radiation. We study this stimulated emission process in flames and in room-temperature flows of O(2) and N(2)O and compare its behavior with that of fluorescence signals acquired simultaneously. Rapid depletion of the laser-excited state by the stimulated emission process may have an impact on the use of diagnostic techniques based on multiphoton excitation in oxygen and in other species. The strength of the stimulated emission signal suggests that it may have applications as a diagnostic technique.

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Laser-Induced Fluorescence (LIF) has been used for the detection of ammonia molecules, NH3, in flames at atmospheric pressure. Excitation was accomplished by a two-photon step from the ground state, X, to the C′ state, with the detection of the fluorescence radiation from the C′ state to the A state. This excitation scheme was also utilized for the simultaneous flame detection of OH and NH3. NO could also be simultaneously detected with the laser beam resulting from frequency mixing of the IR beam and the doubled dye laser beam from a Nd:YAG-based laser system.

Nitrogen atoms, in a flame or produced by photodissociation of nitrogen-containing molecules, were excited through a two-photon absorption process at 211 nm, and the near IR fluorescence was detected. Both in flame and in photodissociation experiments of cold gases, fluorescence, which was resonantly enhanced at the atomic nitrogen wavelength but which originated from excited oxygen atoms, was also identified. This phenomenon, the photochemical effects and those of molecular absorption, stimulated emission, and the mixing of gases are discussed in connection with the detection of atomic nitrogen in flames.

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