Photochemical effects in 243-nm two-photon excitation of atomic hydrogen in flames

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Abstract
This paper describes photochemical effects observed during two-photon 1S–2S excitation of atomic hydrogen in flames using 243-nm laser radiation. An I 4 intensity dependence is observed in regions of the flame where the natural atomic concentration is low, suggesting an I 2 photochemical production mechanism, which we believe is due to two-photon excitation of water molecules, which then predissociate to form H and OH fragments. In a measurement of OH created in the flame by the 243-nm beam, we observe the same I 2 intensity dependence with the laser detuned from the atomic hydrogen 1S–2S resonance, but an apparent I 3,4 dependence is observed when the laser is tuned to the resonance. We believe that a second photochemical mechanism contributes at the resonance, namely, two-photon excitation of H, followed by collisional energy transfer to water molecules, which then fall apart into H and OH fragments. We model this process and show that a combination of I 2 and I 4 dependences can lead to an intensity dependence that mimics a single I 3,4 dependence over a limited range of intensities.