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

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.