This paper describes photochemical effects observed during two-photon 1S–2S excitation of atomic hydrogen in flames using 243-nm laser radiation. An I4 intensity dependence is observed in regions of the flame where the natural atomic concentration is low, suggesting an I2 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 I2 intensity dependence with the laser detuned from the atomic hydrogen 1S–2S resonance, but an apparent I3,4 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 I2 and I4 dependences can lead to an intensity dependence that mimics a single I3,4 dependence over a limited range of intensities.
© 1989 Optical Society of America
Original Manuscript: September 20, 1988
Published: March 15, 1989
J. E. M. Goldsmith, "Photochemical effects in 243-nm two-photon excitation of atomic hydrogen in flames," Appl. Opt. 28, 1206-1213 (1989)