Abstract

Before multiphoton detection schemes for atoms and molecules can be placed on a quantitative footing, cross sections for the multiphoton transitions must be known. We have measured the relevant cross sections for atomic oxygen, an important species in flame diagnostics. The technique of two-photon excited fluorescence was used to measure the cross section for the 2p³p₂ → 3p³p_2,1,0 transition at 226 nm. Excitation was monitored by observing fluorescence at 845 nm from the 3p ³p_2,1,0 → 3s ³S₁ transition. The fluorescence collection system was calibrated absolutely using spontaneous Raman scattering from molecular hydrogen. The spatial and temporal profiles of the exciting dye laser pulses were carefully measured in a gently focused excitation geometry (peak intensity ~10 MW/cm²) allowing the two-photon cross section to be determined. The cross section for absorption of a third, identical photon (2 + 1 photoionization) was then determined by measuring the absolute number of ions produced per laser pulse. The measured cross sections are α = 2.5 × 10⁻²⁸ cm⁴/W and σpi = 8.1 × 10⁻¹⁹ cm². Using a square pulse in space and time, we calculate that ~ 1% ionization is possible for a laser fluence at 226 nm of 1 J/cm² in a 10-ns long pulse.

© 1985 Optical Society of America