Abstract

We are engaged in experiments to probe highly excited autoionizing levels of atomic oxygen using a double-resonance laser ionization scheme. Oxygen atoms are prepared by subjecting O₂ to a microwave discharge at a pressure of ~0.2 Torr. A pump laser provides 225.7-nm photons to populate the 3p levels by two-photon absorption, also ionizing a small fraction of the excited atoms by absorption of a third photon. A second probe laser delayed by ~10 ns is scanned over energies of interest, and the ion current enhancement is recorded. We have observed ion signals from the 3d′(³S°, 3P°, ³D°) autoionizing states at ~124,000 cm⁻¹. Absolute ionization cross sections are obtained by reference to the known value of 5.3 × 10⁻¹⁹ cm² for 225.7-nm photons. A variety of autoionizing line shapes are observed, and further work is in progress to identify the nature of this behavior. We are also measuring cross sections for nonresonant ionization as the probe laser is tuned through the ionization continuum. The continuum data are useful for guiding the development of accurate theoretical descriptions of the final state in the ionization process.

© 1987 Optical Society of America