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Precision photoionization cross sections for He have been measured from threshold to 120 eV with an accuracy ranging from 1 to 2%. In addition, a critical review of the published data has been made and recommended photoionization cross sections from the ionization threshold to 8 keV are presented with an estimated uncertainty of +or-10% above 500 eV. The effect of coherent (Rayleigh) and incoherent (Compton) scattering is discussed.

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The cross sections for double photoionization of helium and the ratios of double to single ionization have been measured from the double-ionization threshold to 820 eV. The results are in very good agreement with several recent calculations. ͓S1050-2947͑98͒05403-1͔ PACS number͑s͒: 32.80.Fb

Relative photoionization cross sections for O + and O 2+ produced by the Auger decay of a 1s hole in atomic oxygen were measured by using synchrotron radiation between 525 and 553 eV. Energies and quantum defects of the members of the two Rydberg series converging to [1s]2s 2 2p 4 ( 4 P) and [1s]2s 2 2p 4 ( 2 P) ionization thresholds were determined. In addition, the 2 P and 4 P ionization thresholds were calculated from the two Rydberg series. The 182 meV resolution of the monochromator allowed a detailed study over both thresholds revealing evidence for post-collision interaction and allowing a comparison of the ionization continuum above both 2 P and 4 P thresholds with that of the ionization continuum above the Ar L 2-3 edges. This comparison indicates that the lifetimes of the Ar(2p) and O(1s) hole states are approximately the same.