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.

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

The photoionization cross sections for Ar ϩ through Ar 4ϩ , produced by the Auger decay of a 2 p hole in argon, have been measured between 242 eV and 253 eV by the use of synchrotron radiation. The high resolution of the monochromator has allowed a detailed study of the postcollision interactions that occur in this spectral region. The concept of photoelectron recapture by Ar 2ϩ to produce the Ar ϩ continuum is studied. The relative values of the quantum-mechanical calculations of the photoelectron recapture probability are shown to be in excellent agreement with the present data. The magnitude and shape of the Ar 2ϩ continuum has been explained on the basis that about 67% of the recaptured photoelectrons produce excited states of Ar ϩ which subsequently reemit the electrons by autoionization. ͓S1050-2947͑96͒02308-6͔

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