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The independent particle approximation is shown to break down for the photoionization of both inner and outer nᐉ ͑ᐉ . 0͒ electrons of all atoms, at high enough energy, owing to interchannel interactions with the nearby ns photoionization channels. The effect is illustrated for Ne 2p in the 1 keV photon energy range through a comparison of theory and experiment. The implications for x-ray photoelectron spectroscopy of molecules and condensed matter are discussed. [S0031-9007 (97)03382-6] PACS numbers: 32.80.Fb

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We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He"*""""/He"*" = 1.6%±0.3% at /zv==2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Aberg, who predict 1.7%.