Threshold Behaviour and Resonances in the Photoionization of Atoms and Molecules

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Abstract
Threshold and near threshold satellite intensities are examined with respect to threshold behaviour, specifically for certain electron correlations contributing to the intensity of these satellites. It is assumed that the different dominant electron correlations give rise to at least four types of threshold behaviour, making it possible to differentiate to a first approximation among them. These correlations are subdivided phenomenologically into two classes: "intrinsic" and "dynamic" correlations. Experimental evidence is presented for both inner- and outer-shell photoionization, substantiating this semiempirical concept. Our results are also compared with first ab initio calculations. The effect of resonances is emphasized as part of the threshold behaviour but also with respect to their deexcitation into various decay channels. Multielectron processes such as resonant shake-off, as well as two step autoionization and spectator Auger decay, are shown to play important roles in the decay of resonances and satellite states.