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Abstract

Threshold and near-threshold photoelectron spectra were taken around the Ar K edge and up to 150 eV above threshold in order to study the behaviour of the (1s3p)4p shake-up satellite in the energy region where the transition between the sudden limit and threshold takes place. The threshold spectrum is compared to photoelectron and absorption data exhibiting features of both. The satellite intensity at zero kinetic energy is 3.8% of the 1s main line. The whole data set indicates a smooth increase to the higher kinetic energy values of about 9% around 50 eV above threshold. This behaviour can be well described qualitatively by the adiabatic transition model of Thomas, although the sudden limit value of this model seems to be reached too slowly and is, as a consequence of this, too high. Nevertheless, this strongly supports the idea that the strongest K-shell satellites are predominantly 'shake-up' in character, exhibiting a smooth transition to the sudden-limit intensity in the near-threshold region. Furthermore, other related phenomena, such as post-collision interaction and enhanced intensity of zero-kinetic-energy electrons above threshold, are observed and briefly discussed.