THRESHOLD AND NEAR-THRESHOLD PHOTOELECTRON-SPECTROSCOPY AROUND THE ARK EDGE

Threshold and near-threshold photoelectron spectra were taken around t he Ar K edge and up to 150 eV above threshold in order to study the be haviour 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 e xhibiting features of both. The satellite intensity at zero kinetic en ergy is 3.8% of the 1s main line. The whole data set indicates a smoot h increase to the higher kinetic energy values of about 9% around 50 e V 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 conse quence of this, too high. Nevertheless, this strongly supports the ide a that the strongest K-shell satellites are predominantly 'shake-up' i n character, exhibiting a smooth transition to the sudden-limit intens ity in the near-threshold region. Furthermore, other related phenomena , such as post-collision interaction and enhanced intensity of zero-ki netic-energy electrons above threshold, are observed and briefly discu ssed.