Helium double photoionisation and Compton scattering: A showcase for electron correlation

The present status of double photoionisation studies is comprehensively rev iewed. Recent findings are described which shed some light on the transitio n from the correlated motion of the two electrons near threshold to the sha keoff-like behaviour at higher photon energies. For extremely high photon e nergies, where Compton scattering becomes the dominant process, new results for the He2+/He+ ratio between 6 and 120 keV are presented. The results co nfirm the prediction of Bergstrom et al. that the ratio reaches an intermed iate maximum between 12 and 15 keV, before declining towards the asymptotic limit. Furthermore, this asymptotic limit seems not to be reached even at energies as high as 120 keV.