Valence double photoionization of beryllium

We have measured the double-to-single photoionization ratio of beryllium (1s(2)2s(2)) between 32 and 80 eV and compare our data with the electron-impact ionization cross section of Be+. In this energy region only simultaneous but not sequential emission of both 2s electrons can take place. We also compare our data with the double-to-single photoionization ratios of He, H-2 and Li and find surprisingly good agreement of the photon-energy dependence if the ratio is scaled appropriately. The double-to-single photoionization ratio is lower for Be than for He due to a weaker electron-electron interaction between the valence electrons.