Abstract
Photoionisation cross sections for excited nl states (n+ have been computed in the framework of a single-electron model by the use of a central potential. The evolution of non-hydrogenic behaviour of photoionisation near threshold is studied along a given series. Moreover, isoelectronic variations from Rb to Sr+ as well as systematic trends along the sequence of alkali atoms Li through Cs are discussed. The non-hydrogenic traits, mainly the occurrence of minima in photoionisation cross sections, are analysed in terms of quantum defect theory.