Autoionization in atomic and molecular physics

The important roles of ionized atoms and molecules in many aspects of physical and chemical phenomena is being increasingly recognized. The role of the autoionization process is here considered and some of its characteristics examined in the light of perturbation theory. Although there is no fundamental difference between autoionization in atoms and in molecules, the two cases are dealt with separately since molecular systems lead to different types of autoionization and experimentally observable effects. Experimental methods used to investigate the effects, which are here discussed, include absorption and emission spectroscopy, photoionization investigations, photoelectron spectroscopy and electron impact studies. (U.K.)