Spectroscopy and electronic structure of ion-pair states

Abstract

Ion-pair states are a distinctive sub-set of the valence states of molecules. In a few cases they constitute the ground electronic state, but more usually their term values lie close to the lowest Rydberg states. Their unusual spectroscopic properties and the techniques used to access them are discussed with particular reference to the diatomic halogens. Their electronic structure is best approached from two complementary points of view, as a single molecular orbital configuration or the product of separated ion configurations, according to the ionic separation. The consequences of this dual character for absorption and fluorescence spectroscopy are outlined. Finally, the state-specific coupling of Rydberg and ion-pair states is surveyed.