Giant Resonances in the Transition Regions of the Periodic Table

Abstract

In the transition regions of the periodic table of the elements, atomic d or f orbitals undergo a fairly sudden change from hydrogenic to fully collapsed form. This transition involves a large reduction in the mean orbital radius - by about 95% for the 4f orbital - and results in corresponding qualitatitive changes in physical processes sensitive to orbital size (e. g. excitation cross sections, bonding character). It is caused by a shift, as the nuclear charge Z increases, in the close balance between repulsive centrifugal and attractive atomic forces on the electron. The balance can also be tilted within a given element in the transition region, for instance by a change in the occupancy of its core or valence orbitals, or by the formation of a molecular bond. Transition region elements are thus characterized by an unusual sensitivity of gross orbital properties to external perturbations; and, from the standpoint of theoretical representation, to the effects of electron correlation, LS term dependence, and special relativity.