ATOMIC SPECTRA AND INSTABILITIES OF VALENCE

The concept of valence is discussed in relation to recent studies of t he subvalence spectra of free atoms. It is argued that valence can be profoundly modified by accidental degeneracies which occur as a result of centrifugal barrier effects. Examples of ''atomic hybridisation'' are given, and their effects on observed spectra are discussed. Anothe r kind of valence change occurs when a radial wavefunction is poised o n the verge of orbital collapse. An example is given where a change of valence can be triggered in an atomic cluster by varying the cluster size. The origin of the effect is traced to properties of the inner sh ort-range well in the effective atomic potential when the centrifugal term is included. It is concluded that more account needs to be taken of the underlying atomic effects and especially radial properties when setting up theories of valence for transition elements.