MULTICHANNEL RYDBERG SPECTROSCOPY OF COMPLEX ATOMS

Multichannel atomic spectra frequently exhibit such extraordinary visu al complexity that they appear at first glance to be uninterpretable. The present review discusses how to unravel such spectra through the u se of theoretical multichannel spectroscopy to extract the key dynamic al implications. Moreover, this class of techniques permits a quantita tive prediction or reproduction of experimental spectra for some of th e more challenging atomic systems under investigation. It is shown tha t multichannel spectroscopy marries the techniques of multichannel qua ntum-defect theory to the eigenchannel R-matrix method (or related met hods). It has long been appreciated that multichannel quantum-defect t heory can successfully use a collision-theory framework to interpret e normously complicated Rydberg spectra. However, the capabilities of mu ltichannel quantum-defect theory have increased dramatically during th e past decade, through the development of nearly ab initio methods for the calculation of the short-range scattering parameters that control the interactions of closed and open channels. In this review, emphasi s is given to the alkaline-earth atoms, for which many different obser vables have been successfully compared with experiment over broad rang es of energy and resolution. Applications of the method to describe th e photoionization spectra of more complex open-shell atoms are also di scussed.