Multichannel atomic spectra frequently exhibit such extraordinary visual complexity that they appear at first glance to be uninterpretable. The present review discusses how to unravel such spectra through the use of theoretical multichannel spectroscopy to extract the key dynamical implications. Moreover, this class of techniques permits a quantitative prediction or reproduction of experimental spectra for some of the more challenging atomic systems under investigation. It is shown that multichannel spectroscopy marries the techniques of multichannel quantum-defect theory to the eigenchannel $R$-matrix method (or related methods). It has long been appreciated that multichannel quantum-defect theory can successfully use a collision-theory framework to interpret enormously complicated Rydberg spectra. However, the capabilities of multichannel quantum-defect theory have increased dramatically during the 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, emphasis is given to the alkaline-earth atoms, for which many different observables have been successfully compared with experiment over broad ranges of energy and resolution. Applications of the method to describe the photoionization spectra of more complex open-shell atoms are also discussed. [S0034-6861(96)00504-1]

DOI:https://doi.org/10.1103/RevModPhys.68.1015