Excited electron orbit collapse and atomic spectra

The reasons for the two-well potential in free atoms and the collapse of the orbit (or wave function) of an electron are discussed. Theoretical data on the dependence of the collapse on the atomic number, the degree of ionization, the configuration, the many-electron quantum numbers, etc., are put in systematic form. A similar effect occurs for a free electron moving in an atomic field. Various manifestations of the collapse and the potential barrier in the energy spectra are discussed: an anomalous behavior of the quantum defect, a nonmonotonic variation in the type of binding in an isoelectronic series, a broadening of the energy spectrum, and a pronounced mixing of configurations. Effects of the collapse and the barrier in photoabsorption spectra are also discussed: a suppression of the Rydberg series, a shift of the absorption edge, the appearance of giant absorption resonances corresponding to transitions in the discrete spectrum, and the alignment of ions during photoionization of atoms. Experimental data which furnish evidence that the interaction of slow electrons with atoms is quasiresonant in the case of a two-well potential are reviewed briefly.