Narrow chaotic compound autoionizing states in atomic spectra

Simultaneous excitation of several valence electrons in atoms gives rise to a dense spectrum of compound autoionizing states AIS . These states are almost chaotic superpositions of large numbers of many-electron basis states built of single-electron orbitals. The mean level spacing D between such states is very small e.g., D<0.01 eV for the numerical example of J^π=4^- states of Ce just above the ionization threshold . The autoionization widths of these states estimated by perturbations, γ=2π|W|², where W is the Coulomb matrix element coupling the AIS to the continuum, are also small, but comparable with D in magnitude: γ∼D. Hence the nonperturbative interaction of AIS with each other via the continuum is very essential. It suppresses greatly the widths of the autoionizing resonances (Γ≈D²/3γ<<D), and leads to the emergence of a ‘‘collective’’ doorway state which accumulates a large share of the total width. This state is in essence a modified single- particle continuum decoupled from the resonances due to its large width. Narrow compound AIS should be a common feature of atomic spectra at energies sufficient for excitation of several electrons above the ground- state configuration. The narrow resonances can be observed as peaks in the photoabsorption, or, in electron-ion scattering, as Fano-type profiles on the background provided by the wide doorway-state resonance. It is also shown that the statistics of electromagnetic and autoionization amplitudes involving compound states are close to Gaussian.