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., D0.01 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}=2{π}|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}{}D²/3{γ}{}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. {} 1996 The American Physical Society.