NARROW CHAOTIC COMPOUND AUTOIONIZING STATES IN ATOMIC SPECTRA

Simultaneous excitation of several valence electrons in atoms gives ri se to a dense spectrum of compound autoionizing states (AIS). These st ates are almost chaotic superpositions of large numbers of many-electr on basis states built of single-electron orbitals. The mean level spac ing D between such states is very small (e.g., D<0.01 eV for the numer ical example of J(pi)=4(-) states of Ce just above the ionization thre shold). The autoionization widths of these states estimated by perturb ations, gamma=2 pi\W\(2), where W is the Coulomb matrix element coupli ng the AIS to the continuum, are also small, but comparable with D in magnitude: gamma similar to D. Hence the nonperturbative interaction o f AIS with each other via the continuum is very essential. It suppress es greatly the widths of the autoionizing resonances (Gamma similar or equal to D-2/3 gamma much less than 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 cont inuum decoupled from the resonances due to its large width. Narrow com pound AIS should be a common feature of atomic spectra at energies suf ficient for excitation of several electrons above the groundstate conf iguration. The narrow resonances can be observed as peaks in the photo absorption, or, in electron-ion scattering, as Fano-type profiles on t he background provided by the wide doorway-state resonance. It is also shown that the statistics of electromagnetic and autoionization ampli tudes involving compound states are close to Gaussian.