Double-electron photoionization of outer electrons in complex atoms.

The aim of this talk is to demonstrate that$^{ }$double-electron photoionization of outer electrons in complex atoms proceeds via virtual creation of the nearest in energy $s$-vacancy instead of direct elimination of two outer electrons. As a result, the elimination of outer electrons by a single photon takes place not because solely of the interaction between ionized electrons, but due to a more complex mechanism, namely that with active participation of the closest $s- $electron. The suggested mechanism becomes particularly important at high photon energies$\omega $. It is known that there the one-electron photoionization cross-section $\sigma _s^+ (\omega )$ for an $s- $electron decreases as $\sigma _s^+ (\omega )\sim 1/\omega ^{7/2}$with $\omega $ growth. In the most studied case of helium double-electron ionization its cross-section $\sigma _s^{++} (\omega )$ decreases also as $\sigma _s^{++} (\omega )\sim 1/\omega ^{7/2}$ thus leading to a known fact that the ratio $R_{ss/s} =\sigma _s^{++} (\omega )/\sigma _s^+ (\omega )$ is $\omega $ independent at$\omega \to \infty $. The suggested mechanism changes the situation dramatically leading for two outer $p$-electrons photoionization to the ratio $R_{pp/p} \sim \omega \to \infty $ at$\omega \to \infty $, because $\sigma _{pp}^{++} (\omega )\sim 1/\omega ^{7/2}$while $\sigma _p^+ (\omega )\sim 1/\omega ^{9/2}$. This feature could be observed experimentally if to study two outgoing electrons in coincidence.