Characterization of high-energy photoionization in terms of the singularities of the atomic potential. II. BeyondK-shell ionization in a many-electron atom, using the example of a two-electron atom in an excited state

Abstract

We describe single photoionization of excited states of two-electron atoms by photoabsorption at high incident photon energies $\ensuremath{\omega}$ (but still $\ensuremath{\omega}\ensuremath{\ll}m).$ Our description of photoionization from excited states of the simplest many-body system is also, however, applicable for the characterization of high-energy photoionization of a many-electron atom from any subshell. We are using an approach [Suri\ifmmode \acute{c}\else \'{c}\fi{} et al., Phys. Rev. A 67, 022709 (2003)] based on asymptotic Fourier-transform (AFT) theory, in which the matrix elements for photoabsorption processes at high energies are understood in terms of the singularities of the many-body Coulomb potential. We obtain the dependence of the total cross section for single ionization of a two-electron atom in any initial state on photon energy. This energy dependence, for a general initial two-electron state, is generally different from the predictions of independent-particle approximation, and it is in qualitative agreement with recent experimental observations of L-shell photoionization in Ne and M-shell ionization in Ar. As in ground-state ionization, the energy dependence of the dominant contribution to the matrix element is connected, through AFT, with the $e\ensuremath{-}N$ singularity; and it is determined by the amplitude of the lowest angular momentum ${l}_{\mathrm{min}}$ with which one electron can approach the $e\ensuremath{-}N$ singularity. When ${l}_{\mathrm{min}}=0,$ as in the cases considered experimentally, this gives a factor for the dominant part of the total cross section, which is the same as for the ground state, $(1/{\ensuremath{\omega}}^{7/2}).$ The final state interaction reduces this energy dependence by just one additional factor of $1/\ensuremath{\omega}.$