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Double ionization of Li-like ions by high-energy photons

The processes of double ionization by absorption or inelastic (Compton) scattering of a photon from atomic systems are of fundamental interest. This is due to the fact that the two-electron emission is exclusively caused by the electron-electron interaction, the accurate treatment of which is still one of the main tasks for different theoretical approaches. As a target, it is usual to use the neutral He atom in the ground state, which is supposed to be the simplest atomic system, where double ionization can occur [1, 2, 3]. Up to now no experimental data are available concerning double ionization of He in metastable states. This is connected with difficulties in preparation of any amount of such targets. However, these processes could serve as a testing ground for the study of inter-shell electron correlations. In contrast to neutral He atoms, the use of multicharged ions as targets could provide important information about most generic features of ionization processes. In particular, it concerns the study of the universal scalings in the behavior of ionization cross sections and double-to-single ionization ratios. There also exist parametrical relationships between the cross sections for ionization of multicharged ions with different number of electrons. This opens wide perspectives for the investigation of both innerand intershell electron correlations, since the experiments on stable targets can be used to test the theoretical predictions for ionization of excited states. We have investigated the double ionization of threeelectron ions by photons with high but still nonrelativistic energies ω characterized by I ω m, where I is the ionization threshold energy and m is the electron mass (h̄ = 1, c = 1) [4, 5]. More precisely, the energy domain ω I is asymptotic for the double-to-single ionization ratio only in the case of photoabsorption. In Compton scattering, the asymptotic regime is reached at higher energies ω η, where η is the average momentum of the bound electron. For photoabsorption and Compton scattering on lithiumlike ions, the universal scaling behavior of the double-tosingle ionization ratios are given by [4, 5]