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Without a formal derivation, we propose a formula for the total andsingle-differential cross in the problem of one-photon double ionization of anatom. The formula is benchmarked against accurate experimental data for thetotal cross section of helium. Furthermore, a direct comparison with ab initiocalculations for the double ionization of Li+ suggests that the framework isvalid for the entire helium isoelectronic sequence. To this end, we introduce aformula for the double ionization of lithium, as well as for the tripleionization of lithium and beryllium.

One-photon double ionization of helium: a heuristic formula for the cross section