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We develop an approximate model for the process of direct (nonsequential)two-photon double ionization of atoms. Employing the model, we calculate(generalized) total cross sections as well as energy-resolved differentialcross sections of helium for photon energies ranging from 39 to 54 eV. Acomparison with results of \textit{ab initio} calculations reveals that theagreement is at a quantitative level. We thus demonstrate that this complexionization process is fully described by the simple model, providing insightinto the underlying physical mechanism. Finally, we use the model to calculategeneralized cross sections for the two-photon double ionization of neon in thenonsequential regime.

Nonsequential Two-Photon Double Ionization of Atoms: Identifying the Mechanism