We report a differential momentum measurement of nonsequential double ionization (NSDI) of Mg in strong laser fields, by combining laser ablation with the supersonic molecular beam technique. The experimental result shows that the momentum distribution of ${\mathrm{Mg}}^{2+}$ ions along the laser polarization exhibits a single-peaked structure. This finding, confirmed by our fully classical model, provides direct experimental evidence that the NSDI of Mg proceeds predominantly via the recollision excitation with a subsequent ionization pathway. Furthermore, we find a distinct discrepancy in the shape of the momentum distribution obtained from the experiment and the classical model. The discrepancy mainly arises from the inability of the model to account for the returning electron impact ionization and excitation cross sections. Our experimental approach can be readily extended to other atomic species that are solid at room temperature. It also has important implications for probing electron entanglement in NSDI of alkaline-earth metal atoms.

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• Atomic & molecular processes in external fields
• Multiphoton or tunneling ionization & excitation