We present a demonstration of two-photon double ionization of neon using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a photon energy regime where both direct and sequential mechanisms are allowed. For an APT generated through high-order harmonic generation (HHG) in argon we achieve a total pulse energy close to $1\mu \mathrm{J}$, a central energy of 35 eV, and a total bandwidth of $\sim 30$ eV. The APT is focused by broadband optics in a neon gas target to an intensity of $3\times {10}^{12}\mathrm{W}{\mathrm{cm}}^{-2}$. By tuning the photon energy across the threshold for the sequential process the double ionization signal can be turned on and off, indicating that the two-photon double ionization predominantly occurs through a sequential process. The demonstrated performance opens up possibilities for future XUV-XUV pump-probe experiments with attosecond temporal resolution in a photon energy range where it is possible to unravel the dynamics behind direct versus sequential double ionization and the associated electron correlation effects.

• Received 9 February 2016

DOI:https://doi.org/10.1103/PhysRevA.93.061402

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Published by the American Physical Society