Understanding strong-field double ionization of many-electron systems is an important fundamental problem with potential implications for molecular imaging within this regime. Using mid-IR radiation, we unambiguously identify the transition from nonsequential ($e, 2e$) to sequential double ionization in Xe at an intensity below ${10}^{14}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$. Ionization from excited orbitals is found to be decisive at low intensities, but we demonstrate that such mechanisms are unimportant in the sequential regime. We utilize these facts to successfully image a molecular dication using laser-induced electron diffraction. This methodology can be used to study molecular dynamics on unprecedented few-femtosecond time scales.

• Received 5 February 2016
• Revised 19 July 2016

DOI:https://doi.org/10.1103/PhysRevA.96.033401