Mechanisms of Strong-Field Double Ionization of Xe

We perform a fully differential measurement on strong-field double ionization of Xe by 25 fs, 790 nm laser pulses in intensity region $(0.4--3)\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$. We observe that the two-dimensional correlation momentum spectra along the laser polarization direction show a nonstructured distribution for double ionization of Xe when decreasing the laser intensity from $3\ifmmode\times\else\texttimes\fi{}{10}^{14}$ to $4\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$. The electron correlation behavior is remarkably different with the low-$Z$ rare gases, i.e., He, Ne, and Ar. We find that the electron energy cutoffs increase from $2.9{U}_{p}$ to $7.8{U}_{p}$ when decreasing the laser intensities from the sequential double ionization to the nonsequential double ionization regime. The experimental observation indicates that multiple rescatterings play an important role for the generation of high energy photoelectrons. We have further studied the shielding effect on the strong-field double ionization of high-$Z$ atoms.