We present a joint experimental and theoretical investigation of nondipole effects in strong-field double ionization of xenon atoms driven by circularly polarized 800 nm laser fields. We observe that the ${\mathrm{Xe}}^{2+}$ ion momentum distribution in the laser polarization plane is Gaussian. The correlated two-electron momentum distribution along a specific direction in the polarization plane reveals a distinct four-lobe structure. We also measure the photoelectron momentum shift of double ionization along the laser propagation direction, which is associated with the nondipole effect. We find that the nondipole effect shows a remarkable enhancement in momentum shift for low-energy electrons, which deviates from the expectation within the picture of sequential double ionization. We develop a nondipole-corrected two-electron classical trajectory Monte Carlo (CTMC) model that reproduces the experimental observations. The abnormal momentum shift of low-energy electrons originates from recollision-induced linear momentum redistribution. Furthermore, the sensitivity of the nondipole effect to the recollision allows us to resolve distinct angular emission patterns between sequential and nonsequential double ionization. These results indicate that the nondipole effect plays a prominent role in electron correlation, and they offer new insights into many-body linear momentum transfer in intense laser-matter interactions.

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• Electronic excitation & ionization
• Multiphoton or tunneling ionization & excitation