We investigate strong-field ionization of helium atoms exposed to an intense infrared (IR) laser field assisted by a weak extreme-ultraviolet (XUV) field, where the XUV photon energy is below the ionization potential of helium, prohibiting direct single-photon ionization. Despite its low intensity, the XUV field enhances the ionization probability by nearly 3 orders of magnitude compared to the case with the IR field alone. By varying the XUV intensity, we demonstrate that this enhancement stems from strong-field ionization assisted by single-XUV-photon absorption. Using the strong-field approximation model, we disentangle the contributions of direct and rescattered electrons to the photoelectron energy spectrum. Our results reveal that direct ionization is primarily enhanced near the zero crossing of the IR field, whereas rescattered electron ionization is enhanced near the field maximum.

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• Atomic & molecular processes in external fields
• Single- and few-photon ionization & excitation