data of Single-photon-assisted strong-field ionization

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