Characterization of XUV+IR ionization using the circular dichroic phase

A strong helicity dependence of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT) with circularly polarized XUV and IR pulses was reported by Han et al. [Nat. Phys. 19, 1649 (2023)1745-247310.1038/s41567-023-02188-z and arXiv:2302.04137]. They attributed a circular dichroic phase in RABBITT to the helical structure of the photoelectron wave packets in the final state. We exploit this effect to determine the magnitude and phase of two-photon XUV+IR ionization amplitudes. In s-electron targets (H, He, Li), such a determination is fully ab initio and requires no further approximations. In heavier noble gases like Ar, characterization of two-photon ionization amplitudes can be made from the circular dichroic phase with minimal and very realistic assumptions.