Multi-sideband interference structures by high-order photon-induced continuum-continuum transitions in helium

Following up on a previous paper on two-color photoionization of Ar(3p)[Bharti et al., Phys. Rev. A 103 (2021) 022834], we present measurements andcalculations for a modified three-sideband (3-SB) version of the"reconstruction of attosecond beating by interference of two-photontransitions" (RABBITT) configuration applied to He(1s). The 3-SB RABBITTapproach allows us to explore interference effects between pathways involvingdifferent orders of transitions within the continuum. The relative differencesin the retrieved oscillation phases of the three sidebands provide insightsinto the continuum-continuum transitions. The ground state of helium has zeroorbital angular momentum, which simplifies the analysis of oscillation phasesand their angle-dependence within the three sidebands. We find qualitativeagreement between our experimental results and the theoretical predictions formany cases but also observe some significant quantitative discrepancies.