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 and
calculations for a modified three-sideband (3-SB) version of the
"reconstruction of attosecond beating by interference of two-photon
transitions" (RABBITT) configuration applied to He(1s). The 3-SB RABBITT
approach allows us to explore interference effects between pathways involving
different orders of transitions within the continuum. The relative differences
in the retrieved oscillation phases of the three sidebands provide insights
into the continuum-continuum transitions. The ground state of helium has zero
orbital angular momentum, which simplifies the analysis of oscillation phases
and their angle-dependence within the three sidebands. We find qualitative
agreement between our experimental results and the theoretical predictions for
many cases but also observe some significant quantitative discrepancies.Comment: 9 pages, 6 figure