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Following up on a previous paper on twocolor photoionization of Ar(3p) [D. Bharti et al., Phys. Rev. A 103, 022834 (2021)], we present measurements and calculations for a modified threesideband (3SB) version of the reconstruction of attosecond beating by interference of twophoton transitions (RABBITT) configuration applied to He(1s). The 3SB 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 continuumcontinuum 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.more » « lessFree, publiclyaccessible full text available February 1, 2025

Abstract We report on a series of detailed BreitPauli and Dirac Bspline Rmatrix (DBSR) differential cross section (DCS) calculations for excitation of the
and$$5\,^2\textrm{S}_{1/2} \rightarrow 5\,^2\textrm{P}_{1/2}$$ $5{\phantom{\rule{0ex}{0ex}}}^{2}{\text{S}}_{1/2}\to 5{\phantom{\rule{0ex}{0ex}}}^{2}{\text{P}}_{1/2}$ states in rubidium by 40 eV incident electrons. The early BP computations shown here were carried out with both 5 states and 12 states, while the DBSR models coupled 150 and 325 states, respectively. We also report corresponding results from a limited set of DCS measurements on the unresolved$$5\,^2\textrm{S}_{1/2}\rightarrow 5\,^2\textrm{P}_{3/2}$$ $5{\phantom{\rule{0ex}{0ex}}}^{2}{\text{S}}_{1/2}\to 5{\phantom{\rule{0ex}{0ex}}}^{2}{\text{P}}_{3/2}$ states, with the experimental data being restricted to the scattered electron angular range 2–$$5\,^2\textrm{P}_{1/2,3/2}$$ $5{\phantom{\rule{0ex}{0ex}}}^{2}{\text{P}}_{1/2,3/2}$ . Typically, good agreement is found between our calculated and measured DCS for excitation of the unresolved$$10^\circ $$ ${10}^{\circ}$ states, with best accord being found between the DBSR predictions and the measured data. The present theoretical and experimental results are also compared with predictions from earlier 40 eV calculations using the nonrelativistic DistortedWave Born Approximation and a Relativistic DistortedWave model.$$5\,^2\textrm{P}_{1/2,3/2}$$ $5{\phantom{\rule{0ex}{0ex}}}^{2}{\text{P}}_{1/2,3/2}$Graphic abstract