Heterodyne analysis of high-order partial waves in attosecond photoionization of helium

Partial wave analysis is key to interpretation of the photoionization of atoms and molecules on the attosecond timescale. Here we propose a heterodyne analysis approach, based on the delay-resolved anisotropy parameters to reveal the role played by high-order partial waves during photoionization. This extends the Reconstruction of Attosecond Beating By Interference of Two-photon Transitions technique into the few-photon regime. We demonstrate that even for moderate (~ 1TW/cm²) intensities, near-infrared-assisted photoionization of helium through Rydberg states results in a tiny contribution from the g₀ wave, which has a significant impact on the photoelectron angular distributions via interference with the s- and d₀-waves. This modulation also causes a substantial deviation in the angular distribution of the recovered spectral phase shift. Our analysis provides an efficient method to resolve isolated partial wave contributions beyond the perturbative regime, and paves the way towards understanding resonance-enhancement of partial waves.