Angle-resolved measurement of Spin-orbit Time Delays in Photoionization using under-threshold RABBITT

In this work, we used attosecond RABBITT to study the angle-resolved photoionization dynamics in Krypton. The 4p valence orbital in Kr has a spin-orbit (SO) split of 0.66 eV. We measure the photoionization time delay between these two channels (J = ½ and J = 3/2) using an XUV-APT pump and a time-delayed NIR probe. With time-resolved photoelectron momentum imaging technique and supporting ab-initio TDSE simulations, we show the complex photon-energy and angular dependence of two-photon time delays where, for an appropriately chosen photon energy, under-threshold RABBITT (uRABBITT) and relativistic spin-orbit coupling effects coexist. We compared the photoionization time delays observed in the uRABBITT channel with the conventional RABBITT channel and find that the former shows a significantly larger two-photon photoionization time-delay due to the presence of discrete Rydberg excitations between the two SO-split thresholds. By tuning the fundamental photon energy, we observe an extreme sensitivity and sign flip of the measured photoionization time delays across the resonances in the J = ½ channel. On the other hand, J = 3/2 channel shows a smooth structureless photon energy dependence of photoionization time delay. The presence of autoionizing resonances also shows complex angular behavior in the uRABBITT channel whereas the conventional RABBITT channel does not have much angular character at all.