Impact of autoionizing resonances and spin-orbit coupling on near threshold RABBITT measurements

Experimental measurement of photoionization time delays continues to be an important application of ultrafast photoelectron spectroscopy. A RABBITT measurement extracts relative time delays between two pathways by introducing a probe photon to mediate a two-photon transition, typically continuum-continuum, to a common final energy. However, in the low-energy region near threshold, one of the pathways can originate from bound states below the ionization threshold or from autoionizing states between spin-orbit split thresholds. We explore this case in Argon, where we studied the RABBITT signal resulting from the interference of the 9th (below threshold) and 11th (above threshold) harmonic pathways to an energetic region between spin-orbit thresholds that is dense with autoionizing resonances. Specifically, in the case of Argon, we conducted angle-resolved RABBITT photoelectron measurements in a dense, highly structured quasi-continuum in between the (²P_1/2) and (²P_3/2) spin orbit split thresholds. Interferometric pathways in this region feature transitions between bound - quasi-bound and bound-continuum states. Similar measurements were conducted in Krypton to obtain the relative photoionization phase between the two spin-orbit split channels in the low energy region of the continuum.