Realistic RABBITT: extending the ‘reconstruction of attosecond beating by interference of two-photon transitions’ technique beyond idealised conditions

The “reconstruction of attosecond beating by interference of two-photon transitions” (RABBITT) technique has been pivotal in the advancement of attosecond physics, allowing recovery of the temporal profile of attosecond pulse trains comprising bursts of only around 100-250 attoseconds (1-2.5 x 10^-16 s).<br><br>Since its introduction in 2001, for which Pierre Agostini jointly received the 2023 Nobel Prize in Physics, the RABBITT technique has been continually improved, extended and repurposed, becoming a versatile tool for both the characterisation of attosecond scale light and probing photoionisation processes.<br><br>In this thesis, we present several advancements to the RABBITT technique.<br><br>We demonstrate how component spin-orbit contributions to RABBITT spectra and phases may be resolved theoretically using R-Matrix with Time-dependence theory (RMT), and apply this approach to an argon autoionising state to investigate a proposed numerical method for post-hoc isolation of spin-orbit components which may be applied to experimental RABBITT spectra.<br><br>We fill a gap in current RABBITT theory by extending RABBITT to account for Higher-Order Processes (RABBITT-HOP) while explicitly accounting for atomic phase shifts, and conduct an investigation of high-lying helium Rydberg states at laser intensities where these higher-order processes cannot be neglected.<br><br>Through this investigation, comprising both RMT simulation and experimental implementation by our collaborators in the State Key Lab for Precision Spectroscopy group at East China Normal University, we demonstrate explicitly the presence of these higher order effects and their impact on the selection of partial-waves which may be accessed by the ionised electron at these higher intensities.<br>