Interpreting Attoclock Measurements of Tunnelling Times

Recent experiments based on the attoclock setup opened the opportunity to time-resolve ionization dynamics in strong IR laser fields. However, reconstruction of ionization times from experiment poses theoretical challenges. We have used the combination of ab-initio calculations with analytical R-Matrix theory to resolve these challenges, without ad-hoc assumptions about the ionization process. Ionization in IR fields can be described as electron tunnelling under the barrier created by the field and the core potential. Thus, attoclock setup addresses the long standing question of tunnelling times: does an electron take a finite real time to pass under a barrier? For the hydrogen atom, our results show that optical tunnelling is instantaneous. Rather than real time delays, we show that it is the long-range Coulomb interaction with the core that is responsible for the angular shifts we see in the photoelectron spectra. We also consider multielectron systems and show how our method can be used to extract strong-field ionization delays associated with multielectron dynamics.