Rich physics emerges as the result of interplay between discrete and continuous quantum states. A process of reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) reveals a similar phenomenon. In RABBITT, primary XUV driven ionization is aided by secondary IR photon absorption or emission. The latter processes involve transitions between continuous states. We demonstrate that when RABBITT crosses the ionization threshold and proceeds via a discrete bound state, its phase makes a sudden jump which can be related to the phase of the continuous transitions above the threshold. Up to now, this phase remained undetermined experimentally and could only be estimated theoretically. The under-threshold RABBITT provides alternative means of assessing this quantity.

• Received 4 August 2020
• Accepted 24 December 2020

DOI:https://doi.org/10.1103/PhysRevA.103.L011101