Under-threshold RABBITT in argon

The process of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT) can involve a transition from under the ionization threshold. Such an under-threshold RABBITT (or uRABBITT) was demonstrated experimentally and analyzed theoretically in He and Ne. In the present work, we explore an analogous process in the argon atom. The uRABBITT in Ar reveals the familiar physical effects: a phase transition across the threshold and the symmetry modification of the photoelectron momentum distribution. It can also be used for mapping the electronic structure of the target atom bound states.