Unveiling Rabi dynamics through angle-resolved photoelectron momentum distributions using an ω-2ω pulse pair

We present an interferometric method for studying the Rabi dynamics in atoms by employing an w-2w extreme ultraviolet pulse pair generated from the seeded free-electron laser. By solving the time-dependent Schrodinger equation (TDSE) for hydrogen atoms, we study the photoelectron spectrum that emerges when the w pulse triggers Rabi oscillations between the ground state and the first excited state. The interference between the one-photon and two-photon ionization pathways in the photoelectron signal gives access to the phase difference between the one- and two-photon transition amplitudes. Compared to the cases without Rabi oscillations, an additional it phase jump is observed in the energy domain. Based on perturbation theory, we demonstrate that this phase jump directly reflects the ultrafast buildup of Rabi oscillations in the time domain. The present w-2w scheme can be generalized and applied to other more complex atoms or molecules provided that the populations of bound states can be efficiently and coherently modulated using the free-electron laser.