Probing the amplitudes and phases of quantum beats in spin-orbit split continua*

Electron ion core interactions influence wavepacket dynamics of excited states and manifest in the photoionization to spin-orbit slit ionization thresholds. We used an XUV pulse to excite a ²[P_1/2]^o5s-3d Rydberg wavepacket in Argon atom, which is then ionized with an IR pulse in a two-photon process leaving the core in either J=1/2 and J=3/2 state, where the interaction of the outgoing electron with the ion core alters the angular momentum for one of the channels. We analyze the photoelectron spectra and angular distributions as a function of XUV-IR delay. We observe a strong quantum beat corresponding to the ²[P_1/2]^o5s-3d energy gap in both ionization channels.  Interestingly, the beats in two channels are out-of-phase, and the results also exhibit beats at unexpected photoelectron energies. The phase difference potentially contains information on the relative photoionization delays into two channels. Theoretical analysis of the two-color ionization shows remarkable agreement with the observed experimental amplitude and phases. In addition, we present angular distribution analysis which points to the small role of higher order Raman-like processes in photoionization signal. Our experimental and theoretical results offer a platform to study electron-ion core interactions in this and other systems.