Probing ultrafast excited-state dynamics using EUV-IR six-wave-mixing emission spectroscopy

We demonstrate application of six-wave-mixing (SWM) spectroscopy in the study of excited states of neon. By combining extreme ultraviolet (EUV) excitation with multi-color near-infrared (NIR) and mid-infrared (MIR) laser fields, we obtained strong emission signals from several channels. Specifically, the EUV pulse excites the system to a 3d state, while the synchronous NIR pulse transfers the electronic population to neighboring dark states. The delayed MIR pulse probes the dynamics of these states, and we observe quantum beats stemming from the interferences between two pathways separated by the spin-orbit splitting. The non-commensurate photon energies of light pulses results in background-free spectral emission that can be isolated from the main EUV spectrum. The multi-color excitation provides access to the optically dark states which cannot be accessed by a one-photon transition from the ground state. Finally, the tunability of the MIR photon energy allows us to control the electronic couplings between states. The SWM spectroscopy thus offers a powerful tool to probe the excited states wave packet dynamics in atomic and molecular systems.