New insights into the process of resonant high harmonic generation in plasma plume

The underline physics of resonance high harmonic generation in plasma plumes is still not well understood. A simple model, which is an extension of the well-known three step model predict the capture of the re-colliding electron into an autoionization state and only then return to the ground state while emitting the HHG photon. Another explanation is a better phase matching due to strong variation of the refractive index near resonance. Here we first rule out the second option. Next, we theocratically study the physical mechanism of high-order harmonic generation (HHG) from chromium ions irradiated by intense laser pulses. By performing a combination of state-of-the-art ab-initio and model simulations that agree with well-known experimental observations, we uncover the fundamental physical picture for HHG in Cr⁺. We found that the main contribution to resonance enhancement originates from initially occupied spin-down 3p states, while spin-up 3d electrons negligibly contribute to resonance HHG. We studied the phase and emission time of each harmonic and compare it to experimental results.