Influence of excited states in high-order harmonic generation at intense mid-infrared field

We present a comprehensive study on the role of various excited states in high-order harmonic generation of hydrogen atoms driven by a long-wavelength (1500 nm) laser field. By numerically solving the time-dependent Schrödinger equation (TDSE) and performing a time-frequency analysis, we investigate the influence of individual excited states on the harmonic spectrum. Our results reveal that the 2s excited state primarily contributes to the enhancement of high-energy harmonic yields by facilitating long electron trajectories, while the 2p excited state predominantly suppresses harmonic yields in the lower-energy region (20th–50th orders) by altering the contributions of electron trajectories. Our results highlight the critical role of the excited states in the HHG process, even at longer laser wavelengths.