High-order harmonic generation and its suppression in H3+ in strong laser fields

High-order harmonic generation in H₃⁺ in a strong midinfrared laser pulse is studied by numerically simulating a four-dimensional time-dependent Schr\"odinger equation including two active electrons. Once one electron is tunneling out, the other bound electron hops in H₃²⁺. At rescattering, if the bound electron may hop to the opposite site compared to that at the ionization moment, the high-order harmonic generation is suppressed and a valley is formed in high-order harmonic spectra. Such harmonic suppression can be used to extract the ultrafast electron localization of the bound electron in H₃²⁺, as well as timing the rescattering moment. Our findings thus shed light on the intricate interplay between electrons, and provide valuable insights into fundamental ultrafast mechanisms.