Dark and bright autoionizing states in resonant high-order harmonic generation: Simulation via a one-dimensional helium model

We study the role of dark and bright autoionizing states (AISs) in photoionization and high-order harmonic generation (HHG) using a one-dimensional helium model. This model allows numerical integration of the time-dependent Schrodinger equation beyond the single-electron approximation, completely taking into account electronic correlation. We find the level structure of the system and the spatial distribution of the electronic density for several states, including AIS. Studying the HHG efficiency as a function of the detuning from the resonances with AISs, we find the HHG enhancement lines. The shapes of these lines are different from the corresponding Fano lines in photoelectronic spectra, in agreement with experimental studies on HHG in helium. Moreover, we simulate HHG under the conditions when the fundamental frequency is close to the even-order multiphoton resonance with the dark AIS. We find enhanced generation of the neighboring odd harmonics. The details of the enhancement lines for these harmonics can be understood by taking into account the temporal delay between the nonresonant and resonant XUV emissions; this delay is defined by the AIS lifetime. Finally, our simulations show that resonances with dark and bright AISs enhance HHG to a similar extent.