Dark and bright autoionizing states in resonant high harmonic generation: simulation via 1D helium model

TL;DR

This study uses a 1D helium model to simulate how dark and bright autoionizing states influence high harmonic generation, revealing resonance effects, enhancement lines, and the role of AIS lifetime in HHG efficiency.

Contribution

It introduces a detailed 1D helium model that accounts for electronic correlation and investigates the impact of autoionizing states on HHG, including resonance effects and enhancement mechanisms.

Findings

HHG enhancement lines are observed near AIS resonances.

Dark AIS can significantly enhance neighboring odd harmonics.

HHG enhancement due to dark and bright AIS is comparable.

Abstract

We study the role of dark and bright autoionizing states (AIS) in photoionization and high harmonic generation (HHG) using a 1D helium model. This model allows numerical integration of the time-dependent Schr\"odinger equation beyond the singe-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 AIS we find the HHG enhancement lines. The shapes of these lines are different from the corresponding Fano lines in the photoelectronic spectra, in agreement with the experimental studies 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 the enhanced generation of the neighbouring odd harmonics. The details of the enhancement lines for these harmonics can be understood taking into account the temporal delay between the emission of the non-resonant and resonant XUV; this delay is defined by the AIS lifetime. Finally, our simulations show that the HHG enhancement due to the dark and the bright AIS is comparable in the studied system.