Role of excited states in helium-like ions on high-order harmonic generation

We theoretically investigate high-order harmonic generation (HHG) of helium (He), lithium cation (Li+), and beryllium dication (Be2+) using the time-dependent Hartree-Fock method to solve the three-dimensional time-dependent Schrodinger equation. It is found that the intensity of the HHG increases significantly from a certain harmonic order below the ionization threshold, and the initial position of the enhancement does not depend on the intensity or the wavelength of the driving laser field. Further analysis shows that excited states play an important role on this enhancement, consistent with the excited-state tunneling mechanism [Phys. Rev. Lett. 116 123901 (2016)]. Our results unambiguously show that excited-state tunneling is essential for understanding the enhancement of HHG. Accordingly, a four-step model is herein proposed to illustrate the multiphoton excitation effect in helium-like ions, which enriches the physics of HHG enhancement.