Very-high-order harmonic generation from Ar atoms and Ar⁺ ions in superintense pulsed laser fields: An ab initio self-interaction-free time-dependent density-functional approach

We present an ab initio nonpertubative investigation of the mechanisms responsible for the production of very-high-order harmonic generation (HHG) from Ar atoms and Ar⁺ ions by means of the self-interaction-free time-dependent density-functional theory recently developed. Further, by introducing an effective charge concept, we can study at which laser intensity the contribution to the high-energy HHG from Ar⁺ ions precede over the Ar atoms. Comparing the HHG behavior from Ar atoms and Ar⁺ ions in the superintense laser field, we conclude that the high-energy HHG observed in the recent experiment originated from the ionized Ar atoms.