Influence of ac Stark shift on below-threshold harmonic generation of H molecular ions in intense laser fields

Abstract

The effects of laser-induced ac Stark shift on the high-order harmonic generation below the ionization threshold of H molecular ions are investigated. The results show that several sub-peak harmonics generated in the vicinity of odd harmonics are clearly observed. Combined with the synchrosqueezing transform of the quantum time-frequency spectrum and a two-state model, we confirm that the transition between the first excited state related to ac Stark-shift and the ground state contributes to the sub-peak harmonics generation. In addition, we find some fine oscillatory structures of the sub-peak harmonic due to spectral interference. The sub-peak harmonics generation and spectral oscillations can be controlled by the laser intensity and the pulse duration. Our study provides a new alternative way to measure the energy difference of ac Stark shift between the excited state and the ground state of atoms and molecules in intense laser fields.