Charge-resonance effect on harmonic generation by symmetric diatomic molecular ions in intense laser fields

We develop an analytic theory for the harmonic generation of symmetric diatomic molecular ions beyond the two-level model, emphasizing the influence of the charge-resonance (CR) states that are strongly coupled to electromagnetic fields for the case of large internuclear distances. Taking into account the continuum states that are ignored in the two-level model and become important for the intense laser case, our model is capable of producing the spectrum for the whole range of harmonic orders that consists of a molecular plateau due to the CR transition and an atomiclike plateau for long-wavelength excitation. Our analytic results are in good agreement with the numerical results from directly solving the Schroedinger equation. Our theory also identifies the crucial role of the CR states in the fine structure of the harmonic spectrum and shows that harmonic generation in molecular systems can be effectively controlled by adjusting the internuclear distance