Field-induced orbital distortion in high-order-harmonic generation from aligned and oriented molecules within adiabatic strong-field approximation

Abstract

We describe high-order-harmonic generation (HHG) within the adiabatic strong-field approximation (ASFA) where the ground state and its energy adiabatically follows the instantaneous external field and within the Stark-shift-corrected SFA (SSFA), where only the energy shift is accounted for. We show that the molecular polarizability reflects the significance of field-induced orbital distortion in the HHG process. We show that for CO${}_{2}$, which possesses a relatively low polarizability, the two-center interference minimum can be clearly seen in both the ASFA and the SSFA. This finding is in agreement with experimental data at large wavelength. Moreover, we introduce a method for analyzing the recombination events. This method relies on averaging the recombination matrix elements weighted with the photon emission probability of each harmonic. In the case of CO${}_{2}$ this method confirms that the interference minimum is determined by recombination to the two O atoms. We use the example of N${}_{2}$O, which has a moderate polarizability, to show that the number of centers taking part in the creation of the interference minimum may change depending on the intensity. Finally, we show that in the short-pulse limit, the minimum in the HHG spectrum from oriented N${}_{2}$O strongly depends on the molecular orientation and carrier-envelope phase.