Multichannel molecular high-order harmonic generation from HeH2+ with the combination of a chirped laser and a unipolar pulse

• We investigate the MHOHG by the combination of a chirped and a unipolar laser pulse.
• The three-step model is generalized to a four-step model in MHOHG.
• We illustrate the processes of MHOHG from HeH2+ in the combined field.
• An ultrabroad supercontinuum spectrum and a two-plateau structure can be observed.

The high-order harmonics from the asymmetric molecule HeH2+ with the combination of a chirped laser and a unipolar pulse is investigated by solving non-Born–Oppenheimer approximation time-dependent Schrödinger equation. We illustrate the multichannel molecular high-order harmonic generation by using a chirped laser pulse with carrier envelope phase (CEP) φ=0φ=0 plus a unipolar pulse, and a chirped laser pulse with CEP φ=πφ=π minus a unipolar pulse, respectively. Due to the presence of a permanent dipole moment, the three-step model is generalized to a four-step model in molecular high-order harmonic generation (MHOHG). The results show that an ultrabroad supercontinuum spectrum with a spectral width of about 65 orders from 163 orders to 228 orders and a two-plateau structure can be observed. Furthermore, the time–frequency analysis shows that the short quantum trajectory is left and the long one is suppressed with the combination of a chirped laser and a unipolar pulse.