We investigate the below-threshold harmonic (BTH) generation of the ${\mathrm{H}}_2^{+}$ molecule driven by a femtosecond infrared (IR) laser pulse in the presence of an extreme-ultraviolet (XUV) field—a process referred to as XUV-induced Rabi flopping. Rabi flopping (RF) is not confined to the three-level system interacting solely with an XUV pulse; it also manifests itself in the dressed cascade multilevel system when an IR field is introduced alongside the XUV pulse. By constructing a five-level Hamiltonian coupled with the molecular Perelomov-Popov-Terent'ev model and comparing with ab initio simulations, we reveal that both the one-photon RF between the two lowest states and the multiphoton RF involving higher excited states occur simultaneously in the dressed cascade multilevel system. Distinct odd and even harmonics emitted from dressed excited states due to multiphoton RF, as well as Rabi splittings, are observed in the BTH spectra. The laser-induced coherence of dressed states leads to quantum interference among excitation pathways that govern the optical response, providing a different approach for coherently controlling BTH generation in the RF regime under combined IR-XUV laser irradiation.

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• Coherent control
• Multiphoton or tunneling ionization & excitation
• Ultrafast phenomena