The light-induced-phase (LIP) model offers an elegant interpretation of how a dressing laser field modulates the line shape of an absorption resonance. However, its central assumption of adiabatic evolution—no population change—limits its validity to weak fields, precluding its application in the strong-field regime, where pronounced diabatic dynamics occur. To overcome this limitation, we generalize the conventional LIP model within the instantaneous-Floquet-state (IFS) formalism. The validity of this Floquet-generalized LIP (FGLIP) model is quantified by a Floquet adiabaticity parameter $\xi$ and demonstrated across a wide range of laser parameters. The deviation of resonance line shape from the FGLIP model is due to amplitude modification and phase shift arising from diabatic transitions between IFSs, namely diabatic corrections. Numerical simulations of helium $1s2p$ absorption show that the resonance line shapes are quantitatively reproduced, via incorporating diabatic corrections in the FGLIP model, from adiabatic $(\xi \ll 1)$ to highly diabatic regimes. Moreover, by leveraging $\xi$ as a control criterion, we demonstrate complete cyclic modulation of the helium $1s2p$ resonance line shape—traversing from symmetric Lorentzian to asymmetric Fano and back—via coherent accumulation of a $2\pi$ LIP shift by tuning the intensity of the dressing infrared laser pulse, all within the adiabatic regime. This work provides both a fundamental understanding of Floquet diabatic dynamics and a practical tool for achieving coherent optical control of quantum systems.

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• Lineshapes & shifts
• Optical transient phenomena
• Strong-field-induced spectra