Symmetry and Vibrational Control of Multi-Channel HHG in Methane

High-harmonic generation (HHG) driven by tailored bicircular laser fields has emerged as a powerful tool for probing structural and dynamical properties of complex molecules. We compute HHG spectra of methane (CH4) interacting with intense ω/2ω counter-rotating bicircular fields using an all-electron approach based on the time-dependent Schrödinger equation. The emitted harmonics are explained as the interplay between the dynamical symmetries of the driving field and the tetrahedral spatial symmetry of the molecule. We unravel the critical role of molecular normal modes in modulating population dynamics across distinct ionization and recombination channels. The appearance of forbidden harmonics under asymmetric stretching of methane is analyzed, showing a clear interplay between such harmonics and the spatial displacement. This work provides insights for experimental investigations of aligned molecular systems, with a view toward a pump–probe setup.