HOMO-driven spectral minima and multi-orbital effects in high-harmonic generation from C ₂₀ fullerene

We report a time-dependent density functional theory study of high-order harmonic generation (HHG) from C ₂₀ , the smallest fullerene cage structure, driven by an intense mid-infrared laser field. The harmonic spectrum exhibits two pronounced minima at approximately 8 eV and 40 eV, both of which remain robust against variations of the laser wavelength and intensity, thereby excluding an origin from laser-driven dynamical interference. By combining orbital-resolved HHG spectra with transition-dipole-moment analysis, we show that these two minima have distinct physical origins. The minimum near 40 eV is dominated by the highest occupied molecular orbital (HOMO) and is accompanied by a corresponding suppression in the HOMO transition dipole, indicating a Cooper-like minimum associated with the intrinsic electronic structure of the HOMO. In contrast, the minimum near 8 eV does not arise from the HOMO transition dipole itself, but instead originates from coherent interference between the HOMO and HOMO-1 channels. We further find that the HOMO contribution alone cannot reproduce the overall HHG spectral intensity, whereas the coherent superposition of the HOMO and HOMO-1 channels recovers both the total harmonic yield and the main spectral profile. These results demonstrate that HHG from C ₂₀ is governed by the combined effects of orbital-specific recombination structures and inter-orbital coherence, highlighting the distinct roles of intrinsic electronic structure and multi-orbital interference in cage-structured fullerenes.