The spectral minimum in high-order harmonic generation (HHG) from aligned ${\text{CO}}_2$ molecules has attracted significant interest and has been extensively studied. However, experimental investigations have largely been limited to short laser wavelengths, and theoretical interpretations often focus solely on the single-molecule response. In this work, we present a systematic study of the wavelength-dependent spectral minimum at the macroscopic level, extending the driving-laser wavelength up to 1800 nm. We first simulate experimentally measured HHG spectra of aligned ${\text{CO}}_2$ molecules, and the good agreement between the simulation and experiment confirms the accuracy of the quantitative rescattering model and macroscopic propagation theory. Our simulations reveal that, at shorter wavelengths, the spectral minimum shifts with increasing laser intensity, whereas at longer wavelengths, its position remains stable regardless of intensity. This behavior arises from differences in the harmonic-intensity distributions of short and long electron trajectories associated with the highest occupied molecular orbital (HOMO) and HOMO-2 orbitals at the single-molecule level. After macroscopic propagation, phase-matching effects further amplify the intensity gap between the HOMO and HOMO-2 contributions as the wavelength increases, thereby reducing the influence of the HOMO-2 orbital on the spectral minimum. We also find that at 1800 nm, the spectral minimum remains stable when we vary both the gas-target position and the degree of molecular alignment. This study provides an effective and robust approach for controlling the spectral structure of molecular high harmonics and offers valuable insights into multielectron effects in molecules.

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• Atomic & molecular processes in external fields
• High-order harmonic generation
• Ultrafast phenomena
• Molecular orbital theory