In bulk carbonic acid (e.g., in interstellar icy dust grains) cluster formation produces two distinct UV-Vis spectral features redshifted by ≈2 eV (25 nm) and by ≈5 eV (80 nm) compared to gas phase single molecule carbonic acid 's adiabatic ionization energy. Theory and experiment were used to investigate the origin of these shifts.

The UV–Vis spectra of H₂CO₃ are investigated in a combined experimental and theoretical approach. A sample of solid H₂CO₃, prepared by electron irradiation of water–carbon dioxide ice, shows characteristics of both amorphous and crystalline H₂CO₃ in the infrared spectrum. To rationalize the experimentally observed redshift in the UV–Vis spectra between monomer and bulk H₂CO₃, a systematic computational study is devised using time-dependent density functional theory. H₂CO₃ is investigated from the monomer to (H₂CO₃)_n clusters, with n up to 66; in addition regular oligomer arrangements derived from previously proposed ambient-pressure H₂CO₃ crystal structures are also examined. The calculations explain the UV–Vis absorption of solid carbonic acid, which is redshifted by ≈2 eV and ≈5 eV compared to the experimentally observed adiabatic ionization energy of the H₂CO₃ monomer. It is highlighted how these shifts emerge due to 1) increasing cluster size, 2) nonplanar arrangements, and 3) noncovalent interactions between H₂CO₃ chains and sheets. The study aims to establish spectrum-to-structure relationships and serves as computational reference data for astrochemical applications in the absence of experimental laboratory data of H₂CO₃ oligomers.