The Origin of 1.6 Terahertz Absorption of Ice

In the terahertz spectroscopy of bimolecular solutions, freezing reduces the background absorption. For example, the absorption coefficient of room temperature water is 237 $^{-1}$ at 1 THz, whereas that of ice is 6.8 cm $^{-1}$ at 173 K. Lower background absorption highlights any spectral features originating from the biomolecule. A reliable baseline spectrum of ice is thus crucial. Despite the interest in biomedical applications, theoretical models and experimental data related to ice are still lacking in the literature, thus hindering progress in the cryogenic measurement of biological samples. We previously reported a highly consistent absorption spectrum of ice in the 0.2 to 2 THz range, which has a profile with a noticeable change of gradient at 1.6 THz. This work extends our investigation into the origin of this feature. Specifically, we investigate whether it is due to scattering by air bubbles, or whether it is an inherent nature of the molecular structure of ice.