Optical constants of Ih, Ic, and amorphous H$_2$O ices in the THz and IR ranges

Abstract

Direct measurements of optical constants in the THz spectral region for astrophysically relevant H$_2$O ice samples are scarce. Extrapolation of optical properties in the THz spectral region from IR data can introduce uncertainties into astrophysical models. We measured the optical properties of water ice samples in the Ih and Ic forms as well as amorphous solid water (ASW) in the THz region in order to derive broad optical constants using literature and experimental data in the THz-IR range. In our experiments, the Ih, Ic, and ASW ices were grown by vapour deposition onto a cold substrate and measured by THz pulsed spectroscopy. Their THz optical properties were retrieved, compared with the THz-IR literature data, and approximated using the multiple-Lorentz model. From the existing literature data on the Ih, Ic, and ASW ices, we selected samples with the highest optical constants and classified them as compact. Their optical properties were merged in the frequency range of $\nu = 0.3$-$120$~THz (the wavelength range of $\lambda = 1$~mm-2.5$~\mu$m). The underlying absorption bands were attributed to vibrational modes and approximated using the multiple-Lorentz model while accounting for anharmonicity. Discrepancies primarily arising in low-absorption regions between the experimental data and broadband models were attributed to factors such as the model's complexity and the baseline-subtraction procedure. The THz response of all ices is formed by the low-frequency wings of the IR bands and the single broad low-intense THz peak around $1.8$~THz, which is very similar for all phases. The opacity calculation for dust grains covered by H$_2$O ice mantles based on experimental data shows discrepancies with data derived by extrapolation. The inferred THz-IR optical constants of water ice are important for future observations and modelling of cold clouds and protoplanetary disks.