Broadband spectroscopy of astrophysical ice analogues

V. Optical constants of Ih, Ic, and amorphous H₂O ices in the terahertz-infrared range

¹ Prokhorov General Physics Institute of the Russian Academy of Sciences, 119991 Moscow, Russia
² Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, Garching 85748, Germany
³ Federal Institute of Education, Science and Technology of Rio de Janeiro (IFRJ), Nilópolis Campus - Rio de Janeiro - Brazil
⁴ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

^★★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 February 2026
Accepted: 23 April 2026

Abstract

Context. Knowledge of the terahertz (THz) - infrared (IR) optical properties of astrophysical ices is important for understanding the dust continuum emission and radiative transfer in dense and cold interstellar environments. Water ice plays a dominant role in the aforementioned phenomena due to its prevalence and the high dipole moment of the H₂O molecule, resulting in high absorptivity and emissivity. Direct measurements of optical constants in the THz spectral region for astrophysically relevant H₂O ice samples are scarce. Extrapolation of optical properties in the THz spectral region from IR data can introduce uncertainties into astrophysical models.

Aims. 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.

Methods. 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.

Results. 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 ν = 0.3-120 THz (the wavelength range of λ = 1 mm-2.5 μm). 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 H₂O ice mantles based on experimental data shows discrepancies with data derived by extrapolation.

Conclusions. The inferred THz-IR optical constants of water ice are important for future observations and modelling of cold clouds and protoplanetary disks.