Broadband spectroscopy of astrophysical ice analogues. II. Optical constants of CO and CO2 ices in the terahertz and infrared ranges

Context. Broadband optical constants of astrophysical ice analogues in the infrared (IR) and terahertz (THz) ranges are required for modeling the dust continuum emission and radiative transfer in dense and cold regions, where thick icy mantles are formed on the surface of dust grains. Such data are still missing from the literature, which can be attributed to the lack of appropriate spectroscopic systems and methods for laboratory studies.
Aims: In this paper, the THz time-domain spectroscopy (TDS) and the Fourier-transform IR spectroscopy (FTIR) are combined to study optical constants of CO and CO₂ ices in the broad THz-IR spectral range.
Methods: The measured ices were grown at cryogenic temperatures by gas deposition on a cold silicon window. We developed a method to quantify the broadband THz-IR optical constants of ices, based on the direct reconstruction of the complex refractive index of ices in the THz range from the TDS data and the use of the Kramers-Kronig relation in the IR range for the reconstruction from the FTIR data. Uncertainties introduced by the Kramers-Kronig relations were eliminated by merging the THz and IR spectra. Finally, the reconstructed THz-IR response was analyzed using classical models of complex dielectric permittivity.
Results: The complex refractive index of CO and CO₂ ices deposited at the temperature of 28 K was obtained in the range of 0.312.0 THz and fitted using the analytical Lorentz model. Based on the measured dielectric constants, opacities of the astrophysical dust with CO and CO₂ icy mantles were computed.
Conclusions: The method developed in this work can be used for a model-independent reconstructions of optical constants of various astrophysical ice analogs in a broad THz-IR range. Such data can provide important benchmarks for interpreting broadband observations from existing and future ground-based facilities and space telescopes. The reported results will be useful in modeling sources that exhibit a drastic molecular freeze-out, such as the central regions of prestellar cores and mid-planes of protoplanetary disks, as well as CO and CO₂ snow lines in disks.