Infrared optical constants and band strengths of interstellar ice analogues: H₂O:CO₂ and H₂O:CO₂:CO mixtures at 10 K

¹ Instituto de Estructura de la Materia, IEM-CSIC, Calle Serrano 121, 28006 Madrid, Spain
² Institut des Sciences Moléculaires d’Orsay, UMR8214, CNRS, Université Paris-Saclay, 91405 Orsay, France
³ CNRS, Aix-Marseille Université, Laboratoire PIIM, Marseille, France

^★ Corresponding authors: belen.mate@csic.es

Received: 8 August 2025
Accepted: 24 September 2025

Abstract

Context. The high-quality infrared spectra of interstellar ices provided by the James Webb Space Telescope have evinced the need for accurate infrared (IR) optical constants of ice mixtures that could be used to model the observed band profiles including scattering and absorption contributions.

Aims. This article aims to provide IR optical constants and band strengths of ice mixtures containing H₂O, CO₂, and CO with mixing ratios comparable to those observed in interstellar ices, particularly in cold molecular clouds.

Methods. Single species ices of H₂O, CO₂, and CO as well as ices of binary H₂O:CO₂ and ternary H₂O:CO₂:CO mixtures were produced via background deposition on a cold substrate at 10 K. Visible refractive indices (at 633 nm) were retrieved from double interferometry measurements during ice growth. Ice densities were estimated using the Lorentz-Lorenz relation. The IR spectra (5000500 cm⁻¹) of pure species and mixtures were employed to derive band strengths and optical constants. The optical constants were derived using a code based on the Kramers-Kronig causal relation and on Fresnel coefficients.

Results. The intensity of the imaginary part, k(ν), of the IR refractive index of porous single-species ices grown in this work is smaller than that of more compact ices available in the literature. Significant changes are observed in the optical constants of the mixtures as a function of mixing ratio. In general, the k(ν) intensity of a given IR band follows the evolution of the partial molecular density of the band carrier. The nature of intermolecular interactions and the ice morphology also affect the profile and intensity of the reported optical constants, but the various effects cannot be easily disentangled. The IR band strengths of some of the strongest H₂O and CO₂ absorptions were found to vary by as much as 25% in some of the binary and ternary mixtures with respect to those of the pure species ices.