How interstellar ice growth shapes the CO band

We investigate how the infrared absorption profile of the solid-state CO stretching mode reflects the chemical and structural environments produced during ice growth under astrophysically relevant conditions. Using two complementary deposition protocols, classical isothermal codeposition at 12 K and a temperature-decreasing ramp that simulates accretion during grain cooling, we produce a set of mixed and layered CO ices containing

H₂O, NH₃, CH₄, CO₂, and CH₃OH. The resulting laboratory CO spectra are decomposed into a library of Gaussian components, each associated with a specific chemical and structural microenvironment. Within this framework, variations in the CO line profile can be related to differences in ice structure and local chemical composition. We apply this laboratory-based approach to JWST observations of two molecular cloud sight lines and one protoplanetary disc.