Broadband infrared spectroscopy of methanol isotopologues in pure, H₂O-rich, and CO-rich ice analogues

Deuterium fractionation is highly efficient during the early stages of star formation, particularly in starless and prestellar cores where temperatures are low (<10 K) and molecular freeze-out onto dust grains is significant. Methanol forms early in these environments following CO freeze-out via successive hydrogenation reactions on grain surfaces, while the production of deuterated methanol requires elevated gas-phase D/H ratios generated through dissociative recombination of deuterated H₃⁺. Consequently, large abundances of deuterated methanol are observed towards young stellar objects where prestellar ices have recently sublimated. Here, we present laboratory broadband infrared spectra of methanol and its isotopologues in astrophysical ice analogues, complemented by anharmonic vibrational calculations used to guide band assignments. Experiments were performed at the CASICE laboratory using a Bruker Vertex 70v spectrometer coupled to a closed-cycle helium cryostat, with isotopologue ices deposited at 10 K under high-vacuum conditions. Infrared transmission spectra were recorded over 6000 to 30 cm−1 (1.67 to 333 µm) and compared with spectra of pure isotopologue ices. Distinctive mid-infrared band patterns are identified for each deuterated species. In particular, CH₂DOH exhibits a characteristic doublet at 1293 cm−1 and 1326 cm−1 (7.73 µm and 7.54 µm), while CHD₂OH shows a similar doublet at 1301 cm−1 and 1329 cm−1 (7.69 µm and 7.52 µm), both remaining largely invariant across all studied ice mixtures. These robust spectral signatures provide reliable tracers for identifying deuterated methanol in JWST observations and for constraining astrochemical gas–grain models of deuterium enrichment prior to star and planet formation.