CO₂ Infrared Spectra on Silicate Dust Grain Analogs: Implications for JWST Observations

Carbon dioxide is one of the three most abundant species within the ice mantles around dust grains inside molecular clouds. Since a substantial number of interstellar grains are made of siliceous materials, we have studied the infrared profile of CO 2 deposited on top of a bare and ice-coated amorphous silicate (MgFeSiO 4 ) film using reflection absorption infrared spectroscopy (RAIRS). In contrast to a metal surface, the CO 2 IR profile shows a relaxation of the metal surface selection rule in the presence of the bare MgFeSiO 4 dust grain analog, which brings the IR profile closer to the observational spectra while maintaining the sensitivity of RAIRS. Experiments with the underlying CO and CH 4 ices show that their presence facilitates structural changes toward crystalline ice for the deposited CO 2 at much lower temperatures than on the polar ice layers. Warming-up experiments of CO 2 showed that it tends to stay on the silicate surface for much longer than on the gold surface without the silicate layer. We noticed for the first time a split in the 13 CO 2 IR feature on the pure or ice-covered silicate grain as a marker for the onset of diffusion. The laboratory 13 CO 2 profile then closely resembles recent JWST observations of this feature around young and embedded protostars, suggesting that it can be linked to the observed feature.