2025ApJ...988L..27S

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 CO₂ deposited on top of a bare and ice-coated amorphous silicate (MgFeSiO₄) film using reflection absorption infrared spectroscopy (RAIRS). In contrast to a metal surface, the CO₂ IR profile shows a relaxation of the metal surface selection rule in the presence of the bare MgFeSiO₄ 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 CH₄ ices show that their presence facilitates structural changes toward crystalline ice for the deposited CO₂ at much lower temperatures than on the polar ice layers. Warming-up experiments of CO₂ 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 ¹³CO₂ IR feature on the pure or ice-covered silicate grain as a marker for the onset of diffusion. The laboratory ¹³CO₂ 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.