Structure, spectroscopy and dynamics of layered H2O and CO2 ices

Molecular dynamics simulations of structural, spectroscopic and dynamical properties of mixed water–carbon dioxide (H₂O–CO₂) ices are discussed over temperature ranges relevant to atmospheric and astrophysical conditions. The simulations employ multipolar force fields to represent electrostatic interactions which are essential for spectroscopic and dynamical investigations. It is found that at the water/CO₂ interface the water surface acts as a template for the CO₂ component. The rotational reorientation times in both bulk phases agree well with experimental observations. A pronounced temperature effect on the CO₂ reorientation time is observed between 100 K and 200 K. At the interface, water reorientation times are nearly twice as long compared to water in the bulk. The spectroscopy of such ices is rich in the far-infrared region of the spectrum and can be related to translational and rotational modes. Furthermore, spectroscopic signatures mediated across the water/CO₂ interface are found in this frequency range (around 440 cm⁻¹). These results will be particularly important for new airborne experiments such as planned for SOFIA.