Infrared Spectroscopic Study of Methane Ice, Pure and in Mixtures with Polar (H₂O) and Nonpolar (N₂) Molecules

Mid-infrared studies of fundamental modes of ices of pure CH₄ and its mixtures with polar (H₂O) and nonpolar (e.g., N₂) molecules are essential in order to learn the state of aggregation and thermal history of ices present in the interstellar medium and outer solar system bodies. Such data will be useful in the interpretation of observational data from the James Webb Space Telescope. Using an ultrahigh vacuum apparatus, we conducted reflection-absorption infrared spectroscopy measurements in the mid-IR range of pure methane ice and methane-containing ice mixtures of interest to interstellar and solar system ice chemistry, e.g., with H₂O and N₂ molecules. We found that nuclear spin conversion (NSC) in solid methane and its crystalline structures is affected─in different ways─by the presence of H₂O and N₂. Specifically, we found a relationship between the thickness and the solid-state ordering transformation in methane thin films. This new study of the NSC of pure CH₄ ice and of the CH₄:H₂O ice mixture at 7 K is carried out in relation to the segregation of H₂O using the ν₁ and ν₂ IR inactive modes of methane. The diffusion of N₂ and CH₄ in the CH₄:N₂ ice mixture with temperature cycling has been studied to obtain the relationship between IR features and the state of aggregation of the ice.