Infrared Spectroscopic Study of Methane Ice, Pure and in Mixtures with Polar (HO) and Nonpolar (N) Molecules.

Mid-infrared studies of fundamental modes of ices of pure CH and its mixtures with polar (HO) and nonpolar (e.g., N) 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 HO and N molecules. We found that nuclear spin conversion (NSC) in solid methane and its crystalline structures is affected─in different ways─by the presence of HO and N. 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 CH ice and of the CH:HO ice mixture at 7 K is carried out in relation to the segregation of HO using the ν and ν IR inactive modes of methane. The diffusion of N and CH in the CH:N ice mixture with temperature cycling has been studied to obtain the relationship between IR features and the state of aggregation of the ice.