Interaction of CH4 and H2O in ice mixtures

Abstract

Ice mixtures of methane and water are investigated by means of IR spectroscopy in the 14–60 K range. The spectroscopic research is focused on the symmetry-forbidden ν₁ band of CH₄ and the dangling bond bands of water. The ν₁ band is visible in the spectra of the mixtures, revealing a distorted methane structure which co-exists with the normal crystalline methane. The water dangling bond bands are found to increase their intensity and appear at red-shifted frequency when distorted methane is present. Methane adsorbed on water micropores or trapped inside the amorphous solid water structure is assumed to be responsible for these effects. CH₄ mobility in water ice depends on the deposition method used to prepare the samples and on the temperature. After warming the samples to 60 K, above the methane sublimation point, a fraction of CH₄ is retained in the water ice. An adsorption isotherm analysis is performed yielding the estimation of the desorption energy of CH₄ on H₂O amorphous surfaces.