Interaction of CH4 and H2O in ice mixtures.

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 nu(1) band of CH(4) and the dangling bond bands of water. The nu(1) 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. CH(4) 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 CH(4) is retained in the water ice. An adsorption isotherm analysis is performed yielding the estimation of the desorption energy of CH(4) on H(2)O amorphous surfaces.