SPECTROSCOPIC EFFECTS IN CH4/H2O ICES

Abstract

Recent observations of CH4 in different astrophysical objects encourage laboratory research on methane/water ice mixtures. An IR spectroscopy laboratory investigation is presented on these systems. Co-deposited samples are formed by vapor deposition of CH4 and H2O on a cold substrate, in a wide range of stoichiometries, from very diluted mixtures to CH4/H2O = 2.5 values. Samples are prepared at 14 K and at 40 K, and their temperature behavior is studied when they are warmed up to 60 K. The spectroscopic analysis is centered on the methane features, and also on the water dangling bonds (DBs) that appear in the spectra of the mixtures. The IR forbidden ν1 band shows up in the spectrum (3.44 μm), indicating some form of distorted methane. The combination bands ν3 + ν4 and ν1 + ν4 are seen at 2.32 and 2.38 μm, and the ν2 + ν3 band weakly at 2.21 μm. Whereas ν3 is not shifted in spectra of mixed samples, the wavenumber peak of ν4 and its combination bands vary in a 6 cm–1 range, providing a possible estimation for the relative methane concentration in the sample. Bands in the spectra of mixtures are always broader than their counterparts in pure CH4 ice. The intensity of ν4 appears to increase in mixed samples with respect to the pure solid. Raising the temperature of the ices up to 60 K liberates part of the methane, but a fraction is retained with a maximum value of ~7% ± 2%. This limit may provide information on the temperature properties of astrophysical objects. The different spectral characteristics of water DBs with increasing methane proportion in mixed samples can also furnish information to estimate the stoichiometry of the mixture.