Aims. We studied the interaction between CO 2 (guest) and H 2 O (host) molecular ices. Methods. Ices of CO 2 and H 2 O are prepared by four different deposition techniques: sequential deposition (amorphous water ice followed by addition of CO 2 ), co-deposition (both gases added simultaneously), inverse sequential deposition (carbon dioxide ice followed by addition of water) and crystalline sequential deposition (crystalline water ice is prepared first and CO 2 is added afterwards). Samples are deposited at 80 K and are studied by temperature programmed desorption and transmission infrared spectroscopy. Results. Two slightly different varieties of association of CO 2 and H 2 O are revealed from the different spectroscopic properties of the asymmetric stretching band of 12 CO 2 and 13 CO 2 . The two varieties are found to co-exist in some of the samples at 80 K, whereas only the so-called internal CO 2 remains after heating at 105 K. At 80 K carbon dioxide is able to adhere to a crystalline water ice surface. Activation energies for the desorption of CO 2 from amorphous (E d = 20.7 ± 2 kJ mol −1 ) and crystalline (E d = 19.9 ± 2 kJ mol −1 ) water ice are derived from measurements of the sticking of CO 2 as a function of ice temperature. Conclusions. These findings may have implications for the study of icy bodies of the Solar System.

The solid phases of methanol were investigated using IR spectroscopy and numerical calculations with the SIESTA method. Improved spectra are reported of amorphous methanol at 90 K, and in particular of the alpha and beta phases at 130 and 165 K, respectively, with assignments of bands not previously measured. The main features of the spectra of each phase are discussed and compared. A study of spectral changes with temperature leads to the conclusion that the metastable phase previously reported might be a mixture of the two known stable phases. Such a mixture could explain all spectral features observed in this investigation. The theoretical calculations provide reasonable agreement with the experimental data for most of the parameters, but predict H-bondings stronger than those observed. Differences between the spectra of the alpha and beta phases are predicted with similar characteristics to the experimental results.

Ice mixtures of CO2 and H2O are studied using Fourier transform reflection-absorption infrared (RAIR) spectroscopy. Mixtures are prepared by sequential deposition or co-deposition of the two components from the gas phase onto an Al plate kept at 87 K inside a low-pressure chamber. Two CO2 structures are found in most experiments: a crystalline form similar to pure CO2, which evaporates when warming at 105 K, and a noncrystalline species which remains embedded in amorphous water ice after warming. Significant spectral variations are found depending on the deposition method and the thickness of the solid. Features characteristic of the RAIR technique appear in the spectral regions of the normal modes of the bending and asymmetric stretching CO2 vibrations. Simulations using Fresnel theory and Mie scattering are carried out with acceptable agreement with the experimental spectra of solids of variable thickness, from approximately 1 microm to the limit of nanoparticles. Theoretical calculations of a pure CO2 crystal are performed. The relaxed geometry of the solid and its vibrational spectrum are determined and compared to the experimental results.

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