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The conversion from neutral to zwitterionic glycine is studied using infrared spectroscopy from the point of view of the interactions of this molecule with polar (water) and non-polar (CO 2 , CH 4 ) surroundings. Such environments could be found on astronomical or astrophysical matter. The samples are prepared by vapour-deposition on a cold substrate (25 K), and then heated up to sublimation temperatures of the co-deposited species. At 25 K, the neutral species is favoured over the zwitterionic form in non-polar environments, whereas for pure glycine, or in glycine/water mixtures, the dominant species is the latter. The conversion is easily followed by the weakening of two infrared bands in the mid-IR region, associated to the neutral structure. Theoretical calculations are performed on crystalline glycine and on molecular glycine, both isolated and surrounded by water. Spectra predicted from these calculations are in reasonable agreement with the experimental spectra, and provide a basis to the assignments. Different spectral features are suggested as probes for the presence of glycine in astrophysical media, depending on its form (neutral or zwitterionic), their temperature and composition.

Carbon dioxide (CO 2 ) is one of the most relevant and abundant species in astrophysical and atmospheric media. In particular, CO 2 ice is present in several solar system bodies, as well as in interstellar and circumstellar ice mantles. The amount of CO 2 in ice mantles and the presence of pure CO 2 ice are significant indicators of the temperature history of dust in protostars. It is therefore important to know if CO 2 is mixed with other molecules in the ice matrix or segregated and whether it is present in an amorphous or crystalline form. We apply a multidisciplinary approach involving IR spectroscopy in the laboratory, theoretical modeling of solid structures, and comparison with astronomical observations. We generate an unprecedented highly amorphous CO 2 ice and study its crystallization both by thermal annealing and by slow accumulation of monolayers from the gas phase under an ultrahigh vacuum. Structural changes are followed by IR spectroscopy. We also devise theoretical models to reproduce different CO 2 ice structures. We detect a preferential in-plane orientation of some vibrational modes of crystalline CO 2 . We identify the IR features of amorphous CO 2 ice, and, in particular, we provide a theoretical explanation for a band at 2,328 cm −1 that dominates the spectrum of the amorphous phase and disappears when the crystallization is complete. Our results allow us to rule out the presence of pure and amorphous CO 2 ice in space based on the observations available so far, supporting our current view of the evolution of CO 2 ice.astrochemistry | solid state morphology C arbon dioxide (CO 2 ) has come to play a fundamental role in several aspects of the Earth's geophysics (1, 2), but it is also a key element in astrophysical research (3, 4). In the interior of dense interstellar clouds, as well as in the envelopes around young stars, dust grains are covered by ice mantles formed by frozen volatile molecules, with water being the most abundant molecular species, followed by carbon monoxide (CO), CO 2 , methanol, methane, and others (5, 6). The structure of CO 2 in the icy phase of the interstellar grains is still an open question. Is CO 2 mixed up with other frozen components, or is it segregated in multilayer structures (7)? Has it attained a crystalline arrangement, or does it have an amorphous structure (8)? Because solid CO 2 is an indicator of the temperature history in the envelopes of young stars (9, 10), it is important to address these questions. Most of the available information on these systems comes from spectroscopic observations. Thus, many laboratory experiments have been performed on low-temperature CO 2 , both as a single species and mixed with other components, using IR spectroscopy as the main detection tool (11-15). In the context of solid-state physics, the existence of transverse optical (TO) and longitudinal optical (LO) modes in amorphous materials was questioned because the origin of this effect was linked to long-range order in crystals, but it was proved that longitudinal mode...

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