Experimental Investigation on the Morphology of Interstellar Ice Analogues

Spectroscopic observations of cold and dense interstellar clouds show the presence of "dirty ice" mantles on dust grains, mainly composed by water molecules. These ices are enriched by the presence of other simple species that are either formed by surface reactions or accreted from the gas phase. While there is quite a general consensus that interstellar water ice is mainly amorphous, its morphology (porous or compact) still remains poorly known. Morphology is important due to its influence both on the catalytic efficiency of grain surfaces and on the release to the grain of the fraction of the formation energy of species, as shown by laboratory simulations of molecular hydrogen formation. Ice porosity may be identified through the weak infrared absorption features (~ 2.7 μm) showing the presence of dangling bonds on the pore surface. To our knowledge, there has been to date no detection of such absorptions in the infrared spectra of interstellar ices, perhaps suggesting that they may have a compact nature. It has been already investigated that interstellar porous ice may be compacted by the transient heating of stellar radiation and cosmic ray bombardment. In this thesis I report an experimental work, performed using FORMOLISM (the experimental apparatus at the University of Cergy-Pontoise - France), that shows relevant changes in the ice morphology following atomic hydrogen exposure. In particular, it is shown that a thin highly porous ice film is gradually changed into a more compact structure. This is probably due to the transient heating caused by the energy released to the ice during H2 formation. Such a process may also produce in the interstellar space compact amorphous ice mantles concurrently with the other envisaged processes. Moreover, I have experimentally analyzed the morphology of the just formed water ice. Analysing one of the possible mechanism of water formation (the pathway H + O2) under conditions mimicking those found in a molecular cloud, we have found that the water just synthesized has a non-porous structure. Indeed, the layers of water formed in this way show the kinetic characteristics typical of a compact (non-porous) ice, as for instance the D2 TPD peak position