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Background

Solid materials in space, namely ices, silicates and carbons, are present in different environments such as the interstellar medium, comets, asteroids and outer Solar System objects.

Refractory dust particles (average radius $\sim$0.05 $\mu$m) made of silicates or carbonaceous material are released from stars (mainly in the red giant phase) in whose atmospheres they are formed, into the interstellar medium (ISM). Occasionally diffuse clouds in the ISM (n$_H$$\sim$1-10$^3$ cm$^{-3}$, T$\sim$100 K; where n$_H$ is the total number of H atoms, i.e. H+2H$_2$, H$_2$ being the numeric density of the hydrogen molecules) contract to form dense molecular clouds (n$_H$$\geq$10$^4$ cm$^{-3}$, T$\sim$10-20 K). In these regions the numeric density of dust particles (grains) is n$_d \sim$ 10$^{-12}$ n$_H$. The temperature of the dust in dense clouds is as low as 10-20 K and thus virtually all atoms and molecules (with few exceptions such as He and Ne) that impinge on the grains stick on the surface to form ice mantles with an average radius estimated to be of the order of 0.1 $\mu$m. As the cloud contracts, atomic hydrogen is converted into molecular hydrogen, through H+H combination on grains and the consequent release of H$_2$ in the gas phase. This process has important consequences on the chemistry of icy mantles: when H dominates, hydride species such as H$_2$O, CH$_4$, NH$_3$, CH$_3$OH are expected to form leading to a mantle dominated by polar molecules. When H$_2$ dominates, molecules such as the observed CO and the inferred O$_2$ and N$_2$ accrete on grains to form an outer shell of apolar ices. When a star is observed from behind a dense molecular cloud (field star), its light is absorbed by the matter in the cloud and the analysis of the observed spectrum gives information on the composition of the cloud. While the electromagnetic radiation in the UV-Vis spectral range is completely absorbed in dense molecular clouds, it is possible to observe in the IR range the vibrational absorption spectrum due to the presence of refractories and ices along the line of sight.

Dense molecular clouds, after further contraction, are the places where stars are born. The observation of protostars (stars still embedded in their placental cloud) is a further probe of the presence of ices in the clouds. In this case the almost blackbody continuum emitted from the young object is absorbed by grains whose temperature changes as a function of the distance from the object. These observations, mainly obtained by IR spectroscopy, may reveal the evolution of ices due to thermal and/or energetic processing (e.g. interaction with UV photons and/or stellar particle winds and cosmic rays).

Ices are also present on many objects in the Solar System such as the satellites of the external planets (Jupiter and beyond), the planet Pluto, the so called trans-Neptunian objects (TNOs, a class of numerous small objects not yet well investigated), and comets. In this case the study of the composition of the ices is based on the study of the electromagnetic radiation coming from the Sun and reflected by the surface to the observer.

Energetic (keV-MeV) particles and UV photons impinging on solid surfaces made of refractory (carbonaceous and/or silicates) materials and/or ices are present in a variety of environments in space including the interstellar medium and planetary system. The study of the effects of ion irradiation and UV photolysis is based on laboratory simulations of relevant targets bombarded with fast charged particles and by Lyman-$\alpha$ photons under physical conditions as similar as possible to the astrophysical ones. Two main effects occur: (1) material is eroded from the target (sputtering) and (2) physico-chemical modifications are induced, including the formation of different molecules.

Fast ions penetrating solids deposit energy in the target by elastic interactions with target nuclei and by inelastic collisions causing ionizations and excitations. Thus chemical bonds are broken along the path of the incoming ion and physico-chemical modifications occur, including the formation of molecules originally not present in the target. These molecules include species that can be both more volatile than the parent ones and less volatile. When carbon is an important constituent of the irradiated target it gives rise to a refractory residue which is left over after warming up to room temperature. This residue has a complex structure, and after prolonged irradiation evolves to form hydrogenated amorphous carbon. In the case of UV photolysis, the energy is released to the target material through single photo-dissociations, photo-excitations or ionization events per incoming photon. Also in this case new molecular species are formed.

Different techniques have been used, by different groups, to characterize the physico-chemical effects induced by energetic ions and UV photons. In our laboratory we have been using, for about 20 years, in situ IR and Raman spectroscopies.

A number of different refractory materials (carbonaceous and/or silicates) and frozen gases have been irradiated to study their chemical and/or structural evolution. Usually samples are prepared at low temperature (10-20 K) and their spectral characteristic recorded before, during and after processing with energetic ions (3 to 400 keV) and UV photons (Lyman-$\alpha$, 121.6 nm=10.2 eV). Targets are subsequently warmed-up and spectra are taken at increasing temperatures (20 to 300 K).

This research, in the past 20 years, has been financially supported by ASI, CNAA, CNR, INAF, MIUR, OACt, Regione Sicilia and University of Catania.

Figure 1.26: On the left side a schematic view of the experimental apparatus used for in situ Raman spectroscopy of ion irradiated samples is shown. On the right side details of the vacuum chamber in the ``Raman configuration''. In order to obtain infrared spectra the glass objective is removed. A hole in the sample holder allows the infrared beam to transmit through the substrate and the sample.
\includegraphics[width=70mm]{lasp/raman.eps} \includegraphics[width=60mm]{lasp/raman_chamber_detail.eps}


next up previous contents index
Next: Experimental facilities Up: Laboratory of experimental astrophysics Previous: Laboratory of experimental astrophysics   Contents   Index
Innocenza Busa' 2005-11-14