The Laboratory Approach

Since the two main families of solids present in space are carbon- and silicon-based materials, laboratory experiments are focused on various forms of natural or laboratory synthesised pure and hydrogenated carbons and crystalline and amorphous silicates. In the latter case, main attention is devoted to olivine (Mgx,Fe^x)2Si04 (Forsterite x = 1; Fayalite x = 0) and pyroxene (Mgx,Fei.x)Si03 (Enstatite x = 1; Ferrosilite x = 0), which appear promising candidates to be present in space.

2.1. Techniques

To produce micron/sub-micron sized grain analogues, vapour condensation, pyrolysis or grinding techniques are applied to pre-selected targets or natural pure minerals. The obtained samples can be subject to various kinds of processing which simulate mechanisms active in space. Thermal annealing is relevant in stellar outflows, as well as in the Oort cloud, while UV irradiation may be efficient to modify the structure and the chemical properties of dust during the permanence in the ISM. The ion bombardment may have a different impact during some critical evolution stages of materials: ISM, pre-comet phase, in comets and in the interplanetary medium. Moreover, it must be considered that materials are continuously in interaction with the gaseous components, so that at least the interaction with hydrogen must be accounted for properly.

At all different stages of material production/processing detailed analyses must be performed on the samples to identify the details of their status and to check the efficiency of the applied processes. The characterisation can rely on different complementary analytical techniques such as: scanning and transmission electron microscopy (useful to monitor morphology, surface status and structure), electron diffraction and energy loss spectroscopy (to identify the crystalline degree/structure), energy dispersive X-ray analysis (to determine the elemental abundance). Finally, absorption, emission, reflectance and Raman spectroscopic analyses in the widest spectral range (from the far UV to the mm) are applied to correlate composition, structure and morphology of samples with their spectral behaviour. Of course, those mentioned above are some of the most useful techniques applied so far for characterisation of materials. As long as new and more sophisticated methods are made available by the development of technologies, the investigation may become more accurate.

2.2. Results

The most common feature of grains produced by vapour condensation of various species is a fluffy structure formed by round-shaped grains in an amorphous status, as shown in Figure 1. The size distributions are skew, with average radii about 5 nm (for amorphous carbon grains) and about 30 nm (for amorphous silicate grains).

Carbon presents different structural properties and spectral features depending on the production conditions. In particular, amorphous carbon grains condensed by arc discharge in an inert argon atmosphere (ACAR grains) show a wide UV absorption peak around 240 nm. Grains condensed by the same technique, but in H2 atmosphere (ACH2 grains), display no UV "bump". This spectral difference testifies a different internal structural organisation of the grains, due to the interaction of C atoms with hydrogen. In a hydrogen-free structure, C atoms can achieve a short-scale arrangement in (1-2 nm) aromatic networks, while in the presence of hydrogen, C-H bonds prevent their formation, giving rise to a less organised structure [13]. Interesting enough, ACH2 grains after thermal annealing up to 1050 K [14], UV irradiation up to 4 x 1022 eV cm" [15] or ion bombardment up to 660 eV/C-atom [16] always display the appearance of the UV feature, indicating an aromatisation process, also due to the hydrogen release. On the other hand, exposing hydrogen-free carbon to interaction with atomic H up to 7 x 1019 atoms cm"2 produces the appearance of a relevant 3.4 jam absorption feature clearly due to the formation of C-H aliphatic bonds [17].

Figure 1. Transmission electron micro-graphs of laboratory dust analogues. Top: carbon grains produced by vapor condensation in an arc discharge between two carbon electrodes in an H2 atmosphere at 10 mbar [11], Bottom: amorphous silicate (forsterite) grains condensed after laser ablation of a natural forsterite target in O2 atmosphere at 10 mbar [12].

Figure 1. Transmission electron micro-graphs of laboratory dust analogues. Top: carbon grains produced by vapor condensation in an arc discharge between two carbon electrodes in an H2 atmosphere at 10 mbar [11], Bottom: amorphous silicate (forsterite) grains condensed after laser ablation of a natural forsterite target in O2 atmosphere at 10 mbar [12].

The laboratory results mentioned above tell us that aromatic and aliphatic carbon forms compete to determine the arrangement of carbon nano-grains: the dominance of one of the two depends on the formation conditions (e.g., presence of hydrogen) and on the relative dominance of processing mechanisms, as well as interaction with hydrogen. All these phenomena are active, at different rates, in space environments. As a consequence the actual status of cosmic carbon is tightly related to the local conditions and it can be predicted on the basis of laboratory results.

For silicates, the bands falling around 10 and 20 |im, due to Si-0 stretching and O-Si-O bending are diagnostic of chemical composition and crystalline degree. Sharp vs. broad features identify crystalline vs. amorphous silicates. The Fe/Mg ratio determines the detailed position of the band sub-structures (Figure 2).

Figure 2. Infrared absorption spectra of crystalline, amorphous and thermally processed fayalite (top panel) and enstatite (bottom panel) [12,18].

Thermal annealing of amorphous silicates produces crystallisation (Figure 2). By analysing the progressive transformation of the broad into sharp features, it is possible to derive the so-

called "activation energy" that relates time and temperature needed to perform a complete amorphous to crystalline transition. In general, the values obtained by various authors for different silicates [12,18,19], although not fully in agreement probably due to the use of slightly different samples, provide values of the activation energy of the order of some 10,000 K. This result may have important implications when applied to the astrophysical conditions, as it implies that about 106 yr are needed to crystallise silicates at ~ 1000 K. These conditions appear critical to achieve in space.

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