Chemistry Reference
In-Depth Information
Linear carbon chains with sp hybridization have been proposed to be
present in the interstellar medium [2-4]. These chains can be constituted by
alternate single (C-C) and triple (C
C)
bonds (poly-cumulenes) and have been proposed as building blocks for
carbynes, a carbon allotrope [5].
In the laboratory, the study of the properties of pure polyynes and poly-
cumulenes is inhibited because of their extreme reactivity with oxygen and
the formation of cross linked chains. In space, refractory dust made of
silicates or carbonaceous material are formed in the atmosphere of evolved
stars and released into the interstellar medium (ISM). In dense molecu-
lar clouds (n
Hydrogen
C) bonds (polyynes) or double (C
ΒΌ
10
4
cm
3
, T
10-20K) atoms and molecules that
impinge on the grains stick on them and may react to form ice mantles.
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 probe of the presence of ices in the clouds: the almost
black-body continuum emitted from the young object is absorbed by grains
whose temperature changes as a function of the distance from the object.
These observations, which are mainly obtained by IR spectroscopy, may
reveal the evolution of ices due to thermal and/or energetic (e.g. interaction
with UV photons and/or stellar particle winds and cosmic rays) processing
(e.g. Cox and Kessler [6]).
Carbon-based materials and C-rich 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 (a class of
numerous small objects not yet well investigated), and comets [7]. Energetic
(keV to MeV) particles impinging on solid surfaces made of refractory
(carbonaceous and/or silicates) materials and/or ices are present in a variety
of environments in space. These include the interstellar medium and the
planetary system. Ion irradiation produces several effects, whose study has
been based on laboratory simulations of relevant targets bombarded with
fast charged particles under physical conditions as similar as possible to the
astrophysical ones. Fast ions penetrating solids deposit energy in the target
by elastic interactions with target nuclei and by inelastic collisions causing
ionizations and excitations. Thus, bonds are broken along the path of the
incoming ion and physico-chemical modifications occur, including the for-
mation of molecules originally not present in the target which can be both
more volatile or less volatile than the irradiated ones. When carbon is
an important constituent of the irradiated target it gives rise to a refractory
residue which is left over after warming up. That residue has a complex
structure, which after prolonged irradiation evolves to form a hydrogenated
amorphous carbon that we call IPHAC (ion produced hydrogenated
amorphous carbon). IPHAC formation has been observed in several kinds
of carbon-containing targets (polystyrene, polypropylene, graphite, dia-
mond, etc.) and even in frozen gases (C
6
H
6
,CH
4
,C
4
H
10
, etc.). This occurs
for a combination of bombarding ions (H, He, Ar, Kr, etc.) and ion energies
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