  Silicon is present in interstellar dust grains, meteorites and asteroids, and
to date thirteen silicon-bearing molecules have been detected in the gas-phase
towards late-type stars or molecular clouds, including silane and silane
derivatives. In this work, we have experimentally studied the interaction
between atomic silicon and hydrogen under physical conditions mimicking those
at the atmosphere of evolved stars. We have found that the chemistry of Si, H
and H$_2$ efficiently produces silane (SiH$_4$), disilane (Si$_2$H$_6$) and
amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely
detected towards the carbon-rich star IRC\,+10216, while disilane has not been
detected in space yet. Thus, based on our results, we propose that gas-phase
reactions of atomic Si with H and H$_2$ are a plausible source of silane in
C-rich AGBs, although its contribution to the total SiH$_4$ abundance may be
low in comparison with the suggested formation route by catalytic reactions on
the surface of dust grains. In addition, the produced a-Si:H dust analogs
decompose into SiH$_4$ and Si$_2$H$_6$ at temperatures above 500 K, suggesting
an additional mechanism of formation of these species in envelopes around
evolved stars. We have also found that the exposure of these dust analogs to
water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups
at the expense of SiH moieties, which implies that, if this type of grains are
present in the interstellar medium, they will be probably processed into
silicates through the interaction with water ices covering the surface of dust
grains.
