  Observations of diffuse clouds showed that they contain a number of simple
hydrocarbons (e.g. CH, C$_2$H, (l- and c-)C$_3$H$_2$, and C$_4$H) in abundances
that may be difficult to understand on the basis of conventional gas-phase
chemical models. Recent experimental results revealed that the
photo-decomposition mechanism of hydrogenated amorphous carbon (HAC) and of
solid hexane releases a range of hydrocarbons into the gas, containing up to 6
C-atoms for the case of HAC decomposition. These findings motivated us to
introduce a new potential input to interstellar chemistry; the "top-down" or
degradation scheme, as opposed to the conventional "build-up" or synthesis
scheme. In this work, we demonstrate the feasibility of the top-down approach
in diffuse clouds using gas-grain chemical models. In order to examine this
scheme, we derived an expression to account for the formation of hydrocarbons
when HACs are photo-decomposed after their injection from grain mantles. Then,
we calculated the actual formation rate of these species by knowing their
injected fraction (from experimental work) and the average rate of mantle
carbon injection into the ISM (from observations). Our preliminary results are
promising and reveal that the degradation scheme can be considered as an
efficient mechanism for the formation of some simple hydrocarbons in diffuse
clouds. However, an actual proof of the efficiency of this process and its rate
constants would require comprehensive experimental determination.
