  The origin of the diffuse interstellar bands is one of the longest-standing
mysteries of the interstellar medium is explored within the framework of The
Heterogeneous dust Evolution Model at the IaS (THEMIS). The likely nature of
the diffuse interstellar band carriers and their evolution is here explored
within the framework of the structures and sub-structures inherent to doped
hydrogenated amorphous carbon grains in the interstellar medium. Based on the
natural aromatic-rich moieties (ashphaltenes) recovered from coal and oil the
likely structure of their interstellar analogues is investigated within the
context of the diffuse band problem. It is here proposed that the top- down
evolution of interstellar carbonaceous grains, and in particular a-C(:H)
nano-particles, is at the heart of the formation and evolution of the diffuse
interstellar band carriers and their associations with small molecules and
radicals such as C2, C3, CH and CN. It is most likely that the diffuse
interstellar bands are carried by dehydrogenated, ionised, hetero- cyclic,
aromatic rich moieties that form an integral part of the contiguous structure
of hetero-atom doped hydrogenated amorphous carbon nano-particles and their
daughter fragmentation products.
