The nature and evolution of a-C(:H) nanoparticle substructures and speculations on the origin of the 3-4μm emission bands

The nature and evolution of hydrocarbonaceous grains within interstellar and circumstellar media is still far from resolved, perhaps owing to the complex nature of their seemingly simple binary atomic compositions. This work explores the fine details of the composition of amorphous hydrocarbon nanoparticles, a-C(:H), as well as the evolution of their inherent substructures under extreme conditions, focusing on the characteristic CH_n bands in the $3-4,μ$m wavelength region. Particular attention is paid to the role of dehydrogenation and its effects on the sp^3 and sp^2 hybridisations that can lead to an extensive conjugated domain functionalisation of the contiguous structural network within a-C(:H) nanoparticles. Qualitatively, this approach is able to explain the origin and evolution, including the appearance and disappearance, of emission bands observed in the $3-4,μ$m wavelength regime without a significant aromatic moiety content within the structures. A diatomic a-C(:H) phase is likely at the heart of the dust evolution in the interstellar medium and circumstellar and photodissociation regions observed at short wavelengths. It appears that we have some way to go in fully understanding these complex materials. Much laboratory work is required to elucidate their chemical and structural evolution at nanoparticle sizes under extreme conditions.