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

TL;DR

This work investigates amorphous hydrogenated carbon (a-C(:H)) nanoparticles as a key component of interstellar dust, arguing that extreme irradiation drives dehydrogenation and transformation toward highly conjugated, strained networks that can produce the $3-4\,\mu$m CH bands without requiring aromatic PAHs. By constructing analogues from small conjugated cycles and examining the evolution of CH and CC bands, the authors propose that the observed $3.3\,\mu$m emission may originate from end-of-road olefinic/sp$^{2}$ networks rather than planar PAHs, and they connect this to broader mid-infrared features through a coupled evolution of composition, structure, and size within the THEMIS framework. The paper outlines conceptual pathways for dehydrogenation, presents experimental recommendations for studying sub-nanometre carbon networks under EUV irradiation, and discusses the astrophysical consequences for dust classification, band-ratio diagnostics, and JWST-era interpretation of diffuse ISM and PDR spectra. Overall, the study challenges the dominance of PAHs in the 3–4 μm regime and emphasizes the pivotal role of a-C(:H) sub-nanostructures and their evolution in shaping interstellar dust spectra and evolution.

Abstract

The nature and evolution of hydrocarbonaceous grains within interstellar and circumstellar media is still far from resolved, perhaps owing to the rather complex nature of their seemingly simple binary atomic compositions. This work explores the fine details of amorphous hydrocarbon nanoparticle, a-C(:H), composition and the evolution of the inherent sub-structures under extreme conditions, focusing on the characteristic CH$_n$ bands in the 3-4 micron wavelength region. Particular attention is paid to the role of dehydrogenation and its effects on the sp^3 and sp^2 hybridisations, leading 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 micron wavelength regime without a significant aromatic moiety content within the structures. A diatomic a-C(:H) phase is likely at the heart of the observed dust evolution in the interstellar medium, and circumstellar and photodissociation regions, as observed at short wavelengths. It appears that we have some way to go in fully understanding these complex materials. Much laboratory work will be required in order to elucidate their chemical and structural evolution at nanoparticle sizes under extreme conditions.

Figures (5)

• Figure 1: The spectra of a selected set of conjugated cyclic and aromatic molecules. Upper panel, conjugated, non-planar molecules 1,3-cyclohexadiene (blue), 1,4-cyclohexadiene (violet), and cyclooctatetraene (green) (see also Fig. \ref{['fig_cyclic_structures']}). Lower panel, planar, aromatic molecules naphthalene (green), pyrene (rose), antthracene (cobalt), and phenanthrene (blue). The black lines are the normalised spectra of the blended sub-components in the relative fractions $\frac{1}{2}, \frac{1}{4}, \frac{1}{4}$ (upper) and $\frac{1}{6}, \frac{1}{3}, \frac{1}{6}, \frac{1}{3}$ (lower), respectively. The yellow and grey boxes give the usual ranges for the functional groups: aromatic and olefinic CH, olefinic cis $^{\rm H}$$>$C$=$C$<^{\rm H}$, the aliphatic CH$_2$ symmetric and antisymmetric modes, and tertiary ($3^\circ$) aliphatic CH.
• Figure 2: A dehydrogenation scenario for the cyclic structures within a-C(:H), starting with a cyclohexane-like ring and evolving through cyclohexene-like, cyclohexadiene-like and ending with an aromatic benzene-like ring. The H atoms inside (outside) the rings are tertiary, $3^\circ$ sp$^3$ (sp$^2$) CH groups and the paired H atoms are sp$^3$ CH$_2$ groups.
• Figure 3: The structure of cyclic molecules containing C=C bonds. Benzene is the only planar, aromatic structure and all other molecules have only sp$^3$$>$CH$_2$ and/or sp$^2$ cis olefinic $^{\rm H}$$>$C$=$C$<^{\rm H}$ groups.
• Figure 4: The $5-15\mu$m spectrum of the same species, and in the same proportions, as in Fig. \ref{['fig_spectra']}. The vertical long-dashed black lines indicate the band centres of the interstellar emission bands. The short-dashed (shaded area) and solid green lines show the a-C(:H) modes (range) assigned to CH and CC bonds, respectively 2005AA...432..895D2008AA...490..665P1986AdPhy..35..317R.
• Figure 5: 2D examples (thin lines) of fully conjugated 3D systems (thick lines, with the dotted bonds and arrowed/shadowed bonds connected), which exhibit only cis --HC$=$CH--, aromatic CH and $>$C$=$ functional groups, the latter are sp$^2$ C atoms bonded to three other C atoms.