Abstract
Context. The UV bump observed in the interstellar medium extinction curve of galaxies has been assigned to π → π⋆ transitions within the sp2 conjugated network of carbon grains. These grains are commonly thought to be graphitic particles or polycyclic aromatic hydrocarbons. However, questions are still open regarding the shape and degree of amorphization of these particles, which could account for the variations in the 2175 Å astronomical bump. Optical spectra of graphitic and onion-like carbon structures were previously obtained from dielectric constant calculations based on oscillating dipole models. In the present study, we compute the optical spectra of entire populations of carbon clusters using an explicit quantum description of their electronic structure for each individual isomer. Aims. Our aim is to determine the optical spectra of pure carbon clusters Cn=24,42,60 sorted into structural populations according to specific order parameters, namely asphericity and sp2 fraction, and to correlate these order parameters to the spectral features of the band in the region of the UV bump. Our comparison involves data measured for the astronomical UV bump as well as experimental spectra of carbon species formed in laboratory flames. Methods. The individual spectrum of each isomer is determined using the time-dependent density functional tight-binding method. The final spectrum for a given population is obtained by averaging the individual spectra for all isomers of a given family. Our method allows for an explicit description of particle shape, as well as structural and electronic disorder with respect to purely graphitic structures. Results. The spectra of the four main populations of cages, flakes, pretzels, and branched structures (Dubosq et al. 2019, A&A, 625, L11) all display strong absorption in the 2–8 eV domain, mainly due to π → π⋆ transitions. The absorption features, however, differ from one family to another and our quantum modeling indicates that the best candidates for the interstellar UV bump at 217.5 nm are cages and then flakes, while the opposite trend is found for the carbonaceous species formed in flame experiments; the other two families of pretzels and branched structures play a lesser role in both cases. Conclusions. Our quantum modeling shows the potential contribution of carbon clusters with a high fraction of conjugated sp2 atoms to the astronomical UV bump and to the spectrum of carbonaceous species formed in flames. While astronomical spectra are better accounted for using rather spherical isomers such as cages, planar flake structures are involved as a much greater component in flame experiments. Interestingly, these flake isomers have been proposed as likely intermediates in the formation mechanisms leading to buckminsterfullerene, which was recently detected in space. This study, although restricted here to the case of pure carbon clusters, will be extended towards several directions of astronomical relevance. In particular, the ability of the present approach to deal with large-scale molecular systems at an explicit quantum level of electronic structure and its transferable character towards different charge states and the possible presence of heteroatoms makes it the method of choice to address the important case of neutral and ionic hydrogenated compounds.
Highlights
The UV bump, first observed in the 1960s (Stecher 1965a), is a broad ultraviolet absorption bump observed on the interstellar medium extinction curves from the Milky Way to high-redshift galaxies, with a stable position centered at 217.5 nm, showing variations in intensity and width in different lines of sight (Bless & Savage 1972; Fitzpatrick & Massa 1986, 1990, 2007; Gordon et al 2003; Elíasdóttir et al 2009; Clayton et al 2015; Zafar et al 2018; Fitzpatrick et al 2019, and references therein)
We focused on the UV bump only, i.e., we did not try to fit the far UV rise or the energy range lower than 4 eV (3.2 μm−1) as the lowestenergy part exhibits a shoulder near 2.8 μm−1 partly assigned to the contribution of larger grains in the dust distribution in astrophysical models
The optical response of pure carbon clusters Cn with n = 24, 42, and 60 was computationally determined using a multiscale approach based on the systematic identification of important families of isomers and the calculation of individual absorption spectra by means of time-dependent density functional tight-binding theory
Summary
The UV bump, first observed in the 1960s (Stecher 1965a), is a broad ultraviolet absorption bump observed on the interstellar medium extinction curves from the Milky Way to high-redshift galaxies, with a stable position centered at 217.5 nm, showing variations in intensity and width in different lines of sight (Bless & Savage 1972; Fitzpatrick & Massa 1986, 1990, 2007; Gordon et al 2003; Elíasdóttir et al 2009; Clayton et al 2015; Zafar et al 2018; Fitzpatrick et al 2019, and references therein) This spectral feature has long been assigned to π→π∗ transitions specific to sp conjugated carbon bonding of graphite particles or polycyclic aromatic hydrocarbons (Mishra & Li 2015, 2017). Graphite’s properties were included in astrophysical dust models such as the one developed by Draine & Lee (1984), the Drude model appearing as a convenient way to model the astronomical UV bump (Fitzpatrick & Massa 1986, 2007).
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