Quantum modeling of the optical spectra of carbon clusters structural families and relation to the interstellar extinction UV-bump

• Published in: Astronomy and astrophysics (Berlin) Vol. 634
• Main Authors: Dubosq, Clement, Calvo, F., Rapacioli, Mathias, Dartois, E., Pino, T., Falvo, C., Simon, Aude
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.02.2020
• Subjects: Astrophysics; Atomic and Molecular Clusters; Atomic Physics; Chemical Physics; Chemical Sciences; Instrumentation and Methods for Astrophysic; or physical chemistry; Physics; Sciences of the Universe; Theoretical and; carbon cluster; interstellar extinction; optical spectra; carbonaceous grains; td-dftb; modelling; dftb
• DOI: 10.1051/0004-6361/201937090
• ISSN: 0004-6361

Context. The UV-bump observed in the interstellar medium extinction curve of galaxies has been assigned to π→π transitions within the sp 2 conjugated network of carbon grains. These grains are commonly admitted to be graphitic particles or polycyclic aromatic hydrocarbons. However, questions are still open regarding the shapes and amorphisation degree of these particles, which could account for the variations of 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 have computed the optical spectra of entire populations of carbon clusters using an explicit quantum description of their electronic structure for each individual isomer. Aims. We aim at determining the optical spectra of pure carbon clusters Cn=24,42,60 sorted into structural populations according to specific order parameters, namely asphericity and sp 2 fraction, and correlated 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-based 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) 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 nm are cages, 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 playing a lesser role in both cases. Conclusions. Our quantum modeling shows the potential contribution of carbon clusters with a high fraction of conjugated sp 2 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 flakes 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 has recently been 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 a method of choice to address the important case of neutral and ionic hydrogenated compounds.