Mapping the structural diversity of C60 carbon clusters and their infrared spectra

The current debate about the nature of the carbonaceous material carrying the infrared (IR) emission spectra of planetary and proto-planetary nebulae, including the broad plateaus, calls for further studies on the interplay between structure and spectroscopy of carbon-based compounds of astrophysical interest. The recent observation of C60 buckminsterfullerene in space suggests that carbon clusters of similar size may also be relevant. In the present work, broad statistical samples of C60 isomers were computationally determined without any bias using a reactive force field, their IR spectra being subsequently obtained following local optimization with the density-functional-based tight-binding theory. Structural analysis reveals four main structural families identified as cages, planar polycyclic aromatics, pretzels, and branched. Comparison with available astronomical spectra indicates that only the cage family could contribute to the plateau observed in the 6-9 micron region. The present framework shows great promise to explore and relate structural and spectroscopic features in more diverse and possibly hydrogenated carbonaceous compounds, in relation with astronomical observations

RASPT2 Analysis of the F – (H 2 O) n =1–7 and OH – (H 2 O) n =1–7 CTTS States

International audienceWe analyze the electronic structure of the lowest excited states of the F–(H2O)n=1–7 and OH–(H2O)n=1–7 anionic clusters in the framework of RASPT2 theory. At the ground-state geometry, these clusters can bind the excess electron in the first excited singlet and triplet states for n ≥ 3 for F– and n ≥ 2 for OH–. The geometry relaxation of the F–(H2O)n=1–7 clusters in their lowest-energy triplet state produces two series of minima. A first series is made of a F radical weakly bound to a negatively charged water cluster to form F-(H2O)n–. A second series associated with hydrogen transfer from a water molecule to the fluorine atom is built on a HF molecule and a OH radical bound to a negatively charged water cluster to form OH-HF-(H2O)n−1–. This second series provides the lowest-energy isomers of F–(H2O)n for the excited state. These two series of minima are inherited from the neutral fluorine water cluster structure only weakly perturbed by the excess electron. They are similar to the OH–(H2O)n isomers obtained for the lowest-energy triplet state, which are also made of a neutral OH radical inserted in the water molecule network of a (H2O)n– cluster. For all of these clusters in the lowest-energy excited state, the excess electron is localized outside of the cluster near unbound hydrogen atoms. Its binding energy is well correlated to the electric dipole of the cluster, and a lower limit of 4.1 D is necessary to bind it to the cluster. The two series of F–(H2O)n isomers offer two very different routes for geminate recombination observed in water solutions. Our calculation suggests that the recombination takes place with the OH radical left after hydrogen transfer rather than with the F radical

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

International audienceContext. 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

Analysis of nanoscale fluid inclusions in geomaterials by atom probe tomography: Experiments and numerical simulations

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Radiative relaxation in isolated large carbon clusters: Vibrational emission versus recurrent fluorescence

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Analysis of nanoscale fluid inclusions in geomaterials by atom probe tomography: Experiments and numerical simulations

International audienc