Speaker
Description
The lack of natural sources of carbon fullerenes on Earth and within the solar system is one of the puzzles of astrochemistry, astrophysics and planetary science. Recently, Sittler et al. [1] have hypothesized that the ionospheric formation of these exotic species in the Titan atmosphere can occur from simple molecules such as methane. This can change our understanding of the carbon cycle throughout the universe and shed light on complicated astrochemistry reactions beyond the known fullerenes factories, such as dying stars and planetary nebulae [2]. We believe that, with a large number of exoplanets already discovered and cataloged, it is possible to search for the fingerprints of this allotrope of carbon within their atmospheres. Our observation strategy will first search for exoplanets with thick methane atmospheres. Within these candidates, we search for the UV/VIS absorption of fullerenes. Cataldo et al.[3] have recorded and measured the UV/VIS spectra of C60 fullerene in a solvent under laboratory conditions. In the visible wavelength range, the features at 404 (Strong), 440 (weak), and 670 nm (weak), together with five sub-features at 500, 530, 570, 600, and 628 nm, were also reported [3]. By absorbing a vacuum UV photon (7.80eV), neutral fullerene is ionised to C60+ [4]. This cation shows a strong absorption band at 823.1 nm [3]. The other member of the fullerene family, C70 and its cations C70+ also exhibit absorption bands within UV/VIS wavelength range. Their relatively strong absorption bands are (466.9 & 544.9 nm) and (445.5 & 641.8 nm) for C70 and C70+ species, respectively [3]. Considering these data, we are proposing the spectroscopic observation of these exotic species and perhaps other members of the fullerene family in the atmosphere of the targeted exoplanet systems. Our proposal benefits from reliable theoretical calculations of UV/VIS spectra when experimental data is not available. The theoretically calculated value of one of the weak allowed transitions of C60 (Figure 1) at the VIS wavelength range of 604.28 nm (corresponding experimental value at 600 nm) by applying EDF2/6-31G* quantum chemical model is presented in
Figure 2. The theoretical approach enables us to identify spectral signatures (weak/strong) of other fullerenes from the Roman spectrometer in the absence of experimental data.
References
[1] Edward C. Sittler; John F. Cooper; Steven J. Sturner; and Ashraf Ali. “Titan’s ionospheric
chemistry, fullerenes, oxygen, galactic cosmic rays and the formation of exobiological molecules
on and within its surfaces and lakes”. Icarus 344 (2020), p. 113246.
[2] SeyedAbdolreza Sadjadi; Quentin Andrew Parker; Chih-Hao Hsia; and Yong Zhang. “A Theoretical Study of Infrared Spectra of Highly Positively Charged C60 Fullerenes and Their Relevance to Observed UIE Features”. In: ApJ 934 (2022), p. 75.
[3] Franco Cataldo; Susana Iglesias-Groth; and Yaser Hafez. “On the Molar Extiction Coeffiecints of the Electronic Absorption Spectra of C60 & C70 Fullerene Radical Cation”.Eur. Chem
. Bull 2 (2013), p. 1013.
[4] SeyedAbdolreza Sadjadi and Quentin Andrew Parker. “It remains a cage: ionization tolerance
of C60 fullerene in planetary nebulae”. In: FNCN 29 (2021), p. 620.
