The fingerprint of protonated buckminsterfullerene

In the EUROPAH network, we investigate not only PAHs, but also other carbonaceous species that are relevant in an astrophysical context, such as fullerenes.

The most famous member of this molecular family is the buckminsterfullerene. This unique molecule consists of 60 carbon atoms in the shape of a soccer ball and it has the highest possible molecular symmetry.

C60 was discovered in 1985 by Sir Harold W. Kroto and his co-workers who were awarded the Nobel Prize in Chemistry for the achievement. Since its discovery, the physico-chemical properties of C60 have been thoroughly studied and its spectroscopic properties have been characterized. It belongs to the icosahedral point group that imposes selection rules that leave almost all vibrational modes inactive. The buckyball therefore has a very sparse IR spectrum.

The recent detection of C60 and C70 in space has drawn attention again to the fullerenes. Due to their electronic structure, they can easily form other fullerene-related species, such as protonated C60, which can be also present in the interstellar medium. Furthermore, it was Sir Kroto himself who contemplated that C60H+ is likely the most abundant form of C60 in the ISM.

At Radboud University, we carry out experiments on protonated fullerenes using the FELIX free electron laser. We combine mass spectrometry with IR spectroscopy and measure the IR wavelength-dependent fragmentation of ions. In this way, we can record spectra of gaseous ionized species, which is nearly impossible with direct absorption measurement.

With this technique, we managed to obtain the vibrational spectrum of C60H+ for the first time. Adding one proton to the carbon cage breaks the high symmetry, so that all vibrational modes become IR active. This results in a very rich vibrational spectrum in contrast to that of neutral C60. Thus, we can see a beautiful textbook example of the effect of symmetry breaking on molecular spectroscopy. 

Comparing the fingerprint of C60H+ to emission spectra from nebulae associated with high C60 abundances, we conclude that C60H+ is a plausible contributor to their IR emission. However, protonated C60 alone does not explain all spectral features. We speculate that a mixture of fullerenes in cationic and protonated form would reproduce the observed spectra.

For further reading, you can find our research on protonated C60 in Nature Astronomy.

This post was written by Julianna Palotás who is doing her PhD at the Radboud University in Nijmegen.
Credit for the picture of the Orion nebula: Daniël Rap

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