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A study on charged fullerenes embedded in helium nanodroplets confirms their existence in interstellar space


Irradiation with pulses of infrared light of fullerene ions embedded in liquid helium nanodroplets has revealed transitions between the solid, liquid and superfluid phases of the helium layers surrounding the charged fullerene. The accuracy of these measurements, performed in an environment similar to that found in interstellar space, confirms the presence of fullerenes in space and opens the door to the characterization of diffuse interstellar clouds associated with other species rich in carbon.

Helium nanodroplets provide a unique tool for the study of properties of molecules under conditions similar to those found in interstellar space: low temperatures and almost total absence of interaction with the environment. The latter is the consequence of the superfluidity of liquid helium, which prevents embedded molecules to experience any friction when they move inside. However, the introduction of charged molecular species inside these nanodroplets produces substantial modifications in the latter, especially in the first layers of solvation around the charged molecule.

Using photoelectron spectroscopy techniques in combination with molecular dynamics calculations, the properties of the first helium layers that form around the C60+ fullerene have been studied in the framework of a wide international collaboration involving researchers from Universidad Autónoma de Madrid, IMDEA-Nanoscience and IFIMAC Condensed Matter Physics Center. The results demonstrate the appearance of phase transitions in the first helium layers as the number of helium atoms increases. In particular, it is observed a transition from a solid phase of helium (usually referred to as the “Atkins snowball”, see figure), to a liquid phase, when the second layer begins to form, and finally to the superfluidity phase. All this happens before reaching a hundred helium atoms. The precision required to reveal such changes is of the order of 0.05 nanometers, which explains why such phenomena have gone unnoticed to date.

Given the insulation and low temperature environment provided by helium nanodroplets, the method has confirmed the assignment of two diffuse interstellar bands (DIB) to the presence of the C60+ ion in space, which demonstrates the important role of fullerenes as a carbon reservoir in the universe. The work published in the journal Nature Communications shows that, with the accuracy achieved in this study, the absorption spectra of other astrophysically relevant species can be determined, such as smaller fullerenes, polycyclic aromatic hydrocarbons and their derivatives.



Figure 1. The measurement principle. Fullerene C60+ (whose carbon atoms are represented by black spheres), surrounded by an “Atkins snowball” formed by a layer of helium atoms (transparent spheres surrounding the C60+), is irradiated by a laser pulse of infrared light, which leads to the evaporation of helium atoms. The wavelength of the absorbed light depends on the number of helium atoms initially adsorbed on the C60+.



Atomically resolved phase transition of fullerene cations solvated in helium droplets
M. Kuhn, M. Renzler, J. Postler, S. Ralser, S. Spieler, M. Simpson, H. Linnartz, A.G.G.M. Tielens, J. Cami, A. Mauracher, Y. Wang, M. Alcamí, F. Martín, M.K. Beyer, R. Wester, A. Lindinger & P. Scheier
Nat. Commun. 7, 13550 doi: 10.1038/ncomms13550 (2016)


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