A collaboration¹ between researchers at the Universities of Leicester and Innsbruck has developed a completely new way of forming charged molecules² which offers tremendous potential for new areas of chemical research. Professor Andrew Ellis from our Department of Chemistry has been working for several years with colleagues at the Institute of Ion Physics in Austria on exploring the chemistry of molecules inside liquid helium. The team's latest and most startling discovery is that helium atoms can acquire an excess negative charge which enables them to become aggressive new chemical reagents.

 

Helium is a famously unreactive gas but when cooled to just above absolute zero it becomes a superfluid, a strange form of liquid. (Among other bizarre properties, liquid helium can flow upwards³ because it has zero viscosity⁴ and its capillary⁵ action is stronger than gravity.) The Anglo-Austrian team manufacture droplets⁶ of superfluid liquid helium by subjecting helium gas to a combination of high pressure and low temperature and then force it through a pinhole just 5 µm in diameter into a vacuum chamber⁷. These droplets provide a controlled environment into which molecules can be added to study chemistry.

 

The molecules in this case were fullerenes, a class of large carbon molecules, named after their geometrical similarity to the geodesic spheres developed by architect Buckminster Fuller in the 1950s. The droplets of helium were passed through a cell containing C60 or C70 fullerenes and the resultant mixture was hit by an electron beam of energy between 0 and 150 eV.

 

What Professor Ellis and his colleagues discovered was that clusters of five or more fullerene molecules became dianions (gained a double negative charge) when targeted by a beam of about 22 eV. Dianions are not uncommon⁸ in chemistry but they are normally very unstable⁹ and short-lived outside of common chemical solutions (such as water). The creation of relatively¹⁰ stable fullerene dianions in liquid helium opens up a whole new research area for chemists.

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