Liquid helium offers a gateway to a whole new branch of chemistry

When liquid helium is cooled down from 4.2K to the lambda point at 2.2K the liquid for a brief moment boils up violently.Suddenly, boiling ceases. This is the transition from the normal phase to the superfluid phase. (Photo credit: Wikipedia)

This is really just the beginning of a new branch of chemistry . . .

A collaboration between researchers at the Universities of Leicester and Innsbruck has developed a completely new way of forming charged molecules 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 upwards because it has zero viscosity and its capillary action is stronger than gravity.) The Anglo-Austrian team manufacture droplets 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 chamber. 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 uncommon in chemistry but they are normally very unstable and short-lived outside of common chemical solutions (such as water). The creation of relatively stable fullerene dianions in liquid helium opens up a whole new research area for chemists.

So how have these dianions come about? Adding two electrons sequentially to something is difficult because of Coulomb’s Law: the negative charge of the first electron will tend to repel the second electron. What has evidently happened is that two electrons have attached themselves to a fullerene molecule simultaneously. The key question is where do these two electrons come from and why don’t they repel each other?

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