London

Researchers at the Ecole Normale Supérieure (ENS) in Paris have coaxed two helium atoms together to create a super-sized molecule up to 100 nanometres long — ending years of argument over whether such large molecules are possible.

Helium is an inert gas whose atoms don't like to form bonds with others. But under particular circumstances, the atoms can be brought together to form diatomic (He2) molecules, says Jérémie Léonard, a graduate student at the ENS. Now, in work that will appear in a forthcoming issue of Physical Review Letters, his group has created a helium molecule far larger than any other ever made (J. Léonard et al. preprint at http://arxiv.org/cond-mat/0304446).

To create the molecules, Léonard and his colleagues cooled helium gas in a magnetic atom trap to extremely low temperatures — about 10 microkelvin — and exposed it to infrared light. The infrared photons push the electrons to one side of the positively charged helium nuclei and make the atoms mildly attractive to each other, momentarily creating hundreds of thousands of loosely bound diatomic molecules.

Because the bond between the two component atoms is so weak, the molecules are up to a tenth of a micrometre long — thousands of times larger than most simple molecules — and only hold together for about 50 nanoseconds. But that's still long enough for researchers to study the quantum behaviour of helium atoms and the way that they interact with others.

“There has been a lot of speculation over whether these molecules exist or not,” says Peter van der Straten, a physicist at Utrecht University in the Netherlands, who used a similar technique to make smaller helium molecules in 2000 (N. Herschbach et al. Phys. Rev. Lett. 84, 1874–1877; 2000).

Van der Straten says he is impressed by the ENS group's work because the team managed to detect the minute amounts of heat given off by the giant helium molecules as they broke apart. This is the first time such detection has been achieved, he says.