When it comes to charges, molecules of hydrogen are just too symmetrical. It makes them impossible to control with electric fields because their lack of polarity means that thermal motions always outweigh any applied electrical force. Water and other asymmetric molecules, by contrast, can be pulled to and fro by an electric field. Remember the electrostatic trick with a hair comb and a dribbling bathroom tap from childhood?

In the 1980s, it was suggested that hydrogen molecules could be rendered polar by flinging one of its electrons into a high-energy orbital, so disrupting the molecule’s symmetry. The electron would then feel the pull of an electric field to a different degree at one end of the molecule and so drag it along like a puppet on a string. Until now, no one had become puppet master of the hydrogen molecule.

Now, researchers at ETH Zurich [Hogan et al. Phys. Rev. Lett., (2009) 103, 123001] have found a way to dangle hydrogen molecules, H2, on an electron string without the excited electron simply dropping back to the ground state before the researchers can achieve anything useful with their molecular puppet.

Stephen Hogan, Christian Seiler, and Frederic Merkt looked at several of hydrogen’s excited molecular orbitals in detail and picked out the one that had the potential for longevity. They then used circularly polarized laser light to “cool” the hydrogen molecules, slowing them to speeds between static and 500 metres per second, which essentially brings them close to absolute zero in terms of thermal energy. In a three-dimensional electrostatic trap they could then take on the role of puppet master of the excited state for some 50 microseconds; ample time for a detailed study.

Indeed, in this cold and controlled state, the researchers suggest that it should be possible to study molecular collisions at very low energies. They add that carrying out precision spectroscopic measurements should also be possible on this the apparently simplest of molecules. The study also paves the way for exploring the properties of molecular gases held in so-called quantum degeneracy, where each molecule is in the exact same energy state as its neighbours.

Merkt and colleagues point out that all atoms and molecules have the potential to be controlled in this way. They conclude that their method might therefore be used to prepare cold, stationary samples of a wide range of molecules.