A novel means by which atoms can lose excess energy has recently been observed. This development may have considerable implications for many fields.

Predicted in 1997, the idea of interatomic Coulombic decay, or ICD, is best seen through the example in which it has just been measured. Consider two neon atoms, sitting about six atomic radii apart (0.34 nm), weakly bound by van der Waals interactions. A tightly bound electron is knocked loose from one of them.

As might be expected, a less tightly bound electron drops down to fill its place, but the energy released is transferred to the neighbouring neon atom where it kicks out a low-energy electron. The resulting two adjacent, singly charged Ne1+ ions repel each other and are emitted back-to-back. Together with the coincident electron, they provide a unique fingerprint for ICD.

Recent work by T Jahnke and colleagues, conducted at the J W Goethe-Universität Frankfurt am Main and the Max-Planck-Institut für Plasmaphysik in Garching, shows the effect unambiguously. The experiment used a beam line at the BESSY synchrotron radiation facility in Berlin, in single bunch operation, together with the COLTRIMS (cold target recoil ion momentum spectroscopy) technique. The researchers detected the energy of two Ne1+ fragments in coincidence with the ICD electron, yielding a clean, background-free experimental spectral distribution of the ICD electrons.

The observation of this process is currently generating a great deal of interest. ICD is expected to occur most commonly in hydrogen-bonded systems such as water, and could explain the hitherto mysterious presence of low-energy electrons in irradiated solutions of biomolecules.

Further reading

T Jahnke et al. 2004 Phys. Rev. Lett. 93 163401.

Compiled by Steve Reucroft and John Swain, Northeastern University