Image: Gehard Meyer/Free University of Berlin

Imagine two buoys in a bay, far apart but tied to each other with a rope. In certain waves, the rope becomes alternatively slack and taut as the buoys lean toward and pull away from each other. That rope, it turns out, is not unlike an indirect interaction that takes place between distant atoms dropped onto a metal surface. The atoms--called adatoms--experience a force that oscillates between attraction and repulsion, mediated by the standing waves that the surface electrons in the metal generate. That interaction stirs up rings of attraction and repulsion surrounding each adatom (see scanning tunneling microscope image at right).

In the October 2nd issue of Physical Review Letters, Karl-Heinz Rieder of the Free University of Berlin and his colleagues report the first measurements of this strange, long-range interaction. Their findings confirm predictions about the force made back in 1978 by Nobel laureate Walter Kohn of the University of California at Santa Barbara and K. H. Lau. They predicted--and now Rieder has shown--that if the surface electrons are in specific quantum states, this interaction will cover very long distances; it will be oscillatory; and the period of the oscillations will vary with the wavelength of the surface electrons. The results, researchers hope, may help clarify how atomic-scale structures form on surfaces.