On the surface. Scientists saw indium atoms (white dots) take large jumps across a copper surface.

Atomic Sliding Puzzle

Atoms on the surface of a material are constantly moving about, like tiles on a sliding puzzle, say physicists in the Netherlands. Their surprising discovery, reported in the 19 February issue of Physical Review Letters, might force scientists to rethink the mechanisms of many chemical reactions, such as those responsible for depleting the ozone layer.

"The surface, up until now, was considered to be a pretty static thing," says Raoul van Gastel, a physicist at the University of Leiden in the Netherlands. Scientists assumed that at room temperature, atoms in the outer layer of a chunk of (say) copper stayed more or less locked in place--although, because all copper atoms are identical, it was hard to tell for sure.

So when van Gastel and his colleagues sprinkled indium atoms on a surface of pure copper and observed it with a scanning-tunneling microscope, they didn't expect to see much happening--perhaps just a slow, atom-by-atom diffusion of indium throughout the copper. Most of the time, nothing did happen. But every so often, some indium atoms would suddenly leapfrog several atom-widths away. The team concluded that a handful of holes were skittering along the surface, drastically rearranging copper atoms in their path. This sliding-puzzle-like activity seems to explain the behavior of the indium atoms. "It matches perfectly," van Gastel says.

"This observation is a surprise," says physicist Miquel Salmeron of the University of California, Berkeley. The evidence is still indirect: The presumed vacancies moved too fast for the team to spot them directly. Still, "it's nice to see," Salmeron says. He believes that these vacancies might help chemists get a handle on processes that take place at surfaces. By ignoring the atomic dancing going on there, chemists may have seriously underestimated how quickly surface atoms can move about, making it impossible to understand the rates and mechanisms of many chemical reactions, such as those on ice crystals in the upper atmosphere that lead to depletion of the ozone. "I'm still surprised, after so many years, we've ignored important effects on surfaces," says Salmeron.