Hollow excitement energizes atomic physics.

Hollow excitement energizes atomic physics 

Researchers have discovered that a close encounter between a highly charged, slowly moving ion and a metal surface often yields a neutral atom flush with excess energy -- an intriguing product they describe as a "hollow" atom. In this unusual, ephemeral atomic state, nearly every electron present occupies one of the atom's outer shells, leaving the inner shells empty.

"That's a very exotic atomic state, which no one has ever been able to study before," says Fred W. Meyer of the Oak Ridge (Tenn.) National Laboratory. "Right now, we're investigating the details of how a hollow atom is formed, how fast it's formed and how fast it decays. Our emphasis is on trying to understand the fundamental physics to find out what's going on."

The discovery of this remarkable atomic state hinged on the development several years ago of a new type of ion source capable of stripping atoms of nearly all their electrons without imparting extremely high speeds to the resulting ions. "That development initiated a whole research program in which people started looking at collisions between low-energy, multicharged ions and metal surfaces," Meyer says. For anyone with access to such an ion source, "it's a very active field of research."

These investigations revealed that a highly charged ion -- in many cases, little more than a bare nucleus -- traveling toward a metal surface can extract and capture a large number of electrons from its target in an extraordinarily short time. Indeed, enough electrons make the jump to neutralize the bombarding ion before it crashes into the metal surface.

Measurements of the characteristic X-rays emitted by electrons shifting from high-energy, outer shells to low-energy, inner shells allow researchers to track the process. Recent studies of this radiation, conducted at a laboratory n Grenoble, France, suggest that argon ions can capture 16 or 17 electrons from a silver surface in just a few femtoseconds. At Oak Ridge, a more direct measurement of neutralization times based on observations of highly charged nitrogen ions striking a gold surface gives similar values.

"We know from the [X-ray! energy spectrum that the ion must have been neutralized," Meyer says. "From the speed of the ion and the distance at which these things start to happen, we can infer that a multicharged ion captures a bunch of electrons extremely quickly."

But that leaves a disturbing puzzle. Conventional theoretical models predict that after electrons pass from the metal to highly excited states of the projectile ion, they trickle down to lower energy levels in a cascade of small steps, each step taking an appreciable amount of time. The observations, however, indicate that the whole neutralization process occurs several orders of magnitude faster than this complex mechanism would suggest.

The effort to reconcile theory and observation raises some fundamental questions about atomic physics, Meyer says. For example, to what extent is the structure and decay scheme of excited ions or atoms perturbed by their closeness to a surface? Perhaps the incoming ion and the metal surface combine to form a hybrid structure inadequately described by any present atomic model. Another possibility is that strong inter-actions among electrons during the capture process may propel some of them directly into low-lying states.

Resolving the mystery will require more detailed experimental and theoretical investigations, Meyer says. Eventually, such studies may also lead to a better understanding of how the leakage of ions from magnetically confined plasmas affects the walls of devices built to contain nuclear fusion.