Atomic shadow-puppetry reveals structure.

Atomic shadow-puppetry reveals structure

Those educational films shown in grade school would seem incomplete without little fingers jutting in front of the projector to animate the screen with wiggly rabbits and flying birds. When properly staged, atoms accomplish a remarkably similar effect, according to five chemists who say they have learned both to produce atomic shadow-shows and to intepret them as structural revelations of a solid materialshs topmost atomic or molecular layers.

The group already hs used the new technique -- called angular distribution Auger microscopy, or ADAM -- to map surfaces of pure metals with and without other atomic or molecular coatings. "It produces very sharp and straightforward images of atomic structure," says research leader Arthur T. Hubbard of the University of Cincinnati.

The first scientists to apply ADAM to their work wil be solid-state physicists interested in the details of how atomic layers stack into, say, semiconductor devices such as thin-film lasers, Hubbard predicts. The technique should also prove useful for studying polymer films, catalysts and even dynamic phenomena such as atomic vibrtions, he adds.

French researcher Pierre Auger discovered the underlying principle in 1925. Bombarding an atom with radiation, such as X-rays or high-energy electrons, tends to dislodge and expel an electron circling in one of the atom's inner orbitals. A less tightly bound electron orbiting farther away then falls into the more internal vacancy while the atom ejects a third, "Auger" electron. Since atoms of particular elements eject electrons at characteristic energies, measuring the energies of the fleeing Auger electrons identifies the parent atoms. Scientists have used Auger electrons since the mid-1960s to determine the elemental compositions of materials.

Many researchers have noted that the number of Auger electrons measured varies as the electron detector's angular view of the sample changes. They have attributed these variations to several factors, including diffractions effects and quantum mechanical fluctuations of individual atoms.

By building a novel instrument capable of measuring Auger electrons from any angular perspective, the Cincinnati chemists say they have uncovered a more likely origin for the angular distribution of Auger electrons. "The distributions are composed of 'silhouettes' of surface atoms 'backlit' by emission from atoms deeper in the solid," they write in the Jan. 12 SCIENCE. Computer simulations based on this unconventional interpretation of Auger signals agree closely with data from actual ADAM analyses, they report.

In one example, Hubbard's group analyzed a platinum base coated with a single layer of silver atoms, which in turn was topped with iodine atoms. They used ADAM to map the angular distribution of silver's Auger electrons. "We used the silver monolayer as the light bulbs and the iodine atoms as the shadow-creating scatterers," Hubbard says. Since virtually any element can serve as an emitter or a scaterrer, he says the technique should have wide applications in analyzing solids and should also complement other surface-analysis techniques such as scanning tunneling microscopy.

"This thing ought to be checked out a lot more carefully," cautions chemical physicist William F. Egelhoff of the National Institute of Standards and Technology in Gaithersburg, Md. He says the silhouette interpretation flies in the face of several independent quantum mechanical explanations of the distribution of Auger electrons. But chemist Neal R. Armstrong of the University of Arizona in Tucson says chances are good tht Hubbard is right and that ADAM will emerge as another useful analytical tool.