Electron holography on a crystal canvas.

Electron holography on a crystal canvas

Surface scientist expend a great deal of effort pinpointing the locations of atoms on or near a material's surface--a task they would find easier and more revealing if they could obtain three-dimensional images with enough resolution to depict the atoms' precise locations. That capability now seems within reach.

For the first time, a team of researchers has reconstructed a surface's three-dimensional crystal structure from the pattern generted by electrons emitted from surface atoms. The results prove that a diffraction pattern produced by such electrons can be interpreted as a hologram -- the electron-generated equivalent of the visible-light holograms so often used today as security features on credit cards.

"Experimentalists have been seeing these [diffraction] patterns for years. They had jsut never thought of interpreting them as holograms," says physicist Dilano K. Saldin of the university of Wisconsin-Milwaukee. He and his colleagues describe their reconstruction technique inthe Aug. 20 PHYSICAL REVIEW LETTERS.

When an atom near the surface emits an electron, that electron may come directly to the surface or it may bounce off a neighboring atom before emerging. Because electrons also behave like waves, electrons traveling along paths of different lengths would overlap at the detector, producing a distinctive interference pattern. The intensity of that diffraction pattern would vary from place to place, depending on the angle at which the electrons leave the surface.

Saldin's group developed a computer-based technique for analyzing such intensity patterns to extract information about the crystal's atomic arrangement and to construct its three-dimensional structure. The researchers say their tecnique is powerful enough to handle diffraction data produced by a variety of methods currently used by scientists to probe the nature of surfaces, including photelectron nd Auger spectroscopy.

"We can do the reconstruction in a few minutes on a personal computer," Saldin says.

The resulting images shows the relative positions of a typical atom and its nearest neigbors. Because individual atoms yield only one electron, the hologram and its subsequent reconstruction represent averages over all electron-emitting atoms and their nearest neighbors. Thus, the new technique works best when all the electron-emitting atoms sit in roughly the same surroundings, as they would in a near-perfect crystal.

Surface scientists have several methods for punching electrons out of specific types of surface atoms. Each method produces a distinctive electron diffraction pattern amenable to holographic reconstruction. In many cases, researchers can focus on one element, which allows them to work out how its particular atoms are arranged.

Saldin and his colleagues have successfully tested their reconstruction scheme on diffraction patterns created by electrons scattering from coper surfaces.