Atomic refraction: bending matter waves.

When a light beam passes obliquely from air into water, its path doesn't continue along the same straight line it followed in air. It refracts, changing its direction of travel at the air-water interface.

The light wave also slows down as it enters the water. The ratio of the speed of light in a vacuum to its speed in water represents the water's index of refraction.

Now, researchers have determined the index of refraction not of light waves, but of beams of sodium atoms as they travel through different gases. Such measurements provide new insights into the forces that govern collisions between sodium atoms and various types of gas atoms.

David E. Pritchard of the Massachusetts Institute of Technology, Jorg Schmiedmayer of the University of Innsbruck in Austria, and their coworkers report their findings in the Feb. 13 Physical Review Letters.

The researchers took advantage of the fact that sodium atoms can, under certain circumstances, behave more like waves than particles. Thus, two beams of sodium atoms can combine to produce an interference pattern. The pattern's "dark" areas indicate an absence of atoms where the matter waves cancel each other out, and "bright" areas show a surfeit of atoms where they reinforce each other (SN: 9/7/91, p.158).

"Atom interferometers are becoming a powerful tool in the field of atomic physics," the researchers note.

To measure the index of refraction that arises from sodium matter waves traveling through a gas, Pritchard and his colleagues split a sodium beam in two, then sent the two components along separate paths before recombining them (see diagram). By sending one of the components through a given gas, the researchers could determine its index of refraction from the change in the resulting interference pattern.

The researchers investigated the effect on sodium of the gases helium, neon, argon, krypton, and xenon. From their data, they calculated the refractive index of each gas, which provided new details about the long-range forces between the gas atoms and sodium atoms.

Of the gases studied, they discovered that helium atoms behave most like hard spheres in their interactions with sodium. These atoms exert the weakest long-range attraction. Xenon, on the other hand, has a strong long-range interaction, which plays a significant role in sodium-xenon collisions.

Significant discrepancies between the results of this experiment and predictions based on direct collision experiments suggest the need to refine how the long-range interactions between these atoms are characterized, the researchers suggest.

Pritchard and his collaborators hope to improve the sensitivity of their method to extract other details of the interatomic forces governing collisions between different atoms.