Bouncing cold hydrogen atoms to a focus.

Bouncing cold hydrogen atoms to a focus

When a slow-moving hydrogen atom splashes down on a liquid-helium surface, it usually bounces away in much the same way as a light beam reflects from a mirror. Physicists have now used that effect to focus a beam of hydrogen atoms by letting the atoms reflect from a concave, hemispherical mirror coated with liquid helium. The achievement opens up the possibility of manipulating atomic hydrogen beams in the same way as researchers manipulate light beams today.

"We believe the present work clearly demonstrates that the hydrogen atom could play a prominent role in particle optics experiments," says J.T.M. Walraven of the University of Amsterdam in the Netherlands. He described the research at last week's American Physical Society meeting in Anaheim, Calif.

According to classical theory, incoming hydrogen atoms cooled to temperatures of much less than 1 kelvin should stick to a liquid-helium surface. Quantum theory, however, allows several other possibilities. If the liquid helium is cold enough, the incoming atoms may bounce away as if the liquid's surface were a perfect mirror. Alternatively, if the surface happens to be rough -- perhaps because of tiny ripples induced by the approaching particles -- the atoms may scatter in widely varying directions.

To focus hydrogen atoms, Walraven and his colleagues use a quartz mirror ground and polished to optical precision, then coated with a liquid-helium film about 125 angstroms thick. The hydrogen atoms, cooled to less than 600 millikelvins and stored in a special reservoir, fan out from an orifice 0.5 millimeter in diameter.

To determine how perfectly the liquid-helium mirror reflects hydrogen atoms, the researchers measure how long it takes for the reservoir to lose its hydrogen atoms. When the mirror's center coincides with the center of the orifice, all the escaping hydrogen atoms should reflect back into the reservoir, meaning the reservoir would never empty.

Walraven's group finds that when the orifice sits far from the mirror's center, the reservoir's decay time is only slightly longer than that expected in the absence of a mirror. Moving the mirror to bring the orifice into focus increases the decay times dramatically.

The results show that under optimal focusing conditions, at least 80 percent of the atoms are reflected perfectly. The researchers attribute much of the reflectivity loss to uncertainties in the mirror's position.

However, a small proportion of that loss may arise from scattering caused by a slight surface roughness. Because of an attractive force between incoming hydrogen atoms and the liquid helium, an approaching atom tends to pull on the surface, sometimes generating one or more quantized ripples. These "ripplons" could deflect a certain number of atoms, causing them to go off in unpredictable directions.

"This experiment demonstrates that it's possible to make high-reflectivity mirrors for cold hydrogen atoms, especially if we go to lower temperatures," Walraven says. Such precise control of atomic hydrogen beams could help scientists investigate a variety of quantum-mechanical effects.