Light goes on for antimatter-rich [H.sub.2]O.

Scientists are old hands at making water heavy. They combine an atom of oxygen with two deuterium atoms, which are hydrogen atoms containing a neutron as well as a proton. Conceiving of a new, remarkably light variety of water, a pair of chemists calculate that it is possible to replace the hydrogen protons with antimatter particles known as positrons. These are the positively charged antimatter counterparts of electrons.

Studies of "positronic water" could deepen understanding of how clusters of charged particles interact, says David M. Schrader of Marquette University in Milwaukee. Scientists already use penetrating beams of positrons to study materials. If positronic water indeed forms in this bombardment, they say, it may alter the radiation pattern emitted by the material and give additional information to researchers determining the material's physical properties.

By predicting the behavior of molecules whose atoms will be much more mobile than those of ordinary water, Schrader and Nan Jiang, also of Marquette, are subjecting theories of molecular energies and chemical reactions to an extreme test, says Alec T. Stewart of Queen's University at Kingston, Ontario. "This sort of stuff goes one step at a time, and this is another good step."

More than 40 years ago, experimenters succeeded in coaxing electrons and positrons into a short-lived coexistence as positronium, a hydrogenlike atom with a positron replacing the proton. They have since induced positronium to form two-atom molecules with ordinary elements, such as hydrogen and chlorine.

Positronium differs from antihydrogen, which is the full antimatter complement to hydrogen. Scientists first created antihydrogen nearly 3 years ago by joining positrons with antiprotons, which are as massive as ordinary protons but negatively charged (SN: 11/30/96, p. 340).

In the Dec. 7 Physical Review Letters, Schrader and Jiang describe their quantum-mechanics calculations predicting that two positroniums plus oxygen can form a fleetingly stable molecule. The bonds between positronium and oxygen would have only about one-fourth the strength of hydrogen-oxygen bonds and would not form a fixed angle. Moreover, the positronic water molecule would be about 75 percent bigger than a molecule of ordinary water, they conclude.

Don't expect ever to sip a glass of positronic water, however.

"First, it would explode right away" because of antimatter-matter annihilations, Schrader says. The researchers estimate that each molecule would last only 220 picoseconds before its positrons and electrons would collide and vanish in a blaze of gamma rays.

Even if positronic water could somehow be kept intact, the scientists can't yet predict important characteristics that depend on interactions between atoms, such as whether it would form a liquid at room temperature. "There is no reason to believe it would be similar" to ordinary water, Schrader says.

Researchers now face the formidable challenge of forcing two positronium atoms into the same molecule, says Lester D. Hulett Jr. of Oak Ridge (Tenn.) National Laboratory. "A three-body collision [of two positronium atoms and an oxygen atom] is not all that likely," he says.

If seekers of positronic water, however, can use the intense positron beam at Lawrence Livermore (Calif.) National Laboratory or the more powerful ones being built in Japan, they hope to detect the first positronic water molecules within a decade, Schrader says.