First helium dimer: a truly supercool giant.

All of the approximately 100 chemical elements in nature form pure molecules made up of one type of atom - except for the inert gas helium. Helium has the reputation of a loner, a lightweight atom that never settles down to bond with others of its own kind. Scientists have speculated that helium atoms do pair up to form diatomic molecules, but only at extremely low temperatures.

Try one-thousandth of a kelvin, a smidgen above absolute zero.

Using a new apparatus that can attain such frigidity, W. Ronald Gentry and his colleagues at the University of Minnesota in Minneapolis have detected helium dimers for the first time. Their observation proves the existence of such molecules - ending a 65-year debate - and validates recent calculations predicting that the extreme low temperature was necessary to coax the atoms to cozy up to one another.

While no longer an oddball among the elements, helium nevertheless forms an unusual molecule. The dimer is held together by the weakest chemical bond ever measured. "Helium doesn't form ordinary bonds;' says Gentry. "The only thing that holds the molecule together is the long-range force caused by the electrons being unevenly distributed around the nucleus."

Oddly, the second smallest atom also forms the world's largest diatomic molecule, rivaling biological macromolecules in size, he notes. Between the dimer's two atoms lies a gulf of 55 angstroms instead of the usual two to three.

James B. Anderson of Pennsylvania State University in University Park comments that Gentry's group beautifully demonstrated that they had indeed detected the fragile molecule and not some experimental artifact. "While the theory has been solid for a few years;' Anderson says, "it wasn't clear that a helium dimer would actually be seen this century."

In the Feb. 15 .JOURNAL OF CHEMICAL PHYSICS, Gentry and his co-workers describe how they cooled the helium by squirting a pulsed beam of high-pressure helium gas into a vacuum chamber. There, the helium expanded as its pressure plummeted by 12 orders of magnitude. A mass spectrometer detected the dimers.

The team wiII now use the apparatus to further study the new helium molecule. Says Gentry, "These very-low-temperature beams are so interesting because they allow us to probe the extreme quantum mechanical behavior of matter."