Crystalline hydrogen gets its first X ray.

For decades, theoretical physicists have explored solid hydrogen-the simplest of elements-with their minds. Experimental physicists, however, have had a difficult time of it, largely because X rays tend to fly past hydrogen atoms with little interaction. Now, a team of U.S. and French physicists announces it has overcome this challenge and has peered at a single crystal of hydrogen.

"To take a direct look at solid hydrogen is a real breakthrough," says Paul Loubeyre of the University of Paris. "It was considered impossible." In the Oct. 24 Nature, he and his coauthors describe how they carefully squeezed hydrogen gas until it became solid, then captured its image at the European Synchrotron Radiation Facility, the world's brightest X-ray instrument.

The researchers injected hydrogen and helium gas into a tiny sealed vise, or diamond anvil, made of two diamonds that can be squeezed together. For several days, the physicists slowly closed the vise, increasing the pressure on the gases to force them into a fluid state. Because of electrical interactions, hydrogen and helium repel one another.

At a pressure of more than a million times atmospheric conditions, the hydrogen solidified into a single crystal, pushed into the center of the vise by the liquid helium surrounding it. The researchers also created a crystal of deuterium, a heavier isotope of hydrogen.

"It's a really clever idea, to grow the crystal in helium," says Isaac F. Sil- vera of Harvard University. The more compressible fluid cushioned the crystal, preventing it from shattering.

"The single crystal is the reason we could take an X ray," says Russell J. Hemley of the Carnegie Institution of Washington (D.C.), a coauthor of the report. To capture an image, the researchers fired an X-ray beam into the diamond anvil. The radiation passed through one diamond, diffracted off the atoms in the hydrogen crystal, exited through the second diamond, and finally reached a detector. "Diamond is transparent not only to the eyes but to X rays," says Hemley.

Although the imprisoned crystal was little more than 3 micrometers thick, the physicists were able to align the X-ray beam with the immobilized hydrogen.

Instead of slipping past randomly moving hydrogen atoms, the beam interacted with the electrons confined within the fixed crystalline pattern.

As pressures approached 1.1 million atmospheres, the scattering of the X-ray beam off the hydrogen atoms revealed that the crystalline structure was becom- ing increasingly ordered, compressing 25 percent more than expected.

Building on earlier experiments (SN: 4/20/96, p. 250), the researchers hope someday to squeeze the atoms tightly enough to convert from crystalline to metallic form, in which electrons can move freely among the atoms. Metallic hydrogen may be a superconductor at room temperature.

"It's the simplest system in nature," says Silvera. "But it's also one of its great challenges."