New path toward X-ray lasers found.

Researchers have long sought an efficient means of generating intense bursts of short-wavelength X-rays to create a laser. Such a source could be used to image matter, allowing the study of living biological tissue and other materials on an atomic or molecular scale.

This possibility now appears closer with the discovery of a new method for producing X-rays in clusters of atoms. Charles K. Rhodes and his coworkers at the University of Illinois at Chicago have shown that clusters of xenon atoms irradiated by extremely short, powerful pulses of ultraviolet light can generate X-rays having wavelengths ranging from 2 to 3 angstroms.

"The work demonstrates an important new mechanism for the production of a highly desired form of excited matter," Rhodes says.

The team's measurements also provide crucial information indicating how the process can be greatly improved. "These findings give promise for the construction of the brightest light sources ever produced at any wavelength," Rhodes says.

The researchers report their results in the Aug. 25 Nature.

Groups throughout the world have expended a great deal of effort trying to develop an X-ray laser. However, the success of these efforts has generally been limited to "soft" X-rays, having wavelengths of 100 angstroms or more. In one scheme, researchers at the Lawrence Livermore National Laboratory in Livermore, Calif., have obtained X-ray wavelengths as short as 45 angstroms.

To achieve even shorter wavelengths, the trick is to find a way of concentrating power so that a source, such as an ultraviolet laser, can effectively deliver energy in an extremely brief burst to a restricted volume of the X-ray-generating medium.

Taking a unique approach, Rhodes and his collaborators discovered that intense ultraviolet light focused on a gas consisting of clusters of xenon atoms can cause the ejection of one or more tightly bound electrons - normally found in orbits close to an atomic nucleus - while leaving behind an outer shell of weakly bound electrons. The result is a highly excited form of matter called a hollow atom. When electrons refill the vacancies, they generate X-rays.

The effect occurs only for clusters of atoms. The intense electric field of the incoming ultraviolet light induces the outer electrons of a cluster's atoms to oscillate collectively These coordinated motions facilitate the transfer of energy - a large number of ultraviolet photons - directly to an atom's inner electrons.

By a remarkable coincidence, the same effect is also responsible for channeling the ultraviolet radiation, allowing the delivery of energy directly to the atoms of a cluster. The combination of channeling with the efficient ejection of inner electrons by photons of ultraviolet light produces an ideal environment for generating X-rays.

"These two, seemingly unrelated phenomena work together to create this concentration of power," says Bernd Crasemann of the University of Oregon in Eugene. "It could be a significant factor in the development of an X-ray laser."

The researchers have found that they can maximize the emission of X-rays by adjusting the size of the atomic clusters appropriately This result suggests the possibility of designing a new class of molecular materials optimized for the efficient production and amplification of X-rays, Rhodes says.

"If the physics works the way we want it to, we could expect an X-ray laser soon," Rhodes adds. "We're now setting up the key set of experiments to evaluate [the physics]. The next goal is to prove amplification."

"It's an approach that could conceivably be extended and made practical," Crasemann comments. "One needs to make more measurements and really confirm what's been found."