Elusive physics problem is solved

Collaborating scientists from the Lawrence Livermore and Lawrence Berkeley national laboratories and the University of California at Davis reported in the December 24, 1999, issue of Science that they have solved a fundamental but elusive problem in atomic physics. They have calculated the final energies and directions of three charged particles that collide, break up, and scatter when a hydrogen atom ionizes. Being able to predict what happens during ionization is important because the phenomenon pervades a wide range of physical processes. For example, ionization by electron impact is responsible for the glow of fluorescent lights and for the ion beams that engrave silicon chips.
The collaborating team-physicist Tom Rescigno of Lawrence Livermore and computer scientist Bill McCurdy of Lawrence Berkeley, along with doctoral candidate Mark Baertschy of UC Davis and postdoctoral fellow William Isaacs of Lawrence Berkeley-were dealing with the simplest example of electon-impact ionization, which still is an intrinsically difficult problem. It requires calculation of energies and directions for a final state in which all three particles are moving away from each other. The problem had resisted solution for some 40 years.
Earlier, others had developed analytic solutions for particle interactions in an isolated hydrogen atom; they helped to establish the new quantum theory in the early part of the 20th century. In the 1950s, the bound states of the helium atom, with its two electrons, were computed accurately and established the theoretical framework for modern quantum chemistry. But scattering problems are much more difficult, and complete solutions for three or more scattering particles seemed intractable until large-scale computing power became available.
The ionization of a hydrogen atom begins with an electron incoming at a certain velocity. It interacts with the atom so that two electrons fly out at an angle to each other, leaving a proton behind. In their breakthrough calculations, the scientific team employed a transformation of Schrödinger's equation, which yields probabilities of finding particles in a certain state. Using computationally intensive processes, they extracted all the dynamic information of the particle interactions, which was then used to obtain specifics about the energies and directions of outgoing electrons.
In solving the problem, the collaborators noted that the large-scale computers they used, which are usually found tackling problems concerning very large and therefore very complex systems, had instead been used "to answer a basic physics question for one of the simplest systems imaginable in physics and chemistry."
Contact: Thomas N. Rescigno (925) 422-6210 (rescigno1@llnl.gov).

The Lab's contribution to an orbiting observatory

One of the world's most powerful x-ray telescopes, the X-Ray Multi-Mirror Newton Observatory (XMM), was launched in December 1999 by the European Space Agency. From space, the telescope has been sending back images and spectra of stars exploding and spewing out materials to form new stars, or twin stars chasing one another at such great speeds that they generate a dynamo that twists the stars' magnetic fields, causing intense stellar flares and storms.
The XMM has two spectrometers that split incoming x radiation into different wavelengths. The resulting x-ray spectra reveal the elemental makeup of objects being observed as well as the temperatures, densities, and velocities of the emitting material.
The images, which surpass even those possible using the Hubble Space Telescope, reveal giant Neptunian storms driven by prevailing winds of 1,800 kilometers per hour. Scientists are using information provided by the telescope to study the planet's storms and their evolution, a first step toward understanding Neptune's weather and climate.
Reflective grating arrays inside the spectrometers perform the x-ray splitting. They were designed, prototyped, and fabricated by Lawrence Livermore researchers. The arrays function like a glass prism that splits ordinary visible light into a rainbow of hues. They consist of 182 gold plates with extremely tiny grooves (on the order of millionths of centimeters) that must be precisely constructed and aligned. The plates must remain fixed to within 20 millionths of a centimeter in order to diffract x rays to a common detector. The delicate arrays must be mounted in the spacecraft in a way that can withstand the tremendous shock and vibration of launch without losing alignment.
"It was an incredible challenge to design and develop not only the reflective grating plates in the arrays but also the large, lightweight support structures that hold the grating arrays in the spacecraft," said Todd Decker, project engineer on Livermore's work for the XMM.
The work came to Livermore via Columbia University physics professor Steve Kahn, principal investigator for the reflection grating array, which is funded by NASA. Kahn came to Livermore for help because of the Lab's expertise in precision engineering.
Contact: Todd A. Decker (925) 422-2022 (decker1@llnl.gov).
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