Advanced photons in science

by David Bradley

David Bradley worked with Argonne National Laboratory on a series of articles for the annual report of the ANL's Advanced Photon Source. Elemental Discoveries presents a sneak preview of what's on offer reporting on how X-rays can reveal the natural world's inner secrets.

Are films ferroelectric? | Gold nanocrystals | Photosynthetic system | Catalytic clues | SAXS and the water channel Digging in the dirt | Folding Protein Sensors | X-ray movies

Dissecting the atom

When a photon with low energy hits an atom it can usually knock just one electron from the atom. Higher energy photons, such as hard x-rays, on the other hand, can knock out two or more electrons at a time. Jon Levin and Brad Armen of the University of Tennessee, at Knoxville are working with Linda Young, Elliot Kanter, Bertold Krässig, and Stephen Southworth of the Argonne National Laboratory using the BESSRC-CAT wiggler beamline 11-ID- D at the Advanced Photon Source to knock out electrons from krypton atoms to help them understand the fundamental physical processes of ionization and decay that can occur in atoms.

There are numerous processes that can occur, such as electron excitation to different empty levels in the atom, the recapture of electrons before they escape an ion's domain, and the 'bubbling' up to the outer shells of empty spaces where an electron might otherwise reside in an atom known as holes. These various phenomena provide different routes to the residual ion, which might ultimately have lost one or two or even all of its original electrons.

Using this beamline, the ANL and Knoxville team is delving into the depths of an atom's electron shells to discover what happens when high-energy photons ionize the atom. The charge on such ions depends strongly on the energy of the x-ray photons. Moreover, the properties of the ion can reveal new information about how atoms are organized and might one day lead to new applications in laser optics for medical and analytical applications.

The researchers have studied the effects of x-ray photons on atoms of the noble gas krypton, which provides a replenishable, non-corrosive target of non- interacting atoms. Krypton has atomic number 36, and so there are 36 electrons in various shells around the atomic nucleus. An x-ray photon hitting a krypton atom can eject an electron to produce a krypton ion. This residual krypton ion also has a hole in a deep inner shell, which means it is in a highly energetic state and can release the absorbed photon energy via several paths. For instance, the hole can bubble up through the atom's energy levels triggering the loss of additional electrons. These various energy decay paths involve a combination of radiative (fluorescence) and non-radiative (Auger) processes. The researchers hope their studies will ultimately lead to a predictive and quantitative map of these decay paths.

Earlier research examined the ion charge state distribution with the incident x-ray energy close to the so-called K-edge. The K-edge is the energy required to eject the deepest electron, a 1s electron, from the atom's innermost shell). Near the vicinity of the K-edge, it is possible for the 1s electron to be excited to a vacant orbital, such as the "5p" (6p, 7p...); the researchers anticipated that the decay paths would be different here. In this earlier work, the researchers monitored the ion charge state for events where K-fluorescence occurred.

Their current work builds on this with a new method that can allow them to observe individual decay paths in isolation. They have now better defined the decay path using a high-resolution x-ray detector, which can distinguish between decay paths proceeding through a 2p or a 3p hole.

The new experiments are feasible because the researchers are using a coincidence technique between fluorescence x-rays and ions. The Figure shows a map of the decay paths they obtained using this technique. On the basis of these results, they have developed a theory - the "spectator cascade decay" (SCD) model - to explain their findings. In the SCD model, an excited electron acts as a spectator to the decay of the krypton ion's core. The model provides the means to predicting ionization effects, not only for krypton, but for many other atoms, which takes us one step closer to understanding complex vacancy decay patterns in atoms. The understanding of such decay phenomena may suggest techniques to control or prevent the decay of highly energetic states, which could be exploited in a new type of laser that uses higher photon energy. Such lasers could have a variety of biomedical and analytical applications.