Can scientists image the motion of electrons inside atoms? It's a challenging problem, but solving it would help us develop a more complete understanding of things like chemical reactions and the interactions of light and matter. So researchers are using a variety of techniques to probe the internal structure of atoms, seeking to test theories and find new, potentially interesting phenomena.
The latest in this line of work is an effort by chemists Henri J. Suominen and Adam Kirrander, who propose using X-ray lasers to study the electron dynamics in noble gas atoms. In a new paper inPhysical Review Letters, they outline how this process should work: exciting the electrons into energy states where they are weakly bound to their atoms, then scattering specially prepared X-ray photons off those atoms. Studying the scatter pattern should allow researchers to reconstruct the electron dynamics in some detail.
While this proposed experiment will not be easy to perform and is sensitive to known issues with X-ray lasers, it could also lead to direct measurements of electron motion inside atoms—a significant accomplishment.
The figure shows a visualization of an argon atom in a Rydberg state (three spheres) at different points in time, after bombardment by X-ray photons. The circular patterns at left are those formed by the X-rays after they scatter. The three images in sequence show the evolution of the atom in space and time.
Rydberg atoms have the electrons in their outer layers excited until the electrons are only weakly bound to the nucleus, making the atoms physically very large. The increased size allows light to scatter off the outermost electrons without much interference from the nucleus or from the inner core of electrons. In other words, it's a way to isolate the electron dynamics from other messy phenomena. Noble gases like argon are particularly useful for this, since they are largely non-reactive chemically and relatively easy to model theoretically.
Electrons in Rydberg states also have much slower reaction times: picoseconds (10-12 s, or trillionths of a second) as opposed to femtoseconds (10-15 s) or less: still really short, but a factor of a 1000x improvement is nothing to sneeze at.
That leads to the second aspect of the proposed experiment: using X-ray lasers, which interact with the electrons on shorter time scales than their reaction times.
X-ray lasers (including the Linac Coherent Light Source [LCLS] at Stanford's SLAC laboratory) are highly tunable, producing any of a variety of wavelengths in controlled bursts of photons. An X-ray laser can capture electron behavior in both space and time, minimally disturbing the atoms in the process. That's in contrast with infrared or other types of light, which can strongly interact with electrons, changing the experimental outcome.