Watching electrons move within an atom

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 in Physical 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.

Rydberg states meet X-ray lasers

We all know the cartoon picture of an atom: planet-like electrons zooming around nucleus in loops, creating a sort of star-shaped pattern. However, real atoms are defined by electron quantum states, which may be spheres, squashed spheroids, or somewhat fantastical shapes like barbells and doughnuts. These quantum states dictate the interactions among atoms, electrons, and photons; as such, they determine the chemical and optical properties of matter.

That's why many scientists are interested in the dynamics of electrons within atoms—knowing where and how quickly electrons move. As is the case with many quantum systems, the trick lies in getting the measurements, even beyond intrinsic limits defined by the Heisenberg uncertainty principle. Performing experiments requires introducing other particles that interact with the atoms, which can alter the systems we want to study. For that reason, the authors of the new paper propose several workarounds such as using Rydberg states of noble gas atoms and probing them with X-ray lasers.

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.

In their paper, the authors used computer calculations to simulate the behavior of argon atoms in Rydberg states from first principles. They were aided by the fact that the outermost electrons would be nearly free, which simplified the calculations greatly. Then they simulated the patterns that would result from scattering X-ray photons of various wavelengths off the atoms, including how the electron states evolved in time. (See the video below for an example from these simulations.)

The diffraction pattern produced by the X-rays depends on the quantum wave structure in the atom, so as the electrons fluctuate between energy states, so does the pattern. That would enable researchers to see changes in electron behavior in both space and time—an exciting possibility.

To get it right, however, we would still have to correct known problems in preparing Rydberg states and using X-ray lasers, but modern methods have come some way toward addressing these issues.

While Rydberg states aren't representative of most atoms under ordinary circumstances, the ability to effectively create movies of electron behavior in real time would be a significant advance. It could assist in the design of new photovoltaic materials and in understanding many other systems that involve the interaction between light and matter.

Physical Review Letters, 2013. DOI: 10.1103/PhysRevLett.112.043002  (About DOIs).