Monday, October 09, 2017

Caught in the Act: The Quick Escape of Electrons

When hit with an energetic particle of light, an electron orbiting the nucleus of an atom can break free in less than one quadrillionth of a second. Exactly what happens during this fraction of a second is difficult to capture, but there is a lot to be gained by doing so. Mapping the interactions between an escaping electron and the other particles inside of an atom will bring us closer to being able to control the behavior of an electron or other subatomic particle inside of an atom—and maybe even bring us closer to creating new states of matter.

In research published last week in the journal Nature Physics, scientists from The Ohio State University and the University of Virginia present an important step forward in this quest. Led by Dietrich Kiesewetter (Ohio), Louis DiMauro (Ohio) and Bob Jones (Virginia), the team modified an existing method for studying interactions involving free electrons—electrons not bound to a nucleus—so that it could be used to study electrons on their way "out the door". Using this modified method, they were able to measure how an electron’s momentum changes over the fraction of a second it takes to escape a host atom.

This cartoon illustrates the RABBITT+ method. An attosecond pulse (shown in blue) shines from right to left onto the atom. The atom-photon interaction, contained in the bubble, releases an electron (purple) that moves from left to right toward the detector (not shown). The physics being probed is at the earliest time within the bubble. The red wave depicts an infrared optical field with characteristics tied to the attosecond pulses.
Image Credit: Bob Jones, University of Virginia.
When a photon is absorbed by a bound electron, the electron gets a boost in momentum, the so-called Einstein photoelectric effect. In the simplified version of this story, the change in momentum is directly related to the time it takes the electron to leave the atom—the larger the momentum, the greater the speed. Therefore, by measuring the energy of an emitted electron (which is closely related to its momentum) you can determine the time it took to escape. This sounds relatively straightforward, but the reality is not quite so simple.

The length of time it takes an electron to escape from an atom depends not only on the momentum boost, but also on the type of atom and where the electron is located within the atom. These last two factors govern the forces an electron feels from the nucleus and other electrons inside of the atom. These forces slow down and speed up the electron along its escape route, becoming less influential as the distance between the electron and particles grows. In order to see these changes, the escape needs to be studied in time intervals of attoseconds. An attosecond is just 1×10−18 seconds—a billionth of a billionth.

In this new research, the team shined light of different energies on a gas of atoms, ranging from light with an energy close to the threshold required for an electron to break free to light with an energy far beyond the threshold. The electrons emitted by near-threshold-energy photons are most sensitive to the internal forces. By comparing the energy of the ejected electrons in the two cases, the researchers were able to determine the changes in momentum that an electron experience along the way due to these internal interactions.

The technique they used relies on a set of closely spaced attosecond pulses of extreme ultraviolet light, resulting in a train of closely spaced electrons emitted from atoms in a gas. The information collected by the experiment isn’t in a one-to-one ratio, meaning that you don’t get an individual measurement for each electron. Instead, you get a continuum of information with a certain pattern. This pattern reflects the average momentum and change in momentum for an electron in attosecond intervals of time. This information can then be tied back to the electron’s path, reflecting the characteristics of the internal interactions.

Using this technique, dubbed RABBITT+, scientists can now experimentally study questions like, “How does an electron emitted from a helium atom differ from one emitted by a neon atom?” With the results, they can start mapping the details of interactions that happen over unbelievably tiny time scales in the equally tiny space occupied by atoms. These scales may be hard to fathom, but the reality is that the processes taking place within them contain information on one of the most fundamental and important interactions in our world—the way light interacts with matter.

Kendra Redmond

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