Entangled view of vacancies

Electronic Materials

June 25, 2008

What was the initial location of the electron before it was ejected from the dumbbell-shaped N2 molecule by the high-energy photon (blue)? Diagram shows the probability distribution of the photoelectron in the case of its prior localization (left) or for the Auger electron (right). As both electrons form an entangled state, the Auger electron is also localized. (Credit: Markus S. Schöffler.)

Researchers believe that they may have resolved the long standing debate between physicists and chemists about what happens to a vacancy created by a high-energy photon [Schöffler et al., Science (2008) 320, 920].

When atoms form a molecule, the outer shell valence electrons are shared – or delocalized. But what about the core electrons surrounding the nuclei – are they localized or delocalized? Researchers from Johann Wolfgang Goethe-Universität in Germany, State University of Aerospace Instrumentation in Russia and Auburn and Kansas State Universities set out to explore this issue by creating a valence in N2 using the synchrotron radiation produced by the Advanced Light Source at Lawrence Berkeley National Laboratory.

The synchrotron creates a high-energy photon that, when absorbed by the N2, ejects a photoelectron from the inner core shell of the molecule, leaving behind a vacancy. When a valence electron drops down to fill the vacancy, an Auger electron is emitted. By detecting the Auger electron, the researchers can tell whether this short-lived vacancy remains in the vicinity of one nucleus – is localized, or spreads out along the molecular axis – is delocalized.

As is so often the case at the quantum level, the answer depends on how you look at it. Depending on the direction in which the photoelectron or Auger electron is emitted, the state of the vacancy can be thought of as either localized or delocalized. This strange result arises from quantum entanglement – the behavior of the photoelectron and the Auger electron is interdependent – and is the first demonstration of the effect in electrons.

In the end, say the researchers, the answer as to whether a vacancy is localized or not can only be determined by considering the whole system.

In the future, the researchers plan to apply the approach to other systems. “I think it would be very interesting to compare the results in larger homo nuclear systems as fullerenes,” Markus S. Schöffler told Materials Today.

Cordelia Sealy