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Electron timed in hop between atoms

July 20, 2005 By Mark Peplow This article courtesy of Nature News.

Clocked speed should shed light on chemical theories.

Life moves pretty fast if you're an electron, and scientists have now timed one as it hops from one atom to another.

Given that the movement of electrons underlies all chemical reactions, the team hopes that its technique could help to design catalysts or even speed developments in a futuristic form of computing known as spintronics.

Wilfried Wurth, a physicist from the University of Hamburg, Germany, and his team used short X-ray pulses to watch an electron moving away from a sulphur atom stuck to a ruthenium metal surface. They saw that it took just 320 quintillionths of a second to make the jump, that's 320 attoseconds or 320*10-18 seconds. The speed fits with theories of electron movement.

Fast work

Researchers have managed to measure even quicker electron processes in the past, such as the movement of an electron within a single atom. "The electron of the hydrogen atom takes just 150 attoseconds to make it around the nucleus," says Alexander Föhlisch, part of the Hamburg team.

But this is the first time they have spied on an electron moving between two different atoms that are tightly bound to each other. Watching this sort of process has much more relevance to real-world chemistry, says Föhlisch.

"You're looking at the building blocks of chemistry in real time," says Peter Knight of Imperial College London, who uses very short laser pulses to freeze-frame electrons in similar ways.

Shifting clouds

Watching an electron move is tricky, given that electrons are not single objects. Instead chemists think of them as existing within clouds of probability, and have to track such 'clouds' as they move between atoms.

To do this, the team used X-rays to boost the energy of an electron close to the core of a sulphur atom. This pushed the electron towards the ruthenium surface, leaving the remaining electrons in the sulphur atom to shuffle around. By watching how the X-ray pulses were absorbed, the researchers tracked how long it took for the electron's cloud of probability to shift entirely from the sulphur to the ruthenium. They report the results in Nature1.

The team chose to study a sulphur atom because the element is notorious for clogging up catalysts based on metals such as ruthenium. Sulphur in gasoline can deactivate the metals in a vehicle's catalytic converter, for example.

Catalysts and computers

Chemists use computer simulations to predict how catalysts will behave, so measurements of electron motion may improve these models and help to build better catalysts, says Knight.

The scientists have added to their technique by using a polarized X-ray wave that rotates as it travels, in order to excite electrons that 'spin' in a specific direction.

Föhlisch thinks that electrons with different spins will move through magnetic materials at different speeds. Clocking these speeds is important for scientists working on 'spintronic' computing, in which the binary data in your computer is carried in the spin state of electrons, hopefully leading to faster, smaller processors.

"Of course it's also just fun to know how fast the electron actually goes," laughs Föhlisch.


  1. Fohlisch A., et al. Nature, 436. 373 - 376 (2005).


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