Materials Today

Volume 15, Issue 5, May 2012, Pages 188


Capturing the movement of atoms: Tools and Techniques

  • Nina Notman

US researchers have ‘filmed’ the motion of vibrating nuclei in molecules with the help of an ultrafast camera. Multiple snapshots of the structural changes that oxygen and nitrogen under go when ionised were taken at time intervals of just a few femtoseconds (10−15 S).

The work, published in Nature [Blaga C. I., et al., Nature (2012) doi: 10.1038/nature10820], utilises the technique of laser-induced electron diffraction (LIED). The first step involves ionising a diatomic molecule using an ultrafast laser. The laser knocks a single electron out of the outer shell of one of the molecule's two atoms. And the freed electron initially flies away from the atom, before turning around and coming back to hit it.

The scattered signal of the electron as it recollides with the moving ionised molecule is then recorded. Because the scattering, or rather diffraction, pattern depends on the location of the atom's two nuclei, it is possible to determine the exact location of them both at the time of collision.

To make a video, multiple images taken at different timeframes are needed. To do this, the wavelength of the laser was changed. “In our LIED, the advancement of time is provided by changing the wavelength of the [laser]. The time at which the image is taken is directly proportional to the wavelength, thus increasing the wavelength captures the image at a later time,” explains team leader Louis DiMauro from Ohio State University.

Full-size image

The principle of LIED.

Courtesy, Cosmin Blaga, Ohio State University.

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The technique is so precise that even nuclei movements of 0.1 angstrom can be detected; meaning a molecule's movement over time can be captured in unprecedented detail.

“Ultimately, we want to really understand how chemical reactions take place,” says DiMauro. He also wants to move on from just assessing diatomic molecules to studying the reaction between more complex molecules, but he notes that this is a big step to take. “Looking at two atoms – that's a long way from studying a more interesting molecule like a protein.”

Additionally, DiMauro is planning to use the technique to control reactions on an atomic scale. By adjusting the laser that launches it, it is possible to control the trajectory an electron takes as it comes back towards the molecule. “The next step will be to see if we can steer the electron in just the right way to actually control a chemical reaction,” he explains. “Demonstrating this control will probably take a few years but our initial work provides a roadmap.”