Time for resolution: Single molecule

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  • Published: Sep 1, 2014
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Time for resolution: Single molecule

You say you want the resolution

Seeing a single molecule vibrate through time- resolved coherent anti-Stokes Raman scattering Credit: Steven Yampolsky et al/Nature

For the first time, chemists have succeeded in measuring the vibrational movements of a single molecule with time resolution on the femtosecond scale using time-resolved coherent anti-Stokes Raman scattering. The work shows that the vibration of a single molecule can differ wildly from the behaviour of larger mass of molecules, it also led to the development of a new method for identifying single molecules by optical means.

Eero Hulkko of the University of Jyväskylä, Vartkess Apkarian of the University of California, Irvine, and a team led by Eric Potma report details of their findings and the new technique in the journal Nature Photonics. They used femtosecond visible laser pulses to see the motion of individual molecules of the organic compound bipyridylethylene (BPE) indirectly as the scattering of the light pulses in time-resolved coherent anti-Stokes Raman scattering (tr-CARS). The technique requires 90-nanometre gold nanoparticles to act as "plasmonic nanoantennae" to amplify the signals from the molecule to a detectable level.

One molecule at a time

"Detecting one molecule through scattering of light is extremely difficult”, explains Hulkko, "which is why we needed to amplify the signal." He explains that the vibrations of a single molecule are governed by the probabilistic rules of quantum mechanics. In order to be detectable using tr-CARS, the molecule must exist in two, or more, quantum states simultaneously; it's almost the classic metaphor of Schrodinger's sarcastic cat incarnate. In this coherent superposition of vibrational states, a wave packet, phase correlation can be observed; that is something that is lost, through dephasing, when one has molecular clusters.

Use the key, unlock the door

The key to success in the current work was that the team could create a wave packet of the single molecule vibrations and monitor it for 10 picoseconds and show that the time-dependent motion of the wave packet corresponds to the actual vibrations of the molecule. Their tests essentially prove that the "breathing" vibrations of a single molecule do not lose their phase correlation. Then, using simulations, they could explain the observations and show that pure dephasing is a characteristic of a clutch of molecules rather than a single molecule. The fact that a single molecule does not undergo dephasing was previously not fully appreciated.

"Observing the breathing of a single molecule takes us one step closer to seeing real chemistry at work at the level of a single molecule," Apkarian suggests. The technique opens up new possibilities for quantum computing using molecules and quantum information transfer. Fundamentally, single molecule observation means individual photon observations.

"The next step will be the optimization of the method to allow for more and longer measurements. This will make it possible to more precisely study the fluctuations of the molecule. Ultimately, we would like to be able to follow the making or breaking of a chemical bond on a single molecule as seen from the perspective of the vibrating chemical bond itself," Potma told SpectroscopyNOW.

Related Links

Nature Photonics, 2014, online: "Seeing a single molecule vibrate through time- resolved coherent anti-Stokes Raman scattering"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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