Proton dynamics
Protons seen moving at record resolutions
`It's like chopping up the 630 million kilometres from here to Jupiter into pieces as wide as a human hair,' comments Dr John Tisch of Imperial College's Laser Consortium.
Last week a team of physicists combining laser scientists from Imperial College and the Max Planck Institute for Nuclear Physics in Germany revealed that they have pushed back the time scale boundaries with which the motion of nuclear particles can be tracked.
The ground-breaking technology has increased the dynamical resolution to once every 100 attoseconds. An attosecond is one billion billionth of a second.
The protons have been seen to travel at an average speed of 1.33km/second.
Although the protons cannot be seen directly a hypothetical `snapshot' is taken at every 100 attosecond interval. These snapshots are actually emitted x-ray photons.
A laser fires extremely short pulses of light at a molecule in order to tear out an electron with extremely high power and then fire it back in the manner of an `electron boomerang'. Energy is flung back out of the molecule in the form of x-ray photons when the electron returns to its original position.
Meanwhile the molecule in deficit of an electron attempts to rearrange itself into its new equilibrium position (see figure).
The return of the electron stops it in its tracks and the resulting photons reveal to the scientists how much the protons moved in this time interval.
The experiment relies on the fact that the intensity of emitted light will vary according to the position of the nucleus components.
The method can be viewed as a `pump-probe' technique. The laser `pumps' an electron out of the molecule and the emitted photon's provide a `probe' into the instantaneous positions of the nucleons.
The scientists used a clever trick of measuring two different isotopes to get around the problem that factors other than the proton dynamics can affect the intensity of photon energy emitted.
For example, H2 and D2 differ only by a neutron thus by measuring the difference in their results the nucleon dynamics alone can be isolated. In the future the team would like to eliminate the need for this comparison.
Previous work by scientists in Ottawa could only pin down the proton positions at intervals of 1 femtosecond but this new research slices this time interval into 10 more segments.
The key to the accuracy of the new technique lies in the `chirped' nature of the emitted light. Chirping can be thought of as temporarily spreading out different frequencies in the laser pulse.
The research opens the door for even more exciting developments in molecular science. Once we know how the proton moves, it is only a matter of time before the scientists can control the movements themselves.
`Control of this kind underpins an array of future technologies, such as control of chemical reactions, quantum computing and high brightness x-ray light sources for material processing,' says Prof John Marangos, Director of the Laser Consortium in the Blackett Lab.
The research will allow greater understanding of exactly how protons respond to ionisation, a subject about which little is known.
Current spectroscopic methods do not reveal how a molecule rearranges itself on a nuclear level. Lead author of the original paper Dr Sarah Baker comments:
"We are very excited by these results, not only because we have `watched' motion occurring faster than was previously possible, but because we have achieved this using a compact and simple technique that will make such study accessible to scientists around the world."
