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Filming molecules of hydrogen

German researchers have shot photos of a molecule of deuterium using ultrashort laser pulses. In order to do this, they've built a very special camera with an average laser pulse duration of only six to seven femtoseconds. The scientists think these lasers will help them to control the chemical reactions of larger molecules.
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Written by Roland Piquepaille, Inactive on

I doubt that you'll be able to take pictures of hydrogen molecules with your digital camera anytime soon, but German researchers at the Max Planck Institute for Nuclear Physics have done it. They've shot photos of a molecule of deuterium using ultrashort laser pulses. In order to do this, they've built a very special camera with an average laser pulse duration of only six to seven femtoseconds. The scientists think these lasers will help them to control the chemical reactions of larger molecules. But read more...

Before going further, let me refresh your memory about what is a femtosecond. It doesn't last very long: only 10-15 second or one billionth of one millionth of a second. And in the seven femtoseconds "exposure time" that these researchers have reached, light travels only two thousandths of a millimeter.

So what have done these scientists at the Max Planck Institute for Nuclear Physics?

To shorten the "exposure time", researchers at the Max Planck Institute for Nuclear Physics developed pump-probe apparatus with an average laser pulse duration of only six to seven femtoseconds, allowing molecular motion to be measured continuously for the first time.

Below are two images taken by this very special camera. On the left, you can see the development of the wave packet after 400 femtoseconds. And on the right is a snapshot of the hydrogen molecule at the same time. (Credit: Max Planck Institute for Nuclear Physics)

Images of a molecule of deuterium over time

This image has been extracted from a short movie showing "the quantum mechanical wave patterns of a vibrating and spinning molecule." (AVI format, 37 seconds, 3,73 MB) (Credit: Max Planck Institute for Nuclear Physics)

Now, are you ready for some heavily technical details?

For the measurement, the researchers used deuterium molecules, a compound of two heavy hydrogen atoms. They are not energetically excited, and are therefore in the quantum mechanical ground state. The first pump laser pulse removes an electron from a deuterium molecule and it is ionised. Adjusting to the new situation, the two nuclei of the ionised deuterium molecule move further apart and vibrate around a new resting position. The pump pulse also makes the molecule rotate.
With the subsequent probe laser pulse the scientists remove the second electron from the molecule; as there are now no more electrons available for fusion and the positively charged nuclei repel each other, the remains of the molecule "explode"; the closer the two nuclei are to each other when the second ionisation takes place, the more violent the explosion. Using a "reaction microscope" which they developed some time ago, the researchers measure the energy of the two deuterium nuclei from which they calculate the distance between them and their positions at the moment of explosion.

In terms of photography -- and even of optics -- this certainly is a milestone. But what will it be useful for?

The scientists want to manipulate and control the chemical reactions of larger molecules in this way. Experiments of this kind are already being carried out on methane molecules in the laboratory in Heidelberg.

For more information, this research work has been published in Physical Review Letters under a very evocative title, "Spatio-Temporal Imaging of Ultrafast Molecular Motion: 'Collapse' and Revival of D2+ Nuclear Wave Packet" (Volume 97, Number 19, November 6, 2006). Here are two links to the abstract and to a preprint version of the full paper (PDF format, 14 pages, 480 KB).

Sources: Max Planck Society news release, via EurekAlert!, November 8, 2006; and various websites

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