Camera for the fastest films
The complex is several kilometers in size and the object of interest is about one tenth of a billionth of a meter. This is what the dimensions look like when researchers want to observe how atoms react to laser beams.
In the textbook, everything is rigid. Atoms sit as solid balls in well-defined places and build an orderly lattice or molecule. In theory, the electrons whiz around on the energy surface of quantum physics and wriggle atomic nuclei around statistical rest positions. In practice, nobody has ever seen whether all this really happens like that.
The problem is that the world of atoms is monstrously small - say in the order of 0.000 000 000 1 meter - and even monstrously fast. To express how quickly molecules adapt to stress, we need seconds with about eleven zeros after the decimal point. Another zero is added for the vibrations of atoms, and the collision of two electrons in solids requires another zero. Extending the period of a normal second to the age of the earth (roughly 4.6 billion years) for size comparison, the actions of the atoms would take place on the order of hours.
Scientists have been getting along wonderfully with the tiny dimensions for several decades. Their secret recipe is X-rays. Their wavelengths are so short that they already react to individual atoms and produce strange spot patterns on the detector behind a sample, from which specialists can determine the mean position of the atoms. We owe this technique numerous colorful models of molecules, crystals and whole viruses. So far, however, research has not been on the heels of the ultra-fast era with atomic precision. But that could soon change. A large team led by David Mark Fritz from the Stanford Linear Accelerator Center has now solved the problems that make it so difficult to take a quick look at the smallest details.
On the one hand, the scientists faced the problem of generating sufficiently strong and short X-ray pulses. A common source of this is electron beams, which are forced into curved paths by magnetic fields in a device called an undulator, emitting radiation of all kinds. The electrons, on the other hand, usually come from huge circular particle accelerators, but their quality was not good enough for the upcoming ultrafast measurements. Instead, the team used the Stanford Linear Accelerator Center's three-kilometre linear accelerator-a prototype that brought the breakthrough in electron quality.
However, the reliability of the X-rays was lacking. It is not possible to predict exactly when this will appear. The pulses arise spontaneously and are therefore somewhat random. However, if you want to observe how the atoms "do something", they must of course do it at the right time - i.e. when the measuring X-ray pulse is on its way. The right synchronization was required, in the space of thousandths of a billionth of a second.
Fritz's physicists finally used the disadvantage of the unreliable pulses to their advantage for their measurements. Instead of wanting to start the X-ray bombardment at a fixed time, they just let it happen on its own whim - and thereby indirectly controlled the other parts of the experiment: They used an electro-optical crystal that changes its properties when the electron beam passes. While the latter triggers the X-rays shortly afterwards, a laser registers the reaction of the crystal and transmits the signal to the actual experimental setup around a hundred meters away. In this way, with a long series of pulses, they obtained as many still images of slightly different states of the sample. Put in the correct time sequence, this results in a movie of the atomic movements.
In their first experiments, the scientists followed the reactions of bismuth atoms when their electrons were excited with laser light. A comparatively unspectacular start in view of the projects with which researchers will soon be knocking on the door. How chemical reactions take place, what superconductors do at the atomic level, how thin films are deposited and what tricks are used in the first steps of photosynthesis can only be really understood in moving images. The camera for it is finally available.
