Using X-Ray Lasers to See
Agreement reached on a Max Planck research group at the University of Hamburg for the Centre of Free Electron Laser Studies
Scientists have been unable to view the exchange of electrons between molecules because it all happens too quickly - until now. Researchers will soon be using extremely short and intense laser pulses of X-ray light, radiated by very fast free electrons, to observe chemical reactions. An example is the Free Electron Laser (XFEL) at the German Electron Synchrotron (DESY) in Hamburg. The Max Planck Society is establishing a research group at the University of Hamburg that will join forces with University and DESY working groups to form the Centre of Free Electron Laser Studies (CFEL). The Max Planck research group will investigate the potential and limits of the X-ray laser starting 2008. For this purpose, the group will receive from the MPS and the federal state of Hamburg approximately eight million euros each year for five years. This was decided by the Senate of the Max Planck Society today in Frankfurt am Main.
The X-ray laser that DESY is currently constructing will allow scientists completely new insight into the world of molecules and will contribute new knowledge to many scientific disciplines including medicine, biology, physics, atmospheric research, chemistry and materials science. "This is because the X-ray laser has very high time and space resolution," according to Professor Robert Schlögl, Chairman of the Chemistry, Physics and Technology Section of the MPS. He was a member of the commission that developed the scientific concept for the Max Planck research group at CFEL. This body also included Professor Jürgen Lüthje, President of the University of Hamburg, Professor Jochen Schneider, Research Director at DESY, Professor Joachim Ullrich, Director at the Max Planck Institute for Nuclear Physics and Professor Martin Stratmann, Managing Director at the Max Planck Institute for Iron Research.
The light from the X-ray laser flashes only for a few femtoseconds. A femtosecond is a millionth of a billionth of a second. Even light travels only a few micrometers in this time. The pulses allow the chemical reactions in which electrons regroup to be observed. They jump so quickly from one reaction partner to another that their trace becomes indistinct in longer light pulses. Lasers that fire femtosecond pulses already exist, but their energy is not sufficient to make electrons in the atoms of different elements visible.
X-ray laser pulses provide detailed images of molecules and the surfaces of solids because the wavelength of X-ray light is roughly equivalent to the distances between atoms in these samples. "As the pulses are also very intense, a single molecule might produce a diffraction pattern," said Schlögl. A diffraction pattern arises when X-ray light interacts with atoms. To generate the effect with conventional X-ray sources, scientists have to shine light through millions of molecules. Biomolecules in particular often have only tiny traces which they will in future be able to analyse with the CFEL.
In order to generate the special X-ray laser light, scientists must resort to special materials and methods: They "chase" electrons through a tube that is 3.4 kilometres long.. When the electrons have almost reached the speed of light, magnets force them to execute a slalom over the final third of the course. As they waver from side to side, the electrons increasingly gain energy - and must also release it - these are the X-ray pulses. A synchrotron accelerating electrons on a circular track also functions on the same principle. However, the scientists can persuade only slaloming electrons to emit laser pulses, that is, coherent and very intense light.
The Federal Ministry for Education and Research decided as early as 2003 to fund XFEL after the Scientific Counsel made a very strong case for the project. It is expected to cost a total of 900 million euros, 60 percent of which will come from the Federal Republic of Germany. The remaining 40 percent will be provided by the 13 European partner countries, which include France, England and Italy. German organizations using the X-ray laser for research will include the Max Planck Society, the Helmholtz Association, which owns DESY, and various universities.
The scientists at the Centre of Free Electron Studies (CFEL) will investigate how to exploit the full potential of XFEL. In addition to the Max Planck Research Group, two departments and a theoretical group from DESY, five working groups from the University of Hamburg and a detector group from DESY and the University will be working at CFEL. The Max Planck Research Group will include two scientific departments, two experimental junior research groups and one theoretical junior research group. Furthermore, several Max Planck Institutes will send scientists to be part of an Advanced Study Group, which will work at CFEL starting at the end of this year.
In a Max Planck Research Group, the MPS and the university in question work intensively together. The Research Group is, however, not part of the MPS but is part of the respective university in which the MPS participates. Correspondingly, the universities and the MPS select the heads of the research groups by mutual agreement - in the case of CFEL, DESY is also involved. The funding from the Max Planck Society for the research groups is limited to five years.
The Max Planck Research Group at the Centre for Free Electron Laser Studies will research the basic principles of spectroscopy with the X-ray laser. Although the outlook for an X-ray laser is very promising, scientists have limited experience with an instrument of this nature. They have been experimenting for some time with lasers which work on the same principle and deliver UV pulses, however, X-ray lasers are not yet available. It is, therefore, not yet clear how samples will react to the energy packages from the X-ray laser: "They create a plasma in the sample for a very short time," says Schlögl. In a plasma the electrons separate from the atomic nuclei. "We must first find out whether we will still be able to view the material in the same way afterwards," says Schlögl. The high-energy radiation could also destroy the molecules - at least when the pulse lasts too long. "Just recently an experiment revealed that molecules were destroyed if they are exposed to a pulse for longer than 30 femtoseconds." This means that the researchers have to ensure that the pulses are as short as possible.
Although such short pulses do not do any damage to the subject of the investigation, the scientists are immediately confronted with the next task: How is it possible to detect a light signal if it doesn’t even leave a trace in the molecule being investigated? The semiconductor laboratory at the Max Planck Institutes for Physics and Extraterrestrial Physics is addressing this problem. The researchers at this laboratory, which equips satellites with detectors, have already constructed the first detectors for particularly high-energy X-ray light. "The detectors work on the same principle as a CCD camera," says Schlögl: light triggers an electrical impulse in the detectors by generating an electron cloud in the semiconductor. However, the detectors need to be even faster and more sensitive for XFEL. For this too the scientists already have an idea: "We will work with metastable electron clouds." These trigger a signal with a pulse of just a few femtoseconds; it is this principle that ensures the excellent "vision" of satellites. However, the detectors for XFEL are set to become even better. "Astrophysicists could benefit from this as well," explains Schlögl.
2009 will see the first experiments by the scientists in the new Max Planck Research Group. These will not be carried out on the free electron laser in Hamburg, however, but on SLAC, the first laser of its kind, which is currently under construction in Stanford, USA. In return, the German researchers will make a detector available to the American scientists. XFEL will probably not emit its first light until 2012, but it will radiate shorter, higher-energy and more intense pulses than SLAC. A few years later, an X-ray laser on the BESSY synchrotron in Berlin will produce even shorter pulses, although these will not be as energy-rich. "The X-ray laser on BESSY is therefore even more suitable for investigating chemical reactions," says Schlögl: "XFEL will therefore yield more information about the structure since it produces harder, higher-energy X-ray radiation."