No rose-coloured glasses here

What do you associate with the Day of Light?

Light is essential for life, and for researchers it is also a wonderful tool for understanding materials.

At the X-ray free-electron laser SwissFEL, you work with X-rays, which we can't see with our eyes. Is "light" really the right term for this?

For a physicist there is no difference between visible and invisible; we also refer to infrared and ultraviolet, as well as X-rays, as light. Our detector – the eye – is precisely tuned to the wavelength spectrum of the sun. For us, SwissFEL is something like a very precise and powerful sun for which we use appropriate detectors, just with shorter wavelengths.

What makes X-ray light so valuable for research?

Under a microscope, you can discern objects that are not any smaller than the wavelength of the light. With X-rays, SwissFEL’s  wavelength is very short and corresponds roughly to the distance between atoms. So with that we can make out atomic structures; visible light would be much too coarse. That sounds simpler than it turns out to be in practice. It is important that the X-ray light should have excellent quality in terms of intensity, focus, and coherence. Coherence means that all of the light waves oscillate in phase with each other. You can imagine it like a tandem bicycle on which the riders not only pedal at the same speed, but also always have the right pedal up at the same moment.

There are several experiment stations at SwissFEL. What are they there for?

All X-ray light is not the same. We distinguish between hard X-rays with high energy, which we generate in the Aramis beamline, and soft X-rays with low energy, which we generate in Athos. We feed three experiment stations with Aramis, where we use the very short wavelength to investigate the atomic structure of matter. Athos, on the other hand, delivers lower-energy X-ray light, which is ideal for spectroscopic studies. In addition, with Athos we have developed special systems, called CHIC and APPLE-X, that allow us to manipulate the electron beam of SwissFEL and generate FEL radiation "à la carte" with unique properties. This is unique to Athos, and it enables us to carry out our research under optimal conditions.

What exactly are you using it to investigate?

In spectroscopy, we are looking at the electrons in the atoms. They are responsible not only for the chemical behaviour of substances, but also for electronic properties of matter such as superconductivity. The soft X-rays from Athos are ideal for studying the electronic structure of atoms and matter. So we can study chemical states. A special feature is that SwissFEL provides us with pulsed X-ray light. The pulses are incredibly short, in the range of femtoseconds or even less – that is, a millionth billionth of a second. This is exactly the time span in which electrons jump from one state to another and carry out a chemical reaction. We can look at chemical reactions in real time, just as if we were shooting a "molecular movie" with an extremely fast high-speed camera. Incidentally, this is a capability that only an X-ray free-electron laser like SwissFEL can offer. It is therefore an important complement to synchrotrons, which achieve a time resolution ranging from milliseconds to nanoseconds.

What is the status of work on the Athos beamline?

We are currently setting up two experiment stations there: Maloja and Furka. Simply put, Maloja is responsible for research on atoms, nanoparticles, and molecules in liquids and gases, while Furka concentrates on solids at very low temperatures, a few degrees Kelvin above absolute zero. We saw the first X-ray light in Maloja last year and have already carried out some test experiments. For example, we shot noble gas into the X-ray beam, which, with its very high intensity and energy, strips almost all electrons from the atoms of the gas through a so-called multi-ionisation process. The first experiments with external users should start this summer. Furka is still under construction, and we will put this experiment station into operation in the summer.

In light of the restrictions due to the coronavirus pandemic, how did you manage all this, and how do you want to run the experiments when the scientists have to adhere to hygiene rules?

We have learned a lot about that over the past few months. Usually four to six researchers work at one end station, and at times there are researchers from external partners as well. You can't all be in the same room at the same time. But the experiment stations are set up in such a way that the only manual operation is exchanging the samples. You can just as easily control the experiments from your kitchen table, for example starting and stopping the experiments or turning and adjusting the sample. All of this is computer-controlled.

Back to UNESCO's Day of Light: The motto this year is "Trust in Science." How important is the public's trust in science?

Very important. At PSI, we don't set ourselves apart in an ivory tower or view the world through rose-coloured glasses – we provide useful answers for society. To do this, we have to be transparent and show the relevance of our research to solving societal challenges. When the coronavirus became an issue, for example, we very quickly launched a Covid-19 research programme. But we also contribute to the future with our normal research. For example, researchers at PSI have observed molecular processes in bacterial cells in action, which promises progress in the development of new methods in neurobiology. And colleagues in my laboratory have developed a new method for tracking thermal and magnetic changes inside solids. This is of interest, for example, to achieve higher storage density in computer hard drives. In the future, this could also help with the development of quantum computers and many other technologies. SwissFEL is a kind of Swiss Army knife for light – universal, high-performance, and versatile, like no other research facility in the world.

Text: Bernd Müller