Through several grants from the Carlsberg Foundation, it has been possible to establish a unique world-class facility for research on molecular ions with radiation from the Danish synchrotron ASTRID2. The facility is now operational and will start to deliver results in a broad spectrum of research fields, ranging from fundamental questions in atomic physics, to energy flow in biological molecules and processes of importance in climate science. In the future, the facility will also become available for international users.
By Lars H. Andersen, Dr. Scient, Professor, Head of Institute, Department of Physics and Astronomy, Aarhus University & Henrik B. Pedersen, PhD, Associate Professor, Department of Physics and Astronomy, Aarhus University
ASTRID - a Danish Accelerator Facility
Denmark only has a few large-scale research facilities and Danish scientists often travel abroad to carry out research, for instance to CERN in Switzerland or DESY in Hamburg.
The Accelerator facility ASTRID (Aarhus Storage Ring In Denmark) at Aarhus University is the exception. Here, scientists from around the world come to Denmark and perform their research. Today, the most recent generation of the facility, ASTRID2, is a unique light source in the so-called extreme ultra violet (XUV) region of the spectrum.
The ASTRID accelerator facility at Aarhus University with AMOLine and the XRING laboratory. Electrons are injected into ASTRID, where they are accelerated to 580 MeV before transferred to the ASTRID2 synchrotron. Inside ASTRID2, synchrotron radiation is generated in an undulator and send through AMOLine for wavelength selection with a monochromator. The resulting light from AMOLine is used for ion research at three different stations in the XRING laboratory.
Adapting to New Scientific Challenges
Since the initial construction in the early 1990s, the ASTRID facility has progressively adapted to meet new scientific questions. In its first decade, ASTRID could uniquely accelerate heavy ions and electrons. The special ability to accelerate and store ions made ASTRID a world-leading laboratory for studying the properties of small molecules and clusters: ground-breaking results were obtained on interactions between electrons and ions, on internal energy flow in molecular systems, laser cooling, and negative ions.
Gradually, the scientific focus of ASTRID shifted towards the use of accelerated electrons emitting high energy synchrotron radiation. In 2009, the construction of a new facility began, and only three years later, in 2012, ASTRID2 delivered light for the first time.
The ASTRID2 facility was set up parallel with huge international facilities, for instance at DESY in Hamburg and at MAXlab in Lund, and at first sight it could seem pointless for a Danish facility to compete on this scene. However, the combination of the modern extremely brilliant light source in the XUV range with specialised experimental stations, actually allows science at ASTRID2 which is not possible anywhere else in the world.
“The combination of the modern extremely brilliant light source in the XUV range with specialised experimental stations, actually allows science at ASTRID2 which is not possible anywhere else in the world” – Lars H. Andersen
Combining Competences at Aarhus University
Ions – charged atoms or molecules – are an extremely important object of research for investigating fundamental properties of matter. Due to their charge, ions can be influenced with electric and magnetic fields which universally makes it possible to perform extremely well-controlled experiments, even on complex systems.
Beyond the role as a controllable research object, ions are present in many places in nature, where they are often key players in chemical reactions. This is true for reactions in biological media (e.g. cells), liquids (e.g. the oceans), and natural gas phases, like the Earth’s atmosphere. Even in extremely dilute gases found in the interstellar medium, ions are observed and act as essential players for the evolution of these environments.
At Aarhus University there is a strong community around research using ions, for instance exploiting conventional lasers, and lasers at international facilities like the FLASH in Hamburg to investigate dynamical properties of molecules. Researchers at Aarhus University have been world-leading in the innovative development of ion beam facilities: the first electrostatic storage ring ELISA [1] was built there, and a dedicated new facilities to study photo-physics of molecular ions were established recently [2-3]. Here, primarily the visible spectrum is studied with standard laser technology (see e. g. Ref. [4]).
“The Carlsberg Foundation stimulated the union of two strong competences at Aarhus University – the synchrotron radiation and the ion research.” – Lars H. Andersen
With large grants in 2011-13, the Carlsberg Foundation made the construction of a new experimental facility (AMOLine) at ASTRID2 possible. This delivers monochromatic high intensity XUV light to a novel ion station (the XUV Storage RING (XRING) laboratory). Thereby, the Carlsberg Foundation stimulated the union of two strong competences at Aarhus University – the synchrotron radiation and the ion research. The ion facility (XRING) is presently in operation and the photon delivery system (AMOLine) is in its very final commissioning phase.
Facing Research Frontiers on a Broad Palette
The grants from the Carlsberg foundation have enabled investigations on ions at the forefront of molecular science. Our future research will to a large degree be defined by the novel possibilities provided by the facility. The new facility will certainly also promote careers of scientists at all academic levels in the future.
From the onset, the envisioned scientific case for the AMOLine and the XRING laboratory was broad, and the combined laboratory now aims at addressing basic scientific questions as well as questions with a more direct societal impact.
How Is an Electron Detached from a Negatively Charge Ion?
This is among the most classical problems in atomic physics and the answer relates directly to the development of theory. In the energy regime corresponding to infrared and visible light, experimentalist have addressed and refined the answer to this question for decades. However, in the ultra violet and extreme ultra violet region, where new facets of the detachment process are indeed foreseen, only a few isolated studies exist, for instance from free electron lasers [5]. The new experimental facility at ASTRID2 allows such fundamental process, like electron detachment, to be investigated in an unprecedented detail that will almost certainly challenge our present description of small quantum systems.
What Is the Role of Ionisation and Ions in the Atmosphere?
To understand and combat climate change it is the responsibility of science to provide accurate descriptions and models the climate system of the Earth. The atmosphere is an enormously complicated physical system, influenced by a wealth of natural and anthropogenic processes. It is interesting to note that several large scale phenomena of the climate, like water- and airflows circuits are well represented in atmospherics models, while microscopic processes like ionisation and chemical reactions, and hence their impact on the climate system, are much more sparsely characterised. With the new ion-beam facility at ASTRID2, we aim to make progress in this field. The figure displays first results obtained at the new facility on the ionisation of the α-pinene molecule.
Result from the investigation of the photofragmentation of α-pinene (molecular structure as indicated) with the new experimental setup at ASTRID2, showing (a) the photofragmentation pattern at a photon energy at 17 eV, and (b)-(d) yields as a function of photon energy for three selected fragment masses. α-pinene is emitted from coniferous trees and indeed have the characteristic odor of the pine tree. α-pinene belongs to the class of so-called monoterpenes (C10H16) which constitutes an important group of volatile organic compounds (VOC) that are emitted into the atmosphere from vegetation and which subsequently, through oxidation with ozone or ionisation in a partly unknown sequence of processes, leads to formation of secondary organic aerosols.
How Does Energy Flow in Biological Molecules?
Standard laser systems perform well down to about 200 nm (6 eV). Molecules, however, continue to absorb light at higher photon energies, where the level density of excited states becomes increasingly higher. To explore the effect that UV and VUV radiation has on biological molecules, we are constructing a new electrospray-ion source that delivers gas-phase biological molecular ions that will be exposed to the energetic radiation from ASTRID2. The molecular response will be ionisation and fragmentation in a competition that depends on how the molecules share the energy between the various degrees of freedom (electronic versus nuclear). For large bio-molecules, the systems may indeed start to act as their own heat-bath and hence efficiently drain the initially driven mode of excitation for excess energy, and will in this way become ‘self-protective’. Such studies may become of use in the development of future sunscreens.
Acknowledgement
We wish to thank senior scientist Søren Vrønning Hoffmann, beam line scientist Nykola Jones as well as the staff of the construction and mechanical workshops at Department of Physics and Astronomy for their skillful work throughout the project.
References
[1]. S. P. Møller, Nucl. Instrum. Methods Phys. Res., Sect. A 394, 281 (1997).
[2]. H. B. Pedersen, A. Svendsen, L. S. Harbo, H. V. Kiefer, H. Kjeldsen, L. Lammich, Y. Toker, and L. H. Andersen, Rev. Sci. Instrum. 86, 063107 (2015).
[3]. A. Svendsen, R. Teiwes, H. V. Kiefer, L. H. Andersen, and H. B. Pedersen, Rev. Sci. Instrum. 87, 013111 (2016).
[4]. A. Svendsen, H. V. Kiefer, H. B. Pedersen, and L. H. Andersen, J. Am. Chem. Soc. 139, 8766 (2017).
[5] L. S. Harbo, A. Becker, S. Dziarzhytski, C. Domelse, N. Guerassimova, A. Wolf, and H. B. Pedersen, Phys. Rev. A, 86, 023409 (2012).