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Successful experiments with stored ions in DESIREE

In November 2021, we reported on Michael Gatchell's experiment, which showed that fragments from collisions in the interstellar medium can contribute to the formation of new, larger molecules. Here, DESIREE Director Henning Schmidt reports on some other recent successes.

Forsningsteam vid DESIREE

Tell us about your background and when you came to Fysikum
- I went to Aarhus University in Denmark in 1986 and I wanted to become an upper secondary school teacher. Only after years of study did I realise that postgraduate studies were an option and I then obtained my PhD in 1994. I came to Stockholm in 1995 as a postdoc with Henrik Cederquist in the Atomic Physics Division. I was an Assistant Professor 1998-2003 in the Molecular Physics Division, which is now part of Chemical Physics. In 2006, I was awarded a lectureship in atomic physics and I was promoted to professor in 2011.

What is your current role at Fysikum?
- I am a professor and head of the atomic physics division . In addition, I am the director of DESIREE, which is a research infrastructure funded by the Swedish Research Council. The research there is in AMO (Atomic, Molecular and Optical) Physics and many local members of the divisions of Atomic Physics, Chemical Physics, Instrumentation Physics and the Technical Division participate. In addition, of course, to all our external users from Sweden and the rest of the world. I have no teaching of my own at the moment, but I chair the department's undergraduate teaching committee.

What kind of research do you do at DESIREE?
- The research is focused on experiments with stored ions. DESIREE consists of two electrostatic storage rings where ion beams can be stored for a long time thanks to the excellent vacuum, which resultds from the whole vacuum chamber being cooled to 13 Kelvin. DESIREE was built from scratch locally here at Fysikum, and initially at the Manne Siegbahn-Laboratory. In recent years, we have been achieving more and more results leading to a number of interesting publications. In November 2021, you could read about Michael Gatchell's results published in Nature Communications. That experiment showed that fragments from collisions in the interstellar medium can survive there and help to form new larger molecules in this very thin gas in the space between the stars.

Moa Kristiansson och Dr. Gustav Eklund
Moa Kristiansson och Dr. Gustav Eklund

Tell us about the latest successes with DESIREE
- Now in January, results from two completely different DESIREE experiments have been published and, coincidentally, they came on the same day. One is a lifetime experiment where negative ions of bismuth were studied the other is an experiment with merged beams of positive and negative ions.

Negative ions can lose the extra electron and their charge
- The principle is simple. Some of the bismuth ions are created in an excited state where their extra electron is more loosely bound than in the ground state. In DESIREE, they are then exposed to laser light of a wavelength chosen such that only the ions remaining in the excited state can have their extra electron removed and thus lose their charge. The neutral atoms formed in this process leave the ring and are counted by a particle detector. Over time, the excited ions decay to the ground state and we can then see this by the count rate of the above mentioned photodetechment process decreasing with time after the ions were stored. This allows us to determine the average lifetime of the excited state. Only in cold storage rings like DESIREE can one store these negative ions long enough to measure such long lifetimes (16 seconds for Bi-). The work is part of Moa Kristiansson's PhD thesis and the specific experiment with bismuth was performed in collaboration with, among others, Wesley Walter from Denison University, Ohio and Dag Hanstorp, Gothenburg and the results were published on 18 January in Phys. Rev. A Letters, see link below.

Negative hydrogen ions and positive magnesium ions
- DESIREE is built as two storage rings to allow the study of reactions between oppositely charged particles at low energies. The first results were only published in 2020, so this is still new ground for us to break. The latest publication is about Mutual Neutralization (MN) between negative hydrogen ions, where the heavier hydrogen isotope deuterium is used for practical reasons, and positive magnesium ions: Mg+ + D- → Mg + D. In DESIREE we can measure not only that an MN reaction has taken place but also in which quantum state the neutral Mg atom is formed. The experiment had been proposed by Paul Barklem and Jon Grumer from Uppsala University and was carried out by Dr Gustav Eklund, as part of his thesis work. Paul and Jon are interested in these processes because they also take place in the atmospheres of stars and there they affect the relationship between the abundance of magnesium in the star (in this case), and the strength of absorption of certain wavelengths seen in the stellar spectra. For magnesium, different theoretical descriptions led to different predictions for the distribution on the final states of the MN process, which meant that our results were indeed needed to decide which was the best computational approach in this case. The results were published in Phys. Rev Letters on 18 January, see link below.

What experiments will you perform in the future?
- DESIREE is a research infrastructure to which you can send proposals for new experiments and, for this reason among others, there is never a shortage of new ideas to explore. In our own group, we have been pushing forward the experiments on merged beams, focusing on atomic ions for the first few years of this type of experiment. At the same time, we have carried out many experiments with both small and large complex molecules whose properties we have studied through spontaneous decays and their interactions with laser light. One of the major future steps is to upgrade the instrumentation around DESIREE so that we can take on even more challenging opportunities to study interactions between different complex ions in the two rings. We have just received a new grant from the Council for Research Infrastructures (RFI) of the Swedish Research Council to carry out some of the necessary upgrades. We think that there will be exciting experiments to perform at DESIREE for many years to come.

That's why you should study at Fysikum!
- You should study at Fysikum because we offer a really good education where the main emphasis is on learning physics as an academic subject with the theoretical and experimental tools to go with it. Despite the academic starting point, there is no shortage of job opportunities even outside academia for recent physics graduates. Fysikum has a very strong research environment under constant development, which is of course a good reason to do research here, but which is also very much a good reason to choose our courses. The proximity to top-class research in most branches of physics is a strength and something we should exploit even further in our education. We are working on that!

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Paper: Bi-experiments
Paper: Mutual neutralisation between negative hydrogen ions
Contact Henning Schmidt
More about the Atomic Physics Division