Collisions of highly charged ions and cold atoms

In the past few decades, kinematically complete high-resolution studies of ionization processes have been studied extensively using COLTRIMS. The targets are provided by supersonic expansions of gas jets, which limits the resolution to the thermal spread of the target atoms and the available atomic target to species without active electrons. These limits can be overcome by combining the novel technique of RIMS with state-of-the- art laser cooling, where the target atoms can be trapped and cooled to very cold temperatures well below 1mK in magneto-optical traps (MOTRIMS). As compared to COLTRIMS targets, systems with one or more active electrons can be explored. This is in particular interesting for multiple charge exchange experiments, since the energy difference between the transferred electrons is non negligible. Furthermore, the ability to state-prepare and polarize the target atoms leads to the possibility of exploring new physics, as for example the initial state dependence in multi-electron threshold ionization of the target atoms.


With our current setup densities of up to a few 1011 atoms/cm-3 can be achieved. Therefore a dark spontaneous force optical trap loaded by a 2D MOT is used which not only overcomes the density limit of a normal magneto optical trap but also reduces loading times of the trap to as low as 300ms. This allows measurements of processes with very low probability such as multi electron charge transfer, which are otherwise disguised by the far more dominant single charge transfer channel. To resolve the dynamics of such processes a new recoil ion momentum spectrometer has been build (see figure).

The whole setup has been tested using a pulsed laser beam. The inset of the figure shows the recoil ions' angular momentum distribution depending on the polarization of this pulsed laser. It can be clearly seen that the very small momentum transferred to the ion during the ionization process can be well resolved. The determined resolution of the recoil ions' momentum is 0.10 a.u. which is sufficient to study multiple charge transfer in highly charged ion – atom collisions. With these measurements also the target could be characterized in great detail and the use of the 2D MOT as an independent target has been explored.

Recently we got new funding from the BMBF in the framework of FLAIR at GSI. This allows us as a next step to upgraded the target by implementing a dipole trap where the atoms are trapped at the focus of a far detuned, intense laser beam. This technique allows the reach even higher densities and by letting the warmest atoms evaporate from the trap a Bose-Einstein-Condensate (BEC) can be reached. This way a completely new target will be provided where not only the interactions between single atoms and ions but also collective effects which are only present in BECs can be investigated. In addition, using a dipole trap allows to trap atoms without the use of a magnetic field which has several advantages. Firstly the trap can be run continuously whereas in the present setup the magnetic field as well as the MOT lasers have to be switched of several milli-seconds before any recoil momentum can be measured with high accuracy. Secondly it is possible to state prepare the atoms in the dipole trap which makes it possible to explore the dependence of multiple charge transfer on the polarization of the target.

Latest news and research results

Download the latest poster from the MOTRIMS group [pdf]

A detailed description of our atom trap and imaging system has been published in Applied Physics B17.01.2017

In order to precisely measure the density distribution in our dark sponataneous froce optical trap (DarkSPOT), we developed a novel imaging system which is based on saturation absorption imaging. The details of this setup and the characterization of our atom cloud were published as part of the topical collection “Enlightening the World with the Laser” - honoring T. W. Hänsch.

B. Höltkemeier et al., A dense gas of laser-cooled atoms for hybrid atom–ion trapping, Applied Physics B 123.1, or see our full list of publications
The dynamics of an ion in a radio-frequency trap has been investigated07.12.2016

This paper is a follow-up on our recent publication in PRL (see news from 10.06.2016) investigating sympathetic cooling of a trapped ion using a neutral buffer gas. We provide a detailed analysis of the ions energy distribution and it dependence on different parameters such as the rf-frequency and voltage of the trap as well as the mass and spatial distribution of the neutral buffer gas. We also provide an estimate for the optimal atom-to-ion mass ratio in order to achive the maximum cooling rate.

B. Höltkemeier et al., Dynamics of a single trapped ion immersed in a buffer gas, Phys. Rev. A 94, 062703 , or see our full list of publications
Bastian Höltkemeier finishes his PhD!27.10.2016

Bastian Höltkemeier from the HAITRAP team successfully defended his thesis... Congratulations!

After the defense there was a little celebration in the CQD lounge where Bastian received his PhD hat (see picture). Part of the hat is an actual ion trap which Bastian can use to also trap ions at home! Definitely monkey approved!

B. Höltkemeier, Sympathetic cooling of ions in a hybrid atom ion trap, University Library Heidelberg, or see our full list of publications

The MOTRIMS team

Photo of the MOTRIMS project team.
Prof. Matthias Weidemüller

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Publications by the MOTRIMS project

2017B. Höltkemeier, J. Glässel, H. Lopez, M. Weidemüller, A dense gas of laser-cooled atoms for hybrid atom-ion trapping, Applied Physics B123, 51 (2017)

2016 B. Höltkemeier, P. Weckesser, H. Lopez, M. Weidemüller, Buffer-Gas Cooling of a Single Ion in a Multipole Radio Frequency Trap Beyond the Critical Mass Ratio, Phys. Rev. Lett. 116, 233003 (2016)

B. Höltkemeier, P. Weckesser, H. Lopez, M. Weidemüller, Dynamics of a single trapped ion immersed in a buffer gas, Phys. Rev. A 94, 062703 (2016)

2015 H. Lopez, B. Höltkemeier, J. Glässel, P. Weckesser, M. Weidemüller, T. Best, E. Endres, R. Wester, Sympathetic cooling of OH- ions using ultracold Rb atoms in a dark SPOT, Proceedings of the International School of Physics Enrico Fermi 189 (2015)

2013 Simone Götz, Bastian Höltkemeier, Thomas Amthor, Matthias Weidemüller, Photoionization of optically trapped ultracold atoms with a high-power light-emitting diode, Review of Scientific Instruments 84, 043107 (2013) [pdf]

2012 Simone Götz, Bastian Höltkemeier, Christoph S. Hofmann, Dominic Litsch, Brett D. DePaola, Matthias Weidemüller, Versatile cold atom target apparatus, Review of Scientific Instruments 83, 073112 (2012)

2011 S. Trippel, M. Stei, C. Eichhorn, R. Otto, P. Hlavenka, M Weidemüller, R. Wester, Nanosecond Photofragment Imaging of Adiabatic Molecular Alignment, J. Chem. Phys. 134, 104306 (2011)

2010 C Greve, M Kröner, S Trippel, P Woias, R Wester, M Weidemüller, Storage of protonated water clusters in a biplanar multipole rf trap, New J. Phys. 12, 065035 (2010)

J, Mikosch, M. Weidemüller, R. Wester, On the dynamics of chemical reactions of negative ions, International Reviews in Physical Chemistry 29, 589-617 (2010)