BESSY II Experiment Cools 10 Million Ions to 7.4 K

Scientists at BESSY II at Helmholtz Zentrum Berlin, have developed an experimental method that cooled 10 million ions to 7.4 K for the first time. The new ion trap they created provides an opportunity to use cryogenic X-ray spectroscopy to study the magnetism and ground states of molecular ions. It is also, according to the research team, the foundation needed to develop new materials for energy-efficient information technologies.

“Until now, everyone assumed it would not be possible to reach lower temperatures at such a high density of ions with a quadrupole ion trap. But it can be done,” says Tobias Lau, a researcher at Helmholtz-Zentrum Berlin (HZB). He says the RF electromagnetic field used in the new method doesn’t just trap the stored ions, but also “jiggles” them so they gain energy and increase in temperature.

The research team introduced helium at high pressure as a buffer gas to draw off this additional energy. “You have to imagine this as kind of a cold syrup that damps the macro motion of the particles, slowing their rotation and translation,” says Vicente Zamudio-Bayer from the University of Freiburg.

Previously it was only possible to cool down about 1,000 ions to 7.5 K using buffer gas. But a thousand ions, the scientists say, are not enough for spectroscopic analyses.

The researchers used the UE52-PGM station at BESSY II, allowing them to vary polarization of soft X-ray radiation in the experiments. The set-up at this beamline facilitates X-ray spectroscopy of cryogenic ions under externally applied magnetic fields, meaning a sample can be analyzed in an externally applied magnetic field using circularly polarized X-rays (X-ray magnetic circular dichroism/XMCD). The process, according to the researchers, yields information about the magnetic moments of the electrons subdivided into both spin and orbital contributions. “We were able for the first time to experimentally determine the magnetic moments of nickel demarcations thanks to the especially low temperatures,” Lau says.

Work on the ion trap is part of a larger project of HZB and the University of Freiburg funded by the German Federal Ministry of Education and Research. The goal is even lower temperatures, ranges where the magnetic effects show up more clearly. “We hope we will soon get to 5 K,” says Zamudio-Bayer.