The collaboration went to Berlin with a miniaturized electron beam ion trap (EBIT) from MPIK to the BESSY II synchrotron source, with the goal of precisely measuring this X-ray absorption of atomic oxygen in the laboratory. The device, PolarX-EBIT, can prepare ions in highly charged states, such as for example N6+ or O6+ with only one or two electrons such as hydrogen or helium. Highly charged ions have spectral lines with a pattern similar to that of atoms with the same number of bound electrons, but energetically transferred to the X-ray range. It was a matter of using the very precisely theoretically known X-ray lines of these hydrogen-like and helium-like ions as standards for the energy calibration of the synchrotron radiation. This new method therefore surpasses the accuracy of all calibration methods used to date.
A gas absorption cell from NASA's inventory connected to the PolarX-EBIT allowed the MPIK doctoral student Steffen Kühn and his team colleagues to measure the previously used X-ray absorption lines for gases such as molecular oxygen and nitrogen, or atomic neon, simultaneously with the calibration lines. With this type of measurement setup and the high accuracy of the theoretically calculated energies for the ions stored in the PolarX-EBIT, many systematic uncertainties of earlier methods could be excluded.
The data analysis showed that the energy values of the absorption lines of molecular oxygen used very frequently in the worldwide synchrotron community for calibration were off by 0.45 eV at 540 eV, i.e., almost by one part in thousand. The new value implies a “deceleration” of atomic oxygen in the interstellar space of our galaxy by 250 km/s, whereby this element falls back into the “permissible” and typical range of about +/− 100 km/s.
With the demonstrated method, the uncertainties can be reduced even further. The new “standards” are so precise that other previously unknown problems of monochromators calibrations became obvious. It is also interesting to note that space-borne X-ray telescopes already use such lines of highly charged ions from cosmic sources as energy references. Now, the present experiment has eliminated the previously grossest incongruence of interstellar oxygen. In the future, it will offer more urgently needed exact X-ray energy references not only for astrophysics, but also for research at synchrotrons, in order to meet the ever-increasing calibration demands of many applications.
High-Precision Determination of Oxygen Kα Transition Energy Excludes Incongruent Motion of Interstellar Oxygen, M. A. Leutenegger et al., Phys. Rev. Lett. 125, 243001 (2020), DOI: 10.1103/PhysRevLett.125.243001