Transitions between levels of cooled relativistic ions measured by laser spectroscopy


D. Winzen et al / Scientific Reports, 2021
D. Winzen et al / Scientific Reports, 2021

Physicists accurately measured the wavelengths of the transitions 2S1/2−2P1/2 and 2S1/2−2P3/2 lithium-like carbon ions C3+dispersed to relativistic speeds in the cumulative ring. To do this, they directed ultraviolet radiation towards the ion beam and detected the subsequent fluorescence of ions, and to adjust to the resonance they changed not the wavelength of radiation and the speed of the beam. The work is published in Scientific Reports.


Spectroscopy of ions, dispersed to speeds close to the speed of light, is fraught with certain difficulties. First of all, this is due to the fact that the ions are not in one place in the form of gas, as is usually the case in classical spectroscopy, and quickly fly on the cumulative ring. Another difficulty is related to the emergence of the strongest Doppler shift in wavelengths, as well as distortions arising from Lorenz's transformations.


There is, however, another difficulty. For accurate experiments with relativistic beams (not just spectroscopic), they should be cold enough, which is understood as a small scattering of pulses of ions in a beam. At the moment, the most common method of cooling fast ions in storage rings is the method of electronic cooling, which is to mix a beam of "hot" ions beam of "cold" electrons. Laser cooling is expected to be more effective for this purpose, but this method requires knowing the exact wavelengths of transitions in the ions. At the same time, for some ions, in particular, for lithium-like carbon ions, C3+there are discrepancies in literary data.



To resolve these contradictions, and to test radiation summing and detection systems in an experimental cumulative ring installed at the Helmholtz Center for the Study of Heavy Ions in Darmstadt, a team of physicists from Germany and China led by Danyal Winters conducted a series of experiments to refine the wavelengths of transitions 2S1/2−2P1/2 and 2S1/2−2P3/2 y ions 12C3+. The ions accelerated in the cumulative ring to speeds equal to almost half the speed of light and cooled with an electronic cooler. In a separate area of the ring, the ions collided with laser radiation with a wavelength of 257 nanometers, and re-emitted photons were entered into the registration system.


A schematic image of an experimental cumulative ring with an enlarged area in which fluorescence of ions is detected. / D. Winzen et al / Scientific Reports, 2021
A schematic image of an experimental cumulative ring with an enlarged area in which fluorescence of ions is detected. / D. Winzen et al / Scientific Reports, 2021

The peculiarity of the experiment was that the exact velocity of the ions depended on the voltage applied to the electronic cooler. This allowed for a thin setting of the laser wavelength in the ion center system, as due to the doppler shear it shifts to the short-wave area according to the Lorenz factor. Near the resonances, the authors changed the voltage in the cooler with a step of one volt, scanning the study area three times. By the number of counts on the ultraviolet radiation detector, they concluded about the intensity of absorption and subsequent fluorescence.


Left: the number of counts on the detector (blue) and the voltage on the cooler (red) depending on the scanning time. The fading peaks of registration are due to a decrease in the number of ions in the beam over time. Right: same, but raw data from the detector is replaced by the normalized intensity of fluorescence. / D. Winzen et al / Scientific Reports, 2021
Left: the number of counts on the detector (blue) and the voltage on the cooler (red) depending on the scanning time. The fading peaks of registration are due to a decrease in the number of ions in the beam over time. Right: same, but raw data from the detector is replaced by the normalized intensity of fluorescence. / D. Winzen et al / Scientific Reports, 2021

Thus, by changing the voltage on the electronic cooler, physicists connected it through the current with the speed of the ions and, therefore, with the wavelength in the mass center system. As a result of wavelengths, transitions to levels 2P1/2 and 2P3/2 155,0779 (12)sys(1)stat and 154,8211 (12)sys(2)stat nanometers accordingly.

Spectral circuit to navigate 2S1/2−2P3/2. The approximation is made by The Voigt function. / D. Winzen et al / Scientific Reports, 2021
Spectral circuit to navigate 2S1/2−2P3/2. The approximation is made by The Voigt function. / D. Winzen et al / Scientific Reports, 2021

It is worth noting a lot of work to account for all sorts of errors conducted in this experiment. As a result of their careful accounting, physicists have concluded that the measured wavelengths are in good agreement with the data obtained earlier in experiments on interferometry and plasma spectroscopy, as well as with theoretical predictions. They also noted that several improvements in the pilot installation and additional calibrations would improve the accuracy of the experiment in the future.

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