In spectroscopy, there is a problem of expanding the spectra of atoms due to their mutual collisions, which continuously change their energy levels.
Spectroscopy specialists from Germany, Italy and Switzerland at the Max Planck Institute for Quantum Optics, under the leadership of Masaki Hori, tried to fight this expansion. In 2013, at the CERN laboratory, they launched antiprotons into liquid helium. The project was designed to test whether helium bath spectroscopy is possible at all. Anna Soter was in the group, and she was interested in how this mixture would react to different helium temperatures. She convinced colleagues to spend precious antimatter on measurements at different temperatures.
“It was a random idea on my part,” says Sauter, now a professor at the Swiss Federal Institute of Technology in Zurich. “People weren’t sure it was worth spending antiprotons on this.”
The experimenters found that the widths of the spectral lines of antiprotons in helium, with decreasing temperature, begin to narrow, reaching sub-GHz values.
It should be noted here that the theorists invented that antiprotons launched into liquid helium allegedly replace an electron in the outer orbital of helium, and a certain atom of “antiprotonic helium” is obtained, and they work with the spectra of this particular exotic atom. Under normal conditions, the antiproton experiences a cascade descent to lower orbitals, while photons are released, and this ends with an interaction with the nucleus, where the antiproton annihilates with the formation of charged pions. According to the ideas of these theorists in liquid helium, the antiproton in the outer orbit of “antiproton helium” is protected from its nucleus by an electron shell, and there is reason to expect that the widths of the corresponding lines will be narrow.
To test this hypothesis, the authors stimulated transitions of antiprotonic helium from the states (n, l) = (37, 35) and (39, 35) to the state (38, 34) with two lasers with wavelengths of 726 and 597 nm, respectively. In the end, the laser-activated antineutron reached helium nuclei and annihilated, which was recorded by acrylic Cherenkov detectors.
The scientists tuned the laser frequencies slightly in the vicinity of the resonance and monitored the number of detector counts for gaseous and liquid helium. In the first case, the behavior of the spectral lines showed the usual dependence on pressure: as pressure increased, the width increased, blurring the hyperthin quadruplet caused by the spin-spin interaction between an electron and an antiproton.
In liquid helium, which has a superfluid phase transition (helium-II) at 2.17 kelvin, things didn’t work out that way. If the dependence of widths on temperature in helium-I had an exponential character, then in helium-II it had a minimum in the range of 1.7-1.9 kelvin. The resonance width at 729 nanometers was about 0.9 gigahertz, and at 597 nanometers it was about 1.1 gigahertz, which is an order of magnitude smaller than predicted by calculations based on impact broadening in binary collisions. The same was observed for line shifts. See Fig. one.
The results of these experiments are published in Nature.
Of course, an antiproton cannot occupy the entire electron orbital, which is huge for it, and cannot move along it like a planet. (What the theorists apparently imagined.) The antiproton remains between the helium atoms. He himself receives photons from the laser and gives them. That is why in superfluid helium (helium-II) at 2.17 kelvin, it emits in the range of 1.7-1.9 kelvin at any temperature.