Spin Dynamics in Photoeffect

Photoelectric effect is one of the dominant processes in which the radiation interacts with atomic matter. Biological and electronic vision and detection of radiation in a wide energy range relies on it. In the Universe, where most of the energy is shifted by the photons, it plays a critical role of radiation absorption and ionization. On the fundamental level the interpretation of the photoelectric effect was a precursor for the development of quantum mechanics. However, after a century of research it is still not well understood. Photoeffect in heavy atoms is strongly affected by relativistic and spin effects. The spin of the electron couples to the magnetic fields generated in the process. It was observed that at high energies such effects dominate the dynamics. We report on the experimental observation of the spin precession in the strong magnetic field induced in the photoelectric process. We measured this process at the ESR storage ring in time reverse - the electron radiatively recombined into a bound state of the initially bare uranium ion. The spin state of the final ion was determined via its radiative decay. The two x-rays were observed in coincidence. This was the first time that correlated photons emitted by a heavy ion were observed. The experimental results agree with the full-order relativistic calculations that show a strongly pronounced spin-induced dynamics.