In contrast to its apparent simplicity (that brought Einstein his Nobel Prize), the photoelectric effect, when an electron is knocked out from its parent atom by a photon, is quite complicated to analyze in general, especially when the atom contains a large number of electrons.
Like the many-body problem in classical mechanics, the quantum many-body problem is very difficult to conceptualize and remains a serious challenge for theory. Hence, the principal role in this field is played by experiment. The latter, however, faces its own difficulties when it comes to unraveling data associated with the atomic photoeffect itself from a variety of other effects due to essentially irrelevant phenomena.
Not the least among the latter phenomenon is related to the spin (and thus the magnetic moment) of the atomic nucleus. It may be thought of as the quantum generalization of the angular momentum in classical mechanics, which is calculated as the product of the linear momentum (mass times velocity) of a particle and its position vector relative to the axis of rotation. Each proton and each neutron in the nucleus possess their own magnetic moment. While these moments tend to largely compensate each other, the resultant moment does not always have to vanish. Any residual moment, even though relatively small and hence its interaction with the electronic shell is labeled as "hyperfine", may dramatically influence the process of the photoelectron emission. A non-zero nuclear spin spoils the picture, in particular when the atom is excited, which is the reason for this case being of such strong interest for physicists.
A collaboration of seven physicists from Italy, France, Germany, and Russia chose to perform their study on xenon -- the element previously used to resolve mysterious features in the atomic photoeffect. Being a noble gas, xenon is very convenient for such studies: it does not form chemical bonds and does not contaminate the apparatus with its compounds. Even more important in the choice was that, among all the noble gases, only xenon has stable isotopes with both zero and non-zero nuclear magnetic moments. Furthermore, xenon is an interesting atom on his own rights, due to the large number of electrons and the associated complicated dynamics of its electron shells.
The experimental design suggested isotope separation with the help of a mass-spectrometer. Subsequently, each of the isotopes was excited with synchrotron radiation and simultaneously irradiated with a wavelength-tunable laser beam. All ejected electrons were counted and sorted by energy and scattering angle.
All this is easy to say, but reality is much more complicated. The first targets excited by synchrotron radiation were obtained in the late 1990s, but the principal difficulty was to combine two radiation beams, laser and synchrotron. Moscow theorist A. N. Grum-Grzhimailo, one of the collaborators, says that only a few people in the world are currently capable to solve this problem. One of them -- Michael Meyer from the European XFEL GmbH based in Hamburg (Germany) -- contributed to the experiment described above. The actual experiment was carried out using a unique beamline with variable polarization, maintained by the group of Laurent Nahon at the French synchrotron SOLEIL.
The role of the two theorists from the Institute of Nuclear Physics,, A. N. Grum-Grzhimailo and E. V. Gryzlova (the winner of the UNESCO 2012 L'Oreal award for "Women in Science"), was like the song goes: "paint on... later on, I will explain it all" (a reference to "The Painting Artists" by B. Okudzhava).
The task was to provide a theoretical interpretation for the photoeffect on the excited xenon atom, isolated from the influence of the nuclear magnetic moment. Nobody expected a quiet and peaceful life within the collective of 54 electrons, but the gradual improvement of the existing theoretical models finally led to success in describing the pure atomic photoelectric effect. This work, A. N. Grum-Grzhimailo says, is paving the way for a large class of studies with artificially disabled nuclear magnetic moments and for complicated atomic processes with isotope selection that we could previously not even think about.