New experiments with helium atoms make it possible to switch the electron correlation on and off as desired.
Apart from the hydrogen atom, which consists of only one proton and one electron, the helium atom is the simplest atom of our world. The helium atom consists of a doubly charged nucleus and two orbiting electrons. The existence of two electrons leads to a new point of view with far-reaching consequences, namely the concept of electron correlation. In the journal Physical Review Letters  the experimental observation of the controlled emergence of electron correlation in helium atoms is reported. Photoionization of helium was studied under conditions in which the electron correlation can be arbitrarily turned on and off. For off-line correlation, helium behaves like a hydrogen atom. By contrast, for activated correlation, the dynamics of the ionization process are strongly determined by the interaction between the two electrons.
In the experiment, helium atoms were ionized by the absorption of a single photon in the ultraviolet spectral range. This was possible because the atoms were brought into a long-lived excited state by shocks with high-energy electrons in a discharge source. The energy of the exciting photon was adjusted so that it was just sufficient for the ionization of the atom. Thus, 99.9% of the photon energy was used to overcome the binding energy of the electron and only 0.1% released to the freed after ionization electron as kinetic energy. The resulting photoelectrons were thus very slow. In the experiment, they were accelerated to a two-dimensional detector where their impact locations were measured. The impact sites map the velocities of the electrons in the detector plane.
As impressively demonstrated in the famous double-slit experiment on the interference of single electrons, which was voted "The Most Beautiful Physics Experiment" a few years ago in a vote by "Physicsworld", electrons have both particle and wave character. Responsible for this is quantum mechanics. The wave properties of matter are described by a wavelength named after the French physicist de Broglie, which can be assigned to any moving particle. The lower the kinetic energy of the electron, the greater the de Broglie wavelength. If the energy of the electron is small enough, the de Broglie wavelength becomes observable in the macroscopic world. In the photoionization experiments published this week, the wave nature of the slow electrons leads to the observation of a series of interference fringes, with constructive and destructive interferences alternating on the detector (see Figure 1).
This interference phenomenon has been measured more and more accurately by experiments of our team in recent years. In fact, our previous experiments have revealed the existence of two different mechanisms for the generation of interference. In experiments with hydrogen atoms, it has been shown that the interference may be related to the nodular structure of the wave function, which was excited by photoabsorption in the atom. In experiments with larger atoms with many electrons, such as the precisely measured xenon atoms, it has been shown that the interferences can also be the result of differences in the length of possible paths of the electron to the detector. Put simply, two paths that differ by an integer number of de Broglie wavelengths become constructive interference, two paths that differ by a half-integer number of de Broglie wavelengths will lead to destructive interference.
As shown in the current study, helium atoms show both mechanisms. Interestingly, a small change (<< 1%) in the strength of an applied external electric field is sufficient to alter the observed interference pattern. As it turns out, "hydrogen-like" helium atoms, where the nodal structure of the wave function determines the interference pattern, can be transformed into "xenon-like" helium atoms, where the emerging electron correlation destroys the "hydrogen-like" wave function.
In this way, the helium atom becomes a wonderful nano-laboratory for controlled switching on and off of the electron correlation.