The onset of “electron correlation” in the helium atom has been observed for the first time by an international team of researchers. Using the “photoionization microscopy” technique that the team developed in 2002, the researchers have now turned their quantum microscope on the helium atom. The team also found that it was able to tune these electron correlations at will.
The helium atom comprises a doubly charged nucleus surrounded by two electrons, and is nature’s second simplest atom after the hydrogen atom, which consists of one proton and one electron. The existence of exactly two electrons in helium provides physicists with the perfect laboratory to test “electron correlations”, which occur when the properties of electrons are influenced by their interactions with other electrons. This is important because the electrons in most materials, such as superconductors, interact so strongly with each other that it is impossible to predict their properties by simply studying the behaviour of individual electrons.
Strongly correlated
Proper descriptions of electron correlation are highly sought after but are notoriously difficult to achieve, explains Marc Vrakking of the Max-Born-Institute in Berlin, who was the lead researcher of the new work. “For example, the ‘density functional theory’ [a computational quantum-mechanical modelling method that looks at the electronic structure of many-body systems] would be a perfect theory that would be able to solve just about any problem of chemical interest, if only it were known how to include the effect of electron correlation correctly. Whole armies of theoreticians are working on and struggling with this,” he laments.
Many phenomena in atomic physics can be successfully understood without taking the correlations into account. For example, understanding how atoms or molecules ionize when they are illuminated by high-energy photons can be done by only considering the response of an electron in a single orbit, neglecting its interactions with other electrons in the atom or molecule. Vrakking told physicsworld.com that working out exactly when electron correlation becomes important in such systems is a very active field of research. “There is a lot of research aimed at observing the onset of electron correlation, to try and understand it in a way that hopefully can later be transferred to more complex systems, where the inclusion of electron correlation effects is indispensable,” he says.
In the new work, Aneta Stodolna, of the FOM Institute for Atomic and Molecular Physics in the Netherlands, along with Vrakking and other colleagues in France, Germany and the US, studied the photoionization of helium. Similar to the method perfected by the team last year while studying the hydrogen atom, the experiment begins with helium atoms that are excited by colliding them with energetic electrons, thereby putting the helium into a long-lived excited state. The helium atoms are then ionized by the absorption of a single ultraviolet photon, the energy of which is tuned such that it is only just enough to ionize the helium – 99.9% of the photon’s energy is used to overcome the ionization potential of the atom and just 0.1% of the photon’s energy is converted into photoelectron kinetic energy. The very slow photoelectrons are then accelerated towards a 2D detector, where their position is captured. This provides a measure of the velocity of the electron in the plane of the detector.
Whole armies of theoreticians are working on and struggling with [electron correlations]
Marc Vrakking, Max-Born-Institute, Berlin
Electrons exhibit wave–particle duality, and the lower the kinetic energy of the electron, the larger is its De Broglie wavelength. In fact, for low enough kinetic energies, the De Broglie wavelength becomes observable on macroscopic length scales. In the helium photoionization experiments, the wave-like nature of the slow electrons allowed the researchers to observe a series of interference rings, with constructive and destructive interferences that alternated at their detector.
In the hydrogen experiments that the team carried out last year, the interference patterns were connected to the nodal patterns of the atomic wavefunctions that were excited when the atom absorbed a photon. Previous research carried out by Vrakking’s team with xenon atoms found that the interference patterns can also be seen due to differences in the pathlengths of electrons travelling to the detector. But surprisingly, with helium, both effects seemed to come into play.
Stark appearances and unexpected states
When atoms are placed in electric fields, there is a shifting and splitting of their spectral lines, which is known as the “Stark effect”. With an increase in the electric field, some Stark states are shifted towards higher excitation energies; these are referred to as blueshifted Stark states. “To ionize an atom from that state, you’ll need laser light, which has shorter wavelengths (i.e. more energy) compared with the case without the electric field. Shorter wavelengths mean that the colour of the laser light will be ‘more blue’,” explains Stodolna. Conversely, states that are shifted towards lower energies require longer wavelengths, and so less energy to be excited. Therefore, the colour of laser light is tuned more towards the red, and this is known as a redshifted Stark state.
Vrakking and colleagues did not expect to see any red states in their experiment because these have very short lifetimes and so cannot be identified when the photoionization yield is measured as a function of photon energy. Rather, many blue states show up in the experiment, and the majority of the interference measurements that the team made were indeed for these blue states. But the researchers also observed some irregular measurements. “At some quite rare positions, we could suddenly see a red state, and we observed a ring pattern in accordance with the quantum number of that red state. We could determine that this was the result of an interaction of this very short-lived red state with a nearby blue state. This interaction resulted in a situation where the two electrons in the helium atom, which normally strongly interact with each other, suddenly did not really interact with each other anymore, and thereby the helium atom started behaving like a hydrogen atom,” explains Vrakking.
Moreover, the team observed that it could control the dynamics of the helium atoms by applying tiny changes (much less than 1%) to the strength of the external electric field. Indeed, when the electron correlations are turned off, the helium atom behaves just like a hydrogen atom. When turned on, its dynamics is strongly affected by the interaction between the two electrons.
Vrakking believes that the team’s work with the helium atom has shown how it can be used as an excellent model system for those keen to study the onset of electron correlation in simple systems.
The research is published in Physical Review Letters.
