Researchers are implementing atomic switches

Double ionization of lithium by ultraviolet photons succeeded

Observed momentum distribution of the emitted photoelectron after single ionization of lithium atoms: ionization from the 2s shell (a), 1s shell (b) and 1s shell with simultaneous excitation of another electron (c). © MPI for Nuclear Physics
Read out

For the first time, researchers at the Free-Electron Laser FLASH at the Research Center DESY in Hamburg have realized a kind of switch for a correlated atomic process. The probability of double ionization of lithium atoms upon irradiation with UV light could be controlled by targeted manipulation of one of the participating atomic electron orbitals. Only the spatial orientation of the orbital was changed.

How complex systems arise from the interaction of simple components is one of the fundamental questions of physics. An essential role is played by the mutual influence of the particles, called correlation, which ultimately leads to the whole being more than the simple sum of its parts. Even the three-body problem reveals the mathematical difficulties involved.

Manipulate multi-particle processes in a controlled manner

One goal of modern atomic physics is not only to understand multi-particle processes better, but also to manipulate them in a controlled manner. This was achieved by researchers working with Alexander Dorn at the Max Planck Institute for Nuclear Physics in Heidelberg on the example of the double ionization of lithium by ultraviolet photons, in which they specifically prepared the spatial structure of the atom.

The measurements were made possible by an unprecedented combination of three state-of-the-art techniques: The photons of an energy of 85 eV, which trigger the reaction, were provided by the new free-electron laser in Hamburg. These encounter ultracold lithium atoms cooled to very low temperatures - 0.1 degrees above absolute zero - in a magneto-optical trap, trapped by laser light forces. There they can be specially prepared by further lasers. Finally, this trap is located in a so-called reaction microscope, which allows the simultaneous and very efficient detection in principle of all reaction products, the electrons and the ion with high resolution.

From single to double ionization

The figure shows as an example the observed velocity distribution of the electron after single ionization of lithium by UV photons: The different ring-shaped patterns correspond to the ionization from the outermost 2s shell (a), from the 1s shell (b) and from the 1s shell with simultaneous excitation of one of the remaining electrons on the 2p-shell (c). The basis is the photo effect, which was first correctly interpreted by Einstein in 1905, where the entire energy of a single light quantum (photon) is first transferred to exactly one electron. display

However, this can transfer part of its energy through the mutual electrical repulsion-indicated by the dashed oblique line-to another electron and, as in case (c), excite it into a bound state-a correlated process, In exactly the same way, this electron can also get so much energy that it also leaves the atom, so double ionization occurs.

Illustrative representation of the double ionization of prepared lithium atoms (a) by vertically polarized UV light (b). The probability of emission of the second electron on the 2p shell depends on whether its orbital (red lobes) is aligned parallel or perpendicular to the light polarization. MPI for nuclear physics

Excitation by an optical laser

Dorn and his colleagues have now excited the 2s electron into a 2p orbital with an optical laser, whereby its spatial orientation can be specifically planned (red lobes). In the second step, an electron was liberated from the 1s shell by irradiation with polarized UV laser pulses. As already evident from the measurement for single ionization, the electron is preferably emitted in the direction of the electric laser field (E) (blue lobes). The researchers now have the choice of aligning the prepared 2p orbital parallel or perpendicular to the laser field, which greatly influences the probability of double ionization - when aligned in parallel, it is increased, in the other case clearly suppressed ckt.

Successful pilot experiment

This effect occurs only at very low energies of the emitted electrons, that is close to the energy threshold for double ionization; he disappears at higher energies. Right at the threshold, the two electrons are completely correlated, they have to tune their energy and angle exactly to escape both from the potential well of the ion: they prefer to escape in exactly opposite directions - what at parallel alignment preferably works.

According to the researchers, this pilot experiment demonstrates that the correlated state change of several electrons in bound systems, here in the double ionization of atoms, is completely controlled by laser preparation and, at suitably chosen conditions, practically can be switched on and off. This newly developed method not only provides far-reaching insights into quantum-dynamic electron correlation in atomic systems, but researchers are hoping for such correlated effects in other quantum systems.

(idw - Max Planck Institute for Nuclear Physics, 24.09.2009 - DLO)