In 2009, the journal Nature Physics called it the "Ionization Surprise". So far, physicists believed that they would understand the ionization of atoms by strong laser fields well. But when they ionized rare-gas atoms with relatively long-wavelength (a few μm) laser light, they were able to observe unexpectedly slow electrons that could not be explained by current theories. In the current issue of Physical Review Letters, scientists from the University of Rostock, the Max Planck Institute for Nuclear Physics in Heidelberg and the Max Born Institute have now explained this observation.
The ionization of atoms by strong laser fields plays an important role in ultrashort pulse laser laboratories today. The process is the basis for important processes such as the generation of high harmonic photons, which in turn enable the production of attosecond laser pulses (1 as = 10-18 s). With the help of this technique, tomographic methods can be developed that allow the observation of ultrafast electron and atomic motions on a time scale from attoseconds to a few femtoseconds (1 fs = 10-15 s). Physicists have been using theoretical methods for decades to describe high-field laser ionization. They are usually based on the so-called "strong field approximation" (SFA). This assumes that after ionization, the motion of the free electron is largely determined by the electric field of the ionizing laser, while the Coulomb force plays little role between the electron and the remaining ion.