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.
The Stark field approximation has served scientists well over the years and has contributed to the understanding of many experimental observations of ionization with strong laser fields. But only until now! In a noteworthy publication, in 2009, US and German scientists reported a new phenomenon in high-field ionization: they observed a pronounced signal peak in the kinetic energy distribution of the photoelectrons at very low energies, involving up to 50 percent of the emitted electrons. Remarkably, the physical cause could not be determined.
In the new publication, the scientists from Rostock, Berlin and Heidelberg now show that the low electron energies are caused by the Coulomb attraction between the flying electron and the remaining ion. They have developed a novel theoretical description of the strong field ionization process, which corresponds to the old Starkfeld approximation at the beginning of the ionization process, but later calculates the orbit of the electron in a combined Coulomb and laser field. This approach was able to convincingly reflect the low photoelectron energies and show that they are caused by the oscillation of the electron in the oscillating laser field. In this process, the electrons are brought into the vicinity of the ion, whereby the electron trajectories are significantly disturbed. As a result, the electrons only barely escape the attraction of the ion.
The Coulomb-corrected SFA formalism, based on the interference of quantum orbits described above, not only solved the mystery of the "ionization surprise," but was also helpful in a related work on the occurrence of holographic structures in the strong-field ionization Week in Science Express appears.