Ionization by strong laser fields: Understanding the 'Ionization Surprise'

Ionization of atoms by strong laser fields plays an important role in today´s ultrafast laser laboratories. It is at the basis of important techniques such as high-harmonic generation, which allows the generation of attosecond (1 as = 10^-18 s) laser pulses, and furthermore allows the development of tomographic methods that make it possible to observe ultrafast electronic and atomic movements on the attosecond to few femtosecond (1 fs = 10^-15 s) timescale. Theoretical methods for describing strong laser field ionization were already developed a few decades ago. They are commonly based on the so-called "strong-field approximation" (SFA), which argues that after ionization the motion of the ionized electrons is largely determined by the electric field of the ionizing laser, and hardly by the Coulomb force that the electron and the ion left behind exert on each other.

For several decades the strong-field approximation has served scientists well and has allowed them to understand many observations that were experimentally made in connection with the strong field laser ionization. That is to say, until now. In a remarkable paper last year, scientists from the US and Germany reported the observation of a new phenomenon in strong-field laser ionization, namely a very pronounced peak at low energies in the photoelectron kinetic energy distribution, that contained as many as 50% of the emitted photoelectrons. Remarkably, its physical origin could not be identified.

In the new paper, the Rostock, Heidelberg and MBI scientists argue that it is the failure to include the Coulomb attraction between the departing electron and the ion left behind that is at the root of the low energy feature. They developed a novel theoretical description of strong-field ionization process, which in its initial stages mimics the traditional SFA approach, but then switches to calculating trajectories that the electrons follow in the combined Coulomb + laser field. This approach convincingly reproduces the low energy feature, and shows that it is caused by electrons that are pushed back-and-forth by the oscillatory laser field. In this process the electrons are brought into close proximity to the ion, which strongly disturbs the electron orbit, leading to a situation where the electron can just barely escape the attraction of the ion.

The ´Coulomb-corrected´ SFA formalism based on interfering quantum orbits described above, not only solves the mystery of the "Ionization Surprise" but was also instrumental in related work on the appearance of holographic structures in strong-field ionization, which appears in the journal Science in December.