Multiple ionisation represents the very first step in intense laser-plasma interaction with (neutral) matter. Its understanding in terms of the fundamental underlying processes is important and has gone far beyond the intuitive sequential field ionisation, but is presently still largely limited to non-relativistic intensities.
In particular, most of the present models  (e.g. the most successful "rescattering model" for multiple ionization; see subproject "ion momentum spectroscopy") are expected to fail in the relativistic regime due to relativistic kinematics of the free electron, and the influence of magnetic forces. On the other hand, the interaction between correlated electron dynamics in tightly bound inner shells and relativistic laser fields is still unexplored. Hence, the dominant ionisation mechanisms in this regime still need to be identified on the basis of new experimental data and theoretical models at relativistic intensities.

Within the project B8 of the Sonderforschungsbereich Transregio18 in collaboration with G.G. Paulus , we aim at the new regime of collective tunnel ionization in few-cycle intense laser pulses. During one optical laser cycle we expect to liberate more than one electron per laser cycle.

Former research had been focussed on:

• The role of rescattering at laser intensities above 10¹⁶W/cm²
• Validity of single particle theories for tunneling in multi-electron systems
  • How fast is the core relaxation between succesive ionization steps?
  • Dependence on the magnetic quantum number?
• Strong field ionization of inner shells by circularly polarized light

Experimental setup

High intensity laser

Typical ion yield spectrum