Atomic ionization in super-strong laser fields
Recent years have witnessed a remarkable progress in high-power short-pulse laser techniques. Modern laser systems provide peak light intensities of the order of 1020 Wcm-2 or above in pulses shorter than 100 fs. The field strength at these intensities is a hundred times the Coulomb field binding the ground state electron in the hydrogen atom. These extreme photon densities allow highly nonlinear multi-photon processes such as above-threshold ionization, high harmonic generation, laser-induced tunneling, Coulomb explosion, multiple ionization and others, where up to a few hundred photons can be absorbed from the laser field.
In parallel with these experimental developments, massive efforts have been undertaken to unveil the precise physical mechanisms behind multi-photon ionization (MPI) and other strong-field ionization phenomena. It was shown convincingly that multiple ionization of atoms by an ultra-short intense laser pulse is a process in which the highly nonlinear interaction between the electrons and the external field is closely interrelated with the few-body correlated dynamics. A theoretical description of such processes requires development of new theoretical methods to simultaneously account for the field nonlinearity and the long-ranged Coulomb interaction between the particles.
In our group, we developed explicitly time-dependent, non-perturbative methods to treat MPI processes in many-electron atoms. These methods are based on numerical solution of the time-dependent Schrödinger equation for a target atom or molecule in the presence of an electromagnetic and/or static electric field. Projecting this solution onto final field-free target states gives us probabilities and cross-sections for various ionization channels.
By using such a procedure, we can obtain quite a detailed information on the ionization process. As an example, we show in the figure the momentum distribution of photoelectrons ejected from the lithium atom by a femtosecond laser in the near infrared spectral range. Comparison with experiment is very convincing.
M. Schuricke, J. Steinmann, Ganjun Zhu, I. Ivanov, A. Kheifets, A. N. Grum-Grzhimailo, K. Bartschat, A. Dorn, and J. Ullrich
Strong Field Ionization of Lithium
Phys. Rev. A 83, 023413, 2011
I. A. Ivanov and A. S. Kheifets
Above-threshold ionization structures in photoelectron momentum distributions for single ionization of He by strong electromagnetic field
Phys. Rev. A 80, 063418, 2009