Kiel 2011 – scientific programme

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SYCP: Symposium Charged particles in ultra-short fields

SYCP 1: Charged particles in ultra-fast fields

SYCP 1.5: Invited Talk

Thursday, March 31, 2011, 16:30–17:00, HS D

Multiple ionization of atoms by FEL radiation: a time-dependent density functional perspective — •Dieter Bauer — Universität Rostock, Institut für Physik, 18051 Rostock, Germany

One of the most fundamental quantum processes in intense laser-matter interaction is the nonlinear photoeffect where several photons are absorbed by the emitted electron. Multiphoton and tunneling ionization for laser wavelengths around 800nm have been studied since many years. Now, with more and more short-wavelengths free electron lasers (FEL) coming into operation a new parameter regime is increasingly accessible. On one hand the situation at short wavelengths seems to simplify because the ponderomotive energy (typically considered to be a measure for "nonperturbativeness") is negligible. On the other hand FEL experiments showed [1,2] that surprisingly high charge states are also generated in this regime. A purely sequential ionization scenario was shown to underestimate the maximum charge state [3]. Hence, electron correlation or even collective effects must play a role. Moreover, while at long wavelengths the outcome of ionization experiments is largely species-independent and rather scales with the laser parameters only, at short wavelengths the electronic structure of the target system is important [2].

Because the time-dependent Schrödinger equation for atoms in intense laser fields can be solved only for at most two active electrons in full dimensionality alternative methods for strongly-driven many-electron systems are needed. One of the most successful may-particle method in electronic structure theory is density functional theory (DFT). It has been extended for the treatment of time-dependent problems (TDDFT) but is mostly applied in the linear-response regime there [4]. In principle, TDDFT is also applicable to matter in strong fields (nonlinear TDDFT) but benchmark calculations showed that for strongly correlated processes standard exchange correlation potentials are insufficiently accurate and often the density functionals for the observables are unknown in terms of the density or the Kohn-Sham orbitals alone [5-7]. In the talk I will discuss recent progress in the understanding of TDDFT, show comparisons of TDDFT predictions with exact results for highly correlated model systems, and present the outcome of TDDFT simulations of rare gas atoms in strong EUV pulses.

[1] A.A. Sorokin et al., Phys. Rev. Lett. 99, 213002 (2007). [2] M. Richter et al., Phys. Rev. Lett. 102, 163002 (2009). [3] S.V. Popruzhenko, V.D. Mur, V.S. Popov, and D. Bauer, Phys. Rev. Lett. 101, 193003 (2008). [4] M.A.L. Marques et al., Time-Dependent Density Functional Theory (Springer, Heidelberg, 2006). [5] M. Lein and S. Kümmel, Phys. Rev. Lett. 94, 143003 (2005). [6] F. Wilken and D. Bauer, Phys. Rev. Lett. 97, 203001 (2006). [7] M. Ruggenthaler and D. Bauer, Phys. Rev. Lett. 102, 233001 (2009).