/de/research/projects/2.2/topics/Topic 1/EAA/neutrals/FTI/index.htm
2.2 Strong-field Few-body Physics
Project coordinator(s): H. Rottke, F. Morales
Frustrated tunnel ionization
Excitation of atoms in the tunnel regime- the frustrated tunnel ionization mechanism (FTI)

Excitation of atoms is typically associated with the absorption of photons and consequently, described in terms of a (multi) photon picture. We measure excited neutrals atoms after strong field interaction at intensities well within the tunnel regime. Using a direct detection technique we measure excited neutral He yields and He+ yields and determine their ratio to be about 5-20%. An extension of the famous three step model of strong field dynamics including Coulomb interaction between the ionic core and the tunnel electron shows that the electron gets recaptured if it does not gain enough drift energy to escape the ionic core potential after the laser is over. That is true particularly for those electrons starting near the maximum of an optical field cycle, where the field enveloppe is also maximum and for a limited range of the lateral momentum. This what we call „frustrated tunnel ionization (FTI)“ leads to excitation of the neutral atom. Classical Monte Carlo simulations predict the n state (and also l state) distribution of the final states. The n distribution can be compared with ab initio fully quantum mechanical calculations showing striking agreement. Further experimental evidence for this mechanism to be in effect is given by a polarization dependent ion and excited neutral yield measurement for He. (Nubbemeyer et al. PRL 2008)

As main result one can state that the FTI mechanism describing excitation in the tunnel regime completes the three step model. The FTI process should be very general and as is shown in the following has also been observed in the strong field fragmentation of small molecules and should also been observable in ionic atoms.

see also