Today laser pulses with electric fields comparable to or higher than the electrostatic forces binding valence electrons in atoms and molecules have become routine. They are standard tools with applications in laser acceleration of electrons and ions, generation of short wavelength emission from plasmas and clusters, laser fusion, etc. Intense fields are also naturally created during laser filamentation in the air or due to local field enhancements in the vicinity of metal nanoparticles.
Fig. 1: (a) Binding potential well without (dashed line) and with (solid line) external superatomic field. (b) In the superatomic field the electron becomes almost free and its motion is dominated by the oscillation in the field with amplitude α0=E2/ω2, where E and ω are the laser field strength and frequency, respectively. (c) Sketch of the KH potential. (d) Angle and energy-resolved photoelectron spectrum of potassium interacting with 800 nm, 1.4·1013 W/cm2, 65 fs laser pulse (pz and pρ are electron momenta along and perpendicular to the laser polarization). (e) Comparison of ab initio photoelectron spectrum (black solid line) and spectrum resulting from ionization of the KH atom initially prepared in the states KH 4s (red dashed line) and KH 4p (blue dashed line) for electrons ejected along laser field.Most prominent lines are assigned based on symmetries of KH bound states and net number of absorbed photons. (A strong signal coming from the 3d KH state is due to the 2-photon population from the initial 4s KH state during turn-on of the KH harmonics)
One would expect that very intense laser fields would always lead to fast ionization of atoms or molecules. However, recently the experimental team at the MBI observed acceleration of neutral atoms at the rate of 1015 m/s2 when exposing these atoms to very intense infrared (IR) laser pulses . Thus, substantial fraction of atoms remained stable during the pulse. What is the structure of these exotic laser-dressed atoms surviving superatomic fields? Can it be directly imaged using modern experimental tools?
Using ab-initio calculations for the potassium atom, we have shown  how the electronic structure of these stable “laser-dressed” atoms can be unambiguously identified and imaged in the angle resolved photoelectron spectra obtained with standard femtosecond laser pulses and velocity map imaging techniques, see e.g. recent experiments [3,4]. We find that the electronic structure of these atoms follows the theoretical predictions made over 40 years ago by W. Henneberger , that have so far remained unconfirmed experimentally and thus not generally accepted. We have also shown that the so-called Kramers-Henneberger (KH) atom is formed and can be detected even before the onset of the stabilization regime. Our findings open the way to visualizing and controlling bound electron dynamics in strong laser fields and reexamining its role in various strong field processes, including microscopic description of high-order Kerr non linearities and their role in laser filamentation .
Interview in Phys.org (English)
Interview in Verbundjournal 88/2011, p. 20, (German)