Today, laser pulses with electric fields comparable to or higher than the electrostatic forces that bind valence electrons in atoms or molecules have become routine tools with numerous applications. These include the laser acceleration of electrons and ions, the generation of short-wave emission in plasmas or clusters, laser fusion, and many more. Intense fields are also generated during laser filamentation in the air or by local field enhancement near metal nanoparticles. First, one would expect that these strong fields always lead to rapid ionization of the atoms or molecules. Recently, however, a group of MBI experimenters observed the acceleration of neutral atoms of up to 1015 m / s2 during the interaction of these atoms with very intense infrared laser pulses . Thus, a substantial proportion of the atoms remained stable during the pulse. The question then arises of the structure of these exotic laser-dressed atoms that survive superatomic fields. Can this be imaged directly with modern experimental methods? Using ab-initio calculations for the potassium atom,  we show how the electronic structure of these "laser-dressed" atoms can be uniquely identified and mapped in angle-resolved photoelectron spectra using standard femtosecond laser pulses and velocity-map-imaging methods (see eg current experiments [3,4]). We find that the electronic structure of these atoms corresponds to the theoretical predictions formulated by W. Henneberger more than 40 years ago , which until now have remained experimentally unconfirmed and thus not universally accepted. We also show that the so-called Kramers-Henneberger (KH) atom forms, and can even be detected before the onset of the stabilization region. Our results open the possibility of visualizing and controlling the dynamics of bound electrons in strong laser fields and examining their influence on various strong-field effects, including the microscopic description of high-order Kerr nonlinearities and their role in laser filamentation .
Visualization of the Kramers-Henneberger atom
Fig. 1 Direct visualization of the exotic Kramers-Henneberger atom in a photoelectron spectrum. Angular and energy resolved photoelectron spectrum of potassium with a 800 nm, 1.4 · 1013 W / cm2 and 65 fs laser pulse. (pz and pρ are the electron pulses along and perpendicular to the laser polarization axis.)
1. Eichmann et al., “Acceleration of neutral atoms in strong short-pulse laser fields”, Nature, 461, 1261-1264 (2009).
2. Felipe Morales, Maria Richter, Serguei Patchkovskii and Olga Smirnova,
“Imaging the Kramers-Henneberger atom”, PNAS, doi:10.1073/pnas.1105916108.
3. Wollenhaupt M., Krug M., Köhler J., Bayer T., Sarpe-Tudoran C. & Baumert T., “Photoelectron angular distributions from strong-field coherent electronic excitation”, Appl. Phys. B, 95, 245 (2009).
4. Schuricke M., Zhu G., Steinmann J., Simeonidis K., Ivanov I., Kheifets A., Grum-Grzhimailo A. N., Bartschat K., Dorn A. & Ullrich J., “ Strong-field ionization of lithium”, Phys. Rev. A, 83, 023413 (2011).
5. W. Henneberger, “Perturbation method for atoms in intense laser ﬁelds”, Phys. Rev. Lett., 21, 838 (1968).
6. Béjot et al., “Higher-Order Kerr Terms Allow Ionization-Free Filamentation in Gases”, Phys. Rev. Lett., 104, 103903 (2010).
Dr. Felipe Morales Moreno, (030) 6392 1358, Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
Dr. Maria Richter, (030) 6392 1239, Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
Prof. Dr. Olga Smirnova, (030) 6392 1340, Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy