Description
Today laser pulses with electric fields comparable to or
higher than the electrostatic forces binding valence
electrons in atoms and molecules have become a routine tool
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. One would expect that very intense fields
would always lead to fast ionization of atoms or molecules.
However, recently observed acceleration of neutral atoms [1]
at the rate of 1015 m/sec2 when exposed to very intense
infrared (IR) laser pulses demonstrated that a substantial
fraction of atoms remained stable during the pulse. What is
the structure of these exotic laser-dressed atoms surviving
super-atomic fields? Can it be directly imaged using modern
experimental tools? Using ab-initio calculations for the
potassium atom, we show [2] how the electronic structure of
these stable "laser-dressed" atoms can be unambiguously
identified and imaged in 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 [5], that have so far remained unconfirmed
experimentally and thus not generally accepted. We also show
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 for
visualizing and controlling bound electron dynamics in
strong laser fields and reexamining its role in various
strong field processes, including the microscopic
description of high order Kerr non-linearities and their
role in laser filamentation [6].
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, accepted.
3. M. Wollenhaupt, M. Krug, J. Köhler, T. Bayer, C.
Sarpe-Tudoran and T. Baumert, “Photoelectron angular
distributions from strong-field coherent electronic
excitation”, Appl. Phys. B, 95, 245 (2009).
4. M. Schuricke, G. Zhu, J. Steinmann, K. Simeonidis, 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).
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).
