/de/research/projects/2.2/topics/Topic 2/noble gas dimers/index.htm
2.2 Strong-field Few-body Physics
Project coordinator(s): H. Rottke, F. Morales
Subproject
"Strong field ionization of noble gas dimers"

A. von Veltheim, M. Baggash, H. Rottke

General aspects

An atom or molecule exposed to a high intensity laser pulse becomes excited or eventually photoionized even if the energy of the photons is much less than the ionization potential and the first bound excited state of the system. With rising light intensity even multiple ionization is observed and for molecules in addition dissociation into charged fragments. In the long wavelength limit the photoionization mechanism can well be understood in semicalssical terms. Photoionization becomes electric field ionization in the strong, oscillating electric field of the light pulse which easily reaches inneratomic levels at light intensities beyond 1013 W/cm2. Multiple ionization in such a light pulse may tehn not necessarily be a succession of independent electric field ionization steps where one electron after the other is removed. It was found that they may leave the atomic system in a highly correlated way (see our publication).

Experimental access to strong field processes

The appropriate method to get insight into the strong field induced, possibly highly correlated, ionization and, in case of molecules, dissociation dynamics is a complete analysis of the final momenta all charged particles, electrons and ions, have gained after the interaction of the light pulse with the atom or molecule. This gives detailed insight into the mechanisms leading to excitation, multiple ionization and the mutual interaction of the electrons and ions (their correlation) while they leave the atom/molecule. Today, this kind of analysis is usually done with the help of a reaction microscope.

The Momentum Spectrometer (Reaction Microscope)


In the center of the reaction microscope a cold supersonic atomic/molecular beam is intersected by a focused laser beam. Ions and electrons created in the focal spot are extracted by a weak homogenous electric field. At the end of two drift tubes they reach position sensitive detectors. For each particle, electron and ion, all three Cartesian components of the momentum it gained in the interaction with the laser pulse can be reconstructed with high accuracy from its time of flight and the position where it hits the detector. The electric field together with a homogeneous magnetic field for the photoelectrons is able to guide all charged particles from the laser focal spot to the detectors.

Strong field ionization of noble gas dimers

Why are noble gas dimers of interest?

- They constitute diatomic molecules with large internuclear separation (R ~ 3.5 - 4.5 Å)
- The molecule consists of two nearly unperturbed atoms (weak polarization forces form the bond)
- The first ionization potential is nearly equal to that of the atomic contituents

- They practically form a double slit atomic electron emitter (two-center interference of emitted electron waves)

- In the singly charged ion electron charge oscillations can be induced by the applied strong electric field of the laser pulse

- Double ionization (DI) may proceed via different pathways:

  • two-site ionization followed by Coulomb explosion
  • one-site DI followed by charge transfer and Coulomb exposion
  • one-site single ionization with excitation of a second electron at the same site followed by "interatomic Coulombic decay" (ICD, see Averbukh et al., and Jahnke et al.) or autoionization after charge transfer
  • "frustrated" strong field triple ionization (for "frustrated" tunnel ionization see T. Nubbemeyer et al.)

 

Specific experiments with noble gas dimers:

The "double slit", coherent strong field electron emitter
Strong field double ionization of argon dimers