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. Multiple ionization in such a light pulse is usually not a succession of independent electric field ionization steps where one electron after the other is removed. It was found that they leave the atom in a highly correlated way (see our publication).

Experimental access to strong field processes

The appropriate method to investigate the momentum correlation of the photoelectrons and ions (for moleculular excitation) is a complete analysis of the final momenta of all charged particles (electrons and ions) formed 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 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.

The current field of research: Strong field ionization of noble gas dimers

Why are noble gas dimers of interest?

The "double slit" strong field electron emitter

We have investigated the appearance of "double slit" interference in the photoelectron spectra of Ar2 dimers after strong field single ionization. Due to the large internuclear separation R of the dimer (R ~ 3.8 Å) interference appears already in the spectrum of the directly emitted photoelectrons (which did not rescatter on the dimer ion core) having small kinetic energy. The first results can be found in Z. Ansari et al..

Interference in the photoelectron spectrum only appears provided it is principally impossible to localize the atomic constituent of the dimer from which the electon was emitted. In the case of the noble gas dimers this means the ion is formed in a definite "gerade" or "ungerade" state of a charge resonance pair of states. In this case the strong field transition amplitude of the Ar2 dimer also in the strong field ionization limit can be written (at least approximatly) as a product of the atomic transition amplitude to the continuum and an interference term (see Z. Ansari et al.). This allows an extraction of the contribution of interference to the dimer photoelectron spectrum by simply dividing the dimer by the atomic spectrum.

High resolution Ar2 and Ar photoelectron momentum distributions are shown in the figure below. The laser parameters are given in the figure. Distinct differences between the atomic and dimer spectra are foung in the vicinity of zero momentum. This localization of the differences is mainly due to the fact that the dimer axis is not aligned with respect to the direction of polarization of the laser beam.

In order to extact the contribution of two center interference to the dimer spectrum we divide the spectra point wise and extract cuts trough this spectrum along the pz-axis. The result is shown in the next figure for three cuts at different pr.

The smooth lines indicate fits to the experimental interference factor assuming a certain distribution of the ion over different accessible states with definit symmetry with respect to inversion. Part of the momentum dependence of the ratios is well reproduced by the fits. Mainly the "ungerade" fraction of the charge resonance ionic states contributes thus allowing interference in the photoelectron spectrum to appear since in this case an identification of the emission center is in principle impossible. Only a narrow spike in the ratio at zero momentum cannot be attributed to two-slit inteference. This spike can also easily identified in the dimer photoelectron spectrum above.

Present status:

For more details on the role of two-slit interference in the photoelectron spectra of molecules at large internuclear separation ionized by strong laser pulses see Z. Ansari et al..