We study the strong-field ionization dynamics of nanometer-sized clusters, which are fundamentally different from the strong-field ionization dynamics of atoms or small molecules. While tunnel ionization can also occur in clusters, much more efficient ionization processes take place once pairs of free electrons and holes are generated. We have developed an ionization ignition scheme, in which an intense HHG pulse generates electron-hole pairs, and further ionization takes place by a below-threshold NIR laser pulse via avalanching processes, leading to very high (up to 15+) ionization degrees of the atoms in the cluster. The ionization ignition method provides unprecedented temporal and spatial control over strong-field processes in dielectric clusters and solids. In the future, the development of an intense attosecond source will allow us to control these processes with attosecond time resolution.
For small clusters, consisting of 2 – 4 atoms or molecules, which are weakly bound by polarization forces and have large separations between their atomic/molecular constituents, we are investigating the effect of the cluster structure on strong field ionization in the near and mid infrared spectral range. The structure affects photoelectron momentum distributions through interference phenomena which can be traced back to the large separation between the cluster constituents. Besides interference phenomena, charge transfer phenomena within the cluster, initiated by strong field ionization, influence its breakup after photoionization. This breakup is investigated through multiple ionization of the small cluster followed by Coulomb explosion with the charged fragments and their momenta allowing to reconstruct the charge transfer. The investigations are done using Cold Target Ion Momentum Spectroscopy (COLTRIMS), which allows a kinematically complete analysis of all charged final products, ions and photoelectrons.
Another area of interest in this topic is the study of conical intersections and light-induced conical intersections (LICIs). Conical intersections play a profound role in the relaxation dynamics of polyatomic molecules, and as such are strongly implicated in the photo-protection of key bio-molecules such as DNA. Conical intersections do no exist in diatomic molecules, but are created when diatomic molecules are placed in an external field. Dynamics around both conical intersections and LICIs will be theoretically and experimentally studied.
Fig. 1: Nano-fireworks in an argon nanoparticle are ignited by a moderately intense and invisible XUV laser pulse. A subsequent visible laser pulse heats the nanoparticle very efficiently, resulting in its explosion. Electrons and ions move in different directions and send out fluorescence light in various colors. Without the XUV pulse the nanoparticle would remain intact.