Complexity in the ionization dynamic of Helium dimers

Helium dimers (He$_2$) are a Van-der-Waals bound system and the most extreme quantum matter in AMO physics with a binding energy below 0.1 $\mu$eV (1.1 mK). Its internuclear distance varies from 2 to several hundreds Angstrom (larger than a C60 fullerene). Their existence was predicted theoretically by Slater in 1928 and the experimental prove followed 1994 by Sch\"{o}llkopf in a diffraction experiment. Single and double charged projectiles with energies of 25 - 150 keV/u were used to fragment the Helium dimer into two He$^+$-ions. Using the COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) imaging technique we measured the three dimensional momentum vector of all fragments (He$^+$-ions and emitted electrons). We found different fragmentation/decay mechanisms, one involving the inter atomic coulombic decay (ICD).