By measuring the momentum vectors of all heavy fragment of a molecule in coincidence with one or more photoelectron or Auger electron COLTRIMS allows to study the ionization process in the molecular frame of reference. The fragmentation process of the remaining molecular ion is often rapid enough, that the molecule has no time to rotate before fragmentation. Then the molecular orientation can be inferred from the fragment direction for each ionization event.

We have used this technique to study double photo ionization of D2 and of K-shell ionization of several small molecules (CO, N₂, C₂H₂, C₂H₄, C₂H₆). The experiments are performed by the COLTRIMS collaboration @ ALS.