Fragmentation of Small Multi-Electron Molecular Ions in Cold Electron Collisions

The fragmentation of the small multi-electron molecular ions HF+, DF+ and D3O+ was investigated in slow electron collisions leading to dissociative recombination. Momentum spectroscopy of the collision products from stored beams of each ion species merged with an intense cold electron beam has been performed at the ion-storage ring TSR, Heidelberg, utilizing two different imaging detectors. For the first time, a rotationally and fine-structure resolved kinetic energy release spectrum of the DR process was measured. The unique structure of HF+ combined with an optimal control of the experimental parameters allowed to derive the dissociation energies of HF+ and DF+ with a precision sufficient to investigate the Born-Oppenheimer breakdown in the electronic ground states of the two isotopes. The measured kinetic energy release distribution for the different dissociation channels in the D3O++ DR yields information on the internal states of educt and products. In addition, a strongly asymmetric energy partitioning among the deuterium products of the three-body channel OD + D + D is observed for high kinetic energy releases, indicating a sequential decay via an excited water molecule. Including ab-initio calculated adiabatic potential energy surfaces, a mechanism describing the interrelated dissociation into D2O + D and OD + D + D is proposed