How molecules and clusters explode

The multiple ionization and fragmentation of small molecules, e.g. N ₂ and H ₂O, by fast H ⁺, He ⁺, Ar ^q+ and O ^q+ ions was studied utilizing a position- and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, a kinematically complete image of the molecular break-up can be derived for each individual event. Of special interest are the "Coulomb explosion" processes like N ₂ → N ^p+ + N ^q+ or H ₂O → H ⁺ + H ⁺ + O ^q+. In these cases the pure Coulomb explosion model — though successful in case of H ₂ — turns out to insufficient to explain the measured energy and angular spectra. In case of H ₂O better agreement is achieved with ab initio MCSCF calculations of the intermediate H ₂O ^{( q+2)+} parent ion.