Kinetic Energy Release in the Dissociation of Some Simple Molecular Ions. Water and Hydrogen Sulfide

The technique of ion kinetic energy spectroscopy has been applied to a study of S+. formation from H2S+. and O+. formation from H2O+.. Unimolecular formation of S+. occurs by predissociation of the first excited state via the repulsive 4A2 state of H2S+.. Above the classical crossover region this reaction proceeds rapidly on the mass spectrometer time scale but tunneling through the barrier occurs slowly and gives rise to metastable ions which fragment with conversion of all the available potential energy to kinetic energy. Collisional excitation of ground state H2S+. yields excited ions which rapidly dissociate via the 4A2 repulsive surface to give substantially excited (v=2) H2. This reaction occurs with the partitioning of some 30% of the available energy into translational energy of the products. The heat of formation of S+., determined from the appearance potential, requires only slight correction for the excess energy term arising from the potential energy difference between the crossover region and the ground state of the products because the repulsive surface is unusually flat. Ground state H2O+. ions undergo collision-induced excitation with loss of 22± 4 eV of kinetic energy to give a high energy excited state which fragments directly to give O+. (2D or 2P) and vibrationally excited H2. Appearance potential measurements do not provide a reliable assignment of the products because of the excess energy terms. These results are in contrast to earlier conclusions that fragmentation upon electron impact yields two hydrogen atoms.