Positive ions of titanium/oxygen clusters have been investigated with mass spectrometry and collision-induced dissociation. The cluster ions were produced by sputtering titanium dioxide powder and titanium foil (exposed to oxygen) with an energetic (8 keV) xenon atom beam. Titanium/oxygen cluster ions sputtered from the foil were seen to have the form [Ti"0M]+, where n equals 1 to 8 and 8 ranges from 0 to 4. Sputtering titanium dioxide powder yields abundant [ , ^]"1" clusters, where n equals 1 to 7 and <5=1,2, and 3. Titanium foil sputtered in a high-pressure (0.1-0.2 Torr) fast-atom bombardment (HPFAB) ion source with 02 produces [Ti"02"_,]+ cluster ions as the only abundant cluster species. The collision-induced dissociation spectra of [Ti"02^]+ clusters show mainly losses of Ti, TiO, and Ti02 moieties. Mass-selected [Ti"02^]+ cluster ions also react hyperthermically with 02 in the collision quadrupole of a tandem mass spectrometer to form [TÍ"02,h]+ clusters. A simple pair-potential ionic model was used to calculate energy-optimized geometric structures of the clusters. Distinct structural differences between the Ti"02n_2 and Ti"02lH clusters were not found, whereas the lowest energy Ti"02" isomers were found to have pendant and terminal oxygen atoms. For both titanium/oxygen and cobalt/oxygen clusters, the pair-potential model identifies the most abundant clusters as those having the lowest calculated energy per atom.
Ion/molecule reaction products of cobalt cluster ions have been characterized using mass spectrometric techniques. Atomic and bare-metal cluster ions were desorbed from foils by particle bombardment within a high-pressure (0.1–0.2 Torr) ion source. Sputtered metal cluster ions react with O2 to produce abundant stoichiometric or nearly stoichiometric cobalt(II) oxide cluster ions. The positive cluster product ions consist of three types: oxygen-deficient [Co(CoO)x]+ clusters, oxygen-equivalent [(CoO)x]+ clusters, and (in less abundance) metal-deficient [(CoO)xO]+ clusters. Tandem mass spectrometry and collision spectroscopy provide structural information about the more abundant cobalt cluster product ions. A major collision-induced fragmentation pathway for the oxygen-equivalent [(CoO)x]+ clusters is the loss of a CoO moiety to form [(CoO)x−1]+ fragments. A major collision-induced fragmentation pathway for the oxygen-deficient [Co(CoO)x]+ clusters is the loss of a cobalt atom to yield [(CoO)x]+ fragments. Geometric structures of the cobalt/oxygen cluster ions were calculated using a Coulomb plus Born–Mayer pair-potential model. The oxygen-equivalent cluster structures were found to be ‘‘globular’’ cages, rings, or ladders. The oxygen-deficient cluster structures were found to be strained and ‘‘angular’’ with protruding cobalt atoms. The structures are discussed in terms of the observed collision-induced fragmentations. The fragmentations are rationalized using an ‘‘instantaneous’’ dissociation model of the collision-induced dissociation of the cluster ions. Preliminary trajectory calculations using classical dynamics support the use of this instantaneous dissociation model. The role of cluster ion structure in reactivity and collision-induced dissociation is discussed in terms of the experimental data and theoretical structures.
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