FORMATION AND FRAGMENTATION OF GAS-PHASE TITANIUM OXYGEN CLUSTER POSITIVE-IONS

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 [TinO2n-δ]+, where n equals 1 to 8 and 8 ranges from 0 to 4. Sputtering titanium dioxide powder yields abundant [TinO2n-δ]+ clusters, where n equals 1 to 7 and δ=1,2, and 3. Titanium foil sputtered in a high-pressure (0.1-0.2 Torr) fast-atom bombardment (HPFAB) ion source with O2 produces [TinO2n-1]+ cluster ions as the only abundant cluster species. The collision-induced dissociation spectra of [nO2n-δ]+ clusters show mainly losses of Ti, TiO, and TiO2 moieties. Mass-selected [TinO2n-δ]+ cluster ions also react hyperthermically with O2 in the collision quadrupole of a tandem mass spectrometer to form [TinO2n-1]+ clusters. A simple pair-potential ionic model was used to calculate energy-optimized geometric structures of the clusters. Distinct structural differences between the TinO2n-2 and TinO2n-1 clusters were not found, whereas the lowest energy TinO2n 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. © 1990, American Chemical Society. All rights reserved.