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Abstract
Transition-metal oxide cation clusters, MnO+m (M = V, Nb, Ta, Cr, Fe), are producedin a molecular beam using laser vaporization in a pulsed nozzle cluster source and detectedwith time-of-flight mass spectrometry. The mass spectrum for each metal exhibits a limitednumber of stoichiometries for each value of n, where m n. The cluster cations are massselected and photodissociated using the second (532 nm) or third (355 nm) harmonic of aNd:YAG laser. All of these clusters require multiphoton conditions for dissociation, consistentwith their expected strong bonding. Dissociation occurs by either elimination of oxygenor by fission processes producing stable cation species and/or eliminating stable neutrals,repeatedly producing clusters having the same specific stoichiometries. In oxygen elimination,vanadium, chromium and iron species tend to lose units of O2, whereas niobium and tantalumlose O atoms. For each metal increment, n, oxygen elimination proceeds until a terminalstoichiometry is reached. Clusters having this stoichiometry do not eliminate more oxygen,but rather undergo fission, producing smaller MnO+m species. The smaller clusters producedas fission products represent the corresponding terminal stoichiometries for those smallern values. This behavior suggests that these clusters have stable bonding networks at theircore, but additional excess oxygen at their periphery. Chromium and iron also shows astrong preference for eliminating specific stable neutral clusters such as FeO, CrO3, Cr2O5,or Cr4O10. Specific cation clusters are identified to be stable because they are producedrepeatedly in the decomposition of larger clusters. These combined results determine thatM2O+4 , M3O+7 , M4O+9 , M5O+12, M6O+14, and M7O+17 have the greatest stability for V, Nb, andTa oxide clusters. The Cr clusters determined to have the greatest relative stability areCr2O+4 , Cr3O+6 , Cr3O+7 , Cr4O+9 and Cr4O+10 and the most stable iron clusters are n = mclusters of FenO+m, where n=2-13.The most stable cation clusters have been calculated using density functional theory tobe ring or cage structures comprised on M-O-M-O networks. The inferred neutral clusterseliminated are also noteworthy as their stoichiometries are found in the corresponding bulkmaterials. In addition our data implies that the vanadium group and iron oxide clustersexhibit oxidation states which are commonly found in the corresponding bulk oxides. Thevanadium group clusters imply oxidation states of +4 and +5 whereas the iron clusters suggestthe commonly found +2 and +3 states. In contrast, the most stable chromium clusterssuggest +4 and +5 oxidation states which are not commonly found in the correspondingsolid oxide materials. These results have provided insights into the similarities and differencesbetween the oxide clusters and their corresponding nanoparticle and bulk oxides whichare useful for nanomaterial isolation experiments.