Photodissociation studies of transition metal oxide cluster cations
Molek, Karen Virginia Sinclair
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Transition-metal oxide cation clusters, MnO+ m (M = V, Nb, Ta, Cr, Fe), are produced in a molecular beam using laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. The mass spectrum for each metal exhibits a limited number of stoichiometries for each value of n, where m ≥ n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic of a Nd:YAG laser. All of these clusters require multiphoton conditions for dissociation, consistent with their expected strong bonding. Dissociation occurs by either elimination of oxygen or 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 tantalum lose O atoms. For each metal increment, n, oxygen elimination proceeds until a terminal stoichiometry is reached. Clusters having this stoichiometry do not eliminate more oxygen, but rather undergo fission, producing smaller MnO+ m species. The smaller clusters produced as fission products represent the corresponding terminal stoichiometries for those smaller n values. This behavior suggests that these clusters have stable bonding networks at their core, but additional excess oxygen at their periphery. Chromium and iron also shows a strong 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 produced repeatedly in the decomposition of larger clusters. These combined results determine that M2O+ 4 , M3O+ 7 , M4O+ 9 , M5O+ 12, M6O+ 14, and M7O+ 17 have the greatest stability for V, Nb, and Ta oxide clusters. The Cr clusters determined to have the greatest relative stability are Cr2O+ 4 , Cr3O+ 6 , Cr3O+ 7 , Cr4O+ 9 and Cr4O+ 10 and the most stable iron clusters are n = m clusters of FenO+ m, where n=2-13. The most stable cation clusters have been calculated using density functional theory to be ring or cage structures comprised on M-O-M-O networks. The inferred neutral clusters eliminated are also noteworthy as their stoichiometries are found in the corresponding bulk materials. In addition our data implies that the vanadium group and iron oxide clusters exhibit oxidation states which are commonly found in the corresponding bulk oxides. The vanadium group clusters imply oxidation states of +4 and +5 whereas the iron clusters suggest the commonly found +2 and +3 states. In contrast, the most stable chromium clusters suggest +4 and +5 oxidation states which are not commonly found in the corresponding solid oxide materials. These results have provided insights into the similarities and differences between the oxide clusters and their corresponding nanoparticle and bulk oxides which are useful for nanomaterial isolation experiments.