Photodissociation of vanadium, niobium, and tantalum oxide cluster cations

Transition-metal oxide clusters of the form MnOm+(M=V,Nb,Ta) are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. Consistent with earlier work, cluster oxides for each value of n produce only a limited number of stoichiometries, where m>n. The cluster cations are mass selected and photodissociated using the second (532nm) or third (355nm) harmonic of a Nd:YAG (yttrium aluminum garnet) 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, repeatedly producing clusters having the same specific stoichiometries. In oxygen elimination, vanadium 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,...