Effect of sequential oxidation on the electronic structure of tungsten clusters

Using photoelectron spectroscopy and first principles molecular orbital calculations, we report the first observation of an abrupt change in the electronic structure of W₄O_m^- (m≤6) clusters at m=5 which is well below the bulk stoichiometric composition of tungsten oxide (WO₃). The signature of this onset is established from an anomalous increase in the vertical detachment energies (VDE), adiabatic electron affinities (AEA), and the energy gaps between highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO). These changes are also accompanied by the cleavage of 50% of the W-W bonds and localization of electron charge density in W₄O₅ cluster.