Photodissociation of Iron Oxide Cluster Cations

Abstract

Iron oxide cluster cations, Fe(n)O(m)(+), are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. The mass spectrum exhibits a limited number of stoichiometries for each value of n, where m > or = n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic of a Nd:YAG laser. At either wavelength, multiple photon absorption is required to dissociate these clusters, which is consistent with their expected strong bonding. Cluster dissociation occurs via elimination of molecular oxygen, or by fission processes producing stable cation species. For clusters with n < 6, oxygen elimination proceeds until a terminal stoichiometry of n = m is reached. Clusters with this 1:1 stoichiometry do not eliminate oxygen, but rather undergo fission, producing smaller (FeO)n(+) species. The decomposition of larger clusters produces a variety of product cations, but those with the 1:1 stoichiometry are always the most prominent and these same species are produced repeatedly from different parent ions. These combined results establish that species of the form (FeO)n(+) have the greatest stability throughout these small iron oxide clusters.