Observation and the Origin of Magic Compositions of ConOm- Formed in Oxidation of Cobalt Cluster Anions

To obtain atomistic insights into the early stage of the oxidation process of free cobalt cluster anions Co-n(-), the reaction of Co-n(-) (n <= 10) with varied pressure of O-2 was studied experimentally and theoretically. Population analysis of the oxidation products ConOm- as a function of m revealed two types of magic compositions: the population decreases abruptly upon addition of a single O atom to and removal of a single O atom from the magic compositions. Magic compositions of the former type were further divided into oxygen-rich (n:m similar to 3:4) and oxygen-poor (n:m similar to 1:1) series. The oxygen-rich compositions most likely correspond to fully oxidized states, since the compositions are comparable to those of Co3O4 in the bulk. Their appearance is ascribed to the significant reduction of binding energies of O atoms to fully oxidized clusters. In Contrast, oxygen-poor compositions correspond to the intermediates of the full oxidation states in which only the surface is oxidized on the basis of theoretical prediction that oxidation proceeds by bonding O atoms sequentially on the surface of Co-n(-) while retaining its morphology. Their appearance is ascribed to the kinetic bottleneck against internal oxidation owing to significant structural change of the Co-n(-) moiety. In contrast, magic compositions of the latter type are associated with the abrupt increase of survival probability as anionic states during the relaxation of internally hot Co oxide clusters based on the m-dependent behaviors of adiabatic electron affinities determined by photoelectron spectroscopy.