Photodissociation of Cerium Oxide Nanocluster Cations

Cerium oxide cluster cations, CexOy+, are produced via laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. The mass spectrum displays a strongly preferred oxide stoichiometry for each cluster with a specific number of metal atoms x, with x <= y. Specifically, the most prominent clusters correspond to the formula CeO(CeO2)(n)(+). The cluster cations are mass selected and photodissociated with a Nd:YAG laser at either 532 or 355 nm. The prominent clusters dissociate to produce smaller species also having a similar CeO(CeO2)(n)(+), formula, always with apparent leaving groups of (CeO2). The production of CeO(CeO2)(n)(+) from the dissociation of many cluster sizes establishes the relative stability of these clusters. Furthermore, the consistent loss of neutral CeO2 shows that the smallest neutral clusters adopt the same oxidation state (IV) as the most common form of bulk cerium oxide. Clusters with higher oxygen content than the CeO(CeO2)(n)(+) masses are present with much lower abundance. These species dissociate by the loss of O-2, leaving surviving clusters with the CeO(CeO2)(n)(+) formula. Density functional theory calculations on these clusters suggest structures composed of stable CeO(CeO2)(n)(+), cores with excess oxygen bound to the surface as a superoxide unit (O-2(-)).