Photodissociation of Manganese Oxide Cluster Cations

Manganese oxide cluster cations are produced by laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. The mass spectrum contains intense peaks for stoichiometries corresponding to (MnO) _n⁺. Multiphoton photodissociation of these clusters yields smaller ions with the same stoichiometric ratio, either by sequential elimination of MnO units or by various fission processes with roughly equal efficiencies. Fragmentation of clusters containing excess oxygen also yields (MnO) _n⁺ fragments. These apparently stable fragments are investigated further using density functional theory to determine their likely structures. The lowest energy structure for Mn₂O₂⁺ is found to be a planar ring, and those for Mn₄O₄⁺ and Mn₆O₆⁺ are cuboids. Mn₃O₃⁺ is predicted to have a six-membered ring structure and Mn₅O₅⁺ has a fused cube/ring configuration similar to the structure of the oxygen evolving center of Photosystem II. Open-shell, high-spin configurations on individual manganese atoms couple antiferromagnetically and ferromagnetically to produce low-spin and high-spin configurations on different sized clusters.