Time-of-flight mass spectrometry was used to investigate negative Mn-O clusters produced by 532 um laser vaporization of MnCO3 solid pellet. Five series MnxOy, x = 1-33, y - x= 0,1,2,3,4 cluster ions were observed. For cluster ion MnxOy with x >5, the mean valences of Mn atom in cluster MnxOy, ions is 2.4±0.1, and is almost unchangeable with the size of cluster from x=5-25. The relative intensity of the observed peaks with the same x but different y, can be well fitted by a binomial distribution, for x= 5-12 clusters. All the observed information indicate a gas-phase MnO, MnO2 molecule aggregation mechanism for the production of these oxygen-rich cluster ions.
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