Mass-spectrometric study of formation and stability of manganese and manganese oxide cluster anions

We present a mass-spectrometric investigation of the reactions of manganese cluster anions Mn-x(-) (in the size range x = 2-12) with molecular oxygen in the gas phase. The manganese and manganese oxide cluster anions are stud ied in a pulsed supersonic molecular beam experiment and generated in a con ventional laser vaporization cluster source with an additional flow tube re actor. Relative reactivities are determined at near room temperature as a f unction of oxygen partial pressure, cluster size, and composition. We obser ved for the first time the controlled formation of at least four stable man ganese oxide series: MnxO-, MnxO2-, MnxO3-, and MnxO4- (x = 2-12). The reac tivities decrease rapidly with cluster size reaching a minimum around Mn, a nd Mn;. They increase smoothly with further increasing cluster size and sho w no significant pressure dependence for clusters of x > 5. The adsorption of molecular oxygen on the cluster surface most probably takes place in a d issociative way resulting in the formation of distinct Mn-O bonds. The Mn-5 (-) cluster exhibits a special behavior. It has the highest intensity in ma ss spectra and shows the lowest reactivity towards oxygen. Another interest ing feature is the evolution of an intense Mn2O- peak with increasing oxyge n pressure, (Int J Mass Spectrom 185/186/187 (1999) 673-683) (C) 1999 Elsev ier Science B.V.