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

Abstract

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 studied in a pulsed supersonic molecular beam experiment and generated in a conventional laser vaporization cluster source with an additional flow tube reactor. Relative reactivities are determined at near room temperature as a function of oxygen partial pressure, cluster size, and composition. We observed for the first time the controlled formation of at least four stable manganese oxide series: Mn x O −, Mn x O 2 −, Mn x O 3 −, and Mn x O 4 − ( x = 2–12). The reactivities decrease rapidly with cluster size reaching a minimum around Mn 4 − and Mn 5 −. They increase smoothly with further increasing cluster size and show no significant pressure dependence for clusters of x > 5. The adsorption of molecular oxygen on the cluster surface most probably takes place in a dissociative way resulting in the formation of distinct Mn–O bonds. The Mn 5 − cluster exhibits a special behavior. It has the highest intensity in mass spectra and shows the lowest reactivity towards oxygen. Another interesting feature is the evolution of an intense Mn 2O − peak with increasing oxygen pressure.