Mass-spectrometric study of the formation, evaporation, and structural properties of doubly charged MgO clusters

Doubly charged (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}^{2+}$ (n=4–172) and (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}_2^{2+}$ (n=12–24) clusters were produced in a gas-aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. The mass spectrum exhibits an interesting dependence on the source conditions, such that it can be changed from one comprised of singly charged clusters to one comprised almost entirely of doubly charged clusters, simply by changing the flow rate of the carrier gas. The abundance of patterns of (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}^{2+}$ clusters shows that the clusters have cubic structures resembling pieces of the MgO fcc crystal lattice, with the most stable structures being cuboids, and cuboids with an O-atom vacancy or complete terrace. The abundances of (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}_2^{2+}$ clusters can also be explained in terms of cubic structures, but in this case the O-atom vacancies may be occupied by one or two excess electrons, analogous to a solid-state F center. Simple ionic-model calculations of the structures and relative stabilities of (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}^{2+}$ clusters are in good agreement with those inferred from the mass spectra. The (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}^{2+}$ clusters evaporate primarily by losing magnesium and oxygen in equal amounts, similar to the congruent vaporization of solid MgO, yielding clusters of the same series. The mechanism of evaporation is probably a stepwise one in which the movement of atoms or molecules from corners, kinks, and ledges into an adsorbed layer exerts at least partial control over the evaporation rate. At small sizes the doubly charged clusters also appear to undergo Coulomb explosion, in which they fragment to form (MgO${)}_{\mathit{n}}^{+}$ and (MgO${)}_{\mathit{n}}$ ${\mathrm{Mg}}^{+}$ clusters.