The ionization energies of dust-forming metal oxide clusters

Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigate the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They include clusters of magnesia (MgO)_n, silicon monoxide (SiO)_n, alumina (Al₂O₃)_n, and titania (TiO₂)_n with stoichiometric sizes of n=1−8. The magnesia, alumina and titania clusters show relatively little variation in their ionization energies with respect to the cluster size n, ranging from 7.1−8.2 eV for (MgO)_n, 8.9−10.0 eV for (Al₂O₃)n, and 9.3−10.5 eV for (TiO₂)_n. In contrast, the (SiO)_n ionization energies decrease with size n, starting from 11.5 eV for n=1, and decreasing to 6.6 eV for n=8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral-neutral reactions