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 investigated the vertical
and adiabatic ionization energies of four different metal-oxide clusters by
means of density functional theory. They included clusters of magnesia
(MgO)$_n$, silicon monoxide (SiO)$_n$, alumina (Al$_2$O$_3$)$_n$, and titania
(TiO$_2$)$_n$ with stoichiometric sizes of $n$=1$-$8. The magnesia, alumina,
and titania clusters showed relatively little variation in their ionization
energies with respect to the cluster size n: 7.1$-$8.2 eV for (MgO)$_n$,
8.9$-$10.0 eV for (Al$_2$O$_3$)$_n$, and 9.3$-$10.5 eV for (TiO$_2$)$_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.
