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 (Al₂O₃)_n, and titania (TiO₂)_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, from 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.

中文翻译：

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形成粉尘的金属氧化物簇的电离能

恒星尘埃颗粒主要由在富氧 AGB 恒星的星周包层中形成的无机无机物质组成。然而，尘埃合成的初始阶段或其成核尚不清楚。特别是，以分子簇为代表的成核物质的化学性质是不确定的。我们通过密度泛函理论研究了四种不同金属氧化物簇的垂直和绝热电离能。它们包括氧化镁 (MgO) _n、一氧化硅 (SiO) _n、氧化铝 (Al ₂ O ₃ ) _n和二氧化钛 (TiO ₂ ) _{n 的}簇化学计量尺寸为 n = 1–8。氧化镁、氧化铝和二氧化钛团簇的电离能相对于团簇大小 n 的变化相对较小：(MgO) _n为 7.1–8.2 eV，(Al ₂ O ₃ ) _n为 8.9–10.0 eV ，以及 9.3 (TiO ₂ ) _n为 –10.5 eV 。相比之下，(SiO) _n电离能随着尺寸 n 减小，从 n = 1 的 11.5 eV 开始，n = 8 时降低到 6.6 eV。因此，我们将中性金属氧化物簇的稳定性限制设置为通过辐射或高温持续电离，并通过中性-中性反应进行成核。