Origin of O $1\mathit{s}$ Core-Level Shifts on Oxygen Adsorbed $\mathrm{Si}\left(111\right)-(7\times 7)$

Density functional calculations are used to examine the chemical and structural origin of O $1s$ core-level shifts measured on the initial oxidation stage of $\mathrm{Si}\left(111\right)-(7\times 7)$. Our analysis of metastable core-level peaks leads to a conclusive identification of the long-sought metastable oxidation species as a tetrahedral ${\mathrm{SiO}}_4$ unit, formed by two successive ${\mathrm{O}}_2$ adsorptions on a Si adatom. The origin of a higher-binding core-level shoulder is clarified by the presence of a threefold-coordinated subsurface O atom, introduced as a decay product of the metastable ${\mathrm{SiO}}_4$ unit. The present study provides a detailed atomic-scale picture of the initial oxidation process of $\mathrm{Si}\left(111\right)-(7\times 7)$.