Accommodation of two excess electrons in sodium chloride cluster anions

We report photoelectron spectra of sodium chloride cluster anions, (NaCl${)}_{\mathit{n}}$ ${\mathrm{Na}}^{\mathrm{-}}$(n=1–21), which contain two excess electrons. We observe three distinct types of spectra, corresponding to three different modes for accommodating the two electrons. Our evidence suggests that, in some clusters, the electrons form spin pairs that either occupy a single anion vacancy as an F’ color center or localize on a ${\mathrm{Na}}^{+}$ cation to form a ${\mathrm{Na}}^{\mathrm{-}}$ anion. In other clusters, the two electrons separately occupy a pair of anion vacancies as a double F color center.