Photoelectron spectroscopy of sodium chloride anions with two excess electrons

Photoelectron spectroscopy measurements of (NaCl)_nNa^? (n=1–13, except n=10) are reported. The observed electron energy spectra fall into three distinct types, reflecting different correlations between the two excess electrons and ions. Depending on the host clusters' structures, the experimental evidence indicates that the two excess electrons in these sodium chloride clusters could be spin paired, forming a bipolaron or a Na^? anion. The two excess electrons could also be spin parallel, forming a double F-center state.