Electron scattering on ${\mathrm{NO}}_2^{-}$, ${\mathrm{NO}}_2^{-}·({\mathrm{H}}_2\mathrm{O})$, and ${\mathrm{NO}}_2^{-}·({\mathrm{H}}_2\mathrm{O}{)}_2$ was performed in two storage rings. We confirm the presence of earlier reported ${\mathrm{NO}}_2^{2-}$ dianion resonances and show that they remain when water is attached. Importantly, hydration tunes the energy: each water molecule lowers the ground state energy by $0.8\pm 0.3\mathrm{eV}$ relative to the monoanion. ${\mathrm{NO}}_2^{2-}$ is observed to decay by two-electron emission, possibly in combination with fragmentation. ${\mathrm{NO}}_2^{2-}·({\mathrm{H}}_2\mathrm{O})$ mainly decays into ${\mathrm{NO}}_2^{-}+{\mathrm{H}}_2\mathrm{O}+e^{-}$.

• Received 12 November 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.223401