We present a study of the production of metastable negative ions by cesium sputter sources. We report measurements of relatively intense (∼0.5 nA) beams of ${\mathrm{N}}_2^{\mathrm{-}}$ and ${\mathrm{CO}}^{\mathrm{-}},$ two ions with extremely low (<-2.3 eV) electron affinities, and show that they form metastable negative ions with comparatively long lifetimes. Surprisingly, the current of ${\mathrm{N}}_2^{\mathrm{-}}$ is comparable in intensity to ${\mathrm{N}}_3^{\mathrm{-}},$ a molecular negative ion with a binding energy of 2.8 eV. The existence of metastable ${\mathrm{N}}_2^{\mathrm{-}}$ has a twofold significance. First, ${\mathrm{N}}_2^{\mathrm{-}}$ may play an important role in atmospheric chemistry. Second, ${\mathrm{N}}_2^{\mathrm{-}}$ is an excellent test of calculations on many-body systems involving electron-correlation effects. We verified the identification of the ions discussed in this paper using the techniques of accelerator mass spectrometry and Coulomb-explosion imaging. Using an in-beam decay technique, we determined lower bounds for the lifetimes of the most stable states of ${\mathrm{N}}_2^{\mathrm{-}},$ ${\mathrm{CO}}^{\mathrm{-}},$ and ${\mathrm{CO}}_2^{\mathrm{-}}:$ $180\pm 30,$ $140\pm 25,$ and $170\pm 30\mu \mathrm{s},$ respectively. We also report intensities of ${\mathrm{Be}}^{\mathrm{-}}$ and beryllium cluster negative ions, ${\mathrm{Be}}_n^{-},$ for $n=2\mathrm{}–6.$

• Received 15 October 1996

DOI:https://doi.org/10.1103/PhysRevA.60.3786