Negative ions of molecular hydrogen. The long-standing puzzle of how a hydrogen molecule can hold on to an extra electron for so long has finally been experimentally resolved. The negative molecular ion H2 is the simplest of all molecular anions. It’s unstable, but the details of its unstable states have been the subject of much confusion. They matter beyond the confines of molecular theory because an H2 collision complex is thought to be relevant to the formation of the first generation of stars. A clear signal of metastable H2 with a lifetime of microseconds was found in a 2005 mass-spectrometer experiment. Theory, however, predicted that no H2 state could survive longer than femtoseconds unless the separation between the protons was much bigger than the 0.7-Å spacing of the neutral H2 ground state. Now for the first time, the details of the...

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