Mass spectrometric observation of the stable negative molecular ions HI− and H2I−

Using a 100 in. radius double focusing mass spectrometer we have detected the negative molecular ions HI− and H2I− produced in a Pennig ionization discharge source. The ions are identified by an absolute precision mass measurement, and their flight time through our machine is ≊10−4 sec, indicating the ions are stable with respect to autodetachment. This observation determines the electron affinity of HI to be positive and ≳0.007 eV.