Benchmark energies of negative ion states of argon

The excitation energy of the and negative ion states of argon, observed in elastic electron scattering have been determined to be 11.1030 and 11.2763 eV, respectively, with an uncertainty of less than 1 meV. This precision has been achieved by accurately fitting, with a minimum of assumptions, a concurrently measured near-threshold electron-impact excitation cross section of the metastable state of argon, which is of known excitation energy. These measurements provide benchmark energies for theoretical comparison and experimental calibration.