Benchmark energies of negative ion states of argon

The excitation energy of the a1 and a2 (3p)5(4s)22 P3/2,1/2) 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 (3p)54s 3 P2 metastable state of argon, which is of known excitation energy. These measurements provide benchmark energies for theoretical comparison and experimental calibration. (Author)