Benchmark comparisons for near-threshold electron-impact excitation of Ar metastable levels

Near-threshold electron-impact excitation of the metastable levels (3p 5 4s) 3 P 0 and (3p 5 4s) 3 P 2 of Ar is calculated in a 41-state semi-relativistic R -matrix (close-coupling) approach and the negative-ion resonance structure in the cross sections is analysed. For the first time, angle-differential excitation cross sections of the Ar metastable states at low energies are compared with the available experimental data. Drastic changes in the shape of the excitation functions with scattering angle reflect the contributions of different partial-wave symmetries and negative-ion resonances to the cross section.