B-spline Breit–Pauli R-matrix calculations for electron collisions with argon atoms

We have applied the Breit–Pauli B-spline R-matrix method described earlier (J. Phys. B: At. Mol. Opt. Phys. 37 (2004) 2173) to the treatment of e–Ar collisions. An individually optimized, term-dependent set of non-orthogonal valence orbitals is used to account for the strong term dependence in the one-electron orbitals. Concentrating on the near-threshold regime for excitation of the 3p54s manifold, we obtain excellent agreement with benchmark experimental data for the production of metastable atoms and for excitation of the VUV-emitting J = 1 levels. A detailed partial-wave resolved resonance analysis shows that many of the structures listed by Buckman and Clark (Rev. Mod. Phys. 66 (1994) 539) are composed of overlapping resonances. These, together with their principal components and decay channels, are identified and, in some cases, alternative classifications are suggested.