Electron excitation cross sections for the 3s²3p^{2 3}P → 3s3p^{3 5}S⁰ transition in S²⁺

Experimental and theoretical collisional excitation cross sections are reported for the 3s²3p² 3P → 3s3p^{3 5}S⁰ spin-forbidden transition in S²⁺. The transition energies (wavelengths) are 7.171 eV (1728.9 Å, J = 2 → 2) and 7.237 eV (1713.1 Å, J = 2 → 1). In the experimental approach, use is made of the electron energy-loss method with merged electron and ion beams. The metastable fraction in the S²⁺ beam is assessed and minimized. A novel "electronic aperture" is used to limit the trajectories of elastically scattered electrons into the trochoidal electrostatic analyzer. Theoretically, the R-matrix method in a 17 state and a 27 state calculation is used to obtain cross sections from threshold to 15.0 eV. Good agreement is found between experiment and the results of the 27 state theory. The presence of two strong resonances near threshold (7.5-9.0 eV) and a third resonance above threshold (11.5 eV) is verified by experiment.