Near-threshold electron-impact excitation of the 3$^3$S and 3$^1$S states of Helium.

A joint experimental and theoretical study of near-threshold electron-impact excitation of the 3$^3$S and 3$^1$S states in helium is reported. A high-resolution electron spectrometer was used to study integral cross sections in the energy region of the $n=3-5$ negative-ion resonances. Photons were detected from the decay of the two states and the observed intensities were normalised to theoretical predictions from a new $B$-spline $R$-matrix (close-coupling) method, which allows for non-orthogonal orbitals to improve the target description. Remarkable agreement between experiment and theory is demonstrated in both the overall energy dependence of the cross section and the fine details of a wealth of resonance structures. A detailed list of the resonances in the individual partial waves, along with their widths, will be presented.