Electron scattering at a threshold above a resonance: helium

Excitation of the lowest triplet state (2 3 S) near threshold arises from interactions of short range, because of the electron-exchange needed to bring about the change of target spin. The short-range interaction is described by the simplest possible R-matrix which reproduces the 2 S resonance observed about 0.5 eV below threshold (i.e. a simple pole term without a background). Only s-waves are considered. The observed singularity ('cusp') in the elastic differential cross section, and the observed nearly vertical onset of the inelastic cross section at threshold are accounted for if one includes the effect of polarization of the atom on a slow outgoing electron which has been scattered inelastically.