Threshold effects in positron scattering from isoelectronic series of atoms and molecules

The observation of features, often referred to as "Wigner cusps" in a scattering cross section at the opening of a new scattering channel has been widely reported in electron scattering, and has been attributed to strong coupling between the two scattering channels. Recently a series of cusps were observed in positron scattering from the noble gases [A. C. L. Jones, Phys. Rev. Lett. 105, 073201 (2010)10.1103/PhysRevLett.105.073201]. In this case, the strong opening of the positronium formation cross section resulted in a cusplike feature, or bump, in the total elastic scattering cross section. The positronium formation channel is typically the largest partial scattering cross section in the region of the electronic excitation and ionization thresholds in most atoms and molecules, and it is plausible that similar features may exist in other atomic and molecular species. We have undertaken measurements of the isoelectronic targets of helium - H2, and neon - H2O, NH3, and CH4, in an effort to better understand the mechanisms leading to these threshold features.