Quantitative Theory and Accurate Experiments: Low-energy Electron Scattering by He and H2 as Case Studies

Accurate low-energy electron scattering data are needed in many fields of physics. However, accurate experiments are difficult to design and to carry out. By 1967 low-energy electron–He cross sections had been measured by two different techniques, designed to provide accurate data. Unfortunately, the data differed by amounts well outside the estimated error bars. Despite the relative simplicity of the He atom, decisive theoretical calculations on the e–He system could not be done with methods available in 1967. After a decade of development of theoretical methodology it became possible in 1979 to carry out calculations with absolute estimates of residual error limits. The results were found to agree closely with the momentum transfer cross section deduced from electron swarm data and with recent beam data by improved techniques, but were inconsistent with the original beam data of 1965. More recently, a similar conflict exists between data measured for electron-impact vibrational excitation of the hydrogen molecule by electron swarm and beam techniques. This conflict has persisted despite great progress in beam scattering techniques and in theoretical methods. A brief review of the relevant electron scattering theory will be given.