QUANTITATIVE THEORY AND ACCURATE EXPERIMENTS - LOW-ENERGY-ELECTRON SCATTERING BY HE AND H-2 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 dat a. Unfortunately, the data differed by amounts well outside the estima ted error bars. Despite the relative simplicity of the He atom, decisi ve theoretical calculations on the e-He system could not be done with methods available in 1967. After a decade of development of theoretica l methodology it became possible in 1979 to carry out calculations wit h 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 recen tly, a similar conflict exists between data measured for electron-impa ct vibrational excitation of the hydrogen molecule by electron swarm a nd beam techniques. This conflict has persisted despite great progress in beam scattering techniques and in theoretical methods. A brief rev iew of the relevant electron scattering theory will be given.