Electron impact excitation of the 1 1S→2 3S transition in helium: differential cross-section determinations at high incident kinetic energies

Differential cross sections have been determined to within plus-or-minus8% accuracy for the 1 1S→2 3S transition in helium at 200, 300, 400, and 500 eV incident electron energies and scattering angles out to 35degree. Relative measurements were made absolute by both direct and indirect normalization to highly precise elastic differential cross sections. Serious deviations from the predictions of first-order plane wave theory are found throughout the range of experimental parameters employed in the present work. Further consideration suggests that such discrepancies persist to incident energies as high as 1000 eV raising serious doubts about the validity of the Born--Oppenheimer approximation