A collision cross section study of the 1 1S → 2 1P and 1 1S → 2 1S transitions in helium at kinetic energies from 200–700 eV. Failure of the Born approximation at large momentum changes

TLDR: Differential collision cross sections have been determined for the 1 1S → 2 1S and 1 1 S → 2 P transitions in helium at scattering angles within the range 7.5°−35° and kinetic energies of 200, 300, 400, 500, and 700 eV as discussed by the authors.

Abstract: Differential collision cross sections have been determined for the 1 1S → 2 1S and 1 1S → 2 1P transitions in helium at scattering angles within the range 7.5°−35° and kinetic energies of 200, 300, 400, 500, and 700 eV. Multiple scattering errors have been eliminated by determining the ration of inelastic to elastic cross section as a function of pressure (the dependence is linear) and extrapolating to zero pressure. The error in the absolute cross sections is less than 5% in most instances. Where large corrections for multiple scattering are necessary the error bound is 6%. Generalized oscillator strengths (f) have been calculated from the data for both transitions and compared with the Born approximation calculations (fB) of Kim and Inokuti [Phys. Rev. 175, 176 (1968)]. At a scattering angle of 30° the ratio fB/f (which would be unity if the Born approximation were accurate) decreases from the value 0.86 at 200 eV to 0.16 at 500 eV for the transition 1 1S → 2 1P. At smaller values of the squared momentu...