Anomalous behavior in differential cross sections for the 2 3S excitation in helium at minute scattering angles for fast electron collisions

Differential cross sections (DCS) for the 1 1S-2 3S excitation in helium have been measured with a high angular resolution of about 1 degree as a function of the scattering angle θ from 12 degree to 0 degree for the incident energies of 200 and 500 eV. For the incident energy of 500 eV, the DCS increases drastically as the angle decreases in the angular range smaller than 4 degree and reaches a value 0.075 a02/sr at θ=0 degree, the increase being nearly 3 orders of magnitude. The DCS at zero scattering angle for impact energies from 200 to 800 eV have been measured. The DCS's are found to be about 1 order of magnitude greater than those of published measurements by Skerbele, Harshbarger, and Lassettre [J. Chem. Phys. 58, 4285 (1973)] and Klump and Lassettre [J. Chem. Phys. 62, 1838 (1975)]