MEASUREMENTS OF THE ELECTRON-IMPACT DIFFERENTIAL CROSS-SECTIONS AND GENERALIZED OSCILLATOR-STRENGTHS FOR EXCITATION OF THE 2-S-1 AND 3-S-1 STATES IN HELIUM AT SMALL SCATTERING ANGLES

Differential cross sections (DCS's) for excitation of the 2 S-1 and 3 S-1 states in helium have been measured at small scattering angles fro m O degrees to 15 degrees for the electron-impact energies from 100 to 500 eV. Measurements were performed with a high angular resolution, b etter than 1 degrees, and with an accuracy of the angle position of 0. 2 degrees. Distinct forward peaking features have been observed at min ute scattering angles lower than about 3 degrees for impact energies h igher than 200 eV. Experimental DCS's as functions of the scattering a ngle are compared with theoretical calculations based on various kinds of approximation method. Effective generalized oscillator strengths ( GOS's) for the 2 S-1 and 3 S-1 excitations were deduced from the DCS's for impact energies 100-500 eV as a function of the squared momentum transfer K-2. A systematic discrepancy between the measured effective GOS's and the theoretical GOS calculated by the first Born approximati on has been displayed at the low limit of K-2. The effective GOS's app ear to approach the theoretical GOS very slowly as the impact energy i ncreases, however, the GOS for low values of K-2 Still does not agree with the theoretical GOS up to the impact energy 800 eV for the 2 S-1 excitation.