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

摘要

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 from O degrees to 15 degrees for the electron-impact energies from 100 to 500 eV. Measurements were performed with a high angular resolution, better than 1 degrees, and with an accuracy of the angle position of 0.2 degrees. Distinct forward peaking features have been observed at minute scattering angles lower than about 3 degrees for impact energies higher than 200 eV. Experimental DCS's as functions of the scattering angle 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 approximation has been displayed at the low limit of K-2. The effective GOS's appear to approach the theoretical GOS very slowly as the impact energy increases, 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. [References: 36]