TESTS OF BORN APPROXIMATIONS - DIFFERENTIAL AND TOTAL 23S 21P AND 21S CROSS SECTIONS FOR EXCITATION OF HE BY 100- TO 400-EV ELECTRONS

The angular dependence of 2S3, 2P1, and 2S1 excitation of He for incident electron energies from 100 to 225 eV (2S3) and 400 eV (2P1 and 2S1) has been measured. Apparent generalized oscillator strengths f(K) and differential cross sections for the transition 1S1→2P1 are obtained by normalizing to Schiff and Pekeris's optical oscillator strength. From the experimental intensity ratios 2S32P1 and 2S12P1 we then calculate differential 1S1→2S3 and 1S1→2S1 cross sections. The differential cross sections are integrated to get total cross sections. The f(K) found here for 2P1 and 2S1 excitation decrease faster with increasing momentum transfer K than in earlier studies. Departures from the Born approximation appear only below 200 eV for 2P1 excitation, but occur at higher energy and are larger (especially for large momentum transfers) for 2S1 excitation. The angular dependence found for 2S3 excitation disagrees strongly with the Ochkur (Bonham) approximation. Our total 2S3 cross sections are much lower than all other existing theoretical and experimental data. © 1968 The American Physical Society.