Dipole term and first derivative at K=O of the generalized oscillator strength of He by keV electron impact

An approximate method is described for obtaining the derivative to K2 of the generalized oscillator strength for keV electron scattering at zero momentum transfer, over a large range of energy losses. The measured data enables one to reduce the systematical uncertainty in the derivation of optical oscillator strengths to below the 1% level. Results are presented for He over the spectral range of 19 to 65 eV. The data for the derivative are in satisfactory agreement with earlier electron scattering results at lower impact energy and extend over a sufficient range to allow the application of a sum rule for this term of the generalized oscillator strength.