Electron-impact excitation at small scattering angles: The Lassettre limitand attendant normalization of measured relative differential cross sections - art. no. 012709

The recent generalized Lassettre expansion (GLE) [Phys. Rev. Lett. 81, 961 (1998)] employing only a single Regge pole is used to demonstrate the appli cability of the Lassettre limit theorem over the entire electron impact ene rgy a without the involvement of the nonphysical region of the apparent gen eralized oscillator strength (AGOS). At forward scattering the GLE yields t he unique long-sought-after normalization cm-ve to the optical oscillator s trength of the measured relative electron differential cross sections (DCS' s) through the AGOS. Optically allowed transitions in H, He, Xe, and N2O ar e used to illustrate the normalization procedure. We conclude that the GLE together with the momentum dispersion method of Haffad et al. [Phys. Rev. L ett 76, 2456 (1996)] now constitute a set of invaluable methods for use in guiding electron DCS measurements, including the identification of incorrec tly normalized and/or spuriously behaved data in the difficult to measure s mall angular regime. It is hoped that this paper will inspire more measurem ents of DCS's at zero-angle scattering, since currently very few such measu rements are available.