Principal-quantum-number dependence of coherence and correlation parameters for excitation of helium by electrons

Electron-photon coherence and correlation parameters for electron-impact excitation of the n ${}_{}{}^1{}_{}{}^{}P$ (n=2–8) states of helium at 80 eV are predicted to be essentially independent of the principal quantum number (n), and the available experimental data for n=2 and 3 show (semiquantitatively) the same behavior. The n independence of these parameters predicted by first-order many-body theory can be explained using basic concepts associated with quantum-defect theory and, if verified by further experimental results, has important consequences for understanding the physics of the electron-atom scattering process.