Differential cross sections for electron-impact excitation of krypton at low incident energies:: I.: Excitation of the 4p55s configuration

The electron-impact excitation of the individual levels that constitute the 4p(5)5s configuration of Kr is experimentally and theoretically investigated at incident electron energies of 20.0, 15.0, 13.5 and 12.0 eV, for scattering angles ranging from 10 degrees to 135 degrees. High resolution electron energy-loss spectroscopy is used to obtain spectral intensities for the excitation of each of the four 4p55s levels from the ground state. The intensities lead to three differential cross section ratios. Absolute electron-impact excitation cross sections are then determined by normalization to elastic scattering cross sections using the conventional inelastic to elastic normalization method. The present theoretical cross sections are calculated using two different methods, namely the R-matrix method and the unitarized first-order many-body theory. Comparisons between the experimental and the theoretical results show some good agreement, but reveal areas where significant improvement of the present models is needed. Additionally, it is shown that in the present case, just as in general for the rare gases, differential cross section ratios provide a sensitive test of theoretical models as well as unique insights concerning relativistic effects in the scattering process. Comparisons with existing models and other experimental data are also presented.