Electron impact excitation of 11S→21P and 11S→31P of helium: excitation cross sections and polarisation fractions obtained from XUV radiation

The excitation of the 2 1 P and 3 1 P states of helium by electrons has been studied in the energy range from near threshold to 2 keV. This was done by observing the XUV radiation produced at emission angles of 55 degrees C and 90 degrees C to the incident electron beam. Fano plots of the relative data were fitted by straight lines in the high-energy region. The measurements were brought on an absolute scale by normalising the 'relative' slopes of the fits to the slopes predicted by the Bethe approximation. For energies exceeding 350 eV the total cross sections are in agreement with the Bethe approximation. In the lower energy region (30-300 eV) the total cross sections are in good agreement with recent distorted-wave calculations. By combining the results obtained at 55 degrees C and at 90 degrees C to the electron beam, total cross sections from excitation of the magnetic sublevels were obtained. Polarisation fractions were derived from the experimental results in the energy range 30-300 eV.