Measurement of the cross sections for electron-impact excitation of the holmium atom

The cross sections for the excitation of 39 spectral lines of the holmium atom in the spectral interval 300--700 nm have been measured by the method of intersecting electron and atomic beams. Optical excitation functions have been measured for electron energies from the threshold to 250 eV. The largest excitation cross sections, up to 2.7 x 10-16 cm2, are found for two transitions with wavelengths of 405.4 and 410.4 nm. Most of the optical excitation functions which were found increase near the threshold and have broad, rounded maxima at an electron energy of 18--20 eV, but a group of lines in the spectral region 530--630 nm has sharp maxima at approx.6 eV. Fourteen transitions, 12 of which turn out to be resonant transitions, are identified on the basis of the measured excitation thresholds