Excitation of Mg+, Ca+, Sr+ and Ba+ resonance levels in electron-ion collisions

A systematic study of electron excitation of alkaline earth ions at low collision energies (2-100 eV) was carried through for the first time, using the technique of crossed, modulated, charged-particle beams. The cross sections for excitation of the resonance lines and senior terms of the subordinate series was measured for Mg+, Ca+, Sr+, And Ba+ ions, and the polarization of the strong component of the resonance doublet was measured for the last three ions. The cross sections for excitation of the resonance levels of the ions were measured; their values near the threshold were found to be 2.2+-0,4, 2.9+-0.5, 3.2+-0.6, and 4.4+-0.7 in units of 10-15cm2 for Mg+, Ca+, Sr+, and Ba+, respectively. The experimental results are compared with various theoretical calculations of the cross sections for excitation of the resonance levels