A measurement of the cross section for electron impact ionisation of singly charged tungsten ions

Abstract

The crossed ion and electron beams technique has been used to make an absolute measurement of the cross section for the ionisation process e+W+ to 2e+W2+ at energies ranging from below threshold up to 750 eV. The compounded random and systematic errors are estimated to be less than about 5% at energies in excess of 18 eV. The W+ ions are probably mainly in the 6D ground state but with a significant fraction in the 4D metastable state. Other metastable ions may account for a few per cent of the total. Comparison of the present data with the scaled Born prediction of McGuire taking into account both 6s outer electrons and 5d inner-shell electrons shows quite good agreement above about 50 eV, but the theory seriously underestimates the cross section at lower energies. The measured ionisation threshold for the ground-state ion is 16.1+or-1.0 eV, compared with the 17.7 eV given by Finkelnburg and Humbach (1955) from a study of screening constants. There are no other known measurements of the ionisation threshold.