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

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+ → 2e + W2+ at energies ranging from below threshold up to 750 eV. The W+ ions are probably mainly in the 6D ground state but with a significant fraction in the 4D metastable state. Comparison of the present data with the scaled Born prediction of earlier workers 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 +- 1.0 eV, compared with the 17.7 eV from a previous study of screening constants. (author)