The influence of hyperfine structure on the determination of the ionisation potential of Tl I

New high-resolution spectra of Tl I are reported in the wavelength range 2100-2000 AA using the first order of a 3 m 6000 line/mm holographic grating. Synchrotron radiation from the 500 MeV accelerator in Bonn provided the background source of continuum. The observations comprise 6s 2 6p 2 P 1/2 to 6s 2 ns 2 S 1/2 (12<n<48) and 6s 2 nd 2 D 3/2 (12<n<70) spectra extending to much higher values of n than previous work. The authors have resolved the hyperfine structure of the ground state which is shown to have a significant effect on the determination of the ionisation potential.