Although the Zeeman effect of the Th I, Th II and Th III spectra had been investigated by Lier (39.1), the preliminary analysis of Th II was performed by MacNally et al. (42.1) from extended Zeeman data obtained by using the M.I.T. Bitter electromagnet at high fields. In this first work, the system of transitions 6d²7s + 6d³ + 6d7s² - 5f6d7s + 6d²7p + 6d7s7p and a second system built on 5f7s² and 5f6d7s involved 219 levels and 1091 classified lines from 814 to 215 nm but could not be connected to the first one. In a further step, MacNally observed the infrared spectrum (1123 - 866 nm) and determined the energy of 5f7s² 2F_5/2 (4432.29 cm^-1) relative to the ground level 5d²7s ⁴F_3/2 (45.1).

An independent series of investigations at the Zeeman laboratory of Amsterdam led to similar conclusions and the finding of additional levels (43.1; 44.1; 46.3). Ten levels were later reported by Charles (58.1). The first extensive parametric study of the Th II configurations in several mixed groups led Minski (69.3) to discover 28 levels in the vicinity of predicted energies.

The comprehensive work of Zalubas and Corliss (74.9) was based on new observations of about 14000 Th II lines from 200 to 2500 nm and includes revised energies, 108 new levels and a list of 6500 classified lines.

The thorium spectrum emitted by a hollow cathode was remeasured by Palmer and Engleman by means of Fourier transform spectrometry (83.9) and the energy levels are now accurate to about 0.003 cm^-1 (83.9; unp.2). A systematic study of isotope shifts DT(232-230) from enriched ²³⁰Th I₄, samples in electrodeless discharge tubes was used to derive isotope shift values for 184 even- and 167 odd levels relative to the ground state (84.3). These accurate values (rounded-off to the nearest 0.001 cm^-1) are quoted here with a few additional results (unp.18) and replace earlier measurements of (53.1; 58.1; 60.5).

The hyperfine constants of a few low lying levels of ²²⁹Th have been derived from the analysis of a hollow cathode spectrum recorded by Fourier transform spectrometry (74.6).