The spectrum of thorium plasma produced by laser irradiation on a thorium nitrate crystal shows a strong emission between 9.5 and 11.5 nm. It is attributed to overlapping transition arrays 5d-5f and 5d-6p in ion stages of thorium from Th VIII to Th XVI (81.1). No detailed assignments are given.

At longer wavelengths similar experiments led McCormack and O'Sullivan (93.2) to identify ten lines of Th XII, eight of which being transitions to the ground level 5d¹⁰ 7s ²S_1/2. The derived level energies have an uncertainty of about 150 cm^-1.