A list of 848 low-excitation spectral lines of Tm III has been obtained by utilizing the sliding-spark light source at a peak current of 6 A. An analysis of these lines yielded 108 energy levels belonging to the configurations 4ƒ<sup>13</sup>, 4ƒ<sup>12</sup>5<i>d</i>, 4ƒ<sup>12</sup>6<i>s</i>, and 4ƒ<sup>12</sup>6<i>p</i>. The classified lines and level values are presented as well as a theoretical interpretation of these configurations.
JACK SUGAR, "Spectrum of Doubly Ionized Thulium (Tm III)," J. Opt. Soc. Am. 60, 454-466 (1970)