The hyperfine-structure separations of Tl203 and Tl205 in the 72S½ state were obtained from the 3776-A resonance line with a grating spectrograph. These measurements were carried out because previously quoted values resulted from a study of the 5350-A transition ending on the 62P½ state, the hyperfine structure of which was inferred to be 8 mk, while recent atomic-beam magnetic-resonance experiments showed Δv(62P½) ≍ 17 mk. Our results for Δv(72S½) are 405±5 mk and 412±4 mk for Tl203 and Tl205.
R. J. HULL and H. H. STROKE, "Hyperfine Structure of Thallium203 and Thallium205 in the 7 2S½ State," J. Opt. Soc. Am. 49, 1088-1089 (1959)