Study of transitions in thulium atoms in the 410 — 420-nm range for laser cooling

The possibility of laser cooling of thulium atoms is considered. The hyperfine structure of almost cyclic 4f¹³6s² (J_g = 7/2) ↔ 4f¹²5d_3/26s² (J_e = 9/2) and 4f¹³6s² (J_g = 7/2) ↔ 4f¹²5d_5/26s² (J_e = 9/2) transitions at 410.6 and 420.4 nm, respectively, is studied by the method of sub-Doppler saturation spectroscopy in counterpropagating laser beams. The hyperfine splitting of excited levels involved in these transitions is measured and the natural linewidths of these transitions are determined. The structure of the neighbouring 4f¹³6s6p (J_e = 5/2) and 4f¹²5d_5/26s² (J_e = 7/2) levels is studied for the first time by this method. The decay probabilities of the J_e = 9/2 levels via channels removing atoms from the cooling cycle are calculated. It is found that the branching ratio for the strong transition at 410.6 nm (A = 6 × 10⁷ s^-1) is smaller than 2 × 10^-5, which makes this transition most promising for laser cooling. The laser cooling of atoms in a Zeeman cooler at this transition is simulated. The possibility of using a laser-cooled cloud of thulium atoms to study the metrological transition at 1.14 μm is discussed.