Measurement of absolute frequencies of transitions in thulium atoms

Clock transition at $1.14 \mu \mathrm{m}$ in neutral thulium is a promising candidate for compact and robust transportable optical clock due to its extremely low sensitivity to external electric and magnetic fields. Cold Tm atoms are now used in optical lattice clocks, quantum simulators and for the study of Bose-Einstein condensation and cold collision physics. In this work, we discuss experimentally measured absolute frequencies of the two transitions at 410.6 nm and 530.7 nm, used for laser cooling of thulium atoms, transition at 418.8 nm used for optical pumping and clock transition at 1140 nm. Using accurate laser spectroscopy in an ensemble of cold thulium atoms, we measured frequencies of these transitions using an optical frequency comb, which was referenced to an active hydrogen maser. Transition frequencies were measured with an uncertainty at one tenth of their natural linewidth, which is sufficient for cooling and pumping of thulium atoms. The uncertainty of the clock transition frequency is 30 Hz, which was limited by the maser frequency calibration with GNSS. Our results can find application in existing experiments with cold thulium atoms and especially in new projects.