Abstract
The Doppler-free spectroscopy of atomic thallium 6P1/2 ! 7S1/2 transitions have been observed by centering the Lamb dip using two counter-propagating laser beams perpendicular to the atomic beam, and the absolute frequencies of these transitions have been measured to an accuracy of 0.4 MHz (1 ppb) using the optical femtosecond comb. The hyperfine splittings and isotope shifts are derived from our results, which are in agree with the previous experimental results, and improved by a factor of 2–3.