Frequency Metrology on single trapped ions in the weak binding limit:   The 3s1/2-3p3/2 transition in 24-Mg+
  We demonstrate a method for precision spectroscopy on trapped ions in the limit of unresolved motional sidebands. By sympathetic cooling of a chain of crystallized ions we suppress adverse temperature variations induced by the spectroscopy laser that usually lead to a distorted line profle and obtain a Voigt profile with negligible distortions. We applied the method to measure the absolute frequency of the astrophysically relevant D2 transition in single 24-Mg+ ions and find 1072082934.33(16)MHz, a nearly 400fold improvement over previous results. Further, we find the excited state lifetime to be 3.84(10) ns. 
M. Herrmann, V. Batteiger, S. Kn\"unz, G. Saathoff, Th. Udem, T.W.   H\"ansch