Frequency metrology on single trapped ions in the weak binding limit: The 3s-3p fine structure doublet in Mg+

Absolute frequency measurements on single trapped ions reached tremendous accuracies by probing narrow transitions in tightly confined ions (resolved sideband regime) [1,2]. A variety of interesting transitions may hardly be studied in this regime, because their transition linewidths well exceed common (secular) trap frequencies. Accurate spectroscopy in the so-called weak binding limit is challenging, since an interrogating laser induces detuning dependent heating and cooling which distorts the line profile. We present both theory and experimental demonstration of a spectroscopic method (Fig. 1) which essentially removes these limitations and allows us to observe a well understood line shape with high signal-to-noise ratio (Fig. 2) [3].