Absolute frequency measurements of the D lines in ⁹Be⁺ using a single trapped ion.

Optical frequencies of the D lines in ⁹Be⁺ were measured using single, laser cooled ions stored in a radio-frequency (rf) Paul trap. The spectroscopy laser was stabilized to an optical frequency comb and referenced to UTC(NIST). From these measurements, we extract the ²P fine-structure interval and ²P_1/2 hyperfine constant with results which agree well with theory. The D2 measurement helps to resolve a 7σ discrepancy between previous trap-based measurements and one performed using co-linear laser spectroscopy. Effects relevant to precision measurements of strong transitions in ion traps are discussed, including quantum interference and motional heating due to photon scattering under intermediate confinement in the Lamb-Dicke regime.